Synthesis and sensing applications of a new fluorescent derivative of cholesterol

Article abstract of DOI:10.1039/c5nj02601f

A fluorescent 4-chloro-7-nitro-1,2,3-benzoxadiazole (NBD) derivative of cholesterol (CTN) was designed and synthesized. The compound forms a 1: 1 non-fluorescent complex with Hg²⁺ in a solvent mixture of water and ethanol. Based on this discovery, a detection method with a detection limit (DL) of ～0.08 μM for Hg²⁺ was developed. The presence of other commonly found metal ions has little effect on the detection. Though Fe²⁺ and Cu²⁺ have some interference, it could be avoided by a simple oxidation or reduction procedure. Further studies revealed that the complex can be used for the turn-on detection of some organophosphorus pesticides (OPs), such as DDVP, glyphosate, chlorpyrifos, diazinon and phoxim, either in solution or film state. The DLs for the five OPs are 0.015, 0.018, 0.087, 0.098 and 0.113 μg mL^-1 in solution, respectively. In addition, the compound as developed is interesting and attractive because it can sense the heavy metal ion and the OPs in a fast and sensitive manner. It is therefore believed that the developed method possesses the potential to be used for rapid on-site detection of hazardous chemicals.

Full text of DOI:10.1039/c5nj02601f

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Synthesis and sensing applications of a new
fluorescent derivative of cholesterol†
Cite this:DOI: 10.1039/c5nj02601f
Yanchao Lu, Qingqing Sun, Baolong Hu, Xiangli Chen, Rong Miao* and Yu Fang*
¨
A fluorescent 4-chloro-7-nitro-1,2,3-benzoxadiazole (NBD) derivative of cholesterol (CTN) was designed
2+
and synthesized. The compound forms a 1 : 1 non-fluorescent complex with Hg in a solvent mixture of
water and ethanol. Based on this discovery, a detection method with a detection limit (DL) of B0.08 mM
2+
for Hg was developed. The presence of other commonly found metal ions has little effect on the
2+
2+
detection. Though Fe and Cu have some interference, it could be avoided by a simple oxidation or
reduction procedure. Further studies revealed that the complex can be used for the turn-on detection
of some organophosphorus pesticides (OPs), such as DDVP, glyphosate, chlorpyrifos, diazinon and
Received (in Montpellier, France)
2
4th September 2015,
phoxim, either in solution or film state. The DLs for the five OPs are 0.015, 0.018, 0.087, 0.098 and
Accepted 4th December 2015
À1
0
.113 mg mL in solution, respectively. In addition, the compound as developed is interesting and
attractive because it can sense the heavy metal ion and the OPs in a fast and sensitive manner. It is
therefore believed that the developed method possesses the potential to be used for rapid on-site
detection of hazardous chemicals.
DOI: 10.1039/c5nj02601f
www.rsc.org/njc
operation, poor stability and unsatisfactory sensitivity and these
shortcomings make them inadequate for rapid on-site detection.
1
. Introduction
Organophosphorus pesticides (OPs) including phosphate esters
Compared to other commonly found methods, fluorescence
and other relevant compounds are widely used in agriculture, techniques, generally speaking, possess a number of advantages,
animal husbandry and aquaculture industry to increase the such as higher sensitivity, lower cost, and much greater probability,
19
output of the relevant production through the protection of for the design of sensing materials or methods. Therefore, great
1
–3
plants or animals from insects, fungi and other pests.
OPs efforts have been devoted to the development of new fluorescent
are not only found in soil and water sources, but also accumulate in sensors, in particular metal complex-based fluorescent sensors,
2
0–22
agricultural products, aquatic organisms, domestic animals and for the detection of OPs over the last few years.
