An Ultrasensitive and Selective Metal-Organic Framework Chemosensor for Palladium Detection in Water

Article abstract of DOI:10.1021/acs.inorgchem.8b02871

A new europium-based metal-organic framework, termed KFUPM-3, was constructed using an allyloxy-functionalized linker. As a result of coordinative interactions between the allyloxy moieties and Pd²⁺, highly selective changes in both the absorpt

Full text of DOI:10.1021/acs.inorgchem.8b02871

Communication
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
An Ultrasensitive and Selective Metal−Organic Framework
Chemosensor for Palladium Detection in Water
Aasif Helal,*,† Ha L. Nguyen,†,‡ Amir Al-Ahmed,§ Kyle E. Cordova,†,‡ and Zain H. Yamani†
†Center of Research Excellence in Nanotechnology and §Center of Research Excellence in Renewable Energy, King Fahd University
of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
‡Department of Chemistry and Berkeley Global Science Institute, University of California, Berkeley, Berkeley, California 94720,
United States
S
* Supporting Information
ensure a selective and reversible recognition site for palladium.
ABSTRACT: A new europium-based metal−organic
framework, termed KFUPM-3, was constructed using an
allyloxy-functionalized linker. As a result of coordinative
interactions between the allyloxy moieties and Pd2+, highly
selective changes in both the absorption and emission
spectra of KFUPM-3 were observed. Accordingly,
KFUPM-3 was demonstrated to have an ultrasensitive
Pd2+ detection limit (44 ppb), regenerative properties
without loss in performance, detection of palladium in
different oxidation states and in the presence of other
competitor metal ions, and fully functional sensing
capabilities over a wide pH range.
Through a facile solvothermal synthesis, the resulting MOF,
termed KFUPM-3, was highly crystalline, and structural analysis
revealed a two-dimensional sql topology. As expected, highly
selective changes in both the absorption and emission spectra of
KFUPM-3 were confirmed upon exposure to aqueous solutions
containing palladium at various concentrations. Furthermore, it
was demonstrated that KFUPM-3 had an ultrasensitive Pd2+
detection limit of 44 ppb, demonstrated regenerative properties,
was capable of selectively detecting palladium in different
oxidation states and in the presence of other heavy metal ions,
and operated over a wide pH range.
The first step in realizing a new MOF chemosensor centered
on the design of a linker that was capable of selectively binding
palladium. After protecting the carboxylic acid of the
commercially available 2,5-dihydroxyterephthalic acid, we
n the design of new chemosensors for palladium detection,
Ithere are five criteria that must be met: (i) high-intensity performed a Williamson ether synthesis to introduce allyloxy
groups at the 2 and 5 positions of the terephthalic acid (see
sections S1 and S2). This was followed by a saponification to
deprotect the allyloxy-functionalized linker in order to achieve
the targeted 2,5-bis(allyloxy)terephthalic acid (H2BAT) in 85%
yield. The synthesized H2BAT was fully characterized by NMR
(1H and 13C) and elemental analysis (see section S2). With
H2BAT in hand, MOF synthesis was then performed under
solvothermal conditions. Specifically, Eu(NO3)3·5H2O and
H2BAT were dissolved in a 5:2:1 (v/v) solution of N,N′-
dimethylformamide (DMF), ethanol, and water, respectively,
and the resulting solution was heated at 80 °C for 3 days to
produce KFUPM-3 as colorless, rhombohedral-shaped single
crystals (see section S2).
fluorescence; (ii) selectivity for palladium over other potential
contaminants; (iii) ability to operate with a low detection limit;
(iv) reversibility such that the chemosensor can be regenerated;
(v) stability in aqueous media under different pH conditions.1
Considering these criteria, metal−organic frameworks (MOFs)
are attractive in that their constituent building blocks can be
designed and functionalized in a rational manner.2−5 Specifi-
cally, MOFs constructed from lanthanide-containing secondary
building units (SBUs) are ideal especially when stitched
together by organic linkers with fluorogenic properties.6 In
this way, direct energy transfer from appropriately designed
organic linkers in their excited state to the lanthanide SBUs
results in “luminescence sensitization”.7 Furthermore, the
organic linkers can be functionalized with moieties that
selectively and reversibly interact with palladium, even in
environments where low concentrations are present.8,9 This
allows for the MOF backbone to be equipped with both
recognition of palladium via coordinative binding and a
fluorogenic site that correspondingly responds to this binding.
