Synthesis of a fluorinated graphene oxide-silica nanohybrid: Improving oxygen affinity

Article abstract of DOI:10.1039/c6ra02585d

An easy method to achieve a fluorinated graphene oxide-silica nanohybrid (GOSF) is presented. Graphene oxide (GO) was synthesized by Hummer's modified method, the GO-silica nanohybrid (GOS) was obtained via Fischer esterification, the fluorinated moiety (

Full text of DOI:10.1039/c6ra02585d

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Synthesis of a Fluorinated Graphene Oxide-Silica Nanohybrid:
Improving Oxygen Affinity.
A. Maio,a D. Giallombardo,a R. Scaffaro,a A. Palumbo Piccionellob and I. Pibirib
Received 00th January 20xx,
Accepted 00th January 20xx
An easy method to achieve a fluorinated graphene oxide-silica nanohybrid (GOSF) is presented. Graphene oxide (GO) was
synthesized by Hummers modified method, the GO-silica nanohybrid (GOS) was obtained via Fischer esterification, the
fluorinated moiety (3-pentadecafluoroheptyl,5-perfluorophenyl-1,2,4-oxadiazole) was introduced by nucleophilic
substitution operated by the hydroxyl functionalities onto the GOS surface. Full characterization of the new materials
confirmed the formation of covalent bonds between the graphene oxide /silica hybrid matrix and the fluorinated moieties.
The proposed methodology offers an easy way to get fluorinated carbon/silica hybrid nanomaterials avoiding the harsh
reaction conditions usually involved in the preparation of fluorinated materials, and allowing to selectively immobilize
specific fluorotails. Moreover, performed oxygen uptake and release kinetics showed that the introduction of fluorinated
moieties increases the oxygen exchange, making the material interesting for perspective applications in biomedical field,
as oxygen delivery system, as filler for biocompatible materials, and in the preparation of membranes for purification of
water.
DOI: 10.1039/x0xx00000x
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their differences, all the revisions to former models agree on
considering GO as a family of closely related materials, or a
composite material itself, which composition, dependent on
Introduction
Graphene oxide (GO) is one of the most extensively studied
forms of functionalized graphene as it is readily available by
exfoliating oxidized graphite,1 and it is the most common
intermediate in the synthesis of reduced graphene.1-4
The coexistence of aromatic and non-aromatic domains in a
double honeycomb composed by both sp2 and sp3 hybridized
carbon atoms, as well as the presence of a variegated
assortment of functional moieties, defects, holes and cavities,
contribute to the attainment of unique and intriguing
properties from a chemical-physical point of view.5
The versatility of GO makes it particularly suitable for the
synthesis of graphene-based materials, owing to the possibility
to further functionalize GO lamellae with a large variety of
compounds. In fact, one can either covalently attach specific
moieties onto GO by exploiting its highly reactive oxygen
moieties (e.g. carboxyl, epoxy and hydroxyl) or physically
immobilize planar drugs via π-π stacking interactions, thus
involving graphenic aromatic domains.1-5
Nevertheless, some issues still must be solved. Recent studies
on GO structural modelling focused on its metastability, thus
suggesting that GO has not to be considered as a static
compound with a given set of functional moieties.6-7 Despite
several conditions used within synthesis and purification steps,
evolves over time.8-9
However, GO-based materials currently find applications in
many areas, ranging from solar cells, resonators, electronic
devices, supercapacitors, catalyst supports, electronic
components to biotechnology, including drug delivery, and
biosensing.10-15
In this context, GO-silica nanohybrids are currently used in
many fields, ranging from optoelectronics to medicine,7-9
because of the potentialities for electrical applications, their
chemical inertia and cytocompatibility.
On the other side growing interest of the materials community
has been posed on the functionalization of GO with fluorine.
For instance, fluorinated GO has been proven to be the first
carbon material for Magnetic Resonance Imaging without the
addition of magnetic nanoparticles,16 moreover, it has proven
to absorb NIR-laser energy and efficiently transform it into
heat, thus making promises to be used as a contrast agent for
MRI, ultrasound and photoacoustic imaging, and also as
targetable drug carrier and NIR laser inducible hyperthermic
material that can ablate thermosensitive cancer cells. 17-18
The introduction of fluorine on the basal planes of graphene
has been realized by several methodologies,19 such as F
plasma, or exposure to F2 or XeF2 at high temperature.
