A highly selective naked-eye probe for hypochlorite with the p-methoxyphenol-substituted aniline compound

Article abstract of DOI:10.1016/j.tetlet.2010.09.041

A highly selective naked-eye detection of ClO^- is successfully established with probe 1 by taking advantage of the oxidation transformation of p-methoxyphenol into benzoquinone with ClO^- and the ICT absorption within the electron donor-acceptor compound. The color of the solution of probe 1 was changed, obviously upon addition of ClO^- and ClO^- with concentration as low as 1.74 μM can be analyzed in aqueous solution with probe 1. Moreover, the interferences of other anions can be neglected.

Full text of DOI:10.1016/j.tetlet.2010.09.041

Tetrahedron Letters 51 (2010) 6052–6055
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Tetrahedron Letters
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A highly selective naked-eye probe for hypochlorite with the
p-methoxyphenol-substituted aniline compound
Kai Cui a,b, Deqing Zhang a, , Guanxin Zhang a, Daoben Zhu a
⇑
a Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b Graduate School of Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
A highly selective naked-eye detection of ClOÀ is successfully established with probe 1 by taking advan-
tage of the oxidation transformation of p-methoxyphenol into benzoquinone with ClOÀ and the ICT
absorption within the electron donor–acceptor compound. The color of the solution of probe 1 was chan-
Article history:
Received 16 July 2010
Revised 11 September 2010
Accepted 14 September 2010
Available online 17 September 2010
ged, obviously upon addition of ClOÀ and ClOÀ with concentration as low as 1.74
l
M can be analyzed in
aqueous solution with probe 1. Moreover, the interferences of other anions can be neglected.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Hypochlorite
Naked-eye detection
Oxidation
Intramolecular charge-transfer
Hypochlorite anion (OClÀ) and hypochlorous acid (HOCl) are the
biologically important reactive oxygen species (ROS).1 In the phys-
iological pH solution HOCl is partially dissociated into OClÀ. In
organisms hypochlorite is generated from the reaction of H2O2
and chloride ion catalyzed by myeloperoxidase (MPO) enzyme.2
Hypochlorite possesses important antibacterial properties and
plays an essential role in the prevention of microorganism invasion.
But, abnormal level of hypochlorite can lead to tissue damage and
diseases such as atherosclerosis, arthritis, and cancers.3 This may re-
late to the fact that hypochlorite in the physiological condition can
react with DNA, RNA, fatty acids, cholesterol, and proteins. However,
the exact action mechanism of hypochlorite in these diseases is not
fully understood. Therefore, development of convenient and specific
probes for OClÀ is highly desired for investigations of the functions
of OClÀ in biological systems. Additionally, hypochlorite and hypo-
chlorous acid are widely used in our daily lives as a disinfectant
and as a bleaching agent. Simple detection methods for OClÀ and
HOCl in the environment are also needed.
in the presence of HOCl. Very recently, Ma et al.9 have designed a
selective fluorescent probe for HOCl with the ferrocene–anthra-
cene dyad. The fluorescent probes are sensitive but the signal read-
out still needs the aid of the bulk instruments. In comparison,
colorimetric probes, in particular the naked-eye detection methods
have obvious advantages. In this regard, selective naked-eye probe
for OClÀ (and HOCl) is not available, to the best of our knowledge.
Herein we want to describe a new highly selective naked-eye
probe for the detection of OClÀ with the p-methoxyphenol-substi-
tuted aniline compound (1, Scheme 1).
The molecular design rationale for this naked-eye probe 1 is illus-
trated in Scheme 1 and explained as follows: (1) it is known that
benzoquinone-substituted aniline is colorful (from purple to blue
dependent on the chemical structure and the surrounding medium)
dueto theintramolecular charge-transfer(ICT)betweenthequinone
and aniline units.10 But, the ICT absorption disappears and it be-
comes almost colorless when the quinone unit is reduced to hydro-
quinone or substituted hydroquinone; (2) p-methoxyphenol can be
Recently, several sensitive and selective probes have been
reported for the detection of OClÀ (and HOCl) by taking advantage
of the strong oxidation capacity of OClÀ (and HOCl). For instance,
the groups of Nagano,4 Ma,5 Shin6 and Libby7 have independently
reported Rhodamine (or Fluorescein)-based fluorescent probes
for OClÀ (and HOCl). Yang et al.8 have recently described a highly
selective BODIPY-based fluorescent sensor for HOCl by making
use of the transformation of p-methoxyphenol into benzoquinone
O
O
O
O
O
N
O
O
N
O
2
2
2
2
D
OCl -
ICT
⇑
Corresponding author.
E-mail address: dqzhang@iccas.ac.cn (D. Zhang).
Scheme 1. Design rationale for probe 1.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.041

