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Jul 26, 2010 - ... of Diamond Nanoparticles Using “Click” Chemistry. Alexandre Barras, Sabine Szunerits, Lionel Marcon, Nicole Monfilliette-Dupont, an...
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Functionalization of Diamond Nanoparticles Using “Click” Chemistry Alexandre Barras, Sabine Szunerits, Lionel Marcon, Nicole Monfilliette-Dupont, and Rabah Boukherroub* Institut de Recherche Interdisciplinaire (IRI, USR 3078), Parc de la Haute Borne, 50 Avenue de Halley, BP 70478, 59658 Villeneuve d’Ascq, France Received April 28, 2010. Revised Manuscript Received July 13, 2010 The paper reports on covalent linking of different alkyne-containing (decyne, ethynylferrocene, and N-propargyl1-pyrenecarboxamide) compounds to azide-terminated nanodiamond (ND) particles. Azide-terminated particles (NDN3) were obtained from amine-terminated nanodiamond particles (ND-NH2) through the reaction with 4-azidobenzoic acid in the presence of a carbodiimide coupling agent. Functionalized ND particles with long alkyl chain groups can be easily dispersed in various organic solvents without any apparent precipitation after several hours. The course of the reaction was followed using Fourier transform infrared (FT-IR) spectroscopy, UV/vis spectroscopy, fluorescence, cyclic voltammetry, thermogravimetric analysis (TGA), and particle size measurements. The surface loading of pyrene bearing a terminal acetylene group was found to be 0.54 mmol/g. Because of its gentle nature and specificity, the chemistry developed in this work can be used as a general platform for the preparation of functional nanoparticles for various applications.

1. Introduction Carbon-based nanomaterials such as fullerenes, carbon nanotubes, and carbon nanoparticles are receiving much attention due to their remarkable mechanical, electrical, and thermal properties.1 The importance of carbon materials in biological applications has also been recognized.2 Recently, other carbon-based nanomaterials, particularly diamond particles (often referred to as nanodiamond), have started to emerge as alternative candidates for similar and many other applications.3 Because of its optical transparency, chemical inertness, hardness, and high specific area, nanometer-sized diamond particles have attracted great attention for their promising applications in the field of nano- and biotechnology.4-6 Independently of nanodiamond (ND) applications, it is generally necessary to modify the particle surface according to the specific requirement of the application. The development of different strategies for the surface functionalization of nanodiamond has thus big interest from both fundamental and applied research view points.1 Various strategies for the modification of nanodiamond surface have been proposed. Fluorination of nanodiamond powder using an elemental fluorine/hydrogen mixture at temperatures varying from 150 to 470 °C was reported by Liu et al.7 The resulting fluorinated ND powder was used as a precursor for the preparation of alkyl, amino, and amino acid ND derivatives by nucleophilic displacement of the fluorine. The as-modified NDs exhibited an improved dispersion in organic solvents and a reduced particle *To whom correspondence should be addressed: Tel þ33 3 62 53 17 24; Fax þ33 3 62 53 17 01; e-mail [email protected].

(1) Dai, L. Carbon Nanotechnology: Recent Developments in Chemistry, Physics, Material Science and Device Applications; Elsevier: Amsterdam, 2006. (2) Chao, J.-I.; Perevedentseva, E.; Chung, P.-H.; Liu, K.-K.; Cheng, C.-Y.; Chang, C.-C.; Cheng, C.-L. Biophys. J. 2007, 93, 2199–2208. (3) Krueger, A. Adv. Mater. 2008, 20, 2445–2449. (4) Kossovsky, N.; Gelman, A.; Hnatyszyn, A. J.; Rajguru, S.; Garrell, R. L.; Torbati, S.; Freitas, S. S. F.; Chows, G.-M. Bioconjugate Chem. 1995, 6, 507–511. (5) Zhang, X.-Q.; Chen, M.; Lam, R.; Xu, X.; Osawa, E.; Ho, D. ACS Nano 2009, 3, 2609–2616. (6) Yeap, W. S.; Tan, Y. Y.; Loh, K. P. Anal. Chem. 2008, 80, 4659–4665. (7) Liu, Y.; Gu, Z.; Margrave, J. L.; Khabashesku, V. N. Chem. Mater. 2004, 16, 3924–3930.

