Human Vision Simplifies Sensing Technique

Dr. James P. Smith

A fluorescence detection method that uses the human eye as a key component promises to take fluorescence analysis out of the laboratory and into the field.

Developed by scientists at the Center for Fluorescence Spectroscopy at the University of Maryland School of Medicine, the technology was detailed in the April 1 issue of Analytical Chemistry. The technique includes the use of polarizers, filters and a light-emitting diode (LED) or a laser excitation source.

"It's a very simple idea," said Zygmunt Gryczynski, one of the developers. "But to our knowledge, no such device has been used up to now. Our goal was to develop a simple method, easy to use by any individual, and relatively inexpensive."

A fluorescence sensing technique that relies on the human eye to determine polarization can detect submicromolar concentration of rhodamine 800 and indocyanine green, dyes that could be used for monitoring drugs in patients with chronic diseases.

The idea is indeed a simple one. Incident light fluoresces a transparent reference material, and the fluorescent light is polarized. The transmitted incident light then fluoresces the sample, whose fluorescent light is not polarized. Next, a filter removes the incident light and selects the wavelengths needed for analysis. Light then passes through a dual polarizer, half polarized vertically and half horizontally. The eye views the light through an adjustable polarizer, which is rotated until both sides of view are of equal brightness. The angle of rotation is related by a calibration curve to the concentration of the analyte.

The reference material is not necessarily the same as the analyte. Polarization of the reference fluorescence is achieved with a polarizer or by doping an oriented stretched film with a fluorophore. The only electronics are the power supply and excitation source. An LED and small batteries are adequate. Investigators showed the system's effectiveness by analyzing rhodamine B in ethanol, using a 543-nm HeNe laser. They found a working range up to 15 µmol concentration with a precision of 0.5 to 1 µmol. In another component of the study, fluorescein solution was used to measure pH in the range of six to eight, with a precision of ±0.1 pH units.

Fluorescence under skin

This technology has several potential applications, including the monitoring of glucose, pH, oxygen, calcium and pharmaceuticals. For example, a red-light laser would be able to excite fluorescence under the skin, enabling control of transdermal drug dosing.

"We consider this a generic method," Gryczynski said. "It is not designed for any specific application. Instead, it is easily modified for use in a variety of applications."