Insights into nanoparticle surface bonding and coating architecture via multinuclear NMR

Abstract

Tantalum oxide nanoparticles (TaO_x NPs) are promising as high-Z-contrast agents for computed tomography (CT) due to their profound imaging benefits relative to those of clinical iodinated contrast media (ICM) at the X-ray tube voltages ≥100 kVp required for most patients. Furthermore, TaO_x NPs have prevailed through extensive non-GLP and GLP (good laboratory practice) preclinical development, including in vivo/vitro safety testing and imaging efficacy studies. This is due in part to innovative structural engineering of the NPs' core size and coating, which has been shown to provide favorable pharmacokinetics and promote rapid renal clearance, with negligible organ retention. In this study, a carboxybetaine zwitterionic siloxane polymer (CZ) coating for a lead candidate TaO_x NP is thoroughly characterized using multinuclear/multidimensional nuclear magnetic resonance (NMR) spectroscopic techniques. ¹H and ¹H/¹³C heteronuclear multiple bond correlation NMR spectroscopies are used to confirm the CZ coating's structure, and in combination with ²⁹Si NMR, the architecture of the siloxane coating bound to the TaO_x NPs' surface is described. Of particular significance, ²⁹Si NMR spectra were used to identify the T-region bonding modes of the CZ coating and show the superiority of diafiltration over dialysis for purification of the TaO_x NPs. Through a spectral comparison, a cyclic siloxane impurity in the TaO_x NP product purified through dialysis was found to be absent in the product purified through diafiltration. Finally, the ¹H Carr-Purcell-Meiboom-Gill (CPMG) NMR pulse sequence was used in a novel manner to probe the distance-dependent interactions between the ¹H spins of the CZ coating and the TaO_x NPs' surface.

Keywords: 29Si NMR; X-ray computed tomography; diagnostic imaging agent; nanoparticle; siloxane coating; tantalum oxide.

Figure 1 •

The ¹H NMR spectrum of the 3 nm CZ TaO_x NPs in D₂O. Letters a–e in the inset NP graphic denote ¹H positions along the CZ backbone and correspond with peaks a–e in the NMR spectrum.

Figure 2 •

The HMBC spectrum for the CZ TaO_x NPs in D₂O showing ¹H (F1 axis) and ¹³C (F2 axis) 2-to-3 bond correlations. The red-letter labels a–e for the NP graphic at the top of the figure denote ¹H positions along the CZ backbone and correspond with peaks a–e of the F1 axis. The red-letter labels A–D for the second NP graphic within the figure denote ¹³C positions along the CZ backbone and correspond to peaks A–D. Black letter pairings on the plot label peaks associated with ¹H/¹³C interactions, e.g., label Ae in the bottom left-hand corner identifies the peak that arises from the correlation between ¹H e and ¹³C A. Due to limitations of the low-pass J-filter, single-bond correlations, Fa, Ce, and D’d”, are observed [71].

Figure 3 •

(a) Silicon T-structures with the T descriptor referring to the central Si atom featuring a fully drawn zwitterionic group. (b) Deconvoluted and curve-fitted ²⁹Si NMR spectrum for dialysis-purified CZ TaO_x NPs. (c) Deconvoluted and curve-fitted ²⁹Si NMR spectrum for CZ TaO_x NPs purified through diafiltration.

Figure 4 •

(a) Overlay of ¹H NMR and ¹H CPMG NMR spectra (in D₂O); the inset highlights the CPMG-facilitated decrease in peak broadening (arrow). (b) Black circular data points: the ΔFWHM between the ¹H NMR and ¹H CPMG NMR spectra for resonances a–e (black points) vs. the peak assignment (line-fit, R² = 0.9924). Orange triangular data points: natural log (ln) of the ΔFWHM for peaks a–e plotted vs. the peak identity (R² = 0.9763).