O. M. Piciu

Size-dependent trajectories of DNA macromolecules due to insulative dielectrophoresis in submicrometer-deep fluidic channels

In this paper, we demonstrate for the first time that insulative dielectrophoresis can induce size-dependent trajectories of DNA macromolecules. We experimentally use lambda (48.5 kbp) and T4GT7 (165.6 kbp) DNA molecules flowing continuously around a sharp corner inside fluidic channels with a depth of 0.4 mum. Numerical simulation of the electrokinetic force distribution inside the channels is in qualitative agreement with our experimentally observed trajectories. We discuss a possible physical mechanism for the DNA polarization and dielectrophoresis inside confining channels, based on the observed dielectrophoresis responses due to different DNA sizes and various electric fields applied between the inlet and the outlet. The proposed physical mechanism indicates that further extensive investigations, both theoretically and experimentally, would be very useful to better elucidate the forces involved at DNA dielectrophoresis. When applied for size-based sorting of DNA molecules, our sorting method offers two major advantages compared to earlier attempts with insulative dielectrophoresis: Its continuous operation allows for high-throughput analysis, and it only requires electric field strengths as low as approximately 10 Vcm.

Measuring the wavelength-dependent divergence of transmission through sub-wavelength hole-arrays by spectral imaging

We present a study on the far-field patterns of light transmitted through sub-wavelength metallic hole-arrays. Spectral imaging measurements are used here on hole arrays for the first time. It provides both spatial and spectral information of the transmission in far-field. The visibility of the images, measured in two illumination modes: Köhler and collimated, is calculated for different planes in and out of focus. The transmission under collimated illumination reveals that 75% of the beam if non-divergent. The results are in agreement with the low divergence measured by Lezec [Science 297, 820 (2002)].

Continuous Electrodeless Dielectrophoretic Separation in a Circular Channel

We present a novel continuous electrodeless separation structure based on dielectrophoresis (DEP). The non-uniform electric field is generated by applying voltage over a circular channel. Driven by the electro-osmotic flow, the particles with different dielectric properties move continuously to the different location across the channel as they flow due to the different DEP force, thus continuously separated into the different outlets. The finite element modelling and simulation results show it can separate particles of different dielectric properties in both spatial and time domain. Compared with the previously reported dieletrophoretic separation using electrode arrays [1-10], this structure is more easily fabricated, mechanically robust and chemically inert. And compared with the previously reported electrodeless dielectrophoretic separation methods [11-14], this structure achieves higher throughput and continuous separation.

Structured illumination microscopy using extraordinary transmission through sub-wavelength hole-arrays

A new microscopy method for multi diffraction-limited spot illumination is based on extraordinary light transmission through a periodic metal grid (typical period of 600 nm) of sub-wavelength holes (150 nm). Multiple spots illuminate a fluorescently labeled sample and the emission is collected by far-field optics. Theoretical comparison with a confocal microscope reveals equivalent spot sizes and a scanning method with the advantage of multiple illumination spots. The system is used to measure the actual transmitted field with a fluorescent sample in far-field. The obtained results are consistent with the theoretical prediction and provide a proof of concept of the midfield microscope.

Fabrication and optical characterization of nano-hole arrays in gold and gold/palladium films on glass:

This paper reports on the improved fabrication process and the optical characterization of different nano-hole arrays in thin metal films that are to be integrated into a novel atto-litre titre plate device for high-speed molecular analysis, such as DNA hybridizations and protein immunoassays. The optical detection is based on the enhanced optical transmission that was recently discovered when light passes through periodically distributed sub-wavelength apertures in optically thick metal films. The transmitted light has also small angular diffraction and well-defined spectral features. Using electron-beam (e-beam) lithography (EBL) and lift-off technique, various array structures with hole diameters ranging between 100 nm and 200 nm and different pitches were fabricated in a 200-nm thick layer of gold (Au), palladium (Pd), and gold/palladium (Au/Pd = 60/40) alloy on glass. Introducing Pd to Au, the grain size of the material is decreased, getting a more well-defined shape of the holes. The transmitted spectrum was measured through periodically and randomly distributed nano-holes in Au. Transmitted spectra were compared as well through similar subwavelength hole arrays in Au, Pd, and Au/Pd alloy. Moreover, the fluorescence of Rhodamine G6 (0.05 µM) was measured when using the transmitted light through periodical cavities in Au as the illumination source. It reveals a nine-fold increase in the fluorescent signal.

PDMS-Glass bonded Lab-on-a-chip device for Single Cell Analysis

Current paper describes the testing, alignment and bonding of a PDMS-glass bonded microfluidic device used for single cell analysis. The channels were successfully tested using carboxylate modified polystyrene beads for any leakage. Polystyrene beads of 2 mum have been successfully trapped within the reservoir of the device by well known dielectrophoresis technique and the same technique will be used to trap a typical yeast cell. Electrode configuration is the notable factor in which pre-sorter electrodes were introduced to bring the far-off particles to the trapping range. Currently work is being carried out on the viability of the yeast cell as PDMS by itself is hydrophobic. Channels are being tested by coating with a layer of PVA (polyvinyl alcohol) which can increase the time of hydrophilicity. At the same time the responses of the gold electrodes coated with PVA are studied as well.

Measuring the near-field of extraordinary transmission through a periodic hole-array

The knowledge of the near-field of extraordinary transmission through hole-arrays is mostly theoretical; there is less experimental validation of the theory. We study the near-field properties by measuring fluorescent molecules that are immersed in a solution and their Brownian motion. The measurements are performed by filling the space above the hole-array with fluorescent solution and exciting these molecules through the hole-array. By measuring both the fluorescence and the direct exciting light, it is possible to learn about the near-field properties.

Atto-liter periodical cavities for optical molecular detection

A process technology for fabrication of high quality nano-hole arrays in thin gold layers on glass substrates, suitable for biomolecular applications is presented. Making use of a bi-layer resist system, electron-beam lithography and a lift-off technique, different arrays of square-shaped nano-holes have been obtained. The holes dimensions are between 100nm and 200nm, with different lattice constants in each case. The arrays were furthermore optically characterized, showing uniformity and increased intensity of the out-coming light