Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 682–686
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Dansyl-8-aminoquinoline as a sensitive pH fluorescent probe with dual-responsive ranges in aqueous solutions Min Zhang a, Shuyu Zheng a, Liguo Ma b,1, Meili Zhao a, Lengfang Deng a, Liting Yang a, Li-Jun Ma a,⇑ a b
School of Chemistry and Environment, South China Normal University, Shipai, Guangzhou 510631, PR China College of Chemical Engineering, China University of Petroleum, Beijing 102200, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A new fluorescence probe (DAQ) for
A sensitive pH fluorescent probe (dansyl-8-aminoquinoline, DAQ) has been synthesized. The probe could selectively and sensitively respond to pH with dual-responsive ranges from 2.00 to 7.95 and from 7.95 to 10.87 in aqueous solution, as it showed pKa values of 5.73 and 8.56 under acid and basic conditions, respectively.
pH was synthesized. The probe exhibits a specific selectivity for pH in aqueous solution. The probe could sensitively respond to pH with dual-responsive ranges.
Fluorescence pH Probe 300
300 7.95
pH
200
HN
in acidic solution
150
O
2.00
S
in basic solution
O
pKa′ = 8.56
pKa = 5.73
100
200
pH
150 10.85
100 50
50 0 400
Intensity
Intensity
250
7.95
250
N
0 400
450
500 550 Wavelength/nm
600
650
450
500 550 Wavelength/nm
600
650
N
DAQ
a r t i c l e
i n f o
Article history: Received 27 June 2013 Received in revised form 26 November 2013 Accepted 8 December 2013 Available online 18 December 2013 Keywords: Dansyl group Aminoquinoline Fluorescent pH probe Dual-responsive range
a b s t r a c t A sensitive pH fluorescent probe based on dansyl group, dansyl-8-aminoquinoline (DAQ), has been synthesized. The probe showed dual-responsive ranges to pH changes, one range from 2.00 to 7.95 and another one from 7.95 to 10.87 in aqueous solution, as it showed pKa values of 5.73 and 8.56 under acid and basic conditions, respectively. Furthermore, the pH response mechanism of the probe was explored successfully by using NMR spectra. The results indicated that the responses of DAQ to pH changes should attribute to the protonation of the nitrogen atom in the dimethylamino group and deprotonation of sulfonamide group. Ó 2014 Published by Elsevier B.V.
Introduction The pH value is an important parameter in a lot of systems such as chemical reactions and biological processes, which needs to be monitored and controlled [1–3]. Currently, there are many ⇑ Corresponding author. Tel.: +86 20 39310251; fax: +86 20 39310187. E-mail addresses: [email protected] (L. Ma), [email protected] (L.-J. Ma). 1 Tel.: +86 10 89739075; fax: +86 10 89733089. 1386-1425/$ - see front matter Ó 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.saa.2013.12.062
available methods for pH measurement, such as microelectrodes [4], nuclear magnetic resonance (NMR) [5], absorption and fluorescent spectroscopy [6–9]. Among these methods, fluorescent detection is gaining much more attention recently due to the advantages including rapid response time, nondestructive testing, and excellent pH sensitivity [10,11]. Many fluorescent pH probes have been reported, and some of them widely used in analytical chemistry, physiology and bioscience [12–15]. However, the traditional fluorescent probes can only be used in either acidic or basic condition,
M. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 682–686
they could not be utilized in both acidic and basic pH ranges. Therefore, there is great demand for the development of pH fluorescent probes that can cover larger pH ranges. Dansyl group is a frequent strategy used for the construction of fluorescent chemosensors due to its characteristic photophysical properties [13,16–19]. In our previous work, a water-soluble and structurally simple fluorescent probe (dansyl acid) was synthesized, which showed sensitive response to pH changes from 7.00 to 2.00 with high selectivity in aqueous solution due to the protonation of nitrogen atom in the dimethylamino group [13]. However, it could not be utilized under basic condition, which is limited by the simply structure. In order to monitor broader range of pH changes, we developed a sensitive dansyl-based fluorescent probe, dansyl-8-aminoquinoline (DAQ). The incorporation of a sulfonamide group into the probe enable the deprotonation of the 8amino group under basic condition [20], offering potential for the monitoring of pH changes in the basic range. The experiment results showed that the fluorescent probe could display two pKa in aqueous solutions reversibly within the pH ranges from 2.00 to 7.95 and from 7.95 to 10.87. The results indicated that DAQ is a sensitive pH probe with dual-responsive ranges, and could be used under both acidic solution and basic solution. To the best of our knowledge, less attention has been paid to design a kind of fluorescence pH probe with dual-responsive ranges. Experimental Materials All chemicals were purchased from commercial sources and used without further purification.
