Investigation of Adsorption Behavior of Anticancer Drug on Zinc Oxide Nanoparticles: A Solid State NMR and Cyclic Voltammetry (CV) Analysis

The present study aims to comprehend the adsorption behavior of a set of anticancer drugs namely 5-fluorouracil (5-FU), doxorubicin and daunorubicin on ZnO nanoparticles (ZnO NPs) proposed as drug delivery systems employing solid state (ss) NMR, FTIR and Cyclic Voltammetry (CV) analysis. FTIR and 1H MAS ssNMR data recorded for bare ZnO nanoparticle confirmed the presence of adsorbed –OH groups on the surface. 13C CP-MAS NMR spectra recorded for free and ZnO surface adsorbed drug samples exhibited considerable line broadening and chemical shift changes that complemented our earlier report on UV-DRS and XRD data of surface adsorption in case of 5-FU. Moreover, a remarkable enhancement of 13C signal intensity in case of loaded 5-FU was observed. This clearly indicated rigid nature of the drug on the surface allowing efficient transfer of 1H polarization from the hetero nitrogen of 5-FU to ZnO to form surface hydroxyl (–OH) groups and the same has been observed in the quantum chemical calculations. To further analyze the motional dynamics of the surface adsorbed 5-FU, longitudinal relaxation times (T1) were quantified employing Torchia method that revealed significant enhancement of 13C relaxation rate of adsorbed 5-FU. The enhanced rate suggested an effective role of quadrupolar contribution from 67Zn to the 13C relaxation mechanism of ZnO_5-FU. The heterogeneous rate constant (khet), average free energy of activation (∆G≠) and point of zero charge (PZC) measured for free and drug loaded ZnO NPs samples using CV further support the SS-NMR results.