An amplified electrochemical proximity immunoassay for the total protein of Nosema bombycis based on the catalytic activity of Fe3O4NPs towards methylene blue

Proximity hybridization induced bipedal DNA walker for label-free electrochemical detection of apolipoprotein A4 based on DNA meditated Ag nanoparticles growth

Apolipoprotein A4 (Apo-A4) is considered as a prospective molecular biomarker for diagnosis of depression due to its neurosynaptic toxicity. Here, we propose a neighboring hybridization induced catalyzed hairpin assembly (CHA) driven bipedal DNA walker that mediates hybridization of Ag nanoparticles (Ag NPs) with DNA probes for highly sensitive electrochemical quantitative detection of Apo-A4. Driven by CHA, this bipedal DNA walker can spread all over the surface of the sensor, induce the HP1-HP2 double chain structure, make the surface of the sensor negatively charged, and adsorb a large number of Ag ions. After chemical reduction with hydroquinone, the Ag NPs formed provide signal tracers for electrochemical dissolution analysis of the target. The Ag NPs formed by chemical reduction of hydroquinone can provide signal traces for electrochemical stripping analysis of target thrombin. The linear range of this method is from 10 pg mL-1 to 1000 ng mL-1, and the detection limit is 5.1 pg mL-1. This enzyme-free and labeling detection method provides a new strategy for rapid clinical detection of Apo-A4 and accurate identification of depression.

Recent advances in electrochemical proximity ligation assay

Proximity ligation assay (PLA) is a vigorously developed homogeneous immunoassay assisted by DNA combining dual recognition of target protein by pairs of proximity probes, in which the detection of protein is tactfully converted to the detection of DNA. The booming developments in PLA have enabled a variety of ultrasensitive assays for the detection of protein and this concept of PLA is also extended to the detection of nucleic acids and some small molecule. The association between PLA and electrochemical method, defined as electrochemical proximity ligation assay (ECPLA), has gained much interests in disease diagnosis, food safety and environmental assays with the advantages, such as broad range of targets, simplicity, low cost and rapid response. In this review, we took a different perspective to present the history of PLA, the classical ECPLA biosensing methodology as well as the developments of ECPLA based on several key parameters, such as sensitivity, selectivity, reusability and generalization. In addition, the developments of PLA with electrochemiluminescence as readout are also presented. Finally, perspective and some unresolved challenges in ECPLA that can potentially be addressed have also been discussed.

Electrochemical Immunosensors with PQQ-Decorated Carbon Nanotubes as Signal Labels for Electrocatalytic Oxidation of Tris(2-carboxyethyl)phosphine

Nanocatalysts are a promising alternative to natural enzymes as the signal labels of electrochemical biosensors. However, the surface modification of nanocatalysts and sensor electrodes with recognition elements and blockers may form a barrier to direct electron transfer, thus limiting the application of nanocatalysts in electrochemical immunoassays. Electron mediators can accelerate the electron transfer between nanocatalysts and electrodes. Nevertheless, it is hard to simultaneously achieve fast electron exchange between nanocatalysts and redox mediators as well as substrates. This work presents a scheme for the design of electrochemical immunosensors with nanocatalysts as signal labels, in which pyrroloquinoline quinone (PQQ) is the redox-active center of the nanocatalyst. PQQ was decorated on the surface of carbon nanotubes to catalyze the electrochemical oxidation of tris(2-carboxyethyl)phosphine (TCEP) with ferrocenylmethanol (FcM) as the electron mediator. With prostate-specific antigen (PSA) as the model analyte, the detection limit of the sandwich-type immunosensor was found to be 5 pg/mL. The keys to success for this scheme are the slow chemical reaction between TCEP and ferricinum ions, and the high turnover frequency between ferricinum ions, PQQ. and TCEP. This work should be valuable for designing of novel nanolabels and nanocatalytic schemes for electrochemical biosensors.

