Recent Applications and Future Trends of Nanostructured Thin Films-Based Gas Sensors Produced by Magnetron Sputtering

Abstract

The field of gas sensors has been developing for the last year due to the necessity of characterizing compounds and, in particular, volatile organic compounds whose detection can be of special interest in a vast range of applications that extend from clinical evaluation to environmental monitoring. Among all the potential techniques to develop sensors, magnetron sputtering has emerged as one of the most suitable methodologies for the production of large-scale uniform coatings, with high packing density and strong adhesion to the substrate at relatively low substrate temperatures. Furthermore, it presents elevated deposition rates, allows the growth of thin films with high purity, permits a precise control of film thickness, enables the simple manufacturing of sensors with low power consumption and, consequently, low costs involved in the production. This work reviewed all the current applications of gas sensors developed through magnetron sputtering in the field of VOCs assessment by gathering the most relevant scientific works published. A total of 10 compounds were considered for this work. Additionally, 13 other compounds were identified as promising targets and classified as future trends in this field. Overall, this work summarizes the state-of-the-art in the field of gas sensors developed by magnetron sputtering technology, allowing the scientific community to take a step forward in this field and explore new research areas.

1. Introduction

Volatile organic compounds (VOCs) are defined as “… any organic compound …, having, at 293.15 K, a vapor pressure of 0.01 kPa or more, or having a corresponding volatility”, meaning that these compounds exhibit a gaseous state at around room temperature [1,2]. Additionally, there are other compounds that, besides not fitting in the definition abovementioned, exhibit characteristics similar to VOCs and are equally dangerous when present in the air. Ammonia is one of the main examples of these analytes. Due to this characteristic, VOCs and similar compounds have been characterized as one of the primary sources of indoor and outdoor air pollution, as well as major contributors to the development of a large number of pathologies [3,4].

The presence of volatile compounds in both interior and exterior environments is directly related to daily use objects and activities. Personal care products, creams, perfumes, detergents, pesticides, tobacco, food, building materials, paints, furniture, and many other ordinary objects are among the main sources of VOCs [5,6]. Cooking, smoking, and cleaning, in turn, constitute the activities responsible for significative emissions of these types of pollutants. In this way, the environments of both private and public locations, namely houses or personal vehicles, shops, public transportation, schools, hospitals, and many others, are commonly rich in VOCs and other volatile compounds, representing a risk to human health [7,8].

In terms of environmental pollution, the aforementioned compounds are known for their contributions to the well-known “syndrome of sick building”, in which the indoor air composition of closed spaces is often hazardous to people in both short- and long-term exposure scenarios [9,10]. Their utilization in a large number of industrial applications is an example of this syndrome. The indoor air of factories and production lines is often filled with VOCs, creating a hazardous environment for chronically exposed employees [11,12]. Another two examples of locations in which the presence of large concentration levels of VOCs can have significative consequences are hospitals and schools [13,14]. The exposure to these particularly toxic compounds to people undergoing any kind of medical treatment can lead to a worsening of the health condition and affect the outcome of the treatment. In regard to schools, the presence of volatile compounds may represent an unnecessary danger for younger children since their respiratory systems are still under development, and possible health problems at such young ages can be even more critical [15,16].

Due to their volatile feature, VOCs and similar compounds can effortlessly cross biological membranes, such as alveolar or pulmonary tissues, ocular tissues, and even cutaneous tissues. These interactions lead to processes of mutation by oxidative stress in the human cells and, consequently, can cause a large number of health conditions and pathologies [17,18]. These diseases range from simpler conditions like allergies, skin irritation and pruritus, to more complex pathologies like asthma, chronic obstructive pulmonary disease, and a few other inflammatory and respiratory conditions [19,20]. Some VOCs have even been identified as carcinogenic. Benzene, toluene, ethylbenzene, xylenes, and formaldehyde, for example, have been deeply studied regarding their toxicity and direct responsibility in the development of severe forms of lung, oral, breast, and gastric cancers [21,22].

Several methodologies and analytical technologies have been developed and employed in the field of volatile compounds detection, identification, and quantification. Among these, one can highlight chromatographic techniques, like liquid and gas chromatography [23,24], and spectrometric techniques like mass spectrometry [25], infrared spectroscopy [26], and ion mobility spectrometry [27,28]. Despite their wide-reaching utilization, all these techniques exhibit major limitations that, in some situations, bound their utilization in real scenarios, namely the cost of operation, the necessity of complex processes of sample preparation, the lack of libraries for identification of the analytes and of calibration curves for purposes of quantification, the impossibility of performing in situ analyses, and some others [29,30].

Aiming to tackle the aforementioned limitations, the development of gas sensors for the assessment of VOCs and compounds with similar behavior has been rising as a viable and promising solution [31,32]. Various types of sensors have been created by academics, namely optical, electrochemical, acoustic, chemical resistive, and many others. Among these, chemical resistive gas sensors based on conducting metal oxides (MOs) have played a major role due to their sensing capacities, and specifically due to their physicochemical stability, good sensitivity, rapid response time and recovery rate, small dimensions, low detection limits, simple manufacturing, low power consumption and low costs [33,34,35].

Several MOs are used in the production of gas sensors for the detection of gases, such as tungsten trioxide (WO₃) [36], titanium dioxide (TiO₂) [37], nickel oxide (NiO) [38], ferric oxide (Fe₂O₃) [39], copper oxide (CuO) [40], indium oxide (In₂O₃) [41], and tin oxide (SnO₂) [42]. All of them exhibit proved capacities for the detection, identification, and quantification of volatile organic compounds in fields of applications that extend from environmental to health scenarios [43,44]. The most common methods used to prepare metal oxide-based films include the sol–gel process [45], pulsed laser deposition [46], ion beam techniques [47], chemical vapor deposition [48], and DC- or RF- magnetron sputtering [49,50]. Among them the magnetron sputtering method offers several advantages, namely high adhesion of the films, facility of sputtering any metal, alloy or compound, high-purity films, ability to coat heat-sensitive substrates, and excellent uniformity on large-area substrates, which explains the numerous applications of the sputtered films [51,52,53].

Many studies related to this technique deal with the influence of the main deposition–condition parameters like sputtering pressure, applied power, deposition time, substrate temperature, distance sputtering target–substrate, and oxygen partial pressure, which can be tuned to control the structure, composition, and other relevant properties of the MOs films. For this reason, it is crucial to correlate their properties with the deposition parameters in order to engineer the sensors for the desired application [54,55].

Despite not being the scope of this work, the fundamentals of the magnetron sputtering methodology can be briefly outlined. Magnetron sputtering is a sophisticated physical vapor deposition (PVD) technique widely used for producing thin films with precise control over their composition, thickness, and microstructure. The process takes place within a vacuum chamber at low pressure, where a target material (cathode) is bombarded by high-energy ions, typically generated from an inert gas like argon. A magnetic field is applied in the vicinity of the target, which traps the electrons and increases plasma density near the target, enhancing the efficiency of the sputtering process. As the argon ions collide with the target, atoms are ejected from its surface. The sputtered neutral particles, atoms, groups of atoms or even molecules, are propelled through the vacuum chamber in a straight line until they interact with some other particle or a surface. When the interaction with the surface of the substrate occurs, it condenses, and a thin film grows. The use of a magnetic field differentiates magnetron sputtering from traditional sputtering, offering a higher control over the deposition rate and consequently of the films’ properties. Figure 1 depicts the process of thin films deposition through magnetron sputtering [56].

Advanced variations of the process, such as reactive sputtering and high-power impulse magnetron sputtering (HiPIMS), further expand its capabilities, enabling the production of thin films with superior properties for applications ranging from electronics and optics to sensors. Both technologies are advanced techniques for thin film deposition, each with a distinct process operation and applications. Reactive sputtering involves the introduction of a reactive gas, like oxygen or nitrogen, into a sputtering chamber along with an inert gas (normally argon). This reactive gas interacts with the sputtered material (typically a metal) to form a compound, such as an oxide or nitride, which is deposited as a thin film on a substrate. It is commonly used for producing various compound films and requires careful control regarding the gas flow and process parameters to prevent issues like target poisoning, where the reactive gas forms a compound layer on the target surface, reducing the sputtering efficiency [57,58].

HiPIMS, on the other hand, is a more advanced form of sputtering that employs high-power pulses to ionize a large fraction of the sputtered material. These pulses, which are delivered in short bursts, create a highly energetic plasma that significantly boosts the ionization of the target material. This increased ionization allows for better control of the film deposition, leading to denser films with more uniformity and superior adhesion and, consequently, higher mechanical properties. HiPIMS can be used for both simple metallic films or complex coatings, and its ability to produce highly dense films makes it ideal for applications requiring hard and durable layers [59,60].

One of the main differences between the two techniques is the film quality. HiPIMS tends to produce films with higher quality due to the high level of ionization, which results in fewer defects, better adhesion, and higher density. In contrast, while reactive sputtering can also produce compound films with good properties, it requires precise management of reactive gas flow to avoid target poisoning, which can compromise the film quality and the process stability. Moreover, reactive sputtering can achieve relatively higher deposition rates as the process is primarily governed by the sputtering of the target material and its interaction with the reactive gas. In contrast, HiPIMS presents lower deposition rates because a significant portion of the energy is used to ionize the sputtered material, which reduces the overall rate of material that reaches the substrate. This drawback is often acceptable in HiPIMS, where film quality and properties are more important than the rate of deposition. The choice between the two techniques depends on the specific requirements of the application, such as film quality, deposition rate, and material properties [58,60].

Once the production procedure has finalized, the sensors are exposed to the target compounds and the variation of a specific parameter is assessed during that exposure. One of the most common parameters evaluated during the experiment is impedance. The sensors are assembled in the interior of a dedicated vacuum chamber of a system of impedance spectroscopy, and the impedance variance of the thin films during the interaction with the volatile compounds is registered. Not being the scope of this work, detailed information on the working principle of the sensors can be found elsewhere [29,61]; nonetheless, a summarized schematic is included in Figure 2, where the exposure of the gas sensors developed through magnetron sputtering to generic VOCs and the assessment of the impedance variation can be seen.

Considering all the mentioned facts, this work aims to address the most relevant contemporary applications in the field of gas sensors produced by magnetron sputtering for VOCs assessment. To do so, a deep bibliographic search was conducted in order to collect the most significant papers in the field. Main applications, information regarding the production, characteristics, and performance of the sensors of each paper are properly addressed under each one of the main subchapters dedicated to specific VOCs. Finally, future trends in the application of thin films gas sensors produced by magnetron sputtering are equally reviewed.

