A novel SWCNT platform bearing DOTA and β-cyclodextrin units. "one shot" multidecoration under microwave irradiation

Article abstract of DOI:10.1039/c4ob00611a

The functionalization of single-walled carbon nanotubes (SWCNTs) via microwave-assisted grafting reactions enables efficient multidecoration in a single step. A novel water-soluble SWCNT platform was prepared via the simple 1,3-dipolar cycloaddition of azomethine ylides under dielectric heating. Thanks to a single grafting reaction the CNT surface binds in a 1:1 ratio an amino acidic β-cyclodextrin (β-CD) derivative and the DOTAMA moiety (1,4,7,10-tetraazacyclododecane-N,N′,N′′,N′′ ′-tetraacetic acid monoamide). This novel "one shot" synthesis, compared with multistep functionalizations, preserves the SWCNT's structural integrity (TEM images). Besides thermogravimetric analyses, the determination of the amount of β-CD and DOTA moieties grafting onto the SWCNT's surface was performed on the basis of phenolphthalein and gadolinium complexation, respectively. This journal is

Full text of DOI:10.1039/c4ob00611a

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A novel SWCNT platform bearing DOTA and
β-cyclodextrin units. “One shot” multidecoration
under microwave irradiation†
Cite this: Org. Biomol. Chem., 2014,
12, 4708
E. Calcio Gaudino,a S. Tagliapietra,a K. Martina,a A. Barge,a M. Lolli,a E. Terreno,b
D. Lemboc and G. Cravotto*a
The functionalization of single-walled carbon nanotubes (SWCNTs) via microwave-assisted grafting reac-
tions enables efficient multidecoration in a single step. A novel water-soluble SWCNT platform was
prepared via the simple 1,3-dipolar cycloaddition of azomethine ylides under dielectric heating. Thanks to
a single grafting reaction the CNT surface binds in a 1 : 1 ratio an amino acidic β-cyclodextrin (β-CD)
derivative and the DOTAMA moiety (1,4,7,10-tetraazacyclododecane-N,N’,N’’,N’’’-tetraacetic acid mono-
amide). This novel “one shot” synthesis, compared with multistep functionalizations, preserves the
SWCNT’s structural integrity (TEM images). Besides thermogravimetric analyses, the determination of the
amount of β-CD and DOTA moieties grafting onto the SWCNT’s surface was performed on the basis of
phenolphthalein and gadolinium complexation, respectively.
Received 22nd March 2014,
Accepted 6th May 2014
DOI: 10.1039/c4ob00611a
www.rsc.org/obc
the tube, or otherwise using covalent functionalization onto
Introduction
the conjugated skeleton via various reactions.11 As CNTs have
The use of carbon nanotubes (CNTs) as multi-functional nano- strong aromatic character, it is possible to adsorb other
materials has attracted much attention from both academics aromatic molecules onto the sidewalls using stacking inter-
and industry thanks to their intriguing electronic,1 optical2 actions. Non-covalent derivatization is potentially reversible
and mechanical properties.3 Their exclusive morphology and and fully preserves the unique electronic properties of the
structure make them suitable candidates for numerous appli- material and avoids sidewall damage. While being counted
cations ranging from nanoelectronics and sensorics4 to biome- among suitable procedures for covalent bond formation, cyclo-
dical applications,5 first of all drug delivery6 and imaging.7 addition reactions also represent an effective method for pre-
The main drawback of CNTs is their insolubility in aqueous paring functionalized CNT based materials. They generally
media and organic solvents, resulting in their aggregation into occur between two unsaturated entities and give a cyclic
micrometer-sized bundles. This is caused by the presence of product in an atom-economic manner. They do not require
van der Waals inter-tube forces.8 Therefore, significant efforts any extra reagents and catalysts, which often make these pro-
have been made in the last few years to find appropriate cesses expensive and complicated.12 They include a wide range
methodologies that solubilise pristine CNTs via chemical of reactions, such as the 1,3-dipolar cycloaddition of azo-
functionalization,9 making them biocompatible and possibly methine ylides,13 CuI-catalyzed azide–alkyne cycloaddition
biodegradable.10
reactions,14 [4 + 2] Diels–Alder reactions,15 [2 + 1] cycloaddition
In general, CNT functionalization can be achieved using reactions,16 and other types of cycloadditions.17
two main approaches: non-covalently via supramolecular
Besides their strength and toughness, CNTs have shown
adsorption or by wrapping different functional molecules onto excellent electrical and thermal conductivity.18 Extensive
investigations into CNT behaviour under microwave (MW)
irradiation have paved the way for important applications in
aircraft stealth technology and other military equipment.
