Pore-filled nanoporous silica colloidal films with enantioselective permeability

Article abstract of DOI:10.1002/zaac.201300408

We prepared nanoporous silica colloidal films whose nanopores were filled with polymer brushes containing chiral selector moieties in the side chains, or with a chiral polypeptide. Both types of polymers were grown inside the nanopores using surface-initiated polymerization and thus were covalently attached to the nanopore surface. We studied the enantioselective permeation in the resulting porefilled films and found that they possess a relatively high enantioselectivity. Increasing the number of chiral selectors inside the nanopores and preventing through solution diffusion by pore-filling with polymers did not lead to increased enantioselectivity compared to that observed for silica colloidal films whose nanopores were surfacemodified with a monolayer of chiral selectors. This suggests that the enantioselectivity depends solely on the energy difference between the enantiomer/selector complexes and that through solution diffusion plays a minor role in the transport of enantiomers, which occurs predominantly through site hopping.

Full text of DOI:10.1002/zaac.201300408

ARTICLE
DOI: 10.1002/zaac.201300408
Pore-Filled Nanoporous Silica Colloidal Films with Enantioselective
Permeability
[a]
[a]
[a]
[a]
Patricia A. Ignacio-de Leon, Julie A. Cichelli, Alexis E. Abelow, and Ilya Zharov*
Keywords: Mesoporous materials; Silica; Nanopores; Polymers; Chiral resolution
Abstract. We prepared nanoporous silica colloidal films whose nanop-
ores were filled with polymer brushes containing chiral selector moie-
ties in the side chains, or with a chiral polypeptide. Both types of
ores and preventing through solution diffusion by pore-filling with
polymers did not lead to increased enantioselectivity compared to that
observed for silica colloidal films whose nanopores were surface-
polymers were grown inside the nanopores using surface-initiated po- modified with a monolayer of chiral selectors. This suggests that the
lymerization and thus were covalently attached to the nanopore sur-
face. We studied the enantioselective permeation in the resulting pore-
filled films and found that they possess a relatively high enantio-
selectivity. Increasing the number of chiral selectors inside the nanop-
enantioselectivity depends solely on the energy difference between the
enantiomer/selector complexes and that through solution diffusion
plays a minor role in the transport of enantiomers, which occurs pre-
dominantly through site hopping.
tically active polyelectrolyte multilayer membranes[27] and
nanotube membranes surface-modified with antibodies
have also been shown to separate enantiomers.
Introduction
[
28,29]
The ability to obtain enantiomerically pure compounds is of
high importance in the preparation of new drug molecules.
Although advances in stereoselective synthesis provide new
and efficient ways to obtain a desired enantiomer, chiral sep-
aration of racemic mixtures[ is still the most widely used
industrial technique to produce single enantiomers. Despite
their utility, these separation techniques, which include
crystallization and chromatography, are labour, material and
energy-intensive and are usually difficult to scale up. Mem-
brane-based separations[
method for chiral separations. Chiral membrane separations of
enantiomer mixtures are based on the difference in non-cova-
[
1]
For practical applications, chiral membranes should possess
a high surface area, low mass transfer resistance, good me-
[
2]
[30]
chanical strength and high selectivity.
In this respect, nan-
3]
oporous silica colloidal membranes may provide an ideal me-
dium for chiral separations. Indeed, silica colloidal crystals are
formed via self-assembly of silica spheres into a close-packed
[31]
face-centered cubic (fcc) lattice with a void fraction of 0.26
and contain ordered arrays of three-dimensional interconnected
nanopores whose size can be controlled by changing the size
of the silica spheres used to assemble the colloidal crystal,
4–8]
provide an attractive alternative
whose molecular flux remains significant even when the pore
lent binding energy between each enantiomer and a selector
[32]
size is sufficiently small to impart chemical selectivity,
and
moiety,[
9–11]
or on the preferential inclusion into a cavity
whose surface can be easily modified through standard silox-
[3]
within a porous organic or inorganic material.
Presently used enantioselective membranes[
consist of a porous material supporting either a liquid phase
containing chiral selector molecules, or carrying surface-bound
selector moieties. The latter, much more robust enantioselec-
tive membranes contain selector moieties introduced by coat-
[33]
[34,35]
ane chemistry
to impart permselectivity.
Silica colloi-
12–15]
most often
[32]
dal membranes can be prepared as thin films,
sus-
[36,37]
[38,39]
pended,
and free-standing membranes.
