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biosensors Article A Multipurpose and Multilayered Microneedle Sensor for Redox Potential Monitoring in Diverse Food Analysis Samuel M. Mugo 1,* , Dhanjai 2,* , Weihao Lu 1 and Scott Robertson 1 1 2 Abstract: This work presents a multipurpose and multilayered stainless steel microneedle sensor for the in situ redox potential monitoring in food and drink samples, termed MN redox sensor. The MN redox sensor was fabricated by layer-by-layer (LbL) approach. The in-tube multilayer coating comprised carbon nanotubes (CNTs)/cellulose nanocrystals (CNCs) as the first layer, polyaniline (PANI) as the second layer, and the ferrocyanide redox couple as the third layer. Using cyclic voltammetry (CV) as a transduction method, the MN redox sensor showed facile electron transfer for probing both electrical capacitance and redox potential, useful for both analyte specific and bulk quantification of redox species in various food and drink samples. The bulk redox species were quantified based on the anodic/cathodic redox peak shifts (Ea/Ec) on the voltammograms resulting from the presence of redox-active species. The MN redox sensor was applied to detect selected redox species including ascorbic acid, H2O2, and putrescine, with capacitive limits of detection (LOD) of 49.9, 17.8, and 263 ng/mL for each species, respectively. For the bulk determination of redox species, the MN redox sensor displayed LOD of 5.27 × 103, 55.4, and 25.8 ng/mL in ascorbic acid, H2O2, and putrescine equivalents, respectively. The sensor exhibited reproducibility of ~1.8% relative standard deviation (%RSD). The MN redox sensor was successfully employed for the detection of fish spoilage and antioxidant quantification in king mushroom and brewed coffee samples, thereby justifying its potential for food quality and food safety applications. Lastly, the portability, reusability, rapid sampling time, and capability of in situ analysis of food and drink samples makes it amenable for real-time sensing applications. Keywords: electrochemical microneedle sensor; MN redox sensor; food safety; H2O2; putrescine; food quality monitoring 1. Introduction Reactive oxygen species (ROS) are natural by-products of mitochondrial aerobic respiration and cellular metabolism [1–3]. A balance of metabolic redox species comprising ROS and antioxidants maintain the cellular physiological metabolic functions in plants and animals at a steady state [4]. Due to both endogenous and exogenous causes, an imbalance of these redox species can lead to cellular oxidative stress [5,6]. To maintain physiological metabolic balance, the natural antioxidant defense system can inhibit the oxidation of ROS, thus preventing the release of free radicals [7–9]. Common antioxidants, such as ascorbic acid and thiols, participate in the termination of harmful chain oxidative processes caused by these free radicals [10]. Biogenic polyamines such as spermine, spermidine, and putrescine are by-products produced by the bacteria-catalyzed degradation of fruits, vegetables, and meats, and are an index of food freshness and quality [10,11]. Measuring the content of oxidants and antioxidants (reductants) in various foods can be a useful index for spoilage and fruit ripening [12], viral and bacterial infections onset, and as functional foods/nutraceuticals characterization [7]. In general, colorimetric assays utilizing 2,2-Diphenyl-1-picrylhydrazyl Department of Physical Sciences, MacEwan University, Edmonton, AB T5J 4S2, Canada Department of Chemistry, University of Allahabad, Prayagraj 211 002, India * Correspondence: mugos@macewan.ca (S.M.M.); dhanjai83@gmail.com (D.) Citation: Mugo, S.M.; Dhanjai; Lu, W.; Robertson, S. A Multipurpose and Multilayered Microneedle Sensor for Redox Potential Monitoring in Diverse Food Analysis. Biosensors 2022,12,1001. https://doi.org/ 10.3390/bios12111001 Received: 28 September 2022 Accepted: 5 November 2022 Published: 10 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). Biosensors 2022, 12, 1001. https://doi.org/10.3390/bios12111001 https://www.mdpi.com/journal/biosensorsPDF Image | Microneedle Sensor for Redox Potential Monitoring
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