A highly selective fluorescent sensor for the detection of Al3+ and CN- in aqueous solution: Biological applications and DFT calculations

Article abstract of DOI:10.1039/c6nj01544a

A new "turn-on fluorescence type" chemosensor 1 (N′1,N′2-bis((E)-2-hydroxybenzylidene)oxalohydrazide) with a simple structure was devised and synthesized. In aqueous solution, receptor 1 could efficiently detect both Al³⁺ and CN^- at

Full text of DOI:10.1039/c6nj01544a

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A highly selective fluorescence sensor for Al3+ and CN- in aqueous sDoOluI: 1t0i.o10n39:/C6NJ01544A
biological applications and DFT calculations
Tae Geun Jo,a Jae Jun Lee,a Eunju Nam,b Kwon Hee Bok,a Mi Hee Lim,b* Cheal Kim*
aDepartment of Fine Chemistry and Department of Interdisciplinary Bio IT Materials,
Seoul National University of Science and Technology, Seoul 139-743, Republic of Korea.
Fax: +82-2-973-9149; Tel: +82-2-970-6693; E-mail: chealkim@seoultech.ac.kr
bDepartment of Chemistry, Ulsan National Institute of Science and Technology (UNIST),
Ulsan 44919, Republic of Korea. Fax: +82-52-217-5409; Tel: +82-52-217-5422; E-mail:
mhlim@unist.ac.kr
Abstract
A
new “turn-on fluorescence type” chemosensor
1
(N'1,N'2-bis((E)-2-
hydroxybenzylidene)oxalohydrazidehas) with a simple structure was devised and
synthesized. In aqueous solution, the receptor 1 could efficiently detect both Al3+ and CN-
at two different wavelengths. The limit of detection for Al3+ (2.0 μM) is below the World
Health Organization (WHO) guideline for drinking water (7.41 μM). To utilize a practical
and biological application, the abilities of 1 for monitoring Al3+ were tested in real water
samples and living cells. In addition, 1 showed a highly selective fluorescence
enhancement for CN- in the presence of other anions without any interference. The sensing
mechanisms of 1 for Al3+ and CN- were supported by theoretical calculations.
Keywords: Aluminium, Cyanide, Fluorescence sensor, Theoretical calculations, Cell
imaging.
1

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1. Introduction
The development of highly sensitive and selective chemosensors for detecting metal ions
and anions has always been of especial interest, because they play important roles in
biological, chemical, ecological and endocrine systems.1-5 These ions are also related to
diverse environmental and health problems.6-9 Therefore, qualitative and quantitative
analysis of various ions have received considerable attention.10-12 Among the various metal
ions, Al3+ is the most abundant metal and the third most prevalent metallic elements in the
earth’s crust,13,14 and its compounds are extensively used in chemical industry production,
food additives, pharmaceuticals, water purification and industrial processes such as
production of light alloys.15 However, as a non-essential element in human body, Al3+ can
be a competitive inhibitor of several essential elements, viz. Mg2+, Ca2+ and Fe2+ due to its
similarities in ionic size and charge.16 Thus, the unregulated amounts of aluminium in the
body may cause various problems, such as Al-related bone disease (ARBD),
encephalopathy, myopathy, Parkinson disease and Alzheimer diseases.17-19 The World
Health Organization (WHO) has recommended the maximum limit of aluminium
concentration to be at 200 μg L-1 (7.41 μM) in drinking water.20 Therefore, it is important
to develop accurate probes for detecting Al3+ because of its critical impact on human health.
In fact, various sensors for Al3+ have been developed and reported21-26 so far, but that there
is still a lot of challenges, such as the sensing ability in aqueous solution.
Among the biologically and industrially crucial anions, cyanide has received the
attention,27-32 because it is well known as one of the most fatal toxic anions. Its toxicity is
caused by its propensity to bind to the iron in cytochrome c oxidase, interfering with
electron transport and resulting in hypoxia.33-36 Despite these toxicities, various industrial
processes such as raw materials for synthetic fibers, resins, herbicides, and the gold-
extraction process require the presence of cyanide.37,38 For this reason, considerable
attention has been focused on developing reliable and efficient sensors for detecting the
presence of cyanide.39-42
In particular, among various methods for detection of analytes, fluorescence technique,
which has selective and sensitive ability of detecting important chemical species, is a very
2

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attractive research topic in the molecular imaging fields.43-45 Therefore, fluorescence
indicators have been extensively utilized as a versatile instrument in analytical chemistry,
biochemistry, and cell imaging.46-48
The salicylidene Schiff bases can be easily synthesised by condensing of salicylaldehyde
and amine derivatives,49 and are known to be able to strongly bind the metal ions, resulting
in different optical properties according to a type of the metal ions.50-54 In addition, Schiff
bases containing phenolic groups are sometimes used as sensors for anions because of the
ability of the phenolic-OH group to interact with anions through hydrogen bonding even
in aqueous solution.55-60 Considering this facts, we designed 1 as a dual-target sensor that
could strongly coordinate to metal ions through Schiff base (imine) and interact with the
targeted anions through hydrogen bond of the phenolic-OH groups.
