Functionalization of sol gel bioactive glasses carrying Au nanoparticles: selective Au affinity for amino and thiol ligand groups

It is demonstrated here that bioactive glasses containing Au nanoparticles (AuNPs) can be selectively functionalizedwith small molecules carrying either amino or thiol groups by simply varying the temperature and pH of thefunctionalization batch. The results evidence the following. (i) At room temperature (RT), no functionalization ofAu-free glass occurs, whereas in the case of glasses containing AuNPs, stable linkages form only with amino groups, as inthis condition Au does not bind with either thiol or hydroxyl groups. The RT functionalization with cysteine and cystineconfirms the preferential functionalization through the amino groups, while the -SH groups are oxidized to S-S bridges.(ii) The functionalization with cysteine and cystine, compared at pH = 5, 9, and 12, is shown not to take place at pH = 5and to be hindered by the glass matrix dissolution at pH = 12 (with consequent release of AuNPs), while the best resultsare obtained at pH = 9. (iii) For the effect of reaction temperature, at 4 °C it is possible to obtain a strong Au-Sinteraction, whereas at RT, a weak Au-N linkage is formed. These results should allow production, in a selective way, ofdifferent bonds exhibiting different strengths and, consequently, different release times in solution, with a wide range ofpossible applications (for instance, weak Au-N bonds in the case of drug delivery, strong Au-S bonds in proteinimmobilization).