Magnetic Resonance Study of Gadolinium-Grafted Nanodiamonds

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Magnetic Resonance Study of Gadolinium-Grafted Nanodiamonds Alexander M. Panich, Alexander I. Shames, Nikolaj A. Sergeev, Vladimir Yu. Osipov, Alexander E. Alexenskiy, and Alexander Ya. Vul J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05403 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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The Journal of Physical Chemistry

Magnetic Resonance Study of Gadolinium-Grafted Nanodiamonds A. M. Panich,*a A. I. Shames,a N. A. Sergeev,b V. Yu. Osipov, †c, A. E. Alexenskiy,c A. Ya. Vul'c a

Physics Department, Ben-Gurion University of the Negev, 8410501 Be'er Sheva, Israel, Institute of Physics, University of Szczecin,70-451 Szczecin, Poland c Ioffe Physical-Technical Institute, St. Petersburg 194021, Russia b

Abstract

We report on EPR,

13

C and 1H NMR study of detonation nanodiamond particles with surface

grafted by paramagnetic gadolinium ions obtained by ion exchange with hydrogen atoms of carboxyl groups through the reaction of aqueous nanodiamond suspension with an aqueous solution of gadolinium nitrate. Our findings give clear evidence that Gd3+ ions are chemically bound to the nanodiamond surface and interact with electron and nuclear spins of the diamond nanoparticle, which results in acceleration of electron and nuclear spin-lattice relaxations. A model of positioning of Gd3+ ions on the DND surface terminated by oxygen-containing groups is suggested. The distance between the Gd ion and nanodiamond surface is estimated by relaxation measurements as 0.32 nm. Biomedical applications of the studied nanomaterials are discussed.

* Corresponding author: e-mail [email protected], Phone: +972 8 6472458, Fax: +972 8 6472904 † e-mail [email protected], Phone/Fax : + 7 812 297 00 73

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1. Introduction Detonation nanodiamond (DND) particles reveal both significant scientific interest and perspective technological applications due to small primary particle size (~4 to 5 nm) with narrow size distribution, facile surface functionalization and high biocompatibility. They show great potential for a variety of applications in different areas, such as quantum computing,1 drug delivery2–5 and medical imaging.6, 7 For example nanodiamonds (ND), which may carry out at least one fluorescent nitrogen vacancy (NV- center) per particle, are potential cellular biomarkers.8 NV- centers may work as a basic units of a quantum computer and have potential applications

in

novel

electronics

and

computational

science

including quantum

cryptography and spintronics. The chemically active surface of DND enables conjugation with a variety of functional groups that allows preparation of nanoparticles with controlled chemical, electronic and physical properties.2,

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The DND surface with negative zeta-potential is mainly

functionalized by carboxyl and hydroxyl groups that enable its grafting by metal cations.9-11 Inclusion of various transition elements onto the DND surface would yield promising materials for advanced applications in molecular electronics, high density magnetic recording and materials engineering. Such nanoparticles can be used as magnetic resonance imaging (MRI) contrast agents to enhance the local signal intensity in cells and tissues that are magnetically similar but histologically distinct since they reduce the relaxation time of water protons in tissues under examination. Particularly Gd3+ ions are perspective for this purpose as they have a large unpaired electron spin of S = 7/2 and fast spin dynamics in the gigaHertz frequency range. Manus et al.

7

reported on synthesis of an amine-functionalized Gd(III) contrast agents

conjugated to the DND surface. Such agents consist of the paramagnetic metal species with one or more coordination sites available for water to interact with the unpaired electrons of the Gd(III) ion, resulting in a decrease of the proton spin-lattice relaxation time. Nakamura et al.12 developed

a

fabrication

of

DND

particles

modified

with

an

organogadolinium

(diethylenetriaminepentaacetic acid, DTPA) moiety by chemical modification for use as MRI ACS Paragon Plus Environment

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contrast agent. Zhao et al.

