Coating Fe3O4 quantum dots with glutamic acid to achieve enhanced catalysis for facile and sensitive detection of chromium(VI) in water.

A universal green synthesis approach for Glu-Fe3O4 quantum dots (QDs) with ultrasmall size (∼3.3 nm) and enhanced catalysis was demonstrated using glutamic acid (Glu) as a dopant. Abundant carboxylic groups and amino groups on the surfaces with a zeta potential of -6.28 mV resulted in greatly improved environmental stability. The Glu-Fe3O4 QDs exhibited oxidoreductase-like catalytic activity. The steady-state kinetic assays indicated that the QDs had a high binding affinity for the substrate TMB. A facile and sensitive colorimetric platform was developed using the Glu-Fe3O4 QDs for the detection of Cr(VI) in naturally sourced water and waste water, which showed a detection limit of 31.02 nM, much lower than the maximum Cr(VI) level permitted in drinking water by the World Health Organization (WHO) and the industrial water threshold. This universal green synthesis approach and the unique catalytic activity of Fe3O4 QDs open a new horizon in materials chemistry and the development of colorimetric biosensors for environmental protection and public health.

Enzyme-free electrochemical biosensor based on bio-barcode amplification for ultra-sensitive detection of microRNA.

The rapid and accurate detection of miRNAs is of great significance for early diagnosis and treatment of cancer. Hence, a novel enzyme-free and label-free electrochemical biosensor based on bio-barcode amplification for detecting miRNAs was presented. Sandwich structures constructed of magnetic nanoparticles modified with DNA probes, gold nanoparticles with numerous barcoded DNA strands that hybridized with target miRNAs were fabricated as the amplifier. The released barcoded DNA strands then acted as the secondary targets and triggered the electrochemical sensor with a significant electrochemical response. A highly sensitive (detection limit of 0.24 fM) and selective electrochemical miRNA detection was realized, which has great potential for application in miRNA-related clinical diagnosis and biochemical research.

A dual-mode sensor based on colorimetric and Tyndall effect of gold nanoparticles for ultra-sensitive detection of Hg2.

Mercury ion (Hg2+) is the most widespread and highly toxic environmental pollutant, which exerts numerous adverse effects on environmental and human health. There is an urgent need to develop a convenient method for detecting Hg2+. Herein, a novel dual-mode sensor based on colorimetric and Tyndall effect of gold nanoparticles was developed for ultra-sensitive determination of Hg2+. In this strategy, a polyA-modified Au probe with high uniformity was assembled successfully. Both modes were based on Hg2+-induced aggregation of the probes. Hg2+ was reflected according to the color variations of solution and the Tyndall effect of Au reporter. With the aid of a laser pointer, the Tyndall mode demonstrated about 615-fold improvement on sensitivity compared with the colorimetry way. Taking advantages of low cost and convenient assembly of polyA-based Au probe and the combination of colorimetry and Tyndall effect, our strategy determined the Hg2+ with high sensitivity and wide range. By changing probes or nanoparticles, the proposed strategy is expected to be a universal platform for detecting other analytes in environmental and even biological samples. A novel dual-mode sensor based on colorimetric and tyndall effect of gold nanoparticles for ultra-sensitive determination of Hg2+ was exploited.

A novel AuNP colorimetric sensor based on a polyadenine probe for ultra-sensitive detection of Ag.

Silver(I) ions (Ag+) are harmful to humans and can be bioaccumulated in organisms. Although numerous methods for Ag+ analysis have been established, new strategies are still in urgent need. Here, we propose a colorimetric sensor based on polyadenine (polyA)-mediated DNA-functionalized gold nanoparticles (AuNPs) for the specific measurement of Ag+ ions. In this strategy, a polyA-modified Au probe with high uniformity was assembled successfully. The method was based on Ag+-induced aggregation of the probe. Ag+ was reflected according to the color variations of solution. Taking advantage of the low cost and convenient assembly of the polyA-based Au probe, our strategy determined Ag+ with high sensitivity and wide range. In addition, by changing probes or nanoparticles, the proposed strategy is expected to be a universal platform for detecting other analytes in environmental and even biological samples.

Ultrasensitive detection of DNA methyltransferase activity: a novel dual-amplification fluorescence technique.

DNA methyltransferase (MTase) is an important regulatory enzyme in various biological processes. However, current methods for investigating MTase activity are still limited in terms of sensitivity and/or generality. Herein, we proposed a dual amplification fluorescence strategy for the ultrasensitive detection of DNA adenine methylation methyltransferase (Dam MTase) activity based on strand displacement amplification (SDA) coupled with rolling circle amplification (RCA). In this study, the hairpin probe could not be cleaved by Nt.AlwI nicking endonuclease (Nt.AlwI) in the presence of Dam MTase, and the subsequent SDA-RCA reaction was blocked, resulting in a weak fluorescence signal. Moreover, the blocking effect was more pronounced at a higher concentration of Dam MTase. This assay provides a very low detection limit (down to 0.0067 U ml-1), as well as good selectivity against other types of MTases (e.g., CpG methyltransferase (M.SssI MTase)). In addition, the analytical mode improves the generality and can be extended to the detection of other types of DNA MTases.

Methylation-blocked cascade strand displacement amplification for rapid and sensitive fluorescence detection of DNA methyltransferase activity.

DNA methylation catalyzed by DNA adenine methylation methyltransferase (Dam MTase) is strongly connected with a variety of biological processes, hence, monitoring Dam MTase activity is of great importance. Here, we developed a rapid and sensitive fluorescence sensing strategy for the detection of Dam MTase activity based on methylation-blocked enzymatic recycling amplification. In this fluorescence sensing system, Dam MTase-induced methylation blocked the subsequent reactions. In contrast, in the absence of Dam MTase, the unmethylated probe initiated the cascade strand displacement amplification for significant signal amplification. Under optimized conditions, this method has a lower detection limit of 0.67 U/mL and a shorter assay time (90 min) compared with previously reported similar methodologies.

Signal-on electrochemical DNA (E-DNA) sensor for accurate quantification of nicking-assisted rolling circle amplification (N-RCA) products with attomolar sensitivity.

Rolling circle amplification (RCA) has become an increasingly important amplification technique in nucleic acid analysis, immunoassay, and molecular diagnosis due to its high specificity and sensitivity. However, the accurate quantification of RCA products via the extensively used fluorescent signaling method has been challenged primarily by the non-specific and sequence-independent binding of the fluorescent dyes to DNA. Here, we have developed a signal-on E-DNA sensor for accurate quantification of the RCA products with high specificity and sensitivity. A restriction enzyme was introduced to cleave the long tandem repeat sequences generated in the RCA reaction into many short monomers. The short monomers were then used as secondary targets to trigger the E-DNA sensor to produce an amplified redox current and thus the resulting RCA products were detected. The method was successfully applied to the detection of miR-7a with high specificity and the detection limit was as low as 0.59 fM.