Surface Acoustic Wave Gas Sensor Research Articles

Analysis of the acoustoelectric response of SAW gas sensors using a COM model

Surface acoustic wave (SAW) gas sensors based on the acoustoelectric effect exhibit wide application prospects for in situ gas detection. However, establishing accurate models for calculating the scattering parameters of SAW gas sensors remains a challenge. Here, we present a coupling of modes (COM) model that includes the acoustoelectric effect and specifically explains the nonmonotonic variation in the center frequency with respect to the sensing film’s sheet conductivity. Several sensing parameters of the gas sensors, including the center frequency, insertion loss, and phase, were experimentally compared for accuracy and practicality. Finally, the frequency of the phase extremum (FPE) shift was determined to vary monotonically, and the range of selectable test points was wide, making the FPE an appropriate response parameter for leveraging in SAW gas sensors. The simulation results of the COM model were highly consistent with the experimental results. Our study is proposed to provide theoretical guidance for the future development of gas SAW sensors.

Indium-Doped SnO2 Based Surface Acoustic Wave Gas Sensor with Ba0.6Sr0.4TiO3 Film

SnO2-based gas sensors have been widely synthesized and used for the detection of various hazardous gases. However, the use of doped SnO2 in sensing applications has recently attracted increased interest due to the formation of a synergistic effect between the dopant and the host. Moreover, in the case of a surface acoustic wave (SAW) sensor, the piezoelectric material used in the fabrication of the sensor plays a crucial role in defining the response of the SAW sensor. As a ferroelectric material, barium strontium titanate (Ba0.6Sr0.4TiO3) has recently been studied due to its intriguing dielectric and electromechanical properties. Its high acoustic velocity and coupling coefficient make it a promising candidate for the development of acoustic devices; however, its use as a piezoelectric material in SAW sensors is still in its infancy. In this paper, we present the design, fabrication and validation of an indium doped SnO2-based SAW gas sensor on Ba0.6Sr0.4TiO3 thin film for room temperature (RT) applications. Pulsed laser deposition was used to deposit thin films of Ba0.6Sr0.4TiO3 and indium-doped SnO2. Different characterization techniques were employed to analyze the morphology and crystallization of the films. The performance of the fabricated sensor was validated by exposing it to different concentrations of ethanol and then analyzing the recorded frequency shift. The sensor exhibited fast response (39 s) and recovery (50 s) times with a sensitivity of 9.9 MHz/Δ. Moreover, the sensor had good linear response and reproducibility. The fabricated indium-doped SnO2-based SAW gas sensor could be suitable for practical room temperature applications.

Sensitive Materials Used in Surface Acoustic Wave Gas Sensors for Detecting Sulfur-Containing Compounds.

There have been many studies on surface acoustic wave (SAW) sensors for detecting sulfur-containing toxic or harmful gases. This paper aims to give an overview of the current state of polymer films used in SAW sensors for detecting deleterious gases. By covering most of the important polymer materials, the structures and types of polymers are summarized, and a variety of devices with different frequencies, such as delay lines and array sensors for detecting mustard gas, hydrogen sulfide, and sulfur dioxide, are introduced. The preparation method of polymer films, the sensitivity of the SAW gas sensor, the limit of detection, the influence of temperature and humidity, and the anti-interference ability are discussed in detail. The advantages and disadvantages of the films are analyzed, and the potential application of polymer films in the future is also forecasted.

A passive wireless surface acoustic wave (SAW) sensor system for detecting warfare agents based on fluoroalcohol polysiloxane film

