pH-Switchable Electrochemical Sensing Platform based on Chitosan-Reduced Graphene Oxide/Concanavalin A Layer for Assay of Glucose and Urea

Current advancements and prospects of enzymatic and non-enzymatic electrochemical glucose sensors

This review discusses the most current developments and future perspectives in enzymatic and non-enzymatic glucose sensors, which have notably evolved over the preceding quadrennial period. Furthermore, a thorough exploration encompassed the sensor's intricate fabrication processes, the diverse range of materials employed, the underlying principles of detection, and an in-depth assessment of the sensors' efficacy in detecting glucose levels within essential bodily fluids such as human blood serums, urine, saliva, and interstitial fluids. It is worth noting that the accurate quantification of glucose concentrations within human blood has been effectively achieved by utilizing classical enzymatic sensors harmoniously integrated with optical and electrochemical transduction mechanisms. Monitoring glucose levels in various mediums has attracted exceptional attention from industrial to academic researchers for diabetes management, food quality control, clinical medicine, and bioprocess inspection. There has been an enormous demand for the creation of novel glucose sensors over the past ten years. Research has primarily concentrated on succeeding biocompatible and enhanced sensing abilities related to the present technologies, offering innovative avenues for more effective glucose sensors. Recent developments in wearable optical and electrochemical sensors with low cost, high stability, point-of-care testing, and online tracking of glucose concentration levels in biological fluids can aid in managing and controlling diabetes globally. New nanomaterials and biomolecules that can be used in electrochemical sensor systems to identify glucose concentration levels are developed thanks to advances in nanoscience and nanotechnology. Both enzymatic and non-enzymatic glucose electrochemical sensors have garnered much interest recently and have made significant strides in detecting glucose levels. In this review, we summarise several categories of non-enzymatic glucose sensor materials, including composites, non-precious transition metals and their metal oxides, hydroxides, precious metals and their alloys, carbon-based materials, conducting polymers, metal-organic framework (MOF)-based electrocatalysts, and wearable device-based glucose sensors deeply.

Nanomaterial-Based Electrochemical Nanodiagnostics for Human and Gut Metabolites Diagnostics: Recent Advances and Challenges

Metabolites are the intermediatory products of metabolic processes catalyzed by numerous enzymes found inside the cells. Detecting clinically relevant metabolites is important to understand their physiological and biological functions along with the evolving medical diagnostics. Rapid advances in detecting the tiny metabolites such as biomarkers that signify disease hallmarks have an immense need for high-performance identifying techniques. Low concentrations are found in biological fluids because the metabolites are difficult to dissolve in an aqueous medium. Therefore, the selective and sensitive study of metabolites as biomarkers in biological fluids is problematic. The different non-electrochemical and conventional methods need a long time of analysis, long sampling, high maintenance costs, and costly instrumentation. Hence, employing electrochemical techniques in clinical examination could efficiently meet the requirements of fully automated, inexpensive, specific, and quick means of biomarker detection. The electrochemical methods are broadly utilized in several emerging and established technologies, and electrochemical biosensors are employed to detect different metabolites. This review describes the advancement in electrochemical sensors developed for clinically associated human metabolites, including glucose, lactose, uric acid, urea, cholesterol, etc., and gut metabolites such as TMAO, TMA, and indole derivatives. Different sensing techniques are evaluated for their potential to achieve relevant degrees of multiplexing, specificity, and sensitivity limits. Moreover, we have also focused on the opportunities and remaining challenges for integrating the electrochemical sensor into the point-of-care (POC) devices.

