Biosensor for the oxidative stress biomarker glutathione based on SAM of cobalt phthalocyanine on a thioctic acid modified gold electrode

Self-assembled monolayer (SAM) of cobalt teraaminophthalocyanine (CoTAPc) was developed on thioctic acid (TA) dithiol modified gold electrode and electrochemically evaluated as a glutathione (GSH) selective biosensor. The CoTAPc-TA-Au modified electrode was developed by the covalent immobilization of the CoTAPc as the electrochemical mediator onto previously prepared gold electrode modified with TA (TA-Au) via amid bond formation with the carboxylic group of TA, producing well-organized SAM of the mediator. For comparison, another electrode modified with 3-mercaptopropionic acid (MPA) as a monothiol linker instead of TA was similarly prepared. The electrode surface modification was characterized using SEM, AFM, CV, and EIS. The contact angle measurements of the surface confirmed the formation of CoTAPc SAM on both TA and MPA modified electrodes. The CoTAPc-TA-Au modified electrode showed enhanced catalytic activity for GSH oxidation compared to that of CoTAPc-MPA-Au, indicating that the TA dithiol allowed for more coverage of the catalyst layer on the electrode surface with stronger binding. The experimental parameters controlling the voltammetric processes like scan rate and pH of sample solution were optimized. Using DPV technique, the proposed sensor exhibited a linear response of oxidation peak current vs. GSH concentration, over the concentration range between 10 and 100 μmol L ⁻¹ with a LOD of 1.5 μmol L ⁻¹ for the CoTAPc-TA-Au modified electrode compared to 5.5 μmol L ⁻¹ GSH, for the CoTAPc-MPA-Au electrode. The proposed sensor was utilized for detection of glutathione in some hemolyzed blood samples.