Novel polypeptide/thiol - SBA-15 hybrid materials synthesized via surface selective grafting

Article abstract of DOI:10.1039/b927487a

Novel hybrid materials are synthesized through the surface selective grafting of poly-l-lysine and thiols from SBA-15. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b927487a

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Novel polypeptide/thiol—SBA-15 hybrid materials synthesized
via surface selective graftingw
Jonathan D. Lunn and Daniel F. Shantz*
Received (in Berkeley, CA, USA) 5th January 2010, Accepted 3rd March 2010
First published as an Advance Article on the web 26th March 2010
DOI: 10.1039/b927487a
Novel hybrid materials are synthesized through the surface
selective grafting of poly-L-lysine and thiols from SBA-15.
Though we found in initial investigations that amines graft
easily to the external surface of SBA-15 through traditional
reaction in toluene, thiols were much less reactive, requiring
high temperatures, long reaction times, and the addition of
water (ESIw). Additionally, we found that traditional ethanol
extraction ineffectually removes the Pluronic template. Here,
The continued interest in hybrid materials has led to intense
efforts in developing new synthesis paradigms for making
materials which advantageously combine the features of both
soft and hard matter. In this vein, ordered mesoporous silica
16,17
we present a modified approach adapted from the literature
1,2
(OMS)–organic hybrids have attracted considerable attention.
using microwaves that rapidly and effectively grafts both
silanes and extracts the Pluronic template.
Given the well-defined textural properties of OMS and the
ability to functionalize silica using known silane chemistries,
these appear to offer numerous opportunities for developing
new hybrid materials.
SBA-15 was synthesized as described previously. External
surface functionalization was achieved by reacting as-made
SBA-15 in neat organosilane while stirring and irradiating
with microwaves (300 W) for 15 minutes.z The Pluronic
template was then removed by a rapid three step ethanol
extraction which was assisted by microwaves (100 W). Extraction
was followed by a traditional post-synthetic grafting of
We recently reported the synthesis of OMS–polypeptide
hybrids via the use of surface-tethered amines as initiators
for the ring-opening polymerization of N-carboxyanhydride
3
NCA) amino acids. Building on that report, here we
(
ꢀ
0.5 mmol g functionalized SiO (FS) of the other silane. In
2
1
demonstrate the ability to selectively functionalize the outer
or inner surface of SBA-15 with poly-L-lysine while grafting a
different functional group, here thiols, to the opposite surface.
We believe the ability to graft different organic architectures
surface-selectively will extend the scope of OMS hybrids in a
diverse field of scientific interests. In the current work, amines
and thiols were chosen given their potential interest in
this manner, two samples were prepared: one with amines
externally and thiols internally (X) and another with thiols
externally and amines internally (I). Poly-Z-L-lysine (PZK)
was grafted from the amine-functionalized surface, deprotected
(PK) using HBr, and neutralized (Fig. 1). High and low
ꢀ
1
monomer loading (10 and 5 mmol NCA g functionalized
SiO ) syntheses were performed to observe the effect of the
4
–6
7
catalysis
and molecular gating, and their well-developed/
2
orthogonal chemistries that permit subsequent reactions/use
as chemical handles.
polypeptide loading: PZK-X-10 and PZK-I-10 and PZK-X-5
and PZK-I-5, for high and low, respectively.
Recently, there has been increasing interest in developing
such multifunctional materials. An array of different approaches
have been reported in the literature. One method utilizes the
OMS-templating agent as a barrier to selectively graft organics
XPS spectra taken after the second functionalization step
show the effective incorporation of both silanes (Fig. 2C). As
expected due to the surface sensitivity of XPS, the peaks at the
characteristic binding energies of the externally grafted silane
are more intense than those of the internal group. Additionally,
NHS-fluorescein (amine selective) and BODIPY-TMR
thiosulfate (thiol selective) dyes were used to visualize the
degree of surface segregation of the amines and thiols by
Confocal Microscopy. Despite limits in optical resolution,
images of these samples show amine and thiol-selective dyes
7
–10
to the outer surface followed by template extraction.
A
second approach uses sequential co-condensation in radially
growing OMS nanoparticles to incorporate one or more
1
1–13
functional groups.
Other methods employing diffusion-
1
4
15
limited deprotection and surface tension differences have
also been reported. Though likely less domain specific
than ‘‘designer’’ methods used for OMS nanoparticles, the
‘
‘barrier’’ approach yields high external organosilane concen-
trations and uses traditional OMS synthesis protocols
applicable to a wide variety of common systems. For these
reasons, it is the method of choice here.
Artie McFerrin Department of Chemical Engineering,
Texas A&M University, 3122 TAMU, College Station, Texas, USA.
E-mail: shantz@chemail.tamu.edu
w Electronic supplementary information (ESI) available: Detailed
synthesis procedures and analytical techniques; XPS and IR spectra;
adsorption isotherms; SEM images; initial thiol grafting results; and
results from polylysine-only hybrids prepared via traditional amine
grafting. See DOI: 10.1039/b927487a
Fig. 1 Idealized pictorial representations of PK-X (left) and PK-I
2 2 2 2 2
(right); R = –CH CH CH CH NH .
2
926 | Chem. Commun., 2010, 46, 2926–2928
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Table 1 Adsorption and TGA data
