Molecular Fluorescence Spectroscopy

Molecular fluorescence spectroscopy measures light emitted by molecules after they absorb radiation, providing highly sensitive and selective quantitation.

Definition

Molecular fluorescence spectroscopy is a luminescence-based analytical method that quantifies analytes from the intensity of light they emit on returning from an excited electronic state to the ground state.

Scope

This topic covers photoluminescence methods used in analysis—principally fluorescence, with related phosphorescence and chemiluminescence. It treats the excitation and emission process, the instrumentation of a spectrofluorometer with its separate excitation and emission wavelength selectors, the factors that govern fluorescence intensity and quantum yield, and the quenching and inner-filter effects that complicate quantitation.

Core questions

• How do absorption and emission combine to produce a fluorescence signal, and why is it so sensitive?
• What molecular features make a compound strongly fluorescent?
• How do quenching and inner-filter effects distort fluorescence measurements?
• When is fluorescence preferable to absorption for trace analysis?

Key theories

Excitation and emission with Stokes shift

A molecule absorbs a photon to reach an excited singlet state, loses energy non-radiatively to the lowest vibrational level, then emits a photon of longer wavelength; this Stokes shift separates emission from excitation and underlies the sensitivity and selectivity of fluorescence, depicted compactly by the Jablonski diagram.

Mechanisms

Absorption of ultraviolet or visible light raises a molecule to an excited singlet state. Vibrational relaxation brings it to the lowest excited level, from which it can emit a fluorescence photon of lower energy. Because emission is measured against a near-dark background rather than as a small change in a large transmitted signal, fluorescence can reach far lower detection limits than absorption. Intensity is proportional to concentration at low absorbance but is reduced by quenching and by inner-filter absorption at higher concentration.

Clinical relevance

Fluorescence methods are central to bioanalysis and clinical diagnostics, including immunoassays, nucleic-acid quantitation and sequencing detection, flow cytometry, and environmental trace analysis, owing to their high sensitivity and the availability of selective fluorescent labels.

History

George Stokes described and named fluorescence in the mid-19th century, observing the characteristic shift to longer wavelengths. The energy-level picture organized by Jabłoński in the 1930s clarified the competing radiative and non-radiative pathways, and the development of sensitive photomultiplier-based spectrofluorometers established fluorescence as a leading trace-analysis technique.

Key figures

• George Gabriel Stokes
• Aleksander Jabłoński
• Joseph R. Lakowicz

Seminal works

• lakowicz2006
• skoog2017

Frequently asked questions

Why is fluorescence usually more sensitive than absorption?

Fluorescence is measured as light emitted against a dark background, so even a faint signal stands out, whereas absorption requires detecting a small decrease in a large transmitted beam, which limits how low a concentration can be measured.

What is the inner-filter effect?

At higher analyte or matrix absorbance, some of the exciting light and some of the emitted light are reabsorbed before reaching the detector, so fluorescence intensity no longer rises linearly with concentration and can even fall.