Non-blinking dendritic crystals from C-dot solution
Non-blinking dendritic crystals from C-dot solution
Carbon letters. 2015. Sep, 16(3): 211-214
Copyright © 2015, Korean Carbon Society
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : August 21, 2014
  • Accepted : May 03, 2015
  • Published : September 15, 2015
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About the Authors
Ashmi, Mewada
N. S. N. Research Centre for Nanotechnology and Bio-nanotechnology, Ambernath, India
Ritesh, Vishwakarma
Nagoya Institute of Technology, Japan
Bhushan, Patil
N. S. N. Research Centre for Nanotechnology and Bio-nanotechnology, Ambernath, India
Chinmay, Phadke
N. S. N. Research Centre for Nanotechnology and Bio-nanotechnology, Ambernath, India
Golap, Kalita
Nagoya Institute of Technology, Japan
Maheshwar, Sharon
N. S. N. Research Centre for Nanotechnology and Bio-nanotechnology, Ambernath, India
Madhuri, Sharon
N. S. N. Research Centre for Nanotechnology and Bio-nanotechnology, Ambernath, India

Bio-imaging and drug carriers for delivery have created a huge demand for crystals. Crystals are fascinating materials that have been grown for a long time but obtaining biocompatible fluorescent crystals is a challenging task. We report on the growth of fluorescent crystals using a carbon dot (C-dot) solution by a hydrothermal process. The crystallization pattern of these C-dots exhibited a unique dendritic structure having a feather-like morphology. The growth temperature and pressure were maintained at 60℃ and 200 mmHg, respectively, for crystal growth. A green fluorescence (under UV light) that was observed in the C-dot solution was retained in the crystals formed from the solution. Cytotoxicity studies on Vero cells revealed the crystals to be extremely biocompatible. These fluorescent crystals are extremely well suited for biomedical and optoelectronic applications.
1. Introduction
Since the accidental discovery of carbon dots (C-dots) during the purification of single walled carbon nanotubes [1] , they have been studied widely for a vast array of applications [2] . Zero dimensional C-dots are known for their size dependent fluorescence properties [3] , bio-compatibility and high water solubility [4] , chemical inertness, and low toxicity [5] . These properties of C-dots make them suitable for bio imaging [6] , drug delivery [7] , photo catalytic activity, and other practical applications [2] .
The attractive optical properties of C-dots have motivated researchers to use these tiny carbon nanoparticles to grow fluorescent crystals. Organic nonlinear optical crystals have many applications in biological labelling [8 , 9] . Fluorescent crystals can be used for bio sensing, optical data storage, document security, detecting forgery [10] , and photo induced electron transfer property for light harvesting [11 , 12] . By exploiting their exceptional properties, C-dots can replace semiconductor metal quantum dots such as CdSe, CdTe, and CdS, which involve extremely harmful precursors [13 , 14] as well as complex procedures for their synthesis.
Numerous methods of crystal growth such as high temperature growth and low temperature growth [15 , 16] have been reported for different types of semi-conductor crystals. In the present work, C-dots were synthesized using a microwave assisted method using Sorbitol as a precursor followed by their crystallization. The formation of dendritic crystals from C-dots was observed and explained by a straightforward method using C-dots as compared to the exacting conditions in chemical vapour deposition and electrochemical methods. Fluorescent studies have been performed to assess the optical properties of carbon dendrites. To the best of our knowledge this is the first report on the synthesis of fluorescent crystals grown in a controlled environment using a C-dot solution.
2. Experimental
C-dots were synthesized as per our previous report using Sorbitol by microwave assisted heating [17] . The dark brown C-dot solution was then purified by dialysis against nanopure water using a pre-activated dialysis bag (MW cut-off 12-14 kD).
Spectral properties of C-dots were studied by a UV-visible spectrophotometer (Lambda-25, Perkin Elmer, USA) and fluorescence spectroscopy (Perkin Elmer) using a standard quartz cuvette with a path length of 1 cm.
Morphological characteristics of C-dots were studied using transmission electron microscopy (TEM; Carl Ziess, GmbH, Germany). The sample was loaded on a copper grid for the TEM analysis.
The purified C-dots were loaded on clean quartz plates and subjected to vacuum drying under pressure (200 mmHg) for 8 hours at 60℃ (Ganesh Scientific Ltd., India) to initiate crystallization. The morphological studies after crystallization were carried out using an inverted optical fluorescence microscope (Carl Ziess-Axiovert 40).
In order to study the cytotoxicity, C-dot crystals were weighed and re-dissolved in sterile D/W. Cytotoxicity experiments were carried out as per our previous publication using a MTT assay [7] .
3. Results and Discussion
Although the purified (dialyzed) solution was light brown, it exhibited deep green fluorescence under UV light (365 nm), as seen in the inset of Fig. 1a . The origin of fluorescence in C-dots is ascribed to the presence of surface defects and functional groups exposed on the surface (e.g., hydroxyl and carboxylic acid functional groups).
