Graphene oxide-labeled sandwich-type impedimetric immunoassay with sensitive enhancement based on enzymatic 4-chloro-1-naphthol oxidation

The influence of charge on the translation of the sandwich ELISA approach to electronic biosensors

The sandwich approach, whereby an antigen is captured by a primary antibody and detected by a secondary antibody, is commonly used to improve the selectivity and sensitivity of enzyme-linked immunosorbent assays (ELISA). This work details the experimental factors that impact the reliable translation of this sandwich approach to two commonly used electronic biosensors, namely potentiometric and impedimetric biosensors. Previous studies have demonstrated the Debye screening limitations associated with potentiometric biosensors. However, the correlation between the ionic strength of the measurement buffer and the impedimetric biosensing response has not been studied. Potentiometric biosensors were able to successfully detect the primary antibody and the target antigen by decreasing the ionic strength of the phosphate buffered saline (PBS) measurement buffer from 1x PBS to 0.01x PBS. However, the secondary antibody used for the selective signal amplification was not reliably detected. Therefore, the sandwich approach is not viable for potentiometric sensing at biologically relevant ionic strengths, due to the Debye screening effect. Alternatively, decreasing the ionic strength of the measurement buffer allowed for the successful translation of the sandwich approach to impedimetric biosensors. Impedimetric biosensing literature typically attributes a measured increase in the charge transfer resistance to an increase in the thickness of the immobilized biolayer. However, this work highlights the influence that both the charge and thickness of the biolayer have on the transport of the redox couple. Decreasing the ionic strength of the measurement buffer lowers the molecular charge screening effect. This permits the transport of a positively charged redox probe through a negatively charged immobilized biolayer via migration and diffusion. The results demonstrate that the use of a buffer at a lower, yet biologically relevant ionic strength allows for the successful translation of the sandwich approach to impedimetric biosensors.

Perovskite-based electrochemiluminescence analysis of H2O2

The detection of hydrogen peroxide (H2O2) represents an extensive requirement across various domains, including food, environmental, and medical fields. This study introduces a highly sensitive technique for the quantification of H2O2, integrating the electrochemiluminescence properties of perovskite with bio-catalyzed precipitation. A water-soluble perovskite-based electrochemiluminescence (ECL) biosensing interface was constructed, wherein H2O2 catalyzes a precipitation reaction that leads to the formation of an insoluble precipitate on the electrode surface. This occurrence effectively quenches the electrochemiluminescence signal of the perovskite, thus facilitating the quantitative detection of H2O2. The modified perovskite demonstrated excellent ECL performance, offering a stable signal source, while the bio-catalyzed precipitation reaction significantly amplified the quenching effect, thereby enhancing detection sensitivity. This strategy exhibits excellent stability and sensitivity, presenting a promising method for the detection of hydrogen peroxide, which holds great potential for applications in various fields.

A simple electrochemical immunosensor based on worm-like platinum for highly sensitive determination of alpha-fetoprotein

Alpha-fetoprotein (AFP) has become a specific tumor marker of primary liver cancer in clinical diagnosis. In this work, we prepared worm-like platinum (WL Pt) nanomaterial via chemical etching without organic solvents and ultra-high temperature. Due to its small particle size and the formation of surface vacancies during the etching process, it had a large specific surface area, and thus exhibited superior electrocatalytic activity for the reduction of hydrogen peroxide. Combining the signal amplification based on hydrogen peroxide reduction and the specific recognition of antigen with antibody, we constructed a simple label-free electrochemical immunosensor with a sandwich-like structure. The developed electrochemical immunosensor showed a wide linear range (0.0001-100 ng mL^-1), a low detection limit (0.028 pg mL^-1), good selectivity and stability. Further, the immunosensor was comparable with enzyme-linked immunosorbent assay (ELISA) and had a good accuracy for AFP detection in human serum samples proving the feasibility of potential application, which is expect to become one of the most promising method in early diagnosis and clinical analysis for liver cancer.