For
poultry through the food chain. OPs are severely toxic to human example, El-Asfoury et al. prepared a luminescent Eu-complex,
and animal nervous systems, and can persist in the environment. [Eu(TAN) (Phen)(H O) ]Cl, and used it as a sensor for OPs
The pollution and harm caused by OPs have raised great concern detection in the aqueous phase, where the OPs tested include
4–6
2
2
2
2
3
over the last few decades, and their control and monitoring are Malathion, Endosulfan, Heptachlor and Chlorpyrifos. The
widely recognized as important issues for public health. Methods luminescence emission of the complex was quenched by
for their sensitive, fast and simple detection are urgently needed to Malathion and enhanced by the others. Corresponding detection
À1
satisfy their in situ, online analysis requirements.
limits (DLs) were 0.47, 1.02, 0.66 and 0.64 mmol L for the OPs,
To our knowledge, during the past few decades, a variety of respectively. Pitchumani and co-workers created a per-6-amino-
methods, including GC/MS, HPLC, immunoassay, quantum b-cyclodextrin:Eu(III) complex and studied its sensing ability for
2
4
dot-based molecularly imprinted polymers (QD-MIP), colorimetric OPs. It was reported that the emission of the complex was
7
–18
methods, and photoelectron-chemical sensors,
have been highly sensitive and selective to the presence of Fenitrothion, a
À12
developed for the detection of OPs. However, most of them typical OP pesticide, with a DL of 1 Â 10
M. As a coin always
suffer from some disadvantages, such as sophisticated instru- has two sides, the great selectivity of a method may ensure its
mentation, complicated specimen preparation, time-consuming usability in the quantitative analysis of a specific analyte within
laboratories, but, at the same time, limit its usability in the on-
site mass screening of OPs. Therefore, the development of more
powerful fluorescent sensors for OPs is still needed.
Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education),
2
+
It is well known that the determination of Hg is of great
School of Chemistry and Chemical Engineering, Shaanxi Normal University,
Xi’an 710119, P. R. China. E-mail: yfang@snnu.edu.cn
2
+
importance, because Hg is the most toxic among the heavy
metal ions and it causes prenatal brain damage, vision and
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj02601f
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2
5,26
hearing loss, and even death.
In fact, various methods for
the sensitive and selective detection of the heavy metal ions
2
7
have been reported.
Thanks to the specific interaction
2
+
between Hg and N atoms, and the fact that the triazole ring
is recognized as a potential cation and anion binding site,
different fluorescent sensors based on the combination of
triazole and fluorescent units have been created.
example, the performance of a conventional and unqualified
rhodamine-based fluorescent Hg sensor was greatly improved by
Tian and co-workers via the introduction of triazole structures.
Similarly, Wu and co-workers prepared two pyrene-appended and
2
8–32
For
Scheme 1 The structure of CTN.
2+
33
2. Experimental section
2.1. Reagents and materials
34
triazole-based fluorescent sensors. The ligands as produced
showed high selectivity for Hg over other metal ions, and the
2+
Diethanolamine (Alfa aesar, 499.0%), 4-chloro-7-nitro-1,2,3-
benzoxadiazole (Alfa aesar, 498.0%) cholesterol chloroformate
(Alfa aesar, 498.0%), and propargylamine (Alfa aesar, 499.0%)
were used directly without further purification. Dichloro-
methane, chloroform and DMF were distilled from calcium
hydride under nitrogen prior to use. All other reagents were of
analytical grade and used without further purification or treat-
ment. Water used throughout this study was acquired from a
Milli-Q reference system. All metal salts were dissolved in a
mixture solvent of ethanol and water (v/v = 10 : 1, 10 mM HEPES
buffer, pH 7.4) to obtain 0.25 mM stock solutions. The OPs were
used directly without further purification. The stock solutions
DLs of the two sensors were 10 and 15 mM, respectively. However,
2
+
for Hg sensing by the fluorescence method, the challenge
remains mainly with the slow, irreversible and less sensitive
response as well as tedious operation in testing. Compared to
pyrene, used in the aforementioned chemical sensors, NBD is
less used in sensor development, even though it possesses a
number of advantages such as visible light excitation, longer
wavelength emission, greater environmental sensitivity and
35–38
higher fluorescence quantum yield.
Based on these properties,
the combination of the NBD and triazole structures may provide
2+
novel fluorescent Hg or other heavy metal ion sensors. Recently,
À1
2+
(0.5 mg mL ) of the pesticides were prepared using ethanol as
a solvent.
