used reducing agent to produce rGOs. As rGOs feature a negligible amount of
oxygen-containing—epoxy, hydroxyl, and carboxyl—functional groups, surface
modifications on rGOs are performed through non-covalent interactions between
aromatic ring molecules and rGOs’ basal plane. Namely, Shi et al. reported in 2013
the grafting of hydrophobic alkyl chain-terminated PEGs on rGOs to achieve higher
dispersibility [19].
Recently, several groups utilized proteins as novel reducing agents, which can
assure better stability and biocompatibility as well. In 2011, Liu et al. utilized
gelatin as the reducing agent, which yielded well-dispersed rGOs in physiological
conditions [20]. Gelatin-rGO composite was employed as a novel in vitro imaging
agent, which was successfully internalized in cells without significant toxicity. In
2013, Sheng et al. reported BSA-assisted reduction of GOs, where BSA functions
as a surfactant as well [21]. Resulting BSA–rGOs was employed as in vivo imaging
probes as it exhibited great solubility in physiological conditions.
4.2.4 Photoluminescent Nano-GOs and GQDs
While GOs are known for their ability to quench fluorescence in close proximity,
GQDs exhibit faint, yet non-negligible photoluminescence. Typically, the quantum
yields (QYs) of solvothermally synthesized GQDs (5–10 nm) are below 20%
[22–24]. In 2011, Zhu et al. synthesized GQDs by solvothermally cut GOs in
dimethylformamide [23]. The resulting GQDs showed great dispersibility in
physiological conditions, with an average size of 5.3 nm and distinct green
fluorescence with 11.4% QY. The authors employed these GQDs for bio-imaging
studies without any significant toxicity. In 2012, Zhang et al. reported an electrochemical oxidation of graphite to produce GQDs [24]. Electrolysis of graphite
under alkaline conditions and subsequent treatment with hydrazine yielded GQDs
with green photoluminescence, with 14% QY. They also utilized GQDs for
bio-imaging studies on various types of stem cells without any appreciable toxicity.
Interestingly, the optical properties of GQDs can be modulated by various
surface modifications. In 2012, Li et al. reported microwave-assisted method to
produce GQDs, with subsequent reduction using NaBH 4 [25]. Initially,
as-synthesized GQDs exhibited green-yellow fluorescence with 11.7% QY. On the
other hand, reduced GQDs showed blue fluorescence with an increased QY of
22.9%. Photoluminescence was found to be altered by either reduction or grafting
of exogenous molecules [25, 26]. They showed that both NaBH 4 –assisted
reduction of GQDs and grafting of alkylamines yielded blue-shifted emission, with
increased QYs. In 2013, Wu et al. synthesized nitrogen-doped GQDs via bottom-up
method from L-glutamic acid as the precursor, yielding about 5 nm-sized GQDs
[27]. The authors employed the n-doped GQDs as an effective in vitro and in vivo
imaging agent with a QY reaching 54.5%.
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J. M. Yoo et al.
oxygen-containing—epoxy, hydroxyl, and carboxyl—functional groups, surface
modifications on rGOs are performed through non-covalent interactions between
aromatic ring molecules and rGOs’ basal plane. Namely, Shi et al. reported in 2013
the grafting of hydrophobic alkyl chain-terminated PEGs on rGOs to achieve higher
dispersibility [19].
Recently, several groups utilized proteins as novel reducing agents, which can
assure better stability and biocompatibility as well. In 2011, Liu et al. utilized
gelatin as the reducing agent, which yielded well-dispersed rGOs in physiological
conditions [20]. Gelatin-rGO composite was employed as a novel in vitro imaging
agent, which was successfully internalized in cells without significant toxicity. In
2013, Sheng et al. reported BSA-assisted reduction of GOs, where BSA functions
as a surfactant as well [21]. Resulting BSA–rGOs was employed as in vivo imaging
probes as it exhibited great solubility in physiological conditions.
4.2.4 Photoluminescent Nano-GOs and GQDs
While GOs are known for their ability to quench fluorescence in close proximity,
GQDs exhibit faint, yet non-negligible photoluminescence. Typically, the quantum
yields (QYs) of solvothermally synthesized GQDs (5–10 nm) are below 20%
[22–24]. In 2011, Zhu et al. synthesized GQDs by solvothermally cut GOs in
dimethylformamide [23]. The resulting GQDs showed great dispersibility in
physiological conditions, with an average size of 5.3 nm and distinct green
fluorescence with 11.4% QY. The authors employed these GQDs for bio-imaging
studies without any significant toxicity. In 2012, Zhang et al. reported an electrochemical oxidation of graphite to produce GQDs [24]. Electrolysis of graphite
under alkaline conditions and subsequent treatment with hydrazine yielded GQDs
with green photoluminescence, with 14% QY. They also utilized GQDs for
bio-imaging studies on various types of stem cells without any appreciable toxicity.
Interestingly, the optical properties of GQDs can be modulated by various
surface modifications. In 2012, Li et al. reported microwave-assisted method to
produce GQDs, with subsequent reduction using NaBH 4 [25]. Initially,
as-synthesized GQDs exhibited green-yellow fluorescence with 11.7% QY. On the
other hand, reduced GQDs showed blue fluorescence with an increased QY of
22.9%. Photoluminescence was found to be altered by either reduction or grafting
of exogenous molecules [25, 26]. They showed that both NaBH 4 –assisted
reduction of GQDs and grafting of alkylamines yielded blue-shifted emission, with
increased QYs. In 2013, Wu et al. synthesized nitrogen-doped GQDs via bottom-up
method from L-glutamic acid as the precursor, yielding about 5 nm-sized GQDs
[27]. The authors employed the n-doped GQDs as an effective in vitro and in vivo
imaging agent with a QY reaching 54.5%.
84
J. M. Yoo et al.