Fluorinated graphene has also been obtained by exfoliation
method from graphite fluoride. However, plasma and high
temperature can damage the material and are not
recommended. Electrochemical processes have been also
a. Department of Civil, Aerospace, Environmental, Materials Engineering, University
of Palermo, Viale delle Scienze – Parco d'Orleans II, Ed. 6, 90128, Palermo, Italy
b. Department of Biological, Chemical and Pharmaceutical Sciences and
Technologies, University of Palermo, Viale delle Scienze – Parco d'Orleans II, Ed.
17, 90128, Palermo, Italy
c. telephone: +39 091 238 63723; email: roberto.scaffaro@unipa.it
d.
telephone: +39 091 238 97545; email: ivana.pibiri@unipa.it
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reported for the synthesis of graphene oxyfluoride, or the use Flash Silica gel (200-400 mesh), light petroleum (fraction
View Article Online
DOI: 10.1039/C6RA02585D
boiling in the range of 40–60 °C) and ethyl acetate were
of extremely corrosive reagent hydrogen fluoride.
A possible alternative to the direct introduction of fluorine is purchased by Merck.
the possibility to introduce perfluorinated moieties, in this Dialysis was performed by Spectra/Por® Standard RC tubing;
case, besides the vantage of mild conditions, the added value MWCO 12-14 kDa.
is the peculiar ability to dissolve large amounts of All the reactants were used as received without any further
physiologically important gases, such as oxygen and carbon purification.
dioxide. Functional studies have shown that perfluorinated
molecules improve O2 solubility of approximately 50 times, in
Synthetic procedures
fact they are known as blood substitutes.20
Synthesis of GO: GO was synthesized by oxidation of neat
Furthermore, the direct fluorination of graphene oxide can
graphite, according to a Hummers modified method.2 Briefly,
lead to a wide distribution of fluorinated moieties, due to the
the graphite powder and KMnO4 were premixed in the solid
aforementioned presence of a variegated family of oxygen
state and then added to a mixture of H2SO4 and H3PO4, the
reaction was stopped with H2O2 30 wt. % in H2O, and ice. For
the work up, several centrifugations were carried out with HCl,
water and C2H5OH. Other details can be found elsewhere
groups in GO, which, as obvious, are prone to react in different
ways. Using a GO decorated with silica nanoparticles, instead,
can be a successful strategy where the selective attachment of
a specific type of fluorinated moieties is preferred.
2,31,32
.
Aim of this work was to explore a new way for the introduction
of perfluoroalkyl chains onto the surface of a GO-Silica
nanohybrid material (GOS) and study the interesting features
of the so-obtained new material, in this specific case by
analyzing its oxygen affinity.
Synthesis of GOS: For the synthesis of GOS nanohybrids, GO
(0.12 g) and silica nanoparticles (0.12 g) were dispersed in
water (100 mL) with 2% HCOOH, sonicated at 50 °C for 1 hour
and then transferred to a Teflon-coated crystallizer, where the
dispersion was magnetic stirred at 80 °C for 3 hours, thereafter
the temperature was increased up to 120 °C. Of course, as long
as the solvent was present, the temperature remained
constant at 100 °C, after about 25 minutes the total
evaporation of water occurred and the gel-like slurry was
maintained at 120 °C for 35 minutes. Finally, a brown film was
peeled off, dispersed in THF under ultra-sonication treatment,
thoroughly washed and dialyzed for 24 h in order to remove
the silica not covalently bonded. Characterization details are
reported elsewhere.15
Synthesis of 3-pentadecafluoroheptyl-5-pentafluorophenyl-
1,2,4-oxadiazole (1): the synthesis has been achieved as
previously reported33-36 by mixing pentadecafluoroheptyl
amidoxime (1.28 g, 3 mmol), pyridine (0.26 g, 3.3 mmol) and
pentafluorobenzoyl chloride (0.76 g, 3.3 mmol), stirring in
toluene (30 mL) at 0 °C for 10 h. Evaporation of the solvent
gave a residue that was treated with water and refluxed for 2
Both graphene based materials21 and silica based
nanohybrids22 are frequently reported as biocompatible
materials. The idea to combine the intriguing features of these
two materials, furthermore introducing perfluorinated
moieties in order to maximize the ability of the material to
uptake and release oxygen, would greatly increase the interest
towards this novel nanohybrid in the biocompatible materials
field, since it is known that oxygen is essential for cellular
metabolism and organ functioning and the lack of sufficient
oxygen at the cellular level causes severe damage to cells and
tissues.23
In this context, the introduction of perfluorinated 1,2,4-
oxadiazoles derivatives is interesting, due to their wide
application in synthesis24-28 and in medicinal chemistry,26-28and
also as advanced materials.29
Their biocompatibility and inertness, associated to the ability
to dissolve a larger amount of oxygen make perfluoroalkylated
oxadiazoles very interesting moieties to be introduced in
materials for biomedical and eco-compatible applications.
h.