Earlier, we described[40,41] nanoporous silica colloidal films
[42]
modified with a monolayer of chiral selectors.
These films
exhibited good chiral selectivity, transporting one enantiomer
of a pair up to 4.5 times more efficiently than the other, while
maintaining a high molecular flux. We demonstrated that the
[16]
ing the pores with a thin layer of a chiral polymer,
lently grafting the selector to the membrane,[
by cova-
may be pre-
17–21]
pared as a porous polymeric films that incorporate chiral selec-
transport in these films occurs via a facilitated mecha-
tors,[
22–24]
or by molecularly imprinting a polymer.
[25,26]
Op-
[28,29]
nism,
with chiral selectors serving as fixed-site carries
that allow enantiomers to hop from one selector to the next
*
Prof. Dr. I. Zharov
and thus cross the membrane.[
43,44]
The stereoselective non-
Fax: +1-801-581-8433
E-Mail: i.zharov@utah.edu
covalent interactions between enantiomers and surface-bound
chiral selectors result in higher binding energy for one enantio-
mer in a pair, leading to it being transported more efficiently
across the chiral membrane (i.e. the enantiomer interacting
stronger with surface-immobilized chiral selectors is trans-
ported with a higher rate).[
[
a] Department of Chemistry
University of Utah
3
15 South 1400 East
Salt Lake City, UT 84112, USA
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300408 or from the au-
thor.
40]
Z. Anorg. Allg. Chem. 2014, 640, (3-4), 649–654
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
649

P. A. Ignacio-de Leon, J. A. Cichelli, A. E. Abelow, I. Zharov
ARTICLE
In the present work, we explored silica colloidal films in
which the entire nanopore space is filled with a polymer carry-
ing multiple chiral selectors. As was demonstrated in our pre-
vious study,[ the maximum coverage of monolayer chiral
selector moieties on the surface of silica spheres was 6–8 mo-
41]
2
lecules/nm . While the grafting density of a polymer is un-
likely to exceed this number, the presence of side-chains con-
taining chiral selector moieties may significantly increase the
total number of the sites capable of chiral recognition on the
surface of the colloidal, which in turn may lead to increased
enantioselectivity. In addition, filling the colloidal nanopores
with a polymer would prevent through solution diffusion ac- Figure 2. TEM images of initiator-modified SiO
2
spheres (left) and
spheres with poly(1S) grown for 25 min. Scale bar = 100 nm.
SiO
2
ross the membrane, which may further improve the selectivity.
We have chosen atom transfer radical polymerization
(ATRP) to produce polymer brushes inside the nanopores since
this technique has been widely used to grow polymer brushes
[
45–49]
covalently attached to various surfaces.
We demonstrated
previously[ that the growth of polymer brushes inside the
48]
pores of the colloidal crystal does not affect its lattice.
Results and Discussion
We designed and synthesized the methacrylic monomer 1S
(Figure 1) containing a chiral selector moiety based on the chi-
ral selector that exhibited good selectivity in our previous stud- Scheme 1. Preparation of chiral polymer-modified silica spheres by
[
40,41]
ies.
In order to model the polymerization reactions that surface-initiated ATRP.
would take place inside the nanopores of a colloidal film we
modified 200 nm silica spheres with chlorobenzyl ATRP initia-
tor moieties (Figure 2) following the previously reported pro-
[
47]
cedure (Scheme 1).
The surface coverage for the initiator
2
was estimated as ca. 4 molecules/nm based on the weight loss
of 2% measured by thermogravimetric analysis (Figure 3). The
initiator-modified silica spheres were used in the surface-initi-
ated ATRP of the chiral monomer 1S.
Figure 3. TGA of unmodified (1), initiator-modified (2) and poly(1S)-
modified SiO spheres with polymer grown by surface-initiated ATRP
for 25 min (3).
2
The thickness of the polymer brushes was also measured as
a function of polymerization time using dynamic light scat-
tering (DLS). DLS measurements for silica spheres modified
with the poly(1S) grown for 5 min gave the polymer brush
thickness of 26 nm. A longer polymerization time (25 min)
resulted in polymer brush thickness of 48 nm. The difference
Figure 1. Chiral monomers used in ATRP modification of colloidal
films and chiral probe used for cyclic voltammetry measurements.
