Selective fluorescence assay of aluminum and cyanide ions using chemosensor containing naphthol

Article abstract of DOI:10.1039/c4ra01245c

The selective assay of aluminum and cyanide ions is reported using fluorescence enhancement and quenching of a phenol-naphthol based chemosensor (PNI) in aqueous and nonaqueous solvents, respectively. PNI gave no significant fluorescence in water. The binding properties of PNI with metal ions were investigated by UV-vis, fluorescence, and electrospray ionization mass spectrometry in a Bis-Tris buffer solution. The addition of aluminum ions switches on the fluorescence of the sensor PNI in water, comparable to relatively very low fluorescence changes in the presence of various other metal ions. The complex stability constant (K_a) for the stoichiometric 1:1 complexation of PNI with aluminium ions was obtained by fluorimetric titrations and NMR experiments. However, upon treatment with cyanide ions, the fluorescence of PNI was selectively turned off and the yellow solution of PNI turned to red in methanol. Other comparable anions, such as F^-, Cl^-, Br^-, I^-, CH₃COO^-, and H ₂PO₄^-, afforded no apparent fluorescence quenching. The interaction of PNI with cyanide ions was studied by NMR experiments.

Full text of DOI:10.1039/c4ra01245c

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PAPER
Selective fluorescence assay of aluminum and
cyanide ions using chemosensor containing
naphthol†
Cite this: RSC Adv., 2014, 4, 18094
Soojin Kim,‡a Jin Young Noh,‡b Sol Ji Park,a Yu Jeong Na,b In Hong Hwang,b
c
b
a
*
*
Jisook Min, Cheal Kim and Jinheung Kim
The selective assay of aluminum and cyanide ions is reported using fluorescence enhancement and
quenching of a phenol–naphthol based chemosensor (PNI) in aqueous and nonaqueous solvents,
respectively. PNI gave no significant fluorescence in water. The binding properties of PNI with metal ions
were investigated by UV-vis, fluorescence, and electrospray ionization mass spectrometry in a Bis–Tris
buffer solution. The addition of aluminum ions switches on the fluorescence of the sensor PNI in water,
comparable to relatively very low fluorescence changes in the presence of various other metal ions. The
complex stability constant (Ka) for the stoichiometric 1 : 1 complexation of PNI with aluminium ions was
obtained by fluorimetric titrations and NMR experiments. However, upon treatment with cyanide ions,
the fluorescence of PNI was selectively turned off and the yellow solution of PNI turned to red in
methanol. Other comparable anions, such as Fꢀ, Clꢀ, Brꢀ, Iꢀ, CH3COOꢀ, and H2PO4ꢀ, afforded no
apparent fluorescence quenching. The interaction of PNI with cyanide ions was studied by NMR
experiments.
Received 20th February 2014
Accepted 7th April 2014
DOI: 10.1039/c4ra01245c
www.rsc.org/advances
by metal ions and may result in bad inuence to the central
nervous system in humans.1–4 The average daily human intake
of aluminum ions recommended by WHO is about 3–10 mg.5,6
In the study of chemosensors for aluminum ion, Schiff base
complexes showed various applications, such as analytical,
biological, and clinical elds. Recently, Schiff base-type che-
mosensors have also been reported as a major class for sensing
aluminum ions.7–12
Cyanide ions are extremely toxic to mammals and lead to
death above a certain level. WHO recommends the cyanide
concentration in drinking water to be kept lower than ca. 2
mM.13 Chemicals containing cyanide ions are widely used in
electroplating, gold mining, and polymer production.14–16
Recently, some chemosensors specic for cyanide have been
reported based on fatal toxicity of cyanide in biological systems
and environment.17–30 More examples of selective chemosensors
for cyanide are needed to develop fast and accurate detection of
cyanide in various situations.