683
anhydrous acetone, followed by the addition of NEt3 (0.1 mL). Thereafter, dansyl chloride (0.1 g, 0.37 mmol) in 5.0 mL acetone was added to the above acetone solution dropwise, and the resulting mixture was stirred for 30 min at 0 °C in an ice-water bath, and then under room temperature for 20 h. Thereafter, the solution was washed by saturated NaHCO3 solution and water until to neutral. The organic solution was filtered after drying over anhydrous MgSO4. After removing the solvent, the resulting DAQ was further purified using column chromatography on silicagel (elution with dichloromethane), resulting in 43.5% yield. The structure of DAQ was characterized with 1H NMR and ESI-MS (m/z): 377.46; calcd. for C21H19N3O2S, 376.41. A absinthe-green single crystal of DAQ was obtained from a HCl/CH3CN (1:1, v:v) solution and characterized by X-ray crystallography [21]. As shown in Fig. 1, DAQ exhibited a characteristic chair conformation with the two aryl rings being almost perpendicular to each other. Such a conformation, which is conserved for different substituted aromatic rings, could expose the nitrogen and oxygen atoms and provide properties on H+ coordination [13,17,18]. Measurements of fluorescent spectra All fluorescent experiments were carried out in buffer solution (DMSO 2%) at 25 °C. And the buffered solutions with various pH values were prepared by mixing 3.0 mM homologous sodium salt: acetate (pH = 2.00–5.37), phosphate (pH = 6.26–7.95), borate (pH = 8.45–9.06) and carbonate (pH = 9.48–10.87). Prior to fluorescence measurement, solutions were kept at room temperature for 24 h. A fixed excitation wavelength at 340 nm was used in the emission spectra. The concentration of DAQ in all the fluorescent experiments is 20.0 106 mol L1 in buffered solutions.
Instruments Mass spectra were carried out by using LCQDACAXPMAX mass spectrometer (Finnigan). 1H NMR spectra were recorded on a Varian NMR Systems 400 MHz spectrometer using TMS as the internal standard. Single-crystal X-ray intensity data were measured at room temperature (298 K) on a Bruker SMART APEX II diffractometer. All fluorescent emission spectra were recorded with a Hitachi F-2500 fluorescence spectrophotometer. UV–vis absorption spectra were determined on a Shimadzu UV-1700 spectrophotometer at room temperature. All the pH values of aqueous solutions were measured precisely with a PHS-3B digital pH meter. Synthesis and characterization of DAQ The DAQ was synthesized through a one-step reaction using dansyl chloride and 8-aminoquinoline (see Scheme 1). 8-Aminoquinoline (0.053 g, 0.37 mmol) was dispersed in 10.0 mL
4 2
+ H 3C
N
6 HN
N
acetone, NEt3
r.t.
O S O 8′
2′
0
3′
CH 3
Dansyl chloride
As shown in Fig. 2A, the fluorescent spectra of DAQ in 3.0 mM buffered solution displayed a strong characteristic emission band of dansyl group at 500 nm, which may overlap the characteristic emission of the quinoline group whose fluorescent emission was very weak [20]. With the pH value gradually decreased from 7.95 to 2.00, the fluorescent intensity of DAQ was quenched to some extent. When pH value dropped to 2.00, the fluorescent emission of DAQ was completely quenched (resulting in a 99.5% quenching, see Fig. 2A), and the green fluorescent color disappeared (see inset in Fig. 2A). The result indicated that DAQ was a sensitive fluorescent switch to pH/H+ within the pH range from 7.95 to 2.00. In addition, the quenched fluorescent intensity of DAQ could be
7
N
NH 2
O S O
Fluorescent response of DAQ to pH changes
5
3 Cl
Results and discussion
8-aminoquinoline
DAQ
Scheme 1. The synthetic route to DAQ.
6′
4′ H 3C
7′
N
CH 3
1′ Fig. 1. The X-ray crystal structure of DAQ.
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M. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 682–686
(A)
300
(B)
250
250 pH
200 150
Intensity
Intensity
300
7.95
7.95
2.00
200 150
100
100
50
50
0
0
400
450
500
550
600
650
(C)
250 pH
intensity
300
10.87
200 pKa=5.73
150
pKa'=8.60
100 50 0
400
450
500
550
600
2
650
3
4
5
6
Wavelength/nm
Wavelength/nm
7
8
9
10 11
pH
Fig. 2. Fluorescence spectra of DAQ (20.0 lM) in 3.0 mM buffer solution at various pH values (A) (downward, 7.95, 7.79, 7.29, 6.76, 6.53, 6.26, 5.37, 4.86, 4.20, 3.50, 3.00, 2.50, 2.00), (B) (downward, 7.95, 8.45, 8.67, 8.82, 9.06, 9.48, 9.67, 10.45, 10.87). (C) The variation of the peak fluorescence intensity of DAQ (500 nm) in the pH range from 2.00 to 10.87. Insets: fluorescence photographs of DAQ at pH values of 2.00 (left) and 7.95 (right), and pH 7.95 (left) and 10.87 (right), corresponding to (A) and (B), respectively. Illumination wavelength: 365 nm UV light.
recovered when the pH was re-modulated from 2.00 to 7.95. The fluorescent response to pH was probably induced by a protonation of the nitrogen atom of the dimethylamino group of DAQ [13]. As shown in Fig. 2B, the fluorescent emission band of DAQ was also quenched when the pH value increased from 7.95 to 10.87 (the intensity at 500 nm was decreased by 96.9%), while the green fluorescent color changed to colorless (see inset in Fig. 2B). This revealed that DAQ was a sensitive fluorescent switch to pH/H+ within the pH range from 7.95 to 10.87. Analysis of fluorescent intensity changes as a function of pH using the Henderson–Hasselbach-type mass action equation could yield two pKa (Fig. 2C) [22], with one pKa of 5.73, indicating the probe’s potential application in acidic organelles [13–15], and another pK a0 of 8.56, indicating the probe’s potential application in chemical systems [15]. The results indicated that the probe DAQ can respond to pH with dual-responsive range from 2.00 to 7.95 and from 7.95 to 10.87, which can expand the applicable pH ranges of the probes.