Herein, we report synthesis, characterization, and sensing properties of the chemosensor
1 for Al3+ and CN- which was based on the combination of 2-hydroxybenzaldehyde and
oxalohydrazide. 1 showed a selective enhancement of fluorescence in the presence of Al3+
ions in a near-perfect aqueous solution. In particular, 1 could be used to detect Al3+ in real
water samples and biological systems. In addition, 1 could also detect CN- by fluorescence
enhancement among various ions in aqueous media. Moreover, the sensing mechanisms
for the detection of Al3+ and CN- were proposed and supported by the theoretical
calculation methods.
2. Experimental
2.1 Reagents and apparatus
Unless otherwise specified, all the solvents and reagents (analytical grade and
spectroscopic grade) were obtained commercially and used without further purification. 1H
13
NMR and C NMR measurements were performed on a Varian 400 MHz and 100 MHz
spectrometer and chemical shifts (δ) were recorded in ppm. Absorption spectra were
recorded at 25°C using a Perkin Elmer model Lambda 25 UV/Vis spectrometer.
Electrospray ionization mass spectra (ESI-mass) were collected on a Thermo Finnigan (San
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Jose, CA, USA) LCQTM Advantage MAX quadrupole ion trap instrument. The spray
voltage was set at 4.2 kV, and the capillary temperature was set at 80 °C. Elemental analysis
for carbon, nitrogen, and hydrogen was carried out by using a Flash EA 1112 elemental
analyzer (thermo) in Organic Chemistry Research Center of Sogang University, Korea.
2.2 Synthesis of receptor 1
A solution of 2-hydroxybenzaldehyde (0.98 g, 8.0 mmol) was added to a solution
containing oxalohydrazide (0.12 g, 2.0 mmol) in methanol, followed by addition of three
drops of phosphoric acid into the reaction mixture. The reaction mixture was stirred for 3
d at room temperature. A white precipitate produced was collected by filtration, washed
several times with methanol, and dried in a vacuum to obtain the pure white solid. The
yield was 75.4%. 1H NMR (400 MHz, DMSO-d6) δ: 12.60 (s, 2H), 10.98 (s, 2H), 8.81 (s,
2H), 7.54 (d, 2H), 7.32 (t, 2H), 6.92 (m, 4H); 13C NMR (100 MHz, DMSO-d6, ppm): 157.50,
155.75, 151.13, 131.86, 129.34, 119.32, 118.48, 116.36 ppm. ESI-MS m/z [1-H+]-: calcd,
325.09, found, 325.10. Anal. Calcd for C16H14N4O4 (M.W. = 326.10): C, 58.90; H, 4.32; N,
17.17 %. Found: C, 58.85; H, 4.43; N, 17.12 %.
2.3 Fluorescence titraions of 1
For Al3+; sensor 1 (0.5 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 5 μL of
the sensor 1 (3 mM) was diluted to 2.995 mL with bis-tris buffer solution (10 mM, pH 7.0)
to make the final concentration of 5 μM. Al(NO3)3 (0.10 mmol) was dissolved in bis-tris
buffer (5 mL) and 3.0-48.0 μL of the Al3+ ion solutions (20 mM) were added to the solution
of 1 (5 μM) prepared above. After mixing them for a few seconds, fluorescence spectra
were taken at room temperature.
For CN-; sensor 1 (0.5 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 30 μL
of the sensor 1 (3 mM) was diluted to 2.97 mL with bis-tris buffer/DMSO (1/1, v/v) to
make a final concentration of 30 μM. Tetraethylammonium cyanide (TEACN, 0.2 mmol)
was dissolved in bis-tris buffer (5 mL) and 3.6-72.0 μL of the CN- solutions (100 mM)
were added to the solution of 1 (30 μM) prepared above. After mixing them for a few
seconds, fluorescence spectra were taken at room temperature.
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2.4 UV-vis titrations of 1
For Al3+; sensor 1 (0.5 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 40 μL
of the sensor 1 (3 mM) was diluted to 2.96 mL with bis-tris buffer solution (10 mM, pH
7.0) to make the final concentration of 40 μM. Al(NO3)3 (0.5 mmol) was dissolved in bis-
tris buffer (5 mL) and 4.8-57.6 μL of the Al3+ ion solutions (100 mM) were transferred to
the solution of 1 (40 μM) prepared above. After mixing them for a few seconds, UV-vis
spectra were taken at room temperature.
