Smartly designed photoreactive silica nanoparticles and their reactivity

Article abstract of DOI:10.1039/c1jm00055a

Monodisperse, colloidal silica nanoparticles (NPs) are being widely investigated due to a variety of applications in various fields of chemistry. Many works utilize incorporation of various functional groups to silica NPs for their further modifications. However, at present no benzophenone (BPh) or phenyl azide (PA) containing silica NPs exist. Upon UV irradiation BPh and PA form highly reactive species that react with any organic material. Here we present a convenient method for the preparation of novel hybrid photoreactive silica NPs (denoted as SiO₂@photoreactive group) prepared by co-condensation of photoreactive organosilanes and tetraethyl orthosilicate (TEOS) to obtain SiO₂@PA and SiO₂@BPh NPs. The reactivity of these two types of silica NPs is compared to that of perfluorinated phenyl azide (PFPA) based SiO₂ NPs. The reactivity evaluation is carried out by the reaction of the three types of SiO₂ NPs with highly inert poly(2-chloro-paraxylelene) films. It is found that, in contrast to what is stated in the literature, PA is much more reactive than PFPA, when dealing with solid state photochemical reactions. Next, photoreactive silica NPs on polymer films are used as an intermediate functional phase for a second modification step using silane-based chemistry. A successful incorporation of amine functionality onto silica NPs is achieved by their reaction with 3-aminopropyltriethoxysilane (APTES) and is verified by fluorescence microscopy. This strategy provides a general and versatile route to efficient functionalization of silica by light.

Full text of DOI:10.1039/c1jm00055a

As featured in:
Light-mediated research from Jean-Paul Lellouche’s
laboratory, Institute of Nanotechnology & Advanced
Materials, Bar-Ilan University, Ramat-Gan, Israel.
Title: Smartly designed photoreactive silica nanoparticles and
their reactivity
The design and fabrication of novel nanosized materials possessing
light-sensitive properties has always been the subject of intense
activity and interest amongst the scientific community. Accordingly,
innovative UV-photoreactive silica (SiO2) nanoparticles emerge as
an excellent material candidate for the surface functionalization
and nanostructuration of biocompatible polymeric coatings
opening a quite wide and generic avenue in this challenging
domain.
See A. Peled et al.,
J. Mater. Chem., 2011, 21, 11511.
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PAPER
Smartly designed photoreactive silica nanoparticles and their reactivity†
*
Anna Peled, Maria Naddaka and Jean-Paul Lellouche
Received 5th January 2011, Accepted 28th February 2011
DOI: 10.1039/c1jm00055a
Monodisperse, colloidal silica nanoparticles (NPs) are being widely investigated due to a variety of
applications in various fields of chemistry. Many works utilize incorporation of various functional
groups to silica NPs for their further modifications. However, at present no benzophenone (BPh) or
phenyl azide (PA) containing silica NPs exist. Upon UV irradiation BPh and PA form highly reactive
species that react with any organic material. Here we present a convenient method for the preparation
of novel hybrid photoreactive silica NPs (denoted as SiO2@photoreactive group) prepared by co-
condensation of photoreactive organosilanes and tetraethyl orthosilicate (TEOS) to obtain SiO2@PA
and SiO2@BPh NPs. The reactivity of these two types of silica NPs is compared to that of
perfluorinated phenyl azide (PFPA) based SiO2 NPs. The reactivity evaluation is carried out by the
reaction of the three types of SiO2 NPs with highly inert poly(2-chloro-paraxylelene) films. It is found
that, in contrast to what is stated in the literature, PA is much more reactive than PFPA, when dealing
with solid state photochemical reactions. Next, photoreactive silica NPs on polymer films are used as an
intermediate functional phase for a second modification step using silane-based chemistry. A successful
incorporation of amine functionality onto silica NPs is achieved by their reaction with
3-aminopropyltriethoxysilane (APTES) and is verified by fluorescence microscopy. This strategy
provides a general and versatile route to efficient functionalization of silica by light.
were introduced into silica NPs for example amine,3 fluorescent
dyes,4 carbazole,5 alkyne,6 atom transfer radical polymerization
(ATPR) initiator,7 etc.
