Alex Terray

Refractive-index-driven separation of colloidal polymer particles using optical chromatography

Separation of equivalently sized polystyrene, n=1.59, poly(methylmethacrylate), n=1.49, and silica, n=1.43, beads has been accomplished using optical chromatography. The optical separations were performed using a glass flowcell that permits the optical trapping laser to be lightly focused into the fluid pathway against the fluid flow. Separation occurs due to the balance of fluid and optical forces; particles come to rest when the force due to the fluid flow equals the radiation pressure force. The ability to optically separate particles based upon their refractive index opens avenues for the characterization of colloidal samples based upon chemical characteristics, in addition to size.

On-line sample pre-concentration in microfluidic devices: a review.

On-line sample preconcentration is an essential tool in the development of microfluidic-based separation platforms. In order to become more competitive with traditional separation techniques, the community must continue to develop newer and more novel methods to improve detection limits, remove unwanted sample matrix components that disrupt separation performance, and enrich/purify analytes for other chip-based actions. Our goal in this review is to familiarize the reader with many of the options available for on-chip concentration enhancement with a focus on those manuscripts that, in our assessment, best describe the fundamental principles that govern those enhancements. Sections discussing both electrophoretic and nonelectrophoretic modes of preconcentration are included with a focus on device design and mechanisms of preconcentration. This review is not meant to be a comprehensive collection of every available example, but our hope is that by learning how on-line sample concentration techniques are being applied today, the reader will be inspired to apply these techniques to further enhance their own programs.

Enhanced optical chromatography in a PDMS microfluidic system

The purely refractive index driven separation of uniformly sized polystyrene, n = 1.59 and poly(methylmethacrylate), n = 1.49 in an optical chromatography system has been enhanced through the incorporation of a custom poly(dimethysiloxane) (PDMS) microfluidic system. A customized channel geometry was used to create separate regions with different linear flow velocities tailored to the specific application. These separate flow regions were then used to expose the entities in the separation to different linear flow velocities thus enhancing their separation relative to the same separation in a constant velocity flow environment. A microbiological sample containing spores of the biological warfare agent, Bacillus anthracis, and a common environmental interferent, mulberry pollen, was investigated to test the use of tailored velocity regions. These very different samples were analyzed simultaneously only through the use of tailored velocity regions.

Cascade optical chromatography for sample fractionation

Optical chromatography involves the elegant combination of opposing optical and fluid drag forces on colloidal samples within microfluidic environments to both measure analytical differences and fractionate injected samples. Particles that encounter the focused laser beam are trapped axially along the beam and are pushed upstream from the laser focal point to rest at a point where the optical and fluid forces on the particle balance. In our recent devices particles are pushed into a region of lower microfluidic flow, where they can be retained and fractionated. Because optical and fluid forces on a particle are sensitive to differences in the physical and chemical properties of a sample, separations are possible. An optical chromatography beam focused to completely fill a fluid channel is operated as an optically tunable filter for the separation of inorganic, polymeric, and biological particle samples. We demonstrate this technique coupled with an advanced microfluidic platform and show how it can be used as an effective method to fractionate particles from an injected multicomponent sample. Our advanced three-stage microfluidic design accommodates three lasers simultaneously to effectively create a sequential cascade optical chromatographic separation system.

Particle separation and collection using an optical chromatographic filter

An optofluidic design has been used to completely separate and collect fractions of an injected mixture of colloidal particles. A three-dimensional glass microfluidic device was constructed such that the fluid was directed though a 50-μm-diameter channel. A laser was introduced opposite the flow and its spot size adjusted to completely fill the channel. Thus, for a given laser power and flow rate, certain particles are completely retained while others pass through unhindered. Separation efficiencies in excess of 99% have been attained for a mixture of polymer and silica beads.

Sample concentration using optical chromatography

Optical chromatography is a technique for the separation of particles that capitalizes on the balance between optic and fluidic forces. When microscopic particles in a fluid flow encounter a laser beam propagating in the opposite direction, they are trapped axially along the beam. They are then optically pushed upstream from the laser focal point to rest at a point where the optic and fluidic forces on the particle balance. Because optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, both coarse and fine separations are possible. We describe how an optical chromatography beam directed into a tailored flow environment, has been adapted to operate as an optical filter for the concentration / bioenrichment of colloidal and biological samples. In this work, the demonstrated ability to concentrate spores of the biowarfare agent, Bacillus anthracis, may have significant impact in the biodefense arena. Application of these techniques and further design of fluidic and optical environments will allow for more specific identification, concentration and separation of many more microscopic particle and biological suspensions.

