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Miniaturization and wireless continuous glucose monitoring are key factors for the successful management of diabetes. Electrochemical sensors are very versatile and can be easily miniaturized for wireless glucose monitoring. The authors report a microneedle-based enzyme-free electrochemical wireless sensor for painless and continuous glucose monitoring. The microneedles (MNs) fabricated consist of a 3 × 5 sharp and stainless-steel electrode array configuration. Each MN in the 3 × 5 array has 575 µm × 150 µm in height and width, respectively. A glucose-catalyzing layer, porous platinum black, was electrochemically deposited on the tips of the MNs by applying a fixed cathodic current of 2.5 mA cm -2 for a period of 200 s. For the non-interference glucose sensing, the platinum (Pt)-black-coated MN was carefully packaged into a biocompatible ionomer, nafion. The surface morphologies of the bare and modified MNs were studied using field-emission scanning electron microscopy (FESEM) and energy-dispersive X-ray analysis (EDX). The wireless glucose sensor displayed a broad linear range of glucose (1→30 mM), a good sensitivity and higher detection limit of 145.33 μA mM -1 cm -2 and 480 μM, respectively, with bare AuMN as a counter electrode. However, the wireless device showed an improved sensitivity and enhanced detection limit of 445.75, 165.83 μA mM -1 cm -2 and 268 μM, respectively, with the Pt-black-modified MN as a counter electrode. The sensor also exhibited a very good response time (2 s) and a limited interference effect on the detection of glucose in the presence of other electroactive oxidizing species, indicating a very fast and interference-free chronoamperometric response.

Introduction: Alzheimer's disease (AD), a heterogeneous pathological process representing the most common causes of dementia worldwide, has required early and accurate diagnostic tools. Neuropathological hallmarks of AD involve the aberrant accumulation of Amyloid beta (Aβ) into Amyloid plaques and hyperphosphorylated Tau into neurofibrillary tangles, occurring long before the onset of brain dysfunction. Areas covered: Considering the significance of Aβ and Tau in AD pathogenesis, these proteins have been adopted as core biomarkers of AD, and their quantification has provided precise diagnostic information to develop next-generation AD therapeutic approaches. However, conventional diagnostic methods may not suffice to achieve clinical criteria that are acceptable for proper diagnosis and treatment. The advantages of nanomaterial-based biosensors including facile miniaturization, mass fabrication, ultra-sensitivity, make them useful to be promising tools to measure Aβ and Tau simultaneously for accurate validation of low-abundance yet potentially informative biomarkers of AD.., Expert Opinion: The study has identified the potential application of advanced biosensors as standardized clinical diagnostic tools for AD, evolving the way for new and efficient AD control with minimum economic and social burden. After clinical trial, nanobiosensors for measuring Aβ and Tau simultaneously possess innovative diagnosis of AD to provide significant contributions to primary Alzheimer's care intervention.

Rheumatoid arthritis (RA) is a chronic autoimmune disease that produces a progressive inflammatory response that leads to severe pain, swelling, and stiffness in the joints of hands and feet, followed by irreversible damage of the joints. The authors developed a miniaturized, label-free electrochemical impedimetric immunosensor for the sensitive and direct detection of arthritis Anti-CCP-ab biomarker. An interdigitated-chain-shaped microelectrode array (ICE) was fabricated by taking the advantage of microelectromechanical systems. The fabricated ICE was modified with a self-assembled monolayer (SAM) of Mercaptohexanoic acid (MHA) for immobilization of the synthetic peptide bio-receptor (B-CCP). The B-CCP was attached onto the surface of SAM modified ICE through a strong avidin-biotin bio-recognition system. The modified ICE surface with the SAM and bio-molecules (Avidin, B-CCP, Anti-CCP-ab and BSA) was morphologically and electrochemically characterized. The change in the sensor signal upon analyte binding on the electrode surface was probed through the electrochemical impedance spectroscopy (EIS) property of charge-transfer resistance (R ct ) of the modified electrodes. EIS measurements were target specific and the sensor response was linearly increased with step wise increase in target analyte (Anti-CCP-ab) concentrations. The developed sensor showed a linear range for the addition of Anti-CCP-ab between 1 IU mL -1 → 800 IU mL -1 in phosphate buffered saline (PBS) and Human serum (HS), respectively. The sensor showed a limit of detection of 0.60 IU mL -1 and 0.82 IU mL -1 in the PBS and HS, respectively. The develop bio-electrode showed a good reproducibility (relative standard deviation (RSD), 1.52%), selectivity and stability (1.5% lost at the end of 20th day) with an acceptable recovery rate (98.0% → 101.18%) and % RSD's for the detection of Anti-CCP-ab in spiked HS samples.

