Fluorescence turn-on sensors for HSO4-

Article abstract of DOI:10.1039/b918324h

A coumarin-based derivative (1), a highly selective and sensitive turn-on fluorogenic probe for the detection of HSO₄^- ions in aqueous solution, has been designed and synthesized. Various spectroscopic and DFT calculations revealed t

Full text of DOI:10.1039/b918324h

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ꢀ
Fluorescence turn-on sensors for HSO4 w
Hyun Jung Kim,a Sankarprasad Bhuniya,a Rakesh Kumar Mahajan,*b Rajiv Puri,b
Hongguang Liu,c Kyoung Chul Ko,c Jin Yong Lee*c and Jong Seung Kim*a
Received (in Cambridge, UK) 7th September 2009, Accepted 1st October 2009
First published as an Advance Article on the web 16th October 2009
DOI: 10.1039/b918324h
A coumarin-based derivative (1), a highly selective andꢀsensitive
turn-on fluorogenic probe for the detection of HSO4 ions in
aqueous solution, has been designed and synthesized. Various
spectroscopic and DFT calculations revealed that H-bonding
between the phenolic –OH and imine nitrꢀogen of 1 played a
crucial role in its high selectivity for HSO4
.
Chemosensors based on ion-induced fluorescence changes
are predominantly attractive because of the simplicity and low
detection limit of fluorescence detection methods.1–3 In the midst
of it all, fluorescence ‘turn-on’ sensors for detection of anions of
biological importance still remain a challenging aspect given that
general anions can act as fluorescence quenchers.4,5 Hence, much
attention has been paid to the design of ‘turn-on’ anionic
fluorescence sensors. Among the anions, hydrogen sulfate
(HSO4ꢀ) is of particular interest owing to its established role
in biological and industrial areas.6 This amphiphilic anion
eventually dissociates at high pH to generate toxic sulfate ion
(SO42ꢀ),7 causing irritation of the skin and eyes and even
respiratory paralysis. For these reasons, an improved method
for the detection and sensing of HSO4ꢀ ions with high selectivity
is of current interest in the chemosensor research field. Only a few
examples of fluorogenic chemosensors showing absorbance or
Fig. 2 (a) Fluorescence spectra of 1 (3.0 mM) lex = 355 nm upon
addition of TBA+ salts of Fꢀ, Clꢀ, Brꢀ, Iꢀ, CH3CO2ꢀ, HSO4
,
ꢀ
H2PO4ꢀ, NO3ꢀ, and OHꢀ (50 equiv.) in 1 : 1 CH3CN–H2O (v/v).
(b) Fluorescence titration spectra of 1 (3.0 mM) in 1 : 1 CH3CN–H2O
(v/v) upon addition of TBA+ HSO4ꢀ (0, 0.1, 0.3, 0.5, 1, 3, 5, 6, 7, 8, 10,
20, 30, 50, 100 equiv.) with excitation at 355 nm.
(Fig. S1,w S hereafter denotes Supporting Information). More
interestingly, we observed a remarkable fluorescence increment
ꢀ
fluorescence changes upon HSO4 complexation have been
reported;8 however, selectivity remains questionable.
ꢀ
of 1 at 485 nm upon addition of HSO4 to a solution
We herein report a new coumarin-based fluorescent sensor (1)9
in Fig. 1 that ꢀexhibits a unique fluorescence change in the presence
of the HSO4 ion and with high selectivity over other anions.
To establish the mechanism of the fluorescence changes upon
addition of HSO4ꢀ, two more analogues (2 and 3) were also
prepared and their fluorescent properties examined.9
of CH3CN–H2O (1 : 1) (Fig. 2a), whereas no meaningful
fluorescence changes were noticed with other anions. The
ꢀ
titration spectra of 1 with the HSO4 ion at 485 nm show a
13-fold fluorescence enhancement (Fig. 2b). Job’s plot analysis
of the fluorescence titration spectra exhibited a maximum at a
0.5 mol fraction of HSO4ꢀ, indicating formation of a 1 : 1
complex between 1 and the HSO4ꢀ ion (Fig. S2w). On the basis
of both the 1 : 1 stoichiometry and fluorescence titration data
Compound 1 shows a characteristic UV-Vis absorbance band
centered at 355 nm in 50% aqueous CH3CN. In the presence of
the HSO4ꢀ ion, the absorption band at 355 nm shifted to 370 nm
ꢀ
from Fig. 2, the association constant of 1 for HSO4 was
calculated to be 4.86 ꢁ 104 Mꢀ1 10
.
