1. Introduction
The advent of graphene, a perfect two dimensional (2D) material, composed of single-atom-thick sheets of sp2 bonded carbon atoms packed into a honeycomb lattice, has opened up the exciting new horizon of the carbon era in the field of science and technology. From its discovery in 2004 by Geim and Novoselov [1], graphene has attracted increasing attention due to its excellent properties and applications in diversified fields [2,3]. Owing to its structural features, graphene is characterized by a number of unique and extraordinary structural, optical, and electronic properties (Table 1) [4] with mesmerizing transport phenomena such as the quantum Hall effect [5], optical transmittance, and fluorescence quenching ability [6]. Graphene is a zero-band-gap semiconductor and demonstrates high electron mobility under ambient conditions, [7] which is advantageous in sensors, super capacitors, and electrocatalysis application. The high optical transparency of graphene nanocomposites pushes forward the fabrication of transparent conductive films [8,9] for application in solar cells, advanced electronics etc. All of these properties make graphene an ideal building block in the fabrication of nanocomposites. Graphene nanocomposites also show high thermal conductivity that provides excellent thermal stability, which is important in some electronic devices or catalytic reactions that release heat, such as fuel cells and lithium-ion batteries.
Before graphene, another carbon nanomaterial, carbon nanotubes (CNTs), were of great interest in the fabrication of nanocomposites in biosensor applications [16,17], however, the preference for this material seems to have declined with the emergence of graphene due to its easy availability and some other advantageous properties in comparison to CNTs [18]. Graphene has a unique basal plane structure to load microspheres of several hundred nanometers in diameter, which presents a benefit over CNTs for nanomaterial decoration (Figure 1) [19]. Its 2D structures make it plausible to synthesize graphene-based nanocomposites by novel synthesis methods such as thermal decomposition of intercalated graphene precursors, which is a challenge in the case of CNT-based nanocomposites [20,21]. The higher surface area of graphene improves interfacial contact with the other components in comparison to CNTs and can prevent the accumulation of secondary components, thus preserving some unique properties in the nanoscale level [22]. In addition, graphene has no metallic impurity, which is the major drawback of CNTs in biosensor applications, and hence can be easily integrated into complex sensors or other devices through conventional microfabrication approaches. Conversely, the one-dimensional nature of CNTs creates difficulty in controllably assembling integrated electronic architectures on them.
Nanocomposites consist of multiphase materials wherein one phase (dispersed phase) in nanosize form is dispersed in a second phase (matrix/continuous phase), with the ensuing combination of the individual properties of the component materials [25]. Graphene-inorganic metal and metal oxide nanocomposites are now substrates of interest due to their advantageous properties in diversified fields of application. In some instances, these composites not only overcome the limitations of the usage of a single component in biosensor applications but also provide higher effective surface area, excellent catalytic properties, higher specificity, limit of detection (LOD), etc. For example, individual sheets of graphene have a tendency towards irreversible self-agglomerations [26] by van der Waals and π-π stacking interactions, which may partially reduce their electrochemical properties. The addition of a second component (noble metal nanoparticles) acts as a nano-spacer and conductor, hence increasing the graphene interlayer distance to minimize the agglomeration, making both faces accessible and improving the electrical conductivity [27,28].
Direct immobilization of biomolecules (proteins) onto CNTs [29] or graphene oxide (GO) [30] has been proved unstable, therefore frequently applied washing steps in biosensor fabrication can readily remove proteins. Consequently, this presents undesirable effects, such as poor reliability/repeatability and non-specificity of the sensor. Graphene-nanoparticle hybrid structures offer a number of highly desirable and markedly advantageous additional unique physicochemical properties and functions in bio-applications in comparison to either material alone [31]. Among the noble metal nanoparticles, AuNPs are one of the most studied nanomaterials, due to their remarkable surface chemical properties [32], higher chemical stability, excellent catalytic activity [33], biocompatibility [34], and other notable properties. These properties make AuNPs a model component for the detection of DNA [35,36,37] and proteins [38], rapid identification of microorganisms [39], and differentiation of cancer patients from healthy individuals [40].
Therefore, it is highly expected that extraordinary outputs can be achieved using the fabricated graphene-AuNPs composites in numerous applications. In this regard, AuNPs/reduced graphene oxide (rGO) composites comply by offering around 2.3 times superior electrocatalytic current density [41], and stronger Raman signals from Rhodamine 6G (R6G) absorbed on the nano-composites than individual pure AuNPs [42]. In addition, the presence of Au and Ag nanostructures (AgNS) on graphene increases the SERS by factors of about 45 and 150, respectively, than graphene alone [43]. This review emphasizes the wide-ranging synthesis and fabrication procedures of graphene-AuNPs hybrids, their application as a fundamental component for the electrochemical and SERS-based biosensor, as well as SERS-measured bioimaging.
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