New sensor passes litmus test

Scientists at the University of Illinois at Urbana-Champaign in the US have developed a biosensor that acts like litmus paper and changes colour in the presence of certain chemicals.

Scientists at the University of Illinois at Urbana-Champaign have developed a biosensor that functions in much the same fashion as a strip of litmus paper. The colourimetric sensor is said to be based on DNA-gold nanoparticle chemistry, and could be used for sensing a variety of environmental contaminants.

Using gold nanoparticles laced with DNA, Illinois chemistry professor Yi Lu and graduate student Juewen Liu were able to hybridise the nanoparticles into aggregate clusters that have a characteristic blue colour. In the presence of a specific metal ion, the catalytic DNA will break off individual gold nanoparticles, resulting in a colour shift to red. The intensity of the colour depends upon the initial concentration of contaminant metal ions.

By applying the DNA-gold nanoparticle solution to a substrate, the researchers can create a biosensor that functions in the same manner as litmus paper. ‘These simple colourimetric sensors eliminate the need for additional instrumentation, and are well suited for on-site, real-time detection and quantification,’ Lu said.

To obtain the necessary catalytic DNA for their biosensors, Lu and Liu used in vitro selection. Simple and cost-effective, the selection process can sample a very large pool of DNA (up to 1,000 trillion molecules), amplify the desired sequence by the polymerase chain reaction and introduce mutations to improve performance.

While most DNA is double stranded, the catalytic DNA Lu and Liu use has a single strand that can wrap around like a protein. In that single strand, the researchers fashioned a specific binding site that can only accommodate certain metal ions.

‘In addition to lead, the selection process can be customised to select catalytic DNA that would be active for other metal ions, such as mercury, cadmium and zinc,’ Lu said.

The dynamic response of the sensor solution can be tuned over a wide range by introducing inactive catalytic DNA into the mix, Lu said. Incorporating more of the inactive DNA shifts the sensor’s sensitivity to higher contaminant concentrations without saturation. By using various combinations of active and inactive catalytic DNA, the sensor could be packaged as a colourimetric array to detect different concentrations of contaminants.

‘There are many old houses around the world that still contain leaded paint,’ Lu said. ‘According to the US Environmental Protection Agency, leaded paint test kits that are currently available have shown high rates of both false positive and false negative results when compared to laboratory results. Our catalytic DNA-gold nanoparticle sensor can overcome these shortcomings.’

Lu is also working with colleagues at the US National Science Foundation’s Nanoscale Science and Engineering Center for Directed Assembly of Nanostructures to further develop the biosensor technology.

‘Our ultimate goal is to develop a microchip array with different colour schemes for simultaneously detecting many different metal ions,’ Lu added.