Deep Ultraviolet 222 nm All-Solid-State Laser Experiments
Figure 5.20 Laser spot of 222 nm pulsed laser output at the highest average power (see color plate) At the maximum average output power of 35 mW for the 222 nm pulsed laser, the stability of the laser output power was measured. As shown in Figure 5.21, the output power stability over 2 hours was within 2%.
Figure 5.21 Stability of 222 nm pulsed laser output power
5.3 Bacterial Inactivation Experiments Using 222 nm Pulsed Laser
5.3.1 Principle and Application Advantages of Far-UVC Inactivation of Bacteria
In 1903, Niels Finsen was awarded the Nobel Prize for discovering that ultraviolet light can kill bacteria [2]. Over the following century, ultraviolet disinfection became widely popular and has been extensively used for sterilizing objects, hospital wards, and other public places. Amid the ongoing COVID-19 pandemic, countries worldwide urgently need new methods to inactivate viruses in the air.
Optical sterilization and disinfection mainly work through two mechanisms: photochemical and photothermal effects.
(1) Photochemical effect: When bacterial genetic material (DNA/RNA) is irradiated by ultraviolet light, it strongly absorbs the UV radiation, leading to the formation of pyrimidine dimers (such as thymine dimers) and their isomers in the nucleic acid structure, as illustrated in Figure 5.22. These photoproducts disrupt the bacteria's metabolic functions, preventing reproduction and eventually causing death [3]. This photochemical mechanism is the primary disinfection principle of traditional UV light. However, for fungi and spore-forming microorganisms, UV light struggles to penetrate their dense cell walls, so DNA cannot effectively absorb the radiation, resulting in lower inactivation efficiency against these organisms.
Figure 5.22 Schematic diagram of thymine dimerization in double-stranded DNA under UV irradiation (see color plate)
(2) Photothermal effect: Pulsed light irradiation can rapidly increase the surface temperature of microbial cells, destroying the cell wall, evaporating cellular fluid, and completely disrupting the cell structure, leading to death. The photothermal effect occurs when light energy absorbed by the material is converted into heat. When microorganisms are exposed to intense pulsed light at close range, they absorb a large amount of optical energy in a very short time, causing a sharp rise in surface temperature that irreversibly damages the surface structure. Because the entire photothermal process is extremely brief, the interior of the irradiated object experiences virtually no temperature rise, thus preserving nutrients. According to the photothermal disinfection mechanism, pulsed intense light can effectively kill all types of microorganisms.
Pulsed UV light combines both photochemical and photothermal effects for sterilization, so in theory, its inactivation efficiency is higher than that of either mechanism alone.
Traditional UV germicidal lamps are mercury vapor lamps, which emit a peak wavelength of 254 nm. This wavelength is harmful to human cells and tissues, and in severe cases can cause skin cancer [4] and cataracts [5]. Research over the past decade has shown that ultraviolet light in the 200–230 nm band (known internationally as "far-UVC") can inactivate bacteria, airborne influenza viruses, SARS-CoV-2, and other pathogens without damaging human cells [6–9]. During the 2022 Beijing Winter Olympics, far-UVC was widely used for sterilization and disinfection in China and was referred to as a "light vaccine."
Compared with the typical 254 nm germicidal UV, the biophysical reason far-UVC is harmless to human cells lies in the strong absorption peak of proteins in this wavelength range [10]. In this experiment, a spectrophotometer was used to measure the protein absorption spectrum, as shown in
Figure 5.23. It can be seen that proteins have very low absorption at 254 nm but strong absorption in the 200–230 nm far-UVC range.
Far-UVC can penetrate microorganisms (typical diameters of bacteria and viruses are ~1 μm and ~0.1 μm, respectively) [11], which are much smaller than typical human cells (10–25 μm). Far-UVC is strongly absorbed by proteins in the cytoplasm of human cells and attenuates sharply before reaching the cell nucleus [6]. For human skin, the outermost layer is the stratum corneum, composed of dead, anucleate keratinocytes. The primary function of the stratum corneum is to protect underlying tissues. Most far-UVC radiation is absorbed by proteins in the cytoplasm of stratum corneum cells and cannot penetrate to reach critical basal cells or melanocytes beneath [12], as shown in Figure 5.24. For the human eye, the UV-sensitive tissue is the lens, which is located behind the cornea (corneal thickness ≈ 500 μm) [13]. The penetration of far-UVC through the cornea to the lens is essentially zero [14].
Figure 5.23 Protein absorption spectrum Figure 5.24 Penetration and absorption of far-UVC in human skin (descriptive reference; original figure not shown here)