Xie and co-workers reported a highly selective Hg sensor via the
39
combination of these two moieties. Inspired by the binding of
2+
OPs to heavy metals and the advantages of the Hg sensors, we
thought an effective method would be to design a special molecule,
2.2. Measurements
1
13
2+
H NMR and C-NMR measurements were conducted on a
Fourier Digital NMR spectrometer (Bruker Avance 400 MHz) at
which exhibits fluorescence change when it binds to Hg , and the
subsequent addition of OPs would recover the fluorescence change.
1
2+
room temperature, and the H NMR titration was also conducted
This well-designed molecule would facilitate both Hg and OPs
2+
3 3
with CDCl /CD OD as a solvent. For characterization studies,
however, CDCl was used instead of the mixture solvent. FTIR
3
sensing. To our knowledge, this is the first attempt at using a Hg
complex for OPs sensing.
spectra were recorded on a Bruker VERTEX 70 V infrared spectro-
meter in an attenuated total reflectance (ATR) mode. KBr pellets
were obtained by mixing a small amount of the compound with
anhydrous KBr powder. UV-Vis absorption spectra were obtained at
room temperature using a spectrophotometer (Lambda 950,
Perkin-Elmer, USA). Fluorescence emission spectra were obtained
at room temperature on a time-correlated single photon counting
Edinburgh FLS 900 fluorescence spectrometer using an excitation
wavelength of 450 nm. Fluorescence decay curves were obtained
using an Edinburgh FLS 920 machine.
In addition, a previous study by us shows that fluorescent
sensing films have obvious advantages such as little contam-
ination of the detection system, reversibility, reproducibility
4
0
and convenience for device making. Hydrophilicity is an
important factor for a fluorescence sensing film to be used in
an aqueous solution. Molecules that are very easily dissolved in
water should be avoided in a film that is aimed at being used in
water phase detection. Cholesterol (Chol), a remarkable molecule,
is sparingly soluble in water, but it dissolves well in most organic
solvents. Introduction of (Chol) into the molecules will not only
facilitate the molecular synthesis, but also regulate the hydrophility
of the molecules. Therefore, a molecule with favorable solubility
in water or organic solvent would be acquired and this would
be of benefit for further application in fluorescent sensing
3
. Results and discussion
3.1. Synthesis and characterization
film construction. Based on the aforementioned factors, a new The synthesis of the NBD derivative of chlosterol, CTN, is
fluorescent ligand CTN (Scheme 1) containing one NBD unit, two schematically shown in Scheme S1 (ESI†), and the details are
Chol residues, and two triazoles in the linkers was designed and described in the ESI.† The intermediates and the final product,
1
13
synthesized. As expected, the as-prepared ligand CTN functions CTN, were fully characterized by H NMR, C NMR, IR and ESI-MS
2
+
as an excellent fluorescence quenching probe for Hg . More- (Fig. S9–S20, ESI†).
2
+
over, the formed CTN–Hg complex was used for efficient
detection of OPs with satisfactory sensitivity and selectivity in
3.2. Absorption and fluorescence of CTN
either solution or film state. This paper reports the details of the The UV-Vis absorption of CTN was studied in ethanol, and the
investigations.
results are shown in Fig. S1 (ESI†). With reference to the traces
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shown in the figure, it is seen that CTN in the ethanol solution that the maximum emission of each solvent under study is
displays three absorption bands appearing at 227, 325 and related to its polarity, and the emission is successively red shifted
4
50 nm, respectively. Among them, the two bands appearing at along with the increasing polarity of the solvents; the shift started
longer wavelengths are the characteristic absorptions of the from 525 nm in dichloromethane to 545 nm in DMF.
