Nanoparticle-Based Sandwich Electrochemical Immunoassay for Carbohydrate Antigen 125 with Signal Enhancement Using Enzyme-Coated Nanometer-Sized Enzyme-Doped Silica Beads

An Enzyme-Free Sandwich Amperometry-Type Immunosensor Based on Au/Pt Nanoparticle-Functionalized Graphene for the Rapid Detection of Avian Influenza Virus H9 Subtype

Avian influenza virus H9 subtype (AIV H9) has contributed to enormous economic losses. Effective diagnosis is key to controlling the spread of AIV H9. In this study, a nonenzymatic highly electrocatalytic material was prepared using chitosan (Chi)-modified graphene sheet (GS)-functionalized Au/Pt nanoparticles (GS-Chi-Au/Pt), followed by the construction of a novel enzyme-free sandwich electrochemical immunosensor for the detection of AIV H9 using GS-Chi-Au/Pt and graphene-chitosan (GS-Chi) nanocomposites as a nonenzymatic highly electrocatalytic material and a substrate material to immobilize capture antibodies (avian influenza virus H9-monoclonal antibody, AIV H9/MAb), respectively. GS, which has a large specific surface area and many accessible active sites, permitted multiple Au/Pt nanoparticles to be attached to its surface, resulting in substantially improved conductivity and catalytic ability. Au/Pt nanoparticles can provide modified active sites for avian influenza virus H9-polyclonal antibody (AIV H9/PAb) immobilization as signal labels. Upon establishing the electrocatalytic activity of Au/Pt nanoparticles on graphene towards hydrogen peroxide (H₂O₂) reduction for signal amplification and optimizing the experimental parameters, we developed an AIV H9 electrochemical immunosensor, which showed a wide linear range from 10^1.37 EID₅₀ mL^-1 to 10^6.37 EID₅₀ mL^-1 and a detection limit of 10^0.82 EID₅₀ mL^-1. This sandwich electrochemical immunosensor also exhibited high selectivity, reproducibility and stability.

Overview on the Design of Magnetically Assisted Electrochemical Biosensors

Electrochemical biosensors generally require the immobilization of recognition elements or capture probes on the electrode surface. This may limit their practical applications due to the complex operation procedure and low repeatability and stability. Magnetically assisted biosensors show remarkable advantages in separation and pre-concentration of targets from complex biological samples. More importantly, magnetically assisted sensing systems show high throughput since the magnetic materials can be produced and preserved on a large scale. In this work, we summarized the design of electrochemical biosensors involving magnetic materials as the platforms for recognition reaction and target conversion. The recognition reactions usually include antigen-antibody, DNA hybridization, and aptamer-target interactions. By conjugating an electroactive probe to biomolecules attached to magnetic materials, the complexes can be accumulated near to an electrode surface with the aid of external magnet field, producing an easily measurable redox current. The redox current can be further enhanced by enzymes, nanomaterials, DNA assemblies, and thermal-cycle or isothermal amplification. In magnetically assisted assays, the magnetic substrates are removed by a magnet after the target conversion, and the signal can be monitored through stimuli-response release of signal reporters, enzymatic production of electroactive species, or target-induced generation of messenger DNA.

Development of electrochemical aptasensors detecting phosphate ions on TMB substrate with epoxy-based mesoporous silica nanoparticles

This study, it is aimed to develop an electrochemical aptasensor that can detect phosphate ions using 3.3'5.5' tetramethylbenzidine (TMB). It is based on the principle of converting the binding affinity of the target molecule phosphate ion (PO₄^3-) into an electrochemical signal with specific aptamer sequences for the aptasensor to be developed. The aptamer structure served as a gate for the TMB to be released and was used to trap the TMB molecule in mesoporous silica nanoparticles (MSNPs). The samples for this study were characterized by transmission electron spectroscopy (TEM), Brunner-Emmet-Teller, dynamic light scattering&electrophoretic light scattering, and induction coupled plasma atomic emission spectroscopy. According to TEM analysis, MSNPs have a morphologically hexagonal structure and an average size of 208 nm. In this study, palladium-carbon nanoparticles (Pd/C NPs) with catalytic reaction were used as an alternative to the biologically used horseradish peroxidase (HRP) enzyme for the release of TMB in the presence of phosphate ions. The limit of detection (LOD) was calculated as 0.983 μM, the limit of determination (LOQ) was calculated as 3.276 μM, and the dynamic linear phosphate range was found to be 50-1000 μM. The most important advantage of this bio-based aptasensor assembly is that it does not contain molecules such as a protein that cannot be stored for a long time at room temperature, so its shelf life is very long compared to similar systems developed with antibodies. The proposed sensor shows good recovery in phosphate ion detection and is considered to have great potential among electrochemical sensors.

