A cerium-based metal-organic framework having inherent oxidase-like activity applicable for colorimetric sensing of biothiols and aerobic oxidation of thiols

Article abstract of DOI:10.1039/c7ce01053b

A cerium-based metal-organic framework (MOF; 1) with a UiO-66 (UiO: University of Oslo) framework topology was synthesized solvothermally by employing 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid as a ligand. The MOF was thoroughly characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction, infrared spectroscopy, and thermogravimetric and N₂ sorption analyses. The activated material (1′) retained its structural integrity in water, acetic acid and 1 M HCl solution. XPS investigation reveals the presence of both Ce(iii) and Ce(iv) ions in 1. Owing to the presence of mixed-valence cerium ions, 1′ was able to oxidize the chromogenic peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) or 2,2′-azinobis(3-ethylbenzothizoline-6-sulfonic acid) (AzBTS) in the absence of an external oxidizing agent. Thus, it showed inherent oxidase-like catalytic properties. Inspired by the excellent oxidase-mimicking activity of 1′, a protocol was developed for the rapid colorimetric sensing of biothiols in NaAc buffer (0.2 M, pH = 4). The sensing ability of 1′ towards cysteine was also demonstrated in human blood plasma. Furthermore, the redox-active cerium ions enabled 1′ to exhibit excellent heterogeneous catalytic performance in aerobic oxidation catalysis of thiol compounds. The material is reusable (both as a sensor and as a catalyst), low-cost and highly stable, which renders it a promising candidate for the monitoring of biothiols in immunoassays and medical diagnosis as well as for industrial oxidation catalysis.

Full text of DOI:10.1039/c7ce01053b

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Tiddo J. Mooibroek, Antonio Frontera et al.
Towards design strategies for anion–π interactions in crystal engineering
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A cerium-based metal-organic framework having inherent
oxidase-like activity applicable for colorimetric sensing of
biothiols and aerobic oxidation of thiols
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
c
*b
DOI: 10.1039/x0xx00000x
Rana Dalapati, Balasubramanian Sakthivel, Manoj K. Ghosalya, Amarajothi Dhakshinamoorthy,
*
a
and Shyam Biswas
www.rsc.org/
A cerium-based metal-organic framework (MOF; 1) with UiO-66 (UiO: University of Oslo) framework topology was
synthesized solvothermally by employing 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid as a ligand. The MOF
was thoroughly characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction, infrared spectroscopy,
2
themogravimetric and N sorption analyses. The activated material (1') retained its structural integrity in water, acetic acid
and 1M HCl solution. XPS invetsigation reveals the presence of both Ce(III) and Ce(IV) ions in 1. Owing to the presence of
mixed-valence cerium ions, 1' was able to oxidize the chromogenic peroxidase substrate 3,3′,5,5′-tetramethylbenzidine
(TMB) or 2,2′-azinobis(3-ethylbenzothizoline-6-sulfonic acid) (AzBTS) in the absence of an external oxidizing agent. Thus, it
showed inherent oxidase-like catalytic properties. Inspired by the excellent oxidase-mimic activity of 1', a protocol was
developed for the rapid colorimetric sensing of biothiols in NaAc buffer (0.2 M, pH = 4). The sensing ability of 1' towards
cysteine was also demonstrated in human blood plasma. Furthermore, the redox-active cerium ions enabled 1' to exhibit
excellent heterogeneous catalytic performance in aerobic oxidation catalysis of thiol compounds. The material is reusable
(both as sensor and catalyst), low-cost and highly stable, which renders it a promising candidate for monitoring of biothiols
in immunoassays and medical diagnosis as well as for industrial oxidation catalysis.
Introduction
disadvantages like absence of rapidity, selectivity and
sensitivity. Moreover, the bio-enzymes easily undergo
denaturation or chemical changes upon heating. In addition,
the preparation, storage and purification of enzymes require
Colorimetric sensing technique has gained immense attention
in biology and analytical chemistry owing to its easy visual
readout and rapid visual determination with naked eyes or
7
,8
1
too much time. Hence, numerous analogous materials such
9
low-cost portable instruments. Colorimetric biosensors are
10 11
Au nanoparticles, graphene oxide and carbon
as Fe
1
3
2
O
4
,
capable of showing response towards target biomolecules
dots
have been investigated. Since the catalytic
(
cysteine, glutathione, homocysteine, thiamine, glucose,
2
performances of these materials are analogous to those of
natural enzymes, their enzyme-mimic characteristics have
been utilized for the sensing of biological analytes. The major
advantages of these analogous materials include facile and
controlled synthesis having high yield, lower cost, adjustable
catalytic performances, and tunable structure and
ascorbic acid, etc.) through a change in colour. Detection of
these species is highly desirable, since they play vital roles in
different physiological processes such as metabolism, protein
3
synthesis, detoxification and reversible redox reaction. For
4
the sensing of biomolecules, optical, electrochemical and
5
6
bio-electrochemical sensing techniques are extensively
13
composition. Because of these benefits, enzyme-mimetic
nanomaterials have attracted widespread attention. These
materials exhibit similar catalytic activity as natural oxidases or
peroxidases and hence they have been efficiently utilized as
biosensors for applications in environmental monitoring and
employed. However, these methods have considerable
2
biomedical diagnosis.
