Chiral permselectivity in nanoporous opal films surface-modified with chiral selector moieties

Article abstract of DOI:10.1039/b617607k

The chiral permselectivity in thin opal films modified on the silica surface with chiral selector moieties was studied as a function of opal film geometry, supporting electrolyte concentration, solvent polarity, and chiral selector and linker structure. While opal film thickness, supporting electrolyte concentration and linker length and structure did not have a significant influence on the chiral permselectivity, the nanopore size, solvent polarity and selector structure had a pronounced effect. These observations are in agreement with the chiral selectivity mechanism in the opal films in which the permeating enantiomers are transported with different rates through the surface utilizing non-covalent interactions between the chiral permeant molecules and surface-bound chiral selectors. The chiral selectivity (transport rate ratio for S and R enantiomers) achieved in the present study was 4.5, which is one of the highest reported for chiral membranes. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b617607k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Chiral permselectivity in nanoporous opal films surface-modified with chiral
selector moieties{
Julie Cichelli and Ilya Zharov*
Received 4th December 2006, Accepted 5th March 2007
First published as an Advance Article on the web 21st March 2007
DOI: 10.1039/b617607k
The chiral permselectivity in thin opal films modified on the silica surface with chiral selector
moieties was studied as a function of opal film geometry, supporting electrolyte concentration,
solvent polarity, and chiral selector and linker structure. While opal film thickness, supporting
electrolyte concentration and linker length and structure did not have a significant influence on
the chiral permselectivity, the nanopore size, solvent polarity and selector structure had a
pronounced effect. These observations are in agreement with the chiral selectivity mechanism in
the opal films in which the permeating enantiomers are transported with different rates through
the surface utilizing non-covalent interactions between the chiral permeant molecules and surface-
bound chiral selectors. The chiral selectivity (transport rate ratio for S and R enantiomers)
achieved in the present study was 4.5, which is one of the highest reported for chiral membranes.
that nanotube membranes surface-modified with antibodies
allow enantioselective separations.24,25
Introduction
The past decade has seen increasing interest in the preparation
To be practical, chiral membranes require a high surface
of enantiomerically pure compounds. This interest was
area, low mass transfer resistance, good mechanical strength
intensified by an FDA statement concerning the development
and selectivity. Nanoporous opal membranes may provide an
of chiral drugs as single enantiomers.1 Separation of racemic
ideal medium for chiral separations. Indeed, synthetic opals
mixtures (e.g., by crystallization) is presently the most widely
form via self-assembly of silica nanospheres into a close-
used method of producing single enantiomers in industry. A
packed face-centered cubic (fcc) lattice with a void fraction of
0.26 independent of the size of the spheres.26 The opals contain
variety of other chiral separation techniques exists including
chromatography and electrophoresis, but they only allow a
highly ordered arrays of three-dimensional interconnected
small amount of material to be separated per run, their scale-
nanopores whose size can be controlled by changing the size of
up is expensive and such methods are not energy- and cost-
the silica spheres used to assemble the opal, and whose surface
efficient. Membrane separations, on the other hand, constitute
can be easily modified27 to introduce permselectivity.28,29
a promising method that can be operated under ambient
Importantly, the molecular flux normal to the (111) plane of
conditions.2–4
an opal layer remains significant even when the pore size is
sufficiently small to impart chemical selectivity.30
they are based on the difference in non-covalent binding
Recently, we reported31 on the chiral permselectivity of opal
The common feature of chiral membrane separations is that
energy between each enantiomer and a selector molecule,5–7 or
films surface-modified with
a chiral selector moiety 1
on the preferential inclusion into a cavity within a polymer
or
porous material.8 Presently used enantioselective
(Chart 1).32 We studied the transport of chiral molecules
a
through the chiral opal films using cyclic voltammetry of
membranes9–12 usually consist of a non-selective porous
material either supporting a liquid phase containing chiral
selector molecules,11 or carrying selector moieties on its
surface. In the latter type of enantioselective membranes,
which are much more robust compared to the liquid
membranes, selector moieties can be introduced by coating
the pores with a thin layer of a chiral polymer,13 by covalently
grafting the selector to the membrane,14–18 by preparing a
porous polymeric film that incorporates chiral selectors,19–21
or by molecularly imprinting a polymer.10,18,22 Recently,
optically active polyelectrolyte multilayer membranes have
been shown to separate enantiomers.23 It has also been shown
Department of Chemistry, University of Utah, Salt Lake City, UT,
USA. E-mail: i.zharov@utah.edu; Fax: +1 801 581 84333;
Tel: +1 801 587 9335
{ This paper is part of a Journal of Materials Chemistry issue
highlighting the work of emerging investigators in materials chemistry.
