Simple pyridyl-salicylimine-based
fluorescence
“turn-on
” sensors for distinct detections of Zn
2+, Al
3+and OH
ions in mixed aqueous media
†
Muthaiah Shellaiah, Yen-Hsing Wu and Hong-Cheu Lin*
Simple pyridyl-salicylimine derivatives (F1, F2 and F3) are reported for thefirst time as fluorescence “turn-on” sensors for distinct detections of Zn2+, Al3+and OHions in mixed-aqueous media CH
3CN/H2O with
volume ratios of 6/4 and 3/7 (at pH ¼ 7 and 25 C) via internal charge transfer (ICT), chelation enhancedfluorescence (CHEF), and deprotonation mechanisms. F1 and F2 show diverse turn-on sensing applications to Zn2+, Al3+and OHions, but F3 exhibited thefluorescence turn-on sensing to Al3+and
OHions in CH3CN/H2O (6/4; vol/vol). F1+Zn2+and F2+Zn2+complexes revealed the reversibilities and
ratiometric displacements of Zn2+ with ethylene diamine tetra acetic acid (EDTA) and Al3+ ions,
respectively, in CH3CN/H2O (6/4; vol/vol). On the other hand, F1, F2 and F3 in CH3CN/H2O (3/7; vol/vol)
showed sensitivities only to Al3+ions but negligible selectivities to OHions. Stoichiometry of all sensor
complexes were calculated as 1 : 1 by job's plots based on UV/Vis and PL titrations. The complex formation and binding sites of all sensor materials were well characterized by1H,13C NMR, and mass (FAB) spectral analysis. Detection limits were calculated from standard deviations and linear fitting calculations. The association constant (log Ka) values of sensor complexes were evaluated from the
fluorescence binding isotherms. The fluorescence decay constant (s) values were estimated from time resolvedfluorescence studies. Time, temperature, pH and solvent concentration effects towards sensor responses were fully investigated in this report.
Introduction
The design and synthesis of new molecular sensors towards biologically and environmentally important species are always essential for practical research in various elds of science.1 Among the available detection methods, chemosensors based
on ion-induced uorescence changes are predominantly
attractive in terms of sensitivity, selectivity, response time, simplicity, high degree of specicity and low detection limit.2 Due to the uorescence quenching effects3 of biologically
important ions, the developments of uorescence turn-on
sensors still remains a challenging task. Hence, several molec-ular turn-on sensors4were reported for a variety of cations and anions based on photo induced electron transfer (PET), internal charge transfer (ICT), chelation enhanceduorescence (CHEF), and deprotonation mechanisms. Among them, PET5exhibited various changes of emission intensities with some or no spec-tral shis, whereas ICT6 caused both intensity changes and
spectral shis, and CHEF7 also provided uorescence
enhancements with or without any spectral changes.
In the midst of the important heavy metal ions in the human body, zinc is the second most abundant metal ion and is actively involved in diverse biological activities, such as structural and catalytic cofactors, neural signal transmitters or modulators, regulators of gene expression and apoptosis.8Minute quantities of zinc are necessary for the living organism, but excessive amounts may damage the organism.9Additionally, to the best of our knowledge, some available Zn2+sensors10have difficulty in distinguishing Zn2+ from Cd2+, since cadmium is in the same group of the periodic table and has similar properties. Therefore, the design of a highly selective and sensitiveuorescence sensor for Zn2+ detection without interference from other metal ions, especially Cd2+, is one of the most important objectives. On the other hand, aluminum is the third most prevalent (8.3% by weight) metallic element on the earth and its soluble form (Al3+)
is highly toxic to plant growth.11Intemperance of Al3+deposition
in the brain is believed to cause neurodementia, such as Par-kinson's disease, Alzheimer's disease, and dialysis encephalop-athy.12 However, owing to the weak coordination and strong hydration ability of Al3+ in water, it is easily interfered by the variations of the pH values in solution and the coexistence of interfering ions.13 In comparison with transition-metal ions, Department of Materials Science and Engineering, National Chiao Tung University,
Hsinchu 30049, Taiwan (ROC). E-mail: [email protected]
† Electronic supplementary information (ESI) available: For synthesis and sensor characterization by UV/PL,13C NMR and mass (FAB) spectral evidences. See DOI: 10.1039/c3an36840h
Cite this:Analyst, 2013, 138, 2931
Received 12th December 2012 Accepted 5th March 2013 DOI: 10.1039/c3an36840h www.rsc.org/analyst
PAPER
Published on 05 March 2013. Downloaded by National Chiao Tung University on 28/04/2014 02:04:07.
View Article Online
scarce examples ofuorescence sensors have been reported for Al3+ so far and most of them have synthetic difficulties with limited sensitivities or selectivities. Therefore, it is highly desir-able to develop more sophisticated and selective Al3+ sensors
which can be easily synthesized and handled.14
Similarly, hydroxide ions are ubiquitous in nature, and their properties are important in chemical, biological,
environ-mental, and atmospheric processes.15 Hydroxide is used
worldwide in many industrial processes, and rapid and reliable methodologies for the sensing of hydroxide ions for quality control purposes and monitoring during industrial processing are required.16Problems arise quite simply due to the corrosive nature of the alkali and glass, so pH electrodes become insen-sitive and unstable at high concentrations.17Hence, selective sensors of hydroxide ions at higher pHs are favorable. Owing to the importance of Zn2+, Al3+ and OH ions, many sensory reports for them are separately available as mentioned previ-ously, but sensor probes with dissimilar responses to those analytes are cost-effective and highly desirable for real time applications. However, developments of such sensors are chal-lenging tasks and also have the synthetic difficulties.18In these considerations, Schiff bases19were reported as sensory mate-rials for various analytes with least synthetic difficulties, but only a few of them were accounted for by multiple analyte recognitions.20
Herein, for the rst time we report pyridyl-salicylimine21 Schiff base derivatives (F1, F2 and F3) as uorescence “turn-on” sensors for distinct detections of Zn2+, Al3+ and OHions in mixed-aqueous media [CH3CN/H2O (6/4 and 3/7; vol/vol), pH¼
7 and at 25C] via ICT, CHEF and deprotonation mechanisms as illustrated in Fig. 1.
Results and discussion
Synthesis and photophysical propertiesThree pyridyl-salicylimine Schiff base derivatives F1, F2 and F3 (Fig. 1) were easily synthesized via one-pot aldehyde–amine condensation22reaction as noticed in Scheme S1,† in the pres-ence of methanol with excellent yields and high purities. The photophysical properties and sensor responses of F1, F2 and F3 are shown in Table 1. The absorption and PL maxima of F1, F2
and F3, are 344, 346, 343 nm, and 424, 427, 432 nm, respec-tively. The quantum yield (F) measurements were carried out at different mixed-aqueous media (CH3CN/H2O) concentrations.
Even though, the quantum yields of F1, F2 and F3 were evi-denced that they can be used for sensor applications in CH3CN/
H2O at 6/4, 1/1 and 3/7 vol ratios, but we tend to choose higher
and lower vol ratios (6/4 and 3/7) of mixed-aqueous media. Hence, initially we carried out the sensor titrations in CH3CN/
H2O (6/4; vol/vol), and then we extended to CH3CN/H2O (3/7;
vol/vol). Similarly, the pH measurements of F1, F2 and F3 (Fig. S10 and S11; see the ESI†) suggested that they can be utilized for the sensor titrations from pH ¼ 0 to pH ¼ 8. However, TRPL studies of F1, F2 and F3 at acidic pHs affected theiruorescence decay constants (Fig. S12 and Table S2; see the ESI†). Therefore, we performed all of our UV-Vis/PL titra-tions in CH3CN/H2O (6/4 and 3/7; vol/vol), pH¼ 7 and at 25C.
On the other hand, to evaluate the sensor responses, the1H and
13C NMR titrations were carried out by dissolving F1, F2 and F3
in CD3CN and other ions (Zn2+, Al3+and OH) in D2O.
