Highly Selective and Sensitive Electrochemical Immunoassay of Cry1C Using Nanobody and π–π Stacked Graphene Oxide/Thionine Assembly

Ce-MOF@Au-Based Electrochemical Immunosensor for Apolipoprotein A1 Detection Using Nanobody Technology

Apolipoprotein A1 (Apo-A1) is a well-recognized biomarker in tissues, closely associated with cardiovascular diseases such as atherosclerosis, coronary artery disease, and heart failure. However, existing methods for Apo-A1 determination are limited by costly equipment and intricate operational procedures. Given the distinct advantages of electrochemical immunosensors, including affordability and high sensitivity, along with the unique attributes of nanobodies (Nbs), such as enhanced specificity and better tissue permeability, we developed an electrochemical immunosensor for Apo-A1 detection utilizing Nb technology. In our study, Ce-MOF@AuNPs nanocomposites were synthesized by using ultrasonic methods and applied to modify a glassy carbon electrode. The Nb6, screened from an Apo-A1 immunized phage library, was immobilized onto the nanocomposite material, establishing a robust binding interaction with Apo-A1. The recorded peak current values demonstrated a logarithmic increase corresponding to Apo-A1 concentrations ranging from 1 to 100,000 pg/mL, with a detection limit of 36 fg/mL. Additionally, the developed immunosensors demonstrated high selectivity, good stability, and reproducibility. Our methodology was also effectively utilized for serum sample analysis, showing good performance in clinical assessments. This electrochemical immunosensor represents a promising tool for Apo-A1 detection, with significant potential for advancing cardiovascular disease diagnostics.

Challenges for Field-Effect-Transistor-Based Graphene Biosensors

Owing to its outstanding physical properties, graphene has attracted attention as a promising biosensor material. Field-effect-transistor (FET)-based biosensors are particularly promising because of their high sensitivity that is achieved through the high carrier mobility of graphene. However, graphene-FET biosensors have not yet reached widespread practical applications owing to several problems. In this review, the authors focus on graphene-FET biosensors and discuss their advantages, the challenges to their development, and the solutions to the challenges. The problem of Debye screening, in which the surface charges of the detection target are shielded and undetectable, can be solved by using small-molecule receptors and their deformations and by using enzyme reaction products. To address the complexity of sample components and the detection mechanisms of graphene-FET biosensors, the authors outline measures against nonspecific adsorption and the remaining problems related to the detection mechanism itself. The authors also introduce a solution with which the molecular species that can reach the sensor surfaces are limited. Finally, the authors present multifaceted approaches to the sensor surfaces that provide much information to corroborate the results of electrical measurements. The measures and solutions introduced bring us closer to the practical realization of stable biosensors utilizing the superior characteristics of graphene.

Ultrasensitive Electrochemical Immunosensors Using Nanobodies as Biocompatible Sniffer Tools of Agricultural Contaminants and Human Disease Biomarkers

Nanobodies (Nbs) are known as camelid single-domain fragments or variable heavy chain antibodies (VHH) that in vitro recognize the antigens (Ag) similar to full-size antibodies (Abs) and in vivo allow immunoreactions with biomolecule cavities inaccessible to conventional Abs. Currently, Nbs are widely used for clinical treatments due to their remarkably improved performance, ease of production, thermal robustness, superior physical and chemical properties. Interestingly, Nbs are also very promising bioreceptors for future rapid and portable immunoassays, compared to those using unstable full-size antibodies. For all these reasons, Nbs are excellent candidates in ecological risk assessments and advanced medicine, enabling the development of ultrasensitive biosensing platforms. In this review, immobilization strategies of Nbs on conductive supports for enhanced electrochemical immune detection of food contaminants (Fcont) and human biomarkers (Hbio) are discussed. In the case of Fcont, the direct competitive immunoassay detection using coating antigen solid surface is the most commonly used approach for efficient Nbs capture which was characterized with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) when the signal decays for increasing concentrations of free antigen prepared in aqueous solutions. In contrast, for the Hbio investigations on thiolated gold electrodes, increases in amperometric and electrochemical impedance spectroscopy (EIS) signals were recorded, with increases in the antigen concentrations prepared in PBS or spiked real human samples.

A Nanobody‐on‐Quantum Dot Displacement Assay for Rapid and Sensitive Quantification of the Epidermal Growth Factor Receptor (EGFR)

Biosensing approaches that combine small, engineered antibodies (nanobodies) with nanoparticles are often complicated. Here, we show that nanobodies with different C‐terminal tags can be efficiently attached to a range of the most widely used biocompatible semiconductor quantum dots (QDs). Direct implementation into simplified assay formats was demonstrated by designing a rapid and wash‐free mix‐and‐measure immunoassay for the epidermal growth factor receptor (EGFR). Terbium complex (Tb)‐labeled hexahistidine‐tagged nanobodies were specifically displaced from QD surfaces via EGFR‐nanobody binding, leading to an EGFR concentration‐dependent decrease of the Tb‐to‐QD Förster resonance energy transfer (FRET) signal. The detection limit of 80±20 pM (16±4 ng mL⁻¹) was 3‐fold lower than the clinical cut‐off concentration for soluble EGFR and up to 10‐fold lower compared to conventional sandwich FRET assays that required a pair of different nanobodies.

