NMR and EPR Characterization of Functionalized Nanodiamonds

Article pubs.acs.org/JPCC

NMR and EPR Characterization of Functionalized Nanodiamonds Charlène Presti,† Aany S. Lilly Thankamony,‡,# Johan G. Alauzun,† P. Hubert Mutin,† Diego Carnevale,§ Cédric Lion,∥ Hervé Vezin,⊥ Danielle Laurencin,*,† and Olivier Lafon*,‡ †

Institut Charles Gerhardt de Montpellier, UMR 5253, CNRS-UM-ENSCM, Université de Montpellier, 34095 Montpellier, France Unité de Catalyse et de Chimie du Solide (UCCS), CNRS UMR 8181, ∥Unité de Glycobiologie Structurale et Fonctionnelle (UGSF), CNRS UMR 8576, and ⊥Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), CNRS UMR 8516, Université Lille Nord de France, 59652 Villeneuve d’Ascq, France § Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, EPFL, Batochime, 1015 Lausanne, Switzerland ‡

S Supporting Information *

ABSTRACT: We investigated the potential of solid-state NMR using magic angle spinning (MAS) with and without dynamic nuclear polarization (DNP) and electron paramagnetic resonance (EPR) for the characterization of functionalized nanodiamonds (NDs). We showed that conventional 1H, 31P, and 13C solid-state NMR spectra allow differentiating in a straightforward way NDs from commercial sources and custom-made NDs bearing aromatic or aliphatic phosphonate moieties at their surface. Besides, the short nuclear relaxation times prove the close proximity between the endogenous paramagnetic centers of NDs and the grafted organic moieties. EPR spectra confirmed the presence of these paramagnetic centers in functionalized NDs, which are centered on dangling bonds as well as a few N0 defects, corresponding to the substitution of carbon atoms by nitrogen ones. Hyperfine sublevel correlation spectroscopy indicates that the N0 paramagnetic centers are mostly located in the disordered shell of NDs. Preliminary DNP-enhanced NMR experiments at 9.4 T and 100 K under MAS have shown a lack of significant DNP enhancement, which can be attributed to the short relaxation times of the unpaired electrons and the nuclei in NDs. When using exogenous polarizing agents, the endogenous unpaired electrons contribute to a leakage of polarization. Furthermore, low temperatures lead to a broadening of NMR signals. It therefore appears that conventional direct excitation remains the NMR method of choice for the characterization of functionalized NDs.

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INTRODUCTION Nanodiamonds (NDs), that is, diamond particles with a diameter smaller than 100 nm, are receiving considerable attention due to their remarkable hardness and thermal conductivity, luminescence properties, chemical inertness, and biocompatibility.1−6 For instance, they have been proposed as fillers to reinforce the mechanical properties of polymers and ceramics,7,8 as catalyst supports for metal nanoparticles (NPs),9 or as nanovectors for various drugs.4,6,10 Depending on the preparation and purification methods used, the surface of NDs can be terminated by a variety of functional groups, including carboxylic groups, hydroxyls, ketones, and lactone moieties.5,11−14 To attain the targeted properties, the surface of nanodiamonds most often needs to be functionalized, so that the interactions between the NPs and their environment are optimal. Several methods have been proposed to tune the surface properties of NDs by covalent grafting of organic moieties.14−20 To date, the characterization of NDs functionalized by organic fragments has involved a variety of methods, such as X-ray photoelectron,21 UV,22 and FTIR16,20,23−26 spectroscopies. Solid-state NMR spectroscopy has been attempted,27−29 but the use of this technique is still limited despite the high specific surface area of NDs (∼150−300 m2·g−1), which should be favorable for detecting the surface functional groups. So far, solid© 2015 American Chemical Society

state NMR studies of NDs have mainly focused on the investigation of the core and surface structure of NDs30 prepared according to various techniques such as detonation,31−39 shock compression,32,40 explosive compaction of graphite,41 highpressure and high-temperature synthesis,32 laser ablation,42,43 and in some cases post-treated by exchange of surface ions,44,45 reduction,33,46 ozonation,47 fluorination,32,48−50 or phosphorylation.51 In terms of NMR characterizations of NDs, one of the important features is the presence of endogenous paramagnetic centers in the core of the particles. Schmidt-Rohr and co-workers studied in detail the structure of detonation NDs using 1H and 13 C solid-state NMR experiments, showing that these spherical NPs with a diameter of ∼5 nm contain paramagnetic centers that are mainly distributed within a disordered shell, at distances between 0.4 and 1 nm from the surface.35 There are several consequences of the presence of these paramagnetic centers for studying the functionalities at the surface of NDs by NMR: (i) the relaxation of the NMR-active nuclei is strongly accelerated32,34,35 and (ii) solid-state NMR sequences commonly used Received: March 5, 2015 Revised: May 4, 2015 Published: May 4, 2015 12408