Reagentless Affimer- and antibody-based impedimetric biosensors for CEA-detection using a novel non-conducting polymer

Polyoctopamine (POct), an amine-functionalised non-conducting polymer, as the transducer layer in an electrochemical biosensor, is presented. This polymer offers versatile covalent coupling either through thiol linker conjugation, carboxyl or aldehyde functional groups without the requirement of pre- or post-surface activation. The colorectal cancer biomarker carcinoembryonic antigen (CEA) was selected as the target analyte, whilst an antibody and a synthetic binding protein, an Affimer, were used as distinct bioreceptors to demonstrate the versatility of polyoctopamine as a transducer polymer layer for oriented immobilisation of the bioreceptors. The electrodeposited polymer layer was characterised using cyclic voltammetry, electrochemical impedance spectroscopy, and on-sensor chemiluminescent blotting. The performance of optimised POct-based biosensors were tested in spiked human serum. Results showed that the electropolymerisation of octopamine on screen printed gold electrode generates a thin polymer film with low resistance. Close proximity of the immobilised bioreceptors to the transducer layer greatly enhanced the sensitivity detection. The sensitivity of the smaller monomeric bioreceptor (Affimer, 12.6 kDa) to detect CEA was comparable to the dimeric antibody (150 kDa) with limit of detection at 11.76 fM which is significantly lower than the basal clinical levels of 25 pM. However, the Affimer-based sensor had a narrower dynamic range compared to the immunosensor (1-100 fM vs. 1 fM - 100 nM, respectively). All electrochemical measurements were done in less than 5 min with small sample volumes (10 μl). Hence, polyoctopamine features a simple fabrication of impedimetric biosensors using amine-functionalisation technique, provides rapid response time with enhanced sensitivity and label-free detection.

Handheld pH meter-assisted immunoassay for C-reactive protein using glucose oxidase-conjugated dendrimer loaded with platinum nanozymes

A simple and feasible pH meter-based immunoassay is reported for detection of C-reactive protein (CRP) using glucose oxidase (GOD)-conjugated dendrimer loaded with platinum nanozyme. Initially, platinum nanozymes were loaded into the dendrimers through an in situ synthetic method. Then, GOD and monoclonal anti-CRP antibody with a high molar ratio were covalently conjugated onto carboxylated dendrimers via typical carbodiimide coupling. The immunoreaction was carried out with a competitive mode in a CRP-coated microplate. Along with formation of immunocomplex, the added glucose was oxidized into gluconic acid and hydrogen peroxide by GOD, and the latter was further decomposed by platinum nanozyme, thus accelerating chemical reaction in the positive direction. The produced gluconic acid changed the pH of detection solution, which was determined using a handheld pH meter. Under optimum conditions, the pH meter-based immunoassay gave a good signal toward target CRP from 0.01 to 100 ng mL-1. The limit of detection was 5.9 pg mL-1. An intermediate precision ≤ 11.2% was acquired with batch-to-batch identification. No nonspecific adsorption was observed during a series of procedures to detect target CRP, and the cross-reaction against other biomarkers was very low. Importantly, our system gave well-matched results for analysis of human serum samples relative to a referenced ELISA kit.Graphical abstract.

Ultrasensitive Electrochemical Impedance Detection of Mycoplasma agalactiae DNA by Low-Cost and Disposable Au-Decorated NiO Nanowall Electrodes

Nanostructured electrodes detecting bacteria or viruses through DNA hybridization represent a promising method, which may be useful in on-field applications where PCR-based methods are very expensive, time-consuming, and require trained personnel. Indeed, electrochemical sensors combine disposability, fast response, high sensitivity, and portability. Here, a low-cost and high-surface-area electrode, based on Au-decorated NiO nanowalls, demonstrates a highly sensitive PCR-free detection of a real sample of Mycoplasma agalactiae (Ma) DNA. NiO nanowalls, synthesized by aqueous methods, thermal annealing, and Au decoration, by electroless deposition, ensure a high-surface-area platform for successful immobilization of Ma thiolated probe DNA. The morphological, chemical, and electrochemical properties of the electrode were characterized, and a reproducible detection of synthetic Ma DNA was observed and investigated by impedance measurements. Electrochemical impedance spectroscopy (EIS) ascribed the origin of impedance signal to the Ma DNA hybridization with its probe immobilized onto the electrode. The electrode successfully discriminates between DNA extracted from healthy and infected sheep milk, showing the ability to detect Ma DNA in concentrations as low as 53 ± 2 copy number μL^-1. The Au-decorated NiO nanowall electrode represents a promising route toward PCR-free, disposable, rapid, and molecular detection.

