ORIGINAL PAPER Semi-quantitative determination of cationic surfactants in aqueous solutions using gold nanoparticles as reporter probes Chi-Lap Kuong & Wei-Yu Chen & Yu-Chie Chen Received: 24 August 2006 / Revised: 21 November 2006 / Accepted: 23 November 2006 / Published online: 23 January 2007 # Springer-Verlag 2007 Abstract Concentrations of cationic surfactants in aqueous solutions have been estimated on the basis of changes in the color of gold nanoparticles, used as reporter probes. We have shown that the colors of gold nanoparticles with anionic protective groups on their surfaces shift from red to indigo/purple and then back to red in a range of cationic surfactant solutions in which concentrations vary from very low to above the theoretical CMCs. The color changes occur near the theoretical CMCs, presumably because the presence of surfactant micelles in the solution prevents the gold nanoparticles from aggregating. We have used gold nanoparticles as reporter probes to determine the concen- trations of cationic surfactants in products such as hair conditioners, which often contain large amounts of alkyl- trimethylammonium halides. Although this approach can only provide an estimate, it can be performed simply by addition of a given amount of gold nanoparticles to a series of diluted solutions, without the need for instruments or labor-intensive procedures. Keywords Gold nanoparticles . Cationic surfactants . Iron oxide particles . Chemical sensor . SALDI MS Introduction Gold nanoparticles have been widely used as sensing probes for different types of analyte, including ions [1–5], gases [6, 7], and biomolecules [8–13]. They can be chemically modified to enable recognition of specific targets. Recognition may result in aggregation of gold nanoparticles and a consequent color change of the solution, usually taken as evidence of the presence of the target species. Excess ions present in a gold nanoparticle suspension can also cause aggregation and fusion of gold nanoparticles. A red shift of the wavelength of the maximum absorption band of the suspension is commonly observed. Conventionally, the critical micelle concentration (CMC) of surfactants is determined by light scattering [14], surface tension [14], and spectroscopy [15, 16]. When spectroscopy is used, organic dyes are usually added to the surfactant solution to indicate the CMC point on the basis of the abrupt change of the absorption spectra of the dyes [15, 16]. Because gold nanoparticles have visible colors, they can potentially be used as reporter probes for determination of the CMCs of surfactants. Gold nanoparticles with anionic protection groups on their surfaces tend to aggregate with a color shift from red to indigo or purple if excess cations, for example cationic surfactants, are present in solution. The presence of cations in the solution results in uneven charge distribution on their surfaces, which may cause further aggregation as a result of electrostatic interaction. We suspect that the presence of surfactant micelles in the solution might prevent aggregation of the nanoparticles. Thus, as the concentration of surfactants approaches their CMC in a gold nanoparticle suspension, the color might remain red. It is of interest whether or not the color of a surfactant solution containing gold nanoparticles will remain Anal Bioanal Chem (2007) 387:2091–2099 DOI 10.1007/s00216-006-1046-5 C.-L. Kuong : W.-Y. Chen : Y.-C. Chen (*) Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan e-mail: yuchie@mail.nctu.edu.tw Y.-C. Chen Institute of Molecular Science, National Chiao Tung University, Hsinchu 300, Taiwan