Although MOFs have been designed with pyridinyl donors that
effectively coordinate palladium, alkene functional groups are
better suited because of their π-donor capabilities, which allow
for selective and reversible coordination.10−14
Single-crystal X-ray diffraction analysis revealed that KFUPM-
3 crystallized in the P1 (No. 2) space group with unit cell
̅
parameters of a = 18.07 Å, b = 18.98 Å, c = 22.53 Å, α = 106.8°, β
= 113.3°, and γ = 96.9° (Figure 1a,b and section S2 and Table
S1). Through structural analysis, it was evident that the
europium-based SBU contained two crystallographically in-
dependent Eu atoms, which are linked together by a μ3-O
originating from a carboxylate of the BAT2− linker. Each Eu
atom adopts a tricapped trigonal-prismatic geometry from the
coordination of eight carboxylates and one DMF guest
molecule. The dinuclear Eu−O SBU is surrounded by six
BAT2− linking units, in which the directionality of these linkers
Herein, we report the design and synthesis of a new MOF
chemosensor that meets the aforementioned criteria for
palladium detection. Our strategy focused on the construction
of a MOF using Eu3+ to form the SBUs, which then were stitched
together by organic linkers that integrated allyloxy moieties to
Received: October 11, 2018
© XXXX American Chemical Society
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DOI: 10.1021/acs.inorgchem.8b02871
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Figure 1. (a and b) Single-crystal structure of KFUPM-3 constructed from H2BAT and Eu3+ building blocks. (c) Structure of KFUPM-3, which adopts
a two-dimensional-layered sql network. Atom colors: Eu, blue polyhedral; C, gray; O, red. H atoms are omitted for clarity.
can be reduced to four total points of extension. As a result, the
structure is extended in two dimensions, adopts the sql topology
with a 4^4 tiling (Figure 1c), and has square-shaped pore
windows of ∼4 Å size. Because of van der Waals forces, the
interlayer spacing of KFUPM-3 is ∼3.5 Å.
took place (see section S3). Fluorescence spectroscopy
measurements were then carried out at an excitation wavelength
of 336 nm (see section S5). At 336 nm, energy transfer occurs
from the BAT2− linker excited states to the ground state of the
Eu3+ ions, with fluorescence emissions resulting from f−f
transitions. Emission peaks were observed at 578, 592, 616, 648,
and 695 nm, which correspond to 5D0 → 7F0, 5D0 → 7F1, 5D0 →
7F2, 5D0 → 7F3, and 5D0 → 7F4 transitions, respectively (Figure
After solvent exchange and activation, powder X-ray
diffraction (PXRD) analysis confirmed phase purity upon a
comparison of the experimental diffraction patterns and the
diffraction pattern simulated from the single-crystal structure
(see section S3). Fourier transform infrared spectroscopy
measurements for KFUPM-3 demonstrated the presence of
the characteristic stretching vibrational bands of the allyloxy
moiety as well as shifted absorption bands for the carboxylate
moiety upon coordination (see section S3). The thermal
stability of KFUPM-3 was proven via thermal gravimetric
analysis, in which two main weight losses were observed: (i) 8 wt
% in the temperature range of 200−300 °C, which was assigned
to the loss of coordinated DMF and water within the pores; (ii)
58 wt % in the temperature range of 300−800 °C, which was
assigned to framework decomposition. The remaining residue
(34 wt %) was attributed to Eu2O3 (see section S3). A N2
adsorption isotherm at 77 K revealed that KFUPM-3 was
microporous with a Brunauer−Emmett−Teller surface area of
227 m2 g−1 and a pore volume of 0.159 cm3 g−1 (see section S3).
