Dual sensing of copper ion and chromium (VI) oxyanions by benzotriazole functionalized UiO-66 metal-organic framework in aqueous media

Article abstract of DOI:10.1016/j.jphotochem.2019.112238

A new dual chemosensor UiO-66-NH-BT (BT=1-methylenebenzotriazole) based on the UiO-66 (University of Oslo) framework, containing benzotriazole functionalized dicarboxylate struts was synthesized and characterized. This isoreticular Metal-Organic Framework (MOF) was found to be a very selective and ultrasensitive for copper ion and chromium oxyanions in aqueous media. It showed a detection limit of 16.9 ppb (0.266 μM) for Cu²⁺ ion, 280 ppb (1.3 μM) for Cr₂O₇ ^2? and 47.7 ppb (0.411 μM) for CrO₄ ^2? anions. The quenching constants (K_sv) for Cu²⁺, Cr₂O₇ ^2?, CrO₄ ^2? was found to be 1.1x10⁵ M^?1, 3.9x10³ M^?1, and 6.7x10³ M^?1 respectively. The covalently bonded benzotriazole moiety with the UiO-66 framework not only produces an emission peak at 491 nm but also act as an intrinsic binding site for both cations and anions. The nature of the coordinative interaction between the analytes and the UiO-66-NH-BT has also been elaborated with the help of ICP and FTIR. This chemosensor also demonstrated a regenerative property without the loss in performance for five consecutive cycles.

Full text of DOI:10.1016/j.jphotochem.2019.112238

Journal Pre-proof
Dual Sensing of Copper Ion and Chromium (VI) Oxyanions by
Benzotriazole Functionalized UiO-66 Metal-Organic Framework in
Aqueous Media
Aasif Helal, M. Nasiruzzaman Shaikh, Md. Abdul Aziz
PII:
S1010-6030(19)31459-5
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112238
JPC 112238
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
24 August 2019
3 November 2019
12 November 2019
Please cite this article as: Helal A, Nasiruzzaman Shaikh M, Abdul Aziz M, Dual Sensing of
Copper Ion and Chromium (VI) Oxyanions by Benzotriazole Functionalized UiO-66
Metal-Organic Framework in Aqueous Media, Journal of Photochemistry and amp;
Photobiology, A: Chemistry (2019), doi: https://doi.org/10.1016/j.jphotochem.2019.112238
This is a PDF file of an article that has undergone enhancements after acceptance, such as
the addition of a cover page and metadata, and formatting for readability, but it is not yet the
definitive version of record. This version will undergo additional copyediting, typesetting and
review before it is published in its final form, but we are providing this version to give early
visibility of the article. Please note that, during the production process, errors may be
discovered which could affect the content, and all legal disclaimers that apply to the journal
pertain.
© 2019 Published by Elsevier.

Dual Sensing of Copper Ion and Chromium (VI) Oxyanions by
Benzotriazole Functionalized UiO-66 Metal-Organic
Framework in Aqueous Media
Aasif Helal,a* M. Nasiruzzaman Shaikh, and Md. Abdul Aziz
aCenter of Research Excellence in Nanotechnology, King Fahd University of Petroleum and
Minerals,
Dhahran 31261, Saudi Arabia
Corresponding author
Tel: +966 013 860 7532; E-mail address: aasifh@kfupm.edu.sa (A. Helal)
Graphical abstract
Fluorescence quenching of benzotriazole functionalized UiO-66 by copper and chromium
oxyanions.
Highlights
 A new dual chemosensor based on the UiO-66 (University of Oslo) Metal –Organic
Framework, containing benzotriazole functionalized dicarboxylate struts.
 This MOF based chemosensor is highly selective and ultrasensitive to Cu2+ ion and
chromium oxyanions (Cr2O72- and CrO42- anions) in aqueous media.
1

2-
 This sensor showed a detection limit of 16.9 ppb for Cu2+ ion, 280 ppb for Cr2O7
and 47.7 ppb (0.4 M) for CrO42- anions.
 This chemosensor has quenching constants (Ksv) of 1.1x105 M-1, 3.9x103 M-1, and
2-
6.7x103 M-1 for Cu2+, Cr2O7 , and CrO42- respectively.
 This sensor also demonstrated a regenerative property for five consecutive cycles.
Abstract: A new dual chemosensor UiO-66-NH-BT (BT=1-methylenebenzotriazole)
based on the UiO-66 (University of Oslo) framework, containing benzotriazole
functionalized dicarboxylate struts was synthesized and characterized. This isoreticular
Metal-Organic Framework (MOF) was found to be a very selective and ultrasensitive for
copper ion and chromium oxyanions in aqueous media. It showed a detection limit of
16.9 ppb (0.266 M) for Cu2+ ion, 280 ppb (1.3 M) for Cr2O72- and 47.7 ppb (0.411
2-
M) for CrO42- anions. The quenching constants (Ksv) for Cu2+, Cr2O7 , CrO42- was
found to be 1.1x105 M-1, 3.9x103 M-1, and 6.7x103 M-1 respectively. The covalently
bonded benzotriazole moiety with the UiO-66 framework not only produces an emission
peak at 491 nm but also act as an intrinsic binding site for both cations and anions. The
nature of the coordinative interaction between the analytes and the UiO-66-NH-BT has
also been elaborated with the help of ICP and FTIR. This chemosensor also demonstrated
a regenerative property without the loss in performance for five consecutive cycles.
Keywords: Dual chemosensor, Isoreticular, Metal-Organic Framework, Ultrasensitive,
Benzotriazole.
1. Introduction
2