For CN-; sensor 1 (0.5 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 30 μL
of the sensor 1 (3 mM) was diluted to 2.97 mL with bis-tris buffer/DMSO (1/1, v/v) to
make a final concentration of 30 μM. TEACN (1.0 mmol) was dissolved in bis-tris buffer
(5 mL) and 18-144 μL of the CN- solutions (200 mM) were transferred to the solution of 1
(30 μM) prepared above. After mixing them for a few seconds, UV-vis spectra were taken
at room temperature.
2.5 Job plot measurements
For Al3+; sensor 1 (0.5 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 1.0 mL
of the sensor 1 (3 mM) was diluted to 29.0 mL with bis-tris buffer solution (10 mM, pH
7.0) to make the final concentration of 100 μM. 5.0, 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, 0.5
and 0 mL of the receptor 1 solutions were taken and transferred to vials. In the same way,
Al(NO3)3 (0.1 mmol) was dissolved in bis-tris buffer (1 mL) and 0.03 mL of the Al3+
solution was diluted to 29.97 mL of bis-tris buffer solution (10 mM bis-tris, pH 7.0) to
make the concentraiton of 100 μM. 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 mL
of the Al3+ solutions were added to each diluted 1 solution, seperately. Each vial had a total
volume of 5 mL. After shaking them for a few seconds, UV-vis spectra were taken at room
temperature.
For CN-; sensor 1 (0.5 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 1 mL
of the sensor 1 (3 mM) was diluted to 29.0 mL with bis-tris buffer/DMSO (1/1, v/v) to
make the final concentration of 100 μM. 5.0, 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, 0.5 and 0
mLof the receptor 1 solutions were taken and transferred to vials. In the same way, TEACN
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(0.01 mmol) was dissolved in bis-tris buffer (1 mL) and 0.3 mL of the CN- solution was
diluted to 29.7 mL with bis-tris buffer/DMSO (1/1, v/v) to make the concentraiton of 100
μM. 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 mL of the CN- solutions were added
to each diluted 1 solution, seperately. Each vial had a total volume of 5 mL. After shaking
them for a few seconds, fluorescence spectra were taken at room temperature.
2.6 Competition with other metal ions or anions
For Al3+; sensor 1 (0.5 mg, 0.0015 mmol) was dissolved in DMSO (0.5mL) and 5 μL of
the sensor 1 (3 mM) was diluted to 2.995 mL with bis-tris buffer solution (10 mM, pH 7.0)
to make the final concentration of 5 μM. MNO3 (M = Na, K, 0.02 mmol) or M(NO3)2 (M
= Mn, Co, Ni, Cu, Zn, Cd, Mg, Ca, Pb, 0.02 mmol) or M(NO3)3 (M = Fe, Cr, Al, Ga, In,
0.02 mmol) or M(ClO4)2 (M = Fe, 0.02 mmol) was dissolved in bis-tris buffer (1 mL). 39.0
μL of each metal solution (20 mM) was taken and added to 3 mL of the solution of receptor
1 (5 μM) to give 52 equiv. Then, 39.0 μL of Al3+ solution (20 mM) was added to the mixed
solutions of each metal ion and 1 to make 52 equiv. After mixing them for a few seconds,
fluorescence spectra were obtained at room temperature.
For CN-; sensor 1 (0.5 mg, 0.0015 mmol) was dissolved in DMSO (0.5 mL) and 30 μL
of the sensor 1 (3 mM) was diluted to 2.97 mL with bis-tris buffer/DMSO (1/1, v/v) to
make a final concentration of 30 μM. Stock solutions (100 mM) of the tetraethylammonium
-
-
salts of F-, CN-, Cl-, Br- and I-, the tetrabutylammonium salts of AcO-, H2PO4 , BzO-, N3
and SCN-, and NaSH were separately dissolved in bis-tris buffer (1 mL). 64.8 μL of each
anion solution (100 mM) was taken and added to 2.94 mL of the solution of receptor 1 (30
μM) to give 72 equiv. Then, 64.8 μL of TEACN solution (30 mM) was added to the mixed
solutions of each anion and 1 to make 72 equiv. After mixing them for a few seconds,
fluorescence spectra were obtained at room temperature.
2.7 pH effect test
A series of buffers with pH values ranging from 2 to 12 was prepared by mixing sodium
hydroxide solution and hydrochloric acid in bis-tris buffer.
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For Al3+; after the solution with a desired pH was achieved, sensor 1 (0.5 mDgO,I:01.00.1003195/C6NJ01544A
mmol) was dissolved in DMSO (0.5 mL), and 5 μL solution of the sensor 1 (3 mM) was
diluted to 2.995 mL with bis-tris buffer (10 mM, pH 7.0) to make the final concentration
of 5 μM. Al(NO3)3 (0.10 mmol) was dissolved in bis-tris buffer (5 mL) and 39.0 μL of the
Al3+ ion solution (20 mM) was added to the solution of 1 (5 μM) prepared above. After
mixing them for a few seconds, fluorescence spectra were taken at room temperature.
