All the chemicals used were of analytical grade or above unless specifically noted, and were used as received without further purification. LMG (purity 98%), LCV (purity 99%), MG, CV, acetonitrile (chromatographic grade), and NaBH4 were obtained from Shanghai Civic Chemical Technology Co., LTD (Shanghai, China). Doubly distilled water was used throughout this work. Stock solutions of LMG (330.5 mg L−1), LCV (373.5 mg L−1), MG (364.9 mg L−1) and CV (408.0 mg L−1) were prepared in acetonitrile. These
