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Special Report:
ECEs and Biomedicine

April 2004

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Quantum dots & gold nanoparticles...
Developing no-kill sensors for live cell monitoring

As researchers in many fields seek to understand cellular function, there is a growing need for nondestructive, real-time measurement of chemicals in living cells.

An ECE team, headed by Kathleen Meehan, is pursuing several methods of using light for live cell monitoring. The team is developing optical biosensors for applications ranging from monitoring the health of wastewater treatment bacteria to tracking the path of disease-causing viruses through an organism.

Live cell monitoring is important in many research efforts for both efficiency and accuracy, according to Meehan. Current methods often involve killing the sample cells in order to monitor and to measure the chemicals that represent the activity being studied. “Wiping out the population at various intervals is very time consuming,” she said, “and it doesn’t give real-time information – only glimpses.”

‘If we go too short, like blue and ultraviolet, the proteins in the cells absorb the energy, and the cell basically cooks. It’s like frying an egg...’
– Kathleen Meehan

Avoiding Cooking the Cells
Developing optical biosensors involves both devising the correct light source and tailoring the sensing technique. The light source needs to be selected so that excessive energy does not cause cell damage.

“Live cells are very susceptible to excess energy,” she explained. “This limits the wavelength and power we can use. If we go too short, like blue and ultraviolet, the proteins in the cells absorb the energy, and the cell basically cooks. It’s like frying an egg when the protein in the egg white turns from clear to white.” With bacteria cells, for example, the wavelength must be longer than 500 nm to avoid damaging and killing the cell.

The light source is closely tied to the sensing technique and Meehan’s team is working with several different mechanisms including gold nanoparticles, quantum dots, and fluorescent dyes.

The Gold Standard
Environmental engineers are seeking to develop inline monitors to detect the fouling before the remediation fails. In collaboration with Nancy Love, a civil and environmental engineering professor, Meehan’s team is exploring the use of gold nanoparticles to monitor changes in the acidity of the fluid inside the wastewater treatment bacteria and the potassium concentration outside the cells. When exposed to toxins, the internal acidity of the bacteria changes, releasing a large fraction of the potassium into the water around them. When this happens, they stop absorbing the organics and the treatment system must be taken off line.

Love and Meehan are working to develop an early warning system based on monitoring the potassium levels of the bacteria using techniques that rely on gold nanoparticles.

Gold is being considered in the sensor because of its chemical stability and its unique optical properties. Plus, it is biocompatible – cells don’t die on contact with it. One sensor configuration involves using gold nanoparticles as intracellular probes. While gold is generally very reflective, gold nanoparticles have a narrow absorption band in the visible wavelength region. This narrow band absorption is caused by the surface plasmon effect.

Undergraduate Research: Top left - Illuminating Gold Nanoparticles: Undergraduate Joseph Hernandez (EE ‘06) measures the response of a solution of gold nanoparticles. Gold is considered a good material for biosensors because of its stability and optical properties.

Right: Building Gold Nanoparticles for Live Cell Monitoring: Ajay Bhatia (EE ’04) prepares an e-beam evaporator for depositing gold molecules on an optode film. As an undergraduate research assistant, Bhatia developed a process for forming alumina nanopores that will be used to precisely space the gold nanoparticles. With precise placement, the gold should boost the fluorescence of a sensor.

Bottom left: Closeup of the wells.

The wavelength of the peak in the narrow band absorption is determined by the size of the gold nanoparticles and the index of refraction of the fluid around them. The index of refraction changes with the acidity of the fluid inside the cells. The acidity of the cell can be determined by measuring which wavelength of light is absorbed by the gold nanoparticles. So, a shift in the absorbed wavelength means there has been a change in the pH of the cell.

Enhancing Fluorescence
Another configuration under development incorporates gold nanoparticles with fluorescent dyes in order to get enhanced signals. “These dyes are not very efficient and we are not talking about large changes in potassium outside of the cells, so the signal to noise ratio is very low,” Meehan said. “We hope that by coupling the surface plasmon effect of the gold particles with fluorescence, we can boost the signal by a factor of 25 to 1000 — the more light the better.”

Feeding Quantum Dots to Bacteria
Gold is not the only substance under study for the sensors. Meehan is also working with Ken Meissner of the Applied Bioscience Center on developing quantum dots as intracellular probes. Quantum dots are nanometer-sized pieces of semiconductor material. They are introducing the dots into the fluid around the cells and attempting to trigger the cells into ingesting the dots.

“We are looking at how the optical properties of quantum dots are influenced by the chemical composition of the cells. The intensity of the light they emit should be a function of the chemicals inside the cell,” Meehan said.

The quantum dot sensors will be used in collaboration with biologists Jill Sible and John Tyson, who are studying the chemical reaction pathways for cell apoptosis, which is the natural process of cell death. “We hope to make the quantum dots sensitive to certain chemicals so that we can monitor when the chemicals appear in the cell. This will give us more quantitative data than gold nanoparticles,” Meehan explained.

Whether customizing gold particles, quantum dots, or laser wavelengths, the biosensor work involves trying out different ideas to optimize the solution. “That is what I like about this,” Meehan said. “All the work is application driven and we are learning a lot of biology, which has always been a strong interest of mine. This is fun.”

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