Facile phenylboronate modification of silica by a silaneboronate

Article abstract of DOI:10.1021/la3040837

Macroscopic and colloidal silica surfaces were readily modified with alkoxysilaneboronate, IV, yielding silica surfaces with covalently bonded phenylboronic acid groups. XPS and neutron activation confirmed the presence of boron. The ability of these surfaces to specifically interact with polyols was demonstrated with polyol-coated latex and ARS, a dye that specifically couples to boronic acid groups immobilized on colloidal or macroscopic silica. This is a new, direct approach for introduction of phenylboronic acid groups onto silica surfaces.

Full text of DOI:10.1021/la3040837

Article
pubs.acs.org/Langmuir
Facile Phenylboronate Modification of Silica by a Silaneboronate
Robert Pelton,*,† Yuguo Cui,† Dan Zhang,† Yang Chen,‡ Kate L. Thompson,§ Steven P. Armes,§
and Michael A. Brook‡
‡
†Department of Chemical Engineering and Department of Chemistry and Chemical Biology, McMaster University,
1280 Main Street West, Hamilton, Ontario, Canada, L8S 4L7
§Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, United Kingdom
S
* Supporting Information
ABSTRACT: Macroscopic and colloidal silica surfaces were readily modified with alk-
oxysilaneboronate, IV, yielding silica surfaces with covalently bonded phenylboronic acid groups.
XPS and neutron activation confirmed the presence of boron. The ability of these surfaces to
specifically interact with polyols was demonstrated with polyol-coated latex and ARS, a dye that
specifically couples to boronic acid groups immobilized on colloidal or macroscopic silica. This
is a new, direct approach for introduction of phenylboronic acid groups onto silica surfaces.
reaction with silanes.16 However, we found only two references
INTRODUCTION
■
to preparation of silanes bearing boronic acid groups. Glad and
Polymer- and surface-immobilized phenylboronic acid and related
co-workers prepared N-(2-hydroxy-3-[3-(tripropoxysilyl)pro-
boronates have been extensively reported in the scientific and
poxylpropyl))3-aminobenzene boronic acid propyl ester.17,18
patent literature, describing a wide range of phenomena and
They described the use of this silane in the preparation of
applications, including sensing,1−4 chromatographic separation,5,6
molecularly imprinted materials. More recently, Xu et al.
pH-dependent gelation,7 pH-dependent flocculation,8 and
described the use of essentially the same silane for preparing
lectin biomimetics.9 All applications are based upon the ability
silica monoliths bearing boronic acid groups.19
of organoboronates to condense with diols and polyols.10,11
Herein, we describe a one-step procedure for phenylboronate
The boronate condensation reaction is spontaneous in water
derivatization of silica surfaces with a new silaneboronate (see
under ambient conditions; however, the complexes are weak,
IV in Scheme 1). We demonstrate the approach by modi-
pH dependent, and sensitive to the diol/polyol stereochemistry.
fication of quartz crystal microbalance silica sensors and
Indeed, it is the labile, environmentally sensitive nature of these
colloidal silica. The resulting surfaces were smooth, uniform,
diol−boronate complexes that makes them interesting and useful.
Herein, we describe a new, one-step approach to grafting phenyl-
boronic acid groups onto silica
and rich in immobilized phenylboronic acid groups capable of
specific interactions with polyols and carbohydrates.
Boronic acid groups immobilized on divinylbenzene beads or
EXPERIMENTAL SECTION
Materials. Triethoxysilane, (+)-dimethyl L-tartrate, 4-vinyl phenyl-
silica particles are commercially available.6 In addition, boronate
■
vinyl monomers for copolymerization12 and bifunctional
organo boronates suitable for bioconjugation are also available.
