Surface Functionalization of Detonation Nanodiamonds by

DNDs were first modified by phenylphosphonic dichloride, and the grafting modes and hydrolytic stability under neutral conditions were investigated us...
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Surface Functionalization of Detonation Nanodiamonds by Phosphonic Dichloride Derivatives Charlene Presti, Johan G. Alauzun,* Danielle Laurencin, and P. Hubert Mutin* Institut Charles Gerhardt Montpellier, UMR 5253 CNRS/ENSCM/UM2/UM1, Université Montpellier 2, CC1701, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France S Supporting Information *

ABSTRACT: A new method for the functionalization of detonation nanodiamonds (DNDs) is proposed, on the basis of surface modification with phosphonic dichloride derivatives. DNDs were first modified by phenylphosphonic dichloride, and the grafting modes and hydrolytic stability under neutral conditions were investigated using 1H, 13C, and 31P solid state NMR spectroscopy, Fourier transform infrared spectroscopy, as well as elemental analysis. Then, in order to illustrate the possibilities offered by this method, DNDs functionalized by mesityl imidazolium groups were obtained by postmodification of DNDs modified by 12-bromododecylphosphonic dichloride. The oxidative thermal stability of the functionalized DNDs was investigated using thermogravimetric analysis.

1. INTRODUCTION Nanodiamonds have been receiving considerable attention owing to their remarkable hardness and thermal conductivity, biocompatibility, and tunable surface chemistry, which make them attractive materials for a wide range of applications1 in biology,2−5 tribology,6 catalysis,7 or as fillers in nanocomposites.8 Detonation nanodiamonds (DNDs) can be produced in industrial quantities at low cost by detonation of oxygen-deficient explosives.9−11 According to infrared, Raman, and X-ray photoelectron spectroscopy studies,12−14 the surface of purified, pristine DNDs is covered by a variety of oxygen-containing groups, such as hydroxyl, carboxyl, anhydride, ester (lactone), or ketone groups. Tailoring the surface chemistry of DNDs is crucial for many applications, and several approaches have been explored to obtain more chemically homogeneous DND surfaces or to functionalize them by covalently grafted organic groups.15 DNDs predominantly terminated by carboxyl,16,17 hydroxyl,18−20 C−F,21 or C−H22 groups have been obtained using various oxidation or reduction reactions. Covalent grafting of organic groups has been achieved by reaction of amines16 or chitosan derivatives23 with carboxylic acid groups, or of alkoxysilanes,5 acyl chlorides,20 or dopamine derivatives24 with hydroxyl groups. Furthermore, a variety of organic groups has been grafted to DNDs by reacting fluorinated DNDs with amines or alkyllithium salts,21 or by treating hydrogenated DNDs with alkenes or diazonium salts.22,25 ATRP initiators were also grafted using esterification chemistry on carboxylated DNDs17 allowing the synthesis of ultradispersed nanodiamond (UDD)/polymer brush materials. We have previously reported the surface modification of DNDs by phosphorylation using POCl3,26 which reacts with hydroxyl groups to form surface phosphate esters. The resulting © 2014 American Chemical Society

phosphorylated DNDs showed drastically increased thermal stability in air. In the present work, we report the grafting of organic groups to DNDs by reaction of phosphonic dichloride compounds with the surface hydroxyl groups and formation of phosphonate esters. In the first part of this Article, high purity DNDs were reacted under different conditions with phenylphosphonic dichloride (PhPOCl2), and the nature and hydrolytic stability of the surface species were investigated using a combination of 31P MAS NMR and Fourier transform infrared spectroscopy (FTIR). In the second part, in order to illustrate the versatility of our method, the DNDs were grafted with 12-bromododecylphosphonic dichloride (Br(CH2)12POCl2), and then postmodified by reaction with a mesityl imidazole derivative (Figure 1), leading to DNDs functionalized by mesityl imidazolium groups. Such mesityl imidazolium groups tethered to silica have been used as precursors to N-heterocyclic carbene (NHC) ligands to prepare highly active Ru-NHC heterogeneous catalysts.27,28 In the third part of this work, the influence of the functionalization of DNDs by organophosphorous groups on their thermal stability in air was investigated using thermogravimetric analysis (TGA).

