Chemosensors, Volume 6, Issue 4 (December 2018) – 25 articles

The SiO₂ZrO₂ composite films were prepared by means of sol-gel technology and characterized by scanning electron microscopy, energy dispersive X-ray (EDX) analysis, and X-ray diffraction. The presence of the stable monoclinic ZrO₂ with an impurity of tetragonal phases is shown. The film surface is characterized by the presence of ZrOCl₂·6H₂O or ZrCl(OH)/ZrCl(OH)₂ grains. The crystallite size negligibly depends on the annealing temperature of the film and amount to 10–12 nm and 9–12 nm for the films thermally treated at 200 °C and 500 °C, respectively. The film’s resistance is rather sensitive to the presence of NO₂ impurities in the air at a low operating temperature (25 °C). Accelerated stability tests of the initial resistance showed high stability and reproducibility of the sensor based on the SiO₂ZrO₂ film thermally treated at 500 °C. Full article

Microelectronic devices have great potential to be integrated into the Internet of Things, bringing benefits to the environment, society, and economy. Especially, microscaled chemical sensors for environmental monitoring are of great interest since they can be manufactured by cost, time, and resource efficient inkjet printing technology. The aim of the present literature review is a reflection of state-of-the-art inkjet-printed chemiresistive sensors. It examines current material approaches used to realize printed chemiresistors, especially the challenges in the realisation of accurate electrode patterns as well as the deposition of various sensing materials by inkjet printing technology. The review will be completed by an overview of current research activities dealing with the integration of chemiresistive sensors into wireless applications. The result of this review confirms that during the last decades, the number of publications covering inkjet-printed chemical, especially chemiresistive, sensors and their introduction into the Internet of Things is growing. Furthermore, it reveals the need for further research regarding material science and printing technology compatibility to achieve reliable and reproducible chemiresistive sensors. Full article

Refluxed zinc oxide (ZnO) nanoparticles (NPs) were prepared and attached to carboxylic acid functionalized multi-walled carbon nanotubes (COOH-MWNTs) via sonication. Practical optimization of electrocatalysts using sonication to disentangle a carbon nanotube composite for monitoring uric acid (UA) is shown. Monitoring UA is important for the management of medical disorders. Selection of sonication time is a crucial step in producing the desired composite. We report, for the first time, the practical use of Raman spectroscopy to tune the sonication involved in tethering ZnO NPs to the multi-walled carbon nanotube (MWNT) surface. Maximum current for detecting UA, using chronoamperometry and cyclic voltammetry, correlated with the highest sp²-hybridized carbon signal, as seen in the integrated Raman G band peak areas denoting maximum COOH-MWNT disentanglement. An array of ZnO/COOH-MWNT composites were prepared ranging from 60 to 240 min sonication times. Optimum sonication (150 min) corresponded with both maximum measured current and MWNT disentanglement. The sensor was able to quantitatively and selectively measure UA at clinically relevant concentrations (100–900 μM) with rapid current response time (< 5 s). Full article

The anticipated diagnosis of various fatal diseases from the analysis of volatile organic compounds (VOC) biomarkers of the volatolome is the object of very dynamic research. Nanocomposite-based quantum resistive vapor sensors (vQRS) exhibit strong advantages in the detection of biomarkers, as they can operate at room temperature with low consumption and sub ppm (part per million) sensitivity. However, to meet this application they need to detect some ppm or less amounts of biomarkers in patients’ breath, skin, or urine in complex blends of numerous VOC, most of the time hindered by a huge amount of water molecules. Therefore, it is crucial to analyze the effects of moisture on the chemo-resistive sensing behavior of carbon nanotubes based vQRS. We show that in the presence of water molecules, the sensors cannot detect the right amount of VOC molecules present in their environment. These perturbations of the detection mechanism are found to depend on the chemical interactions between water and other VOC molecules, but also on their competitive absorption on sensors receptive sites, located at the nanojunctions of the conductive architecture. This complex phenomenon studied with down to 12.5 ppm of acetone, ethanol, butanone, toluene, and cyclohexane mixed with 100 ppm of water was worth to investigate in the prospect of future developments of devices analysing real breath samples in which water can reach a concentration of 6%. Full article

Thin films of fluorinated chromium phthalocyanine were prepared using spin coating techniques and annealed at 100, 200, 300, and 400 °C. The prepared films were investigated using UV-Visible absorption spectroscopy, Raman spectroscopy, and atomic force microscopy. The band gap characteristics were evaluated to study the difference electronic transitions between the prepared thin films under different annealing temperatures. Films were exposed to ammonia vapor in a concentration range of 40–100 ppm to demonstrate the gas sensing activity of prepared devices. Resistance versus voltage behavior was investigated upon the exposure of ammonia gas and the samples show an increase in the resistance towards the existence of ammonia molecules. The dependency of the sensors on time was studied to evaluate the response and recovery time, which were found to be 10 and 13 s respectively. Full article

