Development and evaluation of piezoelectric-polymer thin film sensors for low concentration detection of volatile organic compounds related to food safety applications

In this study, the regioregular poly (3-hexyl thiophene) (rr-P3HT) based piezoelectric sensors were developed and evaluated to detect alcoholic volatile organic compounds (VOCs) associated with spoiled and Salmonella typhimurium contaminated packaged beef headspace. The drop coating technique was used to deposit thin films of rr-P3HT on both the sides of quartz crystal microbalance (QCM) electrode. The QCM polymer sensors were found to provide repeatable and reproducible sensor response to alcohol VOCs with a fast recovery (<2 min) at room temperature (25 °C). The principal component analysis on the sensors sensitivities was performed to discriminate the sensed alcohol VOCs, namely: 3-methyl-1-butanol from 1-hexanol. The QCM polymer sensors demonstrated selective response to low concentration of 3-methyl-1-butanol (average estimated lowest detection limit (LDL): 4.35 ppm) and to 1-hexanol (average estimated LDL: 3.20 ppm). The 30 days storage study performed on QCM sensors showed identical sensitivity responses for sensing 3-methyl-1-butanol and 1-hexanol at low concentrations.     

Formaldehyde sensors based on nanofibrous polyethyleneimine/bacterial cellulose membranes coated quartz crystal microbalance

A novel formaldehyde sensor based on nanofibrous polyethyleneimine (PEI)/bacterial cellulose (BC) membranes coated quartz crystal microbalance (QCM) has been successfully fabricated. The nanoporous three-dimensional PEI/BC membranes are composed of nanofibers with diameter of 30-60 nm. The sensor showed high sensitivity with good linearity and exhibited a good reversibility and repeatability towards formaldehyde in the concentration range of 1-100 ppm at room temperature. Moreover, the results showed that the sensing properties were mainly affected by the content of PEI component in nanofibrous membranes, concentration of formaldehyde and relative humidity. Additionally, the nanofibrous PEI/BC membrane coated QCM sensors exhibited a good selectivity to formaldehyde when tested with competing vapors. The simple and feasible method to prepare and coat the PEI/BC sensing membranes on QCM makes it promising for mass production at a low cost.     

Formaldehyde sensing properties of electrospun NiO-doped SnO2 nanofibers

Formaldehyde is a kind of hazardous gases dangerous to human health. Hence, gas sensor is an essential device to monitor formaldehyde in air, especially in indoor ambient. Semiconductor metal oxides are studied as gas-sensing material to detect most of key gases for decade years. For the purpose of actual application and meeting a variety of conditions, diverse additives added into host material are expected to improve the performance of gas sensors. The formaldehyde gas-sensing characteristics of undoped and NiO-doped SnO₂ (NSO) nanofibers synthesized via a simple electrospinning method were investigated in this study. It is noticed that the addition of NiO causes the distortion at the surface of SnO₂ nanofibers, which is responsible to adjust activation energy, grain sizes and chemical states of host material. The sensors fabricated from NSO nanofibers exhibited good formaldehyde sensing properties at operating temperature 200 °C, and the minimum-detection-limit was down to 0.08 ppm. The response time and recovery time of the sensors were about 50 s and 80 s to 10 ppm formaldehyde, respectively. The sensor shows a good long-term stability in 90 days. The simple preparation and excellent properties significantly advance the viability of electrospun nanofibers as gas sensing materials. The sensing mechanisms of NSO nanofibers to formaldehyde were discussed. The results indicated that NSO nanofibers could be used as a candidate to fabricate formaldehyde sensors in practice.     

Polymer-grafted QCM chemical sensor and application to heavy metal ions real time detection

A flow type quartz crystal microbalance (QCM) chemical sensor was developed for monitoring heavy metal ions in aqueous solutions (that is suitable for environmental monitoring). The sensor is based upon surface chelation of the metal ions at multifunctional polymer modified gold electrodes on 9 MHz AT-cut quartz resonators, functioning as a QCM. New processes have been developed which enable to obtain surface-modified gold electrodes with high heavy metal ions complexing ability. These polymer grafted QCM sensors can selectively adsorb heavy metal ions, such as copper, lead, chrome and cadmium, from solution over a wide range from 0.01 to 1000 ppm concentration by complexation with functional groups in the polymers. Cations typically present in natural water did not interfere with the detection of heavy metals. X-Ray Reflectivity (XRR) and Total Reflection X-ray Fluorescence (TXRF) were carried out to characterise the unmodified and modified gold surfaces as well as to verify the possibility to selectively bond and remove metal ions.     

