David Erickson

Optical manipulation of nanoparticles and biomolecules in sub-wavelength slot waveguides

The ability to manipulate nanoscopic matter precisely is critical for the development of active nanosystems. Optical tweezers are excellent tools for transporting particles ranging in size from several micrometres to a few hundred nanometres. Manipulation of dielectric objects with much smaller diameters, however, requires stronger optical confinement and higher intensities than can be provided by these diffraction-limited systems. Here we present an approach to optofluidic transport that overcomes these limitations, using sub-wavelength liquid-core slot waveguides. The technique simultaneously makes use of near-field optical forces to confine matter inside the waveguide and scattering/adsorption forces to transport it. The ability of the slot waveguide to condense the accessible electromagnetic energy to scales as small as 60 nm allows us also to overcome the fundamental diffraction problem. We apply the approach here to the trapping and transport of 75-nm dielectric nanoparticles and λ-DNA molecules. Because trapping occurs along a line, rather than at a point as with traditional point traps, the method provides the ability to handle extended biomolecules directly. We also carry out a detailed numerical analysis that relates the near-field optical forces to release kinetics. We believe that the architecture demonstrated here will help to bridge the gap between optical manipulation and nanofluidics.

Zeta-potential measurement using the Smoluchowski equation and the slope of the current-time relationship in electroosmotic flow

The zeta -potential of a solid-liquid interface is an important surface characterization quantity for applications ranging from the development of biomedical polymers to the design of microfluidic devices. This study presents a novel experimental technique to measure the zeta -potentials of flat surfaces. This method combines the Smoluchowski equation with the measured slope of current-time relationship in electroosmotic flow. This method is simple and accurate in comparison with the traditional streaming potential and electrophoresis techniques. Using this method the zeta -potentials of glass and poly(dimethylsiloxane) (PDMS) coated surfaces in KCl and LaCl3 aqueous solutions were measured using several flow channels ranging from 200 to 300 microm in height. The zeta -potential was found to vary from -88 to -66 mV for glass surface and -110 to -68 mV for PDMS surfaces depending on the electrolyte and the ionic concentration. The measured values of the zeta -potential are found to be independent of the channel size and the applied driving voltage and generally are repeatable within +/-6%.

Nanoscale optofluidic sensor arrays

In this paper we introduce Nanoscale Optofluidic Sensor Arrays (NOSAs), which are an optofluidic architecture for performing highly parallel, label free detection of biomolecular interactions in aqueous environments. The architecture is based on the use of arrays of 1D photonic crystal resonators which are evanescently coupled to a single bus waveguide. Each resonator has a slightly different cavity spacing and is shown to independently shift its resonant peak in response to changes in refractive index in the region surrounding its cavity. We demonstrate through numerical simulation that by confining biomolecular binding to this region, limits of detection on the order of tens of attograms (ag) are possible. Experimental results demonstrate a refractive index (RI) detection limit of 7 x 10(-5) for this device. While other techniques such as SPR possess a equivalent RI detection limit, the advantage of this architecture lies in its potential for low mass limit of detection which is enabled by confining the size of the probed surface area.

Optofluidic trapping and transport on solid core waveguides within a microfluidic device

In this work we demonstrate an integrated microfluidic/photonic architecture for performing dynamic optofluidic trapping and transport of particles in the evanescent field of solid core waveguides. Our architecture consists of SU-8 polymer waveguides combined with soft lithography defined poly(dimethylsiloxane) (PDMS) microfluidic channels. The forces exerted by the evanescent field result in both the attraction of particles to the waveguide surface and propulsion in the direction of optical propagation both perpendicular and opposite to the direction of pressure-driven flow. Velocities as high as 28 mum/s were achieved for 3 mum diameter polystyrene spheres with an estimated 53.5 mW of guided optical power at the trapping location. The particle-size dependence of the optical forces in such devices is also characterized.

Nanomanipulation Using Silicon Photonic Crystal Resonators

Optical tweezers have enabled a number of microscale processes such as single cell handling, flow-cytometry, directed assembly, and optical chromatography. To extend this functionality to the nanoscale, a number of near-field approaches have been developed that yield much higher optical forces by confining light to subwavelength volumes. At present, these techniques are limited in both the complexity and precision with which handling can be performed. Here, we present a new class of nanoscale optical trap exploiting optical resonance in one-dimensional silicon photonic crystals. The trapping of 48 nm and 62 nm dielectric nanoparticles is demonstrated along with the ability to transport, trap, and manipulate larger nanoparticles by simultaneously exploiting the propagating nature of the light in a coupling waveguide and its stationary nature within the resonator. Field amplification within the resonator is shown to produce a trap several orders of magnitude stronger than conventional tweezers and an order of magnitude stiffer than other near-field techniques. Our approach lays the groundwork for a new class of optical trapping platforms that could eventually enable complex all-optical single molecule manipulation and directed assembly of nanoscale material.