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(a) UV-visible spectra of purified carbon dots (C-dots); inset shows photoluminescence spectra and C-dot solution in ambient and UV light (b) transmission electron microscopy image of C-dot; inset shows selected area electron diffraction pattern of C-dots.
The optical spectra of purified C-dots displayed in Fig. 1a reveal two distinct peaks at 229 and 266 nm, which arise due to π→π* and n→ π* transitions associated with C=C and carbonyl/hydroxyl groups, respectively [18] . Photoluminescence (PL) spectra of C-dots show a sharp peak at 543 nm when excited at 350 nm (inset of Fig. 1a ). The appearance of PL may be attributed to the oxygen containing moieties derived from the precursor and functional groups on C-dots [2] . Fig. 1b shows a transmission electron micrographic image of a single C-dot of 7 nm diameter. The C-dots show lattice spacing of 3.44 Å (marked in Fig. 1b ). This inter-atomic spacing is equal to the (001) plane spacing of graphite, which confirms the graphitic nature of C-dots and is denoted in the surface area electron diffraction pattern (inset of Fig. 1b ).
After vacuum drying of the C-dot solution on a quartz plate, a typical feather-like pattern of crystals was observed ( Fig. 2a ) under an optical microscope. The crystals showed the same green fluorescence when exposed to UV light ( Fig. 2b ). A dialysed NaOH and ethanol solution, which was used during the fabrication of the C-dots, was also vacuum dried under the same conditions noted in the Materials and Methods section. Fig. 2c shows that the pattern of crystallization is different and small irregularly shaped crystals were observed. Also, no fluorescence was observed under UV light (data not shown). This clearly confirms the deep green fluorescence.
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Optical microscopic image of crystallised carbon dots in (a) ambient light (b) under UV-light and (c) crystals of dialysed NaOH-ethanol mixture (which did not show fluorescence).
The arrangement of crystals is a stochastic process. The process explained below can be speculated on the basis of classical nucleation theory [19] . The solution contains smaller size C-dots, which have higher free energy with chemical potential µ as compared to a supersaturated solution (chemical potential = µ s ). This C-dot solution was kept at temperature of 60℃ and pressure of 200 mmHg until it was completely dried. During this process, the density of C-dots in the solution increases as the water evaporates. At the point of critical supersaturation where µ > µ s , crystal nucleation is initiated because supersaturated liquids are highly unstable ( Fig. 3 ). As per thermodynamics, the Gibbs free energy of the solution decreases due to evaporation, which initiates the nucleation of C-dots to form crystals [20] . The entropy of the solution decreases, and hence fusion takes place, causing disorder in the surroundings to balance the entropy of the universe [19] . Further research is under way to optimize and realize the effects of parameters such as temperature, pressure, duration etc. on the crystallization.
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Schematic representation of formation of fluorescent dendritic structures.
Furthermore, the re-dissolved crystals of C-dots were found to have excellent biocompatibility on Vero cells. The percentage survival of cells was found to be more than 80% at all concentrations used (0.5-2 mg/mL). An initial concentration of 0.5 mg/mL showed negligible cell death whereas 88.7% live cells were observed at the highest concentration used, i.e. 2 mg/mL ( Fig. 4 ). Our results are consistent with previous reports on the non-toxic nature of C-dots [21 , 22] . These C-dots crystals hence prove to be harmless and can be used in various biological applications.
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Cytotoxicity studies of carbon dots (C-dots) on Vero cell lines.
4. Conclusion
A simple hydrothermal method was used to grow fluorescent crystals from a C-dot solution at 60℃ and pressure of 200 mmHg. During evaporation of the C-dot solution fascinating structures were formed that showed a consistent feather-like dendritic pattern. Even after the formation of crystals the C-dots retained their fluorescent property. The fluorescence property of the crystals is expected to open up a new area of research in optoelectronics.
The authors wish to acknowledge the financial support provided by the authorities of SICES, Ambernath, and especially to Mr. K.M.S. Nair and Mr. K.M.K Nair. We give special thanks to IIT Bombay, Mumbai for conducting the high-resolution TEM analysis.