Long-term monitoring of environmental warfare agengts is a challenge for chemical gas sensors. To address this issue, we developed a 433 MHz passive wireless surface acoustic wave (WSAW) gas sensor for dimethyl methylphosphonate (DMMP) detection. This WSAW gas sensor includes a YZ lithium niobate (LiNbO3) substrate with metallic interdigital transducers (IDTs) etched on it, and an antenna was placed near the IDT. A DMMP-sensitive viscoelastic polymer fluoroalcoholpolysiloxane (SXFA) film was prepared on a LiNbO3 substrate, and mode modeling coupling was used to optimize the design parameters. The sensor can function properly in an environments between −30 °C and 100 °C with humidity less than 60% RH. When the wireless transmission distance was within the range of 0–90 cm, the sensor noise increased with distance, and the stability was less than 32°/h. While optimizing the film thickness of SXFA, a relationship was observed between sensor sensitivity and film thickness. When the film thickness of SXFA reached 450 nm, the optimal value was reached. At a distance of 20 cm between the transmitting and receiving antennas, DMMP was detected at different concentrations with the developed WSAW gas sensor. The lower detection limit of DMMP was 0.48 mg/m3, the sensitivity of the sensor was 4.63°/(mg/m3), and repeatable performance of the sensor was confirmed.

A multimodal lithium niobate-based SAW gas sensor for accurate detection of carbon dioxide at elevated temperature

Carbon dioxide (CO2) detection is crucial for safety control, monitoring of the environment, and other industrial applications. The diverse fields of applications make the detection of CO2 a challenging task. In this paper, a study on a multimodal surface acoustic wave (SAW) CO2 sensing system was conducted to scrutinize the sensitivity of the lithium niobate-based multimodal SAW sensor towards CO2 with temperature compensation. The study focused on developing and evaluating a SAW temperature-compensated gas sensor using time-of-flight measurements. The sensor exhibited good repeatability and sensitivity toward different concentrations of CO2.

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.

Real-time detection of acetone gas molecules at ppt levels in an air atmosphere using a partially suspended graphene surface acoustic wave skin gas sensor.

To improve the quality of modern life in the current society, low-power, highly sensitive, and reliable healthcare technology is necessary to monitor human health in real-time. In this study, we fabricated partially suspended monolayer graphene surface acoustic wave gas sensors (G-SAWs) with a love-mode wave to effectively detect ppt-level acetone gas molecules at room temperature. The sputtered SiO2 thin film on the surface of a black 36°YX-LiTaO3 (B-LT) substrate acted as a guiding layer, effectively reducing the noise and insertion loss. The G-SAWs exhibited enhanced gas response towards acetone gas molecules (800 ppt) in a real-time atmosphere. The high sensitivity of the G-SAW sensor can be attributed to the elasticity and surface roughness of the SiO2 film. In addition, the G-SAW sensor exhibited rapid response and recovery at room temperature. This study provides a potential strategy for diagnosing different stages of diabetes in the human body.

Nanostructured Silicon-Based Surface Acoustic Wave Sensor for CO2 Gas Detection

Surface acoustic wave (SAW) gas sensors with nanostructured material-based sensing layers are desirable in gas sensing owing to their high sensitivity, and their fast response and recovery time. In this work, a novel SAW gas sensor with a nanostructured silicon (Si)-based sensing layer with four pairs of aluminum-based input and output interdigital transducer (IDT) is developed and fabricated. A finite-element analysis (FEA) is used to determine the sensitivity while varying the height of the nanostructured Si. As a proof of concept, the SAW sensor is tested for carbon dioxide (CO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{2}}$ </tex-math></inline-formula> ) gas concentrations, ranging from 500 to 2000 ppm. The results demonstrate that the developed SAW sensor with a nanostructured Si-based sensing layer showed a maximum frequency shift of 4.62177 kHz with a response and a recovery time of 31 and 40.5 s, respectively, for a 2000-ppm CO2 gas concentration. The developed nanostructured Si as a sensing layer for the SAW gas sensor demonstrated higher sensitivity than previously reported devices and has promising potential to be used as a sensing layer for next-generation MEMS SAW gas sensors.

Design and analysis of high-sensitivity multilayered structure surface acoustic wave sensor for dichloromethane gas detection

Micro- and nano-sensors are key devices used in various fields from medical to environmental science. With the progress of micro- and nano-fabrication technology and the use of newly designed materials, sensor design has been significantly improved, leading to the development of high-performance gas sensors. A novel multilayer structure gas sensor based on surface acoustic wave (SAW) is proposed for the detection of volatile harmful gas dichloromethane. The gas sensing characteristics of single-layer and multilayer SAW sensors were studied by using finite element method. The influence of geometric parameters on the sensor performance was analyzed and the optimal structure was determined. The propagation characteristics of SAW were obtained by transient simulation of the whole structure. The sensitivity of the sensor is evaluated by placing the sensor in different concentrations of dichloromethane gas. The results show that the sensitivity of the PIB/AlN/Diamond/Si SAW gas sensor is as high as 66.9 ppm/Hz under the optimal geometry structure, which is nearly 30 times higher than that of other single-layer and multilayer SAW sensors. This work provides a good solution for the design of high-sensitivity SAW sensors.