a
b
(BJH)/ Organic Lysine /
S(a
s
)/
V
p
/
cm g
d
p
2
m g
ꢀ1
3
ꢀ1
ꢀ1
Sample
nm
(wt%)
mmol g FS
SBA-15
X
I
9.4
19.6
24.5
68.9
53.7
—
—
—
6.04
5.74
660
651
—
33
—
—
34
—
204
60
—
256
0.66
0.65
n.p.
0.02
n.p.
n.p.
0.02
n.p.
0.18
0.03
n.p.
0.23
7.5
7.4
—
5.6
—
—
5.9
—
PZK-X-10
PK-X-10
PK-X-10 H
PZK-X-5
PK-X-5
PZK-I-10
PK-I-10
PK-I-10 H
PZK-I-5
PK-I-5
63.8
49.7
68.0
52.7
4.66
4.68
5.18
4.66
6.2
6.2
—
61.8
45.8
3.73
3.07
6.6
a
b
= 0.9; n.p.=non-porous. Calculated from
Determined at p/p
0
TGA data after subtracting organosilane and residual Pluronic
contributions.
Fig. 2 Confocal images of (A) I and (B) X dyed with NHS-fluorescein
green, amine selective) and BODIPY-TMR thiosulfate (red, thiol
(
selective). The scale bar represents 5 mm. (C) XPS spectra of I (top)
and X (bottom).
occupying separate domains (Fig. 2A and B), consistent with
the XPS data and suggesting successful selective functionalization
in both materials.
Fig. 3 SEM images of PZK-X-5 (left) and PZK-I-5 (right). The white
As in our previous work, we found that the ring-opening
polymerization of N-carboxyanhydride amino acids proceeds
from the surface of amine-functionalized SBA-15, demonstrating
that the amines are accessible and the thiols do not inhibit the
polymerization. Selectively-grafted samples with only amines
show similar results (ESIw). IR (ESIw) indicates that signifi-
cant amounts of polypeptide are formed in both sets of mate-
scale bar represents 1 micron.
estimated values for complete deprotection and neutralization
as noted by the calculated mmol lysine per gram FS in Table 1.
Complementary to SEM, XPS shows a dramatic decreasing
trend in the C/Si and N/Si atomic ratios from PZK-X-10 to
PZK-I-5 (Fig. 4A). PZK-X-10 has C/Si and N/Si ratios of 425
and 28, respectively, whereas, PZK-I-5 has ratios of 60 and 6.
The much lower contribution of N and C in the PZK-I-5
spectra provides strong evidence for more internally confined
polypeptide. PZK-X-5 and PZK-I-10 show similar ratios,
again suggesting that excess peptide is exiting the pores in
PZK-I-10. Another notable difference is observed in the
spectra of the deprotected samples: the S 2s and S 2p peaks
are visible for the internally grafted samples as opposed to the
externally grafted samples where none are observed (Fig. 4B).
ꢀ
1
rials, evident from the amide peak intensities (B 1650 cm and
ꢀ
1
ꢀ1
B1550 cm ) relative to the Si–O–Si bending peak (B 1100 cm ).
It is similarly apparent that the polymer can be deprotected
with ease in HBr, shown by the disappearance of the carbamate
ꢀ
1
peak (B 1700 cm ) (Fig. 5).
TGA shows that, with the exception of PZK-I-5, much
larger amounts of peptide are formed compared to the
B3.5 mmol lysine per gram FS maximum achieved in our
previous study (Table 1). For the externally grafted samples,
this result may be explained by a lack of pore wall confinement
on polymer growth. As might be expected taking this into
consideration, the externally grafted peptide samples for both
monomer loadings show larger amounts of polymer relative
to their internally grafted counterparts. SEM show large
amounts of polymer on the external surface of the elementary
particles (d B 0.5 mm) for these samples (ESIw and Fig. 3). For
the internally grafted samples, polymer chains close to the
pore mouth likely continue to grow outward without external
interference leading to larger than expected polymer loadings;
SEM, in support, shows polymer on the outer surface of
PZK-I-10 (ESIw). SEM images of PZK-I-5 (Fig. 3), in contrast,
show smaller and more defined particles indicative of much
less or no polymer on the outer surface. Weight losses recorded
for the deprotected samples are in excellent agreement with
Fig. 4 (A) Plot of N/Si and C/Si atomic ratios for poly-Z-L-lysine
grafted samples; (B) XPS spectra of PK-I-5 (top) and PK-X-5
(bottom).
This journal is ꢁc The Royal Society of Chemistry 2010
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This work was funded by the Rober A. Welch Foundation
Grant A-1638). The authors acknowledge the Materials
(
Characterization Facility at Texas A&M for access to their
XPS and FE-SEM instruments. Additionally, the authors
thank Mrs Nataly Garcıa Vargas for her SEM work.
´
Notes and references
z A comparison of the as-made functionalized samples with as-made
SBA-15 indicates some Pluronic is likely extracted into the silane
phase during functionalization; however, it does not seem to have a
large effect on the resulting samples and its impact is probably limited
to pore mouth functionalization. Polylysine-only hybrids prepared
using traditional grafting in toluene lead to qualitatively similar
porosity and surface composition trends (ESIw).
Fig. 5 IR of neutral and protonated (A) PK-X-10 and (B) PK-I-10.
ꢀ1
The dashed lines mark the amine absorption band at B3060 cm
.
y Protonation of the side chain amines is reversible as they were
initially charged following deprotection, then neutralized.
As there are numerous studies on polyelectrolyte brush
1
8
swelling, we were interested in looking at the effects of side
chain protonation on the deprotected samples. PK-I-10 and
PK-X-10, neutralized after deprotection, were treated with
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.1 M HCl and sonicated for 10 minutes. Nitrogen adsorption
of PK-I-10 H shows a drastic decrease in porosity (Table 1).
Similarly, though there was very little initial porosity, PK-X-10
H shows absolutely no porosity (Table 1).
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ꢀ
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19
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928 | Chem. Commun., 2010, 46, 2926–2928
This journal is ꢁc The Royal Society of Chemistry 2010