(3eo) Separation of Macromolecules by Photonic Crystal Defects Chromatography (PCDC)

Size Exclusion Chromatography (SEC), also known as Gel Filtration Chromatography, is currently one of the most prevalent separation and characterization techniques for synthetic and natural polymers, including proteins and carbon nanotubes. The basic separation mechanism is an entropic exclusion related to the size of the molecules compared to pore size. The application of SEC for the separation of complex mixtures is severely impaired by its poor separation resolution, large reagent consumption and intrinsic difficulties for the integration into automated multistep analysis. The underlying problem of SEC and other gel-based conventional separation techniques is a lack of engineering control of the molecular separation process in terms of both pore geometry and surface interaction. Further development of artificial sieving structures with tailored surface functionalities (e.g. by modifying surface hydrophobicity and surface charge density) has been considered. However, since an engineering control of the interaction potential is still missing, these methods are subject to undesired surface adhesion and clogging by macromolecules.

Recently, the barrier of optical nanomanipulation has been surpassed by utilizing the field amplification and localization in photonic crystal (PC) cavities. PCs are composite dielectric structures possessing a periodic spatial variation in their refractive index on the order of the wavelength of light. Introducing defects into the periodic lattice of a PC structure induces formation of localized photonic modes within the otherwise forbidden photonic band gap. This leads to a significant enhancement of the optical gradient trapping force. Such devices are capable of trapping nanoparticles with radii down to 10 nm with a low input power of 5 mW in aqueous media. PCDC utilizes differences in the interaction of analyte molecules in a flow (mobile phase) with localized optical electric fields (OEFs) (representing the stationary phase). Macromolecules interact with the optical fields depending on their intrinsic properties such as size, shape and refractive index. The lateral concentration distribution depends on the interaction strength between the solute molecules and the gradient of the localized field. The stronger the field gradient the stronger the interaction strength of a given solute molecule and the more nonuniform the concentration profile normal to the surface of the PC structure. Separation is achieved by confining the distribution of solute molecules to different lateral positions in a non-uniform flow (i.e. Poiseuille flow). This separation mechanism is known as field flow fractionation (FFF). The lateral position of a given solute's average concentration determines its residence time in the channel.

In this presentation, the separation mechanism for OEF-FFF is outlined and the relevant dimensionless parameters are analyzed considering a discreet number of OEFs confined to a microchannel whereby a plug of dispersed solutes is undergoing Poiseuille flow. There are three relevant dimensionless parameters that determine the selectivity, resolution, and dispersion or quality of separation: the dimensionless field strength, the normalized field decay length, and the Peclet number. The first two dimensionless groups determine the selectivity and resolution of separation, while the Peclet number is responsible for overall dispersion and quality of separation. A detailed numerical analysis of these dimensionless groups considering experimentally feasible axially non-uniform OEFs is discussed and the results are compared to an analytical analysis of a simpler axially uniform OEF. An experimental design is outlined and preliminary experimental results are correlated to the theoretical results.