13

prepared a ND-polyglycerol-Gd(III) conjugate system that

demonstrated relatively high water proton relaxivity of this complex in aqueous solution. Another effect of the transition metal ions is that they, being attached to the diamond surface or positioned close to it, at distances < 0.4 nm, can affect spin relaxation of the paramagnetic centers inside the diamond particle, e.g., of the color NV- centers. Tetienne et al.14 treated the 10 nm ND in aqueous solution of gadolinium perchlorate molecules Gd(ClO4)3 and showed NV- centers feel the magnetic noise induced by the external Gd3+ ions and that significant level of Gd spin noise at the frequency of the NV- center’s zero-field splitting (2.87 GHz) reduces the NV- spin-relaxation time T1, depending on the proximity and concentration of Gd. Pelliccione et al.15 attached gadolinium to a silicon atomic force microscopy (AFM) tip by submerging the cantilever in a chelated Gd solution [gadopentetate dimeglumine in water] for several minutes. They showed that positioning the Gd-coated tip in close proximity to a shallow NV- center can reproducibly reduce its electron spin-lattice relaxation. Hydrophilicity of DND particles causes some water adsorption on the nanodiamond surface, which in turn makes water protons being responsive to grafting paramagnetic ions. This yields a potential use of such DND as a magnetic contrast agent in a number of specific biomedical applications, in which permeability of 5 nm particles through submicron / nanometer tissue pores is important. We have recently reported on preparation and investigation of Cu- and Co-grafted DND, in which the transition metal ions are conjugated to the DND surface via exchange with protons of surface carboxyl (–COOH) groups.9-11 Such chemical modification results in increase in the relaxation rate, revealing appearance of paramagnetic Cu2+ and Co2+ complexes at the DND surface and their interaction with the

13

C and 1H nuclear spins. Those complexes are destroyed

on annealing the samples at 900 oC. Following this investigation, adsorption of several transition metal and inorganic ions from aqueous solutions on DND was studied, aiming usage of these materials as selective adsorbents for application in solid phase extraction and chromatography.16


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Nano-diamond particles functionalized with amide–thiourea ligands for adsorption of uranium ions were synthesized.17 In the present paper we report on preparation and electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) study of highly purified DND particles whose surface is chemically grafted by gadolinium. We note that Gd3+ ion is the most promising for MRI applications owing to its large magnetic moment of ~7 µ B (here µ B is the Bohr magneton), and therefore it is conventionally used in MRI.

2. Experimental The samples were fabricated in the Ioffe Physical-Technical Institute.9-11 An industrial DND powder produced by Federal State Unitary Enterprise “Technolog” (St. Petersburg, Russia) from TNT/RDX (60/40) mixture by was used as an initial material. To remove the non-diamond carbon, a standard sample purification that includes oxidation and consequent etching in HF and KOH with multiple washing in purified water followed by centrifugation and drying was carried out. The subsequent deagglomeration process included annealing in air and dispersing of DND particles in deionized water under ultrasonic treatment followed by centrifugation, as it is described in details in ref 18. Surface grafting of DND particles by gadolinium ions was performed in two steps. At first, a stable aqueous suspension of highly purified DND particles was prepared by the previously developed method.18 The average particle size in suspension determined by the dynamic light scattering (DLS) method using a Zetasizer Nano spectrometer (Malvern Instruments Ltd, UK) was 5 nm with the full width at half-maximum of the size distribution function was ~1.5 nm (see Figure S3 in Supporting Information). X-ray diffraction (XRD) data yield the size of coherent-scattering region of the DND particles as ~ 4.3 nm. The pH of aqueous suspension of DND particles was 7.1. More details of DND sample characterization such as XRD pattern, FTIR and Raman spectra, zeta potential and particle size distribution are presented in Supporting Information. ACS Paragon Plus Environment

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Most of the particles exist in suspension in the monodisperse form. The surface modification was performed by mixing of 19 mL of the aqueous DND suspension in a concentration of 0.42 wt % with a certain volume of aqueous solution of gadolinium nitrate hexahydrate Gd(NO3)3·6H2O (Sigma Aldrich 211591) in concentration of 0.189 g/L. Gadolinium ion concentration in solution, prepared for DND surface modification, was 4.18×10-4 mol/L. After draining the solution was stirred for 10 minutes at room temperature and then dried in a desiccator under a "technical vacuum", i.e., under the residual gas pressure of 5 - 10 mm Hg during 1 day. As a result, up to 80 mg of dry product were obtained for each sample. It was suggested that the dissociated Gd cations in this solution undergo ion exchange with the hydrogen atoms of carboxyl groups on the DND surface and thus become to be chemically bound to the nanoparticle surface. Variations of the volume of the aqueous solution of gadolinium nitrate used in mixing from 0.6 to 40 mL allowed preparing DND powders with a nominal gadolinium concentration from 0.049 to 3.28 wt %. Obtaining higher than 3.5 wt % of gadolinium concentration by this method was not possible due to particle coagulation and solid precipitation. We studied an initial (non-grafted) and four gadolinium-grafted DND samples with 0.049, 0.49, 1.64 and 3.28 wt.% of Gd, respectively.19 The actual concentrations of Gd3+ ions were determined by superconducting quantum interference device (SQUID) magnetometer Quantum Design MPMS-7 following the method described in ref 19 and were found to be 1.53 × 1018 , 1.61×1019 , 5.14 × 1019 , and 7.85 × 1019 spin/g, respectively. The experimental magnetization curves taken at 2 K were fitted using Brillouin formula and Gd3+ ion concentration as a fitting parameter.19 We note that the compounds prepared are stable and did not undergo noticeable changes (over 10%) for several years of storage. X-band (microwave frequency ν = 9.4 GHz) continuous wave EPR measurements on a series of initial and Gd3+-grafted polycrystalline detonation nanodiamond (DND) samples were carried out using a Bruker EMX - 220 spectrometer equipped with Agilent 53150A frequency counter at room temperature (RT, T ~295 K). Precise determination of g-factors was done by ACS Paragon Plus Environment

