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Article

Ceramic Nanotubes—Conducting Polymer Assemblies with Potential Application as Chemosensors for Breath Ammonia Detection in Chronic Kidney Disease

by
Alexandru Florentin Trandabat
1,
Romeo Cristian Ciobanu
1,*,
Oliver Daniel Schreiner
1,2,
Thomas Gabriel Schreiner
1,2 and
Sebastian Aradoaei
1
1
Department of Electrical Measurements and Materials, Gheorghe Asachi Technical University, 700050 Iasi, Romania
2
Department of Medical Specialties III, Faculty of Medicine, University of Medicine and Pharmacy “Grigore T. Popa”, 700115 Iasi, Romania
*
Author to whom correspondence should be addressed.
Chemosensors 2024, 12(9), 198; https://doi.org/10.3390/chemosensors12090198
Submission received: 1 August 2024 / Revised: 10 September 2024 / Accepted: 19 September 2024 / Published: 23 September 2024
(This article belongs to the Section Materials for Chemical Sensing)
Browse Figures
Versions Notes

Abstract

:
This paper describes the process of producing chemosensors based on hybrid nanostructures obtained from Al2O3, as well as ZnO ceramic nanotubes and the following conducting polymers: poly(3-hexylthiophene), polyaniline emeraldine-base (PANI-EB), and poly(3, 4-ethylenedioxythiophene)-polystyrene sulfonate. The process for creating ceramic nanotubes involves three steps: creating polymer fiber nets using poly(methyl methacrylate), depositing ceramic films onto the nanofiber nets using magnetron deposition, and heating the nanotubes to 600 °C to burn off the polymer support completely. The technology for obtaining hybrid nanostructures from ceramic nanotubes and conducting polymers is drop-casting. AFM analysis emphasized a higher roughness, mainly in the case of PANI-EB, for both nanotube types, with a much larger grain size dimension of over 5 μm. The values of the parameter Rku were close or slightly above 3, indicating, in all cases, the formation of layers predominantly characterized by peaks and not by depressions, with a Gaussian distribution. An ink-jet printer was used to generate chemiresistors from ceramic nanotubes and PANI-EB structures, and the metallization was made with commercial copper ink for printed electronics. Calibration curves were experimentally generated for both sensing structures across a wider range of NH3 concentrations in air, reaching up to 5 ppm. A 0.5 ppm detection limit was established. The curve for the ZnO:PANI-EB structure presented high linearity and lower resistance values. The sensor could be used in medical diagnosis for the analysis of breath ammonia and biomarkers for predicting CKD in stages higher than 1. The threshold value of 1 ppm represents a feasible value for the presented sensor, which can be defined as a simple, low-value and robust device for individual use, beneficial at the patient level.

