The antioxidant activity of the biohybrides based on carboxylated/hydroxylated carbon nanotubes-flavonoid compounds

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Abstract
This study reveal the relationship between SWCNTs (SWCNTs, carboxylated respective, hydroxylated) with a series of flavonoids (quercetin, rutin) to design new bionanohybrids with higher antioxidant capacity. The chemiluminescence assay proved that the carbon nanotubes have antioxidant activity as electron donor or as proton donor (SWCNTs -hydroxylated and carboxylated) in the redox transition reaction with free radicals. The bi-ohybrids "flavonoids-carbon nanotubes" shown a high antioxidant activity, evaluated by the amplification factor, ranging from 3 up to 4 times higher than single flavonoids. These results open the perspective to design functional carriers based on carbon nanotubes to transport low soluble antioxidants to specific target with improved scavenging rate against the free radicals (reactive oxygen species)
Digest Journal of Nanomaterials and Biostructures Vol. 8, No. 1, January - March 2013, p. 445 - 455
THE ANTIOXIDANT ACTIVITY OF THE BIOHYBRIDES BASED ON
CARBOXYLATED / HYDROXYLATED CARBON NANOTUBES-FLAVONOID COM-
POUNDS
C. NICHITAa,b, I. STAMATINa*,
aUniversity of Bucharest, Faculty of Physics, 3Nano-SAE Research Centre, MG-
38, Bucharest-Magurele, Romania
bNational Institute for Chemical-Pharmaceutical Research and Development, 112
Vitan Street, 031299, Bucharest, Romania
This study reveal the relationship between SWCNTs (SWCNTs, carboxylated respective,
hydroxylated) with a series of flavonoids (quercetin, rutin) to design new bionanohybrids
with higher antioxidant capacity. The chemiluminescence assay proved that the carbon
nanotubes have antioxidant activity as electron donor or as proton donor (SWCNTs -
hydroxylated and carboxylated) in the redox transition reaction with free radicals. The bi-
ohybrids “flavonoids- carbon nanotubes” shown a high antioxidant activity, evaluated by
the amplification factor, ranging from 3 up to 4 times higher than single flavonoids. These
results open the perspective to design functional carriers based on carbon nanotubes to
transport low soluble antioxidants to specific target with improved scavenging rate against
the free radicals (reactive oxygen species)
(Received February 11, 2013; Accepted March 4, 2013)
Keywords: Carbon nanotubes, Flavonoids, Antioxidant activity, Chemiluminescence
assay, Free radicals
1. Introduction
The carbon nanomaterials once discovered they are seen as potential support in biomedi-
cine, therapeutics, food, drug delivery, sensors. The oldest application of the carbon materials in
medicine is the active carbon (medicinal charcoal), known by the ancient civilizations as absorbent
against acidity or as food supplement. Each nanocarbon, after discovering, was used in bio-
applications with hope to find solution to improve health, probes for the materials investigation or
biocompatible nanodevices. So, since the discovery of fullerenes or 'buckyballs' (C60) in 1985 [1]
and the carbon nanotubes (CNT, 1991) [2], a large class of applications were envisaged and their
principles are demonstrated in biomedicine and pharmacology [3,4,5]. The uniqueness comes from
its geometry and structure: single- or multi-rolled graphene (polyaromatic with large delocalized
-electron systems) reaching few nanometers in diameters and length up to a few micrometers.
