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P Solid-State NMR Relaxation in the Zirconium Phosphate Network in the Presence of Paramagnetic Centers: A Detailed Relaxation Study in Static and Rotating Samples of Layered Zirconium Phosphate Materials

Vladimir I. Bakhmutov* and Abraham Clearfield Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States ABSTRACT: The careful study of MAS effects on the spin−lattice and spin−spin phosphorus NMR relaxation in two layered zirconium phosphate materials, α-ZrP and mPEG-PO3/Zr(IV)/DOX/ZrP, has clearly shown that the fast spin-diffusion and limited spindiffusion mechanisms are completely negligible in spite of the presence of paramagnetic centers detected in these materials by EPR. The 31P T1 relaxation is always nonexponential in static and spinning samples as well. The 31P T1 relaxation in α-ZrP follows a stretched exponential law to show the presence of a 31P T1 distribution. The 31P T1 time remarkably depends on spinning rates only in a 7 mm NMR probe head due to the frictional heating effects in 7 mm rotors but not due to the spin-diffusion mechanism. In accord, the 31P T1 and T2 times are independent of MAS rates in the experiments performed with 2.5 mm rotors. All the 31P T1 and T2 relaxation data support well the interpretation of the 31P T1 times in the zirconium phosphate network by dipolar proton−phosphorus interactions and justify the application of the 31P T1 and 31P T1/T2 approaches to probing molecular mobility in zirconium phosphate materials.

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INTRODUCTION Constant attention of researchers focused on the chemistry of layered zirconium phosphates1−8 is explained by their important industrial applications as catalysts, biosensors and fuel cells. Because of intercalation of inorganic/organic components7 and biomolecules,6 the interlayer distance of 7.6 Å in initial α-zirconium phosphate, Zr(HPO4)2·H2O, increases allowing uptake of various cationic complexes and pillaring agents. This approach leads to molecular systems interesting in the context of their applications for drug delivery in nanomedicine.4−6,9 One of the important aspects in the chemistry of zirconium phosphate materials is study of molecular dynamics in the zirconium phosphate network which can play a decisive role in heterogeneous catalysis, proton conductivity of the materials, and also in interactions between the phosphate network and intercalated compounds controlling their releasing. Recently,10 the 31P NMR spin−lattice relaxation times (31P T1) measured in a series of modified and intercalated zirconium phosphate materials structurally characterized by solid-state NMR in ref 11 have allowed us to recognize in these systems internal rotations around P−O bonds in P−OH groups. On the base of the developed 31P T1/31P T2 approach, the rotational correlation times have been determined at room temperature.10 In addition, internal rotations in HPO4− groups and also proton transfer between them have been quantitatively characterized in α-zirconium phosphate by variable-temperature measurements of 31P T1 and 1H T1ρ times to give finally motional activation energies.12 The phosphorus T1 relaxation data10,12 have been © XXXX American Chemical Society

quantitatively interpreted in terms of dipole−dipole protonphosphorus interactions operating in the zirconium phosphate network. At the same time, the variable-temperature measurements of proton relaxation times of α-zirconium phosphate in the rotating coordinate system, 1H T1ρ, have indicated a relaxation contribution via paramagnetic centers observed at lowest temperatures.12 The γ constant for protons is larger by the factor of 2.5 than that for phosphorus nuclei. In addition, the character of 31P relaxation treatments and good agreements between the theory and experiments10,12 did not show the presence of a significant paramagnetic contribution. Nevertheless, experimental evidence has not been reported. Potentially paramagnetic centers in samples can cause phosphorus relaxation by spin diffusion,13,14 which is not governed by motions in the phosphate network. The aim of the present work is investigation of this problem, solution of which is important not only for zirconium phosphate systems but also for other complex materials containing noncontrolled paramagnetic impurities. Finally the solution is also important in a pedagogical aspect. Here we study α-zirconium phosphate, Zr(HPO4)2·H2O, named as α-ZrP, and material m-PEG-PO3/ Zr(IV)/DOX/ZrP obtained with α-ZrP, treated by Zr(IV) ions and monomethoxy-poly(ethylene glycol)-monophosphate (mPEGPO3) and intercalated with doxorubicin hydrochloride (DOX). According to the solid-state NMR criteria established Received: February 14, 2017 Revised: March 16, 2017 Published: March 16, 2017 A