Modeling of DNA hybridization kinetics for spatially resolved biochips

The marriage of microfluidics with detection technologies that rely on highly selective nucleic acid hybridization will provide improvements in bioanalytical methods for purposes such as detection of pathogens or mutations and drug screening. The capability to deliver samples in a controlled manner across a two-dimensional hybridization detection platform represents a substantial technical challenge in the development of quantitative and reusable biochips. General theoretical and numerical models of heterogeneous hybridization kinetics are required in order to design and optimize such biochips and to develop a quantitative method for online interpretation of experimental results. In this work we propose a general kinetic model of heterogeneous hybridization and develop a technique for estimating the kinetic coefficients for the case of well-spaced, noninteracting surface-bound probes. The experimentally verified model is then incorporated into the BLOCS (biolab-on-a-chip simulation) 3D microfluidics finite element code and used to model the dynamic hybridization on a biochip surface in the presence of a temperature gradient. These simulations demonstrate how such a device can be used to discriminate between fully complementary and single-base-pair mismatched hybridization using fluorescence detection by interpretation of the unique spatially resolved intensity pattern. It is also shown how the dynamic transport of the targets is likely to affect the rate and location of hybridization as well as that, although nonspecific hybridization is present, the change in the concentration of hybridized targets over the sensor platform is sufficiently high to determine if a fully complementary match is present. Practical design information such as the optimum transport speed, target concentration, and channel height is presented. The results presented here will aid in the interpretation of results obtained with such a temperature-gradient biochip.

A method for nanofluidic device prototyping using elastomeric collapse

Nanofluidics represents a promising solution to problems in fields ranging from biomolecular analysis to optical property tuning. Recently a number of simple nanofluidic fabrication techniques have been introduced that exploit the deformability of elastomeric materials like polydimethylsiloxane (PDMS). These techniques are limited by the complexity of the devices that can be fabricated, which can only create straight or irregular channels normal to the direction of an applied strain. Here, we report a technique for nanofluidic fabrication based on the controlled collapse of microchannel structures. As is demonstrated, this method converts the easy to control vertical dimension of a PDMS mold to the lateral dimension of a nanochannel. We demonstrate here the creation of complex nanochannel structures as small as 60 nm and provide simple design rules for determining the conditions under which nanochannel formation will occur. The applicability of the technique to biomolecular analysis is demonstrated by showing DNA elongation in a nanochannel and a technique for optofluidic surface enhanced Raman detection of nucleic acids.

Optofluidic ring resonator switch for optical particle transport

In this work, we demonstrate an optofluidic switch using a microring resonator architecture to direct particles trapped in the evanescent field of a solid-core waveguide. When excited at the resonant wavelength, light inserted into the bus waveguide becomes amplified within the ring structure. The resulting high optical intensities in the evanescent field of the ring generate a gradient force that diverts particles trapped on the bus to the ring portion of the device. We show that this increase in optical energy translates to an increase of 250% in the radiation pressure induced steady-state velocity of particles trapped on the ring. We also characterize the switching fraction of the device, showing that 80% of particles are diverted onto the ring when the device is at an on-resonance state. The optofluidic switch we present here demonstrates the versatility in exploiting planar optical devices for integrated particle manipulation applications.

Forces and Transport Velocities for a Particle in a Slot Waveguide

Optofluidic transport seeks to exploit the high-intensity electromagnetic energy in waveguiding structures to manipulate nanoscopic matter using radiation pressure and optical trapping forces. In this paper, we present an analysis of optical trapping and transport of sub-100 nm polystyrene and gold nanoparticles in silicon slot waveguides. This study focuses on the effect of particle size, particle refractive index, and slot waveguide geometry on trapping stability and the resulting transport speed. Our results indicate that stable trapping and transport can be achieved for objects as small as 10 or 20 nm in diameter with as much as a 100 fold enhancement in trapping stiffness over the state of the art.

Surface-Enhanced Raman Scattering Based Ligase Detection Reaction

Genomics provides a comprehensive view of the complete genetic makeup of an organism. Individual sequence variations, as manifested by single nucleotide polymorphisms (SNPs), can provide insight into the basis for a large number of phenotypes and diseases including cancer. The ability rapidly screen for SNPs will have a profound impact on a number of applications, most notably personalized medicine. Here we demonstrate a new approach to SNP detection through the application of surface-enhanced Raman scattering (SERS) to the ligase detection reaction (LDR). The reaction uses two LDR primers, one of which contains a Raman enhancer and the other a reporter dye. In LDR, one of the primers is designed to interrogate the SNP. When the SNP being interrogated matches the discriminating primer sequence, the primers are ligated and the enhancer and dye are brought into close proximity enabling the dyes Raman signature to be detected. By detecting the Raman signature of the dye rather than its fluorescence emission, our technique avoids the problem of spectral overlap which limits number of reactions which can be carried out in parallel by existing systems. We demonstrate the LDR-SERS reaction for the detection of point mutations in the human K-ras oncogene. The reaction is implemented in an electrokinetically active microfluidic device that enables physical concentration of the reaction products for enhanced detection sensitivity and quantization. We report a limit of detection of 20 pM of target DNA with the anticipated specificity engendered by the LDR platform.