Graphene-based optical biosensors and imaging

Graphene is the fundamental building element of many carbon allotropes including graphite, charcoals, carbon nanotubes, buckminsterfullerene, and other buckyballs, etc. Graphene comprises a single layer of six-atom rings in a honeycombed network and can be conceptually considered as a true planar aromatic macromolecule.1 Interestingly the theory of graphene was explored in 1947 and scientists believed that graphene could not exist by itself due to thermodynamic instability at the nanometer scale. Though scientists investigated the reduced graphene oxide in 1962, the key advance in graphene synthesis and study was explored in 2004 by scientists at Manchester University peeling a single layer of graphene using adhesive tape and a pencil, which is often referred to as a scotchtape or drawing method. In 2005, they collaborated with another group from Columbia University and found that the quasiparticles in graphene were massless Dirac fermions. Later more important unusual properties of graphene have been unveiled such as the quantum Hall effect, the bipolar-transistor effect, the ballistic transport of charges, large quantum oscillations, etc., which led to the immediate boost of further researches and applications based on graphene. Today graphene is attracting increasing attention from the physical, chemical, and biomedical fields as a novel nanomaterial with many exceptional features including unparalleled thermal conductivity (5000 W/m/K), excellent electrical conductivity (1738 S/m), high surface-to-volume ratio (2630 m²/g), remarkable mechanical strength (about 1100 GPa), and biocompatibility. Recently, graphene and functionalized graphene have been successfully used in many biomedical and bioassay applications and show promising potentials in these fields. These applications utilized the advantages of graphene in electrical and optical properties to construct a variety of graphene-based biosensors including electrochemical biosensors, field-effect transistor (FET) biosensors, optical biosensors, etc. Compared to carbon nanotubes, graphene lacks some bandgapinduced optical properties, but its chemical versatility and tunability nature, fluorescence, effective quencher, Raman, and NIR activity make it favorable, for fabricating optical biosensors. Recently, graphene-based optical nanobiosensors have been constructed to detect DNA/RNA, proteins, small molecules, and ions using different signal platforms, including fluorescence, colorimetry, SPR, SERS, etc. This chapter focuses on the recent progress in graphene-based optical biosensors and discusses the design, fabrication, and application of these optical nanobiosensors. In the end, we will discuss the future opportunities and challenges in this field. Since the basics of graphene, including its synthesis and properties, has been described in Chapter 13, this chapter will not deal with it.