Antifouling Aptasensor Based on Self-Assembled Loop-Closed Peptides with Enhanced Stability for CA125 Assay in Complex Biofluids

A brief and universal ultralow fouling sensing platform capable of assaying targets in complex biofluids was developed based on designed antifouling peptides that could form a loop-closed structure with enhanced stability. The newly designed peptide with thiol groups in its two terminals self-assembled onto a gold nanoparticle (AuNP)-modified electrode surface to form a stable loop structure, which displayed excellent antifouling performance, outstanding stability under enzymatic hydrolysis, and satisfactory long-term antifouling capability in complex biofluids (clinical human serum). The antifouling and highly sensitive electrochemical aptasensor was constructed via one-step co-immobilization of the designed peptides and aptamers onto the electrode surface modified with electrodeposited poly(3,4-ethylenedioxythiophene) (PEDOT) and AuNPs. The developed peptide-based aptasensor exhibited a decent response for the analysis of the cancer antigen 125 (CA125), with a relatively wide linear range (0.1-1000 U mL^-1) and a low limit of detection (0.027 U mL^-1), and was capable of detecting CA125 in clinical serum samples with acceptable accuracy. This antifouling strategy-based self-assembled peptide with a loop-closed structure provided a potential path for the development of various low-fouling biosensors for application in complex biological fluids.

Electrochemical determination of T2 toxin by graphite/polyacrylonitrile nanofiber electrode

Fabricating graphite electrode corrected with nanofiber by electrospinning as a considerable procedure for utilization in the fluid materials, milk, and syrup for detection of T2 mycotoxin is a significant technique. The modern biosensor was fabricated at normal degrees of room and utilized via buffer Britton-Robinson (B-R) in pH = 5 to refine the chemico-mechanical specifications. The electrochemical manner of the modified surface was surveyed using the scanning electron microscopy (SEM), cyclic voltammetry (CV), square wave voltammetry (SQWV), electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV). The corrected electrode displayed a linear reply to T2 toxin in two distinct concentration ranges of 30-100 nM with correlation coefficients of 0.99. The greatest signals in the square wave spectrums for the B-R buffer created on the uttermost signals of the obtained streams were pH = 5 and 0.5 M of KNO3 for T2 toxin. The modified electrode has a big signal, broad dynamic concentration and high sensitivity and selectivity.

Boronate ester bond-based potentiometric aptasensor for screening carcinoembryonic antigen-glycoprotein using nanometer-sized CaCO3 with ion-selective electrode

Phenylboronic acid-functionalized nanometer-sized CaCO₃ particles (PBA-CaCO₃) were designed to determine the carcinoembryonic antigen (CEA) glycoprotein with a portable Ca²⁺ ion-selective electrode (Ca-ISE) through a typical boronate ester bond. CaCO₃ nanospheres were conjugated to 3-aminophenylboronic acid by amine-epoxy reaction, whereas target CEA was captured into the aptasensing interface by the immobilized thiolated aptamer on gold substrate. Upon PBA-CaCO₃ introduction, 3-aminophenylboronic acid labeled to CaCO₃ microsphere specifically recognized with CEA glycoprotein based on sugar-boronic acid interaction to form a sandwiched complex. The carried CaCO₃ was dissolved under acidic conditions to release Ca²⁺ ion with a portable Ca-ISE readout. Thanks to the specific boronate ester bond between PBA and 1,2-diols, the synthesized PBA-CaCO₃ exhibited good conjugation properties for CEA glycoprotein. Under optimum conditions, Ca-ISE-based aptasensing platform exhibited good electrode potential response for evaluation of target CEA, and allowed detection of CEA at a concentration as low as 7.3 pg mL^-1. Importantly, Ca-ISE-based aptasensing system is readily extended to detect other disease-related glycoproteins by controlling the corresponding aptamer.

Distance-dependent visual fluorescence immunoassay on CdTe quantum dot-impregnated paper through silver ion-exchange reaction

A paper-based visual fluorescence immunoassay is presented for the detection of matrix metalloproteinase-7 (MMP7) that is related to renal cancer. The method is based on the distance-dependent fluorescence quenching of CdTe quantum dots (QDs) on a nitrocellulose membrane by Ag⁺ following a sandwich-type immunoreaction on microtiter wells using silver nanoparticle (AgNP)-labeled secondary antibody- and primary antibody-coated microtiter wells. The silver nanoparticles captured in the well are dissolved with HNO₃, while the quenching effect of QDs is based on silver ion-exchange reaction under 365-nm excitation light irradiation. Increasing concentration of released Ag⁺, thus higher concentration of the protein, leads to an increased distance of quenching on the nitrocellulose membrane. The paper-based immunoassay by combination of AgNP-assisted ion-exchange reaction with QD gives good distance-dependent responses and allows the detection of MMP7 at a concentration as low as 7.3 pg mL^-1. The coefficients of variation are less than 6.9% and 12.4% for intra-assay and inter-assay, respectively. High specificity and long-term stability are achieved during the assay. Importantly, the testing of human serum samples using our strategy shows well-matched results with commercial human MMP7 ELISA kits. Graphical abstract A distance-dependent visual immunoassay is developed for the determination of serum matrix metalloproteinase-7 on CdTe quantum dot-impregnated paper with silver ion-exchange reaction.

