Control of the surface charges of Au-Ag nanorods: Selective detection of iron in the presence of poly(sodium 4-styrenesulfonate)

In this article, we report a simple approach for selectively sensing Fe²⁺ ions using CTAB-stabilized Au-Ag nanorods (CTAB-Au-Ag NRs) in the presence of poly(sodium 4-styrenesulfonate) (PSS). The prepared CTAB-Au-Ag NRs exhibit an intense longitudinal surface plasmon resonance absorption (>10⁹ M^-1 cm^-1 at 827 nm) in the near-infrared region. As a result of attractive electrostatic interactions between PSS and CTAB, agglomeration of the CTAB-Au-Ag NRs induces a change in the absorption at 827 nm. From £potential measurements, we found that the degree of agglomeration was highly dependent on the surface charge density of the CTAB-Au-Ag NRs. Because Fe²⁺ (Fe³⁺) ions selectively interact with PSS, the degree of agglomeration-and, thus, the change in absorption at 827 nm-is dependent on the concentration of Fe²⁺ (Fe³⁺) ions. To improve the selectivity of the present sensing system, Fe³⁺ ions were reduced to Fe²⁺ ions in the presence of ascorbic acid prior to analysis. The concentrations of CTAB-Au-Ag NRs and PSS are both important parameters in determining the sensitivity and selectivity of the present approach toward sensing Fe²⁺ ions. Under the optimum conditions [34 pM CTAB-Au-Ag NRs, (5 × 10^-6)% PSS, pH 7.2], the limit of detection for Fe²⁺ ions at a signal-to-noise ratio of 3 was 1.0μM. We applied this nanosensor system to the determination of Fe²⁺ in ferritin and in aqueous environmental samples; this approach has the advantages of simplicity, accuracy, and precision (the relative standard deviation from five runs with each sample was below 3%).