Screen-printed electrode designed with MXene/doped-polyindole and MWCNT/doped-polyindole for chronoamperometric enzymatic glucose sensor

The enzymatic glucose sensors as modified by MXene-dPIn and MWCNT-dPIn on a screen-printed carbon electrode (SPCE) were investigated. Herein, MXene was molybdenum carbide (Mo3C2) which has never been utilized and reported for glucose sensors. The biopolymer type to support the enzyme immobilization was examined and compared between chitosan (CHI) and κ-carrageenan (κC). MWCNT-dPIn obviously showed a larger electroactive surface area, lower charge transfer resistance and higher redox current than Mo3C2-dPIn, indicating that MWCNT-dPIn is superior to Mo3C2-dPIn. For the chitosan-based sensors, the sensitivity value of CHI-GOD/Mo3C2-dPIn is 3.53 μA mM-1 cm-2 in the linear range of 2.5-10 mM with the calculated LOD of 1.57 mM. The sensitivity value of CHI-GOD/MWCNT-dPIn is 18.85 μA mM-1 cm-2 in the linear range of 0.5-25 mM with the calculated LOD of 0.115 mM. For the κ-carrageenan based sensors, κC-GOD/MWCNT-dPIn exhibits the sensitivity of 15.80 μA mM-1 cm-2 and the widest linear range from 0.1 to 50 mM with the calculated LOD of 0.03 mM. The presently fabricated sensors exhibit excellent reproducibility, good selectivity, high stability, and disposal use. The fabricated glucose sensors are potential as practical glucose sensors as the detectable glucose ranges well cover the glucose levels found in blood, urine, and sweat for both healthy people and diabetic patients.

Noninvasive glucose monitoring using portable GOx-Based biosensing system

Wearable biosensors have gained huge interest due to their potential for real-time physiological information. The development of a non-invasive blood glucose device is of great interests for health monitoring in reducing the diabetes incidence. Here, we report a sandwich-structured biosensor that is designed for glucose levels detection by using sweat as the means of monitoring. The Prussian blue nanoparticles (PBNPs) and carboxylated carbon nanotubes (MWCNT-COOH) were self-assembled on the electrode to improve the electrochemical performance and as the sensor unit, glucose oxidase (GOx) was immobilized by chitosan (CS) as the reaction catalysis unit, and finally encapsulated with Nafion to ensure a stable performance. As a result, the GOx/PBNPs/MWCNT-COOH sensor displays a low detection limit (7.0 μM), high sensitivity (11.87 μA mM-1 cm-2), and excellent interference resistance for a full sweat glucose application range (0.0-1.0 mM) for both healthy individuals and diabetic patients. Additionally, the glucose sensor exhibits stable stability for two weeks and can be successfully applied to screen-printed carbon electrodes (SPCE), demonstrating its great potential for personalized medical detection and chronic disease management.

A biosensor based on Fe3O4@MXene-Au nanocomposites with high peroxidase-like activity for colorimetric and smartphone-based detection of glucose

A novel magnetic nanozyme Fe₃O₄@MXene-Au nanocomposite, which possessed higher peroxidase-like activity than that of Fe₃O₄ nanoparticles and Fe₃O₄@MXene nanocomposites, was developed. The outstanding magnetic properties of the nanozyme endowed it with the ability of simple and rapid separation, achieving great recyclability. Based on Fe₃O₄@MXene-Au nanocomposites and glucose oxidase (Glu Ox), a highly selective colorimetric biosensor for glucose detection was developed. Fe₃O₄@MXene-Au nanocomposites can catalyze H₂O₂ produced from glucose catalyzed by glucose oxidase to ·OH and oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized TMB (oxTMB) with a significant absorbance at 652 nm. The linear range of glucose was 0-1.4 mM under optimal conditions, with a limit of detection (LOD) of 0.11 mM. Glucose in human whole blood was successfully detected with satisfactory recoveries. Furthermore, a facile agarose hydrogel detection platform was designed. With smartphone software, glucose detection can be realized by the agarose hydrogel platform, demonstrating the potential in on-site and visual detection of glucose.

Novel biosensor platform for glucose monitoring via smartphone based on battery-less NFC potentiostat

Near-field communication (NFC) was used to control a portable glucose biosensor for diabetes diagnosis. The system comprised a smartphone and an NFC potentiostat connected to a screen-printed carbon electrode (SPCE) modified with Prussian blue-graphene ink and functionalized with gold nanoparticles-embedded poly (3,4ethylene dioxythiophene):polysulfonic acid coated with glucose oxidase (GOx-AuNPs-PEDOT:PSS/PB-G). GOx catalyzed the glucose redox reaction while the conductivity and sensitivity of the AuNPs-PEDOT:PSS composite enhanced electron transfer to the PB-G, which was used as a mediator. The fabrication process was characterized by scanning electron microscopy (SEM) with energy dispersibe x-ray analysis (EDX). The platform was electrochemically characterized by electrical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The NFC biosensing device was then applied to quantify glucose in human blood serum by amperometry. The linear concentration range and detection limit for glucose were 0.5-500 μM and 0.15 μM, respectively. The accuracy of the device was good and results were in agreement with the results obtained from the standard hospital method. This NFC glucose sensing device can be a simple, sensitive, selective and portable platform for medical diagnosis.

Non-invasive in-vivo glucose-based stress monitoring in plants

Plant stress responses involve a suite of genetically encoded mechanisms triggered by real-time interactions with their surrounding environment. Although sophisticated regulatory networks maintain proper homeostasis to prevent damage, the tolerance thresholds to these stresses vary significantly among organisms. Current plant phenotyping techniques and observables must be better suited to characterize the real-time metabolic response to stresses. This impedes practical agronomic intervention to avoid irreversible damage and limits our ability to breed improved plant organisms. Here, we introduce a sensitive, wearable electrochemical glucose-selective sensing platform that addresses these problems. Glucose is a primary plant metabolite, a source of energy produced during photosynthesis, and a critical molecular modulator of various cellular processes ranging from germination to senescence. The wearable-like technology integrates a reverse iontophoresis glucose extraction capability with an enzymatic glucose biosensor that offers a sensitivity of 22.7 nA/(μM·cm²), a limit of detection (LOD) of 9.4 μM, and a limit of quantification (LOQ) of 28.5 μM. The system's performance was validated by subjecting three different plant models (sweet pepper, gerbera, and romaine lettuce) to low-light and low-high temperature stresses and demonstrating critical differential physiological responses associated with their glucose metabolism. This technology enables non-invasive, non-destructive, real-time, in-situ, and in-vivo identification of early stress response in plants and provides a unique tool for timely agronomic management of crops and improving breeding strategies based on the dynamics of genome-metabolome-phenome relationships.

