Proposed Triaxial Atomic Force Microscope Contact Free Tweezers for Nanoassembly

We propose a triaxial atomic force microscope contact-free tweezer (TACT) for the controlled assembly of nanoparticles suspended in a liquid. The TACT overcomes four major challenges faced in nanoassembly, as follows. (1) The TACT can hold and position a single nanoparticle with spatial accuracy smaller than the nanoparticle size (~5 nm). (2) The nanoparticle is held away from the surface of the TACT by negative dielectrophoresis to prevent van der Waals forces from making it stick to the TACT. (3) The TACT holds nanoparticles in a trap that is size-matched to the particle and surrounded by a repulsive region so that it will only trap a single particle at a time. (4) The trap can hold a semiconductor nanoparticle in water with a trapping energy greater than the thermal energy. For example, a 5 nm radius silicon nanoparticle is held with 10 kBT at room temperature. We propose methods for using the TACT as a nanoscale pick-and-place tool to assemble semiconductor quantum dots, biological molecules, semiconductor nanowires, and carbon nanotubes.Engineering and Applied Science

Triaxial AFM Probes for Noncontact Trapping and Manipulation

We show that a triaxial atomic force microscopy probe creates a noncontact trap for a single particle in a fluid via negative dielectrophoresis. A zero in the electric field profile traps the particle above the probe surface, avoiding adhesion, and the repulsive region surrounding the zero pushes other particles away, preventing clustering. Triaxial probes are promising for three-dimensional assembly and for selective imaging of a particular property of a sample using interchangeable functionalized particles.Engineering and Applied Science

Microwave Dielectric Heating of Drops in Microfluidic Devices

We present a technique to locally and rapidly heat water drops in microfluidic devices with microwave dielectric heating. Water absorbs microwave power more efficiently than polymers, glass, and oils due to its permanent molecular dipole moment that has a large dielectric loss at GHz frequencies. The relevant heat capacity of the system is a single thermally isolated picoliter drop of water and this enables very fast thermal cycling. We demonstrate microwave dielectric heating in a microfluidic device that integrates a flow-focusing drop maker, drop splitters, and metal electrodes to locally deliver microwave power from an inexpensive, commercially available 3.0 GHz source and amplifier. The temperature of the drops is measured by observing the temperature dependent fluorescence intensity of cadmium selenide nanocrystals suspended in the water drops. We demonstrate characteristic heating times as short as 15 ms to steady-state temperatures as large as 30 degrees C above the base temperature of the microfluidic device. Many common biological and chemical applications require rapid and local control of temperature, such as PCR amplification of DNA, and can benefit from this new technique.Comment: 6 pages, 4 figure

Self-Driving Capacitive Cantilevers for High-Frequency Atomic Force Microscopy

We demonstrate a simple way to actuate an atomic force microscope cantilever at high frequencies by electrically driving a thin-film capacitor on its surface. Capacitive driving directly actuates the vibrational mode of the cantilever, removing the effects of unwanted mechanical modes present in conventional driving systems and removing the need for a drive piezoelectric. Practical vibration amplitudes are attainable at drive voltages <5 V. We capacitively drive the first mechanical resonance of a tapping mode cantilever (243 kHz) and a high-frequency cantilever (1.5 MHz) with vibration amplitudes in agreement with our model of capacitive driving.Physic

High Spatial Resolution Kelvin Probe Force Microscopy With Coaxial Probes

Kelvin probe force microscopy (KPFM) is a widely used technique to measure the local contact potential difference (CPD) between an AFM probe and the sample surface via the electrostatic force. The spatial resolution of KPFM is intrinsically limited by the long range of the electrostatic interaction, which includes contributions from the macroscopic cantilever and the conical tip. Here, we present coaxial AFM probes in which the cantilever and cone are shielded by a conducting shell, confining the tip-sample electrostatic interaction to a small region near the end of the tip. We have developed a technique to measure the true CPD despite the presence of the shell electrode. We find the behavior of these probes agrees with an electrostatic model of the force, and we observe a factor of 5 improvement in spatial resolution relative to unshielded probes. Our discussion centers on KPFM, but the field confinement offered by these probes may improve any variant of electrostatic force microscopy.Engineering and Applied Science

The Importance of Cantilever Dynamics in the Interpretation of Kelvin Probe Force Microscopy

A realistic interpretation of the measured contact potential difference (CPD) in Kelvin probe force microscopy (KPFM) is crucial in order to extract meaningful information about the sample. Central to this interpretation is a method to include contributions from the macroscopic cantilever arm, as well as the cone and sharp tip of a KPFM probe. Here, three models of the electrostatic interaction between a KPFM probe and a sample are tested through an electrostatic simulation and compared with experiment. In contrast with previous studies that treat the KPFM cantilever as a rigid object, we allow the cantilever to bend and rotate; accounting for cantilever bending provides the closest agreement between theory and experiment. We demonstrate that cantilever dynamics play a major role in CPD measurements and provide a simulation technique to explore this phenomenon.Physic

