Zinc oxide nanorods for the solid phase microextraction of volatile aldehydes

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Zinc oxide (ZnO) nanoparticles have been successfully employed in different application fields due to their inherent characteristics such as wide band-gap or thermal stability. As with other nanomaterials, zinc oxide may exist in many structural forms depending on the synthetic process. In this sense, ZnO nanorods can be easily obtained in the laboratory by a hydrothermal treatment using zinc nitrate and hexamethylenetetramine as precursors. The concentrations of the precursors, as well as the temperature and incubation time, play a key role in the final physical characteristics of the product as it is indicated in Figure 1. Figure 1A shows a scanning electron micrograph of the ZnO nanoparticles before the hydrothermal reaction while Figures 1B-C illustrate the final product using different starting equimolar concentrations of the precursors (0.1 mM, 1mM and 10 mM, respectively). The average width and length of the nanorods increase with the initial concentration of precursors. However, the total effective surface area increases with the concentration from 0.1 to 1 mM, decreasing for higher concentrations since the nanorods tend to stack up (1).

Figure 1. Effect of the precursors´concentration on the structure of the ZnO nanorods. For details, see text. This image is reproduced from reference 1.
 ZnO nanorods are promising sorptive materials in solid phase microextraction (SPME) attending to their high surface-to-volume ratio, their thermal and mechanical stability and their adsorption capacity towards volatile compounds. In fact, they have already been proposed as sorptive phases coated to an inert silica support (2). However, this type of support presents a low mechanical stability and it has increasingly been replaced by a stainless steel, at least in lab-made fibers. In a recent article, the use of stainless steel wires coated with ZnO nanorods has been proposed for the extraction of aldehydes from oily food (3). The ZnO nanorods are synthesized following the previously described hydrothermal reaction under optimal conditions. In this sense, the precursors´ concentrations are a critical, affecting directly to the extraction capabilities of the final product. The optimal nanorod array presents a diameter range of 300-500 µm and a thickness coating of ca. 3-5 µm. This thin coating allows the rapid extraction of the analytes (10 min) but reduces the overall extraction capacity of the fiber. However, similar extraction performance is observed for the proposed coating when it is compared with commercial CAR/PDMS fibers.

In the article, the readers will find the synthetic procedure, the modification of the SPME manifold to avoid the shaving of the coating as well as the characterization and performance of the new coating.

References:

(1) Enhanced visible light photocatalysis through fast crystallization of zinc oxide nanorods. Link to the article
(2) ZnO nanorod array solid phase micro-extraction fiber coating: fabrication and extraction capability. Link to the article
(3) ZnO nanorod coating for solid phase microextraction and its applications for the analysis of aldehydes in instant noodle samples. Link to the article

Special acknowledgements to: Beilstein Journal of Nanotechnology. This open access journal permits unrestricted use, distribution, and reproduction in any medium, of its material. Figure 1 has been reproduced from one article which is also referenced in this post (1).

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