Integrated hand-held electrochemical sensor for multicomponent detection in urine

Electrochemical sensors have shown great advantage and application potential in point-of-care testing (POCT) related scenarios. However, some fatal problems plague its widespread utilization, which include the susceptibility of sensors to interference in real samples (e.g. pH), the contradiction between the limited objects detectable for most sensors and the requirement of multi-target analysis in most cases, and the complicated procedures in sensor preparation as well as in routine use. This paper contributed a tip-like electrochemical sensor prototype. By integrated with a commercial pipettor, it fulfilled semi-automatic assay procedure of sampling, detection and rinsing, thus saving operational time and manual work. The tip sensor owns the property of simple fabrication and is free from any modification of extra bio/chem materials. Moreover, built on multiple electrochemical signal outputs including open circuit potential, peak current and potential of specific electrochemical reaction, this work established a novel multi-component sensing strategy, wherein detection of uric acid (UA), urea and pH in urine samples was realized by using one single working electrode. The detection range for the above targets is 5.0~600 μM for UA, 4.0~8.0 for pH and 0.5~7.0 mM for urea with the detection limits (S/N = 3) of 0.05 μM for UA and 5.4 μM for urea, and the sensitivity of pH assay is 73 mV/pH. Notably, as variation of sample pH has impact on electrochemical analysis, the pH-related parameter was introduced for calibration to diminish such interference. The developed tip sensor and the novel sensing strategy may open a new window for electrochemical technology and broaden its application in POCT.

Stimulus Response of GQD-Sensitized Tb/GMP ICP Nanoparticles with Dual-Responsive Ratiometric Fluorescence: Toward Point-of-Use Analysis of Acetylcholinesterase and Organophosphorus Pesticide Poisoning with Acetylcholinesterase as a Biomarker

In this study, by rationally designing the stimulus response of graphene quantum dot (GQD)-sensitized terbium/guanine monophosphate (Tb/GMP) infinite coordination polymer (ICP) nanoparticles, we have constructed a smartphone-based colorimetric assay with ratiometric fluorescence, which could be applied for the detection of acetylcholinesterase (AChE) and organophosphorus pesticides (OPs) directly. First, GQDs with abundant functional groups were chosen as the guest, which not only could be used as one of the signal readouts but also served as the antenna ligand to further sensitize the fluorescence of the host Tb/GMP. Upon being excited at 330 nm, the green fluorescence of the Tb/GMP host is highly enhanced, while the blue fluorescence of GQDs is suppressed due to the confinement of the ICP host. With the presence of thiocholine (TCh), an enzymatic product hydrolyzed from acetylthiocholine (ATCh) by AChE, the competitive coordination of Tb³⁺ between GMP and TCh results in the collapse of the ICP network and thereby the release of GQDs into the solution; thus, the fluorescence of Tb/GMP turns off and the fluorescence of GQDs turns on. The dual-responsive ratiometric fluorescent intensity change leads to the corresponding green-to-blue fluorescent color change obviously, which constitutes a novel mechanism for the colorimetric analysis of AChE. Moreover, when OPs are subsequently introduced, the activity of AChE is blocked, thus preventing the stimulus response of GQD@Tb/GMP ICP nanoparticles, leading to the fluorescent color change from greenish-blue to green, which could also be employed for OP detection. Benefitting from the high sensitivity, good reliability, and the obvious color changes, the method demonstrated here is a promising candidate to realize smartphone-based point-of-use applications, which is of great importance for timely clinical diagnosis and treatment of OPs related to health issues with AChE as an exposure biomarker in less industrialized countries, in remote settings, or even in home care services.

Switchable Graphene-Based Bioelectronics Interfaces

Integration of materials acts as a bridge between the electronic and biological worlds, which has revolutionized the development of bioelectronic devices. This review highlights the rapidly emerging field of switchable interface and its bioelectronics applications. This review article highlights the role and importance of two-dimensional (2D) materials, especially graphene, in the field of bioelectronics. Because of the excellent electrical, optical, and mechanical properties graphene have promising application in the field of bioelectronics. The easy integration, biocompatibility, mechanical flexibility, and conformity add impact in its use for the fabrication of bioelectronic devices. In addition, the switchable behavior of this material adds an impact on the study of natural biochemical processes. In general, the behavior of the interfacial materials can be tuned with modest changes in the bioelectronics interface systems. It is also believed that switchable behavior of materials responds to a major change at the nanoscale level by regulating the behavior of the stimuli-responsive interface architecture.

