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In this work, a label-free electrochemical biosensor is fabricated for the detection of nereistoxin (NRT)-related insecticides based on NRT binding-triggered conformational switching of DNAzyme and micropipette tip-based miniaturized homogeneous electrochemical device. NRT-related insecticides are first converted into NRT with two sulfhydryl groups in a pretreatment step. Then, the thymine-Hg2+-thymine (T-Hg2+-T) mismatch is used to induce the programmed assembly of DNAzyme subunits into functional catalytic structures, thereby activating the Mg2+-DNAzyme and the subsequent hemin/G-quadruplex DNAzyme, respectively. The electrochemical signals generated by the catalytic activity of hemin/G-quadruplex DNAzyme are recorded using a miniaturized electrochemical device. After introduction of the converted NRT, the catalytic functions of Mg2+-DNAzyme are switched-off by uptaking Hg2+ from Mg2+-DNAzyme to disband its catalytic structure, and block the electrochemical signal generation pathway. With such design, a highly sensitive “signal-off” strategy is proposed for bensultap (BST) detection, with a 6.9 ng/L detection limit and 0.01–2000 μg/L linear range. Our strategy also demonstrates the applicability of BST detection in agricultural products.

In this work, an electroanalytical platform for nereistoxin (NRT)-related insecticide detection is proposed on the basis of NRT induced DNA conformational switching and exonuclease III (Exo III) assisted target recycling. NRT-related insecticides were first hydrolyzed and converted into NRT with two thiol groups (–SH). Then, a cytosine–Ag+–cytosine (C–Ag+–C) mismatched base pair was adopted to induce a blunt-ended hairpin configuration of HP DNA. In the presence of converted NRT, it could take up Ag+ from HP DNA to change its conformation from a hairpin to single-stranded structure (HP ssDNA). Thereafter, the obtained HP ssDNA was further hybridized with an H1 hairpin probe on the electrode surface to trigger the Exo III cleavage process, releasing HP ssDNA for recycling leaving the G-quadruplex fragment of H1, which was used for hemin/G-quadruplex complex formation. The reversible redox reaction of Fe(III)/Fe(II) of hemin gave a remarkable electrochemical response for quantitative determination of the NRT-related insecticides. As an analytical model, a low detection limit of 3.9 ng L−1 and a wide linear range of 0.01–1500 μg L−1 with excellent selectivity were achieved for cartap detection. The proposed method also displayed great applicability for cartap detection in agricultural products.

This work reports on a new electrochemical sensing strategy for quantitative monitoring of inorganic mercury(II) (Hg2+) in the contaminated water by immobilization of poly(p-aminobenzene sulfonic acid)-functionalized single-walled carbon nanotubes on a glassy carbon electrode. The electrochemical behavior of the resulting sensor was assessed and optimized. Under optimal conditions, the sensor displayed good electrocatalytic responses to Hg2+ reduction ranging from 1.0 ?mol L-1 to 0.5 mmol L-1 with a detection limit of 0.5 ?mol L-1. The selectivity against other potentially interfering ions and the applicability in contaminated water samples have also been evaluated.

In this work, a novel sandwich-type impedimetric biosensor was fabricated for highly sensitive and selective detection of lipopolysaccharide (LPS) from E. coli O55:B5. LPS aptamer I was immobilized onto electrodeposited gold nanoparticles (Au) modified electrode for capturing LPS in buffer and serum. LPS aptamer II and rolling circle amplification (RCA) primer were conjugated with gold nanoparticles (AuNPs) to use as signal tags. LPS was sandwiched between AuNPs/primer/aptamer II bioconjugates and aptamer I. This was followed by extension of RCA, which resulted in long poly-thymine (T) single strand (ssDNA) sequences to adsorb copper ions (Cu2+) into the skeleton of poly-T sequences. Thus, poly-T ssDNA directly served as the specific template for copper nanoparticles (CuNPs) formation with the aid of ascorbic acid (VC). Finally, the formed CuNPs@ssDNA complex promoted the oxidation of pyrocatechol violet (PCV) to insoluble bulky oligomer (precipitation) in the presence of H2O2. The reaction significantly enhanced the resistance signal of LPS quantitative determination. The impedimetric aptasensor exhibited a wide dynamic working range of 0.01 pg/mL ˜ 100 ng/mL with a detection limit of 4.8 fg/mL. This method also showed good selectivity and reproducibility, and demonstrated the applicability of LPS detection in human serum samples, offering a promising avenue for sensitive LPS detection.

