Faradaic Electrochemical Impedance Spectroscopy Research Articles

Selective detection of alpha synuclein amyloid fibrils by faradaic and non-faradaic electrochemical impedance spectroscopic approaches

This study utilized faradaic and non-faradaic electrochemical impedance spectroscopy to detect alpha synuclein amyloid fibrils on gold interdigitated tetraelectrodes (AuIDTE), providing valuable insights into electrochemical reactions for clinical use. AuIDE was purchased, modified with zinc oxide for increased hydrophobicity. Functionalization was conducted with hexacyanidoferrate and carbonyldiimidazole. Faradaic electrochemical impedance spectroscopy has been extensively explored in clinical diagnostics and biomedical research, providing information on the performance and stability of electrochemical biosensors. This understanding can help develop more sensitive, selective, and reliable biosensing platforms for the detection of clinically relevant analytes like biomarkers, proteins, and nucleic acids. Non-faradaic electrochemical impedance spectroscopy measures the interfacial capacitance at the electrode–electrolyte interface, eliminating the need for redox-active species and simplifying experimental setups. It has practical implications in clinical settings, like real-time detection and monitoring of biomolecules and biomarkers by tracking changes in interfacial capacitance. The limit of detection (LOD) for normal alpha synuclein in faradaic mode is 2.39-fM, The LOD for aggregated alpha synuclein detection is 1.82-fM. The LOD for non-faradaic detection of normal alpha synuclein is 2.22-fM, and the LOD for nonfaradaic detection of aggregated alpha synuclein is 2.40-fM. The proposed EIS-based AuIDTEs sensor detects alpha synuclein amyloid fibrils and it is highly sensitive.

Non-Invasive Detection of Cytokines in Saliva Towards Asthma Phenotyping Using an Electrochemical Sensor

Introduction: Asthma is a complicated and heterogeneous condition of the lungs which is usually associated with the chronic inflammation of the airways. Traditionally, classification of asthma has been based on clinical attributes such as the symptoms observed and response to treatment. However, with the growing understanding of the heterogeneity of asthma as a spectrum with varying underlying causes, asthma phenotyping has been on the horizon. Cytokines are the chemical messengers that play a crucial role in the immune system. They mediate communication between the cells in the immune system and as such have a direct effect on the inflammation process. The role of some cytokines has been established in the pathogenesis of asthma. Due to the involvement of cytokines in the modulation of asthma, they can be excellent target for asthma phenotyping. This is also helpful because identification and quantification of these inflammatory mediators can allow for more precise medicine for more targeted treatment. To aid the identification and quantification of cytokines in the phenotyping of asthma, we have created a customizable (currently for IL-8 and IL-10) multi-marker electrochemical platform for the detection of cytokines non-invasively in saliva. The developed electrochemical platform can detect cytokines rapidly and non-invasively within reported physiological ranges of the given cytokines. Evaluating the presence and quantity of the cytokines will be useful in the phenotyping and treatment plans adopted by physicians.Materials and Methods: The electrochemical sensor consists of 6 multiplexed gold electrodes on a PCB substrate. IL-8 and IL-10 antibodies were immobilized onto the sensor surface using a gold-thiol linker chemistry. Sensor response was evaluated with non- faradaic Electrochemical Impedance Spectroscopy (nf-EIS). Pooled human saliva was spiked with increasing doses of IL-8 and IL-10 in their respective physiological ranges to develop a dose response curve. Spike and recovery experiments were conducted to validate the efficacy of dose response curve.Results and Discussion: Sensor response to both IL-8 and IL-10 spiked doses in pooled saliva showed a dose-dependent response to increasing concentrations of IL-8 and IL-10 respectively as illustrated by the Nyquist plots in Fig.1. The change in impedance with increasing doses of IL-8 and IL-10 in pooled human saliva lends to credence to the establishment of the presence and quantification of cytokines in saliva that can be used by clinicians and scientists alike in the further development of personalized medicine for severe asthmatics unresponsive to traditional asthmatic treatments.Conclusion: This study presents a noninvasive electrochemical sensor for rapid and early detection of cytokines in pooled human saliva towards asthma phenotyping. The sensor exhibits a dose dependent response to IL-8 and IL-10, markers implicated in pathogenesis of asthma. Sensor capability was demonstrated with spike and recovery experiments illustrating the ability of the developed electrochemical sensor to detect and quantify cytokines in human saliva. Figure 1

Impedimetric Bacterial Detection Using Random Antimicrobial Peptide Mixtures.

