Graphene Paper Decorated with a 2D Array of Dendritic Platinum Nanoparticles for Ultrasensitive Electrochemical Detection of Dopamine Secreted by Live Cells

To circumvent the bottlenecks of non‐flexibility, low sensitivity, and narrow workable detection range of conventional biosensors for biological molecule detection (e.g., dopamine (DA) secreted by living cells), a new hybrid flexible electrochemical biosensor has been created by decorating closely packed dendritic Pt nanoparticles (NPs) on freestanding graphene paper. This innovative structural integration of ultrathin graphene paper and uniform 2D arrays of dendritic NPs by tailored wet chemical synthesis has been achieved by a modular strategy through a facile and delicately controlled oil–water interfacial assembly method, whereby the uniform distribution of catalytic dendritic NPs on the graphene paper is maximized. In this way, the performance is improved by several orders of magnitude. The developed hybrid electrode shows a high sensitivity of 2 μA cm^?2 μm ^?1, up to about 33 times higher than those of conventional sensors, a low detection limit of 5 nm, and a wide linear range of 87 nm to 100 μm . These combined features enable the ultrasensitive detection of DA released from pheochromocytoma (PC 12) cells. The unique features of this flexible sensor can be attributed to the well‐tailored uniform 2D array of dendritic Pt NPs and the modular electrode assembly at the oil–water interface. Its excellent performance holds much promise for the future development of optimized flexible electrochemical sensors for a diverse range of electroactive molecules to better serve society.     

Reduced Graphene Oxide/Pt Nanoparticles/Zn‐MOF‐74 Nanomaterial for a Glucose Biosensor Construction

In this study, a new glucose biosensor was fabricated by immobilizing glucose oxidase (GOx) on platinum nanoparticles (Pt NPs) decorated reduced graphene oxide (rGO)/Zn‐MOF‐74 hybrid nanomaterial. Herein, the biosensor fused the advantages of rGO with those of porous Zn‐MOF and conductive Pt NPs. This has not only enlarged the surface area and porosity for the efficient GOx immobilization and faster mass transport, but also provided favorable electrochemical features such as high current density, remarkable electron mobility through metal nanoparticles, and improved electron transfer between the components. The GOx‐rGO/Pt NPs@Zn‐MOF‐74 coated electrode displayed a linear measurement range for glucose from 0.006 to 6 mM, with a detection limit of 1.8 μM (S/N: 3) and sensitivity of 64.51 μA mM^?1 cm^?2. The amperometric response of the enzyme biosensor demonstrated the typical behavior of Michaelis‐Menten kinetics. The obtained satisfying sensitivity and measurement range enabled fast and accurate glucose measurement in cherry juice using the fabricated biosensor. The water‐stable Zn‐MOF‐74 demonstrated higher enzyme loading capacity and can be potent supporting material for biosensor construction.     

Graphene as signal amplifier for preparation of ultrasensitive electrochemical biosensors

Early diagnosis of diseases with minimal cost and time-consumption has become achievable due to recent advances in the development of biosensors. These devices use biorecognition elements for the selective interaction with an analyte and the signal read-out is obtained via different types of transducers. The operational characteristics of biosensors have been reported as improving substantially when a diverse range of nanomaterials is employed. This review presents the construction of electrochemical biosensors based on graphene, atomically thin 2D carbon crystals, a nanomaterial currently the subject of intensive studies. Here, the most attractive directions for graphene applications in biosensor preparation are discussed, including novel detection and amplification schemes exploiting graphene’s unique electrochemical, physical and chemical properties. There is probably a very bright future for graphene-based biosensors, but much further work is required to fulfill the high expectations.     

