Disclosed herein are second near-infrared (NIR-II) fluorescent composite and its production method. The method mainly includes the steps of, mixing a gold nanocluster having a plurality of a thiol-based compound on its outer surface and alpha-glycerylphosphorylcholine (alpha-GPC) in a solvent to form a mixture; replacing the solvent with an inert gas; and heating the mixture at a temperature about 100-200° C. in the presence of the inert gas until at least a portion of the gold nanocluster is encapsulated by a capping layer consisting of alpha-GPC, thereby producing the NIR-II fluorescent composite. The thus-produced NIR-II fluorescent composite is characterized by having an emission wavelength covering NIR-II region detectable by specialized camera. Also encompassed in the present disclosure is a method for conducting in vivo bioimaging of a target area in a subject. The method includes administering the present NIR-II fluorescent composite to the target area; and detecting the fluorescence emitted therefrom at a wavelength between 900 to 1700 nm.