Dual-Target Electrochemical Sensor Based on 3D MoS2-rGO and Aptamer Functionalized Probes for Simultaneous Detection of Mycotoxins

A dual-target aptamer functionalized probes (DTAFP) was applied for the detection of aflatoxin B1 (AFB1) and zearalenone (ZEN) simultaneously, which has not been reported. Meanwhile, two functional materials for signal amplification of the DTAFP were synthesized: 1) a three-dimensional molybdenum disulfide-reduced graphene oxide (MoS₂-rGO) as a favorable loading interface; 2) a double-probes gold nanoparticles (AuNPs) modified by Thionin (Thi) and 6-(Ferrocenyl) hexanethiol (FC6S) as distinguishable and non-interfering signals. Mycotoxins on the electrode surface release into solution under the function of the DTAFP, leading a reduction of the differential peak impulse in signal response. Under the optimum conditions, the aptasensor exhibited a detection range of 1.0 pg mL⁻¹–100 ng mL⁻¹ for AFB1 and ZEN, with no observable cross reactivity. In addition, the aptasensor performed excellent stability, reproducibility, specificity, and favorable recovery in the detection of edible oil. This work demonstrated a novel method for the construction of a simple, rapid, and sensitive aptasensor in the detection of multiple mycotoxins simultaneously.

Nanobody-based label-free photoelectrochemical immunoassay for highly sensitive detection of SARS-CoV-2 spike protein

Until now, COVID-19 caused by SARS-CoV-2 is engulfing the worldwide and still ranging to date, continuing to threaten the public health. The main challenge facing COVID-19 epidemic is short of fast-response and high-efficiency methods to determine SARS-CoV-2 viral pathogens. Herein, a nanobody-based label-free photoelectrochemical (PEC) immunosensor has been fabricated for rapidly detecting SARS-CoV-2 spike protein. As a small-size and high-stability antibody, nanobody was directly and well immobilized with Au nanoparticles and TiO₂ spheres by the interaction. Au deposited TiO₂ nanomaterial possessed 8.5 times photoelectric performance in comparison with TiO₂ in the presence of electron donor owing to surface plasma resonance effect of Au. Based on the steric hindrance effect, this immunoassay platform realized the linear detection from 0.015 to 15000 pg mL⁻¹, and a limit of detection was low as 5 fg mL⁻¹. The label-free PEC immunoassay design provides a new idea for convenient, rapid, and efficient test of SARS-CoV-2 spike protein and broadens further application of nanobody as an identification agent to specific biomarkers.

Poly(Thionine)-Modified Screen-Printed Electrodes for CA 19-9 Detection and Its Properties in Raman Spectroscopy

Polythionine (PTH) is an electroactive compound known for its excellent electron transfer capacity. It has stable and redox centers in its structure, and it can also be generated by electropolymerization of thionine (TH). Due to its properties, it has been used in a large number of applications, including the construction of electrochemical biosensors. In this work, PTH is explored for its ability to generate electrons, which allows it to act as an electrochemical probe in a biosensor that detects CA 19-9 on two different substrates, carbon and gold, using differential pulse voltammetry (DPV) as a reading technique in phosphate buffer (PhB). The analytical features of the resulting electrodes are given, showing linear ranges from 0.010 to 10 U/mL. The Raman spectra of PTH films on gold (substrates or nanostars) and carbon (substrates) are also presented and discussed as a potential use for SERS readings as complementary information to electrochemical data.

Electrochemical Signal Substance for Multiplexed Immunosensing Interface Construction: A Mini Review

Appropriate labeling method of signal substance is necessary for the construction of multiplexed electrochemical immunosensing interface to enhance the specificity for the diagnosis of cancer. So far, various electrochemical substances, including organic molecules, metal ions, metal nanoparticles, Prussian blue, and other methods for an electrochemical signal generation have been successfully applied in multiplexed biosensor designing. However, few works have been reported on the summary of electrochemical signal substance applied in constructing multiplexed immunosensing interface. Herein, according to the classification of labeled electrochemical signal substance, this review has summarized the recent state-of-art development for the designing of electrochemical immunosensing interface for simultaneous detection of multiple tumor markers. After that, the conclusion and prospects for future applications of electrochemical signal substances in multiplexed immunosensors are also discussed. The current review can provide a comprehensive summary of signal substance selection for workers researched in electrochemical sensors, and further, make contributions for the designing of multiplexed electrochemical immunosensing interface with well signal.