Metal−Organic Frameworks (MOFs) with porous structure and high surface area has been
extensively used for the capture and storage of carbon dioxide, hydrogen and methane [1-
3]. The finely tuned pores with functional sites have enabled us to use MOF in
developing materials for separations of small and large molecules, sensing of different
analytes, drug delivery, carbon dioxide conversion and heterogeneous catalysis [4-9].
These pore environments can be engineered by using functionalized linkers for different
potential applications [10]. The functionalization of the linkers can be achieved in two
ways postsynthetic modification (PSM), [10 -13] or postsynthetic exchange (PSE), [14]
and by direct solvothermal synthesis using presynthetically functionalized organic
linkers. Thus these isoreticular MOFs have gained extensive importance as the
functionality and metrics of an extended porous structure can be need fully modified
without changing its underlying topology [15].
Copper is an omnipresent metal ion and an essential trace element for the growth
and development of human beings. Thus low concentration of copper effects the enzyme
activity owing to the redox-active nature [16] while excessive accretion of copper cause
damage to the liver and kidney, hepatolenticular degeneration (Wilson’s disease),
Alzheimer’s diseases, Menkes syndrome, neutropenia and myelopathy [17]. Thus the
U.S. Environmental Protection Agency (EPA) has set the limit of copper concentration in
drinking water to be 1.3 ppm (∼20 μM) [18]. There are several analytical techniques for
detection of copper ions, such as inductively coupled plasma mass spectrometry (ICP-
MS), inductively coupled plasma atomic emission spectrometry (ICP-AES), activated
carbon or natural zeolite adsorption etc [17,19]. MOFs and their composites have also
been used for the detection of copper ions in aqueous solution [20-23]. But these methods
3

have their own disadvantages such as low selectivity, tedious and troublesome analysis
methods, complicated synthetic procedure and lack of practical applicability.
Chromium as an oxidant has large industrial applications such as chromium
electroplating, metallurgy, pigment production, leather tanning, detergent manufacturing,
welding, steel manufacturing, and wood preservation [24]. This chromium exists as
oxyanions in its most stable oxidation states of Cr(VI) ions in the environment as highly
soluble, non-biodegradable pollutants [25,26]. Cr(VI) is a carcinogenic species with
higher oxidation potential and diffusible cell membrane size [26]. Chromate ion can
cause allergic reaction in human and prolonged exposure results in chrome ulcer, contact
dermatitis, and irritant dermatitis [27]. Due to the exposure potential and toxicity the U.S.
Environmental Protection Agency (EPA) has set the maximum concentration level of
Cr(VI) in drinking and ground water to be 0.1 ppm (100g/L) and 0.05 ppm (50 g/L)
respectively [28]. Several feasible analytical techniques have been employed for the
detection of these chromium species such as inductively coupled plasma mass
spectrometry, atomic absorption spectrometry, etc [27,29,30]. However, these methods
have their own impediment such as selectivity, prolonged sample preconditioning, use of
expensive and complicated instruments, and lack of practical relevance. Several MOF
based optical chromate sensors are also reported, but most of them lack the selectivity
with other anions and their detection ability in water [25, 26, 31-33].
Zirconium based MOFs were first synthesized by Cavka et al which exhibited a
high thermal, chemical and acidic stability due to the very strong metal ligand bond that
arises from high oxidation state of Zr(IV) which results high charge density and bond
polarization [34-36]. The benzene ring of the UiO-66 could be easily functionalized
4

without disturbing the high thermal and chemical stability of the MOF. This unique
feature of UiO-66 makes it a suitable candidate to be used in various applications. In case
of most Zirconium based MOFs the fluorescence originates from the organic linkers. Due
to the Zirconium clusters amino benzene ring of the framework in UiO-66-NH2 produce
weak emission, it can be functionalized with heteroaromatic rings to produce a strong
emission as well as good binding sites for analytes [37].
In this paper, we report the synthesis and characterization of a benzotriazole
functionalized UiO-66-NH2 nanocrystalline MOF (UiO-66-NH-BT) and its application in
the sensing of heavy metal copper and chromium (VI) oxyanions. A new benzotriazole
functionalized 2-aminoterphthalic acid linker (H2L) was used to prepare the UiO-66-NH-
BT by hydrothermal methods. This MOF was then characterized with powdered XRD,
FTIR, SEM, TGA, BET, and digestion NMR. The absorption and the emission properties
of this MOF were studied and were found to be highly selective for copper ion and
chromium oxyanions in aqueous media (Figure 1).
UiO-66-NH2
UiO-66-NH-BT
Figure 1. Schematic representation of UiO-66-NH2 and UiO-66-NH-BT.
2. Experimental
5

2.1. Materials and General Methods
Chemicals used in this work: 2 aminoterephthalic acid (98% purity), benzotriazole
(99% purity), formaldehyde (99% purity), methanol (99.9% purity), ethanol (97.2%
purity), N,N-dimethylformamide (DMF; 99.8% purity), dichloromethane (99.8% extra
dry grade), potassium hydroxide (98% purity), all other nitrates or chlorides of the metal
salts, and the sodium or potassium salts of the anions were purchased from Sigma
Aldrich Corporation. NMR solvents: dimethyl sulfoxide-d6 (DMSO-d6; 99.9% purity)
were purchased from Cambridge Isotope. All chemicals were used without further
purification. Water used in this work was double distilled and filtered through a Millipore
membrane. The solutions of metal ions were prepared from their nitrate and chloride salts
and anions were prepared from their sodium and potassium salts (analytical grade)
followed by subsequent dilution to prepare the working solutions.
Instrumentation: 1H and 13C NMR spectra were recorded on a Bruker AM-400
spectrometer using Me4Si as the internal standard. Elemental microanalyses (EA) were
performed using a PerkinElmer-EA 2400 elemental analyzer. Powdered X-ray diffraction
patterns of the samples were recorded using a Rigaku MiniFlex diffractometer, which
was equipped with Cu-K radiation. The data were acquired over the 2 range of 5° and
30°. The FT-IR spectra of UiO-66-NH-BT were obtained using a Nicolet 6700 Thermo
Scientific instrument in the range of 400-4000 cm-1, using KBr. Thermogravimetric
analysis (TGA) of the samples were performed using a TA Q500. In this study, an
activated sample of UiO-66-NH-BT (10 mg) was heated in alumina pan under airflow
(60 mL min-1) with a gradient of 10 C min-1 in the temperature range of 30 - 800 C. The
BET surface areas of the MOFs were calculated by using Micromeritics ASAP 2020
6

