Recent interest in quantum dots (QDs) stems from the plethora of potential applications that arises from their tunable absorption and emission profiles, high absorption cross sections, resistance to photobleaching, functionalizable surfaces, and physical robustness. The emergent use of QDs in biological imaging exploits these and other intrinsic properties. For example, quantum confined Stark effect (QCSE), which describes changes in the photoluminescence (PL) of QDs driven by the application of an electric field, provides an inherent means of detecting changes in electric fields by monitoring QD emission and thus points to a ready mean of imaging membrane potential (and action potentials) in electrically active cells. Here we examine the changing PL of various QDs subjected to electric fields comparable to those found across a cellular membrane. By pairing static and timeresolved PL measurements, we attempt to understand the mechanism driving electric-field-induced PL quenching and ultimately conclude that ionization plays a substantial role in initiating PL changes in systems where QCSE has traditionally been credited. Expanding on these findings, we explore the rapidity of response of the QD PL to applied electric fields and demonstrate changes amply able to capture the millisecond timescale of cellular action potentials.
Analysis of the intrinsic scatter and fluorescence profiles of marine algae can be used for general classification of
organisms based on cell size and fluorescence properties. We describe the design and fabrication of a Microflow
Cytometer on a chip for characterization of phytoplankton. The Microflow Cytometer measured distinct side-scatter and
fluorescence properties of Synechococcus sp., Nitzschia d., and Thalassiosira p. Measurements were confirmed using
the benchtop Accuri C6 flow cytometer. The Microflow Cytometer proved sensitive enough to detect and characterize
picoplankton with diameter approximately 1 mm and larger phytoplankton of up to 80 mm in length. The wide range in
size discrimination coupled with detection of intrinsic fluorescent pigments suggests that this Microflow Cytometer will
be able to distinguish different populations of phytoplankton on unmanned underwater vehicles. Reversing the
orientation of the grooves in the channel walls returns the sample stream to its original unsheathed position allowing
separation of the sample stream from the sheath streams and the recycling of the sheath fluid.
A multi-analyte diagnostic system based on a novel microflow cytometer is under development as a portable, fielddeployable
sensor for environmental monitoring and for rapid point-of-care and on-site diagnosis of exposure to
biothreat agents. The technology relies on a unique method for ensheathing a sample stream in continuous flow past an
illuminated interrogation region. This sheathing approach efficiently focuses particles in the interrogation region of the
fluidic channel and minimizes clogging by complex samples. Fluorescently coded microspheres provide the capability
for highly multiplexed assays. In this report, separation of six microsphere sets was demonstrated with determination of
immunoassays on three of the six sets; comparison to the commercial platform was made.
The increasing demand for portable devices to detect and identify pathogens represents an
interdisciplinary effort between engineering, materials science, and molecular biology. Automation
of both sample preparation and analysis is critical for performing multiplexed analyses on real world
samples. This paper selects two possible components for such automated portable analyzers:
modified silicon structures for use in the isolation of nucleic acids and a sheath flow system suitable
for automated microflow cytometry.
Any detection platform that relies on the genetic content (RNA and DNA) present in complex
matrices requires careful extraction and isolation of the nucleic acids in order to ensure their
integrity throughout the process. This sample pre-treatment step is commonly performed using
commercially available solid phases along with various molecular biology techniques that require
multiple manual steps and dedicated laboratory space. Regardless of the detection scheme, a major
challenge in the integration of total analysis systems is the development of platforms compatible
with current isolation techniques that will ensure the same quality of nucleic acids. Silicon is an
ideal candidate for solid phase separations since it can be tailored structurally and chemically to
mimic the conditions used in the laboratory.
For analytical purposes, we have developed passive structures that can be used to fully ensheath one
flow stream with another. As opposed to traditional flow focusing methods, our sheath flow profile
is truly two dimensional, making it an ideal candidate for integration into a microfluidic flow
cytometer. Such a microflow cytometer could be used to measure targets captured on either
antibody- or DNA-coated beads.