DOI: 10.1021/acs.jpcc.7b01466 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Stylized structure of the layered m-PEG-PO3/Zr(IV)/DOX/ZrP material where DOX molecules are located in the interlayer space, PEG is bonded to the surface and groups P−O−H experience an internal rotation.

description of the relaxation character, recycle (relaxation) delays and maximal τ values were adjusted to 400 s. In the context of relaxation measurements, a 7 mm NMR probe has been used not only to reduce the experimental time (signal-tonoise ratios) but also to check an influence of rf-coils on perfectness of pulses. A standard Hahn-echo pulse sequence was used to determine 31 P T2 times at variation in echo delays. The Hahn-echo MAS NMR experiments were synchronized with spinning rates. The experimental 31P relaxation curves represented as “signal intensity versus τ time” have been treated with a standard nonlinear fitting computer program based on the Levenberg− Marquardt algorithm. According to the statistics, the errors in T1 time determinations were less 15%. The latter was confirmed by independent reproductions of the relaxation experiments. Since the synchronization with spinning rates automatically dictates minimal echo delay times, the Hahn-echo experiments have been treated with exponentials providing relatively large errors (20−25%). EPR data were collected with a Bruker ELEXSYS II E 500 spectrometer operating at 9.75 GHz (X-band) and using the Xepr software. Since the aim of these EPR experiments was observation of paramagnetic species, they have been performed only at room temperature.

in ref 11, the structure of m-PEG-PO3/Zr(IV)/DOX/ZrP material can be represented as shown in Figure 1.

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EXPERIMENTAL SECTION Materials. The α-zirconium phosphate was synthesized following the reflux method reported by Sun and co-workers.15 The synthetic procedure has been fully described in ref.12 The m-PEG-PO3/Zr(IV)/DOX/ZrP material has been prepared by J. L. Colón and co-workers. Details of the synthesis and characterizations of the material by different methods will be soon presented as a separate work. Powders of the phosphates were dried and investigated by solid-state NMR. Instrumentations. The X-ray powder diffraction (XRPD) measurements were performed from 2 to 40° (in the 2θ axis) using a Bruker D8 Advance X-ray power diffractometer with Cu Kα radiation (λ = 1.5406 Å) with Bragg−Brentano assembly and operated at a potential of 40 kV and a current of 44 mA. Bragg’s law (nλ = 2d sin θ) was used to determine the interlayer distance in the zirconium phosphate layers taking into account the first order diffraction peak, where λ is the wavelength of the X-ray source, d is the interlaminar distance between planes in the unit cell, and θ is the diffraction angle. The XRPD patterns of α-ZrP and m-PEG-PO3/Zr(IV)/DOX/ZrP show peaks corresponding to a d-spacing of 7.6 and 20.3 Å, correspondently. The 31P{1H} static and MAS NMR spectra at different spinning rates were obtained with a Bruker Avance-400 spectrometer (162 MHz for 31P nuclei) equipped with standard 2.5 and 7 mm NMR probe heads. Standard 2.5 and 7 mm zirconium oxide rotors have been applied. The 31P chemical shifts are referred to external H3PO4. The 31P T1 relaxation times were measured by inversion− recovery (180°−τ−90°) experiments where radio frequency (rf) pulses were well calibrated and τ delays were widely varied in account for rough 31P T1 estimations. Relaxation (recycle) delays were adjusted to provide full nuclear relaxation in each cycle. The inversion−recovery method was used instead the saturation-recovery method to obtain a more detailed description of recovery curves and to recognize better a character of the phosphorus NMR relaxation. For the best