ꢀ
The competitive binding aptitude of 1 with HSO4 over
other anions was studied as shown in Fig. 3. The fluorescence
intensity of 1 was quenched upon addition of various anion
mixtures, including TBA+ salts of: Fꢀ; Clꢀ; Brꢀ; Iꢀ;
CH3CO2ꢀ; H2PO4ꢀ; NO3ꢀ; OHꢀ. Upon gradual addition of
HSO4ꢀ to the anion mixture, the fluorescence of 1 was revived,
Fig. 1 Structures of fluorescent sensors 1–3.
a Department of Chemistry, Korea University, Seoul 136-701,
Republic of Korea. E-mail: jongskim@korea.ac.kr;
Fax: +82-2-3290-3121; Tel: +82-2-3290-3143
ꢀ
suggesting that 1 possesses an excellent selectivity for HSO4
over competitive anions.
b Department of Chemistry, Guru Nanak Dev University,
Amritsar 143005, India. E-mail: rakesh_chem@yahoo.com;
Fax: +91-183-2258820
Furthermore, the fluorescence changes of 1 (3.0 mM) in the
absence and presence of 30 equiv. of HSO4ꢀ, in different pH
environments, were studied and are described in Fig. S3.w
Compound 1 gave an insignificant response to fluorescence
changes under different pH conditions. However, upon addition
c Department of Chemistry, Sungkyunkwan University,
Suwon 440-746, Republic of Korea. E-mail: jinylee@skku.edu
w Electronic supplementary information (ESI) available: Experimental
details. CCDC 746885. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b918324h
ꢀ
of the HSO4 ion, we found a distinct increase in the emission
ꢂc
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hydroꢀgen (–OH) of 1, another between the hydrogen of the
HSO4 and the oxygen (–OH) of 1, and a third between
ꢀ
the hydrogen of the HSO4 and the nitrogen of 1. Thus,
ꢀ
apparently 1 can interact strongly with the HSO4 ion.
Natural bond orbital (NBO) analyses (Fig. S8w) show a
notable change in the charge of the N atom when 1 forms
a complex wꢀith HSO4ꢀ. The charge of the N atom of 1, 2,
and 1-HSO4 is ꢀ0.527, ꢀ0.446, and ꢀ0.456, respectively.
From these data, we noticed that the N atom of 1 possessed
greater electron density even though it was exhibiting intra-
molecular H-bonding with the phenolic –OH group. In similar
H-bonding systems, the electrons usually transfer from
the nitrogen to the hydrogen, and as a consequence, the charge
density on the N is reduced. This unusual negative charge
allocation on the N of 1 leads to fluorescence quenching;
Fig. 3 Fluorescence spectra of 1 (3.0 mM) in 1 : 1 CH3CN–H2O (v/v)
in the presence of HSO4ꢀ (50 equiv.) and TBA+ salts of various anions
(Fꢀ, Clꢀ, Brꢀ, Iꢀ, CH3COOꢀ, H2PO4ꢀ, NO3ꢀ, OHꢀ, 50 equiv.,
respectively) with excitation at 355 nm.
bands of 1 at 485 nm between pH 4 and 10. Such broad pH
spans in an aqueous miliꢀeu make it very useful in several
applications, such as HSO4 detection in wastewater, industrial
ꢀ
nevertheless, it showed affinity to form a complex with HSO4
via an H-bond and consequently enhanced the fluorescence.
The ‘Off–On’ fluorescence mechanism of 1 is also explained
by the widely accepted photo-induced electron transfer (PET)
(Fig. S9w) mechanism.12 Table 1 lists the dominant electron
transitions with the two strongest absorptions, considering
oscillator strength.
trade analysis, and physiological treatment.
ꢀ
To obtain insight into the binding mode of 1 with HSO4
,
1H-NMR titration experiments were carried out (Fig. S4w).
We found that the phenolic proton (Ha) had intramolecular
H-bonding with the imine nitrogen at 12.6 ppm, which was
conclusively evidenced by the X-ray crystal structure (Fig. S5w).z
However, in the presence of HSO4ꢀ, the corresponding
–OH peak disappeared, delineating that the intramolecular
H-bonding of 1 is interrupted by coordination of HSO4ꢀ with 1.
To examine the role of intermolecular H-bonding between
the phenolic –OH and imine moiety and the role of the
coumarin unit in the fluorescence changes of 1, 2 was prepared
without the hydroxy group and 3 with a pyrenyl moiety instead
of coumarin. Upon addition of HSO4ꢀ to 2 and 3, the observed
fluorescence change of 2 was marginal and non-selective toward
the anions, implying that the H-bonding between the phenolic
–OH and the imine N in 1 plays a crucial role in the high
The nonbonding lone pair electrons of the nitrogen correspond
to the HOMO in 1, HOMO-1 in 2, and HOMO-2 in
1-HSO4ꢀ, while the occupied p-bonding (Fig. S10w) orbital
characters of CQN are HOMO-1 in 1, HOMO in 2, and
HOMO-4 in 1-HSO4ꢀ, respectively. Since these p-bonding
orbitals and the nitrogen lone pair orbitals possess small energy
differences, the electronic transition from the nitrogen lone pair
to the occupied p-bonding orbital was assumed to trigger the
PET process, and thus lead to fluorescence quenching.