A novel Fe-hemin-metal organic frameworks supported on chitosan-reduced graphene oxide for real-time monitoring of H2O2 released from living cells

Herein, a kind of novel hemin-based metal organic frameworks (Fe-hemin-MOFs) with unique peroxidase-like bioactivity was developed for the first time. The synthesized Fe-hemin-MOFs exhibited satisfactory catalytic activity toward hydrogen peroxide (H₂O₂). When it was further supported on Chitosan-reduced graphene oxide (CS-rGO), amplified electrochemical signal could be obtained. The Fe-hemin-MOFs/CS-rGO composite was used to construct a novel H₂O₂ electrochemical sensor. The electrocatalytic reduction of H₂O₂ displayed two segments linearity range from 1 to 61 μM and 61-1311 μM, as well as a low detection limit of 0.57 μM. Furthermore, the proposed sensor was successfully used for real-time monitoring of H₂O₂ released from living cells, which extended the practical application of MOFs-based sensors in monitoring the pathological process in living cells.

An AgNP-deposited commercial electrochemistry test strip as a platform for urea detection

We developed an inexpensive, portable platform for urea detection via electrochemistry by depositing silver nanoparticles (AgNPs) on a commercial glucose test strip. We modified this strip by first removing the enzymes from the surface, followed by electrodeposition of AgNPs on one channel (working electrode). The morphology of the modified test strip was characterized by Scanning Electron Microscopy (SEM), and its electrochemical performance was evaluated via Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). We evaluated the performance of the device for urea detection via measurements of the dependency of peak currents vs the analyte concentration and from the relationship between the peak current and the square root of the scan rates. The observed linear range is 1-8 mM (corresponding to the physiological range of urea concentration in human blood), and the limit of detection (LOD) is 0.14 mM. The selectivity, reproducibility, reusability, and storage stability of the modified test strips are also reported. Additional tests were performed to validate the ability to measure urea in the presence of confounding factors such as spiked plasma and milk. The results demonstrate the potential of this simple and portable EC platform to be used in applications such as medical diagnosis and food safety.

Exploration of Chitinous Scaffold-Based Interfaces for Glucose Sensing Assemblies

: The nanomaterial-integrated chitinous polymers have promoted the technological advancements in personal health care apparatus, particularly for enzyme-based devices like the glucometer. Chitin and chitosan, being natural biopolymers, have attracted great attention in the field of biocatalysts engineering. Their remarkable tunable properties have been explored for enhancing enzyme performance and biosensor advancements. Currently, incorporation of nanomaterials in chitin and chitosan-based biosensors are also widely exploited for enzyme stability and interference-free detection. Therefore, in this review, we focus on various innovative multi-faceted strategies used for the fabrication of biological assemblies using chitinous biomaterial interface. We aim to summarize the current development on chitin/chitosan and their nano-architecture scaffolds for interdisciplinary biosensor research, especially for analytes like glucose. This review article will be useful for understanding the overall multifunctional aspects and progress of chitin and chitosan-based polysaccharides in the food, biomedical, pharmaceutical, environmental, and other diverse applications.

A label-free electrochemical aptasensor based on 3D porous CS/rGO/GCE for acetamiprid residue detection

A novel label-free electrochemical aptasensor was fabricated based on a three-dimensional porous electrode (3D-CS/rGO/GCE) for the detection of acetamiprid residues. The sensing signal was generated by the DNA itself. The porous electrode was prepared by electrodeposition in situ and characterized by scanning electron microscope (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). These results indicated that the porous electrode has a uniform nanoporous structure, high active area, and excellent conductivity, leading to improve the transmission efficiency of current signals. The 3D-CS/rGO/GCE was used to increase a load of acetamiprid aptamer on the electrode. Meanwhile, DNA self-assembly strategy was used to further increase the DNA amounts. Thus the electrochemical current was amplified significantly due to increased phosphate group amounts by the above synergistic effect. The determination of acetamiprid residues using square wave voltammetry (SWV) showed good sensitivity, with the linear range from 0.1 pM to 0.1 μM and the detection limit was 71.2 fM. The label-free electrochemical aptasensor was also used to detect acetamiprid residues in tea samples with satisfactory results.

