Development of a High-Sensitivity Thermal-Analysis Technique for Small-Volume Bio-Samples and Applications

Abstract

This thesis focuses on development of high-sensitivity thermal analysis technique and its application to small-volume bio-samples such as single cell and macromolecule. Thermal analysis of biological sample is important because all the biological activities are thermally active but they properly operate only in a narrow temperature region. To understand how a biological material maintains the optimal temperature in biological system, delineating the energy flow and thermal transport in the material is essential. However, thermal properties of biological materials are hardly measured due to its extremely small size and difficulty in manipulation. Conventional thermal analysis techniques have limitation to reduce the sample volume below microliter level because the signal-to-noise ratio decreases significantly as sample volume decrease. This thesis demonstrates a high-sensitivity alternating current (AC) thermal conductivity measurement instrument based on three-omega method. The proposed device can measure 27 pl sample, which is comparable to the single cell volume. To confine the thermal penetration depth of heat pulse within the thickness of cell, we fabricate nanometer width line heater/sensor by employing the electron beam lithography technique. By using this instrument, we measured the thermal conductivity of HeLa, NIH-3T3 J2, and hepatocyte cell. This measurement provides fundamental property to investigate thermodynamics of biological cell. And we measured thermal conductivity of dead NIH-3T3 J2 and hepatocyte to show that the thermal conductivity can be potentially used as a tool to analyze cell viability and response to the environmental effect. We also demonstrated that he developed thermal analysis technique can be applied as a bio-sensing tool. First, conformational dynamics of protein in aqueous solution was analyzed by the technique. We measured the change in thermal conductivity of aqueous solution of protein at different pH values while varying the temperature and protein concentration. Results suggest that thermal conductivity can be exploited to probe the conformational dynamics of protein. The proposed method of denaturation monitoring requires much simpler experimental setup than conventional method such as differential scanning calorimetry and far-ultraviolet circular dichroism detection. Consequently, it is expected to be useful in lab-on-a-chip (LoC) applications as the probe can be easily miniaturized for integration into LoC devices and allows real-time analysis. Second, probing technique which can assess cell viability and concentration in cell suspension was proposed. The correlation between thermal conductivity and cell concentration or viability was experimentally examined. Furthermore, without data processing to obtain thermal conductivity, the voltage signal that reflects the temperature response of the sensor also can be used as a tool to probe viability and concentration real-time. The proposed AC thermal sensing technique is label-free, non-invasive, long-term, and real-time. Also, a novel method to detect droplets and determine the protein content of droplet in microfluidic system was developed. By measuring the thermal response of droplets and carrying flow in real-time, water droplets in an oleic acid carrying flow can be detected, and the concentration of bovine serum albumin protein in droplet can be estimate. This method has strong potential to detection and characterization technique for droplet-based microfluidic systems.