In spite of this wide range of commercial boron-containing reagents,
we believe there is no convenient, one-step preparation of
boronic acid, platinum(0)−1,3-divinyl-1,1,3,3-tetramethyldisiloxane
(Karstedt’s catalyst, Pt2[(CH2CHMe2Si)2O]3), tetraethyl orthosili-
cate (TEOS), ammonium hydroxide (28−30%), and alizarin red S
(ARS) were all purchased from Sigma-Aldrich. Toluene, dichloromethane,
boronate-modified silica. In particular, we are interested in
hydrogen peroxide (30%), and molecular sieves type 4A were purchased
modification of small, fragile constructs, such as quartz crystal
microbalance sensor surfaces, surface plasmon sensor surfaces
and specific locations in microfluidic devices. In addition, we
are interested in boronate derivatization of colloidal silica by
processes that do not induce colloidal aggregation.
from Caledon Laboratories Inc. Methylene chloride-d2 was purchased
from Cambridge Isotope Laboratories Inc. Tris(hydroxymethyl)-
aminomethane and sodium chloride were purchased from Bioshop
Canada Inc. Sodium hydroxide (0.1 N) and hydrochloric acid (0.1 N)
were purchased from Labchem Inc.
Published approaches to silica−boronate preparation include
adsorption of a organoboronate polymer13,14 or hydrosilylation
of a vinyl boronate to silica.15 One of the easiest and most
widely applied approaches to silica surface modification is the
Received: October 14, 2012
Revised: December 21, 2012
Published: December 26, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Dimethyl Tartrate Protected Phenylboronate Silane, Subsequent Reaction with Silica, and Complexation
with Polyol-Coated Latex
̈
Poly(glycerol monomethacrylate)-stabilized polystyrene latex
(PGMA-PS) was supplied by S. Armes (their designation KLT 156).
Latex preparation and characterization was described previously.20
The hydrodynamic diameter of the particle was of 177 nm, and the
electrophoretic mobility was close to zero. The poly(glycerol mono-
methacrylate) stabilizer chains had a degree of polymerization of 50,
and the stabilizer coverage was 3.2 mg m−2.
Preparation of Dimethyl Tartrate-Protected Phenylboro-
nate Silane, IV. Our approach was to use hydrosilylation to couple
dimethyl tartrate-protected p-vinylphenyl boronic acid to triethoxy-
silane − see Scheme 1. Specifically, a mixture of 4-vinyl phenylboronic
acid (I) (0.5 g) and (+)-dimethyl L-tartrate (II) (0.6019 g) in 100 mL
of dichloromethane in the presence of 20 g of 4A molecular sieves was
stirred at room temperature overnight to give near quantitative yield
of III.
Preparation of Stober Silica.21 TEOS (2.4 mL) was added via a
syringe into a mixture of 80 mL of ammonia (28%) and 56 mL of
methanol under sonication using a Branson 3510R-DTH sonicator
(Branson Ultrasonics Corp.). After 30 min, the system was left for 5 h
under a nitrogen atmosphere at 30 °C. The resulting particles were
first washed with methanol three times and then washed once with
acetone. Each washing cycle included dispersing the particles in
solvents under sonication and centrifuging at 10 000 rpm using an
Allegra 25R centrifuge (Beckman Coulter) to separate the particles
from solvents. Particles were dried at 80 °C.
QCM-D Silica Sensor Silane Treatment. A 2.5% (w/v) silane IV
solution in dry toluene was used to prepare the phenylboronate silane-
coated silicone dioxide (SiO2) sensors. SiO2 sensors were put in a
Spin150-v3 spin coater, and 15 drops of IV were placed on top of the
sensors. The spin coater was spun at 2000 rpm and then placed in an
oven, allowing reaction for 2 h at 50 °C. Sensors were rinsed with dry
toluene once and then three times with dichloromethane . Finally, the
sensors were dried in an oven at 80 °C.
To a mixture of III and 0.6549 mL of triethoxysilane (1.05 equiv) in
100 mL of toluene was added 2 drops of Karstedt’s catalyst. The
reaction mixture was heated for 2 h at 50 °C. Finally, the toluene and
excess triethoxysilane were removed under vacuum, giving IV as a
colorless oil; 1H NMR of the product is shown in Figure 1. Analytical
data for the new compounds are given below.