K. Cui et al. / Tetrahedron Letters 51 (2010) 6052–6055
6053
oxidized by OClÀ into benzoquinone (in solutions which are not
a
highly basic) according to the previous report;8 (3) in order to
improve the solubility of probe 1 in aqueous solution glycol chains
are introduced in probe 1. The results indicate that probe 1 is
colorless and it becomes blue after the addition of ClOÀ, and thus
probe 1 can be used for the naked-eye detection of ClOÀ. The
synthetic approach for probe 1 is outlined in Scheme 2. The synthe-
sis started from the reaction of compound 2 with p-bromoaniline
leading to compound 3. Compound 4 was transformed into com-
pound 5 after sequential reaction with n-BuLi and B(OMe)3. After
Suzuki reaction of 3 with 5, and removal of MOM group, probe 1
was obtained as a pale yellow oil in 37.4% yield. The chemical
structure of probe 1 was established with 1H NMR, 13C NMR, and
HRMS spectral data.11
0.6
0.4
0.2
0.0
1.0 eq NaOCl
0 eq NaOCl
300 400 500 600 700 800
Wavelength (nm)
b
0.08
0.06
0.04
0.02
0.00
y=0.0015x-0.0004, r=0.997
Probe 1 can be dissolved in water and aqueous solutions of
probe 1 with concentrations as high as 0.5 mM can be prepared.
Figure 1a shows the absorption spectrum of probe 1 and those
after addition of different amounts of ClOÀ in PBS buffer (0.1 M,
pH 7.4). Probe 1 absorbs strongly around 310 nm and exhibits al-
most no absorption above 400 nm. However, a new absorption
around 572 nm emerged gradually and the absorption around
310 nm decreased simultaneously. Under the present condition
the reaction of probe 1 with ClOÀ was rather fast and it reached
equilibrium almost immediately after the addition of ClOÀ. Based
on the fact that the corresponding benzoquinone-substituted ani-
line exhibited typical ICT absorption around 572 nm,10 this new
absorption band was likely due to the oxidation of p-methoxyphe-
nol unit in probe 1 into the benzoquinone unit leading to the for-
mation of the corresponding benzoquinone-substituted aniline.
In fact, mass spectral signals at 494.5 (M+2H+H+) and 516.5
(M+2H+Na+), where M corresponds to the molecular weight of
the corresponding benzoquinone-substituted aniline, were de-
tected for the solution of probe 1 after the addition of ClOÀ (see
Fig. S2). In agreement with the absorption spectral change for
probe 1 after introducing ClOÀ, the colorless aqueous solution of
probe 1 became blue (see the inset of Fig. 1a). It should be noted
that the color change for probe 1 after addition of ClOÀ is depen-
dent on the concentration of probe 1; the higher the concentration
of probe 1 the larger the absorbance variation at 572 nm which
leads to more obvious color change. When the concentration of
0
10
20
30
40
50
[NaOCl]/mM
Figure 1. (a) Absorption spectra of probe 1 (50
concentrations of OClÀ in 0.1 M PBS at pH 7.4. Inset: the color change for probe 1
(400 M) after the addition of 1.0 equiv NaOCl. (b) The plot of the absorption
lM) in the presence of different
l
spectra at 572 nm versus concentration of hypochlorite tested.
ClOÀ (see Supplementary data). Figure 1b depicts the linear varia-
tion of the absorbance at 572 nm,
D
A (A À A0), where A0 and A were
the absorbance of the solution at 572 nm in the absence and pres-
ence of ClOÀ versus the concentration of ClOÀ in the solution
À
(y = 0.0015 CNaOCl 0.0004 (n = 9, r = 0.997)). The corresponding
detection limit for ClOÀ was estimated to be 1.74
lM (S/N = 3) with
probe 1. Compared to the reported detection methods5,6,9 for ClOÀ
this naked-eye detection assay is not sensitive. Further optimizing
the assay conditions and even modifying the structural design are
necessary.
The detection of ClOÀ with probe 1 was also examined in aque-
ous solutions of different pH values. Figure 2 depicts the plot of the
variation of the absorbance at 572 nm for the solution of probe 1
probe 1 is over 50
lM the color change can be distinguished for
probe 1 in 0.1 M PBS at pH 7.4 after the addition of 1.0 equiv of
(50 l lM) versus the pH value of the solution.
M) and ClOÀ (50
O
O
NH2
Br
O
N
O
2
2
Obvious absorbance change was detected for solutions with pH
values from 5 to 9 and large absorbance variation was observed
in solutions with pH 6–7. These results are understandable by
considering the following points: (1) at low pH the solution is
acidic and as a result the aniline unit may be protonated which
a
O
+
O
I
2
2
Br
3
O
O
O
O
Br
B(OH)2
b
0.10
0.08
OCH3
OCH3
4
5
0.06
O
O
1 only
1.0 eq NaOCl
O
N
O
2
2
0.04
0.02
0.00
c, d
OH
H3CO
4
5
6
7
8
9
10
1
pH
Scheme 2. The synthetic procedure for probe 1. Reagents and conditions: (a)
K2HPO4, CH3CN, reflux; (b) n-BuLi, B(OMe)3, THF, À78 °C; (c) Pd(PPh3)4, NaHCO3,
toluene/H2O, reflux; (d) CF3COOH, CH2Cl2.
Figure 2. The variation of the absorbance at 572 nm for probe 1 (50 lM) in PBS
(0.1 M) at different pH values in the absence (j) and presence (d) of 1.0 equiv of
ClOÀ.