Gas sensing properties at room temperature of a quartz crystal microbalance coated with ZnO nanorods

Gas sensors based on a quartz crystal microbalance (QCM) coated with ZnO nanorods were developed for detection of NH₃ at room temperature. Vertically well-aligned ZnO nanorods were synthesized by a novel wet chemical route at a low temperature of 90 °C, which was used to grow the ZnO nanorods directly on the QCM for the gas sensor application. The morphology of the ZnO nanorods was examined by field-emission scanning electron microscopy (FE-SEM). The diameter and length of the nanorods were 100 nm and 3 μm, respectively. The QCM coated with the ZnO nanorods gas sensor showed excellent performance to NH₃ gas. The frequency shift (Δf) to 50 ppm NH₃ at room temperature was about 9.1 Hz. It was found that the response and recovery times were varied with the ammonia concentration. The fabricated gas sensors showed good reproducibility and high stability. Moreover, the sensor showed a high selectivity to ammoniac gas over liquefied petroleum gas (LPG), nitrous oxide (N₂O), carbon monoxide (CO), nitrogen dioxide (NO₂), and carbon dioxide (CO₂).     

Enhancing the sensitivity of ionic liquid sensors for methane detection with polyaniline template

Flammable gas sensors are essential in occupational health and safety to prevent fire or explosion in gas facilities and underground mining. Our early study demonstrated that ionic liquid (IL)/quartz crystal microbalance (QCM) gas sensors and sensor arrays were excellent for the detection of various organic vapors at both room temperature and elevated temperatures. In this paper, we developed a general method that significantly enhanced the sensitivity of the IL/QCM sensors for flammable gases detection by immobilizing IL on a conductive polymer polyaniline (PAn) template. Studies were performed to optimize the PAn oxidation states, thickness, and IL concentrations. Results showed that the sensitivity increased with increasing the PAn film thickness and the amount of IL immobilized within the PAn film. The sensitivity depended also on the oxidation state and doping state of PAn. With doped and partially oxidized PAn (emeraldine salt) the IL/QCM sensor showed the best performance. The current detection limit for methane was as low as about 115 ppm at room temperature. The sensitivity also depended on the structure of the IL used. Among the four ILs tested, two of them showed excellent sensitivities after being immobilized in the PAn film.     

Detection of heavy metal ions in aqueous solution by P(MBTVBC-co-VIM)-coated QCM sensor

A novel copolymer P(MBTVBC-co-VIM) was designed and successfully synthesized for the fabrication of copolymer-coated QCM sensors to detect the heavy metal ions in aqueous solution. The copolymer P(MBTVBC-co-VIM) contains many nitrogen (N) and sulfur (S) atoms in the side groups as electron donors, which can easily form complexes with heavy metal ions. The strong interaction between the S atom and Au electrode of quartz crystal further assures the stability of copolymer thin films on the quartz crystal surface in aqueous media. The QCM results indicated that the P(MBTVBC-co-VIM)-coated sensor exhibited high sensitivity, stability and selectivity for the detection of Cu²⁺ in aqueous solution. The lowest detection limit can reach 10 ppm Cu²⁺ in aqueous solution, which resulted in the frequency shift of 3.0 Hz (ΔF₃/3). The P(MBTVBC-co-VIM)-coated QCM sensors had porous surface morphologies as revealed by AFM investigation. Such porous structures enhanced the surface areas of the copolymer thin films, which increased the contacting probability of N and S atoms with heavy metal ions in solution and improved the detection sensitivity of the copolymer-coated QCM sensors.     