1. Introduction

Metal oxides exhibit several critical advantages as gas sensor materials, including simple fabrication at a low cost and applicability to both sensing technologies of oxidizing or reducing gasses [1]. From a structural point of view, most commercial sensors currently present as active detection surfaces of metal oxides in the form of thick films [2]. The considerable difficulties encountered in obtaining sensors based on thin films and ceramic nanotubes prevents their large-scale manufacture. However, their apparent advantages in terms of compactness, precision, and high detection sensitivity encourage researchers to continuously improve the available technologies.
In the last 15 years, some studies have been dedicated to the synthesis of ceramic nanotubes using different technologies [3,4,5,6,7,8,9,10,11,12], but no research has specifically focused on ceramic nanotube composites, despite certain articles discussing carbon–ceramic nanotube composites or ceramic nanoparticles combined with carbon nanotubes, as in references [13,14,15,16,17,18,19,20]. Nanotube-based devices could alter their conductivity because of surface adsorption when they come into contact with chemical substances, similar to findings from extensive research on carbon nanotubes [21]. Adsorption on the surface can also occur with biomolecules like amino acids and proteins, indicating that nanotubes can also be used to detect bioagents [22,23]. Most authors focus on the surface properties of carbon nanotubes; however, none have examined the analogous properties of ceramic nanotubes.
On the other hand, many research results emphasize the advantages of using conducting polymers for sensor applications, alone or as composites with nano-carbon or metallic/ceramic particles, as in [24,25,26,27,28,29,30,31], but, in general, studies on hybrid structures obtained from ceramic nanotubes and conducting polymers are practically non-existent in the literature, despite their potential applications for various types of sensors, but also for photovoltaic energy generation, energy storage in supercapacitors, or other photocatalytic applications.
Resistive gas sensors are flexible and affordable options for identifying a variety of gasses in various scenarios [32,33]. By selecting the appropriate sensing material, resistive gas sensors can be customized to detect a particular gas of interest [34,35,36]. However, their selectivity decreases and their response and recovery times increase [36,37]. Temperature and humidity levels can affect how well resistive gas sensors operate [38]. However, these sensors have a simpler design, which enables easy integration into signal processing systems and efficient mass production. By choosing the right sensing material, resistive gas sensors can be customized to identify a particular gas, such as ammonia. Our paper intends to put the base of a new type of chemiresistors, based on ceramic nanotube composites with conducting polymers, with potential use in breath ammonia detection associated with chronic kidney disease (CKD) [39]. Ammonia breath testing [40,41] was recently appointed as an accurate diagnostic tool, based on the concept that some specific gasses represent by-products of a limited liver and kidney function, which leads to increased blood urea nitrogen (BUN) within the body. The damage to the kidneys may be caused by various conditions, e.g., diabetes, heart disease, age, etc., and a preliminary sign is a metallic taste, which is clearly related to elevated levels of ammonia (NH3) in the mouth. Similarly, this occurrence could be linked to sensitivity to certain foods or food intolerances, highlighting the significance of utilizing these sensors. Regrettably, identifying ammonia in breath is difficult because of its low levels and inherent interferents. Only a few studies have been conducted on the topic of dedicated sensors for detecting ammonia, e.g., [42,43,44]. However, they are not adequate for detecting low concentrations in air, and no commercial biosensor for ammonia has been developed to date. It is known that resistive gas sensors have reduced selectivity and longer response and recovery times. Factors such as temperature, the influence of other exhaled gasses, and humidity may impact the performance of resistive gas sensors for biomedical applications. Yet, for initial inquiries regarding CKD or for regular home monitoring, chemiresistors are deemed effective under room temperature conditions, as long as the readings are not rapidly repeated. The syndrome detection is based on exceeding a threshold value and does not require an exact assessment of the value of the exhaled gas concentration.
The novelty of this paper is mainly related to the development of hybrid nanostructures obtained from ceramic nanotubes and conducting polymers with dedicated sensing features not yet presented in the literature. The surface architecture of ceramic nanotubes represents a critical element that may impact the future development of sensor applications. Another novelty is related to the direct application presented in the paper to evaluate breath ammonia, which can be associated with CKD, under circumstances in which only a few research papers have addressed the development of sensors for this purpose. The sensor principle presented in the paper is more straightforward, cost-effective, and efficient compared to the other currently proposed methods for ammonia detection in CKD, e.g., colorimetry, fluorescence chromatography, thermal decomposition, ion mobility spectrometry, gas chromatography, fluorescence, or photoacoustic detection [45,46,47,48,49].

2. Technology for Obtaining Hybrid Nanostructures from Ceramic Nanotubes and Conducting Polymers

2.1. Technological Equipment

The electrospinning process of polymethyl methacrylate (PMMA) nanofibers was carried out using Neu-Pro-BM equipment from TongLiTech in Wuhan, China.
The Tectra Sputter Coater (Tectra GmbH Physikalische Instrumente, Frankfurt, Germany) was the device employed for radiofrequency (RF) magnetron sputtering.
A specialized furnace, (Nabertherm GmbH, Lilienthal, Germany), operating at temperatures up to 800 °C, was utilized for the calcination of PMMA.