They could have unusual toxicological properties even though the data obtained in vitro systems
are not conclusive [6,7,8]. For systems in vivo some acute inflammatory pulmonary effects in ro-
dents were observed [9]. In the drug delivery, CNTs are used as carrier loaded with suitable drugs
via functional groups. In this respect, new controlled drug delivery systems have devised such as
delivery of therapeutic agents to the desired site, enhancing bioavailability and drug protection to
name few in the nanotherapeutic area [10, 11, 12]. On the other hand new nanoscale materials
have been investigated for drug delivery applications including: nanoparticles, nanotubes, nano-
fibers, dendrimers, liposomes, polymer micelles, nanogels, nanocrystals, viral vectors, and virus-
like particles [13,14,15 ]. Despite of fast advances with nanomaterials in medicine, their toxicolo-
*Corresponding author: office@3nanosae.org
446
gy remains a challenge to further developments in the evaluation of the adverse health effects and
the environmental damage [16,17] or to improve the pharmacological profiles [5a, 12a, 18]. In re-
thinking the nanotherapeutics with nanoparticles a special field is killing of the reactive oxygen
species (ROS) using antioxidants to reduce oxidative stress against human body. The human body
is exposed to an increasing oxidative stress, one of the harmful key issues in health care where ex
cess of ROS (chemical species with unpaired electrons on the molecular orbitals, well-known free
radicals) generated in various pathogenic processes are recognized as an indicator in citotoxiciy
and cellular disorder [19]. At the intracellular level, ROS are balancing by the biochemical antiox
idants such as Glutathione [20]. During the inflammatory processes from endogenous/ exogenous
factors takes place overproduction and releasing of intracellular ROS with depletion in antioxi
dants. The supplements are taken from vegetables, which have a large complex of antioxidants, to
day used in the food processing or as nutraceuticals (commodities derived from food, used in the
medicinal form of pills, capsules, potions and liquids with physiological benefits) [21]. To increase
the antioxidant potential delivered to a target need carriers with molecular recognition and this
could be the carbon nanostructures [12 22]. Earlier studies with fullerene shown a strong antioxi
dant activity [23] and this has been exploited in nanocosmetics for the skin care and dermatology
[24]. Moving us toward carbon nanotubes it is possible to conceive new advanced antioxidants in
combination with well-known natural polyphenols compounds which themselves have various an
tioxidant properties [25]. In the polyphenols class the flavonoids hold a privileged position. The
pharmacological studies have proved that flavonoids possess many beneficial effects on the hu
man health, including cardiovascular protection, anticancer activity, antiulcer effects, anti-allergic,
antiviral, and anti-inflammatory properties [26]. In addition the flavonoids are important compo
nents in the human diet, although they are generally considered as non-nutrients. Sources of flavor
noids are foods, beverages, different herbal drugs, and related phytomedicine [27]. Besides flavor
noids exhibit various biological activities, however, most interest has been devoted to the antioxi-
dant activity, which is due to their ability to scavenge free radicals involved in most of diseases
[25a]. The important structure-activity relationships of the antioxidant activity and the capacity of
flavonoids to act as antioxidants in vitro, have been established and related to the number of hy-
droxyl groups in their molecular structures [28].Regarding the free radicals inhibiting or direct in-
teraction with DNA, enzymes and membrane receptors and hence the antioxidant efficacy of fla-
vonoids in vivo, are aspects less documented, presumably because of the limited knowledge on the
bioavailability and mechanisms of action in human body [29, 30]. Based on these pros and cons
arguments in this study of the antioxidant activity related to the oxidative stress the opportunity to
a deeper understanding via carbon nanotubes properties can be a good challenge. In a general
scheme of the antioxidant activity deals with redox transitions from free radicals (R) to the antiox-
idant molecules (AH) by electron or hydrogen (H++ e-) donating. SWCNTs, hydroxylated and car-
boxylated can perform these redox transitions by both mechanisms (scheme1 a) in similar way as
AH or by neutralizing with forming of intermediate byproducts (SWCNT-COOR). When
SWCNTs are functionalized with antioxidants, AH, the free radicals neutralizing can be performed
by both compounds in a synergic way, here denoted with (SWCNT-OH-A)* or (SWCNT-COOH-
A)* made of orto-quinone from AH and –C=O respective -COOR from nanotubes. In inset
(scheme 1) are represented the direct redox transition for two representative flavonoids (quercetin
and rutin) into a less active “orto-quinone” compound [31]. This contribution presents designing of
biohybrids with quercetin (Q) and rutin (R) supported on SWCNTs, hydroxylated (SWCNT-OH),
carboxylated (SWCNT-COOH) using high density ultrasonic field excitation with subsequent co-
precipitation in dimethyl sulfoxide (DMSO- solvent compatible with the body fluids). DMSO is
appropriate solvent for Q and R (Q is insoluble in water and R-partial soluble) and good dispersant
for SWCNTs. In addition, in high density ultrasonic field takes place a large amount of bubbles
from the cavitation effect where the solvent molecules gain high kinetics energy which by me-
chanical collision amplifies the nanotubes dispersion and the excitation of the solute molecules,
favorable mechanisms to contribute to functionalizing without any intermediary process. If water
is used instead of DMSO is possible to reduce carboxyl or hydroxyl groups due to generation in
the cavitation bubbles very reactive free radicals such as –OH, dioxygen anion (O-2) and H+ [32],
deleterious for antioxidants and SWCNT-OH/COOH. The antioxidant activity for systems
SWCNTs- Quercetin and Rutin was evaluated by chemiluminescence assay and first time demon-
447
strated in vitro the capability of the nanotubes to amplify the scavenging rate related to a given free
radical generator ( -OH) from the hydrogen peroxide decomposition.