instrument. The surface morphology of these materials were discerned using a field
emission scanning electron microscope (FESEM, LYRA 3 Dual Beam, Tescan), which
operated at 30 kV. The FESEM samples were prepared from suspension in ethanol.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS) of the copper and chromate
treated samples of UiO-66-NH-BT were carried in Thermo Scientific XSeries 2 ICP-MS.
The absorption spectra of the MOF were studied using a Jasco V-670 spectrophotometer.
Fluorescence spectra were measured using a Horiba, Fluorolog-3 fluorescence
spectrophotometer, which was equipped with a xenon discharge lamp and 1 cm quartz
cells with slit width 2 nm for both the source and the detector. The quantum yield and
chromaticity studies were done using Fluoromax-4 equipped with Quanta-Phi integration
sphere (Horiba) by using liquid sample holder at room temperature. The solutions at
different pH were prepared under adjustment by a Mettler Toledo pH meter.
Sample preparation for photophysical studies: In a typical luminescence-sensing
experimental setup, 0.001 g of UiO-66-NH-BT powder was dispersed in 1 ml of water. In
a 1 cm quartz cuvette, 3 mL of dispersed aqueous solution of UiO-66-NH-BT was placed
and the absorption and emission responses were measured in-situ after incremental
addition of freshly prepared analyte solutions. The mixtures were sonicated for 5 minutes
after each incremental addition of the analytes for uniform dispersion during the
luminescent measurements. All of the measurements were performed at 298 K.
2.2. Synthesis:
7

Synthesis of the linker:
Scheme 1. Synthesis of the linker H2L.
Synthesis of Compound 2
A solution of 2-aminoterephthalic acid (0.5 g, 2.8 mmol) in methanol (10 mL) and 2
drops of HCl was refluxed for 10 h. The solvent was removed under vacuum and the
residue was diluted with water and extracted with CH2Cl2. The organic layer was dried
over anhydrous Na2SO4 and concentrated. The residue was purified by SiO2 column
chromatography (elution with EtOAc:hexane-1:3) to give 1 in 87% yield. A mixture of 1
(0.45 g, 2.2 mmol), benzotriazole (0.255 g, 2.2 mmol), formaldehyde (37%) (0.065 g, 2.2
mmol) in Ethanol: water (1:3) (10 mL) was stirred for 24 h. The solvent was removed
under vacuum and the residue was diluted with water and extracted with ethyl acetate.
The organic layer was dried over anhydrous Na2SO4 and concentrated. The residue was
purified by SiO2 column chromatography (elution with EtOAc:hexane-1:1) to give 2 in
70% yield. 1H NMR (DMSO-d6) 3.83 (s, 6H), 6.37 (d, J = 5.2 Hz, 2H), 7.19 (d, J = 6.4
Hz, 1H), 7.37 (t, J = 6.4 Hz, 1H), 7.56 (t, J = 6.0 Hz, 1H), 7.83-7.86 (m,1H), 7.88 (s,
1H), 8.00 (d, J = 6.4 Hz, 1H), 8.13 (d, J = 6.4 Hz, 1H), 8.89 (bs, 1H, NH); 13C NMR
(DMSO-d6) 59.68, 115.12, 117.18, 118.59, 121.00, 123.2, 128.22, 131.49,
135.77, 136.23, 138.6, 149.37, 151.8, 169.63, 171.14; Anal. Calcd for C17H16N4O4: C,
60.00; H, 4.74; N, 16.46; Found: C, 60.17; H, 4.91; N, 16.52 (Figure S1, S2).
8

Synthesis of linker 2-((1H-benzo[d][1,2,3]triazol-1-yl)methylamino)terephthalic acid
H2L
Compound 2 (0.3 g, 0.9 mmol) was refluxed with an ethanolic solution of KOH for 12 h.
The solvent was removed under vacuum, the residue was diluted with water, and the pH
was adjusted to 5.0 by addition of HCl (1 M). The off-white solid precipitated was
filtered with a Buchner base washed with water and recrystallized from ethanol to give
linker H2L in 80% yield. 1H NMR (DMSO-d6) 6.37 (d, J = 5.5 Hz, 2H), 7.20 (d, J =
8.0 Hz, 1H), 7.37 (t, J = 7.5 Hz, 1H), 7.53 (t, J = 7.0 Hz, 1H), 7.85 (s, 1H), 7.87 (d, J =
8.0 Hz, 1H), 8.00 (d, J = 8.5 Hz, 1H), 8.09 (d, J = 8.5 Hz, 1H), 9.07 (bs, 1H, NH), 12.95
(s, 2H, COOH); 13C NMR (DMSO-d6) 70.71, 111.57, 113.59, 115.79, 117.66, 119.64,
124.64, 127.86, 132.50, 136.14, 145.86, 148.54, 151.68, 167.29, 169.52; Anal. Calcd for
C15H12N4O4: C, 57.69; H, 3.87; N, 17.94; Found: C, 57.77; H, 3.94; N, 18.07 (Figure S3,
S4).
Synthesis of UiO-66-NH2
UiO-66-NH2 was synthesized by dissolving ZrCl4 (0.125 g, 0.54 mmol) and 2-amino-1,4-
benzenedicarboxylic acid (0.134 g, 0.75 mmol) in DMF (20 mL) with ultrasonic
vibration for 30 min then 0.5 mL of acetic acid was added. The as-obtained mixture was
transferred to a 50 mL vial and heated at 393 K for 24 h. Then the vial was cooled in air
to room temperature. The resulting UiO-66-NH2 was washed three times with DMF (5-10
mL) using a centrifuge (10,000 rpm for 30 min), and then sequentially immersed in
methanol (5-10 mL three times per day) for three 24 h periods. Finally, UiO-66-NH2 was
activated by removing the solvent under vacuum for 24 h at 100 C. FT-IR (KBr, cm-1):
3430, 1662, 1570, 1434, 1383, 1261, 1098, 768, 663. Anal. Calcd for C54H52N8O36Zr6
9

