Visual detection of Hg²⁺ in aqueous solution using gold nanoparticles and thymine-rich hairpin DNA probes

Abstract

We report a sensitive method for visual detection of mercury ions (II) (Hg²⁺) in aqueous solution by using gold nanoparticles (Au-NPs) and thymine (T)-rich hairpin DNA probes. The thiolated hairpin DNA probe was immobilized on the Au-NP surface through a self-assembling method. Another thymine-rich, digoxin-labeled DNA probe was introduced to form DNA duplexes on the Au-NP surface with thymine-Hg²⁺-thymine (T-Hg²⁺-T) coordination in the presence of Hg²⁺. The Au-NPs associated with the formed duplexes were captured on the test zone of a lateral flow strip biocomponent (LFSB) by immunoreaction events between the digoxin on the duplexes and anti-digoxin antibodies on the LFSB. The accumulation of Au-NPs produced a characteristic red band on the test zone, enabling visual detection of Hg²⁺ without instrumentation. A detection limit of 0.1 nM was obtained under optimal experimental conditions. This method provides a simple, rapid, sensitive approach for the detection of Hg²⁺ and shows great promise for point-of-care and in-field detection of environmentally toxic mercury.

Figure 1

(A) Schematic illustration of the preparation of HO-Au-NP conjugates; (B) Schematic illustration of visual detection of Hg²⁺ on lateral flow strip biosensor; and (C) Visual detection of the sample solution with Hg²⁺ and without Hg²⁺ on the lateral flow strip biosensor; (D) Typical photo images and recorded optical responses of LFSBs in the absence (control) and presence of 10-nM Hg²⁺. Assay time: 30 min.

Figure 2

(A) Histogram of the S/N ratio for the LFSB using the DNA detection probes with different numbers of thymine and (B) Effect of the DNA detection probe concentration on the S/N ratio of the LFSB. Hg²⁺ concentration: 50 nM.

Figure 3

(A) Effect of running buffer component on the S/N ratio of the LFSB; (B) Effect of HO-Au-NP conjugate amount on the S/N ratio of the LFSB. The conjugate amount was adjusted by changing the dispensing cycles of the conjugate on the pad; and (C) Effect of the amount of anti-digoxin on the S/N ratio of the LFSB. Hg²⁺ concentration: 50 nM; Assay time: 30 min.

Figure 4

Photo images (left) of the LFSBs with different concentrations of Hg²⁺ and the resulting calibration curve (right). The photo images of the LFSB were recorded with a digital camera, and the optical responses of red bands on the LFSB were recorded with a strip reader. Assay time: 30 min. Running buffer: 10 mM Tris-HCl + 1.75 mM Mg²⁺ + 1/80 SSC.

Figure 5

(A) The optical intensities (peak areas) of the LFSB test zone analyzed with 50 nM Hg²⁺ and 1 μM of other metal ions. Blue bar: individual metal ions in the sample solution; Red bar: the mixture of Hg²⁺ (50 nM) and a corresponding metal ion; (B) The optical intensities of the LFSB test zone with 5 nM Hg²⁺ at different storage times (weeks). Data for each storage period were obtained from six LFSBs and error bars indicate standard deviations.