Microchip electrophoresis based multiplexed assay for silver and mercury ions simultaneous detection in complex samples using a stirring bar modified with encoded hairpin probes for specific extraction

Abstract

It is crucially important to rapidly, simultaneously, and sensitively determine trace amounts of heavy metal ions in complex samples. Herein, a stirring bar modified with two kinds of encoded hairpin DNA probes (H0 and H0′) was used in a multiplexed strategy allowing for specific extraction of Hg2+ and Ag+ coupled to microchip electrophoresis (MCE) separation and LED induced fluorescence (LIF) detection. The extraction step utilizes stir bars, which are functionalized with designed hairpin DNA probes (H0 with TT and H0′ with CC mismatches in stems). This allows the specific capture of Hg2+ and Ag+ through CAg+C and THg2+T interactions. These complexes are then enzymatically degraded by the action of exonuclease III (Exo III). The ions released during this enzymatic reaction can initiate a new cycle of interactions with hairpin structures and enzymatic reactions and so on. This cyclic step is specific to the presence of Hg2+ and Ag+ and represents the first round of amplification of the presence of the selected ions. The resulting single strand DNAs on the stirring bars after enzymatic degradation were used in the second step as primers to trigger the catalytic hairpin assembly (CHA) in the presence of a couple of hairpin structures in solution. Such a reaction allows producing duplexes that can be monitored by MCE-LIF. The fluorescence intensity of CHA products (IP) increased and that of hairpin DNAs (IR) decreased with the increase of target concentrations. The signal ratios (IP/IR and IP′/IR′) consisted of targets. The assay was employed for Hg2+ and Ag+ detection in several mediums including water, milk, and fish samples with complex matrices. The results showed that the assay could avoid matrix interference to increase the sensitivity. Therefore, the multiplexed assay was ideal to simultaneously and quickly detect metal ions in complex samples.