Fullerene has drawn wide interest across many fields due to its favorable electronic and optical properties, which has spurred its use in a myriad of applications. One of the hallmark properties of fullerene is its ability to act as a photosensitizer and efficiently generate 1O2, a form of Reactive Oxygen Species (ROS), upon visible irradiation when dispersed in organic solvents. However, the application of fullerene in environmental systems has been somewhat limited due to fullerene's poor solubility in water, which causes individual fullerene molecules to aggregate and form large colloidal species, quenching much of fullerene's 1O2 production. This is unfortunate given that 1O2 provides many advantages as an oxidant compared to ROS produced from typical advanced oxidation processes, such as OH radicals, due to 1O2's greater chemical selectivity and its ability to remain unaffected by the presence of background water constituents, such as natural organic matter and carbonate. Hence, fullerene materials may hold great potential for the oxidation and disinfection of complex waters. Herein, we chronicle the advances that have been made to propel fullerene materials towards use in emerging water disinfection technologies. Two approaches to overcome fullerene aggregation and the subsequent loss of 1O2 production in aqueous systems are herein outlined: 1) addition of hydrophilic functionality to fullerene's cage, creating highly photoactive colloidal fullerenes; and 2) covalent attachment of fullerene to solid supports, which physically prevents fullerene aggregation and allows efficient 1O2 photo-generation. An emphasis is placed on the inactivation of MS2 bacteriophage, a model for human enteric viruses, highlighting the potential of fullerene materials for light-activated disinfection technologies.
Top1. Introduction
The development of novel materials for the inactivation of waterborne pathogens is in a critical need for much of the world today. Materials that, in response to visible light, can efficiently inactivate viruses and spore forming bacteria that survive in dry conditions would be transformative for many of the challenges in developing countries and in the context of bioterrorism defense. In particular, such materials would be highly useful for advancing solar disinfection (SODIS) and antimicrobial/biocidal coating technology. Semiconductor photocatalysts, such as the archetypical TiO2, have been intensely pursued in hopes of realizing such disinfection technologies,(Min Cho, Chung, Choi, & Yoon, 2005a; Lonnen, Kilvington, Kehoe, Al-Touati, & McGuigan, 2005) but have been somewhat hindered by their inefficient visible light utilization. Hence, many researchers have put forth serious efforts on expanding TiO2’s visible absorption through various doping schemes,(Pelaez et al., 2012; Rehman, Ullah, Butt, & Gohar, 2009) while others have focused on pursing new small-band gap semiconductors such as WO3,(Kim, Lee, & Choi, 2010; Zhu, Xu, Fu, Zhao, & Zhu, 2007) graphitic carbon nitride,(H. Wang et al., 2014; Xu, Wang, & Zhu, 2013) and CdS quantum dots(Bessekhouad, Robert, & Weber, 2004). Moving beyond conventional inorganic photocatalysts, photosensitizing organic dye molecules, which efficiently harvest visible light, have gained recent attention with a particular focus on buckminsterfullerenes.(Chae, Hotze, & Wiesner, 2009; Jaesang Lee et al., 2009) Buckminsterfullerenes, or simply fullerenes, and their functional derivatives have been proposed by researchers as effective antimicrobial agents, via photocatalytic production of singlet oxygen (1O2) and subsequent microbial inactivation (Liyi Huang et al., 2010; Jaesang Lee et al., 2009; Q. Li et al., 2008; George P. Tegos et al., 2005). In contrast with the ubiquitous semiconductor photocatalysts, fullerenes have the advantage of being able to be activated by visible light, especially with functionalization, and covalently anchored to a host material such as polymers. Photocatalysts used for disinfection should be recoverable/reusable, completely conserved (no escape into the environment), activated by visible light, and able to retain their catalytic properties over repeated use. Fullerene derivatives are very attractive as photocatalysts because they can potentially exhibit these essential characteristics when properly functionalized and covalently anchored onto a supporting structure.