Electrochemical Fine-Tuning of the Chemoresponsiveness of Langmuir-Blodgett Graphene Oxide Films

Graphene oxide has been widely deployed in electrical sensors for monitoring physical, chemical, and biological processes. The presence of abundant oxygen functional groups makes it an ideal substrate for integrating biological functional units to assemblies. However, the introduction of this type of defects on the surface of graphene has a deleterious effect on its electrical properties. Therefore, adjusting the surface chemistry of graphene oxide is of utmost relevance for addressing the immobilization of biomolecules, while preserving its electrochemical integrity. Herein, we describe the direct immobilization of glucose oxidase onto graphene oxide-based electrodes prepared by Langmuir-Blodgett assembly. Electrochemical reduction of graphene oxide allowed to control its surface chemistry and, by this, regulate the nature and density of binding sites for the enzyme and the overall responsiveness of the Langmuir-Blodgett biofilm. X-ray photoelectron spectroscopy, surface plasmon resonance, and electrochemical measurements were used to characterize the compositional and functional features of these biointerfaces. Covalent binding between amine groups on glucose oxidase and epoxy and carbonyl groups on the surface of graphene oxide was successfully used to build up stable and active enzymatic assemblies. This approach constitutes a simple, quick, and efficient route to locally address functional proteins at interfaces without the need for additives or complex modifiers to direct the adsorption process.

Innovations in the synthesis of graphene nanostructures for bio and gas sensors

Sensors play a significant role in modern technologies and devices used in industries, hospitals, healthcare, nanotechnology, astronomy, and meteorology. Sensors based upon nanostructured materials have gained special attention due to their high sensitivity, precision accuracy, and feasibility. This review discusses the fabrication of graphene-based biosensors and gas sensors, which have highly efficient performance. Significant developments in the synthesis routes to fabricate graphene-based materials with improved structural and surface properties have boosted their utilization in sensing applications. The higher surface area, better conductivity, tunable structure, and atom-thick morphology of these hybrid materials have made them highly desirable for the fabrication of flexible and stable sensors. Many publications have reported various modification approaches to improve the selectivity of these materials. In the current work, a compact and informative review focusing on the most recent developments in graphene-based biosensors and gas sensors has been designed and delivered. The research community has provided a complete critical analysis of the most robust case studies from the latest fabrication routes to the most complex challenges. Some significant ideas and solutions have been proposed to overcome the limitations regarding the field of biosensors and hazardous gas sensors.

Effects of Mono-Vacancies and Co-Vacancies of Nitrogen and Boron on the Energetics and Electronic Properties of Heterobilayer h-BN/graphene

A study is carried out which investigates the effects of the mono-vacancies of boron (VB) and nitrogen (VN) and the co-vacancies of nitrogen (N), and boron (B) on the energetics and the structural, electronic, and magnetic properties of an h-BN/graphene heterobilayer using first-principles calculations within the framework of the density functional theory (DFT). The heterobilayer is modelled using the periodic slab scheme. In the present case, a 4 × 4-(h-BN) monolayer is coupled to a 4 × 4-graphene monolayer, with a mismatch of 1.40%. In this coupling, the surface of interest is the 4 × 4-(h-BN) monolayer; the 4 × 4-graphene only represents the substrate that supports the 4 × 4-(h-BN) monolayer. From the calculations of the energy of formation of the 4 × 4-(h-BN)/4 × 4-graphene heterobilayer, with and without defects, it is established that, in both cases, the heterobilayers are energetically stable, from which it is inferred that these heterobilayers can be grown in the experiment. The formation of a mono-vacancy of boron (1 V_B), a mono-vacancy of nitrogen (1 V_N), and co-vacancies of boron and nitrogen (V_BN) induce, on the structural level: (a) for 1 V_B, a contraction n of the B-N bond lengths of ~2.46% and a slight change in the interfacial distance D (~0.096%) with respect to the heterobilayer free of defects (FD) are observed; (b) for 1 V_N, a slight contraction of the B-N of bond lengths of ~0.67% and an approach between the h-BN monolayer and the graphene of ~3.83% with respect to the FD heterobilayer are observed; (c) for V_BN, it can be seen that the N-N and B-B bond lengths (in the 1 V_B and 1 V_N regions, respectively) undergo an increase of ~2.00% and a decrease of ~3.83%, respectively. The calculations of the Löwdin charge for the FD heterobilayer and for those with defects (1 V_B, 1 V_N, and V_BN) show that the inclusion of this type of defect induces significant changes in the Löwdin charge redistribution of the neighboring atoms of VB and VN, causing chemically active regions that could favor the interaction of the heterobilayer with external atoms and/or molecules. On the basis of an analysis of the densities of states and the band structures, it is established that the heterobilayer with 1 V_B and V_BN take on a half-metallic and magnetic behavior. Due to all of these properties, the FD heterobilayer and those with 1 V_B, 1 V_N, and V_BN are candidates for possible adsorbent materials and possible materials that could be used for different spintronic applications.

