A highly selective and naked-eye sensor for Hg2+ based on quinazoline-4(3H)-thione

Article abstract of DOI:10.1039/c2nj40400a

A highly selective colorimetric and fluorescent sensor for Hg²⁺, TPS (TPS = 2-(4-(diphenyl-amino)phenyl)quinazoline-4(3H)-thione), based on a 2-substituted quinazoline-4(3H)-thione derivative, was designed and characterized in this paper. TPS displays relatively strong fluorescence centered at about 468 nm. When mixing with Hg²⁺, TPS interacts with Hg²⁺ in a 2:1(TPS-Hg²⁺) stoichiometry via a coordination bond interaction between the sulfur atom and Hg²⁺ with an association constant of 4.17 × 10⁸ M^-2 (R² = 0.96). Upon addition of Hg²⁺, TPS showed a remarkable decrease in fluorescence spectra (468 nm) with a clear color change from yellow to red, so TPS could serve as a naked-eye sensor for Hg²⁺. The sensor allow determination of Hg²⁺ in the working range 2.0 μM-12.0 μM with a detection limit of 1.5 μM. Upon addition of KI to the solution of TPS-Hg²⁺ species, the spectra can be restored to the original spectrum in both absorption and fluorescence and indicates that this sensor could be reused. The sensor exhibited excellent reproducibility, reversibility and selectivity.

Full text of DOI:10.1039/c2nj40400a

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PAPER
A highly selective and naked-eye sensor for Hg2+ based on
quinazoline-4(3H)-thionew
Qunbo Mei,*a Lingxia Wang,a Bo Tian,a Fang Yan,a Bin Zhang,a Wei Huanga
and Bihai Tong*b
Received (in Montpellier, France) 17th May 2012, Accepted 21st June 2012
DOI: 10.1039/c2nj40400a
A highly selective colorimetric and fluorescent sensor for Hg2+, TPS
(TPS = 2-(4-(diphenyl-amino)phenyl)quinazoline-4(3H)-thione), based on a 2-substituted
quinazoline-4(3H)-thione derivative, was designed and characterized in this paper. TPS displays
relatively strong fluorescence centered at about 468 nm. When mixing with Hg2+, TPS interacts
with Hg2+ in a 2 : 1(TPS-Hg2+) stoichiometry via a coordination bond interaction between the
sulfur atom and Hg2+ with an association constant of 4.17 Â 108 MÀ2 (R2 = 0.96).
Upon addition of Hg2+, TPS showed a remarkable decrease in fluorescence spectra (468 nm)
with a clear color change from yellow to red, so TPS could serve as a naked-eye sensor for Hg2+
The sensor allow determination of Hg2+ in the working range 2.0 mM–12.0 mM with a detection
limit of 1.5 mM. Upon addition of KI to the solution of TPS-Hg2+ species, the spectra can be
restored to the original spectrum in both absorption and fluorescence and indicates that this
sensor could be reused. The sensor exhibited excellent reproducibility, reversibility and selectivity.
.
sensor for the determination of mercuric ions has been of
particular interest.6 So far, a number of fluorescence Hg2+
sensors, such as small molecules,7 metal complexes8 and polymers9
have been reported. In recent years, small-molecule-based Hg2+
fluorescent sensors have been focused on because advantages such
as simple operation, high sensitivity, high selectivity, low cost, and
the fact that they do not require sophisticated instruments.10
Current research on Hg2+ sensors based on thione is almost
focused on the desulfurization reaction of carbonyl sulfide by take
advantage of the high affinity of Hg2+ for sulfur.10g,11 Chen12
reported two colorimetric sensors containing both a donor
1,3-dithiole ring and an acceptor carbonyl group or thiocarboyl
group for naked eye detection of Hg2+. The significant color
changes by the addition of the corresponding metal ions makes
them excellent naked eye sensors. Herein, we report a highly
selective fluorescent sensor based on 2-(4-(diphenylamino)-
phenyl)quinazoline-4(3H)-thione (TPS). Upon addition of Hg2+
to the solution of TPS, TPS interacts with Hg2+ in a 2 : 1
(TPS-Hg2+) stoichiometry via a coordination bond interaction
between the sulfur atom and Hg2+. The sensor shows a satisfactory
selective, sensitive, and reversible color and fluorescence
Introduction
Mercury is one of the most dangerous metal elements to
humans and the environment, and exists in nature mainly in three
forms: elemental, inorganic and organic mercury. Generally, the
toxicity of organic mercury is greater than metallic mercury and
inorganic mercury compounds, while elemental and mercury ions
can be converted into organic mercury. Various forms of mercury
and its compounds have high chemical activity. They can associate
strongly with thiol, carboxyl and phosphate in biological bodies
and cause great damage to human health. Because of the high
toxicity and serious effects on human health and the environment
resulting from mercury contamination, the detection of Hg2+ has
been attracting a significant amount of attention.