Enzyme-Antibody-Modified Gold Nanoparticle Probes for the Ultrasensitive Detection of Nucleocapsid Protein in SFTSV

As humans and climate change continue to alter the landscape, novel disease risk scenarios have emerged. Sever fever with thrombocytopenia syndrome (SFTS), an emerging tick-borne infectious disease first discovered in rural areas of central China in 2009, is caused by a novel bunyavirus (SFTSV). The potential for SFTS to spread to other countries in combination with its high fatality rate, possible human-to-human transmission, and extensive prevalence among residents and domesticated animals in endemic regions make the disease a severe threat to public health. Because of the lack of preventive vaccines or useful antiviral drugs, diagnosis of SFTS is the key to prevention and control of the SFTSV infection. The development of serological detection methods will greatly improve our understanding of SFTSV ecology and host tropism. We describe a highly sensitive protein detection method based on gold nanoparticles (AuNPs) and enzyme-linked immunosorbent assay (ELISA)—AuNP-based ELISA. The optical sensitivity enhancement of this method is due to the high loading efficiency of AuNPs to McAb. This enhances the concentration of the HRP enzyme in each immune sandwich structure. The detection limit of this method to the nucleocapsid protein (NP) of SFTSV was 0.9 pg mL⁻¹ with good specificity and reproducibility. The sensitivity of AuNP-based ELISA was higher than that of traditional ELISA and was comparable to real-time quantitative polymerase chain reaction (qRT-PCR). The probes are stable for 120 days at 4 °C. This can be applied to diagnosis and hopefully can be developed into a commercial ELISA kit. The ultrasensitive detection of SFTSV will increase our understanding of the distribution and spread of SFTSV, thus helping to monitor the changes in tick-borne pathogen SFTSV risk in the environment.

A review on impedimetric immunosensors for pathogen and biomarker detection

Since the discovery of antibiotics in the first quarter of the twentieth century, their use has been the principal approach to treat bacterial infection. Modernized medicine such as cancer therapy, organ transplantation or advanced major surgeries require effective antibiotics to manage bacterial infections. However, the irresponsible use of antibiotics along with the lack of development has led to the emergence of antimicrobial resistance which is considered a serious global threat due to the rise of multidrug-resistant bacteria (Wang et al. in Antibiotic resistance: a rundown of a global crisis, pp. 1645-1658, 2018). Currently employed diagnostics techniques are microscopy, colony counting, ELISA, PCR, RT-PCR, surface-enhanced Raman scattering and others. These techniques provide satisfactory selectivity and sensitivity (Joung et al. in Sens Actuators B Chem 161:824-831, 2012). Nevertheless, they demand specialized personnel and expensive and sophisticated machinery which can be labour-intensive and time-consuming, (Malvano et al. in Sensors (Switzerland) 18:1-11, 2018; Mantzila et al. in Anal Chem 80:1169-1175, 2008). To get around these problems, new technologies such as biosensing and lab-on-a-chip devices have emerged in the last two decades. Impedimetric immunosensors function by applying electrochemical impedance spectroscopy to a biosensor platform using antibodies or other affinity proteins such as Affimers (Tiede et al. in Elife 6(c):1-35, 2017) or other binding proteins (Weiss et al. in Electrochim Acta 50:4248-4256, 2005) as bioreceptors, which provide excellent sensitivity and selectivity. Pre-enrichment steps are not required and this allows miniaturization and low-cost. In this review different types of impedimetric immunosensors are reported according to the type of electrode and their base layer materials, either self-assembled monolayers or polymeric layers, composition and functionalization for different types of bacteria, viruses, fungi and disease biomarkers. Additionally, novel protein scaffolds, both antibody derived and non-antibody derived, used to specifically target the analyte are considered.