Chromatography
of
the
residue
gave
3-
pentadecafluoroheptyl-5-pentafluorophenyl-1,2,4-oxadiazole
(1) (1.47 g, 81%): mp, 37–38 °C (from Petroleum). 19F NMR
(CDCl3) δ: 79.68 (t, 3F, J = 13.0 Hz), 113.54 (t, 2F, J = 12.7 Hz),
121.63 (s, 2F), 122.29 (s, 4F), 122.97 to 123.0 (m, 2F), 126.37 to
126.42 (m, 2F), 133.28 to 133.45 (m, 2F), 142.97 (tt, 1F, J = 6.5
and 21.1 Hz), 158.32 to 158.54 (m, 2F). GC–MS m/z: 604 (M+ ,
100%). Anal. Calcd. for C15F20N2O: C, 29.82; N, 4.64; found C,
29.75; N, 4.58.
Synthesis of GOSF: A suspension of GOS (0.080 g) in absolute
DMF (6 mL) was sonicated for 10 minutes. Potassium tert-
butoxide (0.312 g) was added, and the reaction mixture was
stirred at room temperature for fifteen minutes. Compound 1
(1.440 g) was added in one portion, and the resulting
suspension was stirred at room temperature overnight (24
hours). Ethyl acetate (30 mL) was added to dilute the excess of
fluorinated reagent, and the solution was centrifuged for ten
minutes at 4000 rpm. The solid (65.6 mg) was recovered by
Experimental
Materials. Neat graphite (grade Ma 399, 45 μm) was
purchased by NGS Naturgraphit (Germany), fumed silica
nanoparticles, Aerosil® OX50, were provided by Evonik
Industries (Germany).
Pentadecafluoroheptyl amidoxime has been prepared as
reported.30 Pentafluorobenzoyl chloride, potassium tert-
butoxide, H3PO4, H2SO4, KMnO4, HCl, H2O2, toluene, pyridine,
dimethylformamide (DMF), tetrahydrofuran (THF), C2H5OH and
HCOOH were purchased by Sigma Aldrich.
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filtration and purified by repeated rinsing and filtration with argon atmosphere before imaging to avoid electrostatic
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DOI: 10.1039/C6RA02585D
further organic solvent.
discharge during the tests.
The surface composition was studied with a Phenom ProX,
PhenomWorld equipped with EDX probe to detect the
presence of C, O, Si, N and F.
Characterization
Melting point for
1 was determined on a REICHART-
1
XPS investigations were carried out by using a PHI 5000
Versaprobe-II system. The spectrometer was calibrated by
assuming the binding energy (BE) of the Au 4f7/2 line to be
84.0 eV with respect to the Fermi level. Both extended spectra
(survey - 187.85 eV pass energy, 0.5 eV∙step-1, 0.05 s∙step-1)
and detailed spectra (for C 1s and O 1s - 11.75 eV pass energy,
0.05 eV∙step-1, 0.2 s∙step-1) were collected with a standard Al
Kα source working at 300 W. The standard deviation in the BE
values of the XPS line is 0.10 eV. The atomic percentage, after
a Shirley-type background subtraction, was evaluated by using
the PHI sensitivity factors. The peak positions were corrected
for eventual charging effects by considering the C 1s peak at
284.8 eV and evaluating the BE differences.39 The curve fitting
was carried out by means of an iterative least square
procedure making use of Voight functions on Shirley-type
background.15,32,39
Oxygen solubility measurements were performed, as
previously reported,35-36 by using a digital oxymeter, with a
Schott Gerade 120 mm probe having a membrane with an
exterior Teflon layer.
Measures were recorded by placing the electrode tip into the
bulk phase at 80 mm distance from the air–liquid interface.
Data from oxygen saturated aqueous suspensions (20 ml)
containing GOS and GOSF at 0.5 mg mL-1, were taken at
atmospheric pressure. In particular, each suspension was
initially stirred with a magnetic stir bar while pure oxygen was
continuously bubbled. The temperature of each suspension
was adjusted to 25 °C by using a thermostated oil bath. Once
THERMOVAR hot-stage apparatus. H NMR spectra in solution
were recorded on a BRUKER AC 250 E spectrometer and were
taken with TMS as an internal standard. 19F NMR spectra in
solution were recorded on a BRUKER Avance 300 (282.3 MHz)
spectrometer and were taken with CFCl3 as internal standard
(b before the multiplicity stands for broad signal). GC/MS
determinations were carried out on a GC-MS Shimadzu QP-
2010.
