U.S. patent application number 16/987688 was filed with the patent office on 2021-02-11 for fluorescent turn-on chemosensors for detection of aluminum ion and azide.
The applicant listed for this patent is Morgan State University. Invention is credited to Fasil Abebe.
Application Number | 20210041423 16/987688 |
Document ID | / |
Family ID | 1000005050831 |
Filed Date | 2021-02-11 |
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United States Patent
Application |
20210041423 |
Kind Code |
A1 |
Abebe; Fasil |
February 11, 2021 |
FLUORESCENT TURN-ON CHEMOSENSORS FOR DETECTION OF ALUMINUM ION AND
AZIDE
Abstract
Two rhodamine derivatives, L.sub.1 and L.sub.2, bearing
2-methoxy-1-naphthaldehyde and 5-bromo-3-methoxy salicylaldehyde
units were synthesized using microwave-assisted organic synthesis
and used for reversible sequential fluorescence detection of
aluminum ion (Al.sup.3+) and azide (N.sub.3.sup.-) in aqueous
acetonitrile solution.
Inventors: |
Abebe; Fasil; (Elkrdige,
MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Morgan State University |
Baltimore |
MD |
US |
|
|
Family ID: |
1000005050831 |
Appl. No.: |
16/987688 |
Filed: |
August 7, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62883785 |
Aug 7, 2019 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C07D 491/107 20130101;
G01N 33/52 20130101; C07D 311/88 20130101; G01N 21/78 20130101;
C07F 5/069 20130101 |
International
Class: |
G01N 33/52 20060101
G01N033/52; G01N 21/78 20060101 G01N021/78; C07D 491/107 20060101
C07D491/107; C07D 311/88 20060101 C07D311/88; C07F 5/06 20060101
C07F005/06 |
Claims
1. A compound having the formula: ##STR00006##
2. A compound having the formula: ##STR00007##
3. A method for synthesizing the compound L.sub.1 of claim 1,
comprising mixing a compound having the formula ##STR00008## with
2-methoxy-1-naphthaldehyde and ethanol, stirring a resulting
mixture until homogenous, and irradiating the resulting mixture in
a microwave reactor.
4. A method for synthesizing the compound L.sub.2 of claim 1,
comprising mixing a compound having the formula ##STR00009## with
5-bromo-3methoxy salicylaldehyde and ethanol, stirring a resulting
mixture until homogenous, and irradiating the resulting mixture in
a microwave reactor.
5. A method for determining a presence of Al.sup.3+ in a sample,
comprising: contacting the sample with a colorless solution
comprising a compound according to claim 1 and observing whether
the colorless solution turns pink in color, where a change in color
of the solution to pink indicates the presence of Al.sup.3+ in the
sample.
6. A method for determining a presence of N.sub.3.sup.- in a
sample, comprising: contacting the sample with a pink solution
comprising a compound according to claim 2 and observing whether
the pink solution turns colorless, where a change in color of the
solution from pink to colorless indicates the presence of
N.sub.3.sup.- in the sample.
7. A method according to claim 5 wherein the colorless solution
shows no absorption above 450 nm in UV-vis absorption spectra, and
wherein an absorption peak above 525 nm indicates the presence of
Al.sup.3+ in the sample.
8. A method according to claim 6 wherein the pink solution shows an
absorption peak above 525 nm in UV-vis absorption spectra, and
wherein no absorption above 450 nm indicates the presence of
N.sub.3.sup.- in the sample.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to rhodamine Schiff base
compounds for the detection of micromolar levels of Al.sup.3+ ions
and azide (N.sub.3.sup.-).
Description of the Background
[0002] Several approaches discuss detection of aluminum but not in
conjunction with azide. U.S. Pat. No. 9,891,237 relies on a Schiff
base for metal cation detection, but its sensor relies on a form of
benzazole. U.S. Pat. 7,615,377 also uses ligands for detection of
metal ions, and it also is based on fluorescence, but it does not
apply the specific formula towards the same specific ligands. U.S.
Pat. No. 7,906,320 covers a fluorescence-based biosensor that can
specifically detect metals and also discusses quenchers that emit
at specific wavelength ranges. U.S. Pat. No. 7,018,840 refers to
fluorescent metal sensors, with rhodamine complexed with metal ions
through ligand binding but does not list aluminum as one of the
exemplary metal ions. U.S. Pat. No. 5,567,619 detects for aluminum,
among other elements/compounds and does mention some other similar
attributes, such as chelation and certain color indications, but it
is overall more primitive in nature.
SUMMARY OF THE INVENTION
[0003] The present invention relates to sensor compounds
("sensors") that are developed from rhodamine derivatives that may
be used for detecting the presence of Al.sup.3+ and other
metals.
[0004] Widespread use of aluminum in pharmaceuticals, cooking
utensils, aluminum foil, vessels, and trays can result in the
moderate increase in Al.sup.3+ concentration in food, and
potentially damage the central nervous system in humans.