Ferrocene-labeled and purification-free electrochemical biosensor based on ligase chain reaction for ultrasensitive single nucleotide polymorphism detection

Single nucleotide polymorphisms (SNPs) are crucial during the early diagnosis of a given disease as well as in evaluating their response to certain drugs. Thus, this study sought the development of ferrocene (Fc)-labeled electrochemical biosensor for SNP detection. This proposed system involves the ligation of four short probes (e.g., A, B, A', and B', where B' is labeled with an Fc-tag) in the presence of target DNA via ligase chain reaction (LCR), resulting in the formation of Fc-tagged duplex AB-A'B' in 2ⁿ. Subsequently, immobilization of the Fc-tagged duplex AB-A'B' on a single-stranded DNA capture probe (SC-DNA)-carboxyl multi-wall carbon nanotube (MWCNT-COOH) modified glassy carbon electrode (GCE) was accomplished through hybridization. Owing to the specificity of hybridization, and the use of Fc as electrochemical probe for detection of duplex AB-A'B', such strategy realized directly analysis of LCR products without the need for purification. By taking advantage of the thermal stability and high-discrimination ability of HiFi Taq DNA ligase for single-base differences, the specificity of hybridization, the EGFR T790 M mutant DNA (MT-DNA) biosensor was developed to offer a low limit of detection (0.75 aM), a high discrimination of single-base mismatches [as low as 0.01% (molar fraction)], a wide linear range of more than 7 orders of magnitude (1 aM-10 pM), and the recovery rates (95.3%-107.8%) from human serum samples. Thus, the biosensor under development was found to be economical, highly-sensitive, and exceptionally selective for detection of SNPs, and as well as extending the versatile applications of LCR to offer great potential for diagnosis and individual clinical regimens.

Antibody-Oligonucleotide Conjugates as Therapeutic, Imaging, and Detection Agents

Antibody-oligonucleotide conjugates (AOCs) are a novel class of synthetic chimeric biomolecules that has been continually gaining traction in different fields of modern biotechnology. This is mainly due to the unique combination of the properties of their two constituents, exceptional targeting abilities and antibody biodistribution profiles, in addition to an extensive scope of oligonucleotide functional and structural roles. Combining these two classes of biomolecules in one chimeric construct has therefore become an important milestone in the development of numerous biotechnological applications, including imaging (DNA-PAINT), detection (PLA, PEA), and therapeutics (targeted siRNA/antisense delivery). Numerous synthetic approaches have been developed to access AOCs ranging from stochastic chemical bioconjugation to site-specific conjugation with reactive handles, introduced into antibody sequences through protein engineering. This Review gives a general overview of the current status of AOC applications with a specific emphasis on the synthetic methods used for their preparation. The reported synthetic techniques are discussed in terms of their practical aspects and limitations. The importance of the development of novel methods for the facile generation of AOCs possessing a defined constitution is highlighted as a priority in AOC research to ensure the advance of their new applications.

Proximity hybridization triggered rolling-circle amplification for sensitive electrochemical homogeneous immunoassay

A new homogeneous electrochemical immunoassay strategy was developed for ultrasensitive detection of carcinoembryonic antigen (CEA) based on target-induced proximity hybridization coupled with rolling circle amplification (RCA). The immobilization-free detection of CEA was realized by the use of an uncharged peptide nucleic acid (PNA) probe labeled with ferrocene (Fc) as the electroactive indicator on a negatively charged indium tin oxide (ITO) electrode. In the presence of a target protein and two DNA-labeled antibodies, the proximate complex formed in homogeneous solution could unfold the molecular beacon, and a part of the unfolded molecular beacon as a primer hybridized with the RCA template to initiate the RCA process. Subsequently, the detection probe modified Fc (Fc-PNAs) hybridized with the long amplified DNA products. The consumption of freely diffusible Fc-PNAs (neutrally charged) resulted in a significant reduction of the Fc signal due to the fact that long amplified DNA/Fc-PNA products were electrostatically repelled from the ITO electrode surface. The reduction of the electrochemical signal (signal-off) could indirectly provide the CEA concentration. Under the optimal conditions, CEA detection was implemented in a wide range from 1 pg mL^-1 to 10 ng mL^-1, with a low detection limit of 0.49 pg mL^-1. The proposed strategy exhibited advantages of good selectivity, high sensitivity, acceptable accuracy, and favorable versatility of analytes. Moreover, the practical application value of the system was confirmed by the assay of CEA in human serums with satisfactory results.