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aggregation. Ultradispersed diamond/polymer brushes hybrid materials with excellent solution dispersibilities were prepared by atom transfer radical polymerization (ATRP) using the “grafting-from” synthesis strategy.8 Covalent functionalization of diamond nanoparticles through C-C bond formation was achieved by the direct reaction with aryldiazonium salts under vigorous ultrasonication.9 The versatility of the diazonium chemistry was used to introduce brominated aryl groups on hairy diamond nanoparticles, which initiate efficiently atom transfer radical polymerization reaction of tert-butyl methacrylate (tBMA).10 Subsequent hydrolysis of the polymer brushes into poly(methacrylic acid) allowed bovine serum albumin (BSA) covalent immobilization. Zheng et al. described a method to introduce primary alcohol groups on hydroxylated nanodiamond surface using a chemical approach.11 The reactivity of the terminal alcohol groups was exploited to create various functionalities on the ND surface (halides, amines, cyanide, azide, and thiols). The functionalized ND was further used for attaching drug molecules.11 Surface graphitization of NDs, followed by radical-initiated surface grafting of oligomers with various functionalities, including -C(dO)OCH3, -COOH, -NH2, or aliphatic moieties was investigated by Chang et al.12 In this functionalization scheme, NDs graphitization was achieved by thermal treatment (1200 °C) for 1-3 h. To introduce various functionalities, the graphitized NDs were sonicated in a solution containing monomers and a radical initiator, benzoyl peroxide. Surface functionalization of hydrogen-/ deuterium-terminated diamond particles with neat di-tert-amyl peroxide at elevated temperature was reported by Yang et al.13 The modified diamond particles were successfully used for solid (8) Li, L.; Davidson, J. L.; Lukehart, C. M. Carbon 2006, 44, 2308–2315. (9) Mangeney, C.; Qin, Z.; Dahoumane, S. A.; Adenier, A.; Herbst, F.; Boudou, J.-P.; Pinson, J.; Chehimi, M. M. Diamond Relat. Mater. 2008, 17, 1881–1887. (10) Dahoumane, S. A.; Nguyen, M. N.; Thorel, A.; Boudou, J.-P.; Chehimi, M. M.; Mangeney, C. Langmuir 2009, 25, 9633–9638. (11) Zheng, W.-W.; Hsieh, Y.-H.; Chiu, Y.-C.; Cai, S.-J.; Cheng, C.-L.; Chen, C. J. Mater. Chem. 2009, 19, 8432–8441. (12) Chang, I. P.; Hwang, K. C.; Ho, J. A.; Lin, C.-C.; Hwu, R. J.-R.; Horng, J.-C. Langmuir 2010, 26, 3685–3689. (13) Yang, L.; Vail, M. A.; Dadson, A.; Lee, M. L.; Asplund, M. C.; Linford, M. R. Chem. Mater. 2009, 21, 4359–4365.

Published on Web 07/26/2010

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phase extraction. In a different approach, acid-terminated ND powder was converted to the corresponding acyl chloride, and the latter was coupled to primary amines through amide bond formation.14,15 This way, it was possible to functionalize the ND particles with ionic liquids to form a gel.15 Alternatively, the silanization reaction of hydroxyl-terminated ND particles was successfully applied for the preparation of highly dispersed particles in aqueous media.5,16 Surface modification of NDs can also be achieved by adsorption of polymers. Amino-modified diamond by adsorption of polyallylamine onto oxidized diamond followed by thermal curing or chemical cross-linking with a diepoxide was reported by Saini et al.17 In the search for new and versatile strategies to functionalize ND particles and encouraged by the successful demonstration of “click” chemistry to covalently attach acetylene-bearing molecules to azide-terminated boron-doped diamond,18,19 we have adopted this technique for the chemical functionalization of commercially available amine-terminated ND particles. The concept of “click” chemistry was introduced by Sharpless and is based on the copper(I)-catalyzed triazole formation through the classic Huisgen 1,3-dipolar cycloaddition between azides and alkynes.20,21 This cycloaddition reaction is irreversible and proceeds in quantitative yields and high selectivity. It is tolerant to a variety of solvents (including water) and can be performed in the presence of many other functional groups. In this study, we report for the first time on the covalent linking of various compounds bearing a terminal acetylene group (1-decyne, electroactive ethynylferrocene, and fluorescent N-propargyl1-pyrenecarboxamide) to azide-terminated ND particles (primary diameter of 4 nm) using “click” chemistry. Functionalized ND particles with long alkyl chain groups showed a good dispersion in organic solvents without visible precipitation after several hours. The chemically functionalized NDs were characterized using transmission Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy, fluorescence, and electrochemical measurements.