Scheme 1 The principle of the scavenging free radicals (R*) by redox transition reaction via elec-
tron donating or H ( H++ e-) donating from a) SWCNTs, SWCNT-COOH, SWCNT-OH b)
SWCNTs decorated with antioxidants (AH) c) AH- antioxidants. Inset: two flavonoid representa-
tives, Quercetin and Rutin (a glycoside between flavanol quercetin, the aglycon, and disaccharide-
rutinose [33]). The mechanisms of the scavenging radicals as explained in [31] with conversion in
less reactive orto-quinone (see comments in text).
2. Experimental details and comments
2.1. Materials
Carbon nanotubes characteristics: SWCNTs, SWCNT-OH, SWCNT-COOH, purchased
from Shenzhen Nanotech Port Co. Ltd (NTP) Physical characteristics and chemical composition
are given in table 1.
Table 1: Physical and composition characteristics for SWCNTs ( based on supplier data)
Products Diameter Length Purity
( %)
Ash
(%)
Specific
Surface Ar-
ea
( m2/g)
Carboxyl
ratio
(%)wt
Hydroxyl ratio
(%) wt
SWCNT <2nm 5-15um >95% < 5% 500-700
N
/A
N
/A
SWCNT-
COOH
<2nm 5-15um >97 < 2 N/A 2.31*
N
/A
SWCNT-OH <2nm 5-15um >97% <2 N/A N/A 2.97**
*The rate of surface carbon atom carboxylated8-14mol%
**The rate of surface carbon atom hydroxylated : 6-8mol%
448
Our supplemental investigation using TGA and Chemical analysis (CNOS) are in agree-
ment with data supplied in table 1. Raman spectroscopy (Raman spectra were recorded by Jasco
NRS 3100 equipment with dual laser beams, 532 and 785 nm, resolution 4 cm-1, configuration ,
backscattering) shows SWCNTs with radial breathing mode at 260 cm-1 ( close to 2nm diameter
counted from ref [34] and figure 2 from AFM topography). G bands, associated with the tangential
modes of vibrations in the aromatic rings (figure 1) are centered on 1584 cm-1 for all SWCNTs but
D bands and other fingerprints are quite different. SWCNT has D band at 1220 cm-1 and a small
peak at 1308 cm-1. The D modes correspond to the first order resonance Raman scattering process
that reflects the presence of defects on the nanotubes body [35]. The two bands reflect the presence
of two types of defects which vanish for SWCNT-OH and reinforce only one type in SWCNT-
COOH, at 1308 cm-1. At low Raman shift wavelengths are several fingerprints assigned to other
disorder carbon forms, also from defects sites which alter -continuum structure. At the defect
sites, bonds are formed through alteration of the carbon body structure with introduction of penta-
gons and heptagons, known as Haeckelites (in Raman spectra localized in region 1000- 1100 cm-
1[36]).
These defect sites usually come from cleaning process using sulfuric/ nitric acid wet
chemistry. Then the carbon nanotubes become more reactive at these defect sites, consequently,
more prone to functionalizing respective to catch the free radicals on their body. In SWCNT-OH
the small band at 794 cm-1 can be assigned to the bending for –OH and 877 cm-1 in association
with 680 cm-1 are assigned to –COOH groups (indexed with KnowITAll ® Informatics systems,
BioRad Laboratory) in SWCNT-COOH. The Haeckelites sites are insignificant in carboxylated re-
spective hydoxylated carbon nanotubes and can be associated with subsequent chemical treatments
to induce carboxyls and hydroxyl groups.
Fig. 1 RAMAN spectra for SWCNT, SWCNT-OH, SWCNT- COOH, in inset SEM micro-
graph (scale 500nm) for pristine SWCNTs as received. Haeckelites defects (see text) for
SWCNTs are in range 1000-1100 cm-1, hydroxyl groups assigned at 794 cm-1(-O-H bend-
ing), carboxyl groups assigned at 877 cm-1 and 690 cm-1 (indexed with Know ITAll soft
ware).