The oxidation of thiols to disulfides has drawn immense
interest since the disulfides play vital roles in many
1
4
biochemical and industrial processes.
In the industry,
disulfides are used in synthesis of pharmaceuticals,
1
agrochemicals and vulcanization of rubber. Furthermore, the
5
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protection of thiols can be easily achieved through the confirmed by XPS analysis. The activated MOF (1') features an
formation of disulfides and conversely the cleavage of disulfide inherent oxidase-mimic behaviour in NaAc buffer at acidic pH,
1
6
bonds leads to the formation of thiols. The oxidation of thiol since it is capable of performing rapid oxidation of
compounds is an effective method for the industrial chromogenic peroxidase substrates like 3,3′,5,5′-
production of disulfides because of the commercial availability tetramethylbenzidine (TMB) or 2,2′-azinobis(3-
of many thiols and their convenient preparation procedures. ethylbenzothizoline-6-sulfonic acid) (AzBTS) without the need
However, many over-oxygenated by-products such as of any external oxidizing agent (e.g. H ). On the basis of
2 2
O
sulfoxides and sulfones can be produced during the oxidation these outcomes, we have established a colorimetric sensing
of thiols. On the other side, the selective oxidation of thiols to platform for biothiols in NaAc buffer (pH = 4.0) as well as in
disulfides have been so far accomplished through the human blood plasma. Moreover, the heterogeneous catalytic
employment of a variety of reagents such as Ce(IV) salts, activity of 1' for oxidation catalysis of thiol compounds to
permanganates, peroxides, air along with transition-metal disulphides using molecular oxygen has been thoroughly
oxides, sodium chlorite, sodium perborate, halogens, nitric investigated.
1
7
oxide and ferric chloride.
Metal-organic frameworks (MOFs) have received tremendous
attention in last two decades since they have shown
application potentials in chemical sensing, gas adsorption and
Experimental
Materials and synthetic procedures
1
8-20
separation, heterogeneous catalysis and drug delivery.
Until today, few Fe(III)-containing MOFs such as MIL-53, MIL-
8A, MIL-68, MIL-100, MIL-88-NH and MIL-101 have been
2
The synthesis of H DMTDC ligand was accomplished by
4
following a formerly documented protocol. All the other
1
8
2
reagents were procured from commercial vendors. The
reported, which have exhibited inherent peroxidase-like
–1
2
1-28 infrared (IR) spectra in the range 440-4000 cm were collected
with a Perkin Elmer Spectrum Two spectrometer. To describe
absorption bands, the following notations were used: broad
catalytic behaviour applicable for colorimetric biosensing.
These examples also include two Zr(IV)-based MOFs having
2
9-31
Fe(III)-metalated porphyrin ligands.
However, only two
(br), shoulder (sh), weak (w), strong (s), medium (m) and very
M(IV)-based (M = Zr, Ce) single-metal MOFs have been
employed so far to demonstrate the oxidase-like activity for
strong (vs). Thermogravimetric analyses (TGA) were carried
out by a Mettler-Toledo TGA/SDTA 851e thermogravimetric
instrument in a temperature range 25-700 °C with a heating
3
+
4+
biosensing. These MOFs include a mixed-valence (Ce /Ce )
3
Ce-MOF (called MVCM) and Zr(IV)-based MOF UiO-66-NH
2
33
2
.
−
1
rate of 5°C min in air atmosphere. X-Ray powder diffraction
XRPD) measurement were performed with a Bruker D2 Phaser
Among various MOFs that have displayed either oxidase- or
peroxidase-like activity, facile colorimetric sensing for biothiols
(
3
1
X-ray diffractometer (10 mA, 30 kV) utilizing Cu-Kα (λ = 1.5406
Å) radiation. X-ray photoelectron spectroscopy (XPS)
measurement was carried out at room temperature using a
have been demonstrated only for Fe-MIL-88-NH
2
, MVCM and
Zr-UiO-66-NH . On the other hand, only two MOFs namely
2
Fe(BTC) and Fe-MIL-100 have been demonstrated to
accomplish oxidation catalysis of thiols to disulphides using
custom-built
near-ambient
pressure
photoelectron
1
8, 34
spectrometer (Prevac, Poland) and full details are available in
ref. 18 and 19. It is equipped with R3000HP analyser (Scienta)
with a twin-anode and monochromatic (Al-Kα) X-ray source.
However, the experiment was carried out with Mg-Kα (hν =
molecular oxygen.