Flash chromatography was performed by using silica gel
(Merck, 0.040–0.063 mesh) and mixtures of light petroleum
(fraction boiling in the range of 40–60 °C) and ethyl acetate in
various ratios.
FTIR analysis for solid samples was carried out by using a
Perkin-Elmer FT-IR/NIR Spectrum 400 spectrophotometer. The
spectra were recorded in the range 4000-400 cm-1.
Micro-Raman spectroscopy was performed by means of a
Renishaw InVia instrument, with diode laser excitation at 633
nm and spectral resolution 1 cm-1. Measurements in at least
five different sample positions have been repeated for each
treatment. Analysis of the spectra has been done after
subtraction of a linear baseline for the samples GOS and
GOSF37-39, and subsequent normalization of each curve to the
G band.39
13C Cross Polarization Magic Angle Spinning Nuclear Magnetic
Resonance (13C {1H} CP-MAS NMR) spectra have been acquired
at room temperature by a Bruker Avance II 400 MHz (9.4 T)
spectrometer, operating at 100.63 MHz for 13C.‡ All
experiments have been performed with a MAS spinning rate of
8 kHz, 1024 scans a contact time of 0.6 ms, a delay time of 10 s
the suspension reached
a
stable maximum oxygen
1
concentration (saturated solution), bubbling was stopped and
the release of dissolved oxygen was determined by evaluating
the change in the oxygen solubility (desaturation) as a function
of time.
Experiments have been conducted in triplicate and the statistic
analysis according to T-student test allowed to calculate P<
6*10-4.
and an excitation pulse on the H nuclide of 4.5 μs. A standard
sample of adamantane has been used as an external reference
and all samples were placed in a 4 mm zirconia rotors sealed
with KEL-F caps.
13C Direct Polarization (DP) solid state spectra with a MAS
spinning rate of 8 kHz, 1024 scans, a delay time of 10 s and an
excitation pulse on the 13C nuclide of 4.5 μs. A standard sample
of adamantane has been used as an external reference and all
samples were placed in a 4 mm zirconia rotors sealed with KEL-
F caps.
Results and discussion
GO was synthesized by oxidizing neat graphite by a quite
common method2 and decorated with silica nanoparticles via a
rapid and eco-friendly hydrothermal treatment.15
19F Direct Polarization (DP) solid state spectra were acquired
under fast MAS conditions by a spinning rate of 25 kHz.
Spectra were acquired with an excitation pulse on the 19F
nuclide of 4.0 μs, a delay time of 3 s and 8 scans.
As a result, the most reactive sites of GO, such as epoxy rings
and carboxyls, were involved in reactions with silanol groups of
silica, thus allowing the achievement of graphene oxide
nanoplatforms equipped with hydroxyls and silica
nanoparticles. These latter ones, bearing silanols were
selectively exploited to anchor perfluorinated 1,2,4-
oxadiazoles via a facile aromatic nucleophilic substitution
(SNA). The remaining –OH moieties of GO, located in those
zones not covered by silica, were capable to operate the
The morphology of nanohybrids was observed by scanning
electron microscopy, carried out in an ESEM FEI QUANTA 200
microscope on both nanoparticles and nanostructured films. In
this latter case, the samples were cryo-fractured in liquid
nitrogen to investigate the morphology of both longitudinal
and cross section. The samples were attached on an aluminum
stub using an adhesive carbon tape and then sputter coated
with gold (Sputtering Scancoat Six, Edwards) for 90 s under
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covalent binding of
pathway followed is schematized in Figure 1.
1
via the same SNA mechanism. The Magnetic Resonance (13C {1H} MAS NMR and 19F MAS NMR)
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DOI: 10.1039/C6RA02585D
and XPS analysis.
Several characterization techniques were used to demonstrate FTIR spectra of GOS and GOSF are provided in Figure 2,
the effective grafting of fluorotails onto GOS matrix and to together with those of GO and for sake of comparison.
investigate the structure-properties relationships of the Compound shows absorption peaks related to C-F bond
1
1
nanohybrid herein synthesized: more in detail, the arising from perfluoroalkyl chain and perfluorophenyl ring at
characterization of the materials has been performed by FTIR, 1200 cm-1, 1140 cm-1 and 970 cm-1 whereas the double peak
Raman Spectroscopy, SEM-EDX, Magic Angle Spinning Nuclear located at 1570 cm-1 and 1650 cm-1 could be ascribed to C=N
bond of oxadiazole and C=C of phenyl ring.
