Composite Quantum Sensors Based on Fluorescent Nanodiamonds for Intracellular Controlled Heating in Living Cells

Quantum sensors based on fluorescent nanodiamonds (FNDs) have emerged as potential nanosensors for measuring the physical parameters inside single cells due to their remarkable photostability and unique magneto-optical properties. Owing to the physicochemical inertness of FNDs, they can be hybridized with gold nanoparticles (FND–GNPs). This hybrid has unique applications, which cannot be realized by FNDs alone, such as two-in-one nanoheaters/thermometers. Nevertheless, the development of FND–GNP hybrids with highly uniform structures, high dispersity, and diverse functionalities remains challenging. In this study, we have designed FND composite quantum sensors (CQSs). In the first step, GNPs were directly grown from HAuCl4 on the surface of a FND coated with a thin layer of polydopamine (PDA), FND–PDA–GNP. The wet chemistry route provided the highly uniform structure of FND–GNPs in which the size of the GNPs can be controlled by changing the reaction conditions. In the second step, FND–PDA–GNP was further stabilized using an additional PDA layer (FND–PDA2–GNP) and was then modified with hyperbranched polyglycerol (HPG), FND–PDA2–GNP–HPG. The obtained composite exhibits excellent dispersity and aggregation resistance in salt-containing solutions. Finally, the one-pot functionalization of the amine-modified CQS was established from FND–PDA2–GNP, facilitating cell internalization, and thus controlled heating in a living cell was demonstrated.