For CN-; after the solution with a desired pH was achieved, sensor 1 (0.5 mg, 0.0015
mmol) was dissolved in DMSO (0.5 mL), and 30 μL solution of the sensor 1 (3 mM) was
diluted to 2.97 mL with bis-tris buffer/DMSO (1/1, v/v) to make the final concentration of
30 μM. TEACN (0.10 mmol) was dissolved in bis-tris buffer (1 mL) and 64.8 μL of the
TEACN ion solution (100 mM) was added to the solution of 1 (30 μM) prepared above.
After mixing them for a few seconds, fluorescence spectra were taken at room temperature.
2.8 Determination of Al3+ in water samples
Fluorescence spectral measurements of water samples containing Al3+ were carried by
adding 15 μL solution of the sensor 1 (3 mM) and 0.60 mL of 50 mM bis-tris buffer stock
solution to 2.385 mL sample solutions. After mixing them for a few seconds, fluorescence
spectra were taken at room temperature.
2.9 Imaging experiments in living cells
HeLa cells (ATCC, Manassas, USA) were maintained in media containing DMEM, 10%
fetal bovine serum (FBS, GIBCO, Grand Island, NY, USA), 100 U/mL penicillin (GIBCO),
and 100 mg/mL streptomycin (GIBCO). The cells were grown in a humidified atmosphere
with 5 % CO2 at 37 °C. Cells were seeded onto confocal imaging dish (SPL Life Sciences
Co., Ltd., South Korea) at a density of 150,000 cells per 1 mL and then incubated at 37 °C
for 24 h. Cells were first stained with SYTO16 nucleic acid stain (0.5 % v/v final DMSO
concentration; 2.5 M; at 37 °C) for 20 min. Prior to addition of aluminum nitrate
(dissolved in water), cells were washed with 2 mL of 10 mM bis-tris buffer (pH 7.4, 150
mM NaCl). After incubation with aluminum nitrate for 1 h, cells were exposed to 1
(dissolved in DMSO; 1% v/v final DMSO concentration; 20 M; at room temperature) for
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30 min. Imaging was performed with an EVOS FL fluorescence microscope (Life
technologies) using a DAPI light cube (DAPI, 2-(4-amidinophenyl)-1H-indole-6-
carboxamidine; ex = 357 (± 22) nm; em = 447 (± 30) nm) and GFP light cube (ex = 470
(±11) nm; em = 510 (±21) nm).
2.10 1H NMR titrations
For 1H NMR titrations of receptor 1 with Al3+, three NMR tubes of receptor 1 (3.3 mg,
0.01 mmol) dissolved in DMSO-d6 were prepared and then three different concentrations
(0, 0.005 and 0.01 mmol) of Al(NO3)3 dissolved in DMSO-d6 were added to each solution
1
of receptor 1. After shaking them for a minute, H NMR spectra were taken at room
temperature.
For 1H NMR titrations of receptor 1 with CN-, four NMR tubes of receptor 1 (3.3 mg,
0.01 mmol) dissolved in DMSO-d6 were prepared and then four different concentrations
(0, 0.005, 0.01 and 0.02 mmol) of TEACN dissolved in DMSO-d6 were added to each
1
solution of receptor 1. After shaking them for a minute, H NMR spectra were taken at
room temperature.
2.11 Theoretical calculations
All theoretical calculations were performed by using the Gaussian 03 suite.61 The singlet
ground states (S0) were optimized by DFT methods with Becke’s three parametrized Lee-
Yang-Parr (B3LYP) exchange functional62,63 with 6-31G** basis set64,65. In vibrational
frequency calculations, there was no imaginary frequency, suggesting that the optimized
structures represented local minima. To investigate the electronic properties of singlet
excited states, time-dependent DFT (TD-DFT) was performed in the ground state
geometries. The Cossi and Barone’s CPCM (conductor-like polarizable continuum
model)66,67 was used to consider the solvent effect. The 20 singlet-singlet excitations were
calculated and analyzed. The GaussSum 2.168 was used to calculate the contributions of
molecular orbitals in electronic transitions.
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3. Results and discussion
The compound 1 was obtained by the coupling reaction of 2-hydroxybenzaldehyde and
oxalohydrazide with a 75.4% yield in methanol (Scheme 1) and characterized by 1H NMR
and 13C NMR, ESI-mass spectrometry, and elemental analysis.
Scheme 1. Synthesis of sensor 1.