HOMO–LUMO calculations
The HOMO–LUMO calculations of F1, F2 and F3 were carried out by semi-empirical AM1 method23and we found that HOMO and LUMO of F1 and F3 were localized on phenyl and pyridyl rings, respectively, whereas, for F2 both HOMO and LUMO were located equally on phenyl and pyridyl rings as noticed in Fig. S13A–C.† However, the above case was not observed in phenoxides of F1, F2 and F3, in which the phenoxides of F2 and F3 positioned their HOMO and LUMO in phenyl and pyridyl rings, respectively, and F1 sited them only on phenyl rings as shown in Fig. S13D–F.† The localization of electron clouds in F1, F2 and F3 were also affected during the formation of sensor complexes and the formation of phenoxides were highly favourable at higher pHs. Therefore, this calculation provides more support for lateral explanations of sensor complexes of F1, F2 and F3 with Zn2+and Al3+ions as well as phenoxide formed
of F1–F3 with OHions.
Fluorescence titrations on cations and anions
Initially, F1, F2 and F3 in CH3CN/H2O (6/4; vol/vol) were
investigated towards metal ions (Li+, Ag+, K+, Na+, Cs+, Ni2+, Fe2+, Co2+, Zn2+, Cd2+, Pb2+, In3+, Ga3+, Mg2+, Cu2+, Cr3+, Fe3+, Ag2+, Mn2+, Eu3+, Hg2+, Mg2+ and Al3+ in H2O), and then
extended to CH3CN/H2O (3/7; vol/vol). As shown in Fig. 2, F1
and F2 revealed selectivities to Zn2+and Al3+ions with different spectral shis, but F3 showed the selectivity just to Al3+ ions
with no sensor response to Zn2+ ions. Due to the ICT mecha-nism, F1 and F2 exhibited with different spectral shis towards Zn2+ and Al3+ ion PL enhancements. However, in F3 the ICT
found to be inhibited by the presence of methyl group in the third position of the pyridyl unit, and hence provided selectivity
just to Al3+ ions via CHEF mechanism. The PL maxima of
F1+Zn2+, F2+Zn2+, F1+Al3+, F2+Al3+and F3+Al3+appeared at 508, 505, 487, 485 and 477 nm, respectively, with more folds of PL enhancements. As shown in Table 1, the sensor properties and
PL intensities of F1, F2 and F3 to Zn2+ demonstrated a
Fig. 1 Structures and schematic representations of sensor responses of F1, F2 and F3 in (a) CH3CN/H2O (6/4; vol/vol) and (b) CH3CN/H2O (3/7; vol/vol).
decreasing trend as F1+Zn2+> F2+Zn2+> F3+Zn2+(29.9 and 29.2 folds and no sensitivities). In contrast to the Zn2+sensors, the PL intensities of F1, F2 and F3 to Al3+ pronounced a reverse trend as F1+Al3+ < F2+Al3+< F3+Al3+ (29.5, 39 and 44.8 folds). Similarly, as visualized in Fig. 4A and B, F1 and F2 provides the turn-on sensor responses to Zn2+and Al3+with green and blue uorescence with differential spectral shis. On the other hand, F3 exhibited turn-on sensor response to Al3+with blue
uores-cence rather than Zn2+as noticed in Fig. 4C. The above variation
could be well explained on the basis of HOMO and LUMO concept (Fig. S13A–C†), wherein both HOMO and LUMO elec-tron clouds were located in both rings of F2. Comparing F1 and F3, even though they possessed HOMO and LUMO electron clouds correspondingly on phenyl and pyridyl rings, the pres-ence of methyl group in F3 provided entirely different sensor
properties. Further investigations of F1, F2 and F3 in CH3CN/
H2O (6/4; vol/vol) towards various anions (F, Br, Cl, I,
ClO4, BH4, NO3, PO4and OH) in H2O at pH¼ 7, 25C
showed selective sensor responses to OHanions as noticed in Fig. 1 and 3. However, their PL intensity changes towards OH ions varied as shown in Table 1; F1, F2 and F3 revealed 30.1, 19.7 and 22.9 folds, respectively, along with greenuorescence under UV-light irradiations as envisaged in Fig. 4A–C. Forma-tion of phenoxide ions might be the cause for the PL enhancements of F1, F2 and F3, roughly at ca. 500 nm. In addition, the pH value of the above sensor systems to OHions were noticed as 7, even maintained aer the PL excitations, and allowed us to accomplish the further measurements such as pH effects. As shown Fig. S13D–F† in HOMO–LUMO levels F2 and
Table 1 Photophysical and sensor properties of F1, F2 and F3
Compound F Sensor response to Zn2+a Sensor response to Al3+a Sensor response to OHa sa,b,c,e(ns)
F1 (lex¼ 344 nm; lem¼ 424 nm) 0.011a Turn-on (29.9 folds) (lex¼ 344 nm; lem¼ 508 nm) Turn-on (29.5 folds) (lex¼ 344 nm; lem¼ 487 nm) Turn-on (30.1 folds) (lex¼ 344 nm; lem¼ 502 nm) 2.19 0.016b 0.018c 0.033d F2 (lex¼ 346 nm; lem¼ 427 nm) 0.008a Turn-on (29.2 folds) (lex¼ 346 nm; lem¼ 505 nm) Turn-on (39 folds) (lex¼ 346 nm; lem¼ 485 nm) Turn-on (19.7 folds) (lex¼ 346 nm; lem¼ 499 nm) 1.51 0.012b 0.014c 0.030d F3 (lex¼ 343 nm; lem¼ 432 nm) 0.010a NA Turn-on (44.8 folds) (lex¼ 343 nm; lem¼ 477 nm) Turn-on (22.9 folds) (lex¼ 343 nm; lem¼ 500 nm) 1.35 0.013b 0.015c 0.031d aCH
3CN/H2O (10–6/0–4; vol/vol).bCH3CN/H2O (1/1; vol/vol).cCH3CN/H2O (3/7; vol/vol).dCH3CN/H2O (1/99; vol/vol), 9,10-diphenyl anthracene
(DPA) in CH3CN as a reference standard (F ¼ 0.9).eFluorescence lifetimes.
Fig. 2 Sensor responses of (a) F1 in CH3CN/H2O (6/4; vol/vol), (b) F2 in CH3CN/ H2O (6/4; vol/vol) and (c) F3 in CH3CN/H2O (6/4; vol/vol) towards metal ions in H2O.
Fig. 3 Sensor responses of (a) F1 in CH3CN/H2O (6/4; vol/vol), (b) F2 in CH3CN/ H2O (6/4; vol/vol) and (c) F3 in CH3CN/H2O (6/4; vol/vol) towards anions in H2O (each 50 equiv.).
F3-phenoxides were localized on phenyl and pyridyl rings, respectively, but F1-phenoxide was restricted just to the phenyl ring. However, the under-sized difference in PL enhancements of F2 and F3 to OHions was due to the positional change of methyl substituent in the pyridyl unit. Therefore the sensor responses trend of F1, F2 and F3 to OHions were akin to F1+OH> F3+OH> F2+OHas noticed in Table 1. The sensor titrations of F1, F2 and F3 on cations were repeated in CH3CN/
H2O (3/7; vol/vol), and evidenced the sensitivities of them only
to Al3+ions as noticed in Fig. S14 (see the ESI†). UV-Vis/PL titrations on Zn2+ions
By increasing the concentrations of Zn2+0–30 mM (0, 2, 4, 6, 8, 10, 14, 16, 18, 20, 22, 24 and 30 mM in H2O), except F3, the
sensitivities of F1 and F2 (20mM) in CH3CN/H2O (6/4; vol/vol)
towards Zn2+ions were clearly observed in Fig. 5. The
uores-cence spectra of F1 (lem ¼ 508 nm) and F2 (lem ¼ 505 nm)
showed turn-on responses rapidly, and the insets clearly illus-trated that the turn-on properties were saturated at 20mM Zn2+ ions, thereaer further addition of Zn2+ affected the sensor
property. The above statement was further conrmed via stoi-chiometry, binding site, and sensor complex formation studies. In order to establish the specic selectivities of F1 and F2 to Zn2+, we performed the single and dual metal competitive analysis as noticed in Fig. 6. In single metal system (black bars), all the metal ion (Li+, Ag+, K+, Na+, Cs+, Ni2+, Fe2+, Co2+, Zn2+, Cd2+, Pb2+, In3+, Ga3+, Mg2+, Cu2+, Cr3+, Fe3+, Ag2+, Mn2+, Eu3+, Hg2+, Mg2+and Al3+in H2O) concentrations were kept at 20mM,
and for dual-metal (red bars) studies, 20mM of Zn2++ 20mM of other metal ions in H2O and 20mM of Zn2++ 20mM of metal ion
mixtures in H2O were taken. During the dual metal analysis, the
Zn2+ effect at 40 mM was taken and we found that an excess addition of Zn2+ would affect the sensitivities as mentioned before. Furthermore, the sensitivities of F1 and F2 towards Zn2+ ions can be well demonstrated as in Fig. 6, which explains sensing abilities of F1 and F2 in the presence of different metal ion backgrounds. Both systems (single and dual-metal analysis)
conrmed the sensitivities of F1 and F2 to Zn2+ even in the
presence of interfering Cd2+ions. However, in both cases of F1 and F2, the sensor responses were entirely affected by the presence of Al3+ ions rather than the other metal ions. This helped us to perform the ratiometric displacement measure-ments, to establish the distinguishable selectivities of F1 and F2 to both Zn2+ and Al3+ ions. Additional explanations for the
interfering effect of Al3+ions to Zn2+sensor was also provided by
the association constant (log Ka) studies. In addition to
uo-rescence titrations, UV-Vis absorption titrations also revealed the sensitivities of F1 and F2 to Zn2+ ions. Both F1 and F2 exhibited absorption maxima at 344 and 346 nm, respectively, and upon the addition of Zn2+ions 0–30 mM (0, 5, 10, 15, 20, 22, 24, 28 and 30mM) shows the quenching spectra as evidenced in Fig. S15 (see the ESI†).