(Zr6O4(OH)4(2-NH2BDC)6(DMF)2(H2O)2): C, 33.49; H, 2.71; N, 5.79; Zr, 28.27; Found:
C, 33.57; H, 2.94; N, 6.01; Zr, 28.68.
Synthesis of UiO-66-NH-BT
UiO-66-NH-BT was synthesized in a similar way as above by dissolving ZrCl4 (0.125 g,
0.54 mmol) and 2-((1H-benzo[d][1,2,3]triazol-1-yl)methylamino)terephthalic acid (0.234
g, 0.75 mmol) in DMF (20 mL) with ultrasonic vibration for 30 min then 0.5 mL of acetic
acid was added. The as-obtained mixture was transferred to a 50 mL vial and heated at
393 K for 24 h. Then the vial was cooled in air to room temperature. The resulting UiO-
66-NH-BT was washed in the same way as the previous method by three times with
DMF (5-10 mL) using a centrifuge (10,000 rpm for 30 min), and then sequentially
immersed in methanol (5-10 mL three times per day) for three 24 h periods. Finally, UiO-
66-NH-BT was activated by removing the solvent under vacuum for 24 h at 100 C. FT-
IR (KBr, cm-1): 3342, 1700, 1606, 1548, 1482, 1421, 1340, 1250, 1185, 1096, 1043, 772,
651, 582. Anal. Calcd for C96H82N26O36Zr6 (Zr6O4(OH)4(2-NHCH2BT-
BDC)6(DMF)2(H2O)2): C, 42.34; H, 3.04; N, 13.37; Zr, 20.10; Found: C, 42.43; H, 3.31;
N, 13.79; Zr, 20.61.
3. Results and Discussion
3.1. Characterization of the linker H2L
In this work, we have presynthetically modified the 2-aminoterephthalic acid linker by
covalently appending it with a benzotriazole moiety to give H2L (Figure 2). This new
linker was synthesized by reacting methyl ester of 2-aminoterephthalic acid with
benzotriazole in the presence of formaldehyde to give compound 2, followed by the
deprotection of the diacid by alkaline hydrolysis to give H2L in good yield as shown in
Scheme 1. The structures of 2 and H2L were confirmed by 1H NMR, 13C NMR, (Figure
10

S1-S4 of Supporting Information (SI)) and elemental analyses data. The 1H NMR spectra
of 2 and H2L exhibited doublet peak at  6.38 ppm that is assigned to the (-CH2-) protons
that bridges the 2-aminoterephthalic acid with the benzotriazole, another peak is observed
O
OH
NH2
O
OH
N
N
N
H
N
HO
O
HO
O
2-aminoterephthalic acid
H2L
Figure 2. Structures of the linkers.
at 8.9 ppm that corresponds to the NH proton of the 2-aminoterephthalic acid.
3.2. Characterization of UiO-66-NH2 and UiO-66-NH-BT
Figure 3. Powered XRD patterns of UiO-66-NH2 obtained from cif file, UiO-66-NH2 as
synthesized, UiO-66-NH-BT as synthesized, UiO-66-NH-BT after addition of copper,
and UiO-66-NH-BT after addition of chromate ion.
11

The powdered XRD pattern of UiO-66-NH2, was persistent with that documented
in the literature (Figure 3) [34]. UiO-66-NH-BT was also synthesized with the same high
crystallinity and characteristic peaks of UiO-66-NH2 at 2θ = 7.78°, 8.92° (Figure 3). This
establishes the fact that functionalization of the 2-aminoterephthalic acid with
benzotriazole did not disturb the framework and connectivity of UiO-66-NH2.
The IR spectrum of UiO-66-NH-BT (Figure S5, SI) shows a sharp peak at 772
cm-1 that represents the stretching vibration of the C―H bond in the two benzene rings.
The two peaks at 1043 cm-1 and 1096 cm−1 indicate the stretching vibration of two
adjacent N―C bonds on the benzene ring of the benzotriazole. The peak at 1250 cm−1 is
strong, and corresponds to the stretching vibration of ―N=N― of the benzotriazole. The
peaks at 1295 cm-1 and 1349 cm-1 represents the vibrations of the bond between aromatic
carbon and nitrogen of the 2-aminoterephthalate. The peaks at 1606 cm-1 and 1700 cm−1
represent –COO asymmetrical stretching and the peaks at 1482 cm-1 and 1548 cm−1
implies to the –COO symmetrical stretching. The broad peak at 3342 cm−1 corresponds to
the symmetric and asymmetric vibrations of NH bond of the organic linker.
Additionally, the alkaline digestion 1HNMR of the UiO-66-NH-BT confirmed the
presence of the linker H2L in the framework. After digesting the UiO-66-NH-BT with
KOH solution, peaks at chemical shifts matching H2L were observed (Figure S6, SI).
12

Figure 4. TGA of UiO-66-NH-BT and UiO-66-NH2.
The thermal stability of the UiO-66-NH2 and UiO-66-NH-BT materials, were
measured by thermogravimetric analysis (TGA) under an airflow with a heating rate of
5°C min−1 (Figure 4). Both samples show identical thermal behavior with the initial mass
loss of 5.3% and 4.3% for UiO-66-NH2 and UiO-66-NH-BT below 150 °C. This mass
loss is assigned to the removal of trapped solvents molecules from the pores of the
structure. The total weight loss for UiO-66-NH2 and UiO-66-NH-BT are 46.5% and
48.5% respectively occurs at a temperature of 410 °C indicating that both MOFs are
stable up to 410 °C. The final residue of 44.3% and 34.7% for UiO-66-NH2 and UiO-66-
NH-BT respectively corresponds to the zirconium oxides. The ICP-MS of the samples
showed the content of Zirconium in UiO-66-NH2 is 31% while in UiO-66-NH-BT is
25%. These results clearly indicate that the introduction of the benzotriazole functional
groups does not influence the thermal stability.
13