Single-Particle Mobility Analysis Enables Ratiometric Detection of Cancer Markers under Darkfield Tracking Microscopy

Here, we introduced a single-particle mobility analysis-based ratiometric strategy for quantitative detection of disease-related biomarkers using antibody-conjugated gold nanoparticles (AuNPs) as probes under darkfield tracking microscopy (DFTM). On the basis of the capability of discriminating nanoparticles with different hydrodynamic sizes and detecting the changes in hydrodynamic effect, single-particle mobility analysis enables us to determine the amount of aggregated and monodispersed nanoprobes for the sandwich-like immunoassay strategy, making it possible to quantify the biotargets by analyzing the relative changes in the aggregate-to-monomer ratio of nanoprobes. By using capture antibody and detection antibody conjugated AuNPs as nanoprobes, we demonstrated ratiometric detection of carcinoembryonic antigen (CEA) over a linear dynamic range from 50 to 750 pM, which is acceptable for clinical diagnostic analysis of CEA in tumor patients. This ratiometric detection technique exhibited excellent anti-interference ability in the presence of nonspecific proteins or complicated protein mixtures. It can be anticipated that this robust technique is promising for the accurate detection of disease biomarkers and other biomolecules for biochemical and diagnostic applications.

Development of a sensitive electrochemical immunosensor using polyaniline functionalized graphene quantum dots for detecting a depression marker

As a new type of conductive material, polyaniline functionalized graphene quantum dots (PAGD), which were prepared by in-situ polymerization had been used to construct a novel electrochemical immunosensor for early screening of depression markers-heat shock protein 70 (HSP70). Profiting from the huge specific surface area, good bioactivity and excellent structure of PAGD, a variety of heat shock protein 70 (HSP70) was firmly loaded on the surface of PAGD for successful construction of basic electrode (HSP70/PAGD/GCE), which was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), respectively. Due to the HSP70 fixed on the surface of basic electrode and the HSP70 in the samples can competitively combine with the horseradish peroxidase labeled human HSP70 antibody (HRP-Strept-Biotin-Ab). As a result, there is negative correlation between the concentration of HSP70 in samples and the detection signal of the proposed electrochemical immunosensor (HRP-Strept-Biotin-Ab-HSP70/PAGD/GCE) in the test liquid. Under conditions optimized for determining HSP70, wide linearity was obtained in the range of 0.0976-100 ng/mL, with a low detection limit of 0.05 ng/mL at 3σ. Moreover, the proposed electrochemical immunosensors was successfully applied to detect HSP70 in plasma samples, and exhibited good precision, acceptable stability and reproducibility. Therefore, this study provides a novel and convenient method for early clinical screening of depression markers, and also provides a reliable and objective analysis method for the diagnosis of depression at the molecular level.

New photothermal immunoassay of human chorionic gonadotropin using Prussian blue nanoparticle-based photothermal conversion

A new near-infrared-based photothermal immunosensing strategy was developed for the sensitive and feasible detection of human chorionic gonadotropin (HCG) by use of a Prussian blue nanoparticle-based photothermal conversion system. Prussian blue nanospheres synthesized by the one-pot method were used for the labeling of anti-HCG detection antibody. A sandwich-type immunoreaction was initially conducted on a monoclonal anti-HCG antibody-coated microplate with a nanoparticle-labeled signal antibody. Accompanying formation of the sandwiched immunocomplex, Prussian blue nanospheres caused photothermal conversion under 980-nm laser irradiation, thereby resulting in an increase of the temperature of the detection system measured by a portable digital thermometer. The properties and factors influencing the analytical performance of the photothermal immunoassay were studied in detail. Under the optimal conditions, the Prussian blue nanoparticle-based photothermal immunoassay exhibited good temperature responses relative to target HCG concentrations within the dynamic range of 0.01-100 ng mL^-1 at a low detection limit of 5.8 pg mL^-1. This system also displayed good anti-interference behavior with regard to other cancer biomarkers, good reproducibility, and relatively long storage stability. The method accuracy was evaluated for analysis of human serum specimens, giving results that matched well with those obtained with a commercial HCG enzyme-linked immunosorbent assay kit. Importantly, this protocol is promising for advanced development of photothermal immunoassays. Graphical abstract.