Properties of Dendrimer‐Encapsulated Pt Nanoparticles Doped Polypyrrole Composite Films and Their Electrocatalytic Activity for Glucose Oxidation

A type of novel electroanalytical sensing nanobiocomposite material was prepared by electropolymerization of pyrrole containing poly(amidoamine) dendrimers‐encapsulated platinum nanoparticles (Pt‐PAMAM), and glucose oxidase (GOx). The Pt nanoparticles encapsulated in PAMAM are nearly monodisperse with an average diameter of 3 nm, and they provide electrical conductivity. Polypyrrole acts as a polymer backbone to give stable and homogeneous cast thin films, and it also defines the electrical conductivity. Both Polypyrrole and PAMAM can provide a favorable microenvironment to keep the bioactivity of enzymes such as glucose oxidase. The homogeneity of GOx/Pt‐PAMAM‐PPy nanobiocomposite films was characterized by atomic force microscopy (AFM). Amperometric biosensors fabricated with these materials were characterized electrochemically using cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and amperometric measurements in the presence of hydrogen peroxide or glucose. All those show the resultant biosensor sensitivity was strongly enhanced within the nanobiocomposite film. The optimized glucose biosensor displayed a sensitivity of 164 μA mM^?1 cm^?1, a linear range of 0.2 to 600 μM, a detection limit of 10 nM, and a response time of <3 s.     

Amperometric Biosensors Based on Recombinant Bacterial Laccase CotA for Hydroquinone Determination

In this study, stable CotA laccase from Bacillus subtilis 168 was adsorbed on electrode modified with a thiol graphene‐gold nanoparticle (thGP‐AuNPs) nanocomposite film. The novel bacterial laccase biosensor was employed for quantitative detection of hydroquinone (HQ) and the electrochemical properties of this laccase biosensor were investigated. The results indicate that the immobilized CotA shows great oxidation activity towards HQ in the presence of oxygen and the biosensor shows linear electrocatalytic activity in the concentration range from 1.6 to 409.6 μM, with a detection limit of 0.3 μM. Further, the CotA modified electrode, when compared to fungal laccase‐modified biosensors, shows better alkaline stability (retaining approximately 80 % and 70 % of response current at pH 8 and 9, respectively) and reusability (retaining ～87 % of response current after 100 days). The development of this new kind of laccase on a biosensor will offer a novel tool for substance detection applications in hostile environments, especially for industrial pollutants.     

A Novel Bioelectrochemical Sensor Based on Immobilized Urease on the Surface of Nickel Oxide Nanoparticle and Polypyrrole Composite Modified Pt Electrode

Nickel oxide nanoparticle (NiO?NP) and polypyrrole (PPy) composite were deposited on a Pt electrode for fabrication of a urea biosensor. To develop the sensor, a thin film of PPy?NiO composite was deposited on a Pt substrate that serves as a matrix for the immobilization of enzyme. Urease was immobilized on the surface of Pt/PPy?NiO by a physical adsorption. The response of the fabricated electrode (Pt/PPy?NiO/Urs) towards urea was analyzed by chronoamperometry and cyclic voltammetry (CV) techniques. Electrochemical response of the bio‐electrode was significantly enhanced. This is due to electron transfer between Ni²⁺ and Ni³⁺ as the electro‐catalytic group and the reaction between polypyrrole and the urease‐liberated ammonium. The fabricated electrode showed reliable and demonstrated perfectly linear response (0.7–26.7 mM of urea concentration, R²= 0.993), with high sensitivity (0.153 mA mM^?1 cm^?2), low detection of limit (1.6 μM), long stability (10 weeks), and low response time (～5 s). The developed biosensor was highly selective and obtained data were repeatable and reproduced using PPy‐NiO composite loaded with immobilized urease as urea biosensors.     