Acoustic Wave Sensors for Detection of Blister Chemical Warfare Agents and Their Simulants.

On-site detection and initial identification of chemical warfare agents (CWAs) remain difficult despite the many available devices designed for this type of analysis. Devices using well-established analytical techniques such as ion mobility spectrometry, gas chromatography coupled with mass spectrometry, or flame photometry, in addition to unquestionable advantages, also have some limitations (complexity, high unit cost, lack of selectivity). One of the emerging techniques of CWA detection is based on acoustic wave sensors, among which surface acoustic wave (SAW) devices and quartz crystal microbalances (QCM) are of particular importance. These devices allow for the construction of undemanding and affordable gas sensors whose selectivity, sensitivity, and other metrological parameters can be tailored by application of particular coating material. This review article presents the current state of knowledge and achievements in the field of SAW and QCM-based gas sensors used for the detection of blister agents as well as simulants of these substances. The scope of the review covers the detection of blister agents and their simulants only, as in the available literature no similar paper was found, in contrast to the detection of nerve agents. The article includes description of the principles of operation of acoustic wave sensors, a critical review of individual studies and solutions, and discusses development prospects of this analytical technique in the field of blister agent detection.

High-performance p-hexafluoroisopropanol phenyl functionalized multi-walled carbon nanotube film on surface acoustic wave device for organophosphorus vapor detection

A delay line-type surface acoustic wave (SAW) gas sensor based on p-hexafluoroisopropanol phenyl (HFIPPH) functionalized multi-walled carbon nanotube (MWCNT) film is developed to detect organophosphorus dimethyl methylphosphonate (DMMP) vapor (a simulant of chemical nerve agent sarin). Inspired by the transfer process of Cu-based graphene, a uniform and size-controllable HFIPPH-MWCNT film is successfully prepared on the SAW device via a wet-etching transfer method. For the first time, we use the method of measuring the change of the sensor’s insertion loss to achieve the detection of ultra-low concentration DMMP vapor. The designed sensor exhibits a fast response/recovery time about 3 s/50 s, and a low detection limit of 0.1 ppm. Additionally, the stability and selectivity of the sensor and the influence of humidity on its response are evaluated through experiments. The acoustoelectric effect is proved to be the sensing mechanism of the sensor insertion loss response.

Piezoelectric Vibration Energy Harvesters with Distinct Interdigital Electrodes Used for Toxic Gas Detection and in a Numerical Simulation for a Glucose Sensor Application

Introduction: In this paper, a surface acoustic wave sensor (SAW) for dangerous gas sensing applications has been designed and experimented. The Glucose Sensor applications has been simulated to show that the piezoelectric material has a high significance for use in detection. Background: Detection of gases is one of the major applications SAW gas sensor extend its services into the field of medical and even in power plants. A Surface Acoustic Wave (SAW) is an acoustic wave that propagates on the surface of an elastic material (usually a solid), with an amplitude that decreases with the depth of the substrate. Objectives: SAW devices typically use electrodes on a piezoelectric material to convert an electrical signal to a SAW. Methods: This paper reviews the significance of piezoelectric materials and focuses on MEMS based SAW, and we investigate the resonance frequencies of a SAW gas sensor, which consists of an Inter Digitated Transducer (IDT) etched onto a piezoelectric substrate and covered with a thin Poly Isobutylene (PIB) film. Results: The mass of the PIB film increases as PIB selectivity adsorbs CH2Cl2 (Dichloromethane, DCM) in the air. This causes a shift in resonance to a slightly lower frequency. Conclutions: Our characterization of the piezoelectric material have shown a high significance when a sensitive layer of gas is etch for detected the dangerous gases, we used it in another application Glucose Sensor to show the importance of our system. The sensor of the Glucose Gensor application has been designed and simulated by a finite element analysis which was conducted on LiNbO3 pellets. This model is applied to verify the sensing properties of DCM and the glucose. On the other hand, the Glucose Sensor and the toxic gas detection allow measurement of the concentration,whether it is for the dangerous gases or glucose. The concentration of external glucose and the current density versus glucose concentration were measured by a finite element.

