In this study we present a colorimetric detection method for Cr (VI) in aqueous solution based on as synthesized silver nanoparticles (Ag NPs) without surface functionalization. The method principle involves reduction of Cr (VI) to Cr (III) by excess reductant present in as synthesized Ag NP dispersion, and subsequent aggregation of Ag NPs by Cr (III) leading to red-shift of the surface plasmon resonance (SPR) peak. The UV-vis absorption spectra, Zeta potentials, dynamic light scattering measurements, and scanning electron microscopy (SEM) confirmed the aggregation of the Ag NPs. Under the optimized conditions, a good linear relationship (correlation coefficient r = 0.981) was obtained between the ratio of the absorbance at 550 nm to that at 390 nm (A 550/390) and the concentration of Cr (VI) over the range of 10 -3-10 -9 M [50 mg/L to 50 ng/L]. The reported probe has a limit of detection down to 1 nM, which, to the best of our knowledge, is the lowest ever reported for the colorimetric detection of Cr (VI). Furthermore, a remarkable feature of this method is that it involves a simple technique exhibiting high selectivity to Cr (VI) over other tested heavy metal ions. © 2012 Elsevier B.V. All rights reserved.