Photostable methylene blue-loaded silica particles used as label for immunosorbent assay of Salmonella Typhimurium

Salmonella Typhimurium is a major cause of food poisoning. To solve the limitations of the routine enzyme linked immunosorbent assay such as laborious assay procedure, lack of long-term enzyme stability, and insufficient sensitivity, we provided a non-enzymatic colorimetric immunosorbent assay platform to overcome these problems. The highly photostable redox dye particles was constructed by silica particles (diameter = 598 ± 14.4 nm) loaded with methylene blue (Si-MB) and applied to be a label for immunoassay of S. Typhimurium. The sandwich assay format involved incubation of an analyte in a microplate wells modified with monoclonal anti-Salmonella, followed by exposure to a polyclonal anti-Salmonella/Si-MB bioconjugate and then measurement of absorbance at 598 nm. The platform had an assay time of 20 min, could detect heat-killed Salmonella with a limit of detection of 48 CFU mL^-1 , and gave good recoveries in milk. The labels could be stored at 4 °C for 70 days without any deterioration.

Prussian blue nanoparticle-labeled aptasensing platform on graphene oxide for voltammetric detection of α-fetoprotein in hepatocellular carcinoma with target recycling

An enzyme-free electrochemical aptasensing platform based on a graphene oxide nanosheet-modified gold-disk electrode was developed for the voltammetric detection of alpha-fetoprotein (AFP) in hepatocellular carcinoma by using a Prussian blue nanoparticle (PBNP)-labeled aptamer. The electroactive PBNP, a typical signal-generation tag, was utilized for the labeling of the aminated AFP aptamer by using covalent conjugation. The electrochemical sensing platform was prepared in a simple manner on the basis of a π-π stacking reaction between the immobilized graphene oxide and the PBNP-labeled AFP aptamer. Upon target AFP introduction, the analyte reacted with the aptamer, thus resulting in the dissociation of the PBNP from the nanosheets. In the presence of DNase I, the newly formed AFP/aptamer-PBNP complex was cleaved to release target AFP, which could react again with the aptamer on the nanosheets, thereby causing target recycling. During this process, the cleaved PBNP-aptamer was far away from the electrode to decrease the voltammetric signal. Under optimum conditions, the voltammetric peak current of the modified electrode decreased with the increment of the target AFP concentration within the linear range of 0.01-300 ng mL^-1 at a low detection limit of 6.3 pg mL^-1. The precision and reproducibility of the aptasensing protocol were acceptable (CV: <15% for intra-assay and inter-assay). Other possible nontarget biomarkers did not interfere significantly with the voltammetric signal of this system. Human serum samples containing target AFP were assayed with electrochemical aptasensing and a commercial human AFP ELISA kit, and gave well-matched results from these two methods. Importantly, our strategy provides a new horizon for the determination of disease-related proteins.

The synthesis of PEI core@silica shell nanoparticles and its application for sensitive electrochemical detecting mi-RNA

Although the silica-based nanoparticles (NPs) have been widely explored as the labels for sensing different targets, the simple and novel scheme, to impose a large number of signal molecules inside silica NPs, is challenge. Herein, a new scheme for this purpose was developed. This new strategy was based on densely doped polyethyleneimine (PEI) inside silica nanoparticles and forming the PEI@silica nanoparticles. Then, the Cu²⁺ was selected as the electrochemical signal molecule model to be loaded in PEI@silica nanoparticles the based on the strong coordination reaction of Cu²⁺ with PEI and test its signal amplification ability. Our results showed that 7.6 × 10⁵ Cu²⁺signal ions could be loaded in a single PEI@silica nanoparticles. Thereafter, based on the discriminating interaction of this PEI/Cu²⁺/SiO₂ NPs towards both ssDNA probes and ssDNA probe/mi-RNA complex, as well as the specific adsorption effect of this NPs on chemically modified electrode, a highly sensitive electrochemical method for detecting mi-RNA was developed and successfully used to detect mi-RNA in the human serum samples.

Superparamagnetic nanoarchitectures for disease-specific biomarker detection

Synthesis, bio-functionalization, and multifunctional activities of superparamagnetic-nanostructures have been extensively reviewed with a particular emphasis on their uses in a range of disease-specific biomarker detection and associated challenges.

Magneto-controlled flow-injection device for electrochemical immunoassay of alpha-fetoprotein on magnetic beads using redox-active ferrocene derivative polymer nanospheres

A new electrochemical immunosensing protocol by coupling with a magneto-controlled flow-through microfluidic device was developed for the sensitive detection of alpha-fetoprotein (AFP) on magnetic beads (MB) using ferrocene derivative polymer nanospheres (FDNP) as the electroactive mediators.

Multiplexed electrochemical immunoassay for two immunoglobulin proteins based on Cd and Cu nanocrystals

Herein, a simple and feasible electrochemical immunosensing method for simultaneous voltammetric detection of two immunoglobulin proteins, human IgG (HIgG) and rabbit IgG (RIgG), was developed using two distinguishable signal-generation tags on the same electrode. The immunosensor was prepared by immobilizing two Fab antibody fragments on a gold electrode. After this, Cu and Cd nanocrystals, as nanotags, were synthesized and functionalized with identical detection antibodies. Transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the Cu and Cd nanocrystals. The covalently modified electrode with the Fab antibody fragments through the Au-thiolate bond (to dispel the non-specific adsorption) was investigated via scanning electron microscopy (SEM). After the sandwiched immunoreaction, the antibody-modified nanocrystals were captured on the immunosensor, which could be interrogated in pH 3.5 HCl using square-wave anodic stripping voltammetry. Experimental results also indicated that the multiplexed immunoassay enabled the simultaneous detection of HIgG and RIgG in a single run with the similar linear range from 0.01 to 10 ng mL^-1, and the limits of detection (LODs) towards two analytes could be as low as 3.4 pg mL^-1 (at 3σ). Acceptable assay results on precision, reproducibility, specificity, and method accuracy were also acquired. Importantly, the newly designed strategy avoided cross-talk and enzymatic introduction as compared to conventional electrochemical immunoassays, thus exhibiting a promising potential in clinical applications.