Individuals with diabetes can benefit considerably from continuous blood glucose monitoring. To address this challenge, a proof-of-concept was performed for continuous glucose monitoring (CGM) based on an enzymeless porous nanomaterial (pNM)-modified microneedle electrode array (MNEA). The pNM sensing layer was electrochemically deposited on MNs by applying a fixed negative current of -2.5 mA cm -2 for 400 s. The pNM-modified MNEA was packed using a biocompatible Nafion ionomer. The fabricated MNEAs were 600 × 100 × 150 µm in height, width, and thickness, respectively. The surfaces of the modified MNs were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The fabricated MNEAs showed a wide dynamic range (1-30 mM) in phosphate-buffered saline (PBS) and in artificial interstitial fluid (ISF), with good sensitivities (PBS: 1.792 ± 0.25 µA mM -1 cm -2 , ISF: 0.957 ± 0.14 µA mM -1 cm -2 ) and low detection limits (PBS: 7.2 µM, ISF: 22 µM). The sensor also showed high stability (loss of 3.5% at the end of 16 days), selectivity, and reproducibility (Relative standard deviations (RSD) of 1.64% and 0.70% for intra- and inter-assay, respectively) and a good response time (2 s) with great glucose recovery rates in ISF (98.7-102%)., Competing Interests: The authors declare no conflict of interest.

Rheumatoid arthritis and osteoarthritis can be treated through specific drug injection into the intra-articular space. Several failures during drug injection attempts with conventional fluoroscopy and ultrasonography in a small area of the intra-articular space have been reported. In this work we present an innovative impedance measurement-based method/algorithm for needle tip positioning to enhance image-guided intra-articular vaccination treatment. A novel algorithm for detecting the intra-articular space in the elbow and knee joints of a live porcine model is reported. An impedance measurement system was developed for biological tissue measurement. The electrical impedance in the intra-articular space was monitored and the needle tip was examined by ultrasonography. The contrast dye was vaccinated and checked using fluoroscopy to confirm that the dye was properly inoculated in the cavity. The electrical impedance was estimated for various needle inclusion profundity levels in saline solution, which were broadly used to evaluate the proposed device for in vivo examinations. Good efficiency was observed in the impedance-based measurements using a monopolar injection needle for intra-articular therapy. To enhance the needle tip positioning for intra-articular therapy, the intended impedance measurement device with a monopolar injection needle can be used as a complement to existing modalities.

An rGO-Ag@SiO 2 nanocomposite-based electrochemical sensor was developed to detect etidronic acid (EA) using the differential pulse voltammetric (DPV) technique. Rapid self-assembly of the rGO-Ag@SiO 2 nanocomposite was accomplished through probe sonication. The developed rGO-Ag@SiO 2 nanocomposite was used as an electrochemical sensing platform by drop-casting on a gold (Au) printed circuit board (PCB). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) confirmed the enhanced electrochemical active surface area (ECASA) and low charge transfer resistance (R ct ) of the rGO-Ag@SiO 2 /Au PCB. The accelerated electron transfer and the high number of active sites on the rGO-Ag@SiO 2 /Au PCB resulted in the electrochemical detection of EA through the DPV technique with a limit of detection (LOD) of 0.68 μM and a linear range of 2.0-200.0 μM. The constructed DPV sensor exhibited high selectivity toward EA, high reproducibility in terms of different Au PCBs, excellent repeatability, and long-term stability in storage at room temperature (25 °C). The real-time application of the rGO-Ag@SiO 2 /Au PCB for EA detection was investigated using EA-based pharmaceutical samples. Recovery percentages between 96.2% and 102.9% were obtained. The developed DPV sensor based on an rGO-Ag@SiO 2 /Au PCB could be used to detect other electrochemically active species following optimization under certain conditions.

