Tyndall-Effect-inspired assay with gold nanoparticles for the colorimetric discrimination and quantification of mercury ions and glutathione.

This work initially reports a new nanosening method for simple, sensitive, specific, visual detection of mercury (II) (Hg2+) and glutathione (GSH) using the Tyndall Effect (TE) of the same colloidal gold nanoparticle (GNP) probes for efficient colorimetric signaling amplification. For the TE-inspired assay (TEA) method, arginine (Arg) molecules are pre-modified on the GNPs' surfaces (Arg-GNPs). Upon the Hg2+ introduction, it can be specifically coordinated with the terminal -NH2 and -COOH groups of the Arg molecules to make the Arg-GNPs aggregate, producing a significantly-enhanced TE signal in the reaction solution after its irradiation by a 635-nm red laser pointer pen. On the other hand, the introduction of the GSH results in the production of the original Arg-GNPs' weak TE response, as it is able to bind such metal ion via mercury-thiol reactions to inhibit the above aggregation. Under the optimal conditions, the utility of the new TEA method is well demonstrated to quantitatively detect the Hg2+ and GSH with the aid of a smartphone as a portable TE reader during the linear concentration ranges of 50-3000 and 10-3000 nM, respectively. The detection limits for the Hg2+ and GSH are estimated to be as low as ∼3.5 and ∼0.3 nM, respectively. The recovery results obtained from the detection of Hg2+ in the complex tap and pond water samples and the assay of GSH in real human serum and urine samples are also satisfactory.

On-site, rapid and visual method for nanomolar Hg2+ detection based on the thymine-Hg2+-thymine triggered "double" aggregation of Au nanoparticles enhancing the Tyndall effect.

This work describes a new nanosensor for the simple, rapid, portable, colorimetric analysis of mercury(ii) (Hg2+) ions by combining the sensitive Tyndall effect (TE) of colloidal Au nanoparticles (AuNPs) with specific thymine-Hg2+-thymine (T-Hg2+-T) coordination chemistry for the first time. For the TE-inspired assay (TEA), in the presence of Hg2+ in a sample, the analyte can selectively mediate the hybridization of three types of flexible single-stranded DNAs (ssDNAs) to form stable rigid double-stranded DNAs (dsDNAs) via the T-Hg2+-T ligand interaction. Subsequent self-assembly of the dsDNAs with terminal thiol groups on the AuNPs' surfaces led to their "double" aggregation in addition to the lack of sufficient ssDNAs as the stabilizing molecules in a high-salt solution, resulting in a remarkably enhanced TE signal that positively relied on the Hg2+ level. The results demonstrated that such a TEA method enabled rapid naked-eye qualitative analysis of 625 nM Hg2+ within 10 min with an inexpensive laser pointer pen as an inexpensive handheld light source to generate the TE response. Making use of a smartphone for portable TE readout could further quantitatively detect the Hg2+ ions in a linear concentration range from 156 to 2500 nM with a limit of detection as low as 25 nM. Moreover, the developed equipment-free nanosensor was also used to analyze the Hg2+ ions in real samples including tap water, drinking water, and pond water, the obtained recoveries were within the range of 93.68 to 108.71%. To the best of our knowledge, this is the first report of using the AuNPs and functional nucleic acids to design a TE-based biosensor for the analysis of highly toxic heavy metal ions.

A novel strategy for sensitive and rapid detection of ascorbic acid via the Tyndall effect of cobalt hydroxide nanoflakes.

Cobalt oxyhydroxide (CoOOH) nanoflakes, as nanoenzymes and fluorescence quenchers, have been widely used in colorimetric and fluorescent analysis. However, their promising light scattering property-the Tyndall effect (TE)-has never been applied in biosensors and biological analysis to date. Herein, we report for the first time a novel strategy for point-of-care detection of ascorbic acid (AA) with the TE of CoOOH nanoflakes providing colorimetric signaling. In this detection system, CoOOH nanoflakes exhibit a strong red TE signal under the illumination of a hand-held 635 nm laser pointer pen. However, the introduction of AA could induce a significant decrease of the TE because it could reduce CoOOH into Co2+ and results in the degradation of the CoOOH nanoflakes. The changes in the TE intensity could be read-out using a smartphone for the portable quantitative analysis of AA. The results showed that this CoOOH nanoflake-based TE-inspired assay (TEA) exhibited a good linear range from 0.25 µM to 40 µM for AA, with a detection limit of 12 nM. It also showed high selectivity toward AA over common potential interfering species. Importantly, this method possessed the advantages of simple operation, low consumption of time and equipment-free analysis and was successfully applied to the detection of AA in vitamin C tablets.