Fig. 2. (a) Fluorescence spectra and (b) fluorescence intensity ratios (I₆₁₅/I₅₀₃) of CSMPP (10 μM) in buffers (containing 2% DMSO) in the pH range of 2.60–6.80. The inset fluorescence pictures are the solutions at different pH taken under a hand-held UV lamp with an excitation of 365 nm. (c) The calibration curve was fitted based on the relationship between the ratio (I₆₁₅/I₅₀₃), R_min, R_max and the pH, whose equation is presented in the ESI.† (d) The pH reversibility of CSMPP in response to pH 7 and pH 3. The PL intensity ratio (I₆₁₅/I₅₀₃) is plotted as a function of cycle times. (e) The interference of different chemical species with the probe in the buffer of pH 6.8 and pH 3.0. Chemical species: (1) blank; (2) Ca²⁺; (3) Mg²⁺; (4) Cd²⁺; (5) Co²⁺; (6) Fe²⁺; (7) Fe³⁺; (8) Ni²⁺; (9) Cu²⁺; (10) Al³⁺; (11) Zn²⁺; (12) Ag⁺; (13) Mn²⁺; (14) Pb²⁺; (15) K⁺; (16) Na⁺; (17) I⁻; (18) HPO₄²⁻; (19) H₂PO₄⁻; (20) Ac⁻; (21) NO₃⁻; (22) CO₃²⁻; (23) ClO⁻; (24) HS⁻; (25) SCN⁻; (26) S₂O₃²⁻; (27) H₂O₂; (28) tryptophan; (29) glutamine; (30) aspartic acid; (31) glycine; (32) leucine; (33) valine; (34) cysteine; (35) homocysteine; (36) glutathione; (37) arginine; (38) serine; (39) glucose. Condition: probe concentration is 10 μM; the concentration of agents 1–14 is 0.2 mM; the concentration of agents 15–39 is 1 mM; the anion for all cations is Cl⁻, except that NO₃⁻ is for Ag⁺, Mn²⁺ and Pb²⁺. The cation for all anions is Na⁺. The PL emission at 503 nm and 615 nm was recorded under the excitation of 365 nm.