Recombinant antibodies and their use for food immunoanalysis

Antibodies are widely employed as biorecognition elements for the detection of a plethora of compounds including food and environmental contaminants, biomarkers, or illicit drugs. They are also applied in therapeutics for the treatment of several disorders. Recent recommendations from the EU on animal protection and the replacement of animal-derived antibodies by non-animal-derived ones have raised a great controversy in the scientific community. The application of recombinant antibodies is expected to achieve a high growth rate in the years to come thanks to their versatility and beneficial characteristics in comparison to monoclonal and polyclonal antibodies, such as stability in harsh conditions, small size, relatively low production costs, and batch-to-batch reproducibility. This review describes the characteristics, advantages, and disadvantages of recombinant antibodies including antigen-binding fragments (Fab), single-chain fragment variable (scFv), and single-domain antibodies (VHH) and their application in food analysis with especial emphasis on the analysis of biotoxins, antibiotics, pesticides, and foodborne pathogens. Although the wide application of recombinant antibodies has been hampered by a number of challenges, this review demonstrates their potential for the sensitive, selective, and rapid detection of food contaminants.

The role of the surface ligand on the performance of electrochemical SARS-CoV-2 antigen biosensors

Point-of-care (POC) technologies and testing programs hold great potential to significantly improve diagnosis and disease surveillance. POC tests have the intrinsic advantage of being able to be performed near the patient or treatment facility, owing to their portable character. With rapid results often in minutes, these diagnostic platforms have a high positive impact on disease management. POC tests are, in addition, advantageous in situations of a shortage of skilled personnel and restricted availability of laboratory-based analytics. While POC testing programs are widely considered in addressing health care challenges in low-income health systems, the ongoing pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections could largely benefit from fast, efficient, accurate, and cost-effective point-of-care testing (POCT) devices for limiting COVID-19 spreading. The unrestrained availability of SARS-CoV-2 POC tests is indeed one of the adequate means of better managing the COVID-19 outbreak. A large number of novel and innovative solutions to address this medical need have emerged over the last months. Here, we critically elaborate the role of the surface ligands in the design of biosensors to cope with the current viral outbreak situation. Their notable effect on electrical and electrochemical sensors' design will be discussed in some given examples. Graphical abstract.

Thionine-functionalized three-dimensional carbon nanomaterial-based aptasensor for analysis of Aβ oligomers in serum

How to sensitively detect early biomarkers of Alzheimer's disease (AD) is nowadays, one of the major challenges. In this work, Aβ oligomers (AβO), one of the AD biomarkers, was analyzed using an electrochemical aptasensor, which was prepared based on thionine (Th) - functionalized three - dimensional carbon nanomaterials (reduced graphene oxide (rGO) and multi-wall carbon nanotubes (MWCNTs)) immobilized DNA-aptamer. Th, a positively charged planar aromatic molecule, form many π - π conjugated structures with rGO and MWCNTs, then improving the structural stability, electron transfer and the capacitive properties of Th-rGO-MWCNTs nanocomposites. Under the optimal conditions, differential pulse voltammetry (DPV) current responses decreased with the increase of AβO concentration. The obtained AβO aptasensor presented a wide linear range of 0.0443 pM-443.00 pM and limit of detection (LOD) was 10 fM. Meanwhile, AβO aptasensor displayed remarkable stability and selectivity. It has a great potential for early diagnosis of AD in human real serum samples.

Advances in the Immunoassays for Detection of Bacillus thuringiensis Crystalline Toxins

Insect-resistant genetically modified organisms have been globally commercialized for the last 2 decades. Among them, transgenic crops based on Bacillus thuringiensis crystalline (Cry) toxins are extensively used for commercial agricultural applications. However, less emphasis is laid on quantifying Cry toxins because there might be unforeseen health and environmental concerns. Immunoassays, being the preferred method for detection of Cry toxins, are reviewed in this study. Owing to limitations of traditional colorimetric enzyme-linked immunosorbent assay, the trend of detection strategies shifts to modified immunoassays based on nanomaterials, which provide ultrasensitive detection capacity. This review assessed and compared the properties of the recent advances in immunoassays, including colorimetric, fluorescence, chemiluminescence, surface-enhanced Raman scattering, surface plasmon resonance, and electrochemical approaches. Thus, the ultimate aim of this study is to identify research gaps and infer future prospects of current approaches for the development of novel immunosensors to monitor Cry toxins in food and the environment.

Nanobodies targeting immune checkpoint molecules for tumor immunotherapy and immunoimaging (Review)

The immune checkpoint blockade is an effective strategy to enhance the anti-tumor T cell effector activity, thus becoming one of the most promising immunotherapeutic strategies in the history of cancer treatment. Several immune checkpoint inhibitor have been approved by the FDA, such as anti-CTLA-4, anti-PD-1, anti-PD-L1 monoclonal antibodies. Most tumor patients benefitted from these antibodies, but some of the patients did not respond to them. To increase the effectiveness of immunotherapy, including immune checkpoint blockade therapies, miniaturization of antibodies has been introduced. A single-domain antibody, also known as nanobody, is an attractive reagent for immunotherapy and immunoimaging thanks to its unique structural characteristic consisting of a variable region of a single heavy chain antibody. This structure confers to the nanobody a light molecular weight, making it smaller than conventional antibodies, although remaining able to bind to a specific antigen. Therefore, this review summarizes the production of nanobodies targeting immune checkpoint molecules and the application of nanobodies targeting immune checkpoint molecules in immunotherapy and immunoimaging.