2. Gas Sensors Applications

As mentioned, due to their sensing capacities and overall qualities, VOCs sensors produced by magnetron sputtering have been gaining relevance in academia. In fact, this growing interest can be observed in Figure 3, in which the number of scientific papers published and indexed in one of the most relevant indexing databases (Web of Science) under the keywords “gas sensors” and “volatile organic compounds” since 2010 is represented. An evident increase in the number of publications per year can be seen, proving the novelty of the field and the relevancy of this review work.

The relevancy of magnetron sputtering-based gas sensors is of special interest to identify the main field of actuation of this technique, the principal compounds currently studied through these types of sensors, its advantages and disadvantages in comparison to other analytical procedures, and to identify areas of interest whose promising results vaticinate a prosperous future concerning the gas sensors’ utilization. This review paper reports on all these topics.

To gather the most relevant documents in the field, a bibliographic search was conducted in the aforementioned database (Web of Science) through the following method: “magnetron sputtering” and “name of the compound”. A total of 10 VOCs were classified as being the most relevant based on 1044 scientific papers. Figure 4 summarizes the ratio of scientific papers per compound.

As mentioned, a total of 10 compounds were identified as being the most relevant in the field. They are acetone, ammonia, butanol, ethanol, formaldehyde, isopropanol, methane, methanol, propane and toluene. For each compound, the most relevant papers were carefully addressed below.

Table 1. Summary of the characteristic parameters for each one of the addressed compounds.

Compound	Formula	CAS	Vapor Pressure	Note
Acetone	C3H6O	67-64-1	24.6 KPa	VOC
Ammonia	NH3	7664-41-7	850 KPa	-
Butanol	C4H10O	71-36-3	0.6 KPa	VOC
Ethanol	C2H6O	64-17-5	5.8 KPa	VOC
Formaldehyde	CH20	30525-89-4	0.5 KPa	VOC
Isopropanol	C3H8O	67-63-0	4.2 KPa	VOC
Methane	CH4	74-82-8	14.1 MPa	VOC
Methanol	CH4O	67-56-1	12.9 KPa	VOC
Propane	C3H8	74-98-6	289.6 KPa	VOC
Toluene	C7H8	108-88-3	2.9 KPa	VOC

summarizes some of the characteristic parameters of the addressed compounds, namely, their chemical formula, chemical abstracts service (CAS) number, vapor pressure, and a note on their fitting in the VOCs definition.

2.1. Acetone

Acetone is one of the most studied VOCs regarding its clinical applications and suitability to act as a relevant compound for purposes of medical diagnosis [62]. In fact, the presence of acetone in biological samples has been linked to many diseases that range from simpler scenarios to more complex conditions like carcinogenic pathologies. Among the studied conditions, one can mention asthma [63], chronic kidney diseases [64], chronic liver diseases [65], cystic fibrosis [66], diabetes [67], malaria [68], sleep apnea [69], colorectal cancer [70], gastric cancer [71], and even lung cancer [72]. Under the environmental field topic, acetone represents a low risk since it is easily dispersed in both air and water; nonetheless, it is known for its hazardousness in scenarios of chronic exposure. Cutaneous and ocular irritation, loss of consciousness, mucosa irritation, and even pulmonary congestion and edema have been reported [73,74].

Since acetone can represent such a preponderant role in both environmental and medical fields, there is an intense demand for systems capable of detecting it accurately. Thin films-based gas sensors developed through magnetron sputtering have emerged as one of the most promising techniques to tackle the current challenges [75]. Sachdeva et al. (2018), for example, developed WO₃-based sensors through RF-reactive magnetron sputtering for later detection of acetone. The sputtering deposition lasted for one hour and the as-sputtered films were annealed at 500 °C. Via this procedure, the authors successfully developed sensors with 100 nm of thickness, with response and recovery times of five and four minutes, respectively, capable of detecting acetone in the range of ppm_v [76]. Tungsten trioxide was also the MO selected by Sucharitakul et al. (2021) for the fabrication of acetone sensors. The films with 500 nm of thickness were deposited using DC-reactive magnetron sputtering with an applied power of 30 W. Once annealed for four hours at 400 °C, the sensors were exposed to acetone at levels of concentration of 100 ppm_v. The authors were able to accurately detect acetone with very interesting response and recovery times of 24 and 27 s [77]. Finally, Drmosh et al. (2021) could equally detect and quantify acetone successfully using tungsten oxide-based sensors. The thin films with 50 nm of thickness were produced by DC-reactive magnetron sputtering with an applied power of 30 W and annealed at 700 °C. Subsequently, these sensors were exposed to acetone with concentrations ranging between 0.5 and 8 ppm_v [78].

Zinc oxide has also been explored in order to access its sensing capacities. Al-Hardan et al. (2013), for example, prepared ZnO-based thin films with 250 nm of thickness via RF-reactive magnetron sputtering with 150 W for purposes of detecting acetone. The ZnO- based sensors, after being annealed at 500 °C for 6 h, were exposed to acetone samples with a concentration of 500 ppm_v, exhibiting recovery and response times of 70 and 95 s, respectively [79]. In an independent study, the same research group also employed ZnO sensors for the detection of acetone samples. Using the same procedure, but changing the applied power to 200 W, the authors increased the detection limits, and were capable of detecting acetone in concentration levels as low as 15 ppm_v [80]. Finally, zinc oxide was also used as a MO to produce thin films-based sensors by Kim et al. (2017). The authors developed the sensors using RF-reactive magnetron sputtering with an applied power of 150 W which, after annealed at 350 °C, were capable of detecting acetone and ethanol in concentrations ranging between 20 and 100 ppm_v, proving the suitability of the procedure [81].

Dyndalt et al. (2020), in turn, developed gas sensors based on thin films of CuO-Ga₂O₃ developed by fully reactive magnetron sputtering (100% O₂). After annealing at 400 °C for four hours, the sensors were able to detect acetone at concentration levels as low as 0.1 ppm_v [82]. All the reviewed works prove the suitability of the magnetron sputtering method for the development of acetone sensors to be applied in a vast range of applications.

Table 2. Summary of the main parameters used in the acetone sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[76]	WO3	RF	-	80/20%	3 mTorr	32/8 ccm	500 °C	1 h	100 nm
[77]	WO3	DC	30 W	68/32%	5 × 10−3 Torr	-/-	400 °C	4 h	500 nm
[78]	WO3	DC	50 W	80/20%	3.7 × 10−3 Torr	70/- sccm	700 °C	2 h	50 nm
[79]	ZnO	RF	150 W	20/80%	2.2 × 10−2 mbar	-/-	500 °C	6 h	250 nm
[80]	ZnO	RF	200 W	20/80%	2 × 10−2 mbar	-/-	500 °C	6 h	250 nm
[81]	ZnO	RF	150 W	-/-	2 mTorr	10/3 sccm	350 °C	-	-
[82]	CuO-Ga2O3	-	100 W	-/-	2 × 10−2 mbar	-/20 sccm	400 °C	4 h	60–70 nm

summarizes the main parameters of each one of the addressed works.

Table 3. Summary of the performance parameters for each acetone sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[76]	260 °C	15–20 ppmv	5 min	4 min
[77]	350 °C	100 ppmv	24 s	27 s
[78]	300 °C	0.5–8 ppmv	45 s	251 s
[79]	400 °C	500 ppmv	70 s	95 s
[80]	400 °C	15–1000 ppmv	-	-
[81]	200 °C	20–100 ppmv	-	-
[82]	-	0.1–1.25 ppmv	319 s	901 s

summarizes the performance parameters of each sensor.

2.2. Ammonia

Ammonia is a well-known pollutant whose behavior is very similar to the one reported to VOCs, despite not fitting the definition, and whose presence in air has been reported as corrosive. In scenarios of human exposure to ammonia vapors, ocular, cutaneous, and respiratory tract irritation have been reported. In cases of extremely elevated concentration levels, exposure can even lead to death [12,83,84]. In parallel, ammonia has been considered as a potential biomarker for the diagnosis and control of chronic kidney diseases [64] and chronic liver diseases [85]. Its presence in biological fluids, like exhaled air or urine, can be indicative of kidney and liver issues and can represent an open window to the condition of these organs in scenarios of the initial stages of disease development [86].

Being aware of ammonia relevance, Boyadzhiev et al. (2010) used the DC- and RF- reactive magnetron sputtering to deposit TiO₂ thin films-based sensors for its assessment. The initial prototype of the sensors proved to be capable of detecting samples of ammonia with 1000 ppm_v of concentration [87]. In a more recent study, the same research group improved the characteristics of the sensors developed by DC- and RF- reactive magnetron sputtering in order to increase their sensitivity, namely, the deposition of thinner films. With these improvements, the authors could achieve concentration levels as low as 50 ppm_v [88].

Besides titanium dioxide, zinc oxide has equally been used in the development of thin films through magnetron sputtering for gas sensing. Vinoth et al. (2018), for example, produced films of ZnO and yttrium-doped ZnO via RF-magnetron sputtering to be used as gas sensors to detect ammonia in concentration levels ranging from 50 to 200 ppm_v. The magnetron sputtering technique proved to be more than appropriate to produce ammonia sensors since they exhibited response and recovery times as low as 100 s. Additionally, the detection of ammonia was achieved at room temperature [89]. The sensors developed through DC-reactive magnetron sputtering by Fairose et al. (2017) could equally detect ammonia at around room temperature (30 °C). Interestingly, the assembled sensors were capable of sensing the target analyte at concentration levels as low as 1 ppm_v and exhibited response and recovery times as low as 5 and 4 s. These auspicious results prove the suitability of zinc oxide sensors developed by magnetron sputtering for purposes of ammonia assessment [90].

Besides the commonly used TiO₂ and ZnO thin films, other oxides have been also explored regarding their sensing capacity. Dhivya et al. (2014) developed cadmium oxide (CdO) thin films via the DC-reactive magnetron sputtering technique for gas sensors. The depositions were carried out for different deposition times in order to obtain films with distinct thicknesses. This study revealed important results since the produced sensors could detect ammonia in concentration levels as low as 50 ppm_v; however, a negative aspect of the sensors was the optimal sensing temperature, which was defined as 150 °C [91]. Tin oxide was selected by Hien et al. (2014) for the development of gas sensors via RF-reactive magnetron sputtering. At an optimal sensing temperature ranging between 50 and 200 °C, the authors could identify and quantify ammonia samples with concentrations of 50 to 200 ppm_v [92]. Yordanov et al. (2014), in turn, proved that molybdenum oxide (MoO₃)-based gas sensors were fully capable of characterizing ammonia. To do so, the authors developed the sensors through RF- and DC-reactive magnetron sputtering and exposed them to previously prepared samples of ammonia with concentrations of 50 ppm_v. The results showed the stability and capacity of the sensors for long-term measurements; nonetheless, the authors failed to provide all the details in regard to the creation and implementation of the sensors, namely, the response and recovery times, the optimal sensing temperatures, or even the sputtering conditions [93]. Finally, hybrid thin films have equally been explored. The work of Ponmudi et al. (2019) exhibited exactly a successful ammonia characterization with sensors developed through RF-magnetron sputtering of Al₂O₃:Cr₂O₃:CuO (1:1:1) thin films. Once annealed at temperatures ranging between 300 and 1000 °C, the sensors were exposed to samples of ammonia with concentrations of 100 ppm_v. Response and recovery times as low as 7 s were observed, proving the capability of the sensors developed via magnetron sputtering to detect ammonia [94].

