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.

Toward ubiquitous environmental gas sensors—capitalizing on the promise of graphene

Atomically thin sheets of carbon known as “graphene” have captured the imagination of much of the scientific world during the past few years. Although these single sheets of graphite were under our noses for years—within technologies ranging from the humble pencil, which has been around since at least 1565 (Petroski, H. The Pencil: A History of Design and Circumstance; Alfred A. Knopf: New York, 1993), to modern nuclear reactors—graphene was merely considered as part of graphite’s crystal structure until 2004, when Novoselov, Geim, and colleagues (Science 2004, 306, 666−669) first presented some of the surprising electrical properties of graphene layers they had isolated by mechanically peeling sheets off graphite crystals. Today, graphene’s unique electronic structures and properties, bolstered by other intriguing properties discovered in the intervening years, threaten the dominance of carbon nanotubes, a more mature allotrope of carbon, in potential applications from electronics to sensors. In this review, we will consider the promise of graphene for producing small-scale gas sensors for environmental monitoring.

Synthesis, characterization, and multilayer assembly of pH sensitive graphene−polymer nanocomposites

pH sensitive graphene−polymer composites have been prepared by the modification of graphene basal planes with pyrene-terminated poly(2-N,N′-(dimethyl amino ethyl acrylate) (PDMAEA) and poly(acrylic acid) (PAA) via π−π stacking. The pyrene-terminal PDMAEA and PAA were synthesized using reversible addition−fragmentation chain transfer (RAFT) polymerization with a pyrene-functional RAFT agent. The graphene−polymer composites were found to demonstrate phase transfer behavior between aqueous and organic media at different pH values. Atomic force microscopy (AFM) analysis revealed that the thicknesses of the graphene−polymer sheets were approximately 3.0 nm when prepared using PDMAEA (M_n: 6800 and PDI: 1.12). The surface coverage of polymer chains on the graphene basal plane was calculated to be 5.3 × 10⁻¹¹ mol cm⁻² for PDMAEA and 1.3 × 10⁻¹⁰ mol cm⁻² for PAA. The graphene−polymer composites were successfully characterized using X-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared (ATR-IR) spectroscopy, and thermogravimetric analysis (TGA). Self-assembly of the two oppositely charged graphene−polymer composites afforded layer-by-layer (LbL) structures as evidenced by high-resolution scanning electron microscopy (SEM) and quartz crystal microbalance (QCM) measurements.

Direct electro-deposition of graphene from aqueous suspensions

We describe the direct electro-chemical reduction of graphene oxide to graphene from aqueous suspension by applying reduction voltages exceeding -1.0 to -1.2 V. The conductivity of the deposition medium is of crucial importance and only values between 4-25 mS cm^-1 result in deposition. Above 25 mS cm^-1 the suspension de-stabilises while conductivities below 4 mS cm_^-1 do not show a measurable deposition rate. Furthermore, we show that deposition can be carried out over a wide pH region ranging from 1.5 to 12.5. The electro-deposition process is characterised in terms of electro-chemical methods including cyclic voltammetry, quartz crystal microbalance, impedance spectroscopy, constant amperometry and potentiometric titrations, while the deposits are analysed via Raman spectroscopy, infra-red spectroscopy, X-ray photoelectron spectroscopy and X-ray diffractometry. The determined oxygen contents are similar to those of chemically reduced graphene oxide, and the conductivity of the deposits was found to be ~20 S cm ^-1.

Controllable corrugation of chemically converted graphene sheets in water and potential application for nanofiltration

A combination of AFM, SEM and permeation experiments suggests that the amplitude of corrugation of chemically converted graphene (CCG) sheets in water can be readily controlled by hydrothermal treatment, leading to a new class of permeation-tuneable nanofiltration membranes.

Comparison of performance parameters for conventional and localized surface plasmon resonance graphene biosensors

This paper investigates the enhancement of the sensitivity and adsorption efficiency of a localized surface plasmon resonance (LSPR) biosensor that includes a layer of graphene sheet on top of the gold layer. For this purpose, biomolecular interactions of biotin-streptavidin with the graphene layer on the gold thin film are monitored. The performance of the LSPR graphene biosensor is theoretically and numerically assessed in terms of sensitivity and adsorption efficiency under varying conditions, including the thickness of biomolecule layer, number of graphene layers and operating wavelength. Enhanced sensitivity and improved adsorption efficiency are obtained for the LSPR graphene biosensor in comparison with its conventional counterpart. It is found that the LSPR graphene biosensor has better sensitivity with lower operating wavelength and larger number of graphene layers.