Flavonoids: rutin, quercetin (Sigma-Aldrich);
Reagents for chemiluminescence: Luminol (disodium salt 5-amino-2,3-dihydro-1,4-phthalazine-
dione), 3 wt % hydrogen peroxide, buffer TRIS-HCl, (Sigma Aldrich).
Solvents: DMSO (Sigma-Aldrich, analytical grade);
2.2 Equipments
Chemiluminometer (Sirius Luminometer Berthelot - GmbH Germany): for antioxidant activity
mea-surements by chemiluminescence technique (CL).
449
Ultrasound system: Ultrasonic processor, UIP 1000W (Hielscher - Ultrasound Technology) fre-
quency 20 kHz, sonotrode amplitudes up to 170 micron, liquid pressures up to 10 bars. The sono-
trode horn with diameter 22 mm provides a high ultrasonic field density ~ 260W/cm3 (measured
by calorimetric method).
Atomic Force Microscopy: NTEGRA PRIMA Platform (NT-MDT) for study of topography at mo-
lecular scale, tip NGS01, tapping operating mode.
2.3 Methods
a) Functionalizing carbon nanotubes- flavonoids. The SWCNTs are functionalized with 10% wt
flavonoids by excitation in ultrasonic field using DMSO as solvent in two steps. Briefly 0.9 mg
SWCNTs in 5ml DMSO and 0.1 mg falvonoids in 5 ml DMSO are apart ultrasonicated for 4 min
on cold water bath at 100C (to avoid overheating over 50 0C and the biomolecules degradation). In
the next stage the solutions are merged and ultrasonicated for another 4 min. The mixture was left
overnight at room temperature in nitrogen-glove box to perform co-precipitation between nano-
tubes and flavonoids. The precipitate was extracted by centrifugation at 2000g and dried at 600C
for few days up to constant weight under inert atmosphere.
All samples are indexed as SWCNT, SWCNT-OH (hydroxylated), SWCNT-COOH (carboxy- lat-
ed), for quercetin (Q, SWCNT-Q, SWCNT-OH-Q, SWCNT-COOH-Q) and rutin (R, SWCNT-R,
SWCNT-OH-R, SWCNT-COOH-R).
b) The chemiluminescence assay. There are lots of generators of free radicals but the system lumi-
nol - H2O2 at pH>8.5 lead to high reactive oxygen species to be neutralized by the flavonoids at a
given concentration. The rate of the chemiluminescence quenching is direct related with the flavo-
noid structure and its capacity of scavenging being a measure of the capacity to reduce free radi-
cals.
The antioxidant activity counted by the scavenging rate (SR%) of the flavonoid- carbon nanotubes
was evaluated by chemiluminescence assay [CL] using the method described elsewhere [37].
Briefly the chemiluminescent system is made of luminol- H2O2- TRIS-HCl and the scavenging
rate is defined as:
(1)
where: I0 = CL intensity in the absence of samples at t = 5s; Is = CL intensity for sample at t = 5s.
In case of the biohybrids systems the active principles can be activated or inhibited depending of
the carbon nanotube support. Based on this assumption was defined the amplification factor:
 
SWCNT SWCNT
*
() SR
SR

aff
SR %
Fm
mSR % SR %
mm
(2)
where SR is the measured scavenging rate and SR*- the additive scavenging rate counted as indi-
vidual contribution proportional with their concentration, m is the biohybrid weight, mSWCNT & mf
are weights of SWCNTs and flavonoid.
