The ongoing COVID-19 pandemic has raised global concerns over our unpreparedness to contain highly contagious diseases. SARS-CoV-2, the virus responsible for COVID-19, can survive for multiple days on surfaces, and so can many other pathogens. Thus, we need to have at our disposal various technologies to disinfect surfaces in convenient and scalable ways.
Irradiation with ultraviolet-C (UVC) light is a widespread approach to inactivate bacteria and viruses on inanimate surfaces; both doctors and hairdressers have been using such techniques to disinfect their tools for many decades. However, UVC light is dangerous to us humans because our skin cells and certain proteins strongly absorb light of that wavelength, increasing the risk of dermatitis and skin cancer. This glaring limitation calls for a safer method to inactivate pathogens.

Fortunately, a team of scientists from Nagoya City University (NCU), Japan, have recently developed a novel approach for surface disinfection, that relies on pulsed irradiation with harmless visible light instead of UVC light. Their latest study—published online in Scientific Reports on 16th November 2021 ̶ was led by Professor Takahiro Matsumoto of the Graduate School of Design and Architecture and also included Senior Lecturer Ichiro Tatsuno and Professor Tadao Hasegawa of the Graduate School of Medical Sciences at NCU.
The team had previously shown that it is possible to inactivate certain bacteria infused with dye, using a low-power and easily available green pulsed laser. However, further experiments with a wavelength-tunable laser proved that the underlying optical inactivation mechanisms were more complex than they had earlier considered. To tackle this knowledge gap, they carried out a series of experiments and theoretical analyses to elucidate what exactly inactivates microorganisms irradiated by a nanosecond pulsed laser, and how.

First, the researchers investigated how pulsed laser irradiation affects gold nanoparticles, whose thermodynamic and thermophysical properties are well known compared to the complex organic structures found in bacteria and viruses. They found that gold nanoparticles reached temperatures of over 700 °C ̶ and almost instantly melted when irradiated ̶ due to the photothermal effect, by which the energy from absorbed light increases the temperature of a material.
Then, based on these results, the team proceeded to investigate how pulsed laser irradiation can be used to inactivate Escherichia coli (E. coli) bacteria. By modifying the wavelength of the pulsed laser, they observed a complex inactivation behavior that cannot be explained away by the simple absorption of light. In this case, scattering (which refers to phenomena involving the deviation of the path of light waves as they encounter irregularities, like particulate matter, or radiation while passing through a medium) played a key role in defining how much the bacteria were affected by the irradiated light.
Through further experiments using E. coli fused with dyes (which modify their absorption and scattering properties), the researchers showed that the photothermal effect caused by pulsed laser irradiation at the appropriate wavelength is enough to instantaneously inactivate and destroy bacteria. “Using live/dead fluorescence microscopy images to visualize the inactivation of bacteria by pulsed laser irradiation, we verified that our approach is promising for the site-selective inactivation of various pathogenic viruses and bacteria in a safe and simple manner,” highlights Prof. Matsumoto.
Overall, the results presented in this study will pave the way to new techniques and devices for disinfecting surfaces without putting humans at risk. As Prof. Matsumoto concludes: “By using LED lighting technologies, it may be possible to integrate pulsed light disinfection into indoor lighting. Such technology will help us manage new viral threats that are expected to emerge in the future.” Let us hope further developments in this field allow us to make light work of pathogenic microorganisms!