2.2. Materials and Preparation Methods

All chemical components (ceramic powders and polymers) were purchased from Merck (Darmstadt, Germany) and Kurt J. Lesker Company Ltd. (Hastings, UK) and used as received, without any further adjustments.
According to the general technological description in [4,15,50] (Figure 1), the technology for manufacturing Al2O3 and ZnO ceramic nanotubes was based on three stages.
(i) The initial production of polymer fiber meshes made of poly (methyl methacrylate (PMMA), with a molecular weight (Mw) of 300,000, using a 10 wt% solution with dimethylformamide (DMF) as the solvent. To produce freestanding polymer fiber webs, sizable (10 × 10 cm2) square copper frames were employed as collectors within a conventional electrospinning arrangement. These frames were positioned between the syringe needle spinneret and a 20 × 20 cm2 aluminum plate, which functioned as a grounded electrode. The separation distances between the copper frame collectors and the aluminum plate, as well as between the collectors and the spinneret, were approximately 5 cm and 10 cm, respectively. A syringe pump delivered the solution of 10 mL to a blunt needle with a diameter of 0.8 mm at a flow rate of 0.5 mL/h. The drum rotation was 5 rpm. After more tests, the optimum voltage value applied to the needle was 12 kV. Each copper frame was subjected to a collection time of 60 min. The fiber networks showed uniformity in spatial placement and consistency in diameter. However, since ceramic thin films are deposited on PMMA nanofibers, a minimum diameter of 0.3 μm was deemed ideal to ensure the structural stability of the fibers when covered from a mechanical perspective.
(ii) Magnetron deposition of ceramic films
The PMMA nanofibers nets in copper frames were coated with ceramic thin films on both sides using RF magnetron sputtering. In the deposition procedure, ceramic targets measuring 2 inches in diameter and 0.125 inches in thickness were utilized. Regarding the deposition of Al2O3, an RF power of 200 W was utilized on the magnetron, and the deposition duration for each side was 3 h. For ZnO deposition, the RF power applied to the magnetron was 100 W, and the deposition duration for each side was 2 h. In both cases, within the deposition chamber, an argon atmosphere with a purity of 99.99% at a pressure of 5.4 × 10−3 mbar was used as the working gas.
(iii) Thermal treatment of nanotubes
After this process, the PMMA nanofibers nets, coated on both sides with either Al2O3 or ZnO films, were transferred onto a Si/SiO2 substrate and subjected to calcination using a convection oven. The calcination process was conducted at 600 °C for 12 h in ambient air at atmospheric pressure. Following this procedure, three-dimensional web-like networks of Al2O3 and, respectively, ZnO nanotubes were achieved after the complete combustion of PMMA occurring during the calcination process.
The technology of drop-casting was used to obtain hybrid nanostructures from ceramic nanotubes (Al2O3 and ZnO) and the following conducting polymers: poly(3-hexylthiophene) (P3HT), polyaniline emeraldine-base (PANI-EB), and poly(3, 4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT-PS). Five samples of each type were manufactured to compare technological feasibility. The technological process involved the use of the following solutions:
(i) P3HT at a concentration of 15 mg/mL was mixed in chloroform at room temperature using an ultrasonic bath and left for 30 min to ensure even dispersion.
(ii) A solution of 20 mg/mL PANI-EB in N-methyl pyrrolidinone (NMP) was prepared by dissolving it at room temperature using an ultrasonic bath and allowing it to sit for 30 min to ensure even distribution.
(iii) A 1.3 wt% mixture of PEDOT-PS in water was placed in an ultrasonic bath at room temperature for 10 min to ensure even dispersion.
Next, 240 μL of every polymer solution was applied onto ceramic nanotubes (SiO2/Si substrate) using the drop-casting technique with Pasteur pipettes in every instance. Each solvent was evaporated for 60 min in a vacuum, utilizing a Pfeiffer vacuum pump attached to a desiccator.

3. Results and Discussion

3.1. Characterization Equipment

Transmission electron microscopy (TEM) results were obtained using a JEOL 2100 Plus transmission electron microscope operating at an accelerating voltage of 80 kV (JEOL Ltd., Akishima, Tokyo, Japan). Electron diffraction (SAED) analysis was also performed.
Raman spectroscopy was performed using AvaRaman 532 equipment (Avantes B.V., Apeldoorn, The Netherlands).
Fourier-transform infrared spectroscopy (FTIR) was performed using JASCO equipment (Tokyo, Japan), 12000—50 cm−1 spectral range.
X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) were performed with an AXIS Supra+ unit (Kratos Analytical Ltd., Manchester, UK).
Scanning microscopy SEM was performed using Lyra III XMU equipment (TESCAN GROUP a.s., Brno-Kohoutovice, Czech Republic). To evaluate the obtained graphene layer, a progressive morphological analysis was conducted.
Atomic force microscopy (AFM) analysis using a Dimension Edge unit from Bruker in Billerica, MA, USA, was conducted for optical purposes. Average roughness parameters were provided for four scanned zones on every type of sample.

3.2. TEM and Selected Area (Electron) Diffraction (SAED) Analysis

The TEM images for ZnO nanotube composites are presented in Figure 2, Figure 3 and Figure 4. The hexagonal ZnO phase was identified using SAED analysis. Pick intensity is related to the concentration of ZnO nanotubes in the composite assembly. For each diagram, the other picks beyond Zn are specific to the composition of the polymer.

