Fluorescence spectroscopy is an analytical technique that measures the emission of light from molecules after excitation by photons. It is highly sensitive (detection limits down to pM), selective, and widely used in biochemistry, environmental analysis, and materials science.
Principles of Fluorescence
A molecule absorbs a photon and is promoted from the ground state (S0) to an excited singlet state (S1 or S2). Internal conversion rapidly relaxes the molecule to the lowest vibrational level of S1 (Kasha’s rule). Fluorescence occurs when the molecule returns to S0 by emitting a photon, and the emitted light has a longer wavelength (lower energy) than the absorbed light, a phenomenon known as the Stokes shift. This shift arises from energy loss through vibrational relaxation and solvent reorganization, and a large Stokes shift (20-100 nm) allows separation of excitation and emission light by filters or monochromators. The Jablonski diagram illustrates these photophysical processes: absorption, internal conversion, fluorescence, intersystem crossing, and phosphorescence.
Fluorescence Parameters
Key fluorescence parameters include quantum yield, lifetime, and intensity. Quantum yield (Φ) is the ratio of photons emitted to photons absorbed (Φ = kemitted/kabsorbed), with values ranging from less than 0.01 (weakly fluorescent) to near 1.0 (highly fluorescent, e.g., fluorescein at pH 9, Φ = 0.95). Lifetime (τ) is the average time a molecule spends in the excited state before emitting a photon, typically 1-10 ns for organic fluorophores. Fluorescence intensity follows I = I0 × (1 - 10-εbc) × Φ, where ε is molar absorptivity, b is path length, and c is concentration; at low concentrations, intensity is proportional to concentration.
Fluorophores
Fluorophores are categorized as intrinsic or extrinsic. Intrinsic fluorophores are naturally occurring fluorescent molecules such as aromatic amino acids (tryptophan, λex ~280 nm, λem ~350 nm), NADH, flavins, and chlorophyll. Extrinsic fluorophores are synthetic dyes added to non-fluorescent samples, including fluorescein (λex 494 nm, λem 518 nm), rhodamine, cyanine dyes (Cy3, Cy5), and the Alexa Fluor series. Quantum dots are semiconductor nanocrystals with size-tunable emission, broad excitation, narrow emission, and high photostability.
Instrumentation
A fluorescence spectrometer includes a light source such as a xenon arc lamp (broad spectrum, 200-1000 nm) or LEDs for steady-state measurements, and pulsed lasers or LEDs for time-resolved measurements. An excitation monochromator selects the excitation wavelength, and a double monochromator reduces stray light for high-sensitivity measurements. The sample compartment uses a four-sided clear quartz cuvette for right-angle detection geometry, or front-face geometry for highly absorbing or scattering samples. An emission monochromator scans the emission wavelength while a long-pass filter blocks scattered excitation light, and the detector is typically a photomultiplier tube (PMT) for high sensitivity or a CCD for multichannel detection.
Fluorescence Techniques
Several specialized fluorescence techniques exist. Steady-state fluorescence measures emission intensity at a fixed excitation wavelength and is used for concentration determination and emission spectral characterization. Time-resolved fluorescence measures fluorescence decay after a short excitation pulse and is used to determine fluorescence lifetimes, distinguish dynamic from static quenching, and study molecular dynamics. Fluorescence Resonance Energy Transfer (FRET) involves non-radiative energy transfer from a donor fluorophore to an acceptor within 1-10 nm, with efficiency proportional to 1/r6, making it a molecular ruler for distances in proteins and nucleic acids. Fluorescence polarization (anisotropy) measures the rotational mobility of fluorophores and is used in binding studies and immunoassays.
Quenching
Fluorescence quenching can occur through two mechanisms. Dynamic (collisional) quenching occurs when a quencher such as O2, I-, or acrylamide diffuses to and collides with the fluorophore during the excited state, described by the Stern-Volmer equation: F0/F = 1 + KSV[Q]. Static quenching occurs when the quencher forms a non-fluorescent ground-state complex with the fluorophore. The two mechanisms are distinguished by lifetime measurements: dynamic quenching reduces lifetime while static quenching does not.
Applications
Fluorescence spectroscopy is used for quantitative analysis of fluorescent analytes in pharmaceutical and environmental samples, enzyme activity assays using fluorogenic substrates, DNA sequencing and microarray analysis using fluorescent labels, cell imaging and flow cytometry using fluorescent antibodies and dyes, and environmental monitoring of pollutants such as polycyclic aromatic hydrocarbons and humic substances.