Figure 5. N2 adsorption isotherms of UiO-66-NH-BT and UiO-66-NH2. The filled and
open circles represent the adsorption and desorption branches, respectively.
Nitrogen sorption experiments of UiO-66-NH-BT showed that it is Type I and
had a very significant uptake at relative low pressure (P/P0) of 0 to 0.05 which is
indicative of the microporous nature like UiO-66-NH2. The BET surface area and the
pore volume of UiO-66-NH-BT was found to be 384 m2g-1 and 0.275 cm3g-1 respectively.
These were very small as compared to the pristine UiO-66-NH2 (BET surface area= 1067
m2g-1 and pore volume= 0.517 cm3g-1) due to the presence of the appended benzotriazole
moieties that block the pores and reduced the surface area (Figure 5). The SEM image of
the UiO-66-NH-BT shows that they are uniformly distributed bipyramidal shaped particle
with 125-150 nm in size (Figure S7, SI).
14

3.3. Optical properties of UiO-66-NH-BT
The absorbance and emission studies of UiO-66-NH-BT were carried out in aqueous
solution as an emulsion. In general, 2-aminoterephthalate is luminescent but in UiO-66-
NH2 it has a low emission, due to the ligand-to-metal charge transfer (LMCT), on co-
ordination with the Zr-O clusters [37]. But in case of UiO-66-NH-BT the emission peak
is produced at 491 nm, at an excitation wavelength of 360 nm, which is 58 nm red shifted
from the emission peak of UiO-66-NH2 (em=449 nm) (Figure S8, SI). Thus
functionalization of UiO-66-NH2 with benzotriazole produces a redshift and enhanced
emission due to the electron transfer (ET) from the benzotriazole to the benzene ring in
the main structural framework of UiO-66 (Scheme 1) [38,39]. Moreover, the
benzotriazole with the NH- moiety provide selective binding sites for metal ions. The
absolute quantum yield of UiO-66-NH-BT, calculated from the integrated sphere, was
found to be 0.56 and the CIE coordinates of the chromaticity diagram, are persistent with
the experimentally derived emission results (Figure S9, SI).
3.4. Cation sensing properties of UiO-66-NH-BT
In order to check the cation sensing properties, UiO-66-NH-BT was screened with
different cations and the change in its absorbance properties were noted. It was found that
on addition of different cations only copper ion produces a change in the absorbance of
UiO-66-NH-BT. All the optical sensing experiments were carried out using 10-2 M
aqueous solutions of copper salt. Upon slow addition of Cu2+ to a suspension of UiO-66-
NH-BT the absorbance increased and an additional new peak is formed at 284 nm along
with the peaks at 256 and 360 nm (Figure S10). The formation of this new peak indicates
a coordinative interaction of the copper with the nitrogen rich benzotriazole. The
15

stoichiometry of binding between UiO-66-NH-BT and Cu2+ was confirmed by Job’s plot,
which concluded that a 1:1 stoichiometric complex is formed between UiO-66-NH-BT
and Cu2+ (Figure S11, SI) [40,41].
Figure 6. Changes in fluorescence emission spectra of UiO-66-NH-BT with the
incremental addition of Cu(NO3)2 (10-2 M) in water (exe= 360 nm). Inset: Fluorogenic
changes in UiO-66-NH-BT upon addition of Cu2+ in water upon illumination at 365 nm.
In order to further understand the chemosensing properties of UiO-66-NH-BT, the
changes in the fluorescence emission intensity were examined as a function of increasing
Cu2+ concentration at an excitation wavelength of 360 nm. The emission was
quantitatively quenched with increasing concentrations of Cu2+ and was observed to be
completely quenched when a 1:1 UiO-66-NH-BT: Cu2+ molar ratio was achieved (Figure
6, Figure S12, SI). This complete quenching signifies that full complexation occurred
between Cu2+ and the nitrogen rich benzotriazole moiety of the linker. Cu2+ being a
paramagnetic cation, with open shell d-orbitals, inhibit the electron transfer (ET) from the
benzotriazole to the benzene ring in the main structural framework of UiO-66 thus
16

resulting in quenching of fluorescence (Scheme 2). The quenching efficiency was then
elucidated from the Stern–Volmer constant [41,42]. The Stern–Volmer constant, KSV,
obtained from the titration curve shown in Figure 6, was calculated to be 1.1 ×105 (Figure
S13, SI). The sensitivity of UiO-66-NH-BT towards Cu2+ was determined from the
calculated detection limit, which was found to be 16.9 ppb (0.266 M) (Section S6,
Determination of the detection limit, Figure S24). It was found that this detection limit
were quiet comparable with the recently published results on the MOF based copper ion
sensors and is lower than the limit set by the U.S. Environmental Protection Agency
(EPA) (Table S1, SI). The powdered XRD of the copper bounded UiO-66-NH-BT
showed that the crystallinity of the MOF is intact (Figure 3). The selectivity and
tolerance of UiO-66-NH-BT towards Cu2+ as compared to other metal contaminants was
investigated by immersion of UiO-66-NH-BT in solutions of different
Figure 7. Change in the normalized fluorescence emission of UiO-66-NH-BT in water
upon addition of 200 L of different metal cations (10-2 M).
17

