Graphene Oxide-Templated Polyaniline Microsheets toward Simultaneous Electrochemical Determination of AA/DA/UA

Mesoporous Nitrogen-Doped Holey Reduced Graphene Oxide: Preparation, Purification, and Application for Metal-Free Electrochemical Sensing of Dopamine

Holey graphenic nanomaterials with porosity within the basal plane attract significant interest. It is observed that the perforation of graphene can enhance the specific surface area of the nanosheet, ensuring effective wetting and penetration of electrolytes to the electrode surface, facilitating rapid charge transfer, and boosting the electrocatalytic efficacy of the transducers. This study reports the first example of nitrogen-doped holey reduced graphene oxide with a mesoporous morphology of the graphene basal plane (N-MHG). It is shown that N-MHG can be synthesized through a one-step hydrothermal treatment of GO using NH3 and H2O2. A straightforward procedure for the purification of N-MHG has also been developed. AFM, TEM, and Raman analyses have revealed that N-MHG possesses a highly mesoporous network structure with a pore size ranging from 10 to 50 nm. X-ray photoelectron spectroscopy data have indicated a partial reduction of the graphene oxide sheets during the etching process but also show a 3-5 times higher content of C═O and O-C═O fragments compared to rGO. This could account for the remarkable stability of the N-MHG aqueous suspension. An electrochemical sensor for dopamine analysis is assembled on a glassy carbon electrode with N-MHG/Nafion membrane and characterized by cyclic voltammetry and electrochemical impedance spectroscopy.

Utilizing a Disposable Sensor with Polyaniline-Doped Multi-Walled Carbon Nanotubes to Enable Dopamine Detection in Ex Vivo Mouse Brain Tissue Homogenates

Disposable sensors are inexpensive, user-friendly sensing tools designed for rapid single-point measurements of a target. Disposable sensors have become more and more essential as diagnostic tools due to the growing demand for quick, easy-to-access, and reliable information related to the target. Dopamine (DA), a prevalent catecholamine neurotransmitter in the human brain, is associated with central nervous system activities and directly promotes neuronal communication. For the sensitive and selective estimation of DA, an enzyme-free amperometric sensor based on polyaniline-doped multi-walled carbon nanotubes (PANI-MWCNTs) drop-coated disposable screen-printed carbon electrodes (SPCEs) was fabricated. This PANI-MWCNTs-2/SPCE sensor boasts exceptional accuracy and sensitivity when working directly with ex vivo mouse brain homogenates. The sensor exhibited a detection limit of 0.05 μM (S/N = 3), and a wide linear range from 1.0 to 200 μM. The sensor's high selectivity to DA amidst other endogenous interferents was recognized. Since the constructed sensor is enzyme-free yet biocompatible, it exhibited high stability in DA detection using ex vivo mouse brain homogenates extracted from both Parkinson's disease and control mice models. This research thus presents new insights into understanding DA release dynamics at the tissue level in both of these models.

Synthesis of Guanine/Vermiculite Two-Dimensional Nanocomposites for Wireless Humidity Sensing in Nut Storage Environment

Two-dimensional (2D) nanostructures have the advantages of high specific surface area, easy surface functionalization, abundant active sites, and good compatibility with device integration and can be assembled into three-dimensional structures, which are key to the development of high-performance gas sensors. In this study, 2D vermiculite (VMT) nanosheets and guanine (G), two renewable resources with unique chemical structures, were organically combined to fully use the specificity of their molecular structures and functional activities. Driven by the regulation of 2D VMT nanosheets, guanine/vermiculite (G/VMT)-based 2D nanocomposites with controllable pore structure, multiple binding sites, and unobstructed mass transfer were designed and synthesized. The G/VMT nanocomposite material was used as a quartz crystal microbalance (QCM) electrode-sensitive film material to build a QCM-based humidity sensor. G/VMT-based QCM humidity sensor had good logarithmic linear relation (0.9971), high sensitivity (24.49 Hz/% relative humidity), low hysteresis (1.75% RH), fast response/recovery time (39/6 s), and good stability. Furthermore, with a QCM sensor and a specially designed wireless circuit, a wireless humidity detection system transmitting via Wi-Fi allows real-time monitoring of nut storage. This study presents an environmentally friendly, high-performance, miniature 2D nanocomposite sensor strategy for real-time monitoring.

