Preparation and Characterization of Test Strips Composed of Anionic Indicator Dyes and Basic Nano-adsorbents

Abstract

In recent years, we have produced next-generation highly sensitive test papers named ‘dye nanoparticle-coated test strips (DNTSs)’ for harmful ions at ppb level in aqueous samples. The preparative method based on re-precipitation method is simple and feasible, but applicable indicator dyes are limited to be hydrophobic one. In order to fabricate DNTSs with hydrophilic charged indicators, we here report a novel method to prepare DNTSs loaded nano-composites of anionic dyes and ion exchange nano-adsorbents. We tested four popular anionic reagents (tetra-sulfonated TPPS; tri-sulfonated HNB; di-sulfonated Bathophen-S; mono-sulfonated Lumogallion) and two ion exchange nano-adsorbents [weakly basic alumina fiber, AS3 (10 nm×100 nm) and strongly basic trimethyl-modified latex, latex-N(CH3)3+ (100 nm in diameter)]. The preparative method of nano-composite coated DNTSs is also simple; initially an anionic indicator and a nano-adsorbent are mixed in pH-controlled water, and then the dispersion is filtered with a membrane filter having a 0.1 μm pore size under suction. The retained reagent percentage on the membrane strongly depends on ion exchange nano-adsorbent, the charge of an anionic dye, and the reagent concentration, because the nano-composite formation is based on electrostatic interaction. Especially, by using latex-N(CH3)3+, almost 100% coating of anionic dyes was successful in the pH range of 2 – 10. Particle size analysis and zeta potential measurements indicated that nano-composite layers are made in two ways: one is surface filtration of the reagent-adsorbed ion-exchanger; the other is filtration of aggregates of reagent-adsorbed ion-exchangers. The thickness of the nano-composites layer depended on the surface concentration of nano-adsorbent as well as the zeta potential of the nano-composite. The thicknesses of nano-composite layers were 4.55 μm for TPPS/latex-N(CH3)3+ and 3.34 μm for TPPS/AS3 when the surface concentrations of nano-adsorbents were adjusted to be 0.26 mg cm−2 and 0.32 mg cm−2, respectively. Highly negatively charged dyes, like TPPS and HNB, are applicable to the detection of metal ion by filtration enrichment of a sample solution containing high concentration of NaCl; however, mono-anionic or di-anionic dyes, such as Lumogallion and Bathophen-S, are inapplicable due to the leakage of dye or signaling complexes with target ions.