TiO2-coated luminescent porous silicon micro-particles as a promising system for nanomedicine

Porous silicon (pSi) is a sponge-like material obtained by electrochemical etching of a crystalline silicon wafer. Due to quantum confinement effect, this material is photoluminescent and this is a fundamental property in perspective of bioimaging applications. Limitations in NanoMedicine to the use of photoluminescent pSi structures are mainly due to the optical quenching in aqueous environment and to the adverse effects of reactive groups introduced by the etching procedures. In this work, we exploited an inorganic TiO2 coating of the pSi microparticles by Atomic Layer Deposition (ALD) that resulted in an optical stability of pSi particles in a biological buffer (e.g. PBS). The use of a rotary reactor allows a deposition of a uniform coating on the particles and enables a fine tuning of its thickness. The ALD parameters were optimized and the photoluminescence (PL) of pSi-TiO2 microparticles was stabilized more than three months without any significant effect on their morphology. The biocompatibility of the coated microparticles was evaluated by analyzing the release of cytokines and superoxide anion (O2-) by human dendritic cells, which play an essential role in inflammatory and immune response regulation. We demonstrated that the microparticles per se are unable to significantly damage or stimulate human dendritic cells and therefore are suitable candidates for NanoMedicine applications. However, a synergistic effect of the microparticles with bacterial products, which are known to stimulate immune-response, was observed, indicating that a condition unfavorable to the use of inorganic nanomaterials in biological systems is the presence of infection diseases. These results, combined to the proved PL stability in biological buffers, opens the way for pSi-TiO2 microparticles to be a promising material in NanoMedicine, but their ability to increase the immune cell activation by other agonists should be considered and even exploited.