Light-Driven Riboflavin loaded Au/TiO₂ micromotors as a Promising Strategy for Targeted Melanoma Therapy

Light-driven metal-oxide micromotors have emerged as promising theranostic tools for efficient tumor penetration and intracellular drug delivery, enabling more precise targeting and enhanced treatment of malignant cells [1,2]. Titanium oxide (TiO₂) micromotors can overcome tumor microenvironmental barriers under light irradiation and propel themselves using tumor-derived H₂O₂ [3]. This self-propulsion enhances cellular uptake and therapeutic efficacy, although the exact mechanisms underlying their antitumor activity remain incompletely understood [4]. In this study, we characterized the physicochemical properties and investigated the biological effects of newly developed riboflavin (vitamin B2)-loaded gold–titanium dioxide (Au/TiO₂-RbF) micromotors in human A375 melanoma cells in vitro. Structural analysis by SEM revealed that the Au/TiO₂ particles were spherical, uniform, and well dispersed, with an average diameter of approximately 1 µm, consistent with DLS measurements. Elemental mapping using HRTEM/EDS confirmed a homogeneous distribution of Au, Ti, and O throughout the particles. UV–vis spectroscopy of the Au/TiO₂ micromotors showed strong absorption in the UV region (200–400 nm), characteristic of TiO₂, along with an additional peak at 550 nm corresponding to Au. Photoluminescence excitation analysis demonstrated stable and intense UV fluorescence with a maximum emission at 360 nm upon 280 nm excitation, accompanied by visible emission originating from RbF in the 500–600 nm range. Importantly, riboflavin acts as an efficient photosensitizer, and its activation under UVA irradiation leads to enhanced reactive oxygen species (ROS) generation. Using EPR spectroscopy, we demonstrated significantly increased singlet oxygen production from Au/TiO₂-RbF micromotors upon UVA exposure, confirming the photoactivated ROSgenerating capability of the system. To evaluate the biological effects, MTT assays revealed no significant cytotoxicity of the micromotors (5–20 µg/mL) in non-irradiated A375 human melanoma cells after 24 h of treatment. In contrast, UVA irradiation for 2 h triggered dose-dependent cytotoxicity in micromotor-treated cells. Phase-contrast microscopy further revealed morphological changes consistent with apoptotic cell death. Collectively, these results demonstrate that RbF-loaded Au/TiO₂ micromotors induce UVA-triggered, ROS-mediated apoptosis in A375 melanoma cells while exhibiting minimal toxicity in the absence of irradiation. Taken together, our findings highlight the potential of Au/TiO₂-RbF micromotors as a safe and effective light-activated platform for targeted anti-melanoma therapy.