Enhanced sensitivity of langasite-based surface acoustic wave CO gas sensor using highly porous Ppy@PEDOT:PSS hybrid nanocomposite

The potential usage of the nano-porous Ppy@PEDOT:PSS hybrid organic nanocomposite polymers has been widely explored in various applications fields. However, the study of Ppy@PEDOT:PSS based room temperature (RT) CO gas sensing utilizing surface acoustic wave (SAW) sensor is still at infancy. Herein, we report a langasite (LGS) based RT CO SAW gas sensor using Ppy@PEDOT:PSS hybrid organic nanocomposite polymer. The Ppy@PEDOT:PSS/LGS SAW sensor exhibited high sensitivity of ~9.38 KHz positive frequency shift (∆f) as compared to the pristine PEDOT:PSS/LGS SAW (~1.76 kHz negative ∆f) sensor with a short response/recovery times (121 s/138 s), good reproducibility, excellent selectivity and low detection limit (~231 ppb) towards CO (100 ppm) gas at RT. Interestingly, our experimental findings demonstrated that the Ppy@PEDOT:PSS/LGS SAW sensor exhibited the positive ∆f which was attributed to the enhanced elastic loading and decrease in the electrical conductivity under CO gas ambient, whereas the PEDOT:PSS/LGS SAW showed negative ∆f, which was due to the mass loading effect. Moreover, the impressive CO gas sensing performances were attributed to the, high porosity, large surface area, abundant OH and amine functionalizing groups. In addition the synergetic effect of the p-p hybrid heterojunction, which can act as active surface sites that further improving the CO gas molecules absorption, and thus leading to the massive changes in elastic stiffness and the electrical conductivity of the Ppy@PEDOT:PSS film. The scientific findings of this work offers a versatile platform to develop high performance hybrid organic polymer nanocomposites based SAW gas-sensing applications.

Enhanced NO2 sensitivity of SnO2 SAW gas sensors by facet engineering

Surface acoustic wave (SAW) sensors have great advantages in real-time and in-situ gas detection due to the inherent advantages of passive and wireless operation. This inevitably requires the sensor to improve gas sensing performances at room temperature. The facet engineering of metal oxide (MOS) provided an effective way to obtain MOS gas-sensitive materials with superior performance by the facet-dependent properties. Here, we developed a novel strategy to prepare SnO2 quantum wires with different (110) facets ratio by adjusting the synthesis time. The as-prepared SnO2 quantum wires were integrated into SAW devices to optimize the performance of the gas sensor. When the synthesis time was 8 h, the response of the SAW gas sensor was enhanced with the frequency change of 17 kHz, the response and recovery times of 45 s and 96 s, respectively. Our experimental results revealed that the effect of mass loading was the responsible underlying mechanism for the superior NO2 gas sensing performances. In addition, the adsorption and charge transfer properties between the SnO2 surface and NO2 molecules were further discussed by QCM and resistance measurement, respectively. The calculation of density functional theory (DFT) also proves the adsorption energy of NO2 on the SnO2 (110) facet is larger than the (101) facet and (211) facet, indicating that the (110) facet is more beneficial to the adsorption of NO2 molecules. This indicated that the facet engineering of MOS by facet-dependent properties to enhance gas sensitivity may open new opportunities for the design of SAW sensor.

Interface and Sensitive Characteristics of the Viscoelastic Film Used in a Surface Acoustic Wave Gas Sensor.