6054
K. Cui et al. / Tetrahedron Letters 51 (2010) 6052–6055
may weaken the corresponding ICT absorption; (2) at high pH va-
lue HOCl is easily dissociated into ClOÀ and accordingly the oxida-
tion capacity of the solution is weak. This study indicates that
probe 1 can be employed for the detection of ClOÀ in aqueous solu-
tions with pH 5–9.
1 only, F-, Cl-, Br-, I-, ClO3-, ClO4-,
CO32-, SO42-, PO43-, H2PO4-, OH-.
0.6
0.4
0.2
0.0
-
The absorption spectra of probe 1 in the presence of other reac-
tive oxygen species (ROS) and reactive nitrogen specied (RNS)
OCl
Å
including H2O2, 1O2, O2ÁÀ, ROOÅ, NOÅ, and OH were measured in
PBS buffer (0.1 M) at pH 7.4. The absorption spectrum of probe 1
was almost unaltered in the presence of these ROS&RNS. Figure 3a
depicts the absorbance variation at 572 nm for probe 1 in the pres-
ence of ClOÀ (1.0 equiv vs probe 1) and other ROS&RNS (10.0 equiv
for each species vs probe 1). It is obvious that large absorbance
change was only observed for probe 1 after the addition of ClOÀ. This
is consistent with the observation that the solution of probe 1 be-
came blue only after the addition of ClOÀ as shown in Figure 3b,
where the photos of the solutions of probe 1 in the presence of ClOÀ
and other ROS&RNS species were displayed. These results demon-
strate that probe 1 is highly selective toward ClOÀ. Alternatively,
the absorption spectra of probe 1 were also measured after sequen-
tial addition of 10.0 equiv of each ROS&RNS species and 1.0 equiv of
ClOÀ. Absorbance at 572 nm increased significantly after the addi-
tion of 10.0 equiv of NOÅ from sodium nitroferricyanide (III) and
1.0 equiv of ClOÀ, but the absorbance at 572 nm was almost not
enhanced for probe 1 in the presence of 1.0 equiv of ClOÀ and
10.0 equiv of H2O2, O2ÁÀ, RCOOÅ, and HOÅ (see Supplementary data).
This is because of the fact that ClOÀ can react with H2O2, O2ÁÀ, the
precursors for RCOOÅ, and HOÅ,12,13 and as a result ClOÀ was
consumed in the solution and oxidation of p-methoxyphenol cannot
occur accordingly.
300
400
500
600
700
800
Wavelength/nm
Figure 4. Absorption spectra of probe 1 (50 lM) upon addition of NaOCl (1.0 equiv)
and other anions (10.0 equiv) in 0.1 M PBS buffer at pH 7.4.
interferences of other anions for this naked-eye detection of ClOÀ
(HOCl) can be neglected. Additionally, cations such as Fe2+, Fe3+
,
Cu2+, and Zn2+ also show no interferences for the detection of ClOÀ
with probe 1 since the absorption spectrum of probe 1 was almost
not affected by these metal ions (Fig. S1).
In summary, probe 1 was designed and studied for the selective
detection of ClOÀ by taking advantage of the oxidation transforma-
tion of p-methoxyphenol into benzoquinone with ClOÀ and the ICT
absorption within the electron donor–acceptor compound. Absorp-
