chemosensors Article Non-Local Patch Regression Algorithm-Enhanced Differential Photoacoustic Methodology for Highly Sensitive Trace Gas Detection Le Zhang 1,2 , Lixian Liu 1,2,3 , Huiting Huan 1,2,3 , Xukun Yin 1,2 , Xueshi Zhang 1,2 , Andreas Mandelis 3 and Xiaopeng Shao 1,2, *   Citation: Zhang, L.; Liu, L.; Huan, H.; Yin, X.; Zhang, X.; Mandelis, A.; Shao, X. Non-Local Patch Regression Algorithm-Enhanced Differential Photoacoustic Methodology for Highly Sensitive Trace Gas Detection. Chemosensors 2021, 9, 268. https:// doi.org/10.3390/chemosensors9090268 Academic Editor: Chris Blackman Received: 12 July 2021 Accepted: 16 September 2021 Published: 18 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China; lezhangxd@163.com (L.Z.); lixianliu@xidian.edu.cn (L.L.); hthuan@xidian.edu.cn (H.H.); xkyin@xidian.edu.cn (X.Y.); monomp@163.com (X.Z.) 2 Xi’an Key Laboratory of Computational Imaging, Xi’an 710071, China 3 Center for Advanced Diffusion-Wave and Photoacoustic Technologies (CADIPT), Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, ON M5S 3G8, Canada; mandelis@mie.utoronto.ca * Correspondence: xpshao@xidian.edu.cn Abstract: A non-local patch regression (NLPR) denoising-enhanced differential broadband photoa- coustic (PA) sensor was developed for the high-sensitive detection of multiple trace gases. Using the edge preservation index (EPI) and signal-to-noise ratio (SNR) as a dual-criterion, the fluctuation was dramatically suppressed while the spectral absorption peaks were maintained by the introduction of a NLPR algorithm. The feasibility of the broadband framework was verified by measuring the C 2 H 2 in the background of ambient air. A normalized noise equivalent absorption (NNEA) coefficient of 6.13 × 10 −11 cm −1 ·W·Hz −1/2 was obtained with a 30-mW globar source and a SNR improvement factor of 23. Furthermore, the simultaneous multiple-trace-gas detection capability was determined by measuring C 2 H 2 ,H 2 O, and CO 2 . Following the guidance of single-component processing, the NLPR processed results showed higher EPI and SNR compared to the spectra denoised by the wavelet method and the non-local means algorithm. The experimentally determined SNRs of the C 2 H 2 ,H 2 O, and CO 2 spectra were improved by a factor of 20. The NNEA coefficient reached a value of 7.02 × 10 −11 cm −1 ·W·Hz −1/2 for C 2 H 2 . The NLPR algorithm presented good performance in noise suppression and absorption peak fidelity, which offered a higher dynamic range and was demonstrated to be an effective approach for trace gas analysis. Keywords: photoacoustic spectroscopy; gas sensors; multi-component; non-local denoising algorithm 1. Introduction There is an ever-increasing need for non-destructive and rapid monitoring technolo- gies for multiple trace gas species and their concentrations in the fields of environmental protection, medical diagnosis, industrial production, and food safety [1,2]. Photoacoustic spectroscopy (PAS) based technologies [3–6] feature the advantages of fast response, high sensitivity, high selectivity, and a large dynamic detection range and have thus played an important role in multi-component gas sensing. The basic principle of PAS is that the gas molecules absorb the light energy at specific wavelengths and cause the local temperature to increase. Combined with the periodic modulation of the light source, the gas temperature thermally diffuses to generate pressure oscillations and acoustic sig- nals [7]. Up until now, various PAS-based sensor modalities have been developed for multi-gas analysis, such as the use of multi-lasers combined time-division multiplexing methods [8–10], multi-resonators with various frequency demodulation schemes [11], and broadband detection-based thermal emitters or blackbody radiators using several band- pass filters [12]. However, use of multiple lasers yields narrowband wavelength selection for specific gas absorption, limiting the capability to simultaneously detect multiple gases. Chemosensors 2021, 9, 268. https://doi.org/10.3390/chemosensors9090268 https://www.mdpi.com/journal/chemosensors