UV-Vis/PL titrations on Al3+ions
Upon the addition of Al3+0–60 mM (0, 5, 10, 15, 20, 25, 30, 35, 45, 50, 55 and 60mM in H2O), F1 and F2 (20mM) in CH3CN/H2O (6/
Fig. 5 Fluorescence spectral changes of (a) F1 (20mM) in CH3CN/H2O (6/4) (lex¼ 344 nm) and (b) F2 (20 mM) in CH3CN/H2O (6/4; vol/vol) (lex¼ 346 nm) titrated with 0–30 mM of Zn2+ions in H
2O (0, 2, 4, 6, 8, 10, 14, 16, 18, 20, 22, 24 and 30mM were plotted). Insets show PL spectral responses of (a) F1 and (b) F2 as a function of Zn2+.
Fig. 4 Photographs of sensor responses of (A) F1, (B) F2 and (C) F3 upon the addition of Al3+(3 equiv.), Zn2+(1 equiv.) and OH(50 equiv.) under UV-light irradiations.
4; vol/vol) revealed selectivities with appearances of emission peaks (Fig. 7a and b) at 487 and 485 nm, respectively. In the same way, F3 (20mM) in CH3CN/H2O (6/4; vol/vol) also indicated
its sensitivity to Al3+ (0–60 mM with an equal span of 5 mM) through the appearance of peaks at 477 nm, as noticed in Fig. 7c. The insets showed the PL intensity changes with respect to the concentration of Al3+, and also conrmed that an excess
addition of Al3+did not affect the sensitivity. In addition, the
selectivities of F1, F2 and F3, towards Al3+via single (black bars)
and dual (red bars) metal competitive analysis, were carried out, which demonstrated that only Al3+ exhibited the selective sensitivity among the 23 available metal ions (Li+, Ag+, K+, Na+, Cs+, Ni2+, Fe2+, Co2+, Zn2+, Cd2+, Pb2+, In3+, Ga3+, Mg2+, Cu2+, Cr3+, Fe3+, Ag2+, Mn2+, Eu3+, Hg2+, Mg2+ and Al3+in H2O). All
metal ion concentrations were kept as 60mM in H2O for single
metal competitive analysis, whereas for dual-metal systems 60 mM of Al3++ 60mM of other metal ions in H
2O and 60mM of Al3+
+ 60mM of metal ion mixtures in H2O were taken. During the
dual metal analysis, the Al3+effect at 120 mM was taken and we
found that an excess addition of Al3+showed a small increase in the uorescence intensity as noticed in Fig. 8. Similarly, the selective sensor responses of F1, F2 and F3 in CH3CN/H2O (6/4;
vol/vol) to Al3+ions in the presence of other interfering metal ions were evidenced in Fig. 8, and it was also noticed that the presence of Zn2+did not affect their sensitivities. Furthermore,
the PL intensities were found to be increased to several folds in the cases of F2 and F3 with greater selectivities to Al3+ in contrast to Zn2+ions. The selectivities towards Al3+rather than Zn2+ were explained further by ratiometric displacements and competitive binding studies later on. Similar to uores-cence titrations, UV-Vis titrations (Fig. S16, see the ESI†) also conrmed the sensitivities of F1, F2 and F3 in CH3CN/H2O
Fig. 6 Relativefluorescence intensities of (a) F1 (20 mM) and (b) F2 (20 mM) in CH3CN/H2O (6/4; vol/vol) with 20mM Zn2+in H2O in the presence of competing metal ions. Black bars; F1 and F2 (20mM) in CH3CN/H2O (6/4; vol/vol) with 20mM of stated metal ions in H2O. Red bars; F1 and F2 (20mM) CH3CN/H2O (6/4; vol/ vol) with 20mM Zn2++ 20mM of stated metal ions in H
2O (40mM of Zn2+for Zn2+ effect) (mix ¼ combinations of all metal ions except Zn2+and Al3+).
Fig. 7 Fluorescence spectral changes of (a) F1 (1 105M) in CH
3CN/H2O (6/4; vol/vol) (lex¼ 344 nm), (b) F2 (1 105M) in CH3CN/H2O (6/4; vol/vol) (lex¼ 346 nm), and (c) F3 (1 105M) in CH
3CN/H2O (6/4; vol/vol) (lex¼ 343 nm) titrated with 0–60 mM of Al3+ions in H
2O (0, 5, 10, 15, 20, 25, 30, 35, 45, 50, 55 and 60mM were plotted for F1 and F2, along with F3 was plotted with an equal span of 5mM). Insets show PL spectral responses of (a) F1, (b) F2 and (c) F3 as a function of Al3+.
(6/4; vol/vol) to Al3+ with quenching the absorption maxima at 344, 346 and 343 nm, respectively. Upon the addition of 0–40 mM Al3+(0, 2, 5, 10, 15, 20, 22, 24, 28, 32, 36 and 40mM) absorption maxima of F1, F2 and F3 were quenched rapidly up to 20mM and thereaer found to reversible to their original states. Since F1, F2 and F3 in CH3CN/H2O (3/7; vol/vol) also exhibited the
selec-tivities to Al3+ions, theuorescence titrations were performed
further and our observations suggested that their sensing capabilities were not affected any more. Even though the PL intensities of F1, F2 and F3 in CH3CN/H2O (3/7; vol/vol) were
affected little, they reproduced the almost similar uorescence spectral responses in the presence of interfering metal ions as represented by Fig. S17–S19 (ESI†). In addition, the PL sensor responses of F1, F2 and F3 in CH3CN/H2O (6/4; vol/vol) were not
enhanced aer 5 equivalents (see Fig. S53; ESI†). UV-Vis/PL titrations on OHions
Upon the addition of OH0–50 equiv., with an equal span of 5 equiv. in the form of tetrabutyl ammonium salt in H2O, F1, F2
and F3 in CH3CN/H2O (6/4; vol/vol) showed the uorescence
turn-on responses via phenoxide ion formation as depicted in Fig. S20 (see the ESI†), and also visualized green uorescence phenomena under UV-light irradiations (Fig. 4). The specic selectivities of F1, F2 and F3 to OHions were evaluated via single (black bars) and dual-anion (white bars) titrations as noticed in Fig. S21 (see the ESI†). Both systems conrmed sensitivities of F1, F2 and F3 to OHions in H2O. Nevertheless,
while the titrations were repeated with F1, F2 and F3 in CH3CN/
H2O (3/7; vol/vol), we found the fewer folds of PL enhancements
(Fig. S19c; see the ESI†). Since, the OHsensors also revealed
the PL peaks roughly at ca. 500 nm, except the presence of Zn2+ and Al3+, we found none of the other metal ions interfered in the sensory system. However, due to the requirement of 50 equiv. of OHions, the reverse phenomena of interference of OHions in Zn2+or Al3+sensors were not observed. But, because of the
pH changes arising from inorganic metal ion hydroxide salts both Zn2+ and Al3+ sensors were found to be affected. On the other hand, UV-Vis spectra (Fig. S22a–c; see the ESI†) of F1, F2 and F3 in CH3CN/H2O (6/4; vol/vol) towards OHions in H2O
showed the quenching effect. F1, F2 and F3 in CH3CN/H2O (3/7;
vol/vol), exhibited barely 5.3, 4.5 and 4.7 folds of PL enhance-ments, respectively, which was negligible in contrast to F1, F2 and F3 in CH3CN/H2O (6/4; vol/vol). Hence, the OHsensor
systems in CH3CN/H2O (3/7; vol/vol) were not considered
further.