UV–vis response of DAQ to pH changes As shown in Fig. 3A, the absorption spectra of DAQ exhibited three absorption bands at 252, 273 and 334 nm in aqueous solution (pH 7.95). As the solution pHs decreased from 7.95 to 1.51, the absorbance was gradually reduced at first, and then two new bands at 236 and 298 nm began to appear. The new absorption bands were assigned to the characteristic absorption of the dansyl group due to the protonation of dimethylamino group [13,16b,19], which was consistent with the observation of the fluorescent
spectra. The result implied that the protonation of the nitrogen atom in the dimethylamino group induced the fluorescent quenching of DAQ under acidic condition. As the aqueous solution become basic (pH from 7.95 to 12.53, see Fig. 3B), the absorbance spectra of DAQ showed a hypochromicity at first and then another two new bands at 246 and 325 nm appeared, which are characteristic absorption bands of dansyl group [13,16b,20]. The blue shift of the absorption spectrum of dansyl group should be attributed to the deprotonation of the sulfonamide, which can be rationalized by a reduction of the acceptor character of the sulfonyl group [19]. To test the practical application of DAQ, the interference experiment was performed to estimate the influence caused by other ions which may be present in the systems being analyzed. The relative fluorescent intensities of DAQ in the absence or presence of an excess of Ca2+, Fe3+, K+, Mg2+, Zn2+, Al3+ and Pb2+ (100.0 lM) at pH 7.28 and pH 8.67 were shown in Fig. 4. The results indicated that all cations displayed very similar fluorescent intensities as that of the free probe itself. The results indicated that the effect of these metals on pH measurement should be negligible. Therefore, the probe DAQ has high potential for the practical detection of pH changes in aqueous solution.
The response mechanism of DAQ to pH changes The 1H NMR spectra of DAQ at acid and basic conditions were determined in DMSO-d6/D2O (8:3, v/v) (see Fig. 5). Upon the addition of HNO3, the chemical shifts of H(10 ), H(60 ) and H(80 ) in DAQ showed obvious downfield shifts (DdH10 = 0.14 ppm,
0.8
0.8
0.6
Absorbance
(B) 1.0
Absorbance
(A) 1.0
7.95 pH
0.4
1.51
0.6
7.95 pH
0.4
12.53 0.2
0.2
0.0
0.0 300
400
Wavelength
500
600
300
400
500
600
Wavelength/nm
Fig. 3. Absorption spectra of DAQ in acidic solutions (A) (downward, 7.95, 7.20, 6.53, 5.52, 4.56, 3.50, 3.04, 2.51, 2.03, 1.51) and basic solutions (B) (downward, 7.95, 8.52, 9.10, 9.56, 10.13, 10.56, 11.07, 11.53, 12.05, 12.53).
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250
250
200
200
2+
+P b
+A l 3+
n 2+ +Z
+M
+K +
on ly +C a 2+
+P b 2+
+Z
+
+K
+M
+F
on
n 2+
0
+A l 3+
0
g 2+
50
e 3+
50
ly
100
+C a 2+
100
g 2+
150
e 3+
150
+F
Intensity
(B) 300
Intensity
(A) 300
Fig. 4. Fluorescent intensity at 500 nm of DAQ (20.0 lM) in the absence or presence of 100.0 lM Ca2+, Fe3+, K+, Mg2+, Zn2+, Al3+ and Pb2+ in 3.0 mM Na2HPO4/NaH2PO4 buffer solution at pH 7.28 (A) and in 3.0 mM Na2B4O7/H3BO3 buffer solution at pH 8.67 (B).
(A)
(B) (e)
8'
8.7
8.4
(e′) 3‘/4'
(d)
6'
1'
7
4/8'
5
(d′)
(c)
(c′)
(b)
(b′)
(a)
(a′)
8.1
7.8
7.5
ppm
7.2
2.8
2.6
8.7
8.4
8.1
7.8
4' 7'
7.5
ppm
2′′/3/6
7.2
6.9
2.8
2.6
Fig. 5. (A) The 1H NMR spectra of (a) DAQ (11.92 mM), (b) DAQ–HNO3 (10:1, mol/mol), (c) DAQ–HNO3 (5:1, mol/mol), (d) DAQ–HNO3 (2:1, mol/mol), (e) DAQ–HNO3 (1:1, mol/mol); (B) the 1H NMR spectra of (a) DAQ (11.92 mM), (b) DAQ–NaOH (10:1, mol/mol), (c) DAQ–NaOH (5:1, mol/mol), (d) DAQ–NaOH (2:1, mol/mol), (e) DAQ–NaOH (1:1, mol/mol) in DMSO-d6: D2O (8:3, v/v).
