A facile and novel synthetic method for the preparation of hydroxyl capped fluorescent carbon nanoparticles

Colloids and Surfaces B: Biointerfaces 102 (2013) 63–69

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A facile and novel synthetic method for the preparation of hydroxyl capped fluorescent carbon nanoparticles Afroza Khanam, S.K. Tripathi, Debmalya Roy, M. Nasim ∗ Defence Materials & Stores Research & Development Establishment (DMSRDE), DMSRDE PO, G.T. Road, Kanpur 208013, India

a r t i c l e

i n f o

Article history: Received 30 May 2012 Received in revised form 31 July 2012 Accepted 6 August 2012 Available online 1 September 2012 Keywords: Hydroxyl functionalized carbon nanoparticles Surfactant Organic base Fluorescence

a b s t r a c t A new type of fluorescent carbon based nanomaterial has drawn considerable attention due to their unique physicochemical properties. Carboxyl functionalized carbon nanoparticles are well documented in the literature. However, the carbonyl moiety in the carboxyl group considerably reduces the photoluminescence quantum yield. In this study, we present a direct, simple and novel synthetic route to produce hydroxyl functionalized fluorescent carbon nanoparticles derived from candle soot using organic base and surfactant which could be readily scaled up. The functionalization of carbon nanoparticle was confirmed by various spectroscopic techniques. 1 H NMR and FTIR measurements have been used to confirm the presence of sp2 carbon in the form of aryl and hydroxyl moieties. MALDI-TOF Mass and TGA measurements further confirmed the functionalization. Structural characterization of these particles by Raman spectroscopy showed characteristic peaks located at 1333 and 1583 cm−1 corresponding to diamondlike (D) and graphite-like (G) bands of the carbon allotropes respectively. The minimum grain size of 7.3 nm was calculated using Raman spectra of the functionalized carbon nanoparticles which corroborate well with the results of dynamic light scattering (DLS) and TEM studies. UV–vis spectroscopic measurements displayed an absorption band at ca. 245 nm, which was consistent with the optical characteristics of functionalized carbon nanoparticles. PL measurements confirmed that the functionalized carbon nanoparticles have characteristic emission peak and shows fluorescence under blue light excitation. With a combination of free dispersion in water and attractive PL properties, these functionalized carbon nanoparticles hold promise for application in nanotechnology. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Carbon nanostructures are attracting intense interest because of their many unique and novel properties. Carbon based nanomaterials which include carbon nanotubes/nanofibers, fullerenes and carbonaceous nanoparticles have promising applications in nanotechnology, biosensing and drug delivery [1]. The conductivity of C60 fullerene is generally low and this limits its applications in electronic devices. On the other hand, CNTs have very good conductivity and mechanical strength, which makes these materials attractive for electronic devices and for imparting mechanical strength [2]. Despite the huge potentials, the applications of CNTs are still in laboratory scale. This is mainly due to the problem of achieving similar diameter and length distributions of CNTs in batch-to-batch process. The problem of calibration of different aspect ratios CNTs is not a big threat for composite application but achieving uniform distribution is a big challenge [3]. It has been realized that

∗ Corresponding author. Tel.: +91 0512 2451759–78; fax: +91 0512 2450404/2404774. E-mail address: nasim [email protected] (M. Nasim). 0927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfb.2012.08.016

for practical use of the carbon based nanomaterials, we have to look beyond fullerenes and CNTs and carbon nanoparticles are one such potential nanomaterials which have every chance to compete with fullerene and CNTs from application point of view. Carbon soot is an abundant source of the polydisperse and ultrafine particles. It has been popularly used as black ink in paints and in fountain pens for ages. Recently soot originated from carbon nanoparticle has been rediscovered as a new class of carbonaceous nanostructures which possess interesting properties [4,5]. In vitro testing of carbon nanoparticles revealed significant antimicrobial activity against bacteria called ‘Klebsialla pneumonia’ [5]. The synthesis of carbon soot can be carried out in an easy and inexpensive way. Recently, emergence of carbon nanoparticles shows high potential in biological labeling, bioimaging and other different optoelectronic device applications [6]. These carbon nanoparticles are biocompatible, chemically inert [7] and have advantages over conventional cadmium-based quantum dots [8]. The carbon nanoparticles are generally synthesized, purified and functionalized in a solvent medium, although non-solvent based methods have also been reported [9]. Various experimental methods [10] for example, pulsed laser deposition, carbon arc technique and microwave-plasma chemical-vapor deposition, etc.

