An Electrochemical Impedance Immunosensor Based on Gold Nanoparticle-Modified Electrodes for the Detection of HbA1c in Human Blood

Emerging biosensor probes for glycated hemoglobin (HbA1c) detection

Glycated hemoglobin (HbA1c), originating from the non-enzymatic glycosylation of βVal1 residues in hemoglobin (Hb), is an essential biomarker indicating average blood glucose levels over a period of 2 to 3 months without external environmental disturbances, thereby serving as the gold standard in the management of diabetes instead of blood glucose testing. The emergence of HbA1c biosensors presents affordable, readily available options for glycemic monitoring, offering significant benefits to small-scale laboratories and clinics. Utilizing nanomaterials coupled with high-specificity probes as integral components for recognition, labeling, and signal transduction, these sensors demonstrate exceptional sensitivity and selectivity in HbA1c detection. This review mainly focuses on the emerging probes and strategies integral to HbA1c sensor development. We discussed the advantages and limitations of various probes in sensor construction as well as recent advances in diverse sensing strategies for HbA1c measurement and their potential clinical applications, highlighting the critical gaps in current technologies and future needs in this evolving field.

Affinity-based electrochemical sensors for biomolecular detection in whole blood

The detection and/or quantification of biomarkers in blood is important for the early detection, diagnosis, and treatment of a variety of diseases and medical conditions. Among the different types of sensors for detecting molecular biomarkers, such as proteins, nucleic acids, and small-molecule drugs, affinity-based electrochemical sensors offer the advantages of high analytical sensitivity and specificity, fast detection times, simple operation, and portability. However, biomolecular detection in whole blood is challenging due to its highly complex matrix, necessitating sample purification (i.e., centrifugation), which involves the use of bulky, expensive equipment and tedious sample-handling procedures. To address these challenges, various strategies have been employed, such as purifying the blood sample directly on the sensor, employing micro-/nanoparticles to enhance the detection signal, and coating the electrode surface with blocking agents to reduce nonspecific binding, to improve the analytical performance of affinity-based electrochemical sensors without requiring sample pre-processing steps or laboratory equipment. In this article, we present an overview of affinity-based electrochemical sensor technologies that employ these strategies for biomolecular detection in whole blood.

Trends in Quantification of HbA1c Using Electrochemical and Point-of-Care Analyzers

Glycated hemoglobin (HbA1c), one of the many variants of hemoglobin (Hb), serves as a standard biomarker of diabetes, as it assesses the long-term glycemic status of the individual for the previous 90-120 days. HbA1c levels in blood are stable and do not fluctuate when compared to the random blood glucose levels. The normal level of HbA1c is 4-6.0%, while concentrations > 6.5% denote diabetes. Conventionally, HbA1c is measured using techniques such as chromatography, spectroscopy, immunoassays, capillary electrophoresis, fluorometry, etc., that are time-consuming, expensive, and involve complex procedures and skilled personnel. These limitations have spurred development of sensors incorporating nanostructured materials that can aid in specific and accurate quantification of HbA1c. Various chemical and biological sensing elements with and without nanoparticle interfaces have been explored for HbA1c detection. Attempts are underway to improve the detection speed, increase accuracy, and reduce sample volumes and detection costs through different combinations of nanomaterials, interfaces, capture elements, and measurement techniques. This review elaborates on the recent advances in the realm of electrochemical detection for HbA1c detection. It also discusses the emerging trends and challenges in the fabrication of effective, accurate, and cost-effective point-of-care (PoC) devices for HbA1c and the potential way forward.

Printed Antifouling Electrodes for Biosensing Applications

Biosensors based on miniaturized, functional electrodes are of high potential for various biosensing applications, especially at the point-of-care setting among others. However, the sensor performance of such electrochemical devices is still strongly limited, especially due to surface fouling in complex sample fluids, such as blood serum. Electrode coatings based on conductive nanomaterials embedded in antifouling matrices offer a promising strategy to overcome this limitation. However, known composite coatings require long (typically >24 h) and complex fabrication processes, which pose a strong barrier for cost-effective mass manufacturing and successful commercialization. Here, we describe a novel polymer/carbon nanotube (CNT) composite coating that can be produced from an ink containing a photoreactive and antifouling copolymer as well as conductive CNTs using fast and highly scalable printing processes. Coatings were prepared on screen-printed electrodes and characterized using cyclic voltammetry (CV) and protein fouling experiments. The coatings offered an electroactive surface area (EASA) comparable to uncoated screen-printed electrodes and retained >90% of initial EASA after 1 h of exposure to concentrated bovine serum albumin solution, while uncoated electrodes decreased to <20% of initial EASA after the same treatment. Utilizing the universal crosslinking reaction of the polymer coating, antibodies against the inflammatory biomarker C-reactive protein (CRP) were photochemically immobilized on the electrodes. Functionalized electrodes were fabricated in <2 h and were successfully used to quantify nanogram-range concentrations of CRP spiked in undiluted human blood serum using a sandwich-immunoassay with electrochemical read-out, demonstrating the high potential of the platform for biosensing applications.

