Hyphenation of enzyme/graphene oxide-ionic liquid/glassy carbon biosensors with anodic differential pulse stripping voltammetry for reliable determination of choline and acetylcholine in human serum

Acetylcholine (ACh) and its precursor choline (Ch) play important roles in many biological processes. It is expected that Alzheimer's disease is occurred due to the reduction in synthesis of ACh. On the other hand, the increase in the level of ACh results in a depression of heart rate and over production of saliva. Therefore, the quantitative determination of Ch and ACh is very important in biological media. In the current work, sensitive and selective biosensors composed of choline oxidase (ChO) and/or acetylcholine esterase (AChE) on graphene oxide-ionic liquid (GO-IL)/glassy carbon electrode (GCE) hyphenated with anodic differential pulse stripping voltammetry (ADPSV) were firstly established for the determination of ACh and Ch in human serum samples. The molecular bond of ionic liquid 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (AMIM TFSI) with GO was investigated by FT-IR and UV-Vis techniques. Furthermore, the surface topography of ChO/GO-IL and AChE-Cho/GO-IL composites was investigated by SEM and XRD. Then, the electron transfer features of biosensors ChO/GO-IL/GCE and AChE-ChO/GO-IL/GCE were characterized by the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) techniques. The ADPSV was further used for the determination of Ch and ACh. The experimental parameters such as differential pulse working potential, differential pulse scan rate, equilibrium time and long-term stability were further optimized. Detection limits of 0.885 and 1.352 nmol L-1 with excellent linearity (R-2 = 0.9996) over the range of 5-1000 nmol L-1 were obtained for Ch and ACh, respectively. The developed analytical methods showed excellent accuracy and precision for the determination of Ch and ACh in human serum samples avoiding their pretreatment or purification.