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were developed to produce carbon nanoparticles of various configurations and sizes. Xu et al. [11] derived fluorescent carbon dots from single walled carbon nanotubes. Liu et al. [4] had produced carbon nanoparticles from candle soot whereas the multicolor photoluminescent carbon dots have been synthesized using surfactant mediated silica spheres as the carriers and resols as carbon precursor [12]. Fabrication of carbon nanoparticle through polymer nanoparticle as precursor has been explored and reported to possess a potential capability as imaging probes and drug carriers based on their porosity, magnetic property and biocompatibility [13]. Goncalves and Esteves da Silva [14] synthesized and functionalized the carbon nanoparticles with PEG200 and mercaptosuccinic acid for fluorescent carbon dots. The functionalization of carbon nanoparticles with NH2 -polyethylene-glycol (PEG200 ) and N-acetyl-l-cystein (NAC) were used for Hg (II) sensing. Zhang et al. [15] produced luminescent carbon nanoparticles from kerosene soot followed by surface modification with thiocarbamide to improve the properties of carbon nanoparticles. The water-soluble carbon nanoparticles have been synthesized by ultrasonication which showed excellent up-conversion fluorescent properties [16]. Li et al. [17] demonstrated the use of carbon nanoparticles obtained from carbon soot by lighting a candle for Ag+ detection with a detection limit as low as 500 pM. First white light-emitting device have been developed that originated from single carbon dot components with a maximum external quantum efficiency of 0.083% with a current density of 5 mA cm−2 [18]. Organosilane was first used to synthesize highly luminescent (QY = 47%) functionalized amorphous carbon dots which were water soluble, biocompatible and nontoxic to the selected cell lines [19]. Recently hydroxyls-coated carbon dots (QY 5.5%) were synthesized by hydrothermal reaction using water-soluble base. However, it was reported to be a difficult task to control the size and distribution of grain boundary [20]. It has been realized that the surface of the carbon nanoparticles are required to be modified in order to make them compatible with the dispersion matrix. Carbonaceous nanostructures are poorly soluble in aqueous media which severely hinders their application in pharmaceutical and biomedical applications including biosensing and drug delivery where water is the solvent of choice. Normally polymers and surfactants were selectively adsorbed onto the hydrophobic surfaces of the carbon nanostructures that enable better dispersion in aqueous medium [21]. The most of the previous work on water solubilization of carbon nanostructures was based upon surface activation through strong oxidizing acid treatment followed by several steps of derivatization and coupling reactions which utilize expensive polyethylene glycol (PEG)-derived coupling reagents [22]. In all the previously deployed methods, the carbon nanoparticles surface was modified in order to achieve fluorescence. However, a thorough understanding of the fluorescence emanating from carbon nanoparticles is still considered incomplete. In this paper, we utilize a novel surfactant initiated approach to synthesize hydroxyl functionalized fluorescent carbon nanoparticles (FCNPs) derived from candle soot without the complicated purification and further modification of surface (Scheme 1). These carbon nanoparticles exhibit strong PL in visible spectral range and characterized by a battery of modern analytical techniques.

2. Experimental 2.1. Materials The candle soot was collected by putting a clean aluminum sheet on the top of the flame of candles (candles were purchased from local market) at a distance of 10 cm. Sodium methoxide (CH3 ONa), dry benzene, triton X-100 and methanol were purchased

from Sigma–Aldrich Chemical and they were used as received. All the aqueous solutions used in this study were de-ionized water.