tion spectral investigations clearly indicate that probe 1 is highly
selective toward ClOÀ and the interferences of other anions can
be neglected. Of interest is the color change for probe 1 upon
addition of ClOÀ. Thus, a highly selective naked-eye detection of
ClOÀ (HOCl) is successfully established with probe 1 and ClOÀ with
concentration as low as 1.74 lM that can be analyzed in aqueous
The interferences of other anions for the detection of ClOÀ (HOCl)
were also examined. For this purpose, the absorption spectrum of
probe 1 was measured separately in the presence of FÀ, ClÀ, BrÀ,
solution. It is anticipated that probe 1 can be used for practical
detection of ClOÀ by modifying the structure of probe 1 which
may further improve the sensitivity. Furthermore, instead of
aniline, p-methoxyphenol can be connected with electron donor
units which emit intrinsically and accordingly new naked-eye
and fluorescent probes for ClOÀ may be constructed.
IÀ, ClO3À, ClO4À, CO32À, SO42À, PO43À, H2PO4 and OHÀ (10.0 equiv
À
for each anion vs probe 1), respectively. For comparison the absorp-
tion spectrum of probe 1 was also recorded after the addition of
1.0 equiv of ClOÀ. As shown in Figure 4, new absorption around
572 nm was detected for probe 1 only upon addition of ClOÀ and
the absorption spectrum of probe 1 kept almost unchanged in the
presence of other anions. Therefore, it can be concluded that the
Acknowledgments
The present research was financially supported by NSFC,
Chinese Academy of Sciences, and the State Key Basic Research
Program. This work was partially supported by the NSFC-DFG joint
project (TRR61).
a
0.08
Supplementary data
0.06
0.04
0.02
Supplementary data (synthesis and characterization data of
compounds 3, 5, and 1; methods for preparation of ROS&RNS;
absorption spectra, and relevant data for the ensemble) associated
with this article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.09.041.
0.00
1
.-
1only H O
ROO. NO. .OH NaOCl
O
O
2
2
2
2
References and notes
b
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Figure 3. (a) The variation of the absorbance at 572 nm for probe 1 (50 lM) in PBS
(0.1 M) buffer at pH 7.4 after the addition of OClÀ (1.0 equiv) and other ROS&RNS
species including H2O2, 1O2, O2ÁÀ, ROOÁ, NOÅ and ÅOH (10 equiv for each species). (b)
The photos of the solutions of probe 1 (400 lM) in PBS (0.1 M) buffer at pH 7.4 from
left to right: solution of 1 and those after the addition H2O2 (10 equiv), OClÀ
(1.0 equiv), 1O2 (10 equiv), O2 (10 equiv), ROOÅ (10 equiv), NOÅ (10 equiv) and ÅOH
ÁÀ
(10 equiv), respectively.

K. Cui et al. / Tetrahedron Letters 51 (2010) 6052–6055
6055
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113.8, 112.4, 72.2, 70.9, 70.8, 68.7, 59.3, 56.0, 51.2; HRMS calcd for C27H41NO8:
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