Department of Molecular and Medical Pharmacology, David Geffen School of Medicine, University of California, Los Angeles, California, USA.
Anal Chem. 2012 Feb 21;84(4):1915-23. doi: 10.1021/ac202715f. Epub 2012 Feb 1.
We demonstrate a new approach to impedance measurement on digital microfluidics chips for the purpose of simple, sensitive, and accurate volume and liquid composition measurement. Adding only a single series resistor to existing AC droplet actuation circuits, the platform is simple to implement and has negligible effect on actuation voltage. To accurately measure the complex voltage across the resistor (and hence current through the device and droplet), the designed system is based on software-implemented lock-in amplification detection of the voltage drop across the resistor which filters out noise, enabling high-resolution and low-limit signal recovery. We observe picoliter sensitivity with linear correlation of voltage to volume extending to the microliter volumes that can be handled by digital microfluidic devices. Due to the minimal hardware, the system is robust and measurements are highly repeatable. The detection technique provides both phase and magnitude information of the real-time current flowing through the droplet for a full impedance measurement. The sensitivity and resolution of this platform enables it to distinguish between various liquids which, as demonstrated in this paper, could potentially be extended to quantify solute concentrations, liquid mixtures, and presence of analytes.
我们展示了一种新的数字微流控芯片阻抗测量方法，旨在实现简单、灵敏和精确的体积和液体成分测量。该平台只需在现有的交流液滴驱动电路中添加一个串联电阻器，因此实现起来非常简单，对驱动电压的影响可以忽略不计。为了准确测量电阻器上的复杂电压（从而测量通过器件和液滴的电流），所设计的系统基于软件实现的对电阻器上电压降的锁定放大检测，该检测可以滤除噪声，从而实现高分辨率和低限信号恢复。我们观察到皮升级别的灵敏度，电压与体积呈线性相关，扩展到数字微流控设备可以处理的微升级别。由于硬件最小化，该系统坚固耐用，测量结果高度可重复。该检测技术提供了实时流经液滴的电流的相位和幅度信息，从而实现了完整的阻抗测量。该平台的灵敏度和分辨率使其能够区分各种液体，如本文所示，这可能进一步扩展到定量溶质浓度、液体混合物和分析物的存在。