Introduction
Monodisperse, colloidal silica nanoparticles (NPs) prepared
€
1
according to the Stober method were carefully investigated
However, an extensive literature survey showed that only one
work described the preparation of photoreactive SiO2 NPs by
surface silanization with perfluorinated phenyl azide (PFPA)–
silane.8 Photochemistry is a very effective tool for the formation
of highly reactive species that can react with inert materials.
Upon UV irradiation, photoreactive groups such as benzophe-
none (BPh), phenyl azide (PA), and PFPA moieties are known to
generate highly reactive species, i.e., nitrenes (from PA and
PFPA) or excited triplet BPh moieties (from BPh). These highly
reactive species readily react with C–H and C]C bonds of any
organic matter via insertion or abstraction mechanisms.9,10
Herein we present novel hybrid photoreactive silica NPs
(denoted as SiO2@photoreactive group) prepared by co-
condensation of photoreactive organosilanes and TEOS to
obtain SiO2@PA and SiO2@BPh NPs. According to Gann and
Yan8 PA-based organosilanes cannot be used because of poten-
tial tendency of PA to give ring expansion products i.e. dehy-
droazepine, in the presence of nucleophiles.10 For this reason, we
have synthesized SiO2@PFPA NPs and compared their
reactivity to SiO2@PA. The reactivity of these three types of
photoreactive silica NPs was evaluated by their covalent
immobilization into highly chemically stable poly(2-chloro-
paraxylelene) (parylene C) films. The silica functionalized
during the last four decades. There are two basic approaches for
the covalent introduction of chemical functionalities to silica
NPs: surface silanization and co-condensation methods. The
silanization method to produce functional silica NPs was first
reported in 1989 by Philipse and Vrij2 and now frequently used in
various fields of chemistry. In 1993 Van Blaaderen and Vrij3
published the original paper that demonstrated the new concept
of introducing chemical functionalities to silica NPs by co-
condensation of tetraethyl orthosilicate (TEOS) and appropriate
organosilane, yielding stable hybrid organo-silica NPs. Using the
co-condensation method, a variety of chemical functionalities
Department of Chemistry, Nanomaterials Research Center, Institute of
Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat
Gan, 52900, Israel. E-mail: lellouj@ biu.ac.il; Tel: +972 3 5318324
† Electronic supplementary information (ESI) available: List of
abbreviations,
experimental
procedure
for
4-azido-2,3,5,6-
tetrafluorobenzoyl chloride, organic compounds characterization data,
1H- and 13C-NMR spectra, TGA spectra, solid state 13C and 29Si
CP-MAS NMR spectra, UV absorbance spectra, size distribution
graphs, FT-IR spectra, SEM tilt images, AFM images, AFM NP
manipulation images, SEM and AFM of neat parylene C film, XPS
spectra
10.1039/c1jm00055a
and
fluorescence
microscope
images.
See
DOI:
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 11511–11517 | 11511

parylene
C
films were further modified with 3-amino-
vigorous stirring at RT. The NPs were isolated by centrifugation
ꢀ
propyltriethoxysilane (APTES), incubated with a fluorescent tag
and studied by fluorescence microscopy.
(15 000 rpm, 0 C, 20 min). The supernatant was removed and
NPs were redispersed in ethanol (30 mL) using a bath sonicator.
The re-dispersion centrifugation washing was repeated 5 times.
The purified silica NPs were divided in 2 vials. One part was
evaporated and dried in a vacuum oven overnight (30 ꢀC) to
remove solvents. The second part was homogeneously dispersed
in ethanol and stored in the dark in a fridge. Note: in order to
determine the NPs concentration in solution 1 mL of NPs
dispersion was evaporated and dried in a vacuum oven overnight
(30 ꢀC) to remove solvents. Thus all dispersions were diluted with
EtOH to achieve 9 mg mLꢁ1 NPs dispersions. The bare silica NPs
were prepared by the same procedure using TEOS (1.7 mL,
7.6 mmol) without addition of photoreactive silane.