The IR spectra of the polymer-modified silica spheres between the polymer thickness in solution (DLS) and in the
–
1
showed the C-H stretch in 3000–2800 cm region, indicative dry state (TGA and TEM) suggests that the polymer chains are
of the polymer brushes on silica surface. The polymer brush swollen in the solvent. This is important since polymer brushes
was visible in the TEM images of the resulting particles (Fig- can only be used for separations when they swell in the sol-
ure 2), and its thickness after 25 min of polymerization could vents used. For nanoporous colloidal films formed from 200
be estimated, by measuring the low density corona around the nm silica spheres, the distance from the centre of the pore
silica particles, as ca. 10 nm. The TGA analysis (Figure 3) of projection to the nearest silica sphere surface is 16 nm. Thus,
the poly(1S)-modified particles showed 16% weight loss after polymer chains allowed to grow for more than five minutes
25 min of polymerization corresponding to the polymer brush are expected to reach the length sufficient to completely fill
thickness of ca. 12 nm.
the nanopores.
6
50 www.zaac.wiley-vch.de
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 649–654

Pore-Filled Nanoporous Silica Colloidal Films with Enantioselective Permeability
To test the enantioselective permeability of polymer-modi-
fied nanoporous colloidal films, we modified the surface of
silica colloidal films assembled on the surface of Pt electrodes
with initiator moieties and conducted surface-initiated polyme-
rizations of 1S and 1R. Based on the results obtained for sur-
face-initiated ATRP in colloidal solutions, the polymerization
time of twenty-five minutes was chosen for the pore-filling of
the colloidal films. We believe that the moderate initiator sur-
face coverage described above should lead to moderately dense
polymer brushes,[ which in turn should ensure that both the
interior and exterior brush regions are accessible to the
enantiomers permeating through the nanopores.
50]
We found the resulting pore-filled films to be mechanically
stable as was evidenced by our ability to use them repeatedly
in various solutions without cracking or peeling from the glass
support. Also, a significant effort was required to remove the
films from the glass surface, thus suggesting that the polymeri-
zation worked as “chemical sintering“ for the colloidal as-
semblies.
Cyclic voltammetry was used to investigate the enantio- Figure 4. Overlay of the voltammetric responses of a Pt microdisk
electrode carrying a colloidal film pore-filled with (A) poly(1S) and
selectivity of the resulting pore-filled films by measuring the
(
B) poly(1R).
limiting current of the redox-active chiral probe 2 (Figure 1)
for the electrodes carrying poly(1R)- or poly(1S)-filled colloi-
dal films (Figure 4A), in a way similar to that used to investi-
gate the enantioselectivity of colloidal films modified with
monolayers of chiral selectors.[
40,41]
The enantioselectivity of
the poly(1R)-filled colloidal films, defined as the ratio of the
limiting currents for the S and R enantiomers of chiral probe
2, was 2.2. This is similar to the enantioselectivity found for
colloidal films modified with a monolayer of chiral selectors
analogous to 1R.[
40]
We observed a reversal of enantio-
selectivity when the chirality of the selector moieties attached
to the polymer was changed from R to S (Figure 4B). A similar
behavior has been observed previously for colloidal films
[40]
modified with monolayers of chiral selector moieties, which
confirmed that similar phenomena are taking place once the
pores are filled with polymers carrying chiral selectors in their dal film surface with ATRP initiator moiety 1-(trichlorosilyl)-2-[m/p-
side-chains. The selectivities of 1R- and 1S-modified colloidal (chloromethyl)phenyl]ethane.
Figure 5. Overlay of the voltammetric responses of a Pt microdisk
electrode for 2S (black) and 2R (grey) after modification of the colloi-
films were somewhat different, a phenomenon that has been
reported for chiral stationary phases used in liquid chromatog-
raphy[ and in our studies of chiral silica colloidal films.[40]
We repeated the above enantioselectivity experiments with
chiral probe 2 through the colloidal films on its concentration
in the 0.1–18 mM range (Figure 6A). The molecular flux of
permeants increased linearly with the concentration of the chi-
51]
several electrodes, obtaining semi-quantitative reproducibility ral probe until it reached ca. 5 mM (Figure 6B). Further in-
in the absolute and relative changes between the different elec- crease in the probe concentration did not result in higher trans-
trodes. Additionally, control experiments were conducted with port rates, while the enantioselectivity decreased. We can ratio-
electrodes carrying initiator-modified colloidal films and in all nalize these observations in terms of the fixed-site theory of
cases we observed no permselectivity, with both enantiomers facilitated transport.[28,29,43,44] According to this theory, for the
of the chiral probe 2 diffusing through the colloidal film at transport across the membrane by hopping between the fixed-
the same rate, as illustrated in Figure 5. Thus, the observed site carriers high selectivity is to be expected at low permeant
enantioselective permeability can be attributed solely to the concentrations while at high concentrations saturation of the
presence of the polymer chains carrying chiral selector moie- receptors occurs and the transport rate flattens, which corre-
ties and filling the interstitial voids within the silica colloidal sponds to our observations. Our results also suggest that an
films and affecting the diffusion of the enantiomeric perme- optimal ratio of available chiral selectors to the concentration
ants.
