<i>In Operando</i> Characterization of Nanocellulose Based Water Treatment Materials Using Atomic Force Microscopy and Synchrotron Scattering

Nanocellulose in anionic and cationic form can be extracted from biomass using a top-down approach, and the surface chemistry can be tuned to have selective interactions toward water pollutants under aqueous conditions. The versatility of the surface functionalization potential of nanocellulose and its processability into membranes, hydrogel beads, 3D printed filters, electrospun webs, etc., have resulted in promising performance in water treatment. Nanocellulose interactions with pollutants and adsorption can involve multiple mechanisms such as electrostatic interactions, complexation, hydrophobic interactions, hydrogen bonding, precipitation, or nucleation and growth depending on time scales. This is, however, not fully understood, predominantly due to challenges related to characterization under aqueous conditions. In this context, we explored liquid phase atomic force microscopy (AFM), colloidal probe force spectroscopy, and <i>in situ</i> synchrotron scattering methods as advanced characterization tools to extract reliable information on interactions of nanocellulose with metal ions, dyes, pesticides, pharmaceuticals, humic acid, nitrates, PFAS, microplastics, proteins, bacteria, etc., under aqueous conditions. AFM provides information on structure and nanomechanics data on length scales of 1 nm to microns as well as molecular level interactions, whereas scattering methods can detect structures in the range of 1 Å-100 nm. This Account summarizes the research using these techniques under <i>in operando</i> conditions to understand reactions and interactions under aqueous conditions for nanocellulose based systems in the context of water treatment. The use of these techniques to understand the adsorption process, membrane structure, and interactions in wet environments, as well as the synthesis of water treatment materials in aqueous media, is included in this Account. In addition to our work, other relevant reports in the literature are also summarized to demonstrate the possibilities and challenges in this approach. Literature review showed only 6 studies on using AFM/force spectroscopy (4 from our group) and only 3 studies (from our group) on scattering methods on nanocellulose in water treatment, which indicates the challenges and limitations of this approach and also the need for expanding this field. Our works in this field have demonstrated that the advanced characterization methodologies discussed here, viz., atomic force microscopy and X-ray scattering, have significant potential to provide information on nano, molecular, and atomic scales. It is worth mentioning that in order to compensate for the interference with water, which can reduce the accuracy of the data, careful tailoring of experimental design and method development is needed. We also infer that these methodologies and tools, developed to evaluate how the nanocellulose surface interacts/reacts with other hybrid components, biomolecules, and pollutants, can be extended to understand materials and devices (e.g., biomedical implants, conductive material, catalysts, sensors, etc.) driven by surface charge under <i>in situ</i> and <i>in operando</i> conditions.