In the past few years, many analytical methods, including
atomic absorption spectrometry,1 atomic emission spectrometry,2
electrochemical methods3 and fluorescence chemosensors4 etc.
have been applied to detect the presence of Hg2+. Among these
methods, fluorescence chemosensors are widely regarded as one of
the most effective ways due to the advantage of their simplicity.5
The development of highly selective and low cost fluorescent
change response to Hg2+
.
a Key Laboratory for Organic Electronics and Information Displays
(KLOEID), Institute of Advanced Materials (IAM),
Nanjing University of Posts and Telecommunications,
Nanjing 210046, China. E-mail: iamqbmei@njupt.edu.cn
b College of Metallurgy and Resources, Anhui University of
Technology, Ma’anshan, Anhui 243002, P. R. China.
E-mail: tongbihai@ahut.edu.cn
Results and discussion
The synthetic route for TPS is shown in Scheme 1. 2-(4-(diphenyl-
amino)phenyl)quinazoline-4(3H)-thione (TPS) was prepared from
2-(4-(diphenylamino)phenyl)quinazolin-4(3H)-one (TPO) following
a reported procedure.13 TPO reacted with Lawesson’s reagent
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nj40400a
c
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To examine the sensitivity of TPS, the fluorescence emission
spectra of TPS in THF (2.0 Â 10À5 M, lex = 370 nm) in
the presence of increasing amount of Hg2+ (0–2.0 equiv.)
predissolved in acetonitrile were recorded and are shown in
Fig. 2. TPS displays relatively strong fluorescence centered at
about 468 nm in the absence of Hg2+ with lex = 370 nm.
Upon addition of Hg2+, a gradual decrease in the intensity of
fluorescence was observed. The fluorescence titration profile of
TPS versus Hg2+ revealed that the minimum of emission from
TPS was obtained when 40 mM Hg2+ was added to the
solution (Fig. 2, inset). The sensitivity curve indicated that
the sensor TPS, maintained at a concentration of 20 mM, can
be used for the analysis of micromolar concentrations of
Hg2+. The linear response of the fluorescence emission intensity
toward [Hg2+] was obtained in Hg2+ concentration range of
2 Â 10À6 to 12 Â 10À6 M (Fig. S1w), indicating that the sensor
Scheme 1 The synthetic route to TPS.
in dry toluene, and the resultant was purified by column
chromatography with silica gel. Compound TPS was obtained
as a yellow solid with a yield of 65.2%. The structure of TPO
and TPS were identified by NMR spectroscopy and mass
spectrometry (MS).
TPS can quantitatively detect relevant concentrations of Hg2+
.
The linear equation was found to be y = À19.41x + 344.47
(R = À0.99), where y is the fluorescence at 468 nm measured at
a given Hg2+ concentration and x is the concentration of Hg2+
added. The detection limit for Hg2+ was calculated to be
1.5 mM,14 indicating that the sensor TPS is potentially useful
for the quantitative determination of Hg2+ concentration. The
fluorescence decrease was very rapid and reached the minimum
within 1 s upon incubation of TPS with 2.0 equiv. of Hg2+. The
‘on–off’-type weakening in fluorescence may result from the
coordination reaction depicted in Scheme 2. It is well known
that many heavy transition metal (HTM) ions are typical
fluorescence quenchers due to their paramagnetic nature and
the heavy atom effect.15
Fig. 1 shows the absorption spectra of TPS in THF containing
various concentrations of Hg2+ at room temperature. The
absorption spectra of TPS in the absence of Hg2+ has a peak
with a maximum at about 366 nm, which corresponds to the
heterocyclic quinazoline-based p–p* transition, while the triaryl-
amine-based p–p* transition was found centered at 279 nm.
