Development of novel antimicrobial functional materials comprising quaternary ammonium compounds and mesoporous silica

Over the past several decades, mesoporous silica nanoparticles (MSNs) have attracted a tremendous deal of interest in areas such as catalysis, drug delivery systems (DDSs), sensing, environmental remediation, and nanoelectronics due to their unique structures and properties. Since their discovery, research focused on MSN-based drug delivery systems has increased exponentially each year.1 The current body of work utilizes MSNs as a delivery vehicle to host a variety of quaternary ammonium compounds (QACs) for biomedical applications. The results reported herein demonstrate that the combination of porous materials with QACs can have a significant synergistic effect on the antimicrobial properties of the resulting material which paves the way for further studies to build upon this enhancing effect. In chapter 2, benzalkonium chloride (BAC) was used as a template to synthesize mesostructured silica nanoparticles for antimicrobial applications.1 The synthesized material comprised a relatively high density (0.56 g per 1 g of SiO2) of BAC and a high surface area (1500 m2 g-1) after calcination. In the physiologically-relevant pH range of 4.0 to 7.4, BAC was released in a controlled manner showing dependence between pH and rate of release. The material demonstrated inhibition of the gram-positive Staphylococcus aureus and the gram-negative Salmonella enterica at 10 and 130 mg/L, respectively. Taking into consideration a ca. 36 wt.% loading content of BAC within the BAC-SiO2 material, this correlates to an inhibition of S. aureus with 4 mg/L BAC which is a 10-fold enhancement compared to the minimum inhibitory concentration (MIC) of 40 mg/L for pure BAC. These results indicate that either the release of an antimicrobial drug (e.g., BAC) from the MSNs is not mandatory to achieve bactericidal efficacy or the host-guest relationship between BAC and silica MSNs boosts antimicrobial activity. In chapter 3, cetylpyridinium chloride (CPC) was complexed with ZnCl2 to yield cetylpyridinium tetrachlorozincate with a stoichiometry of C42H76Cl4N2Zn where zinc exists as [ZnCl4]2- tetrahedra. This novel material demonstrated parity antimicrobial efficacy as compared to pure CPC despite the fact that ca. 16% of the material is replaced with bacteriostatic ZnCl2. A technique to load the novel material into porous frameworks was proposed and yielded a material with ca. 9.0 and 8.9 wt.% of CPC and Zn, respectively. In chapter 4, a complex comprising chlorhexidine (CHX) with N-cyclohexylsulfamate (i.e., cyclamate) was synthesized yielding a stoichiometry of [C22H32N10Cl2][C7H13O3S]2. This newly discovered material demonstrates excellent antimicrobial activity with a minimum inhibitory concentration (MIC) of 2.5 µg/mL for Streptococcus mutans. Moreover, this material is advantageous as compared to the indispensable chlorhexidine gluconate since the inactive gluconate ion is substituted with the bioactive artificial sweetener (i.e., cyclamate) which has the potential to mitigate the bitter taste commonly associated with chlorhexidine.Ph.D.Includes bibliographical reference