Carbon nanotubes have been attracting considerable interest among material scientists, physicists, chemists, and engineers for almost 30 years. Owing to their high aspect ratio, coupled with remarkable mechanical, electronic, and thermal properties, carbon nanotubes have found application in diverse fields. In this review, we will cover the work on carbon nanotubes used for sensing applications. In particular, we will see examples where carbon nanotubes act as main players in devices sensing biomolecules, gas, light or pressure changes. Furthermore, we will discuss how to improve the performance of carbon nanotube-based sensors after proper modification. Full article

In this study, the gas sensing properties of Co-doped ZnO nanoparticles (Co-ZnO NPs) synthesized via a simple sol-gel method are reported. The microstructure and morphology of the synthesized Co-ZnO NPs were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. Co-ZnO NPs were then used for developing a conductometric gas sensor for the detection, at mild temperature, of low concentration of hydrogen (H₂) in air. To evaluate the selectivity of the sensor, the sensing behavior toward some VOCs such as ethanol and acetone, which represent the most important interferents for breath hydrogen analysis, was also investigated in detail. Results reported demonstrated better selectivity toward hydrogen of the Co-ZnO NPs sensor when compared to pure ZnO. The main factors contributing to this behavior, i.e., the transition from n-type behavior of pristine ZnO to p-type behavior upon Co-doping, the modification of oxygen vacancies and acid-base characteristics have been considered. Hence, this study highlights the importance of Co doping of ZnO to realize a high performance breath hydrogen sensor. Full article

The various forms of carbon nanostructures are providing extraordinary new opportunities that can revolutionize the way gas sensors, electrochemical sensors and biosensors are engineered. The great potential of carbon nanostructures as a sensing platform is exciting due to their unique electrical and chemical properties, highly scalable, biocompatible and particularly interesting due to the almost infinite possibility of functionalization with a wide variety of inorganic nanostructured materials and biomolecules. This opens a whole new pallet of specificity into sensors that can be extremely sensitive, durable and that can be incorporated into the ongoing new generation of wearable technology. Within this context, carbon-based nanostructures are amongst the most promising structures to be incorporated in a multi-functional platform for sensing. The present review discusses the various 1D, 2D and 3D carbon nanostructure forms incorporated into different sensor types as well as the novel functionalization approaches that allow such multi-functionality. Full article

Biomolecule immobilization on bulk silicon dioxide (SiO₂) is an important aspect in the field of Si-based interfaces for biosensing. The approach used for surface preparation should guarantee not only the stable anchoring of biomolecules but also their structural integrity and biological functioning. In this paper, we review our findings on the SiO₂ functionalization process to immobilize a variety of biomolecules, including glucose oxidase, horseradish peroxide, metallothionein, and DNA molecules. Morphological and chemical characterization of SiO₂ surfaces after biomolecule immobilization using techniques already employed in the microelectronic industry are presented and discussed. Optical and spectrophotometric analysis revealed the preservation of biomolecules’ activity once they are anchored on the biointerface. Full article

L-glutamate is one the most important excitatory neurotransmitter at the central nervous system level and it is implicated in several pathologies. So, it is very important to monitor its variations, in real time in animal models’ brain. The present study aimed to develop and characterize a new amperometric glutamate biosensor design that exploits the selectivity of Glutamate Oxidase (GluOx) for l-glutamate, and the capability of a small molecule as propylene glycol (PG), never used before, to influence and extend the stability and the activity of enzyme. Different designs were evaluated by modifying the main components in their concentrations to find the most suitable design. Moreover, enzyme concentrations from 100 U/mL up to 200 U/mL were verified and different PG concentrations (1%, 0.1% and 0.05%) were tested. The most suitable selected design was Pt_c/PPD/PEI(1%)₂/GlutOx₅/PG(0.1%) and it was compared to the same already described design loading PEDGE, instead of PG, in terms of over-time performances. The PG has proved to be capable of determining an over-time stability of the glutamate biosensor in particular in terms of linear region slope (LRS) up to 21 days. Full article

