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 Cu2-xSe and vancomycin-modified NaGdF4:Nd,Yb@NaGdF4 downconversion nanoparticles (DCNPs), the fluorescence of which is weak owing to the spectral overlap of Cu2-xSe absorption with the DCNP NIR emission. The presence of Gram-positive bacteria precisely disconnects the bond between vancomycin-modified DCNPs and Cu2-xSe, 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.
Keywords: Cu2−xSe nanoparticles; Gram-positive bacteria; bacteria-activated fluorescent probe; downconversion nanoparticles; second near-infrared.