The development of simple, easy-to-operate and real-time detection methods for the active ingredients in herbal medicines has aroused growing interest owing to their pivotal health benefits. In this study, a qualitative and quantitative detection method for the flavonoids was developed based on the specific interaction between flavonoids and bovine serum albumin (BSA). A fluorescent gold nanocluster was imbedded into the cavity enclosed in the tertiary structure of BSA, the fluorescence of which can be quenched by the flavonoids with fast response (<5 s). This decrease in fluorescence intensity of BSA-AuNCs as output signal enables the real-time visual inspection of flavonoids. We demonstrated that the present approach was capable of detecting quercetin in serum, plasma, and monitoring the content of flavonoids in proprietary Chinese medicine Rutin Tablets. BSA-AuNCs was the first fluorescent probe for the specific determination of active ingredients in herbal medicines. Hence the reported protein-AuNCs sensing platform can serve as a convenient detection strategy in pharmaceutical analysis. (C) 2018 Elsevier Inc. All rights reserved.