Figure 1. Route to GOSF: GO was synthesized via the modified Hummers method,13 and silica-GO (GOS) composite was prepared by Fischer esterification in water
and 2% of HCOOH at 120 °C between the –COOH and epoxy groups of GO and the –OH moieties of the silica. The functionalization with fluorinated tails has been
realized by aromatic nucleophilic substitution. In particular, a 3-pentadecafluoroheptyl, 5-perfluorophenyl-1,2,4-oxadiazole (1), has been introduced very easily by
nucleophilic substitution operated by the oxygenated functionalities present on the surface of the GO-silica nanohybrid
FTIR spectrum of GO is consistent with literature data1-6,15,31,40
.
phenols, carboxyls and water.1-2 Two well recognizable bands
Although the end-groups refer substantially to –OH, C-O-C and are centred at around 1720 cm-1 and 1624 cm-1, with the
–COOH, the presence of a highly aromatic and variegated former being undoubtedly assigned to carbonyls while the
environment surrounding oxygen moieties determines the latter presumably originating by bending modes of water
coexistence of phenols, epoxy, ethers, lactols, lactones, molecules integrated into GO structure or by C=C of graphenic
aldehydes, ketones, aromatic acids, and many other moieties, sub-lattice.
besides water molecules adsorbed40. Due to these features, At wavenumbers below 1250 cm-1 a bunch of overlapping
GO was found to absorb practically within the entire spectral signals was proposed to be due to C-O-C and C-OH features,
range, especially at lower wavenumbers.40
under the form of either epoxy, cyclic ethers and organic
Nevertheless, the broad absorption band in the range 3750- alcohols. As one can see, the bands assigned to –OH moieties
3200 cm-1 is traditionally assigned to –OH stretching due to (located at 1000 cm-1 and 3000-3600 cm-1) decrease after the
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15
grafting of nanosilica, as well as those attributable to carboxyl overlapping of Si-O-Si, C-O-C and Si-O-C bonds (see blue-
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and epoxy groups, whereas in the range 1200-900 cm-1 it is filled zone in the spectrum), while the peak centred at 1014
DOI: 10.1039/C6RA02585D
possible to detect a variegated absorption band with two main cm-1 can be reasonably assigned to silanols.15
peaks. The one located at 1080 cm-1 can be attributable to the
Figure 2. FTIR spectra of 1 (green), GOS (blue), GOSF (red).
Even in this case (see violet-filled zone), a shoulder can be due binding with 1, instead, -OH signals were found to dramatically
to residual –OH moieties of graphene oxide. The prominent decrease, as well as those referring to silanols. The most
peak at around 800 cm-1 could be ascribed to the formation of prominent band is centred at around 1100 cm-1 and could be
graphenic silicates (-COO-Si) owing to the reaction between ascribed to the formation of Si-O-C bonds due to the grafting
carboxyls of GO and silanols of silica. After SNA covalent of
1
to silica, however the variegated feature and the
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asymmetric shape of the band suggest that signals arising from the oxygen groups of GO either disappear (due VtieowiAtrsticpleaOrntliinael
DOI: 10.1039/C6RA02585D
reduction) or are involved in chemical bonds with silica and 1.
Si-O-Si of the silica, Si-O-C of GO-g-SiO2 and GO-g-
1
,
overlapped. Moreover, even some characteristic absorption Indeed, hydrothermal treatment carried out to prepare GOS,
peaks of C-F fall within the same range, thus making hard and as well as the subsequent synthetic step in alkaline
somehow arbitrary to distinguish each contribution. Within environment, are known to partially restore the graphenic
the range 1700-1550 cm-1 one can observe that even the structure of the GO, moreover in the presence of oxadiazole,
signals coming from GO (1640 cm-1) and those arising from lactones and epoxy groups of GO could react thus further
oxadiazole moiety of
1 combine, thus leading to the altering the graphenic structure. A closer inspection of the
insurgence of another variegated band. Three emergent graphenic skeleton of the ternary nanohybrid can be
bands, located at 1250 cm-1, 790 cm-1 and 690 cm-1, are performed by Raman spectroscopy, particularly suitable when
presumed to respectively indicate the formation of –C-O-aryl, the complexity of the system makes extremely difficult to
aryl-O-aryl bonds, due to the covalent attachment of
graphenic rings, and Si-O-aryl bonds arising from the covalent sensitivity to graphene crystals domains and defects.
attachment of to silica. Based on FTIR results, we can only Figure 3a reports the micro-Raman spectra of GO, GOS and
1 to identify carbon-based nanomaterials, owing to its high
1
assess that –OH moieties decrease after SNA and that most of GOSF.