Cerium oxide nanoparticles (nanoceria) occurs as mixed
3
+
4+
valence state oxides of Ce and Ce , and are able to reversibly
3
5
switch between the two oxidation states. Due to this
phenomenon, nanoceria has featured oxidase-mimic
1
253.6 eV) source. The base pressure in the analysis chamber
−
10
3
6-38 was kept in the 3 to 6 × 10
mbar range. The energy
characteristics beneficial for colorimetric biosensing.
resolution of the spectrometer was set at 0.7 eV with Mg-Kα
radiation at a pass energy of 20 eV. The calibration for the
binding energy (BE) was carried out with the Au(4f7/2) core
level at 84.0 eV. Field emission - scanning electron microscopy
Furthermore, the MVCM material has been recently reported
to sense biothiols colorimetrically due to its oxidase-mimic
3
2
features. On the other hand, few Ce(IV) complexes have
been employed to accomplish oxidation of thiol compounds to
disulphides in the homogeneous phase using molecular
(
FE-SEM) images were collected by Zeiss (Zemini) scanning
electron microscope. UV-vis spectra in the region 250-800 nm
were recorded with Perkin Elmer Lambda 25 UV-vis
3
9
oxygen. In addition, the ability of cerium(IV) ammonium
nitrate (CAN) to act as a homogeneous oxidation catalyst has
been demonstrated in various substrates including alcohols,
thioethers, epoxides, alkylbenzenes and active methylene
a
spectrometer. All solutions for the UV-vis measurements were
prepared by using Milli-Q water. The nitrogen adsorption
4
0
experiments at -196 °C were carried out employing a
compounds. However, until now, there is no report on Ce-
based MOFs showing catalytic activity for colorimetric
biosensing of thiols as well as aerobic oxidation of thiols.
Encouraged by the richness of cerium chemistry in colorimetric
sensing and oxidation catalysis, we have synthesized a Ce-
Quantachrome Autosorb iQ-MP volumetric gas adsorption
equipment. Before the adsorption measurement, the
degassing of the sample was accomplished by heating at 70°C
for overnight in high vacuum. Conversion and selectivity were
determined using Agilent 7820 gas chromatography with FID.
based MOF
b]thiophene-2,5-dicarboxylic acid (H
presence of both Ce(III) and Ce(IV) ions in the framework was
(
1)
incorporating 3,4-dimethylthieno[2,3-
To identify the products, gas chromatography
– mass
2
DMTDC) as ligand. The
2
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spectrometry (GC-MS) measurements were performed with also carried out by replacing the biothiols with the following
Perkin Elmer Clarius 500 spectrometer.
possibly interfering biomolecules: aspartic acid, alanine,
arginine, proline, glycine, isoleucine, lysine, histidine, leucine,
phenylalanine, threonine, methionine, valine, tryptophan and
serine. In order to determine the selectivity of 1' towards
biothiols co-existing with other biomolecules, the above-
mentioned procedure was followed, except the sequential
addition of two amino acid solutions (i.e. interfering
biomolecules followed by biothiols) in the last step instead of
one amino acid (i.e. biothiols) solution.
Synthesis of [Ce
6
O
4
4 6 2
(OH) (DMTDC) ].2DMF.22H O (1)
A mixture of H
2
DMTDC (40 mg, 0.16 mmol) in 2 mL of N,N-
dimethylformamide (DMF) was sonicated until it dissolved
completely. Consequently, ceric ammonium nitrate (CAN) was
added as an aqueous solution (400 µL, 0.533 M). The mixture
was put inside a glass tube, which was sealed. This tube was
heated at 100 °C for 3.5 h by means of a dry block heater. The
mixture was cooled down to ambient temperature and the
light yellow precipitate was filtered off. This precipitate was
Detection of biothiols in human blood plasma
washed twice with DMF and once with acetone. The resulting From the left arm vain, 5 mL of human blood was collected in a
solid was dried in an oven at 70 °C for 6 h and denoted as
tube having ethylenediaaminetetraacetic acid (EDTA) under
Yield of the compound was 48 mg (0.02 mmol, 46%) aseptic conditions. For the separation of plasma, the blood
considering the Ce salt. Anal. calcd for C66
12 was centrifuged at 3000 rpm for 5 min. Afterwards, the straw-
3040.95 g mol ): C, 26.06 H, 3.24 N, 0.921. Found: C, 26.74 H, coloured supernatant human blood plasma (HBP) was
1.
6 2 56
H98Ce N O S
-1
(
-1
3
.18 N, 1.02%. FT-IR (KBr, cm ): 3430 (br), 2927 (w), 2852 (w), collected and employed for the further experiments. 160 µL of
1
647 (s), 1605 (sh), 1495 (m), 1378 (vs), 1268 (w), 1158 (sh), HBP was placed in four microcentrifuge tubes and different
1
103 (m), 1019 (w), 799 (sh), 772 (m), 662 (w), 558 (m), 469 amounts of cysteine solutions were spiked. After that,
(
w).
incubation of the mixture was accomplished at 37 °C for 5 min.
Later, 200 µL of 60% (v/v) acetonitrile was added to this
mixture for precipitating the proteins. Then, the mixture was
centrifuged at 6000 rpm for 5 min for the complete separation
of the proteins. The resulting supernatant solution was used
further for sensing cysteine in a similar method as the sensing
of cysteine in NaAc buffer. A control experiment was also
carried out without spiking the cysteine.
Activation of as-synthesized 1
At first, the as-synthesized material (0.2 g) was stirred in
methanol (20 mL) for 24 h at ambient temperature. Then, the
yellow material was filtered off. The degassing of the solid was
accomplished by heating at 70 °C for 16 h in high vacuum. In
this way, the activated material (denoted as 1') was obtained.
