Synthesis and self-assembly of lipid (DMPC)-conjugated gold nanoparticles

Bio-conjugated nanomaterials play a promising role in the development of novel supramolecular structures, molecular machines, and biosensing devices. In this study, lipid-conjugated gold nanoparticles were synthesized and allowed to form a self-assembled monolayer structure. The nanoparticles were prepared by a phase transfer method, which involved the reduction of potassium tetrachloroaurate (III) by sodium citrate in an aqueous solution and the simultaneous transfer of the reduced species to an organic medium containing DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine). The gold nanoparticles were characterized using UV-Vis spectroscopy and dynamic light scattering (DLS) particle-size analysis. In addition, the resulting nanoparticles were examined using transmission electron microscopy (TEM). The Langmuir-Blodgett (LB) technique was used to assemble the DMPC-capped nanoparticles onto a water subphase at room temperature. The measurement of the compression isotherm confirmed the assemblage of lipid capped gold nanoparticles. This method of synthesis of ordered structures utilizing molecular interactions of lipids will be useful in developing novel metamaterials and nanocircuits.Comment: 7 pages, 5 Figure

Probing the two-dimensional assembly of inorganic complexes and heterocycles for sensing applications

Thesis: Ph. D. in Inorganic Chemistry, Massachusetts Institute of Technology, Department of Chemistry, 2017.Cataloged from PDF version of thesis.Includes bibliographical references.Chemiresistive sensing is a facile, affordable, efficient, and translatable way to detect compounds at concentrations as low as parts per billion; however, the key mechanism of molecular sensing is still unknown. In this thesis, a fundamental approach is used to study molecular assembly and reactivity by looking at the intermolecular interactions of each system presented to determine how specific interactions impact the macroscopic properties or ability of the chemical species to detect incoming analyte gases relevant for chemical sensing. Scanning Tunneling Microscopy (STM) was used as a primary tool to better understand the changes to a sensing system at the molecular level. In a similar vein, several platinum complexes useful as luminescence-based chemical sensors were studied using various liquid crystalline characterization techniques to understand how their intermolecular properties impacted their bulk assembly behavior. The Swager lab has developed chemiresistive and luminescence-based sensors for a wide variety of applications and it is the hope that fundamental studies such as this will help elucidate the molecular basis of the sensing response and in the long term will allow for the development of more sophisticated and predictable sensors.by Markrete Krikorian.Ph. D. in Inorganic Chemistr

Columnar Liquid Crystallinity and Mechanochromism in Cationic Platinum(II) Complexes

Cationic square planar Pt(II) complexes are reported with high degrees of intermolecular association. These complexes display thermotropic columnar liquid crystalline behavior in spite of having only a single side chain. Crystals undergo mechanochromic transformations that can be reversed with solvent.National Science Foundation (U.S.) (Award DMR-1005810)American Society for Engineering Education (Award 2291100

Columnar Liquid Crystallinity and Mechanochromism in Cationic Platinum(II) Complexes

Cationic square planar Pt­(II) complexes are reported with high degrees of intermolecular association. These complexes display thermotropic columnar liquid crystalline behavior in spite of having only a single side chain. Crystals undergo mechanochromic transformations that can be reversed with solvent

Ultratrace Detection of Toxic Chemicals: Triggered Disassembly of Supramolecular Nanotube Wrappers

Chemical sensors offer opportunities for improving personal security, safety, and health. To enable broad adoption of chemical sensors requires performance and cost advantages that are best realized from innovations in the design of the sensing (transduction) materials. Ideal materials are sensitive and selective to specific chemicals or chemical classes and provide a signal that is readily interfaced with portable electronic devices. Herein we report that wrapping single walled carbon nanotubes with metallo-supramolecular polymers creates sensory devices with a dosimetric (time- and concentration-integrated) increase in electrical conductivity that is triggered by electrophilic chemical substances such as diethylchlorophosphate, a nerve agent simulant. The mechanism of this process involves the disassembly of the supramolecular polymer, and we demonstrate its utility in a wireless inductively powered sensing system based on near-field communication technology. Specifically, the dosimeters can be powered and read wirelessly with conventional smartphones to create sensors with ultratrace detection limits