The biosensing of bacterial pathogens is of a high priority. Electrochemical biosensors are an important future tool for rapid bacteria detection. A monolayer of bacterial-binding peptides can serve as a recognition layer in such detection devices. Here, we explore the potential of random peptide mixtures (RPMs) composed of phenylalanine and lysine in random sequences and of controlled length, to form a monolayer that can be utilized for sensing. RPMs were found to assemble in a thin and diluted layer that attracts various bacteria. Faradaic electrochemical impedance spectroscopy was used with modified gold electrodes to measure the charge-transfer resistance (RCT) caused due to the binding of bacteria to RPMs. Pseudomonas aeruginosa was found to cause the most prominent increase in RCT compared to other model bacteria. We show that the combination of highly accessible antimicrobial RPMs and electrochemical analysis can be used to generate a new promising line of bacterial biosensors.

Concentration and diffusion of the redox probe as key parameters for label-free impedimetric immunosensing

Nanoporous surfaces are promising for label-free electrochemical biosensing. We formed nanopores directly on the electrode surface by means of assembling a dense layer of nonconductive nanoparticles. In our model affinity biosensor, covalent attachment of albumin protein on top of 40 nm polystyrene nanoparticles represented a capture of an analyte, resulting in blockage of the nanopores. Different bulk concentrations of the ferro/ferricyanide redox pair were probed by Faradaic electrochemical impedance spectroscopy and fast chronoamperometry. The character of the redox probe permeation towards the electrode surface differed in dependence on its concentration. These data were compared with the theoretical behavior of the free diffusion according to the Cottrell equation. Both the bulk concentration of the redox probe and the timescale of the experiment affected the performance of the electrochemical detection, demonstrating the importance of controlling these parameters in immunosensing applications.

Determining the depth of surface charging layer of single Prussian blue nanoparticles with pseudocapacitive behaviors

Understanding the hybrid charge-storage mechanisms of pseudocapacitive nanomaterials holds promising keys to further improve the performance of energy storage devices. Based on the dependence of the light scattering intensity of single Prussian blue nanoparticles (PBNPs) on their oxidation state during sinusoidal potential modulation at varying frequencies, we present an electro-optical microscopic imaging approach to optically acquire the Faradaic electrochemical impedance spectroscopy (oEIS) of single PBNPs. Here we reveal typical pseudocapacitive behavior with hybrid charge-storage mechanisms depending on the modulation frequency. In the low-frequency range, the optical amplitude is inversely proportional to the square root of the frequency (∆I ∝ f−0.5; diffusion-limited process), while in the high-frequency range, it is inversely proportional to the frequency (∆I ∝ f−1; surface charging process). Because the geometry of single cuboid-shaped PBNPs can be precisely determined by scanning electron microscopy and atomic force microscopy, oEIS of single PBNPs allows the determination of the depth of the surface charging layer, revealing it to be ~2 unit cells regardless of the nanoparticle size.

AptaStrensor (aptamer-based sensor for stress monitoring): The interrelationship between NPY and cortisol towards chronic disease monitoring

Cortisol and NPY are two biomarkers that have been shown to be related with stress, the latter being more associated with neurological stress. The modulation of these biomarkers can be dependent on several factors such as the type and duration of stress one is exposed to. Understanding the role and interplay between these two biomarkers in the modulation of stress would be expedient in the treatment and management of chronic conditions associated with stress. This paper reports the novel use of aptamers for the simultaneous detection of NPY and cortisol in ultra-low volumes (∼2 μL) human sweat, using non- Faradaic Electrochemical Impedance Spectroscopy (nF- EIS). The ions in the sweat rearrange to form an electrical double layer on the electrode surface when the electrode is biased. nF- EIS is able to isolate the modulations in the capacitive region of the electrical double layer as a result of biomarkers binding to their respective aptamers. This paper reports a detection range of 1ng/mL-256 ng/mL for cortisol and 1pg/mL-256 pg/mL for NPY, both covering the physiological ranges for NPY and cortisol respectively in human sweat. In this work, we hypothesize that physical stress would induce an increase in the concentration of cortisol in sweat while expecting to see little to no rise in NPY since NPY has been shown to be modulated in pyschological stress. The developed AptaStrensor platform was demonstrated to detect an upregulation of cortisol in human sweat when subjects were introduced to physical stress (p < 0.05). Concurrently, the developed platform was sensitive enough to detect no significant difference in NPY levels in human sweat when the subjects were subjected to physical stress (p > 0.05). This platform is envisioned to help understand the interplay between cortisol and NPY as a means of monitoring stress-associated chronic conditions.

