Gradient and Superhydrophobic Surfaces in Microextraction Strategies

Gradient and superhydrophobic surfaces are quite fascinating and have been studied in many various fields. The noticeable property of gradient surfaces is the continuous change of chemical or physical properties over the substrate [1]. Until now, various gradient surfaces including the chemical and structural gradients, often with a cumulative nano– to micro–structures and high porosity have been introduced. The special feature of these surfaces is, changing in chemical composition from hydrophilic to hydrophobic which can be extended to the simultaneous and efficient entrapment of various compounds with different polarities. Over the past decades, many publications have been devoted to the fabrication of gradient surfaces, in which the electrochemical methods especially wireless functionalization or bipolar electrochemistry is quite considerable [2]. Bipolar electrochemistry deals with the exposure of an isolated conducting substrate that has no direct connection with a power supply except via an electric field (wireless technique). Briefly, in bipolar electrochemistry, the bipolar electrode acts as an anode and cathode that allows simultaneously oxidation and reduction reactions on the same substrate. A voltage is applied in the bipolar setup between the feeder electrodes that are spaced by a distance in a cell. Therefore, the applied potential Eappl drop linearly through the electrolytic solution and the interfacial difference of potential between the substrate and the solution become the driving force for the bipolar electrochemical reactions [3]. Recently, a novel gradient fiber coating of organoclay–Cu nanoclusters on a copper wire by bipolar electrochemistry is prepared and implemented in headspace in–tube microextraction of CBs from aqueous samples [4].The superhydrophobic materials with rough surfaces and low surface energy are also appropriate candidates for to the increased adsorption of non–polar analytes and holding any organic extractive solvent and influential in subsequent extraction enhancement. More recently, superhydrophobic materials have been successfully used as the extractive phase in needle trap microextraction [5,6] and µ-solid phase extraction [7]. Considering the characteristics of gradient and superhydrophobic surfaces, their synthesis, modifications and applications as extractive phases and/or probes for holding extractive solvents in various microextraction strategies are discussed.