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RESULTS AND DISCUSSION As mentioned above, the general problem at quantitative interpretations of 31P relaxation times in solids is originated from paramagnetic centers that are often present in samples under investigation. For example, unidentified paramagnetic centers have been found in zirconium phosphates.16,17 Figure 2 shows the room-temperature EPR spectra of α-ZrP and mPEG-PO3/Zr(IV)/DOX/ZrP. Both spectra clearly identify the presence of paramagnetic species.10 The EPR spectrum of mPEG-PO3/Zr(IV)/DOX/ZrP material is particularly interesting. As seen, in addition to the broad resonance observed earlier in zirconium phosphates10 it shows the sharp line at the g factor of 2.00. This spectrum is surprisingly identical to that reported for DOX molecules treated with ions Fe3+, where the sharp line observed at the same factor belongs to a DOX B

DOI: 10.1021/acs.jpcc.7b01466 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

the diffusion-limited relaxation curves are nonexponential. In practice, such nonexponential curves can be often approximated to stretched exponentials, f(t) = exp(−t/T1)β with 0.5 < β < 1.22 As it follows from the theory of nuclear relaxation,21 spindiffusion coefficients D(ωR) in eq 2 decrease at mechanical rotations of samples in the magnetic field leading to T1(SD) times dependent on MAS rates ωR: 1/T1(SD) = (1/3)8πNPC1/4D3/4 (ωR )

(2)

Here Np is the paramagnetic center density and C is dipolar coupling. In addition, the diffusion-limited relaxation can be recognized by measurements of spin−spin relaxation times T2(SD) which grow at rotation of samples proportionally to νR2 or νR in eqs 3 and 4 for spinning rate νR measured in Hz and line widths Δν (in Hz) at νR/Δν ≥ 1 or νR/Δν ≫ 1, respectively. Figure 2. Single-scan EPR spectrum recorded for the m-PEG-PO3/ Zr(IV)/DOX/ZrP material at room temperature (the EPR spectrum of α-ZrP recorded with 16 scans is shown in the inset).

T2(SD) = 4/3νR 2/Δν 2τC

(3)

T2(SD) = 4/3νR /Δν

(4)

These theoretical considerations allow us conclude that the P NMR experiments performed in static and differently spinning samples of α-ZrP and m-PEG-PO3/Zr(IV)/DOX/ZrP are capable of discriminating the relaxation mechanisms. In fact, in contrast to spin diffusion, other relaxation mechanisms such as dipolar internuclear interactions or relaxation by chemical shift anisotropy lead to T1 times independent of spinning rates.14 The 31P{1H} static and MAS NMR spectra of m-PEG-PO3/ Zr(IV)/DOX/ZrP and α-ZrP materials (Figure 3 and 4, 31

radical.18 The DOX radical is evidently formed due to an electron transfer from paramagnetic ions in the zirconium phosphate to DOX molecules. Generation of such radicals is well-known at applications of doxorubicin as an anticancer drug.19 A quantitative comparison of the EPR spectra in Figure 2 is obviously problematic. Nevertheless the concentration of paramagnetic centers in m-PEG-PO3/Zr(IV)/DOX/ZrP material is remarkably higher because the EPR spectrum has been recorded with a single scan. In general, nuclear relaxation in rigid solids containing paramagnetic centers (paramagnetic metal ions or even O2) can occur via direct dipolar electron−nucleus interactions20 and/or via diffusion of nuclear spins toward paramagnetic centers due to spin-energy-conserving transitions, i.e., by mutual flops of dipolar-coupled nuclear spins.13,14 Equation 1 shows the dipolar electron−nucleus interactions written commonly for an electron spin of 1/2: 1/T1 = 0.1γI 2γS2ℏ2r −6{τe/(1 + (ωI − ωS)2 τe 2) + τe/(1 + (ωI 2τe 2) + τe/(1 + (ωI + ωS)2 τe 2)} (1)

Figure 3. 31P{1H} MAS NMR spectra of m-PEG-PO3/Zr(IV)/DOX/ ZrP material in a 2.5 mm probe head recorded in a static sample (bottom) and samples spinning at rates of 2 kHz (middle) and 5 kHz (top).

