A water-soluble and retrievable ruthenium-based probe for colorimetric recognition of Hg(II) and Cys

Article abstract of DOI:10.1016/j.saa.2016.04.011

A new ruthenium-based complex 1 [(bis(4,4′-dimethylphosphonic-2,2′-bipyridine) dithiocyanato ruthenium (II))] was developed as a colorimetric probe for the detection of Hg(II) and Cys (Cysteine). The obtained compound 1 can give interconversional color changes upon the alternating addition of Hg(II) and Cys in 100% aqueous solution. The specific coordination between NCS groups with Hg(II) can lead to the formation of 1-Hg^{2 +} complex, which can induce a remarkable spectral changes of probe 1. Afterwards the formed 1-Hg^{2 +} complex can act as effective colorimetric sensor for Cys. Owing to the stronger binding affinity of sulfhydryl group to Hg^{2 +}, Cys can extract Hg^{2 +} from 1-Hg^{2 +} complex resulting in the release of 1 and the revival of absorption profile of the probe 1. By introducing the hydrophilic phosphonic acid groups, the proposed probe exhibited excellent water solubility. The limits of detection (LODs) of the assay for Hg^{2 +} and Cys are calculated to be 15 nM and 200 nM, respectively.

Full text of DOI:10.1016/j.saa.2016.04.011

A water-soluble and retrievable ruthenium-based probe for colorimetric
recognition of Hg(II) and Cys
Yali Cui, Yuanqiang Hao, Yintang Zhang, Baoxia Liu, Xu Zhu, Peng
Qu, Deliang Li, Maotian Xu
PII:
DOI:
Reference:
S1386-1425(16)30171-8
doi: 10.1016/j.saa.2016.04.011
SAA 14374
To appear in:
Received date:
Revised date:
Accepted date:
13 December 2015
2 April 2016
4 April 2016
Please cite this article as: Yali Cui, Yuanqiang Hao, Yintang Zhang, Baoxia
Liu, Xu Zhu, Peng Qu, Deliang Li, Maotian Xu, A water-soluble and retrievable
ruthenium-based probe for colorimetric recognition of Hg(II) and Cys, (2016), doi:
10.1016/j.saa.2016.04.011
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ACCEPTED MANUSCRIPT
A water-soluble and retrievable ruthenium-based probe for colorimetric
recognition of Hg(II) and Cys
Yali Cuia, Yuanqiang Haob, , Yintang Zhangb, Baoxia Liub, Xu Zhub, Peng Qub ,
Deliang Lia,  and Maotian Xub, 
a
Institute of Environmental and Analytical Sciences, College of Chemistry and
Chemical Engineering, Henan University, Kaifeng 475004, Henan, China
b
Henan Key Laboratory of Biomolecular Recognition and Sensing, College of
Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu 476000,
Henan, China
 Corresponding authors. Fax/Tel.: +86 378 3881589, +86 370 2586802.
E-mail addresses: hao0736@163.com (Y. Q. Hao) lideliang@henu.edu.cn (D. L. Li),
xumaotian@sqnc.edu.cn (M. T. Xu).
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Abstract
A new ruthenium-based complex 1 [(bis(4,4'-dimethylphosphonic-2,2'-bipyridine)
dithiocyanato ruthenium (II))] was developed as a colorimetric probe for the detection
of Hg(II) and Cys (Cysteine). The obtained compound 1 can give interconversional
color changes upon the alternating addition of Hg(II) and Cys in 100% aqueous
solution. The specific coordination between NCS groups with Hg(II) can lead to the
formation of 1-Hg2+ complex, which can induce a remarkable spectral changes of
probe 1. Afterwards the formed 1-Hg2+ complex can act as effective colorimetric sensor
for Cys. Owing to the stronger binding affinity of sulfhydryl group to Hg2+, Cys can
extract Hg2+ from 1-Hg2+ complex resulting in the release of 1 and the revival of
absorption profile of the probe 1. By introducing the hydrophilic phosphonic acid
groups, the proposed probe exhibited excellent water solubility. The limits of
detection (LODs) of the assay for Hg2+ and Cys are calculated to be 15 nM and 200
nM, respectively.
