Wonder material graphene has special physicochemical properties, such as high mechanical strength, large surface-to-volume ratio, biocompatibility, superior electrical and thermal conductivity, and advantages such as low cost, ease of production, and safety. As a result, the use of this material in sensor applications has attracted a great deal of attention. Graphene has low charge-transfer resistance and a wide electrochemical potential, which make it almost perfect for fast, multifunctional sensors. The ability to be functionalized is another critical aspect needed in optical biosensors. Graphene, as well as graphene oxide (GO), are resourceful materials for functionalization.
This large number of complimentary characteristics has led to a wide range of analysis on the use of graphene for biosensing applications. Graphene-enhanced surface plasmon resonance (SPR) and graphene field-effect transistors (GFETs) happen to be the most interesting configurations (Figure 1). These types of graphene sensors have been utilized to detect glucose, protein, DNA, and bacteria.
Figure 1. Sketch of GFET sensor (Reproduced from Chem. Sci., 2012,3, 1764, with permission of The Royal Society of Chemistry).
Graphene Field Effect Transistors
GFET is a an alteration of the traditional silicon field-effect transistor, which is ubiquitous in contemporary electronics. In conventional transistors, silicon serves as a thin conducting channel, and its conductivity can be adjusted by applying voltage. GFETs also function in a similar way, but here graphene is used in the place of silicon; graphene gives a relatively thinner and more sensitive channel region.
Since GFETs have the ability to be functionalized and offer a wide electrochemical potential, they are suitable devices for biomolecules to adhere to. Also, since graphene has a high surface-to-volume ratio and extreme thinness, even the lowest concentration of joined molecules can modify the conductivity of the channel region.
GFET biosensors are provided to detect dopamine, hydrogen peroxides, enzymes, as well as reduced b-nicotinamide adenine dinucleotide (NADH) molecules.
Surface Plasmon Polariton Detectors
In yet another configuration, GO or graphene is utilized along with surface plasmon polaritons (SPPs) on metal films to improve the performance of biosensors. Sensors based on SPP leverage the internment of optical waves on the metal surface to form small-volume biological and chemical sensors. The tightly confined surface wave specifies the sensing capacity, improving the sensitivity of optical detection.
Silver and gold are the traditional metals utilized in this kind of sensor, thanks to their complimentary SPP propagation characteristics. Conversely, gold is known to have poor adsorption properties and silver can corrode rapidly. Nevertheless, better adsorption can be realized by placing a graphene layer over the gold, and GO, with its high covalent binding affinity, is especially good at binding proteins.
In this label-free detection system, GO is capable of increasing the sensitivity of SPP sensors. In combination with microfluidics technology, scientists have demonstrated that a single cancer cell can be easily detected. In a recent study, researchers have revealed that GO-based sensor chips could be used to detect the HIV infection. When streptavidin was added to the GO coating, it was observed that the selectivity of the GO SPP sensor improves considerably (Figure 2).
Figure 2. SPP sensor enhanced with graphene (Reproduced from Optics Express 18, 14395 (2010) with permission from the Optical Society of America).
An increasing trend in modern research is adding selectivity to GO SPP sensors; in fact, many reports have been published in this regard. Selectivity can be enhanced using several graphene measurement modes immediately, such as optical, electrical, and mechanical. Apart from SPPs and GFETs, there is a rising trend of other types of graphene-enhanced sensors, such as pressure and micromechanical (MEMS) sensors.
This information has been sourced, reviewed and adapted from materials provided by Graphenea.
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