Bioresponsive-controlled release of methylene blue from magnetic mesoporous silica from the electrochemical detection of telomerase activity

An electrochemical sensing platform was designed to monitor telomerase activity in HeLa cells, using bioresponsively controlled cargo release from magnetic mesoporous silica nanocontainers (MMSNs). The aminated MMSNs were first synthesized by a wet-chemistry method, then methylene blue (indicator) molecules were loaded into the pores with the aid of specifically designed wrapping DNA strands, and then the wrapping DNA-gated MMSNs were immobilized on a magnetic removable screen-printing carbon electrode. Upon target telomerase and dNTP introduction into the detection cell, the wrapping DNA strands on the MMSNs were prolonged to form rigid hairpin-like DNA structures, thus resulting in the dissociation of wrapping DNA strands from the MMSNs. Thereafter, the loaded methylene blue with redox activity was released out from the pores, thereby causing the increase in the electrochemical signal relative to the background signal. Under optimal conditions, an MMSN-based sensing system exhibited good voltammetric responses toward target telomerase activity within the dynamic linear range of 50-5000 cells per mL at a detection limit of 12 cells per mL in the HeLa extract. The reproducibility and generality of our strategy were acceptable by using somatic tumor cell lines. In addition, the inhibition effect of this system was also evaluated by using 3'-azido-3'-deoxythymidine as a telomerase inhibitor, receiving good results in this screening research.

Magnetic bead/capture DNA/glucose-loaded nanoliposomes for amplifying the glucometer signal in the rapid screening of hepatitis C virus RNA

A digital detection strategy based on a portable personal glucometer (PGM) was developed for the simple, rapid, and sensitive detection of hepatitis C virus (HCV) RNA, involving the release of glucose-loaded nanoliposomes due to coupling-site-specific cleavage by the endonuclease BamHI. The glucose-loaded nanoliposomes were synthesized using a reversed-phase evaporation method and provided an amplified signal at the PGM in the presence of HCV RNA. Initially, a 21-mer oligonucleotide complementary to HCV RNA was covalently conjugated to a magnetic bead through the amino group at the 5' end of the oligonucleotide, and then bound to a glucose-loaded liposome by typical carbodiimide coupling at its 3' end. In the presence of the target HCV RNA, the target hybridized with the oligonucleotide to form double-stranded DNA. The symmetrical duplex sequence 5'-GGATCC-3' between guanines was then catalytically cleaved by BamHI, which detached the glucose-loaded liposome from the magnetic bead. Following magnetic separation of the bead, the detached glucose-loaded liposome was lysed using Triton X-100 to release the glucose molecules within it, which were then detected as an amplified signal at the digital PGM. Under optimal conditions, the PGM signal increased with increasing HCV RNA, and displayed a strongly linear dependence on the level of HCV RNA for concentrations ranging from 10 pM to 1.0 μM. The detection limit (LOD) of the system was 1.9 pM. Good reproducibility and favorable specificity were achieved in the analysis of the target HCV RNA. Human serum samples containing HCV RNA were analyzed using this strategy, and the developed sensing platform was observed to yield satisfactory results based on a comparison with the corresponding results from a Cobas^® Amplicor HCV Test Analyzer. Graphical abstract A digital detection strategy utilizing a personal glucometer was developed for the detection of hepatitis C virus RNA. The strategy involved the use of the endonuclease BamHI along with a 21-mer oligonucleotide conjugated to both a magnetic bead and a glucose-loaded nanoliposome. Hybridization of the nucleotide with the target RNA triggered the coupling-site-specific cleavage of the duplex by BamHI, leading to the release of the glucose-loaded nanoliposome. Following separation of the magnetic bead, the free nanoliposome was dissolved, liberating the glucose molecules within it, which in turn were detected as an amplified signal by the glucometer.