Development of a POCT type insulin sensor employing anti-insulin single chain variable fragment based on faradaic electrochemical impedance spectroscopy under single frequency measurement

To improve glycemic control managed through insulin administration, recent studies have focused on developing hand-held point-of-care testing (POCT) electrochemical biosensors for insulin measurement. Amongst them, anti-insulin IgG-based sensors show promise in detecting insulin with high specificity and sensitivity. However, fabrication of electrochemical sensors with IgG antibodies can prove challenging because of their larger molecular size. To overcome these limitations, this study focuses on utilizing the anti-insulin single chain variable fragment (scFv) as a biosensing molecule with single-frequency faradaic electrochemical impedance spectroscopy (EIS). By comparing two different immobilization methods, covalent conjugation via succinimidyl ester and non-covalent poly-histidine chelation, we demonstrated effective modification of the electrode surface with anti-insulin scFv, while retaining its specific recognition toward insulin. Sensor performance was confirmed via the concentration-dependent faradaic electrochemical impedance change using potassium ferricyanide as a redox probe. The optimal frequency for measurement was determined to be the peak slope of the calculated impedance correlation with respect to frequency. Based on the identified optimized frequency, we performed single-frequency measurement of insulin within a concentration range of 10 pM-100 nM. This study can aid in developing a future point-of-care sensor which rapidly and sensitively measures insulin across a dynamic range of physiological concentrations, with label-free detection.

Impedimetric Detection of Albumin-Bound Fatty Acids Using Graphene Oxide Electrode

The fatty acid/albumin (FA/Alb) molar ratio is ≤1 in healthy subjects; this ratio can reach 3–4 in patients with acute myocardial ischemia. We describe the spontaneous desorption–adsorption kinetics of FAs from albumin to a graphene electrode at neutral pH. Albumin-depleted human serum was prepared via ultrafiltration and then mixed with defatted human albumin and sodium oleate at different FA/Alb molar ratios, at a final albumin concentration of 0.6 mM. A commercially available screen-printed graphene oxide (GO)-modified carbon electrode was used for the electrochemical experiments. Frequency-ranged Faradaic electrochemical impedance spectroscopy (EIS) and a single-frequency non-Faradaic impedance measure (chronoimpedance) were used to derive the desorption–adsorption kinetics. The surface of the GO electrode was finally evaluated with the aid of X-ray photoelectron spectroscopy (XPS). With the chronoimpedance experiment, the measured impedance increased accordingly to the FA/Alb ratios. The frequency-ranged EIS showed good linearity between the impedance and the FA/Alb ratio, with a limit of quantification value of 1.06. XPS surface analysis revealed that the FA was adsorbed onto the electrode, with the amount of the adsorbed FA proportional to the FA/Alb ratio. The electrochemical method applied on this peculiar desorption–adsorption kinetics of FAs has the ability to differentiate serum having excess FAs.

Faradaic electrochemical impedance spectroscopy for enhanced analyte detection in diagnostics

Electrochemical impedance spectroscopy (EIS) is a widely implementable technique that can be applied to many fields, ranging from disease detection to environmental monitoring. EIS as a biosensing tool allows detection of a broad range of target analytes in point-of-care (POC) and continuous applications. The technique is highly suitable for multimarker detection due to its ability to produce specific frequency responses depending on the target analyte and molecular recognition element (MRE) combination. EIS biosensor development has shown promising results for the medical industry in terms of diagnosis and prognosis for various biomarkers. EIS sensors offer a cost-efficient system and rapid detection times using minimal amounts of sample volumes, while simultaneously not disturbing the sample being studied due to low amplitude perturbations. These properties make the technique highly sensitive and specific. This paper presents a review of EIS biosensing advancements and introduces different detection techniques and MREs. Additionally, EIS's underlying theory and potential surface modification techniques are presented to further demonstrate the technique's ability to produce stable, specific, and sensitive biosensors.