Proximity hybridization-triggered DNA assembly for label-free surface-enhanced Raman spectroscopic bioanalysis

We have developed a versatile label-free surface-enhanced Raman spectroscopic platform for detecting various biotargets via proximity hybridization-triggered DNA assembly based on the 736 cm^-1 Raman peak of adenine breathing mode. We initially immobilized the first probe to AuNPs and modified the second with poly adenine. Presence of target DNA or protein molecules assembled a sandwich complex that brought the poly adenine close to the AuNPs surface, generating Raman signals, that were proportional to target molecule concentration. These approach exhibits high sensitivity, with a detection limit of 5.4 pM, 47 fM, and 0.51 pg/mL for target DNA, thrombin and CEA, respectively. Owing to a one step proximity dependent complex formation, this technique is simple and can be completed within 40 min, making it a promising candidate for point-of-care testing applications.

Development of a lateral flow test strip for simultaneous detection of BT-Cry1Ab, BT-Cry1Ac and CP4 EPSPS proteins in genetically modified crops

A colloidal gold immunochromatographic strip (ICS) for simultaneous detection of multiple transgenic proteins, including CP4 EPSPS, BT-Cry1Ab and BT-Cry1Ac, was developed in this study. The sensitivity of the strip to the target protein was 5 ng/mL for CP4 EPSPS, 100 ng/mL for BT-Cry1Ab and Cry1Ac, respectively. Parallel analysis for maize, soybean, sugar beet and cotton showed the strip could detect 1% of transgenic content in crops containing BT-Cry1Ab and Cry1Ac, and, at least, 0.1% of content in crops containing CP4 EPSPS. The detection results for seed samples indicated the multicomponent analysis ICS had good accuracy. The analysis could be completed within 10 min and had the advantages of being high-throughput, easy to operate and visual detection. This is the first report of semi-quantitative ICS for detecting three transgenic proteins simultaneously. The developed approach may provide insights into the development of ICS for analyzing simultaneously multiple components in genetically modified crops.

Ultrasensitive Electrochemical DNA Biosensor Fabrication by Coupling an Integral Multifunctional Zirconia-Reduced Graphene Oxide-Thionine Nanocomposite and Exonuclease I-Assisted Cleavage

In this work, a simple but sensitive electrochemical DNA biosensor for nucleic acid detection was developed by taking advantage of exonuclease (Exo) I-assisted cleavage for background reduction and zirconia-reduced graphene oxide-thionine (ZrO2-rGO-Thi) nanocomposite for integral DNA recognition, signal amplification, and reporting. The ZrO2-rGO nanocomposite was obtained by a one-step hydrothermal synthesis method. Then, thionine was adsorbed onto the rGO surface, via π-π stacking, as an excellent electrochemical probe. The biosensor fabrication is very simple, with probe DNA immobilization and hybridization recognition with the target nucleic acid. Then, the ZrO2-rGO-Thi nanocomposite was captured onto an electrode via the multicoordinative interaction of ZrO2 with the phosphate group on the DNA skeleton. The adsorbed abundant thionine molecules onto the ZrO2-rGO nanocomposite facilitated an amplified electrochemical response related with the target DNA. Since upon the interaction of the ZrO2-rGO-Thi nanocomposite with the probe DNA an immobilized electrode may also occur, an Exo I-assisted cleavage was combined to remove the unhybridized probe DNA for background reduction. With the current proposed strategy, the target DNA related with P53 gene could be sensitively assayed, with a wide linear detection range from 100 fM to 10 nM and an attractive low detection limit of 24 fM. Also, the developed DNA biosensor could differentiate the mismatched targets from complementary target DNA. Therefore, it offers a simple but effective biosensor fabrication strategy and is anticipated to show potential for applications in bioanalysis and medical diagnosis.

Electrochemical biosensors: perspective on functional nanomaterials for on-site analysis

BACKGROUND

The electrochemical biosensor is one of the typical sensing devices based on transducing the biochemical events to electrical signals. In this type of sensor, an electrode is a key component that is employed as a solid support for immobilization of biomolecules and electron movement. Thanks to numerous nanomaterials that possess the large surface area, synergic effects are enabled by improving loading capacity and the mass transport of reactants for achieving high performance in terms of analytical sensitivity.

MAIN BODY

We categorized the current electrochemical biosensors into two groups, carbon-based (carbon nanotubes and graphene) and non-carbon-based nanomaterials (metallic and silica nanoparticles, nanowire, and indium tin oxide, organic materials). The carbon allotropes can be employed as an electrode and supporting scaffolds due to their large active surface area as well as an effective electron transfer rate. We also discussed the non-carbon nanomaterials that are used as alternative supporting components of the electrode for improving the electrochemical properties of biosensors.

