One pot synthesis of coordination polymer 2,5-dimercapto-1,3,4-thiadiazole- gold and its application in voltammetric sensing of resorcinol

Article abstract of DOI:10.1039/c4ra02983f

Herein, we report a one pot synthesis of a coordination polymer using 2,5-dimercapto-1,3,4-thiadiazole (DMTD) with gold (Au) and the characterization of the polymer by FTIR, Raman spectroscopy, UV-visible spectroscopy, XPS, XRD, ¹³CNMR, HRTEM, gel permeation chromatography and TGA. Results show that Au ions get coordinated by the diazole and sulfydryl groups of DMTD throughout the polymeric chain. The polymeric layers are stabilized through π-π stacking and hydrophobic interaction. XRD data reveals the crystalline nature of DMTD-Au coordination polymer. The kinetic parameters, such as the energy of activation and the regression value of the DMTD-Au decomposition, have been determined by exploiting thermogravimetric data. HRTEM analysis establishes the nanostructure of the polymeric material DMTD-Au. Its voltammetric study reveals that the synthesized coordination polymer DMTD-Au exhibits excellent electroactivity towards resorcinol and facilitates fast electron transfer kinetics, which is employed for electro-sensing applications. To the best of our knowledge we are the first to report this coordination polymer DMTD-Au modified carbon paste electrode (DMTD-Au/CPE) for the electrochemical detection of resorcinol (RS). The DMTD-Au modified carbon paste electrode has been found to be highly sensitive towards resorcinol due to its strong interaction through intermolecular hydrogen bonding. Under the optimized conditions, the modified electrode exhibit resorcinol oxidation at low over potential (0.55 V) and the anodic peak current shows a linear response with sensitivity and limit of detection of 0.019 μA nM^-1 and 29.77 nM, respectively, at a S/N (signal-to-noise ratio) of 3. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02983f

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One pot synthesis of coordination polymer 2,5-
dimercapto-1,3,4-thiadiazole–gold and its
Cite this: RSC Adv., 2014, 4, 25675
application in voltammetric sensing of resorcinol†
*
Madhu Tiwari, Sandeep Gupta and Rajiv Prakash
Herein, we report a one pot synthesis of a coordination polymer using 2,5-dimercapto-1,3,4-thiadiazole
(DMTD) with gold (Au) and the characterization of the polymer by FTIR, Raman spectroscopy, UV-visible
spectroscopy, XPS, XRD, 13CNMR, HRTEM, gel permeation chromatography and TGA. Results show that
Au ions get coordinated by the diazole and sulfydryl groups of DMTD throughout the polymeric chain.
The polymeric layers are stabilized through p–p stacking and hydrophobic interaction. XRD data reveals
the crystalline nature of DMTD–Au coordination polymer. The kinetic parameters, such as the energy of
activation and the regression value of the DMTD–Au decomposition, have been determined by
exploiting thermogravimetric data. HRTEM analysis establishes the nanostructure of the polymeric
material DMTD–Au. Its voltammetric study reveals that the synthesized coordination polymer DMTD–Au
exhibits excellent electroactivity towards resorcinol and facilitates fast electron transfer kinetics, which is
employed for electro-sensing applications. To the best of our knowledge we are the first to report this
coordination polymer DMTD–Au modified carbon paste electrode (DMTD–Au/CPE) for the
electrochemical detection of resorcinol (RS). The DMTD–Au modified carbon paste electrode has been
found to be highly sensitive towards resorcinol due to its strong interaction through intermolecular
hydrogen bonding. Under the optimized conditions, the modified electrode exhibit resorcinol oxidation
at low over potential (0.55 V) and the anodic peak current shows a linear response with sensitivity and
limit of detection of 0.019 mA nMꢀ1 and 29.77 nM, respectively, at a S/N (signal-to-noise ratio) of 3.