Homogeneous functional self-assembled monolayers: Faradaic impedance baseline signal drift suppression for high-sensitivity immunosensing of C-reactive protein

The limit of detection (LOD) of affinity sensors based on alkanethiol self-assembled monolayers (SAMs) system can be improved by either signal amplification and/or noise reduction. The latter includes baseline signal drift arising, in part, from, monolayer imperfections (and variations in this through repeats) as well as electrical noise of both the sensor and transducer. In reagentless “label free” assays signal drift carries with it the possibility of assay false-positive results (if signal drift is positive) or false-negatives (if specific signal is swamped by downward drift). Faradaic electrochemical impedance spectroscopy (FEIS) sensors based on SAM interfaces have been shown to exhibit considerable baseline signal drift, which consequently affects their assaying capabilities. This study reports on the development of a simple two-step pre-treatment method for generating functional SAMs of 11-mercaptoundecanoic acid (MUA) on Au with a highly suppressed baseline signal drift. For electrochemical characterisation of the SAMs, the charge-transfer resistance (Rct), defect presence (pinhole radius and separation), capacitance, and dielectric constant were evaluated. The interface pre-treatment generates films of substantially improved homogeneity that are subsequently functionalised with anti-human C-reactive protein. CRP, an acute-phase protein, is detectable down to femtomolar levels without any amplification; this is a 2–3 order of magnitude lower detection limit than that typically accessible for analyses of this type. The developed protocols thus present a convenient and general route to suppress baseline signal drifts and improve the limits of detection of affinity sensors based on Faradaic impedance.

