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A Simpler Way to Build an Optical Acidity Detector

Photonics Spectra
May 2008
Technique could advance miniaturization of acidity sensors.

Hank Hogan

A team of researchers in Spain has devised a fluorescent surface molecular sensor for the detection of acidity. The sensor could find application in industry, clinical analysis and environmental sampling.

The team consisted of researchers from the Catalan Institute of Nanotechnology, the Institute of Science and Materials of Barcelona, the Autonomous University of Barcelona and the Center for the Investigation of Nanoscience and Nanotechnology, all in Cerdanyola de Vallès. It used lithographically controlled wetting to apply a pattern of a compound with pH-sensitive fluorescence on the surface of a glass slide.

TWacid_Fig1.jpg

This synthesized molecule switches among cationic, neutral and anionic states, depending on the acidity of the environment (top). The fluorescent properties of the three states differ in an easily distinguishable way (bottom). Images reprinted with permission from Langmuir.


In lithographically controlled wetting, a dilute solution containing suspended particles or dissolved molecules is placed atop a substrate. A thin wire grid or stamp bearing the desired pattern is then brought into contact with the solution.

Capillary forces drive the liquid to gather under the protrusions of the template. The exposed liquid then evaporates through the holes in the template, leaving behind the particles of interest. When the template is removed, the resulting nanopattern on the substrate is a reproduction of the pattern in it.

The technique has a resolution in the hundreds of nanometers and allows feature sizes in the tens of nanometers.

The researchers created a new compound that was fluorescent and that had acid-base sensitivity by starting with commercially available 5-bromovanillin, which could be reversibly interconverted between cationic, neutral and anionic forms. They verified the fluorescence of their compound using a PerkinElmer spectrophotometer, and measured the UV and visible wavelengths using a Varian spectrophotometer.

TWacid_Fig2.jpg

Researchers nanoimprinted patterns of a compound that changes fluorescence depending on the acidity onto glass. The patterns show an easily detected optical change from acidic to neutral to basic environments.


The investigators used lithographically controlled wetting to create patterns of this compound on a glass slide. The pattern consisted of stripes that were 350 nm wide with a center-to-center spacing of 1.4 μm, covering an area of at least 50 μm on a side. Measurements with an Agilent Technologies Inc. atomic force microscope showed that the stripes had an average height of 9 nm.

The researchers measured the emission of the stripes using a homebuilt confocal scanning fluorescence microscope comprising a Z-Laser Optoelektronik green diode laser operating at 532 nm and a PerkinElmer avalanche photodiode.

The researchers found that the sensor quickly responded to various acidity gas flows to which it was exposed. For example, the emission count in neutral air was ~35,000 per second, dropped to ~20,000 for a pH value of about 1 and climbed to ~70,000 for a pH value of about 12. What is more, the original intensity resumed when the pH was again neutral.

They noted that the technique holds promise for miniature acidity sensors. Such devices can test small-volume samples and could be integrated with other sensors into a convenient package.

Langmuir, April 1, 2008, pp. 2963-2966.


GLOSSARY
nanotechnology
The use of atoms, molecules and molecular-scale structures to enhance existing technology and develop new materials and devices. The goal of this technology is to manipulate atomic and molecular particles to create devices that are thousands of times smaller and faster than those of the current microtechnologies.
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