Received 3rd April 2014
Accepted 16th May 2014
DOI: 10.1039/c4ra02983f
www.rsc.org/advances
commercial purposes through facile routes without catalyst at
room temperature.10–12
1. Introduction
In this scenario, 2,5-dimercapto-1,3,4-thiadiazole (DMTD)
may be of signicant interest for constructing CPs with inter-
esting properties because of its four possible bonding sites
(diazole nitrogen atoms and sulfydryl sulfur donors), which
provide feasibility for metal ions to be linked with the ligand and
thus expanding the chain dimensions. Furthermore, azole rings
are superior in terms of hydrogen bonding and p–p stacking.13
We explore the possibility of synthesizing the coordination
polymer DMTD–Au under optimal conditions in the absence of
catalysts or without extra control of other experimental condi-
tions. A simple reaction design is employed, in which the
concentration of Au(III) and DMTD and the reaction conditions
are systematically varied with the progress of the reaction and
product, which is selectivity monitored by several instrumental
techniques. This compound is also compared with the Au(0)–
DMTD composite to understand the difference in the coordi-
nation polymer and the composite of the two components.14
Ultimately, this reaction process leads to an enhanced practical
utility of the synthesized coordination polymer DMTD–Au,
thereby providing a platform for the production of such poly-
mers with physical properties competitive with those of the
existing coordination polymers.
In recent years, the study of coordination polymers (CP) has
emerged as a vibrant area of research. Coordination polymers
are a family of compounds, which are considered to be metal–
organic frameworks with more than two physical/chemical
properties in one (like hybrid materials). CPs are formed by the
incorporation of metal building blocks in multi-dentate organic
or inorganic bridging ligands.1,2 The design of efficient coordi-
nation polymers encapsulated with noble metal ions has
received signicant interest in the past two decades. Coordi-
nation polymers have been widely used for numerous applica-
tions such as sensing,3 water purication,4 hydrogen fuel
storage,5 heterogeneous catalysis or catalyst-supporting energy-
storage devices such as lithium ion secondary batteries6,7 and
double layer capacitors.8,9 Therefore, there is an increasing
demand for the synthesis of such coordination polymers for
School of Materials Science and Technology, Indian Institute of Technology, Banaras
Hindu University, Varanasi-221005, India. E-mail: rajivprakash12@yahooo.com;
Fax: +91-542-2368707
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02983f
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This is the rst time gold salt has been used for the synthesis
of a coordination polymer; the structural features of the poly-
mer have been established and it has been applied in the vol-
tammetric sensing of resorcinol (RS) up to nanomolar level.
Phenolic compounds, which are extensively used in tanning,
cosmetics, dyes, pesticides, avoring agents, medicines, anti- Scheme 1 Reaction scheme for synthesis of co-ordination polymer
DMDT–Au.
oxidants and photography, are highly toxic and poorly degrad-
able in the environment and have therefore been listed as
environmental pollutants by the US Environmental Protection
Agency (EPA) and European Union (EU).15–18 Resorcinol is a
2.3. Instrumentation
phenolic compounds with high toxicity; it can be easily absor-
Infrared spectra in KBr pellets were recorded with a Fourier
bed through the gastric tract and human skin, which can cause
transform infrared (FTIR) spectrometer (Nicolet-6700, USA)
dermatitis, catarrh, convulsion and cyanopathy.19,20 In recent
years, a numerous have been applied for the determination of
RS such as spectrophotometry, high-performance liquid chro-
matography, quartz crystal microbalance, surface Plasmon
resonance and uorescence spectroscopy.21–23 However, these
methods require expensive and sophisticated instrumentation
or complicated sample preparation with a moderate limit of
detection. Electrochemical technique has always offered simple
instrumentation, low cost and portability. In this process, the
concept of a modied electrode is one of the exciting elds of
electroanalytical chemistry.24–26 Several existing electrochemical
methods for RS sensing are based on the immobilization of an
enzyme on a modied electrode. We have demonstrated a
simple non-enzymatic, highly accurate and rapid voltammetric
method with a DMDT–Au modied carbon paste electrode for
the ultra trace level sensing of RS within a wide range of
concentration.