Direct Electron Transfer Type Faradaic Enzyme EIS

Introduction Faradaic electrochemical impedance spectroscopy (Faradaic EIS) has been studied as the principle of biosensors. Considering that Faradaic EIS is a powerful method to investigate the surface of electrodes in details with high sensitivity, variety of Faradaic EIS based biosensors, such as immunosensors or DNA sensors have been reported. These biosensors are based on measuring the charge transfer resistance (Rct) of redox probes at the surface of electrodes. The one of the inherent limitations of currently reported Faradaic EIS based biosensors is the presence of redox probes, which is necessary to monitor the binding of target molecules to ligand which alter the flux of redox probes to the electrode surface, consequently, result the change of Rct. However, the redox probes are generally chemically unstable, and the configuration of continuous monitoring and/or in vivo monitoring type biosensors using redox probe are complicated and difficult. Therefore, the developments of alternative EIS sensor platforms, which make the robust measurements, have been expected.Besides, enzyme-based electrochemical biosensors are categorized into three generations, based on the electron acceptors. In the 1st-generation, oxygen serves as the electron acceptor of the enzyme reaction of oxidase, and the decreasing oxygen concentration or liberating hydrogen peroxide is detected electrochemically. The 2nd-generation principle uses artificial electron acceptors (mediators) for the oxidases or dehydrogenases enzyme oxidative half reactions, instead of oxygen, and the reduced artificial electron acceptors are measured electrochemically. The 3rd-generation principle uses enzymes with the ability of direct electron transfer (DET). Although, Faradaic enzyme EIS sensors have been reported by employing redox mediators or oxygen as the external electron acceptors, there have been no reports on sensors employing DET-type redox enzymes based on Faradaic EIS principle.In this study, we report the construction and characterization of direct electron transfer type Faradaic Enzyme EIS, as the novel biosensor employing DET-type redox enzymes without redox probe. The DET-type Faradaic Enzyme EIS employs flavin adenine dinucleotide (FAD) dependent glucose dehydrogenase (GDH) complex (FADGDH), which is consisted of three subunits: a catalytic subunit which has FAD and 3Fe-4S-type iron-sulfur cluster, an electron transfer subunit which has 3 hemes, and a small subunit as a hitchhiker protein. Thanks to the presence of the multi heme electron transfer subunit, FADGDH is capable of DET (ref.). Method DET-type FADGDH- modified or non-DET-type FADGDH (fungi derived FADGDH)- modified gold disk electrodes (GDEs) were constructed to elucidate its characteristic properties. FADGDHs were modified onto GDEs via self-assembled monolayer (SAM). FADGDHs modified GDEs were evaluated by EIS to analyse faradaic impedance towards different concentrations of glucose, with Ag/AgCl and Pt wire as the reference and counter electrodes, in the absence of redox probe. EIS measurements were performed in certain frequency range by AC voltage with an amplitude of 10 mV superimposed on DC potential. Obtained EIS data were analysed with an equivalent circuit to obtain electrochemical parameters. Result and Discussion When EIS was performed with DET-type FADGDH-modified GDE in the presence of glucose, the obtained semicircles on the Nyquist plots became smaller with increasing depending on glucose concentrations. In contrary, EIS analyses of the non-DET-type FADGDH-modified GDEs revealed that no impedance change was observed by changing glucose concentration. These results indicated that the impedance change with DET-type FADGDH-modified GDE was observed, which results the glucose concentration dependency of Rct values. Considering the observed high sensitivity toward glucose, DET-type Faradaic Enzyme EIS sensor would provide alternative platform for future impedimetric immunosensing system, which does not use redox probe. References Lee et al., Bioelectrochemistry 121, 1-6 (2018)Yamashita et al., Int. J. Mol. Sci. 19, 931 (2018)Shiota et al., Bioelectrochemistry 112, 178-183 (2016)Yamashita et al., Enzyme. Microb. Technol. 52, 123-128 (2013)Kakehi N et al., Biosens. Bioelectron., 22. 2250-2255 (2007)Tsuya et al., J. Biotechnol. 123, 127-136 (2006)Yamaoka et al., Biotechnol. Lett. 26, 1757-1761 (2004)Inose et al., Biochim. Biophys. Acta. 1645, 133-138 (2003) Figure 1

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 (Rct). The Δ(1/Rct) 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.

Cardiac Troponin I: Ultrasensitive Detection Using Faradaic Electrochemical Impedance.

An electrochemical biosensor for the detection of cardiac troponin I, cTnI, an important cardiac biomarker, is described. A combination of a novel monoclonal antibody, mAb20B3, and a novel Ir(III)-based metal complex was used for detection using faradaic electrochemical impedance spectroscopy. A limit of detection of 10 ag/mL was achieved, which is significantly lower than established assays. The ability to detect these ultralow concentrations enables rapid and early stage detection of cardiac events and opens up the possibility of developing a point-of-care device.

CLASP (Continuous lifestyle awareness through sweat platform): A novel sensor for simultaneous detection of alcohol and glucose from passive perspired sweat

Wearable- IOT based low- cost platforms can enable dynamic lifestyle monitoring through enabling promising and exciting opportunities for wellness and chronic- disease management in personalized environments. Diabetic and pre- diabetic populations can modulate their alcohol intake by tracking their glycemic content continuously to prevent health risks through these platforms. We demonstrate the first technological proof of a combinatorial biosensor for continuous, dynamic monitoring of alcohol and glucose in ultra- low volumes (1–5 µL) of passive perspired sweat towards developing a wearable- IOT based platform. Non-invasive biosensing in sweat is achieved by a unique gold- zinc oxide (ZnO) thin film electrode stack fabricated on a flexible substrate suitable for wearable applications. The active ZnO sensing region is immobilized with enzyme complexes specific for the detection of alcohol and glucose through non- faradaic electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA). Biomolecular interactions occurring at the electrode- sweat interface are represented by the impedance and capacitive current changes in response to charge modulations arising in the double layer. We also report the detection of alcohol concentrations of 0.01–100 mg/dl and glucose concentrations of 0.01–50 mg/dl present in synthetic sweat and perspired human sweat. The limit of detection obtained for alcohol and glucose was found to be 0.1 mg/dl in perspired human sweat. Cross- reactivity studies revealed that glucose and alcohol did not show any signal response to cross- reactive molecules. Furthermore, the stable temporal response of the combinatorial biosensor on continuous exposure to passive perspired human sweat spiked with alcohol and glucose over a 120-min duration was demonstrated.

