Development of a Cantilever Based Optical Interfacial Force Microscope for Biological Studies in an Aqueous Environment

A bio-molecular system passes through various metastable energy states until it reaches its lowest energy stable state. Probing these metastable states and finding their relationship with the delicate structures of bio-molecular pairs are crucial for understanding each step of a bio-molecular function. Even with such importance, however, metastable states are difficult to observe with current available techniques (e.g. conventional AFM techniques) because of a relatively short life time and their non-equilibrium nature in a solution phase. Here we designed and developed a cantilever based optical interfacial force microscope (COIFM), a new tool for studying the bio-molecular metastable energy states using the force feedback technique. In order to apply the COIFM to biological systems, we coated the COIFM sensor with a flexible coating material called “silicone elastomer” to isolate electrical signaling for force feedback from charged species in the aqueous environment. The force feedback responses collected such as the square wave response and bandwidth measurement showed that the performance of COIFM in liquid is nearly comparable to that of the COIFM taken in air, indicating that the effect of the elastormer coating on feedback performance was negligible. Force-distance measurements, collected in a conducting PBS liquid, demonstrated its enhanced lateral resolution and force resolution over the conventional IFM by at least an order of magnitude (less than 10 pN). The enhancement agrees with our expectation due to the decreased probe size and due to the sensitive optical detection scheme. This enhanced performance along with COIFM’s excellent distance and force control capability will allow for the investigation of the metastable states along the reaction coordinates