Here r is the electron−nucleus distance, τe is the electron relaxation time, and other constants are well-known.13 As follows from this expression, the electron−nucleus dipolar contribution decreases strongly at increasing the r distance by a factor of r−6. In addition, the T1 time depends on the electron relaxation times, τe. As a result, in such cases, the T1 times are not controlled by molecular motions and practically temperature-independent. Since the phosphorus relaxation in α-ZrP was temperature-dependent,12 the direct electron−nucleus interactions can be ruled out. Nuclear relaxation by spin diffusion, consisting of a transfer of a nuclear spin to a paramagnetic center to relax fast via the strong electron−nuclear dipolar interactions is governed by spin-diffusion coefficients D. Two different cases can be distinguished when nuclei relax by this mechanism: rapid spin diffusion and diffusion-limited relaxation.21 Spin−lattice relaxation by fast spin diffusion is always exponential, whereas

respectively) were the same like reported earlier.10−12 An α-ZrP sample spinning at 5 kHz exhibited a major resonance at −19 ppm corresponding to HPO4− groups in zirconium phosphate layers while two intense lines centered at −19 and −22 ppm (Figure 3) belong to different HPO4− groups in m-PEG-PO3/ Zr(IV)/DOX/ZrP.10,12 Presumably due to low concentrations of paramagnetic centers, the signals in the 31P{1H} MAS NMR spectra are relatively sharp and no paramagnetic shifts are observed. The phosphorus resonances strongly broaden in the static samples and for example, two lines of m-PEG-PO3/ Zr(IV)/DOX/ZrP material transform to a broad resonance with a maximal intensity at δ of −31 ppm (Figure 3). The C

DOI: 10.1021/acs.jpcc.7b01466 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

The results of fitting stretched exponential procedures are listed in Table 1. First, the β parameter changes only slightly Table 1. 31P T1 Relaxation Data for the Phosphorus Resonance at −19 ppm in a Sample of α-ZrP Collected in a 7 mm Probe Head at Different Spinning Rates νRa 31

P T1(β) (s) 13.4 14.4 23.0 27.8 14.0b 16.0b

Figure 4. Fully relaxed 31P{1H} MAS NMR spectrum of α-ZrP recorded in a 7 mm probe head at a spinning rate of 2.05 kHz (top) and the phosphorus partially relaxed spectrum obtained by the inversion−recovery experiment at τ delay of 8 s (bottom).

β

νR (kHz)

0.61 0.59 0.64 0.64 0.56 0.55

0 2.05 4.05 5.05 0 4.0

T1(β) and β are obtained by treatments of the experimental inversion−recovery curves with stretched exponentials. bObtained in a 2.5 mm probe head.12

a

shape of the static resonances is governed by phosphorus chemical shift anisotropy. The phosphorus spin−lattice relaxation curve shown in Figure 5 for the resonance at −19 ppm observed in a sample of α-ZrP spinning at 2 kHz is nonexponential.10,12 As seen, the exponential treatment of the curve actually describes the experiment very poorly. Since the same nonexponential character is observed in static and spinning samples of both materials, fast spin diffusion can be rule out. According to our calculations, the nonexponential relaxation curves obtained for static and spinning samples of α-ZrP are equally treated with stretched exponentials, f(t) = exp(−t/T1)β, and biexponentials (Figure 5) to show practically the same statistics and maximal deviations. In such situations, as it is generally accepted, the stretched exponential relaxation model is more preferable because this model contains less number of variable fitting parameters.