aDipartimento di Scienza e Tecnologia del Farmaco, and Interdepartmental Centre
“Nanostructured Interfaces and Surfaces” – NIS, University of Turin, Via P. Giuria 9,
Dielectric heating takes advantage of the strong MW absorp-
tion exhibited by CNTs,19 and has been extensively applied in
their functionalization via cycloaddition reactions. This
enabling technology ensures efficient heat transfer through
the reaction mixture, enhancing functionalization yields and
rates.20 An example of direct CNT MW functionalization using
10125 Torino, Italy. E-mail: giancarlo.cravotto@unito.it
bDipartimento di Biotecnologie Molecolari e Scienze per la salute, University of
Turin, Via Nizza 52, 10125 Torino, Italy
cDipartimento di Scienze cliniche e Biologiche, University of Turin, Regione Gonzole
10, 10043 Orbassano (TO), Italy
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4ob00611a
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epoxide groups via 1,3-dipolar cycloaddition has previously toxicity. Covalent functionalization, as opposed to non-
been reported by our group.21 This functionalization gave covalent, ensures higher CNT-conjugate stability and better
stable CNT dispersions in a range of polar solvents, including drug release control. So far, covalent CNT functionalization
water and ethanol. As has already been mentioned, soluble or has been introduced via sequential or orthogonal synthetic
dispersible CNTs are currently being explored for use in drug protocols. Triple functionalization has also been reported in
delivery, therapeutic and diagnostic applications. Native and the literature.28 Multi-functionalization generally involves the
functionalized cyclodextrins (CDs) have recently emerged in use of protective groups whose sequential removal may induce
this research field as multifunctional supramolecular agents alterations on the CNT surface. A fundamental requirement
capable of dispersing CNTs via covalent or non-covalent inter- for the rational application of CNT derivatives in biomedical
actions. However, there are many disadvantages associated investigations and nanomedicine is to precisely know the
with these methods, such as time consuming synthetic pro- overall loading of organic moieties bound on the surface and
cedures, poor reproducibility, low immobilization mass of the their relative ratio. This is particularly true in the case of
CDs and structural damage of the CNTs. These disadvantages orthogonal functionalization. These two parameters are funda-
can lead to the loss of their electronic conductivity and mental features of CNT’s use as a diagnostic probe and drug
corrosion resistance.22
carrier.
We therefore introduce a “one shot” double orthogonal
The advantage of tying a CD structure to a CNT surface can
be found in the fact that its cavity remains available for the for- covalent functionalization of SWCNT, promoted by MW
mation of host–guest complexes with organic molecules in the irradiation.
aqueous solution. CD-CNT adducts have been investigated for
Our synthetic strategy is based on the 1,3-dipolar cyclo-
drug delivery, biosensoring and analytical applications invol- addition of in situ generated azomethine ylides which include
ving electrochemistry.23 Moreover, single-walled carbon nano- both a β-CD unit and a DOTAMA tris(t-butyl ester) moiety
tube (SWCNT) gadolinium complexes, called gadonanotubes, (Fig. 1).