3.3. Raman and FTIR, XPS, and EDS Analysis

Figure 5 and Figure 6 show the Raman spectra recorded at an excitation wavelength of 514 nm for Al2O3, as well as ZnO nanotube composites with P3HT, PEDOT:PS, and PANI-EB. The Raman spectrum of P3HT is identified using the following Raman lines located at 729, 1013, 1092, 1184, 1380, 1442, 1515, and 1620 cm−1 attributed to Cα-S-Cα′ bond deformation vibration modes, Cβ-Calchil stretching, Cβ-H bond bending, Cα-Cα′ stretching, Cβ-H bending, Cβ-Cβ stretching, Cα=Cβ stretching, Cα′=Cβ′ stretching, in addition to the quinoid structure [51]. The Raman lines of PANI-EB are identified at 814, 1176, 1247, 1352, 1414, 1501, 1565, and 1610 cm−1, being attributed to the deformation vibration modes of the benzene ring (B) of the bond C-H in the benzene ring, C-N stretch, C-H bond in the quinoid ring, C-C stretch in the quinoid ring, C-H bond in the quinoid ring, C=N stretch, C=C stretch in the quinoid ring, and C-C stretch in the benzene ring [52]. The Raman spectrum of the PEDOT:PSS copolymer is characterized by the following Raman lines located at 439-574 -990, 1257, 1364, 1439, 1502, and 1569 cm−1, which are attributed to the vibrational modes of deformation of the oxyethylene ring, Cα-Cα′ stretching and C-H bending, Cβ-Cβ′ stretching, symmetric C-C stretching, and asymmetric C-C stretching [53]. In conclusion, regardless of the analyzed sample, only the absorption bands of the three polymers were mainly observed in the IR absorption spectra.
The XPS analysis of the nanotube composites is presented in Figure 7 and Figure 8.
As shown in Figure 3, the elements present on the surface of the Al2O3-P3HT sample and identified according to the general spectrum are oxygen, carbon, sulfur, and silicon; the elements present on the surface of the Al2O3-PANI:EB sample and identified according to the general spectrum are sodium, oxygen, nitrogen, carbon, sulfur, and aluminum; the elements present on the surface of the Al2O3-PEDOT:PSS sample and identified according to the general spectrum are sodium, oxygen, nitrogen, carbon, sulfur and aluminum.
As shown in Figure 4, the elements present on the surface of the ZnO-P3HT sample and identified according to the general spectrum are sodium, oxygen, carbon, silicon; the elements present on the surface of the ZnO-PANI:EB sample and identified according to the general spectrum are zinc, oxygen, nitrogen, carbon. The elements present on the surface of the ZnO-PEDOT:PS sample and identified according to the general spectrum are sodium, zinc, oxygen, carbon, sulfur.
Chemical interactions were also observed, but only between PANI-EB and ZnO ceramic nanotubes. In this case, an additional analysis of the EDS characteristic was performed (Figure 3). The chemical interactions are explained by the presence of oxygen in the polymer used and partially due to the large thickness of the samples.

3.4. SEM Analysis

Preliminary proof of the obtained ceramic nanotubes is presented in Figure 9 and Figure 10.
After conducting comprehensive chemical–physical analyses of the technological stages involved in the production of ceramic nanotubes, we observed a uniform dispersion of PMMA fibers within the deposited nets, along with a uniform deposition of ceramic film upon these fibers. Following the thermal process, the images of ceramic nanotubes confirmed the uniformity of the ceramic nanotube structures, which are hollow inside. Figure 11 and Figure 12 show the SEM analysis at 1000x magnification for Al2O3 and ZnO nanotube composites with P3HT, PEDOT:PS, and PANI-EB. The difference in contrast is due to the thickness of the obtained material.
In general, for both cases of ceramic nanotubes, a uniform morphology with a uniform distribution over the surface of composite nets is observed. There are no significant differences among the sizes of covered nanotubes with different polymers. In the ZnO-PANI:EB sample, the chemical interactions between the nanotube and polymer may lead to rare clusters. The SEM image in particular may offer a general view of the sample’s morphology. Still, in the case of applications of sensors, the surface architecture, which is determined using AFM analysis, is decisive.

3.5. AFM Analysis

The AFM optical analysis shows the grain dimension, their distribution vs. surface area, and the general roughness of surfaces.
In the case of optical images (Figure 13 and Figure 14), where the optical images are a little unclear, the composite film is characterized by a variable thickness. Very homogenous films are obtained for both ceramic nanotubes, mainly in the case of composites with PANI-EB.
After analyzing the AFM images (Figure 15, Figure 16, Figure 17, Figure 18, Figure 19 and Figure 20), it is evident that for both ceramic nanotube composites, the grains are generally arranged either in smaller clusters or larger clusters, leading to the formation of zones with a symmetric distribution. Beyond this, the values of the parameter Rku, close to or slightly above 3, indicate, in all cases, the formation of layers predominantly characterized by peaks and not by depressions, with a Gaussian distribution; Table 1.
In the case of Al2O3 nanotube composites with P3HT and PEDOT:PS, lower peaks of under 1 μm were obtained, and overall, the surfaces are smoother, with low roughness (Figure 15 and Figure 16, Table 1).
In the case of Al2O3 nanotubes composites with PANI-EB, very high and agglomerated peaks of about 5 μm are obtained, and still, their distribution remains uniform despite larger roughness (Figure 17, Table 1). The architecture is settled in such a way that the peaks do not form large holes between them. For all composites based on Al2O3, the RSk parameters are positive and relatively high, indicating that the architecture of the surface is characterized more by higher spaces than by lower spaces.
Regarding the composites with ZnO nanotubes, in all cases, we noticed negative RSk parameters but relatively low values, indicating that the architecture of the surface is characterized more by lower spaces than by higher spaces; Table 1. Here, the peaks are higher for each polymer used compared to homolog Al2O3 nanotube composites. The ZnO nanotube composite also achieves the highest peaks with PANI-EB, exceeding 5 μm; Figure 20. Here, the parameters of RMS and Ra present the highest values of all samples, too, meaning the highest roughness; Table 1.
In summary, the AFM analysis emphasized a higher roughness in the case of PANI-EB for both nanotubes, with a much larger grain size dimension but an evenly distributed assortment of grains, with minimal empty space separating them. Structures with symmetrical distribution and high roughness dimensions at the μm scale are seen as ideal for gas sensor applications as they provide a significant active area for interacting with the gas being targeted.

