<p>Sensors that are handy for the common man are highly demanded. Unfortunately, fluorescence sensors alone cannot satisfy this need. Hence here we introduce a dual mode foolproof sensor based on glutathione stabilized copper nanoclusters (GSH-CuNCs) for sensing mercuric ion (Hg²⁺), that can combine the features of both worlds, that is the handiness of a colorimetric sensor and the ultra-sensitivity of a fluorescence sensor. On exciting at 300&#xa0;nm, GSH-CuNCs gave fluorescence emission at 450&#xa0;nm with a stokes shift of 150&#xa0;nm and the emission was dependent of excitation wavelength. GSH-CuNCs was characterized using UV–visible spectroscopy, Photoluminescence (PL), Fourier Transform Infrared Spectroscopy (FTIR), Matrix-Assisted Laser Desorption Ionization–Time of Flight (MALDI TOF), Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Time-Correlated Single Photon Counting studies (TCSPC) studies. It was found that the synthesized probe fits great to serve both as a fluorescence and colorimetric probe in sensing the presence of Hg²⁺. Hg²⁺ caused the quenching of fluorescence of the probe. Ascorbic acid (AsA) was used to turn on the fluorescence of this quenched system. Both quenched (Turn off) and turned on system were also studied by employing PL, TCSPC, TEM and XPS techniques. Colorimetric studies found the change in colour of the probe from light yellow to dark brown. The limit of detection (LoD) of Hg²⁺ was found to be 161.6&#xa0;nM fluorometrically and 1.5&#xa0;µM colorimetrically. LoD of AsA was found to be 39.17&#xa0;nM fluorometrically. This work outshines other similar dual mode sensing techniques for Hg²⁺ in having simple synthesis and sensing strategy with almost similar sensitivity and selectivity. The feasibility of this sensing strategy to be embedded in complex logic devices for multiplex analysis has also been explored.</p>