Highly sensitive detection of glucose via glucose oxidase immobilization onto conducting polymer-coated composite polyacrylonitrile nanofibers

Current study introduces composite polyacrylonitrile - multiwall carbon nanotubes nanofibers (PAN-MWCNTs NFs) coated with conducting polymers (polypyrrole (PPy) or poly(3,4-ethylenedioxythiophene) (PEDOT)) by chemical vapor deposition for efficient glucose detection. The potential of nanofibrous assemblies and nano-conducting elements in biosensing was explored as pre-processing of NFs with MWCNTs and post-processing with CPs were both employed. These 'core-shell' conducting NFs were further employed as platforms for glucose oxidase immobilization for enzymatic detection of glucose. The performance of the biosensors was closely correlated with the concentration of immobilized enzyme and with the type of conducting polymer. The biosensors showed high sensitivities of 92.94 and 81.72 µA/mM.cm^-2 for (PAN-MWCNTs)/ PEDOT and (PAN-MWCNTs)/ PPy accompanied by low limit of detection values of 2.30 and 2.38 µM, respectively. Good operational stability was observed throughout twenty-five consecutive measurements, over 90% activity was maintained for both sensors. This study represents proof of concept for the methodology, showcasing the advantages of nanomaterial synthesis for bio-applications. The work was compared thoroughly with previously reported biosensors showing some of the best results reported to date in terms of analytical characteristics.

Controlled synthesis of Cu-Sn alloy nanosheet arrays on carbon fiber paper for self-supported nonenzymatic glucose sensing

Nanoalloy shows significant advantages and broad application prospects in chemical catalysis, due to the possessed high specific surface energy and abundant active sites can greatly promote their catalytic performance. In this work, morphology-controlled Cu-Sn alloy nanosheet arrays supported on carbon fiber paper (CP) substrate (Cu-Sn/CP) have been developed by a facile one-step electrodeposition technique at room temperature for the first time. Benefiting from the large active surface area, considerable ion transport channels and strong synergistic catalytic effect between Cu and Sn, the as-prepared Cu-Sn/CP served as a self-supported electrode for efficient nonenzymatic glucose sensing. Under optimized conditions, Cu-Sn/CP electrode offers wide linear ranges of 0.0005-2.0 mM and 2.0-10.0 mM, respectively. The detection limit is as low as 0.061 μM (S/N = 3). Cu-Sn/CP electrode also exhibited excellent selectivity and stability. Additionally, the proposed sensor is proven to be suitable for the detection of glucose in human serum samples.

Lignosulfonate-assisted synthesis of platinum nanoparticles deposited on multi-walled carbon nanotubes for biosensing of glucose

It is presented in this work that lignosulfonate (LS) can be preferentially adsorbed on the surface of multi-walled carbon nanotubes (MWCNT) giving rise to the functional platform for platinum nanoparticles (NPt) deposition. The novel MWCNT/LS/NPt hybrid material has been characterized by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS). The morphology of the MWCNT/LS/NPt electrodes has been investigated by atomic force microscopy (AFM). The electrochemical studies of MWCNT/LS/NPt hybrid material revealed strong electrocatalytic properties towards hydrogen peroxide. In addition, the effects of lignosulfonate amount adsorbed at the MWCNT on the voltammetric response of the hydrogen peroxide were discussed and used to select the optimal and effective conditions for the synthesis of the electrode material. An amperometric biosensor for glucose was developed based on the covalent linkage of glucose oxidase (GOx) at the MWCNT/LS/NPt. The enzyme electrode exhibited excellent electrocatalytic activity and rapid response for glucose. The linear range of the glucose determination was 50-1400 µM and LOD was quantified as 15.67 µM.

A feasible electrochemical biosensor for determination of glucose based on Prussian blue - Enzyme aggregates cascade catalytic system

The coral-like gold micro/nanostructures were formed onto carbon cloth followed by a Prussian blue (PB) electrochemical deposition to construct a highly sensitive H₂O₂ biosensor. The SEM image of PB/Au/CC showed the coral-like gold morphology, and EDS and XPS tests also further confirmed the successful loading of Au and PB. The electrochemical tests of PB/Au/CC displayed the electrode possessed excellent performance in sensing H₂O₂, which was quantified in the linear range from 0.002 to 13.97 mM at an applied potential of -0.05 V, with a sensitivity of 454.97 μA mM^-1 cm^-2 and a detection limit of 0.5 μM (S/N = 3). And then a convenient sensing platform was established via the cross-linking enzyme aggregates method, using PB as the mediator to realize the construction of glucose BIOSENSOR GOx_EA@PB/Au/CC. The biosensor responded to glucose in the sensitivity of 70.76 μA mM^-1 cm^-2 within the linear range from 0.05 to 3.15 mM with a detection limit of 10 μM. The sensitivity was much higher than the electrode constructed by the cross-linking enzyme method (GOx@PB/Au/CC), and it was also highly selective, reproducible, and stable. Besides, the proposed biosensor was successfully applied to the glucose determination in real human serum samples, which proved its practicability.