Experimental
Materials
All reagents were obtained commercially from Sigma-Aldrich
unless otherwise noted. Chromatographic purification of products
was accomplished using flash chromatography on Merck silica gel
60 (0.040–0.063) 230–400 mesh ASTM. RT refers to 20–22 ꢀC.
Synthesis of 4-azido-N-(3-(triethoxysilyl)propyl)benzamide 1
(PATES)
The 4-azidobenzoic acid (1.3 g, 8.0 mmol) and 1,10-carbonyl-
diimidazole (CDI) (1.29 g, 8.0 mmol) were dissolved in THF
(35 mL) under nitrogen. After stirring at RT for 2 h 3-amino-
propyltriethoxysilane (APTES) (1.86 mL, 8.0 mmol) was added
to the reaction. The reaction was stirred at the RT overnight. At
reaction completion (TLC checking), the medium was concen-
trated in vacuum affording a yellowish crude solid that was
purified by flash chromatography on silica gel (eluent: acetone/n-
hexane: 85/15) to give 1 (2.35 g, 6.41 mmol) in 80% yield.
Functionalization of parylene C films
4 mm thick parylene C films deposited on glass substrates (7 ꢂ
7 mm) using a chemical vapor deposition (CVD) method were
generously supplied by COMELEC SA Ltd. (Switzerland). The
films were pre-washed for 3 times in EtOH (10 mL) using
a Bransonic cleaner bath sonicator for 10 min and dried under
vacuum at 30 ꢀC for 2 h. 20 mL of ethanolic dispersions (9.0 mg
mLꢁ1) of the corresponding hybrid photoreactive SiO2 NPs or
bare SiO2 NPs were spin coated (2 min, 1500 rpm, N2 atmo-
sphere) on the parylene C films and the films were UV irradiated
under argon atmosphere for 4 h. After irradiation, the films were
washed 3 times in EtOH (10 mL) using the Bransonic cleaner
bath sonicator for 10 min and dried under vacuum at 30 ꢀC
overnight.
Synthesis of 4-azido-2,3,5,6-tetrafluoro-N-(3-(triethoxysilyl)
propyl)benzamide 2 (PFPATES)
The 4-azido-2,3,5,6-tetrafluorobenzoyl chloride (2.37 g, 9.36
mmol) was dissolved in dry THF (15 mL) in a two neck bottom
flask equipped with a drying tube. The 4-dimethylaminopyridine
(DMAP) (2.44 g, 20 mmol) and APTES (4.42 g, 20 mmol) were
dissolved in dry THF (20 mL) and were added to the reaction.
The reaction mixture was stirred at RT for 1 h. The reaction
mixture was concentrated in vacuum affording a yellowish crude
solid that was purified by flash chromatography on silica gel
(eluent: ether/n-hexane: 60/40) to give 2 (1.18 g, 2.7 mmol) in 29%
yield.
General procedure for post-modification of PC–
SiO2@photoreactive group films with APTES
The three types of SiO2 modified films, as well as the PC–SiO2
film, were shaked in a (EtOH/H2O/APTES 2.50 mL/2.50 mL/
10.0 mL) solution for 24 h at RT. The amino modified films were
peeled from the glass support, vigorously washed with EtOH and
dried under air. The self-supported amino modified films were
immersed into a solution (1.0 mg mLꢁ1) of dansyl chloride in
anhydrous DMF for 1 h. Finally, the films were vigorously
washed with DMF and dried under flow of argon.
Synthesis of 4-benzoyl-N-(3-(triethoxysilyl)propyl)benzamide
3 (BPhTES)
The 4-benzoylbenzoic acid (2.26 g, 10.0 mmol) and CDI (1.62 g,
10.0 mmol) were dissolved in dry THF (40 mL) in a two neck
bottom flask equipped with a drying tube. After stirring at RT
for 2 h APTES (2.32 mL, 10.0 mmol) was added to the reaction.
