Dual signaling of hydrazine by selective deprotection of dichlorofluorescein and resorufin acetates

Article abstract of DOI:10.1039/c3ob40091c

The highly selective chemosignaling behaviors for hydrazine by a reaction-based probe of dichlorofluorescein and resorufin acetates were investigated. Hydrazinolysis of latent dichlorofluorescein and resorufin acetate fluorochromes caused prominent chromogenic and fluorescent turn-on type signals. The probes selectively detected hydrazine in the presence of commonly encountered metal ions and anions as background. Dichlorofluorescein and resorufin acetates selectively detected hydrazine with detection limits of 9.0 × 10^-8 M and 8.2 × 10^-7 M, respectively. Furthermore, hydrazine was selectively detected over other closely related compounds, such as hydroxylamine, ethylenediamine, and ammonia. As a possible application of the acetate probes, hydrazine signaling in tap water was tested. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ob40091c

Organic &
Biomolecular Chemistry
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Dual signaling of hydrazine by selective deprotection
of dichlorofluorescein and resorufin acetates†‡
Cite this: Org. Biomol. Chem., 2013, 11,
961
2
Myung Gil Choi, Jung Ok Moon, Jihee Bae, Jung Woo Lee and Suk-Kyu Chang*
The highly selective chemosignaling behaviors for hydrazine by a reaction-based probe of dichlorofluor-
escein and resorufin acetates were investigated. Hydrazinolysis of latent dichlorofluorescein and resorufin
acetate fluorochromes caused prominent chromogenic and fluorescent turn-on type signals. The probes
selectively detected hydrazine in the presence of commonly encountered metal ions and anions as back-
Received 16th January 2013,
Accepted 26th February 2013
ground. Dichlorofluorescein and resorufin acetates selectively detected hydrazine with detection limits of
8
9
.0 × 10− M and 8.2 × 10−7 M, respectively. Furthermore, hydrazine was selectively detected over other
DOI: 10.1039/c3ob40091c
closely related compounds, such as hydroxylamine, ethylenediamine, and ammonia. As a possible appli-
cation of the acetate probes, hydrazine signaling in tap water was tested.
www.rsc.org/obc
Hydrazine is routinely determined by electrochemical analy-
Introduction
1
2
13
14
sis and chromatography, including gas chromatography,
1
5
16
The search for selective and efficient signaling and imaging HPLC, and capillary electrophoresis. Spectrophotometry
systems to detect various chemically and biologically pertinent using colored derivatives, such as p-dimethylaminobenzalde-
1
17
18
species is an important area of supramolecular research.
A
hyde and chlorosalicylaldehyde, is also used to detect
variety of sophisticated systems based on optically responsive hydrazine. Other colorimetric systems have been discussed in
1
9
molecular frameworks have been designed to selectively detect a recent report. However, few fluorometric analysis systems
2
20
or visualize target species. One common strategy is using have been reported despite their sensitivity and specificity.
analyte triggered selective transformations of latent dye deriva- Fluorescence signaling that uses disruption of the internal
2
1
tives to yield signaling fluorochromes based on versatile dyes hydrogen bonding of carbazolopyridinophane, fluorescence
of fluorescein, rhodamine, and resorufin.
Hydrazine has been used as a propellant fuel in rockets and polymers, and intramolecular charge transfer of coumarin-
spacecraft. In industrial applications, hydrazine is used as an like fluorophores have been reported.
oxygen scavenger to prevent corrosion damage in water boilers We recently reported a hydrazine-selective probe based on
3
–5
enhancement of poly(phenylene ethynylene)-type conjugated
2
2
6
23
7
24
for feed and heating systems. Hydrazine is also a key starting hydrazine-induced deprotection of levulinated coumarin. In
material in the organic synthesis of pharmaceuticals, such as this paper, new dual signaling probes were devised for hydra-
8
3
-amino-1,2,4-triazole, maleic hydrazide, and isoniazid, as well zine using selective hydrolysis of dichlorofluorescein and
as pesticides, explosives, and dyes. Hydrazine and its water solu- resorufin acetate derivatives (Scheme 1). Hydrazinolysis of
9
25
tions, however, are highly toxic to humans and animals.
phenol acetates is a well-established process,
thus we
Human occupational data and laboratory animal studies
suggest that hydrazines may cause adverse systemic health
effects including infections of the respiratory tract and
1
0
damage to the lungs, liver, and kidneys. Moreover, the
United States Environmental Protection Agency and the World
Health Organization have classified hydrazines as possible
1
1
cancer-causing environmental contaminants.