2. Experimental Part 2.1. Materials. Acetonitrile (99.8%, CH3CN), copper(I) iodide (CuI), 1-decyne (98%), dichloromethane (g99.8%, CH2Cl2), N,N0 -dicyclohexylcarbodiimide (99%, DCC), 4-(dimethylamino)pyridine (g99%, DMAP), N,N0 -disuccinimidyl carbonate (g95%, DSC), absolute ethanol (g99.8%, EtOH), ethynylferrocene (97%), ferrocenecarboxylic acid, hydrochloric acid (37%, HCl), propargylamine (98%), 1-pyrenecarboxylic acid (97%), magnesium sulfate (MgSO4), tetrabutylammonium hexafluorophosphate (TBAPF6), and triethylamine (g99.5%, TEA) were obtained from SigmaAldrich. 4-Azidobenzoic acid (>97%) was purchased from TCI Europe (Belgium). Milli-Q water (18 MΩ) was used for all experiments. All reagents and solvents were used without further purification. Amine-terminated nanodiamond (ND-NH2) particles were purchased from the International Technology Centre (Raleigh, NC) and display a primary average particle size of 4.0 nm. (14) Mochalin, V. N.; Gogotsi, Y. J. Am. Chem. Soc. 2009, 131, 4594–4595. (15) Park, C.-L.; Jee, A. Y.; Lee, M.; Lee, S.-g. Chem. Commun. 2009, 5576– 5578. (16) Liang, Y.; Ozawa, M.; Krueger, A. ACS Nano 2009, 3, 2288–2296. (17) Saini, G.; Yang, L.; Lee, M. L.; Dadson, A.; Vail, M. A.; Linford, M. R. Anal. Chem. 2008, 6253–6259. (18) Das, M. R.; Wang, M.; Szunerits, S.; Gengembre, L.; Boukherroub, R. Chem. Commun. 2009, 2753. (19) Wang, M.; Das, M. R.; Li, M.; Boukherroub, R.; Szunerits, S. J. Phys. Chem. C 2009, 113, 17082–17086. (20) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004–2021. (21) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596–2599.

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2.2. Synthesis of N-Propargyl-1-pyrenecarboxamide.

1-Pyrenecarboxylic acid (100 mg, 0.41 mmol) and N,N0 -disuccinimidyl carbonate (125 mg, 0.49 mmol) were dissolved in anhydrous CH2Cl2 (15 mL). To this solution, triethylamine (68 μL, 0.49 mmol) and propargylamine (34 μL, 0.49 mmol) were slowly added under stirring. The resulting mixture was kept under stirring at room temperature for 24 h, washed with 5% HCl aqueous solution (20 mL, twice) and H2O (20 mL, twice), and dried over MgSO4. The product was isolated as a yellow solid; yield = 75 mg, 65%. FT-IR: 704, 850, 958, 993, 1072, 1116, 1193, 1206, 1222, 1248, 1506, 1527, 1595, 1631, 1736, 1757, 3040, 3263, 3284 cm-1.