Stoichiometries24of sensor complexes
To ensure the sensor responses of F1, F2 and F3, the stoichi-ometries of F1+Zn2+, F2+Zn2+, F1+Al3+, F2+Al3+and F3+Al3+were
calculated through job's plots as noticed in Fig. S23 (see the ESI†). Regarding F1+Zn2+ and F2+Zn2+, the stoichiometric
calculations were carried out based on their normalized PL intensity changes (see the insets of Fig. 5a and b), in which an excess addition of Zn2+ slightly affected the sensory systems. The job's plots between mole fraction (XM) and normalized PL
intensity changes of F1+Zn2+ and F2+Zn2+ went through
Fig. 8 Relativefluorescence intensities of (a) F1 (20 mM), (b) F2 (20 mM) and (c) F3 (20mM) in CH3CN/H2O (6/4; vol/vol) with 60mM Al3+in H2O in the presence of competing metal ions. Black bars; F1, F2 and F3 (20mM) in CH3CN/H2O (6/4; vol/ vol) with 60mM of stated metal ions in H2O. Red bars; F1, F2 and F3 (20mM) CH3CN/H2O (6/4; vol/vol) with 60mM Al3++ 60mM of stated metal ions in H2O (120mM of Al3+for Al3+effect) (mix ¼ combinations of all metal ions except Zn2+ and Al3+).
maxima at molar fractions of ca. 0.506 (F1+Zn2+) and 0.503 (F2+Zn2+) as shown in Fig. S23a and b (see the ESI†), respec-tively, indicating their 1 : 1 stoichiometric complexes. In a similar manner, the stoichiometries of F1+Al3+, F2+Al3+ and
F3+Al3+ were established by job's plots between X
M and
absorption maximum changes at 344, 346 and 343 nm, respectively. Upon the addition of 0–40 mM Al3+(0, 2, 5, 10, 15,
20, 22, 24, 28, 32, 36 and 40mM), the absorption maxima of F1, F2 and F3 were quenched rapidly up to 20mM, aerward they were found to be restored again. Therefore, the job's plots were
plotted between XM and absorption changes at 344 nm
(F1+Al3+), 346 nm (F2+Al3+), 343 nm (F3+Al3+), where they went through maxima at molar fractions of ca. 0.5 (F1+Al3+), 0.507 (F2+Al3+) and 0.508 (F2+Al3+), respectively, as shown in Fig. S23c–e (see the ESI†), representing their 1 : 1 stoichiometry.
1H and13C NMR titrations on sensor complexes
The sensor properties of F1, F2 and F3 were further conrmed by their binding site analysis via1H and13C NMR titrations.25 For both experiments, the Zn2+and Al3+ions were dissolved in
D2O as well as tetrabutyl ammonium hydroxide (TBAOH) in
D2O, and titrated with F1, F2 and F3 in CD3CN. As publicized in
Fig. 9a–c, the remaining proton environments present in F1, F2 and F3 were related to the addition of Zn2+, Al3+, and OHions, which induced the disappearance of the phenolic –OH (Ha)
signal utterly. However, the1H NMR spectra of OHsensors were discriminated from Zn2+ and Al3+sensors via the entire disappearance of the phenolic–OH (Ha) without affecting the
lingering proton environment. The above observation also well supported the phenoxide ion formation in the OH sensor responses, but to clarify our suspicion, the mass spectral studies were also accomplished subsequently. On the other hand, upon the addition of Zn2+and Al3+ions in D2O to F1, F2
and F3 in CD3CN the phenolic –OH (Ha) totally disappears
with downeld and upeld shiing of residual imine (Hb)
and pyridyl (Hc) protons of F1, F2 and F3 as follows: The
downeld shiing of imine (Hb) protons of F1 (9.49 ppm), F2
(9.45 ppm) and F3 (9.15 ppm) were evidenced for F1+Zn2+(9.97 ppm), F1+Al3+(10.09 ppm), F2+Zn2+(9.95 ppm), F2+Al3+(10.05 ppm) and F3+Al3+(10.05 ppm). In a similar manner, the pyridyl (Hc) protons of F1 (8.57 ppm), F2 (8.38 ppm) and F3 (8.03 ppm)
were downeld shied for F1+Zn2+ (9.12 ppm), F2+Zn2+ (8.93
ppm) and F3+Al3+ (8.23 ppm) but upeld shied for F1+Al3+ (8.12 ppm) and F2+Al3+(7.96 ppm). Due to the higher selectivity and CHEF mechanism to Al3+, F3 provides a different NMR spectrum compared with F1 and F2. Hence, the1H NMR titra-tions conrmed the deprotonation mechanism as well as the involvements of hetero atoms (O, N) towards sensor responses via ICT and CHEF. In addition, the1H NMR spectral titrations of
F1, F2 and F3 with Ga3+for the comparative purpose were also
provided as noticed in Fig. S52 (ESI†). To re-evaluate1H NMR
results, the 13C NMR titrations were carried out in similar conditions and supported the involvement of hetero atoms in Zn2+ and Al3+ sensors, as well as the phenoxide formations through deprotonation of F1, F2 and F3 for OH sensors as shown in Fig. S24–S31 (see the ESI†). The imine group attached
to pyridyl carbons and the –OH group attached to phenyl
carbons of F1 (162.6 and 166.0 ppm), F2 (162.6 and 165.8 ppm) and F3 (162.7 and 165.0 ppm) were downeld shied for F1+Zn2+ (197.4 and 179.7 ppm) and F2+Zn2+(197.4 and 179.9
ppm), respectively. However, in the case of F1+Al3+(198.0 and 165.9 ppm), F2+Al3+(198.0 and 165.7 ppm) and F3+Al3+(197.4 and 165.7 ppm), the–OH group attached to phenyl carbons had less downeld shis in comparison with their individual zinc complexes, and hence 13C NMR spectra became distinguish-able. As found in 1H NMR, F3+Al3+ did not evidence the
Fig. 9 1H NMR spectral changes of (a) F1 (1 equiv.) in CD
3CN (b) F2 (1 equiv.) in CD3CN with Zn2+(1 equiv.), Al3+(1 equiv.), (Al3++Zn2+) [(1 : 1) (each 3 equiv.)] and OH(5 equiv.) ions in D2O, (c) F3 (1 equiv.) in CD3CN with Al3+(1 equiv.), and OH (5 equiv.) ions in D2O.
different spectrum in contrast to F1+Al3+ and F2+Al3+, this
might be because of similar binding site. Even though residual carbon atoms of F1, F2 and F3 showed some downeld and upeld shis, we explained the main affected carbons for simplicity, and both1H and13C NMR titrations ensured the
stoichiometries of sensor complexes. Due to the high concen-tration requirements of NMR measurements,13C NMR
titra-tions of F1, F2 and F3 did not show noticeable changes towards Ga3+, so these related spectra are not presented.
Mass (FAB) spectra26of sensor complexes
The mass spectra of sensor complexes conrmed the binding sites and phenoxides formation along with the stoichiometries as noticed in Fig. S32–S39 (see the ESI†). The phenoxide formations during the OHsensor responses were evidenced by their respective mass peaks [m/z¼ 197 (F1-phnoxide) and m/z ¼ 211 (F2 and F3-phenoxides)] in conjunction with intense TBAOH peaks (Fig. S37–S39; see the ESI†). In the same way, mass spectra of F1+Zn2+ (m/z ¼ 259), F2+Zn2+ (m/z ¼ 273), F1+Al3+(m/z¼ 221), F2+Al3+(m/z¼ 235) and F3+Al3+(m/z¼ 235)
clearly indicated the participation of hetero atoms (O, N) and stoichiometries of the above sensor materials (Fig. S32–S36†). Apart from the peaks of sensor metal complexes, we also found the primitive peaks of F1, F2 and F3 along with their metal ion sources, due to the presence of simple equilibria. In addition to the mass spectra of F1+Zn2+and F2+Zn2+, their binding sites were further inveterated by the reversibilities of the sensor complexes27as revealed in Fig. S40 (see the ESI†). While adding 1 equiv. of EDTA to F1+Zn2+and F2+Zn2+, they were found to be reversible to their original state (F1 and F2). Further investiga-tion also proved that both of them could act as reusable sensor materials up to 10 cycles (Fig. S40e and f†). Hence, the binding sites, stoichiometries, and phenoxide ion formations were well recognized through mass spectral studies.