Site-specifically wired and oriented glucose dehydrogenase fused to a minimal cytochrome with high glucose sensing sensitivity

Direct electron transfer based enzymatic biosensors are highly efficient systems where electrons are transferred directly from the enzyme's electroactive site to the electrode. One way of achieving it is by 'wiring' the enzyme to the electrode surface. The wiring of enzymes to electrode surfaces can be reached in many different ways but controlling its orientation towards the electrode surface is still a challenge. In this study we have designed a Flavin-adenine dinucleotide dependent glucose dehydrogenase that is fused to a minimal cytochrome with a site-specifically incorporated unnatural amino acid to control its orientation towards the electrode. Several site-specifically wired mutant enzymes were compared to each other and to a non-specifically wired enzyme using atomic force microscopy and electrochemical techniques. The surface and activity analyses suggest that the site-specific wiring through different sites maintains the correct folding of the enzyme and have a positive effect on the apparent electrochemical electron transfer rate constant k_ET^app. Electrochemical analysis revealed an efficient electron transfer rate with more than 15 times higher i_max and 10-fold higher sensitivity of the site-specifically wired enzyme variants compared to the non-specifically wired ones. This approach can be utilized to control the orientation of other redox enzymes on electrodes to allow a significant improvement of their electron transfer communication with electrodes.

Rational design of electroactive redox enzyme nanocapsules for high-performance biosensors and enzymatic biofuel cell

The potential application of biodevices based on enzymatic bioelectrocatalysis are limited by poor stability and electrochemical performance. To solve the limitation, modifying enzyme with functional polymer to tailor enzyme function is highly desirable. Herein, glucose oxidase (GOx) was chosen as a model enzyme, and according to the chemical structure of GOx cofactor (flavin adenine dinucleotide, FAD), we customize a biomimetic cofactor containing vinyl group (SFAD) for GOx, and prepared an GOx nanocapsule via in-situ polymerization. The characterization of particle size distribution, TEM, fluorescence and electrochemical performance indicated the successful formation of electroactive GOx nanocapsule with SFAD-containing polymeric network (n (GOx-SFAD-PAM)). The network can act as an electronic "highway" to link the active site with electrode, with capability to accelerate electron transfer as well as enhanced GOx stability. Further investigation of bioelectrocatalysis shows that n (GOx-SFAD-PAM)-based biosensor has low detection potential (-0.4 vs. Ag/AgCl), high sensitivity (64.97 μAmM^-1cm^-2), good anti-interference performance, quick response (3⁓5s) and excellent stability, and that n (GOx-SFAD-PAM)-based enzymatic biofuel cell (EBFC) has the high maximum power density (1011.21 μWcm^-2), which is a 385-fold increase over that of native GOx-based EBFC (2.62 μWcm^-2). This study suggests that novel enzyme nanocapsule with electroactive polymeric shell might provide a prospective solution for the performance improvement of enzymatic bioelectrocatalysis-based biodevices.

One-step modification of nano-polyaniline/glucose oxidase on double-side printed flexible electrode for continuous glucose monitoring: Characterization, cytotoxicity evaluation and in vivo experiment

The single-step modification of the nanostructured polyaniline (PANI)/glucose oxidase (GOD) enzyme on double-sided, screen-printed, flexible electrodes doped with Prussian blue (PB), has been achieved and successfully applied in continuous glucose monitoring in vivo, and its biocompatibility has been evaluated systematically. The proposed fabrication procedure is simple, low cost, and suitable for large-scale production. PB doped with carbon ink catalyzes the reduction of hydrogen peroxide (H₂O₂) in low-voltage conditions, which could help eliminate interferences. And the PANI/GOD nanostructure makes the GOD enzyme more stable for long-term, in vivo monitoring. More importantly, a polyurethane (PU) layer is deposited on the electrode's surface as a diffusion limiting membrane that enhanced the linear range and biocompatibility. In tests in vitro, the proposed biosensor achieved a linear range of 0-12 mM and a good sensitivity of 16.66 μA·mM^-1·cm^-2(correlation coefficient R² = 0.9962) with an excellent specificity to glucose. The biosensor exhibits long-term stability, with a maximum lifespan of 14 days when stored in phosphate buffer solution at 4 °C, and achieves a sensitivity of 120%. The biocompatibilities of the electrode materials have also been systematically evaluated in cytotoxicity and cell adhesion tests to ensure the safety of implantation. In experiments in vivo, the biosensor can successfully monitor the glucose level fluctuation of rats after 24 h following implantation. Overall, the biosensor fabricated with the double-side, screen-printing process, satisfies the glucose monitoring range in vivo and eliminates various types of interference, thus establishing a new, large-scale production procedure for flexible in vivo biosensors.

Graphene oxide-functionalized long period fiber grating for ultrafast label-free glucose biosensor

A label-free glucose biosensor is constructed successfully based on the long period fiber grating (LPFG) functionalized with graphene oxide (GO)-glucose oxidase (GOD) via the chemical crosslink method. GO coated on the surface of LPFG can immobilize GOD by the plentiful binding sites because of its favorable combination of exceptionally high surface-to-volume ratio. The structure and characterization of GOD-GO-modified LPFG are studied by the optical microscope, Fourier transformation infrared spectrometer (FTIR), Raman spectroscopy, scanning electron microscope (SEM) and atomic force microscopy (AFM), respectively. The reaction between GOD and glucose create gluconic acid and H₂O₂, which will lead to an evident shift of LPFG transmission spectrum due to the greater change of the surrounding refractive index (SRI). The GOD-GO-modified LPFG sensor shows a linear response with a response coefficient of 0.77 nm/(mg/mL). This biosensor has good selectivity and can be used for the detection of practical sample. The GOD-GO-modified LPFG biosensor has great prospect in the pharmaceutical research and medical diagnosis fields.

Sensitive amperometric biosensors for detection of glucose and cholesterol using a platinum/reduced graphene oxide/poly(3-aminobenzoic acid) film-modified screen-printed carbon electrode

A facile one-step electrochemical synthesis of a platinum/reduced graphene oxide/poly(3-aminobenzoic acid) (Pt/rGO/P3ABA) nanocomposite film on a screen-printed carbon electrode (SPCE) and its application in the development of sensitive amperometric biosensors was successfully demonstrated herein. The electropolymerization of P3ABA together with co-electrodeposition of rGO and Pt was conducted by cyclic voltammetry, as was the GO reduction to rGO. A Pt/rGO/P3ABA-modified SPCE exhibited excellent electrocatalytic oxidation towards hydrogen peroxide (H₂O₂) and can be employed as an electrochemical platform for the immobilization of glucose oxidase (GOx) and cholesterol oxidase (ChOx) to fabricate glucose and cholesterol biosensors, respectively. Under the optimized conditions at a working potential of +0.50 V, the proposed biosensors revealed excellent linear responses to glucose and cholesterol in the concentration ranges of 0.25-6.00 mM and 0.25-4.00 mM, respectively, with high sensitivities of 22.01 and 15.94 μA mM^-1 cm^-2 and low detection limits (LODs) of 44.3 and 40.5 μM. Additionally, the Michaelis-Menten constant (K_m) of GOx was 3.54 mM, while the K_m of ChOx was 3.82 mM. Both biosensors displayed a good anti-interference ability and clearly exhibited acceptable recoveries for the detection of glucose and cholesterol in a human serum sample (98.2-104.1%).