Advances in Nanomaterial-Based Biosensors for Determination of Glycated Hemoglobin

Diabetes is a major public health burden whose prevalence has been steadily increasing over the past decades. Glycated hemoglobin (HbA1c) is currently the gold standard for diagnostics and monitoring glycemic control in diabetes patients. HbA1c biosensors are often considered to be cost-effective alternatives for smaller testing laboratories or clinics unable to access other reference methods. Many of these sensors deploy nanomaterials as recognition elements, detection labels, and/or transducers for achieving sensitive and selective detection of HbA1c. Nanomaterials have emerged as important sensor components due to their excellent optical and electrical properties, tunable morphologies, and easy integration into multiple sensing platforms. In this review, we discuss the advantages of using nanomaterials to construct HbA1c sensors and various sensing strategies for HbA1c measurements. Key gaps between the current technologies with what is needed moving forward are also summarized.

An advanced molecularly imprinted electrochemical sensor for the highly sensitive and selective detection and determination of Human IgG

An advanced molecularly imprinted electrochemical sensor with high sensitivity and selectivity for the detection of Human immunoglobulin G (IgG) was successfully constructed. With acrylamide imprinting systems, surface imprinting on the nanoparticles CuFe₂O₄ targeted at IgG was employed to prepare molecularly imprinted polymer, which served as recognition element for the electrochemical sensor. Furthermore, the sensor harnessed a molybdenum disulfide (MoS₂)@nitrogen doped graphene quantum dots (N-GQDs) with ionic liquid (IL) nanocomposite for signal amplification. Under optimized experimental conditions, the sensor shortened the response time to less than 8 min, and the response was linear at the IgG concentration of 0.1-50 ng·mL^-1 with a low detection limit of 0.02 ng·mL^-1 (S/N = 3). Our findings suggested that, the sensor exhibited high detectability and long-time stability. The satisfactory results of human serum sample analysis showed that the developed IgG sensor had promising potential clinical applications in detecting IgG content.

A wearable lab-on-a-patch platform with stretchable nanostructured biosensor for non-invasive immunodetection of biomarker in sweat

Conformable, wearable biosensor-integrated systems are a promising approach to non-invasive and quantitative on-body detection of biomarkers in body fluids. However, realizing such a system has been slowed by the difficulty of fabricating a soft affinity-based biosensor patch capable of precise on-body fluid handling with minimal wearer intervention and a simple measurement protocol. Herein, we demonstrate a conformable, wearable lab-on-a-patch (LOP) platform composed of a stretchable, label-free, impedimetric biosensor and a stretchable microfluidic device for on-body detection of the hormone biomarker, cortisol. The all-in-one, stretchable microfluidic device can precisely collect and deliver sweat for cortisol quantitation and offers one-touch operation of reagent delivery for simultaneous electrochemical signal generation and washing. Three-dimensional nanostructuring of the Au working electrode enables the high sensitivity required to detect the pM-levels of cortisol in sweat. Our integrated LOP detected sweat cortisol quantitatively and accurately during exercise. This LOP will open a new horizon for non-invasive, highly sensitive, and quantitative on-body immunodetection for wearable personal diagnostics.

Different strategies for detection of HbA1c emphasizing on biosensors and point-of-care analyzers

Measurement of glycosylated hemoglobin (HbA1c) is a gold standard procedure for assessing long term glycemic control in individuals with diabetes mellitus as it gives the stable and reliable value of blood glucose levels for a period of 90-120 days. HbA1c is formed by the non-enzymatic glycation of terminal valine of hemoglobin. The analysis of HbA1c tends to be complicated because there are more than 300 different assay methods for measuring HbA1c which leads to variations in reported values from same samples. Therefore, standardization of detection methods is recommended. The review outlines the current research activities on developing assays including biosensors for the detection of HbA1c. The pros and cons of different techniques for measuring HbA1c are outlined. The performance of current point-of-care HbA1c analyzers available on the market are also compared and discussed. The future perspectives for HbA1c detection and diabetes management are proposed.