Amyloid Beta Detection by Faradaic Electrochemical Impedance Spectroscopy Using Interdigitated Microelectrodes.

Faradaic electrochemical impedance spectroscopy (f-EIS) in the presence of redox reagent, e.g., [Fe(CN)6]3−/4−, is widely used in biosensors owing to its high sensitivity. However, in sensors detecting amyloid beta (Aβ), the redox reagent can cause the aggregation of Aβ, which is a disturbance factor in accurate detection. Here, we propose an interdigitated microelectrode (IME) based f-EIS technique that can alleviate the aggregation of Aβ and achieve high sensitivity by buffer control. The proposed method was verified by analyzing three different EIS-based sensors: non-faradaic EIS (nf-EIS), f-EIS, and the proposed f-EIS with buffer control. We analyzed the equivalent circuits of nf-EIS and f-EIS sensors. The dominant factors of sensitivity were analyzed, and the impedance change rates via Aβ reaction was compared. We measured the sensitivity of the IME sensors based on nf-EIS, f-EIS, and the proposed f-EIS. The results demonstrate that the proposed EIS-based IME sensor can detect Aβ with a sensitivity of 7.40-fold and 10.93-fold higher than the nf-EIS and the f-EIS sensors, respectively.

Polydopamine-functionalized graphene nanoplatelet smart conducting electrode for bio-sensing applications

The development of a novel polydopamine (PDA)-functionalized graphene nanoplatelets (GNPs)-based disposable sensor is described. The sensor was fabricated by drop-coating PDA@GNPs in polyethylene glycol (PEG) and poly(3,4-ethylenedioxythiophene (PEDOT):poly(styrenesulfonate) (PSS) aqueous suspension onto the working area of a screen-printed electrode (SPE). The final sensor, designated as PDA@GNPs/PPP/SPE, was characterized by scanning electron microscopy (SEM), Raman spectroscopy, Faradaic electrochemical impedance spectroscopy (FEIS) and cyclic voltammetry (CV). Mediated detection of hydrogen peroxide (H2O2) via the redox properties of PDA was achieved. It showed excellent selectivity and sensitivity towards H2O2 with a limit of detection and sensitivity of 0.55 µM (S/N = 3) and 3.0 µA mM−1 cm−2, respectively. Thereafter, glucose oxidase (GOx) was immobilized onto the electrode to develop GOx/PDA@GNPs/PPP/SPE sensor. The glucose biosensor exhibited a limit of detection of 0.25 μM (S/N = 3) and a sensitivity of 0.51 μA μM−1 cm−2; thus, proving its potential suitability for bio-sensing applications.

Selection and Characterization of DNA Aptamers for Electrochemical Biosensing of Carbendazim.

This article reports a novel aptamer-based impedimetric detection of carbendazim, a commonly used benzimidazole fungicide in agriculture. High affinity and specificity DNA aptamers against carbendazim were successfully selected using systematic evolution of ligand by exponential enrichment (SELEX). The dissociation constants (Kds) of the selected DNA aptamers after 10 in vitro selection cycles were characterized using fluorescence-based assays showing values in the nanomolar range. The aptamer which showed the highest degree of affinity and conformation change was used to fabricate an electrochemical aptasensor via self-assembly of thiol-modified aptamer on gold electrodes. The aptasensor exploits the specific recognition of carbendazim by the aptamer immobilized on the gold surface which leads to conformational changes in the aptamer structure. This conformational change alters the access of a ferrocyanide/ferricyanide redox couple to the aptasensor surface. The aptasensor response is thus measured by following the increase in the electron transfer resistance of the redox couple using Faradaic electrochemical impedance spectroscopy. This method allowed a selective and sensitive label-free detection of carbendazim within a range of 10 pg/mL-10 ng/mL with a limit of detection of 8.2 pg/mL. The aptasensor did not show cross reactivity with other commonly used pesticides such as fenamiphos, isoproturon, atrazine, linuron, thiamethoxam, trifluralin, carbaryl, and methyl parathion. Moreover, the aptasensor has been applied in different spiked food matrixes showing high recovery percentages. We believe that the proposed aptasensor is a promising alternative to the currently used methods for carbendazim monitoring.