2. EXPERIMENTAL SECTION All manipulations were carried out under argon atmosphere using the Schlenk technique, to avoid uncontrolled hydrolysis of phosphonic acid dichlorides. 2.1. Materials. Purified detonation nanodiamonds (DNDs, 99% purity, 0.2 wt % of nondiamond carbon) with an average primary Received: May 7, 2014 Revised: July 4, 2014 Published: July 7, 2014 9239

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Figure 1. Surface functionalization of DNDs by phosphonic dichloride derivatives and postmodification by nucleophilic substitution. particle size of 4 nm and a specific surface area of 270 m2 g−1 were purchased from International Technology Center (ITC, Raleigh, NC). They were dried before use for 18 h at 120 °C under reduced pressure (10−2 mbar). Phenylphosphonic dichloride (PhPOCl2, Sigma-Aldrich) was used as received. CH2Cl2 was carefully dried by distillation over anhydrous calcium chloride. 2.2. Characterization. The P content of the samples was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) after a mineralization step under acidic conditions. The N content was measured by combustion, at the Service d’Analyze of the University Montpellier 2. N2 adsorption−desorption isotherms at 77 K were recorded on a Micromeritics Tristar analyzer, and the specific surface area of the samples was determined using the BET method; prior to measurement, the samples (≈100 mg) were degassed at 150 °C for 12 h under vacuum. Thermogravimetric analysis (TGA) was performed under air (50 cm3/min flow) in the 293−1273 K temperature range (heating rate of 5 K/min) using a Netzsch STA 409 PC Luxx thermobalance. 1H−13C cross-polarization magic angle spinning (CP-MAS) solid state NMR spectra were recorded on a Varian VNMRS 600 MHz spectrometer operating at 14.1 T, using a 3.2 mm Varian T3 HXY MAS probe spinning at 18 kHz. There were 90 000 transients recorded for each sample, using a contact time of 200 μs and a recycle delay of 0.6 s. 31P MAS solid state NMR experiments were carried out on a Varian VNMRS 400 MHz spectrometer operating at 9.4 T using a 3.2 mm Varian T3 HXY MAS probe. Single pulse experiments were performed at a spinning speed of 20 kHz with a ∼90° excitation pulse of 3 μs, a recycle delay of 1 s, and 100 kHz spinal-64 1H decoupling. There were 2400 transients recorded for each sample. The Fourier transform infrared (FTIR) spectra were carried out in transmission mode using KBr pellets on a ThermoNicolet Avatar 320 spectrometer. Powder X-ray diffraction (XRD) was performed on a Philips X’pert X-ray diffractometer (Bragg−Brentano geometry) employing Cu Kα1 radiation (λ = 1.540 56 Å) with a voltage of 40 kV and a current of 30 mA. Transmission electron microscopy (TEM) analyses were carried out on a JEOL 1200 EXII microscope operating at 120 kV. The samples for TEM were prepared by casting a drop of the nanoparticle suspension (0.5 mg per mL of H2O) onto copper grids covered with holey carbon support films. 2.3. Experimental Procedures. 2.3.1. Synthesis of 12Bromododecylphosphonic Dichloride. 12-Bromododecylphosphonic dichloride (Br(CH2)12POCl2) was synthesized by an Arbuzov reaction involving 1,12-dodecyldibromide and P(OEt)3, followed by chlorination with POCl3/PCl5 of the resulting diethyl 12-bromododecylphosphonate.29 According to 1H NMR, partial exchange of Br by Cl occurred during the chlorination step, and the final product contained ≈7% of 12-chlorododecylphosphonic dichloride. 31P NMR (CD2Cl2) δ (ppm): 51.4 (s). 1H NMR (CD2Cl2) δ (ppm): 1.32−1.63 (m, 16 H, −CH2−), 1.88 (m, 2H, −CH2−CH2−P), 1.91 (m, 2H, −CH2−CH2− Br or −CH2−CH2−Cl), 2.63 (m, 2H, − CH2−P), 3.46 (t, 1.85 H, −CH2−Br), and 3.58 (t, 0.15 H, −CH2−Cl).

