The advanced development of ammonia gas sensors using various sensing platforms has been widely researched, including quartz crystal microbalance (QCM)-based sensors. This review highlights recent research on ammonia gas sensors based on QCM with an electrospun polymer-based nanofiber active layer. The discussion starts with the principle of QCM as a base sensor, and then follows with the effect of electrospinning parameters on the fabricated nanofiber. Then, the discussion focuses on the potential of fabricated QCM-based sensors in directly detecting ammonia as a biomarker of liver and kidney diseases. The key performance parameters of QCM-based ammonia gas sensors, such as sensitivity, detection range, detection limit, and response/recovery time, are comprehensively discussed in this review. Finally, the potential, current challenges, and future perspectives on the application of QCM-based ammonia gas sensors for direct detection of liver and kidney diseases, such as improving selectivity, humidity resistance, and clinical translation, followed by the potential usage of QCM-based sensor arrays, are considered, offering insight into the future direction of QCM-based sensors in probing human diseases.

Acetic acid is among the most common and damaging airborne contaminants. Exposure to acetic acid gas can irritate the nose and throat, which may harm human health. In light of the increasing use of acetic acid in industry, there is a critical need for an acetic acid detection device that can operate in real time with excellent performance. This study developed an acetic acid gas sensor using a quartz crystal microbalance (QCM) deposited with polyvinylpyrrolidone (PVP) microfiber by a well-recognized electrospinning method. Scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy were employed to observe the morphology and chemical composition of the fabricated microfibers. The obtained sensor has a high sensitivity of (4.144 ± 0.039) Hz·ppm with a low detection limit and rapid response and recovery times of 75 and 66 s, respectively. In addition, the fabricated sensor also exhibits good performance as well as repeatability and long-term stability. Selectivity tests were also conducted for various analytes and exhibited excellent selectivity toward acetic acid. These advancements demonstrate that the PVP microfiber active layers not only address the challenges of detection by enhancing sensitivity but also provide unique mechanism selectivity for acetic acid molecules. However, advanced future research must still be conducted to improve the sensor reversibility, making the sensor more suitable for applications in a real environment.

A method to detect camphor gas is considered indispensable in the pharmaceutical industry. Unfortunately, the available sensors to detect the presence of camphor in the air are very limited and still on a laboratory scale, such as using chromatography-mass spectroscopy (GC-MS). The research's main focus is to obtain a portable sensing system with excellent sensitivity and selectivity. This study explored polyaniline (PANi) concentrations cast over PVAc nanofiber as a matrix to detect camphor gas using a quartz crystal microbalance (QCM) system to measure camphor exposure. Scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) were used to analyze the morphology and chemical composition of the fabricated active layer (, nanofiber with PANi thin film). Increasing the PANi concentration provides more PANi on the sensor surface, thus amassing the active groups to interact with camphor gas molecules. It shows that a sensor with a 0.08% PANi thin film (Nano-PANi8) has a sensitivity of 2.594 Hz ppm, much greater than the sensor without PANi, which is only 0.305 Hz ppm. In addition, the sensor also has good repeatability and rapid response and recovery time of 47 s and 133 s, respectively. Compared to other gaseous compounds, the sensor also has excellent selectivity for camphor and robust long-term stability over three weeks of testing. The produced QCM sensor employing PANi thin film can give a camphor sensor superior performance, including excellent sensitivity, selectivity, and long-term stability. Furthermore, the use of QCM as a base sensor also makes the fabricated sensor portable.

Detecting ammonia at low concentrations is crucial in various fields, including environmental monitoring, industrial processes, and healthcare. This study explores the development and performance of an ultra-sensitive ammonia sensor using carboxylic group-functionalized multi-walled carbon nanotubes (f-MWCNTs) overlaid on polyvinyl acetate nanofibers coated on a quartz crystal microbalance (QCM). The sensor demonstrates high responsiveness, with a frequency shift response of over 120 Hz when exposed to 1.5 ppm ammonia, a sensitivity of 23.3 Hz ppm over a concentration range of 1.5-7.5 ppm, and a detection limit of 50 ppb. Additionally, the sensor exhibits a rapid response time of only 14 s, excellent selectivity against other gases, such as acetic acid, formaldehyde, methanol, ethanol, propanol, benzene, toluene, and xylene, and good stability in daily use. These characteristics make the sensor a promising tool for real-time ammonia detection in diverse applications.

Nanofibers made of different materials have been continuously studied and widely used as membranes due to their simple fabrication techniques and tunable surface characteristics. In this work, we developed polyacrylonitrile (PAN) nanofiber membranes by the electrospinning method and blended them with polysulfone (PSU) to obtain superhydrophobic surfaces on the fiber structures. The scanning electron microscopy (SEM) images show that the fabricated nanofibers have smooth and continuous morphology. In addition, to observe the effect of the PSU-based blending material, Fourier-transform infrared (FTIR) spectra of the samples were acquired, providing chemical compositions of the bare and PSU-blended PAN nanofibers. The fabricated PSU/PAN composite nanofibers have a diameter range of 222-392 nm. In terms of the wettability, the measured water contact angle (WCA) value of the PAN nanofibers was improved from (14 ± 1)° to (156 ± 6)°, (160 ± 4)°, (156 ± 6)°, and (158 ± 4)° after being blended with PSU solutions having concentrations of 0.5, 1, 1.5, and 2 wt %, respectively. This result has proven that the PAN nanofiber surfaces can be tuned from hydrophilic to superhydrophobic characteristics simply by introducing PSU into the PAN solution prior to electrospinning, where a small PSU concentration of 0.5% has been sufficient to provide the desired effect. Owing to its low-cost and highly efficient process, this strategy may be further explored for other types of polymer-based nanofibers.