Table 1 The chemical shifts of hydrogen (d, in ppm) for DAQ in acidic, neutral and basic conditions. DAQ H2 H7 H80 H4 H5 H30 H40 H70 H20 H6 H3 H60 HI0
8.73 8.34 8.34 8.28 8.20 7.53 7.53 7.53 7.53 7.47 7.39 7.14 2.68
d t t d d q q q q q t d s
DAQ–HNO3 (1:1, mol/mol)
Dd (ppm)
DAQ–NaOH (l:l, mol/mol)
Dd (ppm)
8.72 8.34 8.48 8.29 8.23 7.61 7.60 7.57 7.53 7.47 7.40 7.36 2.82
0.01 0.00 0.14 0.01 0.03 0.08 0.07 0.04 0.00 0.00 0.01 0.22 0.14
8.66 8.72 8.22 8.22 8.04 7.49 7.37 7.37 7.07 7.07 7.07 7.07 2.69
0.07 0.34 0.12 0.06 0.16 0.04 0.16 0.16 0.46 0.40 0.32 0.07 0.01
d d d d d m m m d m d q s
s s m m d t s s d d d d s
DdH60 = 0.22 ppm, and DdH80 = 0.14 ppm) respectively (Fig. 5A). The 0 large shift of H(1 ) revealed protonation of the nitrogen atom in dimethylamino group [13], which matched the observation of the fluorescent emission and UV–vis absorption spectra of DAQ to H+ in Figs. 2A and 3A. The changes of chemical shift for other protons on dansyl group possibly due to the density changes in the naphthalene ring which were influenced by the protonated nitrogen atom [13,14]. Meanwhile, there was no significant change in the chemical shifts for protons on the quinoline group (see Table 1),
implying that H+ did not coordinate with the quinoline moiety. On the other hand, the chemical shift of H(10 ) in DAQ did not changed, when sodium hydroxide was added to the solution of DAQ (see Fig. 5B), but the chemical shifts of protons on dansyl and quinoline group showed obvious changes (see Table 1), with H(7) showing a especially large shift (DdH7 = 0.34 ppm). The results suggested that the deprotonation of the sulfonamide group might occur under the basic condition, which could make DAQ nonfluorescent because of intramolecular charge transfer (ICT) effect [14].
Conclusions In summary, a fluorescent probe for pH with simple structure was synthesized, which could selectively and sensitively respond to pH changes in both acidic and basic aqueous solution. Within the dual-responsive ranges from 2.00 to 7.95 and from 7.95 to 10.87, the probe displayed a fluorescent switch to pH changes with two pKa. A pKa value was 5.73 under acidic conditions and the other pK a0 value was 8.56 under basic conditions, indicating the probe’s potential application for studying acidic organelles in living cells and pH changes in chemical systems. The mechanisms of the specific response of DAQ to pH changes were due to the protonation of the nitrogen atom in the dimethylamino group and deprotonation of the sulfonamide group in according to pH changes. With a dual-responsive property to pH changes, the probe could have great potential for the investigation of the pivotal role of H+.
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References [1] H. Izumi, T. Torigoc, H. Ishiguchi, H. Uramoto, Y. Yoshida, M. Tanabe, T. Ise, T. Murakami, T. Yoshida, M. Nomoto, K. Kohno, Cancer Treat Rev. 29 (2003) 541. [2] M. Chesler, Physiol. Rev. 83 (2003) 1183. [3] Z.Q. Hu, M. Li, M.D. Liu, W.M. Zhuang, G.K. Li, Dyes Pigments 96 (2013) 71. [4] D. Ellis, R.C. Thomas, Nature 262 (1976) 224. [5] S.J.A. Hesse, G.J.G. Ruijter, C. Dijkema, J. Visser, Biotechnology 77 (2000) 5. [6] R.G. Zhang, S.G. Kelsen, J.C. LaManna, J. Appl. Physiol. 68 (3) (1990) 1101. [7] Z. Diwu, C.S. Chen, C. Zhang, D.H. Klaubert, R.P. Haugland, Chem. Biol. 6 (1999) 411. [8] S. Charier, O. Ruel, J.B. Baudin, D. Alcor, J.F. Allemand, A. Meglio, L. Jullien, Chem. Int. Ed. 43 (2004) 4785. [9] M. Su, Y. Liu, H. Ma, Q. Ma, Z. Wang, J. Yang, M. Wang, Chem. Commun. (2001) 960–961. [10] J.P. Desvergne, A.W. Czarnik, Fluorescent chemosensors for ion and molecule recognition, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1997. [11] R.P. Haugland, Handbook of fluorescent probes and research products Eugene OR, Molecular Probes 2002. [12] R.P. Haugland, The handbook-a guide to fluorescent probes and labeling technology molecular probes, tenth ed., Eugene, OR, 2005, pp. 935. [13] L.J. Ma, W.G. Cao, J.L. Liu, D.Y. Deng, Y. Q Wu, Y.H. Yan, L.T. Yang, Sens. Actuators, B 169 (2012) 243.
