Surface modification and functionalization of nanoscale metal-organic frameworks for controlled release and luminescence sensing

Article abstract of DOI:10.1021/ja073506r

We describe in this paper a general method for synthesizing a new class of nanocomposites with a nanoscale metal-organic framework (NMOF) core and a silica shell. Silica shells of variable thickness were deposited on the NMOFs that had been surface-modified with polyvinylpyrrolidone (PVP) using a sol-gel procedure. The NMOF core of the nanocomposite could be completely removed (via dissolution) at low pH to afford hollow silica shells with varied thickness and aspect ratios. We also showed that the silica shell of such nanocomposites significantly stabilized the NMOF core against dissolution, thus demonstrating the ability to control the release of metal constituents from such silica-coated NMOFs. The silica shell was further functionalized with a silylated Tb-EDTA monoamide derivative for the luminescence sensing of dipicolinic acid (DPA), which is a major constituent of many pathogenic spore-forming bacteria. Owing to the tunability of NMOF composition and morphology, the present approach should allow for the synthesis of not only interesting nanoshells that are not accessible with presently available templates but also novel core-shell hybrid nanostructures for future imaging, sensing, and drug delivery applications. Copyright

Full text of DOI:10.1021/ja073506r

Published on Web 07/24/2007
Surface Modification and Functionalization of Nanoscale Metal-Organic
Frameworks for Controlled Release and Luminescence Sensing
William J. Rieter, Kathryn M. L. Taylor, and Wenbin Lin*
Department of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599
Received May 17, 2007; E-mail: wlin@unc.edu
Metal-organic frameworks (MOFs) are an interesting class of
materials which have been exploited on the bulk scale for a number
of applications, including gas adsorption and catalysis. We have
recently described a general method for preparing nanoscale MOFs
Scheme 1
1
2
(
NMOFs) using reverse microemulsions and have demonstrated
3
their utility as magnetic resonance imaging contrast agents. Owing
to their tunable nature, we believe that NMOFs have potential in a
variety of other imaging, biosensing, biolabeling, and drug delivery
applications. The successful application of NMOFs in these areas
will critically depend on our ability to modify and functionalize
their surfaces to engender stability, biocompatibility, and specific
functionality. To date, however, there have been no reports on the
surface modification or functionalization of MOF materials.
Coating nanostructures with a silica shell via sol-gel or
microemulsion-based methods has enabled the synthesis of an array
of core-shell nanocomposites, such as silica-coated superpara-
the mixture was stirred for a period of time (up to 7 h) to afford a
silica-coated NMOF, 3, with a desired shell thickness. For example,
an uneven silica shell of 2-3 nm in thickness was deposited on
the surface of 2 after 3 h (Figure 1c), whereas a silica shell thickness
of 8-9 nm was achieved simply by prolonging the reaction time
to 7 h (Figure 1d). For 2 with high aspect ratios (W > 10), the
reaction conditions were slightly modified to minimize the formation
of secondary silica nuclei (Supporting Information). Representative
TEM images for 3 synthesized at W ) 15 with thin (2-3 nm) and
thick (8-9 nm) silica coatings are shown in Figure 1e-g.
4
5
6
7
magnetic metal oxides, quantum dots, gold, carbon nanotubes,
8
and organic polymers. The silica shell as a surface coating offers
several advantages, including enhanced water dispersibility, bio-
compatibility, and the ability to further functionalize the core-
shell nanostructures via the co-condensation of siloxy-derived
molecules. Herein, we describe a general method for synthesizing
a new class of nanocomposites with an NMOF core and a silica
shell. We also demonstrate the ability to control the release of metal
constituents from silica-coated NMOFs and to further functionalize
them for the luminescence sensing of dipicolinic acid (DPA), which
is a major constituent of many pathogenic spore-forming bacteria.
2 2
NMOFs of the composition Ln(BDC)1.5(H O) , 1, where Ln )
Eu3 , Gd , or Tb and BDC ) 1,4-benzenedicarboxylate, were
+
3+
3+
synthesized using a reverse microemulsion system as previously
described (1′ is used to designate Eu-doped Gd(BDC)1.5(H
Scheme 1). This synthesis allowed for the aspect ratios of nanorods
2
O)
2
in
3
1
to be reproducibly tuned from 2.5 to 40 by adjusting the water to
surfactant molar ratio, W. We then used a strategy developed by
van Blaaderen et al. to deposit silica on NMOFs whose surfaces
9
had been modified with polyvinylpyrrolidone (PVP). Treatment
of as-synthesized 1 with PVP (MW ) 40000) in situ for 12 h led
to highly dispersible PVP-coated nanorods. TEM showed that PVP-
functionalized Ln(BDC)1.5(H
W ) 5 had a rod-like morphology with dimensions approximately
00 nm in length by 40 nm in width (Figure 1b). The polydispersity
2 2
O) nanoparticles, 2, synthesized at
1
of nanoparticles of 2 was low, and their particle size and
3
morphology corresponded well to those of 1. Nanoparticles of 2
with an aspect ratio as high as 40 were also synthesized in high
yield at W ) 15.
Nanoparticles of 2 were subsequently coated with silica shells
of variable thickness using a sol-gel procedure. 2 was first primed
with silica by treating with tetraethyl orthosilicate (TEOS, 5 µL
per mg of 2 synthesized at W < 10) in a solution of 4% (v/v)