Introduction
Fluorescent probes for selective detection of various biologi-
cally and environmentally relevant small anions and metal
cations have attracted increasing interest due to the lasting
impact of these ions' toxic effects in biological systems. Due to
the widespread use of aluminum in food and cooking related
goods, the possibility of human exposure to aluminum has
increased. Long-term intake of excess aluminum ions is
possible to spread throughout all tissues in humans and
animals, and eventually accumulates in the bone. The iron
binding proteins in the human body are the major carrier of
Al3+ in plasma. Aluminum ions can enter the brain and reach
the placenta and fetus by such in vivo carriers. Aluminum ions
can stay quite a while in various organs and tissues before they
are excreted through the urine. In addition, aluminum ions
may be one of possible factors of Alzheimer's disease reported
to result from the accumulation of oxidative damage induced
In this study, we report the anion and metal cation-binding
properties of a new imine probe bearing phenol and naphthol
moieties which is the p-conjugated Schiff base receptor. This
imine probe exhibited uorescence enhancement with high
selectivity upon binding to aluminum ions in aqueous solution,
and uorescence quenching with high selectivity to cyanide
anions in methanol. The in vivo application of the probe as an
intracellular sensor of Al3+ is also reported by confocal uo-
rescence microscopy.
aDepartment of Chemistry and Nano Science, Global Top 5 Research Program, Ewha
Womans University, Seoul 120-750, Korea. E-mail: jinheung@ewha.ac.kr; Fax: +82-
2-3277-3419; Tel: +82-2-3277-4453
bDepartment of Fine Chemistry, Seoul National University of Science and Technology,
Seoul, Korea. E-mail: chealkim@snust.ac.kr
cNational Forensic Service, Seoul, 158-707, Korea
† Electronic supplementary information (ESI) available: ESI-MS, Job's plot, and
additional NMR. See DOI: 10.1039/c4ra01245c
‡ Both authors contributed equally to this work.
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Results and discussion
Fluorometric assay of aluminum ion
The chemosensor, ortho-phenol-ortho-naphtholmethylimine
(PNI) was prepared by the reaction of 1-hydroxy-2-naph-
thaldehyde and 2-aminophenol with 61% yield (Scheme 1 and
Experimental section). The binding properties of the PNI
receptor was examined towards various metal ions, such as Ag+,
Na+, K+, Ca2+, Mg2+, Ni2+, Cd2+, Mn2+, Co2+, Cu2+, Zn2+, Hg2+,
Pb2+, Al3+, Sc3+, Cr3+, Fe3+, Ga3+, and In3+ (34 equiv. each) in 50
mM Bis–Tris buffer (pH ¼ 7.0). The uorescence spectral
response in the presence of each metal ion was shown in Fig. 1.
Relative to no uorescence of PNI in aqueous solution, signi-
cant spectral changes were observed in the case of Al3+. No
signicant changes observed with other metal ions demonstrate
the high selectivity of PNI to other metal ions. PNI is even highly
Fig. 1 Emission spectra of PNI (10 mM) in the presence of various metal
ions (340 mM) using El ¼ 450 nm in 50 mM Tris buffer (pH ¼ 7.0).
metal ions. Based on the spectral changes, the inference derived
from the displacement of Al3+
.
selective for Al3+ over Ga3+ and In3+
.
A Job's plot as well as binding analysis using the Benesi–
Hildebrand plot established that a 1 : 1 complex of PNI (Scheme
2 and Fig. S2†). Al3+ could be detected down to 0.13 mM based on
the 3a/slope when 5 mM PNI was employed. The 1 : 1 complex
was supported by the electrospray mass spectrum of PNI and
Al3+ (Fig. S3, ESI†). The ESI-MS peak at m/z ¼ 412.07 (calculated
¼ 412.04) was attributed to {[PNI + Al3+ + 2NO3ꢀ]}+.