These works prove the suitability of the magnetron sputtering procedure for the development of ammonia sensors to be applied in several applications.

Table 4. Summary of the main parameters used in the ammonia sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[87]	TiO2	DC and RF	-	-/64%	-/-	-/-	-	2 h	30–120 nm
[88]	TiO2	DC and RF	12.5 W	-/-	-/-	-/-	-	2 h	30–120 nm
[89]	ZnO, Y-ZnO	RF	-	-/-	-/-	-/-	-	45 min	-
[90]	ZnO	DC	2250 W	-/-	6 × 10−2/ 3 × 10−4 mbar	-/-	300 °C	24 h	-
[91]	CdO	DC	25 W	-/-	3.8 × 10−3 mbar	-/-	-	-	294 nm
[92]	SnO2	RF	50 W	97/3%	10 mTorr	-/-	-	-	100 nm
[93]	MoO3	DC and RF	-	-/-	-/-	-/-	-	-	-
[94]	Al2O3: Cr2O3:CuO	RF	300 W	-/-	5.6 × 10−3 mbar	25/- sccm	300–1000 °C	1 h	358–372 nm

summarizes the main parameters used in the ammonia sensors of each reviewed paper.

Table 5. Summary of the performance parameters for each ammonia sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[87]	Room Temperature	1000 ppmv	-	-
[88]	Room Temperature	50–5000 ppmv	-	-
[89]	Room Temperature	50–200 ppmv	102 s	101 s
[90]	Room Temperature	1–100 ppmv	5–50 s	4–60 s
[91]	150 °C	50 ppmv	50–200 s	50–150 s
[92]	50–200 °C	50–200 ppmv	-	-
[93]	-	50 ppmv	-	-
[94]	Room Temperature	100 ppmv	7 s	7 s

summarizes the performance parameters of each sensor.

2.3. Butanol

Cystic fibrosis is a common pathology whose rapid, non-invasive, and accurate diagnosis has been explored through the identification of biomarkers in human fluids. Butanol has risen as the most promising VOC to perform such a task since different concentration levels of this analyte have been reported in the exhaled air, urine, and even feces of cystic fibrosis patients [95]. Concerning the environmental field, butanol is known for its flammable and corrosive nature. In cases of both short- and long-term exposure, anesthetic effects, nausea, headaches, dizziness, and irritation of the respiratory tract have been reported as main consequences [96,97].

Considering its relevancy, a few works have explored magnetron sputtering as a proper technique to develop butanol sensors. Wang et al. (2024), for example, employed this technique to sputter thin films of Co₃O₄@ZnO, which were then exposed to previously prepared samples (100 ppm_v) of several VOCs including butanol. Since the sensors exhibited response times as low as one second, the samples were successfully assessed during long-term analyses, proving the stability and usefulness of this procedure [98].

TiO₂ nanorod arrays were grown on fluor-doped tin oxide (FTO) via a simple hydrothermal method by Wang et al. (2023) to develop thin film-based sensors. Then Au nanoparticles with an average size of ~6 nm were modified on the surface of TiO₂ nanorods (NRs), Au-TiO₂, using magnetron sputtering technology. Once developed, the authors exposed the sensors to known concentrations of butanol (around 100 ppm_v). For an optimal sensing temperature of 300 °C, the sensors exhibited elevated levels of sensitivity, and the authors registered response and recovery times of 12 and 53 s, respectively, when exposed to the target analyte, proving their suitability and the usefulness of the magnetron sputtering technique [99]. Titanium dioxide films were equally used by Ababii et al. (2019) to assemble butanol sensors. In fact, the authors used the DC-magnetron sputtering technique to deposit different noble metal nanoparticles (Au, Ag, bimetallic Ag–Au, Au–Pt) onto annealed TiO₂ thin films produced via the spray pyrolysis technique, which after were exposed to known concentrations of butanol. The sensors proved to be fully capable of assessing butanol among several other VOCs at an optimal sensing temperature ranging between 200 and 350 °C for concentrations around 100 ppm_v [100].

The magnetron sputtering technique was the technique equally used by Wongrat et al. (2023) to explore the impacts of Cu and Ni in the sensing performance of ZnO-based thin films. To do so, the authors sputtered these elements on top of the thin films of zinc oxide prepared by a direct current heating technique and exposed the sensors to known concentrations of butanol (1000 ppm_v). In both scenarios, the sensors proved to be fully capable of detecting the samples at optimal sensing temperatures ranging between 400 and 450 °C and exhibited response and recovery times ranging from 3 to 4 and from 208 to 747 s, respectively [101].

SnO₂ was the oxide employed during the preparation of butanol sensors by Martínez et al. (2023). To develop these sensors, the authors deposited a thin layer of this oxide using DC-magnetron sputtering, and later applied an additional film of graphene oxide (GO) on top of the initial sputtered film using the layer-by-layer (LbL) technique. These hybrid sensors were capable of successfully assessing concentrations of butanol ranging between 335 and 1676 ppm_v, proving their suitability for this goal [102].

Considering all the aforementioned details, the suitability of magnetron sputtering for the development of butanol sensors is evident. In this way, it is expected that this technique will keep being employed and developed to further increase the applicability of these sensors.

Table 6. Summary of the main parameters used in the butanol sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[98]	Co3O4 @ZnO	-	-	-/-	-/-	-/-	500 °C	1 h	1.2–60 nm
[99]	Au-TiO2	-	-	-/-	-/-	-/-	-	-	-
[100]	M-TiO2 M(Au, Ag, Ag-Au, Au-Pt)	DC	40 W	-/-	136 Pa	48/- sccm	450–600 °C	2 h	12–40 nm
[101]	Cu or Ni-ZnO	-	-	-/-	-/-	-/-	400 °C	4 h	115 µm
[102]	GO-SnO2	DC	-	-/-	1.2 × 10−2 mbar	-/-	-	-	-

summarizes the main parameters of each one of the addressed works.

Table 7. Summary of the performance parameters for each butanol sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[98]	300 °C	100 ppmv	1 s	92 s
[99]	240 °C	50–100 ppmv	13 s	53 s
[100]	200–350 °C	100 ppmv	-	-
[101]	400–450 °C	1000 ppmv	3–4 s	208–747 s
[102]	Room Temp.	335–1676 ppmv	27 s	42 s

summarizes the performance parameters of each sensor.

2.4. Ethanol

Being one of the most common VOCs in the atmosphere, ethanol is a target for many distinct scientific studies. Its presence in both indoor and outdoor air can lead to intoxication and asphyxiation scenarios, as well as dizziness, pruritus, and irritation of the skin [103,104]. In parallel, ethanol has been explored regarding its suitability to act as a disease biomarker. In fact, the presence and eventually adulterated concentration levels of ethanol in samples of biological fluids have been linked to chronic kidney diseases [64], chronic liver diseases [65], cystic fibrosis [105], diabetes [67], breast cancer [106], colorectal cancer [70], lung cancer [107], and even squamous cell cancer [108].

Ethanol sensors produced by magnetron sputtering have been playing a relevant role in the assessment of this compound for a vast range of applications. Chen et al. (2011), for example, developed Ag-ZnO thin films-based sensors via RF-reactive magnetron sputtering to detect ethanol at 100 ppm_v. The authors proved that the sensors were fully capable of assessing ethanol, with being 260 °C the ideal operation temperature [109]. Zinc oxide produced through RF-magnetron sputtering was also used by Tamvakos et al. (2015) to produce ethanol sensors that were able to detect the vapors of this analyte in concentration levels ranging between 10 and 50 ppm_v, which is an interesting interval for medical applications [110]. Finally, ethanol at concentration levels ranging between 50 and 300 ppm_v was successfully detected through the sensors developed by Hassan et al. (2014). To do so, the authors developed pure and 5% Fe-doped ZnO thin films-based gas sensors through RF-magnetron sputtering and exposed them to previously prepared samples. The successful determination of ethanol proved the practicality and suitability of this kind of method in the medical and environmental fields [111].

Besides zinc oxide, other MOs have been employed for purposes of ethanol assessment. Khojier et al. (2023) used TiO₂ thin films to prepare ethanol sensors via RF-reactive magnetron sputtering by changing the applied power. In this work, the authors observed that the change in the applied power between 100 and 300 W led to the modification of the crystallographic structure and surface morphology, and thus promoted a significant effect on the ethanol vapor sensing performance of the devices. These sensors were successfully capable of detecting ethanol in concentration levels as low as 10 ppm_v [112]. CuO has equally exhibited suitable and promising sensing capacities to assess ethanol. Yan et al. (2015) developed sensors based on CuO nanoparticles fabricated via direct thermo-oxidation of sputtered Cu films and then exposed them to previously prepared samples of ethanol. At an optimal operating temperature of 200 °C, the developed sensors were able to detect the target compound in a concentration ranging from 20 to 500 ppm_v and with response and recovery times as low as 4 s [113]. One last MO whose results in the field of ethanol sensing seem to be promising is indium tin oxide (ITO). Pandya et al. (2011) developed ITO-based sensors through RF-magnetron sputtering using distinct applied powers (200 and 300 W) and employed them in the detection of ethanol. The authors successfully detected this volatile compound at 200 ppm_v of concentration and with a response time of 180 s [114].

All the reviewed works demonstrate the suitability of the magnetron sputtering procedure for the development of ethanol sensors whose applications extend from medical to environmental fields.

Table 8. Summary of the main parameters used in the ethanol sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[109]	Ag-ZnO	RF	300 W	-/-	1 Pa	6/20 sccm	-	-	50 nm
[110]	ZnO	RF	70–150 W	-/-	-	-/-	400 °C	1 h	100–200 nm
[111]	ZnO	RF	100 W	-/-	3 × 10−2 mbar	-/-	500 °C	2 h	-
[112]	Fe-TiO2	RF	100–300 W	-/-	-	10/10 sccm	600 °C	-	200 nm
[113]	CuO	-	-	-/-	-/-	-/-	400 °C	3 h	1 µm
[114]	ITO	RF	200–300 W	-/-	20 mTorr	-/-	450 °C	1 h	100 nm

summarizes the main parameters of each one of the addressed works.

