Figure 2

UV–Vis absorption spectra in the 200–500 nm range of l-lysine at pH 2.5 (a), 7.3 (b), 9.7 (c), and 13 (d). The red, green, and blue curves show the absorption spectra of the as-prepared aqueous solution, and after hydrothermal treatment at 130 and 200 °C, respectively. The concentration of l-lysine in the precursor solution was 0.87 M.