Hydrothermal synthesis of polyaniline nanospheres coupled with graphene oxide for enhanced specific capacitance performances

Polyaniline is one of the most common electrode materials for supercapacitors. The morphology of polyaniline directly affects the properties of polyaniline. In this paper, a new method for preparing hollow polyaniline nanospheres is described. Polyaniline-S with solid and hollow structures are successfully synthesized by the hydrothermal method, through varying the amounts of the catalyst and oxidant. The prepared hollow nanospheres have uniform particle size, a smooth surface, and uniform wall thickness. The hollow structure provides rapid permeability to the material, facilitating the transfer and transport of charges and ions in the electrolyte, and it can also act as an ion storage tank to increase the accumulation of ions inside. The specific capacitance of polyaniline-S is high at 235 F g-1 at 0.5 A g-1. To reduce the aggregation of polyaniline-S and improve the electrochemical activity, polyaniline-S, and graphene oxide are composited using the interfacial electrostatic interaction. The content of graphene oxide has a significant influence on the electrochemical performance of the composites. The specific capacitance of the polyaniline-S/ graphene oxide composite with a 10% loading amount of graphene oxide reaches 535 F g-1 at 0.5 A g-1, increase of nearly 128% compared to representing a significant polyaniline-S. The specific capacitance retention rate is 93.6% after 10,000 cycles.

Tailoring Oxygen-Containing Groups on Graphene for Ratiometric Electrochemical Measurements of Ascorbic Acid in Living Subacute Parkinson's Disease Mouse Brains

Ascorbic acid (AA), a major antioxidant in the central nervous system (CNS), is involved in withstanding oxidative stress that plays a significant role in the pathogenesis of Parkinson's disease (PD). Exploring the AA disturbance in the process of PD is of great value in understanding the molecular mechanism of PD. Herein, by virtue of a carbon fiber electrode (CFE) as a matric electrode, a three-step electrochemical process for tailoring oxygen-containing groups on graphene was well designed: potentiostatic deposition was carried out to fabricate graphene oxide on CFE, electrochemical reduction that assisted in removing the epoxy groups accelerated the electron transfer kinetics of AA oxidation, and electrochemical oxidation that increased the content of the carbonyl group (C═O) generated an inner-reference signal. The mechanism was solidified by ab initio calculations by comparing AA absorption on defected models of graphene functionalized with different oxygen groups including carboxyl, hydroxyl, epoxy, and carbonyl. It was found that epoxy groups would hinder the physical absorption of AA onto graphene, while other functional groups would be beneficial to it. Biocompatible polyethylenedioxythiophene (PEDOT) was further rationally assembled to improve the antifouling property of graphene. As a result, a new platform for ratiometric electrochemical measurements of AA with high sensitivity, excellent selectivity, and reproducibility was established. In vivo determination of AA levels in different regions of living mouse brains by the proposed method demonstrated that AA decreased remarkably in the hippocampus and cortex of a subacute PD mouse than those of a normal mouse.

Unveiling the Fundamental Mechanisms of Graphene Oxide Selectivity on the Ascorbic Acid, Dopamine, and Uric Acid by Density Functional Theory Calculations and Charge Population Analysis

The selectivity of electrochemical sensors to ascorbic acid (AA), dopamine (DA), and uric acid (UA) remains an open challenge in the field of biosensing. In this study, the selective mechanisms for detecting AA, DA, and UA molecules on the graphene and graphene oxide substrates were illustrated through the charge population analysis from the density functional theory (DFT) calculation results. Our substrate models contained the 1:10 oxygen per carbon ratio of reduced graphene oxide, and the functionalized configurations were selected according to the formation energy. Geometry optimizations were performed for the AA, DA, and UA on the pristine graphene, epoxy-functionalized graphene, and hydroxyl-functionalized graphene at the DFT level with vdW-DF2 corrections. From the calculations, AA was bound to both epoxy and hydroxyl-functionalized GO with relatively low adsorption energy, while DA was adsorbed stronger to the electronegative epoxy groups. The strongest adsorption of UA to both functional groups corresponded to the largest amount of electron transfer through the pi orbitals. Local electron loss created local electric fields that opposed the electron transfer during an oxidation reaction. Our analysis agreed with the results from previous experimental studies and provided insight into other electrode modifications for electrochemical sensing.