An ultrasensitive photoelectrochemical biosensor for glucose based on bio-derived nitrogen-doped carbon sheets wrapped titanium dioxide nanoparticles

In this work, an ultra-sensing photoelectrochemical (PEC) glucose biosensor has been constructed from the bio-derived nitrogen-doped carbon sheets (NDC) wrapped titanium dioxide nanoparticles (NDC-TiO₂ NPs) followed by the covalent immobilization of glucose oxidase (GODx) on them (designated as a GODx/NDC-TiO₂NPs/ITO biosensor). Initially, the TiO₂ NPs was synthesized by sol-gel method and then NDC-TiO₂ NPs was synthesized utilizing a green source of Prunus persica (peach fruit) through a simple hydrothermal process. The synthesized NDC-TiO₂ NPs composite was characterized by FESEM, HRTEM, Raman spectroscopy, XRD, ATR-FTIR spectroscopy and XPS to determine composition and phase purity. These fabricated GODx/NDC-TiO₂NPs/ITO biosensor exhibited a good charge separation, highly enhanced and stable photocurrent responses with switching PEC behavior under the light (λ > 400 nm). As a result, GODx/NDC-TiO₂NPs/ITO PEC glucose sensor exhibits a good photocurrent response to detection of glucose concentrations (0.05-10 μM) with an ultra-low detection limit of 13 nM under optimized PEC experimental conditions. Also, the PEC glucose sensor revealed a high selectivity, good stability, long time durability, and capability to analyze the glucose levels in real human serum. Also, the further development of this work may provide new insights into preparing other bio-derived carbon nanostructure-based photocatalysts for PEC applications.

Fabrication of a promising immobilization platform based on electrochemical synthesis of a conjugated polymer

Since conjugated polymers are an important class of materials with remarkable properties in biosensor applications, in this study, a novel glucose biosensor based on a conjugated polymer was fabricated via the electropolymerization of the monomer 10,13-bis(4-hexylthiophen-2-yl)dipyridol[3,2-a:2',3'-c]phenazine onto a graphite electrode surface. Glucose oxidase (GOx) was used as the model biological recognition element. As a result of the enzymatic reaction between GOx and glucose, the glucose amount was determined by monitoring the change in the oxygen level associated with substrate concentration via the amperometric detection technique. The proposed system possessed superior properties with K_M^app value of 0.262 mM, 2.88 × 10^-3 mM limit of detection and 105.12 μA mM^-1 cm^-2 sensitivity. These results show that conjugated polymer film provides an effective and stable immobilization matrix for the enzyme. Finally, the biosensor was applied successfully to several commercially available beverage samples for glucose determination proving an inexpensive and highly sensitive system applicable for real time analyses.

In-vitro model for assessing glucose diffusion through skin

Pig ear skin membrane-covered glucose biosensor based on oxygen electrode has been assessed as a tool to evaluate glucose penetration through skin in-vitro. For this, glucose oxidase (GOx) was immobilised on oxygen electrode and covered with the skin membrane. Exposing this electrode to the solution of glucose resulted in glucose penetration though skin membrane, its oxidation catalysed by GOx, consumption of O₂ and decrease of the current of the oxygen electrode. By processing the biosensor responses to glucose, we found that glucose penetration through 250 µm thick skin membrane is slow; 90% of steady-state current response was reached in 32( ± 22) min. Apparent diffusion coefficient for glucose in skin was found to be equal to 0.15( ± 0.07)* 10^-6 cm² s^-1. This value is 45 times lower than glucose diffusion coefficient in water. Tape-stripping of stratum corneum (SC) allows considerably faster glucose penetration. The electrodes covered with tape-stripped skin reached 90% of steady-state current response in 5.0(± 2.7) min. The theoretical estimate of glucose flux through SC was considered exploiting four-pathway theory of transdermal penetration. Theoretical flux values were more that three orders lower than measured experimentally. This high discrepancy might indicate that glucose penetration through healthy human skin could be even slower, allowing much lower flux, than it was found in our study for skin membranes from pig ears.

TMV-Based Adapter Templates for Enhanced Enzyme Loading in Biosensor Applications

Nanotubular tobacco mosaic virus (TMV) particles and RNA-free lower-order coat protein (CP) aggregates have been employed as enzyme carriers in different diagnostic layouts and compared for their influence on biosensor performance. In the following, we describe a label-free electrochemical biosensor for improved glucose detection by use of TMV adapters and the enzyme glucose oxidase (GOD). A specific and efficient immobilization of streptavidin-conjugated GOD ([SA]-GOD) complexes on biotinylated TMV nanotubes or CP aggregates was achieved via bioaffinity binding. Glucose sensors with adsorptively immobilized [SA]-GOD, and with [SA]-GOD cross-linked with glutardialdehyde, respectively, were tested in parallel on the same sensor chip. Comparison of these sensors revealed that TMV adapters enhanced the amperometric glucose detection remarkably, conveying highest sensitivity, an extended linear detection range and fastest response times. These results underline a great potential of an integration of virus/biomolecule hybrids with electronic transducers for applications in biosensorics and biochips. Here, we describe the fabrication and use of amperometric sensor chips combining an array of circular Pt electrodes, their loading with GOD-modified TMV nanotubes (and other GOD immobilization methods), and the subsequent investigations of the sensor performance.

