Conductive Metal-Organic Framework Grown on the Nickel-Based Hydroxide to Realize High-Performance Electrochemical Glucose Sensing

Glucose holds significant importance in disease diagnosis as well as beverage quality monitoring. The high-efficiency electrochemical sensor plays a crucial role in the electrochemical conversion technology. Ni(OH)2 nanosheets are provided with high specific surface area and redox activity that are widely used in electrochemistry. Conductive metal-organic frameworks (cMOFs) perfectly combine the structural controllability of organic materials with the long-range ordering of inorganic materials that possess the characteristic of high electron mobility. Based on the above considerations, the combination of Ni(OH)2 and Ni-HHTP (HHTP=2,3,6,7,10,11-hexahydroxytriphenylene) as an electrode modification material is designed to enhance electrochemical performance. In this work, to improve glucose detection, a sequence of Ni(OH)2@NiCo-HHTP and NiM-LDH@Ni-HHTP (M=Co2+, Mn2+, Cu2+, LDH=layered double hydroxide) are successfully synthesised by doping metals into Ni-HHTP and Ni(OH)2, respectively. As a result, NiCu-LDH@Ni-HHTP showed the best excellent glucose detection sensitivity.

Flexible electrochemical biosensors for healthcare monitoring

As the interest in wearable devices has increased recently, increasing biosensor flexibility has begun to attract considerable attention. Among the various types of biosensors, electrochemical biosensors are uniquely suited for the development of such flexible biosensors due to their many advantages, including their fast response, inherent miniaturization, convenient operation, and portability. Therefore, many studies on flexible electrochemical biosensors have been conducted in recent years to achieve non-invasive and real-time monitoring of body fluids such as tears, sweat, and saliva. To achieve this, various substrates, novel nanomaterials, and detection techniques have been utilized to develop conductive flexible platforms that can be applied to create flexible electrochemical biosensors. In this review, we discussed recently reported flexible electrochemical biosensors and divided them into specific categories including materials for flexible substrate, fabrication techniques for flexible biosensor development, and recently developed flexible electrochemical biosensors to externally monitor target molecules, thereby providing a means to noninvasively examine cells and body fluid samples. In conclusion, this review will discuss the materials, methods, recent studies, and perspectives on flexible electrochemical biosensors for healthcare monitoring and wearable biosensing systems.

A longitudinally expanded Ni-based metal-organic framework with enhanced double nickel cation catalysis reaction channels for a non-enzymatic sweat glucose biosensor

Nickel-based metal-organic frameworks (Ni-MOFs) have attracted increasing attention in non-enzymatic glucose sensing. However, the insufficient active Ni cation sites from a stacked MOF layer, the unclear Ni catalysis mechanism, and the severe liquid alkaline electrolyte remain challenging for practical applications. In this work, the sonication-induced longitudinal-expansion of Ni-MOFs increases the active nickel ion sites, which not only enhances the current response to glucose detection, but also shows the oxidation peak evolution of nickel ions with different sonication times, revealing the mechanism of different glucose detection channels. The Ni-MOF sonicated for 60 min (60 min Ni-MOF) displays enhanced Ni(iii)/Ni(ii) and more significant Ni(iv)/Ni(iii) double nickel cation channels for catalyzing glucose into glucolactone compared to the 0 min Ni-MOF (without sonication), showing optimized glucose detection ability with a high sensitivity of 3297.10 μA mM-1 cm-2, a low detection limit of ∼8.97 μM (signal-to-noise = 3) and a wide linear response range from 10 to 400 μM from the cyclic voltammetry test as well as a high sensitivity of 3.03 μA mM-1 cm-2, a low detection limit of ∼1.16 μM (signal-to-noise = 3) and a wide linear response range from 10 to 2000 μM from the chronoamperometry test. More importantly, an all-solid-state glucose biosensor using a PVA/NaOH solid-state electrolyte and a disposable 60 min Ni-MOF working electrode is assembled for non-enzymatic sweat glucose detection.

