Chemosensors 2022, 10, 213. https://doi.org/10.3390/chemosensors10060213 www.mdpi.com/journal/chemosensors Article Nitrogen Dioxide Optical Sensor Based on Redox‐Active Tetrazolium/Pluronic Nanoparticles Embedded in PDMS Membranes Esteban Araya‐Hermosilla 1, *, Rodrigo Araya‐Hermosilla 2 , Francesco Visentin 3 , Francesco Picchioni 4 , Andrea Pucci 5, * and Virgilio Mattoli 1, * 1 Center for Materials Interfaces @SSSA Istituto Italiano di Tecnologia Viale Rinaldo Piaggio 34, 56025 Pontedera, Italy 2 Programa Institucional de Fomento a la Investigación, Desarrollo e Innovacion (PIDi), Universidad Tecnologica Metropolitana, Ignacio Valdivieso 2409, San Joaquín, Santiago 8940577, Chile; rodrigo.araya@utem.cl 3 Bioinspired Soft Robotics Laboratory, Istituto Italiano di Tecnologia Viale Rinaldo Piaggio 34, 56025 Pontedera, Italy; francesco.visentin@iit.it 4 Department of Chemical Engineering—Product Technology, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands; f.picchioni@rug.nl 5 Department of Chemistry and Industrial Chemistry, University of Pisa, Via Moruzzi 13, 56124 Pisa, Italy * Correspondence: esteban.araya@iit.it (E.A.‐H.) andrea.pucci@unipi.it (A.P.); virgilio.mattoli@iit.it (V.M.); Tel.: +39‐050‐2219‐270 (A.P.) Abstract: Anthropogenic toxic vapour and gases are a worldwide threat for human health and to the environment. Therefore, it is crucial to develop highly sensitive devices that guarantee their rapid detection. Here, we prepared redox‐switchable colloids by the in‐situ reduction of 2,3,5‐tri‐ phenyl‐2H‐tetrazolium (TTC) into triphenyl formazan (TF) stabilised with Pluronic F127 in aqueous media. The colloids were readily embedded in polydimethylsiloxane (PDMS) to produce a selective colour‐switchable membrane for nitrogen dioxide (NO2) detection. We found that the TTC reduc‐ tion resulted in the production of red‐coloured colloids with zeta potential between −1 to 3 mV and hydrodynamic diameters between 114 to 305 nm as hydrophobic dispersion in aqueous media sta‐ bilised by Pluronic at different molar concentrations. Moreover, the embedded colloids rendered highly homogenous red colour gas‐permeable PDMS elastomeric membrane. Once exposed to NO2, the membrane began to bleach after 30 s due to the oxidation of the embedded TF and undergo a complete decolouration after 180 s. Such features allowed the membrane integration in a low‐cost sensing device that showed a high sensitivity and low detection limit to NO2. Keywords: redox‐active organic nanoparticles; pluronic F‐127; sensing membranes; wearable opti‐ cal device 1. Introduction Toxic vapours and gases produced by human activities are a worldwide threat for human and environmental health [1,2]. Nitrogen dioxide (NO2) is a hazardous pollutant found mainly in industrialised and urbanised areas due to the oxidation of nitric oxide in the atmosphere [3]. It is a polar and acid oxide molecule that reacting with airborne water produces nitric acid, one of the components of acid rain [4]. At the industrial level, NO2 is used as an intermediate in the production of nitric acid [5]. Regrettably, NO2 is a corrosive gas that can causes severe damage to the skin, eyes, and respiratory tract [6,7], and can also be a factor of lung cancer [8]. Hence, it is important to develop cost‐effective and highly sensitive wearable sensor devices that can sense NO2 at low concentrations. Citation: Araya‐Hermosilla, E.; Araya‐Hermosilla, R.; Visentin, F.; Picchioni, F.; Pucci, A.; Mattoli, V. Nitrogen Dioxide Optical Sensor Based on Redox‐Active Tetrazolium/Pluronic Nanoparticles Embedded in PDMS Membranes. Chemosensors 2022, 10, 213. https://doi.org/ 10.3390/chemosensors10060213 Academic Editor: Zhihong Liu Received: 25 January 2022 Accepted: 3 June 2022 Published: 6 June 2022 Publisher’s Note: MDPI stays neu‐ tral with regard to jurisdictional claims in published maps and institu‐ tional affiliations. Copyright: © 2022 by the authors. Li‐ censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con‐ ditions of the Creative Commons At‐ tribution (CC BY) license (https://cre‐ ativecommons.org/licenses/by/4.0/).