Progress on the influence of non-enzymatic electrodes characteristics on the response to glucose detection: a review (2016–2022)

Glucose sensing devices have experienced significant progress in the last years in response to the demand for cost-effective monitoring. Thus, research efforts have been focused on achieving reliable, selective, and sensitive sensors able to monitor the glucose level in different biofluids. The development of enzyme-based devices is challenged by poor stability, time-consuming, and complex purification procedures, facts that have given rise to the synthesis of enzyme-free sensors. Recent advances focus on the use of different components: metal-organic frameworks (MOFs), carbon nanomaterials, or metal oxides. Motivated by this topic, several reviews have been published addressing the sensor materials and synthesis methods, gathering relevant information for the development of new nanostructures. However, the abundant information has not concluded yet in commercial devices and is not useful from an engineering point of view. The dependence of the electrode response on its physico-chemical nature, which would determine the selection and optimization of the materials and synthesis method, remains an open question. Thus, this review aims to critically analyze from an engineering vision the existing information on non-enzymatic glucose electrodes; the analysis is performed linking the response in terms of sensitivity when interferences are present, stability, and response under physiological conditions to the electrode characteristics.

Fabrication of Enzyme-Free and Rapid Electrochemical Detection of Glucose Sensor Based on ZnO Rod and Ru Doped Carbon Nitride Modified Gold Transducer

Over 3 in 4 adults with diabetes live in low- and middle-income counties and health expenditure also increased 316% over the last 15 years. In this regard, we fabricate low cost, reusable and rapid detection of diabetes sensor based on zinc oxide rod inserted ruthenium-doped carbon nitride (ZnO-g-Ru-C₃N₄) modified sensor device. Developed sensor device physically and electrochemically characterized using X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), chronoamperometry (CA) and differential pulse voltammetry (DPV). Sensing device as an effective enzyme-free glucose detection with high sensitivity (346 μA/mM/cm²) over the applied lower potential of +0.26 V (vs. Ag/AgCl), fast response (3 s) and broad linear range of (2-28) mM, coupled with a lower limit of detection (3.5 nM). The biosensing device gives better anti-interference ability with justifiable reproducibility, reusability (single electrode re-use 26 times in physiological buffer and 3 times in serum) and stability. Moreover, the real-time applicability of the sensor device was evaluated in human blood, serum and urine samples.

Engineering single-atom catalysts toward biomedical applications

Due to inherent structural defects, common nanocatalysts always display limited catalytic activity and selectivity, making it practically difficult for them to replace natural enzymes in a broad scope of biologically important applications. By decreasing the size of the nanocatalysts, their catalytic activity and selectivity will be substantially improved. Guided by this concept, the advances of nanocatalysts now enter an era of atomic-level precise control. Single-atom catalysts (denoted as SACs), characterized by atomically dispersed active sites, strikingly show utmost atomic utilization, precisely located metal centers, unique metal-support interactions and identical coordination environments. Such advantages of SACs drastically boost the specific activity per metal atom, and thus provide great potential for achieving superior catalytic activity and selectivity to functionally mimic or even outperform natural enzymes of interest. Although the size of the catalysts does matter, it is not clear whether the guideline of "the smaller, the better" is still correct for developing catalysts at the single-atom scale. Thus, it is clearly a new, urgent issue to address before further extending SACs into biomedical applications, representing an important branch of nanomedicine. This review begins by providing an overview of recent advances of synthesis strategies of SACs, which serve as a basis for the discussion of emerging achievements in improving the enzyme-like catalytic properties at an atomic level. Then, we carefully compare the structures and functions of catalysts at various scales from nanoparticles, nanoclusters, and few-atom clusters to single atoms. Contrary to conventional wisdom, SACs are not the most catalytically active catalysts in specific reactions, especially those requiring multi-site auxiliary activities. After that, we highlight the unique roles of SACs toward biomedical applications. To appreciate these advances, the challenges and prospects in rapidly growing studies of SACs-related catalytic nanomedicine are also discussed in this review.

The Influence of Metal-Doped Graphitic Carbon Nitride on Photocatalytic Conversion of Acetic Acid to Carbon Dioxide

Metal-doped graphitic carbon nitride (MCN) materials have shown great promise as effective photocatalysts for the conversion of acetic acid to carbon dioxide under UV–visible irradiation and are superior to pristine carbon nitride (g-C₃N₄, CN). In this study, the effects of metal dopants on the physicochemical properties of metal-doped CN samples (Fe-, Cu-, Zn-, FeCu-, FeZn-, and CuZn-doped CN) and their catalytic activity in the photooxidation of acetic acid were investigated and discussed for their correlation, especially on their surface and bulk structures. The materials in the order of highest to lowest photocatalytic activity are FeZn_CN, FeCu_CN, Fe_CN, and Cu_CN (rates of CO₂ evolution higher than for CN), followed by Zn_CN, CuZn_CN, and CN (rates of CO₂ evolution lower than CN). Although Fe doping resulted in the extension of the light absorption range, incorporation of metals did not significantly alter the crystalline phase, morphology, and specific surface area of the CN materials. However, the extension of light absorption into the visible region on Fe doping did not provide a suitable explanation for the increase in photocatalytic efficiency. To further understand this issue, the materials were analyzed using two complementary techniques, reversed double-beam photoacoustic spectroscopy (RDB-PAS) and electron spin resonance spectroscopy (ESR). The FeZn_CN, with the highest electron trap density between 2.95 and 3.00 eV, afforded the highest rate of CO₂ evolution from acetic acid photodecomposition. All Fe-incorporated CN materials and Cu-CN reported herein can be categorized as high activity catalysts according to the rates of CO₂ evolution obtained, higher than 0.15 μmol/min⁻¹, or >1.5 times higher than that of pristine CN. Results from this research are suggestive of a correlation between the rate of CO₂ evolution via photocatalytic oxidation of acetic acid with the threshold number of free unpaired electrons in CN-based materials and high electron trap density (between 2.95 and 3.00 eV).