metal ions (e.g. Na+, K+, Ca2+, Mg2+, Sr2+, Rb2+, Cs2+, Fe2+, Fe3+, Co2+, Cu2+, Ni2+, Zn2+,
Cd2+, Hg2+, Ag+, Pd2+, Al3+, Ga3+, and Pb2+) (Figure S14). As shown in Figure 7, the
fluorescence emission remained same for all metal ions tested, except for Cu2+ where the
emission was completely quenched. This high selectivity for Cu2+ ion is due to the fact
that copper being the highest occupant on the Irving–Williams series leads to the
formation of the most stable complex with the benzotriazole as compared to other metal
ions. To explore the possibility of using UiO-66-NH-BT in practical purpose competitive
binding experiments with to 200 L of different metal ions (10-2 M) in the presence of
200 L Cu2+ ion was carried out which exhibited that none of the metal ions interfere
with the quenching of UiO-66-NH-BT by Cu2+ (Figure S15, SI).
3.5. Anion sensing properties of UiO-66-NH-BT
Anion sensing properties of UiO-66-NH-BT was also investigated with the aqueous
potassium or sodium salts solutions (10-2 M) of the anions, F−, Cl−, Br−, I−, CH3COO-,
−
2−
−
3-
−
2−
2−
NO3 , SO4 , HSO4 , PO4 , MnO4 , CrO4 , and Cr2O7 . Screening with these anions
2−
showed that optical properties of UiO-66-NH-BT are selectively sensitive to CrO4 , and
Cr2O72− anions. The absorbance studies showed that the peak of UiO-66-NH-BT at 360
2−
nm is redshifted to 372 and 382 nm for Cr2O7 and CrO42− ions respectively (Figure S16
and S17, SI). The Job's methods confirmed that the stoichiometry of binding of UiO-66-
2−
NH-BT with Cr2O7 and CrO42− ions are 1:1 (Figure S18 and S19, SI). The fluorescence
emission spectroscopy analysis of UiO-66-NH-BT with the chromium oxyanions showed
that Cr2O72− ion produces partial quenching (Figure 8) while the CrO42− ion produce a
complete quenching (Figure 9) of the emission spectra indicating that a partial and full
complexation was realized between the chromate oxyanions and the benzotriazole.
18

Figure 8. Changes in fluorescence emission spectra of UiO-66-NH-BT with the
incremental addition of K2Cr2O7 (10-2 M) in water (exe= 360 nm).
Figure 9. Changes in fluorescence emission spectra of UiO-66-NH-BT with the
incremental addition of K2CrO4 (10-2 M) in water (exe= 360 nm).
19

Figure 10. Change in the normalized fluorescence emission of UiO-66-NH-BT in water
upon addition of 200 L of different anionic species (10-2 M).
Moreover, the powdered XRD demonstrate that complete complexation of the chromate
with the MOF did not produce any change in the crystallinity of the UiO-66-NH-BT
2−
(Figure 3). The quenching constant Ksv calculated for Cr2O7 and CrO42− were 3.9x103
and 6.7x103 respectively (Figure S20 and S21, SI). The sensitivity of UiO-66-NH-BT
2−
2−
towards the chromium oxyanions (Cr2O7 and CrO4 ) was calculated to be 280 ppb (1.3
M) and 47.7 ppb (0.411 M) respectively (Section S6, Determination of the detection
limit Figure S25 and S26). It was found that this detection limit were quiet comparable
with the recently published results on the MOF based chromium oxyanion sensors and is
lower than the limit set by the U.S. Environmental Protection Agency (EPA) (Table S2,
SI).In order to check the selectivity of the material it was tested with aqueous solution of
2−
2−
different anions. It was realized that only the chromium oxyanions (Cr2O7 and CrO4 )
20

causes quenching of the fluorescence emission of UiO-66-NH-BT (Figure 10, Figure S22
of SI). The competitive binding experiments showed that none of the anions interfere
with the quenching effects of CrO42− even when added in excess (Figure S23, SI). The
UV-vis absorption spectrum (Figure S27 of SI) shows that two possible mechanisms are
responsible for quenching with the chromate ions. Firstly the wide absorption bands of
the chromate ions aqueous solution covers the absorption bands of the UiO-66-NH-BT
thus there is a competition of absorption of the excitation wavelength energy between the
chromate ions and the MOF. This will prevent the energy transfer to the framework that
will result in quenching. Secondly there exists an overlap between the aqueous solution
of chromate ions and the emission spectrum of the UiO-66-NH-BT (Figure S27 of SI)
which indicates that there is energy transfer between the chromate ion, bonded by
coordinative interaction to the nitrogen of the triazole ring, and UiO-66-NH-BT that leads
to quenching [43,44]. A successful chemosensor should be effective in aqueous media
with widely varying pHs. The emission properties of UiO-66-NH-BT remained unaltered
within a pH range of 6.0-10.0 (Figure S28, SI). Beyond this range, the emission
properties are lost probably due to the decomposition of the structure of UiO-66-NH-BT.
Scheme 2. Schematic illustration of the mechanism of fluorescence in UiO-66-NH-BT.
21

2-
The interaction between the UiO-66-NH-BT and the analytes (Cu2+/CrO4 ) were
studied with the help of ICP and FTIR. UiO-66-NH-BT was accumulated after sensing
experiments and then wash with water to remove the analytes adhered to the surface of
the MOF, and then dried. The ICP analysis of the washed and dried samples concluded
the presence of Cu and Cr in addition to the metal of the framework. It was found that
0.517 mg of Cr and 13.675 mg of Cu was recovered per 100 mg of the Zirconium. FTIR
analysis of the washed and dried UiO-66-NH-BT samples obtained after sensing
experiments offered us with significant information about the nature of the interaction
2-
between the host framework and the analytes (Cu2+/CrO4 ). The coordination of the Cu2+
ion with the nitrogen of the benzotriazole in UiO-66-NH-BT is mirrored in the shifting of
the –N=N– stretching vibration from 1253 to 1243 cm-1. The C-H stretching of the
aromatic ring of the benzotriazole at 1097 cm-1 and 1043 cm-1of the pristine MOF is
shifted to 1088 cm-1 and 1034 cm-1 respectively after interaction with the Cu2+. The Cu2+
ion also binds with the lone pair of the NH moiety of the framework thus shifting the N-
H stretching frequency of the pristine UiO-66-NH-BT from 3348 cm-1 to 3282 cm-1
(Figure S29 and S30, SI). In the case of the chromate ions we can see new peaks are
formed at 626, 738, and 1316 cm-1 that corresponds to the bending vibrational modes of
O-Cr-O bond, that confirms the incorporation of the chromate ion within the framework.
There is a shift in the –N=N– stretching from 1253 cm-1 to 1242 cm-1 due to the binding
of the chromium ion with the nitrogen rich benzotriazole. The shift in the NH peak of the
pristine UiO-66-NH-BT from 3348 cm-1 to 3313 cm-1 is due to the H-bonding of the
chromate oxygen with the NH proton of the framework (Figure S31 and S32, SI). Thus
while ICP confirmed the presence of the analytes within the framework of UiO-66-NH-
22

