Isothermal Titration Calorimetry in Nanoliter Droplets with Sub-Second Time Constants

Abstract

I reduced the reaction volume in microfabricated suspended-membrane titration calorimeters to nanoliter droplets and improved the sensitivities to below a nanowatt with time constants of around 100ms. The device performance was characterized using exothermic acid-base neutralizations and a detailed numerical model. The finite element based numerical model allowed us to determine the sensitivities within 1% and the temporal dynamics of the temperature rise in neutralization reactions as a function of droplet size. The model was used to determine the optimum calorimeter design (membrane size and thickness, junction area, and thermopile thickness) and sensitivities for sample volumes of 1 nl for silicon nitride and polymer membranes. I obtained a maximum sensitivity of 153 pW/√Hz for a 1 µm SiN membrane and 79 pW/√Hz for a 1 µm polymer membrane. The time constant of the calorimeter system was determined experimentally by using a pulsed laser to increase the temperature of nanoliter sample volumes. For a 2.5 nanoliter sample volume, I experimentally determined a noise equivalent power of 500 pW/√Hz and a 1/e time constant of 110ms for a modified commercially available infrared sensor with a thin-film thermopile. Furthermore, I demonstrated detection of 1.4 nJ reaction energies from injection of 25 pl of 1 mM HCl into a 2.5 nl droplet of 1 mM NaOH.