Metal Oxide-Based Composites in Nonenzymatic Electrochemical Glucose Sensors

Grain boundary/doping/architecture engineering in hierarchical N-doped CuO microflowers derived from Cu-based metal-organic framework architectures for highly efficient nonenzymatic glucose detection

Glucose detection is essential in clinical medicine, and the reasonable design of metal oxide electrocatalysts plays a crucial role in developing efficient nonenzymatic glucose (NEG) sensors. Herein, grain boundary/doping/architecture engineering is used to tailor the structures of CuO nanomaterials and tune their surface/electron-transfer properties toward enhanced electrocatalytic oxidation of glucose. Hierarchical N-doped CuO microflowers (N-CuO-MF) are synthesized using a facile hydrothermal method, followed by calcination. N-CuO-MF consist of ultrathin nanoflakes (ca. 20 nm), endowing them with a large specific surface area. Moreover, the nanoflakes are composed of ultrasmall nanoparticles, resulting in abundant grain boundaries. Notably, N-CuO-MF are derived from a precursor of Cu-based metal-organic framework (Cu-MOF) architectures, which is fabricated through a bottom-up route using glycerol as the capping agent/solvent and 1-hexadecyl-3-methylimidazolium bromide ([C16mim]Br) as the template/N source. Glycerol competitively coordinates with Cu2+, leading to the formation of 2D subunits. Moreover, [C16mim]+ cations attach to the subunit surfaces via electrostatic interaction, thus achieving Cu-MOF with a 3D hierarchical structure. As expected, the synergistic effect of rich grain boundaries, N doping, ultrathin nanoflakes, and hierarchical architecture enhances the adsorption of glucose on the electrode surfaces, accelerates electron transfer, and exposes more active sites for glucose oxidation. Accordingly, N-CuO-MF exhibit wide linear ranges, high sensitivity, fast response time, low detection limit, excellent selectivity, and good stability. Owing to their highly efficient electrocatalytic properties, N-CuO-MF could be explored as potential electrocatalysts in NEG sensors for rapid diagnostic tests and health monitoring.

Single-atom nanozymes for enhanced electrochemical biosensing: A review

Enzyme-based electrochemical biosensors have broad and significant applications in biomedical, environmental monitoring, and food safety fields. However, the application of natural enzymes is limited due to issues such as poor stability, complex preparation, and high cost. Single-atom nanozymes (SAzymes), with their unique catalytic properties and efficient enzyme-like activities, present a promising alternative in the field of electrochemical biosensing. Compared to traditional enzymes, SAzyme offer enhanced stability and controllability, making them particularly effective in complex detection environments. This work presents the first systematic review of the progress made since 2018 in the use of SAzymes as alternatives to natural enzymes in electrochemical biosensors, and presents the latest advancements in this area. The review begins with a discussion of various enzyme-like activities of single-atom materials, including peroxidase (POD)-like, oxidase (OXD)-like, catalase (CAT)-like, and superoxide dismutase (SOD)-like activities. It then explores the advantages of SAzymes in improving the performance of electrochemical biosensors from multiple perspectives. The review also summarizes the applications of SAzyme-based electrochemical sensors for reactive oxygen species (ROS), metabolites, neurotransmitters, and other analytes, highlighting specific examples to elucidate underlying catalytic mechanisms and understand fundamental structure-performance relationships. In the final section, the challenges faced by SAzyme-based electrochemical biosensing are discussed, along with potential solutions.

Spinel CuCo2O4 Microflowers with Highly Exposed High-Index (112) Facets for Electrocatalytic Glucose Oxidation

The design of nanomaterials enclosed by high-index facets plays a critical role in the surface-sensitive properties. Herein, hierarchical CuCo2O4 microflowers (CCO-F) with highly exposed high-index (112) facets are rationally designed via a solvothermal method followed by calcination. CCO-F are composed of 30 nm-thick nanoflakes and have an ultrahigh specific surface area of ca. 205.48 m2 g-1. Additionally, nanoparticle-assembled CuCo2O4 microspheres (CCO-S) are prepared, and these nanoparticles are partly enclosed by the (110) facets. Abundant octahedrally coordinated Co3+ (Co3+Oh) and tetrahedrally coordinated Cu2+ (Cu2+Td) exist in the (112) facet, especially Co3+Oh (with a density of 0.063 Å-2); moreover, the (112) facet possesses a high surface Gibbs free energy (2.7367 J m-2). Therefore, CCO-F have significant advantages of adsorbing glucose and conducting the subsequent redox reactions. In addition, the hierarchical microstructure promotes the reaction kinetics of CCO-F. Benefiting from these features, the CCO-F-modified electrode exhibits high sensitivities (1351.2 and 598.7 μA mM-1 cm-2), wide linear ranges (1 μM-3 mM and 4-10 mM), rapid response time, low detection limit, excellent stability, and good selectivity. This work indicates that the exposure of a high percentage of high-index facets is an effective approach to exploring high-performance electrocatalysts for glucose detection.

