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Posted: Dec 08, 2009

Simple nanotechnology paper sensor for detecting toxins in water

(Nanowerk Spotlight) Safe drinking water has been and increasingly will be a pressing issue for communities around the world. In developed countries it is about keeping water supplies safe while in the rest of the world it is about making it safe (see: "Water, nanotechnology's promises, and economic reality"). The potential impact areas for nanotechnology in water applications are divided into three categories – treatment and remediation, sensing and detection, and pollution prevention (read more: "Nanotechnology and water treatment"). Within the category of sensing and detection, of particular interest is the development of new and enhanced sensors to detect biological and chemical contaminants at very low concentration levels. Testing of water against a spectrum of pathogens can potentially reduce the likelihood of many diseases from cancer to viral infections.
"Despite the fact that there are potentially many methods to evaluate water safety, finding a simple, rapid, versatile, and inexpensive method for detection of toxins in everyday items is still a great challenge," Nicholas Kotov tells Nanowerk. "Ideally, water in many areas around the globe needs to be tested daily. This is very hard to do because of the complexity of the methods used. Our new technique allows, in principle, to test every glass of drinking water. This can be equally useful to a farmer, tourist or soldier. Since we use electrical methods of detection, the devices can be small,light, and inexpensive."
Kotov, a professor in the departments of Chemical Engineering, Materials Science and Engineering and Biomedical Engineering at the University of Michigan, together with collaborators from Jiangnan University in PR China, has demonstrated that highly sensitive sensors for every-day environmental assessment can be made from carbon nanotubes in a very simple way: simply by impregnating paper with several layers of single-walled carbon nanotube (SWCNT) dispersion containing antibodies.
"The change of electrical response of the paper reflects the contents of the analyte" explains Kotov. "For us, one of the most surprising results was that the sensitivity of these devices is exceptionally high and comparable with the best biochemical techniques such as ELISA or mass-spectrometry. At the same time, the analysis time is much shorter – at least 28 times – and does not require specialized training."
The conductivity of carbon nanotube-impregnated filter paper
The conductivity of SWCNT-impregnated filter paper. (Image: The Kotov Group, University of Michigan)
In previous work, Kotov's group successfully demonstrated that SWCNTs can be used to prepare smart electronic textiles with biosensing functionalities detecting a target protein very sensitively and specifically ("Nanotechnology e-textiles for biomonitoring and wearable electronics"). His new work now is predicated on the hypothesis that similar technology can be used to satisfy the outlined requirements for toxin detection in food and water, attain the required detection limits, and be highly competitive in terms of quantification and specificity with well-established much more complex technologies.
For their sensor, the researchers dip-coated regular filter paper strips with a SWCNT dispersion and dried it in air; dip-dry cycles were repeated until the desirable electrical parameters of the sensor were obtained. The dispersion was a PSS-water solution with a concentration of 50 mg/mL. To 6 microliters of this solution they added ten mL of antibodies to Microcystin-LR – one of the most common and the most dangerous toxins produced by the cyanobacteria – to obtain an antibody concentration of 10 µg/mL. Under these conditions, the SWCNT/antibody ratio was about 5000:1.
Kotov mentions that the team also tested multi-walled carbon nanotubes in an analogous process but the corresponding MWCNT-paper composites displayed lower stability under current, lower conductivity, and lower overall robustness than SWNT electrodes.
For sensing, the team employed the standard three-electrode electrochemical station to measure changes in electrical properties of the SWCNT-paper strips, which were used as work electrodes.
"The standard electrochemical setup gives more accurate results than a simple clamping of the SWCNT-paper material between two electrodes due to interfacial potential drops at electrode-SWNT interfaces of different nature including the Schottky barrier" says Kotov. "From the detection results, it is clear that that the presence of the target analyte, i.e., Microcystin-LR in this case, reduces the current through the electrode and, hence, the conductivity of SWCNT-paper composite."
Kotov points out that sensitivity and selectivity of electrochemical detection can be influenced very strongly by media conditions, and thus, the basic variables, namely, the buffer composition, pH, and temperature, need to be optimized before the evaluation of the methodís performance is undertaken especially for comparison with other techniques.
" We believe that our carbon nanotube sensor could be a potential and powerful method for monitoring of the environmental water resource" says Kotov. "Importantly, this method is very versatile and can be easily extended to many other harmful chemicals or toxins in the water or food by changing the antibody in the sensor electrode."
The next step for the team is to expand the array of biological sensors based on this technique and further simplify both detection and sensor preparation with simultaneous improvement in sensitivity/selectivity.
These findings have been reported in the November 20, 2009 online issue of Nano Letters ("Simple, Rapid, Sensitive, and Versatile SWNT-Paper Sensor for Environmental Toxin Detection Competitive with ELISA).
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