fredag 17 september 2010

Electronic senses act as police dogs?.

Why is Life depending on carbon and DNA?

Maybe a part of the answer is revealed in US? A new chemical sensor based on just two materials, graphene and DNA, has been unveiled. Scientists believe that it could be used to make an electronic "nose". The moleculeas are recognized electromagnetically, not chemically, as the 2004 Nobelprizewinner said, Linda Buck and Richard Axel. "Receptors are proteins that nestle in the cell's surface and bind specific chemicals in much the same way that a key fits into a lock. When activated by a chemical, receptors trigger molecular signals within a cell that alter the cell's metabolism. In the case of taste receptors, chemicals impinging on the taste buds trigger nerve impulses that travel to the brain, where taste information is processed. " I have earlier written about odor and perception.

Fig. Converting the current noise power density to current variation yields a best‐possible detection threshold of ~ 0.1% of the baseline current using a 1 Hz bandwidth. Assuming a linear response to DMMP concentration, this implies a detection limit of 0.4 ppm for a single device for DMMP.

Each sensor reacts to a specific molecule, just like the olfactory receptor proteins in mammal noses do. But to fabricate thousands of different sensors is expensive and complicated. Now a simple way of sensing chemicals by showing that the electronic properties of DNA-coated graphene change in when exposed to certain molecules.

Graphene is a two-dimensional material with exceptional electronic properties and enormous potential for applications. It is a sheet of carbon just one atom thick, made into a transistor. Each transistor was then soaked in a solution of a specific sequence of single-stranded DNA, which self-assembles into a pattern on the surface of the graphene. DNA is made from four different bases – adenine (A); cytosine (C), thymine (T); and guanine (G) – and an example of a sequence used is GAG TCT GTG GAG GAG GTA GTC. "We only tested a few sequences but the number of possible sequences is essentially endless." The researchers selected their DNA sequences based on the ability of the sequence to work as a chemical sensitizing agent – a role very different from the function of DNA in living organisms. Each sequence behaves a little differently on the surface of graphene because it has a different shape, pH and hydrophilic properties.

This means that every sequence interacts differently with different volatile organic chemicals (VOCs).

The chemical sensors consisting of a single-walled carbon nanotube field effect transistor (swCN-FET) with a nanoscale layer of single stranded DNA (ssDNA) adsorbed to the tube's outerwall. The current through the swCN-FET shows a characteristic response to gaseous analytes. This response varies depending on the base sequence of the adsorbed ssDNA. These sensors have been able to detect methanol, trimethylamine, propionic acid, dimethyl methylphosphonate (a simulant of sarin), and dinitrotoluene (a derivative of TNT) at the ppm level. The response and recovery of this biosensor is on the order of seconds.

When the DNA/graphene reacts with a chemical in its environment, the resistance of graphene changes. This change, which can be as large as 50%, can easily be measured.

Electronic noses can maybe make the security jobs like dogs do.

DNA-decorated graphene chemical sensors. Ye Lu, B. R. Goldsmith, N. J. Kybert and A. T. C. Johnson. Appl. Phys. Lett. 97, 083107 (2010); doi:10.1063/1.3483128
DNA-decorated Carbon Nanotubes for Chemical Sensing, C. Staii, M. Chen, A. Gelperin, and A.T. Johnson, Nano Lett. 2005, 5, 1774-1778
Johnson Group,

"The Nobel Prize in Physiology or Medicine 2004". 17 Sep 2010

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