Abstract
Orientational dependence of the IR absorbing amide bands of silk is demonstrated from two orthogonal longitudinal and transverse microtome slices with a thickness of only ∼100 nm. Scanning near-field optical microscopy (SNOM) which preferentially probes orientation perpendicular to the sample’s surface was used. Spatial resolution of the silk–epoxy boundary was ∼100 nm resolution, while the spectra were collected by a ∼10 nm tip. Ratio of the absorbance of the amide-II C-N at 1512 cm − 1 and amide-I C=O β -sheets at 1628 cm − 1 showed sensitivity of SNOM to the molecular orientation. SNOM characterisation is complimentary to the far-field absorbance which is sensitive to the in-plane polarisation. Volumes with cross sections smaller than 100 nm can be characterised for molecular orientation. A method of absorbance measurements at four angles of the slice cut orientation, which is equivalent to the four polarisation angles absorbance measurement, is proposed.
Highlights
Nanofabrication with a resolution of few nanometers has become common using electron emission [1] and thermal probes [2,3]
Comparable IR absorbance spectra of silk were obtained using three different methods [7]: (i) a table-top Fourier transform infrared (FTIR) in transmission, (ii) a synchrotron-based attenuated total reflection (ATR) FTIR, and (iii) an atomic force microscopy (AFM) with a tip that responds to the IR absorbed light, known as nano-IR [8]
Whether it was possible to determine optical anisotropy due to molecular orientation using a point-like excitation source was the motivation of this study
Summary
Nanofabrication with a resolution of few nanometers has become common using electron emission [1] and thermal probes [2,3]. Structural anisotropy of materials underpin their optical, thermal and mechanical properties, and have to be determined at the highest transverse and longitudinal resolutions [4,5]. A fibril of silk tens-of-nanometers in diameter defines its thermal properties [6]. Comparable IR absorbance spectra of silk were obtained using three different methods [7]: (i) a table-top Fourier transform infrared (FTIR) in transmission, (ii) a synchrotron-based attenuated total reflection (ATR) FTIR, and (iii) an atomic force microscopy (AFM) with a tip that responds to the IR absorbed light, known as nano-IR [8]. The AFM-based nano-IR technique acquires structural information at the nanoscale, the area under the AFM tip forms a volume with a lateral cross-section of ∼20 nm. The far-field absorbance is defined by κ, while the near-field ATR-FTIR mode is affected by n due to the Snell’s law [9,10]
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