Department of Chemistry, Chung-Ang University, Seoul 156-756, Korea.
E-mail: skchang@cau.ac.kr; Fax: +82-2-825-4736; Tel: +82-2-820-5199
†
Dedicated to Professor Andrew D. Hamilton on the occasion of his 60th
birthday.
Electronic supplementary information (ESI) available: Additional chemosignal-
‡
1
13
ing behaviors of dichlorofluorescein and resorufin acetates and H and C NMR
spectra of F1, F2 and R2. See DOI: 10.1039/c3ob40091c
Scheme 1 Signaling of hydrazine by fluorescein acetate F1.
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postulated that hydrazine could readily remove the acetate
moiety of the fluorochrome to yield a deprotected analogue. As
expected, hydrazinolysis regenerated the phenolic moiety of
fluorochromes of dichlorofluorescein and resorufin, and dual
signaling was realized as in probes designed to visualize hydro-
2
6–28
gen peroxide or perborate.
Results and discussion
Signaling by dichlorofluorescein acetates (F1 and F2)
Dichlorofluorescein was converted to its acetates to construct
the latent probe (Fig. 1). Acetate and chloroacetate were tested
as protecting groups because the hydrolysis of fluorescein acet- hydrazine or representative metal ions and anions. [F1] = 5.0 × 10 M, [Hydra-
Fig. 2 UV-vis (a) and fluorescence spectra (b) of F1 and F1 in the presence of
−6
n+
n−
−4
zine] = [M ] = [A ] = 5.0 × 10 M. In a mixture of DMSO and tris buffer solu-
ates strongly depends on the substituents of the ester func-
2
9
tion (pH 8.0, 10 mM) (1 : 1, v/v). λex = 480 nm.
tion.
First, we investigated hydrazine detection by
dichlorofluorescein diacetates F1 and F2 by observing the
absorbance at 512 nm versus time. Compounds F1 and F2
exhibited hydrazine-selective signaling in 50% aqueous DMSO
(Fig. S1, ESI‡). Hydrazine hydrolyzed chloroacetate F2 almost
instantaneously resulting in a constant signal. However, spon-
taneous hydrolysis of probe F2 was also significant. On the
other hand, the diacetate F1 selectively detected hydrazine
within 10 min, and the stability of F1 was sufficient for hydra-
zine signaling. Therefore, the hydrazine signaling behaviors
were further investigated using relatively optimized structure
F1 in terms of signaling speed and stability of the probe itself.
In the UV-vis spectrum of F1, there was no appreciable
absorption above 450 nm in 50% aqueous DMSO (Fig. 2a).
1
Fig. 3 Partial H NMR spectra of F1 in the absence and presence of hydrazine.
−2
−2
Treating F1 with 100 equiv. of hydrazine created a strong [F1] = [DCF] = 1.0 × 10 M, [Hydrazine] = 5.0 × 10 M in D
2 6
O–DMSO-d
(1 : 1, v/v). DCF: 2’,7’-dichlorofluorescein.
absorption band at 512 nm, characteristic of 2′,7′-dichloro-
fluorescein, with a concomitant change in color from colorless
to greenish yellow (inset of Fig. 2a). The absorbance at 512 nm
changed almost 250-fold in the presence of hydrazine. Conver- (Fig. 2b). Treatment with 100 equiv. of hydrazine created a pro-
sely, various metal ions and anions did not affect the absorp- minent emission at 534 nm due to the ring-opened fluorescein
tion spectrum. The absorbance ratio A/A at 512 nm ranged structure. Concomitantly, the fluorescence color of the solu-
o
+
2+
from 0.78 (Li ) to 2.8 (Cu ) for metal ions and from 0.99 tion changed from dark to green under UV illumination
4
−
−
(P
2
O
7
3
) to 1.9 (N ) for anions (Fig. S2, ESI‡). This indicates (inset of Fig. 2b). The fluorescence intensity ratio of F1 at
that under signaling conditions, the possible interference of 534 nm (I/I ) in the presence and absence of hydrazine was
o
ester hydrolysis assisted by common metal ions or anions is larger than 920 (Fig. S3, ESI‡). Other metal ions and anions
not a practical problem in hydrazine sensing by F1.
The fluorescence detection of hydrazine by F1 was also pro- between 0.66 (Fe ) and 3.6 (Co ) for metal ions and between
showed relatively small and constant I/I ratios at 534 nm,
o
3
+
2+
nounced. Diacetate F1 showed very weak emission over 0.89 (SO3 ) and 2.9 (N3−) for anions.