[0005] Novel and unobvious rhodamine Schiff base sensors L.sub.1
and L.sub.2 are described herein that are able to detect micromolar
levels of Al.sup.3+ ions by the chelation-enhanced fluorescence
(CHEF) process. Also of note, Al.sup.3+ complexes L.sub.1-Al.sup.3+
and L.sub.2-Al.sup.3+ behave as highly selective chemosensors for
N.sub.3.sup.- ions by quenching of the fluorescence in
acetonitrile/water (CH.sub.3CN/H.sub.2O) medium at 25.degree.
C.
[0006] The rhodamine derivative sensors L.sub.1 and L.sub.2 bearing
2-methoxy-1-naphthaldehyde and 5-bromo-3-methoxy salicylaldehyde
units were designed and synthesized from the parent rhodamine B and
aromatic aldehydes in a two-step Schiff base condensation, using
microwave-assisted organic synthesis (MAOS) and utilized towards
sequential fluorescence detection of aluminum ion (Al.sup.3+) and
azide (N.sub.3.sup.-) in aqueous acetonitrile solution. Aluminum
ion (Al.sup.3+) triggers the formation of highly fluorescent
ring-open spirolactam.
[0007] A mixture of ethanol with compound 2 and
2-methoxy-1-naphthaldehyde or with compound 2 and 5-bromo-3-methoxy
salicylaldehyde was placed in a reaction vial and then stirred
before being placed in a biotage microwave reactor. The closed
reaction vessel in both cases was run under pressure and irradiated
for 10 minutes. After cooling to room temperature, the resulting
solid was filtered and washed three times with cold ethanol. After
drying, the resulting sensor yield was measured--the L.sub.1 sensor
yielded 92%, while the L.sub.2 sensor yielded 88%.
[0008] Absorption spectra studies showed that on incremental
addition of Al.sup.3+ ions, the absorption intensity at 315 nm
increased gradually and a new absorption peak at 565 nm with a
shoulder at 525 nm was generated by ring opening with a visual
color change from colorless to pink. The well-defined isosbestic
points at 340 and 375 nm clearly indicates the formation of a new
complex species between L.sub.1 and Al.sup.3+. Absorption spectra
of sensors recorded with the continuous addition of Al.sup.3+
showed a continuous increase in the absorption at 565 nm and that
was employed to calculate binding constants for L.sub.1 and L.sub.2
with Al.sup.3+ using the Benesi-Hildebrand method.
[0009] The plot of absorbance of L.sub.1 at 565 nm as a function of
mole fraction of added Al.sup.3+ metal ion reveals that these
probes bind to the metal ion in 1:1 stoichiometry. The fluorescence
spectrum of sensors L.sub.1 and L.sub.2 showed a peak at 585 nm
upon the addition of Al.sup.3+ corresponding to the delocalization
in the xanthenes moiety of rhodamine.
[0010] The fluorescence and colorimetric response of the
L.sub.1-Al.sup.3+ and L.sub.2-Al.sup.3+ complexes were quenched by
the addition of N.sub.3.sup.-, which extracted the Al.sup.3+ from
the complexes and turned off the sensors, confirming that the
recognition process is reversible. The recognition ability of the
sensors was confirmed by fluorescence titration, Job's plot, 1H-NMR
spectroscopy and density functional theory (DFT) calculations.
[0011] When L.sub.1-Al.sup.3+ is used as the sensor for
N.sub.3.sup.-, high concentration of CN-interference must be
eliminated by using mesoporous carbon based adsorbent. The addition
of N.sub.3.sup.- to the L.sub.1-Al.sup.3+ solution led to a change
in color of the solutions from pink to colorless, which was
observed with the naked eye. The addition of N.sub.3.sup.- to the
solution containing L.sub.1-Al.sup.3+ complex resulted in the
reversal of the Al.sup.3+ induced changes in the emission band at
585 nm in the fluorescence emission spectra.
[0012] Gradual addition of N.sub.3.sup.- results in continuous
decrease in the emission intensity at 585 nm. Based on fluorescence
data, the detection limit of L.sub.1-Al.sup.3+ or N.sub.3.sup.- was
calculated as 12 .mu.M. A similar finding was observed for complex
L.sub.2-Al.sup.3+ towards N.sub.3.sup.- ions. The L.sub.2-Al.sup.3+
system revealed remarkably selective fluorescence "off" behavior
exclusively with N.sub.3.sup.-. The limit of detection value for
N.sub.3.sup.- ions was found at 18 .mu.M. These results show that
L.sub.1-Al.sup.3+ and L.sub.2-Al.sup.3+ binds N.sub.3.sup.- ions
with higher selectivity and the process is reversible.