Aerobic oxidation catalysis
Oxidase-like activity of 1'
A typical reaction was performed in which thiophenol (1
mmol) and 2 mL of methanol were poured in a 25 mL round-
bottom flask and catalyst 1' (20 mg) was added to it. The
reaction mixture was stirred at 70 °C in a preheated oil bath
and oxygen from a balloon was purged into it. The progress of
the reaction was regulated by GC through sampling aliquots at
various time gaps. Heterogeneity of the catalytic reaction was
performed as indicated above, except that the catalyst was
filtered after 2 h from the reaction mixture using a 0.2 µm
PTFE filter and then the reaction was continued up to 12 h in
the absence of the catalyst. In addition, the stability of the
catalyst was checked by performing reusability experiments
under the optimized reactions conditions. After the reaction,
For performing the UV-vis experiments, aqueous suspensions
of 1' (concentration: 1 mg/mL) were made by ultrasonic
treatment for 30 min. For examining the oxidase-like activity,
the catalytic oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB)
was investigated to obtain a blue coloured solution. For this
purpose, 120 µL aqueous suspension of 1' and 100 µL of TMB
solution (0.04 mM) were added to 2780 µL of NaAc buffer (0.2
M, pH = 4.0). The influence of reaction time on absorbance
(λmax = 652 nm) of ox-TMB was examined by measuring UV-vis
spectra of the resulting mixture at a regular time interval (2
min) until the absorbance of ox-TMB became saturated. For
investigation of the steady-state reaction mechanism,
additional UV-vis measurements were carried out under
similar conditions as mentioned above, except that various
concentrations of TMB solution were added to the suspension.
1' was filtered, washed twice with methanol and then washed
thoroughly with acetone for the exclusion of all unreacted
compounds from the catalyst. Afterwards, the material was
dried at 80 °C for 16 h in a traditional oven. This dried
compound (1') was used as the catalyst in the subsequent
Detection of biothiols in NaAc buffer
The colorimetric method for the detection of biothiols involve catalytic runs.
the following steps: initially, 2680 μL of 0.2 M NaAc buffer (pH
=
1
4.0), 120 μL of TMB solution (1 mM, ethanol solution) and
-1
00 μL aqueous suspension of 1' (1.0 mg mL ) were mixed
together to get a blue coloured solution of ox-TMB. After the
absorbance of ox-TMB was saturated, 100 μL of a biothiol
solution (0.5 mM) was added and the UV-vis spectrum of the
resulting mixture was recorded. Analogous experiments were
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groups of twelve DMTDC ligands. The chemical structure of the
DMTDC ligand is shown in Fig. 2e. The cubic, 3D framework
is formed by the interconnection of the cluster cores by
H
2
of
1
the carboxylate groups of adjacent ditopic DMTDC ligands. The
structure incorporates two types of cages: larger octahedral
and smaller tetrahedral. Eight tetrahedral cages surround each
central octahedral cage. Both cage types are accessible to
guest molecules via narrow triangular windows. The methyl
groups attached with the DMTDC ligands are located at the
triangular windows. These methyl groups project towards the
inner side of the octahedral cages.
Fig. 1 Simulated (black) and experimental (red) XRPD patterns of 1.
Results and discussion
Preparation and activation procedure
Compound
ammonium cerium(IV) nitrate and 3,4-dimethylthieno[2,3-
]thiophene-2,5-dicarboxylic acid (H DMTDC) ligand in N,N
1 was synthesized by heating a mixture of aqueous
b
-
2
dimethylformamide (DMF) at 100 °C for 3.5 h. In order to
remove the occluded DMF molecules, the as-synthesized
material was first stirred in methanol. Later, the methanol
molecules were removed from the cages by heating the
methanol-exchanged material in high vacuum at 80 °C for 12 h.
In this way, the activated material (termed as 1') was obtained.
Structure description
The experimental XRPD pattern of the as-synthesized form of
1
Fig. 2 Cubic 3D framework structure of 1 in ball-and-stick representation. (a,
matches closely with the theoretical XRPD pattern of its b) Depiction of octahedral (yellow spheres) and tetrahedral (orange spheres)
previously reported, Zr(IV) analogue having UiO-66 type of cages. (c, d) are the magnified views of (a, b), respectively. (e) Structure of
4
2, 43
the H DMTDC ligand. Colour codes: Ce, blue polyhedra; C, grey; O, red; S,
2
framework topology (Fig. 1).
Therefore, both 1 and its
yellow.
Zr(IV) analogue have the same framework topology as that of
4
UiO-66. The XRPD pattern of the as-synthesized compound
can be successfully indexed in a cubic crystal system. The unit
4
Material characterization
cell parameters of
isostructural Zr(IV)-containing material.
structure of the isostructural Zr(IV) material has been analysis. The XPS study reveals that both Ce(III) and Ce(IV) ions
1 (Table S1, ESI†) are very similar with the
4
2, 43
The framework The oxidation state of cerium in
1 was investigated by XPS
4
2, 43
46
described formerly by us.