of the diffusing probe may exist. This ratio could be found
In order to elucidate the mechanism of enantioselective with variation of the grafting densities or the brush lengths of
transport within the colloidal nanopores filled with poly(1S), the polymer modifiers to maintain low through solution dif-
we studied the dependence of the selectivity and flux of the fusion without hindering the transport across the membrane.
Z. Anorg. Allg. Chem. 2014, 649–654
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
651

P. A. Ignacio-de Leon, J. A. Cichelli, A. E. Abelow, I. Zharov
ARTICLE
length after 1 hour of polymerization.[57]. Thus, we prepared
poly(l-alanine)-filled colloidal films and studied their enantio-
selective permeability by cyclic voltammetry, as described
above. Representative voltammograms for both enantiomers of
the chiral probe 2 are shown in Figure 7. The poly(l-alanine)-
filled colloidal films displayed a modest enantioselective per-
meability of ca 1.5, which could be attributed to the inherent
chirality and hydrogen-bonding ability of the polymer and to
the fractional helicity it can adopt on surfaces.[
58]
Figure 6. Limiting current of 2S and 2R for poly(1S)-modified colloi-
dal films for (A) 0.1–18 mM and (B) 0.1–3 mM concentration of 2S
and 2R.
The structure of the chiral selector moiety may have a sig-
nificant effect on the enantioselective membrane permeability
due to the variations in the strength of the non-covalent inter-
actions between the selector and permeating enantiomers.[
To test this notion for the pore-filled colloidal films, we pre-
Scheme 2. Preparation of poly(l-alanine) modified silica spheres.
42]
pared monomers 3R, 4S and 5S, chosen as analogues of chiral
selectors used in our earlier studies.[
40,41]
They formed poly-
mer brushes on silica surface with thickness (Table 1) similar
to that of poly(1R). Poly(3R)-filled colloidal films, most sim-
ilar in structure to poly(1R)-filled films, showed a similar
enantioselectivity, while poly(4S)- and poly(5S)-filled films
showed a lower enantioselectivity, likely due to their reduced
ability to interact via hydrogen bonding, van der Waals interac-
tions and π–π stacking. Overall, these permselectivity values
are comparable to those observed in monolayer-modified col-
loidal films,[
40,41]
antibody-modified nanotubes membranes,
[52]
and compare favorably to the results obtained for optically
active polyelectrolyte membranes.
Figure 7. Overlay of the voltammetric responses of a Pt microdisk
electrode for 2S and 2R after pore-filling of the colloidal film with
poly(l-alanine).
[53]
Table 1. Thickness of polymers formed by ATRP on the surface of
silica spheres measured by DLS, and enantioselectivity (ilim(S)/ilim(R)
)
obtained by cyclic voltammetry for pore-filled nanoporous silica col-
loidal films.
Conclusions
monomer
polymer thickness, nm
enantioselectivity
We prepared nanoporous silica colloidal films pore-filed
with polymers containing chiral selector moieties in the side
chain, and with a chiral main chain. We used surface-initiated
polymerization to prepare the polymers, which thus were sur-
face-immobilized. Overall, pore-filled silica colloidal films
showed enantioselective permeability similar to that of the col-
1R
1S
3R
4S
5S
48 Ϯ 13
48 Ϯ 13
87 Ϯ 14
34 Ϯ 9
2.20 Ϯ 0.04
0.55 Ϯ 0.01
1.80 Ϯ 0.03
0.83 Ϯ 0.01
0.59 Ϯ 0.05
93 Ϯ 5
Finally, we prepared poly(l-alanine)-filled colloidal films. loidal films functionalized with a monolayer of chiral selec-
This chiral polypeptide has been prepared on the surface of tors. We did not observe significant differences in chiral selec-
porous silica and used as a stationary phase for chiral HPLC tivity for different chiral polymers. Thus, we conclude that the
separations.[
54,55]
The grafting of poly(l-alanine) on the sur- enantioselective permeability in pore-filed films depends so-
face of the silica colloidal films has been achieved by surface- lely on the energy difference between the enantiomer/selector
initiated polycondensation of l-alanine N-carboxyanhydride complexes, which similar in monolayer- and polymer-modified
from the aminated silica surface,[ as shown in Scheme 2. colloidal films. While the capacity for chiral recognition within
Our previous studies of this polymerization reaction on silica the polymer-filled colloidal films might have increased, the
spheres revealed that poly(l-alanine) brushes reach ca. 35 nm relative permeation rates for enantiomers remain the same.
