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Brightly luminescent copper nanoclusters (CuNCs) were prepared via a facile one-step synthesis in organic phase, and a novel luminescent nanoswitch on the basis of CuNCs through alternation of the physical states between aggregation and dispersion in response to specific external stimuli was designed. Two states including aggregation state and disaggregation state corresponding to fluorescence on and off signaling can be readily switched in a reversible way based on the aggregation-induced emission and disaggregation-induced quenching mechanism, respectively. This reversible nanoswitch can be controlled by the external stimulus water or N,N'-dicyclohexylcarbodiimide (DCC). The bright luminescence due to aggregation of CuNCs in organic solvents can be effectively quenched by the introduction of a small amount of water, where a disaggregation-induced quenching takes place. This specific behavior is capable to quantify an ultralow level (ppm) of water in aprotic solvents. The excellent reversibility of the nanoswitch enables one to monitor water content in a continuous and recyclable way. [ABSTRACT FROM AUTHOR]