Scanning electron microscopy images display a crystal
morphology of flat rectangular sheets varying from 10 to 15
μm in size (see section S3).
After a successful structural characterization, we turned our
attention to investigating the photophysical properties of
KFUPM-3 (see section S4). As such, the UV−vis absorption
spectrum for KFUPM-3 displayed two peaks, centered at 252
and 336 nm, which resulted from π−π* transitions of the
benzene ring and allylic bond. Optical sensing measurements
were then performed using 10−2 M aqueous solutions of Pd2+.
Upon the slow addition of Pd2+ to a suspension of KFUPM-3,
the absorbance increased and an additional peak, centered at
276 nm, was observed (see section S4). This new peak suggested
that a possible coordinative interaction was occurring between
Pd2+ and the allyloxy moiety of KFUPM-3. This response was
found to be highly selective because no such changes were
observed when other metal ions (e.g., Na+, K+, Ca2+, Mg2+, Sr2+,
Rb+, Cs+, Fe2+, Fe3+, Co2+, Cu2+, Ni2+, Zn2+, Cd2+, Ag+, Al3+,
Ga3+, Ir3+, Rh3+, Pt2+, and Pb2+) were tested. The stoichiometry
of the binding event was then verified by Job’s method, which
concluded that a 1:2 stoichiometric complex was formed
between KFUPM-3 and Pd2+, respectively (see section S4).8,9 It
is worth noting that the crystallinity of KFUPM-3, as proven by
PXRD analysis, was retained after the binding event with Pd2+
2).12,13 The most intense fluorescent peak is the D0 → F2
5
7
Figure 2. Changes in the emission spectrum of KFUPM-3 in water
upon the incremental addition of PdCl2 in water (λex= 336 nm). Inset:
Mechanism of palladium binding and fluorogenic change from orange
(KFUPM-3) to colorless (KFUPM-3-Pd2+) upon the addition of Pd2+
(λex = 336 nm).
transition originating from an Eu3+ ion with anti-inversion
symmetry.15,16 It is also noted that emission of the BAT2− linker
does not appear in the emission spectrum of KFUPM-3, which
indicates that an antenna effect is present. Finally, the absolute
quantum yield for KFUPM-3 was calculated from the integrated
sphere to be 0.48, and the CIE coordinates, obtained from the
chromaticity diagram, agree with the experimentally derived
emission results (see section S5).
In order to understand the sensitivity of the chemosensing
properties of KFUPM-3, changes to the fluorescence emission
intensity were investigated as a function of increasing Pd2+
concentration. As expected, fluorescence was quantitatively
quenched upon exposure to increasing concentrations of Pd2+
and observed to be completely quenched when a 2:1 Pd2+/
KFUPM-3 molar ratio was achieved. This complete quenching
indicates that full complexation was realized between Pd2+ and
the two allyloxy moieties on the BAT2− linker (Figure 2)
because of the strong interaction between the π−d orbital of
B
DOI: 10.1021/acs.inorgchem.8b02871
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Inorganic Chemistry
Communication
alkene and palladium. It is noted that Pd2+ complexation within
the channels of KFUPM-3 perturbs the electronic structure of
the BAT2− linker, which affects the excited state of the linkers.
