Subwavelength Raman imaging of biological samples using near-field spectroscopy

Scanning near-field optical microscopy and Raman scattering were combined to obtain subwavelength molecular resolution. Near-field microscopy allows to overcome the diffraction limit valid for all lens or mirror based optical instruments and lateral resolutions well below 100 nm have been claimed. Raman spectroscopy yields information on the vibrational states of a molecule and therefore allows to distinguish between different chemical compounds easily. This is an advantage to the more widely used near-field fluorescence microscopy. To enhance the notoriously weak near-field Raman signal the sample was brought onto a Raman enhancing surface (silver coated Teflon nanospheres). Additionally brilliant cresyl blue acted as a Raman label for our DNA samples. On such samples Raman images with a resolution better than 100 nm have been obtained. A single near-field SERS spectrum was measured in approximately 60 seconds. The acquisition time currently depends critically on the transmission of the near-field probes. Nevertheless, it is possible to measure whole near-field Raman images in a reasonable time. From these Raman images a preliminary distinction of different constitutions of adsorbed molecules can be done.