Versatile Electrochemiluminescence Assays for PEDV Antibody Based on Rolling Circle Amplification and Ru-DNA Nanotags

The sensitive and accurate detection methods for PEDV antibody have practical significance for the prevention and treatment of PEDV. In this work, a new multiple pathways signal amplification method was proposed to construct a sensitive electrochemiluminescence (ECL) platform for the detection of PEDV antibody. Using Au NP-modified graphene nanosheet (Au-GN) as the substrate, antibody-antigen reaction as the recognition unit, rolling circle amplification (RCA) for signal enhancement, and assembled cascade Ru-DNA nanotags as signal label, the proposed platform behaved with good specificity and sensitivity. The binding system of biotin-streptavidin, RCA, and Ru(bpy)₃²⁺-doped silica nanoparticles (Ru SNPs) showed remarkable amplification efficiency, low background signal, and little nonspecific adsorption. Moreover, the proposed ECL sensor exhibited good analytical performance for PEDV antibody with a wide linear range from 0.1 pg mL^-1 to 5000 pg mL^-1 with a detection limit of 0.05 pg mL^-1 ( S/ N = 3). The proposed strategy exhibited the advantages of excellent stability and sensitivity for determination of the PEDV antibody, which was easy to prepare and had a good application prospect.

NH2-Ni-MOF electrocatalysts with tunable size/morphology for ultrasensitive C-reactive protein detection via an aptamer binding induced DNA walker-antibody sandwich assay

Here we report a general approach for synthesizing size/morphology-controlled NH₂-Ni-MOFs by optimizing the solvent composition in the reaction system, and their correlating catalytic performances are clearly explored for the first time. Interestingly, the electrocatalytic performance was found to increase 2.7 fold when the particle size is reduced from 1.5 μm for NH₂-Ni-MOF(a) to 300 nm for NH₂-Ni-MOF(c). Subsequently, this work exhibits an example of a high-performance NH₂-Ni-MOF(c) electrocatalyst for application in constructing an electrochemical aptasensor to achieve sensitive C-reactive protein (CRP) detection based on an aptamer binding induced DNA walker-antibody sandwich assay. The proposed aptasensor shows a wide linear range from 0.1 pg mL^-1 to 100 ng mL^-1 with a low detection limit of 0.029 pg mL^-1. The work presented here can thus offer an atypical approach to size- and morphology-controlled MOFs in electrocatalysis and biosensing.

The recent development of efficient Earth-abundant transition-metal nanocatalysts

Whereas noble metal compounds have long been central in catalysis, Earth-abundant metal-based catalysts have in the same time remained undeveloped. Yet the efficacy of Earth-abundant metal catalysts was already shown at the very beginning of the 20th century with the Fe-catalyzed Haber-Bosch process of ammonia synthesis and later in the Fischer-Tropsch reaction. Nanoscience has revolutionized the world of catalysis since it was observed that very small Au nanoparticles (NPs) and other noble metal NPs are extraordinarily efficient. Therefore the development of Earth-abundant metals NPs is more recent, but it has appeared necessary due to their "greenness". This review highlights catalysis by NPs of Earth-abundant transition metals that include Mn, Fe, Co, Ni, Cu, early transition metals (Ti, V, Cr, Zr, Nb and W) and their nanocomposites with emphasis on basic principles and literature reported during the last 5 years. A very large spectrum of catalytic reactions has been successfully disclosed, and catalysis has been examined for each metal starting with zero-valent metal NPs followed by oxides and other nanocomposites. The last section highlights the catalytic activities of bi- and trimetallic NPs. Indeed this later family is very promising and simultaneously benefits from increased stability, efficiency and selectivity, compared to monometallic NPs, due to synergistic substrate activation.

The recent development of efficient Earth-abundant transition-metal nanocatalysts

This review presents the recent remarkable developments of efficient Earth-abundant transition-metal nanocatalysts.