Fibers and Polymers 2015, Vol.16, No.5, 1193-1200
DOI 10.1007/s12221-015-1193-4
ISSN 1229-9197 (print version)
ISSN 1875-0052 (electronic version)
Communication
Synthesis and Characterization of TMU-16-NH2 Metal-organic Framework
Nanostructure Upon Silk Fiber: Study of Structure Effect on Morphine and
Methyl Orange Adsorption Affinity
Amir Reza Abbasi*, Jalal ad-Din Aali, Azadeh Azadbakht1, Ali Morsali2*, and Vahid Safarifard2
Faculty of Chemistry, Razi University, Kermanshah 67194, Islamic Republic of Iran
Department of Chemistry, Faculty of Science, Islamic Azad University, Khorramabad, Islamic Republic of Iran
2
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, Tehran 14155-4838, Islamic Republic of Iran
(Received November 25, 2014; Revised March 12, 2015; Accepted March 17, 2015)
1
Abstract: Thin films of a three-dimensional porous Zn(II)-based metal-organic framework, [Zn (H N-BDC) (4-bpdh)]·
3DMF (TMU-16-NH ), containing azine-functionalized pores, were deposited on surfaces of silk fiber via a stepwise manner.
The effect of sequential dipping steps in growth of TMU-16-NH upon fiber has been studied. These systems depicted a
decrease in the size accompanying a decrease in the sequential dipping steps. The deposition of TMU-16-NH upon silk fiber
was monitored by XRPD and FT-IR spectroscopy. The TMU-16-NH upon silk fiber can serve as a host for encapsulating
morphine (Mph) and methyl orange (MO).
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Keywords: Metal-organic framework, Silk, Nanoparticle, Methyl orange, Morphine
fiber with -COOH surface functionalization. The deposited
films of [Zn2(H2N-BDC)2(4-bpdh)]·3DMF (TMU-16-NH2)
[17] were porous and could be loaded with different guest
species. The shown porosity of the films and the known
ability of MOFs to adsorb different kind of species open a
wide field of possible applications, such as protection layers
for working clothes and gas separation materials in the
textile industry, for synthetic polymers with a MOF layered
on the surface. Due to the synthesis and the additional
further processing fibers made from silk are more suitable
compared with other synthetic and natural fibers. The
surface of silk fibers exhibits very large amounts of reactive
carboxylic groups. These carboxylic groups are already able
to complex Zn(II) ions and act as an anchoring group for the
Zn carboxylate, which is the secondary building unit (SBU)
of TMU-16-NH2 and is deposited with the first pulse of the
stepwise deposition. In general, positive ions are bounded to
the silk at high pH value, since the carboxylic groups are
unprotonated and the electron pair on the carboxylic oxygen
is available for donation to metal ions [18,19].
Introduction
The field of metal-organic frameworks (MOFs), which are
also called porous coordination polymers (PCPs), has been
growing tremendously over the last two decades [1,2]. This
fascinating class of crystalline hybrid materials, which are
formed by association of metal centers or clusters linked by
organic moieties, offers an unique chemical versatility
combined with a designable framework and an unprecedentedly
large and permanent porosity [3-5]. These materials have
enormous potential for many practical structure-related
applications. This includes the more traditional areas of
storage, separation or controlled release of gases, catalysis,
sensing, and drug delivery, as well as the adsorptive removal
of hazardous materials, which are based on the pore size and
shape and the host-guest interactions involved [6-11].
However, the majority of these applications are based on the
ability of MOFs to behave as hosts for certain molecules
[12]. Apart from their use as bulk materials, these frameworks
could be processed as supported homogeneous porous thin
films on various surfaces. Controlling the assembly of
metal-organic frameworks thin films on different substrates
is currently recognized as one of the most important issues in
the synthesis of functional materials [13]. Different strategies
have been developed in the literature to fabricate thin films
of MOFs. These technical approaches can be grouped in
several ways such as surface functionalization [14], layerby-layer (LBL) [15] and electrospun nanofibrous filters [16].
In this work we report the layer-by-layer deposition of a
microporous Zn(II)-MOF material on the surface of natural
Experimental
Materials and Physical Techniques
All reagents and solvents were used as supplied by Merck
Chemical Company and used without further purification.
