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Herein, we report a rapid and sensitive colorimetric detection of Hg2+ by designing a specific DNA probe with phosphorothioate RNA modification (PS-probe) for Hg2+ recognition and utilizing DNA-modified gold nanoparticles (DNA-AuNPs) as the transducer. The distance between two DNA-AuNPs is controlled by a linker DNA, providing the linker DNA-regulated aggregation or dispersion status of AuNPs in solution. Exonuclease III (Exo III) can trigger the recycled digestion of linker DNA strands, inhibiting the reformation of aggregated nanoparticles and hence leading to a color shift from purple to red. However, the Hg2+-induced cleavage of the PS-probe can efficiently prevent the digestion of linker DNA strands by Exo III and hence reassemble the modified AuNPs to form aggregates in purple color. Thus, a positive correlation between the linker DNA strands left and the addition of Hg2+ provides a quantitative basis for Hg2+ sensing. A linear range of A520/A700 versus Hg2+ concentration is achieved in the range 2–100 nM associated with a detection limit as low as 1.30 ± 0.04 nM. Moreover, the biosensor exhibits excellent selectivity for Hg2+. The strong selectivity behavior was confirmed by recoveries ranging from 96 to 114% in real water samples. [ABSTRACT FROM AUTHOR]

Cyanobacterial harmful algal blooms in freshwater systems can produce cyanotoxins, such as microcystins (MCs) and nodularins (NODs), presenting serious threats to human health and ecosystems. Required routine monitoring of cyanotoxins in water samples, as posed by U.S. EPA drinking water contaminant candidate list 5 (CCL5), demands for cost-effective, reliable and sensitive MCs/NODs detection methods. We report the development of a colorimetric paper-based immunochip assisted by nanozyme catalysis with a smartphone readout system for rapid detection of cyanotoxins in water. We show that the introduction of biorthogonal click reaction enables in situ facile self-assembly of multi-layers of peroxidase-like nanozyme onto the anti-MCs/NODs monoclonal antibody. We can detect 13 variants of MCs/NODs even in the sub-microgram per liter range with detection limit of below 0.7 μg/L and satisfactory recovery percentages between 88 and 120% in different water matrices. Our technology shows a good correlation with the well-developed ELISA technology, demonstrating its great potential applications in resource-limited or less-developed regions for on-site and large-scale screening of cyanotoxins in water environment. [Display omitted] • A paper-based biochip with high sensitivity and throughput for cyanotoxins. • A layer-by-layer bioorthogonal click reaction for nanozyme amplification. • Limit of detection below 0.7 μg/L towards 13 variants of cyanotoxins. • Satisfactory recoveries in various water environments. • An entire detection cycle time of less than 60 min. [ABSTRACT FROM AUTHOR]

• Facile and ultrasensitive detection of Hg2+ was achieved with an LOD of 0.2 nM. • Cofactor replacement from Mg2+ to Co2+ removed barrier of unmodified AuNPs aptasensor. • An aptasensor was constructed by Exo-III assisted target recycling amplification. Facile and ultrasensitive detection of Hg2+ in water environment remains challenging. Exonuclease III (Exo-III)-assisted target recycling is one of the most popular amplification strategies. Although the magnesium (II) ions are widely acting as cofactors of Exo-III, we recognized that Mg2+ cofactors would strongly disturb the charge distribution on citrate-stablized gold nanoparticles (in the general sense, unmodified AuNPs) surface, thus generate false positive colorimetric signals. To address this issue, we first put forward the view that the cobalt (II) ions can function as the Exo-III cofactor and successfully construct a novel label-free colorimetric aptasensor for facile and ultrasensitive detection of Hg2+ using Hg2+-triggered Exo-III-assisted signal amplification and unmodified AuNPs as indicators. A hairpin-looped DNA probe was rationally designed with thymine-rich recognition termini and specifically recognized trace Hg2+ by a stable T–Hg2+–T structure. A blue-to-red color change of AuNPs with the addition of Hg2+ provided the quantitative detection of Hg2+ with a limit of detection of 0.2 nM and a linear working range from 0.5 nM to 5.0 nM. The whole testing time for one assay was approximately 40 min. Real water samples, even containing Hg2+ at 1 nM, could be determined by the aptasensor with recovery rates from 97% to 103%. [ABSTRACT FROM AUTHOR]