The surface morphology of viscoelastic-sensitive films significantly affects sensing characteristics of surface acoustic wave (SAW) sensors. Uniformity and compactness of the film surface directly influences detectability of the SAW sensor toward target gases. Viscoelastic fluoroalcoholpolysiloxane (SXFA) was prepared in this work using spin coating technology on an SAW delay line of 200 MHz and then used as coating for detection of dimethyl methylphosphonate (DMMP). Polarizing, atomic force, and scanning electron microscopies confirmed the uniformity of the SXFA surface. The particle diameter in the cluster region was 10-15 μm. The contact angle (5.72-26.69°), surface tension (21.053-29.155 mN/m), Gibbs free energy (-160.68 to -153.45 J/m2), and spreading coefficient (0.3028-6.9453 J/m2) of different concentrations of SXFA were obtained through experiments, and their relation was analyzed using the Young T equation and Gibbs adsorption isotherm. The glass transition temperature (-19.7 °C) and elasticity of SXFA were also discussed. The consistency of sensor preparation was confirmed by detecting DMMP with five SAW sensors prepared simultaneously. Seven consecutive tests showed that the SAW sensor presents satisfactory repeatability (standard deviation, s, 1.134; coefficient of variance, v, 0.065; and population mean deviation, δ, 0.913) at a concentration of 1.71 mg/m3 and acceptable linear relationship at a concentration range of 0.058-1.92 mg/m3, with a sensitivity of around 1.21 mv/(mg/m3). The sensor exhibited outstanding sensitivity and satisfactory linearity and repeatability to DMMP. Meanwhile, the sensing mechanism in gas adsorption was also discussed in terms of LSER formulation and hydrogen bonding formation between SXFA and DMMP.

Highly sensitive g-C3N4 nanosheets as a potential candidate for the effective detection of NO2 gas via langasite-based surface acoustic wave gas sensor

In this work, 2D g-C3N4 nanosheets were employed as a sensitive interface for high-performance NO2 SAW gas sensors, operating at various environmental conditions. The immense sensitivity to NO2 gas is attributed to the enhanced mass loading effect.

Development of a SAW poly(epichlorohydrin) gas sensor for detection of harmful chemicals.

The uniformity and compactness of the surface of a viscoelastic sensitive film are among the most important factors that influence the characteristics of a surface acoustic wave (SAW) gas sensor, directly affecting the detection sensitivity of a SAW sensor on a target gas. In this paper, poly(epichlorohydrin) (PECH) with viscoelastic properties was used as sensitive film for the detection of 2-chloroethyl ethyl sulfide (CEES), a common simulant of the chemical agent mustard gas. Nanoscale films were prepared using a spin coating technology on a SAW delay line of 200 MHz. Films were evaluated using polarizing microscopy and atomic force microscopy and observed with uniform surface states and particle diameter in the cluster region of 4.52-5.22 μm. The interface parameters, including contact angle, surface tension, Gibbs free energy, work of adhesion, work of immersion, and spreading coefficient values were 9.31° to 39.63°, 22.475 to 29.945 mN m-1, -85.70 to -78.08 J m-2, 78.08 to 85.70 J m-2, -42.62 to -35.00 J m-2, and 0.46 to 8.08 J m-1, respectively. These values were obtained by experiments combined with the Young T equation and Gibbs adsorption isotherm, and the surface analysis was carried out theoretically. The glass transition temperature (-22.4 °C), viscosity, pyrolysis, and other physical characteristics of the prepared PECH were discussed. Five SAW sensors prepared at the same time were used to test the repeatability of CEES measurements at one concentration, where the consistency of the sensor preparation was confirmed. At a concentration of 13.6 mg m-3 for CEES, 10 consecutive detection results showed good repeatability (i.e., standard deviation = 0.295, coefficient of variance = 0.021, and population mean deviation = 0.364). At room temperature (20 °C ± 5 °C), different concentrations of CEES were detected using the developed sensor, which showed good linearity in the concentration range of 1.9-19.6 mg m-3 (y = 0.0309 + 1.13x, r = 0.99478). The limit of detection was 0.85 mg m-3, the limit of quantitation was 1.91 mg m-3, and the sensitivity of the SAW sensor was 1.13 mV (mg m-3). The adsorption mechanism related to PECH in the detection of CEES was also discussed.