The reaction was stirred at the same temperature during 24 h. At
reaction completion (TLC checking), the medium was concen-
trated in vacuum affording a yellowish crude solid that was
purified by flash chromatography on silica gel (eluent: acetone/n-
hexane: 85/15) to give 3 (2.54 g, 5.91 mmol) in 59% yield.
Characterization
NMR spectra were recorded on a Bruker DRX spectrometer
(300 MHz, 75.5 MHz for 1H, 13C respectively), and are refer-
enced internally according to TMS (0 ppm) or relative to the
1
solvent. Data for H-NMR are recorded as follows: chemical
shift (d in ppm), multiplicity (s, singlet; br, broad signal; d,
doublet; t, triplet; q, quartet; m, multiplet), coupling constant
(Hz), and integration. Data for 13C-NMR are reported in terms
of chemical shift (d in ppm). HRMS were run on a VG-Fison
AutoSpec Premier High Resolution Spectrometer, manufactured
by Waters (UK). FTIR spectra were recorded on a Bruker
TENSOR 27 spectrometer using Diffuse Reflectance Accessory
EasyDiff (PIKE technologies). Samples were prepared by mixing
material (2%) with KBr (dry, IR grade KBr, Aldrich). The
spectra that are obtained by the diffuse reflection technique
appear different from standard transmission spectra. The peak
General preparation of photoreactive hybrid silica NPs
€
Hybrid silica NPs were prepared according to a Stober method
with some modifications. Photoreactive silane (1 or 2 or 3)
(0.76 mmol) was added to a solution of TEOS (1.55 mL,
6.84 mmol) in EtOH (45 mL) under vigorous stirring at RT. Thus
water (2.75 mL) and NH4OH 28% (1.2 mL) were added to the
reaction. The reaction was allowed to proceed for 6 h under
11512 | J. Mater. Chem., 2011, 21, 11511–11517
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intensities at high wave numbers are weak and the peak line
shapes are rounded. In order to compensate these differences the
spectra were transformed into Kubelka Munk units using FTIR
software. UV absorbance spectra were recorded on a Varian
CARY 100 Bio UV-Visible spectrophotometer. Spectra were
obtained in EtOH. DLS and z-potential measurements were
performed using a Malvern Zetasizer Nano series Nano ZS using
ethanolic dispersions of nanoparticles. TGA was carried out on a
TA Instruments apparatus (1GA Q500 model) using a 25–800 ꢀC
temperature profile (10 ꢀC minꢁ1, air, 100 mL minꢁ1). TEM
images were taken using a 120 kV FEI, Tecnai G12 BIOTWIN.
Samples were prepared on formvar/carbon 400 mesh Cu grids by
dropping an ethanolic solution of the particles. The average size
of NPs was determined by measuring 100 NPs from TEM images
using ImageJ software. Solid state 13C and 29Si CP-MAS NMR
were recorded on a Bruker 500 instrument equipped with a 4 mm
solid state probe operating at 125 MHz for 13C and 500 MHz for
29Si nuclei. Chemical shifts were referenced to external samples of
DSS (29Si) and Adamantane (13C). The 13C signal was enhanced
using cross-polarization techniques, where 1H polarization is
transferred to the 13C nuclei. The presented spectra were collected
using a magic angle spinning rate of 8 kHz. For the 13C a contact
time of 3 ms was applied, with a 2.83 ms pulse, a recycle delay of
2 s and a line broadening of 60 Hz. For the 29Si spectra a contact
time of 5 ms was applied, a pulse length of 2.7 ms was used, with
a delay time of 5 s and a line broadening of 100 Hz. AFM
measurements and imaging were carried out using a Nanoscope
V Multimode scanning probe microscope (Digital Instruments,
Santa Barbara, CA). All images were obtained using the tapping
mode with a single LTESP silicon probe (force constant of
48 N mꢁ1, Digital Instruments, Santa Barbara, CA). The reso-
nance frequency of this cantilever was approximately 130–250
kHz. The scan angle was maintained at 90ꢀ, and the images were
captured in the retrace direction with a scan rate of 0.5 per 1 Hz
(respectively for the scan size 10 000 ꢂ 10 000 nm/2000 ꢂ 2000
nm). The aspect ratio was 1 : 1 and image resolution 512 samples
per line. Before analysis of the images, the ‘‘flatting’’ and ‘‘pla-
nefit’’ functions were applied to each image. Gwyddion software
was used for image processing. SEM images were taken on FEI,
QUANTA 200F or Helios 600. Samples were prepared on Cu
pellets by gluing the glass supported parylene C films on double
side carbon tape and coating them with gold. FE-SEM images of
selected samples were generously performed by JEOL Company
using JSM-7600F. Samples were prepared without gold coating.