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comparison with the reference sample - well purified DND powder with g = 2.0028(2).20 Evaluation of the electron spin-lattice and spin-spin relaxation times (TSLe and TSSe, correspondingly) was done by analysis of the saturation dependencies of the peak-to-peak intensities of the most intensive g = 2.00 line following the technique described in ref 21. Spectra and data processing were done using Bruker’s WIN-EPR and OriginLab software. 13

C and 1H NMR measurements of the powder samples were carried out at room

temperature (RT) using a Tecmag pulse solid state NMR spectrometer and an Oxford superconducting magnet. The measurements were made in the external magnetic field B0 = 8.0 T, corresponding to 13C resonance frequency of 85.62 MHz and 1H resonance frequency 340.54 MHz, respectively. The

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C spectra were measured using Hahn echo, while 1H spectra - using

π

π

solid echo pulse sequence ( ) 0 o − τ d − ( )90 o , a technique that refocuses homonuclear dipole– 2 2 dipole couplings between protons (here subscripts denote the phases of pulses). Both 13C and 1H spin-lattice relaxation times were measured using saturation comb sequences. Magnetization recovery was fitted by a stretched exponential function. All measurements were carried out with non-spinning samples on purpose of (i) preventing hindrance of nuclear spin diffusion which is an important mechanism of the spin-lattice relaxation and (ii) making the data obtained useful for MRI community since MRI examinations are taken in a static mode. The duration of the π/2 pulse was 1.5 µs for 1H and 5 µs for 13C measurements.

3. Results and Discussion 3.1. EPR Black trace in Figure 1(a) represents general view (magnetic field sweep width 1 T) of the RT EPR spectra obtained on the DND sample grafted by the highest Gd load (3.82 wt. %) in comparison with the spectrum of the initial (non-grafted) DND sample recorded at the same instrumental conditions (Figure 1(a), blue open circles). These spectra, being recorded at high sensitivity setup (incident microwave power PMW = 20 mW, 100 kHz magnetic field modulation ACS Paragon Plus Environment

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amplitude Amod = 1 mT, receiver gain RG = 2 × 105 , show superposition of some broad lines and very intensive narrow singlet line which is truncated on this spectrum. Inset in Fig. 1(a) shows the narrow line recorded at lower sensitivity setup (PMW = 0.2 mW, Amod = 0.5 mT, RG = 1× 104 ). DIN of broad line (arb. units / mg) EPR Signal Intensity (arb. units / mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

ν = 9.464 GHz

4

0

* * -4

×1

× 4000

-7

320

330

340

g = 2.85

350

(b)

4

g = 2.0

2 0 -2

0.49 wt% Gd 1.64 wt% Gd 3.28 wt% Gd

g = 5.95

-4

0

200

400

600

800

1000

Magnetic Field (mT)

(c) 0.8 0.4 0.0

0

19

2x10

3+

19

4x10

19

6x10

19

8x10

Gd concentration (spin/g)

Figure 1. (a) General view RT EPR spectra of non-grafted DND (blue open circles) and Gd grafted DND (3.82 wt %, black trace), ν = 9.464 GHz. Inset shows EPR spectrum of the intensive singlet line in the latter sample recorded at total gain reduced 4000 times (see scaling coefficients on the plot). Asterisks point out weak background signals in the spectrum of nongrafted DND; (b) broad lines in spectra of the samples with 0.49, 1.64 and 3.28 wt% of Gd extracted from the corresponding general view spectra. In both (a) and (b) vertical scale is normalized per unit mass; (c) DIN of the broad lines vs. Gd3+ concentration obtained by SQUID magnetometer,19 red dotted line represents best linear fit.