Table 9. Summary of the performance parameters for each ethanol sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[109]	260 °C	100 ppmv	-	-
[110]	200–400 °C	10–50 ppmv	147 s	400 s
[111]	-	50–300 ppmv	20–29 s	38–60 s
[112]	-	10–50 ppmv	9–63 s	12–60 s
[113]	200 °C	20–500 ppmv	4–8 s	4–8 s
[114]	250 °C	200 ppmv	180 s	-

summarizes the performance parameters of each sensor.

2.5. Formaldehyde

Formaldehyde is a well-known pollutant often present in both indoor and outdoor air and deeply studied due to its corrosive characteristics and acute toxicity to human health. Both acute and chronic exposure scenarios are known for provoking a vast range of health conditions that extend from ocular and cutaneous irritation to carcinogenic pathologies like lung and oral cancer. Additionally, several studies have reported the impact of exposure to formaldehyde in the worsening of several other pre-existing pathologies, namely, leukemia and other forms of cancer [115,116,117].

Being aware of formaldehyde toxicity, Zhang et al. (2021) developed aluminum-doped zinc oxide (AZO)-based sensors with different aluminum contents on silicon substrates by magnetron co-sputtering technology in order to be capable of characterizing this volatile compound. The sputtered sensors were capable of fully assessing formaldehyde in concentration ranges as low as 0.1–3 ppm_v at an optimal sensing temperature of 240 °C [118]. Doroftei (2016) also focused his work on the development of sensors obtained through RF-magnetron sputtering using La_0.8Pb_0.2FeO₃ (LPFO) and La_0.8Pb_0.2Fe_0.8Zn_0.2O₃ (LPFO-Zn) perovskites as targets for formaldehyde assessment. The produced sensors were later exposed to previously prepared samples of formaldehyde with concentration levels of 400 ppm_v and they could assess formaldehyde with a response time of 12 s, a “spectacular” behavior, as defined by the author [119]. ZnO sputtered films-based sensors were selected by Chen et al. (2022), which were deposited on a surface acoustic wave (SAW) chip via RF-magnetron sputtering. Sputtering powers of 50, 100, and 150 W were chosen, and three kinds of ZnO-SAW sensors were successfully prepared. After exposing the sensors to concentrations of formaldehyde ranging from 1 to 50 ppm_v, they could accurately detect them at room temperature and with minimum response and recovery times of 60 and 180 s, respectively [120].

Formaldehyde was also the target compound of Castro-Hurtado et al. (2011). The authors developed gas sensors based on NiO thin films deposited on alumina substrates via RF-reactive magnetron sputtering. Once annealed, the sensors were exposed to known concentrations of formaldehyde and proved to be capable of detecting concentrations ranging from 5 to 20 ppm_v. Besides the good sensitivity, the extremely long response (420 s) and recovery (1800 s) times were the major disadvantages of these sensors [121]. In a more recent study, the authors updated some details of the production process, namely, the annealing procedure and the new version of the 150 nm thickness films-based sensors allowed to identify and quantify formaldehyde samples at concentrations as low as 2 ppm_v, similar levels to the ones usually found in biological samples [122]. Finally, NiO was the oxide also selected by Prajesh et al. (2020) to develop thin films-based gas sensors. The authors deposited NiO using the RF-reactive sputtering technique with varying Ar:O₂ ratio and were able to define an optimal sensing temperature of 200 °C for the developed sensors and, impressively, they reported being able to quantify formaldehyde samples at concentration levels of 50 ppb_v [123].

Due to its relevance, formaldehyde has been deeply studied by the scientific community. The reviewed works provide evidence that gas sensors developed through magnetron sputtering are among the most promising tools to characterize this analyte.

Table 10. Summary of the main parameters used in the formaldehyde sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[118]	AZO	-	50–150 W	-/-	5.48 mTorr	30/- sccm	400 °C	3 h	80 nm
[119]	LPFO, LPFO-Zn	RF	75 W	-/-	-/-	-/-	750 °C	1.5 h	330 nm
[120]	ZnO	RF	50–150 W	-/-	-/-	-/-	-	-	0.5 µm
[121]	NiO	RF	400 W	50/50%	5 × 10−3 mbar	-/-	700 °C	4 h	150–300 nm
[122]	NiO	RF	400 W	50/50%	5 × 10−3 mbar	-/-	600–800 °C	4 h	150–300 nm
[123]	NiO	RF	200 W	50–80/20–50%	10 mTorr	-/-	-	-	120 nm

summarizes the main parameters of each one of the addressed works.

Table 11. Summary of the performance parameters for each formaldehyde sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[118]	240 °C	0.1–3 ppmv	94–1089 s	206–254 s
[119]	330 °C	400 ppmv	12 s	25 s
[120]	Room Temp.	1–50 ppmv	60–180 s	180 s
[121]	300–340 °C	5–20 ppmv	420 s	1800 s
[122]	-	2–20 ppmv	-	-
[123]	200 °C	0.3–2.5 ppmv	85–135 s	85–135 s

summarizes the performance parameters of each sensor.

2.6. Isopropanol

Also known as 2-propanol, isopropanol is a volatile organic compound whose presence in biological fluids samples has been studied in order to assess its suitability to act as a biomarker. Interestingly, a total of five health conditions exhibit promising results in terms of being diagnosed and monitored through this biomarker. They are breast cancer [124], chronic liver disease [65], chronic obstructive pulmonary disease [125], diabetes [126], and even lung cancer [127]. On the other hand, in addition to not being particularly hazardous to the environment, isopropanol has been reported as the cause of anesthetic states in cases of human exposure to high concentrations [128,129].

Since isopropanol is a preponderant compound in both environmental and medical fields, the demand for sensors capable of detecting it is crucial. ZnO thin films-based sensors were deposited via RF-magnetron sputtering on thermally oxidized porous silicon substrates for formaldehyde analyses by Al-Salman et al. (2014). The authors successfully characterized a mixture of several VOCs, including isopropanol, at concentration levels as low as 2 ppm_v. Additionally, the authors identified an optimal sensing temperature of 250 °C [130]. In another example of the ZnO utility for isopropanol sensing, Wang et al. (2022) deposited Au-nanoparticles-modified ZnO (Au@ZnO) nanofilms via RF-magnetron sputtering and sequential annealing at 600 °C. These sensors exhibited interesting capacities to detect isopropanol at low concentrations (1 ppm_v), with response and recovery times ranging between 2 and 6 s, and 10 and 56 s, respectively [131].

Despite being less common, other oxides can also be used to develop the thin films for gas sensors. Gao et al. (2024), for example, used the RF-magnetron sputtering technique to deposit Al-doped ZnO/WO₃ heterostructure films for purposes of isopropanol sensing. Samples with concentrations of 1–500 ppm_v were considered during the study. Interestingly, the authors were able to differentiate between isopropanol and four other VOCs (ammonia, ethanol, acetone, and xylene) with the developed sensors and at room temperature, proving the suitability of the system for isopropanol sensing and characterization [132]. Finally, the sensing properties of vanadium pentoxide (V₂O₅) films- based sensors produced via DC-reactive magnetron sputtering were tested by Karthikeyan et al. (2016). By registering the response of the sensors when exposed to the target, the authors characterized isopropanol at concentrations as low as 5 ppm_v and at room temperature [133].

As mentioned, isopropanol is a relevant compound whose detection can be useful in both medical and environmental fields. This chapter demonstrates the suitability of the magnetron sputtering technique for the development of isopropanol sensors to be applied for a vast range of applications.

Table 12. Summary of the main parameters used in the isopropanol sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[130]	ZnO	RF	100 W	-/-	2 × 10−5 mbar	-/-	500 °C	1 h	260 nm
[131]	Au-ZnO	RF	20–50 W	-/-	2.5 Pa	40/- sccm	600 °C	2 h	24 nm
[132]	Al-ZnO:WO3	RF	100 W	-/-	2.2 Pa	25/5 mL/min	400 °C	4 h	600 nm
[133]	V2O5	DC	70 W	-/-	-/-	5/1 sccm	-	-	180–320 nm

summarizes the main parameters of each one of the addressed works.

Table 13. Summary of the performance parameters for each isopropanol sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[130]	250 °C	2–22 ppmv	-	61–102 s
[131]	300 °C	1–100 ppmv	2–6 s	10–56 s
[132]	Room Temp.	1–500 ppmv	5 s	22 s
[133]	Room Temp.	5–200 ppmv	15–28 s	13–40 s

summarizes the performance parameters of each sensor.

2.7. Methane

The potentiality of diagnosing lung cancer in a much faster, accurate, and non-invasive way has been a goal of medicine for many years. Interestingly, the identification of specific compounds in biological fluids whose presence and concentration levels may indicate the existence of this carcinogenic condition has gained relevance over the past decades. Methane is one of the most promising VOCs whose presence in biological samples has been linked to lung cancer [134]. Besides its medical usefulness, methane has been targeted as a relevant VOC due to its flammable and explosive characteristics, not to mention the cases of asphyxiation that have been identified in cases of acute exposure [135,136].

Liang et al. (2016) devoted their work to creating sensors capable of characterizing methane. To do so, the authors produced coatings of Au-decorated vanadium oxide (VO_x) via DC-reactive magnetron sputtering on sapphire substrates and, after annealed, they exposed the sensors to previously prepared samples of methane. Concentrations ranging from 500 to 2000 ppm_v were tested, as the sensors’ optimal response was identified as 1000 ppm_v, at room temperature. Long response (1000 s) and recovery (500 s) times were the most relevant drawbacks of the sensors developed in this work [137].

Titanium dioxide (TiO₂) was the oxide selected by Comert et al. (2016) to develop methane sensors via magnetron sputtering. The authors deposited thin films of this MO onto n-Si substrate at 100 °C using RF-magnetron sputtering and, once annealed at temperatures ranging between 500 and 1000 °C, exposed the sensors to previously developed samples of methane with a concentration of 1000 ppm_v. Interesting response (120 s) and recovery (15 s) times were registered by the authors, proving the suitability of this procedure in the production of methane sensors [138].

In a less recent study, tungsten trioxide (WO₃)-based thin films were deposited via RF-reactive magnetron sputtering on silicon substrates, aiming to develop methane sensors. The films were annealed at temperatures ranging from 350 to 450 °C for 24 h, and after they were exposed to samples of VOCs. During the experiment, the authors could successfully characterize methane samples in a range of concentrations of 100–10,000 ppm_v [139].

