DNA stabilized silver nanoclusters for ratiometric and visual detection of Hg²⁺ and its immobilization in hydrogels

DNA oligomers are particularly interesting templates for making silver nanoclusters (AgNCs) as different emission colors can be obtained by varying the DNA sequence. Many AgNCs have been used as Hg²⁺ sensors since Hg²⁺ induces fluorescence quenching. From an analytical chemistry standpoint, however, these 'light off' sensors are undesirable. In this work, taking advantage of the fact that some AgNCs are not as effectively quenched by Hg²⁺, we design a sensor with AgNCs containing two emission peaks. The red peak is strongly quenched by Hg²⁺ while the green peak shows a concomitant increase, producing an orange-to-green visual fluorescence transformation. Using this AgNC, we demonstrate ratiometric detection with a detection limit of 4 nM Hg²⁺. This sensor is further immobilized in a hydrogel matrix and this gel is also capable of detecting Hg²⁺ with a visual response.