Owing to their extraordinary electrical, chemical, optical, mechanical and structural properties, graphene and its derivatives have stimulated exploding interests in their sensor applications ever since the first isolation of free-standing graphene sheets in year 2004. This article critically and comprehensively reviews the emerging graphene-based electrochemical sensors, electronic sensors, optical sensors, and nanopore sensors for biological or chemical detection. We emphasize on the underlying detection (or signal transduction) mechanisms, the unique roles and advantages of the used graphene materials. Properties and preparations of different graphene materials, their functionalizations are also comparatively discussed in view of sensor development. Finally, the perspective and current challenges of graphene sensors are outlined (312 references).

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Biological and chemical sensors based on graphene materials

Graphene-based materials are gaining heightened attention as novel materials for environmental applications The unique physicochemical properties of graphene, notably its exceptionally high surface area, electron mobility, thermal conductivity, and mechanical strength, can lead to novel or improved technologies to address the pressing global environmental challenges This critical review assesses the recent developments in the use of graphene-based materials as sorbent or photocatalytic materials for environmental decontamination, as building blocks for next generation water treatment and desalination membranes, and as electrode materials for contaminant monitoring or removal The most promising areas of research are highlighted, with a discussion of the main challenges that we need to overcome in order to fully realize the exceptional properties of graphene in environmental applications

Environmental applications of graphene-based nanomaterials.

Graphene materials have been widely explored for the fabrication of gas sensors because of their atom-thick two-dimensional conjugated structures, high conductivity and large specific surface areas. This feature article summarizes the recent advancements on the synthesis of graphene materials for this purpose and the techniques applied for fabricating gas sensors. The effects of the compositions, structural defects and morphologies of graphene-based sensing layers and the configurations of sensing devices on the performances of gas sensors will also be discussed.

Graphene-based gas sensors

Three-dimensional (3D) graphene materials (3DGMs) are of great importance due to their unique properties and practical applications. A number of 3DGMs with novel structures have been developed in recent years. This review presents the current progress of 3DGMs. After introducing the preparation strategies of 3DGMs, we summarize the reported 3DGMs based on their different structures, and then focus on the description of their preparation methods, properties and applications. Lastly, the applications of 3D graphene-based materials in supercapacitors are described.

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Three-dimensional graphene materials: preparation, structures and application in supercapacitors

Most of the metal ions are carcinogens and lead to serious health concerns by producing free radicals. Hence, fast and accurate detection of metal ions has become a critical issue. Among various metal ions arsenic, cadmium, lead, mercury and chromium are considered to be highly toxic. To detect these metal ions, electrochemical biosensors with interfaces such as microorganisms, enzymes, microspheres, nanomaterials like gold, silver nanoparticles, CNTs, and metal oxides have been developed. Among these, nanomaterials are considered to be most promising, owing to their strong adsorption, fast electron transfer kinetics, and biocompatibility, which are very apt for biosensing applications. The coupling of electrochemical techniques with nanomaterials has enhanced the sensitivity, limit of detection, and robustness of the sensors. In this review, toxicity mechanisms of various metal ions and their relationship towards the induction of oxidative stress have been summarized. Also, electrochemical biosensors employed in the detection of metal ions with various interfaces have been highlighted.

A review on detection of heavy metal ions in water – An electrochemical approach

Fast and accurate detection of aqueous contaminants is of significant importance as these contaminants raise serious risks for human health and the environment. Mercury and its compounds are highly toxic and can cause various illnesses; however, current mercury detectors suffer from several disadvantages, such as slow response, high cost, and lack of portability. Here, we report field-effect transistor (FET) sensors based on thermally reduced graphene oxide (rGO) with thioglycolic acid (TGA) functionalized gold nanoparticles (Au NPs) (or rGO/TGA-AuNP hybrid structures) for detecting mercury(II) ions in aqueous solutions. The lowest mercury(II) ion concentration detected by the sensor is 2.5 × 10–8 M. The drain current shows rapid response within less than 10 s after the solution containing Hg2+ ions was added to the active area of the rGO/TGA-AuNP hybrid sensors. Our work suggests that rGO/TGA-AuNP hybrid structures are promising for low-cost, portable, real-time, heavy metal ion detectors.

Hg(II) Ion Detection Using Thermally Reduced Graphene Oxide Decorated with Functionalized Gold Nanoparticles

Graphene has attracted growing interest in the past few years. Growing vertically oriented graphene sheets with a designed pattern is practically attractive for device applications based on graphene. Here we report a patterned synthesis of vertical graphene nanosheets using plasma-enhanced chemical vapor deposition. Both experimental and modeling results suggest that the electric field distribution above the substrate material plays a key role in the graphene coverage. Vertical graphene patterns can thus be designed through artificially designing the surface electric field distribution. A field-effect transistor (FET) sensor device has been demonstrated for detection of low-concentration gases using vertically patterned graphene sheets bridging a metal electrode gap.

Patterning Vertically Oriented Graphene Sheets for Nanodevice Applications

A linker-free reduced graphene oxide (R-GO)-CdSe nanoparticle (CdSe NP) hybrid nanostructure was synthesized using a chemical vapor deposition method. CdSe NPs were selectively deposited on the surface of R-GO with controlled NP size and coverage. The distribution and morphology of CdSe NPs on R-GO were characterized by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The resulting hybrid nanostructure exhibited photoresponse to both laser and simulated sunlight AM 1.5G excitation. The hybrid structure with low CdSe NP coverage showed distinct photoresponse times in air, N2, NH3, and NO2, while high CdSe NP coverage led to nearly constant but three orders of magnitude smaller response time in all gases. Such a difference in photoresponse as a function of NP coverage is attributed to the energy band bending at the interface between the R-GO and the CdSe NP. The selective deposition of CdSe NPs on R-GO and the understanding of the subsequent photoinduced charge tr...

Selective deposition of CdSe nanoparticles on reduced graphene oxide to understand photoinduced charge transfer in hybrid nanostructures.

Carbon nanotubes (CNTs) coated with nanocrystals (NCs) or quantum dots (QDs) form a new class of hybrid nanomaterials that could potentially display both the unique properties of NCs and those of CNTs. The photo-induced charge transfer within the hybrid nanostructure may either increase or decrease the CNT electrical conductivity. The use of a chemical vapor deposition method realizes direct assembly of CdSe NCs onto the external surfaces of single-walled CNTs without applying any organic linkers. The NC loading on the nanotubes can be tuned simply through deposition duration. Upon visible light excitation, photo-generated electrons are injected into CNTs from the excited states of CdSe NCs, which is monitored by the measurement of the photocurrent and field effect transistor (FET) characteristics. The direction of the electron transfer could be controlled with the NC coverage on CNTs and the gate voltage.

Kehung Chen

Papers

Hg(II) Ion Detection Using Thermally Reduced Graphene Oxide Decorated with Functionalized Gold Nanoparticles

Patterning Vertically Oriented Graphene Sheets for Nanodevice Applications

Selective deposition of CdSe nanoparticles on reduced graphene oxide to understand photoinduced charge transfer in hybrid nanostructures.

Controllable photoelectron transfer in CdSe nanocrystal–carbon nanotube hybrid structures