2.3.2. Synthesis of 1-Mesityl-1H-imidazole. 1-Mesityl-1H-imidazole was obtained in two steps from 2,4,6-trimethylaniline as previously described.30 1H NMR (CD2Cl2) δ (ppm): 1.94 (6H, oCH3 Mes), 2.30 (3H, p-CH3 Mes), 6.95 (2H, CH Mes), 6.97 (1H, N− CH−N), 7.42 (1H, Mes−N−CH−CH−N), and 7.95 (1H, Mes−N− CH−CH−N). 2.3.3. Functionalization of the DNDs. The procedure for the surface modification of DNDs by phosphonic dichloride derivatives was adapted from our previous work on the surface phosphorylation of DNDs.26 In a typical procedure, 1.25 mmol of PhPOCl2 (sample DND2.5P) or Br(CH2)12POCl2 (sample DNDBr) was added to a dispersion of DNDs (501 mg) in dry CH2Cl2 (50 mL), and the mixture was stirred for 3 days at 40 °C under argon. Alternatively, 2 equiv of water per PhPOCl2 was added to the CH2Cl2 before the addition of the PhPOCl2, and the reaction was then performed following the exact same procedure. The functionalized DNDs were recovered after centrifugation, and three consecutive washing/ centrifugation cycles under ambient conditions, followed by a drying step at 140 °C under reduced pressure (10−2 mbar) for 24 h. Washing conditions follow: successively 130 mL of technical CH2Cl2 (H2O content 310 ppm), ultrapure water, and acetone; the centrifugation was performed at 20 000 rpm for 10 min. 2.3.4. Postfunctionalization with 1-Mesityl-1H-imidazole. A 300 mg portion of DNDs grafted by Br(CH2)12POCl2 (DNDBr) and 50 mL of toluene were successively introduced in a 100 mL roundbottom flask. A solution of 1-mesityl-1H-imidazole (180 mg, 0.97 mmol, approx 10 equiv relative to the grafted Br(CH2)12-POCl2 groups) in 2 mL of toluene was added, and the mixture was heated at reflux (120 °C) for 3 days.27 The postfunctionalized DNDs (DNDMES) were obtained after two consecutive washing/centrifugation cycles in toluene, one in acetone (20 000 rpm/10 min), and a final drying step at 140 °C under reduced pressure (10−2 mbar) for 24 h. 2.3.5. Hydrolytic Stability Test. Grafted DNDs were put in a 50 mL tube with ultrapure water at the concentration of 10 mg of DNDs per mL. The pH value of the resulting suspensions was in the 6.5−7 range. The tubes were shaken on an orbital shaker at 50 rpm speed, at controlled temperature (22 °C) for 1 day or for 7 days. After this treatment the samples were recovered by centrifugation (20 000 rpm, 10 min) and dried under reduced pressure (10−2 mbar, 140 °C, 12 h).

3. RESULTS AND DISCUSSION The high purity DNDs used in this work consist of aggregated diamond primary nanoparticles about 4 nm in size (Supporting Information Figure SI1). Their BET specific surface area derived from N2 sorption isotherms is 270 m2/g. According to the manufacturer, the average aggregate size (as measured by dynamic light scattering in water suspensions) is 160 nm. The powder X-ray diffraction (XRD) diffractogram (Supporting Information Figure SI2) showed broad Bragg peaks at 2θ = 43.9° (111), 75.3° (220), and 91.5° (311) corresponding to 9240

