Engineering of carbon based nanomaterials by ring-opening reactions of a reactive azlactone graphene platform

Article abstract of DOI:10.1039/c5cc00518c

A reactive azlactone-based graphene nanoplatform was successfully synthesized by the ligation of azido-azlactone with alkyne-terminated graphene via Cu(i)-catalyzed cycloaddition. The reactive azlactone rings, grafted on graphene sheets, were subjected to highly efficient ring-opening reactions with functionalized primary amine derivatives incorporating an aminosilane coupling agent or a biological fragment.

Full text of DOI:10.1039/c5cc00518c

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Engineering of carbon based nanomaterials by
ring-opening reactions of a reactive azlactone
graphene platform†
Cite this: Chem. Commun., 2015,
1, 4846
5
Received 19th January 2015,
Accepted 11th February 2015
a
a
b
b
c
de
a
G. Neri, A. Scala, F. Barreca, E. Fazio, P. G. Mineo, A. Mazzaglia, G. Grassi
a
and A. Piperno*
DOI: 10.1039/c5cc00518c
www.rsc.org/chemcomm
A reactive azlactone-based graphene nanoplatform was successfully Polaroid, Rohm and Haas GmbH; recently, 4,4-disubstituted
5
synthesized by the ligation of azido-azlactone with alkyne-terminated azlactones have been used as a way to modify polymer surfaces. In
graphene via Cu(I)-catalyzed cycloaddition. The reactive azlactone the presence of two substituents at the C4 position, the reactivity as a
rings, grafted on graphene sheets, were subjected to highly efficient nucleophile or azomethine ylide is forbidden and the lactone-based
ring-opening reactions with functionalized primary amine derivatives functional group displays only high reactivity toward nucleophilic
incorporating an aminosilane coupling agent or a biological fragment. agents, such as primary amines, alcohols and thiols by means of a
5
ring-opening addition reaction. Here, we describe an efficient and
In recent years, graphene and its multifunctional derivatives have versatile approach to introduce a pendant oxazolone functionality
attracted intensive research interest due to their unique properties on reduced graphene surfaces. The integration of the azlactone
1
and wide variety of promising applications. However, the existence of functionality into graphene is appealing because no activation or
high p–p stacking and van der Waals forces, due to attraction between pre-treatment is necessary for the post-functionalization of graphene
adjacent layers, facilitates irreversible aggregation of graphene or based materials, since they are more stable to hydrolysis than
even restacking to graphite which greatly limits the applications of activated esters, and no side reactions are expected during function-
graphene in several areas, such as engineering, fuel cell technology, alization. We envisage that the range of applications for which
catalysis, biomedical applications and drug delivery. An appropriate azlactone-functionalized graphene materials can be tailored is both
chemical functionalization of graphene addresses the above- broad and diverse, extending from the design of drug delivery
mentioned challenges tuning the structural, physical, chemical systems, to the fabrication of supports for the immobilization of
and electronic properties of graphene to suit specific applica- biomolecules with direct implications for the development of novel
2
tions. Azlactones (oxazol-5-(4H)-ones) are multifunctional com- biosensors in the context of nanoscale detection of pathogens
pounds that contain numerous reactive sites (the electrophilic and other metabolites related to issues of human health.
centres at C2 and C5; the nucleophilic site at C4; the azomethine Moreover, we believe that the reactive azlactone graphene plat-
3
ylide 1,3-dipole), allowing a wide variety of transformations.
form can be suited for the deposition of graphene films on the
Recently, they were employed as mesoionic azomethine ylides surfaces of amine functionalized polymers (Fig. 1).