CONCLUSION

Although several functional nanomaterials have provided the innovative solid substrate for high performances, developing on-site version of biosensor that meets enough sensitivity along with high reproducibility still remains a challenge. In particular, the matrix interference from real samples which seriously affects the biomolecular interaction still remains the most critical issues that need to be solved for practical aspect in the electrochemical biosensor.

Dual-Mode Electrochemical Assay of Prostate-Specific Antigen Based on Antifouling Peptides Functionalized with Electrochemical Probes and Internal References

Sensitive and selective detection of target analytes in complex biological samples is currently a major challenge. Herein we constructed a dual-mode antifouling electrochemical sensing platform for the detection of prostate-specific antigen (PSA) based on two kinds of antifouling peptides functionalized with a graphene oxide-Fe₃O₄-thionine (GO-Fe₃O₄-Thi) probe and internal reference ferrocene (Fc), respectively. The longer peptide (Pep1) modified with the GO-Fe₃O₄-Thi probe was designed to contain a peptide sequence (HSSKLQK) capable of being recognized and cut by PSA. The GO-Fe₃O₄-Thi probe functions not only as a peroxidase mimick (GO-Fe₃O₄) but also works as an electrochemical probe due to the presence of thionine (Thi). The concentration of PSA can be measured through both the increase of differential pulse voltammetry (DPV) signal change of Thi and the decrease of chronoamperometry (CA) signal of the reduction of H₂O₂ electrocatalyzed by GO-Fe₃O₄. The shorter peptide (Pep2) was tagged with Fc, whose DPV signal remained constant and was independent of the presence of PSA, and it was used as an internal reference to ensure the reliability and accuracy of the measurement. The dual-mode PSA sensor exhibits a wide linear range from 5 pg/mL to 10 ng/mL, with low detection limits of 0.76 and 0.42 pg/mL through DPV and CA modes, respectively. More importantly, owing to the antifouling capability of the designed peptides, the biosensor performances remained operable even in human serum, indicating feasibility of the electrochemical biosensor for practical PSA quantification in complex samples.

Dual-target electrochemical aptasensor based on co-reduced molybdenum disulfide and Au NPs (rMoS2-Au) for multiplex detection of mycotoxins

Multiple mycotoxin contamination has posed health risks in the area of food safety. In this study, co-reduced molybdenum disulfide and gold nanoparticles (rMoS₂-Au) were designed and used for the first time as an efficient platform endowing electrochemical electrodes with superior electron transfer rates, large surface areas and strong abilities to firmly couple with large amounts of different aptamers. After further modification with thionine (Thi) and 6-(Ferrocenyl) hexanethiol (FC6S), a platform enabling sensitive, selective and simultaneous determination of two important mycotoxins, zearalenone (ZEN) and fumonisin B1 (FB1), was achieved. The established aptasensor showed excellent linear relationships (R² > 0.99) when ZEN and FB1 concentrations were in the range of 1 × 10^-3-10 ng mL^-1 and 1 × 10^-3-1 × 10² ng mL^-1, respectively. High sensitivity of ZEN and FB1 with a limit of detection as low as 5 × 10^-4 ng mL^-1 was obtained with excellent selectivity and stability. The effectiveness of the aptasensor was verified in real maize samples, and satisfactory recoveries were attained. The established platform could be easily expanded to other aptamer-based multiplex screening protocols in biochemical research and clinical diagnosis.

A Novel Immunosensing Method Based on the Capture and Enzymatic Release of Sandwich-Type Covalently Conjugated Thionine-Gold Nanoparticles as a New Fluorescence Label Used for Ultrasensitive Detection of Hepatitis B Virus Surface Antigen

A novel ultrasensitive and simple amplified immunosensing strategy is designed based on a surface-enhanced fluorescence (SEF) nanohybrid made from covalently conjugated thionine-gold nanoparticles (GNP-Th), as a novel amplified fluorescence label, and magnetic nanoparticles (MNPs), as a biological carrier, used for hepatitis B virus surface antigen (HBsAg) detection. This immunosensing strategy operates on the basis of the capture and then release of the amplified fluorescence label. Capturing of the antiHBs-antibody (Ab)-modified GNP-thionine hybrid (GNP-Th-Ab) is carried out through the formation of a two-dimensional (sandwich) probe between this amplified label and antiHBs-antibody-modified magnetic nanoparticles (MNP-Ab), in the presence of a target antigen and using an external magnetic force. Afterward, releasing of the captured fluorescence label is performed using a protease enzyme (pepsin) by a digestion mechanism of grafted antibodies on the GNP-thionine hybrid. As a result of antibody digestion, the amplified fluorescent hybrids (labels) are released into the solution. To understand the mechanism of enhanced fluorescence, the nature of the interaction between thionine and gold nanoparticles is studied using the B3LYP density functional method. In such a methodology, several new mechanisms and structures are used simultaneously, including a SEF-based metal nanoparticle-organic dye hybrid, dual signal amplification in a two-dimensional probe between the GNP-thionine hybrid and MNPs, and a novel releasing method using protease enzymes. These factors improve the sensitivity and speed, along with the simplicity of the procedure. Under optimal conditions, the fluorescence signal increases with the increment of HBs antigen concentration in the linear dynamic range of 4.6 × 10-9 to 0.012 ng/mL with a detection limit (LOD) of 4.6 × 10-9 ng/mL. The proposed immunosensor has great potential in developing ultrasensitive and rapid diagnostic platforms.