Copolymer-Induced Intermolecular Charge Transfer: Enhancing the Activity of Metal-Free Catalysts for Oxygen Reduction

Breaking the electroneutrality of sp² carbon lattice is a viable way for nanocarbon material to modulate the charge delocalization and to further alter the electrocatalytic activity. Positive charge spreadsheeting is preferable for catalyzing the oxygen reduction reaction (ORR) and other electrochemical reactions. Analogously to the case of intramolecular charge transfer by heteroatom doping, electrons in the conjugated carbon lattice can be redistributed by the intermolecular charge transfer from the nanocarbon material to the polyelectrolyte. A copolymeric electrolyte, epichlorohydrin-dimethylamine copolymer (EDC) was synthesized. The EDC-modified carbon nanotube (CNT) hybrid was subsequently fabricated by sonication treatment and served as a metal-free carbonaceous electrocatalyst with remarkable catalytic activity and stability. The resultant hybrid presents positive charge spreadsheeting on CNT as a result of the interfacial electron transfer from CNT to EDC. DFT calculations were further carried out to reveal that the enhancement of the wrapped EDC polyelectrolyte originates from the synergetic effect of the quaternary ammonium-hydroxyl covalently bonded structure. The CNT-EDC hybrid not only provides an atomically precise regulation to modulate nanocarbon materials from inactive carbonaceous materials into efficient metal-free catalysts, but it also opens new avenues to develop metal-free catalysts with well-defined and highly active sites.

Coupling Self-Assembly Mechanisms to Fabricate Molecularly and Electrically Responsive Films

Biomacromolecules often possess information to self-assemble through low energy competing interactions which can make self-assembly responsive to environmental cues and can also confer dynamic properties. Here, we coupled self-assembling systems to create biofunctional multilayer films that can be cued to disassemble through either molecular or electrical signals. To create functional multilayers, we: (i) electrodeposited the pH-responsive self-assembling aminopolysaccharide chitosan, (ii) allowed the lectin Concanavalin A (ConA) to bind to the chitosan-coated electrode (presumably through electrostatic interactions), (iii) performed layer-by-layer self-assembly by sequential contacting with glycogen and ConA, and (iv) conferred biological (i.e., enzymatic) function by assembling glycoprotein (i.e., enzymes) to the ConA-terminated multilayer. Because the ConA tetramer dissociates at low pH, this multilayer can be triggered to disassemble by acidification. We demonstrate two approaches to induce acidification: (i) glucose oxidase can induce multilayer disassembly in response to molecular cues, and (ii) anodic reactions can induce multilayer disassembly in response to electrical cues.

A reversible, colorimetric, pH-responsive indole-based hydrogel and its application in urea detection

A new type of pH-responsive indole-based (4-HINF) hydrogel, fabricated by a sol–gel method, was utilized as a platform for colorimetric detection of urea in aqueous solution. The colorimetric sensor was established by virtue of the synergistic effect of cation–π interaction and hydrogen bonding with good regenerative ability. The results exhibited linear response in the range of 0–10 mM with a limit of detection of 10 μM. The prepared 4-HINF hydrogel possessed high selectivity to pH change under complicated environments ensuring further applications in environmental and bio-systems.

A new type of pH-responsive indole-based (4-HINF) hydrogel, fabricated by a sol–gel method, was utilized as a platform for colorimetric detection of urea in aqueous solution.