56]
6
52
www.zaac.wiley-vch.de
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 649–654

Pore-Filled Nanoporous Silica Colloidal Films with Enantioselective Permeability
Moreover, although the interstitial space of the colloidal nan- diameter Cu wire (Alfa Aesar) to a 25μm-diameter Pt wire using Ag
paint (DuPont). The Pt wire was then flame sealed in a glass capillary;
opores is filled with the polymer, dramatically reducing the
the capillary was bent into a U-shape, and the middle was cut orthogo-
possibility of through solution diffusion compared to the
nal to the length of the capillary with a diamond saw to expose the Pt
monolayer-modified colloidal films, the enantioselective per-
disk. The resulting electrodes were polished with Microcut Paper disks
meability does not increase. This suggests that through solu-
(
Buehler), from 240 to 1200 grit in succession, until the surface was
tion diffusion plays a minor role in the transport of enantio-
mers, which occurs predominantly through surface hopping.
Our observations are valuable for the future design of mem-
free from visible defects. The colloidal films were deposited onto the
Pt and the glass shroud by placing the electrodes vertically into 1.5
wt.-% colloidal solution of silica spheres in ethanol. The vials were
branes for chiral separations, and for the optimization of enan- placed under a crystallization dish and the solutions were allowed to
tioselective separations.
evaporate for 3 days in a vibration-free environment.
In present work, we used silica colloidal films deposited on
glass-shrouded platinum microelectrodes in order to investi-
gate the transport of enantiomers through polymer-modified
colloidal nanopores. For the actual chiral separations, such
films can be prepared on a porous support. Alternatively, free
General procedure for surface-initiated ATRP on silica spheres.
The preparation of 200 nm silica sphere using a Stöber method has
[40,41]
been described elsewhere.
A monolayer of ATRP initiators was
introduced on the surface of these silica spheres by treating a suspen-
sion of the particles in toluene with 1.5 equivalents of (1-(tri-
standing silica colloidal membranes[59] can be modified with chlorosilyl)-2-m/p-(chloromethyl)phenyl]ethane). Following 18 h of
chiral selector moieties for chiral separations of enantiomers.
stirring at 70 °C, the initiator-modified spheres were isolated via cen-
The enantioselectivity of ca. 2 obtained for pore-filled silica trifugation, washed by 4 cycles of centrifugation and resuspension in
toluene, methanol, dichloromethane and acetone in order to remove
colloidal films is moderately high but would require their use
any adsorbed initiator, then dried in vacuo. The polymerization solu-
tions were prepared as follows: CuCl (0.50 mg, 4.7 μmol) and
Me TREN (3.2 mg, 14.0 μmol) were dissolved in anhydrous deoxy-
6
in a series of membranes in chiral separations to achieve high
enantiomeric purities. We are presently working on increasing
this selectivity by modifying the structure of chiral selector
moieties present in the polymer side-chains, and on the prepa-
ration of free-standing chiral colloidal membranes.
genated DMF (1.0 mL) and the mixture was sonicated for 3–4 min to
facilitate the formation of the organometallic catalyst. In a separate
vial, the chiral monomer (470 μmol) was added to a suspension of
initiator-modified silica spheres (50 mg) in DMF (4.0 mL). The con-
tents of both vials were degassed via four freeze/pump/thaw cycles
and subsequently combined and sonicated to obtain a uniform suspen-
sion. The reaction mixture was then stirred at 70 °C and aliquots were
taken after 5, 10, 15, 20 and 25 min. Polymer functionalized-silica
nanoparticles were precipitated in methanol, washed with large
amounts of methanol, distilled water and acetone then dried in vacuo.
The resulting particles were then characterized using SEM, TEM and
DLS in addition to TGA and IR spectroscopy.
Experimental Section
Materials and Characterization. All reagents were obtained from
[
60]
commercial sources except for Me
6
TREN.
The preparation of com-
pounds 1–5 and of l-alanine N-carboxy anhydride is described in Sup-
porting Information. Solvents were dried following established pro-
2
cedures. All reactions were performed under the N atmosphere.