̈
Stober Silica Silane Treatment. A 0.05 g amount of Stober silica
̈
(1% w/v) was dispersed (30 min sonication) in toluene to which was
added 0.25 g of IV (5% w/v) in toluene in a 25 mL one-neck flask.
The mixture was heated to 50 °C for 2 h with magnetic stirring. The
product was washed once with toluene followed by 3 washers with
dichloromethane. Each washing cycle involved dispersing the particles
in solvents with sonication and centrifuging at 10 000 rpm. Product
was dried at 80 °C.
Ellipsometry. The thickness of the dry films on the sensors was
measured using an Exacta 2000AP self-nulling ellipsometer, with a
laser wavelength of 632.8 nm, at an incident angle of 70° at room
temperature. An ellipsometric measurement gives two parameters,
polarizer and analyzer, using the Exacta 2000AP Start-up software.
1
Compound III. H NMR (CDCl3) δ: 3.87 (s, 6H), 5.11 (s, 2H),
5.34 (d, 1H, J = 11.0 Hz), 5.85 (d, 1H, J = 17.6 Hz), 6.75 (dd, 1H, J =
11 and 17.6 Hz), 7.45 (d, 2H, J = 8 Hz), 7.86 (d, 2H, J = 8 Hz). 13C
NMR (CDCl3) δ 53.1, 77.9, 115.6, 125.8, 135.5 (very weak), 135.6,
136.7, 141.2, 169.9. ESI-MS (high resolution): 291.1023. Calcd mass:
291.1040.
Compound IV. 1H NMR (CDCl3) δ: 1.13 (m, 2H), 1.24 (t, 9H, J =
7 Hz), 2.72 (m, 2H), 3.81 (m, 12H), 5.09 (s, 2H), 7.23−7.87 (m, 4H).
13C NMR (CDCl3) δ: 15.4, 18.2, 29.2, 53.1, 58.9, 77.9, 127.6, 135.5,
+
149.4, 170.0. ESI-MS: 472.4 (75%, M + NH4 ), 455.4 (20%, M + 1).
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1
Figure 1. H NMR of dimethyl tartrate-protected phenylboronate silane.
Film thickness and refractive index were calculated using a uniform
film model in the Exacta 2000AP Sit software.
integrated to give a measure of the boron content with a detection
limit for boron of 10 ppm.
Fluorescence Microscopy. Silane-treated SiO2 sensors were
immersed in 7.3 × 10−4 M ARS at pH 11 in 5 mM NaCl buffer. A
wet sensor was placed under an Olympus BX51 optical and fluo-
rescence microscope fitted with a Q-Imaging Retiga EXi digital camera:
ImagePro software was used for acquiring images and data.
Fluorescence Spectroscopy. Mixtures of silane-coated silica
particles and ARS were prepared in 0.1 M PBS buffer (pH 7.4) with a
final silica particles concentration of 0.05 g/L and ARS concentration
of 0.05−125 ppm. Fluorescence intensities were measured with a
Safire fluorescence spectrophotometer running XFLUOR4 software
version V4.30 to acquire data. The excitation wavelength was 468 nm,
and the emission wavelength was 517 nm. All measurements were
conducted at room temperature.
XPS. QCM-D sensors were placed in a Quantera II imaging XPS
Spectrometer (PHI Physical Electronics Inc.). Samples were irradiated
with a monochromatic Al Kα X-ray source at a takeoff angle of 45°, a
spot size of 100 μm, and a power of 25 W. For most measurements
three data were collected from three locations on the surface.
QCM-D. Deposition measurements were performed with a Q-Sense
quartz crystal microbalance (E4 model from Q-Sense, Sweden) fitted
with the QCM E4 module holding QSX 303 silica sensors with and
without silane coatings. Solutions were injected into the sensors with
an IPC-N935 tubing pump with planetary drive from Ismatec.