have been used as magnetic labels for cellular MRI.24
The first step in the synthetic pathway involves the double
In this article, we introduce a new hybrid SWCNT platform protection of the tyrosine amino and carboxylic groups as
that binds β-CD units as well as DOTAMA (1,4,7,10-tetraaza- t-butyl ester and t-butyl carbamate (Scheme 1). Protected
cyclododecane-N,N′,N″,N′′′-tetraacetic acid monoamide). The derivative 1 was O-alkylated with propargyl bromide in the
latter, a well known macrocycle ligand, enables the formation presence of Cs2CO3. The propargylated phenol derivative 2 was
of a stable Gd(III) complex which is used in MRI applications. then deprotected under acidic conditions to give the tyrosine
This work aims to design a new multifunctional SWCNT plat- derivative 2a. A MW-assisted copper-catalyzed azide–alkyne
form by exploiting the excellent hosting properties of β-CDs cycloaddition (CuAAC)30 was used to obtain product 3 in good
and the complexation ability of DOTAMA derivatives.
yield (80%, Scheme 1) from the reaction of alkyne 2a with
We have developed a fast methodology which is based on a 6-monoazido-6-deoxy-β-CD (β-CD-N3), which was prepared
MW-assisted 1,3-dipolar cycloaddition used for covalent according to the literature.31 The DOTAMA aldehyde derivative
SWCNT functionalization to achieve SWCNT multimodality. 4 was obtained via a Swern oxidation following the reported
This “one shot” procedure enables the SWCNT surface to be procedure.25
controllably functionalized with two different molecules of
Solutions of the β-CD adduct 3 and the DOTAMA derivative
interest: an amino acidic β-CD derivative and a DOTAMA- 4 (molar ratio = 1 : 2.5) were added to a suspension of SWCNTs
aldehyde derivative.25 The MW-assisted synthetic route of in DMF. The mixture was sonicated for a few minutes in a
β-CD-DOTAMA-SWCNT nanocomposites is depicted in cleaning bath to ensure good CNT dispersion. The mixture was
Scheme 2. The SWCNT hybrid structure’s capacity for suspen- irradiated with MW at 100 °C (Scheme 2) for 2 h and the
sion in polar solvents, including water, is dramatically cooled suspension was then centrifuged.
increased by its high level of surface functionalization. This
The solid residue, consisting of the functionalized SWCNT
platform is thus a promising candidate for use as a drug 5, was washed with THF and acetone and dried under vacuum.
carrier, a MRI contrast agent26 and a theranostic system for
cancer imaging and therapy.27 The toxicity of the above men-
tioned modified SWCNTs was evaluated against five human
cell lines and it was shown that they do not affect cell viability,
even at high doses.
Results and discussion
CNT properties can be modulated by surface functionalization,
depending on the nature and spatial location of functional
groups. The main requirements for CNT-based platform’s use
Fig. 1 Azomethine ylides generated in situ via MW-assisted
as a molecular carrier are solubility, binding versatility and low decarboxylation.29
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Fig. 2 (a) Stability after 15 days at room temp. of a 5 mg mL−1 water dis-
persion (2 min sonication) of adducts 5 and 9. (b) TEM images of func-
tionalized SWCNTs 5.
out under a nitrogen atmosphere. However, the differences in
combustion profiles recorded under air for pristine and modi-
fied nanotubes 5 allow the qualitative evaluation of the
SWCNT surface functionalization to be carried out. Therefore,
the TGA of the adduct 5 showed a weight loss of about 40% at
350 °C, which could correspond to one functional group per
Scheme 1 Synthesis of the amino acidic β-CD derivative (3) by MW-
assisted CuAAC.
The ability of 5 to form a suspension is dramatically 250 SWCNT carbon atoms.
improved compared to pristine SWCNTs. Its suspension in
CNT surface functionalization degree can be accurately elu-
DMF, MeOH or water is stable for several days (Fig. 2a). The cidated via quantitative determination of the β-CD and
morphology of the nanocomposite SWCNT adduct 5 was DOTAMA moieties grafted onto the carbon nanotube. In order
analyzed by TEM and the images show that functionalization to accurately determine the content of β-CD in the SWCNT
does not alter the SWCNT structure (Fig. 2b).
adduct 5, the complexation of phenolphthalein (Php), by β-CD
The functionalization degree of SWCNTs was first evaluated inclusion, was measured by means of UV-vis spectroscopy. At
by TGA measurements via a comparison with pristine SWCNTs pH > 8.4, Php is in a conjugated quinoid form, is of a deeper
(Fig. 4). Heating was carried out under air. TGA analysis can colour (intense purple) than the benzenoid form and shows
provide semi-quantitative information only when it is carried strong absorption at a wavelength of 553 nm. β-CD can alter
Scheme 2 “One shot” double covalent functionalization of SWCNT, promoted by MW irradiation.
