Synthesis and Tracking of Fluorescent and Polymerization-Propelled Single-Molecule Nanomachines

This dissertation describes the synthesis of molecular machines designed to operate on surfaces (nanocars) or in the solution phase (nanosubmarines), and the study of their diffusion using fluorescence techniques. The design of these molecular machines is aimed to facilitate monitoring of their movement and incorporation of a source of energy for propulsion. To complement previous scanning tunneling microscopy studies of the translation of nanocars on surfaces, chapter 1 describes the synthesis of a family of fluorescently tagged nanocars. The nanocars were functionalized with a tetramethylrhodamine isothiocyanate (TRITC) fluorescent dye. Single-molecule fluorescence microscopy (SMFM) studies of one of these nanocars revealed that 25% of the nanocars moved on glass. The SMFM results also suggested that the dye hindered the mobility of the nanocars. Seeking to improve the mobility, chapter 2 presents the synthesis of a new set of fluorescent nanocars, featuring a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dye embedded in their axles. The mobility of these inherently fluorescent nanocars on glass was nearly double than that of their TRITC-tagged predecessors. Their diffusion was also studied on reactive-ion-etched glass, and amino-functionalized glass. The results showed that the mobility is affected by the substrate. To equip the nanocars with an energy input for propulsion, two nanocars functionalized with an olefin metathesis catalyst were synthesized, as described in chapter 3. The catalytic activity of these nanocars toward ring-opening metathesis polymerization (ROMP) in solution was similar to that of their parent catalysts. As an alternative approach to investigate if chemical propulsion through a ROMP process can be achieved at the molecular level, chapter 4 presents the synthesis of a fluorescent ROMP catalyst, termed a nanosubmarine, and the study of its diffusion using fluorescence correlation spectroscopy (FCS). FCS results showed an increase of 20 ± 7% in the diffusion constant of this nanosubmarine in presence of its fuel, cis,cis-1,5-cyclooctadiene. Overall, the work accomplished in this dissertation constitutes a step forward toward development of easily tracked and highly mobile nanocars, and paves the way for the synthesis of truly nanosized chemically propelled molecular machines that operate in the solution phase

Hydrothermal treatments of aqueous cellulose nanocrystal suspensions: effects on structure and surface charge content

Cellulose nanocrystals (CNCs) are ideal rheological modifiers for aqueous oil and gas extraction fluids. CNCs are typically produced with sulfuric acid and their aqueous suspensions have uniform and predictable properties under ambient conditions; however, drastic changes occur at elevated temperatures. Herein, the effects of high temperature treatments (ranging from 80 to 180 °C for 1 h to 7 days) on the properties (including uniformity, colloidal stability, and color) of sulfated, phosphated, and carboxylated CNC suspensions were studied. Additionally, cellulose molecular weight, and CNC surface charge content and crystallinity index were quantified before and after heating. CNCs underwent few morphological changes; their molecular weight and crystallinity index were largely unchanged under the conditions tested. Their surface charge content, however, was significantly decreased after heat treatment which resulted in loss of colloidal stability and aggregation of CNCs. The largest change in suspension properties was observed for sulfated CNCs whereas CNCs with a combination of sulfate and phosphate esters, or carboxylate groups, were less affected and maintained colloidal stability at higher temperatures. In fact, desulfation was found to occur rapidly at 80 °C, while many carboxylate groups persisted at temperatures up to 180 °C; calculated rate constants (based on second order kinetics) suggested that desulfation is 20 times faster than decarboxylation but with a similar activation energy. Overall, this study elucidates CNC suspension behavior after heat exposure and demonstrates routes to produce CNCs with improved high temperature performance

Insight into thermal stability of cellulose nanocrystals from new hydrolysis methods with acid blends

This study provides insight into the thermal degradation of cotton cellulose nanocrystals (CNCs) by tuning their physico-chemical properties through acid hydrolysis using blends of phosphoric and sulfuric acid. CNCs isolated by sulfuric acid hydrolysis are known to degrade at lower temperatures than CNCs hydrolyzed with phosphoric acid; however, the reason for this change is unclear. Although all CNCs are inherently relatively thermally stable, their application in polymer composites and liquid formulations designed to function at high temperatures could be extended if thermal stability was improved. Herein, thermogravimetric analysis was carried out on six types of CNCs (in both acid and sodium form) with different surface chemistry, surface charge density, dimensions, crystallinity and degree of polymerization (DP) to identify the key properties that influence thermal stability of nanocellulose. In acid form, CNC surface charge density was found to be the determining factor in thermal stability due to de-esterification and acid-catalyzed degradation. Conversely, in sodium form, surface chemistry and charge density had a negligible effect on the onset of thermal degradation, however, the DP of the cellulose polymer chains highly influenced stability. The presence of more reducing ends in lower DP nanocrystals is inferred to facilitate thermally-induced depolymerization and degradation. Degree of crystallinity did not significantly affect CNC thermal stability. Studying CNCs produced from single or blends of acids (and changing the counterion) elucidated the thermal behavior of cellulose and furthermore demonstrated new routes to tailor CNCs thermal and colloidal stability