[0013] Accordingly, there is provided according to an embodiment of
the invention, a compound having the formula:
##STR00001##
[0014] There is further provided according to the invention a
compound having the formula:
##STR00002##
[0015] There is further provided according to the invention a
method for synthesizing the compound L.sub.1, comprising
mixing a compound having the formula
##STR00003##
with 2-methoxy-1-naphthaldehyde and ethanol, stirring a resulting
mixture until homogenous, and irradiating the resulting mixture in
a microwave reactor.
[0016] There is further provided according to the invention a
method for synthesizing the compound method for synthesizing the
compound L.sub.2, comprising mixing a compound having the
formula
##STR00004##
with 5-bromo-3methoxy salicylaldehyde and ethanol, stirring a
resulting mixture until homogenous, and irradiating the resulting
mixture in a microwave reactor.
[0017] There is further provided according to the invention a
method for determining a presence of Al.sup.3+ in a sample,
comprising: contacting the sample with a colorless solution
comprising compound L.sub.1 or L.sub.2 and observing whether the
colorless solution turns pink in color, where a change in color of
the solution to pink indicates the presence of Al.sup.3+ in the
sample. According to a further embodiment of the invention, the
colorless solution shows no absorption above 450 nm in UV-vis
absorption spectra, and an absorption peak above 525 nm indicates
the presence of Al.sup.3+ in the sample.
[0018] There is further provided according to the invention a
method for determining a presence of N.sub.3.sup.- in a sample,
comprising: contacting the sample with a pink solution comprising a
compound having the formula
##STR00005##
and observing whether the pink solution turns colorless, where a
change in color of the solution from pink to colorless indicates
the presence of N.sub.3.sup.- in the sample. According to a further
embodiment of the invention, the pink solution shows an absorption
peak above 525 nm in UV-vis absorption spectra, and no absorption
above 450 nm indicates the presence of N.sub.3.sup.- in the
sample.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows the chemical structures and synthetic routes of
L.sub.1 and L.sub.2.
[0020] FIG. 2 shows UV-vis spectra of L.sub.1 (10 .mu.M) with
Al.sup.3+ (0-23 .mu.M) in CH.sub.3CN/H.sub.2O (7:3 v/v)
solution.
[0021] FIG. 3 shows an Absorbance Job's plot for determination of
L.sub.1-Al.sup.3+ complex (10 .mu.M) in CH.sub.3CN/H.sub.2O (7:3
v/v) solution.
[0022] FIG. 4a is a fluorescence spectra of L.sub.1 (10 .mu.M) in
CH.sub.3CN/H.sub.2O (7:3 v/v) solution (.lamda..sub.ex=510 nm).
[0023] FIG. 4b is a fluorescence spectra of L.sub.2 (10 .mu.M) with
metal ions (10 .mu.M) in CH.sub.3CN/H.sub.2O (7:3 v/v) solution
(.lamda..sub.ex=510 nm).
[0024] FIG. 5 is a fluorescence spectral titration of L.sub.1 (10
.mu.M) on the incremental addition of Al(NO.sub.3).sub.3 (23
equivalents) (.lamda..sub.ex=510 nm).
[0025] FIG. 6 shows the effect of pH on fluorescence intensity of
sensors L.sub.1 and L.sub.2 (10 .mu.M).
[0026] FIG. 7a shows a proposed binding mechanism of sensor L.sub.1
towards Al.sup.3+ in the presence and absence of azide
(N.sub.3.sup.-).
[0027] FIG. 7b shows a proposed binding mechanism of sensor L.sub.2
towards Al.sup.3+ in the presence and absence of azide
(N.sub.3.sup.-).
[0028] FIG. 8a shows the fluorescence spectra of L.sub.1-Al.sup.3+
(1:1) with anions (10 .mu.M) (.lamda..sub.ex=510 nm).
[0029] FIG. 8b shows a fluorescence spectral titration of
L.sub.1-Al.sup.3+ (23 equivalents of Al.sup.3+) on the incremental
addition of N.sub.3.sup.- (up to 35 equivalents)
(.lamda..sub.ex=510 nm).
[0030] FIG. 9 shows .sup.1H-NMR spectral changes of L.sub.2 (8 mM)
in DMSO-d.sub.6 and titrated with 0-1.0 equivalents of Al.sup.3+ in
deuterated water.
[0031] FIG. 10a shows optimized structures and energy correlation
of the HOMO-LUMO gap between L.sub.1 and L.sub.1-Al.sup.3+
salt.
[0032] FIG. 10b shows optimized structures and energy correlation
of the HOMO-LUMO gap between L.sub.2 and L.sub.2-Al.sup.3+
complex.
DETAILED DESCRIPTION
[0033] Chemicals and Instruments
[0034] All reagents and solvents were purchased as analytical-grade
and used without further purification unless otherwise stated.