A sensing platform for the in situ, real-time analysis of phosphate in natural waters has been realised using a combination of microfluidics, colorimetric reagent chemistries, low-cost LED-based optical detection and wireless communications. Prior to field deployment, the platform was tested over a period of 55 days in the laboratory during which a total of 2682 autonomous measurements were performed (854 each of sample, high standard and baseline, and 40 × 3 spiked solution measurements). The platform was subsequently field-deployed in a freshwater stream at Lough Rea, Co., Galway, Ireland, to track changes in phosphate over a five day period. During this deployment, 165 autonomous measurements (55 each of sample, high standard, and baseline) were performed and transmitted via general packet radio service (GPRS) to a web interface for remote access. Increases in phosphate levels at the sampling location coincident with rainfall events (min 1.45 µM to max 10.24 µM) were detected during the deployment. The response was found to be linear up to 50 µM PO₄³⁻, with a lower limit of detection (LOD) of 0.09 µM. Laboratory and field data suggest that despite the complexity of reagent-based analysers, they are reasonably reliable in remote operation, and offer the best opportunity to provide enhanced in situ chemical sensing capabilities. Modifications that could further improve the reliability and scalability of these platforms while simultaneously reducing the unit cost are discussed. Full article

Metal oxides (MOx) are a well-established material for gas sensing. MOx-based gas sensors are sensitive to a wide variety of gases. Furthermore, these materials can be applied for the fabrication of low-cost and -power consumption devices in mass production. The market of carbon dioxide (CO ${}_2$ ) gas sensors is mainly dominated by infra-red (IR)-based gas sensors. Only a few MOx materials show a sensitivity to CO ${}_2$ and so far, none of these materials have been integrated on CMOS platforms suitable for mass production. In this work, we report a cupric oxide (CuO) thin film-based gas sensor functionalized with gold (Au) nanoparticles, which exhibits exceptional sensitivity to CO ${}_2$ . The CuO-based gas sensors are fabricated by electron beam lithography, thermal evaporation and lift-off process to form patterned copper (Cu) structures. These structures are thermally oxidized to form a continuous CuO film. Gold nanoparticles are drop-coated on the CuO thin films to enhance their sensitivity towards CO ${}_2$ . The CuO thin films fabricated by this method are already sensitive to CO ${}_2$ ; however, the functionalization of the CuO film strongly increases the sensitivity of the base material. Compared to the pristine CuO thin film the Au functionalized CuO film shows at equal operation temperatures (300 ${}^{\circ }$ C) an increase of sensitivity towards the same gas concentration (e.g., 2000 ppm CO ${}_2$ ) by a factor of 13. The process flow used to fabricate Au functionalized CuO gas sensors can be applied on CMOS platforms in specific post processing steps. Full article

Carbon nanotubes are unique one-dimensional materials which can experience a modification in their optical properties as the chemical composition of their ambient environment varies. One of the ways to interrogate these variations in optical properties is through the use of optical fibres. As such, their integration with optical fibre technology would potentially allow for the development of devices for various chemical sensing applications. Full article

Nanostructured nickel on porous carbon-silica matrix (N-CS) has been synthesized using a sol gel process and subsequent pyrolysis treatment at a temperature of 650 °C. The morphology and microstructure of the N-CS sample has been investigated using XRD (X-ray Diffraction), SEM-EDS (Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy), and BET (Brunauer-Emmett-Teller) analysis. The synthesized nanocomposite has been used for developing NCS-modified screen-printed electrodes (NCS-SPCEs) and was applied in the electrochemical monitoring of glucose. After electrochemical activation, via cycling the modified electrode in a potential window from 0 to 0.8 V in 0.1 M KOH solution, the fabricated NCS-SPCEs electrodes were evaluated for the voltammetric and amperometric determination of glucose. The developed sensors showed good sensing performance towards glucose, displaying a sensitivity of 585 µA/mM cm⁻¹ in the linear range from 0.05 to 1.5 mM, a detection limit lower than 30 µM with excellent selectivity. Full article

The thermal degradation related to stability in long-term storage of a carbon nanotube-based biosensor has been investigated. The effect of storage temperature on detachment and denaturation of glucose oxidase (GOx) biocatalyst has been proved. The carbon nanotubes (CNTs) coated with polyethyleneimine (PEI) as entrapping polymer to attract more GOx to form a durable and layered CNT/PEI/GOx structure is used for long-term storage to minimize GOx detachment from the structure and minimize the possibility of enzyme and protein denaturation. After 120 days, the glucose response of the CNT/PEI/GOx biosensor stored under 4°C is preserved up to 66.7% of its initial value, while under a 25 °C storage the response is maintained up to 41.7%. The enzyme coverage activity of CNT/PEI/GOx stored at 4 °C and 25 °C has decreased by 31.1% and 51.4%, respectively. Denaturation and detachment of GOx are the common causes of thermal degradation in biosensors under improper storage temperatures, but the presence of PEI in the structure can slow-down these phenomena. Moreover, the electrons transfer constant of CNT/PEI/GOx biocatalyst stored at 4 °C and 25 °C were 7.5 ± 0.5 s⁻¹ and 6.6 ± 0.3 s⁻¹, respectively, indicating that also electrons mobility is damaged by detachment and denaturation of enzyme protein and the detection of glucose from the glucose oxidation reaction (GOR) is compromised. Full article

