Engineers create chip-size detector

Researchers have created a portable, chip-size version of a detection system that is commonly used by industry and law enforcement to identify everything from agricultural toxins to DNA.

The miniature detector could move certain types of testing from the lab into the field, saving time and money while increasing security.

The team, which used a newly developed laser-processing technique to create the miniature detector, was supported by the US National Science Foundation and led by a Purdue University engineer who conducted the work whilst at the University of California, Berkeley.

‘Now we have a way of putting all of the critical components on one wafer,’ said Timothy D. Sands, the Basil S. Turner Professor of Engineering in the School of Materials Engineering and the School of Electrical and Computer Engineering at Purdue. ‘It’s much the same in concept as going from separate transistors to an integrated circuit that includes many transistors on a single chip.’

Traditional fluorescence detection systems work by attaching a fluorescent dye to specific molecules in a substance and then shining a laser onto the substance. The laser light is absorbed by the dyed molecules, causing them to emit a certain colour, which is picked up by a sensor. The detection work is normally done using bulky, stationary equipment in a laboratory.

The new device, however, fits on a centimetre-wide chip, promising the development of miniature detectors that can be used in the field. Such portable instruments would be useful for a wide range of applications, from biologists doing basic research to farmers testing crops for toxins.

To create the chips, the team used a technique invented by Sands, Nathan Cheung, a UC-Berkeley professor of electrical engineering and computer science, and former graduate student William Wong, now a researcher at the Palo Alto Research Centre in Palo Alto, California. This technique, known as ‘laser liftoff,’ uses a powerful laser to selectively separate and transfer thin-film components from one substrate to another to build up the successive layers of a ‘system-on-a-chip.’

‘We use lasers to manipulate materials, either to grow them or to process them,’ said Sands, who specialises in making devices by combining entirely different materials in new ways. ‘We can transfer films of materials from one substrate to another, and then use this laser-based assembly process to build up complex systems made of materials from different classes that are not normally compatible.’

Fluorescence detection is commonly used in industry and science.

‘It’s the standard technique,’ Sands said. ‘The idea is that you tag a specific molecule or cell with a dye molecule that will emit light when it’s excited. Then you illuminate your subject that’s been tagged with the dye molecule, causing it to emit light at a longer wavelength.’

The colour of the laser is chosen to efficiently ‘excite’ a specific dye. Shining a blue laser on a certain dye, for example, results in the emission of green light. A green laser might be chosen to excite a dye that emits red light.

DNA is tagged with a specific dye and then a fluid containing the tagged DNA is passed under a laser beam. The light-emission data are collected and analysed, revealing information about the DNA.

‘You also have to have some way of filtering the light,’ Sands said. ‘If, for example, you are using a laser that emits blue light, you can’t allow the blue light to go into your photo-detector because it will wash out the signal from the green light the excited molecules emit.

‘You have to filter out the blue light and just pick up the green.’

The research team had to create a tiny filter that could fit on a chip.

The light-emitting diode (LED) that emits blue light is a thin layer of gallium nitride that’s formed on top of a sapphire crystal. When electricity is passed through the LED, it produces a blue light.

With laser liftoff, the researchers use a device called an excimer laser to shine fast pulses of ultraviolet light onto the sapphire. Each pulse lasts only about 25 nanoseconds. The concentrated energy removes the sapphire substrate on which the blue LED is formed, leaving behind only the thin layer. The LED film is transferred by the laser onto a filtering layer of cadmium sulphide, which screens out blue light. The layered diode and filter are then added to a photo detector on a single chip.

The thin-film LED is five micrometers thick and is produced for less than $1, replacing the bulky and expensive laser in bench-top fluorescence detection instruments, Sands said.

‘Something new that we recently reported was to put two colours, a blue and green LED, on one chip,’ he said.

Adding the green LED also meant adding another filter. Illuminating dyed molecules with a green LED causes the dye molecules to emit a reddish light. The second filter keeps green light from getting into the photo-detector so that it only detects the reddish light emitted by the excited dyed molecules.

At least two colours are considered critical for the analysis of biological and chemical materials.

‘If you wanted to do biochemical detection of anthrax or some other substance, you almost always have to have two colours and your sample is tagged with two dyes,’ Sands said. ‘One serves as a control – to precisely calibrate the measurement – and the other colour is for actually detecting the molecule you are after.

‘We are now arguing that we can combine more than two colours, as well as arrays of LED-filter pairs. We could also build a spectrometer on a chip using an array of ultraviolet LEDs and a series of thin-film filters that absorb different colours.’

The team is in the process of comparing the performance of the integrated microchip to bench-top instruments.

‘Even if the performance never exceeds that of laser-based bench-top systems, the small size of the fluorescence detection microchip suggests a future as a portable, hand-held device for chemical detection and bioassay applications in remote locations,’ Sands said.

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