Direct current (DC) insulator-based dielectrophoretic (iDEP) microdevices have the potential to replace traditional alternating current dielectrophoretic devices for many cellular and biomolecular separation applications. The use of large DC fields suggest that electrode reactions and ion transport mechanisms can become important and impact ion distributions in the nanoliters of fluid in iDEP microchannels. This work tracked natural pH gradient formation in a 100 mum wide, 1 cm-long microchannel under applicable iDEP protein manipulation conditions. Using fluorescence microscopy with the pH-sensitive dye FITC Isomer I and the pH-insensitive dye TRITC as a reference, pH was observed to drop drastically in the microchannels within 1 min in a 3000 V/cm electric field; pH drops were observed in the range of 6-10 min within a 100 V/cm electric field and varied based on the buffer conductivity. To address concerns of dye transport impacting intensity data, electrokinetic mobilities of FITC were carefully examined and found to be (i) toward the anode and (ii) 1 to 2 orders of magnitude smaller than H transport which is responsible for pH drops from the anode toward the cathode. COMSOL simulations of ion transport showed qualitative agreement with experimental results. The results indicate that pH changes are severe enough and rapid enough to influence the net charge of a protein or cause aggregation during iDEP experiments. The results also elucidate reasonable time periods over which the phosphate buffering capacity can counter increases in H and OH for unperturbed iDEP manipulations.
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机译：基于直流电（DC）绝缘子的介电电泳（iDEP）微型设备有可能取代传统的交流电介电泳设备，用于许多细胞和生物分子分离应用。大型直流电场的使用表明，电极反应和离子传输机制可能变得很重要，并且会影响iDEP微通道中纳升液体中的离子分布。这项工作追踪了在适用的iDEP蛋白质操作条件下，一个100毫米宽，1厘米长的微通道中自然pH梯度的形成。使用荧光显微镜，以pH敏感的染料FITC异构体I和pH敏感的染料TRITC为参照，观察到在3000 V / cm电场下1分钟内微通道中的pH急剧下降;在100 V / cm的电场中，在6-10分钟的范围内观察到pH下降，并根据缓冲液的电导率变化。为了解决染料迁移影响强度数据的问题，仔细检查了FITC的电动迁移率，发现其（i）朝阳极方向移动，并且（ii）比H迁移小1至2个数量级，这是导致pH从阳极下降的原因。朝向阴极。 COMSOL离子迁移模拟结果与实验结果在质量上吻合。结果表明，pH变化足够严重且足够迅速，足以影响蛋白质的净电荷或在iDEP实验期间引起聚集。结果还阐明了合理的时间段，在该时间段内磷酸盐缓冲能力可以抵消iDEP操作不受干扰时H和OH的增加。
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