‘Structure - Processing - Property’ Relationships of Cellulose Nanocrystals for Optical and Sensing Applications

Abstract

This research enhanced scientific understanding of the structure-processing-property relationships that govern the assembly of sulfated cellulose nanocrystals (CNCs) into films for photonic and sensing applications. CNCs are one-dimensional crystalline nanomaterials derived from cellulose. Their natural abundance, high strength, and ability to form lyotropic liquid crystalline phases make them an intriguing alternative to synthetic advanced materials. The first part of this research focused on CNC’s geometric polydispersity. The effects of a simple sedimentation technique on CNC size distribution, phase behavior, rheological properties, and photonic film properties were explored. Sedimentation of a primarily isotropic aqueous CNC dispersion resulted in two distinct phases. The top phase was isotropic and consisted of shorter nanocrystals, and the bottom phase was liquid crystalline and contained longer nanocrystals. Based on atomic force microscopy, the average length-to-diameter ratio of CNCs in the top and bottom phases was 70 and 51, respectively. Rheological measurements of nanomaterial aspect ratio are often used to augment atomic force microscopy measurements, but this is challenging for aqueous CNCs. Their ability to flow align can cause erroneous viscometer measurements. The low viscosity of dilute dispersions prevents measurement of the zero-shear viscosity on a rotational rheometer, which prevents the use of rotational rheology methods requiring measurement of a low-shear Newtonian plateau. These challenges were overcome by using Fedor's equation to determine the intrinsic viscosity and Bachelor's and Simha's equation to calculate the associated aspect ratio of the top, bottom, and parent phases. Additional rheological studies coupled with cross-polarized optical microscopy images were used to explore how the size distribution in each phase affected its rheology and phase behavior. As expected, the top phase comprised of shorter rods required a higher concentration for forming liquid crystal domains. The differences in dispersion properties directly affected the properties of dried CNC films. Image processing was used to quantify the relative abundance of tactoids, helices with planar anchoring, and helices with homeotropic anchoring for films made from the parent dispersions and each fraction. The bottom fraction containing the longer rods resulted in a higher abundance of the planar anchoring required for selective reflection. The majority of this research focused on modifying CNCs to enable their use in sensing analytes in aqueous media. This work can be split into three segments: (a) surface functionalization of CNCs, (b) selective sensing of analytes using molecularly imprinted polymers coated onto CNC films, and (c) CNC-based microdevice fabrication and characterization. Commercially available sulfated CNCs exhibit high dispersibility in water, which enables manufacturing CNC materials from aqueous dispersions. However, CNCs’ water dispersibility prevents their use in applications requiring hydrolytic stability. To address this limitation, CNCs were modified 3-aminopropyltriethoxy silane (APTES). The functionalization was confirmed with several analytical techniques, including attenuated total reflectance Fourier transform infrared spectroscopy, thermogravimetric analysis coupled infrared spectroscopy, dynamic light scattering, inductively coupled plasma mass spectroscopy, and ultimate analysis. The change in dispersion characteristics was observed using dynamic light scattering, AFM, and optical microscopy. A 12.6% degree of APTES substitution of CNCs’ available hydroxyl groups significantly improved the hydrolytic stability of CNC films while having a minor impact on the films' mechanical properties. In addition, quartz crystal microbalance with dissipation (QCMD) and multiparametric surface plasmon resonance (MP-SPR) studies showed that the CNC-APTES films had a greater irreversible non-specific binding with carbofuran, a pesticide and emerging contaminant. After overcoming the hydrolytic stability challenge, the need for specificity was achieved by synthesizing molecularly imprinted polymers (MIPs) for the detection of carbofuran, a model pesticide; amoxicillin, a model antibiotic residue; and β lactoglobulin, a model food allergen. Polymer synthesis, templating of the analyte, and analyte removal were characterized using ATR-IR, Raman, and CRAIC microspectrophotometer. CNC-APTES films were coated with MIP for sensing experiments, and their sensing capabilities were tested using QCMD and MP-SPR. The detection of β-lactoglobulin was troublesome due to water swelling of protein molecules. However, carbofuran and amoxicillin could both be detected by their corresponding MIPs. In fact, for carbofuran, the reversible sensitivity was 0.03 ppm, which is lower than that of standard commercial sensors. In addition, the MIP for carbofuran was found to be selective during tests with the herbicide 2,4-dichlorophenoxyacetic acid (2,4D) which has a similar benzene-based structure. In the last segment of the research, CNC-APTES was employed to make micro-devices with the aim of using them as mass-dependent resonating microcantilevers to sense analyte adsorption. These devices were fabricated using clean room microfabrication techniques, including photolithography, shear film cast, electron beam physical vapor deposition (ePVD), etching, device release, critical point drying, and sputtering. While portable detection is ultimately envisioned, laboratory studies employed an atomic force microscope (AFM) to probe the cantilevers’ resonance frequencies. This necessitated coating the beams with a thin layer of gold to enable laser reflection and obtain a measurement of the beam resonance frequency. Together with advances in understanding CNC surface modification for the adsorption of analytes, the device fabrication and characterization pave the way for analyte detection using CNC-APTES based devices.