Rapid visual discrimination of Citri Reticulatae Pericarpium from different origins by fluorescent sensors based on aluminum ions and Ag nanoclusters

Citri Reticulatae Pericarpium (CRP) is a medicinal and food homolog commonly used in Chinese medicine. The quality, price, and content of dihydroflavones (DHFs) and polymethoxyflavones (PMFs) in CRP are directly related to their origin. A novel, fast, and simple visual discrimination method for the origin of CRP is presented here. Al3+ and bovine serum albumin/thiosalicylic acid-modified Ag nanoclusters (BSA/TSA-AgNCs) were screened as fluorescent sensors for targeting DHFs and PMFs, respectively. After 60 batches of CRP extracts from different origins reacted with Al3+ or AgNCs sensors, three types of CRP origin could be discriminated accurately by the naked eye or using partial least squares discriminant analysis (PLSDA), according to the significant differences in fluorescence color. It showed a good discrimination effect when applied to filter-paper-based detection. The sensoring mechanism can be the DHFs in CRP form a complex with Al3+, which increases the molecular conjugation and structural rigidity, resulting in a fluorescence enhancement effect. PMFs are more likely to enter the BSA shell of AgNCs owing to hydrophobic interactions and transfer electrons with TSA modified on the surface of AgNCs, resulting in the photoinduced electron transfer (PET) effect and aggregation of TSA-AgNCs, which eventually leads to fluorescence quenching.