A Litmus Test for Lead

This article originally appeared in The Plain Dealer in Cleveland, Ohio's largest newspaper and the official web site of Winn L. Rosch, on 19 June 2003.
Copyright © 2003 Winn L. Rosch. Copyright © 2003 The Plain Dealer. All Rights Reserved.

Put simply, lead makes you dead. The toxicity of lead and the effects of lead poisoning have been known for more than 2000 years.
Lead also makes you stupid, an effect recognized only in the 19th Century. In particular, the lead in paint impairs the development of the brains of children.

Exactly 99 years ago, the ill effects on children of lead carbonate in house paints was identified. In 1974, the United States forbade the sale of house paint containing lead.
But lead remains a peril to children today. Not only is lead in the environment – for example, lead from about seven decades of auto emissions contaminants soil near highways – but also lead paint still hides beneath layers of paints in older homes and schools.

Worse, these hazards are invisible. You can’t tell lead-based paint from safe paint by looking. You can’t see the contamination in soil. Testing for lead can be complex, expensive, and fraught with false-negative and false-positive identifications.

Lead also makes you stupid, an effect recognized only in the 19^th Century. In particular, the lead in paint impairs the development of the brains of children.

The work of Yi Lu, associate professor of chemistry at the University of Illinois at Urbana-Champaign, could change that. In fact, the new technology he has created holds the potential for testing for a huge array of environmental toxins as quickly and cheaply as using a drug-store pregnancy test or checking for acid with litmus paper.

Lu has developed a means for identifying lead through a simple color change, blue to red, on a test strip. Apply the strip to a suspected sample, and in seconds you’ll know if lead is present.

Combination technology

Lu’s invention is remarkable because it combines two of the hottest new technologies, DNA biotechnology and nanotechnology, extending each into entirely new applications.

Lu looked at sensor technology as a two-part problem. First he needed some means to sense for the chemicals he wanted to test for. Then he looked for some obvious and unambiguous way to see or measure his findings.

To sense chemicals (in this case lead), Lu chose to use single-strand dioxyribonucleic acid, the DNA of genetic fame.

In a double-strand helix, DNA is the essence of all life. In single-strand form, however, DNA operates as an enzyme.

"In 1994, it was discovered that DNA can do more than carry genetic information. As an enzyme, it can recognize metals," said Lu.

Since then, scientists have developed an entire library of DNA enzymes number in the millions.

"We selected a small population of DNA from that huge library to find one that codes for a particular analyte and can catalyze reactions," said Lu.

Once they had found the right variety of DNA, biotechnology made it easy to produce as much as is needed for any application. Lu uses the same polymerase chain reaction that is used to duplicate DNA in genetic research.

Gold standard

Lu next turned to the problem of indicator.

"We asked ourselves, ‘How can we turn the recognition in biology into recognizable signals?’" he said. "So we combined the DNA with another technology where particles can change color dramatically."

Specifically, Lu worked with nanoparticles of gold, tiny lumps of a few dozen molecules of gold measuring only a few billionth of a meter across. When clumped together, these nanoparticles appear blue. When separated, the particles appear red.

Lu linked the nanotech gold with the DNA so that in presence of lead, the DNA cleaved the blue gold into the smaller bits of red gold.

Add the DNA-gold mix to a thin substrate to hold them together, and you have a lead test that works like litmus paper.
Of course, you cannot just lay a piece of paper against dry paint. Although Lu’s test strip can identify lead in drinking water, it cannot detect lead in dry paint. That requires an extra step.

"We extract the lead from the paint in acetic acid — household vinegar — and test the acid," said Lu.

The test promises to be inexpensive enough anyone can check the paint before buying a home or can check the paint in an existing home before sanding it.

"Many commercial lead-testing systems are available, but they give high rate of false positive or false negative responses," said Lu. "We believe that we are much more selective."

Moreover, most lead testing systems are expensive, out of the reach of the lower income families most likely to live in homes contaminated by lead-based paint.

"I did back-of-an-envelope calculations and found we only need microliters of chemicals to do a test. That’s less than a dollar in terms of materials, even in the dime range," said Lu.

By selecting different varieties of DNA the same gold-indicator strategy works for other chemicals, too, making a whole range of pollution test strips possible.

"This technology has the potential for detecting other toxic metal ions such as mercury, and cadmium," said Lu. "We are now working on strategies that go beyond metals to other toxins."