3.1 Fluorescence and absorption studies of 1 toward Al3+
The fluorescence selectivity of sensor 1 toward various metal ions was investigated in
bis-tris buffer solution (10 mM, pH 7.0). Upon excitation at 410 nm, 1 exhibited a weak
emission intensity at 488 nm with a low quantum yield (Ф = 0.011) (Fig. 1). Upon the
addition of 52 equiv of metal ions (Al3+, Ga3+, In3+, Zn2+, Cd2+, Cu2+, Fe2+, Fe3+, Mg2+, Cr3+,
Co2+, Ni2+, Na+, K+, Ca2+, Mn2, Pb2+), only Al3+ induced a significant fluorescence
enhancement at 491 nm with a quantum yield (Ф = 0.120), while other metal ions showed
no obvious change in the spectra. The enhancement of fluorescence intensity could be
explained by chelation-enhanced fluorescence (CHEF). The formation of a chelate
complex between 1 and Al3+ might cause a rigid system, leading to CHEF effect (Scheme
2).69-72
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Fig. 1 Fluorescence spectral changes of 1 (5 µM) in the presence of different metal ions
(52 equiv) such as Ga3+, In3+, Zn2+, Cd2+, Cu2+, Fe2+, Fe3+, Mg2+, Cr3+, Co2+, Ni2+, Na+, K+,
Ca2+, Mn2+ and Pb2+ with an excitation of 410 nm in bis-tris buffer solution.
To study a quantitative investigation for the binding affinity of 1 with Al3+, a
fluorescence titration experiment of 1 was performed with Al3+ ion (Fig. 2). Upon the
addition of Al3+ into 1, the emission intensity at 491 nm steadily increased until the amount
of Al3+ reached 52 equiv. Moreover, UV-vis titration experiment of 1 was conducted. The
addition of Al3+ to a solution of 1 showed that the absorption bands at 298 nm and 475 nm
decreased and a new band at 399 nm increased gradually until the amount of Al3+ reached
48 equiv (Fig. 3). Two isosbestic points were observed at 367 nm and 448 nm, suggesting
that only one product was generated from 1 upon binding to Al3+.
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Fig. 2 Fluorescence spectral changes of 1 (5 μM) in the presence of different
concentrations of Al3+ ions in bis-tris buffer solution. Inset: intensity (at 491 nm) of 1 as a
function of Al3+ equiv.
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Fig. 3 UV-vis absorption spectra of 1 (30 µM) obtained during the titration with
Al(NO3)3 (0.12-1.92 mM) in bis-tris buffer solution.
The Job plot73 referred to a 1:1 complexation stoichiometry between 1 and Al3+ (Fig.
S1), which was further confirmed by ESI-mass spectrometry analysis (Fig. S2). The
positive mass spectrum indicated that the first major peak at m/z = 424.10 was assignable
to 1-Al3+ complex - 2H + DMF [calcd, m/z: 424.10] and the second major peak at m/z =
351.00 was assignable to 1-Al3+ complex- 2H [calcd, m/z: 351.07]. From the fluorescence
titration, the association constant (K) of the 1-Al3+ complex was determined as 4.0 × 103
M-1 through Benesi-Hildebrand equation (Fig. S3), which is within the range of those
(103~109) previously reported for Al3+ binding chemosensors.74-77 The detection limit78 of
receptor 1 as a fluorescence sensor for the analysis of Al3+ ions was found to be 2.01 μM
(Fig. S4). The detection limit was below the WHO guideline for drinking water (7.41 μM),
which indicated that 1 could be a practical detector of aluminum ions in drinking water.
The preferential selectivity of 1 for the detection of Al3+ was studied in the presence of
other competitive metal ions of the same concentration (Fig. S5). In the presence of Ga3+,
In3+, Zn2+, Cd2+, Mg2+, Na+, K+, Ca2+, Mn2+, and Pb2+, there was no obvious interference
with the detection of Al3+, while relatively high interferences were observed in the presence
of Cu2+, Fe2+, Fe3+, Co2+, Ni2+ and Cr3+. Nevertheless, it is worth noting that In3+ and Ga3+
hardly inhibited the fluorescence intensity of the 1-Al3+ complex, while it has been a
challenge to distinguish Al3+ from In3+ and Ga3+ due to the similar properties of all three
ions.79-81 To further check the preferential selectivity of 1 toward Al3+ in the presence of
CN-, the competitive study was carried out in bis-tris buffer solution. As shown in Fig. S6,
no interference was observed with CN- under the condition.
The sensing ability at biologically relevant pH is important for the practicality of the
sensor. Thus, we conducted pH effect test in the pH range of 2 to 12 (Fig. S7). The stable
and strong fluorescence intensity of 1-Al3+ complex remained between pH 4 and 10. These
results indicated that sensor 1 could be a good detector over a wide range of pH.