scanning from 2500 to 400 cmꢀ1. Raman spectra were measured
with a Micro Raman Spectrometer at 180ꢁ scattering geometry
(Renishaw, Germany) with a 514.5 nm line of an Ar+ laser at 50
mW at room temperature. TEM measurements were performed
with a Jeol Jem 2010 electron microscope operating at 200 kV on
a carbon-coated copper grid modied with 5 mL of the polymer
solution in ethanol. 13C NMR spectra of the resulting polymer
were recorded on a JEOL Al300 NMR (300 MHz) in DMSO-d6
solvent and reported in parts per million (d) from residual
solvent peak. Thermogravimetric analysis (TGA) was performed
ꢁ
on a NETZSCH, STA 409 PC analyzer at a heating rate of 20 C
minꢀ1 under a ow of nitrogen. X-ray diffraction patterns for
powder samples of the coordination polymer DMTD–Au and
DMTD were recorded with 18 kW anode powder X-ray diffrac-
tometer rotating from 10ꢁ to 80ꢁ, Rigaku, Japan with Cu-Ka
radiation operating in Bragg–Brentano geometry and tted wiꢀth1
a graphite monochromator in diffracted beam with a 3ꢁ min
scan rate. UV-vis absorption spectra were recorded using a
Perkin Elmer Lambda-25 spectrophotometer using a quartz
cuvette with an optical path length of 1 cm. Elemental analysis
was recorded with X-ray photoelectron (Kratos Analytical
2. Experimental
2.1. Materials
2,5-dimercapto-1,3,4-thiadiazole (DMTD) and HAuCl4 were Instrument, Shimadzu group company, Amicus XPS UK). The
purchased from Sigma-Aldrich (India). Ethanol, resorcinol, number-average molecular weight (Mn) and polydispersity index
sodium dihydrogen phosphate and disodium hydrogen phos- (Mw/Mn) were determined by Youglin ACME 9000 gel perme-
phate (SRL, India) were used as received unless mentioned ation chromatography in DMF at 40 ꢁC with a ow rate of 0.5 mL
otherwise. The glassware used in the synthesis of the polymer minꢀ1 on two polystyrene gel columns [PL gel 5 mm 10E 4 A
˚
was cleaned with freshly prepared aqua regia (3 : 1, HCl–HNO3) columns (300 7.5 mm)], which were connected in series to a
and rinsed comprehensively with ultrapure water (Merck India). gradient pump and a RI detector. The columns were calibrated
against seven poly(methyl methacrylate) (PMMA) standard
samples (Polymer Lab, PMMA Calibration Kit, M-M-10). Elec-
trochemical workstation (model-CHI7041C, CH-Instrument
Inc., USA) was used for electrochemical measurement using a
three-electrode assembly (as shown in Fig. 9) consisting of a
2.2. Synthesis of coordination polymer (DMTD–Au)
In a typical synthesis of coordination polymer (DMTD–Au),
ethanolic solution of HAuCl4 (5.95 mM) was added to 2,5-
dimercapto-1,3,4-thiadiazole (17.7 mM, prepared in ethanol
solution) under vigorous stirring for 18 h at 22 ꢁC under a thin
DMTD–Au carbon paste modied electrode as the working
electrode, Pt foil as the counter electrode and Ag/AgCl as the
stream of nitrogen gas (shown in Scheme 1) to avoid partial
reference electrode for all the electrochemical measurements.
oxidation of the precursor molecule. Initially, the monomeric
Neutral phosphate buffer (pH 7.0) was used as a supporting
electrolyte and the scan rate was maintained at 50 mV sꢀ1 for
solution was clear and transparent, which turned turbid brown
during the course of the reaction, and nally a pale yellow
precipitate was obtained. The pale yellow precipitate was puri-
each measurement.
ed by washing several times with water–ethanol mixture
2.4. Modied carbon paste electrode preparation for RS
sensing
(30 : 70 v/v) to remove un-reacted DMTD and gold salt, and
thereaer dried under vacuum at 28 ꢁC for 15 h. The yield
ꢁ
obtained was 64% and the product was stored below 25 C for The active carbon paste was prepared by mixing a known
further use.
amount of DMTD–Au (2.5% w/w) with graphite powder (67.5%
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w/w) in a pestle and mortar followed by the addition of Nujol oil
(30% w/w). The absence and presence of DMTD–Au in the active
paste resulted in the formation of unmodied CPE and modi-
ed CPE/DMTD–Au systems. The mixture was thoroughly
homogenized and lled manually into the cavity (1 mm diam-
eter) of the electrode body. The electrode surface was smoothed
on clean butter paper applying pressure to achieve better elec-
trical contact.