This perturbation then impacts sensitization of the Eu3+
emissive state by prohibiting energy transfer from the linkers
to the Eu3+ SBUs, which, consequently leads to quenching of
fluorescence. The quenching efficiency was then interpreted by
calculating the Stern−Volmer constant.8,9 On the basis of the
titration curve, shown in Figure 2, the Stern−Volmer constant,
KSV, was calculated to be 7.8 × 104, which is comparable to
values obtained for other suspension-based Pd2+ chemosensors
(see section S5). The sensitivity of KFUPM-3 toward Pd2+ was
demonstrated based on the calculated detection limit. For
KFUPM-3, this detection limit was 44 ppb, which is significantly
lower than the 5−10 ppm threshold for palladium in
pharmaceutical products (see section S5).17 Furthermore, this
detection limit is among the lowest and most sensitive reported
for MOF-based Pd2+ chemosensing materials (Table S2). To
assess changes in the emission spectra in the presence of metal
contaminants, KFUPM-3 was again immersed in solutions of
different metal ions (see section S5). As shown in Figure 3, the
palladium species. Because no additional reagents were added
during the analysis to alter the oxidation states of these species, it
is presumed that KFUPM-3 can interact with palladium
irrespective of the oxidation state (see section S7).
Finally, the reusability of KFUPM-3 as a practical chemo-
sensor was demonstrated. In a typical experiment, dispersion of
KFUPM-3-Pd2+ (KFUPM was pre-exposed to Pd2+) in aqueous
media was treated with 0.1 M ethylenediaminetetraacetic acid.
Fluorescence spectroscopy was then performed, and an
emission signal, with a maximum at 616 nm, was observed to
be recovered over the course of four consecutive cycles. This
study powerfully demonstrates that KFUPM-3-Pd2+ coordina-
tive binding is chemically reversible as opposed to an irreversible
cation-catalyzed reaction (see section S8). Moreover, the
diffraction pattern of KFUPM-3 showed that the crystallinity
of the material was maintained (see section S8).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02871.
Linker synthesis and characterization, full KFUPM-3
synthetic details and characterization, and experimental
conditions for sensing measurements (PDF)
Accession Codes
CCDC 1870943 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
Figure 3. Change in the normalized fluorescence emission of KFUPM-
3 in water upon the addition of 200 μL of different metal cations (10−2
M; λex = 336 nm).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: aasifh@kfupm.edu.sa (A.H.).
emission remained for all metal ions tested, whereas the
emission was quenched when Pd2+ was in the presence of
KFUPM-3. Again, this demonstrates that a highly selective
chemosensing process is occurring for KFUPM-3 with Pd2+.
Competition experiments were then carried out in order to
explore the possibility of using KFUPM-3 as a practical ion-
selective (Pd2+) fluorescent chemosensor. In these studies,
KFUPM-3 was first exposed to 200 μL of various competitor
metal ions (10−2 M), at which time 200 μL of Pd2+ was then
added. Fluorescence emission spectroscopy exhibited no
interference in the emission when competitor metal ions were
present with Pd2+ (see section S5). For environmental and
physiological applications, chemosensors also must operate in
media with widely varying pHs. The emission properties and
crystallinity of KFUPM-3 were effectively maintained within a
pH range of 6.0−10.0 (see section S6). Beyond this range, the
structure of KFUPM-3 decomposes, as proven by PXRD
analysis, and the emission properties are lost (see section S6).
The emission response of KFUPM-3 was also evaluated in the
presence of different palladium species with varying oxidation
states: PdCl2, Pd(PPh3)4, Pd(OAc)2, PdCl2(PPh3)2,
(NH4)2PdCl6, and Pd2(dba)3 in deoxygenated water and
acetonitrile. As such, negligible differences were observed for
the emission response of KFUPM-3 toward these differing
ORCID
Ha L. Nguyen: 0000-0002-4977-925X
Kyle E. Cordova: 0000-0002-4988-0497
Zain H. Yamani: 0000-0002-6031-9385
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge Prof. Omar M. Yaghi (University of California,
Berkeley) for continued support. We acknowledge Profs.
Enrique Gutierrez-Puebla and Angeles Monge Bravo (Instituto
de Ciencia de Materiales de Madrid, Spain) for a productive
discussion in the single-crystal structure solution. We are
grateful to Saudi Aramco for supporting the MOF synthesis and
characterization (Grant ORCP2390). The sensing studies were
supported by King Abdulaziz City for Science and Technology,
National Science, Technology, and Innovation Plan (Grant 15-
NAN4601-04).
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