Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
In connection with continuing study on dual-analyte chemosensors [14-21], we herein report a highly selective and sensitive
fluorescence turn-on PY for the detection of In3+ ion in water and Al3+ ion in methanol, respectively. This chemosensor is based on the
pyrene fluorophore with a α-amino acid moiety, and constitutes a new bifunctional (In3+ / Al3+) probe by the convenient solvent-tuning
fashion. Moreover, this probe can be used in the bioimaging of In3+ ion for the first time. As shown in Scheme 1, the probe PY was
synthesized from 1-hydroxypyrene-2-carbaldehyde [22] and D-phenylalanine. The structure was confirmed by 1H, 13C NMR and ESI
mass spectrometry (Fig. S1-S3 in Supporting information).
First, the fluorescence spectral properties of PY were studied in CH3OH solution at room temperature because the selectivity is not
so good in ethanol. As illustrated in Fig. 1a, PY exhibited a strong, single emission band at 430 nm when excited at 364 nm, which
emission was attributed to pyrene-pyrene static excimer [23]. Upon the addition of different cations (Li+, Na+, H+, Ag+, Mg2+, Sr2+,
Ba2+, Pb2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Fe3+, Cr3+, Al3+, Ga3+, and In3+) to the solution, the fluorescence responses of
PY were different. Almost no obvious changes of the fluorescence of PY were observed when Li+, Na+, Mg2+, Sr2+, Ba2+, Mn2+, Zn2+,
and Cd2+ ions were added to the solution, respectively. Due to the chelation enhanced fluorescent quenching (CHEQ) [24], Pb2+, Fe2+,
Ni2+, Cu2+, Hg2+, Ag+, and Fe3+ quenched the fluorescence of PY severely. But the addition of Cr3+, Co2+, Al3+, Ga3+, In3+, or H+
changed the fluorescence signals of PY remarkably. The characteristic monomer emission bands at approximately 400 and 420 nm of
pyrene moieties appeared, and a new peak from the dynamic excimer of pyrene moieties at round 450 nm was also observed.
Compared to the fluorescence intensity of monomer peak, Al3+ ion caused the significant enhancement of the two monomer peaks.
These results showed that PY was highly selective for Al3+ in CH3OH solution. Then the fluorescence titrations of Al3+ were carried
out (Fig. 1b). With the addition of Al3+ to the solution of PY, the monomer emissions at 400 nm and 420 nm increased significantly
and the excimer emission at 430 nm decreased in some extent. Moreover the total fluorescence intensity at 400 nm of PY was
enhanced 16.7-fold when 40.0 equiv. of Al3+ was present. And the fluorescent quantum yield (Φ) of PY (10.0 μmol/L) increased from
60.9% to 63.5% in the presence of Al3+ (40.0 equiv.). The detection limits [25] were calculated to be 4.13 μmol/L and 8.08 μmol/L at
400 nm and 430 nm, respectively (Figs. S4 and S5 in Supporting information). The Job’s plot supported the formation of a 1:1
stoichiometry complexation between PY and Al3+ (Fig. S6 in Supporting information). Based on the stoichiometry, the association
constant K of the complex was then calculated to be about 5.0×104 L/mol by using the emission changes at both 400 nm and 430 nm
with Benesi–Hildbrand plots (Figs. S7 and S8 in Supporting information). In addition, the peak at m/z 618.0226 for [PY + Al3+
+
2ClO4− – 2H+]– in the negative-ion ESI mass spectrum provides additional evidence for the formation of a 1:1 complex of PY•Al3+ (Fig.
S9 in Supporting information). Further, the short response time showed that PY has high sensitivity for Al3+ (Fig. S10 in Supporting
information). And the competition experiments displayed all of other cations had no obvious interference with the detection of Al3+ ion
(Fig. S11 and S12 in Supporting information). These results indicated that PY could act as a turn-on fluorescent probe for Al3+ in
methanol.
Next, the fluorescence spectral properties of PY were investigated in H2O (0.05% DMSO, v/v) at pH 4.8 (Fig. 2a). The fluorescence
intensity of free PY was quenched significantly by water. And the fluorescence spectrum of free PY was different from that in
methanol. Three emission bands at about 396 nm, 416 nm and 446 nm were observed, which also attributed to the monomer and
excimer emission bands of pyrene. After the addition of different cations to the solution, most of them quenched the fluorescence of
PY in different extent. Only In3+ caused a significant fluorescence enhancement of PY with a new emission band at 410 nm. The high
selectivity of In3+ over Al3+ and Ga3+ was probably attributed to a pH effect in water, since Group IIIA ions have strong hydration
ability. Therefore, the pH effect was studied in detail (Fig. S13 in Supporting information). Within the range of pH 4.0 and 5.5, probe
PY showed high selective recognition of In3+ over Al3+ and Ga3+ by turn-on fluorescence mode. The addition of In3+ ion to the solution
resulted a 2.9-fold fluorescence enhancement at 410 nm (Fig. 2b), which denoted chelation-enhanced fluorescence (CHEF) [26]. The
fluorescent quantum yield (Φ) of PY (10.0 μmol/L) increased from 46.1% to 48.8% in the presence of In3+ (40.0 equiv.). And the
intensity decrease in the two bands at about 396 nm and 446 nm, which is resulting from breaking the monomer-excimer equilibrium
of PY in water solution after the formation of PY•In3+ complex. The Job’s plot (Fig. S14 in Supporting information), Benesi–
Hildbrand plot (Fig. S15 in Supporting information), and the ESIMS (Fig. S16 in supporting information) also indicated 1:1 binding
model between PY and In3+ in NaOAc/HOAc (pH 4.8) buffer solution. By using the emission changes at 410 nm, the association
constant K of PY•In3+ was calculated to be 7.1 × 104 L/mol (Fig. S15 in Supporting information). A peak at m/z 777.7 assigned to [PY
+ In3+ + 2ClO4− + H2O − 2H+]− was observed in ESIMS (Fig. S16 in Supporting information). The corresponding detection limit was
found to be 3.42 μmol/L (Fig. S17 in Supporting information), which is low enough for detection of In3+. PY still showed the short
response time for detection of In3+ (Fig. S18 in Supporting information). And all competitive cations had no obvious interference with
the detection of In3+ ion (Fig. S19 in Supporting information). These results clearly showed that PY can function as a specific turn-on
fluorescent probe for In3+ in NaOAc/HOAc (pH 4.8) buffer solution.
Finally, the fluorescence imaging of PY for sensing In3+ ions in HeLa cells was carried out. When HeLa cells were incubated with
PY (10.0 μmol/L) for 30 min at 37 oC, the weak fluorescence of cells was observed (Fig. 3b). As shown in Fig. 3d, at 37 oC the treated
HeLa cells were incubated with In3+ (200.0 μmol/L) in culture medium for 30 min, the strong blue fluorescence was obtained. These
results indicated that PY is cell membrane permeable and could be used for detecting In3+ within living cells.
Wang, Ya-Wen