NBD unit of the probe, and the one at 227 nm originates from Benefiting from the introduction of Chol units, CTN exhibits
the triazole structure. This assignment was confirmed by the good solubility in commonly found organic solvents, which
results from the absorption spectroscopy measurements of should be favorable for functionality studies. However, the
reference compounds, the intermediate 4 (Fig. S2, ESI†) and a strong aggregation tendency of NBD, as well as Chol, which is
41
NBD derivative of Chol, reported earlier. Beer–Lambert plots of well known to form stacks via van der Waals interaction in
the three absorptions as shown in the inset of Fig. S1a (ESI†) almost all known solvents, as revealed in the studies of numerous
42
disclosed the molar absorption coefficients at the three wavelengths molecular gel systems, suggest that it is highly possible for CTN to
4
50 325
227
4
À1
À1
À1
À1
(
2
e
, e
and e ), which are 2.07 (Æ0.01) Â 10 M cm
,
,
form aggregates provided its concentration is sufficiently high. To
examine the possibility, a series of THF solutions of CTN with
different concentrations were prepared and their fluorescence excita-
4
À1
À1
3
.02 (Æ0.01) Â 10 M cm and 8.29 (Æ0.01) Â 10 M cm
respectively.
The steady-state fluorescence excitation and emission spectra tion and emission spectra were recorded. As shown in Fig. 1, it is
À6
À1
of CTN in the same solution were also recorded, and the results found that at concentrations lower than 1 Â 10 mol L , the
are depicted in Fig. S1b (ESI†). As can be seen from the figure, emission profiles of NBD are characterized by two emissions: one of
the emission is broad, structure-less and appears around which appears around 425 and the other around 527 nm. In
527 nm, which is a typical NBD aggregate emission. Maximum particular, the emission could be dominated by one of the two
excitation of the system appears at 325 nm and 450 nm, which emissions as evidenced by the spectra recorded with solutions of
À7
À5
À8
À1
À1
may correspond to the electron transitions of S - S and S - very low concentrations (1 Â 10 , 1 Â 10 mol L ) and those of
0
2
0
S
1
of the NBD moieties, respectively; this result agrees well with higher concentrations (45 Â 10 mol L ). In addition, the results
the UV-Vis measurements. Considering the excitation efficiency, from concentration-dependent studies clearly demonstrate that the
50 nm was chosen as the excitation wavelength throughout the emission observed around 527 nm does originate from the aggregate
study unless otherwise specified, and the emission intensity of of NBD, a tentative conclusion described earlier. Inspection of the
the system was monitored at 527 nm. excitation spectra indicates that a great change occurs when the
4
À5
À1
compound concentration exceeds 5 Â 10 mol L , which may be a
result of the self-absorption of CTN due to the solution concentra-
tions are too high.
3.3. Solvent effect
To determine the solvent effect on the fluorescence behavior of
CTN, the emission spectra were recorded in a variety of solvents
with different polarities. The solvents employed for study
3.4. Substrate effects
include ethanol, dichloromethane, chloroform, THF, DMF, In the preparation and purification of CTN, it was found that a
pyridine, and tetrachloromethane. The emission spectra as conventional TLC plate containing the compound shows a
obtained are collectively shown in Fig. S3 (ESI†). It is seen that bright fluorescence spot when illuminated with UV light. How-
all the emissions appear around 527 nm and possess similar ever, after spin-coating the solution on a glass plate surface, no
profiles as displayed in the inset of the figure. The differences fluorescence was observed, regardless of which solvent was
among them are the intensities of the emissions and a slight used. There were similar observations when filter paper or
variation in the positions of the emissions. It is worth noting conventional writing paper was used instead of the glass plate.
Fig. 1 Fluorescence excitation and emission spectra of the THF solutions of CTN at different concentrations (lex = 450 nm; lem = 527 nm).
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Interestingly, a bright fluorescence could be observed if expanded
polystyrene (EPS) was chosen as a substrate. This observation
stimulated us to think about the practical applications of the
compound such as the detection of materials of practical signifi-
cance. As for why the emission of the compound showed such a
remarkable substrate effect, the exact reasons were not clear at the
preparation of this paper. In any case, it must be related to the
nature and the aggregation of the compound.
2
Fig. 3 Digital images of the response of CTN solution (EtOH/H O, v/v =
3
.5. Sensing behavior of CTN
10 : 1, 10 mM HEPES buffer, pH 7.4) towards different metal ions, where the
upper row was exposed to day light, but the lower row was illuminated by
UV light (365 nm, CTN 1 equiv., metal ions 5 equiv.).
