3 research outputs found
Multimodal Broadband Vibrational Sum Frequency Generation (MM-BB-V-SFG) Spectrometer and Microscope
A broadband sum frequency generation
(BB-SFG) spectrometer with
multimodal (MM) capabilities was constructed, which could be routinely
reconfigured for tabletop experiments in reflection, transmission,
and total internal reflection (TIR) geometries, as well as microscopic
imaging. The system was constructed using a Ti:sapphire amplifier
(800 nm, pulse width = 85 fs, repetition rate = 2 kHz), an optical
parameter amplification (OPA) system for production of broadband IR
pulses tunable between 1000 and 4000 cm<sup>–1</sup>, and two
Fabry–Pérot etalons arranged in series for production
of narrowband 800 nm pulses. The key feature allowing the MM operation
was the nearly collinear alignment of the visible (fixed, 800 nm)
and infrared (tunable, 1000–4000 cm<sup>–1</sup>) pulses
which were spatially separated. Physical insights discussed in this
paper include the comparison of spectral bandwidth produced with 40
and 85 fs pump beams, the improvement of spectral resolution using
etalons, the SFG probe volume in bulk analysis, the normalization
of SFG signals, the stitching of multiple spectral segments, and the
operation in different modes for air/liquid and adsorbate/solid interfaces,
bulk samples, as well as spectral imaging combined with principle
component analysis (PCA). The SFG spectral features obtained with
the MM-BB-SFG system were compared with those obtained with picosecond-scanning-SFG
system and high-resolution BB-SFG system (HR-BB-SFG) for dimethyl
sulfoxide, α-pinene, and various samples containing cellulose
(purified commercial products, <i>Cladophora</i> cell wall,
cotton and flax fibers, and onion epidermis cell wall)
Effects of Plant Cell Wall Matrix Polysaccharides on Bacterial Cellulose Structure Studied with Vibrational Sum Frequency Generation Spectroscopy and X‑ray Diffraction
The crystallinity, allomorph content,
and mesoscale ordering of
cellulose produced by Gluconacetobacter xylinus cultured with different plant cell wall matrix polysaccharides were
studied with vibrational sum frequency generation (SFG) spectroscopy
and X-ray diffraction (XRD). Crystallinity and ordering were assessed
as the intensity of SFG signals in the CH/CH<sub>2</sub> stretch vibration
region (and confirmed by XRD), while Iα content was assessed
by the relative intensity of the OH stretch vibration at 3240 cm<sup>–1</sup>. A key finding is that the presence of xyloglucan
in the culture medium greatly reduced Iα allomorph content but
with a relatively small effect on cellulose crystallinity, whereas
xylan resulted in a larger decrease in crystallinity with a relatively
small decrease in the Iα fraction. Arabinoxylan and various
pectins had much weaker effects on cellulose structure as assessed
by SFG and XRD. Homogalacturonan with calcium ion reduced the SFG
signal, evidently by changing the ordering of cellulose microfibrils.
We propose that the distinct effects of matrix polysaccharides on
cellulose crystal structure result, at least in part, from selective
interactions of the backbone and side chains of matrix polysaccharides
with cellulose chains during the formation of the microfibril
Dependence of Sum Frequency Generation (SFG) Spectral Features on the Mesoscale Arrangement of SFG-Active Crystalline Domains Interspersed in SFG-Inactive Matrix: A Case Study with Cellulose in Uniaxially Aligned Control Samples and Alkali-Treated Secondary Cell Walls of Plants
Vibrational
sum frequency generation (SFG) spectroscopy can selectively
detect not only molecules at two-dimensional (2D) interfaces but also
noncentrosymmetric domains interspersed in amorphous three-dimensional
(3D) matrixes. However, the SFG analysis of 3D systems is more complicated
than 2D systems because more variables are involved. One such variable
is the distance between SFG-active domains in SFG-inactive matrixes.
In this study, we fabricated control samples in which SFG-active cellulose
crystals were uniaxially aligned in an amorphous matrix. Assuming
uniform separation distances between cellulose crystals, the relative
intensities of alkyl (CH) and hydroxyl (OH) SFG peaks of cellulose
could be related to the intercrystallite distance. The experimentally
measured CH/OH intensity ratio as a function of the intercrystallite
distance could be explained reasonably well with a model constructed
using the theoretically calculated hyperpolarizabilities of cellulose
and the symmetry cancellation principle of dipoles antiparallel to
each other. This comparison revealed physical insights into the intercrystallite
distance dependence of the CH/OH SFG intensity ratio of cellulose,
which can be used to interpret the SFG spectral features of plant
cell walls in terms of mesoscale packing of cellulose microfibrils