Briefly, the cubic framework are present in the framework of
1 (Fig. S1, ESI†). The Ce 3d
structure (Fig. 2) of the title compound comprises hexanuclear core level spectrum was analyzed to explore the oxidation
4
cluster cores having composition [Ce
5
12+
]
6
O
4
(OH)
4
. In these state of Ce. The deconvolution of the Ce 3d core level
cluster cores, the six cerium atoms are situated at the corners spectrum suggests the presence of ten peaks. The six peaks at
of an octahedron. The eight faces of each octahedron are 883.1, 888.5, 898.4, 902.1, 907.5 and 916.8 eV correspond to
bridged by the O atoms of μ
arranged in an alternative fashion. Eight
coordinated with each Ce atom. The latter has square- amount of Ce(III) ion present in
antiprismatic geometry in which the O atoms from the by utilizing the peak area of the deconvoluted core level
3 3
-O and μ -OH groups, which are the Ce(IV) state, whereas the Ce(III) state gives rise to four
4
6-48
O
atoms are peaks at 880.6, 885.6, 900.0 and 904.8 eV.
The relative
1
was determined to be 23.6%
4
9
carboxylate, μ
3
-O and μ
1
3
-OH groups occupy the square faces. spectrum. The reducing ability of the DMF solvent with
cluster core is linked with the carboxylate
2+
Each [Ce (OH) ]
4
6
O
4
4
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ARTICLE
prolonged reaction time at high temperature might be
5
responsible for the reduction of the Ce(IV) ions.
The TG trace of activated 1' contains one weight loss stage
0
below the decomposition temperature which is observed
In the IR spectra (Fig. S2, ESI†) of as-synthesized and activated because of the removal of the adsorbed water molecules from
, absorption peaks with high intensity are observed at around the pores. The activated material adsorbs water (moisture)
1
-1
1
595 and 1385 cm . These peaks occur due to the asymmetric from air during storage under ambient conditions before the
and symmetric –CO stretching vibrations of the framework TG measurement.
DMTDC ligands, respectively.
synthesized contains medium absorption peak at around samples (0.1 g) were stirred in various liquids such as water,
650 cm , which is attributable to the stretching vibrations for 1M HCl and acetic acid (15 mL for each liquid) at ambient
2
5
1, 52
The IR spectrum of as-
For checking the chemical stability of activated 1', the
1
-1
1
the carbonyl group of DMF molecules residing inside the temperature for 12 h. Then, the samples were filtered off and
pores. The IR spectrum of the activated compound does not XRPD experiments were carried out for checking their
contain any absorption peak for the occluded DMF molecules. crystallinity. Fortunately, the crystallinity of the compound was
The results from the FT-IR analysis confirm the complete did not change upon stirring in these liquids, which became
activation of the compound.
obvious from their XRPD patterns (Fig. S4, ESI†). The high
stability of the material in both aqueous and acidic medium
encouraged us to investigate its oxidase-like catalytic activity
in acidic medium.
To check the phase-purity and morphology of 1', FE-SEM
images were collected. The FE-SEM images (Fig. S5, ESI†)
confirm the homogeneous crystalline nature of 1' (consisting
of octahedral particles) and the absence of any impure phase.
2
Fig. 3 N sorption isotherms of 1' collected at -196 °C.
In order to check the permanent porosity of activated 1', N
2
sorption experiments were conducted. In Fig. 3, the N
2
sorption isotherms of 1' are displayed. The N
isotherms follow a type-I behaviour according to the IUPAC
classification. From the N adsorption isotherm, the specific
BET surface area and micropore volume (at p/p = 0.5) of 1'
2
sorption
Fig. 4 UV-vis spectra of 0.04 mM TMB (a) with and (b) without 1' (13.3 µM)
in NaAc buffer (0.2 M, pH = 4) at room temperature. Inset: corresponding
chromogenic change of TMB.
2
0
2
-1
3
-1
were calculated to be 1020
respectively.
m
g
and 0.65 cm g ,
Oxidase-mimic properties
To investigate the thermal stability of
1
and 1'
,
thermogravimetric analyses were performed in the range 25- At first, we have explored the ability of 1' to oxidize peroxidase
00 °C under air atmosphere. From the TG traces (Fig. S3, substrates in acidic medium. For this purpose, we have chosen
ESI†), it can be concluded that the material is stable up to 300 two typical chromogenic peroxidase substrates, namely 2,2-
C. The first weight loss of 13.2 wt% (range: 40-150 °C) in the azinobis (3-ethylbenzothizoline-6-sulfonic acid) (AzBTS) and
TG trace of as-synthesized
occurs because of the removal of 3,3’,5,5’-tetramethylbenzidine (TMB). Either a blue (for ox-
2 guest H
O molecules per formula unit (calcd.: 13.0 wt%). In TMB) or green (for ox-AzBTS) color is produced when these
7
°
1
2
2
the range 150-210 °C, the second weight loss of 5.0 wt% takes substrates are oxidized in aqueous medium. These substrates
place due to the removal of 2 occluded DMF molecules per have been previously employed for examining the peroxidase-
2
3, 24, 26, 28, 53-67
formula unit (calcd.: 4.86 wt%). After 210 °C, the material like activity of several MOF materials
2 2
where H O
starts to decompose on account of the loss of framework is employed as an oxidizing agent. On the other hand, the
present cerium-based 1' is capable of oxidizing both AzBTS and
ligands.
