239 1
Anal. Chern. 1987, 59, 2391-2394
Calculation for Fluorescence Modulation by Absorbing Species and Its Application to Measurements Using Optical Fibers Ping Yuan and David R. Walt*
M a x Tishler Laboratory for Organic Chemistry, Department of Chemistry, T u f t s University, Medford, Massachusetts 02155
A mathematlcai calculation Is described that accounts for the different mechanisms of fluorescence modulatkn by absorbing specles. The calculations reported here provide a significant theoretical bask for developlng fiber-optic chemical sensors wtth various underlying response principles. The caicuiatlons show the importance of two parameters-the characteristic Intermolecular dlstance R , and the effective path length b . The higher the concentratlon of the absorber, the smaller Is the Intermolecular distance R and the more probable resonance energy transfer becomes, resulting in a diminished fluorescence Intensity. Slmilarly, an Increase in b causes a diminutlon of fluorescence by Inner fllter effects. The modulation of the fluorescence signal due to energy transfer and inner fitter effects will greatly expand the number of species that might be analyzed by fluorescence techniques using optlcal fibers.
quencher and F is the fluorescence intensity in the presence of quencher. Static quenching of fluorescence often results from the heavy atom effect, particularly heavy metal ions. Fiber-optic sensors for measuring metal ions have been developed based on immobilized ligands with fluorescent characteristics that change upon complexation with the metal ion (2). Mechanism 2: Dynamic Quenching. In dynamic quenching, the quenching species and the fluorescent molecule undergo a collisional process during the lifetime of the excited state of the fluorescent molecule. For a generalized quencher molecule, Q, the relevant equations are
- -+ +
F + hu F* excitation F* F hv' fluorescence F* + Q F &* quenching This phenomenon is mathematically represented by the Stern-Volmer equation
Fluorescence-based analysis is an extremely sensitive technique capable of measuring low analyte concentrations. It is particularly well suited for sensing using fiber optics. Several approaches have been reported for the preparation of fiber-optic chemical sensors (1-10). These sensors are based on changes in the optical characteristics of a reagent phase attached to the tip of an optical fiber, through which the change is detected. The reagent phase usually consists of a chemically specific reagent immobilized on a stable polymeric support. In this paper, we describe a mathematical calculation accounting for the different mechanisms of fluorescence modulation by absorbing species. The calculations provide a significant theoretical basis for developing fiber-optic chemical sensors and analyses with various underlying response principles.
THEORY The fluorescence intensity or quantum yield of luminescent species may be decreased or even eliminated by interactions with other chemical species. This phenomenon is called fluorescence quenching. Quenching occurs by a variety of mechanisms: Mechanism 1: Static Quenching. Interaction between the fluorescent molecule (F) and the quencher (Q) takes place in the ground state, forming a nonfluorescent complex F+Q=F-Q The efficiency of quenching is governed by the formation constant of the complex K , and the concentration of the quencher [Q] 1 F _ -
F"
1 + &[&I
where Fo is the fluorescence intensity in the absence of
F _
1 1 (3) F" 1 + ~QTO[&] 1 + Ksv[&l where Ksv is the Stern-Volmer constant, Ksv = k Q ~ O KQ, is the diffusion-controlled rate constant (= 1O'O M-' s-'), and T~ is the fluorescence lifetime. Since