3. Results and discussions
3.1. The Chemiluminescence assay
The antioxidant activities for SWCNTs and SWCNTs - flavonoid coumpounds are
summa-rized in figure 2
0s
0
II
%SR 100
I
450
Fig. 2. The scavenging rate for SWCNTs and biohybrids with quercetin and rutin
At the first sight the carbon nanotubes in pristine form have a notable antioxidant activity
increasing from SWCNT< SWCNT-OH< SWCNT-COOH. These results can be assigned to the –
OH and – COOH groups which can perform the scavenging free radicals as proton donating. As
well SWCNTs have an appreciable antioxidant activity (8.7 %, figure 2) and this can be assigned
as electron donating from defect sites (SWCNT*, scheme 1) or from “continuum -conjugated
structure”. In this case SWCNTs can be seen as “collector of free radicals” via a single step reac-
tion: SWCNT+R* SWCNT-R. These results are in agreement with the basic concept related to
the antioxidant activity of the flavonoids and polyphenolic compounds The antioxidants (AH) are
involved in redox transitions with single electron donating (or H atom, equivalent with donating of
a proton and an electron) to the free radical species ( R*) in a general scheme: AH+ R*A*+RH,
i.e the free radicals are neutralized resulting less or weakly active byproducts [38]. Unexpected re-
sults come from biohybrids: only 10 % flavonoids selfassembled on SWCNTs lead to a high anti-
oxidant activity, close to the 100% rutin or quercetin scavenging rate. In table 2 are summarized
the amplification factors for each biohybrid taken in account the additive contribution related to
the experimental values (eq 2). The higher Fa values are for byohybrids made of SWCNTs and
SWCNTs-OH with R and Q. That shows that the reaction kinetics to donate an electron is ampli-
fied by SWCNTs respective to donate protons is amplified by the hydroxyl groups and less by the
acidic groups (-COOH). Therefore, the biohybrids work in synergic way bringing together all de-
localized -electron system to annihilate the hydroxyl radicals in case SWCNTs with R and Q. For
SWCNTs-OH with R and Q it is expected as all –OH groups from molecules and nanotubes to be
consumed in the free radicals annihilation.
451
Table 2- The amplification factor, Fa estimate with (2) using data from Fig. 2
Sample SR* (%) SR (%)
(experimental value)
Fa
SWCNT-R 16.29 60.9 3.73
SWCNT-OH-R 17.64 75.71 4.29
SWCNT-COOH-R 28.71 79.21 2.75
SWCNT-Q 16.89 78.09 4.62
SWCNT-OH-Q 18.24 83.89 4.59
SWCNT-COOH-Q 29.31 88.11 3.006
*See equation 2
3.2 AFM Topography
Complementary techniques such as electron microscopy and atomic force microscopy are
also very helpful in elucidating the structure and topography of the biohybrids or other drug deliv-
ery systems based on carbon nanotubes targeting even cancer therapy [39]. In this respect, AFM
images reveal a series of specific biohybrid features: they are made of single tubes or in small
bundles covered with flavonoids in a proper way due to specific interaction induced by the func-
tionalized method. Function of type and level of defects existing in pristine CNT the droplets have
various sizes but the tendency is to cover large area on its surface or to be localized on the specific
sites.
Figs. 3 and 4 show the quercetin and rutin distribution around of “nanotube wires”. Quer-
cetin selfassembled on SWCNT is distributed in small aggregate droplets in specific locations
along nanotube. which can be associated with defect sites ( figure 3 SWCNT-Q). Also rutin
selfassembled on SWCNTs is aligned in larger droplets along of tubes located on defect sites. The
difference is in that quercetin is more disperse along carbon nanotubes and rutin is more localized
due to the steric interactions with glycosides from aglycon. For SWCNT-OH,: quercetin and rutin
are distributed in randomized droplets due to electrostatic interaction between hydroxyl groups but
still with a high coverage on the carbon nanotubes. SWCNT –OH-Q (Figure 3) has a distribution
along of nanotubes but very localized on the defect sites and on the ends of carbon nanotubes.
SWCNT-OH-R is larger distributed along of nanotubes in very thin droplets encompassing the
carbon nanotubes. By comparison with figure 2 where the scavenging rate is higher for quercetin
than rutin a simple conclusion come from synergic activity between electron donating and proton
donating mediated by hydroxyl groups. In case SWCNTs-COOH the main feature is aggregates of
droplets (Figure 3 and 4) with distinctive features: Q-droplets are well localized and R- droplets
are arranged in forms of stacked scales.
Fig. 3. SWCNTs decorated with Quercetin 10%
452
Fig. 4 SWCNTs decorated with Rutin
These features reinforce several general assumptions: a) The biomolecules are localized on
the defect sites via - bonding followed of Vander-Waals stacking in small bubble aggregates; b)
High specific surface area is a dominant factor to increase the scavenging rate in combination with
synergic contribution from delocalized electronic system and –OH functional groups; c) The acidic
groups seems to improve the nanotube contribution to the amplification factor playing a role of
proton donating; d) The conformational behavior, molecular geometry and electronic structure of
quercetin and rutin as well the steric effects are contributors to the synergic antioxidant activity
nanotubes- flavonoids; e) Quercetin has a nonplanar molecular structure, with cross-conjugation
occurring at the C ring (see scheme 1) which in combination with - electron conjugation system
from carbon nanotubes can work as amplified redox transitions; f) The OH groups from Q and R
(ring B, scheme 1) are in the same levels of energies therefore, the antioxidative process mecha-
nisms, exerted by Q and R as a free radical scavenger, relies on two isoenergetic radicals (scheme
1) with extended electronic delocalization between adjacent rings from carbon nanotubes [30d]. g)
–OH groups are very important in the scavenging rates as proton donating but are not a universal
conjecture in the antioxidant activity. The –OH position in flavonoids and its strain by the other
functional groups in whole biohybrids can contribute to the antioxidant activity amplification.
