Micropipet Tip-Based Miniaturized Electrochemical Device Combined with Ultramicroelectrode and Its Application in Immobilization-Free Enzyme Biosensor

Electrodes and electrocatalysts for electrochemical hydrogen peroxide sensors: a review of design strategies

H₂O₂ sensing is required in various biological and industrial applications, for which electrochemical sensing is a promising choice among various sensing technologies. Electrodes and electrocatalysts strongly influence the performance of electrochemical H₂O₂ sensors. Significant efforts have been devoted to electrode nanostructural designs and nanomaterial-based electrocatalysts. Here, we review the design strategies for electrodes and electrocatalysts used in electrochemical H₂O₂ sensors. We first summarize electrodes in different structures, including rotation disc electrodes, freestanding electrodes, all-in-one electrodes, and representative commercial H₂O₂ probes. Next, we discuss the design strategies used in recent studies to increase the number of active sites and intrinsic activities of electrocatalysts for H₂O₂ redox reactions, including nanoscale pore structuring, conductive supports, reducing the catalyst size, alloying, doping, and tuning the crystal facets. Finally, we provide our perspectives on the future research directions in creating nanoscale structures and nanomaterials to enable advanced electrochemical H₂O₂ sensors in practical applications.

A Homogeneous Label-Free Electrochemical microRNA Biosensor Coupling With G-Triplex/Methylene Blue Complex and λ-Exonuclease-Assisted Recycling Amplification

A novel homogeneous label-free electrochemical biosensor using G-triplex/methylene blue (G3/MB) complex as the signal generator together with an amplification assisted by the λ-exonuclease (λ-Exo) has been successfully constructed for ultrasensitive microRNA (miRNA) detection. An integrated microelectrode was designed to realize the miniaturization of the homogeneous electrochemical assay. Taking advantage of G3, that can specifically bind with MB and decrease its diffusion current, a single-stranded functional DNA hairpin structure was designed as the bio-recognition probe. The probe consisted of G3, eight bases to block G3, and the complementary sequences of the target miRNA. Here we chose miRNA141—a potentially diagnostic biomarker of prostate cancer as the model target. The presence of miRNA141 could hybridize with the probe DNA to form a double-stranded structure with a 5′-phosphorylated terminus. Then λ-Exo was adopted to digest mononucleotides from the 5′-end, leading to the release of G3 part and miRNA141. The released miRNA could hybridize with another probe to trigger the cycling process, while the released G3 could therefore interact with MB to cause a detectable decrease of diffusion current. The proposed strategy showed a low detection limit of 16 fM and an excellent specificity to discriminate single-base mismatches. Furthermore, this sensor was applied to detect miRNA141 from diluted human serum samples, indicating that it has great potential in the application of nucleic acid detection in real samples.

A novel miniaturized homogeneous label-free electrochemical biosensing platform combining integrated microelectrode and functional nucleic acids

A miniaturized platform combining integrated microelectrode (IME) and functional nucleic acids was developed for homogeneous label-free electrochemical biosensing. IME was constructed with a carbon fiber microelectrode and a platinum wire in a θ type glass tube as a two-electrode system for electrochemical monitoring at microliter level. A newly reported G-triplex/methylene blue (G3/MB) complex was used as the signal generator in the homogeneous label-free electrochemical biosensor. G3 has strong affinity with MB and it can cause significant decrease of the diffusion current of MB after binding. Melamine was chosen as the model target. Since melamine can interact with nucleobase thymine (T) to form T-melamine-T structure through complementary hydrogen bonds, a single-strand functional DNA hairpin structure with poly T and G3 elaborately blocked via base pairing was designed. The presence of melamine can trigger the conformation switching of the DNA hairpin to release the G3. The released G3 combined with MB could therefore change the diffusion current, leading to a simple and rapid detection of melamine. The combination of functional DNA hairpin as target recognition element, G3/MB as signal generator, and IME as transducer provided a "Mix and Measure" miniaturized platform for the construction of homogeneous label-free electrochemical biosensors.

Electrochemical micropipette-tip for low-cost environmental applications: Determination of anionic surfactants through their interaction with methylene blue

Miniaturization is one of the main requirements in the design of portable devices that allow in-field analysis. This is especially interesting in environmental monitoring, where the time of the sample-to-result process could be decreased considerably by approaching the analytical platforms to the sampling point. We employed traditional mass-produced and low-cost elements (micropipette tips and pins) in an out-of-box application to generate an innovative and cost-effective platform for analytical purposes. We have designed simple and easy-to-use electrochemical cells inside polypropylene micropipette tips with three stainless-steel pins acting as the working, reference and counter electrodes of a potentiostatic system. The pin acting as working electrode was previously coated with carbon ink, meanwhile the rest were used unmodified. In this way, electrochemical in-the-tip measurements were done directly using low volumes (μL) of sample. The devices showed good reproducibility, with a relative standard deviation of 7% (n = 5) for five different tip-based complete electrochemical cells. As a proof-of-concept, its utility has been probed by the determination of an anionic surfactant (sodium dodecyl sulphate, SDS) in water through its interaction with methylene blue (MB). Two different alternatives were presented based on the: 1) increase in the current intensity of the cathodic peak of MB due to the presence of SDS; 2) electrochemical determination of the MB remaining in the aqueous phase after extraction of the pair SDS-MB to an organic medium.