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diamond. Only traces of graphitic carbon at 2θ ≈ 27° (002) were detected, while no fullerene-like carbon at 2θ ≈ 21° (222) could be found,17,31,32 in agreement with the very low content of nondiamond carbon (0.3%) given by the manufacturer. This was also confirmed by Raman spectroscopy as depicted in Supporting Information Figure SI3. 3.1. DNDs Surface Modification by PhPOCl2. Surface functionalization of the dried DNDs was performed by reaction with PhPOCl2 in dry CH2Cl2 under argon atmosphere. The influence of several reaction parameters on the final P content was examined: influence of the washing conditions, of the amount of PhPOCl2, added water, or triethylamine (Table 1).

during the grafting, P−Cl groups are rapidly hydrolyzed to P− OH groups which cannot react with the surface hydroxyl groups but can condense with other P−Cl groups to form P− O−P bridges.33 Solid state NMR and FTIR spectroscopies were used to characterize the surface species of samples DND2.5P and DND2.5P−W. The 31P MAS solid state NMR (Figure 2) of these samples reveals in both cases a major signal at ≈15 ppm, and a minor resonance at ≈6 ppm.

Table 1. Conditions Used for the Functionalization and P Content of the Modified DNDs sample

PhPOCl2a (mmol/g)

H2Oa (mmol/g)

Et3Na (mmol/g)

P contentc (mmol/g, ±0.05)

DND DND2.5P‑Db DND2.5P DND5P DND5P‑Et3N DND2.5P‑W DND5P‑W

2.55 2.55 5.30 4.75 2.55 4.40

0 0 0 0 5.50 10

0 0 0 14.8 0 0

0 0.95 0.55 0.40 0.40 1.30 1.20

Figure 2. 31P MAS NMR of DNDs functionalized by PhPOCl2.

a

Experimental amount of reactant in mmol per g of DND. bD: modified DNDs washed only with technical CH2Cl2. All other samples were successively washed with technical CH2Cl2, water, and acetone (see Experimental Section). cDetermined by ICP-AES.

The replacement of Cl groups in PhP(O)Cl2 by OH or OR groups leads to an upfield shift of ca. 17 ppm, and the formation of a P−O−P bridge leads to a further upfield shift of ca. 9 ppm. Accordingly, the resonance at ≈15 ppm can be ascribed to orthophosphonate units, PhP(O)(OX)2, where X can be either a hydrogen atom or a carbon at the surface of the DND.34 The resonance at ≈6 ppm may be attributed to pyrophosphonate (diphosphonate) units, [PhP(O)(OX)]2O, with one P−O−P bridge (Supporting Information Table SI1). There is no evidence for longer phosphonate oligomers. This gives us four possible phosphonate and diphosphonate surface units, linked to the surface by one or two P−O−C bonds (Figure 3).

The corresponding modified DNDs are referred to as DNDxP where x is the nominal amount of PhPOCl2 (in mmol) reacted with 1 g of DND. Additional letters (D, W, or Et3N) refer to the washing conditions, to the addition of water or triethylamine during the surface modification, as detailed in Table 1. It must be noted first that no chlorine was detected in all of the samples, showing that all of the residual P−Cl groups (if any) were hydrolyzed to P−OH groups during the washing step, as could be expected from the high sensitivity of phosphonyl chlorides to hydrolysis. When the modified DNDs were washed simply with technical CH2Cl2 (sample DND2.5P‑D), the P content was ca. 40% higher than when they were washed successively with CH2Cl2, water, and then acetone (DND2.5P), showing that washing with technical CH2Cl2 (which contains only 310 ppm water) was not sufficient to remove physisorbed or weakly bound phosphonate species. Thus, all other samples were thoroughly washed with CH2Cl2, water, and then acetone, as detailed in the Experimental Section. Increasing the amount of PhPOCl2 (DND5P) or adding triethylamine (DND5P‑Et3N) to trap the HCl formed had no significant influence on the P content. On the other hand, when ca. 2 equiv of water (relative to the phosphoryl chloride) was added during the synthesis (samples DND2.5P−W and DND5P−W), the P content increased significantly, reaching 1.21.3 mmol/g. In the absence of water, the reaction of PhPOCl2 with the hydroxyl groups at the surface of the DNDs should lead to phenylphosphonate groups linked to the surface by one or two P−O−C bonds (ester bonds). In principle, PhPOCl2 could also react with surface carboxylic acids to form mixed anhydrides linked by C(O)−O−P(O) bonds, but such species are highly sensitive to hydrolysis and would not withstand the washing treatment. In the presence of adventitious water or added water

Figure 3. Possible orthophosphonate (A, B) and pyrophosphonate (C, D) surface sites.