Electrocatalysis of cerium metal-organic frameworks for ratiometric electrochemical detection of telomerase activity

A ratiometric electrochemical biosensor was constructed to detect telomerase activity based on electrocatalysis of cerium-based metal-organic frameworks (CeMOFs) and conformation switch of hairpin DNA. First, the CeMOFs were synthesized using Ce as nodes and 1,3,5-benzenetricarboxylic acid as linker in a green method, and then functionalized with gold nanoparticles. The resulted Au@CeMOF tags demonstrated an excellent electrocatalysis toward hydroquinone oxidation. Meanwhile, a methylene blue (MB) modified hairpin probe was designed with telomerase primer (TP) hybridized "stem" and immobilized on the electrode surface via Au-S chemistry. In the presence of the dNTPs and telomerase, the extended TP can open the hairpin DNA and keep the MB away from the electrode surface, resulting in a decrease of electrochemical signal. In the meantime, the TP-extended part could capture the Au@CeMOF-cDNA tags on the electrode surface via hybridization, leading to the increase electrochemical signal of hydroquinone oxidation catalyzed by Au@CeMOF-cDNA tags. Thus, a ratiometric signal output mode was developed for the electrochemical detection of telomerase activity. This biosensor showed wide dynamic correlation of telomerase activity from 2 × 10² to 2 × 10⁶ cells mL^-1 with the limit of detection of 27 cells mL^-1, and was applied to evaluate telomerase activity in single cell. The ratiometric electrochemical strategy based on the catalysis of MOFs provides a new avenue on signal transduction in telomerase detection.

Molecularly Imprinted Poly(thionine)-Based Electrochemical Sensing Platform for Fast and Selective Ultratrace Determination of Patulin

An innovative approach based on a surface functional monomer-directing strategy for the construction of a sensitive and selective molecularly imprinted electrochemical sensor for patulin recognition is described. A patulin imprinted platinum nanoparticle (PtNP)-coated poly(thionine) film was grown on a preformed thionine tailed surface of PtNP-nitrogen-doped graphene (NGE) by electropolymerization, which provided high capacity and fast kinetics to uptake patulin molecules. Thionine acted not only as a functional monomer for molecularly imprinted polymer (MIP), but also as a signal indicator. Enhanced sensitivity was obtained by combining the excellent electric conductivity of PtNPs, NGE, and thionine with multisignal amplification. The designed sensor displayed excellent performance for patulin detection over the range of 0.002-2 ng mL^-1 (R² = 0.995) with a detection limit of 0.001 ng mL^-1 for patulin. In addition, the resulting sensor showed good stability and high repeatability and selectivity. Furthermore, the feasibility of its applications has also been demonstrated in the analysis of real samples, providing novel tactics for the rational design of MIP-based electrochemical sensors to detect a growing number of deleterious substances.

A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters.

A novel electrochemical strategy based on porous 3D graphene-starch architecture and silver deposition for ultrasensitive detection of neuron-specific enolase

This work was aimed at designing a novel and ultrasensitive electrochemical immunoassay strategy to detect neuron-specific enolase (NSE) with a triple signal amplification strategy.

A graphene oxide/gold nanoparticle-based amplification method for SERS immunoassay of cardiac troponin I

A multiple signal amplification of a SERS biosensor was developed for sensitive detection of cTnI with the aid of GO/AuNP complexes.

Sandwich pair nanobodies, a potential tool for electrochemical immunosensing serum prostate-specific antigen with preferable specificity

Prostate-Specific Antigen (PSA) is a crucial biomarker for screening prostate cancer, but a sensitive and selective immunosensor for rapid quantification of serum PSA remains to be developed. In this study, a sandwich pair of nanobodies (Nbs) (i.e., Nb2 and Nb40) against PSA surface antigen was obtained from an alpaca-derived immune phage display library. A sandwich-type immunosensor for the sensitive and selective detection of PSA in serum samples was ingeniously designed based on the pair of Nbs. The small size of Nb40 allowed high capture densities on the surface of reduced graphene oxide (rGO) nanocomposed with massive Au nanoparticles (rGO@AuNPs), which significantly improved the conductivity and provided a large area to anchor many primary antibodies. The secondary antibody Nb2 fused with streptavidin -binding peptide (SBP) cooperated with Nb40 for PSA sandwiching. Accompanying introduction of horseradish peroxidase-streptavidin (HRP-SA) coupled with Nb2-SBP, the faradaic current was linearly correlated with the logarithm of PSA concentration in a range of 0.1-100 ng mL^-1. More importantly, this immunosensor exhibited excellent selectivity, stability, and reproducibility due to the sandwich pair Nbs. The proposed immunosensor was successfully applied in determining PSA in serum samples and could be used for the sensitive and specific detection of PSA.