[14] Z.P. Liu, C.L. Zhang, W.J. He, F. Qian, X.L. Yang, X. Gao, Z.J. Guo, New J. Chem 34 (2010) 656. [15] B. Tang, X. Liu, K.H. Xu, H. Huang, G.W. Yang, L.G. An, Chem. Commun. (2007) 3726. [16] (a) L.J. Ma, Y. Li, L. Li, J. Sun, C. Tian, Y. Wu, Chem. Commun. (2008) 6345; (b) L.J. Ma, W. Cao, J.L. Liu, M. Zhang, L.T. Yang, Sens. Actuators, B 181 (2013) 782. [17] L. Qiu, P.J. Jiang, W.J. He, C. Tu, J. Lin, Y.Z. Li, X. Gao, Z.J. Guo, Inorg. Chim. Acta 360 (2007) 431. [18] (a) J. Guy, K. Caron, S. Dufresne, S.W. Michnick, W.G. Skene, J.W. Keillor, J. Am. Chem. Soc 129 (2007) 11969; (b) M.H. Lee, H.J. Kim, S. Yoon, N. Park, J.S. Kim, Org. Lett. 10 (2) (2008) 213. [19] R. Métivier, I. Leray, B. Valeur, Photobiol. Sci. 3 (2004) 374. [20] Y. Zhang, X. Guo, W. Si, L. Jia, X. Qian, Org. Lett. 10 (3) (2008) 473. [21] Crystal data for the compound DAQ (C21H19N3O2S): Mw = 377.45, crystal dimensions: 0.22 0.20 0.18 mm3, monoclinic, space group: Pbca, a = 14.573(3) Å, b = 11.299(2) Å, c = 22.392(5) Å, a = 90° b = 90° c = 90° V = 3687.0(13) Å3, Z 8, calculated density 1.360 mg/m3, Absorption coefficient 0.197 mm1, Final R indices [I > 2r(I)] R1 = 0.0524, wR2 = 0.1244, R indices (all data) R1 = 0.1149, wR2 = 0.1578, Temperature 298(2) K. [22] pKa was calculated by the equation: pKa = pH ± lg [(Fmax F)/(F Fmin)]. R.A. Bissell, A.P. de Silva, H.Q.N. Gunaratne, P.L.M. Lynch, G.E.M. Maguire, K.R.A.S. Sandanayake, Chem. Soc. Rev. 21 (1992) 187.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Dansyl-8-aminoquinoline as a sensitive pH fluorescent probe with dual-responsive ranges in aqueous solutions Min Zhang a, Shuyu Zheng a, Liguo Ma b,1, Meili Zhao a, Lengfang Deng a, Liting Yang a, Li-Jun Ma a,⇑ a b
School of Chemistry and Environment, South China Normal University, Shipai, Guangzhou 510631, PR China College of Chemical Engineering, China University of Petroleum, Beijing 102200, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A new fluorescence probe (DAQ) for
A sensitive pH fluorescent probe (dansyl-8-aminoquinoline, DAQ) has been synthesized. The probe could selectively and sensitively respond to pH with dual-responsive ranges from 2.00 to 7.95 and from 7.95 to 10.87 in aqueous solution, as it showed pKa values of 5.73 and 8.56 under acid and basic conditions, respectively.
pH was synthesized. The probe exhibits a specific selectivity for pH in aqueous solution. The probe could sensitively respond to pH with dual-responsive ranges.
Fluorescence pH Probe 300
300 7.95
pH
200
HN
in acidic solution
150
O
2.00
S
in basic solution
O
pKa′ = 8.56
pKa = 5.73
100
200
pH
150 10.85
100 50
50 0 400
Intensity
Intensity
250
7.95
250
N
0 400
450
500 550 Wavelength/nm
600
650
450
500 550 Wavelength/nm
600
650
N
DAQ
a r t i c l e
i n f o
Article history: Received 27 June 2013 Received in revised form 26 November 2013 Accepted 8 December 2013 Available online 18 December 2013 Keywords: Dansyl group Aminoquinoline Fluorescent pH probe Dual-responsive range
a b s t r a c t A sensitive pH fluorescent probe based on dansyl group, dansyl-8-aminoquinoline (DAQ), has been synthesized. The probe showed dual-responsive ranges to pH changes, one range from 2.00 to 7.95 and another one from 7.95 to 10.87 in aqueous solution, as it showed pKa values of 5.73 and 8.56 under acid and basic conditions, respectively. Furthermore, the pH response mechanism of the probe was explored successfully by using NMR spectra. The results indicated that the responses of DAQ to pH changes should attribute to the protonation of the nitrogen atom in the dimethylamino group and deprotonation of sulfonamide group. Ó 2014 Published by Elsevier B.V.