Recent Applications of Carbon Nanomaterials for microRNA Electrochemical Sensing

MicroRNA (miRNA) is an important tumor marker in the human body, and its early detection has a great influence on the survival rate of patients. Although there are many detection methods for miRNA at present such as northern blotting, real-time quantitative polymerase chain reaction, microarrays, and others, electrochemical biosensors have the advantages of low detection cost, small instrument size, simple operation, non-invasive detection and low consumption of reagents and solvents, and thus they play an important role in the early detection of cancer. In addition, with the development of nanotechnology, nano-biosensors show great potential. The application of various nanomaterials in the development of electrochemical biosensor has greatly improved the detection sensitivity of electrochemical biosensor. Among them, carbon nanomaterials which have unique electrical, optical, physical and chemical properties have attracted increasing attention. In particular, they have a large surface area, good biocompatibility and conductivity. Therefore, carbon nanomaterials combined with electrochemical methods can be used to detect miRNA quickly, easily and sensitively. In this review, we systematically review recent applications of different carbon nanomaterials (carbon nanotubes, graphene and its derivatives, graphitic carbon nitride, carbon dots, graphene quantum dots and other carbon nanomaterials) for miRNA electrochemical detection. In addition, we demonstrate the future prospects of electrochemical biosensors modified by carbon nanomaterials for the detection of miRNAs, and some suggestions for their development in the near future.     

Lifetime Improvement of Glucose Biosensor by Epoxy‐Enhanced PVC Membrane

A new approach using epoxy resin to enhance the durability and adhesion of a diffusion‐limiting membrane in amperometric biosensors is described. The polymer membrane was mainly composed of commercially available fast epoxy adhesive ATACS 5104, poly(vinyl chloride) (PVC) and plasticizers such as isopropyl myristate (IMP) and Aliquat 336 (AL). It can be readily deposited on various substrates by using coating and other thin film fabrication methods. The effect of epoxy resin in the membrane composition was investigated using a coil‐type glucose biosensor containing extra enzyme. The ideal membrane was found to include approximately 1/3 epoxy resin, 1/3 plasticizer and 1/3 PVC. Such a membrane was verified to be porous and permeable to small molecules like glucose and can tightly adhere to other beneath layers such as a Nafion membrane, which serves as the interference‐eliminating layer. These epoxy‐based glucose biosensors showed excellent electrochemical response properties including a long lifetime and can be used for microanalysis of solutions and biological fluids. With an additional PU outermost layer, the present glucose biosensors can potentially be used for in vivo measurements.     

Recent Progress on Graphene‐based Electrochemical Biosensors

The detailed records and conclusions on the important advancements in graphene‐based electrochemical biosensors have been reviewed. Due to their outstanding properties, graphene‐based materials have been widely studied for the accurate electrochemical detection of many biomolecules, which is extremely vital to the development of biomedical instruments, clinical diagnosis, and disease treatment. This review discusses the graphene research for the effective immobilization of enzymes, including glucose oxidase, horseradish peroxidase, and hemoglobin, etc., and the accurate detection of biomolecules, including glucose, hydrogen peroxide, dopamine, ascorbic acid, uric acid, nicotinamide adenine dinucleotide, DNA, RNA, and carcinoembryonic antigen, etc. In most of the cases, the graphene‐based biosensors exhibited remarkable performance with high sensitivities, wide linear detection ranges, low detection limits, and long‐term stabilities.     

A Sensitive and Selective Nitrite Detection in Water Using Graphene/Platinum Nanocomposite

A glassy carbon electrode modified with reduced graphene oxide and platinum nanocomposite film was developed simply by electrochemical method for the sensitive and selective detection of nitrite in water. The electrochemical reduction of graphene oxide (GO) efficiently eliminates oxygen‐containing functional groups. Pt nanoparticles were electrochemically and homogeneously deposited on the ErGO surface. Field emission scanning electron microscopy (FE‐SEM), Raman spectroscopy, attenuated total reflectance‐fourier transform infrared spectroscopy (ATR‐FTIR), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were used to examine the surface morphology and electrocatalytic properties of the Pt‐ErGO nanocomposite film‐modified electrode surface. The fabricated nitrite sensor showed good electrochemical performance with two linear ranges; one from 5 to 100 µM (R²=0.9995) and the other from 100 to 1000 µM (R²=0.9972) and a detection limit of 0.22 µM. The proposed sensor was successfully applied for the detection of nitrite in tap water samples which proves performance of the Pt‐ErGO nanocomposite films.     