Electrochemical Detection of H2O2 Released from Prostate Cancer Cells Using Pt Nanoparticle-Decorated rGO–CNT Nanocomposite-Modified Screen-Printed Carbon Electrodes

In this study, we fabricated platinum nanoparticles (PtNP)-decorated, porous reduced graphene oxide (rGO)–carbon nanotube (CNT) nanocomposites on a PtNP-deposited screen-printed carbon electrode (PtNP/rGO–CNT/PtNP/SPCE) for detection of hydrogen peroxide (H2O2), which is released from prostate cancer cells LNCaP. The PtNP/rGO–CNT/PtNP/SPCE was fabricated by a simple electrochemical deposition and co-reduction method. In addition, the amperometric response of the PtNP/rGO–CNT/PtNP/SPCE electrode was evaluated through consecutive additions of H2O2 at an applied potential of 0.2 V (vs. Ag pseudo-reference electrode). As a result, the prepared PtNP/rGO–CNT/PtNP/SPCE showed good electrocatalytic activity toward H2O2 compared to bare SPCE, rGO–CNT/SPCE, PtNP/SPCE, and rGO–CNT/PtNP/SPCE. In addition, the PtNP/rGO–CNT/PtNP/SPCE electrode exhibited a sensitivity of 206 μA mM−1·cm−2 to H2O2 in a linear range of 25 to 1000 μM (R2 = 0.99). Moreover, the PtNP/rGO–CNT/PtNP/SPCE electrode was less sensitive to common interfering substances, such as ascorbic acid, uric acid, and glucose than H2O2. Finally, real-time monitoring of H2O2 released from LNCaP cells was successfully performed by this electrode. Therefore, we expect that the PtNP/rGO–CNT/PtNP/SPCE can be utilized as a promising electrochemical sensor for practical nonenzymatic detection of H2O2 in live cells or clinical analysis.

The design, fabrication, and applications of flexible biosensing devices

Flexible biosensors form part of a rapidly growing research field that take advantage of a multidisciplinary approach involving materials, fabrication and design strategies to be able to function at biological interfaces that may be soft, intrinsically curvy, irregular, or elastic. Numerous exciting advancements are being proposed and developed each year towards applications in healthcare, fundamental biomedical research, food safety and environmental monitoring. In order to place these developments in perspective, this review is intended to present an overview on field of flexible biosensor development. We endeavor to show how this subset of the broader field of flexible and wearable devices presents unique characteristics inherent in their design. Initially, a discussion on the structure of flexible biosensors is presented to address the critical issues specific to their design. We then summarize the different materials as substrates that can resist mechanical deformation while retaining their function of the bioreceptors and active elements. Several examples of flexible biosensors are presented based on the different environments in which they may be deployed or on the basis of targeted biological analytes. Challenges and future perspectives pertinent to the current and future stages of development are presented. Through these summaries and discussion, this review is expected to provide insights towards a systematic and fundamental understanding for the fabrication and utilization of flexible biosensors, as well as inspire and improve designs for smart and effective devices in the future.

Fabrication of Stretchable Copper Coated Carbon Nanotube Conductor for Non-Enzymatic Glucose Detection Electrode with Low Detection Limit and Selectivity

The increasing demand for wearable glucose sensing has stimulated growing interest in stretchable electrodes. The development of the electrode materials having large stretchability, low detection limit, and good selectivity is the key component for constructing high performance wearable glucose sensors. In this work, we presented fabrication of stretchable conductor based on the copper coated carbon nanotube sheath-core fiber, and its application as non-enzymatic electrode for glucose detection with high stretchability, low detection limit, and selectivity. The sheath-core fiber was fabricated by coating copper coated carbon nanotube on a pre-stretched rubber fiber core followed by release of pre-stretch, which had a hierarchically buckled structure. It showed a small resistance change as low as 27% as strain increasing from 0% to 500% strain, and a low resistance of 0.4 Ω·cm-1 at strain of 500%. This electrode showed linear glucose concentration detection in the range between 0.05 mM and 5 mM and good selectivity against sucrose, lactic acid, uric acid, acrylic acid in phosphate buffer saline solution, and showed stable signal in high salt concentration. The limit of detection (LOD) was 0.05 mM, for the range of 0.05⁻5 mM, the sensitivity is 46 mA·M-1. This electrode can withstand large strain of up to 60% with negligible influence on its 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.

