Infrared Scattering-Type Scanning Near-Field Optical Microscopy of Biomembranes in Water

Infrared (IR) absorption spectroscopy detects the state and chemical composition of biomolecules solely by their inherent vibrational fingerprints. Major disadvantages like the lack of spatial resolution and sensitivity have lately been overcome by the use of pointed probes as local sensors enabling the detection of quantities as few as hundreds of proteins with nanometer precision. However, the strong absorption of infrared radiation by liquid water still prevents simple access to the measured quantity: the light scattered at the probing atomic force microscope tip. Here we report on the local IR response of biological membranes immersed in aqueous bulk solution. We make use of a silicon solid immersion lens as the substrate and focusing optics to achieve detection efficiencies sufficient to yield IR near-field maps of purple membranes. Finally, we suggest a means to improve the imaging quality by tracing the tip by a laser-scanning approach.