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In order to examine the applicability of 1 in real samples, we carried out the construction
of a calibration curve for quantitative determination of Al3+ by 1 (Fig. S8). Receptor 1
exhibited a good linear relationship between the absorbance of 1 and the Al3+ concentration
(0.0-70.0 μM) with a correlation coefficient of R2 = 0.9918 (n = 3). This result indicates
that 1 could be appropriate for the quantitative analysis of Al3+. Based on the calibration
curve, the chemosensor 1 was applied for the determination of Al3+ in the tap and drinking
water samples (Table 1). The satisfactory recoveries and R.S.D. values were obtained.
Table 1. Determination of Al3+ in water samplesa
Sample
Al3+ added
Al3+ found
Recovery
(%)
R.S.D. (n = 3)
(%)
(μmol/L)
(μmol/L)
Tap water
0.00
40.0
0.00
40.0
0.00
38.2
0.00
37.4
-
-
98.2
-
0.6
-
Drinking water
97.6
1.7
a Conditions: [1] = 15 μmol/L in 10 mM bis-tris buffer solution (pH 7.0).
In light of the above favorable spectroscopic properties of 1 for detecting Al3+, including
high selectivity, high sensitivity and rapid response in the physiological pH range, we
studied its utility for monitoring Al3+ in biological systems. Cells were treated with Al3+ for
1 h and then 1 was added into them (Fig. 4). The background fluorescence was not observed
in the cells that had not been exposed to Al3+. Compared to the unexposed cells, discernible
fluorescence could be observed in the cells exposed to Al3+ (100 μM). These observations
confirm that 1 can be a potential detector to successfully sense Al3+ in biological systems.
Furthermore, to identify the fluorescent localization in cells, the nuclei of cells were stained
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with SYTO16. As shown in Fig. 4e and f, the overlapped fluorescent images between 1-
Al3+ and SYTO16 suggested that the fluorescence response of 1 to Al3+ occurred in the
overall parts of cells, including nuclei and cytoplasms.
Fig. 4 Fluorescent response to Al3+ in the presence of 1 in HeLa cells. Cells were exposed
to (a, b) 0 or (c-f) 100 μM Al3+ for 1 h prior to addition of 1 (20 μM). (a, c) Bright-field
images of cells without or with Al3+, respectively. Fluorescent responses to Al3+ were
detected by 1 in the (b) absence or (d) presence of Al3+. To investigate the distribution of
fluorescence, (e) nuclei of cells were stained with SYTO16 (2.5 μM). (f) Overlapped
fluorescence of (d) and (e). Conditions: DAPI channel [ex = 357 (± 22) nm; em = 447 (±
30) nm] for (b) and (d); GFP channel [ex = 470 (± 11) nm; em = 510 (± 21) nm] for (e).
The scale bar is 25 μm.
1
We have performed H NMR titrations to further examine the binding mode of 1 with
Al3+ (Fig. S9). Upon addition of Al3+, the proton H1 almost disappeared, and the rest protons,
H2, H3, H4, H5, H6 and H7, showed slightly down-field shifts. These results suggest that the
two nitrogen atoms of the amide moieties might coordinate to Al3+ ion. There was no shift
in the position of proton signals on further addition of Al3+ (>1.0 equiv), which indicates a
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1:1 binding of 1 to Al3+. Based on Job plot, ESI-mass spectrometry analysis, andD1OHI: 1N0.1M039R/C6NJ01544A
titration, we proposed the structure of 1-Al3+complex (Scheme 2).
Scheme 2. Fluorescence enhancement mechanism and proposed structure of 1-Al3+
complex.
To clearly understand the fluorescent sensing mechanism of 1 toward Al3+, geometric
optimizations were conducted for 1 and 1-Al3+ complex by applying the B3LYP/6-31g**
1
with CPCM/water, based on Job plot, ESI-mass spectrometry analysis and H NMR
titration (Fig. 5). The energy-minimized structure of 1 showed a distorted structure with
the dihedral angle of 2N, 3C, 4C, 5N = 44.659o (Fig. 5a). 1-Al3+ complex has a planar
structure with the dihedral angle of 2N, 3C, 4C, 5N = 0o, and Al3+ was coordinated to 2N
and 5N atoms of 1 (Fig. 5b). We further investigated the singlet excitations of 1 and 1-Al3+
complex by TD-DFT methods and compared the UV-vis spectra of 1 and 1-Al3+ with their
TD-DFT calculations (Figs. S10a and S11a). In case of 1, the main molecular orbital (MO)
contributions of the first lowest excited state were determined for HOMO → LUMO and
HOMO-1 → LUMO+1 transitions (352.26 nm, Fig. S10b,c), which indicated
intramolecular charge transfer (ICT) transition from the phenol moieties to the oxalamide
one. For 1-Al3+ complex, the first lowest excited state transition, HOMO-4 → LUMO, was
forbidden. Therefore, radiative-allowed transition was determined for the second lowest
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excited state, HOMO → LUMO+1 transition, which indicated intramolecular charge
transfer (ICT) transition of 1 (368.39 nm, Fig. S11b,c). As shown in Fig S12, there were
no obvious changes in the electronic transitions between 1 and 1-Al3+ complex. Only, the
hypochromic shift (352.26 to 368.40 nm) was observed upon chelating of 1 with Al3+.