The silk fiber was obtained from Guilan Silk Company. The
natural silk fibers were pre washed using an aqueous
solution containing NaOH (pH=9.5), at 25 oC for 5 min,
followed by washed several times with water and dried at
ambient temperature. X-ray powder diffraction (XRPD)
measurements were done on a Philips X’pert diffractometer
with monochromatic Cu Kα radiation. The simulated XRD
*Corresponding author: [email protected]
*Corresponding author: [email protected]
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Amir Reza Abbasi et al.
powder pattern based on single crystal data were prepared
using Mercury software. The samples were characterized
with a scanning electron microscope (SEM, Philips XL 30
and S-4160) with gold coating. The average particle sizes
were prepared using Microstructure measurement software.
In situ fluorescence spectroscopy experiment has been
carried out on a JASCO spectroflurimeter (FP 6200).
Infrared spectra were taken with a FT-IR Bruker, vector 22
spectrometer using KBr pellets in the 400-4000 cm-1 range.
Syntheses of TMU-16-NH2 Upon Silk Surfaces
The ligand 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene (4bpdh) was synthesized according to previously reported
methods [20]. 2.3 g (4.5 mmol) of hydrazine hydrate was
added dropwise to a solution of 4-acetylpyridine (1.089 g,
9.0 mmol) dissolved in ethanol (15 ml). Two drops of formic
acid were added and the mixture was stirred at room
temperature for 24 h. The yellow solid that formed was
filtered and washed several times with ethanol/ether (1:1).
Yield: 0.536 g (50 %). The growth of TMU-16-NH2 upon
silk fiber was achieved by sequential dipping in alternating
bath of aqueous Zn(NO3)2·6H2O (0.297 g, 1 mmol) and a
DMF solution of 4-bpdh (0.119 g, 0.5 mmol) and 2-
aminobenzene-1,4-dicarboxylic acid (H2BDC-NH2) (0.181 g,
1 mmol). Before the experiment began, silk fibers were
immersed in an alkaline solution. In alkaline pH, the surface
of fiber becomes negatively charged due to deprotonation of
the carboxylic group present at the fiber’s surface [21]. The
first layer was fabricated by immersing the silk-COO−
surface into an solution of Zn(II) and then in solution of
ligands (1 cycle). When negative fiber was immersed in an
aqueous solution of zinc(II) nitrate, Zn(II) ions are attracted
to the fiber surface [18]. The dipping step in 4-bpdh/H2BDCNH2 solutions allowed the formation of TMU-16-NH2 and
initiated the formation of new TMU-16-NH2 particles, as
illustrated in Figure 1.
The results show that sequential dipping in alternating
baths of aqueous Zn(II) and 4-bpdh/H2BDC-NH2 leads to a
stepwise deposition of TMU-16-NH2 multilayers. The thickness
of the multilayers was increased with the increase of the
deposition cycles [18]. The dipping step in each Zn(II) and
4-bpdh/H2BDC-NH2 solutions was 2 min followed by some
rinses in pure DMF each for 1 min. In order to investigate
the deposition of the first 15 MOF layers on the surface of
the silk fiber, the substrate was dipped alternatively into SBUand linker solution with washing with DMF in between [14].
Figure 1. (A) The preparation of TMU-16-NH upon silk fiber from raw reagents and (B) the use of TMU-16-NH upon silk for
encapsulating and further releasing therapeutic species.
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Fibers and Polymers 2015, Vol.16, No.5
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Results and Discussion
Fourier Transform Infrared Spectroscopy (FT-IR) and
X-ray Powder Diffraction (XRPD)
Due to the small amount and thickness of the deposited
MOF film, the standard technique of FT-IR is suited for the
investigation of film quality. Therefore, after each deposition
cycle, an absorption spectrum of the dried substrate was
recorded. The increase of the intensity of the vibrational
bands (υ=776 cm-1 and υ=834 cm-1) from TMU-16-NH2
phase is proportional to the number of performed deposition
cycles. It can be concluded that the characterization of MOF
films with a low number of performed deposition cycles can
be better achieved using the FT-IR spectra than XRPD, due
to its limitation in detection of films with a thickness <40 nm
[14,22]. The change of the intensity was observed after the
first deposition cycle on the silk surface. Although the
observed change of the intensity is small, the intensity
increases continuously with each further deposition cycle
(Figure 2). The linear increase of the absorbance indicates a
regular assembly of the cationic and anionic building blocks.
FT-IR for TMU-16-NH2 (KBr, 400-4000 cm-1): 3460 (s,
broad), 3349 (s, broad), 2927 (w), 1663 (s), 1625 (s), 1579
(s), 1502 (s), 1427 (s), 1378 (s), 1257 (s), 831 (s), 768 (s) and
574 (s).