In Table 1, the oscillator strengths for the transitions from the
nitrogen lone pair to the p* orbitals (n-p*) are 0.4084, 0.0346,
and 0.2812 for 1, 2, and 1-HSO4ꢀ, respectively, corresponding to
the transition energies of 386, 314, and 443 nm, respectively. As
mentioned above, these contributions cause the fluorescence
quenching of 1. The other important transitions take place at
341, 357, and 362 nm with oscillator strengths of 0.3488, 0.6360,
and 0.2807 for 1, 2, and 1-HSO4ꢀ, respectively. Thus, when a
355 nm light source is used for excitation, both transitions can
take place for 1 and 2. However, for 1-HSO4ꢀ, the n-p*
transition may be very limited due to the transition energy
(443 nm) being more deviated to 355 nm. For 2, a very weak
oscillator strength for the n-p* transition also restricts the
n-p* transition. Thus, the fluꢀorescence is ‘turned-off’ in 1, while
ꢀ
selectivity of 1 for HSO4 ion over other anions (Fig. S6w).
Similarly, compound 3 alone showed weak fluorescence
(Fig. S7w); however, it increased in the presence of HSO4ꢀ, as
noted for 1. This observation suggests that 6-membered intra-
molecular H-bonding of phenolic –OH with the imine N
initially contributed to the fluorescence quenching of 1, which
was then followed by selective blocking of the H-bonding by
ꢀ
the HSO4 ion to give fluorescence enhancement of 1 and 3.
To clarify the ‘Off–On’ fluorescence mechanism, density
functional theory (DFT) calculations were performed with
the B3LYP exchange functional employing 6-31G* basis sets
using a suite of Gaussian 03 programs.11 Fig. 4 shows thꢀe
optimized structures of 1 and its complex with HSO4
(1-HSO4ꢀ). Compound 1 has an intramolecular hydrogen
bond, whereas the 1-HSO4ꢀ complex has three intermolecular
‘turned-on’ for 2 and 1-HSO4
.
The selective binding affinity toward HSO4ꢀ of 1 encouraged
ꢀ
construction of an HSO4 ion-selective electrode PVC membrane
Table 1 Calculated TDDFT excitation properties containing the first
and second highest oscillator strength of 1, 2 and 1-HSO4 in low
ꢀ
ꢀ
H-bonds, one between the oxygen of the HSO4 and the
excitation energy (l > 310 nm)
Excitation Oscillator
wavelength (nm) strength (au)
Compound Excitation
1
HOMO - LUMO
HOMO-1 - LUMO 341
HOMO - LUMO
HOMO-1 - LUMO 314
HOMO-2 - LUMO 443
HOMO-4 - LUMO 362
386
0.4084
0.3488
0.6360
0.0346
0.2812
0.2807
2
357
ꢀ
ꢀ
1-HSO4
Fig. 4 Optimized structures of compound 1 and 1-HSO4 and
distances of the hydrogen bonds (A).
ꢂc
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ꢀ
Table 2 Composition and potential response characteristics of the HSO4 ion-selective electrode
PVC (mg)
100.7
DOSa (mg)
200.1
ToMA Cla (mg)
1.2
Ionophore 1 (mg)
Linear range (M)
Detection limit (M)
Slope (mV/decade)
5.3
1.0 ꢁ 10ꢀ1
3.75 ꢁ 10ꢀ6
ꢀ57.47
ꢀ1.0 ꢁ 10ꢀ5
DOS: Dioctyl sebacate as plasticizer; ToMA Cl: Tri-octyl methyl ammonium chloride.
a
based on 1 for the application. A sensor membrane incorporating
1 was prepared and assembled as previously reported.13 The
potentiometric cell used was Ag/AgCl (3.0 M KCl)/1.0 ꢁ
ions to 1 resulted in a pronounced ‘Off–On’ pattern in the
fluorescence spectra. In addition, a remarkably selective
potentiometric response was accomplished for the HSO4ꢀ ions
over a variety of other anions using an ion-selective electrode
based on 1, incorporated into a polymeric (PVC) membrane.
Hydrogen sulfate selectivity was due to the formation of a
hydrogen bond between the phenolic –OH and the imine
10ꢀ2
M
TBA+HSO4ꢀ/PVC membrane/Test solution/
(3.0 M KCl) AgCl/Ag. The composition of this membrane is
listed in Table 2.
In preliminary experiments, the ionophore 1 acted as an
efficient ion carrier for the HSO4ꢀ anion in comparison to other
examined anions (Fig. S11wꢀ). Most interestingly, the response
time for sensing the HSO4 was less than 10 s (Fig. S12w)
with a suitable operational pH for the sensor from 1.4 to 4.9
(Fig. S13w).