These results suggested that the sensing mechanism of 1 toward Al3+ was originated from
the rigid structure of 1-Al3+ complex, which might inhibit the non-radiative process. Thus,
the CHEF effect plays an important role in the fluorescence enhancement.
Fig. 5 Energy-minimized structures of (a) 1 and (b) 1-Al3+ from B3LYP level.
3.2 Fluorescence and absorption studies of 1 toward CN-
We also investigated the fluorescence selectivity of 1 toward different anions (OAc-, F-,
-
-
Cl-, Br-, I-, H2PO4 , BzO-, N3 , SCN-, HS- and CN-) in buffer-DMSO (1:1, v/v) solution (Fig.
6). Upon the addition of 72 equiv of each anion to 1, only CN- resulted in a drastic
enhancement of the emission intensity (Ф = 3.41×10-2) at 522 nm with excitation at 377
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nm, while other anions showed no or slight change of the fluorescence spectra as compared
with the fluorescence (Ф = 2.48×10-3) of the pure receptor 1.
CN-
200
150
100
1, 1 + OAc-, F-, Cl-, Br-, I-,
50
0
-
-
H2PO4 ,BzO-, N3 , SCN-, HS-
450
500
550
600
650
Wavelength (nm)
Fig. 6 Fluorescence spectral changes of 1 (30 µM) in the presence of different anions
-
-
(72 equiv) such as OAc-, F-, Cl-, Br-, I-, H2PO4 , BzO-, N3 , SCN-, HS- and CN- with an
excitation of 377 nm in buffer-DMSO (1:1, v/v) solution.
The recognition ability of 1 for CN- was investigated by fluorescence titration
experiment (Fig. 7). The emission intensity of 1 at 525 nm steadily increased until the
amount of CN- reached 72 equiv. The photophysical properties of 1 were examined by UV-
vis titration experiment (Fig. 8). UV-vis spectrum of 1 showed absorption bands at 294 nm
and 340 nm. Upon the addition of CN- to a solution of 1, the absorption bands have red-
shifted to 400 nm, accompanied by the formation of an isosbestic point at 360 nm. These
results suggest that only one product was generated from 1 upon binding to CN-.
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Fig. 7 Fluorescence spectral changes of 1 (30 μM) in the presence of different
concentrations of CN- in buffer-DMSO (1:1, v/v) solution. Inset: intensity (at 522 nm) of
1 as a function of CN- equiv.
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Fig. 8 UV-vis spectral changes of 1 (30 μM) in the presence of different concentrations
of CN- in buffer-DMSO (1:1, v/v) solution.
The Job plot73 referred to a 1:1 complexation stoichiometry between 1 and CN- (Fig.
S13), which was further confirmed by ESI-mass spectrometry analysis (Fig. S14). The
negative-ion mass spectrum indicated that the first major peak at m/z = 325.10 was
assignable to 1-H+ [calcd, m/z: 325.09], and the second major peak at m/z = 388.10 was
assignable to 1-2H+ + CH3CN + Na+ [calcd, m/z: 388.10]. From the fluorescence titration,
the association constant (K) of the 1-CN- was determined as 2.6 × 102 M-1 from Benesi-
Hildebrand equation (Fig. S15). The detection limit74 of receptor 1 as a fluorescence sensor
for the analysis of CN- ions was found to be 27.2 μM (Fig. S16).
The preferential selectivity of 1 as a fluorescent sensor for CN- was studied in the
presence of various competing anions (Fig. 9). For inhibition studies, 1 was treated with 80
-
-
equiv of CN- in the presence of other anions (OAc-, F-, Cl-, Br-, I-, BzO-, N3 , SCN-, H2PO4
and HS-). It showed no interference in the detection of CN-. Thus, 1 could be used as a
selective fluorescent chemosensor for CN- in the presence of the competing anions. On the
other hand, to further check the preferential selectivity of 1 toward CN- in the presence of
Al3+, the competitive study was carried out in buffer-DMSO (1:1, v/v) solution. As shown
in Fig. S17, Al3+ interfered with the detection of CN- under the condition. However, the
addition of EDTA(ethylenediaminetetraacetic acid) into a mixture solution of Al3+ and CN-
recovered the sensing ability of 1 toward CN- (Fig. 17).