To determine the crystal phase of TMU-16-NH2 formed
upon silk fiber, XRPD measurements were carried out over
the diffraction angle (2θ) of 3-35 o. Figure 3 shows the
XRPD patterns; simulated from single crystal X-ray data of
TMU-16-NH2 (a), as-synthesized TMU-16-NH2 (b), pristine
silk fibers (c), TMU-16-NH2 upon silk after applying 10 (d)
and 15 deposition cycles (e). The nine major peaks found at
Figure 2. FT-IR spectra of pure silk fiber and silk fiber containing
TMU-16-NH after applying 5 and 15 deposition cycles; (a) silk
yarn, (b) 5 deposition cycles@silk yarn, and (c) 15 deposition
cycles@silk yarn.
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Figure 3. Simulated pattern based on single crystal data of XRPD
pattern of TMU-16-NH (a), as-synthesized TMU-16-NH (b), the
pure silk fiber (c), TMU-16-NH upon silk after applying 10 (d)
and 15 deposition cycles (e), TMU-16-NH upon silk after the
adsorption (f) and delivery of MO (g).
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4.96 o, 8.36 o, 8.52 o, 9.36 o, 10.04 o, 10.40 o, 16.80 o, 17.12 o
and 25.28 o on the 2 theta scale correspond respectively to
the (002), (110), (111−), (112−), (112), (200), (220), (221)
and (331−) crystal planes. Acceptable matches with slight
difference in 2θ, were observed between the simulated
XRPD pattern and the experimental data (Figure 3(d)) [17].
The results indicated that TMU-16-NH2 formed on the silk
fiber and the crystallinity of the coated [Zn2(H2N-BDC)2(4bpdh)]·3DMF MOF films were increased by increasing the
cycles of layer by layer coating of TMU-16-NH2 on the silk
fibers (Figure 3(d) and (e)) [18]. Figure 3(f) shows the X-ray
powder diffraction patterns of TMU-16-NH2 upon silk
soaked in an aqueous solution (10-4 mol l-1) of methyl orange
(MO) at room temperature for 2 h. It is worth noting that
when the crystals were soaked in the solution for about 2 h,
most of the peaks in the XRPD data distinctly weakened.
The delivery of MO from TMU-16-NH2 upon silk performed in
ethanol at room temperature under continuous stirring was
determined by XRPD (Figure 3(g)). The unusual phenomenon
can be well explained by the high amount of guest in TMU16-NH2, which has a significant impact on the sensitivity of
the X-ray analysis. The obtained pattern match with the
pattern of monoclinic TMU-16-NH2, space groups C2/c
with the lattice parameters a=17.1558 (6) Å, b=13.4604 (4)
Å, c=36.1586 (11) Å and z=8 [17]. The wide peak at 17-23 o
corresponds to the silk substrate [18,19].
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Effects of Sequential Dipping Steps
Particle sizes and morphology of nanoparticles are
depending on sequential dipping [23]. Effect of different
Amir Reza Abbasi et al.
sequential dipping in growth of TMU-16-NH2 upon fiber
were studied at pH=9.5. The results suggest that with
increasing the fiber dipping steps into SBU- and linker
Figure 4. SEM image of the pristine silk fiber.
Figure 5. SEM photographs and the corresponding particle size distribution histograms of TMU-16-NH upon silk after applying 4
deposition cycles.
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TMU-16-NH2 MOFs@Silk Fibers
Fibers and Polymers 2015, Vol.16, No.5
1197
Figure 6. SEM photographs and the corresponding particle size distribution histograms of TMU-16-NH upon silk after applying 10
deposition cycles, corresponding wavelength-dispersive X-ray (WDX) analysis of TMU-16-NH upon silk after applying 10 deposition
cycles.
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solution, growth takes place on more nuclei, the Zn(II) and
4-bpdh/H2BDC-NH2 attraction increases, and subsequently
the concentration and size of TMU-16-NH2 particles upon
silk fiber increases [21].
For the sake of investigating the morphology of the
prepared coating samples, the SEM images of samples were
studied. The SEM images of the non-modified natural fiber
(Figure 4) were compared with after applying 4 (Figure 5)
and 10 deposition cycles (Figure 6) of TMU-16-NH2 upon
silk fiber. The surface of the individual fibers is covered by a
continuous film of separated crystals with an average size of
60 and 123 nm for 4 and 10 deposition cycles, respectively,
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Amir Reza Abbasi et al.
