Membranes with the ability of molecular/ionic separation offer potential in many processes ranging from molecular purification/sensing, to nanofluidics and to mimicking biological membranes. In this work, we report the preparation of a perforative free-standing ultrathin silica membrane consisting of straight and parallel nanochannels with a uniform size (∼2.3 nm) for precise and fast molecular separation. Due to its small and uniform channel size, the membrane exhibits a precise selectivity toward molecules based on size and charge, which can be tuned by ionic strength, pH or surface modification. Furthermore, the ultrasmall thickness (10-120 nm), vertically aligned channels, and high porosity (4.0 × 10(12) pores cm(-2)) give rise to a significantly high molecular transport rate. In addition, the membrane also displays excellent stability and can be consecutively reused for a month after washing or calcination. More importantly, the membrane fabrication is convenient, inexpensive, and does not rely on sophisticated facilities or conditions, providing potential applications in both separation science and micro/nanofluidic chip technologies.

The rapid and sensitive detection of nitroaromatic compounds is of great significance for human health, the environment, and public security. The present work reports on the extraction and electrochemical analysis of trace nitroaromatic compounds, such as explosives and organophosphate pesticides (OPs), using the indium tin oxide (ITO) electrodes modified with a highly ordered and aligned binary assembly of silica mesochannels and micelles (BASMM). With a pore diameter of ca. 2-3 nm, silica mesochannels (SMs) perpendicularly oriented to the ITO electrode surface can provide hard and robust supports to confine the soft cylindrical micelles formed by the aggregation of cationic surfactants, namely, cetyltrimethylammonium bromide (CTAB). Due to the organized self-assembly of hydrocarbon tails of CTAB surfactants, each micelle has a hydrophobic core, which acts as an excellent adsorbent for rapid extraction and preconcentration of trace nitroaromatic compounds from aqueous solutions via the hydrophobic effect. Furthermore, the cylindrical micelles are directly in contact with the underlying electrode surface, to which extracted compounds can freely diffuse and then be reduced therein, thus allowing their determination by means of voltammetry. Using the BASMM/ITO sensor, electrochemical analysis of trace nitroaromatic explosives, including 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol, 2,6-dinitrotoluene, 3-nitrophenol, and nitrobenzene, and OPs, such as paraoxon, methyl parathion, and fenitrothion, was achieved with a fast response, wide linear range, high sensitivity, and low detection limit at the ppb level. TNT and paraoxon in real apple, tea, and water samples were also determined. By combining the heterogeneous extraction and determination in one ordered binary nanostructure, the BASMM sensor provides a very simple, rapid, and cost-effective way for analysis of nitroaromatic compounds and can be extended to a wide range of lipophilic yet redox-active analytes.

A thin film consisting of highly ordered and vertically oriented silica mesochannels (SMCs) was prepared on the indium tin oxide (ITO) coated glass electrode surface by chronopotentiometry. The mesochannel has a uniform pore size of 2-3 nm in diameter and a positively charged surface due to grafted ammonium groups. The electrostatic and steric effects resulted from control of the surface charge and the ionic buffer concentration make the SMCs permselective, favoring the mass transport of oppositely charged species and repelling that of similarly charged ones. By using differential pulse voltammetry (DPV), the SMCs with this charge selectivity can be employed for permselective detection of ascorbic acid (AA) and dopamine (DA) that are oppositely charged compounds. The obtained linear detection range was 49-2651 μM for AA and 20-226 μM for DA, respectively. AA and DA in real samples were also determined by the SMC film modified electrode.

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