To examine the practical application of 1 in a variety of pH environments, the pH effect
test was conducted in the pH range of 2 to 11 (Fig. S18). The fluorescent sensing ability of
1 toward CN- remained between pH 7 and 10, which includes the environmentally relevant
range of pH 7.0-8.4.82,83
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Fig. 9 Competitive selectivity of 1 (30 μM) toward CN- (72 equiv) in the presence of
other anions (72 equiv) with an excitation of 377 nm in buffer-DMSO (1:1, v/v) solution.
1
To understand the binding interaction between sensor 1 and CN-, H NMR study was
initiated (Fig. 10). Upon addition of 0.5 equiv of TEACN, the protons H1 (-NH)
disappeared. These results indicate that the amine protons might interact strongly with the
cyanide through the hydrogen bond. The proton H7 (-OH) shifted from 12.6 ppm to 11.4
ppm (< 2.0 equiv) and gradually disappeared (2.0 equiv). Meanwhile, the aromatic protons
(H3,4,5,6) and the imine protons shifted to upfield. On further addition of CN- (> 2.0 equiv),
no shift in the position of proton signals was observed.
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Fig. 10 1H NMR titration of receptor 1 with CN-.
To clearly demonstrate the fluorescent sensing mechanism of 1 toward CN-, theoretical
calculations were performed for 1 and 1-CN- by applying the B3LYP/6-31g** with
CPCM/DMSO. As shown in Fig. S17, the energy-minimized structure of 1 showed a
distorted structure with the dihedral angle of 4N, 5C, 6C, 7N = 44.788o and the distances
of 1O-2H and 10O-9H are 1.017 Å , respectively (Fig. S19a). 1-CN- has a planar structure
with the dihedral angle of 4N, 5C, 6C, 7N = 0.237o and the distances of 1O-2H and 10O-
9H are 1.038 Å , respectively (Fig. 19b). The singlet excitations of 1 and 1-CN- were
investigated by TD-DFT methods (B3LYP/6-31G**/DMSO), and the calculation data were
well consistent with the experimental data (Fig. S20a and Fig. S21a). In case of 1, the main
molecular orbital (MO) contributions of the first lowest excited state were determined for
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HOMO → LUMO and HOMO-1 → LUMO+1 transitions (352.74 nm, Fig. S20b,c), which
indicated intramolecular charge transfer (ICT) transition from the phenol moieties to the
oxalamide one. For 1-CN-, the main molecular orbital (MO) contribution of the first lowest
excited state was determined for HOMO → LUMO transition (369.69 nm, Fig. S21b,c),
which indicated intramolecular charge transfer (ICT) transition. As shown in Fig. S22,
there were no obvious changes in the electronic transitions between 1 and 1-CN-. However,
the energy gap between the HOMO and LUMO of the 1-CN- is smaller than that of 1,
which is in good agreement with the bathochromic shift in the absorption observed upon
the interaction of 1 with CN-. Moreover, the fluorescence enhancement from 1 to 1-CN- is
due to the presence of intramolecular hydrogen boding between 1 and CN- which induces
rigidity into the system (dihedral angles: 44.788o for 1 and 0.237o for 1-CN-) and decreases
non-radiative decay processes.84,85 These results suggest that the sensing mechanism of CN-
by 1 might occur by the hydrogen bond pathway. Based on Job plot, ESI-mass spectrometry
analysis, 1H NMR titration and theoretical calculations, we propose the sensing mechanism
of CN- by 1 (Scheme 3).
Scheme 3. Proposed sensing mechanism of CN- by 1.
4. Conclusion
We have designed and synthesized a simple fluorescent chemosensor 1 that could
recognize both Al3+ and CN- in aqueous solution. Al3+ induced a significant fluorescence
enhancement of 1 while other metal ions showed no obvious change in the spectra. In
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particular, In3+ and Ga3+ hardly inhibited the fluorescence intensity of the 1-Al3+ complex,
whereas it has been a challenge to distinguish Al3+ from In3+ and Ga3+ due to their similar
properties. The fluorescence enhancement of 1 with Al3+ could be explained by effect of
CHEF with theoretical calculations. Moreover, 1 could successfully monitor Al3+ in real
water samples and living cells. Furthermore, 1 showed a highly selective fluorescence
enhancement toward CN- in the presence of other anions without any interference. The
fluorescence sensing of CN- by 1 was proposed to occur through the hydrogen bonding
between 1 and CN-, which was also supported by TD-DFT studies. On the basis of the
results, we believe that receptor 1 will offer an important guidance to the development of
single receptors for recognizing both cations and anions.
Acknowledgements
Basic Science Research Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and Technology (NRF-
2014R1A2A1A11051794,
NRF-2015R1A2A2A09001301,
and
NRF-
2014S1A2A2028270) are gratefully acknowledged.
Supplementary Information
Supplementary material associated with this article can be found, in the online version.
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