Stock solutions of metal ions were prepared from their nitrate and
chloride salts and anion species from their tetrabutylammonium
salts. Distilled deionized water was used throughout the
experiments. .sup.1H-NMR and .sup.13C-NMR spectra were recorded
using an Avance 400 MHz spectrometer (Bruker Billerica, Karlsruhe,
Germany) with tetramethylsilane (TMS) as internal standard and
deuterated chloroform (CDCl.sub.3) as solvent. NMR spectra were
analyzed using MestReNova software (version 10, Mestrela Research,
Feliciano Barrera-Bajo, Spain). The IR spectrum was obtained using
FT-IR spectrometer (Shimadzu, IRAffinity-1S, Columbia, Md., USA).
High resolution electrospray ionization mass spectrometry (ESI-MS)
was acquired with a Bruker Apex-Qe instrument. All UV-vis
spectroscopy experiments were recorded using a Cary UV/vis
spectrophotometer 5000 (Varian, Walnut Creek, Calif., USA).
Fluorescence emission spectra experiments were measured using a
Cary 60 series spectrometer (Agilent, Walnut Creek, Calif., USA),
with excitation and emission slit widths of 5 nm and excitation
wavelength at 510 nm. MAOS reactions were carried out in a single
mode Biotage Initiator 2.0 (Biotage, Uppsala, Sweden).
[0035] Microwave-Assisted Synthesis and Characterization of L.sub.1
and L.sub.2
[0036] Sensors L.sub.1 and L.sub.2 were synthesized from the parent
rhodamine B and aromatic aldehydes (2-methoxy-1-naphthaldehyde and
5-bromo-3-methoxy salicylaldehyde) in a two-step Schiff base
condensation using MAOS heating protocols, as shown in FIG. 1.
Compound 2 was synthesized according to procedure reported in Xiang
Y, Tong A, Jin P, Ju Y, Org. Lett 2006, 8, 2863.
[0037] Synthesis of Sensor L.sub.1
[0038] Using microwave heating protocol: A mixture of compound 2
(105 mg, 0.230 mmol), 2-methoxy-1-naphthaldehyde (41 mg, 0.220
mmol) and ethanol (2 ml) was placed in a 10 ml reaction vial. The
resulting mixture was stirred to make it homogeneous and it was
placed in the cavity of a biotage microwave reactor. The closed
reaction vessel was run under pressure and irradiated for 10 min at
100.degree. C. After cooling to room temperature, the resulting
solid was filtered and washed three times with cold ethanol. After
drying, the sensor L.sub.1 was isolated to give in 92% yield.
Melting point: 244-246.degree. C.; .sup.1H-NMR (CDC1.sub.3),
.delta. (ppm): 9.63 (1H, s, N.dbd.C--H); 8.77 0 (1H, t, J=7.4 Hz,
H--Ar), 7.74 (1H, d, J=8.4 Hz, H--Ar), 7.71 (1H, d, J=8.0 Hz,
H--Ar), 7.63 (1H, d, J=7.7 Hz, H--Ar), 7.48-7.51 (2H, m, H--Ar),
7.15-7.27 (2H, m, H--Ar),7.12 (1H, d, J=8.4 Hz), 7.09 (1H, d, J=4.9
Hz), 6.63 (2H, d, J=8.8 Hz), 6.44 (2H, d, J=2.2 Hz), 6.28 (2H, dd,
J=8.8 Hz, 2.6 Hz), 3.82 (3H, s, OCH.sub.3), 3.31 (8H, q, J=6.9 Hz,
NCH.sub.2CH.sub.3), 1.14 (12H, t, J=6.9 Hz, NCH.sub.2CH.sub.3).
13C-NMR (CDCl.sub.3), .delta. (ppm): 164.6, 157.8, 153.4, 151.7,
148.8, 147.6 (N.dbd.C--H), 137.6, 133.1, 131.9, 130.3, 129.2,
128.1, 127.0, 126.7, 124.0, 123.2, 116.8, 112.9, 108.1, 107.9,
106.5, 104.6, 79.9, 66.3 (spiro carbon), 56.7, 44.3
(NCH.sub.2CH.sub.3), 12.7(NCH.sub.2CH.sub.3); HRMS (ESI): m/z calcd
for C.sub.40H.sub.40N.sub.4O.sub.3: 625.3173; Found: 625.3176
[M+H]+.
[0039] Synthesis of Sensor L.sub.2
[0040] Using microwave heating protocol: A mixture of compound 2
(100 mg, 0.220 mmol), 5-bromo-3-methoxy salicylaldehyde (51 mg,
0.221 mmol) and ethanol (2 ml) was placed in a 10 ml reaction vial.
The resulting mixture was stirred to make it homogeneous and it was
placed in the cavity of a biotage microwave reactor. The closed
reaction vessel was run under pressure and irradiated for 10 min at
100.degree. C. After cooling to room temperature, the resulting
solid was filtered and washed three times with cold ethanol. After
drying, the sensor L.sub.2 was isolated to give in 88% yield.
