Geoscience Reference
In-Depth Information
1.3.4.4 Fluorescence Quenching
The fluorescence intensities and/or fluorescent lifetimes observed from luminescent spe-
cies can be reduced or eliminated by interactions with other chemical species that increase
the probability of deexcitation through nonradiative pathways. This phenomenon is known
as quenching and can be either static or dynamic. Static quenching occurs when the poten-
tially fluorescing chromophore, in the ground state, reacts with the quenching species to
form a nonfluorescent species. The efficiency of this quenching is dictated by the rate of
formation of the nonfluorescent species and the concentration of the quencher (Lakowitz,
2006
, chapter 9). In dynamic quenching, the interaction with the quenching species is dur-
ing the lifetime of the excited state of the potentially fluorescing species. Dynamic quench-
ing is also known as collisional quenching and its efficiency depends on the viscosity of the
solution, the lifetime of the excited state,
τ
0
, of the fluorophore, and the concentration of the
quencher [Q]. This is summarized in the Stern-Volmer equation.
Φ
Φ
0
1
=
(1.15)
1
+
kQ
q
τ
[]
0
where
k
q
is the rate constant for encounters between quencher and potentially luminescing
species and
Φ
0
and
Φ
are the quantum yields of luminescence in the absence or presence of
concentration of the quencher [Q] respectively.
Perhaps the most well known collisional quencher is molecular oxygen, which is known
to quench the vast majority of all known fluorophores. With this is mind it is generally
accepted that observed fluorescence intensities from natural aquatic systems is quenched
to some degree.
1.3.4.5 Influence of Molecular Structure on Fluorescence
High fluorescence yields are observed in highly conjugated “rigid” molecules. Therefore
conjugated systems form the basis for many chromophores and chromophores are defined
as a chemical group in which the electronic transition responsible for a given spectral band
is approximately localized. Such chromophores are often present in naturally occurring
organic compounds, and involve conjugated ring systems such as C=O and N=N in add-
ition to conjugated C-C bonds (Lakowitz,
2006
, chapter 3). Conjugation can also occur
through carboxyl groups and are also important in conjugated systems. In these systems
the double bond of the carbonyl group (−C=O) is adjacent to the single bond attaching
the hydroxyl group (−OH) to the carbon atom. This functional group is of importance in
many naturally occurring organic acids. The restricted rotational and vibration freedom of
such molecules ensures that the energy gap between the lowest singlet state and the ground
electronic state is too large for deactivation via internal conversion. However, if aromatic
hydrocarbons have freely rotating substituents or lengthy side chains, then the fluorescence
efficiency is greatly reduced due to the increase in rotational and vibrational freedom which
subsequently increases the probability of internal conversion.