2.2. Synthesis and purification of functionalized carbon nanoparticles (FCNPs) The carbon nanoparticles were synthesized by candle soot method as mentioned above and flame soot deposited on the sheet surface was scratched with a previously cleaned spatula and collected in a sterilized glass bottle. Following method was used to prepare FCNPs probably via the thermodynamically stable micellar route. Typically, 0.16 g of powdered material of candle soot and 50-wt% triton X-100 were dispersed in 250 ml of dry benzene in a 500 ml three-necked flask by sonication at 20 kHz for 10 min using Digital Ultrasonicator (Elma). The dispersed solution is then mixed with1 M sodium methoxide (in four times with equal parts within 30 min) at 40 ◦ C and refluxed at 60 ◦ C for 48 h with magnetic stirring at inert atmosphere. The dispersed solution was then allowed to stand for 2 h and supernatant was collected by decantation. The suspension was then subjected to filtration and the residue was washed with methanol for several times. The residue was redispersed in methanol followed by centrifuge until pH become 7. The resulting product is functionalized with both hydroxyl and methoxy functional groups (FCNPs 1). Therefore, further 0.1 g of this product and 50 wt% triton X-100 dispersed in 200 ml methanol in 500 ml three-necked flask by sonication for 10 min and continuously mixed using a magnetic stirrer under inert atmosphere for 12 h at 60 ◦ C. The solution containing functionalized carbon nanoparticles with only hydroxyl groups which was then followed by washing with methanol and combination of centrifuge based separation to remove unused surfactant (Triton X-100). The suspension was then subjected to filtration and then resulting final product was dried under vacuum and the FCNPs 2 sample obtained finally. The brown color solution of FCNPs dispersed in de-ionized water was seen to emit blue luminescence upon irradiation with an UV lamp.

2.3. Instrumentation The average particle size and morphological information of the FCNPs samples were examined with a JEOL-5600 scanning electron microscope and by using a FEI, Tecnai 20 TEM, operating at a voltage 200 kV. The methanol dispersion was sonicated for 5 min at 20 kHz frequency and then drop-cast onto a carbon coated copper grid of 200 mesh size followed by air drying at room temperature before loading into the microscope. The size distribution of functionalized carbon nanoparticles in methanol was determined by dynamic light scattering analysis using a Malvern Instruments (Malvern, UK) Zeta Sizer Nano ZS 90 where disposable polystyrene cells from Sigma were used. The Raman spectra of the powder samples of carbon nanoparticles were recorded using a Renishaw Raman Microscope with Ar-ion laser excitation at 514 nm and at 50 mW power. Fluorescence emission and absorption spectra were recorded with a Horiba Scientific Fluoromax-4 Spectrofluorometer at room temperature using methanol as solvent and tryptophan as standard. FT-IR spectra were recorded in a Nicolet 360 spectrometer. NMR characterization was performed in D2 O as solvent for 1 H NMR (400 MHz) on a Bruker FT-NMR Spectrometer at 298 K. The samples were analyzed using MALDI-TOF Mass Spectrometer. Thermal properties were measured using a Hi-Res TGA 2950-Thermogravimetric Analyzer (TA Instruments) attached to a Thermal Analyst 2100 (Du Pont Instruments) thermal analyzer, at a heating rate of 10 ◦ C/min.

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Scheme 1. Steps in the preparation of fluorescent carbon nanoparticles (FCNPs) from soot.

Fig. 1. (A) FT-IR spectra of CNPs (black line), FCNPs 1 (step 1) (blue line) and FCNPs 2 (step 2) (red line) and (B) Raman spectra of CNPs and FCNPs (FCNPs 2). The Raman spectra were fitted with Gaussian function and the area under the curve for I(D) and I(G) of CNPs and FCNPs were calculated respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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Fig. 2. The particle size analysis of the hydroxyl capped carbon nanoparticles (FCNPs 2) by (A) dynamic light scattering (photon correlation spectroscopy) experiment and (B) transmission electron microscope (TEM) analysis.

3. Results and discussion In our experiment, a facile and novel synthetic method was employed for synthesis of fluorescent hydroxyl functionalized carbon nanoparticles (FCNPs) as illustrated in Scheme 1. The carbon nanoparticles (CNPs) derived from candle soot was functionalized with hydroxyl group as they exhibit strong PL and characterized by various instrumental techniques. As we know that hydroxyl groups are strong electron donors, which benefit the fluorescence emission, while carboxyls have strong electron withdrawing ability with the opposite effect, thus the fluorescence properties of hydroxyls-coated carbon