Finally, cadmium oxide thin films have also been explored concerning their sensing capacities. Dhivya et al. (2015), for example, developed CdO films-based gas sensors using DC-reactive magnetron sputtering by varying sputtering power to be employed in the characterization of methane samples. Depending on the used sputtering applied power, films with thicknesses of 240, 320, and 410 nm were prepared. The fabricated sensors could effectively detect and quantify samples of methane with concentrations of 500 ppm_v, with the optimal sensing temperature defined as 100 °C [140].

All the reviewed works point out the potential of the magnetron sputtering technique for the development of methane sensors to be applied for a vast range of applications.

Table 14. Summary of the main parameters used in the methane sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[137]	Au-VOx	DC	2.2 W	-/-	2 Pa	-/5 slpm	470–500 °C	4 h	150 nm
[138]	TiO2	RF	150 W	-/-	10−3 Torr	-/-	500–1000 °C	-	170 nm
[139]	WO3	RF	-	50/50%	-/-	1/1 sccm	350–450 °C	24 h	-
[140]	CdO	DC	20–50 W	-/-	-/-	15/5 sccm	80 °C	1 h	240–410 nm

summarizes the main parameters of each one of the addressed works.

Table 15. Summary of the performance parameters for each methane sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[137]	Room Temp.	500–2000 ppmv	1000 s	500 s
[138]	50 °C	1000 ppmv	120 s	15 s
[139]	300–370 °C	100–10,000 ppmv	-	-
[140]	100–150 °C	500 ppmv	-	-

summarizes the performance parameters of each sensor.

2.8. Methanol

Methanol has been explored, during the past decades, regarding its suitability to act as a biomarker for a total of three diseases. Its presence in samples of biological fluids, like exhaled air, has been linked to diabetes [67], breast cancer [141], and lung cancer [142]. On the other hand, its presence in environmental air is of special interest since methanol is flammable and can be acutely toxic to human health. In fact, methanol exposure has been reported to cause ocular irritation, headaches, fatigue, drowsiness, central nervous system depression, and optic nerve damage [143,144].

Aiming to explore the characteristics of methanol, Rydosz et al. (2019) employed DC- or RF-reactive magnetron sputtering to sputter three distinct types of thin films, copper oxide, titanium dioxide, and tin dioxide, which were later exposed to the target samples. In addition to methanol, the authors also explored the sensing capacities of the three thin films-based sensors for acetone and ethanol. Interestingly, the study revealed that both SnO₂ and TiO₂ oxides exhibited a similar sensitivity to methanol, to the detriment of CuO. The authors detected concentrations ranging from 0 to 200 ppm_v [44]. Coincidently, copper oxide produced by DC-reactive magnetron sputtering was also used by Parmar et al. (2011) to develop methanol sensors. The authors claimed to be able to characterize methanol samples ranging from 100 to 2500 ppm_v at an optimal temperature of 350 °C and with response and recovery times of 235 s [145].

Vinoth et al. (2017) focused their work on developing sensors for several VOCs, including methanol, via RF-magnetron sputtering. The authors selected undoped and Cd-doped (3%, 10%, and 20%) ZnO (CZO) films as the sensing layers, which were exposed to gas samples with concentrations of 50–200 ppm_v. Despite being capable of detecting and quantifying methanol, the low response (300 s) and recovery (360 s) times were the main weaknesses of these sensors [146]. In a more recent work from the same research group, undoped and Mg-doped (3, 10, and 20 mol%) ZnO thin films were produced by RF-magnetron sputtering and could improve the response and recovery times of the sensors for the same range of concentrations [147]. Zinc oxide (ZnO) and platinum nanoparticles (Pt NPs)-decorated ZnO nanorods (Pt/ZnO NRs) thin films produced by DC-magnetron sputtering were also used by Young et al. (2020) for the development of methanol sensors. The as-sputtered thin films-based sensors after being annealed for 6 h at 90 °C were exposed to known concentrations of methanol, namely, 100, 200, 300, 500, 700, and 1000 ppm_v. For all the samples, the sensors revealed to be capable of detecting and quantifying the target compound, with 270 °C being the optimal sensing temperature [148].

As mentioned, methanol is a preponderant compound in both medical and environmental fields. The magnetron sputtering technique has been capable of tackling the demand for methanol sensors, as reviewed in this chapter.

Table 16. Summary of the main parameters used in the methanol sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[44]	CuO, TiO2, SnO2	DC	50–1000 W	-/-	3 × 10−2 mbar	-/0.9–20 sccm	-	-	200–500 nm
[145]	CuO	DC	-	90/10	3 × 10−2 mbar	-/-	550 °C	2 h	85 nm
[146]	ZnO, Cd-ZnO	RF	-	-/-	-	-/-	950 °C	6 h	96–103 nm
[147]	ZnO, Mg-ZnO	RF	100 W	-/-	0.02 mbar	-/-	950 °C	8 h	-
[148]	ZnO, Pt/ZnO NRs	DC	-	-/-	-/-	-/-	90 °C	6 h	55–58 nm

summarizes the main parameters of each one of the addressed works.

Table 17. Summary of the performance parameters for each methanol sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[44]	Room Temp.	0–100 ppmv	-	-
[145]	350 °C	100–2500 ppmv	235 s	235 s
[146]	-	50–200 ppmv	300 s	360 s
[147]	100 °C	50–200 ppmv	283 s	223 s
[148]	270 °C	100–1000 ppmv	-	-

summarizes the performance parameters of each sensor.

2.9. Propane

Similarly to methane, propane is one of the most promising analytes in the field of biomarkers identification for purposes of diagnosing and monitoring lung cancer [149]. In terms of environmental impacts, propane is known for being hazardous since it is flammable and explosive in fire and high-pressure scenarios. When inhaled, propane acts as an asphyxiant if present in elevated concentration levels. Additionally, its combustion emits other particularly toxic compounds, like benzene, whose consequences to human health are considerably more dangerous than the ones from propane itself [150,151].

Since propane is a major player in both environmental and medical fields, its detection is very important. Being aware of that, Rydosz et al. (2015) developed M-doped CuO-based (M = Ag, Au, Cr, Pd, Pt, Sb, Si) thin films using medium frequency (MF) magnetron co-sputtering to create an array of sensors. The developed sensors were annealed at 400 °C for 4 h and were then exposed to propane samples at concentration levels similar to the ones usually found in breath. The authors successfully characterized the propane samples at an optimal sensing temperature of 250 °C and with response and recovery times of 10 and 24 s [152].

Cr-doped TiO₂ thin films with different applied powers (5 to 20 W) on the Cr target deposited via RF-confocal magnetron sputtering were used as sensorial layers to develop propane sensors by Sertel et al. (2020), after which they were exposed to known concentrations of propane, namely, 250, 500, and 1000 ppm_v. They exhibited very promising results in assessing the target analyte with outstanding values of response (1–3 s) and recovery (1–3 s) times [153].

Magnetron sputtering was also the technique used by the Yu et al. (2016) in the development of ZnO thin films-based propane sensors. The authors prepared zeolitic imidazolate framework-8 (ZIF-8) membranes through hydrothermal synthesis under the partial self-conversion of a sputter-coated ZnO layer on porous α-alumina supports and exposed the sensors to previously prepared samples of propane. According to the authors, the sensors were fully capable of assessing the target analyte [154]. Regmi et al. (2018) also developed propane sensors based on zinc oxide thin films produced by the RF-magnetron sputtering technique at various working pressures (2–8 mTorr) at constant applied power (100 W). The produced sensors were exposed to propane concentrations ranging from 300 to 500 ppm_v. At an optimal sensing temperature of 300 °C, interesting values of response and recovery times, i.e., 30 and 35 s, respectively, were registered [155].

All the reviewed works highlight the potential of the magnetron sputtering technique for the development of propane sensors whose applications extend from the environmental to the medical field.

Table 18. Summary of the main parameters used in the propane sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[152]	M-CuO (M = Ag, Au, Cr, Pd, Pt, Sb, Si)	MF	40 W	90/10%	4 × 10−2 mbar	-/-	400 °C	4 h	50 nm
[153]	Cr-TiO2	RF	150 W	-/-	30 mTorr	-/-	-	-	300 nm
[154]	ZnO to ZIF-8	-	100 W	-/-	15 mTorr	10/- sccm	150 °C	3 h	50 nm
[155]	ZnO	RF	100 W	-/-	2–30 mTorr	-/-	-	-	300–400 nm

summarizes the main parameters of each one of the addressed works.

Table 19. Summary of the performance parameters for each propane sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[152]	250 °C	0–100 ppmv	10 s	24 s
[153]	300 °C	250–1000 ppmv	1–3 s	1–3 s
[154]	Room Temp.	-	-	-
[155]	300 °C	300–500 ppmv	30 s	35 s

summarizes the performance parameters of each sensor.

2.10. Toluene

Toluene is among the most hazardous pollutants present in the atmosphere. Scenarios of both acute and chronic exposure to this volatile compound are proven to be responsible for the development of several forms of cancer and for impacts on neurological systems [156,157]. Remarkably, its presence in biological fluids can be useful under medical optics. In fact, toluene has been explored as a biomarker of chronic kidney diseases [158], chronic obstructive pulmonary disease [159], malaria [160], sleep apnea [69], squamous cell cancer [161], breast cancer [106], and lung cancer [162].

Being aware of the dangerousness of toluene, Prakasha et al. (2024) used the RF-magnetron sputtering technique to grow Au-loaded ZnO and TiO₂ thin films-based sensors. The deposition of a thin layer of ZnO on top of TiO₂ and subsequent annealing resulted in a ZnTiO₃/TiO₂ heterostructure. The Au-ZnTiO₃/TiO₂-based sensors were used to characterize samples of toluene previously prepared with concentrations between 10 and 100 ppm_v. By applying principal component analysis to the data collected from the sensors when exposed to the samples, the authors could accurately differentiate between the target analyte and other VOCs (acetone, ethanol, and methanol) with a total explained variance of nearly 80%, proving the suitability of the magnetron sputtering technique in the development of toluene sensors [163]. GaN-nanowires (NWs)/TiO₂-nanocluster hybrid sensors, where GaN NWs were grown via catalyst-free molecular beam epitaxy followed by the deposition of TiO₂ nanoclusters using RF-magnetron sputtering, were selected by Aluri et al. (2011) to develop their toluene sensors. After exposing the developed sensors to a large number of previously prepared samples of toluene, namely, samples with concentrations ranging from just 50 ppb_v to 10,000 ppm_v, the authors were able to register response and recovery times as low as 60 and 75 s, which were promising results in the field of toluene sensing [164].