Study on Interaction Between TATA-Box Binding Protein (TBP), TATA-Box and Multiprotein Bridging Factor 1(MBF1) in Beauveria bassiana by Graphene-Based Electrochemical Biosensors

The regulation of transcription level is an important step in gene expression process. Beauveria bassiana is a broad-spectrum insecticidal fungi widely used in the biologic control of arthropod. The regulation of its transcription level is a multilevel complex process. Multiprotein bridging factor 1(MBF1) is a transcriptional co-activator that bridges sequence-specific activators and the TATA-box binding protein(TBP), Little is known about the interaction between MBF1, TBP, and TBP binding to DNA(TATA-sequences)in filamentous fungi of Beauveria bassiana, The binding of TBP to TATA-box and TBP to MBF1 was investigated via electrochemical biosensor. Graphene oxide has an electronic mobility that is unattainable for any metal, so it will be highly sensitive as a test electrode. Hence, we developed a simple, sensitive and specific sensor based on an TBP probe and graphene oxide that successfully detected the interaction of TBP and TATA-box or MBF1. From the electrochemical impedance spectroscopy (EIS), we find that the radius will increase when adding TATA-box or MBF1 buffer to the modified TBP protein electrode. When adding no TATA-box or no MBF1, the radius is relatively unchanged. The interaction between TBP and TATA-box or MBF1 was proved based on the results. These data confirmed the specificity of the interactions, (1) our developed graphene-based electrochemical biosensor can be used for monitoring the interaction between TBP and TATA-box or MBF1, (2) TBP can bind to TATA-box, (3) TBP can bind to MBF1, and (4) TBP mediates the interactions of MBF1 to DNA. Therefore, this work provided a label-free, low-cost and simple detection method for the complex process of eukaryotic gene transcription regulation.

A competitive immunoassay for electrochemical impedimetric determination of chlorpyrifos using a nanogold-modified glassy carbon electrode based on enzymatic biocatalytic precipitation

A direct competitive impedimetric immunoassay for chlorpyrifos (CPF) was developed that is based on the specific affinity of immunoassay and the enzymatic biocatalytic precipitation amplification strategy. The CPF antibody (anti-CPF) was anchored onto an electro-deposited nanogold modified glassy carbon electrode surface by adsorption of the Au-NH₂ bond and Au-SH bond. This improved the electrode reactivity and the loading amount of anti-CPF. Abundant horseradish peroxidase (HRP) and bovine serum albumin-CPF (BSA-CPF) were anchored onto spherical gold nanoparticles (AuNPs, 16 ± 2 nm) to form HRP-AuNP-BSA-CPF (analyte competitor). CPF determination was achieved when the competitive immunoassay occurred between CPF and analyte competitor with anti-CPF. In the presence of H₂O₂ and 4-chloro-1-naphthol, an enzyme-mediated biocatalytic precipitation process was triggered and produced an insoluble 4-chloro-1(4H)-naphthalenone. This insoluble substance increased the Faradaic impedance of the base electrode. The impedimetric signal was determined at the formal potential of 220 mV and alternating voltage of 10 mV. This signal decreased with increasing concentration of CPF over a linear range of 1.0 × 10^-3 ng mL^-1~10 ng mL^-1 with a detection limit of 0.070 pg mL^-1. The immunoassay has been tested for determination of chlorpyrifos in complex matrices such as artificially spiked vegetables with recoveries in the range 85 to 110%. The relative standard deviations were less than 7.5%. Graphical abstractSchematic representation of electrochemical impedimetric immunoassay for chlorpyrifos determination before enzymatic biocatalytic precipitation (BCP, red line) process and after BCP process (blue line).

Strip-shaped Co3O4 as a peroxidase mimic in a signal-amplified impedimetric zearalenone immunoassay

An impedimetric immunoassay was designed for ultrasensitive determination of zearalenone (ZEN). It is making use of the peroxidase-like activity of strip-shaped Co₃O₄ (ssCo₃O₄) which catalyzes the oxidation of 4-chloro-1-naphthol to produce an insoluble precipitate in the presence of H₂O₂. The precipitate is electrically nonconductive and accumulates on the electrode, thereby retarding the electron transfer from the redox probe ferro/ferricyanide to the surface of electrode. This amplifies the impedimetric signal in accordance with logarithm of the concentration of ZEN. The electrode was further modified with TiO₂ mesocrystals (TiO₂ MCs) which improve the capture of more analytes and increase the performance of the immunoassay. Under optimized experimental condition, the impedimetric signal increased linearly with the logarithm of the ZEN concentration range between 0.1 fg·mL^-1 to 10 pg·mL^-1. The detection limit is of 33 ag· mL^-1. Graphical abstractThis work describes an impedimetric immunoassay based on the use of strip-shaped Co3O4 that catalyzes the production of an insoluble precipitate in the presence of H2O2 on the surface of a glassy carbon electrode. The effect was used for signal amplification in an electrochemical immunoassay for zearalenone.

