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Sensor Array, Light Source or Both?

Photonics Spectra
Nov 2001
Daniel C. McCarthy

Chemists are not content with detecting one or two chemical species within a sample. They prefer, when possible, to measure everything at once. This demand has generated a number of array-based detector technologies, including optical methods that require the intricate alignment of the light source, sample and detector. Researchers at the State University of New York in Buffalo have demonstrated a potential shortcut.

"It's essentially an LED with holes punched into its surface, and we filled the holes with [analyte selective] chemistry," said Frank V. Bright, a chemistry professor at the university who conducted the research with graduate student Eun Jeong Cho. "The main problem it solves is the need to align things in a detection array. You don't have to do that anymore because the sensing platform and the light source are [the] same thing."

Potential commercial applications for this approach include combinatorial chemistry testing, or drug screening and testing.

After machining the protective optic off blue LEDs from Nichia America, Bright and Cho micromachined small-diameter wells into the surface of each diode, taking care not to contact the PN junction. They made two sets of arrays, one fitting 36 wells of 250-µm diameter onto the LED surface, the other fitting 18 wells of 500-µm width.

Into some wells, they deposited a sol-gel doped with a ruthenium-based luminophore that is sensitive to the presence of oxygen. Other wells were left empty as a control. After the gel cured, the chemists placed the array into a flow cell and excited the luminophores by activating the LED. Then by introducing oxygen into the cell, they quenched the luminophore's fluorescence. An Olympus microscope and a CCD camera from Roper Scientific helped capture images that demonstrated that only the filled microwells were responsive to oxygen.

In early experiments, the researchers encountered background light from the LED. Although a long-pass filter eliminated some of this parasitic light, the LED output still demonstrated an optical "tail" that interfered with readout of the recognition chemistry -- a problem solved by applying a layer of black spray paint to the LED before machining the wells.

Since publishing their results, Bright and Cho have been exploring a different approach. Instead of machining the holes, they have print-ed spots on the LED surface by leveraging contact printing technology from DNA chip manufacture. The spots are the same luminophore-doped sol-gel glass deposited in the wells, except that a higher concentration of luminophore is used to overwhelm the parasitic background light from the LED.

The next step, Bright said, is applying spots with different sensing chemistries to detect the different analytes simultaneously. One benefit to this approach is that it could simultaneously disclose a tandem pattern of behavior between two or more analytes, revealing dynamic chemical relationships in a sample.

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