Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA.
Lab Chip. 2010 Oct 21;10(20):2787-95. doi: 10.1039/c005257d. Epub 2010 Aug 20.
This paper investigated the effects of diffusion between non-conductive sheath and conductive sample fluids in an impedance-based biosensor. Impedance measurements were made with 2- and 4-electrode configurations. The 4-electrode design offers the advantage of impedance measurements at low frequencies (<1 kHz) without the deleterious effects of double layer impedance which are present in the 2-electrode design. Hydrodynamic flow focusing was achieved with a modified T-junction design with a smaller cross-section for the sample channel than for the focusing channel, which resulted in 2D focusing of the sample stream with just one sheath stream. By choosing a non-conductive sheath fluid and a conductive sample fluid, the electric field was confined to the focused stream. In order to utilize this system for biosensing applications, we characterized it for electrical and flow parameters. In particular, we investigated the effects of varying flow velocities and flow-rate ratios on the focused stream. Increasing flow-rate ratios reduced the cross-sectional area of the focused streams as was verified by finite element modeling and confocal microscopy. Antibody mediated binding of Escherichia coli to the electrode surface caused an increase in solution resistance at low frequencies. The results also showed that the diffusion mass transport at the interface of the two streams limited the benefits of increased flow focusing. Increasing flow velocities could be used to offset the diffusion effect. To optimize detection sensitivity, flow parameters and mass transport must be considered in conjunction, with the goal of reducing diffusion of conducting species out of the focused stream while simultaneously minimizing its cross-sectional area.
本文研究了在基于阻抗的生物传感器中，非导电鞘和导电样品流体之间扩散的影响。采用 2 电极和 4 电极配置进行阻抗测量。4 电极设计的优点是可以在低频（<1 kHz）下进行阻抗测量，而不会出现 2 电极设计中存在的双层阻抗的有害影响。通过采用修改后的 T 型接头设计实现了流体动力学流动聚焦，该设计的样品通道的横截面小于聚焦通道，从而实现了二维聚焦，只需一个鞘流即可聚焦样品流。通过选择非导电鞘液和导电样品液，可以将电场限制在聚焦流中。为了将该系统用于生物传感应用，我们对其进行了电和流参数的特性分析。特别是，我们研究了改变流速和流速比对聚焦流的影响。增加流速比会减小聚焦流的截面积，这通过有限元建模和共聚焦显微镜得到了验证。抗体介导的大肠杆菌与电极表面的结合导致溶液在低频下的电阻增加。结果还表明，两流界面处的扩散质量传递限制了增加流动聚焦的好处。可以增加流速来抵消扩散效应。为了优化检测灵敏度，必须结合考虑流动参数和质量传递，目的是减少导电物质从聚焦流中扩散出来的同时最小化其截面积。