2.3. Surface Functionalization of Diamond Nanoparticles. 2.3.1. Formation of Azido-Terminated Diamond Nanoparticles (ND-N3). 4-Azidobenzoic acid (0.20 mmol), DCC (0.22 mmol), and DMAP (0.066 mmol) were dissolved in 5 mL of anhydrous CH3CN. A suspension of ND-NH2 particles in anhydrous CH3CN (10 mg in 5 mL) was added to the solution and stirred at room temperature for 24 h under nitrogen. The azido-terminated diamond nanoparticles were separated by centrifugation at 10 000 rpm, purified through consecutive wash/centrifugation cycles at 10 000 rpm with CH3CN (twice) and ethanol (twice), and finally oven-dried (50 °C).

2.3.2. “Clicking” 1-Alkynyl Derivatives to Azide-Terminated Diamond Nanoparticles. The azide-terminated (ND-N3) particles (10 mg) were dispersed in anhydrous CH3CN (10 mL) and sonicated for 30 min. The “click” reaction was performed by addition of 1-alkynyl derivatives (1-decyne, ethynylferrocene, or N-propargyl-1-pyrenecarboxamide, 2 mM), CuI (3.15 mM), and TEA (43 mM) to the ND-N3 particles and subsequent stirring for 24 h at room temperature. The resulting nanoparticles were separated by centrifugation at 10 000 rpm, purified through consecutive wash/centrifugation cycles at 10 000 rpm with CH3CN (twice) and ethanol (twice), and finally oven-dried (50 °C). 2.4. Instrumentation. 2.4.1. FTIR Spectroscopy. Fourier transform infrared (FTIR) spectra were recorded using a Perkin-Elmer Spectrum One FTIR spectrometer with a resolution of 4 cm-1. Dried ND powder (1 mg) was mixed with KBr powder (100 mg) in an agate mortar. The mixture was pressed into a pellet under 10 tons load for 2-4 min, and the spectrum was recorded immediately. Sixteen accumulative scans were collected. The signal from a pure KBr pellet was subtracted as a background. 2.4.2. UV-vis Spectroscopy. ND particles were dispersed in CH3CN (10 μg mL-1), and UV-vis spectroscopic measurements were carried out on a Perkin-Elmer Lambda 950 dual-beam spectrophotometer operating at a resolution of 1 nm in the region 300-600 nm. 2.4.3. Cyclic Voltammetry (CV) Experiments. Cyclic voltammetry measurements were performed using an Autolab potentiostat 20 (Eco Chemie, Utrecht, The Netherlands) in a classical three-electrode configuration. The gold working electrode (A ≈ 0.10 cm2) was sealed against the bottom of a single-compartment electrochemical cell by means of a rubber O-ring. The electrical contact was made to a copper plate through the bottom of the gold substrate. Platinum and Ag/AgCl electrodes were used as counter and reference electrodes, respectively. CVs were recorded in acetonitrile/TBAPF6 (0.1 M) solutions and after addition of 200 μL of diamond nanoparticles suspended in 0.1 M acetonitrile (1 mg/mL) at a scan rate of 50 mV s-1. 2.4.4. Optical Imaging. ND particles were dispersed in CH3CN (0.5 mg mL-1), and a droplet (5 μL) was deposited on a microscope slide before imaging using an Eclipse 80i (Nikon Instruments, Tempe, AZ) optical microscope equipped with a Coolsnap ES2 camera (Photometrics, Tucson, AZ), a Nikon Ph1 DLL 10x/0.30 Plan Fluor objective, a super-high-pressure mercury arc lamp (Southern Micro Instruments), and a DAPI filter set (excitation 340-380 nm; emission 435-485 nm). 2.4.5. Particle Size Distribution. The particle size of ND colloidal solutions (10 μg mL-1) in different solvents was measured at DOI: 10.1021/la101709q

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Figure 1. Schematic illustration of the chemical functionalization of ND-NH2 particles via an azide-alkyne Huisgen cycloaddition reaction. 25 °C using Zetasizer Nano ZS (Malvern Instruments S.A., Worcestershire, UK) in 173° scattering geometry. 2.4.6. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) measurements were made in Al2O3 crucibles in an atmosphere of nitrogen at a heating rate of 10 °C min-1 using a TA Instruments Q50 thermogravimetric analyzer.