nanoparticles are better than that carboxyl coated [23]. The main objective of present work was to prepare fluorescent hydroxyl functionalized carbon nanoparticles from the combustion soot of candles by means of using organic base and surfactant (Triton X-100) followed by the purification of the florescent carbon nanoparticles by solvent in combination of high speed centrifuge based separation. The structural and optical properties of these functionalized carbon nanoparticles derived from candle soot have been fully characterized using an array of experimental techniques like FT-IR, 1 H NMR, Raman Spectroscopy, Photon Correlation Spectroscopy, TEM, MALDI-TOF Mass, TGA, UV–vis, PL and SEM. The Fourier transform infrared (FT-IR) measurements of FCNPs showed that there were typical peaks around 3419 cm−1 and 1434 cm−1 (Fig. 1A), which were assigned to stretching vibrations and inplane bending vibration of hydroxyl group respectively. The broad peak at 3419 cm−1 indicated the existence of hydrogen bonding of the hydroxyl group in functionalized carbon nanoparticles. The significant reduction of the carbonyl peak intensity at 1602 cm−1 for FCNPs 2 compared to FCNPs 1 manifested the conversion of methoxy group to hydroxyl group. The bands at 2954 and 2831 cm−1 corresponded to the stretching vibrations of C H in the group CH3 or CH2 indicating that the combustion of diolefine in candle soot was partially completed. Raman spectra of CNPs and FCNPs are shown in Fig. 1B. The two bands, labeled D and G, refer to the diamond and graphite allotropes of carbon material respectively [24]. The D band of CNPs and FCNPs were found located at 1341 and 1333 cm−1 respectively and it owes its origin to the breathing modes of sp2 atoms in rings, which refers to disordered graphitic phase. The broader and higher intensity of the D band of FCNPs indicates the functionalization of the carbon nanoparticle surfaces. The G band of CNPs and FCNPs were found near 1589 and 1583 cm−1 respectively and this band arises due to bond stretching of all pairs of sp2 atoms in both rings and chains and indicates the presence of highly oriented pyrolytic graphite (HOPG) type morphology. The grain size of graphite was proposed to be inversely proportional to the integrated intensity ratio between the D peak and G peak [25]. The relationship between the grain size and the intensity ratio is given by La =

Fig. 3. NMR analysis of the reaction yields at the end of step 1 (FCNPs 1) and at the end of step 2 (FCNPs 2).

44 (in Å ) [I(D)/I(G)]

(1)

where La is the grain size and I(D) and I(G) are integrated intensity associated with D peak and G peak respectively (the ratio was 0.41 and 0.60 for CNPs and FCNPs). Using the above formula, the calculated grain size of CNPs and FCNPs of 11 nm and 7.3 nm respectively. This observation is further supported by the particle size analysis using dynamic light scattering (DLS) experiment and TEM studies. The dynamic light scattering (DLS) technique, which is commonly referred to photon correlation spectroscopy or quasi-elastic light scattering, enable the exact calculation of the particle sizes

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Fig. 4. MALDI-TOF spectra of CNPs and FCNPs (FCNPs 2) in the three different matrixes viz. sinnapinic acid (SA); 2,5-dihydroxy benzoic acid (DHBA) and ␣-cyno-4 hydroxycinnamic acid (HCCA).

in a suspension with a broad range of sizes, typically from subnanometer to several micrometers [26,27]. The hydrodynamic diameter of a particle can be determined in a suspension by measuring the Brownian motion of the light scattered particles using the Stokes–Einstein equation dh =

kT 3D

(2)

where dh is the hydrodynamic diameter of the particle, D is the translational diffusion coefficient of the particle in a solvent, k is Boltzmann’s constant, T is the absolute temperature and  is the viscosity of the solvent. A narrow particle size distribution at 7.2 nm was found in the DLS experiment of the functionalized carbon nanoparticles in methanol (Fig. 2A), which corroborates well the results of Raman spectroscopic studies. The functionalized carbon nanoparticles (FCNPs) were further subjected to TEM analysis for determining the morphology of the hydroxyl capped nanoparticles. TEM has been used extensively as a powerful analytical tool in the study of nanoscale materials to study the size, morphology and crystalline lattices. Fig. 2B shows a