4. Analysis of Functionality as Gas Sensors for Ammonia

In the literature, different processes of metallization of materials for chemiresitors are described, e.g., drop-casting, electrodeposition in solution, vacuum deposition, etc., but the majority of them are not suitable for basic sensor applications. In our situation, we utilized an ink-jet printer to apply commercial copper ink for printed electronic purposes. A resistor design was created, featuring an operational area of about 2 cm2 (a relatively large surface because the concentration of potentially exhaled NH3 to be detected was also very low) limited by two metalized regions that created the conductive links. The gas sensor’s performance was evaluated with a test system that resembles the one detailed in [54]. The sensor was placed in a sealed container that only permitted gas exchange through two valves and access to the electric connections. A precision ohmmeter was used to externally measure the sensor’s resistance. Different combinations of NH3 in synthetic air (80% nitrogen and 20% oxygen) were transferred through the closed chamber via a valve and released through another valve to keep the pressure at 1 atm. The precise quantity of NH3 in artificial air was individually examined, sample by sample using an SGT-P portable ammonia gas detector (SENKO Advanced Components, Inc., Yokkaichi, Japan) in order to correlate the sensor resistance and NH3 concentration on calibration curves.
Based on the conclusions related to the largest active area of composites with ceramic nanotubes and on a uniform distribution, hybrid structures of Al2O3 and ZnO nanotubes with PANI-EB were selected. Experimental calibration curves were plotted for both sensing structures for a larger domain of NH3 concentration in air of up to 5 ppm. The limit of detection (LoD) was found to be 0.5 ppm for both ceramic nanotube composites, a very reasonable value for the proposed applications; Figure 21. Under this concentration value, the resistance of both sensors presents extremely high and uncontrollable values, with low credibility to be compared with very low gas concentration, which is also difficult to measure with precision by any commercial ammonia gas detector.
The experimental calibration curve for sensing NH3 is presented in Figure 22. A high degree of correlation can be observed for both experimental calibration curves. The curve for ZnO: PANI-EB presents high linearity and lower resistance values, making it suitable for a broader assessment of NH3 levels in air through a basic signal processing system. In contrast, the curve for Al2O3:PANI-EB can be represented by a polynomial curve of at least a second degree, leading to challenges in signal processing and potentially increasing the cost of the sensor. The explanation for the shapes of the characteristics is related to the electronic features of ceramic nanotubes in relation to the conductive polymer. ZnO is a wide-bandgap semiconductor of the II-VI n-type group, with native doping due to oxygen vacancies or zinc interstitials, compared to Al2O3, which exhibits more dielectric features and consequently determines a higher and nonlinear resistance.
For the application of breath ammonia detection, a threshold value of 1 ppm may indicate the syndrome occurrence, i.e., the occurrence of CKD stages greater than 1 [31,32], a threshold feasible for the presented sensor.
Figure 23 presents a comparison of the sensing structures of Al2O3:PANI-EB and ZnO: PANI-EB, showing the change in resistance over time for four NH3 concentrations (0.5, 1, 2, and 3 ppm). The marker “On” is used to signify the start of measurements using NH3 and customized mixtures of synthetic air until the resistance reaches a stable value, as shown in Figure 23. “Off” refers to the state in which only artificial air is directed to the sensor until it reaches its original resistance level in the air. Both resistance reduction and recovery exhibit a quasi-exponential nature. Initially, one can observe the heightened responsiveness of the ZnO: PANI-EB structure, resulting in a faster reaction.
Figure 24 presents the assessment of the sensing structures’ experimental response (on) and recovery time (off). In general, it was observed that the response and recovery times are shorter for the ZnO: PANI-EB structure. The discrepancy increases with greater NH3 concentrations (2 or 3 ppm).
The response time values around 5 s for ZnO: PANI-EB structure at 0.5 ppm NH3 concentration are highly dependable for swiftly detecting CKD, linked to rapid exhalation of air through the mouth. After identifying the syndrome, its intensity can be reassessed by taking a slow breath out lasting around 10–11 s, which is a reasonable step to take. Regarding the recovery time value, it is deemed viable as, at higher NH3 concentrations, such as 3 ppm, the sensor only takes approximately 17 s to regain its initial resistance. Waiting approximately 1 min between consecutive measurements for medical purposes is reasonable, even when evaluating multiple patients using the same device.
The detection mechanism related to ceramic nanotube and conducting polymers is still under research due to the novelty of such assemblies. However, it can be related to the models presented, e.g., those in [55,56,57]. In principle, the effective absorption of gas molecules is the most significant way to achieve a high sensor response, and this is mainly due—in the case of hybrid assemblies—to the large dimension of grain size, with a pretty symmetrical distribution of grains, and with reduced free space between them (a high surface–volume ratio). On the other hand, the semiconducting properties of ceramic nanotubes and conducting polymer assemblies enhance the affinity to tailored gas molecules due to a higher carrier transport and the synergistic interaction between the components, which can be preliminarily evaluated from the XPS characteristic of ZnO-PANI:EB, i.e., by analyzing the high-resolution spectra for each main deep level—Zn 2p, O 1s, N 1s, and C 1s—in the energy domain related to each level (Figure 8). Even if the role of conducting polymer-based nanostructures is considered decisive for the sensing mechanism of gas sensors, as in [58,59,60], new approaches seem to consider that the architecture of hybrid assemblies of conducting polymers and metal oxide plays a major role in enhancing the affinity for detecting a certain gas [61,62,63,64,65]. The present research confirms this theory, in terms of the combination of these different materials resulting in synergistic outcomes. Combining conducting polymers with inorganic nanomaterials creates an efficient method to enhance the movement of charge carriers within and between polymer chains. It is considered that by introducing metal oxide nanostructures into the polymer matrix, a P-N heterojunction is created at the interfaces, which also leads to the generation of a depletion region in both the polymer and metal oxides, as shown by the AFM analysis. Efficient gas molecule absorption is crucial for achieving a strong sensing response in addition to interfacial interactions, as the physical absorption of gas molecules onto the film is the initial stage of gas detection. A coating with a significant surface area, the high volume of pores, and tailored pore size improve the absorption of gas molecules. The presence of target gas molecules on the surface of the nanocomposite film influences the electron levels in the polymers, resulting in a change in the depletion region width and potentially altering the conductive path of the polymers. The increased sensitivity of the nanocomposite films to the target analyte is caused by the combined impact of the altered conductivity and conductive pathway of the polymers in the films.
Regarding, the features of the presented sensor for ammonia, the reaction and restoration times of the sensing devices align with those of similar gas sensors, e.g., based on semiconductive assemblies found in references [49,66,67,68,69,70]. However, in our situation, the quicker response times are attributed to the direct utilization and increased conductivity of ZnO nanotubes and conducting polymer composites. Yet, the sensor characteristics remain inferior or equivalent at this stage regarding the minimum detection limit to other industrial sensors on the market for NH3 concentrations in air, based on different testing methods (colorimetric, plasmonic, capacitive, electrochemical, etc.), as presented in [71,72,73,74,75,76]. However, practically none of those methods or related sensors are feasible for CKD evaluation in the medical environment. As long as the presented sensor can be tailored for different threshold values of NH3 concentrations in air, with critical values of 1–2 ppm, it can be described as a basic, inexpensive, and durable device for personal use, helpful at the individual level, as it allows for better monitoring of the progression of the syndrome or effectiveness of treatment. Overall, the sensor characteristic is in line with the actual methods for CKD detection and evaluation. It can be customized for various threshold levels of NH3 concentrations in the air based on the type of study and the extent of the syndrome. Utilizing a basic, inexpensive, and durable device for personal use can be advantageous for patients as it allows for a more efficient monitoring of syndrome progression or treatment effectiveness at an individual level.
Due to these preliminary successful results, the sensor features (mainly sensitivity, selectivity, response time, and reproducibility) will be further analyzed in the presence of perturbing factors, also determined by the breathing process. The potential influences of exhaled CO2, exhaled humidity, and eventually, exhaled CH4, will be assessed. The study may present additional relevance for associating related digestive syndromes, e.g., irritable bowel syndrome and exhaling CH4 [77], a concept also suggested in [46] in relation to helicobacter pylori infection.