An analysis of interactions between fluorescently-tagged mutant and wild-type SOD1 in intracellular inclusions

By mechanisms yet to be discerned, the co-expression of high levels of wild-type human superoxide dismutase 1 (hSOD1) with variants of hSOD1 encoding mutations linked familial amyotrophic lateral sclerosis (fALS) hastens the onset of motor neuron degeneration in transgenic mice. Although it is known that spinal cords of paralyzed mice accumulate detergent insoluble forms of WT hSOD1 along with mutant hSOD1, it has been difficult to determine whether there is co-deposition of the proteins in inclusion structures.In the present study, we use cell culture models of mutant SOD1 aggregation, focusing on the A4V, G37R, and G85R variants, to examine interactions between WT-hSOD1 and misfolded mutant SOD1. In these studies, we fuse WT and mutant proteins to either yellow or red fluorescent protein so that the two proteins can be distinguished within inclusions structures.Although the interpretation of the data is not entirely straightforward because we have strong evidence that the nature of the fused fluorophores affects the organization of the inclusions that form, our data are most consistent with the idea that normal dimeric WT-hSOD1 does not readily interact with misfolded forms of mutant hSOD1. We also demonstrate the monomerization of WT-hSOD1 by experimental mutation does induce the protein to aggregate, although such monomerization may enable interactions with misfolded mutant SOD1. Our data suggest that WT-hSOD1 is not prone to become intimately associated with misfolded mutant hSOD1 within intracellular inclusions that can be generated in cultured cells

Screening for electrically conductive defects in thin functional films using electrochemiluminescence

Multifunctional thin films in energy-related devices often must be electrically insulating where a single nanoscale defect can result in complete device-scale failure. Locating and characterizing such defects presents a fundamental problem where high-resolution imaging methods are needed to find defects, but imaging with high spatial resolution limits the field of view and thus the measurement throughput. Here, we present a novel high-throughput method for detecting sub-micron defects in insulating thin films by leveraging the electrochemiluminescence (ECL) of luminol. Through a systematic study of reagent concentrations, buffers, voltage, and excitation time, we identify optimized conditions at which it is possible to detect features with areas ~500 times smaller than the area interrogated by a single pixel of the camera, showing high-throughput detection of sub-micron defects. In particular, we estimate the minimum detectable features to be lines as narrow as 2.5 nm in width and pinholes as small as 35 nm in radius. We further explore this method by using it to characterize a nominally insulating phenol film and find conductive defects that are cross-correlated with high-resolution atomic force microscopy to provide feedback to synthesis. Given the inherent parallelizability and scalability of this assay, it is expected to have a major impact on the automated discovery of multifunctional films.Comment: 24 pages, 5 figures, submitted to Langmui

Performance of Pre-Cut Lettuce Packaged in Biodegradable Film Formed on Commercial Vertical-Form-Fill-and-Seal Machines

The purpose of this study was to determine the feasibility of using biodegradable films suitable for fresh-cut lettuce with commercial vertical-form-fill-and-seal packaging machines (VVFS) equipped with heat-sealing bars. Biodegradable high-density polyethylene (BHDPE) and polypropylene (BPP) films were tested. Commercial bags of pre-cut Romaine sealed in a polyethylene/oriented polypropylene (PE/OPP) bag formed on a WFS machine were used as the control. All bags were held at 4.4°C, 80% RH and assessed for reduction in quality during storage per a commercial (in-house) standard utilized by a large pre-cut salad packer in Salinas, California. When the biodegradable films were sealed with a VFFS machine equipped with a thermal-bar heat sealer, a 52.5% fail rate was observed due to the non-continuity of the seals. Leaks were found when bags were vacuum tested to 14 in Hg absolute for 15 seconds. However, a 45.5% fail rate was also observed for commercial bags made using the same VFFS machine, suggesting similar seal concerns for current industry film structures. Though an attempt was made to only store bags that were sealed properly, bags made from the biodegradable films sealed with the thermal-bar did not perform as well as the commercial packages and the shelf-life of the pre-cut Romaine was shortened. When biodegradable bags were sealed using a bar impulse sealer, hermetic seals were obtained. The Romaine stored in these bags had a similar rate of decay and level of pinking after 14 days storage as Romaine packaged in the commercial PE/OPP bags. These results indicate that the use of commercial impulse sealers, rather than thermal-bar heat sealers, would allow industry to utilize these biodegradable films for pre-cut lettuce mixes