Rapid on-site/in-situ detection of heavy metal ions in environmental water using a structure-switching DNA optical biosensor

Abstract

A structure-switching DNA optical biosensor for rapid on-site/in situ detection of heavy metal ions is reported. Mercury ions (Hg²⁺), highly toxic and ubiquitous pollutants, were selected as model target. In this system, fluorescence-labeled DNA containing T-T mismatch structure was introduced to bind with DNA probes immobilized onto the sensor surface. In the presence of Hg²⁺, some of the fluorescence-labeled DNAs bind with Hg²⁺ to form T-Hg²⁺-T complexes through the folding of themselves into a hairpin structure and dehybridization from the sensor surface, which leads to decrease in fluorescence signal. The total analysis time for a single sample was less than 10 min with detection limit of 1.2 nM. The rapid on-site/in situ determination of Hg²⁺ was readily performed in natural water. This sensing strategy can be extended in principle to other metal ions by substituting the T-Hg²⁺-T complexes with other specificity structures that selectively bind to other analytes.

Figure 1

(A) Schematic of evanescent wave all-fiber optical biosensing platform. (B) Photograph of the biosensing platform for the on-site/in situ detection of heavy metal ions. (Photo by Feng Long).

Figure 2. Schematic of structure-competitive sensing mechanism of Hg²⁺ detection.

Figure 3. Exemplary signal profiles for Hg²⁺ detection with the structure-switching DNA-based optical biosensor.

(A) Experiment with 30 nM fluorescence-labeled non-specific DNA sequence as control. (B) Experiment with 20 nM fluorescence-labeled cDNA that specifically binds to DNA probe immobilized onto the sensor surface and with 1 × PBS introduced. (C) Experiment with the mixture of 20 nM fluorescence-labeled cDNA that specifically binds to DNA probe immobilized onto the sensor surface and 20 nM Hg²⁺ introduced.

Figure 4. S/N ratio (magenta) and sensitivity index (blue) change in relation to the concentration of cDNA.

Figure 5

(A) Exemplary sensor response curves and the signals obtained with various concentration of Hg²⁺. (B) Logarithmic calibration plot for determination of Hg²⁺ using structure-switching DNA based optical biosensor system.

Figure 6. Selectivity of the structure-switching DNA-based Hg²⁺ biosensor.

Figure 7

(A) On-site/in situ detection of Hg²⁺ in the microcosm by optical biosensor and validation by CVAFS. (B) Linear correlation between the optical biosensor and CVAFS.