An electrochemical capacitance immunosensor based on an interdigitated wave-shaped micro electrode array (IDWµE) for direct and label-free detection of C-reactive protein (CRP) was reported. A self-assembled monolayer (SAM) of dithiobis (succinimidyl propionate) (DTSP) was used to modify the electrode array for antibody immobilization. The SAM functionalized electrode array was characterized morphologically by atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDX). The nature of gold-sulfur interactions on SAM-treated electrode array was probed by X-ray photoelectron spectroscopy (XPS). The covalent linking of anti-CRP-antibodies onto the SAM modified electrode array was characterized morphologically through AFM, and electrochemically through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The application of phosphate-buffered saline (PBS) and human serum (HS) samples containing different concentrations of CRP in the electrode array caused changes in the electrode interfacial capacitance upon CRP binding. CRP concentrations in PBS and HS were determined quantitatively by measuring the change in capacitance (ΔC) through EIS. The electrode immobilized with anti-CRP-antibodies showed an increase in ΔC with the addition of CRP concentrations over a range of 0.01-10,000 ng mL -1 . The electrode showed detection limits of 0.025 ng mL -1 and 0.23 ng mL -1 (S/N = 3) in PBS and HS, respectively. The biosensor showed a good reproducibility (relative standard deviation (RSD), 1.70%), repeatability (RSD, 1.95%), and adequate selectivity in presence of interferents towards CRP detection. The sensor also exhibited a significant storage stability of 2 weeks at 4 °C in 1× PBS.

In this study, we developed a small size, low cost, highly sensitive electrochemical biosensor with a low limit of detection by immobilizing specific anti-amyloid-β (aβ) antibody onto a self-assembled monolayer functionalized interdigitated chain-shaped electrode (anti-aβ/EDC-NHS/SAM/ICE). The anti-aβ/EDC-NHS/SAM/ICE specifically detects aβ 1-42 peptide (a peptide 1-42 amino acids long), which is one of main biomarkers of Alzheimer's disease in human serum (HS). Electrochemical impedance spectroscopy (EIS) was used to characterize the impedance change of the anti-aβ/EDC-NHS/SAM/ICE biosensor for aβ 1-42 detection, which provided a wide linear range of detection from 10 -3 -10 3  ng mL -1 , and a low limit of detection of aβ in HS (100 pg mL -1 ) much lower than the limit of detection of CSF aβ 1-42 (∼500 pg mL -1 ), and other biosensors. Therefore, the developed biosensor is sensitive enough to be used for the diagnosis of early stage Alzheimer's disease., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The authors report a label-free and direct detection of rheumatoid factor- Immunoglobulin M (IgM-RF) based on an impedimetric-interdigitated wave type microelectrode array (IDWμE). IDWμE was functionalized with a self-assembled monolayer (SAM) of thioctic acid for antigen immobilization. The SAM functionalized IDWμE were characterized by atomic force microscopy, Energy Dispersive X-Ray Spectroscopy, and X-ray photoelectron spectroscopy. The covalent immobilization of IgG-Fc onto the SAM modified electrode arrays was characterized morphologically via AFM and electrochemically via cyclic voltammetry and electrochemical impedance spectroscopy. Impedimetric measurements in the presence of redox probe (Potassium ferrocyanide and potassium ferricyanide) showed a significant change in both the impedance spectrum and charge transfer resistance upon IgM-RF binding. Impedance measurements were target specific and linear with an increase in IgM-RF concentrations between 1 and 200 IU mL -1 in redox probe and human serum, respectively. The sensor showed detection limits of 0.6 IU mL -1 in the presence of redox probe and 0.22 IU mL -1 in the human serum. The biosensor exhibited good reproducibility (relative standard deviation (RSD), 4.96%) and repeatability (RSD, 2.31%) with an acceptable selectivity towards IgM-RF detection. The sensor also showed a good stability for 3 weeks at 4 °C in 1X PBS. Therefore, the sensitive and stable immunosensor exhibiting IDWμE features can be integrated with a miniaturized potentiostat to develop a sensing system that detects IgM-RF for point-of-care applications., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Electrogenerated chemiluminescence (ECL) is an effective method for detecting a wide range of analytes including metal ions, virulent DNA, pathogenic bacteria, tumor cells and glucose. The attractive features of paper including passive liquid transport and biocompatibility are the main two advantages of using paper as a biosensing platform. To achieve key factors in paper-based sensors, the fabrication procedures and the analysis methods are fine tuned to satisfy the requirements of the ultimate-users. Here, we review various ECL signal amplification labels, inexpensive and portable devices, such as rechargeable batteries, which have replaced traditional instrumentation and different light detection technologies used in paper ECL devices. We also highlight the current trends and developments in ECL paper-based microfluidic analytical devices, as well as recent applications of ECL-based detection methods and inexpensive microfluidic devices. We discuss various paper-based devices, including 3D-origami devices, and devices utilizing self-powered and bipolar electrodes. Significant efforts have also been dedicated towards paper based multiplexing analysis (multi-label, and the multi-analyte strategies) and integration of microfluidic lab-on-paper devices with competences for point-to-care diagnostics. This review finally tabulates systematized data on figures of merit and novel types of ECL labels, used for detection of various biomarkers and analytes., (Copyright © 2018 Elsevier B.V. All rights reserved.)

