2 Fluorescent Chemosensors
Fluorescent chemosensor has been widely explored as a rapid and sensitive analytical tool for detection of pollutants in natural water resources, sewage and
industrial wastewater treatment plants compared to chromatographic techniques
[18, 19]. Fluorescent chemosensors offer selective detection of water pollutants by
monitoring the fluorescence of dissolved organic matter and humic-like fluorescence peaks. In this chapter, the fundamentals, sensing mechanisms and advances
of fluorescent chemosensors and their application for sensing water pollutants are
discussed in detail. Fluorescence is defined as light-matter interaction phenomenon
in which the electronically excited molecule upon absorption of the incident light
emits light in longer wavelengths. The emission of light is dependent on the
interaction of the molecule with the medium and the molecule is termed as fluorophore. The emission of fluorescence could be tuned from visible to NIR wavelengths based on the energy gap of the individual fluorophores [20, 21].
Fluorophore (signaling moiety) and receptor (recognition moiety) [22] are the basic
building blocks of fluorescent chemosensor. They are usually connected to each
other by means of spacer through chemical bonding or integrated together.
2.1 Fluorescence Sensing Mechanisms
Modulation of the photophysical properties of a fluorophore (such as emission
wavelength, lifetime (s), quantum yield (u) and quenching) upon interaction with
the analyte of interest is exploited for sensing applications. The interaction of an
analyte with the fluorophore is influenced by various mechanisms such as electron
transfer, charge transfer, energy transfer, excimer/exciplex formation and aggregation induced quenching/emission [23, 24] as shown in Fig. 1.
In photoinduced electron transfer (PET) process, the electron transfer occurs
between fluorophore and acceptor site (receptor), resulting in non-radiative dissipation of excited-state which can quench the fluorescence. Due to PET, a charge
separation is generated and the species relaxes to the ground state via charge
recombination (Fig. 1). Upon interaction of PET probes with the analyte, fluorescence signal change was large enough to be considered as ‘off–on’ or ‘on–off’
through inhibition of PET process [25, 26]. On the other hand, colorimetric based
intramolecular charge transfer (ICT) probes have been effectively utilized for
sensing. This involves the interaction between analyte and the receptor, resulting in
the distinct color change. Therefore, both PET and ICT probes are recognized as
good candidates for the development of highly sensitive fluorescent chemosensors.
Fluorescence resonance energy transfer (FRET) is the interaction between donor
and acceptor with the distance between them range from 10 to 100 Å resulting in
Fluorescent Chemosensor for Detection of Water Pollutants
149
Fluorescent chemosensor has been widely explored as a rapid and sensitive analytical tool for detection of pollutants in natural water resources, sewage and
industrial wastewater treatment plants compared to chromatographic techniques
[18, 19]. Fluorescent chemosensors offer selective detection of water pollutants by
monitoring the fluorescence of dissolved organic matter and humic-like fluorescence peaks. In this chapter, the fundamentals, sensing mechanisms and advances
of fluorescent chemosensors and their application for sensing water pollutants are
discussed in detail. Fluorescence is defined as light-matter interaction phenomenon
in which the electronically excited molecule upon absorption of the incident light
emits light in longer wavelengths. The emission of light is dependent on the
interaction of the molecule with the medium and the molecule is termed as fluorophore. The emission of fluorescence could be tuned from visible to NIR wavelengths based on the energy gap of the individual fluorophores [20, 21].
Fluorophore (signaling moiety) and receptor (recognition moiety) [22] are the basic
building blocks of fluorescent chemosensor. They are usually connected to each
other by means of spacer through chemical bonding or integrated together.
2.1 Fluorescence Sensing Mechanisms
Modulation of the photophysical properties of a fluorophore (such as emission
wavelength, lifetime (s), quantum yield (u) and quenching) upon interaction with
the analyte of interest is exploited for sensing applications. The interaction of an
analyte with the fluorophore is influenced by various mechanisms such as electron
transfer, charge transfer, energy transfer, excimer/exciplex formation and aggregation induced quenching/emission [23, 24] as shown in Fig. 1.
In photoinduced electron transfer (PET) process, the electron transfer occurs
between fluorophore and acceptor site (receptor), resulting in non-radiative dissipation of excited-state which can quench the fluorescence. Due to PET, a charge
separation is generated and the species relaxes to the ground state via charge
recombination (Fig. 1). Upon interaction of PET probes with the analyte, fluorescence signal change was large enough to be considered as ‘off–on’ or ‘on–off’
through inhibition of PET process [25, 26]. On the other hand, colorimetric based
intramolecular charge transfer (ICT) probes have been effectively utilized for
sensing. This involves the interaction between analyte and the receptor, resulting in
the distinct color change. Therefore, both PET and ICT probes are recognized as
good candidates for the development of highly sensitive fluorescent chemosensors.
Fluorescence resonance energy transfer (FRET) is the interaction between donor
and acceptor with the distance between them range from 10 to 100 Å resulting in
Fluorescent Chemosensor for Detection of Water Pollutants
149