Synthesis and characterization of aqueous Cu₂ZnSnS₄ nanoparticles for antimicrobial applications

Hospital-acquired infection (HAI) is a major public health issue in the United States and in the world. According to a report in 2007 by the Centers for Disease Control and Prevention (CDC), there were approximately 1.7 million HAI and 99,000 deaths from the infection. The estimated annual medical cost to treat HAI is $28 - $45 billion. In addition, HAI's are increasingly resistant to a broad spectrum of antibiotics due in no small part to overuse of antibiotics. As a result, there is an increasing interest in non-antibiotic antimicrobial alternatives. Nanoparticles (NPs) have been shown to have antimicrobial properties and inorganic NPs have the advantages of being not susceptible to degradation by enzymes. Such antimicrobial activities can also be assisted by light for semiconducting NPs such as quantum dots (QDs) due to their photoconductive properties. However, the drawback of commercial quantum dots is that they contain heavy-metal elements such as cadmium (Cd), which is toxic to human and the environment. Although TiO2 NPs are known to exhibit antimicrobial properties and do not contain toxic elements their antimicrobial activities need ultraviolet (UV) light activation which can be harmful to human. The goal of this study is to develop environmentally friendly synthesis route to fabricate non-cytotoxic Kesterite, Cu₂ZnSnS₄ (CZTS) QDs in water and investigate their antimicrobial properties both in a suspension and as a coating. CZTS is a semiconducting inorganic compound with a near infrared (NIR) band gap that can absorb the whole spectrum of light including UV, visible, and near infrared (NIR). The advantage of CZTS is that it does not contain toxic elements and its small band gap may allow easy reactive oxygen species (ROS) generation and enhanced antimicrobial activities due to their capability to be activated by visible and NIR light. We have successfully made CZTS NP suspensions in water that exhibited a wurtzite crystalline structure. Furthermore, these CZTS NPs showed photoconductivities in powder compacts and exhibited photovoltaic properties when used as an inorganic dye in dye-sensitized solar cells. CZTS NPs were also shown to inactivate growth of Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria with white light activation in suspension. While CZTS NPs killed bacteria they were relatively benign to human fibroblast cells (Bj-5ta). Coating studies show that CZTS coating is effective in preventing bacterial growth in light in <30 min, and possibly shorter. Unlike antibiotic coatings which can degrade in days, the inorganic CZTS coating is hardy and durable. These studies indicated the aqueous CZTS QDs may have antimicrobial applications.