Colorimetric Glucose Detection Using Copper Complexes and Filter Paper-Based Simulated Skin Analysis: Influence of Methyl Substituted Phenolato Groups

Copper complexes [Cu₂L_A(OAc)₂] (1) and [Cu₂L_B(OAc)₂] (2) were synthesized using Schiff base ligands H₂L_A and H₂L_B (where H₂L_A= 2,2'-((1E,1'E)-((piperazine-1,4-diylbis(ethane-2,1-diyl))bis(azaneylylidene))bis(methaneylylidene))diphenol), and H₂L_B = 2,2'-((1E,1'E)-((piperazine-1,4-diylbis(ethane-2,1diyl))bis(azaneylylidene))bis(methaneylylidene))bis(4 methylphenol). Complexes 1 and 2 act as copper-based colorimetric sensors for glucose through a cascade reaction mechanism: GOx converts glucose to gluconic acid and generates H₂O₂, which acts as a substrate for the copper compound oxidizing o-phenylenediamine (OPD) to form oxidized OPD (oxOPD) detectable at 424 nm. The effects of incubation time and pH on the catalytic activity of compound 2 were systematically evaluated. Under optimized reaction conditions, the Michaelis constants (K_m) for compounds 1 and 2 were determined to be 0.00267 and 0.00251 M, respectively, values comparable to those reported for various peroxidase systems and previously reported enzyme mimics. Further mechanistic studies indicated that the catalytic activity of these compounds is primarily due to the generation of hydroxyl radicals (·OH). Porous microneedles fabricated with simulated skin were utilized for glucose monitoring. The integration of the copper compounds with glucose oxidase (GOx) produced a distinct colorimetric response, changing from white to yellow in the presence of varying glucose concentrations. This approach highlights the potential for noninvasive and visually detectable glucose sensing. The ground-state geometries of complexes 1 and 2 were optimized using B3LYP-level density functional theory (DFT) calculations, and additional calculations confirmed the structural stability and involvement of the LCu(II)-OOH like species.