Frequency and dissipation changes of the 1st−13th overtones were
monitored using Qsoft401 software version 2.5.2. Qtools software
version 3.0.7 was used to analyze the data. All experiments were
conducted at 22 °C.
Scanning Electron Microscopy. QCM-D surfaces were exam-
ined with a Tescan Vega II LSU scanning electron microscope
(Tescan USA, PA) operating at 20 kV. Sensors were mounted onto
grids and sputter coated with gold.
RESULTS AND DISCUSSION
■
Synthesis of Tartrate-Protected Silaneboronate.
Scheme 1 summarizes our approach to the synthesis of the
silaneboronate, IV; the proton NMR data of the final product is
shown in Figure 1. The tartrate protecting group was necessary;
otherwise, the unprotected boronic acid interfered with the
hydrosilylation catalyst. Note that hydrosilylation produces two
isomers. The dominant isomer is IV; however, NMR indicates
the minor presence of the variation of IV where the silicon is
bonded to the benzylic carbon.
Two types of silica surfaces were treated with the silane,
silica-coated QCM-D sensor surfaces and Stober colloidal silica.
In both cases the silica was exposed to the protected silaneboronate
in dry toluene at 50 °C.
Silaneboronate-Treated QCM-D Silica Sensor. Table 1
summarizes the properties of the silaneboronate coating on the
silica surface before contact with liquid water. Ellipsometry gave
an average thickness of 231 nm and Sauerbrey mass of 280 mg/m2.
Assuming the density of fully condensed silane was close to
that of silica (2.2 g/mL), the pore volume of the silane layer
was 45%. Modified sensors were first exposed to water in the
QCM-D, resulting a frequency gain corresponding to a Sauerbrey
weight loss of 5 mg/m2, which we attributed to tartrate removal.
Electrophoretic Mobility. The electrophoretic mobility of the
Stober silica and phenylboronate silane-coated silica particles was
̈
measured using a Brookhaven Zetaplus zeta-potential analyzer
operating in electrophoretic light-scattering mode with BIC Zetaplus
software (version 2.5). A total of 10 runs (15 cycles per run) were
performed at 25 °C.
Dynamic Light Scattering (DLS). The average diameters of silica
and phenylboronate silane-coated silica particles were measured using
a Brookhaven dynamic light-scattering instrument with a BI-9000AT
autocorrelator and 35 mW laser with a wavelength of 632.8 nm at a
scattering angle of 90°. Sample data were acquired using BIC dynamic
light-scattering software 9kdlsw32 (version 3.34). The cumulative
statistical method was used to analyze the data. Sample temperature
was controlled at 25 1 °C using a NESLAB water bath.
Neutron Activation Analysis. Neutron activation analysis was
used to determine the mass percentage of boron in silane-coated silica
particles. In a typical experiment, 10−20 mg samples (in a precleaned
high-density polyethylene vial and heat sealed) were irradiated for 600 s
in a beam port site (nominal neutron flux 5 × 107 neutron/cm2·s)
and simultaneously counted. Gamma spectra were collected with a
hyperpure germanium detector (efficiency 20%, fwhm 1.9 at 1332 keV)
on a PC-based multichannel analyzer. The photopeak at 477.6 keV was
̈
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slightly more deposition at pH 4 compared to pH 7.4, possibly
reflecting the lower degree of silanol dissociation at lower pH.
