“…In the NBD-Cl method, VP was found to react with NBD-Cl in borate buffer pH 8 yielding a highly fluorescent yellow product with maximum absorption at 436 nm and exhibiting strong fluorescence at 550 nm, after excitation at 480 nm (Fig. 5b) (Miyano et al, 1985;Walash et al, 2011;El-Yazbi et al, 2016;Darwish et al, 2009;Attia et al, 2017;Annenkov et al, 2015;Rageh et al, 2010;Omar et al, 2017;Mohamed et al, 2019). This reaction occurs as NBD-Cl reacts with the amine group in the VP under mild alkaline conditions yielding a highly fluorescent yellow colored derivative as shown in Scheme 1 (Miyano et al, 1985;Walash et al, 2011;El-Yazbi et al, 2016;Darwish et al, 2009;Attia et al, 2017;Annenkov et al, 2015;Rageh et al, 2010;Omar et al, 2017;Mohamed et al, 2019).…”