Aiming to detect and differentiate among four VOCs, acetone, ethanol, isopropanol, and toluene, Al-Salman et al. (2015) prepared solutions containing all compounds with concentrations ranging from 2 to 87 ppm_v. The authors claimed that the ZnO nanostructures-based sensors developed through RF-magnetron sputtering could successfully differentiate among the VOCs and exhibited the best response at a temperature of 250 °C [130]. The toluene sensors developed by Kim et al. (2019) were based on Pt- and Pd-ZnO thin films which were obtained through magnetron sputtering and annealed at temperatures ranging from 550 to 650 °C. The sensors could successfully assess samples of the target analyte in concentrations as low as 0.1 ppm_v [165].

Pt-SnO₂ was the functionalized oxide selected by Kim et al. (2016) to develop their toluene sensors. The SnO₂ nanofibers (NFs) were prepared using an electrospinning process, and after they were functionalized with Pt nanoparticles (NPs) with different thicknesses which were produced through magnetron sputtering by changing the deposition time. Once annealed at 650 °C for two hours, the films were exposed to several concentrations of the target analyte, namely, 1, 5, and 10 ppm_v. The sensors, according to the authors, proved to be suitable in the assessment of toluene in the referred concentration range, exhibiting an optimal sensing temperature of 300 °C [166]. Finally, Mohajir et al. (2022) deposited SnO₂ thin films via conventional and glancing angle deposition reactive sputtering as a way of developing toluene sensors. After annealing for 48 h at two distinct temperatures, 350 and 500 °C, the sensors were exposed to very low concentrations of toluene, namely, 50 to 900 ppb_v. These interesting results are evidence of the potentiality of the sensors developed via magnetron sputtering for the assessment of toluene even at trace levels of concentration [167].

All the reviewed works demonstrate the capability of the magnetron sputtering technology for the development of toluene sensors.

Table 20. Summary of the main parameters used in the toluene sensors of each reviewed paper.

Work	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
				Ratio	Pressure	Flow	Temp.	Time
[163]	Au-ZnTiO3/TiO2	-	-	-/-	-/-	-/-	550 °C	2 h	75–150 nm
[164]	GaN)/TiO2	RF	300 W	-/-	-/-	50/5 sccm	650–700 °C	-	150 nm
[130]	ZnO	RF	100–150 W	-/-	2 × 10−5 mbar	-/-	500 °C	2 h	260 nm
[165]	Pt and Pd-ZnO	-	30 W	-/-	-/-	300/10 sccm	550–650 °C	-	10–30 nm
[166]	Pt-SnO2	-	30 W	-/-	2.65 Pa	-/-	650 °C	2 h	3–20 nm
[167]	SnO2	DC	50 W	-/-	3 × 10−3– 6 × 10−3 mbar	6/10 sccm	350–500 °C	48 h	250 nm

summarizes the main parameters of each one of the addressed works.

Table 21. Summary of the performance parameters for each toluene sensor considering the correspondent reviewed paper.

Work	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[163]	210 °C	10–100 ppmv	4 s	20 s 90 s
[164]	-	50 ppbv–10,000 ppmv	60–180 s	75–150 s
[130]	250 °C	2–87 ppmv	-	98 s
[165]	-	0.1–50 ppmv	-	-
[166]	300 °C	1–10 ppmv	-	-
[167]	300–500 °C	50–900 ppbv	-	900 s

summarizes the performance parameters of each sensor.

2.11. Future Trends

As reviewed above, gas sensors developed through magnetron sputtering have been largely used in the monitoring and characterization of major volatile organic compounds whose applications extend from environmental to medical fields. Nonetheless, several other VOCs with similar relevance in these scientific areas have exhibited the potential to be assessed by sputtered thin films-based sensors. This chapter reviews those compounds and establishes general future trends in the topic of sensors developed via magnetron sputtering. The reviewed VOCs are acetaldehyde, acetophenone, benzene, 2-butanone, butyl acetate, ethyl benzene, hexanal, isoprene, limonene, nonanal, phenol, α-pinene, and xylene.

Table 22. Summary of the characteristic parameters for each one of the addressed compounds in the future trends section.

Compound	Formula	CAS	Vapor Pressure	Note
Acetaldehyde	C2H4O	75-07-0	100 KPa	VOC
Acetophenone	C8H8O	98-86-2	0.041 KPa	-
Benzene	C6H6	71-43-2	10.0 KPa	VOC
2-Butanone	C4H8O	78-93-3	12.1 KPa	VOC
Butyl Acetate	C6H12O2	123-86-4	1.38 KPa	VOC
Ethylbenzene	C8H10	100-41-4	0.9 KPa	VOC
Hexanal	C6H10O	66-25-1	1.4 KPa	VOC
Isoprene	C5H8	78-79-5	61.0 KPa	VOC
Limonene	C10H16	5989-27-5	0.2 KPa	VOC
Nonanal	C9H18O	124-19-6	0.04 KPa	VOC
Phenol	C6H6O	108-95-2	0.04 KPa	VOC
α-Pinene	C10H16	80-56-8	0.4 KPa	VOC
Xylene	C8H10	108-38-3	0.8 KPa	VOC

summarizes some of the characteristic parameters of the addressed compounds, namely, their chemical formula, chemical abstracts service (CAS) number, vapor pressure, and a note on their fitting in the VOCs definition.

2.11.1. Acetaldehyde

Acetaldehyde is ubiquitous in the environment since it can be found throughout the air of all kinds of locations. Nonetheless, despite its ubiquitousness, it can be a hazardous VOC if present in concentration levels above certain limits. In fact, in cases of chronic exposure, acetaldehyde is known for provoking nausea, vomiting, headaches, and even unconsciousness [168,169]. Additionally, several studies have investigated the role of acetaldehyde in the development of carcinogenic diseases [170]. On the other side, this compound has been considered as a potential biomarker for the prognostic and monitoring of several pathologies. Among them, one can mention chronic liver diseases [65], chronic obstructive pulmonary disease [125], cystic fibrosis [105], and prostate cancer [171].

In spite of its relevancy in both medical and environmental fields, few works have been conducted aiming to develop sensors fully capable of assessing acetaldehyde. Aiming to explore this field, Presmanes et al. (2017) developed ZnO:Ga-based sensors via RF-sputtering for gas sensing. The sensors proved to be capable of assessing acetaldehyde in concentrations as low as 500 ppb_v [172]. Cindemir et al. (2016) also developed acetaldehyde sensors via magnetron sputtering. The authors assembled indium–tin (In-Sn) oxide thin films with a wide range of compositions using reactive dual-target DC-magnetron sputtering and assessed their suitability when exposed to previously prepared samples. Interestingly, the sensors could characterize samples of this compound in concentration levels as low as 200 ppb_v [173]. Despite these works, some other techniques have been employed in the development of acetaldehyde sensors, namely, the chemical spray pyrolysis technique [174], successive ionic layer adsorption and reaction (SILAR) technique [175], and others, proving that there is demand for acetaldehyde sensors.

Considering the aforementioned reasons, the development of thin films-based gas sensors using magnetron sputtering technology exhibits a potential to be explored and constitutes a niche of work that both academic and industrial fields should look at.

2.11.2. Acetophenone

Besides its utilization in the manufacturing of personal care products like soap and perfumes, and even as a flavoring agent in food, acute exposure to acetophenone has been linked to skin irritation and transient corneal injury and, in scenarios of the inhalation of elevated concentrations, loss of consciousness [176,177]. Additionally, acetophenone has been studied regarding its role in the medical field. This compound has been explored as a potential biomarker for the monitoring of several chronic liver diseases [178] and even of carcinogenic conditions like breast and squamous cell cancers [124,161].

Considering the reasons pointed out before, there is a growing demand for acetophenone sensors. Magnetron sputtering has played a primary role in the development of these sensors, as depicted in the work of Prasanth et al. (2022). The authors deposited RF- magnetron sputtering oxide thin films (ZnO, AZO and SnO₂)-based sensors that were later exposed to previously prepared samples of acetophenone (concentrations range of 0–250 ppm_v). This VOC was successfully characterized by a developed e-nose at around room temperature and with response and recovery times of 17 and 21 s, respectively [179]. Although sparse, the promising results achieved with sensors produced by magnetron sputtering proved the necessity of further work in the field.

2.11.3. Benzene

Benzene is one of the well-known BTEX (benzene, toluene, ethylbenzene, and xylenes) compounds; in this way, both academic and medical fields are aware of its carcinogenic effect in cases of chronic exposure. On the other hand, short-term exposure leads to blood disorders like anemia, excessive bleeding, damage to the immune system, and facilitates infections [180,181]. Despite its hazardousness to both the environment and human health, it has been explored regarding its suitability to act as a biomarker. Interestingly, the presence of altered levels of benzene in biological samples has been linked to chronic obstructive pulmonary disease [159], lung cancer [107], and malaria [68], making this compound of special interest also to the field of medical diagnostics.

Mohajir et al. (2022), being aware of the relevancy of sensing benzene, used conventional and glancing angle deposition reactive sputtering to deposit SnO₂ thin films on top of ceramic substrates. Then, the films were annealed for 48 h at temperatures ranging between 350 and 500 °C. The annealed sensors were exposed to known concentrations of benzene and were able to characterize the target analyte in levels as low as 50 ppb_v, proving the suitability of the developed sensors [167].

As seen, and despite the evident demand for benzene sensors, there is a lack of work in this field. Sensors developed through magnetron sputtering, in this way, have exhibited very promising results that evidence the potential of this technique to tackle the needs of the field.

2.11.4. 2-Butanone

Chronic kidney disease [182], chronic liver disease [183], cystic fibrosis [66], gastric cancer [184], lung cancer [107], and squamous cell cancer [108], are some of the pathologies that have been linked to the presence of 2-butanone in samples of biological fluids. On the other hand, and despite its clinical usefulness, 2-butanone constitutes a danger to the environment since it is classified as an irritant and flammable. In cases of exposure, consequences like loss of consciousness, weakness, nausea, dizziness, headaches, and ocular irritation have been reported [185,186].

Aiming to develop a 2-butanone sensor, Zhang et al. (2020) produced V₂O₅ thin films via a simple hydrothermal method. Once annealed for 2 h at 500 °C, the developed sensors were exposed to previously prepared samples of 2-butanone with concentrations ranging from 0 to 600 ppm_v. The fabricated sensors successfully characterized the samples of the target compound, exhibiting response and recovery times of 2 and 9 s, respectively, and proving the overall relevancy of developing 2-butanone sensors [187].