Recent Advances on Electrochemical Biosensing Strategies toward Universal Point‐of‐Care Systems

A number of very recently developed electrochemical biosensing strategies are promoting electrochemical biosensing systems into practical point‐of‐care applications. The focus of research endeavors has transferred from detection of a specific analyte to the development of general biosensing strategies that can be applied for a single category of analytes, such as nucleic acids, proteins, and cells. In this Minireview, recent cutting‐edge research on electrochemical biosensing strategies are described. These developments resolved critical challenges regarding the application of electrochemical biosensors to practical point‐of‐care systems, such as rapid readout, simple biosensor fabrication method, ultra‐high detection sensitivity, direct analysis in a complex biological matrix, and multiplexed target analysis. This Minireview provides general guidelines both for scientists in the biosensing research community and for the biosensor industry on development of point‐of‐care system, benefiting global healthcare.     

A Label-Free Amperometric Immunoassay for Thrombomodulin Using Graphene/Silver-Silver Oxide Nanoparticles as a Immobilization Matrix

A simple, sensitive, and label-free electrochemical immunosensor has been developed for the measurement of serum thrombomodulin (TM), an endothelial glycoprotein which is associated with the progression and metastasis of tumors. At first, the graphene nanosheets, which were dispersed in Nafion solution, were used to coat the bare gold electrode. Then, silver-silver oxide nanoparticles (Ag-Ag₂O NPs) were immobilized on the graphene-modified electrode by a one-step electrochemical deposition method. Lastly, a thrombomodulin antibody (anti-TM) was attached via amido-Ag affinity. This strategy combines graphene/Ag-Ag₂O NPs hybrid materials as an immobilization matrix and Ag-Ag₂O NPs also as an electrochemical signal indication reagent. The main advantage of this strategy has two important aspects. One is the high stability and unique electronic properties of the graphene nanostructure. The other is the use of Ag-Ag₂O NPs as the immobilization matrix and redox probes, thus avoiding the laborious labeling protein operation. Using this strategy, the concentration of TM in the range of 0.1 to 20 ng/mL was detected, with a detection limit of 31.5 pg/mL (at 3σ). The proposed methodology demonstrates that the nanocomposite film composed of graphene and Ag-Ag₂O NPs is a potential for biosensor applications.     

Methyl Parathion Degrading Enzyme‐based Nano‐hybrid Biosensor for Enhanced Methyl Parathion Recognition

Enzyme‐based electrochemical biosensors with sufficient sensing specificity are useful analytical tools for detection of biologically important substances in complicated systems. Here, we present the design of a nano‐hybrid biosensor for the specific and sensitive detection of methyl parathion (MP). The nano‐hybrid sensing film was prepared via the formation of Au nanoparticals (AuNPs) on silica nanoparticles (SiNPs), mixing with multiwall carbon nanotube (MWNTs) and subsequent immobilization of methyl parathion degrading enzyme (MPD). The fabrication procedure was characterized by scanning electron images, linear scan voltammetry and electrochemical impedance spectroscopy. The combined MPD exhibited high affinity to it substrate and thus a selective, sensitive, fast and cheap method for determination of MP, quantitatively was proposed. A significant synergistic effect of nano‐hybrid on the biosensor performance was observed in biosensing MP. The square wave voltammetric responses displayed well defined peaks, linearly proportional to the concentration of MP in the range from 0.001 to 5.0 μg/mL with a detection limit of 0.3 ng/mL. The proposed biosensor also showed good precision and reproducibility, acceptable stability and accuracy in garlic samples analysis. It provided a platform for the simple and fast construction of biosensors with good performance for the determination of enzyme‐specific electroactive species.     