The authors report on a microneedle-based amperometric nonenzymatic glucose sensor for painless and continuous monitoring of glucose. It consists of 3 × 5 sharp stainless steel microneedles micromachined from a stainless steel substrate. The microneedles are 600 and 100 μm in height and width, respectively. Nafion and platinum black were sequentially coated onto the tip of gold-coated microneedles and used for nonenzymatic (direct) sensing of glucose. Attractive features of the modified microneedle electrode include (a) a low working potential (+0.12 V vs. Ag/AgCl), (b) a linear response in the physiologically relevant range (1-40 mM), (c) a sensitivity as high as 175 μA mM -1  cm -2 , (d) a 23 μM detection limit, and (e) a response time of 2 s. The sensor also exhibits good reproducibility and stability. The sensor is selective for glucose even in the presence of 10-fold higher concentrations of ascorbic acid, lactic acid, dopamine, uric acid, and acetaminophen. Graphical abstract Schematic representation of the fabrication sequence for a nonenzymatic electrochemical glucose sensor using Nafion and platinum black coated microneedle electrode array. The sensor is based on measuring the faradaic current at +0.12 V vs. Ag/AgCl by the direct electrochemical oxidation of glucose to gluconic acid on the surface of a Pt black sensing layer.

Human serum albumin (HSA)-stabilized Au18 nanoclusters (AuNCs) were synthesized and chemically immobilized on an Indium tin oxide (ITO) plate. The assembly process was characterized by advanced electrochemical and spectroscopic techniques. The bare ITO electrode generated three irreversible oxidation peaks, whereas the HSA-AuNC-modified electrode produced a pair of redox peaks for bilirubin at a formal potential of 0.27V (vs. Ag/AgCl). However, the native HSA protein immobilized on the ITO electrode failed to produce any redox peak for bilirubin. The results indicate that the AuNCs present in HSA act as electron transfer bridge between bilirubin and the ITO plate. Docking studies of AuNC with HSA revealed that the best docked structure of the nanocluster is located around the vicinity of the bilirubin binding site, with an orientation that allows specific oxidation. When the HSA-AuNC-modified electrode was employed for the detection of bilirubin using chronoamperometry at 0.3V (vs. Ag/AgCl), a steady-state current response against bilirubin in the range of 0.2μM to 7μM, with a sensitivity of 0.34μAμM(-1) and limit of detection of 86.32nM at S/N 3, was obtained. The bioelectrode was successfully applied to measure the bilirubin content in spiked serum samples. The results indicate the feasibility of using HSA-AuNC as a biorecognition element for the detection of serum bilirubin levels using an electrochemical technique., (Copyright © 2016 Elsevier B.V. All rights reserved.)