in the functionalization of carbon nanomaterials such as full-
4
erenes and carbon nanotubes. The 4,4-dimethyl-5-oxazolone
was studied in industry for many years by groups at 3M,
a
Dipartimento di Scienze Chimiche, Universit a` di Messina,
V.le F. Stagno D’Alcontres 31, 98166, Messina, Italy. E-mail: apiperno@unime.it;
Fax: +39 090393897; Tel: +39 0906765173
Dipartimento di Fisica e di Scienze della Terra, Universit a` di Messina,
b
V.le F. Stagno d’Alcontres 31, 98166, Messina, Italy
c
Dipartimento di Scienze Chimiche, Universit a` di Catania, V.le A. Doria 6, 95125,
Catania, Italy
d
Consiglio Nazionale delle Ricerche CNR-ISMN c/o Dip. di Scienze Chimiche,
V.le F. Stagno D’Alcontres 31, 98166 Messina, Italy
e
College of Engineering, Mathematics and Physical Sciences, University of Exeter,
Exeter EX4 4QF, UK
†
Electronic supplementary information (ESI) available: Materials, experimental
details, XPS, STEM, Raman and FT-IR analyses. See DOI: 10.1039/c5cc00518c
Fig. 1 Schematic illustration of APDMA potentialities.
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graphene oxide sheets, are hydrophobic with limited aqueous
dispersibility and they easily form irreversible agglomerates or
they even restack to form graphite in aqueous solutions. The
treatment of graphene with diazonium compounds was demon-
strated to be a useful strategy to prevent the aggregation of graphene
sheets and to improve their dispersibility in high polar organic
6
media. Thus, the surfaces of G-red were functionalized by attaching
a p-(2-propylnyloxy)-benzene moiety using p-(2-propynyloxy)-
benzamine (5) and sodium nitrite under acid conditions
(Scheme 2).The stability of suspensions in water of G-red and
G-Alk was investigated; as expected a better dispersibility of
G-Alk was observed (ESI,† Fig. S3). Finally, the click reaction
between APDMA (4) and alkyne-terminated graphene (G-Alk),
7
using CuSO
4
/Na-ascorbate as the metal source, led to the
reactive azlactone graphene platform (RAGP) (Scheme 2).
Once the appropriate RAGP structure is obtained, it is important
to quantify the ring opening efficiency of azlactone in order to
proceed with further functionalizations of the nanoplatform. Thus,
the azlactone monomer (4) was treated with an equivalent of APTS in
Scheme 1 Schematic illustration of ring opening reactions on RAGP with
primary amines to give functionalized graphenes (GF-APTS and GF-GSSG).
A study of the optimal experimental conditions and the efficiency
of the ring-opening reactions was carried out by employing a mono-
mer azlactone unit and aminopropyltriethoxysilane (APTS) as nucleo-
philic agents without catalysts. The synthetic methodology was then
applied on the reactive azlactone graphene platform (RAGP) for the
grafting of primary amine derivatives incorporating an aminosilane
coupling agent (APTS) or a biological fragment (GSSG) (Scheme 1).
Our reactive azlactone graphene platform (RAGP) was built by
click scaffold assembly between two reactive structural fragments,
such as an alkyne and an azide. The graphene was functionalized
with an arm bearing a terminal alkyne moiety while the azide group
was introduced in the azlactone fragment.
1
toluene (ESI,† Fig. S4). FT-IR and H NMR analyses of the crude
reaction mixture confirmed the presence of the coupling product,
with no observable side reactions (ESI,† Fig. S5). The downfield shift
of resonance of amino methylene protons (ESI† peak c, Fig. S5) from
2
.45 ppm to 3.31 ppm, the disappearance of characteristic azlactone
À1
absorption at 1818 cm and the presence of the carbonyl amide
group signal at 1649 cm (ESI,† inset Fig. S5) indicate the formation
À1
of the coupling compound with 100% atom efficiency. Then, the
procedure was applied for the post functionalization of graphene
using APTS or GSSG (Scheme 1). The based and functionalized
graphene materials were characterized by several techniques includ-
ing TGA, TEM, XPS, FT-IR and Raman spectroscopy. Unambiguous
evidence of organic groups grafted on graphene sheets was provided
via thermal gravimetric analyses (TGA). G-red shows much higher
thermal stability with a constant mass loss up to 800 1C; the TGA
curve of G-Alk shows a minor thermal stability with no significant
change in profile (Fig. 2). A different behaviour can be observed in
the RAGP, GF-APTS, GF-GSSG profiles where the rapid weight loss at
about 500 1C can be attributed to the decomposition of the organic
component linked by the azlactone fragment.