Multifunctional nanomaterials for smart wearable diabetic healthcare devices

Wearable diabetic healthcare devices have attracted great attention for real-time continuous glucose monitoring (CGM) using biofluids such as tears, sweat, saliva, and interstitial fluid via noninvasive ways. In response to the escalating global demand for CGM, these devices enable proactive management and intervention of diabetic patients with incorporated drug delivery systems (DDSs). In this context, multifunctional nanomaterials can trigger the development of innovative sensing and management platforms to facilitate real-time selective glucose monitoring with remarkable sensitivity, on-demand drug delivery, and wireless power and data transmission. The seamless integration into wearable devices ensures patient's compliance. This comprehensive review evaluates the multifaceted roles of these materials in wearable diabetic healthcare devices, comparing their glucose sensing capabilities with conventionally available glucometers and CGM devices, and finally outlines the merits, limitations, and prospects of these devices. This review would serve as a valuable resource, elucidating the intricate functions of nanomaterials for the successful development of advanced wearable devices in diabetes management.

MOF-derived NiCo hydroxide for highly efficient non-enzymatic glucose biosensing

An efficient and robust electrocatalyst is significant for glucose biosensing. The emergence of metal-organic framework (MOF) derived materials opens up new avenues for the development of high-performance glucose sensing catalysts. Herein, MOF derived nickel-cobalt hydroxide supported on conductive copper sheet (NiCo-OH/Cu sheet) is prepared at room temperature. The as-obtained NiCo-OH is endowed with three-dimensional network structure which enables the effective exposure of active materials, sufficient contact between glucose molecule and catalyst. The NiCo-OH/Cu sheet is revealed as good glucose electrochemical sensing material with a wide linear range of 0.05∼6.0 mM and a high sensitivity of 1340μA mM-1cm-2. Additionally, the as-fabricated NiCo-OH/Cu sheet displays good anti-interference ability and long-term stability.

Electrochemical nonenzymatic glucose sensors catalyzed by Au nanoclusters on metallic nanotube arrays and polypyrrole nanowires

Monitoring blood glucose level is critical, since its abnormality leads to diabetes and causes death, even though glucose is essential for human living. Herein, the sensing study was performed on electrochemical nonenzymatic glucose sensors, which are composed of an Au nanocluster (AuNC) catalyst deposited on a metallic nanotube array (MeNTA) and polypyrrole nanowire (PPyNW). The AuNC was produced by irradiating a femtosecond pulse laser to the Au precursor solution, and it is a simple and facile method. The successful deposition of AuNC on both MeNTA and PPyNW was confirmed by means of the surface morphology and the Au content increase. On the exploration by cyclic voltammetry in alkaline condition, AuNC/MeNTA electrodes showed better performance than AuNC/PPyNW electrodes: The former was a remarkable electrocatalytic detector towards glucose oxidation with better sensitivity, lower detection limit, wider linear range, and longer-term stability without interference from potential interfering agents such as ascorbic acid, urea, NaCl, KCl, etc. Moreover, nonenzymatic AuNC/MeNTA electrodes exhibited high precision and accuracy in real human blood samples and, thus, can be a promising candidate in glucose sensing applications.

Optimized Copper-Based Microfeathers for Glucose Detection

Diabetes is expected to rise substantially by 2045, prompting extensive research into accessible glucose electrochemical sensors, especially those based on non-enzymatic materials. In this context, advancing the knowledge of stable metal-based compounds as alternatives to non-enzymatic sensors becomes a scientific challenge. Nonetheless, these materials have encountered difficulties in maintaining stable responses under physiological conditions. This work aims to advance knowledge related to the synthesis and characterization of copper-based electrodes for glucose detection. The microelectrode presented here exhibits a wide linear range and a sensitivity of 1009 µA∙cm-2∙mM-1, overperfoming the results reported in literature so far. This electrode material has also demonstrated outstanding results in terms of reproducibility, repeatability, and stability, thereby meeting ISO 15197:2015 standards. Our study guides future research on next-generation sensors that combine copper with other materials to enhance activity in neutral media.

