Post-modification of a MOF through a fluorescent-labeling technology for the selective sensing and adsorption of Ag+ in aqueous solution

Article abstract of DOI:10.1039/c2dt30689a

A new approach inspired by fluorescent labeling technology to fluorescently functionalize MOFs via post-modification is reported. A fluorescein-containing MOF FITC?BTPY-NH₂ was synthesized for selective sensing and adsorption of Ag⁺ in aqueous solution.

Full text of DOI:10.1039/c2dt30689a

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Post-modification of a MOF through a fluorescent-labeling technology for the
selective sensing and adsorption of Ag+ in aqueous solution†
Lejie Zhang,a Yuan Jian,*b Jian Wang,a Cheng He,a Xuezhao Li,a Tao Liua and Chunying Duan*a
Received 27th March 2012, Accepted 4th July 2012
DOI: 10.1039/c2dt30689a
A new approach inspired by fluorescent labeling technology
to fluorescently functionalize MOFs via post-modification is
reported. A fluorescein-containing MOF FITC@BTPY–NH2
was synthesized for selective sensing and adsorption of Ag+
in aqueous solution.
Metal–organic frameworks (MOFs) have attracted considerable
attention and become one of the most widely studied hybrid
porous materials for their large pore volumes, accessible through
Scheme 1 Post-modification of BTPY–NH2 with FITC showing the
dye-functionalized fluorescent MOF FITC@BTPY–NH2.
the porous networks, to topologies resulting from the bonding of
multi-dentate ligands to metal centers.1,2 As MOFs have a
degree of structural predictability and well-defined environments
fluorescence maxima in the visible region, is one of the most
common labels used in biological applications.10 Recently,
for luminophores in crystalline form, they offer a unique plat-
form for the development of solid-state luminescent materials.3
Cohen’s group has shown that the superior chemical stability of
MILs allowed for the synthesis of reactive isocyanate and iso-
thiocyanate groups on the organic components of the frame-
In particular, the combination of synthesis flexibility with respect
to both the organic and inorganic components means MOFs
exhibit several advantages over other luminescent materials.4
Furthermore, the ability to absorb molecules into pores allows
the species to be immobilized in close proximity to luminescent
centers.5 In this context, the combination of luminescence and
accessible porosity within MOFs bring the capability to sensitise
the host–guest chemistry to detectable changes in their lumines-
cence and makes them promising candidates for chemical
sensing applications.6 However, the incorporation of intense
fluorescence within the stable pores of MOFs is still a big
challenge, because most of the bright luminophores are not com-
patible with the MOF synthetic conditions or exhibiting groups
interfere with the formation of the desired MOFs.7
Fluorescent labeling is one of the most common method-
ologies used for bioanalytical purposes.8,9 The organic fluoro-
phores may form covalent or noncovalent linkages with the
sample to be analyzed, producing the respective conjugates or
complexes that show fluorescence from short to long wave-
lengths, depending on the marker used. Of the widely used
labels, fluorescein, a poly-cyclic fluorophore with absorption and
work.11 Under suitable conditions, these reactive groups readily
combine with species diffusing through the porous structure,
generating new functionalized MIL. Bearing these in mind,
herein, we try to label a large porous [Zn6 (btb)4(bipy–NH2)3]
(BTPY–NH2) with fluorescein isothiocyanate (FITC) to obtain a
dye-functionalized MOF FITC@BTPY–NH2 (btb = benzene-
1,3,5-tribenzoate, bipy–NH2
=
3-amino-4,4′-bipy-ridine)
(Scheme 1) .12 Fluorescein and its derivatives were always used
in the fluorescent detection of transition metal ions in aqueous
solutions and living cells,13 and the molecules containing thio-
carbamide moiety have been used to bond thiophile metal ions
(e.g. Ag+, Hg2+).14 We anticipated that our dye-functionalized
MOF could be used in the selective and sensitive sensing of
thiophile metal ions in aqueous media. The new functionalized
MOF is also expected to be used in the separation, removal and
enrichment of heavy-transition metal ions from drinking water
because of its heterogeneous features.15
Crystals of BTPY–NH2 were obtained by the solvothermal
reaction of benzene-1,3,5-tribenzoate (29.4 mg, 0.067 mmol),
3-amino-4,4′-bipyridine (8.6 mg, 0.05 mmol) and Zn(NO3)2·6H2O
(30 mg, 0.1 mmol) in 6 mL of DMF at 85 °C for 3 days. Single-
crystal X-ray diffraction revealed that BTPY–NH2 had a similar
cubic lattice to FJL-1, which was reported by Hong’s group.16 It
comprised Zn(II) paddle-wheel SBUs (Fig. 1), each of which
was linked by four benzene-1,3,5-tribenzoate ligands. The ends
of the axial sites of each SBU were occupied by two different
bipy–NH2 ligands, such that there were two different types of
pores in the crystals of BTPY–NH2. The pore and window sizes
of the bigger pores were about 27 Å and 14 Å in diameter,
aState Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian, 116023, P.R. China. E-mail: cyduan@dlut.edu.cn
bInstitute of Nuclear Physics and Chemistry, China Academy of
Engineering Physics, Mianyang, Sichuan, 621900, P.R. China.