Xu X , Ray R , Gu Y , Ploehn HJ , Gearheart L , Raker K , Scrivens WA 2004 Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments J Am Chem Soc 126 12736 -    DOI : 10.1021/ja040082h
Li H , He X , Kang Z , Huang H , Liu Y , Liu J , Lian S , Tsang CHA , Yang X , Lee ST 2010 Water-soluble fluorescent carbon quantum dots and photocatalyst design Angew Chem Int Ed 49 4430 -    DOI : 10.1002/anie.200906154
Sun YP , Zhou B , Lin Y , Wang W , Fernando KAS , Pathak P , Meziani MJ , Harruff BA , Wang X , Wang H , Luo PG , Yang H , Kose ME , Chen B , Veca LM , Xie SY 2006 Quantum-sized carbon dots for bright and colorful photoluminescence J Am Chem Soc 128 7756 -    DOI : 10.1021/ja062677d
Mewada A , Pandey S , Shinde S , Mishra N , Oza G , Thakur M , Sharon M , Sharon M 2013 Green synthesis of biocompatible carbon dots using aqueous extract of Trapa bispinosa peel Mater Sci Eng C 33 2914 -    DOI : 10.1016/j.msec.2013.03.018
Welsher K , Liu Z , Daranciang D , Dai H 2008 Selective probing and imaging of cells with single walled carbon nanotubes as near-infrared fluorescent molecules Nano Lett 8 586 -    DOI : 10.1021/nl072949q
Esteves da Silva JCG , Gonçalves HMR 2011 Analytical and bioanalytical applications of carbon dots TrAC Trends Anal Chem 30 1327 -    DOI : 10.1016/j.trac.2011.04.009
Pandey S , Mewada A , Thakur M , Tank A , Sharon M 2013 Cysteamine hydrochloride protected carbon dots as a vehicle for the efficient release of the anti-schizophrenic drug haloperidol RSC Adv 3 26290 -    DOI : 10.1039/c3ra42139b
Bruchez M , Moronne M , Gin P , Weiss S , Alivisatos AP 1998 Semiconductor nanocrystals as fluorescent biological labels Science 281 2013 -    DOI : 10.1126/science.281.5385.2013
Jazbinsek M , Mutter L , Gunter P 2008 Photonic applications with the organic nonlinear optical crystal DAST IEEE J Sel Topics Quantum Electron 14 1298 -    DOI : 10.1109/JSTQE.2008.921407
Lu Y , Zhao J , Zhang R , Liu Y , Liu D , Goldys EM , Yang X , Xi P , Sunna A , Lu J , Shi Y , Leif RC , Huo Y , Shen J , Piper JA , Robinson JP , Jin D 2014 Tunable lifetime multiplexing using luminescent nanocrystals Nat Photon 8 32 -    DOI : 10.1038/nphoton.2013.322
Frascella F , Ricciardi S , Rivolo P , Moi V , Giorgis F , Descrovi E , Michelotti F , Munzert P , Danz N , Napione L , Alvaro M , Bussolino F 2013 A fluorescent one-dimensional photonic crystal for label-free biosensing based on bloch surface waves Sensors 13 2011 -    DOI : 10.3390/s130202011
Wang X , Cao L , Lu F , Meziani MJ , Li H , Qi G , Zhou B , Har ruff BA , Kermarrec F , Sun YP 2009 Photoinduced electron transfers with carbon dots Chem Commun 3774 -    DOI : 10.1039/B906252A
Ray SC , Saha A , Jana NR , Sarkar R 2009 Fluorescent carbon nanoparticles: synthesis, characterization, and bioimaging application J Phys Chem C 113 18546 -    DOI : 10.1021/jp905912n
Bourlinos AB , Stassinopoulos A , Anglos D , Zboril R , Karakassides M , Giannelis EP 2008 Surface functionalized carbogenic quantum dots Small 4 455 -    DOI : 10.1002/smll.200700578
Land TA , De Yoreo JJ , Lee JD 1997 An in-situ AFM investigation of canavalin crystallization kinetics Surf Sci 384 136 -    DOI : 10.1016/S0039-6028(97)00187-8
Gentili D , Foschi G , Valle F , Cavallini M , Biscarini F 2012 Applications of dewetting in micro and nanotechnology Chem Soc Rev 41 4430 -    DOI : 10.1039/c2cs35040h
Mewada A , Pandey S , Thakur M , Jadhav D , Sharon M 2014 Swarming carbon dots for folic acid mediated delivery of doxorubicin and biological imaging J Mater Chem B 2 698 -    DOI : 10.1039/C3TB21436B
Pandey S , Mewada A , Oza G , Thakur M , Mishra N , Sharon M , Sharon M 2013 Synthesis and centrifugal separation of fluorescent carbon dots at room temperature Nanosci Nanotechnol Lett 5 775 -    DOI : 10.1166/nnl.2013.1617
Auer S , Frenkel D 2001 Suppression of crystal nucleation in polydisperse colloids due to increase of the surface free energy Nature 413 711 -    DOI : 10.1038/35099513
Vekilov PG 2010 The two-step mechanism of nucleation of crystals in solution Nanoscale 2 2346 -    DOI : 10.1039/c0nr00628a
Baker SN , Baker GA 2010 Luminescent carbon nanodots: emergent nanolights Angew Chem Int Ed 49 6726 -    DOI : 10.1002/anie.200906623
Song Y , Shi W , Chen W , Li X , Ma H 2012 Fluorescent carbon nanodots conjugated with folic acid for distinguishing folate-receptor-positive cancer cells from normal cells J Mater Chem 22 12568 -    DOI : 10.1039/c2jm31582c