representative TEM micrograph of the FCNPs. Due to the very high van-der-wall force of the nanosized functionalized carbon particles, they present in a mostly agglomerated form even after sonication of the methanol solution for 5 min at 40 kHz frequency. However, the distribution of the particles size of the FCNPs on the TEM grid surface is small with the majority of the particles falling within the range of below 10 nm in diameter which supports our findings in Raman and DLS studies. 1 H NMR analysis of the CNPs, FCNPs followed by step 1 and resulting product FCNPs followed by step 2 as mentioned above Scheme 1 were performed in D2 O. In order to follow the reaction samples of FCNPs were analyzed by 1 H NMR (Fig. 3). The well defined peaks at ∼8.4 and 3.4 ppm suggests that both hydroxyl and methoxy groups were present on the surface of the CNPs as mentioned in step 1 of Scheme 1. However, the disappearance of peak at 3.4 ppm and the intense peak at ∼8.4 indicates the almost complete conversion of methoxy group to hydroxyl group as shown in step 2 in above mentioned scheme. MALDI-TOF Mass Analysis of CNPs and FCNPs were performed in three different matrices viz. Sinnapinic acid (SA), 2,5-dihydroxy benzoic acid (DHBA) and ␣-cyno-4-hydroxy cinnamic acid (HCCA)

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Fig. 5. (A) TGA traces of CNPs (black line), FCNPs 1 (step 1) (blue line) and FCNPs 2 (step 2) (red line) and (B) SEM images of CNPs and FCNPs (FCNPs 2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

(Fig. 4). The same results produced in all above mentioned matrices and molecular weight observed during analysis of CNPs and FCNPs were 267 and 267 and 392m/z respectively, compare this observation with elemental and EDX data it can be argued easily that the base-catalyzed reaction follows surface passivation with formation of functionalized carbon nanoparticles with hydroxyl groups. The typical TGA thermograms of CNPs and FCNPs taken at a heating rate of 10 ◦ C/min in air are depicted in Fig. 5A. The TGA trace of CNPs shows one-step degradation which is started at ∼500 ◦ C and degraded completely at ∼590 ◦ C without any residue. However, the TGA trace of FCNPs shows that degradation started at around ∼150 ◦ C due to the oxidation of the functionalized CNPs surface. The major degradation of FCNPs has taken place between temperature ranges of 350–400 ◦ C, which is much below their pristine form. It is also supported by the fact that the functionalization reduces the thermal stability of the materials.

The candle soot is black in color and insoluble in water. This is due to the large grain boundaries and hydrophobic nature of carbon particles. However, when the soot is functionalized with the hydroxyl groups as mentioned above method, the nanometer sized FCNPs is produced due to the capping of the CNPs surface which is readily soluble in water. The surface topography of FCNPs has been observed by scanning electron microscopy (SEM) (see Fig. 5B). SEM images indicate that the particles in CNPs formed large clusters whereas due to the capping by hydroxyl group of the FCNPs surface, the aggregate cluster sizes are much smaller and uniform. The similar types of aggregations were also observed by Lijima et al. [28]. The UV absorption of carbon nanoparticles is caused by electronic transitions between the bonding and antibonding orbitals [29]. The (␴–␴*) transitions are expected to produce a band in the far UV region, arising between 60 and 100 nm, whereas the (␲–␲*)

Fig. 6. (A) UV–vis spectra absorption spectra of CNPs (black line), FCNPs 1 (step 1) (blue line), FCNPs 2 (step 2) (red line) and tryptophan (standard) (dotted black line) in methanol where the concentrations of the solutions were adjusted to have similar absorbance of all variants at 245 nm wavelength and (B) PL spectra of CNPs (black line), FCNPs 1 (step 1) (blue line), FCNPs 2 (step 2) (red line) and tryptophan (standard) (dotted black line) in methanol at the excitation wavelength of 245 nm. The green dotted lines represent the Gaussian nonlinear curve fitting of the data using Origin 8 software for quantum yield measurement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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transitions provide absorption maximum located in the range between 180 and 280 nm. The UV spectra of methanol solutions of carbon nanoparticles and functionalized carbon nanoparticles at the same concentration are depicted in Fig. 6A. The UV absorption spectrum of tryptophan was also compared along with the carbon nanoparticles as standard. No distinct change is observed in the UV–vis–NIR spectra of the pristine and functionalized carbon nanoparticles in solution which is consistent with the earlier observation [30]. The longer background extinction till the visible region is a characteristic feature of carbon nanoparticles and the observation of strong background up to near IR region indicates the nanosized grain boundaries of the functionalized carbon nanoparticles. To further explore their optical properties, PL of the as prepared FCNPs was studied using tryptophan as standard. The absorption maxima of CNPs, FCNPs 1, FCNPs 2 and tryptophan in methanol are around 245 nm and hence the concentrations of the solutions were adjusted to have similar absorbance of all the compounds at this wavelength (Fig. 6A). The detailed photoluminescent spectra at 245 nm excitation wavelength of pristine and functionalized CNPs are shown in Fig. 6B. As expected, due to the modified CNPs surface, the PL spectra of FCNPs (1 and 2) show an additional broad peak near 440 nm along with two regular peaks at 583, 634 nm as appeared in PL spectra of neat CNPs. The emission peak position at 583 nm of the FCNPs is slightly red shifted (∼3 nm) as compared to CNPs, indicating the disturbance of electronic configuration of carbon nanoparticles by functionalization. Several mechanisms have been proposed to explain this optical phenomenon of FCNPs. However, due to the lack of solid experimental evidence, the PL mechanism of FCNPs is still an open question. The area under the PL emission peaks of CNPs and FCNPs were determined by the Gaussian nonlinear curve fitting of the data using Origin 8 software. The quantum yield of the sample is measured by the following formula,