Chart 1 The structure of the chiral selector 1, the chiral probe
molecule 2 and the chiral permselectivity obtained for the chiral
selector 1 attached to the silica surface as shown.
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redox-active chiral probe molecules such as 2 (Chart 1) and its
analogues33 and found a selectivity (molecular flux ratio for S
and R enantiomer, calculated as the ratio of the limiting
currents for S and R enantiomer) of ca. 2.2 (Chart 1) that was
reversed when the configuration of the selector moiety was
changed from R to S. In addition, we studied the dependence
of the flux and selectivity on the concentration of the
permeants and found that at low concentrations the molecular
flux increases linearly with the concentration and becomes
saturated at higher concentrations. Thus, we concluded that
permselectivity in the chiral opal films is governed by surface
transport in a way similar to that demonstrated in polymer
membranes23,34 and surface-modified nanotubes.24
Preparation of the opal films
Silica spheres of 200 ¡ 5 and 440 ¡ 11 nm in diameter were
prepared by rapidly adding a solution of tetraethoxysilane
(TEOS) in absolute ethanol to a stirred solution of ammonia
and water in absolute ethanol at room temperature. The
sphere size is determined by the final concentration of the
components, for 200 nm spheres it was 0.2 M TEOS, 0.4 M
ammonia and 17 M water, and for 440 nm spheres it was 0.2 M
TEOS, 1.1 M ammonia and 17 M water. The size and size
uniformity of the spheres were verified by SEM.
The opal films were deposited onto the glass-shrouded Pt
electrodes by placing them into 3 DR Fisher vials filled with
1.5 or 3.0 wt% colloidal solutions of the spheres in EtOH. The
vials were placed under a crystallization dish and the solutions
were allowed to evaporate at room temperature for 2–3 days in
a vibration-free environment.
In order to gain further insight into the molecular transport
mechanism in chiral nanoporous opal films, and to improve
their selectivity, we carried out a systematic study of the
permselectivity as a function of film geometry, solvent,
supporting electrolyte concentration, the structure of the
linker and of the chiral selector moiety using the chiral probe
molecule 2. The results of this study are described below.
Amine modification of the opal membranes was achieved by
immersing the electrodes with pre-assembled opals in an
unstirred 0.056 M solution of 3-aminopropyltriethoxysilane,
4-aminobutyltriethoxysilane (3), 2-(2-amino-ethylamino)-
ethyl-triethoxysilane (4) or 2-{3-[(2-amino-ethylamino)-
methyl]-phenyl}-ethyltrimethoxysilane (5) in dry acetonitrile
at room temperature for 17 hours (stirring the solution led to
damaging the membranes) followed by soaking the electrodes
in dry acetonitrile for 1 hour. The presence of the surface
amino groups was confirmed by treating the silica spheres with
dansyl chloride, followed by fluorescence measurements.
Experimental
Materials
The following reagents were obtained from Sigma-Aldrich,
Inc. and used without further purification: tetraethoxysilane
(TEOS), 3-aminopropyltriethoxysilane, phenylglycine, 3,5-
dinitrobenzoyl
chloride,
R-(+)-a-methylbenzylamine
hydrochloride, ferrocene aldehyde, R-phenylglycine and
benzoylchloride. N-a-Methyl-D-valine hydrochloride was
received from EMD Biosciences, Inc., and 4-aminobutyl-
triethoxysilane, N-(2-aminoethyl)-3-aminopropyl-triethoxysi-
lane and aminoethyl-(aminomethyl)-phenyl-trimethoxysilane
were obtained from Gelest, Inc. Reagent grade solvents were
used except for absolute ethanol, anhydrous MeOH, acetoni-
trile, dichloromethane and dimethylsulfoxide (DMSO) dis-
tilled from CaH2, and THF distilled from Na/benzophenone.
(3,5-Dinitro-benzoylamino)-phenylacetic acid (1)
Both R and S enantiomers of compound 1 were prepared
following the literature procedure.32 A mixture of phenylgly-
cine (23 mmol, 1.1 equivalents) and 3,5-dinitrobenzoyl
chloride (22 mmol, 1.0 equivalent) in 50 mL of dry
tetrahydrofuran (THF) was stirred for 1 week at room
temperature. The R and S enantiomers of phenylglycine were
utilized individually to prepare the respective enantiomerically
pure compounds. Following 1 week of stirring, the solvent was
evaporated from the creamish cloudy mixture, and the residue
was dissolved in 50 mL of 5% aqueous NaHCO3. The aqueous
solution was extracted with diethyl ether (2 6 100 mL),
acidified to pH = 5.0, and washed with ether until no more
material could be extracted into ether. The organic extracts
were combined and concentrated on the rotary evaporator to
yield a cream-colored solid. Crystallization in a minimal
amount of MeOH afforded the enantiomerically pure com-
pound 1 in 92% yield as a cream-colored solid. NMR and IR
spectra of 1 were identical to those reported in the literature.