3. Results and Discussion 3.1. Preparation of Azide-Terminated Nanodiamond (ND-N3). The amine-terminated ND particles used in this work display a primary mean diameter of 4 nm. Figure 1 illustrates the functionalization steps used to “click” various molecules bearing an alkyne terminal group on amine-terminated ND particles. First, azide termination was achieved by covalent coupling of 4-azidobenzoic acid to the surface amine groups. Different reaction conditions were tested to optimize the coupling yield and avoid particle precipitation. The best results were obtained using a solution of N,N0 -dicyclohexylcarbodiimide (DCC) as a coupling agent in the presence of 4-(dimethylamino)pyridine (DMAP) as a catalyst in acetonitrile. The “click” reaction between azide terminal groups and 1-alkyne derivatives (1-decyne, ethynylferrocene, or N-propargyl-1-pyrenecarboxamide) was promoted by CuI in the presence of triethylamine. The transmission FTIR spectrum of the as-received ND-NH2 particles is displayed in Figure 2a. It is dominated by two peaks at 3421 and 1625 cm-1. The broad band between 3000 and 3600 cm-1 can be assigned to the vibration of surface amino groups or/and adsorbed water molecules, while the peak at 1625 cm-1 is most likely due to a superposition of N-H scissoring mode and OH deformation mode of adsorbed water. Similar vibration modes were reported for amine-terminated ND particles prepared from hydroxyl-terminated ND particles functionalized with 3-aminopropyltrimethoxysilane.5 After reaction of ND-NH2 particles with 4-azidobenzoic acid in the presence of DCC and DMAP, new vibration peaks at 2128, 1600, 1376, 1286, and 1174 cm-1 appeared in the FT-IR spectrum (Figure 2b). This is consistent with the main vibration modes observed in neat 4-azidobenzoic acid transmission FTIR spectrum recorded in a KBr disk (data not shown). The presence of an intense peak at 2128 cm-1, characteristic of νas(N3) stretching mode, and peaks associated with amide(I) and amide (II) vibrations at 1650 cm-1 (overlapped with the peak at 1625 cm-1) and 1546 cm-1, respectively, suggests the success of the coupling reaction. The broad band with a maximum near 3400 cm-1 can be attributed to a combination of stretching modes due to water molecules and/or unreacted -NH2 groups. From the comparison of the relative intensity of the band at 2128 cm-1, associated with the azido group, the coupling reaction appears to be more effective when acetonitrile was used as solvent rather than dichloromethane, dimethylformamide, or dimethyl sulfoxide. 3.2. “Clicking” Different Alkyne-Containing Compounds to ND-N3 Particles. In a next step, the ND-N3 particles were reacted 13170 DOI: 10.1021/la101709q

Figure 2. Transmission FTIR spectra of ND-NH2 (a) and ND-N3 particles (b).

with various alkyne-containing compounds using the copper(I)catalyzed Huisgen 1,3-dipolar cycloaddition. Using CuI catalyst resulted in much better cycloaddition yields, based on the relative intensity of the band associated with the -N3 group (complete disappearance of the νas(N3) band in the FTIR spectrum), as compared to the use of a mixture of CuSO4/sodium ascorbate (presence of residual νas(N3) band). Figure 3a shows the transmission FTIR spectrum of azideterminated ND particles after reaction with 1-decyne (ND-Oc). Additional peaks characteristic of the alkyl chain appeared in the region 2800-3000 cm-1 (C-H stretching modes) and at 1456 cm-1 (methylene bending modes). The essentially quantitative cycloaddition of 1-decyne to the azide terminal groups is furthermore evidenced by the complete disappearance of the νas(N3) stretching mode at 2128 cm-1. Furthermore, to demonstrate the versatility of the azidealkyne 1,3-dipolar cycloaddition, ND-N3 were reacted with electrochemically active ethynylferrocene and fluorescent pyrene functionalized with an acetylene moiety. The FTIR transmission spectrum of the product of the reaction of ethynylferrocene with ND-N3, in acetonitrile in the presence of Cu(I) at room temperature, is displayed in Figure 3c. Clear changes can be seen in the spectrum with the appearance of new peaks at 3103, 1106, and 813 cm-1, characteristic of ferrocene absorption bands.22 Finally, a fluorescent molecule bearing an acetylene moiety was clicked onto ND-N3 in the presence of Cu(I). The FTIR (22) Guan, L.; Shi, Z.; Li, M.; Gu, Z. Carbon 2005, 43, 2780–2785.