5. Conclusions

The process of producing chemiresistors, based on hybrid nanostructures obtained from Al2O3 and ZnO ceramic nanotubes and conducting polymers—poly(3-hexylthiophene), polyaniline emeraldine-base, and poly(3, 4-ethylenedioxythiophene)-polystyrene sulfonate—was technologically described.
The process of producing ceramic nanotubes from Al2O3 and ZnO involved three distinct and repeatable steps: creating polymer fiber nets using a 10 wt% solution of poly(methyl methacrylate) in dimethylformamide (DMF); depositing Al2O3 and ZnO films onto the PMMA nanofiber nets using magnetron deposition; and heating the nanotubes at 600 °C to completely burn off the PMMA support.
The technology for obtaining hybrid nanostructures from ceramic nanotubes, and subsequently conducting polymers, was drop-casting.
AFM analysis emphasized a higher roughness, mainly in the case of PANI-EB for both nanotube types, with a much larger grain size dimension of over 5 μm but with a relatively symmetrical distribution of grains, with reduced free space between them. The values of the parameter Rku were close or slightly above 3, indicating, in all cases, the formation of layers predominantly characterized by peaks and not by depressions, with a Gaussian distribution.
An ink-jet printer was used to generate chemiresistors from ceramic nanotubes: PANI-EB structures, and the metallization utilized copper ink designed for printed electronics. A resistor was created with a functional area of approximately 2 square centimeters. Calibration curves were experimentally created for both sensing structures, covering a wider range of NH3 concentration in air, reaching up to 5 ppm. It was determined that the detection limit is 0.5 ppm. The ZnO:PANI-EB structure showed great linearity and lower resistance values, which makes it suitable for easily measuring NH3 levels in the air on a large scale with a basic signal processing system. For the application of breath ammonia detection, a threshold value of 1 ppm may indicate the syndrome occurrence, i.e., appointment for CKD stages superior to 1, a threshold feasible for the presented sensor. As long as the sensor can be tailored for different threshold values of NH3 concentrations in breath air, it can be defined as a simple, low-value, and robust device for individual use. This is beneficial at the patient level because the evolution of the syndrome or treatment efficiency can be surveyed more effectively.