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TMB without any external oxidizing agent such as H
facts have been verified by the visual color changes of the dye and 0.11 µM s , respectively
molecules in NaAc buffer (pH = 4) in presence of 1' and the natural enzyme called horseradish peroxidase (HRP),
2
O
2
. These plot, the values of
K
m
and
V
max were estimated to be 88 µM
-1
.
m
The K value for 1' is lower than
5
0
3
2
corresponding drastic changes in the UV-vis spectra. The pH-
recently reported Ce-MOF called MVCM
as well as
3
6
dependent investigation disclosed that the optimal pH value nanoceria (Table S2, ESI†). This
K
m
value suggests that the
for the oxidase-like activity of 1' is ca. 4. The pH values higher affinity of 1' towards TMB molecule is higher as compared to
or lower than 4 led to decrease in the oxidase-mimic activity of HRP, MVCM as well as nanoceria.
the MOF towards TMB (Fig. S6, ESI†). For TMB, the formation
of a blue colored solution having absorption maxima at 652
and 370 nm (Fig. 4) confirms the formation of the oxidized
8
6
product (denoted as ox-TMB). In case of AzBTS, the new
absorption bands appeared at around 415 and 644 nm, which
+
corroborates the generation of ABTS˙ radical (Fig. S7, ESI†).
69
The green color was developed within one minute, which
indicates a rapid oxidation of the dye molecule by 1'. We have
systematically studied the influence of catalyst amount on the
catalytic activity of 1'
. The change in the absorbance
(monitored at 652 nm) of TMB as a function of time upon
addition of various amounts of 1' is plotted in Fig. 5. This figure
reveals that the catalytic activity of 1' increases when
increasing amounts of its suspension are added to the reaction
medium. Furthermore, the recyclability of 1' towards the
catalytic oxidation of TMB was investigated up to 4 cycles. The
results confirmed that the MOF material is highly stable and
reusable up to
4 cycles under the optimized reaction
conditions (Fig. S8-S9, ESI†).
Fig. 6 Change in the absorption spectrum of ox-TMB upon gradual addition
of 0.5 mM cysteine solution in NaAc buffer (0.2 M, pH = 4). Inset:
corresponding change in color of the ox-TMB solution.
Fig. 7 Absorbance of ox-TMB (monitored at 652 nm) upon addition of a
potentially competing amino acid, followed by the addition of cysteine in
NaAc buffer (0.2 M, pH = 4).
Fig. 5 Time-dependent variation in the absorbance of TMB (monitored at
6
4
52 nm) upon addition of various amounts of 1' in NaAc buffer (0.2 M, pH =
).
The mechanism of the oxidase-like activity of Cerium-based 1'
4
+
3+
Since 1' displayed oxidase-like activity, the apparent steady- towards TMB may be related to the Ce ↔ Ce spontaneous
3
state kinetic parameters were evaluated for the oxidation of electron shuttling through the framework system. When
2
TMB by 1'. For this purpose, the concentration of TMB was TMB solution was added to the suspension of 1' in NaAc buffer
4
varied. To analyze the kinetic parameters, a typical Michaelis- (0.2 M, pH = 4.0), the Ce ions quickly oxidize TMB to form
+
4
+
Menten equation was employed. Lineweaver–Burk plot (Fig. blue-coloured ox-TMB. During this process, Ce ions are
3
S10, ESI†) was used in order to evaluate the maximum initial reduced to Ce ions. Then, the latter is naturally converted to
+
4
+
m
velocity (Vmax) and Michaelis-Menten constant (K ). From this the Ce ions.
6
| J. Name., 2012, 00, 1-3
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ARTICLE
Detection of biothiols in NaAc buffer
Detection of biothiols in human blood plasma
It has been previously reported that biological thiols (biothiols) have It is well known that human blood plasma contains high
7
3
the capability to reduce the blue-coloured ox-TMB in aqueous concentrations of cysteine (165-335 µm), homocysteine (9-
3
1, 32, 70, 71
74 75
13 µM) and glutathione (14-20 µm). Therefore, the ability
medium, leading to the formation of colourless TMB.
Encouraged by this fact, we have developed a colorimetric of 1' to detect cysteine in human blood plasma (HBP) was
protocol for the sensing of biothiols. The UV-vis spectra of ox- evaluated. The concentration of cysteine in HBP samples was
TMB comprising 1' in NaAc buffer (0.2 M, pH = 4) were determined by utilizing the standard addition method. The
recorded with incremental addition of 0.5 mM solutions of recovery experiments were carried out with two HBP samples
various amino acids (including alanine, cysteine, aspartic acid, spiked with cysteine. As shown in Table 1, the recovery for
arginine, histidine, glycine, leucine, isoleucine, lysine, cysteine measurement ranges from 98 to 102% with low RSD
phenylalanine, methionine, serine, proline, tryptophan (n = 3) values. These results verify the suitability of the
threonine and valine) and other thiol-containing biomolecules developed protocol for the sensing of cysteine in HBP samples.
(such as homocysteine and glutathione). Among the different
amino acids and thiol-containing biomolecules added, a rapid
decolourization of ox-TMB was observed upon addition of
biothiol compounds such as glutathione, cysteine and
homocysteine. It can be seen from Fig. 6 and Fig. S11-S13
Table 1. Concentrations of cysteine found in diluted HBP samples
determined by the standard addition technique.
