Grafting of monoglyceride molecules for the design of hydrophilic and stable porous silicon surfaces

Article abstract of DOI:10.1039/b9nj00469f

Hydrophilic chemically stable porous silicon surfaces are generated by surface functionalisation with polar head terminated lipid biomolecules of the monoglyceride type. Two approaches to anchor the monoglyceride moiety to porous silicon surfaces are presented.

Full text of DOI:10.1039/b9nj00469f

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Grafting of monoglyceride molecules for the design of hydrophilic
and stable porous silicon surfacesw
a
a
a
´
Stephanie Pace, Philippe Gonzalez, Jean-Marie Devoisselle,
Pierre-Emmanuel Milhiet, Daniel Brunel and Frederique Cunin*
b
a
a
´
´
Received (in Montpellier, France) 8th September 2009, Accepted 17th November 2009
First published as an Advance Article on the web 3rd December 2009
DOI: 10.1039/b9nj00469f
Hydrophilic chemically stable porous silicon surfaces are
generated by surface functionalisation with polar head terminated
lipid biomolecules of the monoglyceride type. Two approaches to
anchor the monoglyceride moiety to porous silicon surfaces are
presented.
important role played by the hydrophilic and hydrophobic
properties of porous silicon surfaces on cell adhesion and
viability.13 The most hydrophilic porous silicon surfaces were
obtained by hydrosilylation reactions using undecanoic acid or
oligo(ethylene) glycol ligands. Nevertheless, water contact
angle measurements, which were never below 601, showed
the poor relative hydrophilic properties of the surfaces compared
to ozone-oxidized porous silicon surfaces (contact angle o 61).
Highly hydrophilic porous silicon surfaces are usually generated
by ozone oxidation treatment, but are known not to be stable
in aqueous media.1
Solid supports are widely used in biology for the immobilisation
and study of biological species such as cells, biomolecules or
membranes. These include glass, silicon, polystyrene and mica,
mainly because they are mechanically and chemically stable.
Great attention has been paid to using nanostructured porous
substrates that feature an enriched set of physicochemical and
physical properties (large surface area, available volume, optical
properties), offering increased potential for the development of
biomedical devices. Porous silicon, with flexible structural and
optical properties, and tunable surface chemistry, has shown
high efficiency in chemical and biomolecular sensing,1–8 and is
believed to play an important role in application fields
that include drug delivery,9–12 cell culture13,14 and tissue
engineering.15,16 A major limitation of using porous silicon
in biological-like aqueous media is the poor chemical stability
of Si–H surface species, which leads, considering the high
surface area, to very rapid degradation of the substrate.15
Chemical modification of porous silicon surfaces has proved
efficient for protecting them from chemical degradation.
Controlled oxidation of the surface, silanisation chemistry
and alkylation via Si–C bond chemistry has been developed
to passivate and protect the surface of porous silicon.17–20 The
most efficient methods for porous silicon surface stabilisation
use the attachment of ligands via Si–C bonds, which are more
stable towards nucleophilic attack in aqueous media than Si–O
bonds.1,20 Chemical functionalisation of porous silicon via
surface Si–C bonds leads easily to hydrophobic surfaces due
to the presence of hydrocarbon chains. In many cases, including
cell adhesion enhancement21 and artificial membrane deposition,22
it is strongly desirable to have highly hydrophilic stable
surfaces. In recent work, Sailor and his group reported the
In this Letter, we report the surface functionalisation of
porous silicon with polar head terminated lipid biomolecules
in order to generate highly hydrophilic, chemically-stable
porous silicon surfaces. Monoglyceride molecules were chosen
for this purpose as they contain glycol terminus functions to
impart hydrophilic properties and long carbon chains that
can play the role of hydrophobic spacers to keep aqueous
nucleophiles apart from the surface-reactive silicon hydride
species. In the present work, a monoglyceride molecule containing
an olefin function, an a-monoglyceryl undecylcarboxylate, is
prepared and covalently attached to freshly etched porous
silicon surfaces via Si–C bonds (preparation of the a-mono-
glyceryl undecylcarboxylate is described in the Experimental
section). Two approaches to anchor the monoglyceride moiety
to porous silicon surfaces are reported.