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Figure 4. Cyclic voltammograms of a gold electrode immersed in 10 mM ethynylferrocene in acetonitrile/TBAPF6 (0.1 M) (A) and the same electrode in acetonitrile/TBAPF6 (0.1 M) after addition of 200 μL of a suspension of 1 mg mL-1 NDs-Fc (B). Scan rate = 50 mV s-1.

Figure 3. Transmission FTIR spectra of octyl- (a), pyrene- (b), and ferrocene-terminated (c) ND particles obtained through “click” chemistry of ND-N3 and 1-decyne, N-propargyl-1-pyrenecarboxamide, and ethynylferrocene, respectively.

transmission spectrum of ND-Pyr exhibits absorption peaks, characteristic of pyrene at 3045, 1515, 1233, 843, and 711 cm-1 (Figure 3b). The peak at 843 cm-1 is most likely due to the pyrene ring deformation mode while the broad band at 3045 cm-1 is attributed to the aromatic C-H stretching vibration. In addition, bands around 2925 cm-1 due to saturated C-H bonds were also observed in the FTIR spectrum. Again, the azide region for both ND-Fc and ND-Pyr showed a substantial reduction of the peak at 2128 cm-1, indicating a quasicomplete conversion of the azido group into a surface confined triazole ring. 3.3. Ferrocene-Modified (ND-Fc) Particles: Cyclic Voltammetry Characterization. The ND-N3 functionalized with ferrocene moieties (ND-Fc) were furthermore characterized using electrochemical measurements. Figure 4 exhibits the cyclic voltammograms of a gold electrode in 10 mM ethynylferrocene in acetonitrile/TBAPF6 (0.1 M) and the same electrode after addition of 200 μL of a suspension of 1 mg/mL ND-Fc into the electrolyte. The ND-Fc particles show a quasi-reversible redox behavior with a ΔE = 175 mV and a standard redox potential E° ≈ 0.49 V/Ag/AgCl. The standard redox potential is shifted by about 40 mV to a more negative redox potential, as compared to the redox potential value of ethynylferrocene (E° = 0.43 V/Ag/AgCl, ΔE = 60 mV) recorded in 10 mM ethynylferrocene in acetonitrile/TBAPF6 (0.1 M) using a gold electrode. The shape of the CV did not change over time. Thus, it is believed that the ferrocenemodified ND particles are most likely suspended in solution rather than deposited onto the gold interface. In a control experiment, to a suspension of ND-NH2 particles in anhydrous CH3CN (10 mg in 10 mL) was added a solution of ferrocenecarboxylic acid (Fe-COOH, 2 mM) in acetonitrile and stirred at room temperature for 24 h under nitrogen. The diamond nanoparticles were then separated by centrifugation at 10 000 rpm and resuspended in CH3CN. No electrochemical response could be detected using cyclic voltammetry, under similar experimental conditions used for ND particles modified through “clicking” ethynylferrocene. The detected current corresponds to the backLangmuir 2010, 26(16), 13168–13172

Figure 5. UV-vis spectrum of ND-Pyr particles in acetonitrile. Inset: UV-vis spectrum of N-propargyl-1-pyrenecarboxamide in acetonitrile (50 μM).

