Human serum albumin-stabilized gold nanoclusters act as an electron transfer bridge supporting specific electrocatalysis of bilirubin useful for biosensing applications

•The AuNCs formed in HSA act as an electron transfer bridge for bilirubin oxidation.•The oxidation occurs at a specific moiety of bilirubin on the electrode surface.•The HSA-AuNCs bioelectrode detects free bilirubin sensitively in serum samples.

Human serum albumin (HSA)-stabilized Au18 nanoclusters (AuNCs) were synthesized and chemically immobilized on an Indium tin oxide (ITO) plate. The assembly process was characterized by advanced electrochemical and spectroscopic techniques. The bare ITO electrode generated three irreversible oxidation peaks, whereas the HSA-AuNC-modified electrode produced a pair of redox peaks for bilirubin at a formal potential of 0.27 V (vs. Ag/AgCl). However, the native HSA protein immobilized on the ITO electrode failed to produce any redox peak for bilirubin. The results indicate that the AuNCs present in HSA act as electron transfer bridge between bilirubin and the ITO plate. Docking studies of AuNC with HSA revealed that the best docked structure of the nanocluster is located around the vicinity of the bilirubin binding site, with an orientation that allows specific oxidation. When the HSA-AuNC-modified electrode was employed for the detection of bilirubin using chronoamperometry at 0.3 V (vs. Ag/AgCl), a steady–state current response against bilirubin in the range of 0.2 μM to 7 μM, with a sensitivity of 0.34 μA μM− 1 and limit of detection of 86.32 nM at S/N 3, was obtained. The bioelectrode was successfully applied to measure the bilirubin content in spiked serum samples. The results indicate the feasibility of using HSA-AuNC as a biorecognition element for the detection of serum bilirubin levels using an electrochemical technique.

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