# Insights into nanoparticle surface bonding and coating architecture via multinuclear NMR

## TL;DR

This study uses NMR techniques to analyze the structure and bonding of a polymer coating on tantalum oxide nanoparticles used for CT imaging.

## Contribution

The study introduces a novel use of 1H CPMG NMR to investigate interactions between the polymer coating and nanoparticle surface.

## Key findings

- Multinuclear NMR confirmed the structure of the carboxybetaine zwitterionic siloxane polymer coating.
- 29Si NMR revealed the T-region bonding modes and showed diafiltration is better than dialysis for purification.
- CPMG NMR provided insights into distance-dependent interactions between the coating and nanoparticle surface.

## Abstract

Tantalum oxide nanoparticles (TaOx 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, TaOx 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 TaOx 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 TaOx 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 TaOx NPs. Through a spectral comparison, a cyclic siloxane impurity in the TaOx 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 TaOx NPs’ surface.