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The authors describe a carbon dot (CD) based dual-emission ratiometric optical probe for the on-site visual and fluorometric determination of mercury(II) ions. The nanoparticle (NP) probe was obtained by covalently linking the blue emissive carbon dots to the surface of silica nanoparticles containing red-emissive quantum dots (QDs). The red emitting QDs in the silica matrix are inert to Hg(II) and provide a reliable and constant reference signal. They also reduce their toxicity and improve the optical and chemical stabilities, while the blue emission CDs are very sensitive to Hg(II). With increasing concentration of Hg(II), a solution containing the NP probe undergoes a continuous color change from light purple to red. This can be seen with bare eyes or detected instrumentally by measurement of fluorescence intensity under excitation/emission wavelengths of 350/453 and 658 nm. The probe exhibits high sensitivity to Hg(II), with a detection limit of 0.47 nM (at an S/N ratio of 3). This is much lower than the allowable level of mercury (10 nM, ~10 ppb) in drinking water set by the U.S. Environmental Protection Agency. For practical use, the probe was used to quantify Hg(II) in (spiked) tap water where it gave recoveries between 95 and 106% and relative standard deviations between 1.9 and 3.2%. The probe can also be applied in filter paper-based assays, and this paves the way to point-of-care pollution control. This ratiometric probe is nontoxic and easily operated, and therefore shows potential applications for rapid and low-cost visual identification of Hg(II).

A novel magnetic nanomaterial for use in metal ion based affinity chromatography is described. It is based on the chelation between the phosphate groups of phytic acid (PA) and Ti(IV) ions. Due to the large number (6) of phosphate groups of PA, it has a large capacity for Ti(IV) ions. PA was first immobilized on magnetite nanoparticles (PA-MNPs) and then loaded with Ti(IV) ions to obtain the sorbent (Ti-PA-MNPs). The fraction of Ti(IV) ions on the surface of PA-MNPs that is exposed to the solution binds the phosphate groups of phosphopeptides. The bound phosphopeptides can then be magnetically separated. The method was applied to the enrichment of the phosphopeptides in a β-casein tryptic digest. A tryptic digest of bovine serum albumin (BSA) was added at a molar ratio (β-casein to BSA) of 1:2000 to study selectivity. The phosphopeptides were quantified by mass spectrometry. The limit of detection can be as low as 8 × 10−10 mol L−1. This sorbent has a high absorption capacity (53.5 μg mg-1) and shows good recoveries (90%). As many as 2145 phosphopeptides were isolated from 500 μg tryptic digest of a rat liver lysate after enrichment by Ti-PA-MNPs. This is superior to that (1568 phosphopeptides) of commercial TiO2 kit.

The authors describe a method for highly sensitive and selective determination of the activity of protein kinase (PKA). It is based on the finding that silver nanoclusters (AgNCs) can act as a nucleus to catalyze further deposition of silver nanoparticles. This causes the color of a solution to change from pale yellow to black. In the detection scheme presented here, the substrate peptide is phosphorylated by PKA in the presence of ATP. The resulting phosphopeptides bind to oligonucleotide-stabilized AgNCs in the presence of Zr(IV) ions due to electrostatic interactions between Zr(IV) and the phosphate groups, thereby capping the AgNCs. The silver enhancement process (leading to a color change to black) does not work if the AgNCs are capped. The degree of inhibition is proportional to the activity of the kinase. The color change can be detected visually or photographically in a microplate format by exploiting the changes in the grey values of the digital photos. In addition, the DNA-AgNCs display fluorescence emission at 635 nm when excited at 565 nm. Electrochemical assays were performed (at a working voltage as low as 38 mV vs. Ag/AgCl) by using a glassy carbon electrode modified with a solution containing AgNCs, Zr(IV) ions and the peptide, and immersing it into the silver enhancement solution. The assay is highly sensitive and selective. It was applied to the determination of PKA in lysates of HeLa cells. The detection limits typically are between 32 and 37 U⋅ L-1 based on a signal-to-noise ratio of 3.