DOI: 10.1021/acs.jpcc.5b02171 J. Phys. Chem. C 2015, 119, 12408−12422

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The Journal of Physical Chemistry C

state NMR, EPR, and DNP-enhanced solid-state NMR for the characterization of functionalized detonation NDs. Three types of NDs are compared here, which were functionalized by reaction with phosphonyl dichloride compounds bearing different organic chains, namely, dodecyl, octadecyl, or phenyl

to characterize functionalized NPs, such as cross-polarization (CP), are often challenging for paramagnetic samples, since the fast decay of nuclear coherences and the broadening of NMR spectra due to hyperfine couplings often make coherence transfer ineffective.52−55 The presence of paramagnetic centers in functionalized NDs can be either a curse or a blessing for the NMR characterization of materials. On one hand, the hyperfine interaction between the unpaired electrons and the surrounding nuclei broadens the NMR signals, which decreases the resolution and the sensitivity of this spectroscopy. In particular, this broadening can prevent the observation of nuclei distant by less than a few angstroms from unpaired electrons.56 Moreover, as explained above, the efficiency of CP transfer strongly decreases in the presence of paramagnetic centers. On the other hand, the decrease of the nuclear longitudinal relaxation times, T1n, reduces the recycle delay between consecutive NMR acquisition and hence improves the sensitivity.53 Furthermore, the high polarization of unpaired electrons can be transferred via hyperfine interactions to the nearby nuclei, thus enhancing the NMR signal. In the case of diamond, this polarization transfer has been achieved either by continuous-wave or pulsed microwave (μw) irradiation near the electron paramagnetic resonance (EPR) frequency using the dynamic nuclear polarization (DNP) phenomenon43,57−67 or by optical pumping with visible light of the electronic transition of nitrogen-vacancy (NV) centers.68−71 So far, optical pumping has only been achieved for diamond single crystals. Conversely, DNP-enhanced 13C NMR for static samples has been reported for diamond single crystals,57,58,60−65 and chemical vapor deposited diamond films59,66 at low static magnetic field, B0 ≤ 1.9 T and room temperature (T), as well as for polycrystalline diamond samples of various particle sizes, including NDs, at B0 = 3.35 T and T ≤ 1.9 K.26,31 A DNP signal enhancement larger than 330 has also been measured for a diamond single crystal at B0 = 9.4 T, T = 105 K and under static conditions.72 However, low B0 fields or static conditions do not allow the acquisition of high-resolution NMR spectra, which are required to identify surface functionalities of NDs. Recently, DNP-NMR systems at higher field (up to 18.8 T) and T = 100 K under magicangle spinning (MAS) have emerged and are now commercially available.73−79 High-field DNP-NMR under MAS combines high resolution and high sensitivity, which have led to its application for the characterization of several systems, including cells,80,81 biomolecules,82,83 microcrystalline organic solids,84,85 polymers,86 micro-87−89 and mesoporous90−93 materials, and NPs.94−98 For almost all high-field DNP-NMR studies under MAS reported so far, the source of polarization was exogenous nitroxide mono- or biradicals,99−101 such as 1-(TEMPO-4-oxy)3-(TEMPO-4-amino)propan-2-ol (TOTAPOL) or bis(TEMPO)-bis(ketal) (bTbK), which must be incorporated into the investigated system by different protocols, including dissolution,99 postsynthesis impregnation,90,91 dispersion,95 impregnation followed by evaporation,84 film-casting,86 coprecipitation,102 sedimentation,103 and functionalization.104,105 The riboflavin semiquinone radical in flavodoxin proteins is one of the only reported examples of endogenous polarizing agent for highfield DNP-NMR study under MAS.106 In this context, it was pertinent to test the potential of high-field DNP-NMR under MAS for the characterization of functionalized NDs using only the endogenous paramagnetic centers of NDs or in the presence of exogenous nitroxide biradicals. The purpose of this article is thus to present a detailed investigation of the possibilities offered by conventional solid-

Figure 1. Schematic representation of the functionalized NDs investigated here.