Electro‐oxidized Monolayer CVD Graphene Film Transducer for Ultrasensitive Impedimetric DNA Biosensor

We report the effect of electrochemical anodization on the properties of monolayer graphene as the main aim of this research and consequently using the resulting label‐free impedimetric biosensor for DNA sequences detection. Monolayer graphene was grown by chemical vapor deposition (CVD) with methane as precursor on copper foil, transferred onto a glassy carbon electrode and electrochemically anodized. Raman spectroscopy and X‐Ray photo electron spectroscopy revealed enhancement of defect density, roughness and formation of C−O−C, C−O−H and C=O functional groups after anodization. Amine‐terminated poly T probe was linked covalently to the carboxylic groups of anodized graphene by the zero‐length linker to fabricate the impedance‐based DNA biosensor. The anodized graphene electrode demonstrated a superior performance for electrochemical impedance detection of DNA. The DNA biosensor showed a large linear dynamic range from 2.0×10⁻¹⁸ to 1.0×10⁻¹² M with a limit of detection of 1.0×10⁻¹⁸ M using electrochemical impedance spectroscopy (EIS) method. Equivalent circuit modeling shows that DNA hybridization is detected through a change in charge transfer resistance.     

Coupling of a Reagentless Electrochemical DNA Biosensor with Conducting Polymer Film and Nanocomposite as Matrices for the Detection of the HIV DNA Sequences

Abstract

This paper describes a reagentless electrochemical DNA biosensor applied to the detection of human immunodeficiency virus (HIV) sequences based on electrochemical impedance spectroscopy (EIS). The novel DNA biosensor has been elaborated by means of an opposite‐charged adsorption Au‐Ag nanocomposite to a conductive polymer polypyrrole (PPy) modified platinum electrode (Pt) and self‐assembly the mercapto oligonucleotide probes onto the surface of modified electrode via the nanocomposite. The duplex formation was detected by measuring the electrochemical impedance signal of nucleic acids in phosphate buffer solution (PBS). Such response is based on the concomitant conductivity changes of the PPy film and nanocomposite. The reagentless scheme has been characterised using 21‐mer synthetic oligonucleotides as models: parameters affecting the hybridization assay were explored and optimized. The detection limit is 5.0×10^?10 M of target oligonucleotides at 3σ. The potential for development of reagentless DNA hybridization analysis in the clinical diagnosis is being pursued.     

Polymerization of graphene oxide nanosheet by using of aminoclay: Electrocatalytic activity of its platinum nanohybrids

This study describes the polymerization of graphene oxide (GO) nanosheet to reduced‐GO‐aminoclay (RGC) by covalent functionalization of chemically reactive epoxy groups on the basal planes of GO with amine groups of magnesium phyllosilicate clay (known as aminoclay). The resulting RGC sheets were characterized and applied to support platinum nanostructures at toluene/water interface. Pt nanoparticles (NPs) with diameters about several nanometers were adhered to RGC sheets by chemical reduction of [PtCl₂(cod)] (cod = cis,cis‐1,5‐cyclooctadiene) complex. Catalytic activity of Pt NPs thin films were investigated in the methanol oxidation reaction. Cyclic voltammetry results exhibit that the Pt/reduced‐GO (RGO) and Pt/RGC thin films showed improved catalytic activity in methanol oxidation reaction in comparison to other Pt NPs thin films, demonstrating that the prepared Pt/RGO and Pt/RGC thin films are promising catalysts for direct methanol fuel cell.     

Determination of Explosives Using Electrochemically Reduced Graphene

A graphene‐based electrochemical sensing platform for sensitive determination of explosive nitroaromatic compounds (NACs) was constructed by means of electrochemical reduction of graphene oxide (GO) on a glassy carbon electrode (GCE). The electrochemically reduced graphene (ER‐GO) adhered strongly onto the GCE surface with a wrinkled morphology that showed a large active surface area. 2,4‐Dinitrotoluene (2,4‐DNT), as a model analyte, was detected by using stripping voltammetry, which gave a low detection limit of 42 nmol L⁻¹ (signal‐to‐noise ratio=3) and a wide linear range from 5.49×10⁻⁷ to 1.1×10⁻⁵ M . Further characterizations by electrochemistry, IR, and Raman spectra confirmed that the greatly improved electrochemical reduction signal of DNT on the ER‐GO‐modified GC electrode could be ascribed to the excellent electrocatalytic activity and high surface‐area‐to‐volume ratio of graphene, and the strong π–π stacking interactions between 2,4‐DNT and the graphene surface. Other explosive nitroaromatic compounds including 1,3‐dinitrobenzene (1,3‐DNB), 2,4,6‐trinitrotoluene (TNT), and 1,3,5‐trinitrobenzene (TNB) could also be detected on the ER‐GO‐modified GC electrode at the nM level. Experimental results showed that electrochemical reduction of GO on the GC electrode was a fast, simple, and controllable method for the construction of a graphene‐modified electrode for sensing NACs and other sensing applications.     

