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Schematic overview of an electrochemical sensor for ROS detection, showing different potential sensing materials, transducer, and signal processing. 

Abstract Hydroxyl radicals (•OH) are well known as crucial chemicals for maintaining the normal activities of human cells; however, the excessive concentration of •OH disrupts their normal function, causing various diseases, including liver and heart diseases, cancers, and neurological disorders. The detection of •OH as a biomarker is thus essential for the early diagnosis of these serious conditions. Herein, a novel electrochemical sensor comprising a composite of cerium oxide nanoclusters, gold nanoparticles, and a highly conductive carbon was developed for detecting •OH. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the signals generated by the interaction of the composite with •OH radicals. The CV results revealed that the developed sensor could accurately and selectively detect •OH in the Fenton reaction. The sensor demonstrated a linear relationship between the current peak and •OH concentration in the range 0.05 - 0.5 mM and 0.5 - 5 mM with a limit of detection (LOD) of 58 µM. In addition, EIS studies indicated that this electrochemical sensor could distinguish between •OH and similar reactive oxygen species (ROS), like hydrogen peroxide (H2O2). It is also worth mentioning that additional merits, such as reproducibility, repeatability, and stability of the sensor were confirmed. 

Hydroxyl radicals (•OH) are known as essential chemicals for cells to maintain their normal functions and defensive responses. However, a high concentration of •OH may cause oxidative stress-related diseases, such as cancer, inflammation, and cardiovascular disorders. Therefore, •OH can be used as a biomarker to detect the onset of these disorders at an early stage. Reduced glutathione (GSH), a well-known tripeptide for its antioxidant capacity against reactive oxygen species (ROS), was immobilized on a screen-printed carbon electrode (SPCE) to develop a real-time detection sensor with a high selectivity towards •OH. The signals produced by the interaction of the GSH-modified sensor and •OH were characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV curve of the GSH-modified sensor in the Fenton reagent exhibited a pair of well-defined peaks, demonstrating the redox reaction of the electrochemical sensor and •OH. The sensor showed a linear relationship between the redox response and the concentration of •OH with a limit of detection (LOD) of 49 µM. Furthermore, using EIS studies, the proposed sensor demonstrated the capability of differentiating •OH from hydrogen peroxide (H2O2), a similar oxidizing chemical. After being immersed in the Fenton solution for 1 hr, redox peaks in the CV curve of the GSH-modified electrode disappeared, revealing that the immobilized GSH on the electrode was oxidized and turned to glutathione disulfide (GSSG). However, it was demonstrated that the oxidized GSH surface could be reversed back to the reduced state by reacting with a solution of glutathione reductase (GR) and nicotinamide adenine dinucleotide phosphate (NADPH), and possibly reused for •OH detection.