To quantify the photoactivity, the value of the absorbance was monitored at a wavelength of 520 nm (the position of a peak in the absorbance spectra due to the purple DPPH radical). The initial DPPH concentration at t=0 was denoted as c0. As the cumulative irradiation time increased, DPPH concentration (as measured by absorbance) decreased from c0 to lower levels (c). The degradation rate of DPPH was calculated as (c0−c)/c0 as a function of time