Microneedles Mediated Delivery of Nanodiamonds for Quantum Sensing of Free Radicals in Skin Tissue

ABSTRACT Free radicals in the skin are major contributors to the aging process and the development of cancer, making their detection crucial in both research and clinical diagnosis. Real‐time magnetic noise sensing within skin tissues, especially through a quantum sensing technique known as T1 relaxometry, offers a promising approach for monitoring free radical activity. This technique uses fluorescent nanodiamonds (FNDs), whose optical properties change in response to magnetic fluctuations. To effectively deliver FNDs into the epidermis, we developed a dissolving microneedle (DMN) system composed of 10% polyvinylpyrrolidone and 3% hyaluronic acid. The DMNs exhibited strong FND fluorescence intensity concentrated at the microneedle tips (44%), optimizing their deposition within the epidermal layer. Histological analysis confirmed penetration depths of approximately 100–200 µm. To evaluate the sensing performance, T1 relaxometry measurements were conducted using gadolinium chloride solutions at the concentrations of 100 n m and 10 µ m as surrogates for free radicals. Compared to the baseline, the mean T1 values decreased by 35% and 55% for 100 n m and 10 µ m groups, respectively, demonstrating the system's sensitivity. Overall, our results highlight the potential of FND‐loaded DMNs as a minimally invasive platform for high‐resolution monitoring of oxidative stress and related biological processes in skin tissue.