Introduction The pH value is an important parameter in a lot of systems such as chemical reactions and biological processes, which needs to be monitored and controlled [1–3]. Currently, there are many ⇑ Corresponding author. Tel.: +86 20 39310251; fax: +86 20 39310187. E-mail addresses: [email protected] (L. Ma), [email protected] (L.-J. Ma). 1 Tel.: +86 10 89739075; fax: +86 10 89733089. 1386-1425/$ - see front matter Ó 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.saa.2013.12.062
available methods for pH measurement, such as microelectrodes [4], nuclear magnetic resonance (NMR) [5], absorption and fluorescent spectroscopy [6–9]. Among these methods, fluorescent detection is gaining much more attention recently due to the advantages including rapid response time, nondestructive testing, and excellent pH sensitivity [10,11]. Many fluorescent pH probes have been reported, and some of them widely used in analytical chemistry, physiology and bioscience [12–15]. However, the traditional fluorescent probes can only be used in either acidic or basic condition,
M. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 682–686
they could not be utilized in both acidic and basic pH ranges. Therefore, there is great demand for the development of pH fluorescent probes that can cover larger pH ranges. Dansyl group is a frequent strategy used for the construction of fluorescent chemosensors due to its characteristic photophysical properties [13,16–19]. In our previous work, a water-soluble and structurally simple fluorescent probe (dansyl acid) was synthesized, which showed sensitive response to pH changes from 7.00 to 2.00 with high selectivity in aqueous solution due to the protonation of nitrogen atom in the dimethylamino group [13]. However, it could not be utilized under basic condition, which is limited by the simply structure. In order to monitor broader range of pH changes, we developed a sensitive dansyl-based fluorescent probe, dansyl-8-aminoquinoline (DAQ). The incorporation of a sulfonamide group into the probe enable the deprotonation of the 8amino group under basic condition [20], offering potential for the monitoring of pH changes in the basic range. The experiment results showed that the fluorescent probe could display two pKa in aqueous solutions reversibly within the pH ranges from 2.00 to 7.95 and from 7.95 to 10.87. The results indicated that DAQ is a sensitive pH probe with dual-responsive ranges, and could be used under both acidic solution and basic solution. To the best of our knowledge, less attention has been paid to design a kind of fluorescence pH probe with dual-responsive ranges. Experimental Materials All chemicals were purchased from commercial sources and used without further purification.
683
anhydrous acetone, followed by the addition of NEt3 (0.1 mL). Thereafter, dansyl chloride (0.1 g, 0.37 mmol) in 5.0 mL acetone was added to the above acetone solution dropwise, and the resulting mixture was stirred for 30 min at 0 °C in an ice-water bath, and then under room temperature for 20 h. Thereafter, the solution was washed by saturated NaHCO3 solution and water until to neutral. The organic solution was filtered after drying over anhydrous MgSO4. After removing the solvent, the resulting DAQ was further purified using column chromatography on silicagel (elution with dichloromethane), resulting in 43.5% yield. The structure of DAQ was characterized with 1H NMR and ESI-MS (m/z): 377.46; calcd. for C21H19N3O2S, 376.41. A absinthe-green single crystal of DAQ was obtained from a HCl/CH3CN (1:1, v:v) solution and characterized by X-ray crystallography [21]. As shown in Fig. 1, DAQ exhibited a characteristic chair conformation with the two aryl rings being almost perpendicular to each other. Such a conformation, which is conserved for different substituted aromatic rings, could expose the nitrogen and oxygen atoms and provide properties on H+ coordination [13,17,18]. Measurements of fluorescent spectra All fluorescent experiments were carried out in buffer solution (DMSO 2%) at 25 °C. And the buffered solutions with various pH values were prepared by mixing 3.0 mM homologous sodium salt: acetate (pH = 2.00–5.37), phosphate (pH = 6.26–7.95), borate (pH = 8.45–9.06) and carbonate (pH = 9.48–10.87). Prior to fluorescence measurement, solutions were kept at room temperature for 24 h. A fixed excitation wavelength at 340 nm was used in the emission spectra. The concentration of DAQ in all the fluorescent experiments is 20.0 106 mol L1 in buffered solutions.
Instruments Mass spectra were carried out by using LCQDACAXPMAX mass spectrometer (Finnigan). 1H NMR spectra were recorded on a Varian NMR Systems 400 MHz spectrometer using TMS as the internal standard. Single-crystal X-ray intensity data were measured at room temperature (298 K) on a Bruker SMART APEX II diffractometer. All fluorescent emission spectra were recorded with a Hitachi F-2500 fluorescence spectrophotometer. UV–vis absorption spectra were determined on a Shimadzu UV-1700 spectrophotometer at room temperature. All the pH values of aqueous solutions were measured precisely with a PHS-3B digital pH meter. Synthesis and characterization of DAQ The DAQ was synthesized through a one-step reaction using dansyl chloride and 8-aminoquinoline (see Scheme 1). 8-Aminoquinoline (0.053 g, 0.37 mmol) was dispersed in 10.0 mL
4 2
+ H 3C
N
6 HN
N
acetone, NEt3
r.t.
O S O 8′
2′
0
3′
CH 3
Dansyl chloride
As shown in Fig. 2A, the fluorescent spectra of DAQ in 3.0 mM buffered solution displayed a strong characteristic emission band of dansyl group at 500 nm, which may overlap the characteristic emission of the quinoline group whose fluorescent emission was very weak [20]. With the pH value gradually decreased from 7.95 to 2.00, the fluorescent intensity of DAQ was quenched to some extent. When pH value dropped to 2.00, the fluorescent emission of DAQ was completely quenched (resulting in a 99.5% quenching, see Fig. 2A), and the green fluorescent color disappeared (see inset in Fig. 2A). The result indicated that DAQ was a sensitive fluorescent switch to pH/H+ within the pH range from 7.95 to 2.00. In addition, the quenched fluorescent intensity of DAQ could be
7
N
NH 2
O S O
Fluorescent response of DAQ to pH changes
5
3 Cl
Results and discussion
8-aminoquinoline
DAQ
Scheme 1. The synthetic route to DAQ.