Addition of increasing amounts of Hg2+ (0–2.0 equiv.),
predissolved in acetonitrile, to the solution of TPS caused
some changes in its absorption spectra. The absorbance at
366 nm decreased gradually, whereas a new band at 468 nm,
which can be attributed to the metal–ligand (ML) transition,
appeared and increased (Fig. 1), corresponding to a lmax(abs)
red-shift of 102 nm with one isobestic point at 410 nm, which
induced a color change from light-yellow to dark-yellow, and
finally into deep-red (see Fig. 1, inset), indicating that TPS can
serve as a sensitive naked-eye indicator for Hg2+. Meanwhile,
the addition of other metal ions (2.0 equiv.) didn’t have
notable response in the absorption maxima (Fig. 4).
Fig. 3 showed the Job’s plot for the employed dye and
metal. The results revealed complex stoichiometry between
TPS and Hg2+ in THF. From the Job’s plot, a maximum
absence intensity was showed when the molecular fraction of
Hg2+ was close to 0.7, which indicated the 2 : 1 complex
formation between TPS and Hg2+ with a total concentration
of 2.0 Â 10À5 M. On the basis of 2 : 1 stoichiometry, the
Fig. 1 UV-vis spectra of TPS in THF (c = 2.0 Â 10À5 M) in the
presence of increasing amount of Hg2+ (0–2.0 equiv.) predissolved in
acetonitrile. Arrows indicate the absorptions that increase (up) and
decrease (down) during the titration experiments. Inset: Change in the
color of TPS (c = 1.0 Â 10À3 M) after addition of Hg2+. (a): TPS in
THF; (b): TPS plus 0.3 equiv. of Hg2+ in THF; (c): TPS plus 0.5 equiv.
of Hg2+ in THF; (d) TPS plus 1.0 equiv. of Hg2+ in THF; (e): TPS plus
2.0 equiv. of Hg2+ in THF.
Fig. 2 Fluorescence emission spectra of TPS in THF (2.0 Â 10À5 M,
l
ex = 370 nm) in the presence of increasing amount of Hg2+ (0–2.0 equiv.)
predissolved in acetonitrile. Inset: fluorescence intensity of a solution
of TPS contained different concentrations of Hg2+ at lem = 468 nm
(I0 indicates the fluorescence intensity of free TPS and I indicates
the fluorescence intensity upon the introduction of different amounts
of Hg2+).
c
1880 New J. Chem., 2012, 36, 1879–1883
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Scheme 2 The possible binding mechanism of TPS with Hg2+
.
Fig. 5 (Top): Color change of TPS solution (c = 1.0 Â 10À3 M) in
the presence of different metal ions (2 equiv. of each in acetonitrile);
(bottom): Color change of TPS solution containing Hg2+ (2.0 equiv.)
in the presence of different metal ions (2.0 equiv.) in acetonitrile.
Fig. 3 Job’s plot for complex formation of TPS with Hg2+ in THF
medium (labs = 468 nm, the total [TPS] + [Hg2+] = 2.0 Â 10À5 M).
association constant for Hg2+ was estimated to be 4.17 Â 108 MÀ2
(R2 = 0.96) in THF solution by a nonlinear curve fitting16
(Fig. S2w).
Selectivity and interference are the very important para-
Fig. 6 Fluorescence responses of TPS in THF (2.0 Â 10À5 M, lex
=
meters to evaluate the performance of a sensor. Then, various
competing metal ions (Ag+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+
,
400 nm) to various 2.0 equiv. of metal ions. Bars represent the final
(I(468 nm)) emission intensity. The red bars represent the free TPS
solution and the addition of various metal ions (2.0 equiv.) to a
solution of TPS. The green bars represent the change of the emission
that occurs upon the subsequent addition of 2.0 equiv. of Hg2+ to the
above solution.
K+, Mg2+, Na+, Ni2+, Pb2+, Zn2+) were used to investigate
the selectivity of the sensor TPS toward Hg2+. The sensor
TPS displays extraordinarily selective absorption enhance-
ment at 468 nm in the presence of Hg2+, while almost no
changes were observed at 468 nm after adding the other metal
ions (Fig. 4). Furthermore, upon addition of the same amount
of the above-mentioned metal ions, respectively, only Hg2+
induced a distinct color change from yellow to red, as observed
by the naked-eye (Fig. 5, top). The fluorescent responses of
the sensor TPS to Hg2+ and other metal ions were also
investigated. Only the addition of Hg2+ resulted in a prominent
fluorescent change (Fig. 6, the red bars). Additionally, the
effects of coexisting metal ions on Hg2+ determination were
also investigated by competition experiments, as demonstrated
in Fig. 5 (bottom), Fig. 6 (the green bars) and Fig. S6.w When
TPS was treated with 2.0 equiv. of Hg2+ in the presence of
2.0 equiv. of other metal ions, the interference for the detection
of Hg2+ was not observed in the presence of other metal
ions. Thus, TPS can be used as a selective colorimetric and
fluorescent sensor for Hg2+ in the presence of most competing
metal ions. All these selective and interference results indicate
that sensor TPS has high selectivity and strong interaction
toward Hg2+ and could meet the highly selective requirements
for environmental or biological applications.