Measurement system for quartz crystal microbalance sensors

Human olfactory is studied for a long time and in many ways, and many of them are based on gas chromatography technology. They have used gas sensors made of metal-oxide semiconductors. The semiconductor sensors can detect gases as difference in electric resistance by oxidation or reduction of surface on the sensor. Human olfactory is organized by about 2,000 receptors of smell, and many electric sensors are used to emulate the receptors. We consider applying multi-channel QCM for these sensors. However, QCM sensors in a chamber (small box) interfere with each other, and we should examine measures of a design method for anti-interference. In this paper, we propose a design method and build an evaluation system.     

Tethered DNA scaffolds on optical sensor platforms for detection of hipO gene from Campylobacter jejuni

DNA biosensors have gained increased attention over traditional diagnostic methods due to their fast and responsive operation and cost-effective design. The specificity of DNA biosensors relies on single-stranded oligonucleotide probes immobilized to a transduction platform. Here, we report the development of biosensors to detect the hippuricase gene (hipO) from Campylobacter jejuni using direct covalent coupling of thiol- and biotin-labeled single-stranded DNA (ssDNA) on both surface plasmon resonance (SPR) and diffraction optics technology (DOT, dotLab) transduction platforms. This is the first known report of the dotLab to detect targeted DNA. Application of 6-mercapto-1-hexanol as a spacer thiol for SPR gold surface created a self-assembled monolayer that removed unbound ssDNA and minimized non-specific detection. The detection limit of SPR sensors was shown to be 2.5 nM DNA while dotLab sensors demonstrated a slightly decreased detection limit of 5.0 nM (0.005 μM). It was possible to reuse the SPR sensor due to the negligible changes in sensor sensitivity (∼9.7 × 10⁻⁷ ΔRU) and minimal damage to immobilized probes following use, whereas dotLab sensors could not be reused. Results indicated feasibility of optical biosensors for rapid and sensitive detection of the hipO gene of Campylobacter jejuni using specific ssDNA as a probe.     

High-performance liquefied petroleum gas sensing based on nanostructures of zinc oxide and zinc stannate

Toxic and combustible gas detection plays a major role in environmental air quality monitoring. Real-time monitoring of hazardous gases and signal of accidental leakages is of great importance owing to the concern for safety requirements in industries and household applications. A simple and economical method for the fabrication of highly sensitive zinc oxide (ZnO) nanorods based gas sensors for detecting low concentrations of Liquefied Petroleum Gas (LPG) was studied in this work. Platinum (Pt) nanoparticles were deposited on the sensing medium which acts as catalysts to improve the sensor performance. The change in electrical resistance of the metal oxide semiconductor for varying concentrations of LPG was measured. Maximum response of 59% was achieved for 9000 ppm LPG at 250 °C. Further to improve the sensing performance of the sensor towards LPG, surface modification of ZnO nanorods using zinc stannate (Zn₂SnO₄) microcubes was performed. High response of 63% was observed for 3000 ppm LPG at 250 °C. Significant improvement in response of the sensor with Zn₂SnO₄ microcubes on ZnO nanorods was observed when compared to sensor with ZnO nanorods.     

Nano particulate SnO2 based resistive films as a hydrogen and acetone vapour sensor

SnCl₂ (solution) was spin coated on soda lime glass and Al₂O₃ substrate to obtain nano-particulate tin oxide film, directly by sintering at 550 °C for 40 minutes (min). The surface morphology and crystal structure of the tin oxide films were analyzed using atomic force microscopy (AFM) and X-ray diffraction (XRD). The size of SnO₂ nanostructure was determined from UV-vis and found to be ?3 nm. These films were tested for sensing H₂ concentration of 0.1-1000 ppm at optimized operating temperature of 265 °C. The results showed that sensitivity (R_air/R_gas per ppm) goes on increasing with decreasing concentration of test gas, giving concentration dependent changes. Special studies carried out at low concentration levels (0.1-1 and 1-10 ppm) of H₂, give high sensitivity (200 × 10⁻³/ppm) for lowest concentration (0.1-1 ppm) of H₂. The selectivity for H₂ against relative humidity (RH), CO₂, CO and LPG gases is also good. The sensor, at operating temperature of 200 °C, is showing nearly zero response to 300 ppm of H₂, and offering response to acetone vapour of 11 ppm. Selectivity for acetone against RH% and CO₂ was also studied. These sensors can be used as H₂ sensor at an operating temperature of 265 °C, and as an acetone sensor at the operating temperature of 200 °C.     

