We examine the enhancement of optical trapping forces due to plasmon resonances of nanoshells. Nanoshells are nanoscale
particles with a dielectric core and metallic coating that exhibit tunable plasmon resonances. Theory predicts that
the optical trapping force may be three to fifty times larger for trapping-laser wavelengths near resonance than for
wavelengths far from resonance . The resonance absorption of nanoshells can be tuned by adjusting the ratio of the
radius of the dielectric core, r<sub>1</sub>, to the total radius, r<sub>2</sub> . Using back focal plane detection, we measure the trap stiffness
of optical tweezers, from lasers at 973 nm and 1064 nm, for single trapped nanoshells with several different r<sub>1</sub>/r<sub>2</sub> ratios.
Enhanced trapping strengths are not found through these measurements done with single wavelength optical traps. A
tunable-wavelength laser trap will enable more conclusive results.
Photothermal therapy employing nanomaterials is a promising approach to selectively treat targeted tissues with
abnormal characteristics such as tumors. While vital research has focused on the use of these materials in biomedical
applications, net effects of these materials in biological environments are still not well understood. For reliable
biomedical applications, it is crucial to quantitatively evaluate thermal properties of these materials in biological and
physiological environments. To this end, we have developed a highly integrated measurement platform and examined
local thermal properties of single gold shell nanocrystals in biomimetic environments. These nanoshells consist of a
silica core with an outer gold coating. For quantitative measurement of the local thermal profile of gold nanoshells, we
monitor lipid phase transitions triggered by gold nanoshell thermal excitation. Dried lipid layers with adsorbed gold
nanoshells were placed in an aqueous environment. Photothermal excitation of the gold nanoshells induced localized
liposome budding as the lipids were raised above their transition temperature. Single particle tracking of gold
nanoshells in solution and within liposomes revealed larger diffusion rates for the confined nanoparticles, likely due to a
raised local temperature.
We report on improvements and innovations in the use of hydrosomes to encapsulate and study single molecules.
Hydrosomes are optically-trappable aqueous nanodroplets. The droplets are suspended in a fluorocarbon medium that
is immiscible with water and has an index of refraction lower than water, so hydrosomes are stable and optically trapped
by a focused laser beam (optical tweezers). Using optical tweezers, we hold the hydrosomes within a confocal
observation volume and interrogate the encapsulated molecule by fluorescence excitation. This method allows for long
observation times of a molecule without the need for surface immobilization or liposome encapsulation. We have
developed a new way for creating hydrosomes on demand by inertially launching them into the fluorocarbon matrix
using a piezo-activated micropipette. Time-resolved fluorescence anisotropy studies are carried out to characterize the
effects of the hydrosome interface boundary on biological molecules and to determine whether molecules encapsulated
within hydrosomes diffuse freely throughout the available volume. We measured the fluorescence anisotropy decay of
20mer DNA duplexes, and enhanced green fluorescent protein (GFP). We conclude that the molecules rotate freely
inside the nanodroplets and do not stick or aggregate at the boundary.
We investigate near-resonant trapping of Rayleigh particles in optical tweezers. Although optical forces due to a near-resonant laser beam have been extensively studied for atoms, the situation for larger particles is that the laser wavelength is far from any absorption resonance. Theory predicts, however, that the trapping force exerted on a Rayleigh particle is enhanced, and may be three to fifty times larger for frequencies near resonance than for frequencies far off resonance. The ability to selectively trap only particles with a given absorption peak may have many practical applications.
In order to investigate near-resonant trapping we are using nanoshells, particles with a dielectric core and metallic coating that can exhibit plasmon resonances. The resonances of the nanoshells can be tuned by adjusting the ratio of the radius of the dielectric core, r<sub>1</sub>, to the overall radius, r<sub>2</sub>, which includes the thickness of the metallic coating. Our nanoshells, fabricated at Rice University, consist of a silica core with a gold coating. Using back focal plane detection, we measure the trap stiffness of a single focus optical trap (optical tweezers), from a diode laser at 853 nm for nanoshells with several different r<sub>1</sub>/r<sub>2</sub> ratios.