STM Study of Gold(I) Pyrazolates: Distinct Morphologies, Layer Evolution, and Cooperative Dynamics

We describe the first study of trinuclear gold­(I) pyrazolates on the molecular level by time-dependent scanning tunneling microscopy (STM). On the graphite/1-octanoic acid interface dodecyl-functionalized gold pyrazolates formed concentration-controlled morphologies. We found two types of monomeric packing and one dimeric type with two trinuclear gold pyrazolates next to each other on the surface. For an octadecyl-functionalized derivative all studied concentrations resulted in a dimeric morphology. However, different concentrations led to different transient states during the layer evolution. At low concentrations, a transient monomeric state was present with the alkyl chains in a gauche-conformation that subsequently converted to a more optimized anti-conformation. At higher concentrations a less stable “line” polymorph was observed. The confinement of the molecules to the surface led to cooperative dynamics, in which two molecules in a dimer moved as if they were one particle. Furthermore, in a higher level of cooperativity, the rotation of one dimer appears to induce rotations in coupled neighboring dimers

Functionalizing molecular wires: a tunable class of a,u-diphenyl-m, n-dicyano-oligoenes † ‡

We describe the synthesis and characterization of a new class of cyano-functionalized oligoenes and their derivatives. We have made the vinylogous series of a,u-diphenyl-m,n-dicyano-oligoenes (DPDCn) comprised of each odd-numbered member from 3 to 13 linear conjugated olefins. Installing cyano groups onto the oligoene backbone lowers HOMO and LUMO energies by up to $0.7 eV, thereby stabilizing the molecule with respect to oxidative decomposition; this exemplifies a new approach to the stabilization of conjugated oligoenes. UV-vis absorption spectra and redox potentials across the DPDCn series reveal that the molecular band gap ranges from 2.80 to 1.75 eV. This gap can be further tuned by the facile installation of a variety of aryl end-groups. The choice of end-groups also greatly affects the physical properties such as solubility and the solid-state packing. We also present the longest oligoene crystal structure reported to date. Moreover, we find that the prototypical linear structure makes oligoenes suitable as molecular wires and connectors in the bottom-up construction of nanoscale architectures. As a proof of concept, carboxylic acid terminated oligoenes were used to position 10-nm Fe 3 O 4 nanoparticles on a GaAs (100) substrate

Smectic A mesophases from luminescent sandic platinum(II) mesogens

Square planar platinum(II) thienyl pyridyl complexes with board-shaped structures assemble into lamellar (SmA) liquid crystal phases at elevated temperatures. Liquid crystals of this type are expected to have stronger biaxial correlations than typical calamitic mesogens. The mesophase stability improves with decreasing alkyl chain lengths with C₈H₁₇ having the widest range of stability. All complexes are luminescent in solution.National Science Foundation (U.S.) (Grant DMR-1410718

Importance of Direct Metal−π Coupling in Electronic Transport Through Conjugated Single-Molecule Junctions

We study the effects of molecular structure on the electronic transport and mechanical stability of single-molecule junctions formed with Au point contacts. Two types of linear conjugated molecular wires are compared: those functionalized with methylsulfide or amine aurophilic groups at (1) both or (2) only one of its phenyl termini. Using scanning tunneling and atomic force microscope break-junction techniques, the conductance of mono- and difunctionalized molecular wires and its dependence on junction elongation and rupture forces were studied. Charge transport through monofunctionalized wires is observed when the molecular bridge is coupled through a S–Au donor–acceptor bond on one end and a relatively weak Au−π interaction on the other end. For monofunctionalized molecular wires, junctions can be mechanically stabilized by installing a second aurophilic group at the <i>meta</i> position that, however, does not in itself contribute to a new conduction pathway. These results reveal the important interplay between electronic coupling through metal−π interactions and quantum mechanical effects introduced by chemical substitution on the conjugated system. This study affords a strategy to deterministically tune the electrical and mechanical properties through molecular wires