ground current, indicating the absence of any physisorbed ferrocene moieties. 3.4. Pyrene-Modified (ND-Pyr) Particles: UV/vis and Fluorescence Measurements. UV/vis spectroscopy was used to detect the pyrene moieties grafted on the ND particles. Figure 5 shows UV/vis absorption spectrum of pyrene-terminated ND dispersed in acetonitrile (10 μg mL-1). It displays absorption features at 340 and 415 nm, in accordance with the absorption peaks recorded for N-propargyl-1-pyrenecarboxamide in acetonitrile (inset in Figure 5). Furthermore, fluorescence microscopy was performed on NDNH2 and ND-Pyr particles upon excitation at 340-380 nm. Figure 6 shows a net difference in the fluorescence images of ND particles functionalized with pyrene groups as compared to ND-NH2. All these observations along with the absence of residual signal from azido groups in the FTIR spectra confirm clearly the efficient ligation between azido groups on ND particles and alkyne-containing compounds using “click” chemistry. 3.5. Thermogravimetric Analysis. Thermogravimetric analysis was carried out to support the covalent nature of the linking and to determine the surface loading of the ND particles. Figure 7a indicates that the starting ND-NH2 nanoparticles are stable up to 500 °C without any apparent mass loss. On the other hand, the thermogravimetric analysis (Figure 7b) shows the decomposition DOI: 10.1021/la101709q

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Figure 6. Fluorescence images of ND-NH2 (A) and ND-Pyr particles (B).

Figure 8. Photographs of nanodiamond suspensions in dichloromethane (1 mg mL-1) produced with visible light: ND-NH2 (A) and ND-Oc particles (B).

and toluene while the ND-NH2 particles cannot be dispersed in these solvents. The ND-Oc particles dispersed in chloroform and acetone form a brown colloidal solution, which completely precipitates within 2 h while the particles dispersed in toluene, acetonitrile, and ethanol showed no visible precipitation during 4 h. Particle size measurements showed 100-350 nm ND-Oc particle agglomerates in toluene, acetonitrile, ethanol, chloroform, and acetone in contrast to 200-1200 nm agglomerates formed with native ND-NH2 particles.

4. Conclusions Figure 7. Thermogravimetric analysis (TGA) of ND-NH2 (a) and ND-Pyr particles (b).

of the pyrene-functionalized ND particles at temperatures above 250 °C. At 250 °C, most of N-propargyl-1-pyrenecarboxamide was decomposed (data not shown). This is a strong evidence for the covalent grafting of the pyrene moieties onto the ND surface. The weight loss between 200 and 500 °C corresponding to removal of organic groups is around 24%. Thereby, the surface loading of functionalized ND with pyrene groups was found to be ∼0.54 mmol g-1 (assuming that the whole organic clicked fragment is removed from the surface). This value is higher than the surface loading of functionalized ND with oxyhexanol groups (0.13 mmol g-1)11 and in the range of surface loading (0.251.6 mmol g-1) reported for nanodiamond functionalized with various molecules.23 3.6. Solution Properties. Chemical and physical properties of ND particles can be affected by the nature and composition of the capping agent. Because of the long hydrocarbon chains grafted on ND-Oc, their behavior in different organic solvents was investigated. The ND-Oc can be easily dispersed in hydrophobic solvents such as dichloromethane (Figure 8), chloroform, (23) Kruger, A.; Liang, Y.; Jarre, G.; Stegk, J. J. Mater. Chem. 2006, 16, 2322– 2328.

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In conclusion, ND particles functionalized with terminal azido groups were successfully prepared by a simple coupling reaction between ND-NH2 and 4-azidobenzoic in a single step process. The copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition between the -N3 terminal groups on the ND particles and 1-alkynes was used as a facile and efficient way to covalently link various functional molecules onto the ND surface. Comparable results were obtained on hydroxyl-terminated ND-OH particles. The functionalized ND particles can be easily dispersed in various solvents. The technique proposed in this work is straightforward and will allow the preparation of nanocomposite materials with a range of different biomolecular functionalities. The preparation of ND/polymer composite materials and carbohydrate functionalized ND particles using “click” chemistry is in progress in our laboratory. Acknowledgment. We thank Dr. Laurent Heliot (Plate-forme d’Imagie et Biophotonique Cellulaire Fonctionnelle, USR CNRS 3078) and Profs P. Woisel, J. Lyskawa, and S. Degoutin (Laboratoire de Chimie Macromoleculaire, UMR CNRS 8009) for technical support with fluorescence imaging, FTIR spectra, and TGA measurements, respectively. A.B. gratefully acknowledges financial support from The Centre National de Recherche Scientifique (CNRS) and The Universite Lille1 - Sciences et Technologies for a postdoctoral position.

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