6′
4′ H 3C
7′
N
CH 3
1′ Fig. 1. The X-ray crystal structure of DAQ.
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(A)
300
(B)
250
250 pH
200 150
Intensity
Intensity
300
7.95
7.95
2.00
200 150
100
100
50
50
0
0
400
450
500
550
600
650
(C)
250 pH
intensity
300
10.87
200 pKa=5.73
150
pKa'=8.60
100 50 0
400
450
500
550
600
2
650
3
4
5
6
Wavelength/nm
Wavelength/nm
7
8
9
10 11
pH
Fig. 2. Fluorescence spectra of DAQ (20.0 lM) in 3.0 mM buffer solution at various pH values (A) (downward, 7.95, 7.79, 7.29, 6.76, 6.53, 6.26, 5.37, 4.86, 4.20, 3.50, 3.00, 2.50, 2.00), (B) (downward, 7.95, 8.45, 8.67, 8.82, 9.06, 9.48, 9.67, 10.45, 10.87). (C) The variation of the peak fluorescence intensity of DAQ (500 nm) in the pH range from 2.00 to 10.87. Insets: fluorescence photographs of DAQ at pH values of 2.00 (left) and 7.95 (right), and pH 7.95 (left) and 10.87 (right), corresponding to (A) and (B), respectively. Illumination wavelength: 365 nm UV light.
recovered when the pH was re-modulated from 2.00 to 7.95. The fluorescent response to pH was probably induced by a protonation of the nitrogen atom of the dimethylamino group of DAQ [13]. As shown in Fig. 2B, the fluorescent emission band of DAQ was also quenched when the pH value increased from 7.95 to 10.87 (the intensity at 500 nm was decreased by 96.9%), while the green fluorescent color changed to colorless (see inset in Fig. 2B). This revealed that DAQ was a sensitive fluorescent switch to pH/H+ within the pH range from 7.95 to 10.87. Analysis of fluorescent intensity changes as a function of pH using the Henderson–Hasselbach-type mass action equation could yield two pKa (Fig. 2C) [22], with one pKa of 5.73, indicating the probe’s potential application in acidic organelles [13–15], and another pK a0 of 8.56, indicating the probe’s potential application in chemical systems [15]. The results indicated that the probe DAQ can respond to pH with dual-responsive range from 2.00 to 7.95 and from 7.95 to 10.87, which can expand the applicable pH ranges of the probes.
UV–vis response of DAQ to pH changes As shown in Fig. 3A, the absorption spectra of DAQ exhibited three absorption bands at 252, 273 and 334 nm in aqueous solution (pH 7.95). As the solution pHs decreased from 7.95 to 1.51, the absorbance was gradually reduced at first, and then two new bands at 236 and 298 nm began to appear. The new absorption bands were assigned to the characteristic absorption of the dansyl group due to the protonation of dimethylamino group [13,16b,19], which was consistent with the observation of the fluorescent
spectra. The result implied that the protonation of the nitrogen atom in the dimethylamino group induced the fluorescent quenching of DAQ under acidic condition. As the aqueous solution become basic (pH from 7.95 to 12.53, see Fig. 3B), the absorbance spectra of DAQ showed a hypochromicity at first and then another two new bands at 246 and 325 nm appeared, which are characteristic absorption bands of dansyl group [13,16b,20]. The blue shift of the absorption spectrum of dansyl group should be attributed to the deprotonation of the sulfonamide, which can be rationalized by a reduction of the acceptor character of the sulfonyl group [19]. To test the practical application of DAQ, the interference experiment was performed to estimate the influence caused by other ions which may be present in the systems being analyzed. The relative fluorescent intensities of DAQ in the absence or presence of an excess of Ca2+, Fe3+, K+, Mg2+, Zn2+, Al3+ and Pb2+ (100.0 lM) at pH 7.28 and pH 8.67 were shown in Fig. 4. The results indicated that all cations displayed very similar fluorescent intensities as that of the free probe itself. The results indicated that the effect of these metals on pH measurement should be negligible. Therefore, the probe DAQ has high potential for the practical detection of pH changes in aqueous solution.
The response mechanism of DAQ to pH changes The 1H NMR spectra of DAQ at acid and basic conditions were determined in DMSO-d6/D2O (8:3, v/v) (see Fig. 5). Upon the addition of HNO3, the chemical shifts of H(10 ), H(60 ) and H(80 ) in DAQ showed obvious downfield shifts (DdH10 = 0.14 ppm,
0.8
0.8
0.6
Absorbance
(B) 1.0
Absorbance
(A) 1.0
7.95 pH
0.4
1.51
0.6
7.95 pH
0.4
12.53 0.2
0.2
0.0
0.0 300
400
Wavelength
500
600
300
400
500
600
Wavelength/nm
Fig. 3. Absorption spectra of DAQ in acidic solutions (A) (downward, 7.95, 7.20, 6.53, 5.52, 4.56, 3.50, 3.04, 2.51, 2.03, 1.51) and basic solutions (B) (downward, 7.95, 8.52, 9.10, 9.56, 10.13, 10.56, 11.07, 11.53, 12.05, 12.53).