The turn-on uorescence response of PNI was observed for
Al3+ with emission bands at 490 and 515 nm. Upon addition of
34 equiv. Al3+, the uorescence intensity of PNI increased by
100-fold. The uorescence spectra of PNI progressively changed
upon addition of an incremental amount of Al3+ (Fig. 2a). The
uorometric titration curve showed a steady and smooth
increase with increasing Al3+ concentration, which demon-
strated an efficient uorescence response (Fig. 2b). The binding
affinity of PNI towards Al3+ was quantied based on uores-
cence titration experiments, affording the association constant
(Ka) of 2.5 ꢁ 103 Mꢀ1 (ESI, Fig. S1†). Upon binding of Al3+, the
aromatic rings of PNI no more rotate around the imine group,
and two aromatic rings could be on the same plane to make a
good conjugated system through the imine. This conjugation
probably caused the uorescence enhancement. In the case of
other metal ions which can bind to PNI in a similar manner, the
uorescence would be quenched by each metal coordination.
The absorption spectra of PNI also changed upon addition of
Al3+ ions (Fig. 3). The bands at 285, 455, and 473 nm decreased
and no characteristic new bands appeared.
Colorimetric and uorometric assay of cyanide ion
The anion binding affinity of PNI was evaluated by monitoring
their UV-vis and steady-state emission properties as a function
Fig. 4 illustrate the uorescence response of PNI to Al3+ in
the presence of other competing metal ions, respectively. Except
for Cu2+, Cr3+, and Fe3+, backgrounds of most metal ions do not
interfere with the detection of Al3+ by PNI in Tris buffer (pH ¼
7.0). Chemosensors reporting uorescence studies with Al3+ in
the presence of competing metal ions showed inhibition by Ag+,
Cu2+, Cr3+, and Fe3+ 11,12 but PNI was not signicantly inhibited
,
by the other metal ions. In order to understand the interference
by Cu2+, Cr3+, and Fe3+, absorption spectra were obtained using
the mixtures of Al3+ and other metal ions. In the presence of
Cu2+, Cr3+, or Fe3+, the absorption spectrum of PNI and Al3+
were different from that obtained without such interfering
Fig. 2 (a) Changes in emission intensity of 10 mM PNI upon the
Scheme 1 Synthesis of the fluorescent sensor ortho-phenyl-ortho- addition of Al3+ (0–0.6 mM), El ¼ 450 nm in 50 mM Tris buffer. (b)
naphtholmethylimine (PNI).
Binding isotherm monitored by the fluorescence increase at 515 nm.
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Fig. 3 Changes in the UV-vis spectrum of PNI (10 mM) as a function of
the Al3+ concentration in 50 mM Tris buffer.
Fig. 5 (a) Changes in the absorption spectrum of PNI (50 mM) in
methanol with increasing cyanide concentrations. (b) Color changes
on solutions of PNI (50 mM) in the presence of 100 equiv. of anions,
from left to right: PNI alone, CNꢀ, Fꢀ, Clꢀ, Brꢀ, Iꢀ, OAcꢀ, Clꢀ, and
ꢀ
H2PO4
.
Fig. 4 Aluminum ion (34 mM) response for PNI (10 mM) in the absence
and presence of competing metal ions (34 mM) in 50 mM Tris buffer. L
stands for PNI.
Scheme 2 Proposed structure of a 1 : 1 complex of PNI and Al3+
.
of anion concentration in methanol. The changes in the UV-vis
spectrum of PNI were observed as a function of CNꢀ concen-
tration (Fig. 5a). The bands at 286, 320, 457, and 480 nm
decreased and a new band at 300 nm appeared. Two clear iso-
sbestic points observed at 368 and 400 nm indicated a clean
conversion throughout the titration. A photograph of the color
changes induced upon anion treatment to methanol solutions
of PNI is shown in Fig. 5b. An obvious color change from yellow
to red was observed only upon treatment with cyanide. Previ-
Fig. 6 Fluorescence spectral changes of 50 mM PNI obtained at 518
nm upon the addition of 100 equiv. of various anions in methanol. All
fluorescence spectra were acquired with excitation at 450 nm.