PAMAM Dendrimer Modified Reduced Graphene Oxide Postfunctionalized by Horseradish Peroxidase for Biosensing H2O2

In this chapter, we describe the tethering of horseradish peroxidase (HRP) to reduced graphene oxide (RGO) for sensing H₂O₂ in serum. To accomplish this, RGO was synthesized through a green route by reducing graphene oxide (GO) prepared by Hummers method with carrot extract. The RGO was then covalently functionalized by electrochemical amination using fourth generation, amine-terminated PAMAM dendrimers. Subsequently, HRP was postfunctionalized through glutaraldehyde linkage. The synthesized RGO and the functionalization steps were well characterized by spectroscopic, microscopic, and electrochemical techniques. The application of HRP tethered RGO was demonstrated for H₂O₂ sensing in blood serum. This work provides scope for extending this functionalization strategy for other carbonaceous materials as well.

Non-invasive paper-based microfluidic device for ultra-low detection of urea through enzyme catalysis

This work describes the design, fabrication and characterization of a paper-based microfluidic device for ultra-low detection of urea through enzyme catalysis. The microfluidic system comprises an entry port, a fluidic channel, a reaction zone and two electrodes (contacts). Wax printing was used to create fluidic channels on the surface of a chromatography paper. Pre-conceptualized designs of the fluidic channel are wax-printed on the paper substrate while the electrodes are screen-printed. The paper printed with wax is heated to cause the wax reflow along the thickness of the paper that selectively creates hydrophilic and hydrophobic zones inside the paper. Urease immobilized in the reaction zone catalyses urea into releasing ions and, thereby, generating a current flow between the electrodes. A measure of current with respect to time at a fixed potential enables the detection of urea. The methodology enabled urea concentration down to 1 pM to be detected. The significance of this work lies in the use of simple and inexpensive paper-based substrates to achieve detection of ultra-low concentrations of analytes such as urea. The process is non-invasive and employs a less cumbersome two-electrode assembly.

A fluorescent sensor constructed from nitrogen-doped carbon nanodots (N-CDs) for pH detection in synovial fluid and urea determination

Blue luminescent nitrogen-doped carbon nanodots (N-CDs) with pH-dependent properties were prepared from citric acid (CA), glutathione (GSH), and polyethylene polyamine (PEPA) using a two-step pyrolytic route. The N-CDs showed stable and strong emission bands at approximately 455 nm under 350 nm excitation. Moreover, the fluorescence of N-CDs can be gradually decreased by gradually increasing the pH value. A good linear relationship between the fluorescence intensity of N-CDs and the pH range of 3.0–9.0 was obtained. Thus, the response mechanism of N-CDs to pH was systematically investigated. N-CDs possessed –NH₂, –COOH, and –CONH– as active functional groups, which allowed the variable protonation/deprotonation of N-CDs to regulate the fluorescence emission intensities under changed pH values. Furthermore, upon combining urease-catalyzed hydrolysis of urea with increased pH values, a simple but effective fluorescence assay for urea was developed. The analytical performance for urea detection was the linear range of 0 to 10 mM with a detection limit of 0.072 mM. Additionally, the fluorescent sensor based on N-CDs was successfully applied for pH detection in synovial fluid and urea determination in serum.

Blue luminescent nitrogen-doped carbon nanodots (N-CDs) with pH-dependent properties were prepared from citric acid (CA), glutathione (GSH), and polyethylene polyamine (PEPA) using a two-step pyrolytic route.

Smart Nanocomposites of Cu-Hemin Metal-Organic Frameworks for Electrochemical Glucose Biosensing