(Figure 1). In the first part of the manuscript, an analysis of the H, 13C, and 31P NMR spectra of these NDs is performed, with a special emphasis on the particularities related to the NMR characterization of the surface moieties in the case of NDs. The effects of ND paramagnetic centers on the NMR data are clearly demonstrated by the comparison with NMR measurements on diamagnetic titanium oxide NPs functionalized with phenylphosphonate groups. In the second part, we present the results of EPR and DNP-NMR studies of functionalized NDs without and with exogenous polarizing agents. We underscore the difficulties related to the use of this technique for NDs. These difficulties stem from the short electronic and nuclear relaxation times and the polarization leakage via endogenous paramagnetic centers when using exogenous ones. Finally, we conclude on the advantages and challenges of NMR techniques for the characterization of such nanomaterials. 1

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EXPERIMENTAL DETAILS Materials. NDs (>98% purity, 0.2 wt % of nondiamond carbon) with an average primary particle size of 4 nm were purchased from International Technology Center (Raleigh, NC, U.S.A.), and were dried under vacuum (1 × 10−2 mbar) at 140 °C overnight prior to use. Phenylphosphonic dichloride, PhPOCl2, was purchased from Sigma-Aldrich (France; technical grade, 90%). Dodecylphosphonic dichloride CH3-(CH2)11-POCl2 was synthesized from diethyl dodecylphosphonate CH3-(CH2)11PO(OEt)2 using previously established procedures,107 with a yield of 78%. Octadecylphosphonic dichloride CH3-(CH2)17POCl2 was similarly synthesized from diethyl octadecylphosphonate CH3-(CH2)17-PO(OEt)2, with a yield of 41%. Phenylphosphonic acid (PhPO(OH)2, 98% purity) was purchased from Sigma-Aldrich (France) and purified by recrystallization in acetonitrile, and octadecylphosphonic acid (CH3-(CH2)17-PO(OH)2, >95% purity) was purchased from SiKEMIA (Montpellier, France). TiO2 NPs with an average diameter of 12 nm were donated by Saint-Gobain NorPro; their surface functionalization using PhPOCl2 is described in Supporting Information; this functionalized TiO2 phase will be referred to as TiO2‑Ph herein. Functionalization of Nanodiamonds by Phosphorylation. The grafting procedure of phenyl-, dodecyl-, and octadecylphosphonic dichloride was adapted from our previous work on the surface functionalization of NDs by organophosphorous molecules.28 The CH2Cl2 used for the grafting reactions was dried by distillation over P2O5. 12409


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Figure 2. One-dimensional 1H, 31P, and 13C NMR spectra of ND0, NDPh, NDC12, and NDC18 samples. Details on the acquisition conditions can be found in the Experimental Section.

Conventional Solid-State Nuclear Magnetic Resonance. 1H NMR spectra were recorded at room temperature on a Varian 600 MHz (14.1 T) NMR spectrometer at a frequency of 599.82 MHz, equipped with a 3.2 mm Varian T3 HXY MAS probe. The sample was spun at a MAS frequency νr = 18 kHz. For all experiments, the nutation frequency of radiofrequency (rf) pulses on the 1H channel was 100 kHz. The 1H spectra were acquired using a spin−echo sequence to suppress the background signal due to the rotors and the probe. The 1H longitudinal relaxation times, T1(1H), were measured using a spin−echo-detected saturation-recovery sequence,108 involving a burst of presaturation pulses (eight 90° pulses separated by 0.1 ms for the ND samples, and eight 90° pulses separated by 1 ms for TiO2‑Ph), followed by a relaxation delay (ranging from 0.004 to 4 s), and a spin−echo, in which the 5 μs 180° pulse was bracketed by two 55.5 μs delays (each corresponding to one rotor period); eight transients were acquired for each spectrum. The time constant T2′(1H), associated with the nonrefocusable contributions to the dephasing of the proton magnetization, was

