Biomedical Engineering Reference
In-Depth Information
commercially available induction coil heater by using an aqueous solution of cit-
ric acid and a diamine compound (Palashuddin et al. 2014). Generally, the syn-
thetic strategies for C-dots involve the use of a single precursor that acts as both
carbon source and a passivating agent or separately employing a passivating agent
in addition to carbon precursor. Wang et al. reported the synthesis of C-dots from
glycerol with the aid of small amount of an inorganic ion (phosphate solution)
without any surface passivation reagent (Wang et al. 2011). A rapid, one-step
microwave mediated method for synthesizing C-dots using poly(ethylene glycol)
(PEG) as a carbon source as well as a passivating agent has also been reported
(Jaiswal et al. 2012). Yang et al. (2014) employed hydrothermal method to syn-
thesize nitrogen-doped C-dots using ammonium citrate, serving as both carbon
source as well as meeting the requirements of surface passivation. The groups on
the surface of
N
-doped CDs acted as a self-passivation layer. Polyethylene glycol
(PEG), polyethyleneimine (PEI), poly(ethylenimide)-co-poly(ethyleneglycol)-co-
poly(ethyl-enimide) (PPEI), 4,7,10-trioxa-1,13-tridecanediamine (TTDDA) are
some of the commonly employed agents for surface passivation. Nevertheless,
out of the above, the attachment of nitrogen containing moieties onto the surface
of C-dots has been found to generate stronger fluorescence emission. Peng et al.
(2009) prepared C-dots by dehydration and oxidation of carbohydrates with sul-
phuric acid and nitric acid respectively, which were weakly emissive. Quantum
yield increased to 13 % after passivation with TTDDA. Sachdev et al. (2013)
demonstrated a novel one-step method for synthesizing C-dots using chitosan as a
carbon source and PEG as a passivating agent through microwave mediated reac-
tion, but the reported quantum yield was lower. In a study of similar relevance,
the same group made a comparative analysis between PEG and PEI passivated
C-dots synthesized by one-step hydrothermal method, without any post-synthetic
treatment (Sachdev et al. 2014). Consequently, the quantum yield of C-dots syn-
thesized by hydrothermal treatment was more compared to those synthesized by
microwave treatment. These results suggest that the synthesis of brightly fluo-
rescent C-dots is linked to the selection of right synthetic method along with
the passivation polymer. Another interesting scheme of improving the fluores-
cence emission from C-dots involves the surrounding of C-dots by a metal-con-
taining shell or its association with a metal-based nanostructure. For example,
core carbon nanoparticle surface was doped with inorganic salts (ZnO, ZnS, or
TiO
2
) along with the organic functionalization (Baker and Baker 2010; Luo et al.
2013). The resulting C-dots (C
ZnO
-dots, C
ZnS
-dots, or
C
TIO
2
-dots) exhibited much
brighter fluorescence emissions than their undoped counterparts. All the above
investigations point out that C-dots contain a carbon nanoparticle core and sur-
face passivation is responsible for tuning the fluorescence performance. The most
widely accepted emission mechanism in C-dots is the radiative recombination of
surface-confined electrons and holes. Surface passivation schemes make surface
sites more active and emissive to facilitate effective recombinations (Sachdev
et al. 2013; Sun et al. 2006).