Current Status of HbA1c Biosensors

Glycated hemoglobin (HbA1c) is formed via non-enzymatic glycosylation reactions at the α–amino group of βVal1 residues in the tetrameric Hb, and it can reflect the ambient glycemic level over the past two to three months. A variety of HbA1c detection methods, including chromatography, immunoassay, enzymatic measurement, electrochemical sensor and capillary electrophoresis have been developed and used in research laboratories and in clinics as well. In this review, we summarize the current status of HbA1c biosensors based on the recognition of the sugar moiety on the protein and also their applications in the whole blood sample measurements.

Recent Advances in Electrochemical Immunosensors

Immunosensors have experienced a very significant growth in recent years, driven by the need for fast, sensitive, portable and easy-to-use devices to detect biomarkers for clinical diagnosis or to monitor organic pollutants in natural or industrial environments. Advances in the field of signal amplification using enzymatic reactions, nanomaterials such as carbon nanotubes, graphene and graphene derivatives, metallic nanoparticles (gold, silver, various oxides or metal complexes), or magnetic beads show how it is possible to improve collection, binding or transduction performances and reach the requirements for realistic clinical diagnostic or environmental control. This review presents these most recent advances; it focuses first on classical electrode substrates, then moves to carbon-based nanostructured ones including carbon nanotubes, graphene and other carbon materials, metal or metal-oxide nanoparticles, magnetic nanoparticles, dendrimers and, to finish, explore the use of ionic liquids. Analytical performances are systematically covered and compared, depending on the detection principle, but also from a chronological perspective, from 2012 to 2016 and early 2017.

Advances on Aryldiazonium Salt Chemistry Based Interfacial Fabrication for Sensing Applications

Aryldiazonium salts as coupling agents for surface chemistry have evidenced their wide applications for the development of sensors. Combined with advances in nanomaterials, current trends in sensor science and a variety of particular advantages of aryldiazonium salt chemistry in sensing have driven the aryldiazonium salt-based sensing strategies to grow at an astonishing pace. This review focuses on the advances in the use of aryldiazonium salts for modifying interfaces in sensors and biosensors during the past decade. It will first summarize the current methods for modification of interfaces with aryldiazonium salts, and then discuss the sensing applications of aryldiazonium salts modified on different transducers (bulky solid electrodes, nanomaterials modified bulky solid electrodes, and nanoparticles). Finally, the challenges and perspectives that aryldiazonium salt chemistry is facing in sensing applications are critically discussed.

Bioinspired Assemblies and Plasmonic Interfaces for Electrochemical Biosensing

Electrochemical biosensing represents a collection of techniques that may be utilized for capture and detection of biomolecules in both simple and complex media. While the instrumentation and technological aspects play important roles in detection capabilities, the interfacial design aspects are of equal importance, and often, those inspired by nature produce the best results. This review highlights recent material designs, recognition schemes, and method developments as they relate to targeted electrochemical analysis for biological systems. This includes the design of electrodes functionalized with peptides, proteins, nucleic acids, and lipid membranes, along with nanoparticle mediated signal amplification mechanisms. The topic of hyphenated surface plasmon resonance assays is also discussed, as this technique may be performed concurrently with complementary and/or confirmatory measurements. Together, smart materials and experimental designs will continue to pave the way for complete biomolecular analyses of complex and technically challenging systems.

Ochratoxin A Detection on Antibody- Immobilized on BSA-Functionalized Gold Electrodes

Ochratoxin A (OTA)—a toxin produced by Aspergillus carbonarius, Aspergillus ochraceus, and Penicillium verrucosum—is one of the most-abundant food-contaminating mycotoxins. To avoid the risk of OTA consumption for humans and animals, the rapid detection and quantitation of OTA level in different commodities are of great importance. In this work, an impedimetric immunosensor for ochratoxin A (OTA) detection, a common toxic botanical contaminant, was developed via the immobilization of anti-OTA antibody on bovine serum albumin modified gold electrodes. A four-step reaction protocol was tested to modify the gold electrode and obtain the sensing substrate. All the steps of the immunosensor elaboration and also the immunochemical reaction between surface-bound antibody and ochratoxin A were analyzed using cyclic voltammetry and electrochemical impedance spectroscopy. Modification of the impedance due to the specific antigen-antibody reaction at immunosensor surface, was used in order to detect ochratoxin A. Linear proportionality of the charge transfer resistance to the concentration of OTA allows ochratoxin A detection in the range of 2.5–100 ng/mL.

