New tiny sensors detect chemicals in minute samples

Chemists at the University of Buffalo in the US have developed a small, speedy sensor that is able to detect many different chemicals in a sample hundreds or thousands of times smaller than a drop of water.

The National Science Foundation funded research is published in the March issue of Analytical Chemistry, and is available online to subscribers. The research involved investigating xerogels, which are made from porous glasses that react with a special solution to form a rigid, glass-like porous polymer containing a complex network of nanoscopic pores.

Previous work by the university had developed new ways to stabilise and trap proteins within the xerogels, These can be used to flag the presence of key chemicals in a sample.

The provisionally-patented work could provide agricultural, clinical,

environmental and pharmaceutical laboratories with a small, fast and portable detection technique.

“We now understand very well the chemistry involved in making good xerogels that contain active proteins,” said Dr Frank V Bright, co-author and associate chair and professor in the Department of Chemistry in the University of Buffalo’s College of Arts and Sciences.

He explained that traditional xerogel-based sensors are large and designed to detect just one chemical type. The Buffalo researchers wanted to miniaturise the sensor technology so that multiple sensors could be set in a small segment of xerogel, enabling the detection of many different chemicals in a single, small sample.

“The process of having to analyse for different molecules one at a time is amazingly time-consuming, and it turns out to waste a whole lot of the sample,” noted Bright.

Dr Bright and lead author Eun Jeong Cho began the process by micro-machining tiny wells some 1/25,000th of an inch in diameter, on top of an inexpensive light emitting diode. LEDs are made from a semiconducting material that transforms electrical energy into light.

“Using our xerogels in these wells on a LED was a great idea on paper, but the volume of a well turns out to be fairly small, about a billionth of a quart,” noted Bright. “Trying to fill the wells turned out to be a nightmare.”

Co-author Cho suggested they use pin-printing, a technique widely used in genomics, research into species’ genetic make-up. In this, an ultra-thin pin point sucks up tiny amounts of solution by capillary action and deposits, or prints, them onto microscope slides.

Using the type of commercial pin-printer used for exploring DNA, the team found it had solved the problem.

“Pin-printing is like taking a tiny quill pen, dipping it into a solution and

instead of filling wells, we contact-print the sol-gel solution onto the surface directly to form an array of xerogel-based sensors – we no longer need wells at all,” Bright added.

“Because the volume delivered by these pin-printers is less than a trillionth of a quart, the sensors are very small, so we can cram many different sensors in a small footprint and, in principle, detect hundreds or even thousands of chemical species simultaneously,” he said.

The team is now working on ways to pin-print chemical sensors onto the top of an LED to form a completely self-contained sensor array platform.

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