As a first approach, porous Si was thermally hydrosilylated
with a-monoglyceryl undecylcarboxylate. Freshly etched porous
silicon was exposed to a few milligrams of the monoglyceride,
which was dissolved in anhydrous toluene added to the
reaction flask by dynamic distillation under vacuum to avoid
the presence of air. The solution was maintained at 120 1C for
17 h under argon for the hydrosilylation reaction according to
previously described methods.20 The hydrosilylation reaction
between Si–H species and the olefin function of the freshly
synthesised monoglyceride is schematically presented in Fig. 1.
After the reaction, the silicon chip was rinsed thoroughly
with ethanol to remove excess monoglyceride. Fig. 2 shows
FTIR data for (a) a freshly etched porous silicon film and (b)
the same porous silicon surface after the reaction with
a-monoglyceryl undecylcarboxylate. Spectrum (a) exhibits
bands at 912 and 2115 cmꢀ1, assigned to the Si–H2 bending
mode and the Si–Hx stretching mode, respectively. After the
reaction with a-monoglyceryl undecylcarboxylate, bands are
observed in spectrum (b) at 1462, 1743, 2853 and 2930 cmꢀ1
a Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-ENSCM-
´
UM2-UM1, Ecole Nationale Superieure de Chimie de Montpellier,
8 rue de l’Ecole Normale, 34296, Montpellier, France.
E-mail: frederique.cunin@enscm.fr; Fax: +33 467 163 474;
Tel: +33 467 163 444
b Centre de Biochimie Structurale, UMR554 INSERM, UMR5048
CNRS, 29 rue de Navacelles, 34090, Montpellier, France.
Fax: +33 467 417 913; Tel: +33 467 417 917
w Electronic supplementary information (ESI) available: IR and
NMR spectra. See DOI: 10.1039/b9nj00469f
that are due to the dCH deformation vibration mode of a
tet
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Fig.
1
Chemical scheme of a-monoglyceryl undecylcarboxylate
grafting onto a porous silicon surface.
CH2, the nCQO stretching vibration mode of an ester and the
stretching vibration modes of aliphatic C–H bonds, respectively.
In addition, the large band centered at 3494 cmꢀ1 is assigned
to a nO–H stretching vibration mode of the diol, indicating that
the monoglyceride molecule is present at the surface of the
porous silicon. Bands are still observed in spectrum (b) at 909
and 2106 cmꢀ1, indicating that silicon hydride sites are still
present at the surface of the porous film. Complete reaction is
impeded by steric hindrance at the surface, as previously
observed in the literature.23 The presence of weak bands at
3076 and at 1637 cmꢀ1 could be assigned, respectively, to the
Fig. 2 FTIR spectra for (a) a freshly etched porous silicon surface
and (b) a porous silicon surface reacted with a-monoglyceryl undecyl-
carboxylate.
n
C–H stretching vibration mode of the arylic C–H bonds and to
the nCQC stretching vibration mode of the arene CQC bonds
from toluene that has not been totally desorbed after the
reaction. FTIR data of a controlled experiment consisting of
porous silicon refluxed in toluene for 17 h without a-mono-
glyceryl undecylcarboxylate indicates the presence of possible
residual toluene within the pores after rinsing and drying of
the samples (data not shown). Moreover, the previously
mentioned bands at 3076 and 1637 cmꢀ1 could also be
resulting in the formation of monoglyceride anchored onto
the surface of the porous silicon (Fig. 3(b)). After the reaction,
the porous silicon chip was rinsed with ethanol to remove the
excess glycidol.