.sup.1H-NMIR (CDCl.sub.3), .delta. (ppm):11.11 (1H, s, --OH), 8.94
(1H, s, --CH.dbd.N 7.96 (1H, t, J=6.6 Hz, --Ar), 7.49 (2H, m,
--Ar), 6.86 (1H, d, J=6.6 Hz, --Ar), 7.50 (2H, s, --Ar), 6.51-6.43
(4H, m, --Ar), 6.25 (2H, d, J=7.5 Hz, --Ar), 3.82 (3H, s,
--OCH.sub.3), 3.31 (8H, q, NCH.sub.2CH.sub.3), 1.16 (12H, t, J=6.6
Hz, NCH.sub.2CH.sub.3) 13C-NMR (CDCl.sub.3), .delta. (ppm): 163.6,
152.7, 148.5, 146.6 (--CH.dbd.N), 138.5, 138.1, 137.7, 134.0,
128.9, 128.5, 127.5, 123.1, 121.8, 121.3, 108.1, 108.0, 106.5,
104.8, 97.3, 80.9, 65.5 (spiro carbon), 56.1, 43.6
(NCH.sub.2CH.sub.3), 12.4 (NCH.sub.2CH.sub.3). HRMS (ESI): m/z
calcd for C.sub.36H.sub.37BrN.sub.4O.sub.4: 669.2071; Found:
669.2076[M+H]+.
[0041] General Procedure for the Spectroscopic Studies
[0042] All spectroscopic measurements were carried out in aqueous
CH.sub.3CN medium at room temperature. Stock solutions of sensors
L.sub.1 and L.sub.2 (1.times.10.sup.-3 M), selected salts of
cations (1.times.10.sup.-3 M) and anions (1.times.10.sup.-4 M) were
prepared in CH.sub.3CN/H.sub.2O. Thus, L.sub.1-Al.sup.3+ and
L.sub.2-Al.sup.3+ solutions for N.sub.3.sup.- detection were
prepared by addition of 1.0 equivalent of Al.sup.3+ to the solution
of both L.sub.1 and L.sub.2 (20 .mu.M) in Tris-HCl (10 mM, pH=7.2)
buffer containing CH.sub.3CN/H.sub.2O (7:3, v/v) solution. The
resulting solution was shaken well before recording the spectra.
Each fluorescence titration was repeated at least thrice until
consistent values were obtained. Jobs continuous variation method
was used for determining the binding stoichiometry of the
complexation reaction. The association constant (K) was calculated
from absorbance studies by the linear Benesi-Hildebrand equation.
Color changes in solution phase were observed visually under normal
light and under a hand-held UV lamp upon addition of various metal
ions at room temperature.
[0043] Synthesis of Sensors L.sub.1 and L.sub.2
[0044] The synthesis of L.sub.1 and L.sub.2 were prepared in two
steps with 92% and 88% overall yields respectively (FIG. 1). The
results obtained indicate that, unlike classical heating, MAOS
results in higher yields, shorter reaction time, mild reaction
condition, simple work-up procedure and better purity offer
privilege over other methods where complex chromatographic
techniques are required for purification of the target compounds.
The structure of sensors was fully characterized by .sup.1H-NMR,
.sup.13C-NMR, FT-IR and HRMS spectroscopy and all data are in
accordance with the proposed structure.
[0045] Absorption Spectra Studies
[0046] The metal ion sensing of L.sub.1 and L.sub.2 were first
investigated by UV-vis absorption spectra. The colorless solutions
were very weakly fluorescent and showed no absorption above 450 nm,
properties which are characteristic of the predominant ring-closed
spirolactam. The predominant spirolactam form was further confirmed
by observation of the characteristic carbon resonance near 66 ppm
for each of the sensors. The UV-vis spectra of sensors were
recorded in buffer at 25.degree. C. and showed an absorption
maximum at .lamda.=315 nm, which may be attributed to the
intramolecular .pi.-.pi.* charge transfer transition. On
incremental addition of Al.sup.3+ ions, the absorption intensity at
315 nm increased gradually and a new absorption peak at 565 nm with
a shoulder at 525 nm was generated by ring opening with a visual
color change from colorless to pink. The well-defined isosbestic
points at 340 and 375 nm clearly indicates the formation of a new
complex species between L.sub.1 and Al.sup.3+ ion (FIG. 2). The
absorption enhancement is high compared to other metal ions.