Detection limits (LODs)28of sensor complexes
In order to prove the selectivities of F1, F2 and F3 towards discernible detections of Zn2+, Al3+, and OHions, the calcu-lations of detection limits (LODs) were performed through standard deviations and linearttings as shown in Fig. S41 and S42 (see the ESI†) by plotting the relative uorescence intensity (I/I0) changes as a function of concentration. The detection
limits were evidenced as 4.22 107, 4.89 107, 1.69 106, 1.42 106and 1.27 106M, for F1+Zn2+, F2+Zn2+, F1+Al3+, F2+Al3+and F3+Al3+complexes, respectively. In contrast to Al3+, the LODs of Ga3+were found to be 106levels (Fig. S54, ESI†), but no conceivable sensor responses were observed with higher concentrations of Ga3+(Fig. S53, ESI†). Similarly, the LODs of F1+OH, F2+OHand F3+OHwere estimated as 2.79 105, 2.89 105 and 2.78 105M, respectively, and conrmed that they were in an affordable range.
Ratiometric displacements29of Zn2+
Fig. 10 supported the ratiometric uorescence intensity
changes during the addition of Al3+solution to F1+Zn2+(lem¼
508 nm) or F2+Zn2+(lem¼ 505 nm), in which the ratiometric
displacements of Zn2+by Al3+were noticed. While adding 0–80 mM Al3+to F1+Zn2+and F2+Zn2+in the previous processes, both
showed PL quenching up to 20mM, peaks of F1+Al3+(lem¼ 487
nm) and F2+Al3+ (l
em ¼ 485 nm) appeared. In addition, the
above ratiometric displacements were well veried by1H,13C
NMR, mass, and TRPL studies, which were entirely matched with F1+Al3+and F2+Al3+complexes. In1H NMR spectra (Fig. 9a
and b), the imine (Hb) and pyridyl (Hc) protons of [F1+Al3+] +
Zn2+(10.09 and 8.12 ppm) and [F2+Al3+] + Zn2+(10.05 and 7.96 ppm) were totally in line with the imine (Hb) and pyridyl (Hc)
protons of F1+Al3+(10.09 and 8.14 ppm) and F2+Al3+(10.05 and 7.96 ppm). Furthermore, as publicized in 13C NMR spectra (Fig. S43 and S44; see the ESI†) the imine group attached to pyridyl carbons and the–OH group attached to phenyl carbons of [F1+Al3+] + Zn2+ (197.8 and 165.9 ppm, respectively) and [F2+Al3+] + Zn2+(197.8 and 165.7 ppm, respectively) were similar to F1+Al3+ (198.0 and 165.9 ppm, respectively) and F2+Al3+ (198.0 and 165.7 ppm, respectively), which conrmed the ratiometric displacements of Zn2+ by Al3+ in F1+Zn2+ and
F2+Zn2+ complexes. In addition to 1H and13C NMR spectral
studies, mass spectra (Fig. S45 and S46; see the ESI†) of [F1+Al3+]
+ Zn2+ and [F2+Al3+] + Zn2+provided the m/z intense peaks of
F1+Al3+(m/z¼ 221) and F2+Al3+(m/z¼ 235) along with the little intense peaks of F1+Zn2+(m/z¼ 259), F2+Zn2+(m/z¼ 273). The
Fig. 10 Fluorescence spectra of (a) [F1+Zn2+] (20mM F1 in CH
3CN/H2O (6/4; vol/ vol) mixed with 20mM Zn2+in H
2O), and (b) [F2+Zn2+] (20mM F2 in CH3CN/H2O (6/4; vol/vol) mixed with 20mM Zn2+in H
2O) upon the addition of Al3+in H2O (0, 2, 8, 12, 14, 16.18, 20, 24, 30, 38, 48, 56, 62, 68, 76 and 80mM and 0, 2, 8, 12, 14, 16.18, 20, 26, 32, 38, 48, 56, 62, 68, 76 and 80mM, respectively). Inset: ratiometric fluorescence intensity [I487/I508] and [I485/I505] as a function of [Al3+].
above observations veried the ratiometric displacements and the simple equilibrium present in the system. Apart from the PL, 1H, 13C NMR, and mass studies, the decay constant (s) values of [F1+Al3+] + Zn2+ and [F2+Al3+] + Zn2+ derived from
TRPL spectra in Fig. S48c–f† also coincided with F1+Al3+and
F2+Al3+as shown in Tables S1 and S2 (see the ESI†), which was
further explained in the end (TRPL spectra of sensor complexes). As shown in Fig. 10, the displacements of Zn2+by Al3+ was evidenced through their differential spectral shis arising from the ICT mechanism.
Competitive binding analysis
To evaluate the higher binding ability of Al3+ ions compared with Zn2+ions, the competitive binding analyses were utilized as reported in the literature.30 Regarding Zn2+ and Al3+ the
association constants (log Ka) were calculated by plotting
response parameter values (a) as a function of logarithm [Zn2+]
and [Al3+] based on [Zn2+]¼ 1/2KaL(1 a/a2) and [Al3+]¼ 1/
3KaL2(1 a/a3); where L was the ligand anda was dened as a
ratio between the free ligand concentration [L] and the initial ligand concentration [L0]. As evidenced in Fig. S47 (see the
ESI†), the plots between response parameter values (a) and [Zn2+] or [Al3+] for F1, F2 and F3 revealed the association constants (log Ka) of Zn2+ and Al3+ complexed as F1+Zn2+,
F2+Zn2+, F1+Al3+, F2+Al3+and F3+Al3+sensor materials. The log Kavalues of Zn2+in F1+Zn2+and F2+Zn2+were identied as 7.92
and 7.76, respectively, whereas it was found to be larger for Al3+ in F1+Al3+, F2+Al3+ and F3+Al3+ (10.96, 11.64 and 12.38, respectively). Higher log Kavalues of Al3+ions rather than Zn2+
ions well supported the ratiometric displacements of Zn2+by Al3+in F1+Zn2+and F2+Zn2+along with the greater selectivity of F1, F2 and F3 to Al3+ions in contrast to Zn2+ions. Furthermore, at higher concentrations of Ga3+ions (5–10 equiv.) the sensor responses were not enhanced as in the case of Al3+ ions,
therefore the association constant calculations for Ga3+ ions
were not provided. Furthermore, the log Kavalues (Table S1†)
also supported the decay constant (s) values obtained from the time resolved photoluminescence spectra of sensor complexes (F1+Zn2+, F2+Zn2+, F1+Al3+, F2+Al3+and F3+Al3+).
