4.2.1 Enhancing the Dispersibility and Stability Under
Physiological Conditions
In general, GOs are well-dispersed in water for several months without much
aggregation. In physiological conditions, however, they are more prone to aggregate owing to the presence of ionic salts. To enhance the stability under these
conditions, modification with biocompatible polymers and nano-sized GOs are
employed [4–8]. Among many modification methods, grafting of polyethylene
glycol (PEG) is the most frequently used. Many reports have shown that PEG
grafting can improve the dispersibility and stability of GOs in biological buffers and
highly concentrated salts. PEGs can be grafted on the edges of GOs by the reactions
with carboxylic acid and epoxy groups. Other polymers such as polyacrylic acid
(PAA), polyamido amine (PAMAM), and dextran can be also conjugated on GOs
by the same reactions [4–6]. While as-synthesized GOs show a wide range of sizes,
reduction of the average size can be simply achieved by sonication. Many reports
show that high energy sonication can break down large GO flakes into nanometer
scale particles, as small as 10 nm, which show greater dispersibility in many solvents including biological buffers [7, 8].
4.2.2 Graphene as Nanocarriers
Although micrometer-scale GOs are known to be toxic, their large hydrophobic
basal plane and oxygen-containing functional groups on the edges are advantageous
for both covalent and non-covalent chemical modifications with other molecules
including fluorescent organic dyes.
In 2010, Peng et al. employed fluorescein-labeled GOs for intracellular imaging
studies [9]. To prevent the fluorescence quenching and improve dispersibility,
PEGs were grafted to grant enough space between GOs and fluorescein. They
successfully utilized GOs–PEG-fluorescein system for intracellular imaging without
much toxicity. Similarly, Yang et al. utilized cyanine 7 (Cy7)-labeled GOs for
in vivo fluorescence imaging, which also could target tumor [10]. In addition, active
targeting of tumor cells could be done by introducing cancer targeting moieties on
the edges (Fig. 4.1a–c). As folate receptors are overexpressed on tumor cells, folic
acid can be employed as the targeting moiety. Through a simple Ethyl dimethylaminopropyl carbodiimide (EDC) coupling reaction, folic acid can be covalently
bonded to the carboxyl groups of GOs. Amine group-containing antibodies
including Herceptin and transferrin can be also easily conjugated with GOs through
the same reactions, thus enabling antibody-based targeting. Other molecules such as
vascular endothelial growth factor (VEGF), anti-endoglin antibody (TRC105),
b-cyclodextrin, and hyaluronic acid have also been employed on graphene-based
nanomaterials for active targeting imaging studies [11–13]. It should be also noted
that these systems successfully targeted tumor cells without significant toxicity.
82
J. M. Yoo et al.
Physiological Conditions
In general, GOs are well-dispersed in water for several months without much
aggregation. In physiological conditions, however, they are more prone to aggregate owing to the presence of ionic salts. To enhance the stability under these
conditions, modification with biocompatible polymers and nano-sized GOs are
employed [4–8]. Among many modification methods, grafting of polyethylene
glycol (PEG) is the most frequently used. Many reports have shown that PEG
grafting can improve the dispersibility and stability of GOs in biological buffers and
highly concentrated salts. PEGs can be grafted on the edges of GOs by the reactions
with carboxylic acid and epoxy groups. Other polymers such as polyacrylic acid
(PAA), polyamido amine (PAMAM), and dextran can be also conjugated on GOs
by the same reactions [4–6]. While as-synthesized GOs show a wide range of sizes,
reduction of the average size can be simply achieved by sonication. Many reports
show that high energy sonication can break down large GO flakes into nanometer
scale particles, as small as 10 nm, which show greater dispersibility in many solvents including biological buffers [7, 8].
4.2.2 Graphene as Nanocarriers
Although micrometer-scale GOs are known to be toxic, their large hydrophobic
basal plane and oxygen-containing functional groups on the edges are advantageous
for both covalent and non-covalent chemical modifications with other molecules
including fluorescent organic dyes.
In 2010, Peng et al. employed fluorescein-labeled GOs for intracellular imaging
studies [9]. To prevent the fluorescence quenching and improve dispersibility,
PEGs were grafted to grant enough space between GOs and fluorescein. They
successfully utilized GOs–PEG-fluorescein system for intracellular imaging without
much toxicity. Similarly, Yang et al. utilized cyanine 7 (Cy7)-labeled GOs for
in vivo fluorescence imaging, which also could target tumor [10]. In addition, active
targeting of tumor cells could be done by introducing cancer targeting moieties on
the edges (Fig. 4.1a–c). As folate receptors are overexpressed on tumor cells, folic
acid can be employed as the targeting moiety. Through a simple Ethyl dimethylaminopropyl carbodiimide (EDC) coupling reaction, folic acid can be covalently
bonded to the carboxyl groups of GOs. Amine group-containing antibodies
including Herceptin and transferrin can be also easily conjugated with GOs through
the same reactions, thus enabling antibody-based targeting. Other molecules such as
vascular endothelial growth factor (VEGF), anti-endoglin antibody (TRC105),
b-cyclodextrin, and hyaluronic acid have also been employed on graphene-based
nanomaterials for active targeting imaging studies [11–13]. It should be also noted
that these systems successfully targeted tumor cells without significant toxicity.
82
J. M. Yoo et al.