Catalytic oxidation and determination of β-NADH using self-assembly hybrid of gold nanoparticles and graphene

A self-assembly hybrid of gold nanoparticles on graphene modified electrodes for low-potential NADH detection has been achieved. We used the natural polymer chitosan (Chit) to assist the stabilization of graphene in aqueous solution, and immobilize the electronegative Au nanoparticles (NPs) through electrostatic attraction. The synergy of Au NPs with graphene for catalytic oxidation of NADH made the overpotential ca. 220 mV less positive than that on the bare electrode, and remarkably increased the oxidation current. The amperometric sensors based on such modified electrodes for detection of NADH exhibited a good linearity from 1.5 to 320 μM, and showed high sensitivity with a low detection limit of 1.2 μM (S/N = 3). It could also exclude common interfering electroactive compounds like ascorbic acid and possessed good reproducibility and operational stability. Such eminent performance of the Au-RGO/Chit film together with the ability of graphene to significantly enhance the electron transfer between enzymes and the electrode suggested its promise for constructing novel graphene based dehydrogenase biosensors.     

Dual nanocatalysts co-decorated three-dimensional,laser-induced graphene hybrid nanomaterials integrated with a smartphone portable electrochemical system for point-of-care non-enzymatic glucose diagnosis

Non-enzymatic biosensors based on various nanomaterials with large surface-volume ratios and high catalytic efficiencies have been proposed to compensate for the non-stability and high cost of enzymatic biosensors. However, the construction of a stable, highly sensitive, flexible, three-dimensional (3D), microstructured, non-enzymatic biosensor integrated with a smartphone-based portable system has been challenging. Herein, highly conductive laser-induced graphene (LIG) array with a honeycomb-like 3D microstructure co-decorated with copper(I) oxide and gold nanocatalysts was developed via simple and green electro-deposition and chemical reduction approaches for a miniaturized electrochemical flexible non-enzymatic biosensor. SEM, XRD, Raman and XPS analyzations indicated that the Cu₂O and Au nanocatalysts co-decorated three-dimensional, laser-induced graphene hybrid nanomaterials were developed successfully. The signal of the biosensor was improved by more than 10 fold compared to the LIG alone due to the co-decorated with copper(I) oxide and gold nanocatalysts. The fabricated electrochemical biochip was integrated with a smartphone-based microstation for glucose monitoring, presenting a larger linear interval of 1–20 mM with an excellent sensitivity of 236 μA/mM/cm² and a relatively low detection limit of 0.31 μM. Noticeably, the biochip could measure blood sugar on curved surfaces and still deliver stable sensing signals after being bent back-and-forth 25 times. The novel biosensor is a potentially valuable flexible electronic device. The hybrid nanomaterials developed in this work may be applicable to other biosensing, catalytic, and energy devices (supercapacitors and batteries).     

Carbon Nanomaterials in Biosensors: Should You Use Nanotubes or Graphene?

From diagnosis of life‐threatening diseases to detection of biological agents in warfare or terrorist attacks, biosensors are becoming a critical part of modern life. Many recent biosensors have incorporated carbon nanotubes as sensing elements, while a growing body of work has begun to do the same with the emergent nanomaterial graphene, which is effectively an unrolled nanotube. With this widespread use of carbon nanomaterials in biosensors, it is timely to assess how this trend is contributing to the science and applications of biosensors. This Review explores these issues by presenting the latest advances in electrochemical, electrical, and optical biosensors that use carbon nanotubes and graphene, and critically compares the performance of the two carbon allotropes in this application. Ultimately, carbon nanomaterials, although still to meet key challenges in fabrication and handling, have a bright future as biosensors.