18 MΩ⋅cm water was obtained from a Barnstead “E-pure” water puri-
fication system. Transmission electron microscopy (FEI Philips Tecnai General procedure for surface-initiated ATRP in colloidal films.
T-12) and dynamic light scattering (PSS NICOMP 380/ZLS) were used The preparation of colloidal films via vertical deposition of 200 nm
for size characterization of the silica spheres. A Branson 1510 sonic-
ator was used for all sonications. UV/Vis measurements were per-
formed using an Ocean Optics USB2000 or USB4000 instrument.
Thermogravimetric data were collected using a TA Instruments TGA
silica sphere on the surface of platinum microdisk electrodes has been
[
40,41]
described elsewhere.
A monolayer of ATRP initiators was intro-
duced onto the surface of the spheres forming the colloidal film by
immersing the electrodes in a 60 mM solution of 1-(trichlorosilyl)-2-
m/p-(chloromethyl)phenyl]ethane in dry toluene. After 18 h at 70 °C
the initiator-modified electrodes were soaked in dry toluene and aceto-
nitrile to remove any adsorbed initiator and dried. The ATRP solution
was prepared in a three-neck round-bottom flask by dissolving CuCl
2950 Thermogravimetric Analyzer.
Preparation and modification of 200 nm silica spheres in solution.
Silica nanoparticles were prepared following the literature pro-
cedure.[ A solution of tetraethoxysilane (TEOS) in absolute ethanol
was rapidly poured into a stirred mixture of ammonia and water in
absolute ethanol at room temperature. The final concentrations of the
reagents were 0.2 m TEOS, 0.4 m ammonia and 17 m water. The reac-
tion mixture was stirred for 18 hours. The silica spheres were isolated
by repeated centrifugation and resuspension in absolute ethanol. The
diameter of the spheres was found to be 205Ϯ6 nm using dynamic
light scattering (DLS). Amine fictionalization of silica spheres was
achieved by treatment with a solution of 0.056 m 3-aminopropyltriet-
hoxysilane in dry acetonitrile at room temperature for 17 hours fol-
61]
(
6
0.50 mg, 4.7 μmol), Me TREN (3.2 mg, 14.0 μmol) and the chiral
monomer (470 μmol) in dry degassed DMF (5.0 mL). Following fur-
ther degassing by four freeze-pump-thaw cycles, initiator-modified
colloidal electrodes were immersed in the solution and the reaction
mixture was gently stirred at 70 °C for 25 min, after which they were
thoroughly rinsed with DMF, acetonitrile and dichloromethane.
Surface-initiated polymerization of L-alanine in colloidal films. Sil-
ica colloidal films deposited on the surface of Pt microelectrodes were
modified with primary amines by immersing them in a 56 mM solution
lowed by centrifugation and resuspension in acetonitrile. The presence of 3-aminopropyltriethoxysilane in acetonitrile for 17 h under gentle
of amino groups was confirmed by treating the silica spheres with
stirring. Following extensive rinsing and drying the amine-modified
dansyl chloride, followed by fluorescence measurements, as described electrodes were immersed in a 4.7 μM solution of l-alanine N-car-
previously.[
62]
boxyanhydride in DMF (NCA:amine molar ratio of 1:10) under the
nitrogen atmosphere. The reaction mixture was gently stirred at room
temperature for 3 h after which the electrodes were rinsed with distilled
water, soaked in dichloromethane and dried.
Colloidal Film electrodes. Pt microdisk electrodes (25 μm in dia-
meter) shrouded in glass were prepared by first attaching a 1.0-mm-
Z. Anorg. Allg. Chem. 2014, 649–654
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
653

P. A. Ignacio-de Leon, J. A. Cichelli, A. E. Abelow, I. Zharov
ARTICLE
Cyclic Voltammetry Measurements. The flux of permeants across [25] A. Higuchi, M. Tamai, Y. A. Ko, Y. Tagawa, Y. H. Wu, B. D.
Freeman, J. T. Bing, Y. Chang, Q. D. Ling, Polym. Rev. 2010, 50,
13–143.
26] M. Yoshikawa, J. Izumi, T. Kitao, S. Koya, S. Sakamoto, J.
Membr. Sci. 1995, 108, 171–175.