Reversibility is one most important factors for fluorescent
sensors in practical applications.17 4.0 equiv. of KI was
added to the solution of TPS-Hg2+ species to test whether
the proposed complex could be reversed due to the strong
binding ability of KI toward Hg2+. The addition of 4.0 equiv.
Fig. 4 UV-vis spectra of TPS (c = 2.0 Â 10À5 M) in the presence of metal
ions in acetonitrile. Ag+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, K+,
Mg2+, Na+, Ni2+, Pb2+ and Zn2+ ions (2.0 equiv.) were added.
c
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spectra in presence of 0.5 equiv. of Hg2+ in TPS solution.
These spectral changes might suggest that sensor TPS
associates with Hg2+ via a coordination bond interaction
between the sulfur atom of TPS and Hg2+. Upon addition
of KI to the solution of the TPS-Hg2+ species, the spectrum
was restored to the original one of TPS, indicating that the
sensor TPS has not been destroyed when the proposed
complex formed. Based on the results of the Job’s plot, the
reversibility, and the 1H NMR experiments, the plausible
binding mechanism of TPS with Hg2+ is schematically
depicted in Scheme 2.
Fig. 7 Absorbance intensity (468 nm) changes of TPS in THF (2.0 Â
10À5 M) upon alternate addition of Hg2+ (2.0 eq.) and KI (4.0 eq.).
Conclusions
A reversible fluorescent sensor was synthesized with high
sensitivity and selectivity for the detection of Hg2+ using
UV/vis and fluorescent responses. Moreover, the dramatic
color change of the solution made the detection of Hg2+
of KI could restore the original absorption spectra of TPS
(Fig. S7 and S8w). Furthermore, TPS can function as an
absence switch modulated by Hg2+/KI (Fig. 7). In addition,
the fluorescence intensity of 468 nm almost returned to the
original intensity of free TPS (Fig. S9 and S10w) after the
addition of 4.0 equiv. of KI to the complex solution. This
indicated that TPS is a reversible sensor, which can be reused,
and has not been destroyed.
1
possible by the naked eye. Job’s plot and H NMR titration
experiment were carried out to explain the mechanism of the
sensor TPS’s determination of Hg2+. Based on the results of
the Job’s plot and the 1H NMR experiments, the plausible
binding mechanism of TPS with Hg2+ is to form the complex
2TPS:Hg2+. The effects of coexisting metal ions on Hg2+
determination were also investigated by competition experi-
ments. In addition, we anticipate that the sensor can meet the
requirements for environmental or biological applications.
Future efforts include further mechanistic studies, structural
optimization, enhancing the water solubility and extending the
design concept to other types of sensor.
In order to study the detected mechanism of sensor TPS to
Hg2+, a comparison experiment of 1H NMR titration of TPS
with different concentrations of Hg2+ was carried out. Fig. 8
1
shows the results of H NMR titration of TPS with Hg2+ in
DMSO-d6. Treatment of 0.5 equiv. of Hg2+ resulted in a
considerable upfield shift of Ha in the quinazoline group of
TPS (d = 8.55–8.57 ppm) by Dd = 0.32 ppm. The proton
double of Hb in the triphenylamine ring was also shifted
slightly upfield. Upon addition of 1.0–2.0 equiv. of Hg2+
,
the 1H NMR spectra had almost no changes compared to the
Experimental section
General details
All commercially available reactants were purchased and used
as received without further purification. N,N-dimethyl-acetamide
(DMAc) and toluene were dried by standard procedures before use.