Third generation impedimetric sensor employing direct electron transfer type glucose dehydrogenase

Faradaic electrochemical impedance spectroscopy (faradaic EIS) is an attractive measurement principle for biosensors. However, there have been no reports on sensors employing direct electron transfer (DET)-type redox enzymes based on faradaic EIS principle. In this study, we have attempted to construct the 3rd-generation faradaic enzyme EIS sensor, which used DET-type flavin adenine dinucleotide (FAD) dependent glucose dehydrogenase (GDH) complex, to elucidate its characteristic properties as well as to investigate its potential application as the future immunosensor platform. The gold disk electrodes (GDEs) with DET-type FADGDH prepared using self-assembled monolayer (SAM) showed the glucose concentration dependent impedance change, which was confirmed by the change in the charge transfer resistance (R_ct). The Δ(1/R_ct) values were also affected by DC bias potential and the length of SAM. Based on the Nyquist plot and Bode plot simulations, glucose sensing by imaginary impedance monitoring under fixed frequency (5 mHz) was carried out, revealing the higher sensitivity at low glucose concentration with wider linear range (0.02-0.2 mM). Considering this high sensitivity toward glucose, the 3rd-generation faradaic enzyme EIS sensor would provide alternative platform for future impedimetric immunosensing system, which does not use redox probe.

Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials

Electrically-transduced sensors, with their simplicity and compatibility with standard electronic technologies, produce signals that can be efficiently acquired, processed, stored, and analyzed. Two dimensional (2D) nanomaterials, including graphene, phosphorene (BP), transition metal dichalcogenides (TMDCs), and others, have proven to be attractive for the fabrication of high-performance electrically-transduced chemical sensors due to their remarkable electronic and physical properties originating from their 2D structure. This review highlights the advances in electrically-transduced chemical sensing that rely on 2D materials. The structural components of such sensors are described, and the underlying operating principles for different types of architectures are discussed. The structural features, electronic properties, and surface chemistry of 2D nanostructures that dictate their sensing performance are reviewed. Key advances in the application of 2D materials, from both a historical and analytical perspective, are summarized for four different groups of analytes: gases, volatile compounds, ions, and biomolecules. The sensing performance is discussed in the context of the molecular design, structure-property relationships, and device fabrication technology. The outlook of challenges and opportunities for 2D nanomaterials for the future development of electrically-transduced sensors is also presented.

A sensitive impedimetric platform biosensing protein: Insoluble precipitates based on the biocatalysis of manganese(III) meso-tetrakis (4-N-methylpyridiniumyl)-porphyrinin in HCR-assisted dsDNA

In this study, a sensitive biosensing interface for protein was reported based on nonconductive insoluble precipitates (IPs) by the biocatalysis of manganese(III) meso-tetrakis (4-N-methylpyridiniumyl)-porphyrin (MnTMPyP), which was intercalated into formed double-strand DNA (dsDNA) scaffold triggered by hybridization chain reaction (HCR). In the proposed impedimetric aptasensor, carcinoembryonic antigen (CEA) and its aptamer were used as testing model. PtPd nanowires (PtPdNWs) with large surface area and superior conductivity were employed as nanocarriers to greatly immobilize biomolecules (e.g. CEA aptamers). Then, two DNA hairpins H1 and H2 were introduced to trigger HCR with the assistance of DNA initiator, resulting in the formation of a long dsDNA scaffold. Meanwhile, mimicking enzyme MnTMPyP molecules were embedded into the resultant dsDNA, in situ generating the complex MnTMPyP-dsDNA with peroxidase-like activity. Under the biocatalysis of MnTMPyP-dsDNA, 3,3-diaminobenzidine (DAB) was oxidized to form nonconductive IPs. As a result, the electron transfer between electrode interface and redox probe was vastly hindered, leading to the significant amplification of electrochemical impedimetric signal. So, greatly improved analytical performances of the proposed aptasensor were achieved with a detection limit as low as 0.030pgmL(-1). And the successful assay of CEA in human serum samples enabled the developed biosensing platform to have promising potential in bioanalysis.