E-mail: jianyuan839@hotmail.com
†Electronic supplementary information (ESI) available: Crystal structure
data in CIF format, experimental details and the general procedure of
luminescence sensing and absorption. CCDC 873524. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30689a
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Fig. 2 Brightfield images (a)–(c) and confocal laser scanning micro-
graphs at λex = 405 nm (d)–(f) and λex = 488 nm (g)–(i) of
FITC@BTPY–NH2. (d)–(f) and (g)–(i) each show three slices from the
top down. The slices chosen for (d)–(f) and (g)–(i) were in about 75 μm
distance, with crystals of ca. 260 μm dimension in the z direction.
Fig. 1 The structure of BTPY–NH2, viewed along the y axis. The
gray, red, blue and cyan balls represent the carbon, oxygen, nitrogen and
zinc atoms, except the amino groups are drawn in purple. H-atoms have
been omitted for clarity.
respectively, derived from the structure data. PLATON calcu-
lation of BTPY–NH2 indicated that the solvent accessible
volume accounted for 81.2% of the crystal volume, which was a
high level among existing MOFs. The amino group in bipy–NH2
could not be located in the Fourier difference map, owing to the
statistical disorder over four sites coupled with likely confor-
mational disorder at each of these sites. However, 1H NMR spec-
troscopy on digested crystals indicated that this group remained
intact in the MOF.
of FITC in the functionalized MOF was measured at about
5–10% through comparing the UV-vis absorption with that of
pure FITC.17 1H NMR of the digested FITC@BTPY–NH2
suggested that ca.7.5% of the amino groups were converted to
isothiocyanate groups, which indicated that FITC was bound to
BTPY–NH2 covalently.
We also studied FITC@BTPY–NH2 with fluorescence
microscopy (FM) and confocal laser scanning microscopy
(CLSM) (Fig. 2). The blue fluorescence was attributed to bipy–
Furthermore, the characteristic peaks of amino groups could
be observed in the IR spectroscopy (3423 cm−1, 3220 cm−1).
Analysis of the residual electron density within the framework
voids was consistent with its presence. The atoms of this unit
were placed in calculated positions to complete the refinement.
The phase purity of BTPY–NH2 was established by powder
XRD; the observed diffraction pattern closely matched that simu-
lated from the single-crystal structure.
The isothiocyanate groups on FITC molecules can react with
arylamine groups in high yield under mild conditions, which
makes FITC a frequently used fluorescence label in protein
studies. Comparing the dimensions of the FITC molecule (5 ×
10 × 10 Å3) with that of the pore and the window sizes of
BTPY–NH2, it is suggested that molecules of FITC possibly
diffuse into pore channels and react with the amino groups. To
further establish the feasibility of the chemical reaction,
3-amino-4,4′-bipyridine and FITC molecules were mixed in
DMF at room temperature and stirred in the dark. The formation of
the desired conjugate was confirmed by electrospray ionization
mass spectrometry, which exhibited two main peaks corresponding
to [M + H]+, m/z = 561.2 and [M + Na]+, m/z = 583.1.
The post modification of BTPY–NH2 with FITC was then
performed by using similar conditions. The fresh crystals of
BTPY–NH2 were incubated in DMF solutions containing FITC.