A new electrochemical sensing platform was designed for sensitive detection of copper(II) (Cu2+) based on click conjugation of two short oligonucleotides by using methylene blue-functionalized hairpin DNA as the template. The analyte (Cu2+) was in situ reduced to Cu+ by sodium ascorbate, which catalyzed the click conjugation between two single-stranded oligonucleotides one was labelled with a 5′-alkyne and the other with 3′-azide group via the Cu+-catalyzed azide-alkyne cycloaddition. The newly formed long-chain oligonucleotide induced the conformational change of hairpin DNA to open the hairpin, resulting in methylene blue far away from the electrode for the decrease of redox current. Under optimal conditions, the decrease in the electronic signal was directly proportional to target Cu2+ concentration, and allowed the detection of Cu2+ at a concentration as low as 1.23 nM. Our strategy also displayed high selectivity for Cu2+ against other metal ions owing to the highly specific Cu+-catalyzed click chemistry reaction, and was applicable for monitoring of Cu2+ in drinking water with satisfactory results.

This work reports a new electrochemical monitoring platform for sensitive detection of Cu2+ coupling click chemistry with nanogold-functionalized PAMAM dendrimer (AuNP-PAMAM). The system involved an alkyne-modified carbon electrode and an azide-functionalized AuNP-PAMAM. Initially, the added Cu2+ was reduced to Cu+ by the ascorbate, and then the azide-modified AuNP-PAMAM was covalently conjugated to the electrode via Cu+-catalyzed azide-alkyne click reaction. The carried AuNPs accompanying PAMAM dendrimer could be directly monitored by stripping voltammetry after acidic pretreatment. By introduction of high-loading PAMAM dendrimer with gold nanoparticles, as low as 2.8 pM Cu2+ (ppt) could be detected, which was 125-fold lower than that of gold nanoparticle-based labeling strategy. The method exhibited high specificity toward target Cu2+ against other potentially interfering ions, and was applicable for monitoring Cu2+ in drinking water with satisfactory results.

A simple and low-cost DNA sensing platform based on Pb 2+ -specific DNAzyme-modified microplate was successfully developed for highly sensitive monitoring of lead ion (Pb 2+ , one kind of toxic heavy metal ion) in the environmental samples coupling with a portable personal glucometer (PGM)-based detection mode. The detection cell was first prepared simply by means of immobilizing the DNAzyme on the streptavidin-modified microplate. Gold nanoparticle labeled with single-stranded DNA and invertase (Enz-AuNP-DNA) was utilized as the signal-transduction tag to produce PGM substrate (glucose). Upon addition of lead ion into the microplate, the substrate strand of the immobilized DNAzyme was catalytically cleaved by target Pb 2+ , and the newly generated single-strand DNA in the microplate could hybridize again with the single-stranded DNA on the Enz-AuNP-DNA. Accompanying with the Enz-AuNP-DNA, the carried invertase could convert sucrose into glucose. The as-produced glucose could be monitored by using a widely accessible PGM for in situ amplified digital readout. Based on Enz-AuNP-DNA amplification strategy, as low as 1.0 pM Pb 2+ could be detected under the optimal conditions. Moreover, the methodology also showed good reproducibility and high selectivity toward target Pb 2+ against other metal ions because of highly specific Pb 2+ -dependent DNAzyme, and was applicable for monitoring Pb 2+ in the naturally contaminated sewage and spiked drinking water samples.