Figure 7. (A) Photographs showing the visual color change when TMU-16-NH upon silk was immersed in the aqueous solution of MO
(10 mol l ). No further change in color occurred after 7 days, temporal evolution of fluorescence intensity spectra for the loading of Mph
(B) and MO (C) from TMU-16-NH upon silk.
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TMU-16-NH2 MOFs@Silk Fibers
without defects. However, the SEM images with a low
magnification also exhibit areas with big agglomerates of
the deposited MOF. This can be understood by considering
the possible storage effect of the unreacted material between
the fibers. The wavelength-dispersive X-ray (WDX) mapping
of the surface shows the uniform distribution of Zn throughout
the whole substrate surface (Figure 6).
Adsorption Affinity
The porosity of MOF films deposited on substrate surfaces
is an important point concerning the possible use of such
functional materials for different purposes. TMU-16-NH2
has one-dimensional open channels (aperture size of 7.1×
4.6 Å) running along c axis which the internal surface is
decorated by the azine groups of the 4-bpdh ligands [17].
For augment of porosity in TMU-16-NH2 upon silk, we
successfully tested its porosity with guest molecules by
suspending it in an aqueous solution of morphine (Mph) and
methyl orange (MO). The silk fiber containing 1.0 g of
TMU-16-NH2 was immersed in a sufficient amount of an
Fibers and Polymers 2015, Vol.16, No.5
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aqueous solution of Mph or MO (10-4 mol l-1) in a small
sealed flask at room temperature. The dark red solutions of
MO fade slowly to very pale orange (Figure 7A), while the
TMU-16-NH2 upon silk get darker. We tested for the amount
of Mph and MO that can be inserted in the pores. Results
show that 1.0 g of TMU-16-NH2 can absorb approximately
2.00 and 1.271 g of guest in Mph and MO contained TMU16-NH2 upon silk, respectively. The guest content was
estimated by XRPD (Figure 3) and fluorescence intensity
spectra (Figure 7(B) and (C)) [24]. The change of intensity
and width indicates that the resulting solid TMU-16-NH2
upon silk retains the host framework crystallinity as MO/
Mph molecules diffused in. The rapid decline of the
fluorescence intensity spectra also proves that TMU-16-NH2
can absorb Mph faster than MO. The higher sorption for
Mph is attributed to the size and strong hydrogen bonding
interactions. Adsorption of guest was spontaneous and
endothermic, and the entropy (the driving force of the
adsorption) increases with the adsorption of guest. Entropic
hydrophobic interactions occur when a guest replaces the
Figure 8. Temporal evolution of fluorescence intensity spectra for the delivery of Mph (A) and MO (B) from TMU-16-NH upon silk
containing Mph or MO.
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Fibers and Polymers 2015, Vol.16, No.5
water within a cavity. An increase in entropy increases the
favorability of the process.
Release Assays
The encapsulated guest could be easily removed from the
frameworks upon immersion of guest@MOFs in ethanol.
The temporal evolution of fluorescence intensity spectra for
MO in ethanol solution, which shows λmax at 270 nm,
becomes stronger with increasing MO content. The delivery
of MO in ethanol increases with time, indicating that the MO
release is governed by the host-guest interaction. Similar
behaviors were also observed in the delivery of Mph from
TMU-16-NH2 upon silk (Figure 8). The MOF-Mph interaction
is stronger than MOF-MO, so the delivery of MO from
TMU-16-NH2 can be faster than Mph but the amount of
adsorbance is less.
Conclusion
In summary, we report the fabrication of [Zn2(H2N-BDC)2
(4-bpdh)]·3DMF (TMU-16-NH2) metal-organic framework
(MOF) nanostructures upon silk fiber using layer-by-layer
method at ambient pressure and temperature. Due to
existence of -COOH groups on the surface of the silk fibers
no self-assembled monolayer formation was required.
XRPD analyses indicated that the prepared TMU-16-NH2
MOF on silk fibers were crystalline. The deposition of MOF
thin films on natural fiber surfaces might be a new path for
the fabrication of functional materials for different applications,
such as protection layers for working clothes and gas
separation materials in the textile industry. TMU-16-NH2
upon silk may indeed is suitable for applications requiring
frequent loading and unloading of guests.
Acknowledgement
Support of this investigation by Iran National Science
Foundation: INSF (No. 92031906) and Razi University of
Kermanshah are gratefully acknowledged.
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