Photothermal-enhanced synthetic melanin inks for near-infrared imaging

Synthetic melanin has been increasingly exploited as a new class of functional ink materials due to its promising properties such as ultrahigh reflective index and blackness, and good stability in aqueous solution. Whilst the promising photothermal conversion feature of synthetic melanin has been well devoted, the relatively low near-infrared (NIR) light absorption limited their further application as the NIR imaging inks. Herein, we developed a series of synthetic melanin nanoparticles (SMNPs) inks doped with different kinds of metal ions and found that all the products exhibited enhanced light absorption ability and total photothermal effect compared to the undoped sample. Interestingly, it was also found that these properties could be improved correspondingly with the increase of metal ion concentration. This study not only provides a comprehensive understanding of the metal ion-loaded SMNPs with enhanced photothermal effect, but also paves the way towards fabricating new SMNP inks for NIR imaging applications.