Table 1. Properties of the Silaneboronate Film on a QCM-D
Silica Sensor before Immersion into Water
̈
Silaneboronate-Treated Stober Silica. Table 2 summa-
rizes the properties of Stober silica before and after silaneboronate
property
value
2 wt %
source
̈
boron content
thickness
XPS
231.1 nm
ellipsometry
ellipsometry
Table 2. Properties of Stober Silica and Silaneboronate-
Coated Silica in 5 mM NaCl at pH 8.5
̈
refractive index
mass coverage
density
1.1299 0.0047
280 mg/m2
1.21 g/mL
45%
a
QCM Sauerbrey mass
QCM + ellipsometry
silaneboronate-
property
average diameter/nm
Stober silica treated Stober silica
̈
̈
pore fraction
assumed density of solid, fully
hydrolyzed and condensed
silaneboronate was 2.2 g/mL
188.9 1.6
202.2 1.9
−2.34 0.06
3.5%
electrophoretic mobility, 10−8 m2 V−1 s−1 −6.23 0.09
boron (w/w) if 100% conversion
In previous work we have shown that latex particles stabilized
by surface polyol layers will selectively deposit onto cellulose
surfaces bearing phenylboronate groups.22 The poly(glycerol
monomethacrylate) surfaces chains are excellent colloidal
stabilizers, and the only explanation for their pH-dependent
deposition onto the modified cellulose was that deposition was
driven by formation of boronate ester linkages. Therefore, we
believe that the PGMA-coated particles are an excellent probe
for boronate-rich surfaces.
boron (w/w) from neutron activation
0.042%
1.1%
b
boron (w/w) from DLS
0.91%
c
boron (w/w) from XPS
2% 1.1%
1.9%
boron (w/w) from ARS binding, see
Figure 3
a
Example calculations are given in the Supporting Information.
b
c
Assuming core and shell densities were 1.82 g mL−1. 23 On the basis
of 3 measurements.
PGMA-coated latex deposition was monitored by quartz
crystal microbalance measurements. The QCM-D frequency
shift results are shown in Figure 2A at pH 7.4 and Figure 2B
treatment. The diameter increase suggests the presence of a
5 nm silaneboronate layer. The electrophoretic mobility of the treated
particles was significantly less than the starting particles, confirming
that the surface silanols were covered by the silaneboronate.
Five independent approaches were used to estimate the
boron content of the treated silica particles, and the results are
summarized in Table 2. If the all added silane ended up on the
particle surfaces, the theoretical maximum mass fraction of
boron is 3.5%. Neutron activation analysis gave a boron content
of 1.11
0.04%. The corresponding estimate from dynamic
light scattering was 0.91% assuming complete hydrolysis of the
silane and assuming both the silica core and the silica boronate
shell densities were equal to 1.82 g/mL, the reported value for
Stober silica.23
̈
XPS measurements were performed on QCM-D silica
sensors with adsorbed colloidal silaneboronate-treated Stober
̈
silica giving a mean value of 2% but with a broad distribution.
Springsteen and Wang have shown that alizarin red S (ARS)
condenses with boronate ions to give fluorescent products.10
ARS fluorescence characterization of the silaneboronate-treated
Stober silica was preformed two ways: (1) the silaneboronate-
̈
treated Stober silica was deposited onto QCM-D surfaces, and
̈
fluorescence microscopy confirmed that the surfaces were
Figure 2. PGMA-PS deposition onto silica QCM-D sensors before
and after silaneboronate treatment. SEMs micrographs show images of
QCM-D sensor surfaces after the deposition experiment.
at pH 4. At neutral pH, the latex showed strong adsorption
onto the silaneboronate sensor whereas there was little de-
position on the control silica sensor. At low pH, where
boronate ester formation should not occur, there was little
deposition. The surfaces of the QCM-D sensors were examined
at the end of experiments by SEM. Sample micrographs are
shown in Figure 2C−F. The highest coverage corresponded to
the silaneboronate sensor at neutral pH, in agreement the
QCM-D frequency data. There was limited latex deposition on
the untreated silica (Figure 2E and 2F) sensors. There was
Figure 3. ARS binding to silaneboronate-treated silica. Theoretical curve
was fit by varying the boronate−ARS binding constant. Incident wave-
length was 468 nm, and fluorescence intensity was measured at 517 nm.
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file); (2) fluorescence of the silaneboronate-treated Stober silica
̈
dispersions was measured as a function of the added ARS
dosage. Results are shown in Figure 3 together with a curve
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pH and ionic strength; see calculations in the Supporting
Information.10 The content of boron accessible to ARS was
1.9%, which was more than the value from neutron activation
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high binding constant required in our calculation, we have
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CONCLUSIONS
■
The dimethyl tartrate-protected phenylboronate silane IV (see
Scheme 1) readily condenses onto silica surfaces, yielding a
porous silica coating decorated with phenylboronic acid groups.