The optical sensing behavior of CTN to various metal ions was
investigated in EtOH/H O (v/v = 10 : 1, 10 mM HEPES buffer,
2
pH 7.4). With reference to the structure of CTN, it is clearly seen
that the compound is composed of two triazole rings, which are Cd , Pb , and Hg , in a solvent mixture of EtOH/H O
2
+
2+
2+
2
typical chelating structures, two Chol residues and a NBD unit. (v/v = 10 : 1, 10 mM HEPES buffer, pH 7.4). As expected, upon
2
+
Furthermore, the structural units are linked by flexible introduction of the metal ions, only Hg showed significant
chemical bonds; it is the structural nature of the compound alteration of the absorption of CTN in the solvent (Fig. 2a).
that lays the foundation for its sensing applications. Fig. 2a Similarly, among the metal ions tested, the fluorescence emission
2+
2+
2+
and b show the absorption and fluorescence changes of the of CTN was severely quenched by Hg (Fig. 2b). Fe and Cu also
probe, respectively, in the presence of various metal ions, showed some quenching effect on the emission of the compound
2
+
2+
2+
2+
2+
2+
2+
2+
2+
including Mg , Ca , Ba , Mn , Fe , Co , Ni , Cu , Zn , (Fig. 2b), but their quenching efficiencies were much weaker than
2+
Hg . As for other metal ions, there was no obvious quenching
effect on the emission, even in excess.
Further inspection of Fig. 2b reveals that the maximum
emission of CTN shifted from 527 nm to 561 nm upon addition
2
+
of 5 equivalents of Hg into the solution of CTN, which is a
result of the change from a strong fluorophore into a weak
fluorophore. The selective response of CTN to the presence of
2
+
Hg could be directly visualized, as shown in Fig. 3. It is clearly
seen that the ethanol solution of free CTN possesses a light
2
+
yellow color in daylight, but the addition of Hg to the solution
immediately produces a drastic color change from light yellow
to deep orange. In contrast, no significant color change was
observed when an excess of other metal ions being tested were
added. More distinct selective tests were conducted by illuminating
the solutions with UV light (the lower row of the photographs).
2+
Clearly, sensitive and selective naked-eye detection of Hg is easily
performed.
3.6. Interference studies
Tolerance to the presence of other metal ions is a key factor to
determine if a metal ion sensor could find real-life applications.
2
+
Accordingly, the specificity of the CTN probe towards Hg was
systematically studied, and the results are shown in Fig. 4. It is
2
+
2+
seen that with the exception of Fe and Cu , the interference
2
+
from other metal ions to the turn-off sensing of Hg is
2
+
2+
negligible. The presence of Fe and Cu , however, produces
interference to the sensing largely due to the competitive
binding of CTN, as revealed in the spectroscopy studies
Fig. 4a). Considering that Fe and Cu are redox active,
reduction of Cu with Zn and oxidation of Fe with H O2
2
+
2+
(
2
+
2+
2
Fig. 2 (a) UV-Vis spectra of CTN (10 mM) in the presence of 5 equiv. of
different metal ions; (b) fluorescence emission spectra (lex = 450 nm) of
CTN (10 mM) recorded in the presence of a series of metal ions (at five
may screen their interference with the detection. Accordingly, a
further test was conducted and the results are shown in Fig. 4b.
This additional test makes the as-developed method close to
perfect for real-life applications.
equiv.), where the solvent is a mixture of EtOH and H
HEPES buffer, pH 7.4).
2
O (v/v = 10 : 1, 10 mM
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Fig. 5 (a) UV-Vis absorption spectra of the solution of CTN (10 mM) in the
2+
presence of different amounts of Hg , where the solvent used is a mixture
of EtOH and H O (v/v = 10 : 1, 10 mM HEPES buffer, pH 7.4); (b) fluores-
cence emission spectra of CTN solution in the presence of increasing
Fig. 4 (a) Fluorescent responses of the CTN sensing system to the
2+
2
competing metal ions (10 mM, black bars, a and b) and Hg (10 mM, red
bars) in the presence of each competing metal ion (10 mM) and (b) the
2+
2+
amounts (0–5 equiv.) of Hg , where the solvent is a mixture of EtOH and
O (v/v = 10 : 1, 10 mM HEPES buffer, pH 7.4). Inset: Stern–Volmer plot of
fluorescent responses after the reduction of Cu with Zn and oxidation of
2+
H
2
2 2
Fe with H O .
the quenching data.
2
+
3
.7. Composition of the CTN–Hg complex
more than 5% fluorescence quenching. The calculated detection
limit (DL) of the method is B0.08 mM, the details of the
calculation are provided in the ESI.† To our knowledge, the DL
of this method is significantly lower than the values reported in
To further investigate the binding behavior of CTN towards
2
+
Hg , UV-Vis and fluorescence titration experiments were carried
out in EtOH/H O (v/v = 10: 1, 10 mM HEPES buffer, pH 7.4).
Fig. 5a displays the gradual development of the UV-Vis absorp-
2
2
+
34
literature for other fluorescent Hg sensors; for example, He
2
+
2
+
tion spectra of CTN upon increasing Hg concentration; the
whole absorption gradually shifts to longer wavelengths along
et al. reported a NBD and crown ether-based fluorescent Hg
4
3
sensor with a DL of 0.22 mM. In addition, the sensor developed
2
+
with the increasing concentration of Hg . A Job plot revealed
in this study is unique because its response time is less than a
2
+
2
+
that CTN complexes with Hg in a 1 : 1 ratio (Fig. S4a, ESI†). In
addition, a fluorescence titration test was also conducted. Fig. 5b
shows the titration profiles of CTN in the presence of different
few seconds and the formed Hg complex of CTN is soluble,
which allows the complex to be further used for OPs sensing.
2
+
concentrations of Hg . As expected, the fluorescence emission
intensity of the probe decreases and the emission shifts to longer
3.8. Mechanism of the sensing process
2
+
wavelengths along with stepwise addition of Hg . On the other To explore the nature of the quenching, fluorescence lifetime
hand, it can be seen from the Stern–Volmer plot that the measurements were conducted, and the relevant Stern–Volmer
fluorescence quenching of the CTN sensing system is close to plots are shown in Fig. S4b (ESI†). It is seen that the lifetime-
linear at concentrations between 0 and 10 mM (the inset of based plot is almost independent of the concentration of Hg2+
Fig. 5b). Further inspection of the plot reveals that the trace and the ratios of t /t are all very close to the theoretical value of 1,
0
2
+
amount of Hg could cause a notable decrease in the fluores- indicating that the quenching is dominated by static quenching;
cence emission. For example, 0.5 mM of the Hg could induce i.e., the formation of a non-fluorescent CTN–Hg complex is
2
+
2+
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Fig. 6 Partial H NMR spectra of CTN solution in the presence of different
2
+
2+
Fig. 7 Fluorescence recovery of the system of CTN–Hg (EtOH/H O,
equivalents of Hg , where the solvent used is a mixture of CDCl
CD OD.
3
and
2
v/v = 10 : 1, 10 mM HEPES buffer, pH 7.4) upon introduction of DDVP from
3
À1
0
to 5.0 mg mL . Inset: plot of I/I
0
against the concentration of DDVP,
representing recovery of the fluorescence emission.
the nature of the process, a result in support of the tentative
conclusion from the Job plot studies.