Electrochemical nonenzymatic sensing of glucose using advanced nanomaterials

An overview (with 376 refs.) is given here on the current state of methods for electrochemical sensing of glucose based on the use of advanced nanomaterials. An introduction into the field covers aspects of enzyme based sensing versus nonenzymatic sensing using nanomaterials. The next chapter cover the most commonly used nanomaterials for use in such sensors, with sections on uses of noble metals, transition metals, metal oxides, metal hydroxides, and metal sulfides, on bimetallic nanoparticles and alloys, and on other composites. A further section treats electrodes based on the use of carbon nanomaterials (with subsections on carbon nanotubes, on graphene, graphene oxide and carbon dots, and on other carbonaceous nanomaterials. The mechanisms for electro-catalysis are also discussed, and several Tables are given where the performance of sensors is being compared. Finally, the review addresses merits and limitations (such as the frequent need for working in strongly etching alkaline solutions and the need for diluting samples because sensors often have analytical ranges that are far below the glucose levels found in blood). We also address market/technology gaps in comparison to commercially available enzymatic sensors. Graphical Abstract Schematic representation of electrochemical nonenzymatic glucose sensing on the nanomaterials modified electrodes. At an applied potential, the nanomaterial-modified electrodes exhibit excellent electrocatalytic activity for direct oxidation of glucose oxidation.

Influence of diabetes on the foreign body response to nitric oxide-releasing implants

The foreign body response (FBR) to nitric oxide (NO)-releasing subcutaneous implants was compared between healthy and streptozotocin-induced diabetic swine by evaluating inflammation, collagen capsule formation, and angiogenesis. Steel wire substrates were first modified with polyurethane membranes capable of diverse NO-release kinetics (NO fluxes and release durations of 0.8-630.0 pmol cm^-2 s^-1 and 2-13 d, respectively). The NO-releasing materials were implanted in the subcutis for 3, 10, or 25 d for histological and immunohistochemical evaluation of the FBR. A delayed, more severe inflammatory response to control (i.e., non-NO-releasing) implants was observed in diabetic pigs relative to healthy swine. Regardless of the animal disease state, each NO-releasing implant tested elicited reduced inflammation compared to controls at both 3 and 10 d. However, only the NO-release materials capable of releasing low NO fluxes (0.8-3.3 pmol cm^-2 s^-1) for 7-13 d durations mitigated the inflammatory response at 25 d. Using immunohistochemical staining for the endothelial cell surface marker CD-31, we also observed poor blood vessel development at non-NO-releasing implants in diabetic swine. Relative to controls, NO-releasing implants with the longest NO-release duration (13 d) increased blood vessel densities by 47.1 and 70.4% in the healthy and diabetic pigs, respectively. In the healthy model, tissues surrounding the long NO-release materials contained sparse amounts of collagen, whereas implants with shorter NO-release durations (2, 3, and 7 d) were characterized with a dense collagen encapsulation layer, similar to controls. Collagen deposition in diabetic swine was inhibited, and unaffected by NO. These results emphasize several key differences in the FBR in the setting of acute onset diabetes. The observation that NO release counteracts the more severe FBR in diabetic swine while simultaneously promoting tissue integration may help guide the design of medical implants (e.g., glucose sensors) with improved performance for diabetes management.

Smartphone based non-invasive salivary glucose biosensor

The present work deals with the development of a non-invasive optical glucose biosensor using saliva samples and a smartphone. The sensor was fabricated with a simple methodology by immobilization of Glucose oxidase enzyme along with a pH responsive dye on a filter paper based strip. The strip changes color upon reaction with glucose present in saliva and the color changes were detected using a smartphone camera through RGB profiling. This standalone biosensor showed good sensitivity and low interference while operating within 20 s response time. We used various means for improvements such as the use of slope method instead of differential response; use of a responsive pH indicator and made numerous tweaks in the smartphone app. Calibration with spiked saliva samples with slopes for (R + G + B) pixels revealed an exponentially increasing calibration curve with a linear detection range of 50-540 mg/dL, sensitivity of 0.0012 pixels sec^-1/mg dL^-1 and LOD of 24.6 mg/dL. The biosensor was clinically validated on both healthy and diabetic subjects divided into several categories based on sex, age, diabetic status etc. and correlation between blood and salivary glucose has been established for better standardization of the sensor. Correlation of 0.44 was obtained between blood and salivary glucose in healthy individuals whereas it was 0.64 and 0.94 in case of prediabetic and diabetic patients respectively. The developed biosensor has the potential to be used for mass diagnosis of diabetes especially in such areas where people remain prohibited from routine analysis due to high healthcare cost. Apart from that, a smartphone would be the only device the user needs for this measurement, along with a disposable low cost test strip.

Nozzle-jet printed flexible field-effect transistor biosensor for high performance glucose detection

Printable electronics is a subject of great interest for low-cost, facile and environmentally-friendly large scale device production. But, it still remains challenging for printable biosensor development. Herein, we present the fabrication of nozzle-jet printed flexible field-effect transistor (FET) glucose biosensor. The silver source-drain electrodes and ZnO seed layers were printed on flexible substrate by nozzle-jet printer followed by ZnO nanorods (ZnO NRs) synthesis and glucose oxidase (GOx) immobilization. Utilization of nozzle-jet printing methods resulted in highly reproducible electrodes with well-defined vertical grown ZnO NRs for high GOx loading and enhanced glucose sensing performance in a wide glucose detection range. The stability, anti-interference ability, reproducibility, reusability, and applicability in human serum samples were also assessed. Overall, biosensor fabrication using nozzle-jet printer will not only provide large scale production of highly reproducible electrodes but also reduce the fabrication cost. Additionally, printed electrodes can be modified accordingly for different analyte detection.