The article reviews the current status of label-free integrated optical biosensors focusing on the evolution over the years of their analytical performance. At first, a short introduction to the evanescent wave optics is provided followed by detailed description of the main categories of label-free optical biosensors, including sensors based on surface plasmon resonance (SPR), grating couplers, photonic crystals, ring resonators, and interferometric transducers. For each type of biosensor, the detection principle is first provided followed by description of the different transducer configurations so far developed and their performance as biosensors. Finally, a short discussion about the current limitations and future perspectives of integrated label-free optical biosensors is provided. Full article

Drinking water contamination of lead from various environmental sources, leaching consumer products, and intrinsic water-pipe infrastructure is still today a matter of great concern. Therefore, new highly sensitive and convenient Pb²⁺ measurement schemes are necessary, especially for in-situ measurements at a low cost. Within this work dye/ionophore/Pb²⁺ co-extraction and effective water phase de-colorization was utilized for highly sensitive lead measurements and sub-ppb naked-eye detection. A low-cost ionophore Benzo-18-Crown-6-ether was used, and a simple test-tube mix and separate procedure was developed. Instrumental detection limits were in the low ppt region (LOD = 3, LOQ = 10), and naked-eye detection was 500 ppt. Note, however, that this sensing scheme still has improvement potential as concentrations of fluorophore and ionophore were not optimized. Artificial tap-water samples, leached by a standardized method, demonstrated drinking water application. Implications for this method are convenient in-situ lead ion measurements. Full article

The electro-reduction of nitrate ions in artificial sea water was investigated at a gold substrate (E_Au) functionalized by silver nanoparticles (AgNPs). These AgNPs were generated in situ on the gold substrate by the direct decomposition of the metalorganic N,N′-diisopropylacetamidinate silver precursor [Ag(Amd)] in the liquid phase. Very small and well dispersed AgNPs were deposited on the gold electrode and then used as working electrode (E_Au/AgNPs). Square wave voltammetry (SWV) was successfully employed to detect nitrate ions (NO₃⁻) with a detection limit (LOD) of 0.9 nmol∙L⁻¹ in artificial sea water (pH = 6.0) without pre-concentration or pH adjustment. Full article

We report on studies of new gas sensing devices to be used in high humidity environments. Highly thermal-stable, super hydrophobic 2-dimensional (2D) boron nitride nanosheets (BNNSs) functionalized with Pt nanoparticles were prepared and used as an active layer for the prototype. The morphologic surface, crystallographic structures and chemical compositions of the synthesized 2D materials were characterized by using optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM) and atomic force microscope (AFM) and Raman scattering, respectively. The experimental data reveals that high-quality BNNSs were prepared. A pair of Au electrodes were combined with a basic electrical circuit and the 2D sensing material to form high-performance gas sensors for the detection of pollution gases. The present structure is simple and the fabrication is easy and fast, which ensures the creation of a low-cost prototype with harsh (high humidity, high temperature) environment resistance and potential for miniaturization. The responses of the prototype to different target gases with different concentrations were characterized. The influences of the operating temperature and bias voltage effect on sensing performances were also investigated. The fabricated sensors appear to have high selectivity, high sensitivity and fast response to target gases. The sensing mechanism in the present case is attributed to the electron donation from the target gas molecules to the active layer, leading to the change of electrical properties on the surface of BNNS layer. Full article

Recent progress in the area of molecular logic, in particular molecules capable of sensing for acidity and oxidizability, are gathered together in this short review. Originally proposed as AND logic gates that provide a high fluorescence output when simultaneously protonated and oxidized, the concept has been extended from two-input to three-input variants and to include molecules that function as INHIBIT logic gates. Photochemical concepts such as photoinduced electron transfer (PET) and internal charge transfer (ICT) are exploited as favorite design concepts. This review highlights the evolution of Pourbaix sensors with anthracene, pyrazoline, and naphthalimide fluorophores. Future applications abound in various disciplines from corrosion science, material science, geochemistry to cell imaging. Full article