BT the FTIR gave us information about the nature of interaction between the analytes and
the different functional groups of the framework.
3.6. Reusable properties of UiO-66-NH-BT
In order to measure the recyclable sensing ability of UiO-66-NH-BT the fluorescence
sensing experiments were repeated with the same materials recovered after the first set of
experiments followed by washing with 1.0 M aqueous solution of EDTA, water, and then
drying at 100 0C for 1 hour. The recovered UiO-66-NH-BT showed no significant
variation in the fluorescence intensities and sensitivity towards sensing of ions for five
consecutive cycles (Figure S33, SI).
4. Conclusion
In this paper we prepared a benzotriazole appended new linker H2L that was used
to develop isoreticular MOF UiO-66-NH-BT. This presynthetically incorporation of the
benzotriazole moiety in the UiO-66-NH2 framework not only enhanced its intrinsic
fluorescence emission properties but also introduce additional coordinating sites that help
2-
in the selective and sensitive detection of Cu2+ and CrO4 / Cr2O72- ions in aqueous
2-
solution with a detection limit of 16.9 ppb, 280 ppb and 47.7 ppb for Cu2+, Cr2O7 , and
CrO42- respectively. We studied the interaction of the analytes with the functionalities of
the framework and also found that the sensor is recyclable for five consecutive cycles
with the respective analytes.
Conflict of interests
The authors declare that there is no conflict of interests.
Declaration of interests
23

The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We acknowledge Center for Research Excellence in Nanotechnology (CENT) for
continued support. This research was supported by King Abdulaziz City for Science and
Technology – National Science, Technology, and Innovation Plan (NSTIP No. 15-
NAN4601-04).
24

5. References
1. H. Furukawa, K. E. Cordova, M. O’Keeffe, O. M. Yaghi, The Chemistry and
Applications of Metal−Organic Frameworks, Science 341 (2013) 1230444.
2. K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R.
Herm, T. –H. Bae, J. R. Long, Carbon Dioxide Capture in Metal−Organic
Frameworks, Chem. Rev. 112 (2012) 724–781.
3. H. Li, K. Wang, Y. Sun, C. T. Lollar, J. Li, H. –C. Zhou, Recent advances in gas
storage and separation using metal–organic frameworks, Mater. Today 21 (2018)
108-121.
4. C. Wang, D. Liu, W. Lin, Metal−Organic Frameworks as A Tunable Platform for
Designing Functional Molecular Materials, J. Am. Chem. Soc. 135 (2013)
13222−13234.
5. G.-Y. Jeong, A. K. Singh, M.-G. Kim, K.-W. Gyak, U. J. Ryu, K. M. Choi, D.-P.
Kim, Metal-organic framework patterns and membranes with heterogeneous
pores for flow-assisted switchable separations, Nat. Commun. 9 (2018) 3968-
3976.
6. D. Kukkara, K. Vellingiric, K.–H. Kim, A. Deep, Recent progress in biological
and chemical sensing by luminescent metal-organic frameworks, Sens. Actuators
B: Chem. 273 (2018) 1346–1370.
7. A. U. Czaja, N. Trukhan, U. Mꢀller, Industrial Applications of Metal−Organic
Frameworks, Chem. Soc. Rev. 38 (2009) 1284−1293.
25

8. C. A. Trickett, A. Helal, B. A. Al-Maythalony, Z. H. Yamani, K. E. Cordova, O.
M. Yaghi, The chemistry of metal-organic frameworks for CO2 capture,
regeneration and conversion, Nat. Rev. Mater. 2 (2017) 17045-17060.
9. L. Zhu, X.-Q. Liu, H.-L. Jiang, L.-B. Sun, Metal–Organic Frameworks for
Heterogeneous Basic Catalysis, Chem. Rev. 117 (2017) 8129–8176.
10. P. Deria, J. E. Mondloch, O. Karagiaridi, W. Bury, J. T. Hupp, O. K. Farha,
Beyond post-synthesis modification: evolution of metal–organic frameworks via
building block replacement, Chem. Soc. Rev. 43 (2014) 5896−5912.
11. Z. Wang, S. M. Cohen, Postsynthetic modification of metal–organic frameworks,
Chem. Soc. Rev. 38 (2009) 1315−1329.
12. S. –Y. Zhu, B. Yan, A novel sensitive fluorescent probe of S2O82− and Fe3+ based on
covalent post-functionalization of a zirconium(iv) metal–organic framework, Dalton
Trans., 47, (2018) 11586-11592.
13. S. –Y. Zhu, B. Yan, Highly Sensitive Luminescent Probe of Aniline and Trace
Water in Organic Solvents Based on Covalently Modified Lanthanide Metal–
Organic Frameworks, Ind. Eng. Chem. Res. 57 (2018) 16564-16571.
14. S. M. Cohen, The Postsynthetic Renaissance in Porous Solids, J. Am. Chem. Soc.
139 (2017) 2855−2863.
15. R. Banerjee, H. Furukawa, D. Britt, C. Knobler, M. O’Keeffe, O. M. Yaghi,
Control of Pore Size and Functionality in Isoreticular Zeolitic Imidazolate
Frameworks and their Carbon Dioxide Selective Capture Properties, J. Am.
Chem. Soc. 131 (2009) 3875-3877.
26

