Photonic Gauge Has 0.03 °C Resolution

Hank Hogan

Researchers have developed a way to take the temperature of a fluid on a tiny scale and with a light touch. The technique could have applications in microfluidic systems, the labs-on-a-chip that promise to revolutionize analytical chemistry and biochemistry.

A photonic technique based on the temperature-dependence of fluorescence in a dilute fluorophore takes the temperature of a fluid. The approach promises applications in microfluidic systems for analytical chemistry and biochemistry.

The approach is based on the temperature-dependence of fluorescence in a dilute fluorophore. David Ross, a researcher on the project at the National Institute of Standards & Technology (NIST) in Gaithersburg, Md., explained that the fluorophore must have a glow that changes predictably with temperature. Aside from that, there are few requirements to the system.

"It is best to use a neutral, or uncharged, fluorophore when there are strong electric fields in the system," Ross said. "If the fluorophore is charged, its concentration -- and therefore, the intensity -- could be changed by the electric fields."

The process begins with the calibration of changes in the fluorophore across the temperature range of interest by uniformly heating it and measuring the fluorescence intensity with a standard microscope and CCD camera. The technique has an ideal precision of 0.03 °C at room temperature and 0.07 °C at approximately 90 °C, but this varies with the temporal and spatial resolution and with the amount of signal averaging.

With less temperature precision, measurements can be obtained in as little as 33 ms and cover 1 x 1 µm.

Using a mercury arc lamp, filters for emission and excitation, a digital camera from Cohu Inc. of San Diego and the fluorophore rhoda-mine B, the NIST researchers have measured fluid temperatures in 50-µm-wide microchannels. They have estimated that the precision in these experiments was a few degrees centigrade.

The group hopes that the technique will find an application in microfluidic systems. Ross noted that there is plenty of room for refinement. "Increased resolution could also be achieved with better optics and/or a better camera; we just used a standard, inexpensive gray-scale camera," he said.