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A quick and sensitive colorimetric method for the determination of mercury (Hg(II)) has been addressed in this article. Differential centrifugation based monodispersed silver nanoparticles (AgNPs) were synthesized from mature green tea leaves with an average particle size of 14.708 ± 2.4 nm. The obtained AgNPs were analyzed with several spectroscopy and image analysis methods such as UV–Vis spectrophotometry, X-ray diffractometer (XRD), field emission transmission electron microscopy (FETEM), and X-ray photoelectron spectroscopy (XPS). The specific novelty refers to ambient temperature synthesis followed by centrifugal differentiation of highly sensitive, and selective monodispersed AgNPs. The efficacy of the developed sensor was optimum at a pH of 4 and was due to the interfering effect of OH– ions with the Hg(II). The successful Hg(II) detection in aqueous solutions in the 0.001 to 8 mg/L concentration range confirmed upon the applicability of the developed colorimetric sensor in real-time scenarios such as tap water system. Further, AgNPs were analyzed to interact with Hg(II) ions and form a Hg − Ag alloy (amalgam) that allowed a color alteration from brown to no color in the sample. UV − Vis spectrophotometry confirmed that the limit of detection (LOD) and limit of quantification (LOQ) of the sensing system were 0.01 mg/L and 0.04 mg/L, respectively. These findings verily demonstrate the efficacy of the mentioned process to achieve functional AgNPs for the Hg(II) colorimetric sensing application.