Jayne C. Garno

In situ studies of thiol self-assembly on gold from solution using atomic force microscopy

The kinetics and mechanism for the solution-phase adsorption of n-alkanethiols onto gold to form self-assembled monolayers (SAMs) have been monitored in situ using atomic force microscopy (AFM). Time-dependent AFM images reveal detailed structural information about the adsorbed layer during its growth. In 2-butanol, CH3(CH2)17SH molecules initially adsorb on gold with the molecular axis of their hydrocarbon chains oriented parallel to the surface. As the surface coverage increases to near saturation, a two-dimensional phase transition occurs and produces islands composed of molecules with their hydrocarbon axis oriented ∼30° from the surface normal. Continued exposure to the thiol solution results in a greater number of these islands and the growth of these nuclei until a SAM is formed with a commensurate (∛×∛)R30° structure. The growth of the lying-down phase follows a first-order Langmuir adsorption isotherm, while the phase transition is best described by a second-order reaction. The kinetics of the se...

Fabrication of Nanometer-Sized Protein Patterns Using Atomic Force Microscopy and Selective Immobilization

A new methodology is introduced to produce nanometer-sized protein patterns. The approach includes two main steps, nanopatterning of self-assembled monolayers using atomic force microscopy (AFM)-based nanolithography and subsequent selective immobilization of proteins on the patterned monolayers. The resulting templates and protein patterns are characterized in situ using AFM. Compared with conventional protein fabrication methods, this approach is able to produce smaller patterns with higher spatial precision. In addition, fabrication and characterization are completed in near physiological conditions. The adsorption configuration and bioreactivity of the proteins within the nanopatterns are also studied in situ.

Elucidating the role of surface hydrolysis in preparing organosilane nanostructures via particle lithography.

A new method of particle lithography is described for preparing rings or nanoporous films of organosilanes. Millions of exquisitely uniform and precisely spaced nanostructures with designed surface chemistry can be rapidly produced using vapor deposition through mesoparticle masks. Nanoscopic amounts of water are essential for initiating surface hydrosilation. Thus, the key step for preparing covalently bonded nanostructures of organosilanes is to control drying parameters to spatially direct the placement of water on surfaces.

Porphyrin nanoparticles as supramolecular systems

Certain applications of supramolecular porphyrinic systems, such as molecular sieves and photonics, rely on precise nanoarchitectural control of the molecules and/or atoms; therefore they require self-assembled systems of discrete arrays and highly ordered crystals. Other applications, such as oxidation catalysts for simple substrates, may be affected by the use of self-organized materials with less supramolecular order. Colloidal porphyrin nanoparticles can be considered self-organized systems that are governed by the principles of supramolecular chemistry. The formation and potential applications of nanoparticles of these chromophores are discussed in this report with special emphasis on the parameters in the methods used to make these materials, and in terms of the supramolecular chemistry. These principles, concepts, and methodologies are applicable to a wide variety of organic dyes. “Mais Dieu a choisi celuy qui est le plus parfait, c’est a dire celuy qui en meme temps le plus simple en hypotheses, et le plus rich en phenomenes.” Gottfried Leibniz.

Engineering the spatial selectivity of surfaces at the nanoscale using particle lithography combined with vapor deposition of organosilanes

Particle lithography is a practical approach to generate millions of organosilane nanostructures on various surfaces, without the need for vacuum environments or expensive instrumentation. This report describes a stepwise chemistry route to prepare organosilane nanostructures and then apply the patterns as a spatially selective foundation to attach gold nanoparticles. Sites with thiol terminal groups were sufficiently small to localize the attachment of clusters of 2-5 nanoparticles. Basic steps such as centrifuging, drying, heating, and rinsing were used to generate arrays of regular nanopatterns. Close-packed films of monodisperse latex spheres can be used as an evaporative mask to spatially direct the placement of nanoscopic amounts of water on surfaces. Vapor phase organosilanes deposit selectively at areas of the surface containing water residues to generate nanostructures with regular thickness, geometry, and periodicity as revealed in atomic force microscopy images. The area of contact underneath the mesospheres is effectively masked for later synthetic steps, providing exquisite control of surface coverage and local chemistry. By judicious selection in designing the terminal groups of organosilanes, surface sites can be engineered at the nanoscale for building more complex structures. The density of the nanopatterns and surface coverage scale predictably with the diameter of the mesoparticle masks. The examples presented definitively illustrate the capabilities of using the chemistry of molecularly thin films of organosilanes to spatially define the selectivity of surfaces at very small size scales.

Magnetic and Nonmagnetic Nanoparticles from a Group of Uniform Materials Based on Organic Salts

The size and uniformity of magnetic nanoparticles developed from a group of uniform materials based on organic salts (GUMBOS) were controlled using an in situ ion exchange, water-in-oil (w/o) microemulsion preparation. Most of these nanoGUMBOS are in fact ionic liquids (i.e., melting points less than 100 degrees C), while others have melting points above the conventional 100 degrees C demarcation. Simple variations in the reagent concentrations following a w/o approach allowed us to smoothly and predictably vary nanoparticle dimensions across a significant size regime with excellent uniformity. Average sizes of GUMBOS particles ranging from 14 to 198 nm were achieved by manipulation of the reagent concentration, for example. Controllable formation of this new breed of nanoparticles is important for numerous potential applications and will open up interesting new opportunities in drug delivery, magnetic resonance imaging, and protein separations, among other areas.