Selectivity of L.sub.1 was checked in the presence of other metal
ions. No significant change in the UV-vis spectrum was observed
upon the addition of a 10 equivalent excess of other metal ions of
interest: Na.sup.+, K.sup.+, Mg.sup.2+, Ca.sup.2+, Ni.sup.2+,
Zn.sup.2+, Co.sup.2+, Hg.sup.2+, Pb.sup.2+, Fe2+, Fe.sup.3+,
Cr.sup.2+ and Cu.sup.2+. Absorption spectra of sensors recorded
with the continuous addition of Al.sup.3+ showed a continuous
increase in the absorption at 565 nm and that was employed to
calculate binding constants for L.sub.1 and L.sub.2 with Al.sup.3+
using the Benesi-Hildebrand method. The plot of absorbance of
L.sub.1 at 565 nm as a function of mole fraction of added Al.sup.3+
metal ion reveals that these probes bind to the metal ion in 1:1
stoichiometry (FIG. 3). The complex association constant (K)
calculated through the Benesi-Hildebrand equation for Al.sup.3+
with L.sub.1 and L.sub.2 were found to be 3.82.times.104 M.sup.-1
and 2.41.times.104 M.sup.-1, respectively.
[0047] Fluorescence Spectral Response of Sensors
[0048] To further explore the sensing behavior of L.sub.1 for
Al.sup.3+ ion, the fluorescence spectra of L.sub.1 in CH.sub.3CN
with various metal ions were examined. The fluorescence spectra
were obtained by excitation at 510 nm, and both the excitation and
emission slit were 5 nm. The fluorescence intensity of L.sub.1 upon
the additions of metal ions in CH.sub.3CN showed a remarkable
sensitivity and selectivity towards Al.sup.3+, even though there
were relatively small effects with Cu.sup.2+ and Cr.sup.3+ (FIG.
4a). There was a significant emission intensity enhancement with
1.0 equivalent of Al.sup.3+ which indicates sensor L.sub.1 is an
excellent turn-on sensor for Al.sup.3+. This very high fluorescence
enhancement is attributed to the formation of ring-open spirolactum
in the presence of Al.sup.+. This selectivity for Al.sup.3+ ions
over all other ions is due to selective chelate formation with
L.sub.1 to afford an L.sub.1-Al.sup.3+ complex (See FIG. 7a). When
illuminated with a hand-held UV lamp, the addition of Al.sup.3+
ions to sensor solution resulted in orange fluorescence emission
from L.sub.1 solution (FIG. 5). The fluorescence profile of L.sub.2
were very similar to those for sensor L.sub.1: again Al.sup.3+
registered the highest fluorescence enhancement while other metal
ions showed no significant enhancement (FIG. 4b). The fluorescence
spectrum of sensors L.sub.1 and L.sub.2 showed a peak at 585 nm
upon the addition of Al.sup.3+ corresponding to the delocalization
in the xanthenes moiety of rhodamine. It is assumed that the
spirolactam form was opened upon the addition of Al.sup.3+ to
sensors and makes a highly delocalized .pi.-conjugated stable
complexes with Al.sup.3+ through their active donor sites (e.g., N
and O atoms) of receptor part, though other ions failed which
basically indicates that the coordinate moiety of L.sub.1 and
L.sub.2 matches perfectly with Al.sup.3+ ions instead of the other
ions. The detection limits of L.sub.1 and L.sub.2 for Al.sup.3+
ions were estimated based on the fluorescence titration experiment
as 32 .mu.M and 47 .mu.M respectively. Furthermore, the effect of
pH values on the fluorescence of L.sub.1 and L.sub.2 were also
investigated in a pH range from 3 to 10. FIG. 6 shows that for free
L.sub.1 and L.sub.2 at pH<5, due to protonation of the open-ring
of spirolactam, an obvious color change and fluorescence turn-on
appeared. Thus, all the optical measurements were performed in
buffer solution with a pH of 7 to keep the sensors in their ring
closed form.
[0049] Detection of azide (N.sub.3.sup.-)
[0050] Investigation of the reversible binding nature of the
sensors is shown in FIG. 8 and FIGS. 7a and 7b. Due to the high
stability of AlN.sub.3, the L.sub.1-Al.sup.3+ and L.sub.2-Al.sup.3+
complexes serve as a means to detect N.sub.3.sup.-. FIG. 8a shows
the addition of 20 .mu.M of anions N.sub.3.sup.-, CN.sup.-,
ClO.sub.4, CH.sub.3COO.sup.-, HSO.sub.4.sup.-,
H.sub.2SO.sub.4.sup.2-, SCN.sup.-, Cl.sup.-, I.sup.-, F.sup.-, and
OH.sup.- to L.sub.1-Al.sup.3+ (1:1) of which N.sub.3.sup.- alone
quenches the fluorescence, with a slight effect for CN.sup.-,
indicating high selectivity for N.sub.3.sup.-. High concentration
of CN.sup.- contamination is likely to mislead the fluorescent
selectivity of N.sub.3.sup.-. So, when L.sub.1-Al.sup.3+ is used as
the sensor for N.sub.3.sup.-, high concentration of CN.sup.-
interference must be eliminated by using mesoporous carbon based
adsorbent. The addition of N.sub.3.sup.- to the L.sub.1-Al.sup.3+
solution led to a change in color of the solutions from pink to
colorless, which was observed with the naked eye. The addition of
N.sub.3.sup.- to the solution containing L.sub.1-Al.sup.3+ complex
resulted in the reversal of the Al.sup.3+ induced changes in the
emission band at 585 nm in the fluorescence emission spectra.