Metal-Free All-Carbon Nanohybrid for Ultrasensitive Photoelectrochemical Immunosensing of alpha-Fetoprotein

C₆₀ can accept up to six electrons reversibly and show exceptional light absorption over the entire UV-vis spectrum, making it a potential photoactive probe for photoelectrochemical (PEC) bioassay. However, few successful works have been reported to apply fullerenes in PEC biosensing, partially because of the low electronic conductivity and poor interfacial interactions with targeted biomolecules. Herein, we report the addressing of these two obstacles by coupling high conductive graphite flake (Gr), graphene oxide (GO) with sufficient oxygen-containing functional groups, and an alkylated C₆₀ (AC₆₀) into a metal-free all-carbon nanohybrid (AC₆₀-Gr-GO) via harnessing delicate noncovalent interactions among them through a facile mechanical grinding. It was revealed that the as-obtained AC₆₀-Gr-GO nanohybrid not only showed conspicuous enhancement of photocurrent up to 35 times but also offered rich anchors for bioconjugation. With detection of alpha-fetoprotein as an example, the AC₆₀-Gr-GO based PEC immunosensor demonstrated a broad linear detection range (1 pg·mL^-1 to 100 ng·mL^-1) and a detection limit as low as 0.54 pg·mL^-1, superior/competitive to PEC immunosensors for AFP in previous reports. By a proper reinforcement in conductivity and biointerface engineering, this work may provide a new way to use fullerenes as photoactive materials in more general PEC biosensing.

Direct Immunoassay for Facile and Sensitive Detection of Small Molecule Aflatoxin B1 based on Nanobody

Aflatoxin B₁ (AFB₁ ), one of the most toxic mycotoxins, is classified as a group I carcinogen and ubiquitous in various foods and agriproducts. Thus, accurate and sensitive determination of AFB₁ is of great significance to meet the criteria of food safety. Direct detection of AFB₁ is difficult by monoclonal antibody (mAb) with large molecular size (≈150 kD) since the target is too small to produce a detectable signal change. Herein, by combining the electrochemical properties of nanomaterials and the advantages of nanobodies, we developed a direct, highly selective and sensitive electrochemical immunosensor for small molecule detection. The proposed immunosensor had a wide calibration range of 0.01 to 100 ng mL^-1 and a low detection limit of 3.3 pg mL^-1 (S/N=3). Compared with the immunosensor prepared with mAb which was applied in the typical indirect immunoassay, the immunosensor in this work possessed two orders of magnitudes wider linear range and 10-fold more sensitivity. The as-obtained immunosensor was further successfully applied for sensing AFB₁ in real samples. This proposed assay would provide a simple, highly sensitive and selective approach for the direct immunoassay of small molecule AFB₁ , and is extendable to the development of direct immunosensing systems for other small molecules detection by coupling nanocarbon and nanobody.

Development of hierarchical Fe3O4 magnetic microspheres as solid substrates for high sensitive immunoassays

Improving the detection sensitivity of enzyme linked immunosorbent assay (ELISA) is of the utmost importance for meeting the demand of early disease diagnosis. In this work, a sensitive solid substrate for ELISA, i.e., hierarchical iron oxide magnetic microspheres, Fe₃O₄@mSiO₂@poly[poly(ethylene glycol) methacrylate-co-glycidyl methacrylate], was developed via a novel surface-initiated photoiniferter-mediated polymerization (SI-PIMP) strategy. The magnetic microspheres consist of a magnetic Fe₃O₄ core that gives a high magnetic response, a 3D backbone, a mesoporous SiO₂ middle layer, that facilitates microsphere stability and provides anchoring sites, and polymer brushes, that serve as an antifouling and oriented antibody immobilization layer. As a result, the as-prepared microspheres possess a high antibody loading capacity, an enhanced detection signal and a dramatically improved sensitivity, resulting in a 25-fold improvement over conventional ELISA solid substrates.

Simple and rapid monitoring of doxorubicin using streptavidin-modified microparticle-based time-resolved fluorescence immunoassay

Developing a simple analytical method suitable for therapeutic drug monitoring in a clinical setting is key to establishing guidelines on accurate dose administration and the advancement of precision medicine. We devised a simple rapid analytical method through the combination of streptavidin-modified microparticles and a time-resolved fluorescence immunoassay for therapeutic drug monitoring. The analytical performance of this method was investigated and validated using clinical samples. By determination of doxorubicin concentration, the proposed assay has shown a satisfactory linear range of detection (3.8-3000 ng mL-1) with a limit of detection of 3.8 ng mL-1 and an IC50 of 903.9 ng mL-1. The intra and inter-assay coefficients of variation were 4.12-5.72% and 5.48-6.91%, respectively, and the recovery was acceptable. The applicability of the proposed assay was assessed by comparing the determined results with those measured by LC-MS/MS, presenting a satisfactory correlation (R 2 = 0.9868). The proposed assay, which shows satisfactory analytical performance, has great potential for application in the field of TDM in the future.

