UV disinfection technology and UV light source development trends

With the spread of COVID-19 around the world, various protective methods and disinfection technologies have attracted much attention. Medical alcohol, disinfectant, ultraviolet radiation, etc. are all products and means to prevent and control the spread of the new coronavirus. Compared with chemical disinfection technology, what is the performance and safety of ultraviolet radiation disinfection technology? With the continuous development of ultraviolet LED (UV LED) technology, are there any differences in disinfection performance and application scenarios between ultraviolet solid-state light sources and ultraviolet mercury lamps? What are the difficulties and pain points that need to be solved during the advancement of UV LED in the application field? The above issues are closely related to the subsequent development of UV LED. This article will analyze these aspects for your reference.

1. Ultraviolet disinfection technology

Ultraviolet rays are between the visible light band and X-rays. According to wavelength, ultraviolet rays can be subdivided into near ultraviolet (UVA: 320 ~ 400 nm), mid ultraviolet (UVB: 280 ~ 320 nm), deep ultraviolet (UVC: 200 ~ 280 nm) and vacuum ultraviolet (VUV: 10 ~ 200 nm) [1, 2], among which the UVC band has the highest ultraviolet energy, but because it has the shortest wavelength, it is absorbed in the atmosphere, resulting in serious attenuation. The near-Earth solar spectrum almost does not contain ultraviolet light in this band, which is also called the "solar blind" ultraviolet band.

The mechanism of ultraviolet inactivation of microorganisms is not complicated. It mainly uses the absorption of ultraviolet rays by the nucleic acids of microorganisms to destroy their nucleic acid functions and cause the microorganisms to stop replicating, thereby achieving disinfection and purification. It should be pointed out that not the entire UV band has the inactivation function for microorganisms. Only the ultraviolet rays in the 240-260 nm range in the UVC band are easily absorbed by bacteria and effectively act on the DNA of bacteria, interfering with their normal replication and causing bacterial death [3]. UVA and UVB are outside the range of the absorption peak of microorganisms, so the sterilization efficiency is very low and they are part of the ultraviolet rays that are ineffective for disinfection [4].

During the UV disinfection process, no chemicals participate in the reaction, and no disinfection by-products are produced. Data show that under the condition of UVC irradiation intensity of 30 mW/cm2, nearly 100% inactivation of most bacteria can be achieved within 1 s [5]. Therefore, UV disinfection technology is a physical disinfection method that has the advantages of broad spectrum and high efficiency, fast and convenient, environmentally friendly and harmless, simple and practical, and easy to operate [6].

The application of ultraviolet rays in the field of disinfection has a long history. As early as the 19th century, Downes and Blunt [7] mentioned the disinfection effect of ultraviolet rays in their research. Danish scientist Finsen subsequently applied ultraviolet light to the medical and health field, and in 1903 he was awarded the Nobel Prize in Physiology or Medicine. The above shows that ultraviolet disinfection and sterilization technology has long been recognized and applied by humans. It is mentioned in the "Novel Coronavirus Pneumonia Prevention and Control Plan" that the new coronavirus is sensitive to ultraviolet light and heat [8, 9]. Therefore, in epidemic prevention and control, in addition to chemical disinfection technologies such as medical alcohol and chlorine-containing disinfectants, ultraviolet disinfection technology, which is one of the physical disinfection methods, is also recognized by relevant agencies.

At present, the application scenarios of UV disinfection technology are mainly in specific places such as factories and hospitals, and are not popular in families. In addition to objective factors such as the size of UV disinfection equipment, concerns about photobiological safety are also an important reason that affects and limits the application of such products. This concern is primarily due to the damage that UV radiation can cause to human eyes and skin. It should be pointed out that as long as a safe dose is ensured, ultraviolet rays are difficult to cause damage to human skin. Under certain conditions, they are also beneficial to human health. For example, ultraviolet radiation in sunlight can promote the production of vitamin D in the human body. Therefore, infants and young children are encouraged to bask in the sun appropriately. However, excessive sunbathing may cause the skin to darken or even burn. In summary, we believe that in actual use, UV disinfection technology needs to be used scientifically and standardizedly in combination with the characteristics of the UV light source and product instructions. Under the premise of meeting the safe dosage, ultraviolet disinfection method is feasible and beneficial. In addition, with the characteristics of solid-state light sources such as small size, easy integration, and fast switching, combined with mature sensing technology and control technology, safety hazards caused by improper use of ultraviolet light can be effectively avoided. Therefore, in ensuring safety and reliability Under the premise of disinfection, UV disinfection technology can be fully utilized to serve mankind.

2. Comparison of ultraviolet light sources

In the process of inactivating microorganisms, ultraviolet light in the 240-260 nm band mainly plays a role [10, 11]. Common UV disinfection products are mainly based on UV output from low-pressure mercury lamps [12]. With the continuous development of nitride material technology, UV LEDs based on high Al component nitrides have attracted attention. By comparing the characteristics of the two light sources, it helps to understand the performance differences between solid-state light sources and ultraviolet mercury lamps in the field of disinfection.