FTIR spectra confirm the grafting of phenylphosphonate units (Figure 4). The spectrum of the starting DNDs is dominated by vibrations from surface functional groups.1 The broad bands in the 3000−3600 cm−1 region correspond to O− H stretching from adsorbed water and C−OH groups, with weak bands at 2850−2950 cm−1 indicating the presence of aliphatic C−H groups. The band at 1730 cm−1 is assigned to stretching of carbonyl groups, while the band at 1630 cm−1 corresponds mainly to the OH deformation mode of adsorbed water. The strong vibration at 1120 cm−1 has been ascribed to ether (COC asymmetric stretching) and hydroxyl groups (C− OH deformation), and the shoulder at 1045 cm−1 to hydroxyl 9241

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Table 2. Evolution of the P Content of Modified DNDs upon Stirring in Ultrapure Water at 22 °C sample

conditions

P contenta (mmol/g, ±0.05)

DND2.5P

untreated H2O, 24 h H2O, 7 d untreated H2O, 24 h H2O, 7 d

0.55 0.30 0.25 1.30 0.30 0.30

DND2.5P‑W

a

Measured by ICP-AES.

The 31P NMR spectra of the samples treated for 7 days (Figure 5) were nearly identical, indicating the presence of Figure 4. FTIR spectra (transmission, KBr) of DND, DND2.5P, and DND2.5P‑W. The vibrations discussed in the text are indicated by arrows.

groups (COH deformation).12 In the spectra of the modified DNDs, several vibrations characteristic of phosphonate species can be observed such as an intense band at 1140 cm−1 (PO stretching) and overlapping bands in the 900−1100 cm−1 region arising from POC but also POP and/or POH groups.35 Several vibrations characteristic of the phenyl group are also present, such as the aromatic CH stretching band at 3058 cm−1, and the (P)CC bands at 1437, 751, 715, and 695 cm−1. The CO stretching vibration at 1730 cm−1 corresponding to surface carboxyl groups is not modified, confirming the absence of mixed anhydrides.26 In addition, in the case of DND2.5P‑W, broad bands around 2650 and 2310 cm−1 point to the presence of acidic POH groups.35 These bands are less intense in the spectrum of DND2.5P, indicating a lower amount of POH groups. This result combined with the 31P NMR data suggests that POH containing surface species linked to the surface by only one P− O−C bond such as A and C (Figure 3) are more abundant in the sample prepared in the presence of water (DND2.5P‑W) than in the sample prepared in the absence of water (DND2.5P). Accordingly, the higher P content found in DND2.5P‑W is likely related to the limited amount of hydroxyl groups at the surface of the DNDs: only one hydroxyl is consumed to bind A or C species, versus two for B and D species. The low P content found whatever the grafting conditions shows that the grafting density is not limited by steric hindrance but rather by the limited amount of reactive hydroxyl groups at the surface of the DNDs. In such a case, for the same amount of surface hydroxyl it is possible to graft twice as many monodentate species A than bidentate species B that consume the maximum amount of bidentate orthophosphonate species B. This is consistent with the higher P content of the sample prepared in the presence of water, which according to FTIR spectroscopy features mostly A species. The hydrolytic stability of DND2.5P and DND2.5P‑W under neutral conditions (pH 6.5−7) was evaluated by stirring the samples in ultrapure water at 22 °C (see Experimental Section). For both materials, after 1 day of treatment in water, the P content decreased to 0.30 mmol/g (Table 2). Interestingly, the P content did not decrease significantly more after longer treatments (up to 7 days), indicating the presence of a significant amount of quite stable species at the surface of the DNDs (≈0.30 mmol/g), comparable to the loading reported for dopamine-modified DNDs (0.27 mmol/g).24