TRPL spectra and quantum yields (F) of sensor complexes As reported in the literature31and from our results (Fig. S48a– j†), we found that the uorescence decay constants (s) were affected typically by turn-on sensor responses as summarized in Table 1, S1 and S2 (see the ESI†). From the TRPL signals without any sensor responses theuorescence life time values of F1, F2 and F3 were 2.19, 1.51, and 1.35 ns, respectively. During the F1+Zn2+ and F2+Zn2+ sensing processes, the faster decay
components (A1) of F1 and F2 (89.2% and 95.1%) were
decreased to 27.5% and 27.6%, respectively, along with increased values of longer decay components (A2) as shown in
Table S2.† Similar trends were evidenced in F1+Al3+, F2+Al3+,
F3+Al3+, F1+OH, F2+OHand F3+OHsensing responses, and their ultrafast decay time constant (s1) values and longer decay
time constant (s2) values were affected rapidly according to the
results of biexponential decayttings. Except F1+Al3+, F2+Al3+,
and F3+Al3+ sensors, the other sensors (F1+Zn2+, F2+Zn2+, F1+OH, F2+OHand F3+OH) have smallers1and highers2
values, whereas in the previous cases (Al3+ complexes) those values were in the reverse tendency. Based on single exponential decay ttings, the average uorescence life time values of F1+Zn2+, F2+Zn2+, F1+Al3+, F2+Al3+, F3+Al3+, F1+OH, F2+OH
and F3+OHwere estimated as 4.15, 3.83, 11.97, 11.53, 12.16, 3.95, 2.28 and 2.18 ns, respectively. In addition to the above sensory systems, the [F1+Al3+] + Zn2+and [F2+Al3+] + Zn2+sensor materials were also shown in Fig. S48f and g,† in which both of them reproduced the similar TRPL properties (Table S2†) of F1+Al3+ and F2+Al3+. Hence, TRPL properties supported the ratiometric displacement behavior of Zn2+ by Al3+ in F1+Zn2+ and F2+Zn2+and also conrmed the higher binding ability of Al3+ ions. In general, the greater uorescence life time (sAvg)
values of F1+Al3+, F2+Al3+, and F3+Al3+(11.97, 11.53, and 12.16 ns, respectively) sensor materials established better Al3+ selec-tivities of F1, F2 and F3. Similarly, the modest sensor responses of F1+Al3+, F2+Al3+, and F3+Al3+were evidenced by their decay
constants as noticed in Tables S1 and S2.† In addition, the negligible selectivities of F1, F2 and F3 towards Ga3+ were
conrmed by their TRPL results (Fig. S55, ESI†). Furthermore, the quantum yield (F) values32 of F1+Zn2+, F2+Zn2+, F1+Al3+,
F2+Al3+, F3+Al3+, F1+OH, F2+OH and F3+OH sensor complexes reconrmed the sensitivities of F1, F2 and F3. For F1+Zn2+and F2+Zn2+sensors, theF values of F1 and F2 (0.011 and 0.008) were enhanced 25.5 and 24.5 times, respectively, as shown in Table S1.† Similarly, F1+Al3+, F2+Al3+, F3+Al3+,
F1+OH, F2+OHand F3+OHsensor complexes demonstrated 26.4, 27.6, 30.7, 20.0, 15.2 and 17.1 times higherF values than their respective probes F1, F2 and F3 (0.011, 0.008, and 0.01). More interestingly, theF values of [F1+Al3+] + Zn2+and [F2+Al3+] + Zn2+were similar to those of F1+Al3+and F2+Al3+, and veried the higher selectivities towards Al3+ ions. The F values of
F1+Ga3+, F2+Ga3+and F3+Ga3+are also noticed for their modest
sensor responses provided by Ga3+(see Table S1†).
Time and temperature effects33
In general, sensor recognitions are time dependent and in many cases they were rapid, but in some cases they were found to be time consuming. Therefore, the above mentioned sensor complex (F1+Zn2+, F2+Zn2+, F3+Zn2+, F1+Al3+, F2+Al3+, F3+Al3+, F1+OH, F2+OH and F3+OH) recognitions were evaluated with respect to time in seconds as shown in Fig. 11a and b. The Zn2+or Al3+ions in H2O were added to F1, F2 and F3 in CH3CN/
H2O (6/4; vol/vol) as per the stoichiometry (1 : 1), and the PL
intensity changes were analyzed as a function of time/seconds. As envisioned in Fig. 11a, the sensor recognitions of Zn2+were rapid within 20 seconds, thereaer the intensity remains identical. On the other hand, the PL intensities to sensor detections of Al3+ ions were slowly amplied with respect to time (0–180 seconds), as represented by Fig. 11b. In the same way, upon the direct addition of 50 equiv. of TBAOH to F1, F2 and F3, the PL intensity of OHsensor responses were quick (20 seconds) as noted in Fig. 11c. In addition to the individual sensor responses, we also checked the ratiometric sensor
responses of F1+Zn2+and F2+Zn2+by Al3+as a function of time/
seconds. Fig. 11d veried the greater selectivity of Al3+ with
regard to time (0–300 seconds) in ratiometric displacements of Zn2+ (F1+Zn2+and F2+Zn2+). Aer the recognition process, we further extended time effects (0–60 minutes) to F1+Zn2+,
F2+Zn2+, F1+Al3+, F2+Al3+, F3+Al3+, F1+OH, F2+OH and F3+OHsensor complexes, as shown in Fig. S49 (see the ESI†). Aer the sensor detection processes, except the Al3+ sensors
(Fig. S49b†), none of the above sensor responses provided the incredible PL intensity changes up to 1 hour. Owing to the importance of temperature in the sensor responses, we checked the sensitivities of F1, F2 and F3 in CH3CN/H2O (6/4; vol/vol) to
Zn2+, Al3+and OHions at three different temperatures like 25,
35, and 45C. As revealed in Fig. 12a and c, upon increasing the temperature (25–45C) the sensor responses towards Zn2+and
OH showed the decreasing trend. However, Al3+ sensor
responses (Fig. 12b) were increased regarding temperature increment, and hence conrmed the higher selectivities towards Al3+.
Solvent concentration and pH effects
To evaluate our decision to use the CH3CN/H2O (6/4 and 3/7;
vol/vol) aqueous media for sensor titrations, the solvent effects34 on the PL intensities of sensor responses were performed by increasing the H2O amount (0–99%), as exposed in Fig. S50 (see
the ESI†). The PL intensities of probes F1, F2 and F3 (Fig. S50a†) were not affected incredibly up to 40% of H2O and thereaer
showed a little effect up to 70% of H2O but affected further
again within 70–99% of H2O. At the same time, the sensor PL
intensities of F1 and F2 in CH3CN (Fig. S50b†) to Zn2+ were
completely quenched while increasing the portion of H2O more
than 50%, but the PL intensities of sensor responses of F1, F2
and F3 in CH3CN (Fig. S50c†) to Al3+remained identical up to
70% of H2O. However, even at higher proportions of H2O (80–
99%), they illustrated the sensor selectivities towards Al3+ to some folds roughly around ca. 10, 7, and 5 folds (80, 90, and 99%, respectively). The above observations also conrmed that, at higher H2O proportion (>50%), the ICT and cis–trans
inter-conversion were restricted, and provided selectivities just to Al3+ via CHEF. On the other hand, the PL intensities of OHsensors of F1, F2 and F3 in CH3CN (Fig. S50d†) remained similar up to
50% of H2O, but later on they were rapidly quenched. In
general, Fig. S50† veried that F1, F2 and F3 provided the higher selectivities towards Al3+with negligible selectivities to OHat 80–99% semi-aqueous media.
Since previous reports35have noted the necessity of effective pH for the sensor responses, we tend to analyze our sensor systems with various pHs (0–14) as shown in Fig. S10 (see the ESI†). The PL intensities of F1, F2 and F3 in CH3CN/H2O (6/4;
vol/vol) were little quenched at acidic pHs (0–5) and show highly intense peaks at basic pHs (9–14) due to the stable phenoxide formations as noticed in Fig. S11.† Further investigations of uorescence spectra of F1, F2 and F3 in CH3CN/H2O (6/4; vol/
vol) at pH¼ 12 provided intense PL peaks at 525, 527, and 512 nm, respectively, which might arise from their phenoxides as in the case of OHsensors. Moreover, the TRPL decay constant (sAvg) values were decreased for acidic pHs and increased for the
basic pHs as summarized in Table S2,† and hence conrmed that they can be also used as pH sensors to differentiate acidic and basic pHs. Fig. S12† illustrated the TRPL spectra of F1, F2 and F3 in CH3CN/H2O (6/4; vol/vol) and the inset shows the
photographs of acidic, neutral, and basic pHs (2, 7, and 12) under UV-light irradiations, where the greenuorescence was due to phenoxide formations at pH¼ 12. Further analysis of pH effects on the sensor responses were exposed in Fig. S51a–c (see the ESI†), in which F1, F2 and F3 in CH3CN/H2O (6/4 and 3/7;
vol/vol) towards both Zn2+and Al3+sensors were affected rapidly
Fig. 11 PL spectral responses of (a) F1+Zn2+, F2+Zn2+and F3+Zn2+(1 : 1), (b) F1+Al3+, F2+Al3+and F3+Al3+(1 : 1), (c) F1+OH, F2+OHand F3+OH(1 : 50), and (d) [F1+Al3+] + Zn2+and [F2+Al3+] + Zn2+(1 : 1 : 1), as a function of time (seconds).