Characterization
NMR spectra were recorded using a Varian VXL-300 or a
Varian Unity-300 at 300 MHz for 1H and 75 MHz for 13C.
Mass spectra were obtained using a Finnigan MAT95.
Dynamic light scattering (Brookhaven ZetaPALS) and scan-
ning electron microscopy (Hitachi S3000-N) were used to
perform size characterization of the silica spheres. UV spectra
were recorded using an Ocean Optics USB2000 instrument.
Pt microdisk electrodes
Pt microdisk electrodes (25 mm in diameter) shrouded in glass
were prepared by first attaching a 1.0 mm diameter Cu wire
(Alfa Aesar) to a 25 mm diameter Pt wire using Ag paint
(DuPont). The Pt wire was then flame-sealed in a glass
capillary; the capillary was bent to a U-shape and the end cut
orthogonal to the length of the capillary with a diamond saw
to expose the Pt disk. The resulting electrodes were then
polished with microcut paper disks (Buehler), from 240 to
1200 grit in succession, until the surface was free from visible
defects.
Ferrocen-2-ylmethyl-(1-phenyl-ethyl)-amine (2)
A published procedure was used for the preparation of
compound 2.33 Methylbenzylamine hydrochloride (R and S
enantiomers of the amino acid hydrochloride were utilized
individually to prepare the respective enantiomerically pure
compounds), triethylamine, and ferrocene aldehyde (1 equiva-
lent each, 2 mmol) were dissolved in dry CHCl3. Following 3 h of
reflux the reaction mixture was allowed to cool to room
temperature and the solvent was evaporated in vacuo to
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quantitatively yield the imine as a yellow solid. Without further
purification, the imine was dissolved in 20 mL dry of CH3OH
and this solution was cooled to 0 uC with stirring. NaBH4
(4 equivalents, 8 mmol) was carefully added to the reaction
mixture in small portions with stirring at 0 uC. Following the
addition of NaBH4, the reaction mixture was allowed to stir at
0 uC. After 30 min, 20 mL of 1 M aqueous NaOH was added to
the reaction mixture and the organic phase was extracted with
CHCl3 (3 6 100 mL). The combined organic phases were eva-
porated to dryness to provide the pure ferrocene methyl amino
acid 2 as a yellowish-brown oil in 90% yield. The synthesized
compounds required no further purification. NMR and IR
spectra of 2 were identical to those reported in the literature.
temperature for 24 h. The resulting chiral opal electrodes were
then soaked in CH2Cl2 for 1 h.
Surface coverage of chiral selector
To determine the surface coverage of the amine modified opal
film with compounds 1, 6, 7 and 8 they were dissolved in
9.9 mL CHCl3 and 0.1 mL DMSO to afford a stock 1 mM
solution. Their extinction coefficients at 300 nm were deter-
mined by UV measurements at concentrations ranging from
0.1 to 0.5 mM. Next, UV spectra were obtained for colloidal
solution of silica spheres modified with these compounds.
Based on the absorbance at 300 nm, the surface coverage was
estimated using 2.2 g cm23 density for silica spheres. Surface
coverage was calculated to be 6–8 molecules nm22
.
Benzoylamino-phenylacetic acid (7)
CV measurements
A mixture of R-phenylglycine (5.3 mmol, 1.1 equivalents) and
benzoylchloride (5.0 mmol, 1.0 equivalent) in 25 mL of dry
THF was stirred for 1 week at room temperature. The solvent
was then evaporated from the creamish cloudy mixture, and
the residue was dissolved in 20 mL of 1 M aqueous NaOH.
The aqueous solution was extracted with ether (2 6 100 mL)
and ethyl acetate (2 6 100 mL). The organic extracts were
combined and concentrated in vacuo to yield a cream-colored
solid. Crystallization in a minimal amount of methanol
afforded the enantiomerically pure compound 7 in a 83%
yield as a cream-colored solid. 1H NMR (300 MHz, CDCl3): d
8.06 (d, 2H), 7.96 (d, 2H), 7.64–7.35 (m, 8H), 5.77 (s, 1H). 13C
NMR (75 MHz, CDCl3): d 176.9, 172.6, 134.1, 132.2, 130.4,
129.5, 129.3, 129.1, 128.9, 128.7, 127.6, 127.4, 57.2.