ously, some organic dyes have been reported as colorimetric anions examined. The uorescence maximum was shied to
sensors for cyanide ions, in which all probes were covalently 540 nm upon treatment with cyanide. The uorescence
bound with cyanide upon addition.22,23,29
responses of PNI weꢀre unperturbed by Fꢀ, Clꢀ, Brꢀ, Iꢀ,
The uorescence spectrum of free PNI showed an emission CH3COOꢀ, and H2PO4 at 100-fold excess. The titration exper-
band at 518 nm. As shown in Fig. 6, only cyanide ions caused iment showed a progressive intensity decrease at 518 nm
signicant uorescence changes of PNI compared to other (Fig. 7).
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0 and 1 mM Al(NO3)3 for 4 h and then incubated with the che-
mosensor (10 mM) for 20 min. Low background uorescence
was observed in the cells (Fig. 8a) that had not been exposed to
Al(NO3)3. The background uorescence derived from cellular
metal ions, such as Na+, Ca2+, Mg2+, Zn2+, etc., which also
afforded low uorescence with PNI (Fig. 8b). Weak but still
Fig.
7 (a) Fluorescence titrations of 50 mM PNI with tetraethy-
lammonium cyanide (0 to 500 equiv.) in CH3OH. (b) Ratio of fluores-
cence intensities at 518 nm as a function of cyanide concentration.
Further insights into the nature of PNI and cyanide inter-
actions were studied by 1H-NMR titration experiments. Addition
of cyanide to PNI did not show any signicant changes in the
NMR spectra (Fig. S4†). The well resolved resonance signals of
aromatic protons of PNI became a little broad and shied upon
addition of cyanide. Interestingly, the imine proton signal also
appeared no evident shi in the absence and presence of
cyanide. Usually, the neighboring protons to cyanide bonding
covalently to probes were shied signicantly in NMR
spectra.18,21,31 Therefore, the uorescence behavior of PNI
towards cyanide can be explained as a deprotonation process of
phenol and naphthol (Scheme 3). Such
a uorescence
quenching of a probe upon treatment of cyanide was also
proposed due to deprotonation.25
Live cell imaging
We examined the bioimaging application of PNI for mapping
aluminum ions in living cells. HeLa cells were rst exposed to
Fig. 8 Representative fluorescence images of HeLa cells (a) incubated
with PNI only and (b) exposed to 1 mM Al(NO3)3 with 10 mM PNI. The
samples in (b) were incubated with 1 mM Al(NO3)3 for 4 h and exposed
with PNI. Each picture contains DIC images and fluorescent images
Scheme 3 Proposed structures showing the deprotonation reactions (excitation ¼ 480 nm, emission ¼ 520 nm LP). Details are found in
between PNI and CNꢀ.
Experimental section. The scale bar represents 20 mm.
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discernible uorescence was observed in the cells previously 1H), 7.65 (m, 2H), 7.42 (t, J ¼ 8 Hz, 1H), 7.21 (d, J ¼ 8 Hz, 1H),
exposed Al(NO3)3 at 1 mM, compared to the unexposed cells. 7.09 (t, J ¼ 8 Hz, 1H), 7.03 (d, J ¼ 8 Hz, 1H), 6.94 (t, J ¼ 8 Hz, 1H),
These results indicate that the uorescence intensities in the 6.81 (d, J ¼ 8 Hz, 1H). 13C NMR (DMSO-d6, 400 MHz): d 171.08,
exposed cells depended on the Al(NO3)3 concentrations. The 163.64, 146.84, 136.69, 129.98, 128.54, 128.38, 128.27, 128.00,
PNI probe afforded strong uorescence in the presence of 127.81, 127.49, 125.25, 124.42, 123.63, 121.67, 120.44, 118.92,
intracellular Al3+, thus demonstrating its suitability for deter- 115.61, 110.57 ppm. Anal. calcd for C17H13NO2 (263.09): C,
mining the exposure level of cells to aluminum ions.