Self-assembly of octadecyltrichlorosilane: Surface structures formed using different protocols of particle lithography

Summary Particle lithography offers generic capabilities for the high-throughput fabrication of nanopatterns from organosilane self-assembled monolayers, which offers the opportunity to study surface-based chemical reactions at the molecular level. Nanopatterns of octadecyltrichlorosilane (OTS) were prepared on surfaces of Si(111) using designed protocols of particle lithography combined with either vapor deposition, immersion, or contact printing. Changing the physical approaches for applying molecules to masked surfaces produced OTS nanostructures with different shapes and heights. Ring nanostructures, nanodots and uncovered pores of OTS were prepared using three protocols, with OTS surface coverage ranging from 10% to 85%. Thickness measurements from AFM cursor profiles were used to evaluate the orientation and density of the OTS nanostructures. Differences in the thickness and morphology of the OTS nanostructures are disclosed based on atomic force microscopy (AFM) images. Images of OTS nanostructures prepared on Si(111) that were generated by the different approaches provide insight into the self-assembly mechanism of OTS, and particularly into the role of water and solvents in hydrolysis and silanation.

Supramolecular structures in nanocomposite multilayered films

We investigate the multilayered structures of poly(ethylene)oxide/montmorillonite nanocomposite films made from solution. The shear orientation of a polymer-clay network in solution combined with simultaneous solvent evaporation leads to supramolecular multilayer formation in the film. The resulting films have highly ordered structures with sheet-like multilayers on the micrometer length scale. The polymer covered clay platelets were found to orient in interconnected blob-like chains and layers on the nanometer length scale. Inside the blobs, scattering experiments indicate the polymer covered and stacked clay platelets oriented in the plane of the film. The polymer is found to be partially crystalline although this is not visible by optical microscopy. Atomic force microscopy suggests that the excess polymer, which is not directly adsorbed to the clay, is wrapped around the stacked platelets building blobs and the polymer also interconnects the polymer-clay layers. Overall our results suggest the re-intercalation of clay platelets in films made from exfoliated polymer-clay solutions as well as the supramolecular order and hierarchical structuring on the nanometer, via micrometer to the centimeter length scale.

Polythiophenes Containing In-Chain Cobaltabisdicarbollide Centers

New cobalt(III) bis(dicarbollide) complexes covalently linked to two 2-oligothienyl units have been synthesized and electropolymerized in acetonitrile electrolyte in order to produce the corresponding polythiophene films containing in-chain metallic centers. The polymer films electrogenerated from the bithienyl (4b) and terthienyl (4c) derivatives display redox processes attributed to the Co(III)/Co(II) couple at ca. -1.1 V vs SCE and to the p-doping/undoping of the expected quaterthienyl and sexithienyl segments at ca. 0.8 V vs SCE. In contrast, the anodic oxidation of the thienyl (4a) derivative leads to passivation of the electrode surface. As the length of the oligothiophene substituents increases, the metallic and dicarbollide cage carbon atoms contributions in the HOMO decrease dramatically so that the highest occupied frontier orbitals of 4b and 4c can be considered as almost purely oligothiophene-based. From further UV-vis spectroscopy analysis, it is demonstrated that the polymer incorporating the sexithienyl segments is more conjugated than that with the quaterthienyl segments as the absorption maximum for the interband pi-pi* transition was observed at 410 and 448 nm for poly(4b) and poly(4c) respectively. Furthermore, these polymers display a more extended degree of conjugation than the parent oligothiophenes. Such features indicate a significant electronic delocalization through the cobaltabisdicarbollide moiety. Their conducting probe atomic force microscopy characterization indicates that poly(4b) and poly(4c) behave like heavily doped semiconductors rather than pure semiconductors. Mean conductivity values extracted from the current-voltage profiles are 1.4 x 10(-4) and 7.5 x 10(-4) S cm(-1) for poly(4b) and poly(4c), respectively. Such materials are found to be efficient for the electrocatalytic reduction of protons to dihydrogen, as exemplified for poly(4b). The overpotential for hydrogen evolution is significantly decreased by ca. 230 mV with respect to that obtained with the bare electrode (measured for a current density of 1.4 mA cm(-2) in the presence of 20 mM HBF(4)).

Self-Assembled Monolayer Initiated Electropolymerization: A Route to Thin-Film Materials with Enhanced Photovoltaic Performance

Continuing progress in the field of organic polymer photovoltaic (PV) devices requires the development of new materials with better charge-transport efficiency. To improve this parameter, we have investigated surface-attached bilayer polymer PV thin films prepared starting from a covalently attached monolayer of an electroactive initiator using sequential electropolymerization of dithiophene and its derivatives. These systems were found to show significantly increased photocurrent generation quantum yields as compared to systems made through conventional approaches. In addition, the described PV thin films possess remarkable mechanical, air, and photostability. These properties likely arise from the more uniform and better ordered bulk layer morphologies as well as tighter covalently bonded contacts at the interfacial junctions, contributing to improved charge transport. While more studies on the fundamental reasons behind the discovered phenomenon are currently underway, this information can be readily applied to build more efficient organic polymer photovoltaics.