Real-time detection of formaldehyde in the atmosphere remains challenging. The available gaseous formaldehyde sensing methods offer limited sensitivity, selectivity, and robustness. We modified a quartz crystal microbalance (QCM) system for selective detection of formaldehyde in air. The QCM surface was functionalized with polyvinyl acetate (PVAc) nanofibers and doped with 2, 4, and 6 wt% aniline to improve the selectivity and sensitivity of the sensor. The chemical content and morphological structure of PVAc nanofibers doped with aniline were confirmed by Fourier-transform infrared (FTIR) spectroscopy, energy-dispersive X-ray (EDX) spectroscopy, and scanning electron microscopy (SEM). The results showed that the modified QCM sensor had a sensitivity of 0.056 Hz ppm with a response and recovery times of 200 s and 90 s, respectively. It gave limits of detection (LOD) and limit of quantification (LOQ) of 28 ppm and 96 ppm, respectively. Moreover, the modified QCM was selective towards formaldehyde compared to the other gases. The current workplace exposure limit (WEL) for formaldehyde is 2 ppm, with a time-weighted average over eight hours. Future work will focus on improving the reported QCM sensor to meet the required LOD for formaldehyde detection in the environment and industrial sites.

Authentication of meat floss origin has been highly critical for its consumers due to existing potential risks of having allergic diseases or religion perspective related to pork-containing foods. Herein, we developed and assessed a compact portable electronic nose (e-nose) comprising gas sensor array and supervised machine learning with a window time slicing method to sniff and to classify different meat floss products. We evaluated four different supervised learning methods for data classification (i.e., linear discriminant analysis (LDA), quadratic discriminant analysis (QDA), k-nearest neighbors (k-NN), and random forest (RF)). Among them, an LDA model equipped with five-window-extracted feature yielded the highest accuracy values of >99% for both validation and testing data in discriminating beef, chicken, and pork flosses. The obtained e-nose results were correlated and confirmed with the spectral data from Fourier-transform infrared (FTIR) spectroscopy and gas chromatography-mass spectrometry (GC-MS) measurements. We found that beef and chicken had similar compound groups (i.e., hydrocarbons and alcohol). Meanwhile, aldehyde compounds (e.g., dodecanal and 9-octadecanal) were found to be dominant in pork products. Based on its performance evaluation, the developed e-nose system shows promising results in food authenticity testing, which paves the way for ubiquitously detecting deception and food fraud attempts.

The chemical modification of polymer nanofiber-based ammonia sensors by introducing dopants into the active layers has been proven as one of the low-cost routes to enhance their sensing performance. Herein, we investigate the influence of different citric acid (CA) concentrations on electrospun polyvinyl acetate (PVAc) nanofibers coated on quartz crystal microbalance (QCM) transducers as gravimetric ammonia sensors. The developed CA-doped PVAc nanofiber sensors are tested against various concentrations of ammonia vapors, in which their key sensing performance parameters (, sensitivity, limit of detection (LOD), limit of quantification (LOQ), and repeatability) are studied in detail. The sensitivity and LOD values of 1.34 Hz ppm and 1 ppm, respectively, can be obtained during ammonia exposure assessment. Adding CA dopants with a higher concentration not only increases the sensor sensitivity linearly, but also prolongs both response and recovery times. This finding allows us to better understand the dopant concentration effect, which subsequently can result in an appropriate strategy for manufacturing high-performance portable nanofiber-based sensing devices.

The aim of this study is to determine the performance of a lab-made electronic nose (e-nose) composed of an array of metal oxide semiconductor (MOS) gas sensors in the detection and differentiation of () and () incubated in trypticsoy broth (TSB) media. Conventionally, the detection of and is often performed by enzyme link immunosorbent assay (ELISA) and polymerase chain reaction (PCR). These techniques require trained operators and expert, expensive reagents and specific containment. In this study, three types of samples, namely, TSB media, (serotype 4b American Type Culture Collection (ATCC) 13792), and (ATCC) 10876, were used for this experiment. Prior to measurement using the e-nose, each bacterium was inoculated in TSB at 1 × 10-10 CFU/mL, followed by incubation for 48 h. To evaluate the performance of the e-nose, the measured data were then analyzed with chemometric models, namely linear and quadratic discriminant analysis (LDA and QDA), and support vector machine (SVM). As a result, the e-nose coupled with SVM showeda high accuracy of 98% in discriminating between TSB media and , and between TSB media and . It could be concluded that the lab-made e-nose is able to detect rapidly the presence of bacteria and on TSB media. For the future, it could be used to identify the presence of or contamination in the routine and fast assessment of food products in animal quarantine.

Safrole is the main precursor for producing the amphetamine-type stimulant (ATS) drug, -methyl-3,4-methylenedioxyamphetamine (MDMA), also known as ecstasy. We devise a polyacrylonitrile (PAN) nanofiber-based quartz crystal microbalance (QCM) for detecting safrole. The PAN nanofibers were fabricated by direct electrospinning to modify the QCM chips. The PAN nanofiber on the QCM chips has a diameter of 240 ± 10 nm. The sensing of safrole by QCM modified with PAN nanofiber shows good reversibility and an apparent sensitivity of 4.6 Hz·L/mg. The proposed method is simple, inexpensive, and convenient for detecting safrole, and can be an alternative to conventional instrumental analytical methods for general volatile compounds.