Research Highlights
(a) Confocal image of 200-nm red fluorescent microspheres trapped at nanopores by applying a +10-V, 100-kHz asymmetric square wave. The track-etched membrane with the conical nanopores was positioned between two microchannels, and the bottom (horizontal) microchannel was filled with fluorescein (signal in gray). The nanopores show up as dark spots due to scattering. (b) Three-dimensional rendering of the trapped particles in the integrated nanopore/microchannel device.
Integrated Nanopore/Microchannel Devices for ac Electrokinetic Trapping of Particles, M.L. Kovarik and S.C. Jacobson, Analytical Chemistry, 80, 657-664, 2008. [article...]
The unique transport properties which occur in close proximity to nanoscale structures mean that integrating nanofluidic components in microfluidic devices can add functionality. To take advantage of the high electric field strengths generated with conical nanopores, we have fabricated an integrated micro/nanofluidic device for electrokinetic trapping of analyte particles. By applying symmetric and asymmetric square waves of varying frequency, sub-micron particles and C. crescentus bacteria were trapped and concentrated at conical nanopore tips in a track-etched membrane sandwiched between two poly(dimethylsiloxane) microchannels.
In the News... Tech News, Biotechniques, January 2008
Laboratory Automation
Michelle Kovarik, a graduate student in Stephen C. Jacobson’s group in the Department of Chemistry, Indiana University, Bloomington, Indiana, is working on nanofluidics, with a goal of further miniaturizing microfluidic-based separation systems. Although microfluidic separations are now fairly well-characterized, at nanoscale dimensions physical properties of components that were absent or insignificant on a larger scale become more important.
Kovarik says that one of the exciting things about working in laboratory automation is the interaction their group has with colleagues in other departments, for instance, biology and physics. Their group recently moved to a new interdisciplinary science building with a nanoscale characterization facility. “I've had to diversify,” she says. “The collaboration with individuals in other fields changes how people talk about science.”
In the News... Separations Now, February 2008
May the forces be with you
Two US chemists have combined electrophoresis and dielectrophoresis to create a microfluidic device that can trap polymer particles and bacterial cells with a high degree of control and specificity.
Kovarik and Jacobson conclude that combining electrophoresis and dielectrophoresis on the same device in this way delivers great control over both charged and neutral particles, allowing them to be trapped and separated according to their size, charge and dipole moment. Reducing the size of the membrane pores should allow this control to be extended to single biomolecules, they predict.
(a) Transmitted light image of the sample transfer region between the first dimension (1D) and waste 2 (W2) channels in a microfluidic device with 80 μm wide control channels and 32 parallel second dimension (2D) channels. (b) Fluorescence image and (c) COMSOL simulation of sample confinement in the sample transfer region with an electric field strength ratio in the control and 1D channels (EC/E1D) of 2. The arrow depicts flow direction. Scale in (a) applies to all images.
Influence of Channel Position on Sample Confinement in Two-Dimensional Planar Microfluidic Devices, M.A. Lerch, M.D. Hoffman, and S.C. Jacobson, Lab on a Chip, 8, 316-322, 2008.[article...]
Integration of two-dimensional separations onto microfluidic devices requires the ability to efficiently transfer sample between the first and second dimensions. Additionally, each dimension must not influence sample flow in the other. We demonstrated that enhanced sample confinement was achieved using a combination of electrokinetic flow from adjacent control channels and electric field shaping with an array of channels perpendicular to the sample stream. Flow in the sample transfer region was undisturbed by the presence of the parallel channels, enabling sample plugs to be easily routed into the second dimension.
Nanochannels were fabricated in PDMS and reversibly sealed over microscale channels in a glass chip to form the device. Channels are labeled for modified pinched injections.
Attoliter-Scale Dispensing in Nanofluidic Channels, M.L. Kovarik and S.C. Jacobson, Analytical Chemistry, 79, 1655-1660, 2007. [article...]
For nanofluidic devices to be pursued for chemical analysis, methods for precise fluid handling on the nanoscale must be developed. We have investigated the effectiveness of two electrokinetic injection schemes common in microfluidic systems on a nanofluidic chip. Hybrid poly(dimethylsiloxane) and glass devices were used to evaluate both gated and modified pinched injections, with injection volumes as small as 42 attoliters (10-18 L). Because the nanochannels were ~40 μm long, applied potentials from 0 to 10 V generated field strengths up to 1.3 kV/cm in the analysis channel.
In the News... Analytical Chemistry News, March 2007
Reliable Dispensing of Attoliter Volumes of Fluid
In the tiny wonderland where nanowires and nanopores serve as sensors, scientists feel their way through a landscape dominated by unfamiliar forces. Stephen Jacobson has been in the thick of it, hashing out some of the basic infrastructure of this nanorealm. An initial step “is to see how well things from the microfluidic regime scale to the nanofluidic regime,” Jacobson says. “What can you transfer from microfluidics to nanofluidics, and does it work?”