Nanoporous polystyrene fibers functionalized by polyethyleneimine for enhanced formaldehyde sensing

Here we report a novel fabrication approach to highly sensitive formaldehyde sensors by the surface modification of the electrospun nanofibrous membranes. The three-dimensional fibrous membranes comprising nanoporous polystyrene (PS) fibers were electrospun deposition on quartz crystal microbalance (QCM), followed by the functionalization of the sensing polyethyleneimine (PEI) on the membranes. The morphology and Brunauer-Emmett-Teller (BET) surface area of the fibrous PS membranes with fiber diameter of 110-870 nm were controllable by tuning the concentrations of PS solutions. After PEI modification, PEI particles in clusters of varying sizes (50 nm to 1.2 μm) were immobilized onto the surface of the bead-on-string structured nanoporous fibers. The developed formaldehyde-selective sensors exhibited fast response and low detection limit (3 ppm) at room temperature. This high sensitivity is attributed to the high surface-area-to-volume ratio (∼47.25 m²/g) of the electrospun porous PS membranes and efficient nucleophilic addition reaction between formaldehyde molecules and primary amine groups of PEI.     

Mixed self-assembled lipopolymers with spacer lipids enhancing sensitivity of lipid-derivative QCMs for odor sensors

An odor sensing system using a quartz crystal microbalance (QCM) sensor array and pattern recognition technique has been for years a main research topic in our group. For the general field of artificial olfaction using acoustic-wave based sensors such as QCMs it is vital to search for novel sensing materials. Here we present recent results of our ongoing study on application of pegylated lipids as coatings for QCM odor-sensors. The method presented herein is based on self-assembling of lipids and lipid-derivatives on the QCM surfaces. The disulphide-terminated lipids and lipopolymers are co-chemisorbed onto gold electrodes of QCM sensors by simple immersion in ethanolic solutions. This creates porous supports onto which additional layers of lipopolymers are physisorbed. The method allows for fabrication of lipopolymeric QCM odor-sensors with enhanced sensitivity to odorants, capable of very good discrimination among odorant samples—according to the functional group of an odorant.     

Highly stable and sensitive humidity sensors based on quartz crystal microbalance coated with bacterial cellulose membrane

A novel highly stable and sensitive humidity sensor based on bacterial cellulose (BC) coated quartz crystal microbalance (QCM) has been successfully fabricated. The results showed that the sensors possessed good sensing characteristics by increasing more than two orders of magnitude with increasing relative humidity (RH) from 5 to 97%, and the Log(Δf) showed good linearity (20-97% RH). The sensitivity of sensors coated with BC membranes was four times higher than that of the corresponding cellulose membranes at 97% RH. In addition, the sensor sensitivity is greatly enhanced by increasing the coating load of the BC membranes with more absorption sites in the sensing membranes. Moreover, the experimental results prove that the resultant sensors exhibited a good reversible behavior and good long term stability. Herein, not only a novel and low-cost humidity sensor material was exploited, but also a new application area for BC nanofibrous membranes was opened up.     

Chemisorbed PEGylated lipopolymers as sensing film supports for QCM odor sensors

We report application of the PEGylated lipids (PEG lipopolymers) containing disulphide as supports for sensing films for quartz crystal microbalance (QCM) odor sensors. The materials are binding covalently to the surface of gold QCM sensor electrode creating self-assembled cushion. Additional amounts of lipid or lipid-derived materials can be physisorbed on that chemisorbed coating. Both processes do not require much labor and can be performed with minimum instruments in a simple process. The sensors fabricated with the chemisorbed supports are more sensitive to the tested odorants than their non-supported counterparts. Enhanced sensitivity is derived from higher fluidity of the supported films in comparison to the non-supported ones. Discrimination capability among odorants is also better for the sensors with chemisorbed supports than for the non-supported ones—there are no overlaps between the sample groups and the samples are clustered closely within the groups. Overall, the studied supported sensors introduce interesting properties that can be utilized in the odor sensing systems using QCM sensors.     