ꢀ
nitrogen atom in 1 and the HSO4 ions.
This work was supported by the CRI program, SRC
(20090063001), and KRF-2008-313-C00501 of the National
Research Foundation of Korea. JYL acknowledges the
KOSEF (No. R11-2007-012-03002-0) and KRF (KRF-2008-313-
C00388) grants by MEST.
Next, the potentiometric selectivity coefficients over inter-
fering ions were determined using the fixed interference method.
Pot
Accordingly, potentiometric selectivity coefficients ðlog K ꢀ;BÞ
HSO4
Notes and references
can be evaluated by carrying out potential measurements on
solutions containing fixed concentrations of interfering ions
z Single crystal data for 1: C17H10F3NO3, Mw = 333.26, plate orange
crystal, size: 0.30 ꢁ 0.30 ꢁ 0.30 mm3, monoclinic, space group P21/n,
a = 9.855(2) A, b = 5.4280(11) A, c = 26.716(5) A, V = 1428.9(5) A3,
T = 293(2) K, Z = 4, D = 1.549 Mg/m3, r = 0.133 mmꢀ1, F(000) =
(1.0 ꢁ 10ꢀ2 M) and varying concentrations of the HSO4
ꢀ
anion. Table 3 shows the potentiometric selectivity coefficient
680; 7321 reflections measured, of which 2796 were unique (Rint
=
data of the ionophore 1–PVC membrane electrodes for inter-
ꢀ
0.0552). 219 refined parameters, final R1 = 0.0530 for reflections with
I > 2s(I), wR2 = 0.1541 (all data), GOF = 0.925. Final largest
fering anions relative to the HSO4 ions. The selectivity
diffraction peak and hole: 0.192 and ꢀ0.236 e. Aꢀ3
.
coefficient pattern clearly indicates that the electrodes are
ꢀ
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selective for HSO4 over
a number of other anions.
The selectivity coefficient values are obtained for different
ꢀ
secondary anions with the proposed HSO4 ion-selective
electrode, suggesting that these anions do not interfere in the
normal working of the proposed electrode, even when present
at high concentration levels of 1.0 ꢁ 10ꢀ2 M.
ꢀ
In addition, practical application of the proposed HSO4
ion-selective electrode was tested by employing it as an indicator
electrode during the potentiometric titration of the hydrogen
sulfate ion (20.0 mL, 1.0 ꢁ 10ꢀ2 M) against sodium hydroxide
(1.0 ꢁ 10ꢀ2 M) solution. A sharp potential change near the end
point indicated that the proposed electrode could be successfully
used for monitoring potentiometric titrations (Fig. S14w).
In summary, we have designed and synthesized a new,
coumarin-basꢀed, water-compatible, fluorescence ‘turn-on’ sensor
(1) for HSO4 ions. Complexation studies by NMR, UV, and
fluorescence spectroscopy were carried out towards different
anions to demonstrate the sensor’s excellent selectivity for the
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ꢀ
HSO4 ion over other anions tested. In the DFT calculations,
ꢀ
application of the PET mechanism upon addition of HSO4
ꢀ
Table 3 Selectivity coefficients for the HSO4 ion-selective electrode
9 See Electronic Supplementary Information (ESI).
10 Association constants were calculated using the computer program
ENZFITTER, available from Elsevier-BIOSOFT, 68 Hills Road,
Cambridge, UK CB2 1LA; K. A. Connors, Binding Constants,
Wiley, New York, 1987.
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Pot
Pot
Secondary ions (B) LogK
Secondary ions (B) LogK
HSOꢀ4 ;B
HSOꢀ4 ;B
ꢀ
ꢀ
2ꢀ
CH3CO2
Fꢀ
ꢀ3.78
ꢀ2.95
ꢀ3.20
ꢀ4.00
ꢀ3.05
ꢀ3.30
ꢀ4.75
ꢀ1.00
NO2
CO3
ꢀ3.00
ꢀ3.85
ꢀ4.44
ꢀ3.90
ꢀ2.90
ꢀ2.55
ꢀ0.80
ꢀ3.50
SCNꢀꢀ
Citrate
C2O4
2ꢀ
NO3
N3
HCOOꢀ
Brꢀ
ꢀ
2ꢀ
CO3
SO4
Iꢀ
H2PO4
2ꢀ
13 R. K. Mahajan, I. Kaur, R. Kaur, A. Onimaru, S. Shinoda and
H. Tsukube, Anal. Chem., 2004, 76, 7354.
Clꢀ
ꢀ
ꢂc
This journal is The Royal Society of Chemistry 2009
7130 | Chem. Commun., 2009, 7128–7130