A simple and convenient preparation of multicolor carbon dot (CD) solutions was proposed using neutral red as the carbon and nitrogen sources by a layering and extraction procedure. After poly­(vinylpyrrolidone) (PVP) as the cross-linking agent was doped into as-prepared solutions, red-, orange-, and yellow-emitting CD powders were finally fabricated with solid-state fluorescence quantum (QY) yields of 6.33, 16.5, and 43.6%, respectively. Through studies of surface functional groups and photoluminescence (PL) property compared with solution and powder states of CDs, the cross-linking effect emission (CEE) brought by PVP was confirmed in charge of the high solid-state QY of CD powders. In addition, the conjugate structure of neutral red and increasing hydrophobicity are believed to be responsible for the long-wavelength emission of CD powders. Notably, multicolor imaging of latent fingerprints (LFPs) was successfully achieved on complex background surfaces under 365 nm UV illumination. As a result, these carbon-based phosphors are promising in forensic investigation, bioimaging, and sensing fields.

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A series of binuclear (Ce,La) nanocomposite fluorescent powders was elaborately designed for highly-selective recognition of latent fingermarks, which were proved to combine with fingermark residues electrostatically without any damage to touch-DNA.