3. Results and discussion
3.1. Structural, morphological and molecular weight studies
of the synthesized coordination polymer DMDT–Au
The FTIR spectra of DMTD–Au and DMTD were recorded in
400–2500 cmꢀ1 region, as shown in Fig. 1. In the case of DMTD,
the region from 2450–2350 cmꢀ1 contains several weak bands,
which are derived from C–H stretching overtones and Fermi
resonances. Peaks appearing in the region 1300–1205 cmꢀ1 are
assigned to ring modes coupled to (exocyclic) C–C stretch and a
ring stretching mode. The most prominent of these are the
Fig. 2 Raman spectrum of coordination polymer DMTD–Au.
asymmetric stretches (nas) at 1500 cmꢀ1 and the symmetric the absence of an S–S bond in the coordination polymer DMTD–
stretch (ns) at 1407 cmꢀ1 of C]N; the nas and ns modes of C–S–C Au. The results from FTIR and Raman spectroscopy reveal that
stretches have been assigned to peaks at 718 and 658 cmꢀ1
,
gold ions get coordinated between the DMTD molecules
respectively. The peak at 1075 cmꢀ1 is assigned due to an out of throughout the polymer chain.
phase combination of N–N and symmetric C–S–C stretches
The crystal structure of DMTD–Au investigated by powder X-
which get reduced to 1049 cmꢀ1 in DMDT–Au. DMTD–Au shows ray diffraction (XRD) is shown in Fig. 3. Fig. 3(a) and (b) show
a C]N band at 1384 cmꢀ1. Reduction in the intensity of the C] the diffraction pattern of DMTD–Au and DMTD, respectively,
N bond and the stretching frequency of C]N in DMDT–Au while Fig. 3(c) corresponds to the diffraction pattern of fully
indicates the interaction of the nitrogen atom with gold, which reduced gold obtained from JCPDS le CAS number 7440-57-5.
is also conrmed by Raman spectroscopy.
Indexing for the diffraction peaks of DMTD–Au was performed
The Raman spectrum of DMTD–Au, shown in Fig. 2, exhibits by Dicvol soware, which showed that the diffraction features
bands at 380 and 546 cmꢀ1, which arise due to Au–S and Au–N appearing at 2q values of 28.28ꢁ, 40.44ꢁ, 50.10ꢁ, 58.58ꢁ, 66.30ꢁ
linkage, respectively, ensuring the metal–sulphur and metal– and 73.56ꢁ corresponded to the (100), (110), (112), (200), (202)
nitrogen linkage.27 Further, a band at 660 cmꢀ1 in the Raman and (114) planes, respectively, for the tetragonal system. Sharp
spectrum is due to the symmetric C–S–C stretch, while the diffraction peaks show the crystalline characteristics of the
absence of an absorption peak near 522 cmꢀ1 clearly indicates polymer DMTD–Au. The present study reveals not only the
crystalline nature of the coordination polymer but it also
Fig. 3 XRD of DMTD–Au (a), DMTD (b) and gold (c) obtained from
Fig. 1 FT-IR spectra of DMTD–Au (a) and DMTD (b).
JCPDS file CAS number 7440-57-5.
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Fig. 4 XPS spectra of DMTD–Au for Au 4f (a), S 2p (b), N 1s (c) and C 1s (d) regions.
demonstrates the absence of fully reduced gold within the
coordination sphere, supporting the XPS results.
The XPS study of DMTD–Au (Fig. 4) revealed binding energy
values of 85.41 eV and 89.11 eV, as shown in Fig. 4(a), corre-
sponding to Au(I) 4f7/2 and Au 4f5/2, respectively.28,29 The Au 4f7/2
binding energy is shied in DMTD–Au. The direction of this
shi clearly corresponds to Au being in an oxidized state in
DMDT–Au, and its magnitude is similar to that observed for
Au(I). The experimental photoemission curve of S 2p signal in
Fig. 4(b) can be tted in two doublets, i.e., two different atomic
environments of sulfur with 2p3/2 components peaking at 161
and 162.5 eV. The rst one can be assigned to aromatic sulfur
while the second one is related to sulfur attached to a gold
atom.30–32 The nitrogen 1s binding energy (BE) of 400.0 eV in
Fig. 4(c) corresponds to the N–N binding energy.33–35 The BE is
referenced to C(1s) of C]N binding energy near 287 eV (ref. 36)
13CNMR spectra of DMTD–Au in DMSO-d6 at room
in Fig. 4(d) because electronegative substituents decrease the
electron density of carbon atoms, causing a small increase in C
(1s) binding energy.
