Optical Characterization of Lignin Nanoparticles

Sammanfattning: Lignin is one of the main components of wood and plants that acts as a kind of glue providing mechanical strength. It is a main polymer component composed from three phenolic structures, i.e. p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units. It currently draws a lot of attention due to its eco-friendly. Recently, it has been shown that it is possible to produce lignin nanoparticles, small spherical particle that are composed out of lignin, that could possibly be used to replace the hazardous silver nanoparticles that are today used frequently in numerous applications. Lignin nanoparticles could potenitally also be used as functional coatings, as well as biologically degradable adhesives and float switches. Five samples, of nanoparticles, were investigated in this study. The first contained pure lignin nanoparticles, the second pure silver nanoparticles, and the three remaining samples contained lignin-coated silver nanoparticles, extracted from acetone, tetrahydrofuran (THF), and dimetylformamid (DMF) solvents. All samples were characterized using spectroscopic methods, e.g. infrared- and dark-field imaging, as well as UV-Vis-, fluorescence-, and Raman spectroscopy. In this thesis it was shown that lignin-coated silver nanoparticles exhibit surface plasmon resonance which induces a heat effect upon infrared irradiation. To identify the phenolic structures of lignin, UV-Vis spectroscopy was used. It was found that the spectra of the samples exhibited several intense bands. The objective of the UV-Vis spectroscopy was to examine the absorbance characteristics of the lignin-coated silver nanoparticles. Possible surface plasmon resonance wavelengths were determined, and two of the phenolic structures were identified. In this study, Raman spectroscopy was used to define characteristic bands of the samples. This was done to investigate if the lignin nanoparticles have the same characteristics as bulk lignin. Raman spectroscopy provide structural information of lignin. Furthermore, p-hydroxyphenyl, guaiacyl and syringyl structures could be identified with an excitation wavelength of 532nm. A comparison of the spectra of the lignin-containing samples indicated the the Raman features of the specimens were similar meanwhile almost no signs of silver were present, which might show that the particles were fully covered with lignin. Main lignin bands were identified and assigned. The fluorescent properties of the nanoparticles were investigated by obtaining emission spectra for blue-, green- and UV light excitation. The spectra were deconvoluted into their Gaussian components. Emission spectra were obtained for blue-, green- and UV light excitation. It was found that the fluorescence, after UV light exposure, increased with time of exposure. Dark-field microscopy was used to generate light scattering images of the particles. As a result, optical images with different colors (white, yellow, blue and red) could be revealed. The color information, that is related to the size of the particles, was used to estimate ratios of the different particle sizes. The lignin-coated silver nanoparticles, extracted from acetone, exhibited a strong surface plasmon resonance effect, which could be due to the absorbance at 463nm. The lignin-coated silver nanoparticles, extracted from DMF, exhibited a medium surface plasmon resonance effect, which could be due to the absorbance at 362nm. The lignin-coated silver nanoparticles, extracted from the THF solvent, exhibited a weak surface plasmon resonance effect, which could be due to the absorption at 379-380nm. The pure lignin- and silver nanoparticles merely showed bulk heating but no surface plasmon resonance effect could be detected.