A Comprehensive Investigation of Photoactive and Monomeric Chlorophyll a/Amino grafted Mesoporous Silica Nanoparticles Delivery System in Aqueous Solutions

Highly ordered Mesoporous Silica Nanoparticles (MSNs), belonging to MCM-41 family (100 nm wide), with a mean pore size of about 4 nm, are used as carrier for the hydrophobic photosensitizer (PS) Chlorophyll a (Chl) in water medium. In future perspective to study the proposed system in Photodynamic Therapy (PDT) and/or in antimicrobial-PDT (aPDT), advances in Chl studies in aqueous solution, mimicking the biological environment, were presented during this work with the main aim to obtain a supramolecular assembly able to host photoactive Chl in water medium. As synthetized MSNs and amino grafted MSNs (MSNs-NH2) were studied and carefully characterized, with and without Chl, using TGA, SEM, TEM, SAXS, Nitrogen physisorption isotherms, Z-potential investigation, FTIR-ATR and UV–vis absorption/fluorescence analyses, including also the Chl fluorescence lifetime evaluation. To drive Chl inside the pores, the drug loading method by using the adsorption process, was adopted. The NH2 moieties exerted a key role to host Chl inside the MSNs-NH2 pores, allowing to obtain Chl as monomer. Electrostatic interactions through positively charged amino groups were evidenced, along with the hydrogen bond presence involving the pigment hydrated form and the MSNs-NH2. Two Chl populations, i.e. ascribed to the monomeric and dimeric one, were revealed by 1Chl* lifetime measurements and steady state fluorescence emission. The presence of photoactive Chl molecules was demonstrated with the qualitative evaluation of 1O2 by means of chemical probes.