pH-Dependent Morphology of Copper (II) Oxide in Hydrothermal Process and Their Photoelectrochemical Application for Non-Enzymatic Glucose Biosensor  
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Footnoteor non-enzymatic recognition agents [1; 7]; however, with the significant development of nanotechnology, the photoelectrochemical (PEC) analysis method has garnered substantial attention because of its high sensitivity, selectivity, low detection threshold, and cost-effectiveness [8; 9; 10; 11]. They combine photoelectric conversion and electrochemical processing [12], inheriting the advantages of conventional sensors while minimizing excitation source noise through signal separation [8; 13]. These things lead to improvements in the sensitivity and detection limits of the sensing target. Upon light-induced excitation of the nanomaterials acting as photoactive materials, photogenerated electrons participate in redox reactions with target analytes adsorbed on the sensor surface. The resulting change in signal intensity, which is proportional to the adsorbed analyte amount, enables quantitative analysis of its concentration.  
Recent advances in materials technology have promoted the progress of PEC biosensors as a prominent research area [14; 15; 16]. Within the field of glucose biosensors, researchers have actively developed various PEC configurations using various nanomaterials and their structures [17; 18; 19; 20]. In particular, research is moving toward non-enzymatic materials (free enzyme/mimic enzyme/nanoenzymes) [21; 22; 23; 24] due to limitations in the thermal and chemical stability of glucose oxidase \(GOx\), which is an enzyme that is commonly used in a glucose biosensor. [PERSON] et al. used gold nanoparticles \(Au\) NPs that acted as a \(GOx\) mimic enzyme in fluorine-doped tin oxide (\(FTO\)) modified with \(PbS\) quantum dots and \(SiO_{2}\) nanospheres for the application of photoelectrochemical glucose detection [25].  
Enzyme-free glucose PEC sensors have attracted significant attention using metal oxides [26; 27; 28; 29; 30; 31]. Several metal oxides with semiconducting properties, such as \(CuO\), titanium dioxide (\(TiO_{2}\)), zinc oxide (\(ZnO\)), and nickel (II) oxide (\(NiO\)), were used for surface modification [30; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42]. Among them, the p-type \(CuO\), with a narrow bandgap of 1.2-2 eV [43], stands out for its robust catalytic activity under alkaline conditions, making it a promising candidate for glucose oxidation. [PERSON] et al. successfully electrodeposited a \(CuO\) film onto a conductive glass substrate, achieving a broad glucose detection range of up to 29 mM with a detection limit of 59.5 mM at a bias of 0.6 V [35]. A biosensor must discriminate between electroactive interfering species. In a typical physiological sample, interfering species such as ascorbic acid (AA), dopamine (DA), and uric acid (UA) coexist with glucose. \(CuO\) nanomaterials have been shown to have a very high specificity for reactions with glucose in the presence of interfering substances [44; 45]. Many strategies for developing \(CuO\) materials have been reported regarding their synthesis method and morphology for the detection of PEC glucose [27; 35; 36; 46; 47]. However, the hydrothermal synthesis method for \(CuO\) has not yet been widely reported, especially in the process of controlling the morphology of this material.  
This study focused on the impact of pH-controlled hydrothermal synthesis on the morphologies of nanostructured \(CuO\) and their influence on the performance of photoelectrochemical glucose.