Bacterial infections are a serious ongoing threat to human health. As such, within the context of modern technology, there is an urgent need to develop fluorescent probes suitable for diagnosing bacterial infections in vivo. Herein, we synthesize NaGdF 4 :Nd@NaGdF 4 core-shell down-conversion nanoparticles (DCNPs) demonstrating excellent emission intensity in the second nearinfrared (NIR-II) window for use as fluorescent probes to enable high-quality imaging of pathogenic infections. The fluorescent probe is covalently linked with vancomycin (Van), which specifically binds to Gram-positive bacteria such as S. aureus. Both in vitro detection and in vivo imaging results show that the Vanmodified NIR-II fluorescent probe can distinguish Gram-positive from Gram-negative bacteria with a high degree of selectivity. Biocompatibility is verified by MTT assay and histological analyses.

A material system enabling specific NIR-II fluorescence imaging of Gram-positive bacteria is described. The material system is based on the electrostatic binding of Cu 2−x Se and vancomycin-modified NaGdF 4 :Nd,Yb@NaGdF 4 downconversion nanoparticles (DCNPs), the fluorescence of which is weak owing to the spectral overlap of Cu 2−x Se absorption with the DCNP NIR emission. The presence of Gram-positive bacteria precisely disconnects the bond between vancomycin-modified DCNPs and Cu 2−x Se, thus enabling a strong fluorescent signal. In vivo studies show that the material system can be specifically activated at the site of Gram-positive bacterial infection but is essentially nonfluorescent in the area of Gram-negative bacterial infection.

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