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Weak broad lines in the general view spectrum of non-grafted sample are located at gfactors about 4.3 and 2.5 – see peaks marked by asterisks in Figure 1(a). On Gd grafting another broad line having complicated pattern appears. Intensity of this line grows on grafting with the added Gd salt concentration Figure 1(b) shows the broad patterns extracted from the general view EPR spectra of Gd-grafted DND samples by the cubic spline extrapolation of the broad pattern (up to 50 points per spectrum) and subtraction of the truncated narrow component. The spectrum of sample with the lowest Gd concentration 0.049 wt % practically coincides with the background signal of non-grafted sample (not shown) whereas the spectra for higher Gd concentration represent consistent growth of the same broad pattern having characteristic features at geff1 = 5.95 ± 0.05, geff2 = 2.85 ± 0.05 and geff3 = 2.0 ± 0.1. Figure 1(c) demonstrates linear dependence of the double integrated intensity (DIN) of broad lines vs. actual Gd3+ concentration obtained by SQUID magnetometer.19 Intense singlet narrow line with g = 2.0028 ± 0.0002 and quasi-Lorentzian line shape (see magenta trace on the inset to Fig. 1(a)) is observed in EPR spectra of all DND samples under study. Behavior of this line on Gd grafting within the same series of DND samples has been reported in details in ref 19. It was found that the experimental line width ∆Hexpp continuously increases with the Gd concentration. Electron spin-lattice (TSLe) and spin-spin relaxation (TSSe) times were estimated by continuous microwave saturation technique.21 As a matter of fact, the experimental setup in use does not allow reaching incident microwave power levels above 200 mW, which is not enough for precise measurements of fast (shorter than 10-7 s) electron spinlattice relaxation times in DND. Surprisingly, it was found the saturation curves for DND samples showed clear difference with increasing Gd concentration. Analysis of these curves indicates reliably observable (beyond overestimated 30% cumulative experimental and fitting errors) shortening of TSLe from 2.8 × 10 −8 s in non-grafted DND to 6.6 × 10 −9 s in DND grafted by 3.82 wt% of Gd. TSSe = 7.8 × 10 −9 s remains the same for all samples under study. Figure 2 demonstrates dependencies of TSLe and ∆Hexpp on the real Gd3+ concentration.19


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1.1

pp

1.0

-8

2.0x10

∆H

Exp

(mT)

-8

3.0x10

TSL (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9 -8

1.0x10 0.8

0

2x10

19

4x10

19

6x10

19

8x10

19

3+


Figure 2. Concentration dependencies of the line width ∆Hexpp (left axis, red stars) and electron spin-lattice relaxation time TSLe (blue circles, right axis) on Gd3+ concentration. Lines represent best single exponential growth (red dashed line) and decay (blue dotted line) fittings of experimental data.

Weak EPR signals with g ~ 4.3 and 2.5 in the general view spectra of non-grafted samples (marked by asterisks in Fig. 1(a)) are attributed to traces of paramagnetic iron, cooper and nickel impurities whereas broad and more intense patterns in EPR spectra of Gd-grafted samples (Figure 1(a,b)) are direct and unambiguous confirmation of the hypothesis on the formation of stable Gd3+ complexes chemically bound to the DND particle surface.19 Indeed, similar polycrystalline EPR patterns with characteristic effective g-factors 6.0, 2.8 and 2.0 have been observed in glassy or disordered host materials containing 4f 7 Gd3+ and Eu2+ ions with the free ground ion term 8S7/2.22,23 Thus, spectra in Figure 1(a) in their all characteristic features practically coincide with the experimental X-band spectra of soda-silica glass containing Gd3+ ions – see, for instance, Figure 1 in ref 23. Comprehensive ab initio computer simulation of polycrystalline EPR line shapes reported in ref 23 allows attributing such spectra to Gd3+ complexes characterized by a moderate distortion of a cubic, octahedral, or tetrahedral crystal fields. The polycrystalline patterns observed are result of a convolution of (i) a broad and essentially unimodal distribution of zero field splitting parameter D with a maximum in the range ACS Paragon Plus Environment

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0.051 ≤ D ≤ 0.056 cm-1, and (ii) a broad distribution of asymmetry parameter E/D with appreciable probability over the whole range 0.0 ≤ E/D ≤ 0.33. The prominent features g-factors 6.0 and 2.8 are identified with specific EPR transitions that are stationary with respect to D and

