(372a) Development of Highly Sensitive Pico-Calorimetric Sensors Based on Thermoelectric Effect

Microcalorimeters have great potential as biosensors due to their ability to make measurements on very small samples and their short thermal equilibration times. However, they are not yet sensitive enough to analyze nano/micro-scale samples or individual cells, and liquid sample handling remains a critical issue. This work aims to break through these current limitations by measuring pico-Watt power levels quantitatively through use of micromachined sensors and by controlling the wetting and evaporation of liquids. Individual sensors consist of a combination of thermistors and thermopiles on a thin silicon nitride membrane that allow direct differential measurements between a sample and a reference. To maximize sensitivity, sensor geometry and materials were optimized based on the temperature field that develops around a typical sample and on calculations of the corresponding noise equivalent power and minimum detectable power (P_min). Specifically, thermopiles that consist of Constantan and Nichrome couples are capable of sensing below nW level power over a broad range of thermopile lengths, 10 - 10³ µm. Further, thermopile responses at the given power were characterized as a function of thermopile dimensions and liquid volumes. This highly sensitive thermopile-based calorimeter offers particular promise for quantitative measurement of cellular bioenergetics and systems that are limited in analytic volume.