Figure 3. a) micro-Raman spectra of GO, GOSF and GOSF; b) representative multi-peak fitting of a GO spectrum within the wavenumber range 1000-2000 cm-1 (first-
order scattering) for the determination of A, D, G and B modes and the successive calculation of AD/AG and ID/IG. c) Magnification of micro-Raman plots collected
within the range 2000-3200 cm-1 (second-order scattering).
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All the samples exhibited the main characteristic peaks of GOS by nucleophilic substitution, in fully agreement with the
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DOI: 10.1039/C6RA02585D
graphenic materials: the D band (close to the K point in the results of FTIR analysis.
Brillouin zone, arising from the doubly resonant disorder- The effective presence of the fluorinated moieties onto the
induced mode)40-41 is centred at around 1345-1350 cm-1, the sidewalls of GO-silica was evaluated by using NMR equipped
G-mode (at point Γ, originating from in-plane vibration of sp2 with a fluorine-probe. Solid state 19F NMR spectra of GOSF and
carbon atoms)40-41 is located at 1582-1583 cm-1, the modes
1
, in Figure 4b, show the presence of fluorinated moieties in
resulting from their combination and overtone (2D, D+G) are GOSF nanohybrid, in the same chemical shift range observed
centred at around 2700-2900 cm-1.40 Furthermore, two less in the 19F NMR spectrum of compound
1.
intense Raman modes, namely A and B, are detected at 1100 Notably, the band centered at -157 ppm, assigned to F-C (aryl),
cm-1 and 1700-1780 cm-1, and respectively assigned to oxygen- decreases after the functionalization, thus confirming the
bonded carbons (either alcohols or ethers) and highly effective covalent attachment of
oxygenated non-regular rings, either to 5-8-5 rings resulting mechanism.
from a C di-vacancy or to 5-7-7-5 Stone Wales defects.32 The
pictorial representation of these contributions is provided in
Figure 3b, together with an exemplificative multi-peak fitting
of a GO sample. A closer inspection of the results put into
evidence that the samples display no relevant differences
within the spectral region related to first-order Raman
scattering (i.e. the range 1000-2000 cm-1).
1 onto the GOS via a SNA
a
The D/G intensity ratio (ID/IG), which gives information about
the size of sp2 graphenic domains,15,31,32,40-42, was found to be
practically constant (for GOS) or slightly increased (from 1.12
to 1.15 for GOSF) after functionalization step, whereas the
ratio between the integrated areas of D and G (AD/AG) was
equal to 2 for GO, 1.91 for GOS and 1.94 for GOSF. These
features can be explained by considering that the size of
graphenic crystals remains practically unaltered, since the
immobilization of silica and fluorotails onto GO planes involves
only oxygen moieties, as suggested by the depletion of B mode
detected for GOSF and especially GOS, with respect to that of
pristine GO.
b
The most relevant changes are detectable in 2D band (see
Figure 3c), which was found increased after fluorination. Since
2D/G ratio gives information about the exfoliation degree of
graphene oxide,15,37 one can hypothesize that compound
1 is
able to intercalate between stacked lamellae, thus increasing
the interlayer distance as a result of the formation of aryl
ethers.
Solid state 13C NMR spectra of GO, GOS and GOSF are plotted
in Figure 4a
The GOSF spectrum collected in cross polarization C-H
evidenced the presence of carboxylic functionalities at about
160 ppm and C-O at about 70 ppm, comparable to the GO
signals reported in the same Figure. sp2 carbons related to the
graphenic matrix are not present in the Cross Polarization
spectrum because of the high distance between sp2 carbons
and the protons. Direct polarization experiment evidenced the
presence of sp2 carbons as shown in the spectra reported in
Figure 4a. The presence of silica nanoparticles covalently
linked to the GO lamellae, did not allow to register for GOS a
C-MAS spectrum via cross polarization, the spectrum reported
was obtained in direct polarization on 13C. It shows the
presence of a very intense signal located at 110 ppm, related
to the C sp2 resonance, and a broad signal located at 59 and 71
ppm, attributed to C-C-OH and epoxy. This feature is
drastically reduced after functionalization (compare GOS to
Figure 4. a) 13C {1H} MAS NMR spectra of GO (black), GOS (blue) and GOSF (red)
registered in direct (DP) and/or cross polarization(CP) (dotted line); b) 19F MAS
NMR spectra of GOSF (red) and 1 (green).
The morphology of nanohybrids was analyzed by SEM.
Figure 5 reports the micrographs of a GO lamella before (top
left) and after (top right) silica-decoration, whereas the
structure of the lasagna-like self-assembled films is provided
at different magnifications in the underlying micrographs.