27] H. H. Rmaile, J. B. Schlenoff, J. Am. Chem. Soc. 2003, 125,
colloidal films was measured using a 2-electrode cell and an Ag/AgCl
reference/counter electrode. A Par Model 175 Universal Programmer
and Dagan Cornerstone Chem-Clamp potentiostat were used to con-
1
[
[
[
duct the measurements. All solutions were purged with N
dissolved O . The voltammetric responses of the bare, colloidal film-
modified, and pore-filled colloidal film-modified Pt electrodes were
2
to remove
2
6602–6603.
28] S. B. Lee, D. T. Mitchell, L. Trofin, T. K. Nevanen, H. Söderlund,
–3
measured in 1.0 ϫ 10 m dichloromethane solutions of the (R) and
C. R. Martin, Science 2002, 296, 2198–2201.
(S) enantiomers of compound 2 using 0.1 m tetrabutylammonium hexa- [29] B. B. Lakshmi, C. R. Martin, Nature 1997, 388, 758–760.
fluorophosphate (TBAPF) as supporting electrolyte. Measurements [30] D. L. Gin, R. D. Noble, Science 2011, 332, 674–676.
[
[
[
[
31] S. Wong, V. Kitaev, G. A. Ozin, J. Am. Chem. Soc. 2003, 125,
5589–15598.
32] M. R. Newton, A. K. Bohaty, H. S. White, I. Zharov, J. Am.
Chem. Soc. 2005, 127, 7268–7269.
were conducted with several electrodes to test the reproducibility in
the absolute and relative changes in limiting current.
1
Supporting Information (see footnote on the first page of this article):
Synthesis and characterization of compounds 1–5.
33] B. J. Ravoo, D. N. Reinhoudt, S. Onclin, Angew. Chem. Int. Ed.
2
005, 44, 6282–6304.
34] M. R. Newton, A. K. Bohaty, H. S. White, I. Zharov, Langmuir
006, 22, 4429–4432.
2
Acknowledgments
[
[
[
35] J. J. Smith, I. Zharov, Langmuir 2008, 24, 2650–2654.
36] A. K. Bohaty, I. Zharov, Langmuir 2006, 22, 5533–5536.
37] A. K. Bohaty, A. E. Abelow, I. Zharov, J. Porous Mater. 2011,
18, 297–304.
This work was supported by the National Science Foundation grants
CHE-1213628 and DMR-1008251.
[
[
38] A. K. Bohaty, J. J. Smith, I. Zharov, Langmuir 2009, 25, 3096–
3
101.
39] P. A. Ignacio-de Leon, I. Zharov, Chem. Commun. 2011, 47, 553–
55.
References
5
[
1] http://www.fda.gov/cder/guidance/stereo.htm (accessed 08–12–
013).
2] B. L. Feringa, J. Wang, Science 2011, 331, 1429–1432.
[40] J. A. Cichelli, I. Zharov, J. Am. Chem. Soc. 2006, 128, 8130–
2
8131.
[
[
[41] J. A. Cichelli, I. Zharov, J. Mater. Chem. 2007, 17, 1870–1875.
3] Chiral Separation Techniques. A Practical Approach (Ed.: G. Su- [42] W. H. Pirkle, D. W. House, J. M. Finn, J. Chromatogr. 1980, 192,
bramanian), Wiley-VCH, New York, 2001. 143–158.
4] C. A. M. Afonso, J. G. Crespo, Angew. Chem. Int. Ed. 2004, 43, [43] E. L. Cussler, A. Rutherford, A. J. Brown, J. Membr. Sci. 1989,
[
5293–5295.
43, 149–164.
[
[
[
5] J. E. Rekoske, AIChE J. 2001, 47, 2–5.
[44] R. D. Noble, J. Membr. Sci. 1992, 75, 121–129.
[45] Polymer Brushes: Synthesis Characterization, Applications (Ed.:
R. A. Advincula), Wiley-VCH Verlag, New York, 2004.
[46] J. Wang, K. Matyjaszewski, Macromolecules 1995, 28, 7901–
7910.
6] K. B. Jirage, C. R. Martin, TIBTECH 1999, 17, 197–200.
7] M. Yoshikawa, M. Maruhashi, Y. Iwamoto, N. Ogata, Macromol.
Symp. 2007, 249–250, 557–561.
[
8] R. Xie, L.-Y. Chu, J.-G. Deng, Chem. Soc. Rev. 2008, 37, 1243–
1
263.
[47] J. Pyun, T. Kowalewski, K. Matyjaszewski, Macromol. Rapid
Commun. 2003, 24, 1043–1059.
[
9] W. H. Pirkle, T. C. Pochapsky, Chem. Rev. 1989, 89, 347–362.