Enzyme Deposition by Polydimethylsiloxane Stamping for Biosensor Fabrication

High-performance biosensors were fabricated by efficiently transferring enzyme onto Pt electrode surfaces using a polydimethylsiloxane (PDMS) stamp. Polypyrrole and Nafion were coated first on the electrode surface to act as permselective films for exclusion of both anionic and cationic electrooxidizable interfering compounds. A chitosan film then was electrochemically deposited to serve as an adhesive layer for enzyme immobilization. Glucose oxidase (GOx) was selected as a model enzyme for construction of a glucose biosensor, and a mixture of GOx and bovine serum albumin was stamped onto the chitosan-coated surface and subsequently crosslinked using glutaraldehyde vapor. For the optimized fabrication process, the biosensor exhibited excellent performance characteristics including a linear range up to 2 mM with sensitivity of 29.4 ± 1.3 μA mM^-1 cm^-2 and detection limit of 4.3 ± 1.7 μM (S/N = 3) as well as a rapid response time of ~2 s. In comparison to those previously described, this glucose biosensor exhibits an excellent combination of high sensitivity, low detection limit, rapid response time, and good selectivity. Thus, these results support the use of PDMS stamping as an effective enzyme deposition method for electroenzymatic biosensor fabrication, which may prove especially useful for the deposition of enzyme at selected sites on microelectrode array microprobes of the kind used for neuroscience research in vivo.

A stretchable and screen-printed electrochemical sensor for glucose determination in human perspiration

Here we present two types of all-printable, highly stretchable, and inexpensive devices based on platinum (Pt)-decorated graphite for glucose determination in physiological fluids. Said devices are: a non-enzymatic sensor and an enzymatic biosensor, the latter showing promising results. Glucose has been quantified by measuring hydrogen peroxide (H2O2) reduction by chronoamperometry at -0.35V (vs pseudo-Ag/AgCl) using glucose oxidase immobilized on Pt-decorated graphite. The sensor performs well for the quantification of glucose in phosphate buffer solution (0.25M PBS, pH 7.0), with a linear range between 0 mM and 0.9mM, high sensitivity and selectivity, and a low limit of detection (LOD). Thus, it provides an alternative non-invasive and on-body quantification of glucose levels in human perspiration. This biosensor has been successfully applied on real human perspiration samples and results also show a significant correlation between glucose concentration in perspiration and glucose concentration in blood measured by a commercial glucose meter.

Ferrocene-Modified Linear Poly(ethylenimine) for Enzymatic Immobilization and Electron Mediation

Enzymatic glucose biosensors and biofuel cells make use of the electrochemical transduction between an oxidoreductase enzyme, such as glucose oxidase (GOx), and an electrode to either quantify the amount of glucose in a solution or generate electrical energy. However, many enzymes including GOx are not able to electrochemically interact with an electrode surface directly, but require an external electrochemical relay to shuttle electrons to the electrode. Ferrocene-modified linear poly(ethylenimine) (Fc-LPEI) redox polymers have been designed to simultaneously immobilize glucose oxidase (GOx) at an electrode and mediate electron transfer from their flavin adenine dinucleotide (FAD) active site to the electrode surface. Cross-linked films of Fc-LPEI create hydrogel networks that allow for rapid transport of glucose, while the covalently bound ferrocene moieties are able to facilitate rapid electron transfer due to the ability of ferrocene to exchange electrons between adjacent ferrocene residues. For these reasons, Fc-LPEI films have been widely used in the development of high current density bioanode materials. This chapter describes the synthesis of a commonly used dimethylferrocene-modified linear poly(ethylenimine), as well as the subsequent preparation and electrochemical characterization of a GOx bioanode film utilizing the synthesized polymer.

Hierarchically mesostructured porous TiO2 hollow nanofibers for high performance glucose biosensing

Effective immobilization of enzymes on an electrode surface is of great importance for biosensor development, but it still remains challenging because enzymes tend to denaturation and/or form close-packed structures. In this work, a free-standing TiO₂ hollow nanofibers (HNF-TiO₂) was successfully prepared by a simple and scalable electrospun nanofiber film template-assisted sol-gel method, and was further explored for glucose oxidase (GOD) immobilization and biosensing. This porous and nanotubular HNF-TiO₂ provides a well-defined hierarchical nanostructure for GOD loading, and the fine TiO₂ nanocrystals facilitate direct electron transfer from GOD to the electrode, also the strong interaction between GOD and HNF-TiO₂ greatly enhances the stability of the biosensor. The as-prepared glucose biosensors show good sensing performances both in O₂-free and O₂-containing conditions with good sensitivity, satisfactory selectivity, long-term stability and sound reliability. The novel textile formation, porous and hierarchically mesostructured nature of HNF-TiO₂ with excellent analytical performances make it a superior platform for the construction of high-performance glucose biosensors.

High-performance glucose biosensor based on chitosan-glucose oxidase immobilized polypyrrole/Nafion/functionalized multi-walled carbon nanotubes bio-nanohybrid film

A highly electroactive bio-nanohybrid film of polypyrrole (PPy)-Nafion (Nf)-functionalized multi-walled carbon nanotubes (fMWCNTs) nanocomposite was prepared on the glassy carbon electrode (GCE) by a facile one-step electrochemical polymerization technique followed by chitosan-glucose oxidase (CH-GOx) immobilization on its surface to achieve a high-performance glucose biosensor. The as-fabricated nanohybrid composite provides high surface area for GOx immobilization and thus enhances the enzyme-loading efficiency. The structural characterization revealed that the PPy-Nf-fMWCNTs nanocomposite films were uniformly formed on GCE and after GOx immobilization, the surface porosities of the film were decreased due to enzyme encapsulation inside the bio-nanohybrid composite materials. The electrochemical behavior of the fabricated biosensor was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and amperometry measurements. The results indicated an excellent catalytic property of bio-nanohybrid film for glucose detection with improved sensitivity of 2860.3μAmM(-1)cm(-2), the linear range up to 4.7mM (R(2)=0.9992), and a low detection limit of 5μM under a signal/noise (S/N) ratio of 3. Furthermore, the resulting biosensor presented reliable selectivity, better long-term stability, good repeatability, reproducibility, and acceptable measurement of glucose concentration in real serum samples. Thus, this fabricated biosensor provides an efficient and highly sensitive platform for glucose sensing and can open up new avenues for clinical applications.