Gradual addition of N.sub.3.sup.- results in continuous decrease in
the emission intensity at 585 nm (FIG. 8b). Based on fluorescence
data, the detection limit of L.sub.1-Al.sup.3+ for N.sub.3.sup.-
was calculated as 12 .mu.M. A similar finding was observed for
complex L.sub.2-Al.sup.3+ towards N.sub.3.sup.- ions. The
L.sub.2-Al.sup.3+ system revealed remarkably selective fluorescence
"off" behavior exclusively with N.sub.3.sup.-. The limit of
detection value for N.sub.31.sup.- ions was found at 18 .mu.M.
These results show that L.sub.1-Al.sup.+ and L.sub.2-Al.sup.3+ bind
N.sub.3.sup.- ions with higher selectivity and that the process is
reversible. The expected binding mechanism of sensors with
Al.sup.3+ in the presence and absence of azide (N.sub.3.sup.-) is
shown in FIGS. 7a and 7b.
[0051] FT-IR and .sup.1H-NMR Study for Elucidation of Coordination
Mechanism Between Sensors and Al.sup.3+
[0052] To elucidate the coordination mechanism of L.sub.1-Al.sup.3+
and L.sub.2-Al.sup.3+ complexes, the FT-IR spectrum of L.sub.1 and
L.sub.2 were conducted in the absence and presence of Al.sup.3+
ion. The characteristic peak of the amide carbonyl
.gamma..sub.(C.dbd.O) shifted from 1680 cm.sup.-1 to 1614 cm.sup.-1
in the presence of Al.sup.3+, indicating that carbonyl O atoms of
the L.sub.1 and L.sub.2 are involved in the coordination of
Al.sup.3+. .sup.1H-NMR was also performed by adding Al.sup.3+ to
deuterated dimethyl sulfoxide (DMSO-d.sub.6) solution of L.sub.2 as
shown in FIG. 9. The L.sub.2-Al.sup.3+ complexes were prepared by
the additions of 0.25, 0.5 and 1.0 equivalent AlC.sub.36H.sub.2O to
the DMSO solution of L.sub.2. The peaks observed at .delta. 10.10
and .delta. 9.07 are attributable to the phenolic OH and the imine
proton (--CH.dbd.N--) in L.sub.2. Addition of 1 equivalent of
Al.sup.3+ resulted in the disappearance of the hydroxyl proton
indicating the binding of Al.sup.3+ ion through the phenoxide
interaction. Further, small unfilled-shifts from 9.07 to 9.00 ppm
and shortening of imine protons were observed because of the
complex formation between nitrogen atoms and Al.sup.3+. The
formation of the L.sub.2-Al.sup.3+ complex through normal ring
opening was confirmed by performing the .sup.13C-NMR experiment
with L.sub.2 in the absence and presence of Al.sup.3+ ions, from
which it was observed that the signal at .delta.=66 ppm
attributable to the tertiary carbon of the spirolactam ring in
L.sub.2 was absent from the spectrum of L.sub.2-Al.sup.3+ complex.
Therefore, it is understood that the O atom of phenolic OH, N atom
of imine and O atom of spiro ring coordinate to Al.sup.3+ as shown
in FIGS. 7a and 7b.
[0053] Geometry Optimization
[0054] To better understand the nature of the coordination of
Al.sup.3+ with sensors, theoretical calculations on structures
L.sub.1, L.sub.2, L.sub.1-Al.sup.3+ and L.sub.2-Al.sup.3+ were
carried out using Spartan'16 software. Density functional theory
(DFT), employing the B3LYP functional and the 6-31G* basis set was
used to obtain gas phase, optimized geometries of these structures.
The optimized structures of L.sub.1, L.sub.2 and their respective
Al-complexes are depicted in FIGS. 10a and 10b. L.sub.1 and L.sub.2
can undergo rotation of approximately 180.degree. about the N--N
bond, producing two prominent cis and trans conformations. For both
L.sub.1 and L.sub.2, the trans conformation is more energetically
stable than the respective cis one by approximately 11.3 kJ
mol.sup.-1, owing to anti-arrangement of the methoxy (--OMe) group
and the xanthene moiety in trans L.sub.1 and to the
anti-arrangement of the hydroxyl (--OH) group and the xanthene
moiety in trans L.sub.2. Additionally, in trans L.sub.1 the energy
gap between the highest occupied molecular orbital (HOMO) (-4.81
eV) and the lowest unoccupied molecular orbital (LUMO) (-1.34 eV)
is 3.47 eV, and in cis L.sub.1 the gap, HOMO (-5.03 eV) and LUMO
(-1.35 eV), is3.68 eV. In trans L.sub.2, the energy gap, HOMO
(-4.86 eV) and LUMO (-1.29 eV) is 3.57 eV, and in cis L.sub.2 the
energy gap, HOMO (-5.05 eV) and LUMO (-1.22 eV) is 3.83 eV,
suggesting that trans L.sub.1 and trans L.sub.2 are the major
equilibrium conformations available stereochemically for direct
Al.sup.3+ coordination. Also, in trans L.sub.1, the electron
density is delocalized over the entire xanthene moiety with some
found on the spirolactam ring as well as on the imine and the
ortho-methoxy naphthalene moieties (FIG. 10a). In cis L.sub.1, the
electron density is mainly localized on half of the xanthene
moiety. In both trans L.sub.2 and cis L.sub.2, the electron density
is mainly located over the entire xanthene moiety with some found
on the lactam ring nitrogen of both. Moreover, some electron
density is also found on the carbonyl oxygen in trans L.sub.2 but
not on the carbonyl oxygen in cis L.sub.2 (FIG. 10b).