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Scheme 3 Php complexation by β-CD inclusion.
this coloration because Php delocalization is disturbed by the
lactonization of the ionized form upon complexation. As a con-
sequence, the absorbance changes and the pink colour
disappears (Scheme 3).32 The formation of colourless 1 : 1
complexes in alkaline aqueous solution can thus be used for
the quantitative determination of β-CD content in polymeric
materials.33 In fact, a change in absorbance can be quantified
by UV-vis spectroscopy, where the characteristic Php
peak, close to 553 nm, decreases with additional amounts of
β-CD.
Fig. 3 β-CD quantitative determination by phenolphthalein (Php)
inclusion, measured by UV-vis spectroscopy at 553 nm.
The two approaches used to estimate Gd and β-CD content
gave very similar results, indicating that both these methods
can be used to evaluate surface functionalization.
The adduct 5 was dispersed in a Php solution at pH 10.5,
with the aim of quantifying the amount of β-CD grafted onto
the SWCNTs, and the change in Php absorbance was recorded
as UV spectra. As depicted in Fig. 3, Php solution absorbance
at 553 nm decreased significantly in the presence of the deriva-
tive 5. By monitoring the absorbance after 2, 5, 10, 15, 30 min
and 1 h, we observed the saturation point approximately after
15 min. β-CDs grafted onto SWCNTs have been quantified,
and the results were compared with the calibration curve
obtained with free β-CD. The absorbance of the calibration
solutions of β-CD was measured at a wavelength of 553 nm at
room temperature (Fig. 3). The CD content was 95.7 μmol g−1
With the aim of demonstrating that the “one shot” pro-
cedure was the method of choice in terms of yield and
functionalization degree, we compared this protocol with the
sequential synthetic strategy depicted in Scheme 4. SWCNTs
were functionalized via a MW assisted 1,3-dipolar cyclo-
addition using tyrosine and octanal in DMF to obtain the
adduct 6 (Scheme 4), followed by the propargylation of the
phenolic –OH group in the presence of Cs2CO3. The product 7
was analyzed by TGA which showed a covalently bound organic
moiety loading of 1/110 SWCNT carbon atoms. The sequential
click reaction strategy via CuAAC with the 6-monoazido-β-CD
gave the adduct 8 and a second coupling with the azido-
DOTAMA tris(t-butyl ester) derivative34 gave the final product
9. The TGA profile of the latter showed a lower weight loss at
350 °C (20%) than that of the “one shot” obtained compound
5 (Fig. 4). These first qualitative data indicate the lower surface
functionalization degree (about one functional group per 600
carbon atoms of the SWCNT) achieved by this second synthetic
procedure, which causes lower combustion ability of 9 com-
pared to 5. With the aim of quantifying the amount of mole-
cules grafted onto 9, the previously described quantification
methods of β-CD and DOTAMA derivatives were repeated.
The analysis showed slightly lower grafted β-CD content
(12%, referred to the whole portion grafted) compared to the
“one shot” method. The total amount of the second step
grafted DOTAMA derivative was even worse (only 0.8%, referred
to the whole portion grafted), with a β-CD/DOTA ratio of about
15/1.
for the SWCNT derivative 5, which corresponds to
a
functionalization of about 16% by weight (referred to the
whole molecule grafted onto the surface). The inclusion
capacity of SWCNT bound β-CD was roughly the same as the
native β-CD.33a
The amount of DOTAMA derivatives bound to the SWCNT
surface was determined by Gd(III) complexation followed by
acid mineralization and ICP-MS analysis. For this purpose,
adducts 5 were treated with trifluoroacetic acid for ester clea-
vage and the Gd(III) complex was then synthesized via the
addition of a GdCl3 water solution (pH = 5.5) to the depro-
tected solid 5. The suspension was shaken for 12 h and then
the solution was separated from the solid. The solid was
washed three times with water and three times with an EDTA
solution to remove any unreacted Gd(III) ions, and then again
with water.