A Review of Electroactive Nanomaterials in the Detection of Nitrogen-Containing Organic Compounds and Future Applications

Electrochemical and impedimetric detection of nitrogen-containing organic compounds (NOCs) in blood, urine, sweat, and saliva is widely used in clinical diagnosis. NOC detection is used to identify illnesses such as chronic kidney disease (CKD), end-stage renal disease (ESRD), cardiovascular complications, diabetes, cancer, and others. In recent years, nanomaterials have shown significant potential in the detection of NOCs using electrochemical and impedimetric sensors. This potential is due to the higher surface area, porous nature, and functional groups of nanomaterials, which can aid in improving the sensing performance with inexpensive, direct, and quick-time processing methods. In this review, we discuss nanomaterials, such as metal oxides, graphene nanostructures, and their nanocomposites, for the detection of NOCs. Notably, researchers have considered nanocomposite-based devices, such as a field effect transistor (FET) and printed electrodes, for the detection of NOCs. In this review, we emphasize the significant importance of electrochemical and impedimetric methods in the detection of NOCs, which typically show higher sensitivity and selectivity. So, these methods will open a new way to make embeddable electrodes for point-of-detection (POD) devices. These devices could be used in the next generation of non-invasive analysis for biomedical and clinical applications. This review also summarizes recent state-of-the-art technology for the development of sensors for on-site monitoring and disease diagnosis at an earlier stage.

A non-enzymatic glucose sensor based on a mesoporous carbon sphere immobilized Co-MOF-74 nanocomposite

Exploration of credible non-enzymatic glucose sensors with high selectivity and sensitivity is of great significance for early clinical monitoring of glucose concentration and preventing the threat of diabetes to human health. Here, mesoporous carbon (MC) sphere immobilized Co-MOF-74 nanorods (NRs), denoted as Co-MOF-74 NRs/MC, were successfully prepared, in which the nanostructural porous carbon sphere was obtained using cobalt glycolate as the built-in template followed by a subsequent carbonization and acid treatment, and the MC spheres were then in situ deposited on the surface of Co-MOF-74 NRs via a solvothermal method. Benefiting from the good conductivity of the grafted porous carbon spheres and the abundant active sites, as well as the permeability of microporous MOF-74 nanocrystals, the Co-MOF-74 NRs/MC modified glassy carbon electrode (GCE) exhibited effective non-enzymatic glucose sensing performance with a fast response time (less than 3 s) and a glucose sensitivity of 98.0 μA cm-2 mM-1. Furthermore, the Co-MOF-74 NRs/MC/GCE showed a favourable anti-interference capability in the presence of various interferents and good long-term reusability. The applicability of Co-MOF-74 NRs/MC/GCE for glucose sensing in real serum samples was also investigated, verifying the applicability of the electrode for targeted glucose monitoring in practical applications.

In-situ construction of Au/Cu2O nanowire arrays for sensitive glucose sensing

Architecture design is widely regarded as a rational strategy to enhance the sensing performance of electrocatalysts. Herein, the novel three-dimensional hybrids based on Au and Cu₂O were successfully synthesized via steps of in-situ growth, including anodic oxidation, annealing and galvanic displacement. Cu₂O appeared in the morphology of nanowire array on conductive substrate, and was decorated by Au nanoparticles. Benefiting from the unique architecture and binder-free fabrication process, the Au/Cu₂O nanowire arrays possessed high conductivity and abundant exposed active sites, as well as facilitated the direct electron transfer among detection object, electrocatalyst and current collector. Moreover, Au/Cu₂O particles as contrast were fabricated to clarify the effect of structure on sensing ability. The Au/Cu₂O nanowire arrays drove the glucose electro-oxidation reaction with great catalytic activity, in which a potential as low as 0.4 V was needed to reach a high sensitivity of 2.098 mA mM^-1 cm^-2. The excellent selectivity, stability and reproducibility were also obtained by the sensor. Furthermore, the quantitative detection of glucose level in diluted human serum were performed and the satisfactory result make the obtained sensor have the potential for practical applications.