Gas sensing properties of SnO2 nanowires on micro-heater

The SnO₂ nanowires (NWs) network gas sensors were fabricated on a micro-electrode and heater suspended in a cavity. The sensors showed selective detection to C₂H₅OH at a heater power during sensor operation as low as 30-40 mW. The gas response and response speed of the SnO₂ NWs sensor to 100 ppm C₂H₅OH were 4.6- and 4.7-fold greater, respectively, than those of the SnO₂ nanoparticles (NPs) sensor with the same electrode geometry. The reasons for these enhanced gas sensing characteristics are discussed in relation to the sensing materials and sensor structures.     

Micromachined catalytic combustible hydrogen gas sensor

An integrated catalytic combustion H₂ sensor has been fabricated by using MEMS technology. Both the sensing elements and the reference elements could be integrated into the suspended micro heaters connected in a suitable circuit such as a Wheatstone configuration with low power consumption. Two sensitive elements and two reference sensors were integrated together onto a single chip. The size of chip was 5.76 mm² and the catalytic combustion sensor showed high response to H₂ at operating voltage of 1 V. The response and recovery times to 1000 ppm H₂ were 0.36 s and 1.29 s, respectively.     

Enhanced H2S sensing characteristics of Pt doped SnO2 nanofibers sensors with micro heater

Gas sensors were designed and fabricated using oxide nanofibers as the sensing materials on micro platforms using micromachining technology. Pure and Pt doped SnO₂ nanofibers were prepared by electrospinning and their H₂S gas sensing characteristics were subsequently investigated. The sensing temperatures of 300 and 500 °C could be attained at the heater powers of 36 and 94 mW, respectively, and the sensors showed high and fast responses to H₂S. The responses of 0.08 wt% Pt doped SnO₂ nanofibers to 4-20 ppm H₂S, were 25.9-40.6 times higher than those of pure SnO₂ nanofibers. The gas sensing characteristics were discussed in relation to the catalytic promotion effect of Pt, nano-scale morphology of electrospun nanofibers, and sensor platform using micro heater.     

Ethanol sensing using CuO/MWNT thin film

Copper (II) oxide (CuO)/multiwall carbon nanotube (MWNT) thin film based ethanol-sensors were fabricated by dispersing CVD-prepared MWNTs in varying concentration over DC magnetron sputtered-CuO films. The responses of these sensors as a function of MWNT concentrations and temperatures were measured, and compared. The sensing response was the maximum at an operating temperature near 400 °C for all the samples irrespective of the MWNTs dispersed over them. At optimum operating temperature (T_opt) of 407 ± 1 °C, the response is linear for 100-700 ppm range and tends to saturate at higher concentrations. In comparison with bare CuO sample, the response of CuO/MWNT sensing films increased up to 50% in the linear range. The response improvement for 2500 ppm of ethanol was up to 90% compared to bare CuO sample. In addition, the sensing response time also reduced to around 23% for lowest ethanol concentration at T_opt. However, a decrease in the sensor response was observed on films with very high concentrations of MWNTs.     

Electrochemical genosensor based on modified octadecanethiol self-assembled monolayer for Escherichia coli detection

An electrochemical genosensor based on 1-fluoro-2-nitro-4-azidobenzene (FNAB) modified octadecanethiol (ODT) self-assembled monolayer (SAM) has been fabricated for Escherichia coli detection. The results of electrochemical response measurements investigated using methylene blue (MB) as a redox indicator reveal that this nucleic acid sensor has 60 s of response time, high sensitivity (0.5 × 10⁻¹⁸ M) and linearity as 0.5 × 10⁻¹⁸-1 × 10⁻⁶ M. The sensor has been found to be stable for about four months and can be used about ten times. It is shown that water borne pathogens like Klebsiella pneumonia, Salmonella typhimurium and other gram-negative bacterial samples has no significant effects in the response of this sensor.