XPS analysis was performed on a Thermo-VG SIGMA probe.
Contact angle values were measured using a Rame-Hart Model
100 Contact Angle Goniometer. UV irradiations were performed
using a Philips HPK 125w UV lamp. Spin coatings were per-
formed using a ‘‘Specialty Coatings Systems’’ P6700 spin coater.
20 mL of ethanolic dispersions (9 mg mLꢁ1) of the corresponding
hybrid photoreactive silica NPs or bare SiO2 NPs were spin
coated (2 min, 1500 rpm, N2 atmosphere) on the parylene C
films. Fluorescent images were recorded with a Zeiss AxioI-
maging Z1 fluorescent microscope using GFP and CFP filters.
BPh-based triethoxysilane (BPhTES) 311 were synthesized from
the corresponding photoreactive carboxylic acid derivatives by
their reaction with 3-aminopropyltriethoxysilane (APTES) to
form the amide bond.
Hybrid photoreactive SiO2 NPs bearing photoreactive groups
were prepared by a one-pot hydrolytic co-condensation of
photoreactive organosilanes 1–3 (10% molar) and tetraethyl
€
orthosilicate (TEOS) using the well-known Stober sol–gel
method to produce SiO2@PA, SiO2@PFPA, and SiO2@BPh
NPs respectively (Scheme 1). The bare SiO2 NPs were prepared
by the same procedure using TEOS without addition of any
photoreactive organosilane.
The size and morphology of the three types of hybrid photo-
reactive SiO2 NPs as well as of bare SiO2 NPs were determined
using transmission electron microscopy (TEM) and dynamic
light scattering (DLS) measurements. TEM images indicated
that all three types of hybrid photoreactive SiO2 NPs have
a spherical morphology (Fig. 1a–c) with an average size of 52.1 ꢃ
9.0, 80.8 ꢃ 14.6, and 63.9 ꢃ 6.1 nm (Table 1, average measure-
ment of 100 NPs from TEM images using ImageJ software) for
SiO2@PA, SiO2@PFPA, and SiO2@BPh NPs respectively.
In contrast, bare SiO2 NPs possess a larger average size of
141.4 ꢃ 10.6 nm. This phenomenon originates from the co-
condensation process itself. In other words, during hydrolysis of
any photoreactive organosilane the lack of propagating alkoxy
groups hinders the particle growth, which leads to decrease in
sizes.12
The colloidal stability of ethanolic dispersions of hybrid
photoreactive SiO2 NPs was examined by DLS and z-potential
measurements (Table 1). Since DLS provides information con-
cerning the hydrodynamic radius of the particle, the average sizes
of all NPs measured by DLS are larger than the average sizes
determined by TEM. SiO2@PA, SiO2@PFPA, SiO2@BPh NPs,
and bare SiO2 NPs have z-potential values of ꢁ41.2, ꢁ36.5,
ꢁ40.8, and ꢁ48.7 mV respectively, this indicates that all these
NPs form stable dispersions.