aqueous ammonia in ethanol (∼0.2 mg of 2/mL) for 2 h. An
additional aliquot of TEOS (5 µL/mg of 2) was then added, and
Figure 1. (a and c) TEM images of 3 with a 2-3 nm silica shell (W ) 5);
(
b) TEM image of 2 (W ) 5); (d) TEM image of 3 with an 8-9 nm silica
shell (W ) 5); (e and f) TEM images of 3 with a 2-3 nm silica shell (W
)
15); (g) TEM image of 3 with an 8-9 nm silica shell (W ) 15); (h)
TEM image of 8-9 nm silica nanoshells generated from (g). Scale bars
are 50 nm unless otherwise indicated.
9852
9
J. AM. CHEM. SOC. 2007, 129, 9852-9853
10.1021/ja073506r CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
with a silylated Tb-EDTA monoamide derivative, 3′-Tb-EDTM
Scheme 1). Tb3 ions and molecular Tb complexes have been
+
10
11
(
used as sensitive luminescence probes for anthrax and other bacterial
spores by complexing to DPA, which constitutes up to 15% of the
spores’ dry mass. Upon excitation at 278 nm, 3′-Tb-EDTM only
gave Eu luminescence because the Tb-EDTM moiety is essentially
nonemissive. As DPA was added to an ethanolic dispersion of 3′-
Tb-EDTM, the Tb luminescence became clearly visible due to the
formation of the Tb-EDTM-DPA complex. The Tb luminescence
signal provides a sensitive probe for DPA detection, while the Eu
emission from the NMOF core acts as noninterfering internal
calibration. As shown in Figure 2d, the relationship between the
ratio of Tb to Eu emission intensities and DPA concentration
displayed normal saturation behavior. At low DPA concentrations,
the ratio of signal intensities increased linearly; however, the ratio
of intensities began to level off as the Tb-EDTA complexes became
coordinatively saturated with DPA. The DPA detection limit for
this system was estimated to be ∼48 nM. Such a ratiomeric sensing
scheme works equally well in a Tris buffer solution and is able to
selectively detect DPA in the presence of biologically prevalent
interferences such as amino acids (Supporting Information).
In summary, we have developed a general method to coat
NMOFs with silica shells of variable thickness. These shells increase
NMOF core stability and allow for the controlled release of metal
constituents. Silica-coated NMOFs were further functionalized for
the detection of DPA, an important molecular marker in spore-
producing bacteria. Owing to the tunability of NMOF composition
and morphology, the present approach should allow for the synthesis
of novel core-shell hybrid nanostructures for future imaging,
sensing, and drug delivery applications.
Figure 2. TGA curves for 1-3 synthesized at (a) W ) 5 and at (b) W )
5 (black ) 1; blue ) 2; red ) 3 with a 2-3 nm silica shell; green ) 3
with an 8-9 nm silica shell). (c) Time-dependent dissolution curves for 1
red) and 3 (black) with an 8-9 nm silica shell at pH ) 4 and 37 °C. (d)
Dependence of the ratio of Tb to Eu emission intensities for 3′-Tb-EDTM
on DPA concentration (red ) 544 nm/592 nm, black ) 544 nm/615 nm).
The inset shows the linear relationship at low [DPA].
1
(
The compositions of 2 and 3 were further established by TGA
(Figure 2a,b) and powder X-ray diffraction (PXRD). TGA results
showed that 2 had a slight increase of the total weight loss over
that of 1 as a result of the PVP coating (5.5% for W ) 5 and 1.5%
for W ) 15). In contrast, the weight loss for 3 decreased
2
significantly due to the presence of a SiO coating. For 3 synthesized
at W ) 5, a 2-3 nm silica coating led to a 9.6% reduction in weight
loss, whereas an 8-9 nm coating of silica reduced the total weight
loss by 28.5%. For 3 synthesized at W ) 15, a 2-3 nm silica
coating led to a 9.4% reduction in weight loss, whereas an 8-9
nm coating of silica reduced the total weight loss by 25.7%. The
smaller changes to total weight loss for 2 and 3 synthesized at W
Acknowledgment. We acknowledge financial support from
NSF and NIH grants. W.J.R. thanks NSF for a graduate research
fellowship, and W.L. is a Camille Dreyfus Teacher-Scholar.
)
15 were consistent with the reduced surface areas for larger
Supporting Information Available: Experimental procedures and
particles. PXRD studies showed that 2 and 3 exhibited the same
pattern as macroscopic 1, further proving the presence of the
crystalline NMOF core in both PVP- and silica-coated 1. Interest-
ingly, the NMOF core of 3 could be completely removed (via
dissolution) at low pH to afford hollow silica shells with varied
thickness and aspect ratios (Figure 1h). Since the morphologies of
NMOFs can be controlled by exploiting the energetics of different
crystallographic faces, we believe the present approach can be used
to produce interesting nanoshells that are not accessible with
presently available templates.
We have also examined the stabilizing effect of an 8-9 nm silica
shell on the NMOF core. Figure 2c shows the dissolution curves
for 1 and 3 when dialyzed against water at 37 °C. The dissolution
curves for both 1 and 3 at pH ) 4 can be modeled as a zeroth-
order rate law with an apparent rate constant of 0.143 and 0.084
µM/h, respectively. The silica shell of 3 has thus stabilized the
NMOF core against dissolution. A similar stabilizing effect of the
silica shell was also observed at pH ) 5, with an apparent rate
constant of 0.085 and 0.044 µM/h for 1 and 3, respectively. These
results indicate that the rates of cargo release from such core-
shell nanostructures can be readily controlled, presumably by taking
advantage of slow diffusion of metal and organic constituents
through the silica shell.
1
7 figures. This material is available free of charge via the Internet at
http://pubs.acs.org.
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