(0.59−0.64) and thus, the T1 times obtained at different spinning rates can be correctly compared. Second, as earlier,12 these β values are close to 0.67 typical of a Gaussian distribution23 for the spin−lattice relaxation times in powders. The T1 distribution is independently confirmed by different 31P T1 times measured for the isotropic resonance and two mostly intense sidebands24 in the 31P MAS spectrum of α-ZrP spinning at 2 kHz (Figure 4). As seen, the 31P T1 difference is not large. Nevertheless the different T1 times are clearly seen in the partially relaxed spectrum obtained at τ of 8.0 s. Thus, on one hand, our measurements justify the concept of the 31P T1 distribution used in the study of rotations in P−OH groups of α-ZrP material.12 On the other hand, the 31P T1/31P T2 approach10 and the variable-temperature 31P T1 times12 in zirconium phosphate materials have been applied for calculations of rotational correlation times by disregarding the

Figure 5. Experimental 31P inversion−recovery curve (○) obtained for the signal at −19 ppm for α-ZrP in 7 mm probe head at spinning 2.05 kHz treated with exponential (−•−), stretched exponential (--) and biexponential functions(solid). D

DOI: 10.1021/acs.jpcc.7b01466 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 6. Experimental inversion−recovery curve (○) “intensity versus delay time τ” obtained for the 31P resonance at δ of −31 ppm in a static sample of m-PEG-PO3/Zr(IV)/DOX/ZrP. The treatments of the curve with a stretched exponential and a biexponential are shown as (--) and solid line, respectively.

Table 2. 31P T1 and T2 Relaxation Times Obtained for PEG-PO3/Zr(IV)/DOX/ZrP Material in a 2.5 mm Probe Head at Different Spinning Rates νRa

a

T1(short)/fraction (s)

T1(long)/fraction (s)

T2 (ms)

νR (kHz)

Δν (kHz)

0.34/0.25 0.66/0.48 0.07/0.3 0.25/0.2 0.34/0.25 0.03/0.29 0.86/0.26

10.6/0.75 12.1/0.52 10.0/0.7 11.6/0.8 11.7/0.75 11.4/0.71 13/0.74

0.21 2.1 2.1 3.0 3.1 3.0 3.0

0 2.0 3.0 4.0 5.5 6.2 7.3

5.04 0.83 0.79 0.71 0.71 0.69 0.75

T1(short) and T1(long) times are obtained by treatments of the experimental curves with biexponentials, and Δν is the experimental line width.

The 31P inversion−recovery curves obtained for a sample of PEG-PO3/Zr(IV)/DOX/ZrP material, spinning at different rates have been treated with stretched exponentials and biexponentials. In contrast to α-ZrP, the 31P T1 relaxation observed in m-PEG-PO3/Zr(IV)/DOX/ZrP shows a clearly expressed biexponential character (Figure 6) in a full agreement with the data reported in ref 10. One can think that appearance of the 31P T1 component relaxing shortly is caused by a larger concentration of paramagnetic centers in m-PEG-PO3/Zr(IV)/ DOX/ZrP material (see EPR). However, here it should be emphasized that the T1(short) time and the fraction of this component in Table 2 are not stable parameters in calculations of the experiments at different spinning rates. In fact, the T1(short) time and its fraction change strongly from 0.03 to 0.86 s and from 0.20 to 0.48, respectively. According to the experiments carried out in ref 10, these changes are caused by imperfectness of rf-pulses which mostly affect the area of smallest τ delays. On these conditions, the short T 1 components and their fractions play a role of fitting parameters while the T 1 long components are meaningful. This