Variable incidence angle localized surface plasmon resonance graphene biosensor

This paper investigates the enhancement of sensitivity of variable incidence angle LSPR biosensor by monitoring biomolecular interactions of biotin-streptavidin with gold thin film. The investigation is carried out by means of introducing an additional layer of graphene sheet on top of gold layer (graphene biosensor) and using different coupling configuration of laser beam. The sensitivity, which is indicated by the shift of plasmon resonance angle, increases with graphene deposited onto the gold layers and is linearly related with the number of graphene layers. In addition, an investigation of the shift of plasmon dip is carried out for two different analyte interfaces: air and water. It is found that graphene biosensor has better sensitivity for triangular prism, higher prism angle, and water interface. The evaluation approach involves a plot of a reflectivity curve as a function of the angle of incidence while the operating wavelength is kept fixed.

Graphene oxide nanoparticles as a nonbleaching optical probe for two-photon luminescence imaging and cell therapy

Lasting glow: Under femtosecond laser irradiation, graphene oxide nanoparticles (GONs) give strong two-photon luminescence (TPL; see picture). The presence of GONs also induces microbubbling, which causes cell death at an order of magnitude lower laser power than when cells are not labeled. The results show that GONs can be used for TPL-based imaging and photothermal cancer therapy.

Reduced graphene oxide/ZnO composite : reusable adsorbent for pollutant management

Reduced graphene oxide (RGO) coated with ZnO nanoparticles (NPs) was synthesized by a self-assembly and in situ photoreduction method, and then their application for removing organic pollutant from water was investigated. The RGO@ZnO composite nanomaterial has unique structural features including well-dispersed NPs on the surface and dense NPs loading. This composite exhibited a greatly improved Rhodamine B (RhB) adsorption capacity and an improved photocatalytic activity for degrading RhB compared to neat ZnO NPs. These properties made RGO@ZnO reusable for pollutant adsorbent. The composite showed an excellent cycling performance for organic pollutant removal up to 99% recovery over several cycles via simulated sunlight irradiation.

Design and analysis of a multilayer localized surface plasmon resonance graphene biosensor

This paper describes a multilayer localized surface plasmon resonance (LSPR) graphene biosensor that includes a layer of graphene sheet on top of the gold layer, and the use of different coupled configuration of a laser beam. The study also investigates the enhancement of the sensitivity and detection accuracy of the biosensor through monitoring biomolecular interactions of biotin-streptavidin with the graphene layer on the gold thin film. Additionally, the role of thin films of gold, silver, copper and aluminum in the performance of the biosensor is separately investigated for monitoring the binding of streptavidin to the biotin groups. The performance of the LSPR graphene biosensor is theoretically and numerically assessed in terms of sensitivity, adsorption efficiency, and detection accuracy under varying conditions, including the thickness of biomolecule layer, number of graphene layers and operating wavelength. Enhanced sensitivity and improved adsorption efficiency are obtained for the LSPR graphene biosensor in comparison with its conventional counterpart; however, detection accuracy under the same resonance condition is reduced by 5.2% with a single graphene sheet. This reduction in detection accuracy (signal to noise ratio) can be compensated for by introducing an additional layer of silica doped B2O3 (sdB2O3) placed under the graphene layer. The role of prism configuration, prism angle and the interface medium (air and water) is also analyzed and it is found that the LSPR graphene biosensor has better sensitivity with triangular prism, higher prism angle, lower operating wavelength and larger number of graphene layers. The approach involves a plot of a reflectivity curve as a function of the incidence angle. The outcomes of this investigation highlight the ideal functioning condition corresponding to the best design parameters.

Sensitivity analysis of a sub-wavelength localized surface plasmon resonance graphene biosensor

Localized surface plasmon resonance (LSPR) is a promising detection method for label-free sensing of biomolecules. In this paper, a multilayer design for a LSPR biosensor is presented. In the proposed design, a periodic array of dielectric grating is incorporated on top of a graphene layer in the biosensor. The aim is to improve sensitivity of the LSPR biosensor through monitoring biomolecular interactions of biotin-streptavidin. Sensitivity improvement is obtained for the proposed LSPR biosensor compared with conventional SPR counterparts. In addition, to optimize the design, we have investigated grating geometry including volume factor and grating depth. The outcome of this investigation identifies ideal functioning conditions corresponding to the best design parameters.