Liu Xiaoyang, Abbott Nicholas L
Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin 53706-1691, USA.
Anal Chem. 2009 Jan 15;81(2):772-81. doi: 10.1021/ac801933v.
We report the generation and manipulation of spatial gradients in surfactant and micelle concentration across microfluidic channels by combining use of a redox-active surfactant with electrochemical methods. The approach is founded on the observation that 11-ferrocenylundecyltrimethylammonium bromide (FTMA) behaves as a surfactant in aqueous solution (e.g., self-assembles to form micelles at a critical concentration of 0.1 mM in aqueous 0.1 M Li(2)SO(4)) whereas oxidized FTMA remains dispersed in a monomeric state up to concentrations of at least 30 mM. By flowing aqueous FTMA solutions through microfluidic channels (width of 80 microm, depth of 72 microm, and length of 42 mm) and by applying potentials of 0 V (vs Ag|AgCl; cathode) and +0.3 V (vs Ag|AgCl; anode) to gold electrodes lining both side-walls of the microfluidic channels, we measured lateral gradients in concentration of oxidized FTMA and reduced FTMA to be generated across the microfluidic channels by splitting the exiting stream into four channels. These measurements revealed the lateral concentration profile of FTMA to be consistent with the presence of slowly diffusing micelles of FTMA in a spatially localized region near the cathode and monomeric FTMA only near the anode. The lateral concentration profiles of reduced and oxidized FTMA, and thus the patterning of micelles within the microfluidic channels, were manipulated via changes in the inlet FTMA concentration, potentials applied to the electrodes, and flow rate. These experimental measurements were compared to a simple model, which assumed fast electrode kinetics, lateral transport of FTMA by diffusion only (no migration), and local micelle-monomer equilibrium within the bulk solution. This comparison revealed qualitative but not quantitative agreement between model and experiment. Calculations of ionic conductivity and associated experimental measurements support the proposition that Ohmic resistance to the passage of current along the channel (between the working and the counter electrodes) contribute, in part, to the lack of quantitative agreement between the model and the measurements. The capability to generate and manipulate lateral concentration profiles of surfactants and micelles across microfluidic channels, as demonstrated by the results presented in this paper, offers the basis of new principles for continuous separation processes and microanalytical systems, and more broadly, new methods to generate gradients in concentration of analytes that interact with surfactants.
我们报告了通过将氧化还原活性表面活性剂与电化学方法相结合，在微流控通道中生成和控制表面活性剂及胶束浓度的空间梯度。该方法基于以下观察结果：11-二茂铁基十一烷基三甲基溴化铵（FTMA）在水溶液中表现为表面活性剂（例如，在0.1 M Li₂SO₄水溶液中，临界浓度为0.1 mM时自组装形成胶束），而氧化态的FTMA在浓度至少为30 mM时仍以单体状态分散。通过使FTMA水溶液流过微流控通道（宽度80微米、深度72微米、长度42毫米），并对微流控通道两侧壁的金电极施加0 V（相对于Ag|AgCl；阴极）和 +0.3 V（相对于Ag|AgCl；阳极）的电位，我们通过将流出的液流分成四个通道，测量了微流控通道中氧化态FTMA和还原态FTMA浓度的横向梯度。这些测量结果表明，FTMA的横向浓度分布与阴极附近空间局部区域中FTMA缓慢扩散的胶束以及仅阳极附近的单体FTMA的存在一致。通过改变入口FTMA浓度、施加到电极的电位和流速，可控制还原态和氧化态FTMA的横向浓度分布，进而控制微流控通道内胶束的图案化。将这些实验测量结果与一个简单模型进行了比较，该模型假设电极动力学快速、FTMA仅通过扩散进行横向传输（无迁移）且本体溶液中存在局部胶束 - 单体平衡。这种比较揭示了模型与实验之间在定性上而非定量上的一致性。离子电导率的计算及相关实验测量结果支持了这样的观点，即沿通道（工作电极和对电极之间）电流通过时的欧姆电阻部分导致了模型与测量结果之间缺乏定量一致性。如本文结果所示，在微流控通道中生成和控制表面活性剂及胶束横向浓度分布的能力为连续分离过程和微分析系统提供了新的原理基础，更广泛地说，为生成与表面活性剂相互作用的分析物浓度梯度提供了新方法。
