Walter Scott, Jr. College of Engineering

Graduate Exam Abstract

Hua Shao
Ph.D. Final
Oct 31, 2007, 1:30 pm-4 pm
Natural Resources 115
Optofluidic Intracavity Spectroscopic Biosensing System for Single Cell Identification
Abstract: Low cost and label-free identification of single biological cells is of broad
interest in a variety of fields, including clinical diagnostics, drug
discovery, food safety, environmental monitoring, biology, and homeland
security. The goal of this dissertation is to develop a low-cost biosensing
system for label-free identification of single biological cells by combining
optical and microfluidic techniques. Optical refraction of cells is closely
related to cell size, shape, type, and biological states, providing sensitive
probes for cell identification. Biological cells inside an optofluidic cavity
modify the optical cavity modes, providing a probe for optical properties of
the cells that can be used for cell differentiation. In this work, the
perturbation effects on an optofluidic cavity transmission spectra induced by
cells due to their larger refractive indices than the surrounding media were
Passive optofluidic plane-plane Fabry-Pérot (FP) cavities fabricated on Pyrex
glass substrates with a low temperature thermocompressive gold-to-gold
diffusion bonding technique were used in the spectroscopic experiments.
Complete fabrication processes for gold-patterned optofluidic cavities with
HfO2/SiO2 dielectric mirrors using low cost transparency photomasks were
developed and characterized. Critical fabrication steps that affect the
optofluidic cavity’s performance were examined by scanning electron
microscopy, atomic force microscopy, and optical spectroscopy. Theoretical
modeling of cavity finesse reduction due to mirror roughness and tilt shows
good agreement with the measured value. An optofluidic cavity with a cavity
finesse >40 and a mirror tilt <1 degree was demonstrated.
A customized microscopic system was designed to measure the transmission
spectra of the optofluidic cavity. Transmission spectra taken from various
microspheres and biological cells excited by an LED at 890 nm are
quantitatively different in terms of the number, relative position, and
relative wavelength offsets of transverse modes that can be use for cell
recognition. For example, intracavity transmission spectra of canine lymphoma
cells are distinct from noncancerous lymphocytes, indicating the potential of
this technique for cancer detection. The amount of transverse mode shift with
respect to the bare cavity mode provides useful information on the refractive
indices of cells that are important for cell differentiation. High order
transverse modes induced by the transverse optical confinement of spheres and
cells were found to be more effectively excited with tilted illuminations.
Experiments were complemented by theoretical modeling of the transmission
spectra of a cell-loaded FP cavity. Both the longitudinal optical path length
change and the lateral optical confinements were thoroughly studied. Paraxial
Gaussian beam resonator analysis provides insightful understanding of the
effects of cavity length on resonator stability and transverse mode spacing.
The range of cavity lengths for stable operation was found and should be
considered in sensor design. Analysis of transverse mode spacing due to Guoy
phase shift again indicated that a short cavity length would minimize
transverse mode spacing variation with the longitudinal positions of the cell.
A simplified double homogenous sphere model for cancerous cells was developed
to study the spectral changes induced by the nuclei, which are important
factors in pre-cancerous detection. The effective index method allows
extraction of refractive index and nuclear size relationships from wavelength
shifts and shows the impact of transverse mode confinement. Theoretical
extraction from the experimental spectra indicates that the nuclei of
cancerous cells are larger than those of normal ones. However, full numerical
modeling based on the finite-difference time-domain (FDTD) method using
commercial software did not produce consistent results.
Two different fluid flow control mechanisms were explored to stabilize cells
and microspheres within the microfluidic cavity. Pressure driven flow allows a
simple microfluidic system to be used while dielectrophoretic (DEP) trapping
with electrical fields allows cell immobilization in a more controllable way.
Square well shaped dielectrophoretic traps were integrated onto the
microfluidic chip by patterning ~10µm wide gold electrodes using a
transparency mask. DEP trapping of various types of single microspheres and
cells within optofluidic cavities was demonstrated with a 16 MHz, 10 Vpp AC
voltage. Effects of DEP trapping on the bare cavity spectra were investigated
by varying the AC voltages applied between the microelectrodes. The
experimental results indicated that thermal effect was the major factor that
caused a red shift of the bare cavity modes with an increase in trapping
voltage. Future work is suggested including burying gold microelectrodes
beneath the dielectric coatings to reduce the bio-contamination of the
electrodes and investigating spectroscopic properties of DEP trapped
biological cells.
In summary, the biosensing system developed in this work may provide a
low-cost, label-free optical technique for recognizing cells in a microfluidic
Adviser: Dr. Kevin L. Lear
Non-ECE Member: Dr. Charles S. Henry, Chemistry Dept
Member 3: Dr. Carmen S. Menoni
Addional Members:
Program of Study: