Microwave-Assisted Synthesis and Physicochemical Characterization of Tetrafuranylporphyrin-Grafted Reduced-Graphene Oxide

Article abstract of DOI:10.1002/chem.201503887

This work describes the design of a modified porphyrin that bears four furan rings linked by 1,2-bis-(2-aminoethoxy)ethane spacers. This unit is a well-suited scaffold for a Diels-Alder reaction with commercial reduced-graphene oxide, which is also described in this paper. A new hybrid material is obtained, thanks to efficient grafting under microwave irradiation, and fully characterized in terms of structure (UV, TGA, Raman) and morphology (HR-TEM and AFM). Potential applications in photo- and sonodynamic therapy are envisaged.

Full text of DOI:10.1002/chem.201503887

DOI: 10.1002/chem.201503887
Communication
&
Hybrid Materials
Microwave-Assisted Synthesis and Physicochemical
Characterization of Tetrafuranylporphyrin-Grafted Reduced-
Graphene Oxide
[a]
[a]
[a]
[b]
[b, d]
Federica Bosca, Laura Orio, Silvia Tagliapietra, Ingrid Corazzari, Francesco Turci,
[a]
[c, d]
[a]
[a]
Katia Martina, Linda Pastero,
Giancarlo Cravotto, and Alessandro Barge*
derivatives. These systems are characterized by oxygenated
groups; rGO mainly bears these functional groups (OH, COOH)
on its borders, meaning that the aromatic character of the
Abstract: This work describes the design of a modified
porphyrin that bears four furan rings linked by 1,2-bis-(2-
aminoethoxy)ethane spacers. This unit is a well-suited
scaffold for a Diels–Alder reaction with commercial re-
duced-graphene oxide, which is also described in this
paper. A new hybrid material is obtained, thanks to effi-
cient grafting under microwave irradiation, and fully char-
acterized in terms of structure (UV, TGA, Raman) and mor-
phology (HR-TEM and AFM). Potential applications in
photo- and sonodynamic therapy are envisaged.
[
14–23]
planar carbon surface is almost completely maintained.
For these reasons, in this study, rGO has been selected as a suit-
able substrate to be grafted with ad hoc synthesized porphyrin
by a Diels–Alder reaction. The aim of this chemical decoration
is to realize a hybrid nanomaterial for potential application in
[
24]
photo- or sonodynamic therapy.
Indeed, both rGO planar
surfaces can be exploited to accommodate many chemical
structures and the residual functional groups offer the possibil-
[
25]
ity of linking other suitable molecules, thus improving their
ability to be suspended and providing active targeting. Fur-
thermore, the choice of rGO and this specific porphyrin conju-
gation may favour radical delocalization and stabilization. This
latter effect is particularly relevant when these hybrid nanoma-
terials are employed in photo- or sonodynamic therapies, as
these applications require a photo- or a sonosensitizing
Their unique structure confers to carbon-based nanomaterials
particular physical and chemical properties that make them
truly promising systems. Indeed, they have wide-ranging appli-
cations in various fields, such as engineering, biology, chemis-
[1–8]
[25–30]
try, physics, and medicine.
agent.
Graphene is a single-atom-thick layer of graphite, which is
prepared by a number of methods: mechanical, epitaxial, re-
duction of graphene oxide (GO), and solvent dispersion of
Porphyrins are the candidate of choice here as they can be
subjected to excitation by both light and ultrasound irradia-
[
24]
tion while the production of radicals that occurs as a conse-
quence of the relaxation phenomenon can induce cell
[
9]
graphite. It is a nanomaterial characterized by extended p-
conjugation that makes its manipulation difficult because of its
tendency to aggregate. However, this peculiar aromaticity pro-
vides a suitable surface for combining graphene with other
[
31,32]
death.
We designed the structure of compound 4 (Scheme 1) with
the aim of generating a porphyrin-rGO-based hybrid material
with potential applications in photo- and sonodynamic thera-
py. The idea here is that 4 can be linked to rGO by more than
one moiety, thus increasing the probability of the porphyrin
orientating itself coplanar with the rGO surface and maintain-
ing the original graphene sheet geometry (Figure 1). In the
same way, compound 4 could interpose itself between two
rGO foils giving interesting multilayer structures. We selected
the Diels–Alder reaction because, unlike 1,3-dipolar cycloaddi-
tion, it does not require the in situ formation of a reactive in-
termediate. This peculiar behaviour is particularly suitable for
simultaneous multipoint grafting. (Figure 1)
[
10,11]
substrates.
nanomaterials, that is, CNTs and fullerenes,
widespread attention for this reason. GO and reduced GO
Graphene, together with a number of other
[12]
has attracted
[
13]
(
rGO) are two carbon nanomaterials that are direct graphene
[
a] F. Bosca, Dr. L. Orio, Dr. S. Tagliapietra, Dr. K. Martina, Prof. G. Cravotto,
Dr. A. Barge
Department of Drug Science and Technology, University of Turin
Via Giuria 9, 10125 Turin (Italy)
E-mail: alessandro.barge@unito.it
[
b] Dr. I. Corazzari, Dr. F. Turci
Department of Chemistry University of Turin
Via Giuria 9, 10125 Turin (Italy)
Furan-modified porphyrin 4 was obtained from the carboxy-
benzyl monoprotection of 1,2-bis(2-aminoethoxy)ethane (1)
and then further conjugation to meso-tetra(4-carboxyphenyl)-
porphyrin to give product 2. The four amino groups were then
deprotected by hydrogenolysis to give 3 and the desired prod-
uct 4 was obtained following reductive amination with furfural.
The low solubility of 4 makes isolation very easy, while analyti-
cal characterization was a difficult affair.
[
c] Dr. L. Pastero
Department of Earth Sciences, University of Turin
Via Valperga Caluso 35, 10125 Turin (Italy)
[
d] Dr. F. Turci, Dr. L. Pastero
Interdepartmental Center “G. Scansetti”, University of Turin
Via P. Giuria 7, 10125 Turin (Italy)
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/chem.201503887.
Chem. Eur. J. 2016, 22, 1608 – 1613
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Scheme 1. Synthetic pathway for the furane modified porphyrin. a) benzyl chloroformiate, H
phyrin, EDC, DMAP, DCM; c) H , Pd/C, MeOH/dioxane; d) furfural, Na(AcO) BH, MeOH, pH 5.
2
O/EtOH/Dioxane, 08C, pH 7; b) meso tetra(4-carboxyphenyl)por-
2
3
The conditions for the Diels–Alder reaction between com-
mercial rGO (Sigma Aldrich) and furan-modified porphyrin
were optimized using compound 5, which can simulate the
arm of the final compound 4. The reaction was carried out
under microwave (MW) irradiation in a monomode reactor
shift of this maximum peak (Figure 2) is consistent with analo-
gous observations that have already been reported in the liter-
ature [25–30].
The thermal behaviour of 7 was studied by TGA under nitro-
gen in the temperature range of 150–8008C, and compared
with that of both the r-GO/(4) mixture and r-GO pristine
(Figure 3). In the temperature range of 150–2708C (region I), 7
exhibited a mass loss which was significantly higher than that
observed with both pristine rGO and the rGO/4. This process
was assigned, in part, to the thermal decomposition of residual
labile functionalities (e.g. OH, COOH) from rGO structure and,
in part, to the evolution of volatile contaminants derived from
the functionalization procedure.
(
Anton–Parr). Different reaction times, temperatures, and molar
ratios were explored. TGA analyses provided the optimal reac-
tion conditions to give 6 (Figure 1): 4 h, 1308C, and a 5/rGO
weight ratio of 1.2. MW irradiation strongly promoted the
[33]
Diels–Alder cycloaddition
and in spite of reactive groups
available for conjugation (carboxylic residues on rGO surface
and hindered secondary amine on 5) no side reactions could
be detected. Typically the direct MW-assisted amidation with
carboxylic acids and amines requires activators (e.g. HBTU,
HATU, PyBOP, carbodiimides, etc) or at least suitable solid cata-
In the temperature range of 271–8008C (region II), 7 and
rGO/4 mixture underwent a mass loss of approximately 29 and
20%, respectively. In both cases, this datum was unequivocally
ascribed to the thermal decomposition of 4 by comparison
with pristine rGO. Comparing the D-TG curves evidenced the
different nature of the interaction occurring between 4 and r-
GO in the samples 7 and r-GO/4, respectively. In particular the
[
34]
lysts (TiO2).
In accordance with the conditions mentioned above, rGO
was reacted with a DMF solution of 4 to give 7. The UV spec-
trum of 7 shows the characteristic Soret band, which indicates
the presence of a porphyrin on the graphene surface. The red-
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Communication
Figure 1. Synthetic scheme of rGO functionalization by a Diels–Alder reaction.
Figure 2. UV/Vis spectra of 4 (c), (7) (b) and pristine rGO (a) in DMF.
Figure 3. Upper panel: TGA analyses of pristine rGO (b); (7) obtained
after MW irradiation for 4 h, 1308C, of (4)/rGO=1.2 (w/w) (c); rGO/(4)
mixture of (d). The thermal behavior of carbon soot (g) is reported to
prove the inertness of the atmosphere inside the furnace during the heating
ramp. Lower panel: derivative TG curves of pristine rGO (b), rGO/(4) mix-
ture (d) and (7) (c).
process occurring at the higher temperature (minimum peak
at 5208C) with 7 evidenced the presence of stronger interac-
tions that were not observable with the simple mixture. This
result further supports the successful formation of covalent
bonds between porphyrin and rGO in 7.
Raman spectroscopy and high-resolution imaging tech-
niques (TEM and AFM) have been carried out to evaluate the
structural alterations that may possibly be induced on rGO
the carbon lattice (Figure 4Aa). The 2D and 2D’ overtones and
the D+G combination modes produce a large convoluted
[35]
À1
during the functionalization.
Graphene has two main
band at higher frequencies in the 2500–3500 cm region. An
Raman-active bands; the in-phase vibration of the graphite lat-
increase in the relative Raman intensity of the D and D’ bands,
as compared to the G band, is generally observed when the
disorder of graphite layers increases. A representative Raman
spectrum of pristine rGO is reported in Figure 4Aa. The signifi-
cantly high defect content, which is indicated by the high rela-
À1
tice (G band) at approximately 1580 cm and the so-called dis-
À1
order band (D band) at approximately 1350 cm . A D’ band at
À1
approximately 1600 cm , often overlapped with the G band, is
observed when a sufficient number of defects are present in
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Figure 4. A) Raman spectra of rGO and (7) (spectra a and b, respectively),
and B) evaluation of average graphene sheet defectiveness. All spectra were
recorded with an excitation laser wavelength of 523 nm. The position of the
G band peak is indicated by a vertical dashed line. The second-order Raman
lines are also visible.
Figure 5. Representative images of pristine rGO (upper row) and (7) (lower
row) as seen by TEM (A,C), and AFM (B,D). TEM images highlight the exfoliat-
ed flat morphology of the rGO sheets (black arrows) in which some denser
material (white arrows), due to wrinkles in the graphene sheet, occasionally
occurs. AFM images confirm the layered structure and the rhombohedral
shape of the rGO sheets (black arrows). In order to improve the readability,
amplitude images are reported. Some profiles are extracted to illustrate the
height distribution. The thinner structures detectable have a minimum thick-
ness of 2.5 nm (7 monolayers) in both cases. The thickness of the pile-up
structures generally decreases in the loaded sample with respect to the pris-
tine sample. Relative scale bar: 5 nm (TEM) and 1 mm (AFM).
tive intensity of the D band with respect to the G band, is con-
3
sistent with a high number of sp carbon atoms introduced
during the oxidative preparation of the material and only par-
tially reconverted to sp2 during the reduction step. The Raman
spectrum recorded for 7 (Figure 4Ab) is very similar to that of
pristine rGO. This indicates that the functionalization with 4
only slightly altered the rGO defect content. The average ID/IG
ratio, a convenient parameter to estimate the overall defective-
ness of graphitic materials, was calculated on several Raman
spectra collected with rGO and 7 (Figure 4B). The measured
ID/IG ratio was 1.27Æ0.04 and 1.09Æ0.04 for pristine rGO and
HRTEM images of pristine rGO and of 7 are reported in Fig-
ure 5A,C respectively. The structure of the pristine rGO exhibits
large exfoliated planes in which porphyrin grafting may likely
occur. The 2D carbon sheets are occasionally terminated by
wrinkles (white arrows) that indicate areas of higher material
atomic density (darker portions in the bright field TEM image)
and possibly contain a higher number of structural defects.
