Simple colorimetric sensor for Cr(III) and Cr(VI) speciation using silver nanoparticles as a probe

In this study, we present a simple colorimetric probe for detecting chromium species, i.e. Cr(VI) and Cr(III), in aqueous solution using as-synthesized citrate-capped silver nanoparticles without further functionalization. Two types of silver nanoparticles, AgNP-I (10 ml sodium borohydride) and AgNP-II (30 ml sodium borohydride) were synthesized by varying the volume of the reducing agent. AgNP-I was proven to be specific for Cr(III) only whereas AgNP-II would measure total chromium in a binary mixture of Cr(III) and Cr(VI). The probe was tested in a binary mixture at a 1 : 1 ratio, containing the total chromium concentrations in the range of 500-5000 ppb. The sensing method was also successfully applied in a portable colorimeter with similar binary combinations paving the way for further on-site applications.