Long Term Stability of Photoluminescent Mesoporous nsnoSilicon for theranostics applications.

In this study, we report about the effects of biopolymers (PEG and chitosan) functionalization on luminescent meso-porous silicon (pSi). Pholuminescence stability in biological conditions, such as PBS buffer, has been demonstrated for the first time up to 90 days, also maintaining good optical quantum efficiency (from 1.7% to 1.1%). The discovery of photoluminescence (PL) from porous silicon (pSi) and the lack of evidence for its toxicity has stimulated a huge research of effective methods to provide silicon nanostructures functionalization for biomedicine. We recently demonstrated that light emitting pSi particles can be up-taken by human dendritic cells without any toxicity or reduction in cell viability and can be tracked in cells by fluorescence imaging in real time. Moreover, no stimulation of the secretion of pro-inflammatory cytokines has been found [1]. pSi particle size is in the range of 3-10 µm with high porosity: about 30 nm the average diameter. Emission of pSi in the orange-red regime is well-known due to quantum confinement. But this band is sensitive to quenching due to surface oxidation. To avoid it, carboxyl functionalization leads to stabilize emission for years without significant variation in the optical properties. Although pSi microparticles present an optical stability in ethanol for years at room temperature, the applications have been severely limited because of the incompatibility with water solutions, which leads to degradation of the material and to the loss of its optical properties, thus limiting its effective utilization as delivery system [2]. pSi is prepared by anodization of p-type Si wafer in aqueous HF:ethanol solution: pSi microparticles were obtained by sonication and suspended in acrylic acid under illumination thus resulting in carboxyl groups on its surface as shown by FTIR measurements (pSi-COOH sample). Furthermore, polymeric covering carried out by HCl.H2N-PEG-COOH to covalent conjugation of PEG on pSi-COOH in different ratio was done together with Chitosan functionalization. On the other hand, a diblock copolymer made of polylactic-co-glycolic acid and polyethylene glycol (PLGA-b-PEG) was selected for entrapment (pSi-PEG-PLGA). Based on the shrinkage of silicon crystals during chemical reaction with biopolymers, PL blue-shifted but maintaining an optical quantum yield efficiency of about 1-2%, as determined by comparative method. For the first time, experimental results showed that chemically coverage by using biopolymers maintain the PL stability in in vitro (i.e. water and pBS buffer)s for at least 90 days. Moreover, we demonstrated by two photon absorption technique that the system can be efficiently excited at biological window (700-810 nm). The present findings are very promising for several biomedical applications such as bio-imaging, photo-thermal therapy and drug delivery.