Effects of Mono-Vacancies of Oxygen and Manganese on the Properties of the MnO2/Graphene Heterostructure

The effects of the monovacancies of oxygen (VO) and manganese (VMn) on the structural and electronic properties of the 1T−MnO2/graphene heterostructure are investigated, within the framework of density functional theory (DFT). We found that the values of the formation energy for the heterostructure without and with vacancies of VO and VMn were −20.99 meVÅ2 , −32.11meVÅ2, and −20.81 meVÅ2, respectively. The negative values of the formation energy indicate that the three heterostructures are energetically stable and that they could be grown in the experiment (exothermic processes). Additionally, it was found that the presence of monovacancies of VO and VMn in the heterostructure induce: (a) a slight decrease in the interlayer separation distance in the 1T−MnO2/graphene heterostructure of ~0.13% and ~1.41%, respectively, and (b) a contraction of the (Mn−O) bond length of the neighboring atoms of the VO and VMn monovacancies of ~2.34% and ~6.83%, respectively. Calculations of the Bader charge for the heterostructure without and with VO and VMn monovacancies show that these monovacancies induce significant changes in the charge of the first-neighbor atoms of the VO and VMn vacancies, generating chemically active sites (locales) that could favor the adsorption of external atoms and molecules. From the analysis of the density of state and the structure of the bands, we found that the graphene conserves the Dirac cone in the heterostructure with or without vacancies, while the 1T−MnO2 monolayer in the heterostructures without and with VO monovacancies exhibits half-metallic and magnetic behavior. These properties mainly come from the hybridization of the 3d−Mn and 2p−O states. In both cases, the heterostructure possesses a magnetic moment of 3.00 μβ/Mn. From this behavior, it can be inferred the heterostructures with and without VO monovacancies could be used in spintronics.

Recent Advances in Non-Enzymatic Glucose Sensors Based on Metal and Metal Oxide Nanostructures for Diabetes Management- A Review

There is an undeniable growing number of diabetes cases worldwide that have received widespread global attention by many pharmaceutical and clinical industries to develop better functioning glucose sensing devices. This has called for an unprecedented demand to develop highly efficient, stable, selective, and sensitive non-enzymatic glucose sensors (NEGS). Interestingly, many novel materials have shown the promising potential of directly detecting glucose in the blood and fluids. This review exclusively encompasses the electrochemical detection of glucose and its mechanism based on various metal-based materials such as cobalt (Co), nickel (Ni), zinc (Zn), copper (Cu), iron (Fe), manganese (Mn), titanium (Ti), iridium (Ir), and rhodium (Rh). Multiple aspects of these metals and their oxides were explored vis-à-vis their performance in glucose detection. The direct glucose oxidation via metallic redox centres is explained by the chemisorption model and the incipient hydrous oxide/adatom mediator (IHOAM) model. The glucose electrooxidation reactions on the electrode surface were elucidated by equations. Furthermore, it was explored that an effective detection of glucose depends on the aspect ratio, surface morphology, active sites, structures, and catalytic activity of nanomaterials, which plays an indispensable role in designing efficient NEGS. The challenges and possible solutions for advancing NEGS have been summarized.

