Covalent Assembly of Amphiphilic Bola-Amino Acids into Robust and Biodegradable Nanoparticles for In Vitro Photothermal Therapy

Organic photothermal sensitizers, such as indocyanine green (ICG), have been widely explored in photothermal therapy as a good substitute for inorganic materials owing to their advantageous biosafety and strong absorption in the near-infrared region. However, their intrinsic low stability and rapid clearance from the body requires further modification for efficient therapeutic application. In this work, we employed a covalent assembly strategy by covalently cross-linking genipin and a functional bola-amino acid to fabricate stable and degradable nanoparticles capable of loading ICG. The covalent assembly introduced strong covalent interactions in the assembly system together with functional linkers, which led to both enhanced stability and extended functionalities. This is distinguished from the conventional supramolecular strategy that relies only on weak noncovalent interactions. The functional building unit, consisting of phenylalanine and the disulfide bond, enables both good assembly and controllable degradation owing to the disulfide bond that responds to glutathione. The assembled nanoparticles show high stability, negligible toxicity, and considerable biodegradability. After loading ICG, the ICG-loaded nanoparticles possessed high photothermal conversion efficiency, and showed an enhanced photothermal effect in the near-infrared region. This covalent assembly strategy could be extended to various biomolecules containing a primary amino group for the fabrication of efficient and multifunctional nanomaterials used in biomedical applications.