Typically, PhPOCl2 (268 mg, 1.24 mmol) was added to a dispersion of NDs (501 mg) in dry CH2Cl2 (50 mL). The mixture was stirred for 3 d at 40 °C under argon atmosphere. The functionalized NDs (531 mg) were isolated by centrifugation. Three consecutive washing/centrifugation cycles were then performed under ambient conditions; each washing was performed by suspending the functionalized NDs in 40 mL of technical CH2Cl2 (H2O: 310 ppm) in an ultrasonic bath (130 W, 10 min) and then centrifuging them at 20 000 rpm for 10 min. Then, the sample was dried at 140 °C under reduced pressure (1 × 10−2 mbar) for 24 h. A similar protocol was used for the grafting of dodecyl- and octadecyl-phosphonic dichloride. In this manuscript, the different ND samples will be referred to as ND0 for the commercial nanodiamonds and NDPh, NDC12, and NDC18 for the NDs functionalized by phenyl-, dodecyl-, and octadecyl-phosphonic-dichlorides, respectively. The P content, as determined by inductively coupled plasma analysis (ICPOES), was 0.90, 0.70, and 0.48 mmol per gram of NDs, for NDPh, NDC12, and NDC18, respectively. 12410


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were performed at νr = 18 kHz, using a 2.5 μs 90° 1H excitation pulse, followed by a CP contact time, τCP, ranging from 0.1 to 8 ms. During the CP transfer, the rf nutation frequency on the 13C channel was constant and equal to 63 kHz, while the 1H nutation frequency was linearly ramped from 42 to 52 kHz. The 1H → 13C CPMAS experiments on ND samples were acquired using τRD = 0.5 s and 32 000 transients; the CPMAS spectra shown in Figure 2 were acquired using τCP = 0.2 ms. For TiO2‑Ph, CPMAS spectra were acquired with τRD = 0.5 s and a total number of transients of 2400. The 13C chemical shifts were referenced to TMS using the deshielded resonance of adamantane (38.5 ppm) as secondary reference. Electron Paramagnetic Resonance. EPR experiments were performed at 300 and 90 K on ND0, NDC12, and NDC18 using an X-band Bruker Biospin ELEXYS E580E spectrometer operating at 9 GHz. Continuous wave (CW) EPR spectra were recorded at room temperature using an amplitude modulation of 1 G and a microwave power of 5 mW. At 90 K, two-pulses echo field sweep detection was also performed using standard spin echo sequence, π/2-τ−π-echo, with π/2 and π pulse lengths equal to 16 and 32 ns, respectively. The τ value was set to 200 ns. The longitudinal and transverse relaxation times, T1(e) and T2(e), of the unpaired electrons were measured using inversion recovery and spin echo decay sequences, respectively. Hyperfine sublevel correlation spectroscopy (HYSCORE) two-dimensional (2D) spectra were also acquired using the pulse sequence π/2-τ−π/2-t1-π-t2-π/2-τ-echo, whereby an echo is generated at time τ after the last π/2 pulse, τ representing the delay between the first and second π/2 pulses. Because of the fast T2(e) relaxation a τ value of 120 ns was employed to maximize the echo intensity. The echo intensity is measured at each t1 and t2 value, which are varied stepwise at constant τ. This two-dimensional set of echoes yields, after Fourier transform along the t1 and t2 period, a two-dimensional HYSCORE spectrum. Dynamic Nuclear Polarization-Enhanced Nuclear Magnetic Resonance under Magic-Angle Spinning. Synthesis of bTbK. The nitroxide biradical bTbK was synthesized in two steps with a 51% overall yield, according to a procedure adapted from the literature.98,100 The bispiroketal scaffold was first obtained by condensation of pentaerythritol and 2,2,6,6tetramethylpiperidin-4-one in toluene in the presence of paratoluenesulfonic acid. This diamine precursor was then oxidized to bTbK using sodium tungstate dihydrate in 30% aqueous hydrogen peroxide and methanol. Preparation of the Sample for DNP. DNP-NMR experiments were performed on three types of samples: (i) pristine NDC18, (ii) NDC18 dispersed in a frozen 16 mM bTbK solution in 1,1,2,2-tetrachloroethane (TCE), denoted NDC18-TCE* henceforth, and (iii) a frozen solution of 14 mM octadecylphosphonic acid and 16 mM bTbK solution in TCE, denoted C18- TCE* henceforth. The sample NDC18-TCE* was prepared by dispersing at room temperature ∼10 mg of pristine NDC18 into 50 mg of 16 mM bTbK solution in TCE. TCE is advantageous since its 13C NMR signal at 73.8 ppm does not overlap with those of NDC18. We used bTbK since it is soluble in TCE.111 Furthermore, the rigid tether of bTbK constrains the relative orientation of the two TEMPO moieties so as to optimize the cross-effect polarization transfer to protons.100 Other dispersion solvents and nitroxide biradicals were tested, including (i) 16 mM TOTAPOL aqueous solution and (ii) 16 mM TOTAPOL solution in [2H6]-dimethyl sulfoxide (DMSO)/2H2O/H2O mixture (78/14/8 w/w/w). For these other samples, the masses of NDC18 and dispersion

