RESUMO
The lithiation mechanism of Ag2O with one and two Li atoms per unit cell carried out using density functional theory (DFT) studies indicate the adsorption of one Li atom at the tetrahedral interstitial site (TIS) as the most stable one with the formation of the Li-Ag2O compound. PDOS plots depict the hybridization of Li 2s with O 2p and Ag 5s states, resulting in the formation of both Li-O and Li-Ag bonds. Also, the Bader charge analysis validates the bonding of Li at TIS with both Ag and O atoms. The band structure plots showcase a surge in electronic conductivity, accounting for the metallic transition of Ag2O with the addition of Li. The difference in the charge distribution between the pairs of surface and subsurface silver atoms in charge density difference plots (CDDP) reveal the incorporation of Li, invoking charge inequivalence in addition to the symmetry inequivalence in the optimized structure. In the case of two Li atoms, Li placed on the top of oxygen (Otop) and at the subsurface of silver (Agss) was found to be the most stable structure with the formation of two strong Li-O bonds, indicating the feasibility of the conversion mechanism. The increase in the Bader charge of O (-1.28e) as well as CDDP establish the formation of Li2O by the conversion mechanism. Thus, the increase in the Li+ ion concentration becomes the deciding factor for the transition of the lithiation mechanism from Li-Ag2O compound formation to the conversion mechanism. The molecular dynamics (MD) simulations feature Li as an "antisite defect producing guest", leading to the formation of Li-O bonds with a concomitant reduction in the number of Ag-O bonds as a function of time. This confirms a shift in the reaction kinetics from the formation of Li-Ag2O to the conversion mechanism.
RESUMO
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.
