Coated Surface Acoustic Wave Sensors Research Articles

UV light activated g-C3N4 nanoribbons coated surface acoustic wave sensor for high performance sub-ppb level NO2 detection at room temperature

The demand for efficient gas sensors in detecting low ppb-level NO2 gas is increasing rapidly for the human life and environmental protection. In this regard, surface acoustic wave (SAW) based gas sensor with selectively coated one dimensional (1D) graphitic carbon nitride (g-C3N4/GCN) nanostructure interface layer is a viable strategy for the sub-ppb level NO2 gas detection at room temperature (RT∼30 °C). Here, we explore SAW gas sensor employed with 1D GCN nanoribbons (NRs) for NO2 detection under UV-irradiation at RT. Intriguingly, the GCN NRs SAW sensor achieves an ultrahigh frequency response (Δf∼28.13 kHz), short response/recovery times (131 s/116 s), excellent selectivity to NO2 (10 ppm) gas and also records a lowest detection limit (∼42 ppb) under UV activation at RT. The enhanced sensing performances were attributed to the high mass loading effect induced by the enhanced charge transfer between the 1D GCN NRs surface and NO2 gas molecules, boosted by the photoexcited charge carriers, numerous surface sites, and improved specific surface area of 1D GCN NRs. The proposed GCN NRs SAW sensor demonstrates exceptional repeatability, good long-term stability with a minimal loss in its Δf, and exhibited robust sensitivity towards NO2 (10 ppm) under diverse relative humidity conditions (0–80%) at RT. The peculiar sensing results of GCN NRs SAW sensor was elucidates using energy band theory sensing mechanism assisted with resistive type gas sensor. Overall, this study offers significant insights into the development of cost-effective real-time RT based NO2 sensor, paving the way for practical air quality monitoring.

Highly Sensitive and Selective Surface Acoustic Wave Ammonia Sensor Operated at Room Temperature with a Polyacrylic Acid Sensing Layer.

In this study, polyacrylic acid (PAA) films were deposited onto a quartz surface acoustic wave (SAW) resonator using a spin-coating technique for ammonia sensing operated at room temperature, and the sensing mechanisms and performance were systematically studied. The oxygen-containing functional groups on the surfaces of the PAA film make it sensitive and selective to ammonia molecules, even when tested at room temperature. The ammonia molecules adsorbed by the oxygen-containing functional groups of PAA (e.g., hydroxyl and epoxy groups) increase the membrane’s stiffness, which was identified as the primary mechanism leading to the positive frequency shifts. However, mass loading due to adsorption of ammonia molecules is not a major reason as it will result in a negative frequency shifts. When the PAA coated SAW sensor was exposed to ammonia with a low concentration of 500 ppb, it showed a positive frequency shift of 225 Hz, with both good repeatability and stability, as well as a good selectivity to ammonia compared with those to C2H5OH, H2, HCl, H2S, CO, NO2, NO, and CH3COCH3.

Effects of temperature and humidity on the performance of a PECH polymer coated SAW sensor.

The influences of environment, such as temperature, humidity and interfering gases, on the performance of a surface acoustic wave (SAW) sensor in the detection of 2-chloroethyl ethyl sulfide (CEES) were invested. The 150 MHz SAW dual delay lines were used, coated with a poly(epichlorohydrin) (PECH) thin layer, and CEES was detected under different concentrations. Linear correlation between the frequency-shift and the exposure time of the sensor to CEES could be observed, and the limit of CEES could be detected as low as 1.5 mg m−3. Under different temperature (0–50 C°) and humidity (30–80% RH) conditions, CEES was detected by the fabricated SAW sensor coated with PECH, the frequency shifts were measured and the performance of the sensor was evaluated. The results proved that temperature and humidity were the most important factors to influence the performance of SAW sensors; with the decreasing of temperature and the increasing of humidity, there would be larger frequency shifts. In the interference experiments, it was found that most gases existing in the environment in high concentrations would not influence the detection of CEES. Then, the SAW sensor having been fabricated was kept under the conditions of 25 °C and 35% RH for 18 months to further verify the quality, and CEES was detected every so many months. It proved that the performance of the sensor would decrease about 16.39% after 18 months. Although it reflected the attenuation of the sensor to some extent, the sensor was still in good condition. Additionally, the related mechanisms were also discussed.