The AFM investigation of pristine rGO (Figure 5B) confirms
the largely exfoliated nature of the sample by displaying the
occurrence of several thin sheets (arrows) with well-defined ge-
ometry (dotted lines). The lateral extension of the aggregates
ranges between 0.25 and 1 mm in pristine rGO. The pile up of
graphene monolayers is thus confirmed by AFM measure-
ments; sheet thickness distribution (Figure S13, Supporting In-
formation) ranges from 2.5 to 300 nm (between 7 and 870
7
, respectively. The high ID/IG ratio (>1) measured for pristine
rGO may be explained by a large number of polyaromatic do-
mains with smaller overall size and the occurrence of patches
with a highly defective carbon lattice that were induced by the
[36]
oxidative processing of graphene. The slight decrease in ID/
IG ratio observed for the functionalized 7 sample clearly indi-
cates that the adopted synthetic approach followed during the
synthesis does not further modify the hybridization of C. The
observed decrease may be caused by the thermal annealing of
[37]
graphene defects assisted by MW irradiation.
A morphological analysis of rGO and 7 was carried out by
means of high-resolution microscopies, namely TEM and AFM.
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monolayers if considering a theoretical monolayer thickness of
(scan volume 50”50”5 mm). More instrumental detail and synthe-
ses of compounds 1–6 are reported in the Supporting Information.
0
.345 nm).
The nanoscale morphology of functionalized 7 remains
largely unchanged with regards to pristine rGO. The slight in-
crease in denser structures (dark areas) may account for an in-
crease in piled graphene sheet thickness upon conjugation.
This is compatible with the formation of intralayer cross-linking
bridges favoured by the presence of porphyrin. Graphene
monolayers of regular shape and unaltered borders are still
clearly visible (black arrows) both at TEM and AFM resolution.
The AFM analysis of the sample reveals the existence of thin
multilayered structures with a regular outline. The lateral width
of the structures in loaded samples is lower (0.5 mm) than in
the pristine form on average. At the same time, the thickness
of the multilayer stacks decreases, ranging from 2.5 to 95 nm
Synthesis of meso-tetrakis-[4-(furan-2yl-methyl-aminoe-
thoxy-ethoxyethyl-aminocarbonyl)phenyl]porphyrin-grafted
rGO (7)
Compound 4 (93 mg, 0,057 mmol) was dissolved in DMF (2 mL)
and the solution was put into a specific glass vial (10 mL volume)
for Anton–Parr MW reactor, together with a magnetic barr. rGO
(15 mg) was then added, the vial was sealed with its pressure re-
sistant cap and put in the MW cavity. The reaction mixture was
heated under magnetic stirring at 1308 for 4 h. After cooling, the
mixture was centrifuged and the solution separated from solid gra-
phene. The solid was washed several times with DMF and finally
with DCM; then it was dried at 808C in an oven. Grafted rGO 7
was characterized by TGA, Raman spectroscopy, TEM, and AFM
imaging.
(
7 to 275 theoretical monolayers). Despite the thickness value
trend apparently being the opposite of what was expected, it
can be explained by the increased stacking order that occurs
as a consequence of functionalization. The occurrence of well- Acknowledgements
defined geometries is higher in the functionalized sample than
in the pristine, meaning that the order in stacking increases is
due to the development of intralayer cross-linking bridges. On
the other hand, the pile up of layers that arises in the pristine
form derives from a chaotic superposition and folding of the
layers. The shape of a single-layer graphene sheet is preserved
This research was supported by MIUR (fondi ricerca locale
2013) and the Associazione Italiana per la Ricerca sul Cancro
(MFAG 2012, MFAG-13048).
Keywords: Diels–Alder reactions
·
graphene grafting
·
(
dotted lines in Figure 5D) and a more isometric shape is re-
microwaves · modified porphyrin · reduced graphene oxide
ported in the aggregates (ca. 500 nm).
In conclusion, we designed a four-arm star-shaped tetrafur-
anyl porphyrin, a platform well suited for an efficient multi-
point covalent grafting with rGO. The MW-assisted graphene
derivatization by a Diels–Alder reaction with the porphyrin
scaffold did not increase structural defects. The grafted materi-
al appears as a thin sheet with well-defined geometry, charac-
terized by some multilayer cross-linked bridges. This peculiar
connection forces the two structures in a “quasi” coplanar po-
sition. This is an interesting feature for photo- and sonodynam-
ic applications because of the easy delocalization of porphyrin-
induced radicals by p–p interactions on the whole graphene
surface. An increased radical half-life should also enhance the
interaction of this material with biomolecular targets, a condi-
tion required for further development in nanomedicine cancer
therapy.
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Received: September 28, 2015
Published online on January 7, 2016
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