A colorimetric paper sensor for citrate as biomarker for early stage detection of prostate cancer based on peroxidase-like activity of cysteine-capped gold nanoclusters

Citrate is currently considered a preferred biomarker for the early stage detection of prostate cancer. In the present work, based on the highly efficient catalytic properties of gold nanoclusters, a novel system for optical determination of citrate was successfully established under optimized conditions. Cysteine-capped gold nanoclusters (Cys-AuNCs) are shown to have an intrinsic peroxidase-mimetic activity. In the presence of H2O2, Cys-AuNCs nanostructures are able to catalyse the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with high efficiency to produce a blue dye (with an absorbance maximum at 650 nm). Citrate has carboxylic and hydroxyl groups that can bind with free amino and free carboxyl cysteine groups via hydrogen bonds, thus creating a coating on the surface of the gold nanocluster and inhibiting the cluster oxidation activity. Accordingly, a visual, sensitive and simple colorimetric method using Cys-AuNCs as peroxidase mimetic was developed for detecting citrate. A suitable linear relationship for citrate was obtained for the range of 0.5 to 1000 μM. The limit of detection (LOD) of the proposed method was calculated as 0.1 μM and the relative standard deviation (RSD) was obtained to be less than 4.0%. Moreover, the biosensor was used to perform a paper assay on a Y-shaped microfluidic device and make use of the distinctive features of microchannels such as short response time, very low reagent volume required, low fabrication cost etc. A detection limit of 0.4 μM was achieved through the paper test and a good linear range was observed between 1.0 μM-10 mM. The proposed method was further applied to citrate detection in the human urine sample.