Author Contributions

Conceptualization, A.F.T., R.C.C., T.G.S. and O.D.S.; methodology, R.C.C., O.D.S., T.G.S. and S.A.; validation, R.C.C., S.A. and A.F.T.; formal analysis, A.F.T., S.A., T.G.S. and R.C.C.; investigation, R.C.C., O.D.S., T.G.S., A.F.T. and S.A.; data curation, R.C.C., O.D.S., T.G.S. and A.F.T.; writing—original draft preparation, A.F.T. and R.C.C.; writing—review and editing, R.C.C., A.F.T. and S.A.; visualization, R.C.C., O.D.S., T.G.S. and A.F.T.; supervision, A.F.T. and R.C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Chemosensors 12 00198 g001
Figure 1. Technological phases for ceramic nanotube manufacturing.
Figure 1. Technological phases for ceramic nanotube manufacturing.
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Figure 2. TEM image for the ZnO-P3HT composite.
Figure 2. TEM image for the ZnO-P3HT composite.
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Figure 3. TEM image for the ZnO-PANI composite.
Figure 3. TEM image for the ZnO-PANI composite.
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Figure 4. TEM image for the ZnO+PEDOT:PS composite.
Figure 4. TEM image for the ZnO+PEDOT:PS composite.
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Figure 5. Raman spectra of Al2O3 nanotubes composites with P3HT, PANI-EB, and PEDOT:PS.
Figure 5. Raman spectra of Al2O3 nanotubes composites with P3HT, PANI-EB, and PEDOT:PS.
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Chemosensors 12 00198 g006
Figure 6. Raman spectra of ZnO nanotube composites with P3HT, PANI-EB, and PEDOT:PS.
Figure 6. Raman spectra of ZnO nanotube composites with P3HT, PANI-EB, and PEDOT:PS.
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Chemosensors 12 00198 g007aChemosensors 12 00198 g007b
Figure 7. XPS analysis of Al2O3 nanotube composites with P3HT, PANI-EB, and PEDOT:PS.
Figure 7. XPS analysis of Al2O3 nanotube composites with P3HT, PANI-EB, and PEDOT:PS.
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Figure 8. XPS analysis of ZnO nanotube composites with P3HT, PANI-EB, and PEDOT:PS.
Figure 8. XPS analysis of ZnO nanotube composites with P3HT, PANI-EB, and PEDOT:PS.
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Chemosensors 12 00198 g009
Figure 9. PMMA net with Al2O3 ceramic film (500 magnitudes, selected area); Al2O3 ceramic nanotubes after the thermal process (100 k magnitude, with image processing).
Figure 9. PMMA net with Al2O3 ceramic film (500 magnitudes, selected area); Al2O3 ceramic nanotubes after the thermal process (100 k magnitude, with image processing).
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Chemosensors 12 00198 g010
Figure 10. PMMA net with ZnO ceramic film (500 magnitudes, selected area); ZnO ceramic nanotubes after the thermal process (200 k magnitude, with image processing).
Figure 10. PMMA net with ZnO ceramic film (500 magnitudes, selected area); ZnO ceramic nanotubes after the thermal process (200 k magnitude, with image processing).
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Chemosensors 12 00198 g011
Figure 11. SEM image of Al2O3 nanotube composites with P3HT, PEDOT:PS, and PANI-EB.
Figure 11. SEM image of Al2O3 nanotube composites with P3HT, PEDOT:PS, and PANI-EB.
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Chemosensors 12 00198 g012
Figure 12. SEM analysis of ZnO nanotube composites with P3HT, PEDOT:PS, and PANI-EB.
Figure 12. SEM analysis of ZnO nanotube composites with P3HT, PEDOT:PS, and PANI-EB.
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Chemosensors 12 00198 g013
Figure 13. Optical analysis at 500x for Al2O3 nanotube composites with P3HT, PEDOT:PS, and PANI-EB.
Figure 13. Optical analysis at 500x for Al2O3 nanotube composites with P3HT, PEDOT:PS, and PANI-EB.
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Chemosensors 12 00198 g014
Figure 14. Optical analysis at 500x for ZnO nanotube composites with P3HT, PEDOT:PS, and PANI-EB.
Figure 14. Optical analysis at 500x for ZnO nanotube composites with P3HT, PEDOT:PS, and PANI-EB.
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Chemosensors 12 00198 g015
Figure 15. AFM topographic 2D and 3D images and profile lines—Al2O3 nanotube composites with P3HT.