Fig. 12 PL spectral responses of (a) F1+Zn2+, F2+Zn2+and F3+Zn2+, (b) F1+Al3+, F2+Al3+and F3+Al3+, and (c) F1+OH, F2+OHand F3+OH, as a function of temperature (25, 35 and 45C).
at acidic and basic pHs (0–5 and 9–14), but have no effects between 6 and 8 pHs. At the same time, the PL intensities of OHsensor responses of F1, F2 and F3 in CH3CN/H2O (6/4; vol/
vol) were rapidly affected at acidic pHs (0–5), and have no effect at neutral and basic pHs (6–14). In other words, the OHsensor
responses at basic pHs were even better due to the increased stabilities of phenoxides. Therefore, from the pH effect studies we concluded that sensor responses of F1, F2 and F3 towards Zn2+ and Al3+ were effective between 6 and 8 pHs, and OH sensors were effective between 6 and 14 pHs.
Conclusions
In conclusion, three pyridyl-salicylimine derivatives (F1, F2 and F3) were easily synthesized via one-step aldamine condensa-tion, and utilized for therst time as uorescence “turn-on” sensors for distinct detections of Zn2+, Al3+ and OHions in mixed-aqueous media [CH3CN/H2O (6/4 and 3/7; vol/vol), pH¼
7 and at 25C]. F1 and F2 in CH3CN/H2O (6/4; vol/vol) exhibited
uorescence turn-on sensor responses to Zn2+ and Al3+ with
differential spectral shis, but F3 in CH3CN/H2O (6/4; vol/vol)
showed turn-on sensing only to Al3+ions via ICT and CHEF. In
addition, all of them (F1, F2 and F3) in CH3CN/H2O (6/4)
revealed the turn-on sensor responses to OH ions through phenoxide ion formations. Furthermore, F1+Zn2+and F2+Zn2+ sensor complexes in CH3CN/H2O (6/4) evidenced the
revers-ibilities and ratiometric displacements of Zn2+with EDTA and Al3+ ions, respectively. The 1 : 1 stoichiometries of sensor complexes (F1+Zn2+, F2+Zn2+, F1+Al3+, F2+Al3+ and F3+Al3+) were identied from job's plots based on UV/Vis and PL spectral changes. Binding sites of sensor complexes, involvements of hetero atoms (O, N) in sensor recognitions via ICT, CHEF,
deprotonation of phenolic –OH, and phenoxide formations
were well established by1H,13C NMR, and mass (FAB) spectral studies. The typical detection limits (LODs) of F1+Zn2+,
F2+Zn2+, F1+Al3+, F2+Al3+ and F3+Al3+sensor complexes were
calculated as 4.22 107, 4.89 107, 1.69 106, 1.42 106, and 1.27 106M, respectively, by standard deviations and linearttings. Similarly, the LODs of F1+OH, F2+OHand F3+OHwere estimated as 2.79 105, 2.89 105, and 2.78 105M, respectively. The TRPL decay constant (s) and associa-tion constant (log Ka) values conrmed the better selectivities of
F1, F2 and F3 towards Al3+rather than both Zn2+and OHions. In contrast to ion selective electrodes, these sensors can be used for cell image studies in the biological systems (which are underway) involving Zn2+ and Al3+ ions. Since ion selective electrodes are corrosive at higher pH values, F1, F2 and F3 can be utilized as selective sensors at higher concentrations of OH ions via strong green emissions under UV-light irradiations. In addition, they can also be utilized as distinct spectral detections of Zn2+, Al3+and OHions via peak intensity and spectral shis
corresponding to their concentrations.
Acknowledgements
We are grateful to the National Science Council of Taiwan (ROC) through NSC99-2113-M-009-006-MY2 for thenancial support.
We wish to thank Mandapati V. Ramakrishnam Raju for HOMO–LUMO calculations.
Notes and references
1 (a) V. Amendola, L. Fabbrizzi, F. Forti, M. Licchelli, C. Mangano, P. Pallavicini, A. Poggi, D. Sacchi and A. Taglieti, Coord. Chem. Rev., 2006, 250, 273; (b) D. Astruc, E. Boisselier and C. Ornelas, Chem. Rev., 2010, 110, 1857; (c) J. Wu, W. Liu, J. Ge, H. Zhang and P. Wang, Chem. Soc. Rev., 2011, 40, 3483; (d) X. Chen, S. W. Nam, G. H. Kim, N. Song, Y. Jeong, I. Shin, S. K. Kim, J. Kim, S. Park and J. Yoon, Chem. Commun., 2010, 46, 8953.
2 (a) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97, 1515; (b) A. K. Dwivedi, G. Saikia and P. K. Iyer, J. Mater. Chem., 2011, 21, 2502; (c) S. Sumalekshmy and C. J. Fahrni, Chem. Mater., 2011, 23, 483.
3 (a) N. Niamnont, R. Mungkarndee, I. Techakriengkrai,
P. Rashatasakhon and M. Sukwattanasinitt, Biosens.
Bioelectron., 2010, 26, 863; (b) X. Lou, D. Ou, Q. Li and Z. Li, Chem. Commun., 2012, 48, 8462.
4 (a) D. T. McQuade, A. E. Pullen and T. M. Swager, Chem. Rev., 2000, 100, 2537; (b) G. Aragay, J. Pons and A. Merkoci, Chem. Rev., 2011, 111, 3433.
5 (a) M. Sauer, Angew. Chem., Int. Ed., 2003, 42, 1790; (b) A. P. de Silva, T. S. Moody and G. D. Wright, Analyst, 2009, 134, 2385.
6 (a) J. Wu, W. Liu, J. Ge, H. Zhang and P. Wang, Chem. Soc. Rev., 2011, 40, 3483; (b) P. Kaur, D. Sareen and K. Singh, Dalton Trans., 2012, 41, 9607; (c) Y. Chen, C. Zhu, Z. Yang, J. Li, Y. Chiao, W. He, J. Chen and Z. Guo, Chem. Commun., 2012, 48, 5094; (d) R. Satapathy, Y. S. Wu and H. C. Lin, Org. Lett., 2012, 14, 2564.
7 (a) D. A. Pearce, N. Jotterand, I. S. Carrico and B. Imperiali, J. Am. Chem. Soc., 2001, 123, 5160; (b) J. A. Zhou, X. L. Tang, J. Cheng, Z. H. Ju, L. Z. Yang, W. S. Liu, C. Y. Chena and D. C. Bai, Dalton Trans., 2012, 41, 10626; (c) H. Y. Lin, P. Y. Cheng, C. F. Wan and A. T. Wu, Analyst, 2012, 137, 4415; (d) Y. Bao, B. Liu, F. Du, J. Tian, H. Wang and R. Bai, J. Mater. Chem., 2012, 22, 5291; (e) R. Satapathy, Y. S. Wu and H. C. Lin, Chem. Commun., 2012, 48, 5668.
8 (a) J. M. Berg and Y. Shi, Science, 1996, 271, 1081; (b) X. M. Xie and T. G. Smart, Nature, 1991, 349, 521.
9 (a) A. Torrado, G. K. Walkup and B. Imperiali, J. Am. Chem. Soc., 1998, 120, 609; (b) T. Jin, J. Lu and M. Nordberg, NeuroToxicology, 1998, 19, 529; (c) V. Bhalla, Roopa and M. Kumar, Dalton Trans., 2013, 42, 975.
10 (a) Z. Xu, J. Yoon and D. R. Spring, Chem. Soc. Rev., 2010, 39, 1996; (b) Y. Xu, J. Meng, L. Meng, Y. Dong, Y. Cheng and C. Zhu, Chem.–Eur. J., 2010, 16, 12898; (c) X. Liu, N. Zhang, J. Zhou, T. Chang, C. Fang and D. Shangguan, Analyst, 2013, 138, 901; (d) Y. Mikata, A. Yamashita, K. Kawata, H. Konno, S. Itami, K. Yasuda and S. Tamotsu, Dalton Trans., 2012, 41, 4976.
