Cobalt Ferrite Nanoparticles for Three-Dimensional Visualization of Micro- and Nanostructured Cellulose in Paper

Iron cobalt ferrite nanoparticles were used for the detection of micro- and nanostructured cellulose fibers in a cellulose network. Since extraction and modification of these fibers from lignocellulosic plants have undergone a significant increase in efficiency, nowadays composites containing these materials are a major research line in academia and industry. A particularly interesting composite in this context is paper, a nonwoven material where cellulose fibers form a network which is held together mainly by fiber–fiber interactions. In this composite, there is a certain fraction of micro- and nanostructured celluloses present, the so-called fines. Their impact on paper properties is still under debate, and one of the major factors influencing the properties, namely, their distribution inside the paper, remains elusive so far. Here, we present an approach which allows for the detection of these micro- and nanostructured celluloses in paper sheets by combining imaging technologies labeling the fine fibers with inorganic nanoparticles. The addition of these labeled materials during the paper manufacturing process enables imaging in scanning electron microscopy/energy-dispersive X-ray spectroscopy experiments and provides contrast in X-ray microtomography. Using a combination of these two techniques, the location of the fines in the paper sheets was evaluated and quantified, pointing at deposition in pores of the paper as well as at fiber–fiber junctions. We demonstrate that the tensile indices, air permeability, and the water retention value of handsheets were not altered by the addition of labeled fines compared to sheets where nonlabeled fines have been added.