[
10] Chiral Separations (Ed.: B. Chankvetadze), Elsevier, Amsterdam,
[48] O. Schepelina, I. Zharov, Langmuir 2006, 22, 10523–10527.
[49] O. Schepelina, I. Zharov, Langmuir 2007, 23, 12704–12709.
2001.
[11] H. Y. Aboul-Enein, I. Ali, Chiral Separations by Liquid [50] W. J. Brittain, S. Minko, J. Polym. Sci. A Polym. Chem. 2007, 45,
Chromatography and Related Techniques. Marcel Dekker, New
York, NY, 2003.
12] O. A. Hazzazi, G. A. Attard, P. B. Wells, J. Mol. Catal. A 2004, [52] S. B. Lee, D. T. Mitchell, L. Trofin, T. K. Nevanen, H. Söderlund,
3505–3512.
[51] W. H. Pirkle, T. C. Pochapsky, Chem. Rev. 1989, 89, 347–362.
[
2
16, 247–255.
C. R. Martin, Science 2002, 296, 2198–2200.
[53] H. H. Rmaile, J. B. Schlenoff, J. Am. Chem. Soc. 2003, 125,
6602–6603.
[
[
13] M. Yoshikawa, K. Yonetani, Desalination 2002, 149, 287–292.
14] P. Hadik, L.-P. Szabo, Z. Farkas, J. Membr. Sci. 2005, 251, 223–
2
32.
[54] A. Shundo, T. Sakurai, M. Takafuji, S. Nagoaka, H. Ihara, J.
Chromatogr. A 2005, 1073, 169–174.
[55] H. Ihara, N. Nakanishi, T. Sagawa, C. Hirayama, T. Sakurai, T.
Kinoshita, Y. Tsujita, Chem. Lett. 1998, 27, 963–964.
[
[
15] N. H. Lee, C. W. Frank, Polymer 2002, 43, 6255–6262.
16] T. Aoki, M. Teraguchi, S. Kim, T. Kaneko, S. Hadano, Macromol-
ecules 2005, 38, 6367–6373.
[
[
[
[
[
[
[
[
17] J. Randon, F. Garnier, J. L. Rocca, B. Maisterrena, J. Membr. Sci. [56] W. N. E. van Dijk-Wolthuis, L. Water, P. Wetering, M. J.
2
000, 175, 111–117.
Steenbergen, J. J. K. Bosch, W. J. W. Schuyl, W. E. Henninck,
Macromol. Chem. Phys. 1997, 198, 3893–3906.
[57] A. E. Abelow, I. Zharov, Soft Matter 2009, 5, 457–462.
[58] G. E. Job, R. J. Kennedy, B. Heitmann, J. S. Miller, S. M. Walker,
D. S. Kemp, J. Am. Chem. Soc. 2006, 128, 8227–8233.
[59] A. K. Bohaty, I. Zharov, Langmuir 2009, 25, 3096–3101.
[60] G. J. P. Britovsek, J. England, A. J. P. White, Inorg. Chem. 2005,
44, 8125–8134.
18] S. Kiyohara, M. Nakamura, K. Saito, K. Sugita, T. Sugo, J.
Membr. Sci. 1999, 152, 143–149.
19] T. Aoki, S. Tomizawa, E. Oikawa, J. Membr. Sci. 1995, 99, 117–
1
25.
20] C. Minguillon, T. Gumi, C. Palet, Polymer 2005, 46, 12306–
2312.
21] M. Yoshikawa, T. Ooi, J. Izumi, Eur. Polym. Sci. J. 2001, 37,
35–342.
1
3
[61] W. Stober, A. Fink, E. J. Bohn, Colloid Interf. Sci. 1968, 26, 62–
22] T. Aoki, M. Ohshima, K. I. Shinohara, T. Kaneko, E. Oikawa,
Polymer 1997, 38, 235–238.
23] A. Maruyama, N. Adachi, T. Takatsuki, M. Torii, K. Sanui, N.
Ogata, Macromolecules 1990, 23, 2748–2752.
69.
[62] M. R. Newton, A. K. Bohaty, H. S. White, I. Zharov, J. Am.
Chem. Soc. 2005, 127, 7268–7269.
24] E. Yashima, J. Noguchi, Y. Okamoto, J. Appl. Polym. Sci. 1994,
Received: August 15, 2013
54, 1087–1091.
Published Online: November 28, 2013
6
54
www.zaac.wiley-vch.de
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 649–654