E. The results obtained indicate that Gd3+ complexes bound to the DND surface have no specific or narrowly defined site symmetries, with the result that the Gd3+ ions can coordinate themselves with a relatively large number of irregularly disposed ligands. Thus these ions select and "dictate" their own friendly environments on the DND surface fixing them between two neighboring host deprotonated carboxyl groups. Results presented in Figure 2 allow better understanding of mechanisms of the intense singlet line broadening caused by Gd3+ grafting of DND surface.19 Changes in both line width and TSLe are well described by single exponential growth and decay characterized by practically the same exponent. At the same time TSSe remains unchanged. It means that just shortening of electron spin-lattice relaxation time is responsible for the line broadening observed. In the most of cases dipole-dipole interaction (that is responsible for the growth of paramagnetic defects’ line width in DND with Gd3+-grafted cation19) causes inhomogeneous line broadening. However, when dipole-dipole spin interactions undergo some random intermittent modulation, the spins may be divided into two groups. The spins which are located sufficiently far from the isolated spins (for which the modulation is comparable or greater than the broadening produced by the spins in question) provide a contribution to the homogeneous broadening of the EPR lines. The broadening due to the remaining spins (located closer to the isolated spins) retains to some degree the character of inhomogeneous broadening in spite of the presence of spectral diffusion.24 In the case of spins in DND the modulation may be caused by exchange interaction between the spins of intrinsic defects and homogeneous broadening observed evidences that Gd3+ ions responsible for the defects’ line broadening are sufficiently far from the spins of defects. It is consistent with the estimated maximal distance of 0.4 nm from the DND surface reported in ref 19.


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3.2. 13C and 1H NMR 13

C NMR spectra of the samples under study are shown in Figure 3. They reveal a

gradual increase in the line width with increased concentration of Gd. The observed line broadening is evidently caused by interaction of 13C nuclear spins with paramagnetic gadolinium ions. This finding is supported by the correlation of the EPR and 13C NMR line broadenings on Gd concentration shown in Figure 4.

Normalized amplitude (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6

Gd-DND 13 C NMR

1.2 3.28 wt% Gd 0.8

1.64 wt% Gd 0.49 wt% Gd

0.4

0.049 wt% Gd 0 wt% Gd

0.0 300

200

100

0

-100

-200

-300

Chemical shift (ppm) Figure 3. RT 13C NMR spectra of Gd-grafted DND. Spectra are shifted vertically for better presentation.


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1.2 100

Gd-DND

Exp

80

13

∆H pp (mT)

1.1

∆H Exp pp

1.0

60

13

∆ν( C) 0.9

0.8

∆ν( C) (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

0

2x10

19

19

4x10

6x10

19

8x10

19

3+

Gd concentration (spin/g) exp Figure 4. Dependence of EPR line width ∆H pp (open blue circles) and 13C NMR line width ∆ν

(open black triangles) on Gd concentration. Dashed lines are guides for eyes.

The aforementioned EPR data on Gd ions bonding to the DND surface is well supported by our 13C NMR spin-lattice relaxation measurements. The matter is that if the paramagnetic 4fions are bound to the DND surface, an additional relaxation channel appears due to the interaction of carbon nuclear spins with 4f intra-shell electron spins of these ions. Spin-lattice relaxation rate R1 =

1 of a nuclear spin I that interacts with the unpaired electron spin S of a T1

paramagnetic defect is given by expression25,26 R1n (rik ) =

1 2 3τ e 7τ e 1 = γ S2γ I2h 2 S ( S + 1)[ + ]( 6 ) 2 2 2 2 T1n (rik ) 15 1 + ωI τ e (1 + ωe τ e ) d ik

(1)

Here γI and γS are the nuclear and electron gyromagnetic factors, ωI = 2πγIB0 = 5.38 × 108 Hz and

ωe = 1.41 × 1012 Hz are the 13C and electron Larmor angular frequencies in the applied magnetic field B0 = 8 T used in our experiment, respectively, d ik is the distance from the i-th nucleus to k-th paramagnetic center, and τe is the correlation time of the electron spin of paramagnetic center. Therefore the paramagnetic ions bound to the DND surface significantly accelerate