The flux of permeants across the opal film was measured
voltammetrically using a 2-electrode cell and an Ag/AgCl
reference/counter electrode. A Par Model 175 Universal
Programmer and Dagan Cornerstone Chem-Clamp potentio-
stat were used to conduct the measurements. Data were
recorded with a PC using programs written in LabView 7.0.
All solutions were purged with N2 to remove dissolved O2. The
voltammetric response of the bare, opal-modified, and chiral-
modified Pt electrodes was measured in 1 6 1023 M solutions
for R and S enantiomers of compound 2 and ferrocene,
utilizing 0.1 M tetrabutylammonium phosphohexafluoride
(TBAPF) as supporting electrolyte. We found this supporting
electrolyte concentration to be the most suitable for the
measurements since at concentrations below 0.1 M cyclic
voltammograms became extremely noisy. The limiting current
ratio for the enantiomers did not seem to be affected by the
supporting electrolyte concentration. We repeated these
experiments with six different electrodes, obtaining semi-
quantitative reproducibility in the absolute and relative
changes in ilim following deposition of the opal and chemical
modification of the silica surface.
2-Benzoylamino-3-methylbutyric acid (8)
A mixture of N-a-methyl-D-valine hydrochloride (3.7 mmol,
1.1 equivalents) and benzoylchloride (3.5 mmol, 1.0 equiva-
lents) in 25 mL of dry methylene chloride was stirred for 5 min,
followed by the addition of N,N-diisopropylethylamine
(3.5 mmol, 1.0 equivalent). The reaction mixture was stirred
at room temperature for 24 h after which a pink solution was
recovered and the methylene chloride was evaporated. The
residue was dissolved in 15 mL of 1 M aqueous sodium
hydroxide and the aqueous solution was extracted with ether
(3 6 100 mL). The organic layer was evaporated to afford the
crude product as a brown oil. The crude product was purified
via column chromatography using a 50:50 mixture of ethyl
acetate:hexane as the elutent. Chromatography afforded the
enantiomerically pure compound 8 as yellow crystals in a 87%
Results and discussion
Opal film geometry
In order to determine if the film thickness and nanopore
size of the opal films affect the chiral permselectivity, we
assembled opal films using 440 nm diameter silica spheres to
roughly double the pore size compared to those in the opal
formed from 200 nm spheres, and utilized a 3.0 wt%
colloidal solution of 200 nm and 440 nm silica spheres
respectively (as opposed to the 1.5 wt% solution that has
been originally used) to double the thickness of the opal
film. Both types of films were modified with 1R. We obtained
a selectivity of ca. 2.1–2.3 for enantiomers of 2 using
the thicker opal films, and a selectivity of ca. 1.6 for
1
yield. H NMR (300 MHz, CDCl3): d 8.15 (dd, H), 7.96 (d,
2H), 7.70–7.53 (m, 3 H), 4.98 (s, H), 3.66 (m, H), 3.10 (m, H),
1.45 (d, 6H). 13C NMR (75 MHz, CDCl3): d 172.5, 162.6,
134.8, 130.8, 130.6, 162.6, 129.5, 129.1, 128.7, 61.7, 30.5, 29.9,
29.6. MS (CI): m/z 236 ([M + H]+).
Preparation of chiral opal films
enantiomers of
2
using the opal film with larger
Chiral modification of the amine modified opal membranes
was achieved by immersing the electrodes with pre-assembled
amine modified opals in an unstirred, equimolar solution
(0.065 M), of compound 1, 6, 7 or 8 and N-ethoxycarbonyl-2-
ethoxy-1,2-dihydroquinoline (EEDQ) in dry THF at room
nanopores. Based on these results, we conclude that the
film thickness does not significantly affect the permselectivity
of the chiral opals, while increasing the nanopore size
decreases the selectivity to a measurable extent, suggesting
that
a
through-solution diffusion of permeants starts
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playing a part in the overall transport through the opal films
containing larger nanopores.
Solvent polarity
According to the three-point theory of chiral separations,5
the selectivity results from non-covalent interactions, such as
hydrogen bonding and p–p stacking, between the permeating
enantiomers and the surface-bound chiral selector moieties.