Fig. 4 shows FTIR data for (a) a freshly etched porous
silicon film, (b) the same porous silicon surface grafted with
undecanoic acid and (c) the previously undecanoic acid-grafted
porous silicon surface after the reaction with glycidol. After
modification of the surface with undecenoic acid, spectrum (b)
exhibits bands at 1715 cmꢀ1, assigned to the nCQO stretching
vibration mode of the carboxylic acid, and bands at 1462, 2852
and 2925 cmꢀ1 that are assigned to the deformation and
stretching vibration mode of the aliphatic C–H groups. As
assigned to the nC–H stretching vibration mode of the vinyl
tri
C–H bonds, and the band at 1637 cmꢀ1 assigned to the nCQC
stretching vibration mode of the CQC bonds, indicating that
part of the a-monoglyceryl undecylcarboxylate is not covalently
grafted and remains physisorbed at the surface of the
porous silicon after the rinsing step. A control experiment
with lauric monoglyceride (equivalent to a-monoglyceryl
undecylcarboxylate but with no CQC bond) in the presence
of freshly etched porous silicon at 120 1C for 90 min showed
that removing the physisorbed lauric monoglyceride by rinsing
is difficult. Finally, a competitive reaction with the hydro-
silylation is also possible; for example, an etherisation reaction
between the diol from the monoglyceride and the silanol
groups of the partially oxidized porous silicon surface.
In order to prevent undesirable physisorption and the
competitive chemisorption of a-monoglyceryl undecylcarboxylate
onto the porous silicon surface, a second approach has been
investigated where a-monoglyceryl undecylcarboxylate is
grafted onto the porous silicon surface in a two-step reaction.
Freshly etched porous silicon was first thermally hydrosilylated
with undecenoic acid at 120 1C for 90 min under argon
(Fig. 3(a)). Next, the excess of non-grafted undecenoic acid
was easily removed by rinsing with dichloromethane. The
as-modified porous silicon surface was then exposed to glycidol,
which was able to react, in the presence of the triethylamine
catalyst, with the carboxylic acid functions of the grafted acid,
Fig. 3 Chemical anchoring of a monoglyceride molecule in a two-
step reaction onto a porous silicon surface, including (a) grafting of
undecenoic acid onto the porous silicon surface and (b) an addition
reaction of glycidol onto the grafted carboxylic acid function.
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or heterogeneity are minimized.23 Porous silicon surfaces used
in this work exhibit pores of 5 to 8 nm average diameter
(obtained by SEM, data not presented). The theoretical
diameter of the water drops used in this work were calculated
to be 1.56 mm (from the known 2 mL volume of the water
drop), which is 2 to 3 million times larger than the pore
diameters. Because the two considered surface ligands
(undecanoic acid and monoglyceride) are comparable in terms
of chain length and conformation, with molecular weights of
the same order of magnitude (185 and 254 g molꢀ1), we
consider that their surface rugosities are still comparable after
chemical modification, and consequently that the contact
angles of the two surfaces are altered in a comparable way.
The contact angle for the modified surface with the
monoglyceride (undecanoic acid plus glycidol) was measured
as 141, which is lower than for the modified surface with
undecanoic acid only, for which the contact angle was 621,
showing the greater hydrophilicity of the monoglyceride-
terminated surface. This is explained by the higher polarity
of a diol function compared to that of a carboxylic acid
function, as attested by the values of the dipole moments for
ethylene glycol (m = 7.61) compared to acetic acid (m = 5.6).25
The polarity of solvents is also known to be linked to their
dielectric constant, which is 37 and 6.1 for ethylene glycol and
acetic acid, respectively.26 Additionally, besides being more
polar, the diol function is less pH-dependant than a carboxylic
acid function; the pKa value is 4.76 for acetic acid, whereas it is
over 14 for ethylene glycol (14.22).25 The diol-modified porous
silicon surface presented in this work is expected not to be
modified when exposed to varying environmental pH conditions.
Such hydrophilic and inert-to-pH-change surfaces can be
useful in biological and biosensing applications, where hydro-
philic stable surfaces are desirable in various buffered conditions.