One-dimensional nitrogen doped porous carbon nano-array arranged by carbon nanotubes for electrochemical sensing ascorbic acid, dopamine and uric acid simultaneously

In this work, one-dimensional nitrogen doped porous carbon nano-arrays arranged by carbon nanotube (1D CNTs@NPC) were first constructed, using a coating technology at room temperature and followed by high temperature carbonization. It was expected that the resulting glassy carbon electrodes modified by 1D CNTs@NPC (CNTs@NPC/GCE) could express different electrochemical responses to ascorbic acid (AA), dopamine (DA), uric acid (UA), by virtue of the synergistic-improved effect between CNTs and NPC. Under the optimized conditions, there were excellent analytical parameters for CNTs@NPC/GCE to detect AA, DA and UA, i.e. a wide linear range of 40-2100μM for AA, 0.5-49μM for DA and 3-50μM for AA with low detection limits of 0.36μM, 0.02μmol l^-1and 0.57μM respectively. Importantly, the proposed CNTs@NPC/GCE was efficiently applied to determine AA, DA and UA in some real samples with high stability, reproducibility and selectivity. This work will offer an efficient potential for diagnosing ascorbic acid, dopamine or uric acid-related diseases on clinical testing in future.

Fabrication of 3D Ni/NiO/MoS2/rGO foam for enhancing sensing performance

The accurate electrochemical detection of dopamine (DA) is hard to achieve due to the serious interference of a substance with similar redox properties.

Facile Ratiometric Electrochemical Sensor for In Vivo/Online Repetitive Measurements of Cerebral Ascorbic Acid in Brain Microdiaysate

The in vivo monitoring of ascorbic acid (AA) following physiological and pathological events is of great importance because AA plays a critical role in brain functions. The conventional electrochemical sensors (ECSs) usually suffered from poor selectivity and sluggish electron transfer kinetics for cerebral AA oxidation. The exploitation of ECSs adapt to the electrochemical detection (ECD)-microdialysis system, here we reported a facile ratiometric electrochemical sensor (RECS) for in vivo/online repetitive measurements of cerebral AA in brain microdiaysate. The sensor were constructed by careful electrodeposition of graphene oxide (GO) onto glassy carbon (GC) electrodes. Methylene blue (MB) was electrostatically adsorbed onto the GO surface as a built-in reference to achieve ratiometric detection of AA. The subsequent proper electroreduction treatment was able to readily facilitate the oxidation of AA at a relatively negative potential (-100 mV) and the oxidation of MB at separated potential (-428 mV). The in vitro experiments demonstrated that the RECS exhibited high sensitivity (detection limit: 10 nM), selectivity, and stability toward AA determination, enabling the in vivo/online repetitive measurement of cerebral AA in brain microdiaysate with high reliability. As a result, the designed RECS was successfully applied in the ECD-microdialysis system to in vivo/online repetitive monitoring the dynamic change of cerebral AA in the progress of the global cerebral ischemia/reperfusion events. More, the microinjection of endogenous AA and AA oxidase (AAOx) verified the reliability of the proposed RECS for in vivo/online repetitive cerebral AA detection. This proposed sensor filled the gap that no rational electrochemical sensor has been developed for the ECD-microdialysis system since its creation by the Mao group in 2005, which provided a reliable and effective method for brain chemistry research.

Voltammetry at Hexamethyl-P-Terphenyl Poly(Benzimidazolium) (HMT-PMBI)-Coated Glassy Carbon Electrodes: Charge Transport Properties and Detection of Uric and Ascorbic Acid

We describe the voltammetric behavior of an anion-exchange membrane, hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI). The anion-exchange properties of HMT-PMBI chemically modified electrodes were investigated using K4Fe(CN)6 and K2IrCl6 as redox probes. The permselectivity properties of HMT-PMBI chemically modified electrodes were ascertained using tris(2-2')bipyridyl-ruthenium(II) chloride Ru(bpy)32+. Cyclic voltammetry and chronoamperometry were utilized to extract parameters such as the concentration of the redox mediators inside the films and the apparent diffusion coefficients. We found the concentration of K4Fe(CN)6 and K2IrCl6 redox species within HMT-PMBI-coated films to be on the order of 0.04-0.1 mol·dm-3, and values of Dapp ca. 10-10-10-9 cm2·s-1. To evaluate the possibility of using such a polymer coating in electroanalysis, HMT-PMBI-modified electrodes were utilized for the voltammetric detection of uric acid in artificial urine, Surine® and ascorbic acid in Vitamin C samples. The results showed that HMT-PMBI-coated electrodes can detect uric acid in Surine® with a limit of detection (LoD) of 7.7 µM, sensitivity of 0.14 µA·µM-1·cm-2, and linear range between 5 μM and 200 μM, whereas for Vitamin C tablets, the LoD is 41.4 µM, the sensitivity is 0.08 µA·µM-1·cm-2, and the linear range is between 25 μM and 450 μM.