measured using a spin−echo sequence, in which the 180° pulse was bracketed by two rotor-synchronized evolution periods ranging from 1 to 80 rotor periods; eight transients were acquired for each spectrum, with a relaxation delay τRD = 0.5 s for the ND samples, and τRD = 0.6 s for TiO2‑Ph. The 1H spectra shown in Figure 2 were acquired using a spin−echo sequence, in which the 180° pulse is bracketed by two delays of one rotor period (55.5 μs). The 1H chemical shifts were referenced to tetramethylsilane (TMS) using the signal of adamantane (1.8 ppm) as a secondary reference. 31 P NMR spectra were recorded at room temperature on a Varian 400 MHz (9.4 T) NMR spectrometer at a frequency of 161.98 MHz, equipped with a 3.2 mm Varian T3 HXY MAS probe. All 31P NMR experiments were performed at νr = 20 kHz, and SPINAL-64109 proton decoupling with an rf nutation frequency, ν1,dec(1H) = 100 kHz, was applied during acquisition. The 31P longitudinal relaxation times of the ND samples, T1(31P), were estimated using a saturation-recovery sequence, involving a burst of presaturation pulses (20 90° pulses separated by 20 ms), followed by a relaxation delay ranging from 0.05 to 1 s, and a 90° 31P pulse of 3 μs; 32 to 96 transients were acquired for each spectrum. Longer relaxation delays (reaching 400 s) were used in the case of the TiO2-Ph sample. T2′(31P) constants were measured using a spin−echo sequence, in which the 180° pulse was sandwiched by rotor-synchronized evolution periods (ranging from 1 to 100 rotor periods). For the ND samples, 96 transients were acquired for each spectrum, with τRD = 0.5 s, while for TiO2‑Ph, 32 transients were acquired with τRD = 80 s. The 31P MAS NMR spectra shown in Figure 2 were acquired using direct excitation under MAS (DEMAS) sequence (see Figure S1a in the Supporting Information); the number of transients ranged from 32 to 40, and the τRD delay from 0.3 to 0.5 s, depending on the sample. 1H → 31P CP under MAS (CPMAS) experiments (see Figure S1a) were performed on NDPh and TiO2‑Ph. A 2.5 μs 90° proton excitation pulse was used, followed by a CP contact time τCP ranging from 0.1 to 8 ms. During the CP transfer, the rf nutation frequency on the 31P channel was constant and equal to 47 kHz, while the 1H nutation frequency was linearly ramped from 60 to 74 kHz. 1H → 13P CPMAS experiments were acquired with τRD = 0.5 s, and the total number of transients recorded at each contact time was 1000 and 40 for the NDPh and TiO2‑Ph samples, respectively. The 31P chemical shifts were referenced to 85 wt % H3PO4 aqueous solution using the 31P signal (at 2.8 ppm) of synthetic hexagonal hydroxyapatite (Ca10(PO4)6(OH)2) as a secondary reference.110 13 C NMR spectra were recorded at room temperature on a Varian VNMRS 600 MHz (14.1 T) spectrometer at a frequency of 150.83 MHz, equipped with a 3.2 mm Varian T3 HXY MAS probe. For 13C NMR experiments, SPINAL-64 proton decoupling with ν1,dec(1H) = 100 kHz was applied during the acquisition period. For all 13C NMR experiments, the acquisition time was limited to 15 ms, to remain within the rf specifications of the probe, especially for very short recycle delays. DEMAS experiments, such as those shown in Figure 2, were performed at νr = 18 to 20 kHz, using a 2 μs 90° excitation pulse. Depending on the sample, the τRD delay was 0.15 to 1 s, and the number of transients ranged from 2000 to 4000. The longitudinal 13C relaxation times, T1(13C), were estimated using a saturationrecovery sequence, involving a burst of presaturation pulses (ten 90° pulses separated by 0.1 ms for NDPh, and by 5.0 ms for NDC12 and NDC18), followed by a relaxation delay ranging from 0.1 to 2 s, and a 90° 13C pulse of 2.5 μs; 2000 to 4000 transients were acquired for each spectrum. 1H → 13C CPMAS experiments 12411


