We report on a novel nanoparticle platform by electric field assisted assembly, which is capable of manipulating the refractive index distribution through controlling the particle assembly. Two examples based on the control of the scattering properties are presented. We demonstrate lensless imaging in such a system. In addition, we show that random lasing can be enhanced by assembly of anisotropic particles immersed in a gain medium. These examples illustrate that particle assembly technique provides a promising platform for reconfigurable optical applications.
With the increasing interest in simultaneous detection of specific DNA hybridization events, the development of methods to measure multiple DNA interactions at one time is of great importance. Conventional microarrays allow thousands of DNA hybridization interactions to be measured at once, however, this method of detection is limited by high cost as well as the stability and characteristic properties of fluorescent dyes. Here, barcoded nanowires are investigated as replacements for fluorophores on glass surfaces such as those used in microarrays. Potential advantages of nanowires include ease of reflectance-based optical read-out, the large number of tags available, and ability to distinguish multiple hybridizations occurring in a single DNA spot. A method of attaching DNA to glass microscope slides was developed which includes the use of a carboxy terminated silane to derivatize glass slides for DNA attachment. Also determined here is the efficiency of using nanowires as tags in complementary DNA hybridization events. An average of ~5% nonspecific binding was reported for nanowire attachment for all samples.
In recent years there has been a great deal of interest in the measurement of DNA hybridization at surfaces. Surface-confined DNA hybridization has been used to monitor gene expression, to detect the presence of a particular DNA sequence and determine single nucleotide polymorphisms (SNPs). DNA microarrays, which can contain thousands of discrete DNA sequences on a single surface, have become widely used for hybridization studies. While a powerful technique, this technology is limited by the stability of the fluorescent dyes used to label the DNA, and the need to perform measurements ex-situ to reduce the fluorescence background. In this report, we describe the use of colloid-amplified surface plasmon resonance (SPR) to measure DNA hybridization at surfaces. SPR is a surface sensitive technique, which can be used to study hybridization in situ, and the use of colloidal metal tags provides excellent sensitivity. Angle-scanning SPR has been used to study oligonucleotide hybridization to surface confined probes, and work is underway to apply SPR imaging to study DNA hybridization in macro- and microarray formats.
The local structure of biological membranes is critically important to membrane function. Regions of very high positive and negative curvature are found in the membranes of many cells, and rapid changes in membrane curvature are integral parts of many cell activities (e.g. endo/exocytosis, cell crawling, cell division). Our goal is to understand the effects of changes in local membrane structure on membrane properties. Optical tweezers are used to control the local structure of the lipid bilayer by controlling the curvature of the membrane. We use giant (`cell-sized'), thin-walled vesicles as our membrane models. Optically trapped latex microspheres are used to deform the liposome bilayer, forming large areas of altered membrane curvature. In contrast to literature reports in which 514.5 nm light was used in optical trapping, we have not observed adhesion of uncoated latex microspheres to liposome vesicles, nor have we observed signs of rapidly increased osmotic pressure within irradiated vesicles. This indicates that the longer wavelength used in our studies (647.1 nm) is less damaging to biological membranes. Furthermore, optical trapping of vesicles with coexisting gel and fluid phase lipids did not lead to gross changes in domain structure, which would be expected upon laser-induced heating of the membrane.
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