16. L. M. Gaetke, H. S. ChowJohnson, C. K. Chow, Copper: Toxicological relevance
and mechanisms, Arch. Toxicol. 88 (2014) 1929–1938.
17. J. A. Jr. Cotruvo, A. T. Aron, K. M. Ramos-Torres, C. J. Chang, Synthetic
fluorescent probes for studying copper in biological systems, Chem. Soc. Rev. 44
(2015) 4400-4414.
18. 2018 Edition of the Drinking Water Standards and Health Advisories Tables, EPA
822-F-18-001 Office of Water U.S. Environmental Protection Agency
Washington, DC.
19. M. Kobya, E. Demirbas, E. Senturk and M. Ince, Adsorption of heavy metal ions
from aqueous solutions by activated carbon prepared from apricot stone,
Bioresour. Technol., 96 (2005) 1518-1521.
20. F. -Y. Yi, D. Chen, M. -K. Wu, L. Han, H. -L. Jiang, Chemical Sensors Based on
Metal-Organic Frameworks, Chem.Plus.Chem. 81 (2016) 675-690.
21. C. F. Qiao, X. N. Qu, Q. Yang, Q. Wei, G. Xie, S. P. Chen and D. S. Yang, Instant
high-selectivity Cd-MOF chemosensor for naked-eye detection of Cu(II) confirmed
using in situ microcalorimetry, Green Chem., 18 (2016) 951-956.
22. J. Lia, X. Yuan, Y.-N. Wu, X. Ma, F. Li, B. Zhang, Y. Wang, Z. Lei, Z. Zhang,
From powder to cloth: Facile fabrication of dense MOF-76(Tb) coating onto
natural silk fiber for feasible detection of copper ions, Chem. Eng. Sci. 350 (2018)
637-644.
23. L. L. Liu, Y. Z. Yu, X. -J. Zhao, Y. R. Wang, F. Y. Cheng, M. K. Zhang, J. J.
2n−
Shu, L. Liu, A robust Zn(II)/Na(I)-MOF decorated with [(OAc)2(H2O)2]n
27

anions for the luminescence sensing of copper ions based on the inner filter effect,
Dalton Trans., 47 (2018) 7787–7794.
24. B. Dhal, H. N. Thatoi, N. N. Das, B. D. Pandey, Chemical and microbial
remediation of hexavalent chromium from contaminated soil and
mining/metallurgical solid waste: A review, J. Hazard. Mater. 250-251 (2013)
272−291.
25. T. Y. Gu, M. Dai, D. J. Young, Z. G. Ren, J. P. Lang, Luminescent Zn(II)
Coordination Polymers for Highly Selective Sensing of Cr(III) and Cr(VI) in
Water, Inorg. Chem. 56 (2017) 466-4678.
26. B. Parmar, Y. Rachuri, K. K. Bisht, E. Suresh, Mixed-Ligand LMOF
Fluorosensors for Detection of Cr(VI) Oxyanions and Fe3+/Pd2+ Cations in
Aqueous Media, Inorg. Chem. 56 (2017) 10939-10949.
27. R. Rakhunde, L. Deshpande, H.D. Juneja, Chemical speciation of chromium in
water: a review, Crit. Rev. Environ. Sci. Technol. 42 (2012) 776–810.
28. A.A. Mohamed, A.T. Mubarak, Z.M.H. Marstani, K.F. Fawy, A novel kinetic
determination of dissolved chromium species in natural and industrial wastewater,
Talanta 70 (2006) 460–467.
29. S. Mohandossa, J. Sivakamavallib, B. Vaseeharanb, T. Stalina Host-guest
molecular recognition based fluorescence On-Off-On chemosensor for nanomolar
level detection of Cu2+ and Cr2O72− ions: Application in XNOR logic gate and
human lung cancer living cell imaging, Sens Actuators B 234 (2016) 300-15.
28

30. A. Prakash, S. Chandra, D. Bahadur, Structural, magnetic, and textural properties
of iron oxide-reduced graphene oxide hybrids and their use for the
electrochemical detection of chromium, Carbon 50 (2012) 4209−4219.
31. Y. Lin, X. Zhang, W. Chen, W. Shi, P. Cheng, Three Cadmium Coordination
Polymers with Carboxylate and Pyridine Mixed Ligands: Luminescent Sensors
for FeIII and CrVI Ions in an Aqueous Medium. Inorg. Chem. 56 (2017)
11768−11778.
32. R. Lv, J. Wang, Y. Zhang, H. Li, L. Yang, S. Liao, W. Gu, X. Liu, An amino-
decorated dual-functional metal–organic framework for highly selective sensing of
Cr(iii) and Cr(vi) ions and detection of nitroaromatic explosives, J. Mater. Chem. A 4
(2016) 15494-15500.
33. W.-H. Huang, J.-Z. Li, T. Liu, L.-S. Gao, M. Jiang, Y.-N. Zhang, Y.-Y. Wang, A
stable 3D porous coordination polymer as multi-chemosensor to Cr(iv) anion and
Fe(iii) cation and its selective adsorption of malachite green oxalate dye, RSC Adv. 5
(2015) 97127-97132.
34. J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga and K.
P. Lillerud, A New Zirconium Inorganic Building Brick Forming Metal Organic
Frameworks with Exceptional Stability, J. Am. Chem. Soc. 130 (2008) 13850–
13851.
35. M. Zhang, Y.-P. Chen, M. Bosch, T. Gentle, K. Wang, D. Feng, Z. U. Wang and
H.-C. Zhou, Symmetry-Guided Synthesis of Highly Porous Metal–Organic
Frameworks with Fluorite Topology, Angew. Chem. Int. Ed. 53 (2014) 815–818.
29