Transferrin-Coated Nanodiamond–Drug Conjugates for Milliwatt Photothermal Applications

Abstract

Fluorescent nanodiamonds (fNDs) are unique carbon-based nanomaterials due to their outstanding optical and magnetic properties. However, realization of the full potential of fNDs is often limited by their processability because fNDs aggregate strongly in both organic and aqueous solutions. Therefore, robust and potentially universal coating strategies are urgently needed to address these limitations. Derived from mussel foot proteins, the polymerization of l-3,4-dihydroxyphenylalanine (l-DOPA) provides important surface functional groups including amines, carboxylic acid, alcohols, and conjugated Michael acceptors. Herein, l-DOPA is polymerized on fNDs with a high control over the shell thickness. Photoluminescence and optically detected magnetic resonance studies reveal that the unique photophysical properties of fNDs are preserved after thin poly(l-DOPA) film coating. Subsequently, conjugation of transferrin, a heme protein that provides efficient receptor-specific cellular transport, improves the colloidal stability and cellular uptake of the poly(l-DOPA)-coated fNDs. The loading of FDA-approved indocyanine green as a photothermal agent yields an integrated biohybrid material exhibiting an amplified photothermal effect in cells at very low energy intake (≈90 mW cm−2).