Development of graphene based electrochemical biosensors

In this thesis, pencil graphite electrodes (PGEs) with enzyme/electroactive polymer/graphene/tungsten disulfide (WS2) modified nanocomposite surface were prepared for the amperometric determination of glucose and urea. Ortho-phenylenediamine (oPD), pyrrole (Py) and graphene were used to stably and efficiently bind glucose oxidase (GOx) and urease (Urs) enzymes to the electrode surface. Glutaraldehyde (GA) crosslinker was used for stable enzyme immobilization to the PPy-graphene/WS2 nanocomposite surface produced under optimum conditions. So-formed GOx/PPy-graphene/WS2 modified and Urs/PPy-graphene/WS2 modified enzyme electrodes were used as sensitive and selective amperometric glucose and urea biosensors, respectively. Electrochemical behavior of the electrodes was determined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Morphological and surface characterizations of the electrodes were performed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Parameters that affect the working performance of the enzymes such as enzyme concentration, enzyme immobilization time, pH value of the environment and working potential were optimized. The working range of the GOx modified glucose biosensor was 0.05 mM to 7 mM, with a detection limit of 12.3 μM, while the operating range of the Urs modified urea biosensor was from 0.25 mM to 18 mM, with a detection limit of 30.1 μM. Interference studies were performed in the presence of ascorbic acid and urea for the glucose biosensor, whereas for the urea biosensor, they were performed in the presence of glucose and ascorbic acid, and it was determined that the species did not have a significant effect on the amperometric current. Recovery studies were carried out in diluted serum for glucose and diluted urine samples for urea.