In the February 15 issue of Analytical Chemistry (pp 1655–1660), he and graduate student Michelle Kovarik at Indiana University Bloomington address the fundamental issue of reliably delivering fluids at this scale.
“Knowing that you can deliver such small volumes very accurately is important,” says Jacobson. “The reproducibility and the precision of the injections are as good, or nearly as good,” as those of a microfluidic device.
Scanning electron micrograph of a three-dimensional SU-8 feature produced in a single UV exposure using size-dependent transmission properties of the mask.
Fabrication of Three-Dimensional Micro- and Nanoscale Features with Single-Exposure Photolithography, M.L. Kovarik and S.C. Jacobson, Analytical Chemistry, 78, 5214-5217, 2006.[article...]
We present a technique for fabricating three-dimensional micro- and nanoscale features using a single photolithographic exposure. This method takes advantage of the size-dependent transmission properties of small apertures to produce structures of varying height. Other techniques for 3D fabrication require multiple exposures, special means of exposure, or a gray-tone mask. Using this technique, integrated micro- to nanoscale features can be produced in one step. Feature height is controlled by varying the photomask aperture width and exposure energy.
Phase contrast images of a sperm cell moving down toward the extract wall in response to the extract gradient. The sperm cell in each frame is circled for clarity. Images shown were taken at 0.6 second intervals.
Chemotaxis Assays of Mouse Sperm on Microfluidic Devices, S. Koyama, D. Amarie, H.A. Soini, M.V. Novotny, and S.C. Jacobson, Analytical Chemistry, 78, 3354-3359, 2006. [article...]
We have designed a flow-through microfluidic device for studying sperm chemotaxis and have evaluated device performance by measuring the response of mouse sperm to dilutions of mouse ovary extract. Sperm cells entered the chemotaxis chamber from a central channel, and response was evaluated using the ratio of sperm which swam toward the extract stream on one side of the central channel and sperm which swam toward the buffer stream (control) on the other side. Using a microfluidic platform for these studies provides several advantages including precise fluid control, formation of a spatially and temporally stable gradient, and simple differentiation of trapped versus responding cells.
In the News... Science Daily, May 2006
Even When Faint, Ovary Scent Draws Sperm Cells
In this week's Analytical Chemistry, scientists at Indiana University Bloomington report biochemical machinery that allows mouse sperm cells to follow the weakest of scents. Even when ovary extracts were diluted 100,000 times, some sperm cells still found their mark.
"Sperm are known to exhibit chemotaxis toward extracts from various female reproductive organs, but the role of chemotaxis in reproduction is not known," said IUB Associate Professor of Chemistry Stephen C. Jacobson. "The chemicals that actually attract sperm have not been identified. Systematic study of various compounds released by the female reproductive organs under various conditions might further our understanding of these processes."
Scanning electron micrograph of six pillars formed by photopolymerizing a negative tone resist (SU-8) with UV radiation through 770 nm diameter apertures in an aluminum film. The image has been pseudocolored.
Three-Dimensional Mapping of the Light Intensity Transmitted through Nanoapertures, D. Amarie, N.D. Rawlinson, W.L. Schaich, B. Dragnea, and S.C. Jacobson, Nano Letters, 5, 1227-1230, 2005. [article...]
A general method to map the three-dimensional spatial distribution of light emerging from nanoscale apertures is presented that uses photolithographic techniques to create polymer replicas of the intensity distribution. The resulting features varied with aperture diameter and exposure time and showed good correlation with theory. This method provides direct visualization of the intensity distribution in close proximity to nanostructures and overcomes limitations imposed by physical probes where the contribution of the probe to the map requires deconvolution.
In the News... Presstime Bulletin, Photonics Spectra, July 2005, p. 18
Photoresist Replicates Transmission Through Nanoholes
A team of investigators at Indiana University in Bloomington has developed a technique for mapping the 3-D spatial distribution of radiation transmitted through nanoscale apertures in which the transmission is preserved as a polymer replica in a photoresist. The approach, presented in an online edition of Nano Letters on June 4, avoids the problems of methods that employ physical proximity probes, which affect the transmission pattern.
To demonstrate the mapping technique, the scientists fabricated aluminum-coated films with apertures from 110 to 770 nm in diameter by nano-sphere lithography. They coated the films with a 15-µm-thick layer of photoresist and exposed it through holes to 365-nm radiation from a high-pressure mercury lamp, varying the exposure time from 1 to 30 minutes. Postexposure baking and dissolution left free-standing pillars of polymer that replicated the transmission patterns, with pillar heights directly proportional to aperture diameter and exposure time.