Our silaneboronate-treated surfaces demonstrated specific binding
of polyol-coated latex (QCM-D surface) and ARS dye (colloidal
silica), as expected for a surface bearing phenylboronic acid groups.
In our experiments, the coatings thicknesses ranged from 6 nm
for the colloidal silica to 231 nm on the macroscopic the QCM-D
silica sensor. There are no obvious barriers to achieving thinner
coatings. Indeed, we anticipate that our silaneboronate could
effectively functionalize a wide variety of silane-active sub-
strates, including microfluidic devices, optical fibers, and metal
oxide sensor surfaces.
(12) Ivanov, A. E.; Nilsson, L.; Galaev, I. Y.; Mattiasson, B. Boronate-
Containing Polymers Form Affinity Complexes with Mucin and
Enable Tight and Reversible Occlusion of Mucosal Lumen by
Poly(Vinyl Alcohol) Gel. Int. J. Pharm. 2008, 358 (1−2), 36−43.
(13) Ivanov, A. E.; Galaev, I. Y.; Mattiasson, B. Interaction of Sugars,
Polysaccharides and Cells with Boronate-Containing Copolymers:
From Solution to Polymer Brushes. J. Mol. Recognit. 2006, 19 (4),
322−331.
(14) Zhang, D.; Pelton, R. Controlling the Assembly of Nanoparticle
Mixtures with Two Orthogonal Polymer Complexation Reactions.
Langmuir 2012, 28, 3112−3119.
ASSOCIATED CONTENT
* Supporting Information
Fluorescent microscope images of silaneboronate-treated Stober
silica and MathCad modeling ARS binding to silaneboronate-
treated silica. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
̈
(15) Li, X.; Pennington, J.; Stobaugh, J. F.; Schoneich, C. Synthesis of
̈
Sulfonamide- and Sulfonyl-Phenylboronic Acid-Modified Silica Phases
for Boronate Affinity Chromatography at Physiological Ph. Anal.
Biochem. 2008, 372 (2), 227−236.
(16) Plueddemann, E. P. Silane Coupling Agents. Springer: New York,
1991.
AUTHOR INFORMATION
Corresponding Author
*E-mail: peltonrh@mcmaster.ca.
■
(17) Glad, M.; Kempe, M.; Mosbach, K. Selective Affinity Material,
Preparation Thereof by Molecular Imprinting, and Use of the Same.
US 2003/0049870 Al, March 13, 2003.
(18) Glad, M.; Norrlow, O.; Sellergren, B.; Siegbahn, N.; Mosbach,
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Notes
K. Use of Silane Monomers for Molecular Imprinting and Enzyme
Entrapment in Polysiloxane-Coated Porous Silica. J. Chromatogr. A
1985, 347 (0), 11−23.
(19) Xu, Y. W.; Wu, Z. X.; Zhang, L. J.; Lu, H. J.; Yang, P. Y.; Webley,
P. A.; Zhao, D. Y. Highly Specific Enrichment of Glycopeptides Using
Boronic Acid-Functionalized Mesoporous Silica. Anal. Chem. 2009, 81
(1), 503−508.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Sciences and Engineering Research
Council of Canada (NSERC) and Alcon Laboratories for funding
this project. In addition, we thank the Canadian Foundation for
Innovation funded McMaster Biointerfaces Institute for access to
XPS and ellipsometer instruments. Finally, we thank Andrew
Haines for performing some of the measurements.
(20) Thompson, K. L.; Armes, S. P.; York, D. W.; Burdis, J. A.
Synthesis of Sterically-Stabilized Latexes Using Well-Defined Poly-
(Glycerol Monomethaerylate) Macromonomers. Macromolecules 2010,
43 (5), 2169−2177.
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