Thus, solvents capable of disrupting these interactions
might affect the chiral selectivity in opal films. In order to
explore the solvent effect on the chiral permselectivity we
studied the transport of enantiomers of 2 through opal films
modified with 1R in four different solvents: dichloromethane,
acetonitrile (MeCN), THF and DMSO. The selectivities
obtained in these solvents were 2.16 ¡ 0.34, 2.23 ¡ 0.01,
1.47 ¡ 0.01 and 1.31 ¡ 0.02 respectively. No correlation
can be seen between the chiral permselectivity and such
common solvent polarity parameters as dielectric constant or
ET.35 However, it is clear that the selectivities in dichloro-
methane and MeCN are quite similar, while they are
significantly reduced in THF and DMSO to very similar
values. It appears that the hydrogen donor ability of the
solvent plays the most important role, as the donor numbers
for THF and DMSO,35 both oxygen-containing solvents, are
significantly higher than those of dichloromethane and MeCN.
Thus, it is expected that the highest enantiomeric perm-
selectivity in chiral opal films would be achieved in non-polar,
non-donor solvents.
Chart 2 Structures of the linker moieties used to attach the chiral
selector moiety 1 to the opal nanopore surfaces, and the corresponding
chiral permselectivities obtained for the chiral probe 2.
Chiral selector structure
Finally, we explored the influence of the chiral selector
structure on the permselectivity while maintaining the length
of the linker constant at three carbon atoms. The structures of
the chiral selectors are shown in Chart 3 together with the
corresponding selectivities obtained using the chiral probe 2,
and the corresponding voltammograms are shown in Fig. 1.
The surface coverage of the silica spheres modified with the
chiral selector moieties 6–8 in a colloidal solution under the
conditions identical to those used for the opal film modifica-
tion was determined by UV spectroscopy to be ca. 6–8 chiral
moieties per nm2. This value is ca. two times higher than the
typical number of hydroxyl groups on a silica surface,37 and
likely reflects the formation of a homo-condensed or an
anarchically grafted hybrid phase during the amine modifica-
tion of the silica spheres.28,29 As we reported earlier,31 upon
opal film modification the limiting current of ferrocene
decreases by ca. 75%. We attributed this drop in ilim to the
nanopore size reduction as a result of the surface modification
with multiple layers of grafted aminosilane and the chiral
selector moieties. Using the relationship between the limiting
current and opal void fraction,38,39 and straightforward
geometrical considerations,40 we estimated the nanopore size
inside the chiral opal film as 6.4 nm, compared to 16 nm for
the unmodified opal film. The extent to which the surface
Linker structure
Next, we varied the length and structure of the linker36
attaching the chiral selector moiety 1 to the silica surface
with the expectation that placing the chiral selector
moiety away from the surface might improve its ability to
interact with permeating molecules, and thus improve
the selectivity. The structures of the linkers are shown in
Chart
2
together with the corresponding selectivities
obtained using the chiral probe 2. In order to verify that
only the primary amine of the linker was coupled to the
chiral selector moiety in the case of compounds 4 and 5, we
treated them in solution with the chiral selector 1 under
the conditions used for the surface modification, and
determined by NMR that only one chiral selector moiety
was attached. Thus, the linker/selector assemblies 1, 3–5 differ
in the overall length of the chain in all cases, in one additional
secondary amino group in the case of 4, and in the case of 5 in
both an additional secondary amino group and a phenyl
group.
Introducing a slightly longer linker, i.e., 3, does not lead to a
significant increase in chiral selectivity compared to 1.
Furthermore, introducing much longer linkers carrying
additional functional groups capable of hydrogen bond
formation and p–p stacking interactions either does not
increase the chiral selectivity to a large extent (as in the
case of 5), or leads to a significant decrease in the selectivity
(i.e., in the case of 4). The origins of the different
selectivities for the seemingly similar linkers 4 and 5 are not
presently clear.
Chart 3 Structures of the chiral selectors and the corresponding
chiral permselectivities obtained for the chiral probe 2.
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concentration, solvent polarity, and chiral selector and linker
structure. While the opal film thickness, supporting electrolyte
concentration and linker length and structure (with one
exception) did not have a significant influence on the chiral
permselectivity, the nanopore size, solvent polarity and
selector structure had a pronounced effect. These observations
are in agreement with the chiral selectivity mechanism in the
opal films in which the permeating enantiomers are trans-
ported with different rates through the surface utilizing non-
covalent interactions between the chiral permeant molecules
and surface-bound chiral selectors. The chiral selectivity
(transport rate ratio for S and R enantiomers) achieved in
the present study was 4.5, which is one of the highest reported
for chiral membranes.
Acknowledgements
This work was supported by the ACS Petroleum Research
Fund. I. Z. is grateful to the Camille and Henry Dreyfus
Foundation for a New Faculty Award.
Fig. 1 Overlay of voltammetric responses of a Pt electrode for 2S
(black) and 2R (grey) after the opal film surface modification with 6R
(A), 7R (B) and 8R (C).
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