High temperature CO2 sensing and its cross-sensitivity towards H2 and CO gas using calcium doped ZnO thin film coated langasite SAW sensor

Development of indigenous CO2 sensing element operating at high temperature (≥ 350 °C) is highly desirable. Use of langasite (LGS) based surface acoustic wave (SAW) gas sensors is extremely beneficial compared with other commercially available sensors as it can operate at a higher temperature (> 800 °C). In the present work, we have shown CO2 sensing and its cross-sensitivity performance using LGS based SAW sensor. Compared with LGS with other sensing electrodes, calcium doped ZnO thin film (Ca_ZnO) coated LGS substrate shows enhanced CO2 sensing performance. The interdigitated electrodes (IDT) made of Pt was plated and Ca_ZnO thin film was synthesized and deposited on the LGS substrate. Additionally, we have compared the chemo-resistive gas sensing characteristics of Ca_ZnO thin film with the Ca_ZnO coated SAW sensors. Furthermore, temperature effect on CO2 sensing was studied where optimum response (change in resonant frequency ˜2.469 KHz) was observed at 400 °C. Also, the gas adsorption effect which leads to the change of the resonant frequency of the sensors has been deliberated. Besides, we have discussed the predominant effect of sensor conductivity than mass loading which enhances the reduction of the resonant frequency for thin film coated SAW sensors has been deliberated. Additionally, we have demonstrated the cross-sensitivity of H2 and CO gas in CO2 sensing and their mixed gas sensing behavior operating at 400 °C.

Detection of 2,4-dinitrotoluene using hydrogen-bond acidic polymer coated SAW sensor

A strong hydrogen-bond acidic (HBA) polymer linear fluoroalcoholic polysiloxane (PLF), synthesized by one-step hydrosilylation, was introduced for 2,4-dinitrotoluene (2,4-DNT) detection using surface acoustic wave (SAW) devices. FT-IR spectra and 1H NMR spectra characterizations confirmed its molecular structure. Then, PLF was dissolved in chloroform, and spray coated onto a 434 MHz SAW device. And subsequently, the sensitivity and selectivity of the fabricated sensor were studied based on a SAW test platform. To 2,4-DNT of 1 ppm and 100 ppb (for DNT, 1 ppm = 8.1 mg/m3; 1 ppb = 8.1 μg/m3), the responses of the sensor rose 7.7 and 1.5 kHz, respectively. Furthermore, compared with some common interference vapors at 10 mg/m3, the response of the sensor to 2,4-DNT at 1 ppm was 5 times stronger. The sensitive property was attributed to the hydrogen bond interaction between the hexafluoroisopropanol (HFIP) group in PLF and the basic lone electron pairs of nitro-groups in 2,4-DNT. Comparison between the PLF sensor and hydroxyl group functionalized polymer (carbowax and MOBCD)-coated sensors was investigated, and the results showed that the PLF sensor exhibited better sensitivity. In a word, the PLF sensor exhibited fast response, negligible baseline drifts and excellent reversibility, indicating good candidates of HFIP group functionalized HBA polymers for practical nitroaromatic explosives detection.

Hydrogen-bond acidic polymers coated SAW sensors for 2,4-dinitrotoluene detection

Two strong hydrogen-bond acidic (HBA) polymers DKAP and PLF, were coated onto 434 MHz surface acoustic wave (SAW) devices for rapid detection of an explosive compound, 2,4-dinitrotoluene (2,4-DNT).

Effect of metal oxide sensing layers on the distinct detection of ammonia using surface acoustic wave (SAW) sensors

In the present work, SAW sensors coated with various metal oxides as sensitive layer have been exploited for distinct detection of ammonia. Thin films of different metal oxides (ZnO, SnO2, TeO2 and TiO2) of same thickness (40nm) were deposited under optimized parameters using rf sputtering technique for selective detection of liquor ammonia. As deposited ZnO thin film was highly c-axis oriented, whereas SnO2 thin film was polycrystalline. However the coatings of TiO2 and TeO2 thin films were amorphous. SAW sensor coated with ZnO film (ZnO/SAW) was found to be highly sensitive towards liquor ammonia as compared to other metal oxide coated SAW sensors. Effect of the thickness of ZnO sensing layer (20–80nm) has been studied on the sensing response characteristics. Mass loading and elastic loading are identified as the major contributions towards the response of SAW sensor for ammonia and their contributions were evaluated by fitting with the theory.

Utilization of Mass and Elastic Loading in Oxide Materials Based SAW Devices for the Detection of Mustard Gas Simulant

The properties (mass loading and elastic changes) of different oxide materials (ZnO, TeO2, SnO2, TiO2) in thin film form has been explored for the enhanced detection of DBS (di butyl sulphide), a simulant of sulphur mustard gas. All the four oxide materials are deposited on to the surface of SAW (Surface Acoustic Wave) devices to impart sensitivity and selectivity. ZnO and SnO2 films are crystalline whereas TiO2 and TeO2 films are amorphous in nature. All the films are transparent with transparency greater than 75 % in the visible region. The SAW devices coated with different oxide materials were placed in the feedback loop of colpitt oscillator. With the exposure of DBS vapors, differential frequency increases for TiO2 thin films whereas for other oxide coatings (ZnO, TeO2 and SnO2) it decreases. ZnO coated SAW sensor is found to be maximum sensitive to DBS vapors. Investigation of sensing mechanism revealed that mass loading effect is pronounced in TiO2 thin film whereas for other films change in elasticity is dominant. The oxide coatings are very less sensitive to the other interferants.