The achieved results, although scarce, exhibited a promising nature that justifies the investment in the magnetron sputtering technology for purposes of 2-butanone sensing since this VOC assessment is highly relevant for medical purposes. Additionally, there are studies of deposition of V₂O₅ gas sensors using magnetron sputtering for the detection of isopropanol as mentioned in Chapter 2.6, showing the potential of this technique in the development of this oxide for the 2-butanone sensing [188].

2.11.5. Butyl Acetate

Similarly to butanol, butyl acetate has been equally linked to the prognostic and monitoring of cystic fibrosis [189]. Patients suffering from this disease often exhibit altered concentration levels of butyl acetate in the composition of body fluids like breath, urine, or even perspiration. Additionally, butyl acetate has a preponderant role in the environmental field since it is known for being flammable and an irritant. In cases of human exposure, it provokes ocular irritation, nausea, dizziness, and headaches [190,191].

Due to its apparent relevance in both medical and environmental fields, some work is being performed aiming to explore butyl acetate characteristics. Hotovy et al. (2021), for example, developed NiO thin films-based sensors via DC-reactive magnetron sputtering as a way of exploring this compound. The films, whose thicknesses ranged between 25 and 50 nm, were defined as having an optimal sensing temperature of 300 °C when exposed to 3 ppm_v samples of butyl acetate. Response and recovery times of 124 and 102 s were registered by the authors [192]. All the aforementioned results evidence that this is a promising field that deserves to be further explored in order to be considered in the environmental and medical fields.

2.11.6. Ethylbenzene

Being one of the BTEX compounds, ethylbenzene is vulgarly known for its carcinogenic consequences in cases of both acute and chronic exposure. Additionally, it can equally lead to dizziness, irritation, and pruritus in scenarios of short-term exposures [193,194]. In terms of medical usefulness, ethylbenzene has been linked to several pathologies, namely, asthma [195], colorectal cancer, lung cancer [196], malaria [160], and sleep apnea [69]. Once certified, this analyte can eventually act as a biomarker for the diagnosis of these conditions in a rapid, non-invasive, and accurate way.

Aluri et al. (2011), aware of the relevancy and hazardousness of ethylbenzene, have devoted their work to the development of GaN-nanowire/TiO₂-nanocluster hybrid sensors, where GaN nanowires (NWs) were grown using catalyst-free molecular beam epitaxy followed by the deposition of TiO₂ nanoclusters using RF-magnetron sputtering. Interestingly, the developed sensors were able to characterize samples of the target analyte at concentration levels as low as 50 ppb_v, at room temperature, and with response times of 60 s [164]. These results, although scarce, show that the field of developing ethylbenzene sensors through magnetron sputtering is promising and should be explored.

2.11.7. Hexanal

Chronic kidney disease [197], chronic obstructive pulmonary disease [198], cystic fibrosis [66], lung cancer [199], and malaria [160], are among the main pathologies often linked to the detection of hexanal in biological fluids. In this way, this aldehyde has been considered concerning its potential to act as a biomarker. In terms of environmental dangerousness, hexanal can be responsible for fires if present at elevated concentrations due to its flammable nature. Regarding the consequences to human health in cases of exposure, it can cause irritation in the eyes, nose, oral cavities, and skin. Worsening of health conditions has been reported in diabetes and obesity patients exposed to hexanal [200,201].

Intending to assess hexanal and other VOCs, Núñez-Carmona et al. (2021) developed an array of sensors based on SnO₂ and CuO that were produced by rheotaxial growth and the thermal oxidation (RGTO) technique, which involves two phases of deposition: first, the deposition of a metallic thin film via DC-magnetron sputtering from a metallic target to a substrate at higher temperatures and after the fusion of the metal; second, thermal oxidation period in order to produce a metal oxide coating with stable stoichiometry. Later, the sensors were exposed to samples of several VOCs. Hexanal was one of the volatile compounds present in the mixture. Interestingly, the authors could differentiate all the considered compounds through principal component analysis with a total explained variance of more than 80%. Unfortunately, this work does not provide any additional information on the preparation and working details of the sensors [202]. Nevertheless, these promising results prove the suitability of the magnetron sputtering technique for the development of sensors capable of characterizing hexanal.

2.11.8. Isoprene

Isoprene has been considered by the medical field as an analyte capable of acting as a biomarker for a vast range of diseases if detected in biological fluids like breath. Among those diseases, one can list breast cancer [203], chronic kidney disease [64], chronic liver disease [183], chronic obstructive pulmonary disease [159], cystic fibrosis [95], diabetes [67], gastric cancer [71], lung cancer [107], malaria [68], and sleep apnea [69]. Nonetheless, it is equally classified as a hazard to human health. This categorization is due to irritation in the eyes, nose, oral cavities, and respiratory tracts in cases of both chronic and acute exposures. Skin pruritus has equally been reported in the literature [204,205].

Due to its relevancy, the development of sensors capable of detecting isoprene is mandatory. Lin et al. (2024), for example, developed sensors capable of assessing this analyte, namely Si-doped WO₃ films-based sensors which were co-sputtered on glass substrates from two metallic targets (i.e., W and Si). After annealing at 500 °C for 1 h, the sensors were exposed to previously prepared samples of the target analyte and the authors could characterize the samples at concentration levels of 5 ppm_v with very interesting response and recovery times of 1 and 2.5 s, respectively [206]. The results of this work reveal the potential of developed sensors using magnetron sputtering for isoprene sensing and the capacity of the technique to tackle the demand for sensors in this field.

2.11.9. Limonene

Breast cancer [203], chronic kidney disease [158], chronic liver disease [183], chronic obstructive pulmonary disease [207], and squamous cell cancer [208] are well-known pathologies whose precocious diagnostics have been considered through the detection of biomarkers in biological fluids and, specifically, through the assessment of limonene in breath.

Interestingly, limonene is classified as flammable, a hazard to human health and to the environment, and an irritant compound. In fact, this compound has been studied in regard to its potential carcinogenicity. Additionally, it is known for provoking ocular, cutaneous, and gastrointestinal irritation in scenarios of prolonged exposure [209,210].

To assess limonene, again Núñez-Carmona et al. (2021) used the same array of sensors based on SnO₂ and CuO films as in hexanal detection highlighted in Chapter 2.11.7, with limonene being one of the VOCs studied. Interestingly, the authors could differentiate all the considered compounds through principal component analysis with a total explained variance of more than 80%. Unfortunately, as reported before, the work did not disclose any additional information on the preparation and working details of the sensors [202]. Still the results evidence the auspiciousness of the field of limonene sensors developed by magnetron sputtering.

2.11.10. Nonanal

Nonanal has been deeply explored by the medical community due to its apparent suitability to act as a tool of diagnosis. In fact, the presence of nonanal has been linked to a vast range of diseases, namely, asthma [211], chronic kidney disease [182], chronic obstructive pulmonary disease [198], colorectal cancer [212], lung cancer [199], and malaria [160]. Additionally, both short- and long-term exposure to nonanal has been associated with scenarios of strong irritation in the eyes and skin, not to mention its flammable nature [213,214].

With the aim of assessing nonanal, again Núñez-Carmona et al. (2021) used an array of SnO₂ and CuO thin films based-sensors to detect this VOC. Interestingly, the authors achieved a differentiation of all the considered compounds through principal component analysis with a total explained variance of more than 80%. As stated before, the work failed to provide information on the development and working details of the sensors, but the achieved results evidence the potential of this technology to the current demand for nonanal sensors [202].

2.11.11. Phenol

Breast cancer [215], chronic liver disease [183], colorectal cancer [216], cystic fibrosis [95], gastric cancer [217], and squamous cell cancer [218] have been explored in regard to their potential to be diagnosed through the presence of biomarkers in human fluids and, specifically, through the presence of phenol.

Under the environmental topic, phenol is categorized as corrosive, acutely toxic, and hazardous to both health and the environment. In particular, this compound is known for provoking ocular and cutaneous burns, analgesic sensation, heart rate increase, convulsions and, in some cases, even death [219,220].

In regard to the aforementioned hazardousness of phenol, Singh et al. (2015) produced TiO₂ films through the deposition of a Ti film via DC-magnetron sputtering followed by its conversion to TiO₂ film via oxidation to act as a sensorial layer for phenol samples. The thin films, after being annealed at 95 °C for 3 h, were exposed to previously prepared samples of the target analyte which were capable of assessing concentrations ranging between 0.01 and 1 ppm_v with a response time of 250 s [221]. These results prove the novelty and the necessity of developing phenol sensors, and magnetron sputtering arises as a procedure to do so.

2.11.12. α-Pinene

Classified as an irritant, α-pinene can be extremely destructive to mucous membranes and the upper respiratory tract, eyes, and skin. Additionally, cases of vomiting, nausea, headaches, shortness of breath, laryngitis, and coughing have been equally reported [222,223]. In spite of its hazardousness, α-pinene has been studied as a potential biomarker for several pathologies. In fact, its present in biological fluids has been linked to breast cancer [106], chronic liver disease [183], chronic obstructive pulmonary disease [125], and malaria [224].

Magro et al. (2022) developed an array of layer-by-layer thin films as (PAH/GO)₅, (PEI/PSS)₅, (PEI/GO)₅, (PAH/MWCNT)₅, (PAH/MWCNT-COOH)₅, and sputtered thin films as TiO₂ and ZnO, deposited onto ceramic substrates with gold interdigitated electrodes to detect α-pinene in wildfires scenarios. For each metal oxide-based device, the sputtering was performed both in a 100% O₂ and 50:50 O₂/Ar atmospheres, with an applied power of 1000 W during 15 min and 300 W during 5 min for TiO₂ and ZnO, respectively. The e-nose was exposed to the target analyte in the interior of a dedicated vacuum chamber. The authors reported the capacity of characterizing samples with concentrations ranging from 109 to 807 ppm_v [61]. The results achieved by Magro et al. (2022) prove that there is demand for the development of α-pinene sensors through magnetron sputtering.

2.11.13. Xylene

Similarly to many other compounds, xylene is categorized as flammable, an irritant and a hazard to human health. In particular, chronic exposure to this compound can provoke burns in the skin or eyes, dizziness, and asphyxiation. Being one of the BTEX compounds, it is known for its carcinogenic nature and has been linked to the development of diseases like lung, oral, and even breast cancer [193,225]. On the other hand, xylenes have played a major role in the field of biomarkers. In fact, its presence in samples of biological fluids has been correlated to diseases like asthma [226], breast cancer [227], lung cancer [107], sleep apnea [69], squamous cell cancer [208], and tuberculosis [228]. If certified, xylene can eventually act as a non-invasive, accurate, rapid, and painless biomarker to diagnose all these pathologies.