The electrochemical biosensor with enzyme as biorecognition element is traditionally pursued as an attractive research topic owing to their high commercial perspective in healthcare and environmental sectors. The research interest on the subject is sharply increased since the beginning of 21st century primarily, due to the concomitant increase in knowledge in the field of material science. The remarkable effects of many advance materials such as, conductive polymers and nanomaterials, were acknowledged in the developing efficient 3rd generation enzyme bioelectrodes which offer superior selectivity, sensitivity, reagent less detection, and label free fabrication of biosensors. The present review article compiles the major knowledge surfaced on the subject since its inception incorporating the key review and experimental papers published during the last decade which extensively cover the development on the redox enzyme based 3rd generation electrochemical biosensors. The tenet involved in the function of these direct electrochemistry based enzyme electrodes, their characterizations and various strategies reported so far for their development such as, nanofabrication, polymer based and reconstitution approaches are elucidated. In addition, the possible challenges and the future prospects in the development of efficient biosensors following this direct electrochemistry based principle are discussed. A comparative account on the design strategies and critical performance factors involved in the 3rd generation biosensors among some selected prominent works published on the subject during last decade have also been included in a tabular form for ready reference to the readers., (Copyright © 2015 Elsevier B.V. All rights reserved.)

A simple one step method for the alcohol oxidases (AOx) protein mediated synthesis of gold nano-particles (AuNPs) in alkaline (pH 8.5) condition with simultaneous stabilization of the nanoparticles on the AOx protein surface under native environment has been developed. The formation of the AOx conjugated AuNPs was confirmed by advanced analytical and spectroscopic techniques. The significant increase in zeta potential (ζ) value of -57mV for the synthesized AOx-AuNPs conjugate from the AOx (pI 4.5) protein (ζ, -30mV) implied good stability of the in-situ synthesized nano-conjugate. The AOx-AuNPs conjugate showed steady stability in alkaline (upto pH 8.5) and NaCl (up to 10(-1)M) solutions. The efficiency (Kcat/Km) of the AuNP conjugated AOx was increased by 18% from the free enzyme confirming the activating role of the surface stabilized AuNPs for the enzyme. The AuNPs-AOx conjugate was encapsulated with polyaniline (PANI) synthesized by oxidative polymerization of aniline using H2O2 generated in-situ from the AOx catalysed oxidation of alcohol. The PANI encapsulated AuNPs-AOx assembly was stabilized on a glassy carbon electrode (GCE) by chitosan-Nafion mixture and then utilized the fabricated bioelectrode for detection of alcohol amperometrically using H2O2 as redox indicator at +0.6V. The constructed biosensor showed high operational stability (6.3% loss after 25 measurements), wide linear detection range of 10µM-4.7mM (R(2)=0.9731), high sensitivity of 68.3±0.35µAmM(-1) and low detection limit of 7±0.027µM for ethanol. The fabricated bioelectrode was successfully used for the selective determination of alcohol in beverage samples., (Copyright © 2015 Elsevier B.V. All rights reserved.)