High performance langasite based SAW NO2 gas sensor using 2D g-C3N4@TiO2 hybrid nanocomposite

Nitrogen dioxide (NO2) gas has emerged as a severe air pollutant that causes damages to the environment, human life and global ecosystems etc. However, the currently available NO2 gas sensors suffers from insufficient selectivity, sensitivity and long response times that impeding their practical applicability for room temperature (RT) gas sensing. Herein, we report a high performance langasite (LGS) based surface acoustic wave (SAW) RT NO2 gas sensor using 2-dimensional (2D) g-C3N4@TiO2 nanoplates (NP) with {001} facets hybrid nanocomposite as a chemical interface. The g-C3N4@TiO2 NP/LGS SAW device showed a significant negative frequency shift (∆f) of ~19.8 kHz which is 2.4 fold higher than that of the pristine TiO2 NP/LGS SAW sensor toward 100 ppm of NO2 at RT. In addition, the hybrid SAW device fascinatingly exhibited a fast response/recovery time with a low detection limit, high selectivity, and an effective long term stability toward NO2 gas. It also exhibited an enhanced and robust negative frequency shifts under various relative humidity conditions ranging from 20% to 80% for 100 ppm of NO2 gas. The high performance of the g-C3N4 @TiO2 NP/LGS SAW gas sensor can be attributed to the enhanced mass loading effect which was assisted by the large surface area, oxygen vacancies, OH and amine functional groups of the n-n hybrid heterojunction of g-C3N4@TiO2 NP that provide abundant active sites for the adsorption and diffusion of NO2 gas molecules. These results emphasize the significance of the integration of 2D materials with metal oxides for SAW based RT gas sensing technology holds great promise in environmental protection.

A surface acoustic wave hydrogen sensor with tin doped indium oxide layers for intermediate temperatures

High temperature surface acoustic wave (SAW) gas sensors with conducting sensing layers require tuning of the sheet conductivity for optimal response. Conducting metal oxides are attractive sensing materials for their tunable electronic properties and high thermal stability, amongst others. Here, we have investigated the application of indium oxide (IO) and indium tin oxide (ITO) films on langasite (LGS)-based SAW reflective delay line sensor devices for monitoring hydrogen at 350 °C. Specifically, we modeled the effect of the IO and ITO sensing layer thickness on the wave velocity, attenuation, and effective electromechanical coefficient. This was followed by an experimental demonstration of tuning of the ITO film sheet conductivity by controlling the dopant concentration and yielding an improvement in the sensor sensitivity. The current study provides a pathway towards the development of conductivity-based sensing layers for high temperature SAW gas sensors with improved sensitivity.

Surface acoustic wave sensor based on Au/TiO2/PEDOT with dual response to carbon dioxide and humidity

A surface acoustic wave (SAW) gas sensor with an Au/TiO2/poly(3,4-ethylenedioxythiophene) (PEDOT, which is a conductive polymer with photoelectric conversion function) sensing film was constructed for the quantitative detection of water vapor and CO2. The Au/TiO2/PEDOT sensing film was assembled on the delayed region of the 204 MHz SAW delay line, which was used as the base device for the gas sensor. The center frequency of the sensor decreases with an increase in relative humidity (RH), and the center frequency increases with increasing CO2 concentration, so that not only can the two gases be identified, but quantitative analysis can also be performed. The SAW sensor has a response range of 5%–90% for RH and a response range of 500–2000 ppm for CO2 gas. The shifts in center frequency varied linearly with the concentrations, giving rise to the sensitivities of −0.0068 and −0.1880 kHz %−1 for RH and ∼0.003 kHz ppm−1 CO2. The response/recovery time is 9 s/9.2 s for 700 ppm CO2 and 15 s/14 s for 70% RH. The experimental results show that the SAW sensor offers excellent selectivity, wide response range, rapid response, and good stability and repeatability. The mechanism of humidity and CO2 sensing is attributed to the hydrophilic porous structure of the Au/TiO2/PEDOT sensing film, and also to the reversible variation of its viscoelasticity under illumination conditions. The sensor, combined with the communication function of its own SAW device, has several prospective applications in the monitoring of atmospheric conditions.