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identical buildup time constants were measured with and without μw irradiation. We estimated time constants of 4 and 4.5 s for the 1 H polarization buildup of NDC18-TCE* and C18-TCE* samples, respectively. The 1H → 13C CPMAS spectra of NDC18-TCE* and C18-TCE* samples, shown in Figure 7, resulted from the accumulation of 1024 transients. The τRD delay was chosen as 1.3 times longer than the buildup time constant, that is, τRD = 5.2 and 5.9 s so as to optimize the sensitivity.113 Hence, the 1H → 13C CPMAS spectra of NDC18-TCE* and C18TCE* samples were acquired in 90 and 100 min, respectively. The τCP time was 200 and 600 μs for NDC18-TCE* and C18TCE* samples, respectively. The other experimental parameters are identical to those used for the 1H → 13C CPMAS spectra of pristine NDC18. The 13C chemical shifts were referenced to TMS at 0 ppm by using the 13C resonance of TCE as a secondary reference (73.8 ppm).

solution were similar to those used for NDC18-TCE*. However, no DNP enhancement of 1H and 13C NMR signals was measured for NDC18 dispersed in 16 mM TOTAPOL aqueous solution. This absence of enhancement must stem from (i) the formation of ice crystals, which leads to phase separation between ice, NDC18, and TOTAPOL,112 as well as (ii) the poor dispersion of NDC18 in water. For 16 mM TOTAPOL solution in [2H6]DMSO/2H2O/H2O mixture, 1H and 13C NMR signals were enhanced by μw irradiation, but the 13C signal of [2H6]-DMSO at ∼40 ppm overlaps with those of NDC18. The mixture NDC18TCE* was sonicated during 10 min, just before being placed in 3.2 mm sapphire rotors. The sample C18-TCE* was prepared by dissolving ∼5 mg of octadecylphosphonic acid in ∼130 mg of 16 mM bTbK solution in TCE. The aforementioned masses were chosen in such a way that the amount of octadecyl chains was identical in NDC18-TCE* and C18-TCE*. For DNP-NMR experiments, the samples were placed in 3.2 mm sapphire rotors since this material is nearly transparent to microwaves at 263 GHz. Dynamic Nuclear Polarization Experiments. The DNPNMR experiments were performed at EPFL at 9.4 T (400 MHz for 1H) on a Bruker BioSpin Avance III DNP NMR spectrometer equipped with a triple resonance 1H/X/Y 3.2 mm lowtemperature (ca. 100 K) MAS probe and a 263 GHz gyrotron.76 The microwave irradiation was transmitted through a corrugated waveguide to a triple resonance 1H/X/Y MAS probe with 3.2 mm rotors spinning at a MAS frequency νr = 10 kHz. The microwave power delivered to the sample was ∼6 W. The NMR spectra were acquired at a temperature of ∼100 K, which was stabilized using a Bruker BioSpin MAS cooling system. The rotor containing NDC18-TCE* sample was sonicated during 10 min just before its insertion into the low-temperature MAS probe. For pristine NDC18, 1H → 13C CPMAS and 13C DEMAS spectra (see Supporting Information, Figure S1b) were acquired with the μw irradiation “on” and “off”. For both sequences, SPINAL-64 1H decoupling with ν1,dec(1H) = 100 kHz was applied during the acquisition period. For CPMAS experiments, the 1H 90° pulse duration and τCP time were 2.5 and 100 μs, respectively. During the CP transfer, the 1H rf nutation frequency was linearly ramped from 59 to 66 kHz, whereas the rf nutation frequency on 13C channel was constant and equal to 53 kHz. The polarization buildup of protons, which transfer polarization to 13 C nuclei, was measured by varying the recycle delay in 1H → 13 C CPMAS. An identical buildup time constant (70 ms) was measured with and without μw irradiation, and hence an identical τRD delay of 0.5 s was used for 1H → 13C CPMAS with and without μw irradiation. The 1H → 13C CPMAS of pristine NDC18, shown in Figure 6a, resulted from the accumulation of 2048 transients, that is, an experimental time of 22 min. Using the saturation-recovery sequence, we observed a stretched exponential buildup of 13C polarization with a time constant of ∼400 ms. This value was identical with and without μw irradiation. In the saturation-recovery experiment, the 13C magnetization was eliminated at the beginning of the relaxation delay by a burst of a hundred 2.5 μs 90° pulses on the 13C channel. The DEMAS spectra shown in Figure 6b were acquired using τRD = 2 s and a 13 C 90° pulse of 2.5 μs length, and they resulted from the accumulation of 1024 transients, that is, an experimental time of 34 min. For NDC18-TCE* and C18-TCE* samples, the polarization buildup of protons, which transfer polarization to 13C nuclei, was measured in the same way as that of pristine NDC18. Here again,