Highly Reproducible Au-Decorated ZnO Nanorod Array on a Graphite Sensor for Classification of Human Aqueous Humors

Gold-decorated, vertically grown ZnO nanorods (NRs) on a flexible graphite sheet (Au/ZnONRs/G) were developed for surface-enhanced Raman scattering (SERS)-based biosensing to identify trace amounts of human aqueous humors. This Au/ZnONRs/G SERS-functionalized sensor was fabricated via two steps: hydrothermal synthesis-induced growth of ZnO NRs on graphite sheets for nanostructure fabrication, followed by e-beam evaporator-induced gold metallization on ZnONRs/G for SERS functionalization. The thickness of the Au layer and the height of the ZnO NRs for enhancing SERS performance were adjusted to maximize Raman intensity, and the optimized Au/ZnONRs/G nanostructures were verified by the electric finite element computational models to maximize the electric fields. The proposed Au/ZnONRs/G SERS sensor showed an enhancement factor of 2.3 × 10⁶ via rhodamine 6G Raman probe and excellent reproducibility (relative standard deviation of <10%) via Raman mapping of a SERS active area with a square of 100 × 100 μm². To evaluate the actual bioapplicability of point-of-care-testing (POCT) analysis in clinics, SERS data acquisition was performed with an integration time of 1 s from a 1 μL analytic droplet of the sample. The performance of this Au/ZnONRs/G sensor was evaluated using human aqueous humors with cataract and two oxidative stress-induced eye diseases, age-related macular degeneration, and diabetic macular edema. These three eye diseases could be identified without any labeling or modification using the Au/ZnONRs/G SERS sensor and the computational algorithm incorporating a support vector machine and multivariate statistical prediction. Therefore, these findings indicate that our label-free, highly reproducible and flexible Au/ZnONRs/G SERS-functionalized sensor supported by a multivariate statistics-derived bioclassification method has great potential in POCT applications for identifying eye diseases.

Synthesis of nickel(ii) coordination polymers and conversion into porous NiO nanorods with excellent electrocatalytic performance for glucose detection

Porous NiO nanostructures are fabricated by calcinating the Ni(SA)₂(H₂O)₄coordination polymers and used as electrocatalysts for the detection of glucose in a nonenzymatic electrochemical sensor.

Flexible Substrate-Based Devices for Point-of-Care Diagnostics

Point-of-care (POC) diagnostics play an important role in delivering healthcare, particularly for clinical management and disease surveillance in both developed and developing countries. Currently, the majority of POC diagnostics utilize paper substrates owing to affordability, disposability, and mass production capability. Recently, flexible polymer substrates have been investigated due to their enhanced physicochemical properties, potential to be integrated into wearable devices with wireless communications for personalized health monitoring, and ability to be customized for POC diagnostics. Here, we focus on the latest advances in developing flexible substrate-based diagnostic devices, including paper and polymers, and their clinical applications.

Nitrogen-Doped Carbon Dots as A New Substrate for Sensitive Glucose Determination

Nitrogen-doped carbon dots are introduced as a novel substrate suitable for enzyme immobilization in electrochemical detection metods. Nitrogen-doped carbon dots are easily synthesised from polyacrylamide in just one step. With the help of the amino group on chitosan, glucose oxidase is immobilized on nitrogen-doped carbon dots-modified carbon glassy electrodes by amino-carboxyl reactions. The nitrogen-induced charge delocalization at nitrogen-doped carbon dots can enhance the electrocatalytic activity toward the reduction of O₂. The specific amino-carboxyl reaction provides strong and stable immobilization of GOx on electrodes. The developed biosensor responds efficiently to the presence of glucose in serum samples over the concentration range from 1 to 12 mM with a detection limit of 0.25 mM. This novel biosensor has good reproducibility and stability, and is highly selective for glucose determination under physiological conditions. These results indicate that N-doped quantum dots represent a novel candidate material for the construction of electrochemical biosensors.

Plant root nodule like nickel-oxide–multi-walled carbon nanotube composites for non-enzymatic glucose sensors

Herein, in this work we synthesized plant root nodule like NiO–MWCNT nanocomposites by a simple, rapid and solvent-free method using nickel formate as a precursor.