Selective detection of metal ion is a major research interest in the field of sensors development for environmental safety. Herein, a facile, cost-effective and eco-friendly method is developed to synthesis carbon dots (CDs) from plant material (Plectranthus amboinicus) without any harmful chemicals. As-synthesized CDs exhibited bright blue emission under an excitation wavelength of 390 nm. The Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of functional groups with respective binding energies (hydroxyl and carboxyl groups) on the surface of CDs. The interaction between CDs and metal ions is examined by the influence of fluorescent intensity changes with respect to the quenching effect of CDs. When compared to the other metal ions, silver ion (Ag+) had an excellent fluorescent quenching of CDs. The fluorescence intensity of CD exhibited a good linear correlation efficiency for the detection of Ag+ ion from 0-30 nM (R2 = 0.9905) with a detection limit of 10 nM. In addition, an excellent color change is observed (colorimetric analysis and visible eye observation) for silver ion reduction (from Ag+ into Ag0) compared with other metal ions (no specific color changes). This method is simple, sensitive and selective for colorimetric and fluorescence analysis of Ag+ in a water sample.