The azlactone fragment APDMA (4) was prepared by sequen-
tial acylation–cyclodehydration involving a two-step procedure
(Scheme 2). The structures of the obtained compounds were
supported by spectral and analytical data; in particular the
FT-IR spectrum of APDMA shows the strong stretching bands at
À1
À1
À1
2
097 cm (vNQNQN), 1818 cm (vCQO), 1652 cm (vCQN)
À1
and 1200 cm (vC–O–C) (ESI,† Fig. S5). The graphene substrate
(
G-red) was obtained by chemical oxidation of graphite, direct
exfoliation of graphite oxide via ultra-sonication to graphene oxide
GO) and reduction with hydrazine under controlled conditions
ESI,† Fig. S1). The reduced-graphene sheets (G-red), unlike
(
(
Fig. 2 TGA profiles of G-red, G-Alk, RAGP, GF-GSSG, and GF-APTS,
Scheme 2 (a) Synthesis of 2-(4-azidomethyl)phenyl-4,4-dimethylazlactone
APDMA); (b) schematic illustration of RAGP preparation.
under a N
residue (%) and loading (%).
2
atmosphere. The table reports the values, at 500 1C, of weight
(
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Fig. 3 TEM (A–C) and STEM (D, F, H) images with linescans and corresponding total element percentages, detected by peculiar emission lines (for
colours see ESI,† Fig. S9–S11). (A, D) RAGP; (B, F) GF-APTS; (C, H) GF-GSSG. Inset of (A–C) SAED patterns of each sample area; (E) % of O in RAGP; (G) % of
Si in GF-APTS; (I) % of S in GF-GSSG.
The morphology of based and functionalized graphene high amount of Si while GF-GSSG exhibits a low sulphur content.
was investigated by TEM analyses (Fig. 3 and Fig. S10, ESI†). In the XPS spectra the C 1s bands were deconvolved using six
The TEM micrograph of GO showed that GO is partially folded contributions: a main contribution at 284.5 eV attributed to
due to the large area occupied; the images of G-red and G-Alk CQC/C–C in the aromatic ring and four other contributions at
showed homogeneous and quite smooth sheets, with little higher binding energies corresponding to carbon atoms bonded
aggregation (Fig. S10, ESI†). The TEM image of RAGP clearly to oxygen in different surface functionalities (C–OH, C–O–C,
shows a planar backbone conformation and transparency with CQO and OH–CQO) centred at 285.8, 286.6, 287.7, and 288.9 eV,
ripple lines between the layers. The presence of well-defined respectively (Fig. 4). The contribution at about 291.0 eV refers to
8
edges revealed a large number of thin sheets of graphene p–p bonds. As expected, the percentages corresponding to C–C
(Fig. 3). In the amine functionalized graphene materials there and carbon oxidized contributions are almost unchanged
are very noticeable layers of amorphous materials due to the from G-Alk to RAGP sample. On the other hand, the addition
APTES or GSSG residue bound on both surfaces of graphene of APTS induces an evident decrease in the degree of surface
sheets. Elemental mapping information was obtained by STEM carbon oxidation. This behaviour is directly related to the
analyses (Fig. 3 and ESI,† Fig. S11–S13). STEM analyses are symmetrical and shrinked Si 2p and O 1s lineshape features
in good agreement with XPS results which show that the (Fig. 4). Particularly, the position of the Si 2p band at 102.0 eV is
9
graphene nanomaterials have different C, O, Si and S contents, indicative of silicon–oxygen bonds in silanes. The GF-GSSG
as expected on the basis of the chosen functionalization (ESI,† sample is characterized by the presence of S–S bonds at 164.0 eV
Tables S1 and S2); in particular, GF-APTS exhibits a relatively and, to a lower extent, by oxidized sulphur bonds at 168.0 eV.
Fig. 4 (a–d) C 1s bands of XPS spectra of G-Alk, RAGP, GF-APTS, and GF-GSSG; (e–h) O 1s, N 1s, Si 2p and S 2p XPS lineshapes.
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This work was supported by MIUR [project PRIN 20109Z2XRJ_010].
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1
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