The earlier studies show that the number of phenolic –OH groups is not always the only
determining factor on the antioxidant activity. The structure of an antioxidant molecule, the posi-
tions of phenolic –OH groups, presence of other functional groups in the whole molecule, such as
double bonds and their conjugation to –OH groups and ketonic groups, also play important roles in
antioxidant activities [40]. The structure-antioxidant activity relationships of flavonoids and phe-
nolic acids, in general, are influenced by the state of –OH groups. The strain of the phenolic –OH
groups is increasing in the presence of the acidic groups or by other external factors such as defect
sites on the carbon nanotubes, hydrophobicity and polarity of the environment already proved in
different experiments with other polyphenols (rosmarinic acid, sesamol, carnosic acid, caffeic acid
to name few [41]) where phenolic groups are strained by carboxylic or oxygen.
In this respect, the carbon nanotubes revealed multiple facets of the antioxidant mecha-
nisms: 1) A - conjugated electronic system has antioxidant activity as electron donor but not so
efficient to kill free radicals even they carry unpaired electrons (see figure 2, SWCNTs); 2) A pro-
ton donating system coupled with electron donor, i.e. a hydrogen transfer to the free radicals is
more efficient (SWCNT-OH and SWCNT-COOH); 3) Flavonoids (Q&R) perform antioxidant ac-
tivity via proton donating converting to quinone like compounds (scheme 1) with extension of
conjugation on adjoining rings including also carbon nanotubes.
453
4. Conclusions
It is reported a new route to design biohybrids based SWCNTs (carboxylated, respective
hydroxylated) and flavonoid compounds using high mechanical excitation in ultrasonic field. The
excitation in high density ultrasonic field can mediate reactions and functionalizing without wet
chemistry. Therefore, this method is affordable in designing hybrid biosystems for biomedicine
and drugs in clean conditions. The antioxidants - carbon nanotubes are biohybrids with an im-
proved scavenging rate encompassing a more efficient redox transitions via electron-proton donat-
ing to the free radicals. SWCNTs simple or hydroxylated and carboxylated show an intrinsic anti-
oxidant activity in increasing sequence SWCNT-COOH> SWCNT-OH>SWCNT lead to that pro-
ton donating is more efficient than electron donating in the mechanisms of free radicals scaveng-
ing. There is an optimum combination between electron-proton donating when the flavonoids are
selfassembled on carbon nanotubes. The amplification factor reaches high values for SWCNT-Q
(Fa= 4.62) and SWCNT-OH-Q (Fa= 4.59), intermediate values for SWCNT-OH-R (Fa=4.29) and
SWCNT-R
(Fa=3.73). These results can be assigned to the synergic cooperation between –OH groups
from flavonoids and the - electronic conjugate system of the carbon nanotubes. The steric effects
of the glycosides in rutin lead to lower values in the antioxidant activity. The carboxylic groups in-
crease the antioxidant activity but the amplification factor reduced from Fa ~3 for SWCNT-
COOH-Q to Fa=2.75 for SWCNT-COOH-R. These phenomena can be assigned to the specific in-
teraction between hydroxyls- carboxyls- - conjugated system. These results open new approaches
in understanding the antioxidant mechanisms as well in combination with chemiluminescence
could be developed new strategies in signal monitoring of the free radicals involved in different
disease. SWCNT-OH and SWCNT-COOH are more soluble and it is expected to be more compat-
ible with the body fluids, therefore they could be used as nontoxic drug carriers.
Acknowledgments
This work was supported by the strategic grant POSDRU/89/1.5/S/58852, Project “Post-
doctoral programme for training scientific researchers” co-financed by European Social Found
within the Sectorial Operational Program Human Resources Development 2007-2013.
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