The efficiency of UV disinfection technology is mainly affected by the output wavelength of the light source and the dose of UV radiation. In 2011, the research team of the Technical University of Berlin designed and prepared UV LED light source modules based on 269 nm and 282 nm, and used these two different wavelengths of UV solid-state light sources to conduct inactivation experiments on Bacillus subtilis in water. The results show that under the same UV irradiation dose, 269 nm has a more complete inactivation effect on Bacillus subtilis [13]. In 2016, a research team from Seoul National University in South Korea studied the inactivation efficiency of UV LED and low-pressure mercury lamps against Escherichia coli and Salmonella [14]. The results showed that under the same irradiation dose conditions, the killing rate of UV LED with a peak wavelength of 266 nm for both bacteria was significantly better than that of low-pressure mercury lamps. The above experimental results show that there is a wavelength with the highest efficiency for UV inactivation of a specific microorganism.

The output wavelength of UV LED can be set by adjusting the composition of the active area material, and the half-peak width is narrow, around 10 nm. Therefore, the output wavelength of UV LED is arbitrarily adjustable between 200 and 365 nm, covering the range from UVA to UVC. For ultraviolet mercury lamps, this light source has a wide spectrum range and cannot be adjusted. For example, low-pressure ultraviolet mercury lamps mainly output ultraviolet light near 253.7 nm [4]. The wavelengths and irradiation doses required for inactivation of different microorganisms are quite different. Therefore, in actual research on ultraviolet disinfection, it is difficult to use mercury lamps to identify and distinguish which specific wavelength has the best inactivation efficiency for a certain microorganism. In addition to being more flexible in output wavelength, compared with UV mercury lamps, UV LEDs are small in size and can easily be integrated into packages to prepare light source modules containing multiple wavelengths, helping scientific researchers to develop efficient disinfection light sources in a targeted manner.

The dose of ultraviolet radiation is mainly determined by the irradiation intensity and irradiation time of the light source. Researchers have found that using ultraviolet irradiation intensity greater than 90 μW/cm2 for 30 minutes can effectively kill the SARS virus. This dose is the effective dose for the SARS virus [15]. The new coronavirus is also an RNA virus. In theory, ultraviolet light can effectively kill coronavirus. In practice, the deep ultraviolet inactivation dose of the new coronavirus requires further experiments by relevant departments and institutions to clarify. Currently, the inactivation dose for the SARS virus is generally used as a reference. Restricted by factors such as crystal quality, doping efficiency, and light extraction efficiency [1], the quantum efficiency and output optical power of UVC LEDs need to be improved [16]. Under the same irradiation distance, the ultraviolet radiation intensity of UV LED cannot reach the level of ultraviolet mercury lamp for the time being. Therefore, in actual use, it is necessary to appropriately increase the working time of UV LED and shorten the distance between the light source and the irradiated surface to ensure an effective disinfection dose.

In addition to the differences in output wavelength freedom and output light power, UV LEDs and UV mercury lamps also differ in size, startup speed, power consumption, reliability and safety. Compared with LEDs, UV mercury lamps are larger in size, take a long time to warm up, cannot be used immediately, consume high energy, are fragile [1, 11], and contain mercury, which poses a threat to the environment and human health [17]. With the official implementation of the Minamata Convention on Mercury, it is a general trend to replace traditional mercury-containing light sources with cleaner and more efficient UV LED light sources. With the characteristics of UV LED, technical application scenarios that cannot be achieved with traditional UV mercury lamps will also be realized. For example, UV disinfection technology based on UV LED light sources can be combined with personal electronic devices to develop portable UV disinfection products.

3. Problems faced in the development of UV LED

UV LEDs still face many challenges from core materials to device processes. The technical bottlenecks faced in the process of improving UV led performance can be referenced [1]. With technological progress, interdisciplinary and application integration, new application fields continue to emerge, and corresponding standards also need to be improved. China's existing ultraviolet standards and testing methods mainly revolve around traditional mercury lamps, and the applicability of UV LEDs is lacking. For example, the sterilization wavelength of ultraviolet mercury lamps is mainly at 253.7 nm, and the output wavelength of the best inactivation efficiency of UVC LEDs is mainly distributed at 260 ~ 280 nm, which brings differences to the solutions for subsequent applications. Therefore, UV LED light sources urgently need a series of standards from testing to application to support the development of technology. Relevant institutions in China have carried out research on ultraviolet led metrology, standards, testing, etc., and are gradually building a standardized system that matches UVC led applications [18].

4. Concluding remarks

The emergence of COVID-19 has prompted people to pay more attention to health and safety, and the understanding and potential demand for UV disinfection technology are also increasing. Correspondingly, the UV LED industry related to ultraviolet disinfection and sterilization will receive further development. With the deep integration of industry, academia and research, the level of UV LED technology will be further rapidly improved, and UV disinfection technology based on UV LED will also be applied and promoted on a larger scale.