Electronic nose based on the polymer coated SAW sensors array for the warfare agent simulants classification

A reference and three polymer coated surface acoustic wave sensors making an electronic nose were introduced. This system contained the pre-concentration unit in order for detection of low level warfare agent simulants. Principal component analysis (PCA), artificial neural network (ANN) and the combination of these two methods (PCA-ANN) were used for differentiation between the three classes including nerve and mustard simulants and mixtures of them. Different polymers including hexafluoro-2-propanol-substituted polysiloxane SXFA, poly(epichlorohydrin) PECH and phenyl methyl polysiloxane (75% phenyl and 25% methyl) OV 25 were used as chemical interfaces and the obtained results were satisfactory. The loading plot of PCA showed that there were no sensors with the similar loadings and thus these three polymers had the enough and effective separated roles in differentiating between above-mentioned various gas classes. It was also proved that the ANN and PCA-ANN had good capability in pattern recognition of aforementioned gas classes. The fabricated system was successfully used for identification of single and binary samples of the three gas classes above- mentioned.

Development of a preconcentration unit for a SAW sensor micro array and its use for indoor air quality monitoring

A new surface acoustic wave (SAW) sensor system for continuous monitoring of air quality was developed. The system employs a miniaturized array of eight polymer coated SAW sensors, a preconcentration unit (‘trap’), and methods of pattern recognition. Care was taken to minimize both the response times of the sensors and the gas volume of the sensor array. Thus, a small trap with low heat capacity can be used, resulting in low power consumption and rapid thermal desorption. The capabilities of the system are demonstrated by successful discrimination of closely related aromatic compounds (BTXE) in the low- and sub-ppm ranges. Design considerations are made with particular emphasis on the necessities arising from the interplay between sensors, coatings, trap, gas fluidics, and pattern recognition software.

Adhesives: a new class of polymer coatings for surface acoustic wave sensors for fast and reliable process control applications

Fast and reliable on-line detection of organic vapors for control of chemical processes is a challenging application for a new type of analytical instruments: sensor systems based on an array of differently selective chemical sensors. In this work we present the use of mass-sensitive polymer coated surface acoustic wave sensors (SAWs). The sensors were initially coated with a standard set of polymers consisting of a known composition. But this first approach could not meet all requirements. Therefore, a new class of commercially available polymer coating, namely adhesives, was developed. The coating procedure was optimized and the aging process of the adhesives was carefully investigated. As a result the selectivity for ambitious separation problems arising from similar polarity of the components of typical solvent mixtures could be remarkably increased. The system was then applied in a real testing environment application at a chemical plant: the fast on-line control of a preparative reversed phase process HPLC (RP-PHPLC). Data from this industrial application are shown.

Inductively coupled, polymer coated surface acoustic wave sensor for organic vapors

We introduce a new device architecture for polymer coated surface acoustic wave (SAW) sensors for the detection of organic vapors. This new design differs from conventional devices because the sensor is inductively coupled to the rf circuitry so that no bonding wires and sockets are necessary. Two designs of SAW sensors operating in different modes were investigated: a one port device for measurements with a network analyzer or a vector voltmeter and a two port device for oscillator applications.

Performance optimization of surface acoustic wave chemical sensors

Acoustic wave devices coated with a thin layer of chemoselective material provide highly sensitive chemical sensors for the detection and monitoring of vapors and gases. In this work, a variety of coating materials and coating deposition techniques have been evaluated on surface acoustic wave (SAW) devices. A novel thin film deposition technique, matrix assisted pulsed laser evaporation (MAPLE), is utilized to coat high quality polymer films on SAW devices, and conventional pulsed laser deposition is used to deposit a passivation layer of diamond-like-carbon on a SAW device surface to prevent water adsorption. In addition, chemoselective coatings are formed by covalent attachment of functionalized species to the silica surface of SAW devices. The self-assembled monolayer or near monolayer structures are designed to populate the SAW device surface with the desirable hexafluoroisopropanol moeity. The rapid kinetic signals achievable with the various coated SAW sensors during vapor tests are examined as a function of the coating material and the quality of the thin films. In parallel to the thin film deposition, growth, and vapor testing, the electrical characteristics of the SAW sensor have been characterized. The quality factor and residual phase noise of polymer coated SAW devices are examined, and a prediction of the theoretical limit of the phase noise performance of the loop oscillator is made.

