Capillary Electrophoresis Analysis of Metal/Metalloid Oxide Nanoparticles in Water: Method Development for the Enhancement of UV Detection Sensitivity

Abstract

Increasing production and applications of metal/metalloid oxide nanoparticles (NPs) have greatly raised the demand for new analytical techniques capable for trace quantification in water to assess their environmental impacts and health risks. A new analytical method was developed for the sensitive detection of silica (SiO2), titania (TiO2) and zinc oxide (ZnO) as model metal/metalloid oxide NPs. This method was based on the formation of molecular layers and polymeric coatings on the NPs directly in water to selectively add chromophores to their surface for enhanced ultraviolet (UV) light absorption in capillary electrophoresis (CE) analysis. One unique advantage is the ability to identify nanoparticles by observing a stronger peak and/or a shifted migration time. Controlled polymerization of 2-hydroxypropyl methacrylate (HPMA) on SiO2 NPs added a coating of poly-2-hydroxypropyl methacrylate (PHPMA) that increased their UV detection sensitivity by 6±1 folds initially. A second coating with polydopamine produced a larger size of PHPMA-SiO2 NPs, as confirmed by dynamic light scattering (DLS) and transmission electron microscopy, further enhancing their UV detection sensitivity by 12±2 folds. Chitosan coating and HPMA binding on SiO2 NPs produced a significant enhancement of UV detection sensitivity by 50±1 folds. This method was selective for SiO2 in the presence of TiO2 NPs in 10 mM Na2HPO4, a background electrolyte used for CE analysis. Selective enhancement of UV detection sensitivity of TiO2 in the presence of alumina (Al2O3), SiO2, and ZnO NPs in 100 mM Tris was achieved using deoxyribonucleic acid (DNA) and polyethylene glycol (PEG). Single-stranded DNA (ssDNA) exhibited better performance than double-stranded DNA in enhancing the iv sensitivity of UV detection. PEG coating of ssDNA-TiO2 NPs further enhanced the UV detection sensitivity in CE analysis, by providing electrosteric stabilization, up to 13±3 folds for the determination of TiO2 NPs. A monolayer adsorption of dithiothreitol (DTT) and cysteine (Cys) onto ZnO NPs in 10 mM Na2HPO4 improved their detection sensitivity by 28±1 and 25±1 folds, respectively. The selectivity of DTT and Cys towards ZnO was validated in the presence of Al2O3, ceria (CeO2), SiO2 and TiO2 NPs as no changes were observed in the CE-UV peak area of either adsorbates or the NPs. Similar evidence was provided using DLS by determining the hydrodynamic diameters of NPs in the presence of adsorbates. Cys improved the colloidal stability of ZnO NPs by breaking down the aggregates, as evidenced by a reduction of their hydrodynamic diameter.The new approach provides a simple, rapid and efficient CE-based method towards the detection of SiO2, TiO2 and ZnO NPs selectively with enhanced sensitivity.The buffer composition was found to influence the selectivity of various molecular and polymeric coatings. Large enhancement factors have been obtained, improving the UV detection limits of NPs. This work demonstrates the feasibility of a new method for the direct detection of NPs in environmental...