HBP
Without
Spiking
(µM)
Cysteine
Added
(µM)
Cysteine
Found
(µM)
Recovery
(%)
RSD
(%)
(ESI†) that the absorbance of ox-TMB decreased dramatically
Sample
upon gradual addition of 0.5 mM solutions of cysteine,
homocysteine and glutathione. In sharp contrast, negligible
change in the absorbance of ox-TMB was observed in the
presence of other possibly interfering biomolecules (Fig. S14-
S28, ESI†). Even methionine, another sulphur-containing
amino acid, resulted in negligible change in the absorbance of
ox-TMB under the presented conditions. These phenomena
can be understood considering that after oxidation of TMB by
the MOF catalyst to produce bluish coloured ox-TMB, the
biothiols reverses the process by reduction of ox-TMB back to
colourless TMB. The results presented herein suggest that 1'
shows high selectivity towards the sensing of biothiols
Sample 1
Sample 2
65.4
10
9.8
98
1.8
50
10
50.1
0.1
100.2
101
2.1
1.6
1.2
110.5
5
0
51.2
102.4
Catalytic oxidation of thiol compounds
MOFs are considered as promising heterogeneous catalysts for
7
6, 77
organic transformations including oxidation reactions.
MOFs have previously served as heterogeneous catalysts in
the aerobic oxidation of cycloalkanes, hydrocarbons, benzyl
7
8
alcohol, thiols, amines and sulfides. Recently, research
interest in the construction of MOF-type heterogeneous
catalysts for the aerobic oxidation of organic substrates is
(glutathione, cysteine and homocysteine).
In order to investigate the selectivity of 1' towards the
7
6
sensing of biothiols, the UV-Vis spectral response of the MOF
system (i.e., 1' containing ox-TMB in aqueous medium) was
measured towards biothiols in the existence of several
potentially interfering amino acids. It has been observed that
the absorbance of the MOF system changed significantly only
after the introduction of biothiols to the reaction medium,
whereas the introduction of other amino acids caused
negligible changes in the absorbance (Fig. 7 and Fig. S29-S43,
ESI†). Therefore, the selective colorimetric sensing of biothiols
in presence of other potentially competing amino acids can be
achieved by this method.
growing rapidly. Inspired by these observations, we have
investigated the catalytic activity of 1' using thiophenol as a
model substrate under oxygen atmosphere as shown in
Scheme 1.
For determining the limit of detection (LOD) of 1' towards
biothiols, very less concentrated biothiol solutions were slowly
added to the reaction medium and the absorbance of ox-TMB
was monitored. The plots of absorbance of ox-TMB against the
concentration of biothiol solutions revealed linear curves (Fig.
S44-S46, ESI†). The LOD values were estimated from this linear
Scheme 1. Oxidation of thiophenol to 1,2-diphenyldisulfide in presence of
catalyst 1′ and molecular oxygen.
Aerobic oxidation of thiophenol in the absence of 1' exhibited
only 9% conversion at 70 °C in methanol after 12 h (entry 1,
Table 2). In sharp contrast, the oxidation of thiophenol with 1'
using tert-butylhydroperoxide (TBHP) as an oxidant resulted in
5
0
curve by following the previously reported method. The LOD
values for cysteine, homocysteine and glutathione by this
colorimetric biosensing technique were estimated to be 0.150,
6
5% conversion after 12 h in acetonitrile (entry 2, Table 2). On
0.132 and 0.125 µM, respectively. These LOD values are either
the other hand, the oxidation of thiophenol using molecular
oxygen afforded 22% conversion (entry 3, Table 2) in
acetonitrile under same conditions. Interestingly, the aerobic
oxidation of thiophenol afforded quantitative conversion to
similar or lower as compared to the previously reported MOFs
3
1, 33, 72
(Table S3, ESI†)
exhibiting selective detection properties
towards biothiols.
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the corresponding 1,2-diphenyldisulfide in the presence of 1' recyclability of 1' for the oxidation of thiophenol with
as catalyst in methanol (entry 4, Table 2) after 12 h. The time- molecular oxygen was examined under the optimized reaction
st
conversion curve for the oxidation of thiophenol by employing conditions. The conversion of thiophenol observed in the 1 ,
nd
rd
1' (catalyst) and molecular oxygen is shown in Fig. 8. Under 2 , 3 and 4 cycles after 12 h corresponded to 100%, 98%,
th
similar conditions, the oxidation of thiophenol using catalyst 1' 97% and 96%, respectively (Table 2 and Fig. S47, ESI†).
and molecular oxygen in presence of pyridine significantly Furthermore, the crystallinity of the catalyst recovered after
th
reduced the conversion of thiophenol to 27% (entry 5, Table the 4 cycle was very similar with the fresh 1', as corroborated
2). This experiment clearly demonstrates the negative role of by the corresponding XRPD patterns (Fig. S48, ESI†). The
pyridine by acting as a catalyst poison through coordination structural robustness of the MOF catalyst during the catalytic
with the Lewis acid sites, thus inhibiting the coordination of cycles was confirmed by these experiments.
thiophenol with cerium ions. It has been already reported that
ceric ammonium nitrate (CAN) ion is highly effective in the
Table 2. Oxidation of thiophenol to 1,2-diphenyldisulfide in the presence of
catalytic oxidation of thiols and thioethers utilizing molecular
a
1
' under various conditions.