Non-Enzymatic Glucose Sensors Involving Copper: An Electrochemical Perspective

Non-enzymatic glucose sensors based on the use of copper and its oxides have emerged as promising candidates to replace enzymatic glucose sensors owing to their stability, ease of fabrication, and superior sensitivity. This review explains the theories of the mechanism of glucose oxidation on copper transition metal electrodes. It also presents an overview on the development of among the best non-enzymatic copper-based glucose sensors in the past 10 years. A brief description of methods, interesting findings, and important performance parameters are provided to inspire the reader and researcher to create new improvements in sensor design. Finally, several important considerations that pertain to the nano-structuring of the electrode surface is provided.

Laser-driven growth of structurally defined transition metal oxide nanocrystals on carbon nitride photoelectrodes in milliseconds

Fabrication of hybrid photoelectrodes on a subsecond timescale with low energy consumption and possessing high photocurrent densities remains a centerpiece for successful implementation of photoelectrocatalytic synthesis of fuels and value-added chemicals. Here, we introduce a laser-driven technology to print sensitizers with desired morphologies and layer thickness onto different substrates, such as glass, carbon, or carbon nitride (CN). The specially designed process uses a thin polymer reactor impregnated with transition metal salts, confining the growth of transition metal oxide (TMO) nanostructures on the interface in milliseconds, while their morphology can be tuned by the laser. Multiple nano-p-n junctions at the interface increase the electron/hole lifetime by efficient charge trapping. A hybrid copper oxide/CN photoanode with optimal architecture reaches 10 times higher photocurrents than the pristine CN photoanode. This technology provides a modular approach to build a library of TMO-based composite films, enabling the creation of materials for diverse applications.

A Redox Cu(II)-Graphene Oxide Modified Screen Printed Carbon Electrode as a Cost-Effective and Versatile Sensing Platform for Electrochemical Label-Free Immunosensor and Non-enzymatic Glucose Sensor

A novel copper (II) ions [Cu(II)]-graphene oxide (GO) nanocomplex-modified screen-printed carbon electrode (SPCE) is successfully developed as a versatile electrochemical platform for construction of sensors without an additionally external redox probe. A simple strategy to prepare the redox GO-modified SPCE is described. Such redox GO based on adsorbed Cu(II) is prepared by incubation of GO-modified SPCE in the Cu(II) solution. This work demonstrates the fabrications of two kinds of electrochemical sensors, i.e., a new label-free electrochemical immunosensor and non-enzymatic sensor for detections of immunoglobulin G (IgG) and glucose, respectively. Our immunosensor based on square-wave voltammetry (SWV) of the redox GO-modified electrode shows the linearity in a dynamic range of 1.0-500 pg.mL-1 with a limit of detection (LOD) of 0.20 pg.mL-1 for the detection of IgG while non-enzymatic sensor reveals two dynamic ranges of 0.10-1.00 mM (sensitivity = 36.31 μA.mM-1.cm-2) and 1.00-12.50 mM (sensitivity = 3.85 μA.mM-1.cm-2) with a LOD value of 0.12 mM. The novel redox Cu(II)-GO composite electrode is a promising candidate for clinical research and diagnosis.

Copper/reduced graphene oxide film modified electrode for non-enzymatic glucose sensing application

Numerous studies suggest that modification with functional nanomaterials can enhance the electrode electrocatalytic activity, sensitivity, and selectivity of the electrochemical sensors. Here, a highly sensitive and cost-effective disposable non-enzymatic glucose sensor based on copper(II)/reduced graphene oxide modified screen-printed carbon electrode is demonstrated. Facile fabrication of the developed sensing electrodes is carried out by the adsorption of copper(II) onto graphene oxide modified electrode, then following the electrochemical reduction. The proposed sensor illustrates good electrocatalytic activity toward glucose oxidation with a wide linear detection range from 0.10 mM to 12.5 mM, low detection limit of 65 µM, and high sensitivity of 172 μA mM^–1 cm^–2 along with satisfactory anti-interference ability, reproducibility, stability, and the acceptable recoveries for the detection of glucose in a human serum sample (95.6–106.4%). The copper(II)/reduced graphene oxide based sensor with the superior performances is a great potential for the quantitation of glucose in real samples.

Platinum and zinc oxide modified carbon nitride electrode as non-enzymatic highly selective and reusable electrochemical diabetic sensor in human blood

Development of non-enzymatic glucose sensor is essential to reduce the cost of diabetes regular monitoring. Here, graphitic carbon nitride (g-C₃N₄) is modified with platinum and zinc oxide for non-enzymatic electrochemical glucose sensing in physiological conditions for the first time in the literature. The interactions between Pt, g-C₃N₄ and the ZnO are studied using different physicochemical characterization techniques. The Electrochemical glucose sensing at the ZnO-Pt-gC₃N₄ occurs at low applied potential of +0.20 V (vs. Ag/AgCl) with high sensitivity 3.34 μA/mM/cm² and fast response (5 s) time. This sensor exhibited a wide linear range 0.25-110 mM with lower limit of detection of 0.1 µM. The architectured sensor was evaluated in human blood, serum and urine samples. The sensor is 4 time reusable in whole blood without activity deterioration. This reusable surface helps to reduce the cost of strip.