The incorporation of photoreactive organosilanes 1–3 was
studied using solid state 29Si and 13C cross-polarization-magic
angle spinning (CP/MAS) NMR (Fig. 1d and 2, respectively).
The 29Si NMR profiles extended in two regions ꢁ80 to ꢁ120 ppm
Results and discussion
The photoreactive organosilanes PA-based triethoxysilane
(PATES) 1, PFPA-based triethoxysilane (PFPATES) 2, and
Scheme 1 Sol–gel preparation of SiO2@photoreactive NPs.
J. Mater. Chem., 2011, 21, 11511–11517 | 11513
This journal is ª The Royal Society of Chemistry 2011

Fig. 2 Schematic representation of SiO2@PA NP and (a) 13C CP/MAS
NMR spectrum of SiO2@PA; (b) 13C NMR (75 MHz; [D6] DMSO)
spectrum of 1 (PATES).
Fig. 1 TEM images of (a) SiO2@PA, (b) SiO2@PFPA and (c)
SiO2@BPh (all scale bars are 200 nm); (d) 29Si CP/MAS NMR spectra of
the (1) SiO2@PA, (2) SiO2@BPh, and (3) SiO2@PFPA NPs.
and S13†), which originated from the incorporated photoreactive
groups. Finally, the presence of the azide group in SiO2@PA and
SiO2@PFPA NPs was detected by FTIR at 2100 cmꢁ1 (see ESI,
S13†).
Table 1 Characterization of photoreactive and bare silica NPs
TGAa (%) DLSb c/nm z-Potentialc d/mV Sizee/nm
,
,
Sample
Next, parylene C (PC) films were modified with SiO2@PA,
SiO2@PFPA, and SiO2@BPh NPs using UV irradiation to
produce PC–SiO2@PA, PC–SiO2@PFPA, and PC–SiO2@BPh
respectively (Scheme 2). In order to prove that the attachment of
the hybrid photoreactive SiO2 NPs onto PC films arose from the
photochemical reaction a control experiment for PC modifica-
tion using bare SiO2 NPs was performed to yield PC–SiO2.
The topography of the SiO2 functionalized parylene films was
investigated using scanning electron microscope (SEM) and
atomic force microscopy (AFM). It can be clearly seen from
SEM (Fig. 3) and AFM (Fig. 4) images that there is a homoge-
nous coverage of the parylene film by SiO2 NPs for all three types
of photoreactive NPs, while no particles were detected in the case
of the bare SiO2 NPs. Moreover, field emission scanning electron
microscope (FE-SEM) and AFM images demonstrate partial
penetration of NPs into the film for SiO2@PA NPs (Fig. 5 and
S18†). Similar behavior was discovered for SiO2@PFPA and
SiO2@BPh NPs (see ESI, S16–S18†).
SiO2
SiO2@PA
12.96
21.39
SiO2@PFPA 20.09
149.0
107.3
129.6
102.8
ꢁ48.7
ꢁ41.2
ꢁ36.5
ꢁ40.8
141.4 ꢃ 10.6
52.1 ꢃ 9.0
80.8 ꢃ 14.6
63.9 ꢃ 6.1
SiO2@BPh
19.21
a
Weight loss. b Average size. c EtOH dispersion. d z-Potential. e Average
size measurement of 100 NPs from TEM images using ImageJ software.
and ꢁ50 to ꢁ80 ppm, corresponding to siloxane group units Qx
(Qx, Si(OSi)x(OH)4ꢁx
)
and silicon groups Tx (Tx,
CSi(OSi)x(OH)3ꢁx
)
respectively. The signals centered at
ꢁ68 ppm (denoted T) are representative for T units13 confirming
the presence of photoreactive groups on the SiO2 surface.14 The
signals at ꢁ80 to ꢁ120 ppm region are assigned as Q signals,
which originate from the SiO2 matrix. Fig. 2 demonstrates
a good correlation between the 13C CP/MAS NMR spectrum of
SiO2@PA NPs (a) and the 13C NMR (75 MHz; [D6]DMSO)
spectrum of 1 (PATES) (b). It can be clearly seen that all peaks
denoted as 1–6 appear in both spectra. The two peaks appearing
at 17.5 and 58.8 ppm are ascribed to ethoxy groups of 1. In the
spectrum of SiO2@PA NPs they appear due to unhydrolyzed
ethoxy groups in the hybrid SiO2 matrix. The SiO2@PFPA and
SiO2@BPh NPs were also examined by 13C CP/MAS NMR
technique and showed similar characterizations (see ESI, S11†).
