Dielectrophoretic dynamic light-scattering (DDLS) spectroscopy - Analytical Methods & Tools for Lyophilization

Dielectrophoretic dynamic light-scattering (DDLS) spectroscopy
June 18, 2003 (received for review December 10, 2002)
Published online before print August 18, 2003
Folim G. Halaka
PNAS | September 2, 2003
Pyramid Sciences, Inc., P.O. Box 337, Lake Bluff, IL 60044
Communicated by James L. Dye, Michigan State University, East Lansing, MI
Abstract
Dielectrophoretic dynamic light-scattering (DDLS) spectroscopy is presented. DDLS identifies macromolecules based on their dielectric, or polarizability, properties. DDLS measurements are carried out in an oscillating, nonuniform electric field. The field induces macromolecules to undergo dielectrophoretic motion, which is detected by the modulation in the dynamic light-scattering autocorrelation function. The DDLS experimental setup, data analysis, and data on latex particles and yeast cells are presented.
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Identification of macromolecules, particles, and biological cells constitutes an essential part of their production, purification, and eventual utility. Identifying components in solution is important for the biotechnology and chemical industries. In chemical polymerization, for example, control and measurement of the distribution of polymer size are critical to successful manufacturing and product quality. The situation undoubtedly is more complicated in biological systems, with many details affecting biomolecules and cell populations. The present article introduces dielectrophoretic dynamic light-scattering (DDLS) spectroscopy, a tool in the identification of macromolecules based on their dielectric, or polarizability, properties.
When a uniform electric field is applied to polarizable molecules, electrical charge separation, or rearrangement, takes place. After polarization of a molecule and its subsequent reorientation to comply with applied field, equal and opposite forces are exerted on each end of the resulting dipole. For an electrically neutral molecule, no net translation occurs. If the electric field possesses a spatial gradient, unequal forces will be experienced by each end of the dipole, causing the molecule to undergo net translational (dielectrophoretic) motion. These effects have been described for many scientific applications and are richly described in a large number of publications (see, for example, refs. 1-4 and references therein).
The translational motion occurs for a polarizable molecule even when the molecule is, overall, electrically neutral. For the effects discussed above to be observable, the molecular dimensions must be large enough to allow a measurable difference between the forces on each end of the dipole. The term macromolecule is used to recognize this fact.
Additionally, under the influence of an oscillating electric field gradient, the dielectrophoretic motion is frequency-dependent. The response to an oscillating electric field is a function of the time it takes for charges to rearrange (relaxation time). The response depends on the mode of polarization (1-4), and subsequently a particular mode of polarization can be made to prevail by the choice of frequency.
It is the thesis here that polarizable macromolecules present in emulsions, polymer solutions, and cell suspensions can be characterized by their response to the dielectrophoretic effect. It is also demonstrated that this effect can be studied from the modulations in the autocorrelation function in DLS (hence DDLS) measurements as outlined below.
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