2
−
4
90 nm due to the closed spirolactone form of fluorescein
Hydrazine was detected by hydrazinolysis of the phenol
acetate moiety of F1 (Scheme 1). This reaction generated 2′,7′-
dichlorofluorescein, which exhibited its characteristic chromo-
genic and fluorescent behaviors. The F1 transformation to
1
2
′,7′-dichlorofluorescein was evidenced by H NMR, UV-vis,
1
and fluorescence measurements. The H NMR spectrum of the
product obtained by treating F1 with 5 equiv. of hydrazine in
6
50% deuterated aqueous DMSO-d was identical to that of
2
′,7′-dichlorofluorescein in the presence of the same amount
of hydrazine (Fig. 3). Treating F1 with hydrazine shifted the
singlet resonances for 1′,8′-protons and 4′,5′-protons at 7.52
and 6.99 ppm to 6.74 and 6.24 ppm, respectively. Concomi-
tantly, resonance for the methyl protons of acetate at 2.31 ppm
Fig. 1 Structures of dichlorofluorescein and resorufin acetates used for a
hydrazine probe.
2
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Fig. 4 Concentration-dependence of hydrazine detection by F1. [F1] = 5.0 ×
Fig. 5 Time-dependent changes in absorbance of F1 at 512 nm in the pres-
−6
M, [Hydrazine] = from 0 to 2.5 × 10−5 M, in a mixture of DMSO and
1
0
−6
ence of hydrazine and related compounds. [F1] = 5.0 × 10 M, [Hydrazine] =
tris buffer solution (pH 8.0, 10 mM) (1 : 1, v/v). λex = 480 nm.
[
Hydroxylamine] = [Ammonia] = [Ethylenediamine] = [Acetylhydrazine] = 5.0 ×
−4
1
0
M in a mixture of DMSO and tris buffer solution (pH = 8.0, 10 mM)
(1 : 1, v/v).
disappeared. Another resonance at 1.73 ppm developed and is
ascribed to the methyl protons of the acetylhydrazine bypro-
duct (Fig. S4, ESI‡). In addition to this, the UV-vis and fluor-
escence spectra of F1 in the presence of 20 equiv. of hydrazine
were identical to those of 2′,7′-dichlorofluorescein (Fig. S5 and
S6, ESI‡). An interesting observation is that acetylhydrazine,
the product of the reaction between F1 and hydrazine, was not
able to cleave the acetate group under the experimental con-
ditions (Fig. S7, ESI‡).
Diacetate F1 selectively detected hydrazine in the presence
of common metal ions and anions (Fig. S8, ESI‡). With metal
ions or anions as background, the ratio of fluorescence signal-
ing of F1 at 534 nm, I(F1+Hydrazine+ions)/I(F1+Hydrazine), ranged
2
+
between 0.93 and 1.02. Only Cu ions significantly reduced
signaling (I(F1+Hydrazine+Cu(II))/I(F1+Hydrazine) at 534 nm = 0.73),
2
+
Fig. 6 UV-vis (a) and fluorescence spectra (b) of R1 and R1 in the presence of
which might be due to the interaction of Cu ions with
−6
hydrazine or representative metal ions and anions. [R1] = 5.0 × 10 M, [Hydra-
3
0
hydrazine.
n+
n−
−4
zine] = [M ] = [A ] = 5.0 × 10 M. In a mixture of DMSO and tris buffer
solution (pH 8.0, 10 mM) (1 : 1, v/v). λex = 492 nm.
The concentration dependence of hydrazine detection by F1
was assessed by fluorescence titration (Fig. 4). As shown in the
inset of Fig. 4, the fluorescence intensity of diacetate F1 at
5
34 nm was hydrazine concentration-dependent. The relation- chloroacetate R2 derivatives of resorufin were prepared and
−
5
ship was nearly linear up to 2.5 × 10 M hydrazine. The detec- their signaling behavior was tested. As expected, spontaneous
tion limit of F1 for hydrazine was estimated by three times of hydrolysis of chloroacetate R2 was much faster than acetate R1
3
1
−8
standard deviation of the blank signal, and was 9.0 × 10
2.9 ppb), which is sensitive enough for common chemical hydrolysis, the hydrolysis observed at 30 min was minor and
and industrial uses.
did not significantly affect hydrazine detection. Therefore, the
M
(Fig. S9, ESI‡). In fact, while R1 exhibited some spontaneous
(
The hydrazine selectivity of F1 over other closely related signaling behavior toward hydrazine was investigated using
compounds was measured (Fig. 5). Hydroxylamine, one of the the relatively optimized structure R1 in terms of probe stability
most significant materials that interferes with hydrazine analy- and signaling speed.
