Amperometric Determination of Surfactants Using a Gold Electrode Modified with a Self-Assembled Monolayer of Stearylthiol

Abstract

Surfactants, which find many applications as emulsifiers and solubilizers, are also environmental pollutants. Considerable attention has thus been focused on their detection and! or determination.l Highperformance liquid chromatography (HPLC) has been recommended as an effective technique for this purpose. However, except for certain surfactants with characteristic UV absorption2 or fluorescence3, the detection of most surfactants by HPLC has been performed with refractive-index detectors4 whose sensitivity and selectivity are not always satisfactory. Although a postcolumn detector based on ion-pair extraction was shown to give reasonable results5, a simpler and more facile method seems to be worthy of investigation.We recently reported that the structure of a LangmuirBlodgett (LB) membrane cast on a glassy carbon plate is distorted by surfactants to change its permeability to Fe(CN)63-used as a marker ion.6 This finding suggested the possibility of electrochemical detection of surfactants by a solid electrode modified with a lipid membrane. LB membranes, however, are too fragile to be utilized in the flow system of an HPLC detector. On the other hand, alkanethiols form stable self-assembled monolayers (SAMs) on a gold surface.7'8 A novel electrochemical detection of surfactants could thus be attained if the permeability of an SAM to marker ions is affected by surfactants in a manner similar to that of an LB membrane. We discuss in this communication how the electrochemical response of an SAM-coated gold electrode towards marker ions, Fe(CN)63-and Fe3+, depends on the structure of the surfactants as well as their concentrations.The SAM-coated electrode was prepared as follows. A gold disk electrode (1.6 mm) polished with 0.05 µm alumina was treated by multiple triangular potential sweeps (-0.2 -+1.8 V vs. Ag/ AgCI, 10 V s-1) in 1 M H2S04 for 10 min. After being rinsed with distilled water, the electrode was immersed in an ethanolic solution of 10 mM stearylthiol (CH3(CH2)17SH) for 10 h. The thiol-coated electrode was then washed by keeping it in ethanol for 30 min followed by air-drying.The amperometric response of the SAM-coated electrode to the marker ions (5 mM) in aqueous KCl (0.1 M) was examined by measuring the steady-state reduction currents (I and 1°) due to the marker ions at 0.0 V vs. Ag/ AgCI, where I refers to the current in the presence and I° to that in the absence of the surfactants.The I° values were 10 and 50 nA for Fe(CN)63-and Fe3+, respectively. Figures 1 and 2 show the dependence of the current ratio (I/I°) on the concentration of surfactants with hydrophobic C-12 alkyl chain, CH3-

Electrografting: a powerful method for surface modification

“…Surprisingly, the reversible wave of ferricyanide was restored by addition of a surfactant such as trimethyldodecylammonium as was shown for long chain alkane thiols. 92,93 Electrografting of nitrobenzyl alcohol was also demonstrated by the observation of the reversible couple of the nitrobenzyl group after transferring the modified electrode to pure solvent. The water contact angle of carbon surfaces electrografted was measured with different alkanols.…”