Herein, a smart porous material, Cu-hemin metal-organic-frameworks (Cu-hemin MOFs), was synthesized via assembling of Cu²⁺ with hemin to load glucose oxidase (GOD) for electrochemical glucose biosensing for the first time. The formation of the Cu-hemin MOFs was verified by scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, N₂ adsorption/desorption isotherms, UV-vis absorption spectroscopy, fluorescence spectroscopy, thermal analysis and electrochemical techniques. The results indicated that the Cu-hemin MOFs showed a ball-flower-like hollow cage structure with a large specific surface area and a large number of mesopores. A large number of GOD molecules could be successfully loaded in the pores of Cu-hemin MOFs to keep their bioactivity just like in a solution. The GOD/Cu-hemin MOFs exhibited both good performance toward oxygen reduction reaction via Cu-hemin MOFs and catalytic oxidation of glucose via GOD, superior to other GOD/MOFs and GOD/nanomaterials. Accordingly, the performance of GOD/Cu-hemin MOFs-based electrochemical glucose sensor was enhanced greatly, showing a wide linear range from 9.10 μM to 36.0 mM and a low detection limit of 2.73 μM. Moreover, the sensor showed satisfactory results in detection of glucose in human serum. This work provides a practical design of new electrochemical sensing platform based on MOFs and biomolecules.

Graphene oxide-based electrochemical label-free detection of glycoproteins down to aM level using a lectin biosensor

A label-free ultrasensitive impedimetric biosensor with lectin immobilised on graphene oxide (GO) for the detection of glycoproteins from 1 aM is shown here. This is the first time a functional lectin biosensor with lectin directly immobilised on a graphene-based interface without any polymer modifier has been described. The study also shows that hydrophilic oxidative debris present on GO has a beneficial effect on the sensitivity of (8.46 ± 0.20)% per decade for the lectin biosensor compared to the sensitivity of (4.52 ± 0.23)% per decade for the lectin biosensor built up from GO with the oxidative debris washed out.

Rational Design of Bioelectrochemically Multifunctional Film with Oxidase, Ferrocene, and Graphene Oxide for Development of in Vivo Electrochemical Biosensors

This study demonstrates a new strategy to develop in vivo electrochemical biosensors through rational design and simple formation of bioelectrochemically multifunctional film (BMF). The BMF is rationally designed by first efficiently incorporating oxidase, ferrocene mediator, and graphene oxide into polymaleimidostyrene/polystyrene (PMS/PS) matrix to form a homogeneous mixture and then simply formed by drop-coating the mixture onto solid conducting substrate. By using the as-formed BMF, electrochemical biosensors could be constructed with a technical simplicity and high reproducibility. To illustrate the BMF-based biosensors for in vivo applications, we directly couple the biosensors to in vivo microdialysis to establish an online electrochemical system (OECS) for in vivo monitoring of glucose in rat auditory cortex during salicylate-induced tinnitus model. The OECS with the BMF-based biosensor as the detector shows a linear response toward glucose within a concentration range from 50 to 500 μM with a detection limit of 10 μM (S/N = 3). Additionally, the OECS is stable and does not suffer from the interference from the electroactive species endogenously coexisting in the brain microdialysate. With the BMF-based OECS, the basal level of glucose in the microdialysate continuously sampled from rat auditory cortex is determined to be 120 ± 10 μM (n = 5). After the rats were administrated with salicylate to induce transient tinnitus, the microdialysate glucose concentration in the rat auditory cortex remarkably increased to 433 ± 190 μM (n = 5) at the time point of 1.5 h. This study essentially offers a new, technically simple and reproducible approach to development of in vivo electrochemical biosensors, which is envisaged to be relatively useful for understanding of the molecular basis of brain functions.

A simple electrochemical biosensor based on AuNPs/MPS/Au electrode sensing layer for monitoring carbamate pesticides in real samples