diffusion-limited relaxation. Since the nonexponential diffusionlimited relaxation curves can be described by a stretched exponential law at 0.5 < β < 1,22 the character relaxation, itself, is not a reliable criterion. Only spinning effects on the relaxation times can support or refute the above-mentioned assumption. The data in Table 1 show that the 31P T1 times in α-ZrP grow with the spinning rates. Theoretically,21 this unexpected result could correspond to the diffusion-limited phosphorus relaxation in this material. However, as it follows from the data, the T1 values in a static sample and a sample spinning at 2.05 kHz are practically equal. Only at spinning rates of 4.05 and 5.05 kHz, the difference between the static and spinning samples becomes significant. In addition, the spinning effect observed for α-ZrP placed into a 7 mm rotor (Table 1) reduces strongly, when a 2.5 mm rotor was applied: 31P T1(β) = 16 s at spinning rate of 4 kHz versus 14 s in static regime.12 Thus, the situation is not single-valued. To avoid this problem, PEGPO3/Zr(IV)/DOX/ZrP material has been placed into a 2.5 mm rotor for a detailed investigation. E

DOI: 10.1021/acs.jpcc.7b01466 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C approximation is well confirmed by the fact that the 31P T1(long) times measured in a spinning sample of m-PEG-PO3/ Zr(IV)/DOX/ZrP material are in an excellent agreement with those determined for similar zirconium phosphates in ref.10 Then the data in Table 2 show that the 31P T1(long) time practically does not depend on spinning rates: 10.6 s in a static sample and 13.0 s at a spinning rate of 7.3 kHz. This time characterizes phosphorus nuclei more remote from paramagnetic centers and thus no contribution by diffusion-limited relaxation is found. The 31P T2 times measured in m-PEG-PO3/Zr(IV)/DOX/ ZrP material at different spinning rates can be used as the second criterion for the presence of diffusion-limited relaxation (see eqs 3 and 4).21 As follows from the data obtained (Table 2), the 31P T2 time experiences a very small change (in account for errors of 20−25%) at the spinning rates between 2.0 and 4.0 kHz in parallel to the experimental 31P line width, while at the spinning rates between 4.0 and 7.3 kHz, the 31P T2 time does not change in general. Note that eqs 3 and 4 lose the physical sense at spinning rate of 0 kHz. Thus, again, in spite of paramagnetic centers, the diffusion-limited phosphorus relaxation is obviously negligible. Going back to the results in Table 1 observed in a 7 mm rotor versus the data obtained in a 2.5 mm rotor (Table 1, 2), one can conclude that these effects are definitely caused by the well-known frictional heating which increases the local temperature in spinning samples.25 The heating effect depends on the spinning rates and the rotor sizes26 to be maximal in large 7 mm rotors. The heating effect was not possible to measure quantitatively and directly in our case because α-ZrP material soaked in liquid methanol has shown the 1H MAS NMR spectrum, where the signal of the H-bonded alcohol protons, OH, is strongly shifted toward low field due to the presence of water molecules and acidic protons in POH groups of the material to experience a fast proton−proton exchange. At the same time, the internal chemical shift, 1H Δδ(CH3,OH), in the pure liquid methanol placed into in a 7 mm MAS rotor has been measured as 657 and 638 Hz at a spinning rate of 950 to 1430 Hz, respectively, instead 660 Hz in the static sample on our experimental conditions. This 1H Δδ(CH3,OH) decrease corresponds to increasing the temperature at least by 5−7° even for this nonmassive sample. Unfortunately a faster spinning was impossible. Finally, the negligibility of a spin diffusion contribution to phosphorus relaxation in zirconium phosphate materials versus magnesium phosphide Mg3P221 can be explained by the absence of protons in this compound and also by shorter P··· P distances (2.2−2.3 Å) in Mg 3P 2 increasing dipolar phosphorus−phosphorus coupling. These factors in a combination with the absence of high-frequency motions in Mg3P2 open the way for the diffusion-limited relaxation mechanism.

It has been shown that the 31P T1 and T2 relaxation times do not depend on spinning rates in the experiments on 2.5 mmrotors. Thus, the limited spin-diffusion mechanism is completely negligible in zirconium phosphate materials in spite of the presence of paramagnetic centers. The 31P T1 spinning effects observed in the experiments on 7 mm rotors are explained by the frictional heating which changes the local temperature in spinning samples. The data support well the interpretation of the 31P T1 times in the zirconium phosphate network10,12 in terms of dipolar proton-phosphorus interactions and justify the application of 31 P T1 and 31P T1/T2 approaches to characterizations of molecular mobility in zirconium phosphate materials.

