Lanthanide-doped upconversion phosphor (UCP) materials can absorb one or more low-energy photons and subsequently emit one higher-energy, lower-wavelength photon via a unique photoluminescence process called upconversion. UCP nanocrystals that are capable of converting IR radiation to visible light have seen intensive research in the past decade and have already begun to emerge in solar energy and biomedical applications. We herein recognize the great potential for exploiting UCP nanocrystals - that are engineered to convert ordinary visible light into germicidal ultraviolet radiation - in creating highly innovative biocidal surface technologies, i.e., surfaces that inactivate microorganisms when exposed to visible light.
Hypothesis. While a handful of visible-to-UV upconversion materials already exist in the literature, their conversion efficiencies are considered too low for practical biocidal applications. However, based on the recent advances made in IR-to-visible light upconversion technology, they hypothesize that (i) sophisticated material design will lead to higher, more practical upconversion efficiencies and enable UV-emission under low-power sunlight or ambient indoor light conditions; and (ii) such improvements can be achieved through careful selection of lanthanide dopant combinations, low vibrational host matrices, and nano-structural optimization. They further hypothesize that surfaces coated with carefully engineered nanocrystalline UV-emitting phosphors will exhibit biocidal effects, providing an effective, cost-efficient, and sustainable alternative to current antimicrobial surfaces for the deterrence of pathogen transfer.
Objectives. The primary focus of the proposed research is to explore fundamental nanophosphor design strategies that result in efficient upconversion of broadband visible light into UV photons in the germicidal range and, consequently, effective biocidal action when coated onto surfaces.
Approach. Upconversion nanocrystals will be synthesized via sol-gel decomposition and hydrothermal techniques with varying lanthanide dopant combinations, host crystals and nanostructural modifications. The efficiency and mechanisms of energy transfer processes will be gauged using a custom-built high-energy laser photoluminescence spectroscopy. Materials will be characterized using X-ray diffraction analysis, transmission electron microscopy, etc. Biocidal efficacy and biofilm inhibition will be determined via kinetic viability assays and scanning confocal laser microscopy, respectively, using various test microorganisms.
Intellectual Merits. The idea of engineering upconversion nanophosphors for light-activated biocidal surface development has never been explored in the literature. The proposed study will provide fundamental, first-step knowledge in UV-emitting upconversion nanophosphor synthesis strategies, material characterization, and environmental technology applicability. This research will also answer fundamental questions regarding: (i) similarities and differences between the well-understood IR-to-visible upconversion and visible-to-UV upconversion processes; and (ii)design aspects that promote the conversion of a broad range of excitation wavelengths.
Broader Impacts. The advancement of light conversion materials is a critical forefront in sustainable and green technology, as it allows utilization of renewable energy as well as, in this case, decreased reliance on continuous chemical application. Surfaces which can inherently remain pathogen free are a long sought-after tool for inhibiting pathogen transfer in hospitals,food industry, and public areas, while we additionally envision application to solar water disinfection kits for the developing world. Educating undergraduate and graduate students is an integral part of the proposed project and will provide participating students with exceptional interdisciplinary and collaborative learning experiences in applied solid-state physics/chemistry,material science, nanotechnology, and environmental engineering. The project will also leverage an existing high school student summer internship program.
Approximately 98% of solar energy reaching the Earth’s surface is in the form of visible light, presenting tremendous opportunities to develop technologies and strategies utilizing green, sustainable energy sources. In the area of water and surface sterilization, technologies such as semiconductor photocatalysis (such as commonly used titanium dioxide) and solar water disinfection have received widespread scientific and public interests over past decades. Considering the current limitations of these existing - yet continuously evolving - technologies, including low efficiency of visible light utilization, researchers at Georgia Institute of Technology explored an innovative way of using visible light for antimicrobial application; i.e., converting low-energy visible light that is abundant in solar irradiation and indoor lighting into higher-energy ultravilolet (UV) irradiation that is know to kill germs (Figure 1). This somewhat unconventional conversion process is achieved by materials called upconversion (UC) phosphors. These materials are light-absorbing, glass-like crystals doped with rare-earth ions such as praseodymium. Researchers proposed that such conversion of visble light to UV light would offer a new, sustainable approach to environmental technologies such as water disinfection and surface sterilization. Researchers focused on both fundamental phosphor engineering strategies, as well as conducting the first tests on applying UC to antimicrobial surfaces. The work has also furthered the scientific understanding of many aspects of UC materials that will benefit the overall field of applying UC to energy and environmental technology. Through this NSF project, Georgia Tech researchers successfully engineered the new phosphor materials for the purpose of partially converting sunlight or ambient visible light into germicidal UVC radiation. They demonstrated for the first time in the literature that these phosphors, either coated onto surfaces as powders or fabricated in the form of ceramics (Figure 2), exhibit self-sterilizing and anti-biofilm properties upon exposure to visible light. These materials can be useful for various application, such as preventing germ spreading through inanimate surface in public places, hospitals, and food processing facilities.