The chemical stability to corrosion of the monoglyceride-
modified porous silicon surface was tested over time in a
phosphate-buffered saline (PBS) solution and compared to a
non-modified porous silicon surface and an undecanoic
acid-modified porous silicon surface (Fig. 5); it was also
compared with the stability of thermally oxidised and
ozone-oxidised porous silicon surfaces. Thermal oxidation in
air is an efficient method to passivate the surface of porous
silicon and produce a stable oxide surface.27–29 Ozone oxidation
is known to be a classical and simple way to provide highly
hydrophilic surfaces from porous silicon by generating silanol
surface groups.1 As expected, the thermal oxidation treatment
generates a highly stable surface.27–29 The two Si–C modified
surfaces show a slower decrease in their normalised effective
optical thickness in time than the non-modified surface and the
ozone-oxidised surface, as expected with regards to the kinetic
stability properties of the Si–C bond.20
Fig. 4 FTIR spectra for (a) the freshly etched porous silicon surface,
(b) the undecanoic acid-grafted porous silicon surface and (c) surface
(b) reacted with glycidol.
expected, the absence of characteristic bands for CQC bonds
(at 1637 and 3076 cmꢀ1) confirms that undecanoic acid is
covalently attached to the porous silicon surface and that no
undecenoic acid remains physisorbed at the surface after
rinsing with dichloromethane. A subsequent reaction between
the carboxylic acid and the glycidol leads to a shift in the nCQO
stretching band to 1740 cmꢀ1, indicating that the carboxylic
acid has been converted into an ester group. The presence of a
shoulder in the aforementioned band observed at 1710 cmꢀ1 is
consistent with unreacted carboxylic acid functions remaining
after the reaction with glycidol. Finally, the large band
centered at 3398 cmꢀ1 is assigned to the stretching vibration
mode of the diol, confirming the presence of the monoglyceride
molecule covalently attached to the porous silicon surface. No
residual toluene in the pores was observed after rinsing and
drying of the samples. This two-step reaction approach allows
better control of the grafting of the monoglyceride molecules
via Si–C bonds onto the porous silicon.
In order to test the hydrophilicity of the monoglyceride
modified surface obtained in the two-step reaction process,
water contact angle measurements were performed and com-
pared with contact angle values from undecanoic acid-
modified porous silicon surfaces. The obtained results are
summarised in Table 1. It is important to mention here that
contact angle measurements are normally considered for ideal
flat surfaces that are traditionally defined as being smooth,
rigid, chemically homogeneous and non-reactive.24 In the case
of solid surfaces presenting roughness or chemical heterogeneity,
the interpretation of contact angle values are more critical and
need to be considered individually. Nevertheless, the literature
in this field indicates that when the size of the water drop used
for the measurement is sufficiently large compared to the scale
of roughness or heterogeneity, the effects of surface roughness
In conclusion, hydrophilic, chemically-stable, porous silicon
surfaces have been prepared by the covalent attachment of
biocompatible molecules from the monoglyceride family. Two
approaches to anchor the monoglyceride to porous silicon
surfaces have been developed and compared. In the first
approach, the monoglyceride, previously synthesised in the
laboratory, was covalently attached to the porous silicon
surface using hydrosilylation chemistry. In the second
approach, the monoglyceride was covalently attached in a
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Table 1 Water contact angles for modified porous silicon surfaces with undecanoic acid and monoglyceride molecules
Reactant
Y (standard deviation) (1)
Undecylenic acid
62 (3)
a-Monoglyceryl undecylcarboxylate
14 (1)
Experimental
Synthesis
Porous silicon films were prepared from single-crystal p-type
silicon (boron doped, B3 O cm resistivity, h100i orientation,
from Siltronix, Inc.) in an electrochemical etch at a constant
current density of 22.5 mA cmꢀ2 for 5 min in a 1 : 1 48%
aqueous hydrofluoric acid–ethanol solution. Prior to etching,
the silicon chips were washed successively under sonication
with acetone and ethanol for 5 min each. The a-monoglyceryl
undecylcarboxylate was prepared according to a previously
published procedure in our laboratory.30 The synthesis involved