In this study, we examine a means for developing near-IR fluorescent sensors through streamlined, site-specific coupling with peptide-based receptors. As the penultimate step of solid-phase synthesis of a peptide-based receptor, we show a simple means of labeling the N’ terminus with the near IR fluorophore IR-783 to afford a viable fluorescent sensor after cleavage from the resin. The proof-of-concept probe utilized a biotin mimetic peptide sequence as the receptive moiety. Here we revealed a “turn-on” fluorescence enhancement upon binding of the biotin mimetic probe to its intended streptavidin target. Not all peptide-receptive moieties tested were able to generate such an enhancement upon target binding, and as such, the rationale for the observed fluorescence response properties is discussed. Full article

A micro-level technique so-called “microfluidic technology or simply microfluidic” has gained a special place as a powerful tool in bioengineering and biomedical engineering research due to its core advantages in modern science and engineering. Microfluidic technology has played a substantial role in numerous applications with special reference to bioscience, biomedical and biotechnological research. It has facilitated noteworthy development in various sectors of bio-research and upsurges the efficacy of research at the molecular level, in recent years. Microfluidic technology can manipulate sample volumes with precise control outside cellular microenvironment, at micro-level. Thus, enable the reduction of discrepancies between in vivo and in vitro environments and reduce the overall reaction time and cost. In this review, we discuss various integrations of microfluidic technologies into biotechnology and its paradigmatic significance in bio-research, supporting mechanical and chemical in vitro cellular microenvironment. Furthermore, specific innovations related to the application of microfluidics to advance microbial life, solitary and co-cultures along with a multiple-type cell culturing, cellular communications, cellular interactions, and population dynamics are also discussed. Full article

The development of electronic-nose (e-nose) technologies for disease diagnostics was initiated in the biomedical field for detection of biotic (microbial) causes of human diseases during the mid-1980s. The use of e-nose devices for disease-diagnostic applications subsequently was extended to plant and animal hosts through the invention of new gas-sensing instrument types and disease-detection methods with sensor arrays developed and adapted for additional host types and chemical classes of volatile organic compounds (VOCs) closely associated with individual diseases. Considerable progress in animal disease detection using e-noses in combination with metabolomics has been accomplished in the field of veterinary medicine with new important discoveries of biomarker metabolites and aroma profiles for major infectious diseases of livestock, wildlife, and fish from both terrestrial and aquaculture pathology research. Progress in the discovery of new e-nose technologies developed for biomedical applications has exploded with new information and methods for diagnostic sampling and disease detection, identification of key chemical disease biomarkers, improvements in sensor designs, algorithms for discriminant analysis, and greater, more widespread testing of efficacy in clinical trials. This review summarizes progressive advancements in utilizing these specialized gas-sensing devices for numerous diagnostic applications involving noninvasive early detections of plant, animal, and human diseases. Full article

Olfactory systems typically process signals produced by mixtures composed of very many natural odors, some that can be elicited by single compounds. The several hundred different olfactory receptors aided by several dozen different taste receptors are sufficient to define our complex chemosensory world. However, sensory processing by selective adaptation and mixture suppression leaves only a few perceptual components recognized at any time. Thresholds determined by stochastic processes are described by functions relating stimulus detection to concentration. Relative saliences of mixture components are established by relating component recognition to concentration in the presence of background components. Mathematically distinct stochastic models of perceptual component dominance in binary mixtures were developed that accommodate prediction of an appropriate range of probabilities from 0 to 1, and include errors in identifications. Prior short-term selective adaptation to some components allows temporally emergent recognition of non-adapted mixture-suppressed components. Thus, broadly tuned receptors are neutralized or suppressed by activation of other more efficacious receptors. This ‘combinatorial’ coding is more a process of subtraction than addition, with the more intense components dominating the perception. It is in this way that complex chemosensory mixtures are reduced to manageable numbers of odor notes and taste qualities. Full article

Sensitive and specific DNA biomarker detection is critical for accurately diagnosing a broad range of clinical conditions. However, the incorporation of such biosensing structures in integrated microfluidic devices is often complicated by the need for an additional labelling step to be implemented on the device. In this review we focused on presenting recent advances in label-free DNA biosensor technology, with a particular focus on microfluidic integrated devices. The key biosensing approaches miniaturized in flow-cell structures were presented, followed by more sophisticated microfluidic devices and higher integration examples in the literature. The option of full DNA sequencing on microfluidic chips via nanopore technology was highlighted, along with current developments in the commercialization of microfluidic, label-free DNA detection devices. Full article