Thermal properties of nanocapsules measured by scanning force microscopy methods

Abstract

Heated micromechanical cantilevers were used to probe the thermal properties of nanogram samples in two ways: (i) as a micromechanical thermogravimetric balance and (ii) as a local nano-heater for heating individual objects. For micromechanical thermogravimetry, 6ng of polyurethane nanocapsules containing silverazide were freeze dried and attached to the micromechanical cantilever apex. The resonant frequency of the micromechanical cantilever upon heating was recorded. After heating to 250^oC, 25% mass was found remaining on the micromechanical cantilever. In order to image structural changes, the tip of the heatable micromechanical cantilever was positioned on individual nanocapsules weighing less than 6fg, and the capsule was imaged using a scanning force microscope. Then the cantilever was heated and temperature-deflection curves were recorded. Both methods revealed a decomposition temperature of 180^oC for silverazide encapsulated in a polyurethane nanocapsule. Topographic images of individual capsules after decomposition showed a rupture of the shell at one position.