Acetonitrile was of HPLC purity and used without further treat-
ment. All other solvents were used without further purification. The
salts solutions of metal ions were NaNO3, KNO3, Mg(ClO4)2,
AgNO3, Cd(NO3)2Á4H2O, Co(NO3)2Á6H2O, Cr(NO3)3Á9H2O,
Cu(NO3)2Á3H2O, Fe(NO3)3Á9H2O, Hg(ClO4)2Á3H2O, Ni(NO3)2Á
6H2O, Pb(NO3)2, Zn(NO3)2Á6H2O.
The 1H NMR were recorded on a Bruker Ultra Shield Plus
400 MHz spectrometer with TMS as the internal standard and
DMSO-d6 as solvent. Mass spectrometric data were collected
on a Shimadzu GC-MS-QP2010 Plus mass spectrometer.
Melting points were measured on SGW X-4 micro melting
point spectrometer. Elemental analysis was determined in
Elementar Vario MICRO. UV-Vis absorption spectra were
obtained at room temperature on a Shimadzu UV-3600
UV–visible spectrometer. Photoluminescence spectra were
recorded at 298 K by Shimadzu RF-5301PC fluorescence
spectrometer.
Synthesis of compounds TPO and TPS
Fig. 8 Change in partial 1H NMR (DMSO-d6) spectra of TPS upon
2-(4-(diphenylamino)phenyl)quinazolin-4(3H)-one (TPO). A
addition of (1) 0, (2) 0.5, (3) 1.0, (4) 1.5, and (5) 2.0 equiv. of Hg2+
.
mixture of 4-(diphenylamino)benzaldehyde (8.19 g, 30 mmol),
c
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2-aminobenzamide (4.08 g, 30 mmol) and NaHSO3 (5.20 g, 50
mmol) in 30 mL dried N, N-dimethylacetamide (DMAc) was
stirred under N2 for 5 h at 150 1C. The resulting solution was
poured into 1000 mL water and a yellow precipitate was
observed. The precipitate was filtered off and dried to give
TPO as a light yellow solid (11.0 g, 94.3% yield). m.p.:
291–293 1C. 1H NMR (DMSO-d6, 400 MHz) d (ppm):
Notes and references
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7.35–7.39(t, J = 15.76 Hz, 4H), 7.43–7.47(t, J = 14.00 Hz,
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16.88 Hz, 1H), 8.07–8.11(t, J = 16.01 Hz, 3H). 13C NMR
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2-(4-(diphenylamino)phenyl)quinazoline-4(3H)-thione (TPS).
A mixture of P2S5 (5.55 g, 25 mmol) and 2-aminobenzamide
(13.5 g, 125 mmol) were stirred at 150 1C for 2 h. The resulting
solution was cooled to room temperature and a yellow precipitate
was observed. The precipitate was filtered off and washed with
CHCl3 and ether and dried to give Lawesson’s Reagent as yellow
crystals (8.0 g, 79.2% yield).
1.01g (2.5mmol) Lawesson’s Reagent and 1.95 g (5 mmol)
TPO were dissolved in 15 mL toluene. The mixture solution
was stirred at 80 1C for 4 h under N2. Then the solvent was
removed under vacuum to give yellow solid. The crude
product was then purified by silica gel to afford TPS as a
yellow solid (1.32 g, 65.2% yield). m.p.: 210–211 1C. 1H NMR
(DMSO-d6, 400 MHz) d (ppm): 6.95–6.97(d, J = 8.76 Hz,
2H), 7.12–7.18(m, 6H), 7.36–7.40(t, J = 15.61 Hz, 4H),
7.50–7.54(t, J = 14.98 Hz, 1H), 7.69–7.71(d, J = 8.05 Hz,
1H), 7.83–7.87(t, J = 14.91 Hz, 1H), 8.07–8.09(d, J =
8.78 Hz, 2H), 8.55–8.57(d, J = 8.15 Hz, 1H). 13C NMR
(DMSO-d6, 100 MHz) d (ppm): 120.22, 124.36, 125.09,
126.03, 127.72, 127.95, 129.80, 130.18, 130.36, 135.91,
146.67, 150.84, 151.36. GC-MS (m/z): calcd 405; found:
405 for [M+]. Elemental analysis (%) calcd for
C26H19N3S: C, 77.01; H, 4.72; N, 10.36. Found: C, 76.96; H,
4.62; N, 10.32.
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Acknowledgements
We gratefully acknowledge financial support from the
National Basic Research Program of China (973 Program,
2009CB930601), the National Natural Science Foundation of
China (Project No. 50803027, No. 50903001, No. 20905038),
and the Natural Science Fund for Colleges and Universities in
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