Electrochemical impedance spectroscopy versus cyclic voltammetry for the electroanalytical sensing of capsaicin utilising screen printed carbon nanotube electrodes

Screen printed carbon nanotube electrodes (SPEs) are explored as electroanalytical sensing platforms for the detection of capsaicin in both synthetic capsaicin solutions and capsaicin extracted from chillies and chilli sauces utilising both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It is found that the technique which is most applicable to the electroanalytical detection of capsaicin depends upon the analyte concentration: for the case of low capsaicin concentrations, CV is a more appropriate method as capsaicin exhibits characteristic voltammetric waves of peak heights relevant to the capsaicin concentration; but for the case of high capsaicin concentrations where the voltammetric waves merge and migrate out of the potential window, EIS is shown to be a more appropriate technique, owing to the observed linear increases in R(ct) with increasing concentration. Furthermore, we explore different types of screen printed carbon nanotube electrodes, namely single- and multi- walled carbon nanotubes, finding that they are technique-specific: for the case of low capsaicin concentrations, single-walled carbon nanotube SPEs are preferable (SW-SPE); yet for the case of EIS at high capsaicin concentrations, multi-walled carbon nanotube SPEs (MW-SPE) are preferred, based upon analytical responses. The analytical performance of CV and EIS is applied to the sensing of capsaicin in grown chillies and chilli sauces and is critically compared to 'gold standard' HPLC analysis.

Zwitterionic phenyl layers: finally, stable, anti-biofouling coatings that do not passivate electrodes

Organic coatings on electrodes that limit biofouling by proteins but are of sufficiently low impedance to still allow Faradaic electrochemistry to proceed at the underlying electrode are described for the first time. These organic coatings formed using simple aryl diazonium salts present a zwitterionic surface and exhibit good electrochemical stability. The layers represent a low impedance alternative to the oligo (ethylene glycol) (OEG)-based anti-biofouling coatings and are expected to find applications in electrochemical biosensors and implantable electrodes. Two different zwitterionic layers grafted to glassy carbon surfaces are presented and compared to a number of better-known surfaces, including OEG-based phenyl-layer-grafted glassy carbon surfaces and OEG alkanethiol SAMs coated on gold, to allow the performance of these new layers to be compared to the body of work on other anti-biofouling surfaces. The results suggest that phenyl-based zwitterionic coatings are as effective as the OEG SAMs at resisting the nonspecific adsorption of bovine serum albumin and cytochrome c, as representative anionic and cationic proteins at physiological pH, whereas the impedance of the zwitterionic phenyl layers are two orders of magnitude lower than OEG SAMs.

The influence of organic-film morphology on the efficient electron transfer at passivated polymer-modified electrodes to which nanoparticles are attached

The impact of polymer-film morphology on the electron-transfer process at electrode/organic insulator/nanomaterial architectures is studied. The experimental data are discussed in the context of the most recent theory modelling the nanoparticle-mediated electron-transfer process at electrode/insulator/nanomaterial architectures proposed by Chazalviel and Allongue [J. Am. Chem. Soc. 2011, 133, 762-764]. A previous report [Anal. Chem. 2013, 85, 1073-1080] by us qualitatively verified the theory and demonstrates a transition from thickness-independent to thickness-dependent electron transfer as the layer thickness exceeds a certain threshold. This follow-up study explores a different polymer, poly(phenylenediamine), and focuses on the effect of the uniformity of organic film on electron transfer at these hybrid structures. Electron-transfer kinetics of modified surfaces, which were assessed using the redox species Ru(NH3)6(3+) in aqueous solution, showed that a thickness-dependent electron-transfer regime is achieved with poly(phenylenediamine). This is attributed to the sufficiently thin films never being fabricated with this polymer. Rather, it is suggested that thin poly(phenylenediamine) layers have a globular structure with poor film homogeneity and pinhole defects.

Demonstrating the Use of Bisphenol A-functionalised Gold Nanoparticles in Immunoassays

Spherical gold nanoparticles (5-nm diameter) were modified with a small-molecule thiolated bisphenol A (BPA) ligand to achieve an estimated coverage of ~3.3 × 10–10 mol cm–2, or 180 ligands per particle. The modified particles were tested in an enzyme-linked immunosorbent assay (ELISA) format to measure functionality and were shown to bind specifically to anti-BPA antibody while resisting the non-specific adsorption of an antibody with no affinity for BPA. It was found that the use of 10 % ethanol as a co-solvent was required in the ELISA as aqueous buffers alone resulted in poor binding between anti-BPA antibody and the functionalised nanoparticles. This is likely due to the hydrophobic nature of the BPA ligand limiting its solubility, and therefore its availability for antibody interactions, in purely aqueous environments. To our knowledge, this is the first example of a nanoparticle modified with a small organic molecule being used in an ELISA assay.