Fluorescent papain-encapsulated platinum nanoclusters for sensing lysozyme in biofluid and gram-positive bacterial identification

Abstract

Fluorescent metal nanoclusters, a new class of fluorophores, have been widely used in the fields of biosensing, biological imaging, catalysis and advanced therapeutics. In this study, papain-encapsulated platinum nanoclusters (papain-Pt NCs) were synthesized via a biomineralization process, which showed intense green fluorescence with maximum excitation and emission wavelengths at 380 and 490 nm. A panel of characterizations was performed to investigate the optical properties, morphology and surface chemistry of Pt NCs, demonstrating the accurate formation of papain-Pt NCs with good dispersity and stability. As a “turn-off” fluorescence biosensor, papain-Pt NCs exhibited selective and relatively sensitive detection of lysozyme with an excellent linear concentration in the range of 70−1000 nM and a detection limit of 16.94 nM at a signal-to-noise ratio of 3. Meanwhile, Gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus and Microbacterium) cells stained with papain-Pt NCs could emit obvious green fluorescence, whereas their counterparts of Gram-negative bacteria (Escherichia coli, Rhodobacter and Rhizobium) exhibited no fluorescence signal. These results indicate that papain-Pt NCs could be used as fluorescent label-free nanoparticles for the purpose of identifying Gram-positive bacteria. Our current study provides a highly promising fluorescent nanomaterial for lysozyme detection and bacterial differentiation, and opens crucial insights into the design of hybrid bioinspired systems intended for various diseases.