Katherine E. Aidala

Quantitative changes in the elasticity and adhesive properties of Escherichia coli ZK1056 prey cells during predation by bdellovibrio bacteriovorus 109J.

Atomic force microscopy (AFM) was used to explore the changes that occur in Escherichia coli ZK1056 prey cells while they are being consumed by the bacterial predator Bdellovibrio bacteriovorus 109J. Invaded prey cells, called bdelloplasts, undergo substantial chemical and physical changes that can be directly probed by AFM. In this work, we probe the elasticity and adhesive properties of uninvaded prey cells and bdelloplasts in a completely native state in dilute aqueous buffer without chemical fixation. Under these conditions, the rounded bdelloplasts were shown to be shorter than uninvaded prey cells. More interestingly, the extension portions of force curves taken on both kinds of cells clearly demonstrate that bdelloplasts are softer than uninvaded prey cells, reflecting a decrease in bdelloplast elasticity after invasion by Bdellovibrio predators. On average, the spring constant of uninvaded E. coli cells (0.23 +/- 0.02 N/m) was 3 times stiffer than that of the bdelloplast (0.064 +/- 0.001 N/m) when measured in a HEPES-metals buffer. The retraction portions of the force curves indicate that compared to uninvaded E. coli cells bdelloplasts adhere to the AFM tip with much larger pull-off forces but over comparable retraction distances. The strength of these adhesion forces decreases with increasing ionic strength, indicating that there is an electrostatic component to the adhesion events.

Characterizing Pilus-Mediated Adhesion of Biofilm-Forming E. coli to Chemically Diverse Surfaces Using Atomic Force Microscopy

Biofilms are complex communities of microorganisms living together at an interface. Because biofilms are often associated with contamination and infection, it is critical to understand how bacterial cells adhere to surfaces in the early stages of biofilm formation. Even harmless commensal Escherichia coli naturally forms biofilms in the human digestive tract by adhering to epithelial cells, a trait that presents major concerns in the case of pathogenic E. coli strains. The laboratory strain E. coli ZK1056 provides an intriguing model system for pathogenic E. coli strains because it forms biofilms robustly on a wide range of surfaces.E. coli ZK1056 cells spontaneously form living biofilms on polylysine-coated AFM cantilevers, allowing us to measure quantitatively by AFM the adhesion between native biofilm cells and substrates of our choice. We use these biofilm-covered cantilevers to probe E. coli ZK1056 adhesion to five substrates with distinct and well-characterized surface chemistries, including fluorinated, amine-terminated, and PEG-like monolayers, as well as unmodified silicon wafer and mica. Notably, after only 0–10 s of contact time, the biofilms adhere strongly to fluorinated and amine-terminated monolayers as well as to mica and weakly to “antifouling” PEG monolayers, despite the wide variation in hydrophobicity and charge of these substrates. In each case the AFM retraction curves display distinct adhesion profiles in terms of both force and distance, highlighting the cells’ ability to adapt their adhesive properties to disparate surfaces. Specific inhibition of the pilus protein FimH by a nonhydrolyzable mannose analogue leads to diminished adhesion in all cases, demonstrating the critical role of type I pili in adhesion by this strain to surfaces bearing widely different functional groups. The strong and adaptable binding of FimH to diverse surfaces has unexpected implications for the design of antifouling surfaces and antiadhesion therapies.

Nanoscale morphology revealed at the interface between colloidal quantum dots and organic semiconductor films.

The degree of interpenetration at the interface between colloidal quantum dots (QDs) and organic semiconductor molecules commonly employed in hybrid light-emitting devices (QD-LEDs) has been examined using tapping-mode atomic force microscopy. Both phase separation-driven and Contact Printing-enabled QD/semiconductor heterojunction fabrication methodologies lead to significant QD embedment in the underlying organic film with the greatest degree of QD penetration observed for QD monolayers that have been contact printed. The relative performance of QD-LEDs fabricated via three different methods using the same materials set has also been investigated.

Detection of charge storage on molecular thin films of tris(8-hydroxyquinoline) aluminum (Alq3) by Kelvin force microscopy: a candidate system for high storage capacity memory cells.

Retention and diffusion of charge in tris(8-hydroxyquinoline) aluminum (Alq(3)) molecular thin films are investigated by injecting electrons and holes via a biased conductive atomic force microscopy tip into the Alq(3) films. After the charge injection, Kelvin force microscopy measurements reveal minimal changes with time in the spatial extent of the trapped charge domains within Alq(3) films, even for high hole and electron densities of >10(12) cm(-2). We show that this finding is consistent with the very low mobility of charge carriers in Alq(3) thin films (<10(-7) cm(2)/(Vs)) and that it can benefit from the use of Alq(3) films as nanosegmented floating gates in flash memory cells. Memory capacitors using Alq(3) molecules as the floating gate are fabricated and measured, showing durability over more than 10(4) program/erase cycles and the hysteresis window of up to 7.8 V, corresponding to stored charge densities as high as 5.4 × 10(13) cm(-2). These results demonstrate the potential for use of molecular films in high storage capacity nonvolatile memory cells.