Boosted Electrochemical Immunosensing of Genetically Modified Crop Markers Using Nanobody and Mesoporous Carbon

The problems of environmental security and the potential risks of human health caused by transgenic crops have attracted much attention. Recent studies reveal 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) from Agrobacterium sp. strain CP4 protein (CP4-EPSPS), which shows very high resistance to herbicide glyphosate, is a typical biomarker of genetically modified (GM) crops. For this reason, it is highly anticipated to devise a sensitive and convenient strategy to detect CP4-EPSPS protein in crops. Herein, we report a simple electrochemical immunosensor by coupling nanobody, ordered mesoporous carbon (OMC), and thionine (Th). As a capture agent, the nanobody was screened out from an immunized Bactrian camel, and exhibited superior properties with respect to conventional antibody, such as higher stability and stronger heat resistance. Moreover, OMC offered an effective platform with high surface area, electrical conductivity, and biocompatibility, which greatly facilitated the assembly of redox probe Th, and further coupling of large amount of capture nanobodies. As a result, the CP4-EPSPS protein could be determined with high sensitivity and efficiency by differential pulse voltammetry (DPV) in a wide linear range from 0.001 to 100 ng·mL^-1 with a low detection limit of 0.72 pg·mL^-1, which was more than 3 orders of magnitude lower than those of previously reported works. As an example, the proposed electrochemical immunosensor was successfully applied to spiked samples, demonstrating its great potential in CP4-EPSPS screening and detection.

Sensitive detection of Bacillus thuringiensis Cry1B toxin based on camel single-domain antibodies

Bt Cry1B toxin, a residue in insect-resistant transgenic plants, has been identified to be harmful to human health. Therefore, it is urgent to detect the Cry1B toxin level in each kind of transgenic plant. Nbs, with prominently unique physiochemical properties, are becoming more and more promising tools in the detection of target antigens. In this study, an immune phage display library that was of high quality was successfully constructed for the screening of Cry1B-specific Nbs with excellent specificity, affinity, and thermostable. Subsequently, a novel sandwich ELISA for Cry1B detection was established, which was based on the biotin-streptavidin system using these aforementioned Nbs. This established detection system presented a linear working range from 5 to 1000 ng ml^-1 and a low detection limit of 3.46 ng ml^-1 . The recoveries from spiked samples were in the range of 82.51%-113.56% with a relative standard deviation (RSD) lower than 5.00%. Taken together, the proposed sandwich ELISA would be a potential method for the detection of Cry1B toxin in transgenic Bt plants specifically and sensitively.

Quantifying Surface Area of Nanosheet Graphene Oxide Colloid Using a Gas-Phase Electrostatic Approach

We demonstrate a new, facile gas-phase electrostatic approach to successfully quantify equivalent surface area of graphene oxide (GO) colloid on a number basis. Mobility diameter (d_p,m)-based distribution and the corresponding equivalent surface area (SA) of GO colloids (i.e., with different lateral aspect ratios) were able to be identified by electrospray-differential mobility analysis (ES-DMA) coupled to a condensation particle counter (CPC) and an aerosol surface area analyzer (ASAA). A correlation of SA ∝ d_p,m^2.0 was established using the ES-DMA-CPC/ASAA, which is consistent with the observation by the 2-dimensional image analysis of size-selected GOs. An ultrafast surface area measurement of GO colloid was achieved via a direct coupling of ES with a combination of ASAA and CPC (i.e., measurement time was 2 min per sample; without size classification). The measured equivalent surface area of GO was ∼202 ± 7 m² g^-1, which is comparable to Brunauer-Emmett-Teller (BET) surface area, ∼240 ± 59 m² g^-1. The gas-phase electrostatic approach proposed in this study has the superior advantages of being fast, requiring no elaborate drying process, and requiring only a very small amount of sample (i.e., <0.01 mg). To the best of our knowledge, this is the first study of using an aerosol-based electrostatic coupling technique to obtain the equivalent surface area of graphene oxide on a number basis with a high precision of measurement.

Synergy effect of specific electrons and surface plasmonic resonance enhanced visible-light photoelectrochemical sensing for sensitive analysis of the CaMV 35S promoter

A Ag/TiO₂/N-GNR ternary composite-based photoelectrochemical sensor for sensitive analysis of the CaMV 35S promoter.

Ferrocene covalently confined in porous MOF as signal tag for highly sensitive electrochemical immunoassay of amyloid-β

This work provides a universal strategy to attain precise modification of MOF signal tags for sensitive electrochemical detection of proteins.

Thionine–graphene oxide covalent hybrid and its interaction with light

Quenching of the thionine singlet excited state in covalently functionalized graphene oxide with an efficient back electron transfer process.