Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850, United States.
Langmuir. 2013 Jul 2;29(26):8292-301. doi: 10.1021/la400884t. Epub 2013 Jun 20.
Many interfacial chemical phenomena are governed in part by electrostatic interactions between polyelectrolytes and charged surfaces; these phenomena can influence the performance of biosensors, adsorption of natural polyelectrolytes (humic substances) on soils, and production of polyelectrolyte multilayer films. In order to understand electrostatic interactions that govern these phenomena, we have investigated the behavior of a model polyelectrolyte, 15 kbp fluorescently labeled plasmid DNA, near a polarized indium tin oxide (ITO) electrode surface. The interfacial population of DNA was monitored in situ by imaging individual molecules through the transparent electrode using total-internal-reflection fluorescence microscopy. At applied potentials of +0.8 V versus Ag/AgCl, the DNA interfacial population near the ITO surface can be increased by 2 orders of magnitude relative to bulk solution. The DNA molecules attracted to the interface do not adsorb to ITO, but rather they remain mobile with a diffusion coefficient comparable to free solution. Ionic strength strongly influences the sensitivity of the interfacial population to applied potential, where the increase in the interfacial population over a +300 mV change in potential varies from 20% in 30 mM ionic strength to over 25-fold in 300 μM electrolyte. The DNA accumulation with applied potential was interpreted using a simple Boltzmann model to predict average ion concentrations in the electrical double layer and the fraction of interfacial detection volume that is influenced by applied potential. A Gouy-Chapman model was also applied to the data to account for the dependence of the ion population on distance from the electrode surface, which indicates that the net charge on DNA responsible for interactions with the polarized surface is low, on the order of one excess electron. The results are consistent with a small fraction of the DNA plasmid being resident in the double-layer and with counterions screening much of the DNA excess charge.
许多界面化学现象部分受聚电解质与带电表面之间的静电相互作用控制；这些现象会影响生物传感器的性能、天然聚电解质（腐殖质）在土壤中的吸附以及聚电解质多层膜的生产。为了理解控制这些现象的静电相互作用，我们研究了一种模型聚电解质，即 15 kbp 荧光标记的质粒 DNA，在极化氧化铟锡（ITO）电极表面附近的行为。通过全内反射荧光显微镜通过透明电极对单个分子进行成像，原位监测 DNA 在界面处的种群。在相对于 Ag/AgCl 的+0.8 V 的应用电位下，与本体溶液相比，ITO 表面附近的 DNA 界面种群可以增加 2 个数量级。吸引到界面的 DNA 分子不会吸附到 ITO 上，而是保持与自由溶液相当的扩散系数的可移动性。离子强度强烈影响界面种群对施加电位的敏感性，其中界面种群相对于电位的+300 mV 变化的增加幅度从 30 mM 离子强度下的 20%变化到 300 μM 电解质下的 25 倍以上。应用电位下的 DNA 积累用简单的玻尔兹曼模型进行解释，以预测双电层中的平均离子浓度和受应用电位影响的界面检测体积分数。还将 Gouy-Chapman 模型应用于数据，以解释离子种群随距电极表面距离的变化，这表明与极化表面相互作用的 DNA 的有效电荷很低，约为一个额外电子。结果与一小部分 DNA 质粒驻留在双层中，以及抗衡离子屏蔽大部分 DNA 过剩电荷的结果一致。