Figure 5. 31P MAS NMR of samples DND2.5P and DND2.5P‑W after treatment for 7 days in ultrapure water at 22 °C.

orthophosphonate (≈15 ppm) and pyrophosphonate (≈5 ppm) species in a similar 2.5:1 ratio. In addition, bands characteristic of P−OH around 2650 and 2310 cm−1 were not detected in the FTIR spectra of DND2.5P‑W treated for 7 days (Supporting Information, Figure SI4) suggesting that the remaining surface species are mostly B and D species, linked to the surface by two P−O−C bonds. Accordingly, regardless of the preparation method and initial P content, both the amount and the nature of the stable species were similar. This result suggests that although grafting in the presence of water led to a higher grafting density, presumably by increasing the amount of orthophosphonate species linked to the surface by only one P−O−C bond, these species were largely removed during the treatment in water. 3.2. Grafting by Br−CH2)12−POCl2 and Postfunctionalization. DNDs were modified by Br−(CH2)12−POCl2 under the same conditions as DND2.5P, in the absence of added water. 31 P MAS NMR spectroscopy (Supporting Information Figure SI5) confirmed the anchoring of bromo- or chlorododecylphosphonate species to the surface, mostly as orthophosphonate species. The P content (0.40 mmol/g) of the resulting sample DNDBr (Table 3) was slightly lower than the one obtained for DND2.5P (0.55 mmol/g). In the Experimental Section, we mentioned that Br−(CH2)12− POCl2 contained about 7% of Cl−(CH2)12−POCl2 due to partial exchange of Br by Cl during the chlorination step. The low Br/P ratio (≈0.5) in the modified DNDs suggests that further exchange of Br by Cl occurred during the grafting step, upon release of HCl during the grafting. This exchange has no consequence on the next postfunctionalization step with an 9242

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Table 3. Conditions for the Grafting of Br(CH2)12POCl2 and for the Postfunctionalization of DNDBr, P Content, and N Content of the Modified DNDs

a

sample

Br(CH2)12POCl2a (mmol/g)

MesImia (mmol/g)

P content (mmol/g, ±0.05)

N content (mmol/g, ±0.1)

Br content (mmol/g, ±0.05)

DNDBr DNDMES

2.46

0 3.49

0.40 0.40

1.6 1.9

0.21 0.15

Amount of Br(CH2)12POCl2 and 1-mesityl-1H-imidazole (MesImi) reagents used in mmol per g of DND.

H stretch of methylene groups in the alkyl chain. In the spectrum of DNDMES, different vibrations characteristic of the aromatic groups were detected at 1460, 1550, 3097, and 3135 cm−1. The 1H−13C CPMAS NMR spectra were recorded with a contact time of 0.2 ms, selecting primarily carbon sites bonded to protons or close to protons. In the spectrum of the starting DNDs, the broad resonance centered around 40 ppm corresponds mainly to different kinds of C−H carbons, and the resonance at ≈73 ppm to C−OH groups.36 In the spectrum of DNDBr, additional signals corresponding to the methylene groups in the dodecyl chains are detected at 34 ppm (CH2− Br), 30 ppm (C−CH2−C), and 23 ppm (CH2−CH2−P). After postmodification, the relative intensity of the signal centered at 34 ppm decreases, and resonances characteristic of the mesityl groups are detected at 18 ppm (C−CH3) and at 120−140 ppm (aromatic CH), thereby confirming the postfunctionalization of the particles. 3.3. Thermal Stability. In a previous work we had found that phosphate groups grafted at the surface of DNDs significantly improved their oxidative stability.26 In order to see whether grafted phosphonate groups had a similar effect, the thermal stability in air of the DNDs functionalized by various phosphonate groups was investigated using TGA (Figure 7).