77.55; H, 4.98; N, 5.32. Found: C, 77.51; H, 5.01; N, 5.28%.
UV-vis measurements of aluminum ion
Conclusions
Receptor PNI was dissolved in methanol and PNI were diluted
with Bis–Tris buffer to make the nal concentration of 10 mM.
Al(NO3)3$9H2O was dissolved in Bis–Tris buffer. A certain
amount of the Al3+ solution was transferred to each receptor PNI
solution. Aer shaking the vials for a few minutes, UV-vis
spectra were taken at room temperature.
We have described a new uorescent Schiff-base chemosensor
PNI for sensing aluminum and cyanide ions. PNI exhibited high
selectivity for Al3+ in aqueous solution over competing relevant
metal ions and a large turn-on response for detecting aluminum
ions. In addition, the treatment with anions to the methanol
solution leads to different photophysical behaviors. PNI showed
the responses specic for cyanide ions, resulting in readily
distinguished by color. Furthermore, PNI is capable of mapping
aluminum levels in live cells, which might be exploited as
specic and effective sensors for intracellular aluminum ions.
Fluorescence measurements of aluminum ion
Receptor PNI was dissolved in methanol and PNI was diluted in
Bis–Tris buffer to make the nal concentration of 10 mM.
Al(NO3)3 was dissolved in Bis–Tris buffer. The Al3+ solution was
transferred to each receptor PNI solution prepared above. Aer
shaking the vials for a few minutes, uorescence spectra were
taken at room temperature.
Experimental section
Materials and instrumentation
All the solvents and reagents (analytical grade and spectro-
scopic grade) were obtained from Sigma-Aldrich and used as
received. Water was puried with a MilliQ purication system.
The metal ion solutions were prepared with metal nitrate salts
Competition of aluminum ion to other metal ions
M(NO3)x were dissolved in Bis–Tris buffer, respectively. Each
metal solution (10 mM) was added into each PNI solution (10
mM). Then, Al3+ solution was added into the mixed solution of
PNI containing a competing metal ion.
1
in methanol. H and 13C-NMR spectra were recorded on a Var-
ian 400 spectrometer (Palo Alto, CA, USA). Chemical shis (d)
are reported in ppm, relative to tetramethylsilane Si(CH3)4.
Absorption spectra were recorded at 25 ꢂC using a Perkin-Elmer Job plot measurement of aluminum ion
model Lambda 2S UV/Vis spectrometer (Waltham, MA, USA).
Al(NO3)3 dissolved in Bis–Tris buffer was added to each PNI
Emission spectra were recorded on a Perkin-Elmer LS45 uo-
rescence spectrometer. Electrospray ionization mass spectra
(ESI-MS) were collected on a Thermo Finnigan (San Jose, CA,
USA) LCQTM Advantage MAX quadrupole ion trap instrument,
by infusing samples directly into the source at 25 mL minꢀ1 with
a syringe pump. The spray voltage was set at 4.7 kV and the
solution diluted in Bis–Tris buffer. Each vial had a total volume
of 3 mL. Aer shaking the vials for a few minutes, uorescence
spectra were taken at room temperature.
UV-vis and uorescence measurements of cyanide
ꢂ
Tetraethylammonium cyanide in methanol was transferred to
each PNI solution (50 mM). Aer shaking the vials for a few
minutes, UV-vis and uorescence spectra were taken at room
temperature.
capillary temperature at 70 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. To conrm the
accuracy and reliability of the analytical procedure, the
concentrations of various metal ions were checked by induc-
tively coupled plasma (ICP) spectroscopic analysis.
Competition with other anions
Each anion solution (tetraethylammonium salts) was added
into 3 mL of each PNI solution (50 mM). Then, CNꢀ solution was
added into the mixture of each competing anion and PNI.