Keywords: Ruthenium, Mercury, Cysteine, Colorimetric probe, Phosphonic group
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1. Introduction
Development of effective chemoprobes for sensing environmentally and
biologically associated heavy and transition metal ions (ie. Hg2+, Cu2+, Pb2+, and Cr3+)
have attracted considerable attention in recent years [1-5]. Mercury, as one of the most
toxic species in nature, can cause serious environmental and health problems [6, 7]. In
the marine environment, bacteria can transform mercury into methylmercury, a very
potent neurotoxin, which can accumulate in marine food webs [8]. When ingested by
humans, methylmercury triggers several serious disorders including prenatal brain
damage, cognitive and motion disorders, vision and hearing loss, and even death [6,
9-11]. Various traditional techniques such as atomic absorption spectroscopy, atomic
emission spectroscopy and inductively coupled plasma mass spectrometry have been
applied to the detection Hg2+ ions [12-14]. However, these techniques usually involve
sophisticated instrumentation, intricate sample handling protocols, and the need of
professional operations, which limited their applications. For these reasons,
colorimetric chemosensors are especially attractive because they allow naked-eye
viewing the color changes triggered by target analytes without resorting to the use of
sophisticated instruments [15-17]. A desirable colorimetric probe should have strong
absorption band within visible region and respond specifically to the target analyte
[18]. Transitional metal complexes, is a type of potent candidates for colorimetric
applications because of their exceptional light-stability and visible light absorption
ability arised from the ligand-to-metal charge-transfer (LMCT) transitions within the
complexes [19, 20].
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Due to the strong absorptions in the wavelength range of visible light and the
favorable photovoltaic properties, Ru-containing dyes is a kind of useful transition
metal complexes, such as N3 (bis(4,4'-dicarboxylato-2,2'-bipyridine) bis
(isothiocyanato)
ruthenium
(II))
and
N719
(Bis(tetrabutylammonium)
Dithiocyanatobis(2,2'-bipyridine-4-COOH,4'-COO-) ruthenium(II)), have been widely
used as sensitizers for dye-sensitized solar cells [21]. The desirable features of
Ru-based dyes also empower them to be potent chromogenic agents for colorimetric
assays. Coronado et al. discovered that N719 can act effective colorimetric sensors for
Hg2+ based on the interaction between mercury(II) ions and the NCS groups of N719
[22, 23]. Hao et al. found that the N3-Hg2+ complex can serve as a reversible probe
for biothiols based a displacement mechanism [24].
However, all these reported ruthenium-based colorimetric probes suffered the
disadvantage of poor water-solubility which inevitably led to the use of organic
solvents during the detection. Herein, by introducing phosphonic acid moiety which is
highly hydrophilic and has been widely used for design water-soluble compounds
[25-27], into the 2,2'-bipyridine ligands, we developed
a
water-soluble
ruthenium-containing probe 1. The synthesized probe can serve as a dual-functional
sensor for recognition of Hg2+ and subsequently for Cys. Due to the specific
recognition of Hg2+ by NCS group, probe 1 can bind with Hg2+ to form 1- Hg2+
complex, which led to a color change in the dyes. Then in the presence of Cys, Hg2+
can be released from the complex of 1-Hg2+, resulting in the restoration of absorption
profile of probe 1.
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2. Experimental
2.1. General information
All chemical reagents and solvents for synthesis were analytical grade and
commercially available and used as received without further purification unless
otherwise stated. Deionized water (18 MΩ cm-1) from a water purification system
(Simplicity Plus, Millipore Corp., Billerica, MA, USA) was used throughout. The
absorbance spectra were recorded with Shimadzu UV2450 UV–vis
spectrophotometer. NMR experiments were carried out on a 400 MHz Bruker
spectrometer. Mass spectra (MS) were recorded on a mass spectrometer (LCQ Fleet,
Thermo Fisher Scientific).
2.2. Synthesis of probe 1
Scheme 1. Synthesis of probe 1.
2.2.1. Synthesis of ligand 2
Ligand 2 was obtained according to a five-step route according to a literature
procedure [28] with slight modifications (see supplementary information).
2.2.2. Synthesis of (bis(4,4'-dimethylphosphonic-2,2'-bipyridine) dithiocyanato
ruthenium (II)) (1)
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To a solution of 4,4'-Bis(diethylmethylphosphonate)-2,2'-bipyridine 2 (0.228 g,
0.5 mmol) and LiCl (0.225 g, 5 mmol) in dry DMF (10 mL) in a three-necked flask
was added RuCl3 (0.163 g, 0.30 mmol) The mixture was heated at 160-170 oC for 6 h
in the dark under flushed nitrogen. After the solution was cooled to room temperature,
dichloromethane was added and the precipitate was filtered and washed with
dichloromethane. After the compound was dried in a vacuum, the crude residue was
purified by column chromatography on silica gel and eluted with acetone/methanol
(4/6).
