380
Y. Niko and S. Watanabe
supramolecular approach (ensemble approach) is not yet a mature technique, theoretically such fluorescent chemosensors can be prepared by simply mixing a fluorophore
and binding site. Thus, this method could expand the available kinds of fluorophores
and simplify the molecular synthesis. This type of sensor is skipped in this chapter;
however, they are well summarized in the review by Kim and co-workers (Wu et al.
2015).
Another type of supramolecular approach is described in Fig. 11.1c, d. Here,
the assembly/disassembly of a fluorophore in the presence/absence of analyte is
strategically used as a trigger to induce variation in the fluorescent properties of the
fluorophore. Relevant examples will be shown in Sects. 11.4.6 and 11.4.7.
Moreover, as explained in Sect. 11.2.1, some fluorescent chemosensors do not
contain a specific molecule as binding site. One example is when the sensor is required
to detect not specific chemical species but the “phase” in a certain system, as shown
in Fig. 11.1e. Environment-sensitive dyes such as Prodan and Nile Red (Klymchenko
2017) should be good candidates for such an application. The relevant examples will
be shown in Sects. 11.4.8 and 11.4.9.
11.3 Dye Selection in Fluorescent Chemosensors
The choice of fluorophore is probably the most important issue in developing novel
fluorescent chemosensors. Before this chapter, several small organic dyes such as
rhodamines, cyanines, and BODIPY shown in Fig. 11.2a were introduced in detail.
Those dyes are generally known to exhibit large molar absorption coefficients (molar
extinction coefficient ε ~ 100,000 M
−1 cm
−1 ), high fluorescence quantum yield
(Φ ~ 0.50), and small Stokes shift (~25 nm) (Lavis and Raines 2008; Umezawa
et al. 2009). In comparison, Prodan (Weber and Farris 1979), Nile Red (Greenspan
et al. 1985), and 3-hydroxychromones (3HC) (Demchenko et al. 2009) described in
Fig. 11.2b show small molar absorption coefficients and large Stokes shifts. However,
they can vary their fluorescence wavelength (i.e., fluorescence color), quantum yield,
and lifetime in response to the environmental condition (Klymchenko 2017). Thus,
each dye has its fluorescence characteristics, and therefore the fluorophore should be
carefully chosen according to the intended use and/or the available techniques and
instruments.
In this part, several important parameters representing the optical properties of
dyes are listed, and their relation to the performance as fluorescent chemosensors is
mentioned.
11.3.1 Absorption/Fluorescence Wavelengths
Each dye exhibits inherent wavelengths for photon absorption/fluorescence. The first
consideration is determining what to observe and which analytical instruments and
Y. Niko and S. Watanabe
supramolecular approach (ensemble approach) is not yet a mature technique, theoretically such fluorescent chemosensors can be prepared by simply mixing a fluorophore
and binding site. Thus, this method could expand the available kinds of fluorophores
and simplify the molecular synthesis. This type of sensor is skipped in this chapter;
however, they are well summarized in the review by Kim and co-workers (Wu et al.
2015).
Another type of supramolecular approach is described in Fig. 11.1c, d. Here,
the assembly/disassembly of a fluorophore in the presence/absence of analyte is
strategically used as a trigger to induce variation in the fluorescent properties of the
fluorophore. Relevant examples will be shown in Sects. 11.4.6 and 11.4.7.
Moreover, as explained in Sect. 11.2.1, some fluorescent chemosensors do not
contain a specific molecule as binding site. One example is when the sensor is required
to detect not specific chemical species but the “phase” in a certain system, as shown
in Fig. 11.1e. Environment-sensitive dyes such as Prodan and Nile Red (Klymchenko
2017) should be good candidates for such an application. The relevant examples will
be shown in Sects. 11.4.8 and 11.4.9.
11.3 Dye Selection in Fluorescent Chemosensors
The choice of fluorophore is probably the most important issue in developing novel
fluorescent chemosensors. Before this chapter, several small organic dyes such as
rhodamines, cyanines, and BODIPY shown in Fig. 11.2a were introduced in detail.
Those dyes are generally known to exhibit large molar absorption coefficients (molar
extinction coefficient ε ~ 100,000 M
−1 cm
−1 ), high fluorescence quantum yield
(Φ ~ 0.50), and small Stokes shift (~25 nm) (Lavis and Raines 2008; Umezawa
et al. 2009). In comparison, Prodan (Weber and Farris 1979), Nile Red (Greenspan
et al. 1985), and 3-hydroxychromones (3HC) (Demchenko et al. 2009) described in
Fig. 11.2b show small molar absorption coefficients and large Stokes shifts. However,
they can vary their fluorescence wavelength (i.e., fluorescence color), quantum yield,
and lifetime in response to the environmental condition (Klymchenko 2017). Thus,
each dye has its fluorescence characteristics, and therefore the fluorophore should be
carefully chosen according to the intended use and/or the available techniques and
instruments.
In this part, several important parameters representing the optical properties of
dyes are listed, and their relation to the performance as fluorescent chemosensors is
mentioned.
11.3.1 Absorption/Fluorescence Wavelengths
Each dye exhibits inherent wavelengths for photon absorption/fluorescence. The first
consideration is determining what to observe and which analytical instruments and