We demonstrate a novel technique for creating, manipulating, and combining femtoliter to attoliter volume chemical containers. Possible uses include creating controlled chemical reactions involving small quantities of reagent, and studying the dynamics of single molecules. The containers, which we call hydrosomes, are surfactant stabilized aqueous droplets in a low index-of-refraction fluorocarbon medium. The index of refraction mismatch between the container and fluorocarbon is such that individual hydrosomes can be optically trapped by single focus laser beams, i.e. optical tweezers. Previous work on single molecules usually involved the tethering of the molecule to a surface, in order to interrogate the molecule for an extended period of time. The use of hydrosomes opens up the possibility for studying free molecules, away from any perturbing surface. We show that this is indeed true in the case of quantitative FRET with RNA. Furthermore, we demonstrate the controlled fusion of two hydrosomes for studying reactions, such as DNA binding kinetics, and single molecule dynamics under non-equilibrium conditions. We also show the applicability of our technique in analytical chemistry, such as for molecule identification and sorting.
We create long polymer nanotubes by directly pulling on the membrane of polymersomes using
either optical tweezers or a micropipette. The polymersomes are composed of amphiphilic diblock
copolymers and the nanotubes formed have an aqueous core connected to the aqueous interior of the
polymersome. Stabilized membranes of nanotubes and vesicles were formed by the directed selfassembly
of poly(ethylene oxide)-block-polybutadiene, followed by photopolymerization, initiated
by UV light, to a maximum double bond conversion of 15%. The photopolymerized nanotubes are
extremely robust. The applicability of photopolymerization for biophysics and bioanalytical science
is demonstrated by electrophoresing DNA molecules through a stabilized nanotube with an
integrated vesicle reservoir.
We create long polymer nanotubes by directly pulling on the membrane of polymersomes using either optical tweezers or a micropipette. The polymersomes are composed of amphiphilic diblock copolymers and the nanotubes formed have an aqueous core connected to the aqueous interior of the polymersome. We stabilize the pulled nanotubes by subsequent chemical cross-linking. The cross-linked nanotubes are extremely robust and can be moved to another medium for use elsewhere. We demonstrate the ability to form networks of polymer nanotubes and polymersomes by optical manipulation. The aqueous core of the polymer nanotubes together with their robust character makes them interesting candidates for nanofluidics and other applications in biotechnology.
We are developing optically based techniques for the manipulation of nano-containers (containers with sub-picoliter volumes) for handling chemicals in order to perform ultra-small volume chemistry. We are currently investigating three systems, liposomes, polymersomes and hydrosomes, for use as nano-containers. Liposomes and polymersomes are closed structures composed of a lipid and polymer membrane, respectively, that acts as a barrier to separate an aqueous interior environment from an aqueous exterior environment. We are typically working with liposomes or polymersomes that are approximately 10 μm in diameter. Hydrosomes are micron-sized, surfactant-stabilized water droplets that reside in a fluorocarbon environment. The optical techniques we are using include optical tweezers, for trapping and remotely moving the nano-containers, and an "optical scalpel" for localized disruption of lipid and polymer membranes in order to induce fusion of liposomes and polymersomes. In all three systems, we are able to bring together two similar nano-container using optical trapping and subsequently fuse them together, which allows their contents to mix. With the liposomes and hydrosomes we have been able to demonstrate their use for performing a controlled, elementary chemical reaction.
Using optical tweezers and microfluidics, we stretch either the lipid or polymer membranes of liposomes or polymersomes, respectively, into long nanotubes. The membranes can be grabbed directly with the optical tweezers to produce sub-micron diameter tubes that are several hundred microns in length. We can stretch tubes up to a centimeter in length, limited only by the travel of our microscope stage. We also demonstrate the cross linking of a pulled polymer nanotube.
We have developed a new assay in which two mesoscale particles are caused to collide using two independently controlled optical tweezers. This assay involves the measurement of the adhesion probability following a collision. Since the relative orientation, impact parameter (i.e., distance of closest approach), and collision velocity of the particles, as well as the components of the solution, are all under the user's control, this assay can mimic a wide range of biologically relevant collisions. We illustrate the utility of our assay by evaluating the adhesion probability of a single erythrocyte (red blood cell) to an influenza virus-coated microsphere, in the presence of sialic acid-bearing inhibitors of adhesion. This probability as a function of inhibitor concentration yields a measure of the effectiveness of the inhibitor for blocking viral adhesion. Most of the inhibition constants obtained using the tweezers agree well with those obtained from other techniques, although the inhibition constants for the best of the inhibitors were beyond the limited resolution of conventional assays. They were readily evaluated using our tweezers-based assay, however, and prove to be the most potent inhibitors of adhesion between influenza virus and erythrocytes ever measured. Further studies are underway to investigate the effect of collision velocity on the adhesion probability, with the eventual goal of understanding the various mechanisms of inhibition (direct competition for viral binding sites versus steric stabilization). Analysis of these data also provide evidence that the density of binding sites may be a crucial parameter in the application of this assay and polymeric inhibition in general.