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250
250
200
200
2+
+P b
+A l 3+
n 2+ +Z
+M
+K +
on ly +C a 2+
+P b 2+
+Z
+
+K
+M
+F
on
n 2+
0
+A l 3+
0
g 2+
50
e 3+
50
ly
100
+C a 2+
100
g 2+
150
e 3+
150
+F
Intensity
(B) 300
Intensity
(A) 300
Fig. 4. Fluorescent intensity at 500 nm of DAQ (20.0 lM) in the absence or presence of 100.0 lM Ca2+, Fe3+, K+, Mg2+, Zn2+, Al3+ and Pb2+ in 3.0 mM Na2HPO4/NaH2PO4 buffer solution at pH 7.28 (A) and in 3.0 mM Na2B4O7/H3BO3 buffer solution at pH 8.67 (B).
(A)
(B) (e)
8'
8.7
8.4
(e′) 3‘/4'
(d)
6'
1'
7
4/8'
5
(d′)
(c)
(c′)
(b)
(b′)
(a)
(a′)
8.1
7.8
7.5
ppm
7.2
2.8
2.6
8.7
8.4
8.1
7.8
4' 7'
7.5
ppm
2′′/3/6
7.2
6.9
2.8
2.6
Fig. 5. (A) The 1H NMR spectra of (a) DAQ (11.92 mM), (b) DAQ–HNO3 (10:1, mol/mol), (c) DAQ–HNO3 (5:1, mol/mol), (d) DAQ–HNO3 (2:1, mol/mol), (e) DAQ–HNO3 (1:1, mol/mol); (B) the 1H NMR spectra of (a) DAQ (11.92 mM), (b) DAQ–NaOH (10:1, mol/mol), (c) DAQ–NaOH (5:1, mol/mol), (d) DAQ–NaOH (2:1, mol/mol), (e) DAQ–NaOH (1:1, mol/mol) in DMSO-d6: D2O (8:3, v/v).
Table 1 The chemical shifts of hydrogen (d, in ppm) for DAQ in acidic, neutral and basic conditions. DAQ H2 H7 H80 H4 H5 H30 H40 H70 H20 H6 H3 H60 HI0
8.73 8.34 8.34 8.28 8.20 7.53 7.53 7.53 7.53 7.47 7.39 7.14 2.68
d t t d d q q q q q t d s
DAQ–HNO3 (1:1, mol/mol)
Dd (ppm)
DAQ–NaOH (l:l, mol/mol)
Dd (ppm)
8.72 8.34 8.48 8.29 8.23 7.61 7.60 7.57 7.53 7.47 7.40 7.36 2.82
0.01 0.00 0.14 0.01 0.03 0.08 0.07 0.04 0.00 0.00 0.01 0.22 0.14
8.66 8.72 8.22 8.22 8.04 7.49 7.37 7.37 7.07 7.07 7.07 7.07 2.69
0.07 0.34 0.12 0.06 0.16 0.04 0.16 0.16 0.46 0.40 0.32 0.07 0.01
d d d d d m m m d m d q s
s s m m d t s s d d d d s
DdH60 = 0.22 ppm, and DdH80 = 0.14 ppm) respectively (Fig. 5A). The 0 large shift of H(1 ) revealed protonation of the nitrogen atom in dimethylamino group [13], which matched the observation of the fluorescent emission and UV–vis absorption spectra of DAQ to H+ in Figs. 2A and 3A. The changes of chemical shift for other protons on dansyl group possibly due to the density changes in the naphthalene ring which were influenced by the protonated nitrogen atom [13,14]. Meanwhile, there was no significant change in the chemical shifts for protons on the quinoline group (see Table 1),
implying that H+ did not coordinate with the quinoline moiety. On the other hand, the chemical shift of H(10 ) in DAQ did not changed, when sodium hydroxide was added to the solution of DAQ (see Fig. 5B), but the chemical shifts of protons on dansyl and quinoline group showed obvious changes (see Table 1), with H(7) showing a especially large shift (DdH7 = 0.34 ppm). The results suggested that the deprotonation of the sulfonamide group might occur under the basic condition, which could make DAQ nonfluorescent because of intramolecular charge transfer (ICT) effect [14].
Conclusions In summary, a fluorescent probe for pH with simple structure was synthesized, which could selectively and sensitively respond to pH changes in both acidic and basic aqueous solution. Within the dual-responsive ranges from 2.00 to 7.95 and from 7.95 to 10.87, the probe displayed a fluorescent switch to pH changes with two pKa. A pKa value was 5.73 under acidic conditions and the other pK a0 value was 8.56 under basic conditions, indicating the probe’s potential application for studying acidic organelles in living cells and pH changes in chemical systems. The mechanisms of the specific response of DAQ to pH changes were due to the protonation of the nitrogen atom in the dimethylamino group and deprotonation of the sulfonamide group in according to pH changes. With a dual-responsive property to pH changes, the probe could have great potential for the investigation of the pivotal role of H+.
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