In this study, simple on-off fluorescent/UV-visible (UV-Vis) probes were easily prepared using 2-(2-hydroxyphenyl)thiazolidine-4-carboxylic acid (Sen-1) and/or 2-(2-hydroxy-5-nitrophenyl)thiazolidine-4-carboxylic acid (Sen-2) for fast detection of Zn2+ ions. Their sensing properties towards common metal ions were investigated using UV-Vis and fluorescence spectroscopies. Sen-1 and Sen-2 displayed a significant change with the addition of Zn2+ ions in the UV-Vis spectra. The addition of Zn2+ ions induced a 104 nm bathochromic shift for Sen-1. The binding ratio towards Zn2+ metal ions was determined to be 1:1 by using Job plot analysis and fluorescence spectroscopy. The association constant and free energy (Delta G) of Sen-1 and Sen-2 towards Zn2+ ions were calculated by the Benesi-Hildebrand equation. The limit of detection of Sen-1 towards Zn2+ ions is 3.73 x 10(-8) M, which is about 1/100 of the value recommended by the World Health Organization for drinking water. Sen-1 was successfully applied to detect Zn2+ ions in water samples and the fluorescence test strip was prepared for visual detection of Zn2+ ions. Finally, the quantum chemical parameters of Sen-1 and Sen-2, such as highest occupied molecular orbital, lowest unoccupied molecular orbital, and chemical hardness, were investigated by the Becke, three-parameter, Lee-Yang-Parr, Hartree-Fock, and M062x methods.