A sensitive and selective fluorescence "turn-off" sensor to detect heparin using water-soluble silicon nanoparticles (Si NPs) was developed for the first time. The Si NPs were synthesized by a simple one-step procedure, which did not need high-temperature and complex modification. The as-prepared Si NPs featured strong fluorescence, favorable biocompatibility, and robust photo- and pH stability. Significantly, the Si NPs were induced to assemble or aggregate via hydrogen bonding, which resulted in the fluorescence of Si NPs quenched. Under the optimized conditions, the linear range was obtained from 0.02 to 2.0 μg/mL, with a limit of detection of 18 ng/mL (equal to 0.004 U/mL). It was lower than the proper therapeutic level of heparin during cardiovascular surgery and long-term therapy. This proposed method was relatively free of interference from heparin analogues, which commonly existed in heparin samples and could possibly affect heparin detection. Moreover, it did not need to introduce any control medium. As expected, the method was successfully applied to detect heparin in human serum samples with satisfactory recovery ranging from 98.8 to 102.5%. The Si NPs were superbly suitable for cell imaging owing to the negligible cytotoxicity and excellent biocompatibility.

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β-Secretase (BACE1) is an important drug target in the treatment of Alzheimer's disease (AD). Therefore, sensitive detection of BACE1 is essential for AD treatment and drug discovery. In this work, a facile and sensitive fluorescence biosensing platform was developed for BACE1 detection. This sensing platform was constituted based on the interaction between a WS2 nanosheet and a peptide sequence. In the absence of BACE1, a FAM-labeled peptide substrate could be adsorbed on the surface of the WS2 nanosheet, thereby quenching its fluorescence. However, in the presence of BACE1, the hydrolysis of the peptide substrate by BACE1 triggers could occur with the subsequent release of short FAM-linked peptide fragments which could not be adsorbed on the surface of the WS2 nanosheet. This resulted in weak fluorescence quenching, thus restoring the fluorescence signal. By measuring the change in the fluorescence of the FAM-labeled peptide substrate, the fluorescence sensing platform based on the WS2 nanosheet could monitor BACE1. The proposed WS2 nanosheet-based platform exhibited excellent specificity and high sensitivity with a detection limit of 66 pM for BACE1. Importantly, we also demonstrated that this platform was suitable for the screening of BACE1 inhibitors. The proposed sensing platform not only provides a novel strategy for the BACE1 assay but also offers a potential tool for screening drugs.