Development of humidity correction algorithm for surface acoustic wave sensors. Part 2. Mathematical model for water and nitrobenzene co-adsorption on aminopropyltriethoxysilane coated surface acoustic wave sensors

Nitrobenzene and water adsorption isotherms were obtained on an aminopropyltriethoxysilane coated surface acoustic wave sensor (SAWS) at < 5, 17, 33 and 47% relative humidity ( RH) at 6, 13, 25 and 47 °C. The nitrobenzene isotherms were BDDT type 1 (Langmuir) and water isotherms BDDT type 3. The nitrobenzene isotherms were fitted to a series of algorithms based on Langmuir theory and the coefficients of the algorithms derived from the experimental data. The derived model was then used to predict the original isotherms, achieving a good match (better than the ± 12% predicted by error analysis) in the mid-ranges, increasing to as much as minus 35% at 6 °C, 47% RH approaching the saturation vapour pressure of nitrobenzene, and minus 20% at 47 °C, 47% RH approaching the isotherm plateau region. The theoretical model indicates that the nett heat of adsorption of nitrobenzene (isosteric heat of adsorption minus enthalpy of condensation) is the critical parameter affected by RH and temperature. The effect of this on the heat of adsorption is however not readily quantifiable and the corrections needed to model the isotherms had to be derived empirically, rather than based on those two thermodynamic quantities. The results of the work underline the necessity to compensate for changes in RH and temperature, and indicate important selection criteria for coatings to be used with not only an SAWS, but any other sensor which relies on vapour adsorption as part of its response mechanism.

Coated surface acoustic wave sensor employing a reversible mass-amplifying ligand substitution reaction for real-time measurement of 1,3-butadiene at low- and sub-ppm concentrations

Real-time measurement of 1,3-butadiene gas using a surface acoustic wave (SAW) sensor coated with the square-planar Pt(II)-olefin pi-complex PtCl2-(1-hexene) (pyridine) and related complexes is described. Amplification of the sensor response results from displacement of two 1-hexene molecules by each butadiene molecule and formation of the bridged complex [PtCl2(pyridine)]2(1,3-butadiene). Using a 30-MHz SAW oscillator, the rate of frequency change is linearly related to the butadiene air concentration from 150 ppb to > or = 13 ppm and a calculated detection limit of 101 ppb is obtained. Using a 60-MHz oscillator, the detection limit is reduced to 24 ppb. No effect on the sensor response is observed with changes in relative humidity from 5 to 80% or changes in temperature from 25 to 35 degrees C. No interference is observed from several industrially relevant non-olefin organic gases and vapors. Responses are obtained for several olefins, but they interfere with the response to butadiene only at higher relative concentrations. The reagent can be regenerated repeatedly by brief exposure to 1-hexene vapor with retention of the original response characteristics upon subsequent exposure. The potential for using this sensor to monitor occupational 1,3-butadiene exposures is discussed in light of the recently proposed occupational exposure limit of 2 ppm.

Water sorption isotherms on aminopropyltriethoxysilane coated surface acoustic wave sensors

Water sorption isotherms were obtained on surface acoustic wave sensors (SAWS) coated with aminopropyltriethyoxysilane (APTES), and on uncoated SAWS of which the substrate material was polished ST-quartz. The isotherms were obtained at 25°, 30° and 40° over the range 1–80% relative humidity (RH). The isotherms exhibit BDDT type III characteristics typical of weak gas—solid interaction. The isotherms showed good fit to quadratic equations relating frequency change on exposure to humid air with relative humidity. There was no significant hysteresis in the isotherms when the SAWS was taken through a cycle of relative humidity at any of the three temperatures employed. These results are similar to those obtained in earlier work on FPOL and polyvinylpyrollidone coated SAWS. They demonstrate that a correction algorithm based on a quadratic equation should be possible to overcome water vapour response of coated SAWS.

Development of humidity correction algorithm for surface acoustic wave sensors : Part 1. Water adsorption isotherms on coated surface acoustic wave sensors

The adsorption of water vapour onto surface acoustic wave sensors (SAWS) coated with polyvinylpyrrolidone or FPOL (a fluorinated polyether-polyol) was studied at 30, 40 and 50°C. Water adsorption isotherms were obtained which resemble the BDDT type III classification, characteristic of weak gas-solid interactions. At higher temperatures, and lower relative humidity particularly with FPOL, the isotherms demonstrated zero or negative response on exposure to water vapour (i.e., the frequency of the SAWS oscillator increased when exposed to humid air compared with dry air). Although this phenomenon is not yet understood, it may be due to swelling of the polymer, thus affecting its viscoelastic properties. The isotherms can be fitted to a quadratic equation, which is reasonable for a BDDT type III isotherm. It is clear that operation at 40 or 50°C will significantly reduce the effect of water vapour on the SAWS, although it is recognized that the adsorption of target gases may also be decreased.