In order to demonstrate the covalent attachment of the hybrid
photoreactive SiO2 NPs into PC films, NP manipulation by
mechanical pushing using AFM was performed.15 When the
irradiation was performed for 4 h, particles could not be moved
and a scratch was formed on the surface of the parylene film in
the place where the AFM tip moved during the mechanical
manipulation (see ESI, S19–S21†). Scratch formation is evidence
of the physical contact between the tip and the surface of the film
indicating that there was a physical contact between the tip and
ꢀ
The thermogravimetric analysis (TGA) (25–800 C tempera-
ture profile, heating rate of 10 ꢀC minꢁ1, air, see ESI, S10†)
confirms the successful incorporation of photoreactive silanes 1–
3 into the hybrid SiO2 matrix. All hybrid photoreactive SiO2 NPs
had higher weight losses than the bare SiO2 NPs as it can be seen
from Table 1. The weight loss of bare SiO2 NPs originates from
unhydrolyzed alkoxy groups, water and ethanol molecules
trapped in silica matrix. Moreover, all ethanolic dispersions of
SiO2@PA, SiO2@PFPA, and SiO2@BPh NPs have a strong UV
absorbance at 270, 260, and 260 nm respectively (see ESI, S12
Scheme 2 Schematic representation of the covalent modification of
parylene C films using SiO2@PA NPs.
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the case of SiO2@PA and SiO2@PFPA NPs upon UV irradiation
singlet and triplet nitrene species are formed. These reactive
species can react with C–H bonds of the ethyl linker between the
rings in the PC backbone through both abstraction (triplet) and
insertion (singlet) reactions (Scheme 3a).10,16 Only abstraction
reaction leads to the formation of radicals on the parylene
backbone that can lead to chain reactions (cross-linkage and
bond cleavage). In contrast, the insertion reaction is a termina-
tion step. When SiO2@BPh NPs are used, biradicaloid triplet
excited species are formed upon UV irradiation. These triplet
species also react in hydrogen abstraction reactions to form
a ketyl radical and a radical from parylene (Scheme 3b).9,17,18
Such radicals on the parylene backbone enable the partial
penetration of photoreactive SiO2 NPs into the film via cross-
linkage and bond cleavage. Finally and after the particle pene-
tration, bonds can be formed between the radicals on the
parylene film and those on the NP photoreactive groups through
termination reactions (Scheme 3c).
The competition between the triplet and singlet states of the
nitrene species explains its lower reactivity compared to benzo-
phenone, since only the triplet state can lead to the partial
penetration of the particles into the PC film. The lowest reactivity
of the SiO2@PFPA NPs can be attributed both to the inductive
effect of the fluorine groups and to the bigger size of these
particles compared to the other two types.
Fig. 3 SEM images of (a) PC–SiO2@PA, (b) PC–SiO2@PFPA, (c) PC–
SiO2@BPh, and (d) PC–SiO2 (all scale bars are 5 mm).
Finally, in order to fully characterize the chemical and physical
nature of the SiO2 functionalized films, the following analyses
were performed: elemental composition of the surface of the
functionalized films was determined using X-ray photoelectron
spectroscopy (XPS) analysis (Table 2). For the three types of PC–
SiO2@photoreactive group films increase in the atomic concen-
tration of the Si and O elements was detected while the PC–SiO2
had quite similar composition to that of neat PC film. The small
amount of Si, detected in the case of PC–SiO2, is detected as
siloxane bonds and originates from silicon grease that was found
many times in neat parylene films. This contamination was
introduced to the films during the CVD process. In addition, the
detection of fluorine in the case of PC–SiO2@PFPA demon-
strates successful incorporation of SiO2@PFPA into PC films.