Figure 6 shows the morphology of a GOSF micro-device,
constituted by GO lamellae with a discrete distribution of silica
nanoparticles. The detailed micrograph of a zone scarcely
decorated by silica (bottom left) clearly shows that the
graphenic skeleton of GOSF retains the same morphology as
GOSF spectra), thus suggesting that
1 is covalently bonded to
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pristine GO, thus confirming Raman results previously
discussed. The morphology of nanoparticles attached onto
graphenic platforms is provided in the bottom right corner of
Figure 6.
View Article Online
DOI: 10.1039/C6RA02585D
The surface analysis of the nanohybrid, carried out by EDX and
XPS, is provided in Figure 7 a-c.
EDX mapping was conducted in two different zones of GOSF,
characterized by different degree of silica decoration, with the
aim to confirm that fluorotails have been successfully attached
onto both graphene oxide and silica. Figure 7a shows the
elemental mapping of C, Si, N, O, F elements (together with
the combined map) performed in
a
zone densely
functionalized by silica. The results remark the homogeneous
distribution of fluorine and nitrogen, peculiar elements of
compound
1. Figure 7b reports the mapping of the same
elements in a zone scarcely decorated by silica. As one can see,
fluorine and nitrogen are well recognizable even in the
graphenic zones not covered by silica, thus proving that
1 is
bonded even to graphene oxide basal planes, although these
latter regions display the same morphology as pristine GO.
Finally, XPS analysis was performed in order to evaluate the
degree of surface covering after functionalization.
The high resolution XPS spectra collected for GOSF are
reported in Figure 8, together with those of GO and GOS for
sake of comparison.
Figure 5. SEM micrographs of a GO lamella before (top left) and after (top right)
silica-decoration, morphology of lasagna-like self-assembled films at different
magnification (bottom)
Owing to the surface immobilization of 1, both O1s and Si2sp
signals were found to decrease in GOSF with respect to GOS.
The C1s spectrum of GOSF is significantly different from those
of GO and GOS, since it is affected by the remarkable presence
of 1. In fact, after fluorination, C-O and C-O-O contributions
were found to decrease, whereas the peaks associated to C-F
binding (and obviously not present in GO and GOS) are visible
within the range 290-300 eV.
Moreover, it is possible to detect in GOSF the signals of F1s
and N1s, thus further confirming the presence of fluorinated
moieties. More in detail, F1s XPS spectrum of GOSF shows an
asymmetric, strong band located at 688 eV, due to organic
fluorine, i.e. linked to alkyl and phenyl carbon atoms. The less
intense N1s signal is centred at 399-400 eV and due to
oxadiazole ring, The elemental analysis carried out by XPS is
provided in Table 1. According to the results, each molecule of
1
should have a F/N ratio=9.5. After SNA, F/N ratio in GOSF is a
bit lower than in (F/N ratio=9.31), thus confirming the
mechanism proposed. Moreover, the analysis allowed to
evaluate the degree of surface functionalization (X).
1
Figure 6. SEM analysis of GOSF SEM analysis of
micrographs of its hierarchical structure at different magnifications, and a detailed
picture of a fluorinated zone not covered by silica (bottom left).
a GOSF device with detailed
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DOI: 10.1039/C6RA02585D
Figure 7. Surface analysis of GOSF.(a-b): EDX mapping performed in two different zones of GOSF characterized by higher (a), and lower content of silica (b); the scale
bar for the mapped zone of panel (a) and (b) are respectively 3 µm and 30 µm. (c): XPS results of GOSF with C1s and O1s spectra of GO and GOS, and Si2p spectrum of
GOS for comparison.
signals seems to suggests that
graphene and silica.
1 is covalently bonded to both
Table 1. XPS Elemental analysis
In order to obtain some information about the influence of
prepared materials on O2 solubility, oxygen uptake and release
kinetics were performed on oxygen saturated aqueous
solutions containing GOS or GOSF (0.5 mg mL-1) in suspension
at atmospheric pressure by means of a previously reported
saturation method,33-36 at 25°C and at 0.5 mg mL-1. Oxygen
solubility was determined after saturation of the medium and
monitored as a function of time, at atmospheric pressure.
Accordingly to previously reported in vivo experiments,42
saturation curves were fitted by Equation 3, while
desaturation curves were approximated, by Equation 4.
Sample
GO
Silica
GOS
GOSF
1
C (%)
60
3
54.3
60
40.5
O (%)
40
59
37.8
22.3
2.7
N (%)
-
-
Si (%)
-
38
7.9
1.2
-
F (%)
-
-
-
-
1.6
5.4
14.9
51.4
According to Equation 1 and Equation 2 one can express it by
taking into account either F or N atomic concentration.