imidazole (next paragraph), as both Br and Cl groups are good leaving groups in nucleophilic substitution reactions.27 The DNDBr sample was then postfunctionalized by reaction with 1-mesityl-1H-imidazole (DNDMES) in order to illustrate the potential of our functionalization method for attaching more complex functionalities at the surface of NDs (Figure 1). The P content after postfunctionalization was close to the initial one, indicating that the anchoring was stable under the conditions used for this nucleophilic substitution (Table 3). A rough estimation of the yield of the postfunctionalization step can be derived from the N content of the samples (Table 3). Taking into account the presence of N in the pristine DNDs and DNDBr, a yield of ≈40% was found for DNDMES. The postmodification was investigated using FTIR (Figure 6 A) and 1H−13C CPMAS NMR spectroscopies (Figure 6 B). The FTIR spectra of both DNDBr and of DNDMES featured two strong bands at 2855 and 2925 cm−1 corresponding to the C−

Figure 7. TGA in air of neat and modified DNDs.

Instead of the weight gain observed at ca. 350 °C for the starting DND (arising from the oxidation of the surface groups), the modified DNDs showed a weight loss in the 250500 °C range ascribed to the condensation between P−OH groups and to the degradation of the organic groups. The residue at 1000 °C increased with the P content, from 1.8% (unmodified DNDs) to 4%. The oxidation of the diamond core corresponds to the major weight loss in the 450-950 °C range. All the DNDs modified by phosphonate units exhibited an improved thermal stability compared to unmodified DNDs (Figure 7). The thermal stability increased with the P content, as shown by the temperature at which the oxidative weight loss reached 5% (T5%), which varies from 475 °C for the starting

Figure 6. (A) FTIR spectra (transmission, KBr) of DNDs functionalized by reaction with bromododecylphosphonic dichloride (DNDBr) and DNDBr postmodified by reaction with mesityl imidazole (DNDMES). The vibrations discussed in the text are indicated by arrows. (B) 1H−13C CPMAS NMR spectra of DND, DNDBr, and DNDMES. The resonances discussed in the text are indicated by arrows. 9243

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DNDs to 615 °C for DND2.5P−W (Table 4). On the other hand, the oxidative thermal stability does not significantly depend on

P contenta (mmol/g, ±0.05)

T5%b (°C ± 5)

DND DND2.5P DND2.5P‑W DNDBr DNDMES

0 0.55 1.30 0.40 0.40

475 545 615 520 510

a

Determined by ICP-AES. bTemperature corresponding to a 5% weight loss in the oxidation step.

the nature of the organic group, as shown by the similar T5% temperatures found for DNDBr and DNDMES. In the same way, the thermal stability of the phosphonate-modified DNDs is close to the thermal stability of DNDs modified by phosphate groups with a comparable P content.26

4. CONCLUSION We have described a new approach for the covalent functionalization of nanodiamonds by treatment with phosphonic dichloride derivatives. A variety of organic groups can be grafted to the surface either by direct functionalization or using postfunctionalization steps. The bonding of the phosphonate groups to the surface depends on the conditions of reaction and involves mainly orthophosphonate groups linked to the surface by one or two P−O−C bonds. The hydrolytic stability of the modified DNDs was investigated, and a significant amount of phosphonate groups (0.30 mmol/g) remained linked to the surface after 1 week in water at room temperature. The presence of phosphonate groups at the surface of the DNDs greatly increases their oxidative thermal stability, as previously reported for phosphate groups. Future work will concern the application of these new functionalized nanodiamonds, for instance in nanocomposite materials.

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ASSOCIATED CONTENT

S Supporting Information *

TEM image and DRX of the starting DNDs as well as FTIR spectra, 31P MAS NMR, and thermogravimetric analyses of modified nanodiamonds. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Table 4. Thermogravimetric Analysis Data of Starting and Functionalized DNDs sample

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Phone: +33 (0)4.67.14.49.43. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the University of Montpellier 2 for financial support and Guillaume Gracy for his help in the synthesis of phosphonic dichloride and imidazole derivatives. 9244

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