Knowing the importance of xylene, Lee et al. (2011) developed thin films-based sensors to monitor xylenes in volatile samples. The authors produced WO₃ films-based sensors with a thickness of 4.4 µm using RF-reactive magnetron sputtering. The coatings were annealed for ninety minutes at 500 °C. The developed sensors proved to be capable of characterizing samples of xylene in a concentration range of 0–20 ppm_v, and with recovery times of around 30 s [229]. These results demonstrate that the field of xylene sensors is very relevant and that the magnetron sputtering technique is promising for both medical and environmental applications.

The works reviewed in these past chapters highlight the potentialities of magnetron sputtering for the development of gas sensors and, specifically, for VOCs sensing. As addressed, these sensors have a large range of applications that extend from clinical diagnostics to environmental assessments, a fact that evidences the efficiency of these methodologies and reinforces the usefulness of magnetron sputtering to comply with the demand. All the details of the reviewed works are listed in

Table 23. Summary of the main deposition parameters used in the sensors of each reviewed paper.

Work	VOC	Thin Film	DC/RF	Power	Argon/Oxygen			Annealing		Thin Film Thickness
					Ratio	Pressure	Flow	Temp.	Time
[172]	Acetaldehyde	ZnO:Ga	RF	50 W	-/-	2 Pa	-/-	400 °C	4 h	50–100 nm
[173]	Acetaldehyde	In-SnO	DC	11–55 W	-/-	0.58 Pa	25/9 mL/min	400–500 °C	6 h	110–190 nm
[179]	Acetophenone	ZnO, AZO and SnO2	DC	50 W	-/-	-/-	6/10 sccm	-	-	300 nm
[167]	Benzene	SnO2	DC	50 W	-/-	3 × 10−3–6 × 10−3 mbar	6/10 sccm	350–500 °C	48 h	250 nm
[192]	Butyl Acetate	NiO	DC	600 W	30/70%	-/-	-/-	500–600 °C	-	25–50 nm
[164]	Ethyl Benzene	GaN/TiO2	RF	300 W	-/-	-/-	50/5 sccm	700 °C	-	70 nm
[202]	Hexanal	SnO2, CuO	DC	-	-/-	-/-	-/-	-	-	-
[206]	Isoprene	Si-WO3	DC	700 W	-/-	20 mTorr	-/-	500 °C	1 h	-
[202]	Limonene	SnO2, CuO	DC	-	-/-	-/-	-/-	-	-	-
[202]	Nonanal	SnO2, CuO	DC	-	-/-	-/-	-/-	-	-	-
[221]	Phenol	TiO2	DC	-	-/-	5 × 10−6 mTorr	-/-	95 °C	3 h	128 nm
[61]	α-Pinene	TiO2, ZnO	DC	300–1000 W	50/50%	0.13 Pa	-/-	-	-	-
[229]	Xylene	WO3	RF	200 W	50/50%	0.01 Torr	-/-	500 °C	1.5 h	4.4 µm

.

Table 24. Summary of the main performance parameters used in the sensors of each reviewed paper.

Work	VOC	Optimal Sensing Temperature	Detection Range	Response Time	Recovery Time
[172]	Acetaldehyde	Room Temp.–500 °C	500 ppbv	-	-
[173]	Acetaldehyde	500 °C	0.2–25 ppmv	-	-
[179]	Acetophenone	Room Temp.	0–250 ppmv	17 s	21 s
[167]	Benzene	300–500 °C	50–900 ppbv	-	900 s
[192]	Butyl Acetate	300 °C	3 ppmv	124 s	102 s
[164]	Ethyl Benzene	Room Temp.	50 ppbv–1 ppmv	60 s	75 s
[202]	Hexanal	350–400 °C	-	-	-
[206]	Isoprene	325 °C	5 ppmv	1 s	2.5 s
[202]	Limonene	350–400 °C	-	-	-
[202]	Nonanal	350–400 °C	-	-	-
[221]	Phenol	Room Temp.	0.01–1 ppmv	250 s	-
[61]	α-Pinene	Room Temp.	109–807 ppmv	-	-
[229]	Xylene	300 °C	0–20 ppmv	-	30 s

summarizes the performance parameters of each sensor.

By reviewing all the papers previously identified as being the most relevant in the field of sensors developed with magnetron sputtering and consulting the information summarized in all the included tables, one can draw some conclusions. Among all the thin films used to prepare the sensors, a total of eight metal oxides were the most common throughout all the applications. They were aluminum zinc oxide, nickel oxide, vanadium oxide, cadmium oxide, copper oxide, zinc oxide, titanium dioxide, and tungsten trioxide. In terms of the performances of these sensors, two main parameters were considered to be the most relevant. They were the concentration detection range and the response time range.

Considering the detection range, one can infer that the WO₃-based sensors exhibit the widest range of detection. In fact, the detection of concentrations ranging between 0.5 and 10,000 ppm_v has been reported. On the opposite side, AZO-based sensors had the shortest range, exhibiting values between 0.1 and 3 ppm_v. In regard to the response time, whose value should be as low as possible to ensure a rapid reaction to the presence of the target analyte, the sensors developed with AZO thin films exhibited the widest interval. For these types of sensors, response times ranging between 94 and 1089 s were reported. The shortest range of response time was registered for V₂O₅-based sensors. In this case, an interval of 13 s (15–28 s) was reported. Figure 5 summarizes the performance parameters for the eight thin films mainly used in the production of gas sensors through magnetron sputtering.

3. Conclusions

The field of gas sensors has been emerging for the past years due to the necessity of characterizing compounds and, specifically, volatile organic compounds whose detection can be of special interest in a vast range of applications that extend from clinical evaluation to environmental monitoring. Among all the potential techniques to develop sensors, magnetron sputtering has emerged as one of the most suitable methodologies since it enables simple manufacturing of sensors, low power consumption, and low costs involved in production. Additionally, it ensures high deposition rates and high-purity thin films, good adhesion to the substrate, and, consequently, increased mechanical stability, permits a precise control of the film thickness, and as highlighted before, a wide range of applicability.

This work reviewed all the current applications of gas sensors developed through magnetron sputtering in the field of VOCs assessment. By gathering the most relevant scientific works published in one of the most relevant databases, it was possible to conclude that the magnetron sputtering technique has mostly been applied to the development of gas sensors for 10 main VOCs. They are acetone, ammonia, butanol, ethanol, formaldehyde, isopropanol, methane, methanol, propane and toluene. Moreover, 13 other compounds were identified as promising targets and classified as future trends in this field, namely, acetaldehyde, acetophenone, benzene, 2-butanone, butyl acetate, ethylbenzene, hexanal, isoprene, limonene, nonanal, phenol, α-pinene and xylene.

In this review work, it was further revealed that the sputtering parameters played a crucial role in determining the properties and performance of the deposited thin films as gas sensors. Key parameters that were found to significantly influence the sensors characteristics include the following:

• Sputtering power, which changed between DC and RF, typically ranging from 50 to 1000 W, with higher powers generally leading to higher deposition rates and potentially larger grain sizes.
• Working pressure was usually maintained between 0.1 and 10 Pa, with lower pressures often conducing to denser films and higher pressures promoting more porous structures.
• Target-to-substrate distance, commonly set between 5 and 15 cm, influencing the energy of the sputtered particles and therefore the deposition rate.
• Substrate temperature, which varied from room temperature to 500 °C during deposition, affecting the crystallinity and grain size of the films
• Gas flow rates. Typically, argon flow rates of 10–50 sccm were used, with additional oxygen flow (1–10 sccm) for reactive sputtering of the MOs.
• Deposition time, which ranged from a few minutes to several hours, controlling the film thickness, which typically varied from a 10 nm to a 4 μm.

Considering the addressed works, it was possible to verify that the most commonly used MOs for thin films deposition were ZnO and TiO_2. These oxides were tested in regard to their suitability to sense all 10 VOCs considered in this work. Additionally, thin films of WO₃, CuO, and SnO₂ were also considerably used for sensors development via this technique. It was also found that some oxide-based sensors doped with noble metals increased sensing performance for specific VOCs. Most of the developed gas sensors exhibited sensing capacities in the ppm_v range of concentrations, with some exceptions being capable of detecting at ppb_v levels.

Future research on sputtered thin films for sensors should prioritize improving sensitivity, selectivity, and response times by leveraging advanced and nanostructured materials. Nanomaterials like nanoparticles, nanowires, and 2D materials (e.g., graphene) offer a higher surface area for interactions with target molecules, significantly enhancing sensor sensitivity. Researchers could explore how to precisely deposit these materials via sputtering to create uniform, nanostructured films optimized for different sensing applications. Additionally, hybrid materials, such as metal oxides functionalized with organic compounds or polymers, hold promise for improving selectivity, enabling sensors to detect specific analytes more accurately.

Optimizing sputtering parameters dynamically during deposition can further improve sensor performance. By adjusting variables like power, pressure, and gas composition in real-time, researchers can fine-tune film morphology, porosity, and crystalline phases to enhance sensitivity and response times. Multilayer films or heterostructures, where different layers serve distinct functions (e.g., catalytic activity or conductivity), offer another avenue for boosting sensor efficiency. For instance, porous films can facilitate faster diffusion of gases or liquids to the sensing surface, while ultra-thin films reduce response times but may need advanced sputtering techniques to maintain structural stability.

Improving sensor selectivity using dopants and surface functionalization is another critical research direction. Dopants, such as noble metals, can be added during sputtering to enhance the sensor’s ability to detect specific gases or chemicals. Tailoring the concentration and distribution of these dopants can improve the sensor’s response and accuracy. Surface functionalization through post-sputtering treatments or co-deposition methods can allow sensors to selectively bind specific target molecules, improving their ability to operate in complex environments or multi-gas mixtures. Sensor arrays or multi-modal systems, which integrate different sensing mechanisms into a single device, can further improve selectivity through cross-referencing multiple detection signals.

Finally, sensor durability and real-world applicability are key areas for future research. Developing sputtered films that are stable in harsh conditions, such as extreme temperatures or corrosive environments, will be crucial for industrial and environmental monitoring applications. Protective coatings or encapsulation techniques could ensure long-term stability without sacrificing sensor sensitivity. Self-cleaning materials or surfaces that regenerate over time can help sensors maintain their functionality in contaminated environments. Researchers should also conduct extensive field testing under real-world conditions to ensure sensors perform reliably outside of controlled laboratory environments.

Overall, this review paper summarizes the state-of-the-art of the field of gas sensors developed through magnetron sputtering technology through reviewing the most relevant works published in the field, summarizing the main characteristics of the developed sensors, summarizing the applications, and defining future trends for the technique. Moreover, it can be concluded that the detailed analysis of sputtering parameters provides valuable insights for researchers aiming to optimize the gas sensor performance through careful control of the deposition process.