MOF-derived hollow NiCo2O4/C composite for simultaneous electrochemical determination of furazolidone and chloramphenicol in milk and honey

Herein, bimetallic Co/Ni-MOF derived hollow NiCo₂O₄@C composite modified glassy carbon electrode (NiCo₂O₄@C/GCE) is constructed and applied to simultaneously detect furazolidone (FZD) and chloramphenicol (CAP) for the first time. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, nitrogen adsorption-desorption and X-ray photoelectron spectroscopy confirm that NiCo₂O₄@C has hollow and mesoporous structure, abundant carbon matrixes, sufficient oxygen defects and mixed-valence metallic elements. These advantages make NiCo₂O₄@C/GCE show distinguished electrocatalytic performance toward the simultaneous determination of FZD and CAP. The NiCo₂O₄@C/GCE shows wide linear ranges of 0.5-240 µM for FZD and 0.5-320 µM for CAP, low limit of detection of 8.47 nM for FZD and 35 nM for CAP. The mechanism studies show that reductions of FZD and CAP on NiCo₂O₄@C/GCE are both four-electron and four-proton processes. Moreover, the sensor obtains desirable recoveries for the simultaneous determination of FZD (95.85%-103.9%) and CAP (95.72%-104.4%) in milk and honey by standard addition method.

Construction of an MnO2 nanosheet array 3D integrated electrode for sensitive enzyme-free glucose sensing

MnO₂ based electrochemical enzyme-free glucose sensors remain significantly limited by their low electronic conductivity and associated complex preparation. In this paper, an MnO₂ nanosheet array supported on nickel foam (MnO₂ NS/NF) was prepared using a simple hydrothermal synthesis and employed as a 3D integrated electrode for enzyme-free glucose detection. It was found that MnO₂ NS/NF shows high performance with a wide linear range from 1 μM to 1.13 mM, a high sensitivity of 6.45 mA mM^-1 cm^-2, and a low detection limit of 0.5 μM (S/N = 3). Besides, MnO₂ NS/NF shows high selectivity against common interferences and good reliability for glucose detection in human serum. This work demonstrates the promising role of MnO₂ NS/NF as an efficient integrated electrode in enzyme-free glucose detection with high performance.

Voltamperometric Sensors and Biosensors Based on Carbon Nanomaterials Used for Detecting Caffeic Acid-A Review

Caffeic acid is one of the most important hydroxycinnamic acids found in various foods and plant products. It has multiple beneficial effects in the human body such as antioxidant, antibacterial, anti-inflammatory, and antineoplastic. Since overdoses of caffeic acid may have negative effects, the quality and quantity of this acid in foods, pharmaceuticals, food supplements, etc., needs to be accurately determined. The present paper analyzes the most representative scientific papers published mostly in the last 10 years which describe the development and characterization of voltamperometric sensors or biosensors based on carbon nanomaterials and/or enzyme commonly used for detecting caffeic acid and a series of methods which may improve the performance characteristics of such sensors.

A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters.

Progress in utilisation of graphene for electrochemical biosensors

This review discusses recent graphene (GR) electrochemical biosensor for accurate detection of biomolecules, including glucose, hydrogen peroxide, dopamine, ascorbic acid, uric acid, nicotinamide adenine dinucleotide, DNA, metals and immunosensor through effective immobilization of enzymes, including glucose oxidase, horseradish peroxidase, and haemoglobin. GR-based biosensors exhibited remarkable performance with high sensitivities, wide linear detection ranges, low detection limits, and long-term stabilities. Future challenges for the field include miniaturising biosensors and simplifying mass production are discussed.

CeO2-x/C/rGO nanocomposites derived from Ce-MOF and graphene oxide as a robust platform for highly sensitive uric acid detection

Developing suitable substrate materials is of significance in constructing electrochemical biosensors for fast and reliable quantification of molecules of chemical and biomedical interest. For practical applications, biosensors working at low negative potentials have the advantage of high selectivity and sensitivity. In this work, CeO_2-x/C/rGO nanocomposites have been synthesized through the pyrolysis of metal organic frameworks with graphene oxide. The CeO_2-x/C/rGO nanocomposites exhibit excellent catalytic properties towards H₂O₂, which is one of the uricase catalyzed intermediates at low working potentials due to the coexistence of Ce³⁺ and reduced graphene oxide (rGO). A novel biosensor based on the CeO_2-x/C/rGO nanocomposites has been developed and utilized for the detection of uric acid, an important molecule in the biological and medical fields. The biosensor based on the CeO_2-x/C/rGO nanocomposites presents a high sensitivity of 284.5 μA cm^-2 mM^-1 at -0.4 V (vs. SCE), a wide linear range between 49.8 and 1050.0 μM and a low detection limit of 2.0 μM. Moreover, it is found that the amperometric responses are free from interference of ascorbic acid and urea, which shows a great potential for practical applications.