11 E. Delhaize and P. R. Ryan, Plant Physiol., 1995, 107, 315.
12 (a) G. D. Fasman, Coord. Chem. Rev., 1996, 149, 125; (b) Y. W. Wang, M. X. Yu, Y. H. Yu, Z. P. Bai, Z. Shen, F. Y. Li and X. Z. You, Tetrahedron Lett., 2009, 50, 6169; (c) S. H. Kim, H. S. Cho, J. Kim, S. J. Lee, D. T. Quang and J. S. Kim, Org. Lett., 2010, 12, 560.
13 (a) L. Basabe-Desmonts, D. N. Reinhoudt and M. Crego-Calama, Chem. Soc. Rev., 2007, 36, 993–1017; (b) M. Zhang, Y. Q. Liu and B. C. Ye, Chem.–Eur. J., 2012, 18, 2507–2513; (c) X. Sun, Y. W. Wang and Y. Peng, Org. Lett., 2012, 14, 3420. 14 (a) T. H. Ma, M. Dong, Y. M. Dong, Y. W. Wang and Y. Peng, Chem.–Eur. J., 2010, 16, 10313 and references therein; (b) S. Kim, J. Y. Noh, K. Y. Kim, J. H. Kim, H. K. Kang, S. W. Nam, S. H. Kim, S. Park, C. Kim and J. Kim, Inorg.
Chem., 2012, 51, 3597; (c) S. Sen, T. Mukherjee,
B. Chattopadhyay, A. Moirangthem, A. Basu, J. Marek and P. Chattopadhyay, Analyst, 2012, 137, 3975; (d) Y. W. Liu, C. Hung Chen and A. T. Wu, Analyst, 2012, 137, 5201; (e) S. Sinha, R. R. Koner, S. Kumar, J. Mathew, P. V. Monisha, I. Kazia and S. Ghosh, RSC Adv., 2013, 3, 345.
15 J. Du, M. Hu, J. Fan and X. Peng, Chem. Soc. Rev., 2012, 41, 4511.
16 H. Xu and O. A. Sadik, Analyst, 2000, 125, 1783.
17 (a) R. J. Berman, G. D. Christian and L. W. Burgess, Anal. Chem., 1990, 62, 2066; (b) L. R. Allain and Z. Xue, Anal. Chem., 2000, 72, 1078.
18 (a) L. Fabbrizzi, M. Licchelli, F. Mancin, M. Pizzeghello, G. Rabaioli, A. Taglietti, P. Tecilla and U. Tonellato, Chem.–Eur. J., 2002, 8, 94; (b) C. Ma, A. Lo, A. Abdolmaleki and M. J. MacLachlan, Org. Lett., 2004, 6, 3841.
19 (a) Z. B. Li, J. Lin and L. Pu, Angew. Chem., Int. Ed., 2005, 44, 1690; (b) D. Maity and T. Govindaraju, Chem.–Eur. J., 2011, 17, 1410; (c) D. Maity and T. Govindaraju, Inorg. Chem., 2011, 50, 11282.
20 (a) H. Komatsu, D. Citterio, Y. Fujiwara, K. Minamihasi, Y. Araki, M. Hagiwara and K. Suzuki, Org. Lett., 2005, 7, 2857; (b) A. D. Cort, P. D. Bernardin, G. Forte and F. Y. Mihan, Chem. Soc. Rev., 2010, 39, 3863; (c) J. F. Jang, Y. Zhou, J. Yoon and J. S. Kim, Chem. Soc. Rev., 2011, 40, 3416. 21 (a) K. Krzysztof and I. Kuzniarska-Biernacka, Struct. Chem., 2010, 21, 357; (b) R. K. Dubey, P. Baranwal, S. K. Dwivedi and U. N. Tripathi, J. Coord. Chem., 2011, 64, 2649.
22 C. G. Freiherr von Richthofen, A. Stammler, H. Bogge and T. Glaser, J. Org. Chem., 2012, 77, 1435.
23 M. R. Silva-Junior and W. Thiel, J. Chem. Theory Comput., 2010, 6, 1546.
24 (a) S. Muthaiah, Y. C. Rajan and H. C. Lin, J. Mater. Chem., 2012, 22, 8976; (b) Y. C. Rajan, S. Muthaiah, C. T. Huang, H. C. Lin and H. C. Lin, Tetrahedron, 2012, 68, 7926; (c) R. Satapathy, H. Padhy, Y. H. Wu and H. C. Lin, Chem.– Eur. J., 2012, 18, 16061; (d) S. Muthaiah, Y. H. Wu, A. Singh, M. V. R. Raju and H. C. Lin, J. Mater. Chem. A, 2013, 1, 1310.
25 W. Zhou, Y. Li, Y. Li, H. Liu, S. Wang, C. Li, M. Yuan, X. Liu and D. Zhu, Chem.–Asian J., 2006, 1–2, 224–230.
26 (a) J. Yuasa and S. Fukuzumi, J. Am. Chem. Soc., 2006, 128, 15976–15977; (b) X. Zhou, B. Yu, Y. Guo, X. Tang, H. Zhang and W. Liu, Inorg. Chem., 2010, 49, 4002.
27 (a) R. Sheng, P. Wang, Y. Gao, Y. Wu, W. Liu, J. Ma, H. Li and S. Wu, Org. Lett., 2008, 10, 5015; (b) S. Sirilaksanapong,
M. Sukwattanasinitt and P. Rashatasakhon, Chem.
Commun., 2012, 48, 293; (c) P. J. Yang, H. C. Chu, T. C. Chen and H. C. Lin, J. Mater. Chem., 2012, 22, 12358. 28 J. Li, Y. Wu, F. Song, G. Wei, Y. Cheng and C. Zhu, J. Mater.
Chem., 2012, 22, 478.
29 L. Xue, Q. Liu and H. Jiang, Org. Lett., 2009, 11, 3454. 30 (a) G. Grynkiewcz, M. Poenie and R. Y. Tsein, J. Biol. Chem.,
1985, 260, 3440; (b) D. Maity and T. Govindaraju, Chem. Commun., 2012, 48, 1039.
31 (a) M. H. Ha-Thi, M. Penhoat, D. Drouin, M. Blanchard-Desce, V. Michelet and I. Leray, Chem.–Eur. J., 2008, 14, 5941; (b) M. Schaferling, Angew. Chem., Int. Ed., 2012, 51, 3532.
32 (a) Q. Zhao, F. Li and C. Huang, Chem. Soc. Rev., 2010, 39, 3007; (b) H. N. Kim, W. X. Ren, J. S. Kim and J. Yoon, Chem. Soc. Rev., 2012, 41, 3210.
33 (a) P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486; (b) G. Zhang, D. Zhang, S. Yin, X. Yang, Z. Shuai and D. Zhu, Chem. Commun., 2005, 2161; (c) S. K. Arya and S. Bhansali, Chem. Rev., 2011, 111, 6783.
34 (a) H. Tong, Y. Hong, J. W. Y. Lam, Y. Dong, Y. Dong, M. H. lussler, H. H. Y. Sung, Z. Li, I. D. Williams and B. Z. Tang, J. Phys. Chem. B, 2007, 111, 11817; (b) A. Ciupa, M. F. Mahon, P. A. De Bank and L. Caggiano, Org. Biomol. Chem., 2012, 10, 8753; (c) J. Hatai, S. Pal, G. P. Jose, T. Sengupta and S. Bandyopadhyay, RSC Adv., 2012, 2, 7033. 35 (a) K. Cammann, U. Lemke, A. Rohen, J. Sander, H. Wilken and B. Winter, Angew. Chem., Int. Ed. Engl., 1991, 30, 516; (b) B. Ma, F. Zeng, F. Zheng and S. Wu, Chem.–Eur. J., 2011, 17, 14844; (c) M. Hecht, W. Krausb and K. Rurack, Analyst, 2013, 138, 325.
![Fig. 11 PL spectral responses of (a) F1+Zn 2+ , F2+Zn 2+ and F3+Zn 2+ (1 : 1), (b) F1+Al 3+ , F2+Al 3+ and F3+Al 3+ (1 : 1), (c) F1+OH , F2+OH and F3+OH (1 : 50), and (d) [F1+Al 3+ ] + Zn 2+ and [F2+Al 3+ ] + Zn 2+ (1 : 1 : 1), as a function of time](https://thumb-ap.123doks.com/thumbv2/9libinfo/7893964.155749/10.892.477.811.74.382/fig-spectral-responses-al-zn-zn-function-time.webp)