On-the-fly cross flow laser guided separation of aerosol particles based on size, refractive index and density–theoretical analysis

Laser separation of particles is achieved using forces resulting from the momentum exchange between particles and photons constituting the laser radiation. Particles can experience different optical forces depending on their size and/or optical properties, such as refractive index. Thus, particles can move at different speeds in the presence of an optical force, leading to spatial separations. In this paper, we present a theoretical analysis on laser separation of non-absorbing aerosol particles moving at speeds (1-10 cm/sec) which are several orders of magnitude greater than typical particle speeds used in previous studies in liquid medium. The calculations are presented for particle deflection by a loosely focused Gaussian 1064 nm laser, which simultaneously holds and deflects particles entrained in flow perpendicular to their direction of travel. The gradient force holds the particles against the viscous drag for a short period of time. The scattering force simultaneously pushes the particles, perpendicular to the flow, during this period. Our calculations show particle deflections of over 2500 µm for 15 µm aerosol particles, and a separation of over 1500 µm between 5 µm and 10 µm particles when the laser is operated at 10 W. We show that a separation of about 421 µm can be achieved between two particles of the same size (10 µm) but having a refractive index difference of 0.1. Density based separations are also possible. Two 10 µm particles with a density difference of 600 kg/m3 can be separated by 193 µm. Examples are shown for separation distances between polystyrene, poly(methylmethacrylate), silica and water particles. These large laser guided deflections represent a novel achievement for optical separation in the gas phase.

Preparative optical chromatography with external collection and analysis

Optical chromatography, used for particle separation, involves loosely focusing a laser into a fluid flowing opposite the direction of laser propagation. When microscopic particles in the flow path encounter this beam they are trapped axially along the beam and are pushed upstream from the laser focal point to rest at a point where the optical and fluid forces on the particle balance. Because optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, separations are possible. An optical chromatography beam which completely fills a fluid channel can operate as an optically tunable filter for the preparative separation of polymeric/colloidal and biological samples. We show how the technique can be used to separate injected samples containing large numbers of colloids. The power of optical chromatographic separations is illustrated through combination with epi-fluorescence microscopy and sample purification for real-time polymerase chain reaction (RT-PCR) detection of Bacillus anthracis spores.

Numerical simulation of an optical chromatographic separator.

Optical chromatography achieves microscale optical manipulation through the balance of optical and hydrodynamic forces on micron sized particles entrained in microfluidic flow traveling counter to the propagation of a mildly focused laser beam. The optical pressure force on a particle is specific to each particles size, shape and refractive index. So far, these properties have been exploited in our lab to concentrate, purify and separate injected samples. But as this method advances into more complex optofluidic systems, a need to better predict behavior is necessary. Here, we present the development and experimental verification of a robust technique to simulate particle trajectories in our optical chromatographic device. We also show how this new tool can be used to gather better qualitative and quantitative understanding in a two component particle separation.

Single particle analysis using fluidic, optical and electrophoretic force balance in a microfluidic system.

A unique microfluidic system is developed which enables the interrogation of a single particle by using multiple force balances from a combination of optical force, hydrodynamic drag force, and electrophoretic force. Two types of polystyrene (PS) particles with almost identical size and refractive index (plain polystyrene (PS) particle - mean diameter: 2.06 μm, refractive index: 1.59; carboxylated polystyrene (PS-COOH) particles - mean diameter: 2.07 μm, refractive index: 1.60), which could not be distinguished by optical chromatography, reveal different electrokinetic behaviors resulting from the difference in their surface charge densities. The PS-COOH particles, despite their higher surface charge density when compared to the PS particles, experience a lower electrophoretic force, regardless of ionic strength. This phenomenon can be understood when the more prominent polarization of the counter ion cloud surrounding the PS-COOH particles is considered. The surface roughness of the carboxylated particles also plays an important role in the observed electrokinetic behavior.