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A flexible resistive-type polyaniline-based gas sensor was fabricated by simple dip-coating of graphene combined with in situ polymerization of aniline on a flexible waste mask substrate. The prepared polypropylene/graphene/polyaniline (PP/G/PANI) hybrid sensor demonstrated a fast response (114 s) and recovery time (23 s), ppb-level detection limit (100 ppb), high response value (250% toward 50 ppm NH3, which is over four times greater than that of the pristine PANI sensor), acceptable flexibility, excellent selectivity, and long-term stability at room temperature. The morphological and structural properties of the composite sensor materials were characterized by scanning electron microscopy and energy-dispersive spectroscopy characterization, and the surface chemistry of the hybrid sensors was analyzed by Fourier transform infrared spectroscopy. The excellent sensing performance was mainly ascribed to the larger specific surface area and efficient conducting paths of the porous PP/G/PANI network. Moreover, the PP/G/PANI hybrid gas sensor exhibited excellent sensing capability on volatile sulfur compounds contained in human breath, indicating that the hybrid sensor can be applied to breath analysis and kidney disease diagnosis.

This work introduces the concept of a molecularly imprinted gas sensor to monitor the condition of naturally ripened strawberries. Furaneol, 2,5-dimethyl-4-hydroxy-3(2H)-furanone, is considered as an important biomarker related to the strawberry flavor. Identification of furaneol concentration is still a challenge because of its weak adsorption, nonpolar, and unreactive properties. Therefore, no study has been reported yet to measure furaneol gases via a simple chemiresistive mechanism. Herein, we demonstrate the sensor based on molecularly imprinted polymer (MIP)-based polyaniline (PANI). The sensitive and selective detection of furaneol gas with a MIP-PANI gas sensor was observed at room temperature and under different humidity conditions. The comparison between MIP and the nonimprinted (NIP)-based PANI shows a strong interaction between furaneol and the molecularly imprinted polymer. The furaneol gas sensing mechanism is explained based on the interaction between the gas molecules and the charge carriers of MIP-PANI, which results in the functional group change in the carboxylic group. Furthermore, the developed MIP-chemiresistive sensor for real strawberries was compared with a commercial e-nose system. The results show the potential to offer a rapid and cost-effective platform for specific recognition of furaneol.

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Strawberry is one of the beloved fruits worldwide due to its exquisite flavor with highly nutritious compounds which give promising health benefits. [1] With dragging the great attention, efforts to monitor the quality of strawberries and strawberry derivatives are becoming significant to obtain better flavor and nutrition. 2,5-Dimethyl-4-hydroxy-3(2H)-furanone(furaneol) is one of the compounds which plays an important role in a flavor that can be naturally found in various fruits such as strawberry, pineapple, raspberry, and mango. Especially, furaneol composition is reported as a significant factor for maturity condition monitoring of strawberries due to the different amounts at the ripening stage. [2] Furaneol is also known to be an essential signature for controlling the food production process, verifying the food production origin, and regulating the quality of products. Currently, furaneol detection is conducted with time-consuming techniques, such as gas chromatography (GC) and high-performance liquid chromatography (HPLC). For example, Yuan et al. [3] showed the identification of furaneol with HPLC. Also, Buttery et al. [4], and Lopez et al. [5], used GC for the determination of furaneol in foods. Developing a real-time gas sensor for fast, cheap, stable, sensitive detection of furaneol is a significant challenge in the area of the food industry. Chemo-resistive gas sensors can be an attractive approach due to their miniaturization, simple operation, ease of fabrication, and low production cost. For utilizing resistive gas sensors, polyaniline (PANI) got great attention because of its environmental stability, simple synthesis, and ease of tailoring the surface charge characteristics by changing the dopants. However, PANI-based gas sensors are reactive to numerous gases which limits the selective detection of target gas molecules. Many compounds in strawberries may restrict the use of PANI-based sensors. [6] Molecular imprinting technology (MIT), which can create molecularly imprinted polymers (MIPs) obtaining specific binding sites for target molecules, is utilized to achieve selective gas sensing similar to biological antibodies. [7] The improved affinity not only provides selective detection but also enhances response to target gas molecules. In this study, a resistive gas sensor based on synthesized molecularly imprinted polymer and polyaniline is demonstrated for the detection of furaneol gas molecules for its operation in food packaging. Herein, we developed a unique method to synthesize molecularly imprinted polymer and polyaniline (MIP-PANI) nanocomposites with an interfacial polymerization technique. Aniline monomer is polymerized and oxidized simultaneously to tailor the surface charge characteristics forming polyaniline. Then, the molecularly imprinting process on polyaniline is conducted with methacrylic acid (MAA) as a functional monomer, ethylene glycol dimethacrylate (EGDMA) as a crosslinker, furaneol as a template, and benzoyl peroxide as a radical initiator. After the furaneol template was constructed, MIP-PANI presented a high affinity to furaneol target molecules and showed a high sensing response. Their response signals are enhanced tens of times compared to the pure PANI. Furthermore, selective detection of the target furaneol molecules was observed. This study may broaden the application of resistive gas sensors for selective and sensitive detection of volatile organic compounds in food industries. Reference [1] S. Afrin, M. Gasparrini, T.Y. Forbes-Hernandez, P. Reboredo-Rodriguez, B. Mezzetti, A. Varela-Lopez, F. Giampieri, M. Battino, Promising Health Benefits of the Strawberry: A Focus on Clinical Studies, J Agric Food Chem 64 (2016) 4435-4449. [2] C. Aubert, S. Baumann, H. Arguel, Optimization of the analysis of flavor volatile compounds by liquid-liquid microextraction (LLME). Application to the aroma analysis of melons, peaches, grapes, strawberries, and tomatoes, Journal of Agricultural and Food Chemistry 53 (2005) 8881-8895. [3] J.P. Yuan, F. Chen, Separation and identification of furanic compounds in fruit juices and drinks by high-performance liquid chromatography photodiode array detection, Journal of Agricultural and Food Chemistry 46 (1998) 1286-1291. [4] R.G. Buttery, G.R. Takeoka, M. Naim, H. Rabinowitch, Y. Nam, Analysis of furaneol in tomato using dynamic headspace sampling with sodium sulfate, Journal of Agricultural and Food Chemistry 49 (2001) 4349-4351. [5] R. Lopez, M. Aznar, J. Cacho, V. Ferreira, Determination of minor and trace volatile compounds in wine by solid-phase extraction and gas chromatography with mass spectrometric detection, Journal of Chromatography A 966 (2002) 167-177. [6] A.T. Oz, G. Baktemur, S.P. Kargi, E. Kafkas, Volatile Compounds of Strawberry Varieties, Chemistry of Natural Compounds 52 (2016) 507-509. [7] L. Chen, X. Wang, W. Lu, X. Wu, J. Li, Molecular imprinting: perspectives and applications, Chem Soc Rev 45 (2016) 2137-2211.