Three-Dimensional Au/Holey-Graphene as Efficient Electrochemical Interface for Simultaneous Determination of Ascorbic Acid, Dopamine and Uric Acid

The quantification of ascorbic acid (AA), dopamine (DA), and uric acid (UA) has been an important area of research, as these molecules’ determination directly corresponds to the diagnosis and control of diseases of nerve and brain physiology. In our research, graphene oxide (GO) with nano pores deposited with gold nanoparticles were self-assembled to form three-dimensional (3D) Au/holey-graphene oxide (Au/HGO) composite structures. The as-prepared 3DAu/HGO composite structures were characterized for their structures by X-ray diffraction, Raman spectrum, scanning electron microscopy, and transmission electron microscopy coupled with cyclic voltammograms. Finally, the proposed 3DAu/HGO displayed high sensitivity, excellent electron transport properties, and selectivity for the simultaneous electrochemical determination of AA, DA and UA with linear response ranges of 1.0–500 μM, 0.01–50 μM and 0.05–50 μM respectively. This finding paves the way for graphene applications as a biosensor for detecting three analytes in human serum.

Electrochemical sensor based on a three dimensional nanostructured MoS2 nanosphere-PANI/reduced graphene oxide composite for simultaneous detection of ascorbic acid, dopamine, and uric acid

A three dimensional (3D) nanostructured composite based on the self-assembly of MoS₂ nanospheres and polyaniline (PANI) loaded on reduced graphene oxide (denoted by 3D MoS₂-PANI/rGO) was prepared via a feasible one-pot hydrothermal process. The 3D MoS₂-PANI/rGO nanocomposite not only exhibits good functionality and bioaffinity but also displays high electrochemical catalytic activity. As such, the developed 3D MoS₂-PANI/rGO nanocomposite can be employed as the sensing platform for simultaneously detecting small biomolecules, i.e., ascorbic acid (AA), dopamine (DA), and uric acid (UA). The peak currents obtained from the differential pulse voltammetry (DPV) measurements depended linearly on the concentrations in the wide range from 50 μM to 8.0 mM, 5.0 to 500 μM, and 1.0 to 500 μM, giving low detection limits of 22.20, 0.70, and 0.36 μM for AA, DA, and UA, respectively. Furthermore, the 3D MoS₂-PANI/rGO-based electrochemical sensor also exhibited high selectivity, good reproducibility and stability toward small molecule detection. The present sensing strategy based on 3D MoS₂-PANI/rGO suggests a good reliability in the trace determination of electroactive biomolecules.

A three dimensional (3D) nanostructured composite based on the self-assembly of MoS₂ nanospheres and polyaniline (PANI) loaded on reduced graphene oxide (denoted by 3D MoS₂-PANI/rGO) was prepared via a feasible one-pot hydrothermal process.

In situ controllable synthesis of cotton-like polyaniline nanostructures for a H2O2 sensor using an embedded three-electrode microfluidic chip

Fabrication of a thickness controllable polyaniline film based on an embedded three-electrode microfluidic chip for application as a H₂O₂ sensor.