The obtained cis-dichloro-bis(4,4'-diethylmethylphosphonate-2,2'-bipyridine)
ruthenium (II) (54 mg, 0.050 mmol) and KSCN (49 mg, 0.50 mmol) were dissolved in
a mixture of water (5 mL) and ethanol (5 mL) in a three-necked flask. Then the
mixture was heated at reflux for 12 h in the dark under flushed nitrogen. After the
flask cooled to room temperature, acetone was added to the crude reaction mixture
and the precipitate was filtered off and washed with acetone. The compound was then
refluxed in HCl (20%, 10 mL) for 12 h. After removing solvent under vacuum, the
resulting residue was then dissolved in a minimum amount of water and purified by
column chromatography using Sephadex LH 20 as stationary phase and waster as
eluent. 1H NMR (500 MHz, D2O) δ 9.19 (t, J = 5.8 Hz, 2H), 8.39 (s, 2H), 8.24 (d, J =
10.0 Hz, 2H), 7.74 – 7.69 (m, 2H), 7.60 – 7.54 (m, 2H), 7.01 (d, J = 5.8 Hz, 2H), 3.20
(d, J = 20.1 Hz, 4H), 2.95 (d, J = 20.0 Hz, 4H) (Fig. S3). MS [ESI]: m/z, calcd for
[M-NCS]+ 847.94; found 847.96 (Fig. S4).
2.3. UV-vis measurements
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The detection measurements were conducted as follows. First, the stock solution of
the probe 1 was prepared by dissolving the sensor compound in HEPES buffered
aqueous solution (10 mM, pH 7.4). HgCl2 stock solution was prepared in HEPES buffer
(10 mM, pH 7.4). Before the measurements, the probe stock solution and the Hg2+ stock
solution were diluted with HEPES buffered water (pH 7.4), and then the diluted Hg2+
was added into the diluted probe solution; afterward the UV-vis spectra were recorded
at room temperature immediately. All spectrum tests were carried out in a HEPES
bufferred (10 mM, pH 7.4) aqueous solution
2.4. HPLC conditions
Chromatographic analysis was performed using a reversed-phase (RP) HPLC
system (Shimadzu 6AD, Columbia, MO) equipped with a column (Jupiter-10
µm-C18-300Ǻ, dimension of 250 × 4.6 mm i.d.) from Phenomenex (Torrance, CA).
Injection volumes were fixed at 10 μL. The mobile phases were water (mobile phase
A) and methanol (mobile phase B). The flow rate was 1 mL min-1, and the gradient
conditions started at 0 % B, ramped to 50 % B in 20 min. Diode array detector
wavelength was set 308 nm.
3. Results and Discussion
3.1. Synthesis of the probe 1
Synthetic process to prepare probe 1 was depicted in scheme 1. Ligand 2 was
obtained according to a five-step route [28]. This procedure involves oxidation of the
methly groups to carboxylic acids with potassium dichromate. The subsequent
esterification with ethanol, reduction of the ester to the alcohol with sodium
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borohydride, substitution with hydrobromic acid, and reaction with triethyl phosphite
yields the desired compound. The preparation of probe 1 relies on two steps [29]. The
first consists of refluxing 2 equiv of the bidentate ligand 2 with 1 equiv of
Ru(DMSO)4Cl2 in DMF to yield cis-ruthenium bischlorobisbipyridine complex. In the
second step, the chloride ligands are substituted by thiocyanate ligands by heating the
ruthenium bis-chloro complex with KSCN in DMF, and then the diethyl ester
phosphonate groups were fully hydrolyzed by heating the complexes in hydrochloric
1
acid solution. Both of the ligand 2 and the probe 1 were characterized by H NMR
and MS (see supporting information).
3.2. Absorption Spectroscopy of 1 to Hg2+
The absorption response of 1 toward Hg2+ was investigated by UV-vis
spectrophotometric titration in HEPES buffer (10mM, pH 7.4). The UV−vis absorption
spectrum of 1 displayed a strong band with the absorbance peak at 514 nm (ε = 1.4×
105 M-1cm-1), which can be attributed to metal-to-ligand charge transfer transitions
(MLCT) process of ruthenium complex. Upon the addition of increasing amount of
Hg2+, the intensity of the absorption peak of probe 1 at 514 nm decreased gradually,
while a new absorption band at 441 nm increased dramatically. A clear isosbestic point
at 476 nm indicated the conversion of 1 to a new species (1-Hg2+) during the titration
(Fig. 1). The absorbance profile of probe 1 was no longer changed noticeably when the
concentration of Hg2+ reached 30 μM. The 3:2 stoichiometry of the complexation
between 1 and Hg2+ was confirmed with a Job’s plot analysis (Fig. S5), which is in
agreement with previous studies [23, 24]. Therefore the binding mode between probe
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1 and Hg2+ can be speculated that each Hg2+ is tetrahedrally coordinated to three sulfur
atoms from adjacent probe and one Cl- anion.
