292
P. Veerakumar et al.
Diamond consists of sp 3 -hybridized carbons, where the purely covalent chemical
bonds extend three-dimensionally. Diamond is known as an electrically insulator
due to lack of π electrons. On the other hand, the graphene is a 2D single
layer of graphite and known to be very strong conductive material, because the
graphene consists of sp 2 bonds. Structurally, a graphene sheet is a single layer of
carbon atoms packed into 2D honeycomb lattice structure, whereas the CNTs are
considered as rolled-up graphene sheet and their edges of the sheet joint together
to form a seamless cylinder. In the case of 0D nanomaterials, GQDS are formed
by the cutting a graphene monolayer into the small spherical molecules with
the size of below 20 nm. These GQDS are mainly composed of sp 2 hybridized
carbon atoms and crystalline in nature. In contrast, the C-dots composed of sp 3
hybridized carbon atoms predominantly with the various combinations of graphitic
and turbostratic carbons. Moreover, the C-dots exhibit amorphous nature of their
carbogenic cores. Recently, the C-dots and GQDs are getting much popularity
among the 1D and 2D carbon nanomaterials. They can be synthesized from diverse
kind of carbon precursors including organic molecules (citric acid, carbohydrates
derivatives, etc.) and biomolecules (proteins, amino acids, etc.) with facile method
and in shorter time. Another important feature is that C-dots and GQDs have
photoluminescence property inherently as well as they can be tuned by chemical
surface functionalization.
The applications as fluorescence sensors have been extensively studied by using
these carbon nanomaterials due to their inherent fluorescence properties and nontoxicity. Among these nanocarbons, the single- and two-dimensional (1D and 2D)
materials such as CNTs, graphene, and GO were served as sensing platform. The
zero-dimensional (0D) materials like GQDs and C-dots are emerging as promising
fluorescence sensing probe in consequence of several unique features including
strong fluorescence with tunability, high photostability, smaller in size, ease to
be functionalized, biocompatibility, and non-toxicity. Due to these advantages, the
GQDs and C-dots materials were widely utilized as a fluorescent sensing probe in
chemical and biosensors. The most important feature is that these materials exhibit
non/less-toxicity in nature which enhances the potential in their sensor application.
7.5.1 Structural and Fluorescence Analysis of Graphene
Quantum Dots and Carbon Dots
Firstly, Suda et al. have prepared the nanometer-size carbon particles on a Si
substrate using plasma-assisted pulsed laser deposition (PLD) method [129]. The
prepared carbon nanoparticles were in an amorphous state with sp 3 and sp 2 carbon
components. Then in 2006, Sun et al. reported the photoluminescence carbon
nanoparticles which were then named as “carbon dots” [130]. They observed
strong excitation wavelength-dependent fluorescence (Fig. 7.17) which is ascribed
to the emissive surface defects caused by the surface passivation of C-dots with
organic molecules such as diamine-terminated oligomeric poly-(ethylene glycol)
P. Veerakumar et al.
Diamond consists of sp 3 -hybridized carbons, where the purely covalent chemical
bonds extend three-dimensionally. Diamond is known as an electrically insulator
due to lack of π electrons. On the other hand, the graphene is a 2D single
layer of graphite and known to be very strong conductive material, because the
graphene consists of sp 2 bonds. Structurally, a graphene sheet is a single layer of
carbon atoms packed into 2D honeycomb lattice structure, whereas the CNTs are
considered as rolled-up graphene sheet and their edges of the sheet joint together
to form a seamless cylinder. In the case of 0D nanomaterials, GQDS are formed
by the cutting a graphene monolayer into the small spherical molecules with
the size of below 20 nm. These GQDS are mainly composed of sp 2 hybridized
carbon atoms and crystalline in nature. In contrast, the C-dots composed of sp 3
hybridized carbon atoms predominantly with the various combinations of graphitic
and turbostratic carbons. Moreover, the C-dots exhibit amorphous nature of their
carbogenic cores. Recently, the C-dots and GQDs are getting much popularity
among the 1D and 2D carbon nanomaterials. They can be synthesized from diverse
kind of carbon precursors including organic molecules (citric acid, carbohydrates
derivatives, etc.) and biomolecules (proteins, amino acids, etc.) with facile method
and in shorter time. Another important feature is that C-dots and GQDs have
photoluminescence property inherently as well as they can be tuned by chemical
surface functionalization.
The applications as fluorescence sensors have been extensively studied by using
these carbon nanomaterials due to their inherent fluorescence properties and nontoxicity. Among these nanocarbons, the single- and two-dimensional (1D and 2D)
materials such as CNTs, graphene, and GO were served as sensing platform. The
zero-dimensional (0D) materials like GQDs and C-dots are emerging as promising
fluorescence sensing probe in consequence of several unique features including
strong fluorescence with tunability, high photostability, smaller in size, ease to
be functionalized, biocompatibility, and non-toxicity. Due to these advantages, the
GQDs and C-dots materials were widely utilized as a fluorescent sensing probe in
chemical and biosensors. The most important feature is that these materials exhibit
non/less-toxicity in nature which enhances the potential in their sensor application.
7.5.1 Structural and Fluorescence Analysis of Graphene
Quantum Dots and Carbon Dots
Firstly, Suda et al. have prepared the nanometer-size carbon particles on a Si
substrate using plasma-assisted pulsed laser deposition (PLD) method [129]. The
prepared carbon nanoparticles were in an amorphous state with sp 3 and sp 2 carbon
components. Then in 2006, Sun et al. reported the photoluminescence carbon
nanoparticles which were then named as “carbon dots” [130]. They observed
strong excitation wavelength-dependent fluorescence (Fig. 7.17) which is ascribed
to the emissive surface defects caused by the surface passivation of C-dots with
organic molecules such as diamine-terminated oligomeric poly-(ethylene glycol)