A novel electrochemical immunosensor based on PG for early screening of depression markers-heat shock protein 70

In this study, a novel electrochemical immunosensor for early screening of depression markers-heat shock protein 70 (HSP70) was successfully developed based on the porous graphene (PG) with huge specific surface area and excellent structure. Benefiting from the strong adsorption and good bioactivity of PG which was initially prepared via a simple pyrolysis process, a variety of heat shock protein70 (HSP70) can be firmly loaded on the PG to construct the basic electrode (HSP70/PG/GCE)，which was characterized by the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), respectively. Due to the HSP70 fixed on the surface of basic electrode and the HSP70 in the samples can competitively combine with the horseradish peroxidase labeled human HSP 70 antibody (HRP-Strept-Biotin-Ab). As a result, it presented a negative correlation between the concentration of HSP70 in samples and the detection signal of the proposed electrochemical immunosensor (HRP-Strept-Biotin-Ab-HSP70/PG/GCE) in the test liquid. The application of PG with excellent electrical conductivity in construction of immunosensor remarkably improved the sensitivity of the immunosensor for detection of HSP70. The proposed immunosensor demonstrated a wide linear range of 0.0448 ~ 100 ng/mL with a low detection limit of 0.02 ng/mL at 3σ. Moreover, the proposed immunosensor could be applied for the sensitive and efficient detection of HSP70 in real samples with good precision, acceptable stability, reproducibility and satisfactory results. Therefore, the HSP70 immunosensor provides a novel and convenient method for early clinical screening of depression markers-heat shock protein 70.

A voltammetric immunosensor for clenbuterol based on the use of a MoS2-AuPt nanocomposite

An ultrasensitive immunosensor for the direct detection of the illegally used livestock feed clebuterol (CLB) is described. It is based on the use of a glassy carbon electrode modified with an MoS₂-AuPt nanocomposite and on biotin-streptavidin interaction. The use of MoS₂-AuPt accelerates electron transfer, and this leads to a sharp increase in the electrochemical signal for the electrochemical probe hydrogen peroxide. Differential pulse voltammetry was used to record the current signal at a peak potential of -0.18 V (vs SCE). Under optimal conditions, the electrode has a linear response in the 10 pg·mL^-1 to 100 ng·mL^-1 CLB concentration range and a 6.9 pg·mL^-1 detection limit (based on the 3σ criterium). This immunosensor is sensitive, highly specific and acceptably reproducible, and thus represents a valuable tool for the determination of CLB in pork. Graphical abstract Schematic of a voltammetric immunosensor for the determination of clenbuterol (CLB) based on the use of a nanocomposite prepared from molybdenum disulfide and a gold-platinum alloy (MoS₂-AuPt), and making use of the biotin-streptavidin system.

Magnetofluorescent nanocomposites and quantum dots used for optimal application in magnetic fluorescence-linked immunoassay

Magnetofluorescent nanocomposites with optimal magnetic and fluorescent properties were prepared and characterized by combining magnetic nanoparticles (iron oxide@polymethyl methacrylate) with fluorescent nanoparticles (rhodamine 6G@mSiO₂). Experimental parameters were optimized to produce nanocomposites with high magnetic susceptibility and fluorescence intensity. The detection of a model biomarker (alpha-fetoprotein) was used to demonstrate the feasibility of applying the magnetofluorescent nanocomposites combined with quantum dots and using magnetic fluorescence-linked immunoassay. The magnetofluorescent nanocomposites enable efficient mixing, fast re-concentration, and nanoparticle quantization for optimal reactions. Biofunctional quantum dots were used to confirm the alpha-fetoprotein (AFP) content in sandwich immunoassay after mixing and washing. The analysis time was only one third that required in ELISA. The detection limit was 0.2 pg mL^-1, and the linear range was 0.68 pg mL^-1-6.8 ng mL^-1. This detection limit is lower, and the linear range is wider than those of ELISA and other methods. The measurements made using the proposed method differed by less than 13% from those obtained using ELISA for four AFP concentrations (0.03, 0.15, 0.75, and 3.75 ng mL^-1). The proposed method has a considerable potential for biomarker detection in various analytical and biomedical applications. Graphical abstract Magnetofluorescent nanocomposites combined with fluorescent quantum dots were used in magnetic fluorescence-linked immunoassay.

Voltammetric sandwich immunoassay for Cronobacter sakazakii using a screen-printed carbon electrode modified with horseradish peroxidase, reduced graphene oxide, thionine and gold nanoparticles

The authors describe a sandwich-type of electrochemical immunoassay for rapid determination of the foodborne pathogen Cronobacter sakazakii (C. sakazakii). Polyclonal antibody against C. sakazakii (anti-C. sakazakii) and horseradish peroxidase were immobilized on a nanocomposite consisting of reduced graphene oxide, thionine and gold nanoparticles (AuNPs) that was placed on a screen-printed carbon electrode (SPCE). Thionine acts as an electron mediator which also shortens the electron transfer pathway from the conjugated HRP to the electrode surface and amplifies the electrochemical signal. The AuNPs, in turn, improve the electron transfer rate and increase the surface area for capturing antibody. The morphologies of the electrodes were characterized by means of field emission scanning electron microscopy. The electrochemical performance of the immunoassay was evaluated by cyclic voltammetry and differential pulse voltammetry. Under optimal experimental conditions, the electrochemical immunoassay, best operated at a woking potential of -0.18 V (vs. Ag/AgCl) and scan rate of 20 mV/s has a linear response that covers the 8.8 × 10⁴ to 8.8 × 10⁸ CFU·mL^-1 C. sakazakii concentration range, with a 1.0 × 10⁴ CFU·mL^-1 detection limit (at an S/N ratio of 3). The assay was applied to the detemination of C. sakazakii in spiked infant milk powder and gave recoveries ranging from 92.0 to 105.7%. Graphical abstract A sandwich-type electrochemical immunosensor was designed for C. sakazakii based on the use of rGO. TH, HRP, antibody and AuNPs were anchored on rGO. The nanocomposites were used as traces tag and H₂O₂ as enzyme substrates. AuNPs were modified on SPCE by electrodeposition.