Figure 15. AFM topographic 2D and 3D images and profile lines—Al2O3 nanotube composites with P3HT.
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Chemosensors 12 00198 g016
Figure 16. AFM topographic 2D and 3D images and profile lines—Al2O3 nanotube composites with PEDOT:PS.
Figure 16. AFM topographic 2D and 3D images and profile lines—Al2O3 nanotube composites with PEDOT:PS.
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Chemosensors 12 00198 g017
Figure 17. AFM topographic 2D and 3D images and profile lines—Al2O3 nanotube composites with PANI-EB.
Figure 17. AFM topographic 2D and 3D images and profile lines—Al2O3 nanotube composites with PANI-EB.
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Chemosensors 12 00198 g018
Figure 18. AFM topographic 2D and 3D images and profile lines—ZnO nanotube composites with P3HT.
Figure 18. AFM topographic 2D and 3D images and profile lines—ZnO nanotube composites with P3HT.
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Chemosensors 12 00198 g019
Figure 19. AFM topographic 2D and 3D images and profile lines—ZnO nanotubes composites with PEDOT:PS.
Figure 19. AFM topographic 2D and 3D images and profile lines—ZnO nanotubes composites with PEDOT:PS.
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Chemosensors 12 00198 g020
Figure 20. AFM topographic 2D and 3D images and profile lines—ZnO nanotube composites with PANI-EB.
Figure 20. AFM topographic 2D and 3D images and profile lines—ZnO nanotube composites with PANI-EB.
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Chemosensors 12 00198 g021
Figure 21. Limit of detection for sensing NH3.
Figure 21. Limit of detection for sensing NH3.
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Chemosensors 12 00198 g022
Figure 22. Experimental calibration curve for sensing NH3.
Figure 22. Experimental calibration curve for sensing NH3.
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Chemosensors 12 00198 g023
Figure 23. Experimental resistance–time curves for the sensing structures.
Figure 23. Experimental resistance–time curves for the sensing structures.
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Chemosensors 12 00198 g024
Figure 24. Experimental response (on) and recovery time (off) for the sensing structures.
Figure 24. Experimental response (on) and recovery time (off) for the sensing structures.
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Table 1. Average roughness parameters determined by AFM lines—scanned area 40x40 μm.
Table 1. Average roughness parameters determined by AFM lines—scanned area 40x40 μm.
Scanned MaterialRMS (nm)Ra (nm)RSkRKu
Al2O3 nanotubes—P3HT1981230.625.86
Al2O3 nanotubes—PEDOT:PS2731940.163.97
Al2O3 nanotubes—PANI-EB8817020.083.44
ZnO nanotubes—P3HT1191956−0.152.88
ZnO nanotubes—PEDOT:PS265196−0.134.23
ZnO nanotubes—PANI-EB14131086−0.212.89
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Trandabat, A.F.; Ciobanu, R.C.; Schreiner, O.D.; Schreiner, T.G.; Aradoaei, S. Ceramic Nanotubes—Conducting Polymer Assemblies with Potential Application as Chemosensors for Breath Ammonia Detection in Chronic Kidney Disease. Chemosensors 2024, 12, 198. https://doi.org/10.3390/chemosensors12090198

AMA Style

Trandabat AF, Ciobanu RC, Schreiner OD, Schreiner TG, Aradoaei S. Ceramic Nanotubes—Conducting Polymer Assemblies with Potential Application as Chemosensors for Breath Ammonia Detection in Chronic Kidney Disease. Chemosensors. 2024; 12(9):198. https://doi.org/10.3390/chemosensors12090198

Chicago/Turabian Style

Trandabat, Alexandru Florentin, Romeo Cristian Ciobanu, Oliver Daniel Schreiner, Thomas Gabriel Schreiner, and Sebastian Aradoaei. 2024. "Ceramic Nanotubes—Conducting Polymer Assemblies with Potential Application as Chemosensors for Breath Ammonia Detection in Chronic Kidney Disease" Chemosensors 12, no. 9: 198. https://doi.org/10.3390/chemosensors12090198

APA Style

Trandabat, A. F., Ciobanu, R. C., Schreiner, O. D., Schreiner, T. G., & Aradoaei, S. (2024). Ceramic Nanotubes—Conducting Polymer Assemblies with Potential Application as Chemosensors for Breath Ammonia Detection in Chronic Kidney Disease. Chemosensors, 12(9), 198. https://doi.org/10.3390/chemosensors12090198

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