Confocal Raman imaging as a useful tool to understand the internal microstructure of multicomponent aerogels

Abstract

This work shows the characterization of (nano)cellulosic aerogels prepared from Posidonia oceanica waste biomass by means of confocal Raman microscopy (CRM). For this aim, aerogels were prepared by simple freeze-drying of aqueous dispersions of four (nano)cellulosic fractions with different purification degrees, tested at two different concentrations (0.5% and 2%). These were then coated with polylactic acid (PLA) in order to improve their hydrophobicity and subjected to oil sorption-desorption experiments. Both univariate and multivariate analyses, including an approach based on comparing the spectra with those of reference materials and another one based on automatic detection of components, were compared in terms of the quality and the accuracy of the information provided. Univariate analysis only provided accurate information in the simplest systems (native (nano)cellulosic aerogels), while multivariate analyses facilitated the detection of the different components even for the most complex structures. Automatic identification of components was selected as the optimal methodology, although it also underestimated the abundance of the components with the least intense Raman spectra (cellulosic clusters) in the presence of PLA and oil. Comparison with the reference materials resulted in unrealistic images for the most complex systems. Micron-sized regions of concentrated cellulose were detected using CRM, being more abundant in the denser aerogels. Results also confirmed that PLA was preferentially located close to the surface, while oil could penetrate deeper along the matrix. Overall, the results showed the potential of Raman imaging as a novel approach for the characterization of complex biopolymeric aerogels. K E Y W O R D S cellulose, hyperspectral imaging, multivariate analysis, Raman microscopy, renewable 1 | INTRODUCTION Aerogels are extremely light and porous materials with high surface area and low density, which are useful for a wide range of applications due to their high absorption and adsorption capacities. [1-3] Although aerogels have been traditionally made from inorganic materials, [4,5] their application as adsorbent/absorbent structures (pads) in food packaging

Microstructural Analysis of Whey/Soy Protein Isolate Mixed Gels Using Confocal Raman Microscopy

“…An initial analysis was performed by comparing the whole datasets obtained for each hydrogel with the average spectra of the reference gels (WPI100 and SPI100) using the True Component Analysis algorithms of the software. This methodology was previously used for the analysis of other gel systems, such as oleogels [ 37 ] and aerogels [ 34 ]. Figure 4 a shows the obtained images.…”

“…The reference spectra obtained, as described in Section 2.3.3 , were provided as an input in the True Component Analysis function of the WITec Project software to map the distribution of proteins. This method assumes that the Raman spectrum obtained in each point of the sample is a linear combination of the reference spectra [ 34 ].…”

“…In this case, instead of inputting reference spectra to the software, it was asked to select a first component based on the most intense spectrum from the whole dataset and calculate the residual image. If the user considered that the obtained residual image was still structured (i.e., some components were not detected yet), subsequent cycles of adding components and calculating the corresponding residual images were performed, until the residual image showed just noise [ 34 ]. Figure S2 (Supplementary Materials) shows some examples of residual images to illustrate this iterative process.…”

“…The spectral ranges used for this analysis are given in Table 1 and highlighted in grey in Figure 1, and the criteria for their selection are discussed in Sections 3.1 and 3.2. The intensity of each pixel in the images obtained through this method represents the area under the spectra in the selected spectral ranges at each measurement point [34]. The second method considered the full Raman spectra, or part of it, instead of single bands to map the distribution of proteins in the gels.…”

“…Therefore, the microstructural analysis can be performed in a less invasive and less destructive manner, also avoiding potential artefacts derived from the addition of dyes, their solvents, or the staining protocols themselves [ 33 ]. Additionally, while the distribution of only a limited number of compounds of interest can be mapped simultaneously through CLSM in complex matrices (those that can be specifically stained with fluorophores whose emission spectra do not overlap), each pixel of a Raman micrograph contains chemical information from all the components present in that specific spot of the sample [ 34 ]. Due to its immense potential, new applications of Raman microscopy to study the microstructure of food are rapidly emerging [ 35 , 36 , 37 ].…”