The mixture was then shaken in a dark place at room tempera-
ture. The products were washed with DMF scrupulously in order
to remove any physisorbed FITC molecules from the surface of
BTPY–NH2. The identity and phase purity of the BTPY–NH2
crystals before and after modification were confirmed by powder
X-ray diffraction, which showed no lattice distortion. The wt%
NH2 ligands, and a strongly increased green fluorescence (λex =
488 nm) confirms the successful labeling of FITC molecules
inside the MOF crystals. Furthermore, CLSM excites only the
focal layer, thereby providing a layer-by-layer imaging of dye
distribution in and on the MOF crystals. A rather uniform distri-
bution of FITC molecules throughout the crystal resulted.12 It is
suggested that FITC penetrated deeply into the crystal rather
than staying on the external surface owing to the large pores and
windows sizes of BTPY–NH2.
In order to study the recognition property of FITC@BTPY–
NH2, the fluorescence titration of the suspension in water with
various cations was conducted. As shown in Fig. 3, the addition
of 50 μM relevant cations, including Li+, Na+, K+, Mg2+, Ca2+,
Ba2+, Ni2+, Zn2+ and Cu2+ did not cause significant fluorescence
quenching of the hybrid material FITC@BTPY–NH2. Only the
presence of Ag+ and Hg2+ quenched the fluorescence of
FITC@BTPY–NH2 directly, and Ag+ has a higher quenching
efficiency on the fluorescence of FITC@BTPY–NH2 compared
with that of Hg2+. This result is similar to those of the reported
homogeneous sensors containing thiourea groups for sensing
Ag+.14 Meanwhile, it was found that the fluorescence of
FITC@BTPY–NH2 was nearly proportional to the concen-
tration of silver ions in a wide concentration range. The detection
limit of the material towards Ag+ is established at or below
0.1 ppm under the experimental conditions used here (25%
quenching efficiency). To the best of our knowledge, this is the
first heterogeneous probe that can detect less than 0.1 ppm of
Ag+ in aqueous solution. Interestingly, the presence of the above
mentioned metal ions, except Hg2+, did not interfere with the
luminescence responses toward Ag+, demonstrating the
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Notes and references
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Fig. 3 (a) The emission spectra of FITC@BTPY–NH2 (1 mg mL−1
)
upon addition of various amounts of Ag+ in HEPES buffer (pH = 7.0).
(b) Emission intensities at 520 nm FITC@BTPY–NH2 as a function of
the [Ag+] in the range of 0.07–0.75 ppm. λex = 490 nm. (c) Fluorescence
responses of FITC@BTPY–NH2 in HEPES buffer at pH 7.0 towards
metal ions interested. For each sample, a 1 mg mL−1 suspension of the
sensor (black bar) was mixed with 50 μM of the cation of interest (gray
bar) and then subsequently treated with 50 μM AgNO3 (white bar).
selectivity of FITC@BTPY–NH2 towards Ag+ over alkali-,
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Zn2+ and Cu2+. This can be attributed to the thiourea groups,
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Accordingly,
the
adsorption
ability
of
FITC@BTPY–NH2 in solid–liquid phase was estimated in
order to evaluate its potential as a practical absorbent material
for Ag+ in drinking water. Solutions of two different Ag+ con-
centrations (0.18 ppm/50 mL; 0.06 ppm/50 mL) were treated
with 20 mg of FITC@BTPY–NH2 overnight at room tempera-
ture, after filtration the concentrations of residual Ag+ in the
filtrate were analyzed by inductively coupled plasma (ICP)
source mass spectrometry. Most of the Ag+ was adsorbed in both
solutions by FITC@BTPY–NH2, only 0.046 ppm and
0.02 ppm Ag+ remained in the water for the two samples,
respectively. The amount of adsorbed Ag+ for FITC@BTPY–
NH2 is about 0.35 mg per gram. The Ministry of Environmental
Protection of the P.R. China has set a national standard for Ag+
in daily drinking water of 0.05 ppm,21 integrated wastewater dis-
charge of 0.5 ppm.22 Current reported Ag+ adsorbents (e.g. acti-
vated carbon) are often used in separation of silver from ores.
The recovery of ionic silver by these is only useful for ionic
silver concentrations greater than 20 ppm.23 The adsorbed
quality of FITC@BTPY–NH2 is lower than these reported
adsorbents, but it can be useful for reducing the ionic silver con-
centration to below 0.02 ppm. Therefore, FITC@BTPY–NH2 is a
promising material in the recovery and monitoring of waste water.
We acknowledge the financial support from the National
Natural Science foundation of China (21025102 and 21171029).
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This journal is © The Royal Society of Chemistry 2012
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