Fig.
temperature.
5
The HRTEM image in Fig. SI-1† shows that the rigid mole-
cules of the co-ordination polymer DMTD–Au are arrayed in the
coordination polymer sphere with an average diameter of about
170 nm as nanospheres. It may be assumed that the rigid
coordination polymer DMTD–Au molecules are arrayed in
parallel due to the p–p stacking of adjacent molecular chains in
the same plane. The corresponding SAED pattern clearly shows
that the synthesized coordination polymer has crystallinity,
which supports the XRD result, as shown in Fig. 3. The
synthesized coordination polymer DMTD–Au has a prominent
Fig.
6
Proposed structure of synthesized coordination polymer
peak at a chemical shi value of 206.95 ppm (Fig. 5), showing a DMDT–Au.
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strong deshielding of the aromatic carbon as a chemical shi
Considering various characterization techniques, a possible
value of 177 ppm is reported for DMTD.37 Down eld shielding chemical structure of DMDT–Au has been proposed, as shown
in chemical shi value for DMDT–Au signies the interaction of in Fig. 6, which is in accordance with 13CNMR, FTIR, RAMAN,
the metal with the DMDT molecule. It shows that the aromatic HRTEM and XPS results.
carbon atoms are in a strong electronegative environment as in
nature.
The average molecular weight (approximate) and distribu-
tion of coordination polymer DMTD–Au was determined by gel
permeation chromatography using PMMA as a standard. Fig. SI-
2† shows the chromatogram, which clearly indicates that the
average molecular weight of the dominant fraction is located at
around Mn ¼ 370 000 g molꢀ1 (PDI 2.32). The minuscule humps
are because of the fragmentation of polymeric chains and traces
of lower molecular weight polymeric fragments. The UV-vis
spectrum of coordination polymer DMDT–Au is shown in Fig. 7.
The maximum absorption peak at around 433 nm and the
shoulder peak at 323 nm are attributed to p–p* and MLCT
(metal to ligand charge transfer transition) (Au / p*) transi-
tions, respectively.38 Thermal analysis of the polymeric sample
was performed using thermal gravimetric analysis (TGA)
(Fig. 10) under a N2 atmosphere by varying the temperature of
Fig. 7 UV-visible spectrum of coordination polymer DMDT–Au.
Fig. 10 CV for 0.02 M Fe(II)/Fe(III) in 0.1 M PBS (pH 7) at the unmodified
carbon paste electrode (a) and (DMTD–Au) modified carbon paste
electrode (b).
Fig. 8 TGA (a) and DTA (b) plot of DMTD–Au.
Fig. 11 Differential pulse voltammetric response of unmodified
carbon paste electrode (a), (DMTD–Au) modified carbon paste (b) and
DMTD–Au modified carbon paste + RS (c) in phosphate buffer
solution.
Fig. 9 Schematic diagram of the three-electrode system.
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lnln(1/Y) ¼ ꢀ(Ea/R)1/T + constant
where Y ¼ Wt ꢀ WN/W0 ꢀ WN; Y is the fraction of the initial
molecules not yet decomposed; Wt is the weight at time t; WN is
weight at innite time (¼ zero) and W0 is the initial weight. A
plot of lnln(1/Y) vs. 1/T gives an excellent approximation to a
straight line. The slope is related to activation energy. Activation
energy using Broido model, as shown in Fig. SI-3,† is measured
as 25.20 kJ molꢀ1 with an R2 value of 0.984.
3.2. Electroanalysis of CPE/DMTD–Au modied electrode
The cyclic voltammetry of the DMTD–Au modied carbon paste
electrode was examined in the presence of 0.02 M Fe(II)/Fe(III)
redox couple in 0.1 M phosphate buffer solution (PBS) at pH 7.
Three-electrode conguration consisting of the carbon paste
electrode as the working electrode, Pt foil as the counter elec-
trode and Ag/AgCl as the reference electrode was used for RS
sensing, as shown in Fig. 9.