NiMn2O4 spinel binary nanostructure decorated on three-dimensional reduced graphene oxide hydrogel for bifunctional materials in non-enzymatic glucose sensor

Nickel-manganese spinel oxide (NiMn₂O₄) was hybridized with reduced graphene oxide hydrogel (rGOH) via a facile solvothermal process and a highly porous three-dimensional (3D) structure was constructed. NiMn₂O₄/rGOH exhibited excellent electrochemical performance due to the high specific surface area, excellent electrocatalytic activity, and enhanced electrical conductivity due to the synergetic effects between the two components. The NiMn₂O₄/rGOH exhibited excellent glucose sensing performance with high sensitivity (1310.8 μA mM^-1 cm^-2), a wide linear range (2 μM-20 mM), rapid response time (<3.5 s), and anti-interference properties. Furthermore, it also showed excellent supercapacitor performance with a high capacitance (396.85 F g^-1) and excellent energy and power density on account of the large surface area and pseudo-capacitor behavior of NiMn₂O₄. A self-powered glucose sensor can be fabricated with NiMn₂O₄/rGOH as both supercapacitor and glucose sensing electrodes.

Carbon Nanomaterials in Biological Studies and Biomedicine

The "carbon nano-world" has made over the past few decades huge contributions in diverse scientific disciplines and technological advances. While dramatic advances have been widely publicized in using carbon nanomaterials such as fullerenes, carbon nanotubes, and graphene in materials sciences, nano-electronics, and photonics, their contributions to biology and biomedicine have been noteworthy as well. This Review focuses on the use of carbon nanotubes (CNTs), graphene, and carbon quantum dots [encompassing graphene quantum dots (GQDs) and carbon dots (C-dots)] in biologically oriented materials and applications. Examples of these remarkable nanomaterials in bio-sensing, cell- and tissue-imaging, regenerative medicine, and other applications are presented and discussed, emphasizing the significance of their unique properties and their future potential.

Facile synthesis of MnO2-embedded flower-like hierarchical porous carbon microspheres as an enhanced electrocatalyst for sensitive detection of caffeic acid

Tailored designs/fabrications of hierarchical porous advanced electrode materials are of great importance for developing high-performance electrochemical sensors. Herein, we demonstrate a simple and low-cost in situ chemical approach for the facile synthesis of MnO₂-embedded hierarchical porous carbon microspheres (MnO₂/CM). By the characterizations of scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray powder diffraction and energy dispersive spectroscopy, we evidenced that the synthesized product were flower-like carbon microspheres (CM) assembled by the bent flakes with thickness of about several nanometers and MnO₂ nanorods were highly dispersed and successfully decorated on the CM layers, resulting in a rough surface and three-dimensional microstructure. The greatest benefit from the combined porous CM with MnO₂ nanorods is that the MnO₂/CM modified electrode has the synergetic catalysis effect on the electro-oxidation of caffeic acid, leading to the remarkable increase in the electron transfer rate and significant decrease in the over-potential for the caffeic acid oxidation in contrast to the bare electrode and CM modified electrode. This implies that the prepared MnO₂/CM can be employed as an enhanced electrocatalyst for the sensitive detection of caffeic acid. Under the optimum conditions, the anodic peak current of caffeic acid is linear with its concentration in the range of 0.01-15.00 μmol L^-1, and a detection limit of 2.7 nmol L^-1 is achieved based on S/N = 3. The developed sensor shows good selectivity, sensitivity, reproducibility, and also excellent recovery in the detections of real samples, revealing the promising practicality of the sensor for the caffeic acid detection.

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.

Non-enzymatic glucose electrochemical sensor based on silver nanoparticle decorated organic functionalized multiwall carbon nanotubes

Silver nanoparticles were decorated on organic amine chain functionalized multiwall carbon nanotubes to fabricate a sensitive glucose sensor.