[0055] Density functional calculations of molecular interactions of
trans-L.sub.1 and trans-L.sub.2 with aqueous aluminum (Al.sup.3+)
nitrate solution revealed that both sensors are energetically
stabilized on binding with Al.sup.3+ ions. For instance, upon
formation of L.sub.1-Al.sup.3+ salt complex, the HOMO-LUMO energy
gap in trans-L.sub.1 (.DELTA.E=3.47 eV) decreased to .DELTA.E=2.40
eV, and upon formation of L.sub.2-Al.sup.3+ complex, the HOMO-LUMO
energy gap in trans-L.sub.2 (.DELTA.E=3.57 eV) decreased to 2.22
eV. In L.sub.1-Al.sup.3+ salt complex, formulated as [Al (L.sub.1)
NO.sub.3).sub.2(H.sub.2).sub.2] [NO.sub.3], HOMO is primarily
delocalized over the methoxy naphthalene moiety, while LUMO is
primarily delocalized over the xanthene moiety. In
L.sub.2-Al.sup.3+ complex, formulated as Al (L.sub.2)
(NO.sub.3).sub.2(H.sub.2O), HOMO is found over the tricyclic
structure about Al.sup.3+ while LUMO is delocalized over the
xanthene moiety (FIGS. 10a and 10b).
[0056] Vertical electronic excitations of optimized B3LYP/6-31G*
trans-L.sub.1, trans-L.sub.2 and their respective complexes were
computed using time-dependent-density functional theory (TD-DFT)
Spartan'16 software calculations, formalized in water and using a
conductor-like polarizable continuum model (CPCM). In the TD-DFT
UV-vis spectrum of trans-L.sub.1, an absorption band at
.lamda.=379.24 nm with a vertical excitation energy of 3.2693 eV
and corresponding to HOMO-2.fwdarw.LUMO excitation (oscillator
strength=0.4632) dominates. While in the TD-DFT UV-vis spectrum of
trans-L.sub.1-Al.sup.3+ salt complex, an absorption band at
.lamda.=422.57 nm dominates, corresponding to HOMO.fwdarw.LUMO
excitation (vertical excitation energy=2.9341 eV and oscillator
strength=1.0951). In the case of trans-L.sub.2, an absorption band
at .lamda.=344.32 nm dominates, corresponding to HOMO-2.fwdarw.LUMO
excitation with a vertical excitation energy of 3.6008 eV and an
oscillator strength=0.3152. For trans-L.sub.2-Al.sup.3+ complex, an
absorption band at .lamda.=456.19 nm dominates, corresponding to
HOMO-1.fwdarw.LUMO and HOMO.fwdarw.LUMO excitations with a vertical
excitation energy of 2.7178 eV and an oscillator
strength=0.7824.
[0057] Conclusion
[0058] We have developed reversible fluorescent sensors L.sub.1 and
L.sub.2 for the selective and sensitive sequential detections of
Al.sup.3+ and N.sub.3.sup.- via the fluorescence spectral changes.
Upon binding to Al.sup.3+, obvious detectable change in
fluorescence was observed due to the CHEF effect. The in situ
prepared L.sub.1-Al.sup.3+ and L.sub.2-Al.sup.3+ complexes were
used to detect N.sub.3.sup.- via the metal-displacement approach
which displayed an excellent selectivity and sensitivity towards
N.sub.3.sup.-. Thus, upon the addition of N.sub.3.sup.- to
complexes, the intensity of the 585 nm band decreases, indicating
release of L.sub.1 and L.sub.2 from the aluminum complexes.
Stoichiometry and binding mechanisms for both sensors are well
characterized and established by the respective spectroscopic
techniques. These results clearly demonstrate that L.sub.1 and
L.sub.2 sensors described herein will be useful for the analysis of
Al.sup.3+ and N.sub.3.sup.- in environmental samples and biological
studies.
* * * * *