1
5
sis, was tested, along with ethylenediamine and ammonia.
Chromogenic signaling of R1 was monitored by UV-vis
Only hydroxylamine interfered considerably with hydrazine measurements (Fig. 6a). The acetate derivative R1 had a
detection by probe F1. The absorbance ratio at 512 nm, maximum absorption at 453 nm in 50% aqueous DMSO (tris
A
hydroxylamine/Ahydrazine, was 0.18 for hydroxylamine, 0.05 for buffered at pH 8.0). Upon interaction with hydrazine, a pro-
ethylenediamine, 0.003 for ammonia, and 0.003 for nounced chromogenic behavior characteristic of resorufin was
acetylhydrazine.
observed with a color change from yellow to pink, which is
naked-eye detectable (inset of Fig. 6a). That is due to the hydra-
zine-assisted deprotection of the acetate of R1 to resorufin.
Signaling by resorufin acetates (R1 and R2)
Hydrazine was also selectively detected by the hydrolysis of an Other metal ions and anions caused almost no change in the
acetate probe based on another versatile fluorophore, resorufin R1 absorption profile (Fig. S10, ESI‡). On the other hand,
(
Fig. 1). As in the case of fluorescein, acetate R1 and similar to other hydroxyl-protected resorufin derivatives, the
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and resorufin. Pronounced chromogenic and fluorogenic sig-
naling of hydrazine was possible with detection limits of 9.0 ×
−8
−7
10
M for 2′,7′-dichlorofluorescein and 8.2 × 10 M for resoru-
fin, respectively. Both probes were selective for hydrazine over
other common metal ions and anions. Furthermore, hydrazine
was selectively detected over other closely related compounds,
such as hydroxylamine, ethylenediamine, and ammonia. The
probes could be applied to colorimetric sensing of hydrazine
in various laboratory and industrial applications.
Experimental section
General
Fig. 7 Fluorescence detection of hydrazine in distilled water and tap water by
−6
−5
F1. [F1] = 5.0 × 10 M, [Hydrazine] = from 0 to 2.0 × 10 M, in a mixture of
DMSO and tris buffer solution (pH = 8.0, 10 mM) (1 : 1, v/v). λex = 480 nm.
2
′,7′-Dichlorofluorescein, resorufin sodium salt, acetyl chlor-
ide, and chloroacetyl chloride were purchased from Aldrich
Chemical Co. Acetylhydrazine, hydroxylamine, ammonia
water, and ethylenediamine were obtained from TCI. All sol-
vents used for spectroscopic measurements were purchased
fluorescence spectrum of R1 was weak and emission above
5
95 nm was not noticeable in 50% aqueous DMSO (tris
buffered at pH 8.0). Treating R1 with 100 equiv. hydrazine
enhanced the fluorescence intensity at 595 nm by 47-fold
1
from Aldrich Chemical Co. as ‘spectroscopic grade’. H NMR
(Fig. 6b). Concomitantly, the fluorescence color of the solution
13
(
600 MHz) and C NMR (150 MHz) spectra were obtained on a
under UV illumination changed from dark to pink (inset of
Fig. 6b). Responses of other coexisting metal ions and anions
were negligible (Fig. S11, ESI‡).
UV-vis titration of R1 with hydrazine was performed in 50%
aqueous DMSO (pH 8.0 buffer, 10 mM Tris). As the hydrazine
concentration increased, the absorbance of R1 at 453 nm stea-
dily decreased, while the absorbance at 583 nm increased
Varian VNS spectrometer and referenced to the residual
solvent signal. UV-vis spectra were recorded using a Jasco
V-550 spectrophotometer equipped with a Peltier temperature
controller. Fluorescence spectra were measured with a PTI
QuantaMaster steady-state spectrofluorometer. Mass spectra
were obtained on a Micromass Autospec mass spectrometer.