Fig. 1. (A) Absorption spectra changes of probe 1 (50 μM) in the presence of different
concentrations of Hg2+ (0–1 equiv) in HEPES aquous buffer (10 mM, pH 7.4). (B)
Absorption changes of 1 at 514 nm as function the Hg2+ concentration, inset shows the
calibration curve.
Then the changes in the intensity of the absorbance peak (514 nm) of probe 1 were
applied to quantitatively monitor the Hg2+. As shown in Fig. 1B, Abs of probe 1
increases linearly with the concentration of Hg2+ over the range from 0.1 μM to 25.0
μM which can be expressed as a linear equation of Abs = -0.0016 + 0.02098 × [Hg2+]
(R2 = 0.998). All the RSDs (relative standard deviation) at different concentrations are
less than 5.2 % indicating good reproducibility of the probe 1 for Hg2+ detection. The
LOD (limit of detection) for Hg2+ was determined as 15 nM (S / N = 3). And this
value is matchable to the maximum allowable level (10 nM) for Hg2+ in drinking
water set by the US Environmental Protection Agency (EPA) [30], indicating that the
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proposed assay can be a potential tool for environmental monitoring.
To evaluate the selectivity of the proposed probe 1 for Hg2+, optical responses of 1
to other metal cations including Ag+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, K+, Mg2+, Mn2+, Na+,
Ni2+, Pd2+ and Zn2+ were measured. As shown in Fig. 2, only the addition of Hg2+
induce a dramatic change in the absorbance profile of probe 1, and no significant signal
changes were observed upon addition of other metal cations, indicating the specific
coordination between sulfur atom of the probe’s NCS groups with Hg(II). The excellent
selectivity of probe 1 for Hg2+ could also be observed from the photographic images
(inset of Fig. 2), only Hg2+ induced an apparent colour change from red to yellow which
can be easily discriminated with the naked eye. Moreover, competition experiments
were conducted. As shown in Fig. 3, either in the absence or in the presence of other
cations, significant spectral changes were always observed for probe 1 upon addition of
Hg2+. The results indicate that none of these anions interfered with the sensing of Hg2+.
Therefore, probe 1 can act as a highly selective colorimetric probe for Hg2+.
Fig. 2. Absorption response of 1 (50 μM) upon the addition of various ions (100 μM) in
HEPES aquous buffer (10 mM, pH 7.4) (the inset shows their corresponding
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photographic images).
Fig. 3. Absorption responses of 1 (50 μM) in the presence of different ions (100 μM)
(dark bars), followed by addition of Hg2+ (50 μM) (red bars) in HEPES aquous buffer
(10 mM, pH 7.4).
To explore the potential applications of probe 1 for the simple and convenient
monitoring of Hg2+, we employed probe 1 to prepare filter paper-based test strips for
the direct detection of Hg2+ as performed in previous studies [18, 31]. Here, the test
strips were prepared by immersing the filter paper into an aqueous solution of 1 (0.1 M)
and then dried in air. Then the as-prepared test strips were immersed into the aqueous
solution of Hg2+ with different concentrations. As illustrated in Fig. 4, upon the
presence of Hg2+, obvious colour changes of the test strips from red to yellow were
observed and the discernible concentration of Hg2+ can be as low as 20 μM.
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Fig. 4. Photographic images of filter paper-based test strips of 1 for various
concentrations of Hg2+ : (A) blank, (B) 20 μM, (C) 50 μM, (D) 80 μM, (E) 100 μM, (F)
150 μM.
3.3. Absorption Spectroscopy of 1-Hg2+ to Cys
In recent researches, the metal ions (Hg2+, Ag+, or Cu2+)-based complexes have
been exploited for the sensing of biothiols based on the high affinities of sulfur-bearing
groups for these metal ions [32-34]. To test the feasibility of this strategy for the
quantification of Cys in the proposed assay, the in situ generated 1-Hg2+ complex was
titrated with Cys in 100% aquous media. As shown in Fig. 5, the peak absorbance of
1-Hg2+ at 514 nm is increased steadily with the increase in Cys concentration, and on
addition of about 2 equiv of Cys the absorption profile closely matches that of probe 1,
indicating the regeneration of probe 1. The dose-dependent absorbance changes (Abs)
shows a good linearity over a concentration range of 1.0–25.0 μM for Cys, that can be
expressed as Abs = 0.016 × [Cys] / μM - 0.051 (R2 = 0.997), (Fig. 5 inset). The LOD
of 1-Hg2+ complex for Cys was calculated to be 0.2 μM.
