Characterization of a Photoswitchable Spiropyran Surface in Water by Atomic Force Microscopy and Interfacial Force Microscopy
Spiropyran, a molecule that changes its conformational structure depending on its exposure to ultraviolet and visible light, has been recently applied to the control of biological functions such as switchable gating of ion channels. Observing the switchable behavior of spiropyran in "water" is vital as water is the main component of biological systems. To better understand how the spiropyran molecule would undergo photoswitching between polar and non-polar forms in water, we measured interaction forces between an oxidized silica tip and spiropyran tethered to an oxidized silicon surface in water using interfacial force microscopy (IFM). The spiropyran molecules were tethered with a carboxylic acid end group to the oxidized silicon surface using a silane coupling agent of 3-aminopropyldiethoxymethylsilane (ADES) through the formation of an amide linkage. The topographic structure of the tethered surface, observed by atomic force microscopy, showed island structures with a diameter range of 500-1000 nm and a thickness of ~7nm, confirming that the spiropyran molecules were tethered to the top of ADES molecules. The force-distance curve showed an exponentially decaying function with the decay length of 30 nm when the molecules were exposed to ultraviolet light in 0.1 mM KCl solution. When the spiropyran molecules were exposed to visible light, the curve did not exhibit any appreciable exponential decay. The two different types of curves were switchable, depending on UV light and visible light. As the concentration of the KCl in solution increased from 0.1 mM to 3 mM, the decay length decreased from 30 nm to 3 nm. The dependence of the decay length on the ion concentration agreed with the well-known electrical double force behavior between two charged polar surfaces in an ionic solution. This agreement indicated that exposure of UV light created a charged polar form of the spiropyran molecule due to the ring opening, whereas it transitioned back to its native non-polar state due to the ring closure when exposed to visible light.