A simple electrochemical biosensor for quantitative determination of carbamate pesticide was developed based on a sensing interface of citrate-capped gold nanoparticles (AuNPs)/(3-mercaptopropyl)-trimethoxysilane (MPS)/gold electrode (Au). The biosensor was fabricated by firstly assembling three-dimensional (3D) MPS networks on Au electrode and subsequently assembling citrate-capped AuNPs on 3D MPS network via AuS bond. The interface of AuNPs/MPS/Au was negatively charged originating from the citrate coated on AuNPs that would repulse the negatively charged ferricyanide ([Fe(CN)6](3-/4-)) to produce a negative response. In the presence of acetylcholinesterase (AChE) and acetylthiocholine (ATCl), the AChE catalyzes the hydrolysis of ATCl into positively charged thiocholine which would replace the citrate on AuNPs through the strong AuS bond and convert the negative charged surface to be positively charged. The resulted positively charged AuNPs/MPS/Au then attracted the [Fe(CN)6](3-/4-) to produce a positive response. Based on the inhibition of carbamate pesticides on the activity of AChE, the pesticide could be quantitatively determined at a very low potential. The linear range was from 0.003 to 2.00 μM. The sensing platform was also proved to be suitable for carbamate pesticides detection in practical sample.

Recent development of carbon electrode materials and their bioanalytical and environmental applications

New synthetic approaches, materials, properties, electroanalytical applications and perspectives of carbon materials are presented.

Recent Progress in Lectin-Based Biosensors

This article reviews recent progress in the development of lectin-based biosensors used for the determination of glucose, pathogenic bacteria and toxins, cancer cells, and lectins. Lectin proteins have been widely used for the construction of optical and electrochemical biosensors by exploiting the specific binding affinity to carbohydrates. Among lectin proteins, concanavalin A (Con A) is most frequently used for this purpose as glucose- and mannose-selective lectin. Con A is useful for immobilizing enzymes including glucose oxidase (GOx) and horseradish peroxidase (HRP) on the surface of a solid support to construct glucose and hydrogen peroxide sensors, because these enzymes are covered with intrinsic hydrocarbon chains. Con A-modified electrodes can be used as biosensors sensitive to glucose, cancer cells, and pathogenic bacteria covered with hydrocarbon chains. The target substrates are selectively adsorbed to the surface of Con A-modified electrodes through strong affinity of Con A to hydrocarbon chains. A recent topic in the development of lectin-based biosensors is a successful use of nanomaterials, such as metal nanoparticles and carbon nanotubes, for amplifying output signals of the sensors. In addition, lectin-based biosensors are useful for studying glycan expression on living cells.

A regenerated electrochemical biosensor for label-free detection of glucose and urea based on conformational switch of i-motif oligonucleotide probe

Improving the reproducibility of electrochemical signal remains a great challenge over the past decades. In this work, i-motif oligonucleotide probe-based electrochemical DNA (E-DNA) sensor is introduced for the first time as a regenerated sensing platform, which enhances the reproducibility of electrochemical signal, for label-free detection of glucose and urea. The addition of glucose or urea is able to activate glucose oxidase-catalyzed or urease-catalyzed reaction, inducing or destroying the formation of i-motif oligonucleotide probe. The conformational switch of oligonucleotide probe can be recorded by electrochemical impedance spectroscopy. Thus, the difference of electron transfer resistance is utilized for the quantitative determination of glucose and urea. We further demonstrate that the E-DNA sensor exhibits high selectivity, excellent stability, and remarkable regenerated ability. The human serum analysis indicates that this simple and regenerated strategy holds promising potential in future biosensing applications.

Carbon nanomaterial-based electrochemical biosensors: an overview

Carbon materials on the nanoscale exhibit diverse outstanding properties, rendering them extremely suitable for the fabrication of electrochemical biosensors. Over the past two decades, advances in this area have continuously emerged. In this review, we attempt to survey the recent developments of electrochemical biosensors based on six types of carbon nanomaterials (CNs), i.e., graphene, carbon nanotubes, carbon dots, carbon nanofibers, nanodiamonds and buckminsterfullerene. For each material, representative samples are introduced to expound the different roles of the CNs in electrochemical bioanalytical strategies. In addition, remaining challenges and perspectives for future developments are also briefly discussed.

A glucose biosensor based on glucose oxidase immobilized on three-dimensional porous carbon electrodes

A schematic illustration of a 3D-KSC/GOD glucose biosensor is presented.