Micropipette Tip-Based Immunoassay with Electrochemical Detection of Antitissue Transglutaminase to Diagnose Celiac Disease Using Staples and a Paper-Based Platform

In this work, 1-200 μL polypropylene micropipette tips were used as platforms for performing immunoassays after converting their inner surfaces on a capture zone for the analyte of interest. We have used a micropipette-tip immunoelectroanalytical platform for the detection of antitissue transglutaminase (IgA), the main biomarker for celiac disease. Modification of the tip wall with poly-l-lysine allowed adsorption of tissue transglutaminase (tTG), which will capture later anti-tTG (IgA) antibodies developed in celiac-affected people. A sandwich-type format was followed, incubating simultaneously the analyte and the detection antibody, labeled with horseradish peroxidase. With this new application for an extremely common lab material, we can perform quantitative analysis by dispensing the liquid into a low-cost and miniaturized staple-based paper electrochemical platform. The analytical signal was the reduction of the enzymatically oxidized substrate, recorded chronoamperometrically (i-t curve). The intensity of the current obtained at a fixed time after the application of the cathodic potential followed a linear relationship with anti-tTG (IgA) concentration. The relative standard deviation obtained for immunoassays performed in different tips indicates the adequate precision of this new methodology, which is very promising for decentralized analysis. Negative and positive controls produced results that were in accordance with those obtained with spectrophotometric enzyme linked-immunosorbent assays.

Investigation of the interactions between methylene blue and intramolecular G-quadruplexes: an explicit distinction in electrochemical behavior

An explicit difference for binding affinity between MB and different intramolecular G-quadruplexes was quickly and easily obtained by an electrochemical method.

A miniaturized electrochemical device integrating a biconical microchannel and carbon fiber disk ultramicroelectrode

A simple, cheap and practicable miniaturized electrochemical device was developed based on a biconical microchannel.

Methylene blue as a G-quadruplex binding probe for label-free homogeneous electrochemical biosensing

Herein, G-quadruplex sequence was found to significantly decrease the diffusion current of methylene blue (MB) in homogeneous solution for the first time. Electrochemical methods combined with circular dichroism spectroscopy and UV-vis spectroscopy were utilized to systematically explore the interaction between MB and an artificial G-quadruplex sequence, EAD2. The interaction of MB and EAD2 (the binding constant, K ≈ 1.3 × 10(6) M(-1)) was stronger than that of MB and double-stranded DNA (dsDNA) (K ≈ 2.2 × 10(5) M(-1)), and the binding stoichiometry (n) of EAD2/MB complex was calculated to be 1.0 according to the electrochemical titration curve combined with Scatchard analysis. MB was proved to stabilize the G-quadruplex structure of EAD2 and showed a competitive binding to G-quadruplex in the presence of hemin. EAD2 might mainly interact with MB, a positive ligand of G-quadruplex, through the end-stacking with π-system of the guanine quartet, which was quite different from the binding mechanism of dsDNA with MB by intercalation. A novel signal read-out mode based on the strong affinity between G-quadruplex and MB coupling with aptamer/G-quadruplex hairpin structure was successfully implemented in cocaine detection with high specificity. G-quadruplex/MB complex will function as a promising electrochemical indicator for constructing homogeneous label-free electrochemical biosensors, especially in the field of simple, rapid, and noninvasive biochemical assays.

Novel homogeneous label-free electrochemical aptasensor based on functional DNA hairpin for target detection

We first developed a label-free and immobilization-free homogeneous electrochemical aptasensor, which combined a smart functional DNA hairpin and a designed miniaturized electrochemical device. Cocaine was chosen as a model target. The anticocaine aptamer and peroxidase-mimicking DNAzyme were integrated into one single-stranded DNA hairpin. Both aptamer and G-quadruplex were elaborately blocked by the stem region. The conformation switching induced by the affinity interaction between aptamer and cocaine released G-quadruplex part and turned on DNAzyme activity. The designed electrochemical device, constructed by a disposable micropipet tip and a reproducible carbon fiber ultramicroelectrode, was applied to the detection of homogeneous DNAzyme catalytic activity at the microliter level. The aptasensor realized the quantification of cocaine ranging from 1 to 500 μM with high specificity. The clever combination of the functional DNA hairpin and the novel device achieved an absolutely label-free electrochemical aptasensor, which showed excellent performance like low cost, easy operation, rapid detection, and high repeatability.