Ultraviolet (UV) irradiation is particularly effective disinfection method because it is easy to apply, requires no additional chemical input and produces no hazardous by-products. Many studies have been reported using UV LED (light emitting diodes) to inactivate microorganisms such as Escherichia coli. LED (light emitting diodes) which are mercury free and potentially have a much longer lifetime. UV-LED radiation spanned the range of wavelengths between 200 and 400 nm and divided into three groups: UVC (200-290 nm), UVB (290-320 nm) and UVA (320-400 nm).
Irradiation with wavelengths from 300-580 nm was responsible for the reduction of photoreactivation. UVA radiation is poorly absorbed by DNA and is less efficient in inducing damage on DNA, but it still can inactivate microorganisms. UVA inactivation involves an indirect effect by cyclobutene pyrimidine dimers (CPD) formation, reactive oxygen species (ROS) such as hydroxyl radical. This process takes more time than the direct damage produced by UVC radiation.
Many studies have discussed the mechanism of microorganism inactivation by UV-LEDs. Different effects and mechanisms of microorganisms depend on the wavelength. The biological effects of UVC and UVB can directly induce DNA damage, leading to the formation of pyrimidine dimers and preventing them from reproducing without intermediate steps. Direct damage to DNA might be recoverable by DNA-repair mechanisms, such as photo-reactivation and dark repair. DNA repair might be prevented by damaging the repair enzymes, which are proposed to be more vulnerable to high UV intensities.