Fig. 2

Changes of fluorescence intensity after addition of different concentrations of aspartic acid (asp) in 0.01 mol L⁻¹ phosphate buffer solution (pH 7). [HPf] = 2.00 × 10^–7 mol L⁻¹, b upon the addition of [Cu²⁺] = 5.00 × 10^–3 mol L⁻¹, c upon the addition of [Cu²⁺] = 5.00 × 10^–3 mol L⁻¹ and [asp] = 2.00 × 10^–3 mol L⁻¹, d upon the addition of [Cu²⁺] = 5.00 × 10^–3 mol L⁻¹ and [asp] = 4.00 × 10^–3 mol L⁻¹, e upon the addition of [Cu²⁺] = 5.00 × 10^–3 mol L⁻¹ and [asp] = 6.00 × 10^–3 mol L⁻¹, f upon the addition of [Cu²⁺] = 5.00 × 10^–3 mol L⁻¹ and [asp] = 1.00 × 10^–2 mol L⁻¹, g upon the addition of [Cu²⁺] = 5.00 × 10^–3 mol L⁻¹ and [asp] = 3.00 × 10^–2 mol L⁻¹, h upon the addition of [Cu²⁺] = 5.00 × 10^–3 mol L⁻¹ and [asp] = 4.50 × 10^–2 mol L⁻¹ The acidic conditions were avoided during detection, and phosphate buffer solution at pH = 7 was used to allow the stability of the complex. Thus, amino acids act as competitors, as they reduce the quenching effect of Cu²⁺ ions from reacting with pefloxacin due to the development of a Cu–amino acid complex