Black phosphorus (BP) nanostructures with unique layer-dependent properties have been extensively applied in the fields of electronic devices, energy conversion and storage, and nanomedicine. As a narrow band gap semiconductor, they are expected to show strong second near-infrared (NIR-II) fluorescence. However, there is no report on the NIR-II fluorescence of free-standing BP nanostructures, which have great potential in the NIR-II fluorescence bioimaging because of their excellent biocompatibility and biodegradability. Here, for the first time, we report that the BP nanoparticles modified with cholesterol exhibit strong NIR-II fluorescence and can be encapsulated with the PEGylated lipid to form BP@lipid-PEG nanospheres for in vitro and in vivo NIR-II imaging. The resultant BP@lipid-PEG nanospheres exhibit broad emissions from 900 to 1650 nm under excitation by an 808 nm laser and have 8% quantum yield of that of standard dye IR-26. We also show that the NIR-II fluorescence image acquired with emission beyond 1400 nm has the sharpest contrast and can be used to in situ measure the diameter of blood vessels. In addition to NIR-II fluorescence imaging, we also show the potential of BP@lipid-PEG nanospheres in photoacoustic (PA) imaging. Both the long-wavelength NIR-II fluorescence imaging and PA imaging reveal that the as-fabricated BP@lipid-PEG nanospheres can be gradually metabolized by the liver in 48 h, thus making them promising for bioapplications.
Keywords: NIR-II fluorescence; bioimaging; black phosphorus; cholesterol modification; nanospheres.