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RESULTS Conventional Solid-State Nuclear Magnetic Resonance. Nowadays, one of the most common ways to characterize surface-functionalized NPs by solid-state NMR is still the CP sequence. This NMR sequence allows enhancing the signals of poorly abundant nuclei like 13C, by polarization transfer from abundant isotopes like 1H. 1H → 13C CPMAS experiments are thus routinely used to study the grafting of organic fragments at the surface NPs114 like SiO2,114,115 TiO2,114,116 hydroxyapatite,114,117 and even Au.114,118 Furthermore, in the case of functionalized NDs, the 1H → 13C CPMAS sequence selectively enhances the signal of 13C nuclei located near the surface since (i) the protons in NDs are essentially located at the surface35,37 and (ii) the CP transfer is only effective up to a few angstroms and suffers from dipolar truncation.119 In the framework of this study, we thus decided to look into how to best characterize phosphorylated NDs using 1H → 13C and 1H → 31 P CPMAS experiments. In 1H → 13C and 1H → 31P CP experiments, the 1H polarization is transferred to the heteroatom during the contact pulse (Figure S1a). The recycle delay is dictated by the T1(1H) value, and the τCP time is generally chosen so as to maximize the signal of the heteroatom. Below, the results of 1H NMR experiments are first presented, which were performed to determine the T1(1H) values. Then, the 1H → 31P and 1H → 13 C CPMAS studies of functionalized NDs at different τCP times are exposed. Finally, investigations of the same materials using 31 P and 13C DEMAS experiments are reported. 1 H Relaxation. 1H MAS NMR spectra acquired with spin− echo sequences are shown in Figure 2. The spectrum of ND0 exhibits a broad 1H signal centered at ∼3 ppm, which subsumes the signals of C−H and C−OH surface groups as well as that of adsorbed water molecules.35,37 Several novel signals appear on the spectra of the functionalized NDs, which can be ascribed to the aromatic (NDPh) or aliphatic (NDC12 and NDC18) protons of the grafted organophosphorous molecule. In the case of the NDC12 and NDC18 samples, these signals actually obscure those coming from the ND itself. The T1(1H) values are reported in Table 1. For NDC12 and NDC18, T1(1H) values correspond to integrations over the whole signal (due to the lack of resolution between the core ND 1H signals and those from the grafted organophosphorous fragment), while for NDPh, the signals of aromatic protons and those of ND were deconvoluted, and the T1(1H) value reported here corresponds to the aromatic signals only. For functionalized 12412


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The Journal of Physical Chemistry C Table 1. T1 and T2′ Constants Measured for ND0, the Three Functionalized NDs, and TiO2‑Ph T2′(X) (ms)e

T1(X) (s) X

1

H

a

31

P

ND0 NDPh NDC12

0.009 ± 0.001 0.015 ± 0.001b 0.030 ± 0.003b

NMR and EPR Characterization of Functionalized Nanodiamonds

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