Column chromatography was performed with silica gel
(Fig. S12, ESI‡). Thus, the concentration dependence of R1 sig-
(
240 mesh).
naling was analyzed by ratiometry using the ratio of the absor-
bance at 453 and 583 nm. A plot of A583/A453 as a function of
the hydrazine concentration suggests that it could be used as a
Preparation of F1 and F2
calibration curve to determine the hydrazine concentration, up Acetyl chloride (0.21 mL, 3.0 mmol) or chloroacetyl chloride
to 3.0 × 10− M. The detection limit of R1 for hydrazine was (0.24 mL, 3.0 mmol) was added to a suspension of 2′,7′-
5
−
7
31
estimated to be 8.2 × 10 M, which is comparable to the dichlorofluorescein (0.40 g, 1.0 mmol) and cesium carbonate
dichlorofluorescein based probe, F1.
(0.98 g, 3.0 mmol) in DMF (10 mL). The reaction mixture was
stirred at room temperature for 1 h, diluted with water, and
extracted with dichloromethane. The organic phase was sepa-
rated and washed with 0.1 N NaOH solution and water, and
then evaporated to obtain a white solid residue. The product
Hydrazine detection in tap water
Fluorescein-based probe F1 was used to analyze hydrazine in
aqueous solution. Because hydrazine is a suspected carcinogen
and is widely used in various industrial processes, hydrazine
detection in aqueous samples is of interest. We analyzed
hydrazine in tap water and distilled water. An aliquot of hydra-
zine was added to water and the recoveries obtained by F1
signals were compared in tap water and distilled water (Fig. 7).
The analysis of hydrazine in both solutions agreed well at
was purified by column chromatography (silica gel, CH Cl ).
2
2
1
F1: Yield, 90%; H NMR (600 MHz, CDCl
.6 and 0.8 Hz, 1H), 7.77–7.72 (m, 1H), 7.71–7.67 (m, 1H), 7.20
dd, J = 7.7 and 0.9 Hz, 1H), 7.15 (d, J = 0.4 Hz, 2H), 6.87 (d, J =
3
) δ 8.07 (dd, J =
7
(
0
1
3
.4 Hz, 2H), 2.36 (s, 6H); C NMR (150 MHz, CDCl
3
) δ 168.5,
167.9, 151.8, 149.7, 148.4, 135.8, 130.6, 128.9, 125.7, 125.6,
−5
124.0, 122.7, 117.6, 112.7, 80.4, 20.6; HRMS (FAB); m/z calcd
hydrazine concentrations up to 2.0 × 10 M.
+
for C24
Calcd for C24
.92.
F2: Yield, 85%; H NMR (600 MHz, CDCl
H
15Cl
2
O
7
[M + H] : 485.0189, found 485.0193. Anal.
H
14Cl O : C, 59.40; H, 2.91. Found: C, 59.44; H,
2 7
2
Conclusions
1
3
) δ 8.09 (d, J =
We developed new, simple structured probes to detect hydra- 7.5 Hz, 1H), 7.77 (td, J = 7.5 and 1.2 Hz, 1H), 7.72 (td, J = 7.5
zine in aqueous environments and investigated their appli- and 1.0 Hz, 1H), 7.25–7.18 (m, 3H), 6.90 (d, J = 0.4 Hz, 1H),
1
3
3
cations as colorimetric probes. Signaling was effected by 4.37 (s, 4H); C NMR (150 MHz, CDCl ) δ 168.4, 164.5, 151.7,
hydrazinolysis of the phenol acetate moiety to its phenolic ana- 149.6, 147.8, 135.9, 130.8, 129.1, 125.8, 125.5, 123.9, 122.4,
logue. The fluorophores used were 2′,7′-dichlorofluorescein 118.3, 112.5, 80.1, 40.3; HRMS (FAB); m/z calcd for
2
964 | Org. Biomol. Chem., 2013, 11, 2961–2965
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+
C
24
H
13Cl
4
O
7
[M + H] : 552.9410, found 552.9422. Anal. Calcd
6 S. D. Zelnick, D. R. Mattie and P. C. Stepaniak, Aviat. Space
Environ. Med., 2003, 74, 1285.
for C H Cl O : C, 52.02; H, 2.18. Found: C, 52.25; H, 2.26.
2
4
12
4 7
7
I. C. Vieira, K. O. Lupetti and O. Fatibello-Filho, Anal. Lett.,
002, 35, 2221.
Preparation of R1
2
8
9
U. Ragnarsson, Chem. Soc. Rev., 2001, 30, 205.
W. C. Keller, Aviat. Space Environ. Med., 1988, 59, A100.
Resorufin acetate R1 was prepared following the literature
procedure.
3
2
10 A. Umar, M. M. Rahman, S. H. Kim and Y.-B. Hahn, Chem.
Preparation of R2
Commun., 2008, 166.