The dried Gd-SWCNTs were mineralized in nitric acid at
120 °C, and Gd content was determined by ICP-MS measure-
ments. The ICP-MS data indicate a functionalization degree
attributable to a weight increment of about 13% (referred to
the whole molecule grafted) which is comparable to that
obtained by β-CD determination. The ratio between DOTAMA
and β-CD was determined by the reaction mechanism and
is 1 : 1.
Despite the fact that MW irradiation allows high levels of
covalent SWCNT functionalization to occur, prolonged
thermal treatment may affect the CNT surface and damage its
structure due to MW superheating effects on SWCNTs.35 TEM
analysis of the adduct 9 (Fig. 5) highlights the rupture of a
SWCNT tip. This should be compared to the analysis of the
derivative 5 which was obtained using the “one shot” pro-
cedure (Fig. 2).
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Scheme 4 Sequential SWCNT functionalization via MW assisted 1,3-dipolar cycloaddition.
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Fig. 4 Heating rate 10 °C min−1 from room temperature to 800 °C.
Fig. 5 TEM image of functionalized SWCNT 9.
Fig. 6 Effect of SWCNT adducts 5 (white bars) and 9 (black bars) on the viability of A549 (panel A), Huh7 (panel B), Hep2 (panel C), Helf (panel D)
and HeLa (panel E) cell lines, as a function of concentration. The results are expressed as a percentage, by comparing treated cells with cells incu-
bated with culture medium alone. Each point represents the mean S.D. (n = 3).
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Assessment of cell viability
Non-complexed Php was separated from the functionalized
SWCNT derivatives by filtration over a cellulose acetate mem-
brane (0.2 μm) syringe filter (CPS Analitica, Italy).
The effect of SWCNT adducts 5 and 9 on cell viability was eval-
uated on five human cell lines of different histological origin,
namely A549, Huh7, Hep2, Helf and HeLa.
Thermogravimetric analyses were performed using
a
thermogravimetric analyzer TGA 4000 (Perkin Elmer) at 10 °C
min−1 operating under air with alumina crucibles containing
10–20 mg of SWCNT derivatives. Total mass loss was attribu-
ted to functional groups which were covalently attached to the
sidewalls. The number of functional groups was calculated
considering a fixed thermogram temperature, the residual
weight of functionalized SWCNTs (WR), its mass loss (WL) and
the molecular weight of the organic fragment (MF) on its
sidewalls = (%WR/12)/(%WL/MF).
The cells were treated with increasing concentrations of
SWCNT adducts ranging from 0.45 µg mL−1 to 1000 µg mL−1
and cell viability was monitored after 24 hours. The results
shown in Fig. 6 indicate that the two SWCNT adducts did not
affect cell viability at concentrations up to 333 µg mL−1 in all
cell lines tested. A moderate reduction in cell viability was
caused by SWCNT adducts 5 and 9 in Hep2 and by the
SWCNT adduct 5 in Helf at the highest concentration tested.
A “CellTiter 96 Proliferation Assay” (Promega, Madison, WI,
USA) kit for cell viability assays was used according to the
manufacturer’s instructions. Absorbances were measured at
490 nm using a microplate reader (model 680, BIORAD).
Conclusion
We have herein described a “one shot” synthetic protocol
which covalently functionalizes SWCNTs with numerous active
moieties via the MW-assisted 1,3-dipolar cycloaddition of azo-
methine ylides. Besides thermogravimetric analyses, the deter-
mination of the amount of β-CD and DOTA moieties grafted
onto the SWCNT surface was performed on the basis of the
respective complexation of phenolphthalein and gadolinium.
The new polyvalent non-toxic SWCNT platform will be investi-
gated as a multi-carrier for potential diagnostic and theranos-
tic applications.
Acknowledgements
This work was funded by the Compagnia di San Paolo –
University of Turin (Progetti di Ricerca di Ateneo 2011, grant
D15E11001710003).
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Experimental section
Materials and methods
All chemicals were purchased from Alfa-Aesar, Italy and used
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