A New Tactic for Label-Free Recognition of β-Trophin via Electrochemiluminescent Signalling on an AuNPs Supported Immuno-Interface

In this paper, a new strategy is reported for preparing a label-free β-trophin electrochemiluminescent (ECL) immunosensor with good specificity, reproducibility and stability. An aquagel polymer from the hydrolysis of (3-aminopropyl) trimethoxysilane acted as the linker to catch the Au nanoparticles (AuNPs) on the indium-tin oxide (ITO) substrate by a two-step method. The AuNPs play an important role in enhancing ECL and immobilizing the β-trophin antibody. This immunosensor can test for β-trophin using luminol as an ECL probe. The ECL intensity at the resultant sensor, after the direct immuno-interaction, was proportional to the concentration of β-trophin and had a low limit of quantification as 4.2 ng mL⁻¹. After deep discussions on the ECL mechanism of this immunosensor, we found that its sensitivity is greatly affected by the presence of oxygen and improved under deoxygenation. We believe that this sensor can be used for clinical cases.

Genetically functionalized ferritin nanoparticles with a high-affinity protein binder for immunoassay and imaging

Molecular detection of target molecules with high sensitivity and specificity is of great significance in bio and medical sciences. Here, we present genetically functionalized ferritin nanoparticles with a high-affinity protein binder, and their utility as a signal generator in a variety of immunoassays and imaging. As a high-affinity protein binder, human IgG-specific repebody, which is composed of LRR (Leucine-rich repeat) modules, was used. The repebody was genetically fused to the N-terminal heavy-chain ferritin, and the resulting subunits were self-assembled to the repebody-ferritin nanoparticles composed of 24 subunits. The repebody-ferritin nanoparticles were shown to have a three-order of magnitude higher binding affinity toward human IgG than free repebody mainly owing to a decreased dissociation rate constant. The repebody-ferritin nanoparticles were conjugated with fluorescent dyes, and the resulting nanoparticles were used for western blotting, cell imaging, and flow cytometric analysis. The dye-labeled repebody-ferritin nanoparticles were shown to generate about 3-fold stronger fluorescent signals in immunoassays than monovalent repebody. The repebody-functionalized ferritin nanoparticles can be effectively used for sensitive and specific immunoassays and imaging in many areas.

Fully disposable microfluidic electrochemical device for detection of estrogen receptor alpha breast cancer biomarker

A novel fully disposable microfluidic electrochemical array device (µFED) was developed and successfully applied for detection of the biomarker estrogen receptor alpha (ERα). The µFED was constructed using low-cost materials and an inexpensive home cutter printer enabled the manufacture of dozens of µFEDs in less than 2h, at a cost of less than US$ 0.20 in material per device. The µFED incorporates counter and reference electrodes and eight carbon-based working electrodes, which were modified with DNA sequences known as estrogen response elements (DNA-ERE), where ERα binds specifically. Paramagnetic particles heavily decorated with anti-ERα antibody and horseradish peroxidase (MP-Ab-HRP) were used to efficiently capture ERα from the sample solution. The ERα-MP-Ab-HRP bioconjugate formed was injected into the µFED and incubated with the DNA-ERE-modified electrodes, followed by amperometric detection with application of -0.2V vs. Ag|AgCl while a mixture of H₂O₂ and hydroquinone was injected into the microfluidic device. An ultralow limit of detection of 10.0 fg mL^-1 was obtained with the proposed method. The performance of the assay, in terms of sensitivity and reproducibility, was studied using undiluted calf serum, and excellent recoveries in the range of 94.7-108% were achieved for the detection of ERα in MCF-7 cell lysate. The µFED system can be easily constructed and applied for multiplex biomarker detection, making the device an excellent cost-effective alternative for cancer diagnosis, especially in developing countries.