Porous Co3O4 hollow nanododecahedra for nonenzymatic glucose biosensor and biofuel cell

Cobalt oxide hollow nanododecahedra (Co3O4-HND) is synthesized by a facile thermal transformation of cobalt-based metal-organic framework (Co-MOF, ZIF-67) template. The morphology and properties of the Co3O4-HND are characterized by a set of techniques, including transmission electron microscope (TEM), powder X-ray diffraction (XRD), scanning electron microscope (SEM) and Brunner-Emmet-Teller (BET). When tested as a non-enzymatic electrocatalyst for glucose oxidation reaction, the Co3O4-HND exhibits a high activity and shows an outstanding performance for determining glucose with a wide window of 2.0μM to 6.06mM, a high sensitivity of 708.4μAmM(-1)cm(-2), a low detection limit of 0.58μM (S/N=3), and fast response time(<2s). Based on the nonenzymatic oxidation of glucose, Co3O4-HND could be served as an attractive non-enzyme and noble-metal-free electrocatalyst in glucose fuel cell (GFC) due to its excellent electrochemical properties, low cost and facile preparation.

Fabrication of a sensitive amperometric sensor for NADH and H2O2 using palladium nanoparticles-multiwalled carbon nanotube nanohybrid

Palladium nanoparticles decorated multiwalled carbon nanotubes (PdNPs-MWCNTs) were synthesized and simply cast on the surface of a glassy carbon electrode (GCE) to prepare an amperometric sensor. The fabricated sensor (PdNPs-MWCNTs/GCE) showed excellent electrocatalytic activity towards NADH and H2O2 oxidation and H2O2 reduction. A fast, linear and highly sensitive response was observed for NADH in the concentration range between 0.1 and 200 μM with a detection limit (S/N=3) of 32 nM. Also, the sensor exhibited fast and sensitive responses (<2 s) towards H2O2. The sensitivity and detection limit for H2O2 at the operating potential of +0.35 V were 167 nA μM(-1)cm(-2) and 1.2 μM, respectively and better than those obtained at the operating potential of -0.25 V (68 nA μM(-1)cm(-2) and 14 μM). Moreover, further modification of the proposed sensor by glucose oxidase led to the fabrication of a glucose biosensor with satisfactory performance.

Amperometric flow injection analysis of glucose using immobilized glucose oxidase on nano-composite carbon nanotubes-platinum nanoparticles carbon paste electrode

We report a novel amperometric glucose biosensor based on glucose oxidase (GOx) immobilized on a carbon nanotube (CNTs)-poly(diallyldimethyl-ammonium chloride) (PDDA)-platinum nanoparticle (PtNPs) modified carbon-paste electrode (CNTs-PDDA-PtNPs/CPE). The CNTs-PDDA-PtNPs composite materials were characterized by TEM and electrochemical techniques. Cyclic voltammetric results reveal direct electron transfer of the immobilized GOx, indicated by two quasi-reversible redox peaks at a potential of 0.37V (vs. Ag/AgCl) in phosphate buffered solution (PBS) (0.10M, pH 7). The biosensor provides good glucose oxidation activity and retention of GOx electrocatalytic activity due to CNTs-PDDA-PtNPs enhancement of the redox response. The carbon paste electrode was installed as working electrode in a flow through electrochemical cell of a flow injection (FI) system. Glucose was quantified using amperometric measurements at 0.5V vs. Ag/AgCl and PBS carrier (0.10M, pH 7.0) at a flow rate of 1.0mLmin^-1. The linear working ranges for glucose measurements were 0.1-3mM (r²=0.995) and 5-100mM (r²=0.997), with corresponding sensitivities of 0.127 and 0.060 (μAs) mM^-1, respectively. The system provides good precision of 2.8% R.S.D with a calculated detection limit (3S/N) of 15μM. The proposed method was successfully applied to determination of glucose in food and pharmaceutical samples with throughput of 200 samplesh^-1.

Rapid amperometric detection of trace metals by inhibition of an ultrathin polypyrrole-based glucose biosensor

A sensitive and reliable inhibitive amperometric glucose biosensor is described for rapid trace metal determination. The biosensor utilises a conductive ultrathin (55 nm thick) polypyrrole (PPy) film for entrapment of glucose oxidase (GOx) to permit rapid inhibition of GOx activity in the ultrathin film upon exposure to trace metals, resulting in reduced glucose amperometric response. The biosensor demonstrates a relatively fast response time of 20s and does not require incubation. Furthermore, a complete recovery of GOx activity in the ultrathin PPy-GOx biosensor is quickly achieved by washing in 2mM EDTA for only 10s. The minimum detectable concentrations achieved with the biosensor for Hg(2+), Cu(2+), Pb(2+) and Cd(2+) by inhibitive amperometric detection are 0.48, 1.5, 1.6 and 4.0 µM, respectively. Also, suitable linear concentration ranges were achieved from 0.48-3.3 µM for Hg(2+), 1.5-10 µM for Cu(2+), 1.6-7.7 µM for Pb(2+) and 4-26 µM for Cd(2+). The use of Dixon and Cornish-Bowden plots revealed that the suppressive effects observed with Hg(2+) and Cu(2+) were via non-competitive inhibition, while those of Pb(2+) and Cd(2+) were due to mixed and competitive inhibition. The stronger inhibition exhibited by the trace metals on GOx activity in the ultrathin PPy-GOx film was also confirmed by the low inhibition constant obtained from this analysis. The biosensor was successfully applied to the determination of trace metals in tap water samples.

Diamond nanoparticles based biosensors for efficient glucose and lactate determination

In this work, we report the modification of a gold electrode with undoped diamond nanoparticles (DNPs) and its applicability to the fabrication of electrochemical biosensing platforms. DNPs were immobilized onto a gold electrode by direct adsorption and the electrochemical behavior of the resulting DNPs/Au platform was studied. Four well-defined peaks were observed corresponding to the DNPs oxidation/reduction at the underlying gold electrode, which demonstrate that, although undoped DNPs have an insulating character, they show electrochemical activity as a consequence of the presence of different functionalities with unsaturated bonding on their surface. In order to develop a DNPs-based biosensing platform, we have selected glucose oxidase (GOx), as a model enzyme. We have performed an exhaustive study of the different steps involved in the biosensing platform preparation (DNPs/Au and GOx/DNPs/Au systems) by atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM) and cyclic voltammetry (CV). The glucose biosensor shows a good electrocatalytic response in the presence of (hydroxymethyl)ferrocene as redox mediator. Once the suitability of the prototype system to determine glucose was verified, in a second step, we prepared a similar biosensor, but employing the enzyme lactate oxidase (LOx/DNPs/Au). As far as we know, this is the first electrochemical biosensor for lactate determination that includes DNPs as nanomaterial. A linear concentration range from 0.05 mM to 0.7 mM, a sensitivity of 4.0 µA mM(-1) and a detection limit of 15 µM were obtained.