Integration of Faradaic electrochemical impedance spectroscopy into a scalable surface plasmon biosensor for in tandem detection.

We present an integrated label-free biosensor based on surface plasmon resonance (SPR) and Faradaic electrochemical impedance spectroscopy (f-EIS) sensing modalities, for the simultaneous detection of biological analytes. Analyte detection is based on the angular spectroscopy of surface plasmon resonance and the extraction of charge transfer resistance values from reduction-oxidation reactions at the gold surface, as responses to functionalized surface binding events. To collocate the measurement areas and fully integrate the modalities, holographically exposed thin-film gold SPR-transducer gratings are patterned into coplanar electrodes for tandem impedance sensing. Mutual non-interference between plasmonic and electrochemical measurement processes is shown, and using our scalable and compact detection system, we experimentally demonstrate biotinylated surface capture of neutravidin concentrations as low as 10 nM detection, with a 5.5 nM limit of detection.

Functionalized CVD monolayer graphene for label-free impedimetric biosensing

Recent advances in large area graphene growth have led to many applications in different areas. In the present study, chemical vapor deposited (CVD) monolayer graphene supported on glass substrate was examined as electrode material for electrochemical biosensing applications. We report a facile strategy for covalent functionalization of CVD monolayer graphene by electrochemical reduction of carboxyphenyl diazonium salt prepared in situ in acidic aqueous solution. The carboxyphenyl-modified graphene is characterized using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), as well as electrochemical impedance spectroscopy (EIS). We also show that the number of grafted carboxyphenyl groups on the graphene surface can be controlled by the number of cyclic voltammetry (CV) scans used for electrografting. We further present the fabrication and characterization of an immunosensor based on immobilization of ovalbumin antibody on the graphene surface after the activation of the grafted carboxylic groups via EDC/NHS chemistry. The binding between the surface-immobilized antibodies and ovalbumin was then monitored using Faradaic EIS in [Fe(CN)6]3−/4− solution. The percentage change of charge transfer resistance (R ct) after binding exhibited a linear dependence for ovalbumin concentrations ranging from 1.0 pg·mL−1 to 100 ng·mL−1, with a detection limit of 0.9 pg·mL−1. Our results indicate good sensitivity of the developed functionalized CVD graphene platform, paving the way for using CVD monolayer graphene in a variety of electrochemical biosensing devices.

Novel Single Gold Nanowire-based Electrochemical Immunosensor for Rapid Detection of Bovine Viral Diarrhoea Antibodies in Serum

Bovine Viral Diarrheoa (BVD) is a worldwide disease with severe financial implications for the Bovine beef and dairy industries. A key challenge to BVD eradication is that the requirement to send samples to, and receive results from, specialized laboratories slows down the diagnostic process and leads to uncontrolled spread of the virus within a herd until diagnostic confirmation is received. Consequently, rapid identification of BVD is now critical for herd protection and prevention of costly herd outbreaks and new diagnostic devices, suitable for on-farm analysis, that deliver rapid and early identification of animal disease states, are required. We report, here, an electrochemical onchip fully integrated nanowire based immunosensor device for detection of BVD in serum. The capture biomolecule, BVD virus, is covalently immobilized via a carboxylic terminated polymer firstly electrodeposited onto a single nanowire. Electrochemical characterization including faradaic electrochemical impedance spectroscopy and cyclic voltammetry is performed. Label free immunologic detection of antibodies (10 μg/mL, 20 min) is first demonstrated using a bovine serum albumin as a model antigen-antibody system. Then, the immunosensor is applied to detection of bovine viral diarrhoea antibodies (10 μg/mL, 20 min) in both buffer and serum. The sensor clearly discriminates between positive and negative infected bovine sera. This study clearly shows the potential of this chip nanowirebased electrochemical sensor for immunoassays application in serum with a view to developing portable devices for on-farm diagnosis or therapeutic monitoring in animal health applications.