Contact printing of colloidal nanocrystal thin films for hybrid organic/quantum dot optoelectronic devices

Novel thin film optoelectronic devices containing both inorganic colloidal semiconductor quantum dots (QDs) and organic semiconductor thin films have been widely investigated in recent years for a variety of applications. Here, we review one of the most versatile and successful methods developed to integrate these two dissimilar material classes into a functional multilayered device: contact printing of colloidal QD films. Experimental details regarding the contact printing process are outlined, and the key advantages of this QD deposition method over other commonly encountered techniques are discussed. The use of tapping mode atomic force microscopy (AFM) to effectively characterize QD film morphology both on an elastomeric stamp (before contact printing) and as-transferred to the organic semiconductor receiving film (after contact printing) is also described. Finally, we offer suggestions for future efforts directed toward the goal of rapid, continuous QD deposition over larger substrates for the advancement of hybrid optoelectronic thin film devices.

Manipulation of magnetization states of ferromagnetic nanorings by an applied azimuthal Oersted field

We manipulate the magnetic states of ferromagnetic nanorings with an azimuthal Oersted field directed along the ring circumference. The circular field is generated by passing current through an atomic force microscope tip positioned at the center of the ring, and can directly control the chirality of the vortex state. We demonstrate switching from an onion state to a vortex state and between two vortex states, using magnetic force microscopy to image the resulting magnetic states. The understanding of the magnetization switching behavior in an azimuthal Oersted field could improve practical magnetic data storage devices.

Multiple 360° domain wall switching in thin ferromagnetic nanorings in a circular magnetic field

Micromagnetic simulations of the vortex switching process of thin ferromagnetic rings under the application of a circular field, as if created from a current-carrying wire passing through the ring center, reveal that for rings with sub-micron dimensions and thicknesses on the order of the exchange length, the vortex to vortex switching process occurs through the nucleation and annihilation of multiple 360° domain walls (DWs). The DWs can be characterized by their circulation relative to the vortex circulation; the DWs form in pairs with opposite topological indices. The DW with the same circulation annihilates first, which has a smaller energy barrier to overcome before annihilating. The contributions from both the exchange energy and demagnetization energy must be considered to predict which DW will annihilate first. Either wall could be annihilated by offsetting the current toward the wall being targeted.

High-density charge storage on molecular thin films - candidate materials for high storage capacity memory cells

High storage capacity memory cells were fabricated by integrating organic molecules, with dimension ∼1 nm, into flash memory devices. The stored charge within the molecules was directly probed using Kelvin force microscopy, revealing strong confinement of stored charge over time. Molecular memories with increased storage capacity were achieved via addition of dopant molecules.

The creation of 360° domain walls in ferromagnetic nanorings by circular applied magnetic fields

Switching behavior in ferromagnetic nanostructures is often determined by the formation and annihilation of domain walls (DWs). In contrast to the more familiar 180° DWs found in most nanostructures, 360° DWs are the proposed transition state of nanorings. This paper examines the formation of 360° DWs created by the application of a circular magnetic field using micromagnetic simulations. 360° DWs form from pairs of canting moments that are oppositely aligned, which each grow to form rotated domains bounded by two 180° DWs and the 180° DWs combine to form 360° DWs. The resulting 360° DWs occur in pairs of opposite topological winding number due to these domains of opposite canting direction. The final number of DWs formed is greatly impacted by symmetry, both of the ring and of the placement of the circular magnetic field.

A multi-level single-bit data storage device

One method to increase bit density in magnetic memory devices is to use larger structures that have multiple states in which to encode information rather than the typical two state system. A ferromagnetic nanoring with multiple domain walls that annihilate at different applied magnetic fields could serve as such a bit. This paper examines the formation and annihilation of four 360° domain walls (DWs) using micromagnetic simulations. To create the walls, one can apply circular magnetic fields to asymmetric nanoring structures. Nanorings with circular notches on a centered elliptical hole enable the formation of stable DWs in specific locations with known characteristics. By considering the impacts of both domain wall length and topological winding number on domain wall energy, one can create a nanostructure with four stable domain walls that annihilate at different applied magnetic fields. With two stable vortex configurations, such nanorings could theoretically encode up to ten different states.