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nuclear spin-lattice relaxation. However, if magnetic inclusions are contained in a material as a separate phase, such effect will be negligible owing to the aforementioned inversed sixth-power dependence of R1n on d (eq 1). Our measurements of the 13C nuclear spin-lattice relaxation rate R1(13C) (Figure 5a) show that the magnetization recovery in all samples is well described by a stretched exponential function M(t) = M∞{1-exp[−(t/T1)α]}, where M∞ is the equilibrium magnetization and α is the stretched exponential parameter which in the case in question ranges as (0.58 ÷ 0.64) ± 0.03. The measurements show noticeable increase in R1(13C) with increased Gd concentration (Figure 5a). The only reason for the obtained R1’s increase is the aforementioned interaction of the

13

C

nuclear spins with the 4f electron spins of the paramagnetic Gd ions. Taking into account the proportionality of R1(13C) to the inversed sixth power of the distance between the nucleus and paramagnetic ion, such mechanism is effective only in the case that paramagnetic ions are attached to the nanodiamond surface rather than existing as a separate phase in the compound. It allows us to conclude that the paramagnetic gadolinium ions are anchored to the DND surface and presumably form Gd-DND chelate complexes contributing to the 13C spin-lattice relaxation.

160

12

-1

10

13

8 6 4

100

40

60

80

100 120

R 1e, µs

80

140

6

-1

Gd-DND

R1e

60

8

160

-1

13

R1e(µs )

120

10

R1( C), s

140

-1

12

R1( C) (s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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R1( C)

40 20

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2x10

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(a) 19

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8x10

3+



13


-1

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R1dif( C), s

-1

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80

0 0

60

20

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60 -180

R1edif, µs

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100 120

Gd-DND

40

-1

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8

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120

R1diff( C), s

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20

(b)

0 0

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8x10

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Gd concentration (spin/g) Figure 5. (a) Dependence of the experimental values of EPR (open black circles) and 13C NMR (open blue triangles) spin-lattice relaxation rates R1e and R1(13C) on Gd3+ concentration. (b) Dependence of the calculated EPR (open black circles) and

13

C NMR (open blue triangles)

spin-lattice relaxation rates R1ediff and R1diff(13C) caused by Gd ions only on Gd concentration. Insets show correlations between (a) R1e and R1(13C) and (b) R1ediff and R1diff(13C) in DND samples with different Gd3+ concentration. Dashed lines are guides for eyes.

We note that the theory25,26 of nuclear spin-lattice relaxation via paramagnetic impurities reveals linear dependence of the relaxation rate R1 on the paramagnetic ion concentration. Just such dependence was obtained by us in the compounds under study (Figure 5a). Herewith the electron spin-lattice relaxation time of the intrinsic paramagnetic defects in the diamond core and nuclear spin-lattice relaxation time of the core carbons are reduced by 4 and 3 times compared with that of the initial sample, respectively. In our opinion, such moderate reduction indicates that gadolinium ions are positioned at some distance from the diamond core and are attached to the oxygen atoms of the carboxyl groups rather than directly to the carbons of the diamond core. This finding makes realistic our above-mentioned model of substitution of hydrogen atoms of two or three neighboring carboxyl groups by Gd ions in the process of sample preparation. The ACS Paragon Plus Environment

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obtained reduction in the 13C spin relaxation time on Gd doping is twice larger that that obtained by us in Cu- and Co-grafted DND, which revealed only 1.6 times reduction in T1(13C),9-11 although it was expected to be much larger since the spin of Gd3+ is 7 times larger than that for Cu2+ ion and thus SGd(SGd+1)/ SCu(SCu+1) = 21. However, it is worth mentioning that the dependence of the spin-lattice relaxation rate R1(13C) on Gd concentration (Figure 5a) starts from -1 core = 262 ms) in the initial sample, in which the relaxation is R1core n = 3.82 s (corresponding to T1n

caused by interaction of 13C nuclear spins with the paramagnetic centers (mainly dangling bonds carrying unpaired electron spin) positioned inside the diamond core.27 This relaxation is fast itself. Actual contribution of the paramagnetic Gd to nuclear 13C relaxation is 1 1 1 = exp − core , Gd T1n T1n T1n where T1Gd n is the

13

(2)

C spin-lattice relaxation time caused by Gd ions, T1nexp is the experimental

value of T1 and T1core is relaxation time in the initial (non-grafted) DND sample, correspondingly. n Gd −1 The calculated R1Gd in the maximally grafted DND is 16 times larger than that in the n = (T1n )

minimally grafted one (Figure 5b). As mentioned above, nuclear spin-lattice relaxation rate R1 is proportional to the paramagnetic ion concentration. Suggesting that all Gd ions are positioned at the same distance from the DND surface, one can estimate the distance between the Gd ions and DND surface from the 13C spin-lattice relaxation data. For this purpose, let us discuss a model shown in


15


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Figure 6. Sketches of the positioning of a single Gd ion near the DND surface (a) and Gd+3 ions’ fixation on the functionalized (111) diamond surface (shown in grey) via a pair of neighboring deprotonated carboxyl groups. Adsorbed water molecules and interior hydroxyl groups provide the necessary remaining coordination of Gd+3 complex (b).

