A hydrothermal method was applied to the synthesis of green-emitting gold nanoclusters (Au NCs) which are shown to be viable fluorescent probes for 4-nitrophenol (4-NP). The Au NCs were prepared by using thiol-β-cyclodextrin as a template. Under 365 nm excitation, their green fluorescence has a peak at 502 nm, with a narrow emission bandwidth of only 30 nm. The fluorescence and composition of the Au NCs were characterized and the mechanism of the nanocluster formation is discussed. Due to host-guest recognition of β-cyclodextrin and 4-NP, fluorescence is quenched. The probe can selectively recognize 4-NP among other nitrophenols. A fluorometric and colorimetric assay was developed for 4-NP that works in the 0.1 to 100 μM concentration range and has a detection limit of 90 nM (at 3σ). Graphical abstractSchematic representation of hydrothermal synthesis of green-emitting gold nanoclusters using thiol-β-cyclodextrin. Fluorescence is quenched and the absorption of the nanoclusters is increases in the presence of 4-nitrophenol.