ZnO nanocrystals derived from organometallic approach: Delineating the role of organic ligand shell on physicochemical properties and nano-specific toxicity

Abstract

The surface organic ligands have profound effect on modulation of different physicochemical parameters as well as toxicological profile of semiconductor nanocrystals (NCs). Zinc oxide (ZnO) is one of the most versatile semiconductor material with multifarious potential applications and systematic approach to in-depth understand the interplay between ZnO NCs surface chemistry along with physicochemical properties and their nano-specific toxicity is indispensable for development of ZnO NCs-based devices and biomedical applications. To this end, we have used recently developed the one-pot self-supporting organometallic (OSSOM) approach as a model platform to synthesize a series of ZnO NCs coated with three different alkoxyacetate ligands with varying the ether tail length which simultaneously act as miniPEG prototypes. The ligand coating influence on ZnO NCs physicochemical properties including the inorganic core size, the hydrodynamic diameter, surface charge, photoluminescence (quantum yield and decay time) and ZnO NCs biological activity toward lung cells was thoroughly investigated. The resulting ZnO NCs with average core diameter of 4-5 nm and the hydrodynamic diameter of 8-13 nm exhibit high photoluminescence quantum yield reaching 33% and a dramatic slowing down of charge recombination up to 2.4 µs, which is virtually unaffected by the ligand's character. Nano-specific ZnO NCs-induced cytotoxicity was tested using MTT assay with normal (MRC-5) and cancer (A549) human lung cell lines. Noticeably, no negative effect has been observed up to the NCs concentration of 10 µg/mL and essentially very low negative toxicological impact could be noticed at higher concentrations. In the latter case, the MTT data analysis indicate that there is a subtle interconnection between inorganic core-organic shell dimensions and toxicological profile of ZnO NCs (strikingly, the NCs coated by the carboxylate bearing a medium ether chain length exhibit the lowest toxicity level). The results demonstrate that, when fully optimized, our organometallic self-supporting approach can be a highly promising method to obtain high-quality and bio-stable ligand-coated ZnO NCs.

Figure 1

Schematic representation of nanocrystal-ligand interface structure of ZnO NCs prepared by (a) different physical and chemical methods, (b) the classical sol-gel process and (c) the OSSOM approach.

Figure 2

Schematic representation of the synthesis of ligand-coated ZnO NCs from [EtZn(AAA)]-type organometallic precursors and the effect of the ligand’s character on its surface binding mode and ZnO NCs stability.

Figure 3

Representative STEM and HRTEM images of (a–c) ZnO-MAA, (d–f) ZnO-MEAA, (g–i) ZnO-MEEAA NCs.

Figure 4

MTT cell viability evaluation and observation of the morphological changes of A549 (cancer) and MRC-5 (normal) cells after 24 h of incubation with NCs: (a) ZnO-MAA, (b) ZnO-MEAA, (c) ZnO-MEEAA. Experimental data were expressed as the mean of cell viability ± standard deviation (SD) of at least four individual experiments with six replicate wells. Asterisks denote statistical significance at p < 0.05.

Figure 5

The effect of ZnO NCs on intracellular ROS production. The A549 and MRC-5 cells were treated with 5, 15 and 25 µg/mL ZnO NCs for 24 h prior to the ROS determination including addition of DCFH-DA for 30 min followed by fluorescence measurement. The values are represented as mean ± S.D. of three individual experiments.

Figure 6

The apoptosis rate in A549 and MRC-5 cells treated with ZnO-MAA (b), ZnO-MEEAA (c) and untreated with ZnO-AAA NCs cells (d) detected using flow cytometry. The percentage of early and late apoptotic cells is presented in graph (a). Flow charts: lower right quadrant, Annexin V positive and PI negative cells indicates early apoptotic cells; upper right quadrant, Annexin V and PI-positive cells represents necrotic or late apoptotic cells.