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Fig. 5. Absorption spectra of 1-Hg2+ complex in the presence of various
concentrations of Cys (0, 0.1, 0.2, 0.5, 1, 2, 5, 10, 15, 20, 25, 30, 40, and 50 μM) in
HEPES aquous buffer (10 mM, pH 7.4). Inset: calibration curve of Abs vs Cys
concentrations. Error bars represent the standard deviation of three measurements.
Fig. 6. Absorption changes of 1-Hg2+ (50 μM) at 514 nm in the presence of different
amino acids (50 μM) (low bars), followed by addition of Cys (50 μM) (high bars).
In order to evaluate the selectivity of the proposed 1-Hg2+ complex for Cys,
absorbance changes of 1-Hg2+ at 514 nm were recorded in the presence of various other
amino acids. Abs of 1-Hg2+ did not change distinctly in the presence of the other
amino acids. It is probably due to that these amino acids do not have any significant
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interaction with Hg2+. However, in presence of Cys, Abs dramatically changed (Fig.
6). Hence, the proposed 1-Hg2+ has high selectivity for Cys among other amino acids.
To explore whether the 1-Hg2+ complex could maintain its sensing response to Cys in
the presence of various other relevant amino acids, competition experiments of 1-Hg2+
complex were conducted. Fig. 6 (red bars) shows the Abs of 1-Hg2+ to Cys in the
presence of various other species. The results indicated that none of the other amino
acids interfered with Cys detection. All other amino acids only caused very weak
background signal, while upon consequently addition of Cys to each mixture,
immediate absorbance response was achieved. These results strongly suggest that
1-Hg2+ complex is a potent colorimetric assay for Cys monitoring with high specificity.
Scheme 2. The proposed sensing mechanism of probe 1 for Hg2+ and Cys.
3.4. Proposed sensing mechanism
As depicted in Scheme 2, the probe 1 alone displays red color. The addition of
Hg2+ can cause an obvious color change from red to yellow due to the formation of
1-Hg2+ complex. The further addition of Cys could release probe 1 from the
1-Hg2+complex owing to the stronger affinity of Cys to Hg2+, resulting in the
restoration of the absorbance profile of probe. This reversible binding process was
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further investigated and confirmed by HPLC measurements (Fig. 7, black curve). Probe
1 (100 μM) alone showed a main peak with Rt (Retention time) of 13.7 min. After
mixing 1 (100 μM) with Hg2+ (100 μM), the peak at 13.7 min was completely
disappeared, and a new broadened peak at 19.8 min appeared (Fig. 7, red curve). This is
attributed to the conversion of the 1 to 1-Hg2+ complex. After the 1-Hg2+ complex
solution was further treated with Cys (2 equiv), the typical HPLC spectrum of 1 was
entirely restored (Fig. 7, blue curve). Above results revealed that probe 1 can reversibly
coordinate to Hg2+ upon the addition of Cys.
Fig. 7. HPLC chromatograms obtained by injecting a solution of HEPES buffer (10
mM, pH=7.4) containing (black curve) 100 μM probe 1; (red curve) 100 μM 1 and 100
μM Hg2+; (blue curve) 100 μM N3, 100 μM Hg2+ and 200 μM Cys.
4. Conclusions
In summary, we have reported a colorimetric chmosensor 1 for recognition of
Hg2+ and Cys. By incorporating hydrophilic phosphonic groups into the bipyridine
ligands of the ruthenium complex, probe 1 displays excellent water-solubility. Due to
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the specific coordination of Hg2+ to the sulfur atom of the probe' SCN groups, 1 shows
high selectivity toward Hg2+. And this complexation could lead to the absorbance
change of prbe 1 from red to yellow and the formation of 1-Hg2+. Moreover, the in situ
generated 1-Hg2+ complex can act as an effective chemosensor for Cys detection in
aqueous solution.
Acknowledgements
We are grateful to the National Natural Science Foundation of China (Nos. U1404215,
21175091, 21475084, 21276285, 21476266, and 21475085) and Innovation Scientists
and Technicians Troop Construction Projects of Henan Province (No: 41) for support.
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Graphical abstract
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Hightlights
 A novel retrievable ruthenium-based colorimetric probe was developed for
recognition of Hg(II) and Cys based on a displacement mechanism.
 The probe possesses highly sensitivity and selectivity towards the targets.
 The constructed probe displays excellent water-solubility.
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