˚sample = ˚standard × ×

Fsample Fstandard

 2 sample standard

 

(in Å )

×

Asample



Astandard (3)

where ˚ is the quantum yield, F is integrated area under fluorescence spectra, A is absorbance and  is refractive index of the solvent. In our case, the last two terms become one as we have used the same solvent and same concentration for standard and sample. The quantum yield of the pristine CNPs is calculated as 0.041 in the above experimental condition whereas the FCNPs 1 and FCNPs 2 are 0.092 and 0.104 respectively, considering the quantum yield of the standard as 0.14. The result suggests that FCNPs shows strong PL and may be useful for powerful energy transfer component in biological applications. 4. Conclusions We have demonstrated a facile novel alkali assisted synthetic method of high quality FCNPs from candle soot, which exhibit stable and strong photoluminescence. The carboxylic modified CNPs reduce the quantum yield of the PL due to the presence of carbonyl group. The key challenges for hydroxyl functionalization CNPs are the optimum basicity, solubility in the organic medium and to stop the agglomeration of surface functionalized CNPs during the reaction. We have used an organic base and a surfactant to modify the surface of CNPs by hydroxyl groups. By combining the properties of the FCNPs of the free dispersion in water, size dependent optical properties and fluorescence in blue light, FCNPs may provide a new type fluorescent markers or a new approach to