Figure 6(a), in which R is the radius of spherical DND particle, L is the distance between Gd ion and DND surface, r is the distance between a nuclear spin I and the center of the ball, d is the distance between nuclear spin I and Gd ion, and θ is the angle between the Gd-O and OI directions. To find the contribution of the paramagnetic ions to

13

C nuclear spin-lattice

relaxation, one should use eq 1 and calculate the integral

1 T1Gd n

= R1Gd n =

CN SGd V

R

π

2π

r 2 dr ∫0 d 6 ∫0 sin θdθ ∫0 dϕ

(3)

where C=

2 3τ e 7τ e (hγ I γ S ) 2 S ( S + 1)[ + ], 2 2 15 1 + ω I τ e (1 + ωe2τ e2 )


(4)

16

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V=

4πR 3 is the volume of the particle and N SGd is the number of Gd ions grafted to the surface 3

which appears here owing to the proportionality of the relaxation rate to the Gd concentration. Correlation time τe of the electron spin of gadolinium is around 2 × 10 −11 s.28,29 Therefore (ωeτe)2 >>1, and the second term in eq 4 containing the electronic Larmor precession frequency can be neglected compared to the first term. SQUID measurements reveal N SGd ~ 18 Gd3+ ions chemically bound to the surface of the 5 nm DND particle at maximal Gd concentration of 3.28 wt %.19 N SGd for the other samples may be calculated correspondingly. Using the law of cosines, eq 3 may be written as π

2π

1 CN SGd 2 sin θdθ CN SGd = R1Gd r dr d ϕ = n = 2 2 3 Gd ∫0 (r + ( R + L) − 2r ( R + L) cosθ ) ∫0 T1n V ∫0 [ L(2 R + L)]3 R

(5)

The calculation using the nuclear spin-lattice relaxation data obtained and eq 5 yields L = 3.2 Å. This result is in agreement with that estimated by the analysis of EPR line broadening on Gd grafting which shows that the distance between the Gd ion and the DND surface should not exceed 4 Å.19 A realistic sketch of Gd+3 ion positioning on the nanodiamond surface terminated by oxygen-containing groups is shown in Figure 6(b). The model proposed was constructed similar to the models of copper - DND binding studied by the density functional method,

30

which examine coordination of the metal cation by different surface groups. The approach allows us to consider binding of Gd ion with the surrounding atomic groups as a coordination one, and Gd-O bonds as partially ionic. The water molecules surrounding the gadolinium ion provide additional coordination of the ion. Preliminary results of our DFT calculations of this complex yield a lower limit of the Gd-DND distance as 2.74 Å, which corresponds well to the value found from the analysis of the NMR relaxation data. NMR line width is known to be proportional to the nuclear spin-spin relaxation rate. Such a correlation between the 13C line width and spin-spin relaxation rate R2(13C) was obtained in our experiments and shown in Figure 7.


17


1.6 100 13

C NMR Gd-DND

-1

R2 (ms )

1.4

1.2

80

R2 60

1.0

∆ν (ppm)

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∆ν 40

0.8 0

2x10

19

4x10

19

6x10

19

8x10

19

3+


Figure 7. Dependence of

13

C NMR line width ∆ν (open black circles) and

13

C spin-spin

relaxation rate R2 (filled blue circles) on Gd concentration. Dashed lines are guides for eyes.

1

H NMR spectra of DND samples usually reveal two components: a broad component

whose width is caused by dipole-dipole coupling of protons of the surface hydrocarbon and hydroxyl groups and a narrow component attributed to the adsorbed water molecules. However, our 1H NMR spectrum of the initial DND sample under study may be satisfactory described by a single Lorentzian line. This is evidently a consequence of the DND surface treatment before doping (see Section 2), which results in the noticeable increase in the number of the surface carboxyl groups. The protons of these groups undergo fast chemical exchange with the protons of adsorbed water molecules. Such an effect is characteristic of COOH groups due to their acidic dissociation (COO−–H+) and COOH–H2O⇄COO−–H3O+ process.31,32 As it was shown, formation of charged ND–COO−⇄H3O+ structures results in repulsion of DND particles and formation of a stable aqueous suspension of isolated DND.31 The aforementioned proton exchange yields an intense and relatively broad singlet line. The signal of the remaining surface hydrocarbon and hydroxyl groups is masked by this line. This masking effect becomes larger in


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Gd-grafted samples owing to the noticeable 1H line broadening caused by the interaction of proton spins with the 4f- intra shell electron spins of gadolinium – see Figures 8(a, b).

