Metal ion-promoted fabrication of melanin-like poly(L-DOPA) nanoparticles for photothermal actuation

Melanin-inspired polymers are currently the focus of growing interest for a wide range of applications ranging from energy to biomedical area. Whilst researchers have made numerous attempts to prepare and utilize polydopamine nanoparticles (PDA NPs), they have made limited progress in developing and discovering another typical functional mimic of natural melanin, poly (levodopa) (P(L-DOPA)) NPs, probably due to the lack of facile synthetic strategies towards satisfactory structural and functional control of melanin-like NPs. Herein, we reported a one-pot preparation method towards P(L-DOPA) NPs with good yields and controllable size/property in an aqueous solution assisted by various metal ions (i.e., Ni(II), Mg(II), Ca(II), Fe(III), Mn(II), Co (II), Zn(II) and Cd(II)). Interestingly, the resulting P(L-DOPA) NPs exhibited enhanced light absorption and photothermal behaviors compared with well-established PDA NPs, which can be employed to further fabricate kinds of photothermal composite actuators with promising performances such as folding, switching, and forward-moving. This study offers a facile and robust way to synthesize new synthetic melanins beyond PDA, and facilitates further functional discovery and evolution of melanin-inspired polymers and composites.