Amplified inhibition of the electrochemical signal of ferrocene by enzyme-functionalized graphene oxide nanoprobe for ultrasensitive immunoassay

A nanoprobe-induced signal inhibition mechanism was designed for ultrasensitive electrochemical immunoassay at a chitosan-ferrocene (CS-Fc) based immunosensor. The nanoprobe was prepared by covalently loading signal antibody and high-content horseradish peroxidase (HRP) on the graphene oxide (GO) nanocarrier. The immunosensor was prepared through the stepwise assembly of gold nanoparticles (Au NPs) and capture antibody at a CS-Fc modified electrode. After sandwich immunoreaction, the GO-HRP nanoprobes were quantitatively captured onto the immunosensor surface and thus induced the production of a layer of insoluble film through the enzymatically catalytic reaction of the HRP labels. Both the dielectric immunocomplex formed on the immunosensor surface and the enzymatic precipitate with low electroconductivity led to the electrochemical signal decease of the Fc indicator, which was greatly amplified by the multi-enzyme signal amplification of the nanoprobe. Based on this amplified signal inhibition mechanism, a new ultrasensitive electrochemical immunoassay method was developed. Using carcinoembryonic antigen as a model analyte, this method showed a wide linear range over 5 orders of magnitude with a detection limit down to 0.54 pg/mL. Besides, the immunosensor showed good specificity, acceptable reproducibility and stability as well as satisfactory reliability for the serum sample analysis.

Graphene as a signal amplifier for preparation of ultrasensitive electrochemical biosensors

Early diagnostics of diseases performed with minimal money and time consumption has become achievable due to recent advances in development of biosensors. These devices use biorecognition elements for selective interaction with an analyte and signal readout is obtained via different types of transducers. Operational characteristics of biosensors have been reported to improve substantially, when a diverse range of nanomaterials was employed. This review presents construction of electrochemical biosensors based on graphene, atomically thin 2D carbon crystals, which is currently intensively studied nanomaterial. The most attractive directions of graphene applications in biosensor preparation are discussed here including novel detection and amplification schemes exploiting graphene's unique electrochemical, physical and chemical properties. The future of graphene-based biosensors is most likely bright, but there is still a lot of work to do to fulfill high expectations.

A novel signal amplification strategy of an electrochemical immunosensor for human chorionic gonadotropin, based on nanocomposites of multi-walled carbon nanotubes–ionic liquid and nanoporous Pd

A novel electrochemical immunosensor using MWCNTs–BMIMPF₆/NP-Pd as a sensor platform to sequentially immobilize antibodies was developed for hCG detection.

Target-initiated impedimetric proximity ligation assay with DNAzyme design for in situ amplified biocatalytic precipitation

A target-initiated proximity ligation assay protocol with DNAzyme formation was for the first time designed for ultrasensitive impedimetric monitoring of heavy metal ions (silver ions were used in this case) by coupling with an enzymatic biocatalytic precipitation technique.

Designing label-free electrochemical immunosensors for cytochrome c using nanocomposites functionalized screen printed electrodes

We have designed here a label-free direct electrochemical immunosensor for the detection of cytochrome c (cyt c), a heme containing metalloprotein using its specific monoclonal antibody. Two nanocomposite-based electrochemical immunosensor platforms were evaluated for the detection of cyt c; (i) self-assembled monolayer (SAM) on gold nanoparticles (GNP) in polypyrrole (PPy) grafted screen printed electrodes (SPE) and (ii) carbon nanotubes (CNT) integrated PPy/SPE. The nanotopologies of the modified electrodes were confirmed by scanning electron microscopy. Electrochemical impedance spectroscopy and cyclic voltammetry were employed to monitor the stepwise fabrication of the nanocomposite immunosensor platforms. In the present method, the label-free quantification of cyt c is based on the direct electron transfer between Fe (III)/Fe (II)-heme redox active site of cyt c selectively bound to anti-cyt c nanocomposite modified SPE. GNP/PPy and CNT/PPy nanocomposites promoted the electron transportation through the conductive pore channels. The overall analytical performance of GNP/PPy based immunosensor (detection limit 2 nM; linear range: 2 nM to 150 µM) was better than the anti-cyt c/CNT/PPy (detection limit 10 nM; linear range: 10 nM to 50 µM). Further, the measurement of cyt c release in cell lysates of cardiomyocytes using the GNP/PPy based immunosensor gave an excellent correlation with standard ELISA.