Contact angle measurements showed a drastic increase in the
wettability of the PC films upon functionalization. The hydro-
phobic PC became hydrophilic after the incorporation of the
hybrid photoreactive SiO2 NPs while bare SiO2 NPs caused only
a slight decrease in q confirming that no functionalization has
occurred. This slight decrease in q can be attributed to the partial
oxidation of PC during the irradiation due to leftovers of oxygen
as it was also detected by XPS (O 4.73%).
Fig. 4 AFM images of (a) PC–SiO2@PA, (b) PC–SiO2@PFPA, (c) PC–
SiO2@BPh, and (d) PC–SiO2 (all scale bars are 4 mm).
In order to demonstrate the potential of the silica functional-
ized PC films a second step functionalization was performed
using silane based chemistry. APTES was attached to peeled,
self-supported, PC films through formation of siloxane bonds.
The primary amine introduced during this reaction was labeled
with florescent dansyl chloride and detected with fluorescence
microscope (Fig. 6).
Fig. 5 FE-SEM image of PC–SiO2@PA (the analysis was performed
without gold conducting coating).
the bottom part of the particle. In contrast, when the irradiation
was performed for 2 h in the case of SiO2@PFPA NPs, some of
the particles could be moved. It is important to mention that the
highest concentration of particles was observed in the case of
SiO2@BPh NPs. These observations can be explained by the
different possible mechanisms of the photochemical reactions. In
It was already found that SiO2@PA and SiO2@BPh NPs are
more reactive than SiO2@PFPA, thus they were much more
incorporated into PC films. Consequently, a higher amount of
APTES was introduced to those films causing higher fluorescence
intensity in comparison with PC–SiO2@PFPA modified films
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 11511–11517 | 11515

Scheme 3 Proposed mechanisms for the photochemical reaction of SiO2@photoreactive group NPs with PC.
contrast to what was stated in the literature, SiO2@PA NPs are
Table 2 XPS results
Atomic concentration (%)
much more reactive than SiO2@PFPA ones when dealing with
the solid state photochemical reactions described above. More-
over, this is the first report of covalent immobilization of NPs
into biocompatible PC films to form hydrophilic and functional
Contact
angle
Sample
C
Cl
Si
O
F
PC
PC–SiO2
PC–SiO2@PA
PC–
85.84
83.73
75.40
76.26
13.57
10.46
10.74
10.71
0.08
1.08a
4.23
3.42
0.58
4.73
9.63
9.00
0.00
0.00
0.00
0.62
82ꢀ
74ꢀ
38ꢀ
40ꢀ
SiO2@PFPA
PC–SiO2@BPh
76.04
11.23
3.05
9.69
0.00
35ꢀ
a
Siloxane bond.
(see Fig. 6A and C in comparison with Fig. 6B). Fig. 6D shows
the fluorescence microscope image of PC–SiO2, and as it can be
seen no fluorescence is observed. This observation not only
proves the selective reaction of APTES with SiO2 NPs but also
provides additional evidence to the fact that only photoreactive
SiO2 NPs had reacted with PC.
Conclusions
In summary, two types of novel PA or BPh-based highly pho-
toreactive silica NPs were prepared and their reactivity was
compared to that of PFPA-based SiO2 NPs. The reactivity
evaluation was carried out by the reaction of the three types of
SiO2 NPs with highly inert PC films. It was found, that in
Fig. 6 Merged GFP and CFP filters fluorescence microscope images of
(A) PC–SiO2@PA, (B) PC–SiO2@PFPA, (C) PC–SiO2@BPh and (D)
PC–SiO2 films after APTES modification and incubation in dansyl
chloride.
11516 | J. Mater. Chem., 2011, 21, 11511–11517
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J. Mater. Chem., 2011, 21, 11511–11517 | 11517