We report here a fluorescence quenching based non-enzymatic method for sensitive and reliable detection of free bilirubin in blood serum samples using human serum albumin (HSA) stabilized gold nanoclusters (HSA-AuNCs) as fluorescent probe. The fluorescence of the nanoclusters was strongly quenched by bilirubin in a concentration dependent manner by virtue of the inherent specific interaction between bilirubin and HSA. A strong binding constant of 0.55×10(6) L mole(-1) between the HSA-AuNC and bilirubin was discerned. The nano clusters each with size ~1.0 nm (in diameter) and a core of Au18 were homogeneously distributed in HSA molecules as revealed from the respective high resolution transmission electron microscopic and mass spectroscopic studies. The fluorescence quenching phenomena which obeyed a simple static quenching mechanism, was utilized for interference free detection of bilirubin with minimum detection limit (DL) of 248±12 nM (S/N=3). The fluorescence response of HSA-AuNCs against bilirubin was practically unaltered over a wide pH (6-9) and temperature (25-50 °C) range. Additionally, peroxidase-like catalytic activity of these nanoclusters was exploited for colorimetric detection of bilirubin in serum sample with a DL of 200±19 nM by following the decrease in absorbance (at λ440 nm) of the reaction and its rate constant (Kp) of 2.57±0.63 mL μg(-1) min(-1). Both these fluorometric and colorimetric methods have been successfully used for detection of free bilirubin in blood serum samples., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Alcohol oxidase (AOx) with a two-fold increase in efficiency (Kcat/Km) was achieved by physical entrapment of the activator ferrocene in the protein matrix through a simple microwave based partial unfolding technique and was used to develop a 3rd generation biosensor for improved detection of alcohol in liquid samples. The ferrocene molecules were stably entrapped in the AOx protein matrix in a molar ratio of ~3:1 through electrostatic interaction with the Trp residues involved in the functional activity of the enzyme as demonstrated by advanced analytical techniques. The sensor was fabricated by immobilizing ferrocene entrapped alcohol oxidase (FcAOx) and sol-gel chitosan film coated horseradish peroxidase (HRP) on a multi-walled carbon nanotube (MWCNT) modified glassy carbon electrode through layer-by-layer technique. The bioelectrode reactions involved the formation of H2O2 by FcAOx biocatalysis of substrate alcohol followed by HRP-catalyzed reduction of the liberated H2O2 through MWCNT supported direct electron transfer mechanism. The amperometric biosensor exhibited a linear response to alcohol in the range of 5.0 × 10(-6) to 30 × 10(-4)mol L(-1) with a detection limit of 2.3 × 10(-6) mol L(-1), and a sensitivity of 150 µA mM(-1) cm(-2). The biosensor response was steady for 28 successive measurements completed in a period of 5h and retained ~90% of the original response even after four weeks when stored at 4 °C. The biosensor was successfully applied for the determination of alcohol in commercial samples and its performance was validated by comparing with the data obtained by GC analyses of the samples., (© 2013 Published by Elsevier B.V.)

The alcohol oxidase (AOx) cDNA from Aspergillus terreus MTCC6324 with an open reading frame (ORF) of 2001 bp was constructed from n-hexadecane induced cells and expressed in Escherichia coli with a yield of ∼4.2 mg protein g-1 wet cell. The deduced amino acid sequences of recombinant rAOx showed maximum structural homology with the chain B of aryl AOx from Pleurotus eryngii. A functionally active AOx was achieved by incubating the apo-AOx with flavin adenine dinucleotide (FAD) for ∼80 h at 16°C and pH 9.0. The isoelectric point and mass of the apo-AOx were found to be 6.5±0.1 and ∼74 kDa, respectively. Circular dichroism data of the rAOx confirmed its ordered structure. Docking studies with an ab-initio protein model demonstrated the presence of a conserved FAD binding domain with an active substrate binding site. The rAOx was specific for aryl alcohols and the order of its substrate preference was 4-methoxybenzyl alcohol >3-methoxybenzyl alcohol>3, 4-dimethoxybenzyl alcohol > benzyl alcohol. A significantly high aggregation to ∼1000 nm (diameter) and catalytic efficiency (kcat/Km) of 7829.5 min-1 mM-1 for 4-methoxybenzyl alcohol was also demonstrated for rAOx. The results infer the novelty of the AOx and its potential biocatalytic application.