1
1
1 G. Choudhary and H. Hansen, Chemosphere, 1998, 37, 801.
2 C. Batchelor-McAuley, C. E. Banks, A. O. Simm,
T. G. J. Jones and R. G. Compton, Analyst, 2006, 131, 106.
Resorufin chloroacetate R2 was similarly prepared by reacting
resorufin sodium salt (0.24 g, 1.0 mmol) with chloroacetyl
chloride (0.24 mL, 3.0 mmol). The product was purified by
1
13 D. P. Elder, D. Snodin and A. Teasdale, J. Pharm. Biomed.
Anal., 2011, 54, 900.
column chromatography (silica gel, CH
2
Cl
2
). Yield, 80%;
H
NMR (600 MHz, CDCl ) δ 7.82 (d, J = 8.7 Hz, 1H), 7.43 (d, J =
3
14 M. Sun, L. Bai and D. Q. Lui, J. Pharm. Biomed. Anal., 2009,
9
2
2
1
1
.8 Hz, 1H), 7.20 (d, J = 2.2 Hz, 1H), 7.17 (dd, J = 8.7 and
49, 529.
.5 Hz, 1H), 6.86 (dd, J = 9.8 and 2.0 Hz, 1H), 6.33 (d, J =
1
3
15 H. Bhutani, S. Singh, S. Vir, K. K. Bhutani, R. Kumar,
.0 Hz, 1H), 4.34 (s, 2H); C NMR (150 MHz, CDCl ) δ 186.2,
3
A. K. Chakraborti and K. C. Jindal, J. Pharm. Biomed. Anal.,
65.1, 152.7, 149.1, 148.7, 144.3, 135.3, 134.8, 131.5, 131.3,
2007, 43, 1213.
18.7, 109.4, 107.4, 40.7; HRMS (FAB); m/z calcd for
+
16 J. Liu, W. Zhou, T. You, F. Li, E. Wang and S. Dong, Anal.
Chem., 1996, 68, 3350.
C H ClNO [M + H] : 290.0215, found 290.0220. Anal. Calcd
1
4
9
4
for C14
H
8
4
ClNO : C, 58.05; H, 2.78; N, 4.84. Found: C, 58.27; H,
1
1
1
7 M. George, K. S. Nagaraja and N. Balasubramanian,
2
.81; N, 4.74.
Talanta, 2008, 75, 27.
8 X. Chen, Y. Xiang, Z. Li and A. Tong, Anal. Chim. Acta,
Analysis of hydrazine in tap water
2008, 625, 41.
A stock solution of hydrazine (1.0 mM) was prepared in tap
9 M. George, K. S. Nagaraja and N. Balasubramanian, Anal.
Lett., 2007, 40, 2597.
0 A. A. Ensafi and B. Rezaei, Talanta, 1998, 47, 645.
1 A. B. Brown, T. L. Gibson, J. C. Baum, T. Ren and
T. M. Smith, Sens. Actuators, B, 2005, 110, 8.
−
4
water. Stock solutions of F1 (5.0 × 10 M) in DMSO and tris
buffer (1.0 M in tap water) were also prepared. A calculated
amount of hydrazine, F1, and tris buffer was added to a vial
and the resulting solution was diluted to 3.0 mL with distilled
water and DMSO to make a final composition of 1 : 1, v/v. The
2
2
2
2
2 S. W. Thomas III and T. M. Swager, Adv. Mater., 2006, 18,
−
6
final F1 and tris buffer concentrations were 5.0 × 10 M and
1047.
2
1
.0 × 10− M, respectively. All measurements were performed
3 J. Fan, W. Sun, M. Hu, J. Cao, G. Cheng, H. Dong,
K. Song, Y. Liu, S. Sun and X. Peng, Chem. Commun.,
in triplicate.
2012, 48, 8117.
2
2
2
2
4 M. G. Choi, J. Hwang, J. O. Moon, J. Sung and S.-K. Chang,
Org. Lett., 2011, 13, 5260.
5 T. C. Bruice and S. J. Benkovic, J. Am. Chem. Soc., 1964, 86,
Acknowledgements
This work was supported by a fund from the Korea Research
Foundation of the Korean government (2012-0002403).
418.
6 M. C. Y. Chang, A. Pralle, E. Y. Isacoff and C. J. Chang,
J. Am. Chem. Soc., 2004, 126, 15392.
7 B. C. Dickinson and C. J. Chang, J. Am. Chem. Soc., 2008,
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