Biosensors That Protect and Serve

Hank Hogan, hank@hankhogan.com

Between natural and man-made hazards, it’s a dangerous world out there. The good news is that, along with problems, people can provide solutions. Two recent examples show how researchers have turned to biological-based sensors, or biosensors, to sniff out danger and make things a bit safer for us all.

At first it seems as if Dr. Evangelyn C. Alocilja, director of the Nano-Biosensors Lab and an associate biosystems engineering professor at Michigan State University in East Lansing, doesn’t want much.

“Our long-term goal is to design and develop a better detector – a biosensor – for salmonella and other pathogens,” she said.

That’s a goal with public health implications. Salmonella is responsible for 40,000 reported food-poisoning cases annually in the US, with a much larger number unreported. Some pathogens, such as anthrax, also could be used for bioweapons.

A tall order

However, Alocilja wants these new biosensors to be rapid, sensitive, handheld, cheap and field-friendly. Because conventional techniques will not do, she led a team of researchers developing new methods. Recently, they demonstrated a solution that meets her requirements. The result could be a device with a cost per detection of less than $2, far cheaper than current technology.

Gold nanoparticles ~15 nm in diameter – shown through an electron microscope – form part of a salmonella-detecting biosensor. The inset shows the nanoparticles’ absorbance spectrum. Courtesy of Deng Zhang, Michigan State University.

The trick is using two sets of nanoparticles, one group gold or another material and the other group magnetic. Both are coated with target DNA, a function that makes them bind to the pathogen of interest in a sandwich structure. The first group acts as a reporter, via fluorescence or another method. The second provides a means for separation and concentration.

In a demonstration, the researchers used target-specific DNA probes for salmonella. They labeled the gold nanoparticles with fluorescein and mixed them with magnetic ones. After magnetic separation, they measured the resulting fluorescence using a PerkinElmer Inc. multilabel counter. Their detection limit was 1 ng of target gene of salmonella per milliliter of solution, which graduate student Deng Zhang characterized as good, considering the technique’s rapidity and portability. The group reported on its work in the Aug. 19, 2008, online issue of Biosensors and Bioelectronics. Zhang was the lead author.

The fluorescence signal of released bar-code DNA from a biosensor is shown in response to various concentrations of target DNA – in this case, a salmonella gene. Courtesy of Deng Zhang, Michigan State University.

In the future, the reporting method may be changed. Ongoing work by the group is concentrating on disposable carbon electrode-based detectors that generate an electrochemical signal. Because of mass production, these electrodes are cheap. Another advantage is that it’s easier to integrate them into current computer technology.

Tracking weed killer

Herbicides are murder on weeds but not good for animals, beneficial plants or people. Estimates are that as much as 6 percent of applied herbicides end up elsewhere. This pollution is a worldwide problem, with herbicides seen as contributing to struggling fish populations off the West Coast and to dying coral near Hong Kong.

What is needed is an inexpensive and portable way to measure herbicide concentration, enabling people to steer clear of or to remediate bad water or soil. Unfortunately, typical methods require expensive equipment, sample shipment and highly qualified personnel.

Now a group of researchers from Cranfield University in the UK, from the University of Crete in Greece and from the Italian National Research Council in Rome has come up with a solution. The investigators used plant photosynthetic membranes to detect herbicides. The resulting device is easy to handle and sensitive enough for preliminary screening.

This biosensor consists of two active regions, with magnetic beads entrapped in the magnets’ fields. The set of thylakoid-coated beads reacts to a pesticide in the incoming water, while the HRP-coated beads translate that into a signal. Courtesy of Dimitrios Varsamis, Cranfield University.

The biosensor consists of two sets of magnetic polystyrene beads with a connecting channel between them. The first set has plant photosynthetic membranes immobilized on it, with an LED illuminator. Enough herbicide of the right type in incoming fluid disrupts the normal photosynthesis of the membranes; as a result, they produce less hydrogen peroxide in the fluid traveling to the second set of beads. This group produced light with an intensity that varied with peroxide concentration. The researchers captured the chemiluminescence signal using an Ocean Optics Inc. luminometer or a Varian Inc. spectrophotometer.

They assembled the beads in place using magnets. After a one-time use, the beads are flushed away and a new set is magnetically fixed in the right place.

The limits of detection were a few tens of nanomolar concentration, the group reported online June 17, 2008, in a Talanta paper. Lead author and Cranfield graduate student Dimitrios Varsamis said that this limit was subsequently lowered with the use of a Hamamatsu Corp. avalanche photodetector. The device now can detect the total amount of the most frequently used herbicides in a water sample down to levels that meet EU standards.

Varsamis added that the group has successfully transferred these published preliminary results into a microfluidic biosensor. However, more research and development still remains to be done.

“Work would need to be performed on the automation of fluidic handling if it is to be successfully deployed in the field,” he said.