Scanning electrochemical impedance microscopy for investigation of glucose oxidase catalyzed reaction

In this research biointerface based on immobilized glucose oxidase (GOx) was evaluated by scanning electrochemical impedance microscopy (SEIM), which consisted of merged scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). The gluconolactone, which is quickly hydrolyzed to gluconic acid, is produced during the enzyme-catalyzed glucose oxidation reaction. Gluconic acid formed above an enzyme-modified not-conducting plastic surface, was evaluated by EIS technique. A two electrode cell consisting of a scanning probe, which was based on 10 μm diameter ultramicroelectrode and stationary platinum counter/reference electrode was applied for the measurement. Locally measured solution impedance depends on the gluconic acid concentration close to the ultramicroelectrode surface and on the ion diffusion, which is hindered when the electrode is approaching close to the GOx-modified surface. EIS results were evaluated by applying an equivalent circuit consisting of elements representing solution resistance, double-layer capacitance, charge-transfer resistance and Warburg impedance. Solution resistance was calculated and showed to be dependent on the position of ultramicroelectrode. Also it was observed that the thickness of the conducting layer and gluconic acid concentration both are changing in time. The results indicate that here proposed SEIM technique could become a valuable tool for the investigation and characterization of enzyme-modified surfaces of biosensors and biofuel cells.

A paper strip based non-invasive glucose biosensor for salivary analysis

In our present study, we developed an optical biosensor for direct determination of salivary glucose by using immobilized glucose oxidase enzyme on filter paper strip (specific activity 1.4 U/strip) and then reacting it with synthetic glucose samples in presence of co-immobilized color pH indicator. The filter paper changed color based on concentration of glucose in reaction media and hence, by scanning this color change (using RGB profiling) through an office scanner and open source image processing software (GIMP) the concentration of glucose in the reaction medium could be deduced. Once the biosensor was standardized, the synthetic glucose sample was replaced with human saliva from donors. The individual's blood glucose level at the time of obtaining saliva was also measured using an Accuchek(™) active glucometer (Roche Inc.). In this preliminary study, a correlation of nearly 0.64 was found between glucose levels in saliva and blood of healthy individuals and in diabetic patients it was nearly in the order of 0.95, thereby validating the importance of salivary analysis. The RGB profiling method obtained a detection range of 9-1350 mg/dL glucose at a response time of 45 s and LOD of 22.2 mg/dL.

Amperometric glucose biosensor based on layer-by-layer films of microperoxidase-11 and liposome-encapsulated glucose oxidase

An important step in several bioanalytical applications is the immobilization of biomolecules. Accordingly, this procedure must be carefully chosen to preserve their biological structure and fully explore their properties. For this purpose, we combined the versatility of the layer-by-layer (LbL) method for the immobilization of biomolecules with the protective behavior of liposome-encapsulated systems to fabricate a novel amperometric glucose biosensor. To obtain the biosensing unit, an LbL film of the H2O2 catalyst polypeptide microperoxidase-11 (MP-11) was assembled onto an indium-tin oxide (ITO) electrode followed by the deposition of a liposome-encapsulated glucose oxidase (GOx) layer. The biosensor response toward glucose detection showed a sensitivity of 0.91±0.09 (μA/cm2)/mM and a limit of detection (LOD) of 8.6±1.1 μM, demonstrating an improved performance compared to similar biosensors with a single phospholipid-liposome or even containing a non-encapsulated GOx layer. Finally, glucose detection was also performed in a zero-lactose milk sample to demonstrate the potential of the biosensor for food analysis.

A high sensitivity MEA probe for measuring real time rat brain glucose flux

The mammalian central nervous system (CNS) relies on a constant supply of external glucose for its undisturbed operation. This article presents an implantable Multi-Electrode Array (MEA) probe for brain glucose measurement. The MEA was implemented on Silicon-On-Insulator (SOI) wafer using Micro-Electro-Mechanical-Systems (MEMS) methods. There were 16 platinum recording sites on the probe and enzyme glucose oxidase (GOx) was immobilized on them. The glucose sensitivity of the MEA probe was as high as 489 µA mM(-1) cm(-2). 1,3-Phenylenediamine (mPD) was electropolymerized onto the Pt recording surfaces to prevent larger molecules such as ascorbic acid (AA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), and dopamine (DA) from reaching the recording sites surface. The MEA probe was implanted in the anesthetized rat striatum and responded to glucose levels which were altered by intraperitoneal injection of glucose and insulin. After the in vivo experiment, the MEA probe still kept sensitivity to glucose, these suggested that the MEA probe was reliable for glucose monitoring in brain extracellular fluid (ECF).

Simultaneously improving stability and specificity of cell surface displayed glucose dehydrogenase mutants to construct whole-cell biocatalyst for glucose biosensor application

The improved stability and substrate specificity of cell surface displayed glucose dehydrogenase (GDH) mutants by replacing four amino acids from Bacillus subtilis by using site-directed mutagenesis was systematically investigated. A series of mutated GDHs including E170R/Q252L, V149K/E170R/Q252L, E170R/Q252L/G259A and V149K/E170R/Q252L/G259A, were fused to the ice nucleation protein for displaying on cell surface of Eschericia coli. Q252L/E170R/V149K, Q252L/E170R/G259A and Q252L/E170R/V149K/G259A variants were found stable at a wide pH range and shown excellent thermostability. Especially, the Q252L/E170R/V149K/G259A mutant showed half-life of ~3.8days at 70 °C. Q252L/E170R/V149K/G259A variant exhibited the narrowest substrate specificity for d-glucose. The whole cell displayed GDH mutant could be cultured in a large scale with excellent enzyme activity and productivity. In addition, a sensitive and stable electrochemical glucose biosensor can be prepared using the GDH-mutant bacteria modified electrode. Thus, the whole cell biocatalysts are promising candidates for exploitation in a wide range of industrial applications.