In the studies, OPs were added to the compound solution before
2
+
2+
1
Hg , then one equivalent of Hg was added, and then two more
3
.9. H NMR titration assay
2
+
equivalents of Hg were added. The fluorescence emission of
the system was recorded after each addition of the chemicals,
and the results are depicted in Fig. S6 (ESI†). It can be seen that
the addition of OPs and the subsequent addition of equal
2
+
To explore the nature of the complexation of CNT to Hg , a
1
H NMR titration experiment was performed with CDCl3/
CD OD (10 : 1, v : v) as the deuterated solvent, and the results
3
are shown in Fig. 6. As can be seen, with increasing the ratio of
2+
amounts of Hg had little effect upon the fluorescence emission
2
+
Hg to CTN from 0 to 2, the triazole signals of H
from 7.74 ppm to 8.24 ppm and from 7.51 ppm to 8.20 ppm,
respectively. In addition, the H and H signals of the neighboring
‘CH ’’ of the two triazole units also show a downfield shift by
about 0.10 ppm. For NBD, however, no significant shift was
c d
and H shift
of the system. The emission, however, was significantly
2+
2+
quenched when more Hg was added, indicating that Hg
e
f
preferentially combines with the OPs, which explains why intro-
‘
2
2+
duction of OPs turns on the fluorescence emission of the Hg –
CTN system.
2
+
observed even in the presence of 2.0 equivalents of Hg . These
results clearly indicate that triazole is the exact unit within CTN
that directly binds Hg .
Selectivity of the method as developed for the sensing of OPs
was studied by taking commonly found organic pesticides with
no P–O and PQO structures, such as cyfluthrin, carbofuran,
carbendazim, cymperator, and some inorganic phosphates
2
+
3.10. Sensing of OPs
À
2À
(H
2
PO
4
, HPO
4
), as interferences. The results are shown in
Based on the fact that OPs possess similar structures, because Fig. S7 (ESI†). It was demonstrated that the interferences under
2
+
they contain typical bonds in which ‘‘P’’ is one of the two atoms test showed no turn on effect with the Hg –CTN system under
4
4
2+
forming them, they bind Hg in a similar way, leading to the study, even when they were in excess. Interestingly, the addition
release of the ligand, CTN, from the complex and resulting in of any of the OPs into the system with both the complex and
the ‘‘turn-on’’ of the fluorescence emission of the complex. the interferences induced significant fluorescence emission,
2
+
Based on this analysis, CTN–Hg may be used as a sensing suggesting that the method possesses excellent selectivity over
platform for the sensitive detection of OPs.
the interferences studied.
Accordingly, glyphosate, DDVP, chlorpyrifos, diazinon and
More interestingly, the test could be readily realized via
phoxim were employed as representatives of OPs, and their naked-eye observation, as demonstrated by the images shown
2
+
effect upon the emission of the non-fluorescent CTN–Hg
in Fig. 8a. Analysis of the samples shown in Fig. 7 and Fig. S5
complex was systematically investigated by titration of the (ESI†) produced some quantitative results, which are depicted
complex solution with each of the OPs. The results are shown in Fig. 8b. From the data, it is seen that the presence of only
À1
in Fig. 7 and Fig. S5 (ESI†). With reference to the figures, it is 5 mg mL of any of the OPs under study can result in more
found that a continuous and significant increase in and blue than 86% recovery of the initial intensity of the fluorescence
shift of the emission were observed upon addition of any of the emission of the ligand, CTN. In addition, the calculated DLs
five OPs tested into the complex solution, suggesting disaggre- of the method for the five OPs are 0.015, 0.018, 0.087, 0.098 and
À1
gation of the complex.
0.113 mg mL . The visualization test can be also performed
Furthermore, to confirm that the sensing of OPs is via the in the film state. To demonstrate the applicability, a TLC
2
+
2
complexation of Hg , an additional experiment was performed. plate was employed as a substrate, on which the EtOH/H O
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0
.015, 0.018, 0.087, 0.098 and 0.113 mg mL , respectively. It is
anticipated that the compound and the detection methods as
2
+
developed pave the way for efficient sensing of Hg and OPs in
real-life applications.
Acknowledgements
This study was supported by the Natural Science Foundation of
China (21273141, 21527802), the 111 project (B14041), and the
Program for Changjiang Scholars and Innovative Research
Team in University (IRT1070).
Fig. 8 (a) Pictures of the sensing solutions of different compositions
before and after addition of the analytes; (b) histograms representing the
2+
response of the complex of CTN–Hg to the presence of different OPs.
Notes and references
(
v/v = 10 : 1, 10 mM HEPES buffer, pH 7.4) solution of the CTN–
2
+
1
2
3
4
5
6
7
8
9
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1
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À2
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