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high-efficiency catalyst design for applications in bioscience and energy conversion technology. Acknowledgements The authors are grateful to the Director, DMSRDE, Kanpur for help, support and for permitting us to publish the manuscript. The support from Nanoscience and Technology Division, DMSRDE for Raman, DLS and TGA experiments is greatly acknowledged. We also thank to Central Analytical Facility (CAF) Division, DMSRDE for NMR studies. The authors gratefully acknowledge the help from Defence Research Development Establishment (DRDE), Gwalior, for MALDI-TOF, SEM and FT-IR facilities, IIT, Kanpur for PL and UV–vis–NIR analysis and BHU, Varanasi for TEM study. References [1] N.W. Kam, H. Dai, J. Am. Chem. Soc. 127 (2005) 6021; (a) Y. Lin, T.W. Smith, P. Alexandridis, Langmuir 18 (2002) 6147; (b) C.C. Carrion, R. Lucena, S. Cardenas, M. Valcarcel, Analyst 132 (2007) 551; (c) Y.P. Sun, K.F. Fu, Y. Lin, W.J. Huang, Acc. Chem. Res. 35 (12) (2002) 1096; (d) A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (3) (2007) 183. [2] M. Trojanowicz, Trends Anal. Chem. 25 (2006) 480. [3] P.J.F. Harris, Carbon Nanotubes & Related Structures, Cambridge Univ. Press, Cambridge, UK, 1991. [4] H. Liu, T. Ye, C. Mao, Angew. Chem. Int. Ed. 46 (2007) 6473. [5] B. Mohanty, A.K. Verma, P. Claesson, H.B. Bohidar, Nanotechnology 18 (2007) 445102. [6] J.C.G. Esteves da Silva, H. Goncalves, TrAC 30 (2011) 1327. [7] K. Ushizawa, Y. Sato, T. Mitsumori, T. Machinami, T. Ueda, T. Ando, Chem. Phys. Lett. 351 (2002) 105; (a) M.D. Cahalan, I. Parker, S.H. Wei, M.J. Miller, Nat. Rev. Immunol. 2 (2002) 872; (b) L.C.L. Huang, H.C. Chang, Langmuir 20 (2004) 5879; (c) X.L. Kong, L.C.L. Huang, C.M. Hsu, W.H. Chen, C.C. Han, H.C. Chang, Anal. Chem. 77 (2005) 259; (d) X.L. Kong, L.C.L. Huang, S.C.V. Liau, C.C. Han, H.C. Chang, Anal. Chem. 77 (2005) 4273. [8] G.X. Chen, M.H. Hong, T.C. Chong, H.I. Elim, G.H. Ma, W. Ji, J. Appl. Phys. 95 (2004) 1455. [9] H. Hu, B. Zhao, M.E. Itkis, R.C. Haddon, J. Phys. Chem. B 107 (2003) 13838; (a) A. Evelyn, S. Mannick, P.A. Sermon, Nano Lett. 3 (2003) 63; (b) R. Bondyopadhay, E.N. Roth, O. Regev, R.Y. Rozen, Nano Lett. 2 (2002) 25; (c) J. Gavillet, A. Loisean, C. Journet, F. Willaime, F. Ducastelle, J.C. Charlies, Phys. Rev. Lett. 87 (2001) 275504; (d) S. Lijima, Nature 354 (1991) 56. [10] C. Journet, W.L. Le La Chapelle, S. Lafrant, P. Diniard, R. Leek, J.E. Fischer, Nature 388 (1997) 756. [11] X.Y. Xu, R. Ray, Y. Gu, H.J. Ploehn, L. Gearheart, K. Raker, W.A. Scrivens, J. Am. Chem. Soc. 126 (2004) 12736. [12] R. Liu, D. Wu, S. Liu, K. Koynov, W. Knoll, Q. Li, Angew. Chem. Int. Ed. 48 (2009) 4598. [13] W.K. Oh, H. Yoon, J. Jang, Biomaterials 31 (2010) 1342. [14] H. Goncalves, J.C.G. Esteves da Silva, J. Fluoresc. 20 (2010) 1023. [15] S. Zhang, Q. He, R. Li, Q. Wang, Z. Hu, X. Liu, X. Chang, Mater. Lett. 65 (2011) 2371. [16] H. Li, X. He, Y. Liu, H. Huang, S. Lian, S.T. Lian, S.T. Lee, Z. Kang, Carbon 49 (2011) 605. [17] H. Li, J. Zhai, X. Sun, Langmuir 27 (2011) 4305. [18] F. Wang, Y.H. Chen, C.Y. Liu, D.G. Ma, Chem. Commun. 47 (2011) 3502. [19] F. Wang, Z. Xie, H. Zhang, C.Y. Liu, Y.G. Zhang, Adv. Funct. Mater. 21 (2011) 1027. [20] L.L. Qin, L.Y. Fang, Z. Lei, L. Yue, H.C. Zhi, Sci. China 54 (2011) 1342. [21] Y. Lin, T.W. Smith, P. Arevanderidis, Langmuir 18 (2002) 6147. [22] A. Blanco, K. Kostarelos, C.D. Partidos, M. Prato, Chem. Commun. 12 (2005) 376. [23] Q.Y. Xing, W.W. Pei, R.Q. Xu, J. Pei, Fundamental Organic Chemistry, 3rd ed., Higher Education Press, Beijing, 2005, p. 243. [24] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [25] P. Kumar, R. Meena, R. Paulraj, A. Chanchal, A.K. Verma, H.B. Bohidar, Colloids Surf. B: Biointerfaces 91 (2012) 34. [26] M. Kaszuba, D. McKnight, M.T. Connah, F.K. McNeil-Watson, U. Nobbmann, J. Nanopart. Res. 10 (2008) 823. [27] R. Pecora, J. Nanopart. Res. 2 (2000) 123. [28] S. Lijima, M. Yudasaka, R. Yamada, S. Bandow, K. Suenaga, F. Kokai, K. Takahashi, Chem. Phys. Lett. 309 (1999) 65. [29] D. Green, D. McKenzie, P. Lukins, in: J. Pouch, S. Alterovitz (Eds.), Properties and Characterization of Amorphous Carbon Films, Trans. Tech. Pub., Aedermannsdorf, 1990, p. 103. [30] S.N. Baker, G.A. Baker, Angew. Chem. Int. Ed. 49 (2010) 6726.