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AUTHOR INFORMATION

Corresponding Author

*(V.I.B.) E-mail: [email protected]. ORCID

Vladimir I. Bakhmutov: 0000-0002-5011-0385 Abraham Clearfield: 0000-0001-8318-8122 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Robert A. Welch Foundation, Grant A-0673 (Metal Phosphonates as Crystal Engineered Solids and Platforms for Drug Delivery), and the Nuclear Energy University Program (DOE) Grant Award No. DENE0000746 (Mixed Metal Phosphonate−Phosphate Resins for Separation of Lanthanides from Actinides), for which grateful acknowledgement is made. We would like to acknowledge Drs. J. A. Sheikh and J. L. Colón who prepared the m-PEG-PO3/ Zr(IV)/DOX/ZrP material for this study.

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REFERENCES

(1) Kumar, C. V.; Bhambhani, A.; Hnatiuk, N. Layered α-zirconium phosphates and phosphonates. In Handbook Layered materials; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker, Inc.: New York, 2004; pp 313−367. (2) Ferragina, C.; Di Rocco, R.; Giannoccaro, P.; Patrono, P.; Petrilli, L. Intercalation of tris-(2,2′-bipyridyl) ruthenium (II) in α- and γzirconium phosphate: synthesis, thermal behaviour and X-ray characterization. J. Inclusion Phenom. Mol. Recognit. Chem. 2009, 63, 1−9. (3) Santiago, M. B.; Velez, M. M.; Borrero, S.; Díaz, A.; Casillas, C.; Hofmann, C.; Guadalupe, A. R.; Coloń, J. L. NADH Electrooxidation using bis(1,10-phenanthroline-5,6-dione) (2,2′-bipyridine)ruthenium(II)-exchanged zirconium phosphate modified carbon paste electrodes. Electroanalysis 2006, 18, 559−572. (4) Diaz, A.; Saxena, V.; Gonzalez, J.; David, A.; Casanas, B.; Carpenter, C.; Batteas, J. D.; Colon, J. L.; Clearfield, A.; Delwar Hussain, M. D. Zirconium phosphate nano-platelets: a novel platform for drug delivery in cancer therapy. Chem. Commun. 2012, 48, 1754− 1756. (5) Díaz, A.; González, M. L.; Pérez, R.; David, A.; Mukherjee, A.; Báez, A.; Clearfield, A.; Colón, J. L. Direct intercalation of cisplatin into zirconium phosphate nanoplatelets for potential cancer nanotherapy. Nanoscale 2013, 5, 11456−11463. (6) Díaz, A.; David, A.; Pérez, R.; González, M. L.; Báez, A.; Wark, S. E.; Zhang, P.; Clearfield, A.; Colón, J. L. Nanoencapsulation of insulin into zirconium phosphate for oral delivery applications. Biomacromolecules 2010, 11, 2465−2470.

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CONCLUSION The detailed study of phosphorus NMR relaxation has been carried out for two zirconium phosphate materials: α-ZrP and m-PEG-PO3/Zr(IV)/DOX/ZrP. According to the roomtemperature EPR spectra, the materials contain paramagnetic centers. The EPR spectrum of m-PEG-PO3/Zr(IV)/DOX/ZrP shows the presence of a DOX radical. The 31P T1 relaxation is always nonexponential in static and spinning samples of both materials and treated well with stretched exponentials and biexponentials for α-ZrP and mPEG-PO3/Zr(IV)/DOX/ZrP, respectively. F

DOI: 10.1021/acs.jpcc.7b01466 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b01466 J. Phys. Chem. C XXXX, XXX, XXX−XXX