Nanomaterials for Electrochemical Immunosensing

Electrochemical immunosensors resulting from a combination of the traditional immunoassay approach with modern biosensors and electrochemical analysis constitute a current research hotspot. They exhibit both the high selectivity characteristics of immunoassays and the high sensitivity of electrochemical analysis, along with other merits such as small volume, convenience, low cost, simple preparation, and real-time on-line detection, and have been widely used in the fields of environmental monitoring, medical clinical trials and food analysis. Notably, the rapid development of nanotechnology and the wide application of nanomaterials have provided new opportunities for the development of high-performance electrochemical immunosensors. Various nanomaterials with different properties can effectively solve issues such as the immobilization of biological recognition molecules, enrichment and concentration of trace analytes, and signal detection and amplification to further enhance the stability and sensitivity of the electrochemical immunoassay procedure. This review introduces the working principles and development of electrochemical immunosensors based on different signals, along with new achievements and progress related to electrochemical immunosensors in various fields. The importance of various types of nanomaterials for improving the performance of electrochemical immunosensor is also reviewed to provide a theoretical basis and guidance for the further development and application of nanomaterials in electrochemical immunosensors.

Rapid electrochemical quantification of Salmonella Pullorum and Salmonella Gallinarum based on glucose oxidase and antibody-modified silica nanoparticles

In this article, a facile and sensitive electrochemical method for quantification of Salmonella Pullorum and Salmonella Gallinarum (S. Pullorum and S. Gallinarum) was established by monitoring glucose consumption with a personal glucose meter (PGM). Antibody-functionalized magnetic nanoparticles (IgG-MNPs) were used to capture and enrich S. Pullorum and S. Gallinarum, and IgG-MNPs-S. Pullorum and IgG-MNPs-S. Gallinarum complexes were magnetically separated from a sample using a permanent magnet. The trace tag was prepared by loading polyclonal antibodies and high-content glucose oxidase on amino-functionalized silica nanoparticles (IgG-SiNPs-GOx). With a sandwich-type immunoassay format, IgG-SiNPs-GOx were added into the above mixture solution and conjugated to the complexes, forming sandwich composites IgG-MNPs/S. Pullorum and S. Gallinarum/IgG-SiNPs-GOx. The above sandwich composites were dispersed in glucose solution. Before and after the hydrolysis of glucose, the concentration of glucose was measured using PGM. Under optimal conditions, a linear relationship between the decrease of glucose concentration and the logarithm of S. Pullorum and S. Gallinarum concentration was obtained in the concentration range from 1.27 × 10² to 1.27 × 10⁵ CFU mL^-1, with a detection limit of 7.2 × 10¹ CFU mL^-1 (S/N = 3). This study provides a portable, low-cost, and quantitative analytical method for bacteria detection; thus, it has a great potential in the prevention of disease caused by S. Pullorum and S. Gallinarum in poultry. Graphical abstract A schematic illustration of the fabrication process of IgG-SiNPs-GOD nanomaterials (A) and IgG-MNPs (B) and experimental procedure of detection of S. Pullorum and S. Gallinarum using GOD-functionalized silica nanospheres as trace tags based on PGM (C).

Polyion oligonucleotide-decorated gold nanoparticles with tunable surface charge density for amplified signal output of potentiometric immunosensor

Methods based on nanostructures have been developed for potentiometric immunosensors, but most involve low sensitivity or weak signal output and are unsuitable for routine use in diagnosis. Herein, we devise an in-situ signal-amplification strategy for enhanced electrical readout of potentiometric immunosensor toward target prostate-specific antigen (PSA, one kind of cancer biomarkers), based on polyion oligonucleotide-labeled gold nanoparticles (AuNPs). To decrease the background signal, monoclonal anti-human PSA capture antibody was covalently conjugated onto an activated glassy carbon electrode via typical carbodiimide coupling. AuNPs heavily functionalized with the polyion oligonucleotides and polyclonal anti-PSA detection antibodies (pAb₂-AuNP-DNA) were utilized as the signal-generation nanotags. In the presence of target PSA, a sandwich-type immunoreaction was executed between capture antibody and detection antibody on the electrode. The detectable signal derived from the shift in the electric potential as a result of the change in the surface charge before and after the antigen-antibody reaction. With target PSA increased, the captured pAb₂-AuNP-DNA to the electrode accompanying detection antibody increased, thereby resulting in the change of the electrode potential. Due to numerous polyion oligonucleotides with the negative charge, the signal readout amplified. Under the optimal conditions, the shift in the output potential was proportional to the logarithm of target PSA concentration and displayed a dynamic linear range from 0.05 to 20 ng mL^-1 with a detection limit of 13.6 pg mL^-1. An intermediate precision of ≤13.2% was accomplished with the batch-to-batch identification. The selectivity was acceptable. The method accuracy was evaluated for human serum specimens, and gave the consistent results between the potentiometric immunosensor and the referenced enzyme-linked immunosorbent assay (ELISA).

Bioresponsive controlled glucose release from TiO2 nanotube arrays: a simple and portable biosensing system for cocaine with a glucometer readout

Herein, we design a simple and portable biosensing platform for the quantitative detection of cocaine based on target-triggered glucose release from a TiO₂ nanotube array (TiNTA) with a glucometer readout.