Hao Shang

Traveling wave magnetophoresis for high resolution chip based separations

A new mode of magnetophoresis is described that is capable of separating micron-sized superparamagnetic beads from complex mixtures with high sensitivity to their size and magnetic moment. This separation technique employs a translating periodic potential energy landscape to transport magnetic beads horizontally across a substrate. The potential energy landscape is created by superimposing an external, rotating magnetic field on top of the local fixed magnetic field distribution near a periodic arrangement of micro-magnets. At low driving frequencies of the external field rotation, the beads become locked into the potential energy landscape and move at the same velocity as the traveling magnetic field wave. At frequencies above a critical threshold, defined by the beads hydrodynamic drag and magnetic moment, the motion of a specific population of magnetic beads becomes uncoupled from the potential energy landscape and its magnetophoretic mobility is dramatically reduced. By exploiting this frequency dependence, highly efficient separation of magnetic beads has been achieved, based on fractional differences in bead diameter and/or their specific attachment to two microorganisms, i.e., B. globigii and S. cerevisiae.

Transport and functional behaviour of poly(ethylene glycol)-modified nanoporous alumina membranes

The development of hybrid organic–inorganic membranes with a low propensity for protein adsorption and highly uniform nanometre size pores is described. Poly(ethylene glycol) (PEG) monolayers were grafted to nanoporous alumina membranes using covalent silane and physical adsorption poly(ethyleneimine) (PEI) immobilization chemistries. X-ray photoelectron spectroscopy (XPS) and electron microscopy were used to investigate the chemical and physical surface properties of the membranes. The adsorption behaviour of a serum albumin on the membranes was characterized with fluorescence spectroscopy and it was determined that the PEG coating reduced nonspecific protein adsorption to a level too small to be measured. The gas and liquid permeabilities of membranes were measured to determine if the surface chemistries changed the functional behaviour of the membranes. Surprisingly, the silane chemistry produced little change in the permeabilities while polymer adsorption resulted in a total loss of water permeability. The diffusion of ovalbumin through the membranes was also measured and compared with a theoretical value. Diffusion of ovalbumin through the silane-PEG-modified membranes was found to be 50% slower than the unmodified membranes, which suggests that the pores are coated with a dense film of PEG. These results suggest that hybrid organic–inorganic membranes can provide significantly improved functional behaviour over existing organic or inorganic membranes.

Immunoassays in nanoliter volume reactors using fluorescent particle diffusometry.

A model analyte, the M13 virus, was detected through the change in the Brownian motion of a population of microparticles. Epi-fluorescence microscopy was used to simultaneously track antibody-coated and bare microparticles to unambiguously measure the diffusion coefficient and demonstrate multiplexed detection. The sensitivity of the diffusometry assay was high enough that individual virus-to-particle binding ratios could be detected. Analysis of the experimental errors indicated that the primary limitation in the sensitivity of this technique was the variation in the size of the population of microparticles. Analysis of the diffusion measurement results indicated that the change in the drag coefficient of the virus-particle assembly was not a simple sum of the drag coefficients of the individual components and the rate of particle-particle reaction was slower than would be predicted from the uncoupled particle hydrodynamics. The possibility of using diffusometry for sensing and proteomics applications is examined.

Optical Diffusometry Techniques and Applications in Biological Agent Detection

Optical diffusometry is a technique used for measuring diffusion. This work explores the possibility of directly measuring diffusion coefficients of submicron particles for pathogen detection. The diffusion coefficient of these particles is a function of the drag coefficient of the particle at constant temperatures. Particles introduced into a sample containing an analyte bind with the analyte if functionalized with the appropriate antibodies. This leads to an increase in the hydrodynamic drag of the particles and hence a decrease in their diffusion coefficient. This study uses the above principle to effectively measure the diffusion coefficient of the particles using two different experimental approaches. The measured reduction in the diffusion coefficient can be correlated to the amount of analyte present and thus forms the basis of biological agent detection. Sensitivity to experimental conditions is analyzed. It is observed that alternative techniques such as optical trapping hold promise: the diffusive behavior of particles in optical traps is found to be quantitatively different from that of a free particle. Hence preconditions are identified to make optical trapping appropriate for agent detection. DOI: 10.1115/1.2969430

Mesoporous membrane technologies for the collection of airborne biological pathogens

There is an urgent need for efficient, rapid, and inexpensive collection techniques for pathogen detection in environmental samples. For over 40 years membrane filters have been played an important role in the collection of radiological and chemical samples from the environment. Recently inorganic mesoporous alumina membranes have been developed with high densities of highly uniform size pores. Measurements of the physical properties of membranes with 100 nm and 200 nm pores revealed that a transition state hydrodynamic condition exists in the pores that enhanced the permeability of the membranes to gases. These membranes were also found to maintain a high permeability at relative humidities as high as 98% and to be capable of supporting pressures as high as 65 psi. A high density poly(ethylene glycol) monolayer was grafted to the alumina membranes to minimize the adhesion of aerosols to the membranes. This hybrid membrane allowed B. globigii spores to be extracted from aqueous solutions with 96.7% efficiencies. Multi-day collection runs with a prototype collector demonstrated that an instrument based on these membranes could be operated in complex environmental conditions.