Figure 8. (a) Room temperature 1H NMR spectra of Gd-grafted DND. Spectra are shifted vertically for better presentation; dependences of the 1H NMR line width ∆ν (b) and 1H NMR spin-lattice relaxation rates (c) on Gd concentration. Dashed lines are guides for eyes.

Distinguishing between two components was however possible in the spin-lattice relaxation measurements, in which the nuclear magnetization recovery may be satisfactory described by two exponentials. The dependence of these two relaxation times on Gd concentration is shown in Figure 11. Here the faster component attributed to the mobile hydrogen atoms reveals 3.5 times increase in the relaxation rate on Gd doping, while the slower component assigned to the fixed surface hydrocarbon and hydroxyl groups shows 2.2 times increase on doping. We note that aforementioned proton dynamics also contributes to the 1H


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spin-lattice relaxation. Assuming this contribution to be the same for all samples under study, we obtain that paramagnetic Gd ions reduce relaxation time of the protons. Our findings open an opportunity for biomedical applications of the nanomaterials under study, e.g., for the MRI contrast enhancement, as suggested by Manus et al. for the aminefunctionalized Gd(III) contrast agents conjugated to the ND surface.7 In general, the more the ions incorporated onto the DND surface the better the result. In this end, the important issue is that this amount in the case in question does not exceed ~ 3.28 wt %. Our attempt to increase this amount yields salt sedimentation. The matter is that the metal inclusion is determined by the number of the neighboring surface carboxyl groups incorporating Gd3+ ions and by the binding energy of these groups. Thus the ion incorporation is apparently limited by the amount of the appropriate closely spaced surface COOH groups available. It should be noted that among hydrocarbon and hydroxyl groups that each nanodiamond’s surface possesses, the amount of carboxyl groups is not large. Comet et al.33 estimated this amount to be 0.85 sites per nm2, thus a DND particle with diameter of 5 nm reveals ~67 -COOH groups. It means that not more than 33 doubly charged ions (or less than 22 triply charged) may be irreversibly grafted to such a particle. Our magnetic measurements (SQUID) data19 reveal ~ 18 Gd3+ ions incorporated onto the surface of 5 nm DND particle at maximal nominal Gd concentration of ~ 3.28 wt %. Such surface density of 4f- metal cations results in a relatively high spatial density of the magnetic moments of the metal in the 5 nm particle.

Summary We grafted surface of DND particles by chemical attachment of gadolinium ions and studied the interaction of carbon atoms and paramagnetic defects in DND with the guest modifying paramagnetic ions by magnetic resonance techniques. Increase in Gd concentration causes: (i) appearance and strengthening of EPR patterns due to Gd3+ ions, (ii) broadening of the intense singlet EPR lines of intrinsic paramagnetic defects, (iii) acceleration of electron spinlattice relaxation rate of the defects’ spins, (iv) broadening of ACS Paragon Plus Environment

13

C and 1H NMR lines, and (v) 20

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acceleration of

13

C and 1H nuclear spin-lattice relaxation rate. The latter is caused by the

interaction of carbon nuclear spins of the diamond core with grafted Gd3+ ions. All these findings undoubtedly show that Gd3+ ions are chemically bound to the nanodiamond surface forming chelate paramagnetic Gd3+ complexes rather than exist as a separate phase, and shed light on the mechanism of the ions’ incorporation. The grafted Gd3+ ions located at quite short distances by the diamond surface interact with electron and nuclear spins of the diamond core and thus reduce spin-lattice relaxation time of carbon and neighboring hydrogen nuclei and that of electron spins of paramagnetic defects in the core. We believe that our study will result in future progress in fabrication of nanocrystals with functionalized surface and in improvement and optimization of the synthetic control over dopant incorporation.

Acknowledgements Contribution of the Ioffe Institute group was supported by the Russian Science Foundation (project No. 14-13-00795 “Synthesis of Optically Active Materials Based on Nanodiamonds Modified with Ions of 3d–4f Elements”).

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Magnetic Resonance Study of Gadolinium-Grafted Nanodiamonds

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