(
(
1
2
)
)
(
(
3
4
)
)
where:
[O2]∞ = oxygen solubility at t∞;
[O2]load = [O2]0-[O2]∞ where [O2]0 = oxygen solubility at t0;
k = clearance constant (sec).
Where the subscripts F and N stand for the element – fluorine
and nitrogen, respectively – atomic percentage used to
measure the extent of functionalization.
XF (29%) and XN (29.6%) were found substantially coincident.
Based on these findings, the fluorinated moiety attached onto
the surface was around 30% and the decrease of Si and O
The data obtained are reported in Table 2 and in Figure 8.
Data from Table 2 highlight that fluoro-functionalization
increases the dissolved oxygen content at saturation. Notably,
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for GOSF we observe a faster O2 uptake than unfluorinated
GOS and a slower O2 release during desaturation.
Conclusions
View Article Online
In summary, the synthesis of a novel naDnOohI:y10b.r1i0d39m/Ca6tReAr0ia2l58h5aDs
been easily realized by combination of graphene oxide, silica
and perfluorinated heterocyclic moieties.
Moreover, from these data we can observe a higher saturation
value and clearance constant K for GOSF, if compared to
fluorinated polymers35 and perfluorinated surfactants, such as
We adopted an eco-friendly pathway to decorate GO lamellae
with silica nanoparticles, aiming at improving the stability and
biocompatibility of GO and maximizing the –OH content, thus
allowing the selective conjugation of GOS with fluorotails via a
SNA mechanism.
36
structures analogue to 1 and potassium perfluorooctanoate
,
recentely reported in the literature for biomedical
applications.
Data collected confirm the hypothesis of covalent bonding
between the hybrid matrix and the fluorinated moieties.
With respect to the other fluorination routes, exploiting
carboxyl and/or epoxy groups, the method here proposed
allows achieving a significant functionalization degree, since –
OH end groups are more abundant than –COOH in the GO and,
of course, in a GO-silica matrix. Furthermore, this pathway
allows preserving the structure of the perfluorinated moieties,
thus permitting to selectively introduce specific molecules in
order to design the desired final properties. In this case we
have chosen the oxygen affinity, but we could follow an
analogue route to attach different tails for different
applications. Finally, the fluorinated GO-silica herein
synthesized shows a remarkably high oxygen affinity, double-
folded with respect to other fluorinated systems recently
reported in literature for the same purposes33-36, due to the
synergistic effect of silica nanoparticles and perfluorinated
oxadiazole tails. Observed differences, in terms of k values,
suggest the constitution of a fluorinated phase, thus enhancing
the oxygen exchange.
Table 2. Parameters of eq. 1 and 2 determined for desaturation or saturation
curves as a function of time (calculated P < 6x 10-4)
.
Substrate
Saturation
parameters
GOS
GOSF
a)
[O2]∞
K b)
[O2]0
[O2]∞
K b)
40.0±0.3
-39.6±1.2
43.34±0.01
9.49±0.01
1070.4±1.0
45.72±0.02
-27.8±0.8
47.67±0.01
9.41±0.01
1396.6±0.6
a)
Desaturation
parameters
a)
a)expressed in ppm, b) expressed in seconds
Moreover, the presence of graphene oxide leads to the
achievement of free-standing, lightweight thin films, thus
allowing to prepare devices that can be easily re-collected and
recycled after being used.
The success of the present preliminary study promises to use
these novel nanohybrid thin films as oxygen reservoirs for a
wide range of applications, concerning biomedical devices,
such as oxygen delivery systems, water treatment membranes,
fillers for functional nanocomposites.
Acknowledgements
We acknowledge Prof S. Agnello for valuable discussion, Dr. B.
Megna for the micro-Raman tests and Dr. M. Scopelliti for the
XPS analysis.
Funding Sources
University of Palermo, FFR 2012–2013 ATE 0291 “Synthesis
and characterization of organic salts as functional ionic
phases”.
HIPPOCRATES - PON02_00355
Notes and references
‡NMR experimental data were provided by Centro
GrandiApparecchiature-UniNetLab-University of Palermo funded
by P.O.R. Sicilia 2000–2006, Misura 3.15 Quota Regionale.
Figure 8. Oxygen uptake (top) and release (bottom) curves from aqueous solutions
containing composites at 25°C
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TOC
Synthesis of a Fluorinated Graphene Oxide-Silica Nanohybrid:
Improving Oxygen Affinity.
A. Maio, D. Giallombardo, R. Scaffaro* , A. Palumbo Piccionello, I. Pibiri*
An easy method to selectively introduce specific fluorotails into a graphene oxide-silica
nanohybrid. Fluoro-functionalization increases the oxygen exchange at saturation.