(a, b) Time-dependent fluorescence emission spectra of 1 (30 μM) in the presence of 10 equiv of Cys in Hepes buffer/DMSO (1:1, v/v, pH 7.8) at 25 °C. (c) Time-dependent fluorescence emission spectra of 1 (30 μM) in the presence of 10 equiv of Hcy in Hepes buffer/DMSO (1:1, v/v, pH 7.8) at 25 °C. (d) Fluorescence spectra of 1 (30 μM) upon addition of 10 equiv of Cys, Hcy, GSH, Ala, Asn, Arg, Asp, Gln, Glu, Gly, His, Ile, Leu, Lys, Met, Phe, Pro, Ser, Thr, Trp, Tyr, Val in Hepes buffer/DMSO (1:1, v/v, pH 7.8) at 25 °C. (λex = 447 nm, slit: 5 nm/5 nm).