Synthesis of ortho-phenyljulolidineimine (PNI)
A solution of 2-aminophenol (0.12 g, 1.1 mmol) in methanol was
added to a solution containing 1-hydroxy-2-naphthaldehyde
Fluorescent imaging of intracellular Al3+ in cells
(0.18 g, 1 mmol) in methanol. The reaction mixture was stirred Human HeLa cell line were cultured in DMEM (Dulbecco's
for 5 h at room temperature until an orange precipitate Modied Eagle Medium) which were supplemented with 100
appeared. The resulting precipitate was ltered and washed 2 units per ml penicillin, 100 mg mlꢀ1 streptomycin, and 10%
times with ice methanol. The yield: 0.13 g (61%). 1H NMR heat-inactivated fetal bovine serum at 37 ꢂC in a humidied
(DMSO-d6, 400 MHz): 14.61 (d, J ¼ 12 Hz, 1H), 10. 42 (s, 1H), incubator. Cells were seeded on an 18 ꢁ 18 mm cover glass
8.86 (d, J ¼ 12 Hz, 1H), 8.32 (d, J ¼ 8 Hz, 1H), 7.70 (d, J ¼ 8 Hz, (Marienfeld, Lauda-Koenigshofen, Germany) at density 2 ꢁ 105
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cells in culture media. HeLa cells were incubated with 1 mM of 12 H. M. Park, B. N. Oh, J. H. Kim, W. Qiong, I. H. Hwang,
Al(NO3)3 in culture media for 4 hour at 37 ꢂC. Aer washing with
PBS three times to remove the remaining Al(NO3)3, cells were
K.-D. Jung, C. Kim and J. Kim, Tetrahedron Lett., 2011, 52,
5581–5584.
then incubated with 100 mM PNI for 20 min at RT. The treated 13 W. H. Organization, Guidelines for Drinking-Water Quality,
cells were washed with PBS and mounted onto a glass slide with 1996.
ClearMount™ aqueous mounting medium (Invitrogen). The 14 W. Gerhartz, S. Yamamoto, F. T. Campbell, R. Pfefferkorn
uorescent images of the mounted HeLa cells were obtained by
using a confocal laser scanning microscope (CLSM LSM510,
and J. F. Rounsaville, Ullmann's Encyclopedia of Industrial
Chemistry, Wiley-VCH, New York, 1999.
Carl Zeiss) with a 480 nm excitation and LP 520 nm emission 15 S. I. Baskin and T. G. Brewer, Medical Aspects of Chemical and
lters at various magnications (200ꢁ to 400ꢁ).
Biological Warfare, TMM Publications, Washington, 1997.
16 H. Hachiya, S. Ito, Y. Fushinuki, T. Masadome, Y. Asano and
T. Imato, Talanta, 1999, 48, 997–1004.
Acknowledgements
17 (a) S.-Y. Chung, S.-W. Nam, J. Lim, S. Park and J. Yoon, Chem.
Commun., 2009, 2866–2868; (b) Z. Xu, X. Chen, H. N. Kim and
J. Yoon, Chem. Soc. Rev., 2010, 39, 127–137.
18 K.-S. Lee, H.-J. Kim, G.-H. Kim, I. Shin and J.-I. Hong, Org.
Lett., 2008, 10, 49–51.
19 F. Garcia, J. M. Garcia, B. Garcia-Acosta, R. Martinez-Manez,
F. Sanceno and J. Soto, Chem. Commun., 2005, 2790–2792.
20 J. V. Ros-Lis, M. D. Marcos, R. Martinez-Manez, K. Rurack
and J. Soto, Angew. Chem., Int. Ed., 2005, 44, 4405–4407.
21 C.-L. Chen, Y.-H. Chen, C.-Y. Chen and S.-S. Sun, Org. Lett.,
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This work was supported by the National Research Foundation
of Korea Grant funded by the Korea Government (NRF-
2013R1A2A2A03015101, 2012001725, 2012008875), and the
National R&D program of the Ministry of Education, Science
and Technology (MEST), Korea: Development of Molecular
Sensing Technology (Code number 2012-009832).
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