4
0
oxygen. In this context, a control experiment in the presence
of CAN was performed and the observed results are given in
Table 2. It is clear from these data that the reaction proceeds
in the presence of CAN, however, it could not reach
completion because of the absence of adequate Ce(IV) ions in
the reaction medium (entry 6, Table 2). In contrast, the
reaction in the presence of 1' resulted in complete conversion
due to the presence of sufficient amount of Ce(IV) ions as
active sites in the framework, thus proving the merit of
heterogeneity.
Sl. No.
Catalyst
(mg)
---
Oxidant
Solvent
Conversion (%)
1
O
2
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
3
OH
9
b
2
20
TBHP
3
CN
CN
OH
OH
OH
OH
OH
OH
OH
65
3
4
20
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
3
22
20
3
100
27
c
5
20
3
d
6
27
3
68
7
8
9
run-1
run-2
run-3
run-4
3
100
98
3
3
97
1
0
3
96
a
Reaction conditions: thiophenol (1 mmol), solvent (2 mL), 1' (20 mg), 70 °C,
1
2 h.
b
with 1 mmol TBHP.
with 1 mmol pyridine.
with CAN.
c
d
The encouraging results concerning the aerobic oxidation of
thiophenol using 1' as a catalyst prompted us to explore the
versatility of this catalyst with substituted thiophenols. The
results of these experiments are summarized in Table 3.
Thiophenols with electron donating substituents such as
amino, methyl and methoxy at 2-, 4- and 4-positions exhibited
respective disulfides in 71, 97 and 59% yields after 12 h
(entries 2-4, Table 3). On the other hand, thiophenols
Fig. 8. Hot filtration experiment for the aerobic oxidation of thiophenol (a)
containing electron withdrawing groups like fluoro, chloro and
bromo at 2, 3- and 4-positions resulted in 87, 91 and 82%
yields of the corresponding disulfide (entries 5-7, Table 3).
Similarly, cyclohexanethiol gave 26% yield of the
with and (b) without 1'. Reaction conditions: thiophenol (1 mmol), CH
3
OH (2
mL), 1' (20 mg), 70 °C.
Hot filtration experiment was performed in order to verify corresponding disulfide under identical conditions (entry 8,
the heterogeneity of the reaction. Therefore, the oxidation of Table 3).
thiophenol was carried out in presence of 1' and molecular
oxygen under the optimum reaction conditions. The catalyst 1'
was filtered off after 2 h and the reaction was prolonged with
the supernatant liquid for another 10 h. The results obtained
from these experiments are shown in Fig. 8. These results
confirm the heterogeneity of the oxidation catalysis reaction.
However, there was marginal enhancement in the conversion
of thiophenol after removal of the catalyst, which is indicative
of the contribution from blank control. In addition, the
8
| J. Name., 2012, 00, 1-3
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ARTICLE
Table 3. Oxidation of various thiophenols in the presence of 1' and peroxidase substrates: TMB and AzBTS. The oxidase-mimic
a
molecular oxygen.
activity of 1' allowed us to establish a colorimetric sensing
platform for biothiols in NaAc buffer (0.2 M, pH = 4). In
addition, the MOF displayed sensing ability towards cysteine in
human blood plasma. Owing to the existence of framework
Ce(III)/Ce(IV) sites, significant heterogeneous catalytic
performance of 1' was observed in the oxidation of thiol
compounds using molecular oxygen. The MOF material is
reusable, both as a heterogeneous catalyst as well as a
colorimetric biosensor. Moreover, the compound is highly
stable, low-cost and recyclable. These features along with high
biomimetic and heterogeneous catalytic activities make the
present compound suitable for biological sample analysis,
medical diagnostics as well as for industrial oxidation catalysis.
Sl.
No.
Thiol
Disulfide
Yield
(%)
b
1
2
100
71
3
97
Acknowledgements
4
5
59
87
SB would like to acknowledge financial assistance from Science
and Engineering Research Board (SERB; grant no.
EEQ/2016/000012), New Delhi. The Central Instruments
Facility (CIF) of IIT Guwahati is acknowledged facility for
analytical facility. We are thankful to Ministry of Human
Resource Development for Center of Excellence in FAST (grant
no. F. No. 5-7/2014-TS-VII). We are also thankful to Dr.
Chinnakonda S. Gopinath (NCL, Pune, India) for XPS analysis.
AD thanks the University Grants Commission (UGC), New
Delhi, for the award of an Assistant Professorship under its
Faculty Recharge Programme. AD also thanks the Department
of Science and Technology, India, for the financial support
through Extra Mural Research Funding (EMR/2016/006500).
Br
6
7
82
S
S
Br
91
26
8
Notes and references
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4
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.
.
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a
cerium-based MOF
5
.
material
(
1
)
incorporating
3,4-dimethylthieno[2,3-
,
b]thiophene-2,5-dicarboxylic acid as ligand under solvothermal
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Journal Name
Pleas eC dr yo s nt oE tn agd Cj uos mt mm argins
View Article Online
DOI: 10.1039/C7CE01053B
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Table of Contents:
A cerium-based MOF has been reported to exhibit oxidase-
mimic activity for colorimetric sensing of biothiols and aerobic
oxidation of thiols.
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