a reaction between undecenoic acid (Aldrich, 98%) and
glycidol (Aldrich, 96%) in toluene at 120 1C with a solid
catalyst (quaternary ammonium chloride-grafted silica) in
anhydrous toluene for 24 h. The quaternary ammonium
chloride-grafted silica catalyst preparation was adapted from
ref. 26: 0.5 g of (3-(triethoxysilyl)propyl)-trimethylammonium
chloride (65 wt% in water, purchased from Aldrich) was
added to a suspended silica gel (G5H from Grace Davison
with a surface area of 513 m2 gꢀ1) in 30 mL of toluene. The
suspension was stirred at RT for 1 h, then heated and stirred at
120 1C for 3 h in a reactor equipped with a Dean–Stark
apparatus to remove the ethanol and water from the reaction
mixture. The functionalised powder was collected by filtration,
and successively rinsed with toluene, ethanol and diethyl
ether. It was then extracted with a Soxhlet apparatus by a
dichloromethane–diethyl ether mixture (1 : 1). Prior to the
reaction, the catalyst was activated by eliminating water at
150 1C overnight. Following the reaction, the obtained product
was filtered and recrystallized from a diethyl ether–hexane
solution. The obtained a-monoglyceryl undecylcarboxylate
was characterised by mass spectroscopy (FAB: 259 [M + 1]),
FTIR spectroscopy (see S1 in the ESIw) and NMR spectro-
scopy (see S2 in the ESIw). Stability measurements were
performed on a highly doped p-type silicon wafer (boron
doped, 0.0008–0.0012 mO cm resistivity, (100) orientation,
from Siltronix, Inc) etched at 22.5 mA cmꢀ2 for 5 min in a
3 : 1 48% aqueous hydrofluoric acid–ethanol solution. Samples
were mounted in a flow cell and exposed to a PBS (pH 7.4)
solution at a flow rate of 200 mL minꢀ1. Interference spectra
were collected every 30 s for the silicon hydride-terminated,
ozone-oxidised, thermally oxidised and undecanoic acid-modified
surface samples, and every 1 min for the monoglyceride-
modified surface sample. Thermal oxidation was performed
in air at 450 1C for 2 h.
Fig. 5 Stability of modified porous silicon surfaces in PBS (pH 7.4)
for (J) the silicon hydride terminated surface, (K) the ozone-oxidised
surface, (n) the undecanoic acid-modified surface, (&) the thermally
oxidised surface and (’) the monoglyceride-modified surface,
presented as the variation of the normalized effective optical thickness
n
effl/(neffl)0 vs. time (see the Experimental section).
two-step reaction, including the grafting of undecenoic acid
onto the porous silicon surface and an addition reaction of
glycidol onto the grafted carboxylic acid function. The second
approach led to better control of the covalent attachment of
the biomolecule. Aqueous intimate wetting without chemical
degradation over time was achieved. The obtained surfaces
both feature highly hydrophilic properties comparable to
silica-type surfaces and have the high chemical stability that
is classically observed for more hydrophobic, hydrosilylated
porous silicon surfaces. The flexible design of the optical
structures of porous silicon is still possible prior to surface
modification, providing porous silicon with better potential
for biosensing and biological applications, where long term
stability in aqueous media is required.
The authors gratefully acknowledge Prof. Michael J. Sailor
of the University of California, San Diego, USA, and Profs.
Emmanuel Belamie and Michel Granier of the Institut Charles
Gerhardt Montpellier, France for helpful discussions. The
authors thank the Centre National de la Recherche Scientifique
(CNRS) for funding.
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Characterisation
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¨
T. Heikkila, K. Vaha-Heikkila, J. Hirvonen and V.-P. Lehto,
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¨
¨
¨
FTIR data were collected in diffuse reflectance mode with a
Bruker Equinox 55 spectrometer equipped with a DRIFT
(diffuse reflectance infrared Fourier transform) Spectratec
collector system. Contact angle measurements were performed
using Digidrop GBX fast/60 apparatus. Optical reflectivity
spectra were obtained using an Ocean Optics USB2000-VIS-NIR
miniature fiber optic spectrometer. A tungsten lamp was used
for sample illumination. Reflected light from the samples was
collected back along an axis coincident with the surface
normal. The effective optical thickness, neffl, was obtained by
using a fast Fourier transformation of the interference spectra
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