A signicant increase in the peak current of the Fe(II)/Fe(III)
redox couple was observed with the DMTD–Au modied carbon
paste electrode as compared to the unmodied carbon paste
electrode for the same amount of Fe(II)/Fe(III) redox couple, as
shown in Fig. 10, which reveals that the DMTD–Au polymeric
material facilitates electron transfer and can be used as an
excellent electrocatalytic material.
Fig. 12 Schematic representation of the voltammetric sensing
mechanism of resorcinol.
ꢁ
3.3. Voltammetric sensing of resorcinol
the instrument from room temperature to 800 C at a heating
rate of 20 ꢁC minꢀ1. The rst weight loss (2%), which occurred
in the temperature range 40–140 C, accounts for the uncoor-
dinated water molecules, which includes the lattice and
adsorbed water molecules.
It can be seen from Fig. 8 that the degradation of DMTD–Au
starts at 260 ꢁC, which may be due to the cleavage of the metal–
sulphur and metal–nitrogen linkages, indicating good stability of
the synthesized coordination polymer. The corresponding DTA
plot exhibits four peaks at 280 ꢁC, 600 ꢁC, 510 ꢁC and 730 ꢁC.
Broido et al.39 reported a method for the evaluation of the
activation energy related to decomposition. The equation
developed for the calculation of activation energy (Ea) is as
follows:
The synthesized coordination polymer was further utilized for
the voltammetric sensing of resorcinol using the DMTD–Au
modied carbon paste electrode in phosphate buffer solution.
For the voltammetric sensing of resorcinol, we selected a
potential window region of 0 to 0.9 V because the oxidation of
resorcinol is expected in this region. Resorcinol in PBS did not
show any peak at the bare carbon paste electrode at trace
concentration, and no peak of modied carbon paste electrode
was observed, as shown in Fig. 11. Resorcinol addition shows an
anodic peak at a potential of 0.55 V vs. Ag/AgCl over the modi-
ed electrode, as shown in Fig. 11. The electrochemistry
responsible for this electro-sensing of resorcinol is probably
because of the catalytic effect of DMTD–Au, which exhibits high
ꢁ
Fig. 13 Differential pulse voltammogram of RS at various concentrations (112.60 ꢂ 10ꢀ9 to 1.09 ꢂ 10ꢀ6 M) in 0.1 M phosphate buffer (pH 7.0) at
DMTD–Au modified carbon paste electrode (a) and the corresponding standard addition calibration plot of current vs. resorcinol concentration (b).
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electron transfer kinetics via the interaction of diazole nitrogen
with the hydroxyl group of resorcinol through hydrogen
bonding, as shown in the schematic representation in Fig. 12.
The potential utility of the present modied electrode was
further examined by recording the voltammetric response on
the successive addition of resorcinol in PBS (pH 7.0). A
systematic change in current was observed aer each addition
of resorcinol into the supporting electrolyte solution, as shown
in the voltammogram of Fig. 13(a). This DMTD–Au modied
carbon paste electrode does not suffer from electrode fouling
and poisoning during resorcinol detection, which is a major
limitation of other existing modied electrodes. The stability of
the electrode is also very important for biosensing applications
and was tested repeatedly for more than ve times by the same
modied carbon paste electrode. The calibration plot of resor-
cinol concentration vs. peak current is shown in Fig. 13(b) with
the sensitivity and limit of detection of 0.019 mA nMꢀ1 and 29.77
nM, respectively, at an R2 value of 0.999 and a S/N (signal-to-
noise ratio) of 3.
Acknowledgements
Madhu Tiwari and Sandeep Gupta are thankful to CSIR and
UGC, respectively, for research fellowship. We are thankful to
Prof. D. Pandey and Dr A. K. Singh, School of Materials Science
&Technology IIT (BHU) for XRD facility and discussions, Prof. B.
Ray, Chemistry, BHU for GPC of polymer, Prof. Ranjan Kumar,
Physics, BHU for Raman, Prof. A. S. K. Sinha Chemical Engi-
neering, IIT BHU for XPS measurements and Ashish Kumar,
PhD, SMST for fruitful discussions and help.
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polymer DMDT–Au at room temperature. The synthetic method
described here is very simple, does not require a catalyst to
initiate polymerization, and can have commercial applications
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