Nonenzymatic Sweat Wearable Uric Acid Sensor Based on N-Doped Reduced Graphene Oxide/Au Dual Aerogels

Sweat wearable sensors enable noninvasive and real-time metabolite monitoring in human health management but lack accuracy and wearable applicability. The rational design of sensing electrode materials will be critical yet challenging. Herein, we report a dual aerogel-based nonenzymatic wearable sensor for the sensitive and selective detection of uric acid (UA) in human sweat. The three-dimensional porous dual-structural aerogels composed of Au nanowires and N-doped graphene nanosheets (noted as N-rGO/Au DAs) provide a large active surface, abundant access to the target, rapid electron transfer pathways, and a high intrinsic activity. Thus, a direct UA electro-oxidation is demonstrated at the N-rGO/Au DAs with a much higher activity than those at the individual gels (i.e., Au and N-rGO). Moreover, the resulting sensing chip displays high performance with a good anti-interfering ability, long-term stability, and excellent flexibility toward the UA detection. With the assistance of a wireless circuit, a wearable sensor is successfully applied in the real-time UA monitoring on human skin. The obtained result is comparable to that evaluated by high-performance liquid chromatography. This dual aerogel-based nonenzymatic biosensing platform not only holds considerable promise for the reliable sweat metabolite monitoring but also opens an avenue for metal-based aerogels as flexible electrodes in wearable sensing.

Salivary Glucose Detection with Laser Induced Graphene/AgNPs Non-Enzymatic Sensor

The tailoring of novel nanomaterials for sensitive glucose detection through a non-enzymatic mechanism is currently under intensive research. Here, we present a laser-induced graphene (LIG) electrode decorated with silver nanoparticles (AgNPs) as a catalytic element for the direct electrooxidation of glucose. The AgNPs were synthesized through cyclic voltammetry using LIG as a template, resulting in a porous tridimensional assembly with anchored nanostructures. The characterization corroborated the formation of LIG/AgNPs composite with distinctive peaks attributed to Ag₂O and AgO interaction with glucose. The proposed non-enzymatic sensors were successfully applied for non-enzymatic amperometric detection, exhibiting a linear range from 1 to 10 mM in the first peak (+0.7 V) and a narrow range from 1 to 2 mM with higher sensitivity of 52.2 mA/mM and improved LOD of 45 μM in the second peak (+0.55 V). The applicability of the LIG/AgNPs sensor was evaluated with spiked artificial saliva in a PoC format using a smartphone potentiostat, showing an average recovery rate of 91%. The analysis was performed in a portable, mobile, and low-cost fashion using a simulated non-invasive sample, with promising results in clinical ranges.

A dual-functional flexible sensor based on defects-free Co-doped ZnO nanorods decorated with CoO clusters towards pH and glucose monitoring of fruit juices and human fluids

Herein, ZnO nanorods were doped with Co and decorated with CoO clusters through an in situ technique to create a CoO/Co-doped ZnO (CO/CZO) heterostructure at low temperatures (150 °C) on a flexible PET substrate. In the CO/CZO heterostructure, the Co dopant has a low energy barrier to substitute Zn atoms and adsorb over oxygen atoms and their vacancies. Therefore, it decreased the charge density (ND = 2.64 × 1019 cm-3) on non-active sites of ZnO and lowered the charge transfer resistance (317 Ω) at Co-doped-ZnO/electrolyte interface by suppressing the native defects and reducing the Schottky barrier height (- 0.35 eV), respectively. Furthermore, CoO clusters induced a p-n heterostructure with Co-doped ZnO, prevented corrosion, increased the active sites for analyte absorption, and increased the ultimate tensile strength (4.85 N m-2). These characteristics enabled the CO/CZO heterostructure to work as a highly sensitive, chemically stable, and flexible pH and glucose oxidation electrode. Therefore, CO/CZO heterostructure was explored for pH monitoring in human fluids and fruit juices, demonstrating a near-Nernst-limit pH sensitivity (52 mV/pH) and fast response time (19 s) in each human fluid and fruit juice. Also, it demonstrated high sensitivity (4656 µM mM-1 cm-2), low limit of detection (0.15 µM), a broad linear range (0.04 mM to 8.85 mM) and good anti-interference capacity towards glucose-sensing. Moreover, it demonstrated excellent flexibility performances, retained 53% and 69% sensitivity of the initial value for pH and glucose sensors, respectively, after 500 bending, stretching, and warping cycles.