First Model To Compare Performance Of 'Biosensors'

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First Model To Compare Performance Of 'Biosensors'

http://www.medicalnewstoday.com/articles/92962.php

Researchers have developed a new modeling technique to study and

design miniature " biosensors, " a tool that could help industry

perfect lab-on-a-chip technology for uses ranging from medical

diagnostics to environmental monitoring.

The experimental devices represent a new class of portable sensors

designed to capture and detect specific " target molecules, " which

will allow the sensors to identify pathogens, DNA or other

substances.

Now researchers at Purdue University are the first to create " a new

conceptual framework " and corresponding computational model to

relate the shape of a sensor to its performance and explain why

certain designs perform better than others, said Ashraf Alam, a

professor of electrical and computer engineering.

Findings also refute long-held assumptions about how to improve

sensor performance.

The researchers tested and validated their model with experimental

data from various other laboratories.

" Many universities and companies are conducting experiments in

biosensors, " Alam said. " The problem is that until now there has

been no way to consistently interpret the wealth of data available

to the research community. Our work provides a completely different

perspective on how to analyze their data and how to interpret them. "

Research findings are detailed in a paper that appeared in the Dec.

21 issue of the journal Physical Review Letters. The paper was

written by electrical and computer engineering doctoral student

Pradeep Nair and Alam.

Biosensors integrate electronic circuitry with natural molecules,

such as antibodies or DNA, which enable the devices to capture

target molecules. In efforts to design more sensitive devices,

engineers have created sensors with various geometries: some capture

the biomolecules on a flat, or planar surface, others use a single

cylindrical nanotube as a sensing element, and others use several

nanotubes, arranged in a crisscrossing pattern like overlapping

sticks.

Researchers have known for several years that smaller devices are

more sensitive than larger ones. Specifically, the most sensitive

devices are those built on the scale of nanometers, or billionths of

a meter, such as tiny hollow nanotubes made of carbon.

" But we haven't really known why smaller sensors are more

sensitive, " Alam said.

One obstacle in learning precisely why smaller sensors work better

is that the analysis is too computationally difficult to perform

with conventional approaches. The Purdue researchers solved this

problem by creating a model using a mathematical technique called

Cantor transformation, which simplified the computations needed for

the analysis.

" That is the most important aspect of this work, " Nair said. " You

could not effectively analyze the physics behind these biosensors by

using brute force with massive computing resources. It either could

not be done, or you would not be able to get consistent results. "

The new model explains for the first time why a single nanotube

performs better than sensors containing several nanotubes or flat

planar sensors and refutes the predominant explanation for why

smaller sensors work better than larger ones.

" Everyone presumes that the nanometer-scale sensors are better

simply because they are closer to the size of the target molecules, "

Alam said " This classical theory suggests that because larger

sensors dwarf the molecules they are trying to detect, these target

molecules are just harder to locate once they are captured by the

probe. It's like trying to see a small speck on a large surface. But

that same target molecule is no longer a speck if it lands on a

probe closer to its own size, so it's much easier to see.

" What we found, however, was not that smaller sensors are better

able to detect target molecules, but that they are better able to

capture target molecules. It's not what happens after the molecule

is captured that determines how well the sensor works. It's how fast

the sensor actually captures the molecule to begin with that matters

most. "

The distinction is important for the design of biosensors.

The reason smaller sensors capture molecules more effectively is

because using a single nanotube sensor eliminates a phenomenon

called " diffusion slow down. " As a result, target molecules move

faster toward single nanotubes than other structures.

The new model developed by the Purdue researchers determined

that " the smaller the better, " Alam said.

" This acceleration starts coming in when you make sensors on the

size scale of tens of nanometers. That is when you will get a real

advantage. "

Future work will concentrate on applying the model to the

performance of a " fractal sponge, " which is a shape containing many

pores. Such a shape is important for applications in drug delivery

and filtration.