Lateral flow test for visual detection of silver (I) based on cytosine-Ag(I)-cytosine interaction in C-rich oligonucleotides

Abstract

The authors describe an oligonucleotide-based lateral flow test for visual detection of Ag(I). The assay is based on cytosine-Ag(I)-cytosine [C-Ag(I)-C] coordination chemistry to capture gold nanoparticle (AuNP) tags in the test zone. A thiolated C-rich oligonucleotide probe was immobilized on the AuNPs via gold-thiol chemistry, and a biotinylated C-rich oligonucleotide probe was immobilized on the test zone. The AuNPs labelled with C-rich oligonucleotides are captured by Ag(I) ions in the test zone through the C-Ag(I)-C coordination. The resulting accumulation of AuNPs produces a readily visible red band in the test zone. Under optimized conditions, the test is capable of visually detecting 1.0 ppb of Ag(I) which is 50 times lower than the maximum allowable concentration as defined by the US Environmental Protection Agency for drinking water. Hence, the test is inexpensive and highly sensitive. It was applied to the detection of Ag(I) in spiked samples of tap water and river water. In our perception, the test is a particularly valuable tool in limited resource settings.

Keywords: gold nanoparticles; oligonucleotide; silver ion; visual detection; water analysis.

Figure 1

Schematic illustration of visual detection of Ag⁺ on oligonucleotide-based lateral flow test strip. (A) in the presence of Ag⁺; (B) in the absence of Ag⁺.

Figure 2

Typical photo images of oligonucleotide-based lateral flow test strips in the absence of Ag⁺ (0 ppb, control) and presence of 20 ppb Ag⁺.

Figure 3

Photo images (left) of the test strips with different concentrations of Ag⁺ and the resulting calibration curve (right). The photo images of the test strips were recorded with a digital camera, and the intensities of the red bands on the test lines were read by the Image J software. Each data point represents the average value obtained from three different measurements. Assay time: 15 min. Running buffer: 200 mM HEPES.

Figure 4

The intensities of the test zone in the presence of 100 ppb Ag⁺ and 10 ppm of other metal ions. Red bar: 10 ppm individual metal ions in the sample solution; Blue bar: the mixture of Ag⁺ (100 ppb) and a corresponding metal ion. Each data point represents the average value obtained from three different measurements.