Electrochemical Molecular Imprinted Sensors Based on Electrospun Nanofiber and Determination of Ascorbic Acid

In this study, electrochemical molecularly imprinted sensors were fabricated and used for the determination of ascorbic acid (AA). Nanofiber membranes of cellulose acetate (CA)/multi-walled carbon nanotubes (MWCNTs)/polyvinylpyrrolidone (PVP) (CA/MWCNTs/PVP) were prepared by electrospinning technique. After being transferred to a glass carbon electrode (GC), the nanofiber interface was further polymerized with pyrrole through electrochemical cyclic voltammetry (CV) technique. Meanwhile, target molecules (such as AA) were embedded into the polypyrrole through the hydrogen bond. The effects of monomer concentration (pyrrole), the number of scan cycles and scan rates of polymerization were optimized. Differential pulse voltammetry (DPV) tests indicated that the oxidation current of AA (the selected target) were higher than that of the structural analogues, which illustrated the selective recognition of AA by molecularly imprinted sensors. Simultaneously, the molecularly imprinted sensors had larger oxidation current of AA than non-imprinted sensors in the processes of rebinding. The electrochemical measurements showed that the molecularly imprinted sensors demonstrated good identification behavior for the detection of AA with a linear range of 10.0 - 1000 μM, a low detection limit down to 3 μM (S/N = 3), and a recovery rate range from 94.0 to 108.8%. Therefore, the electrochemical molecularly imprinted sensors can be used for the recognition and detection of AA without any time-consuming elution. The method presented here demonstrates the great potential for electrospun nanofibers and MWCNTs to construct electrochemical sensors.

Morphology-dependent Electrochemical Enhancements of Porous Carbon as Sensitive Determination Platform for Ascorbic Acid, Dopamine and Uric Acid

Using starch as the carbon precursor and different-sized ZnO naoparticles as the hard template, a series of porous carbon materials for electrochemical sensing were prepared. Experiments of scanning electron microscopy, transmission electron microscopy and Nitrogen adsorption-desorption isotherms reveal that the particle size of ZnO has big impacts on the porous morphology and surface area of the resulting carbon materials. Through ultrasonic dispersion of porous carbon and subsequent solvent evaporation, different sensing interfaces were constructed on the surface of glassy carbon electrode (GCE). The electrochemical behaviors of ascorbic acid (AA), dopamine (DA) and uric acid (UA) were studied. On the surface of porous carbon materials, the accumulation efficiency and electron transfer ability of AA, DA and UA are improved, and consequently their oxidation signals enhance greatly. Moreover, the interface enhancement effects of porous carbon are also controlled by the particle size of hard template. The constructed porous carbon interface displays strong signal amplification ability and holds great promise in constructing a sensitive platform for the simultaneous determination of AA, DA and UA.

Fabrication of pore-rich nitrogen-doped graphene aerogel

Porosity tuning of NGAs by tailoring GONSs yields the pore-richest NGA with the best mechanical stability and electrocatalytic biosensing activity using the smallest sonicated GONSs and DA with high N content and 3D crosslinking capability.

Hydrothermal synthesis of nitrogen doped graphene nanosheets from carbon nanosheets with enhanced electrocatalytic properties

Nitrogen doped graphene nanosheets (NGS) were synthesized from carbon nanosheets (CNS). The as-synthesized NGS were employed to selectively detect dopamine (DA) and uric acid (UA) with enhanced electrocatalytic activity.

NiCo-embedded in hierarchically structured N-doped carbon nanoplates for the efficient electrochemical determination of ascorbic acid, dopamine, and uric acid

NiCo-embedded hierarchically structured N-doped carbon nanoplates (NiCo-NPs-in-N/C) were facilely synthesized via a one step in situ reduction pyrolysis strategy for efficiently electrochemical determination of ascorbic acid, dopamine and uric acid.

Graphene oxide assisted hydrothermal carbonization of carbon hydrates

Hydrothermal carbonization (HTC) of biomass such as glucose and cellulose typically produces micrometer-sized carbon spheres that are insulating. Adding a very small amount of Graphene oxide (GO) to glucose (e.g., 1:800 weight ratio) can significantly alter the morphology of its HTC product, resulting in more conductive carbon materials with higher degree of carbonization. At low mass loading level of GO, HTC treatment results in dispersed carbon platelets of tens of nanometers in thickness, while at high mass loading levels, free-standing carbon monoliths are obtained. Control experiments with other carbon materials such as graphite, carbon nanotubes, carbon black, and reduced GO show that only GO has significant effect in promoting HTC conversion, likely due to its good water processability, amphiphilicity, and two-dimensional structure that may help to template the initially carbonized materials. GO offers an additional advantage in that its graphene product can act as an in situ heating element to enable further carbonization of the HTC products very rapidly upon microwave irradiation. Similar effect of GO is also observed for the HTC treatment of cellulose.