Nanotubes from a vitamin C-based bolaamphiphile

Article abstract of DOI:10.1021/ja057730x

A bolaform surfactant, 1,12-diascorbyl dodecanedioate (BOLA12), with ascorbic acid units as the polar headgroups was synthesized for the first time. Once dispersed in water above 0.5% w/w, BOLA12 forms hollow nanotubes as revealed by cryo-TEM experiments. These nanostructures transform into clear micellar solutions on heating. X-ray diffraction and SAXS experiments were performed both on the pure solid and on its aqueous dispersions. The critical aggregation concentration and the phase behavior were determined by conductivity and DSC experiments. The latter technique provided also the amount of strongly bound, solvating water molecules that surround the polar headgroups. BOLA12 shows the same reducing properties of ascorbic acid, as indicated by the antioxidant activity evaluated with the DPPH method. This feature was used for the reduction of Pd(II) ions on the surface of the nanoassemblies, which lead to the formation of large bundles homogeneously coated with palladium as observed in SEM micrographs.

Full text of DOI:10.1021/ja057730x

Published on Web 05/16/2006
Nanotubes from a Vitamin C-Based Bolaamphiphile
†
†
‡
†,§
Moira Ambrosi, Emiliano Fratini, Viveka Alfredsson, Barry W. Ninham,
†
,†
†
Rodorico Giorgi, Pierandrea Lo Nostro,* and Piero Baglioni
Contribution from the Department of Chemistry and CSGI, UniVersity of Florence,
0019 Sesto Fiorentino (Firenze), Italy, Physical Chemistry 1, Center for Chemistry and
5
Chemical Engineering, Lund UniVersity, 22100 Lund, Sweden, and Department of Applied
Mathematics, Research School of Physical Sciences and Engineering, Institute of AdVances
Studies, Australian National UniVersity, Canberra, Australia 0200
Received November 14, 2005; E-mail: pln@csgi.unifi.it
Abstract: A bolaform surfactant, 1,12-diascorbyl dodecanedioate (BOLA12), with ascorbic acid units as
the polar headgroups was synthesized for the first time. Once dispersed in water above 0.5% w/w, BOLA12
forms hollow nanotubes as revealed by cryo-TEM experiments. These nanostructures transform into clear
micellar solutions on heating. X-ray diffraction and SAXS experiments were performed both on the pure
solid and on its aqueous dispersions. The critical aggregation concentration and the phase behavior were
determined by conductivity and DSC experiments. The latter technique provided also the amount of strongly
bound, solvating water molecules that surround the polar headgroups. BOLA12 shows the same reducing
properties of ascorbic acid, as indicated by the antioxidant activity evaluated with the DPPH method. This
feature was used for the reduction of Pd(II) ions on the surface of the nanoassemblies, which lead to the
formation of large bundles homogeneously coated with palladium as observed in SEM micrographs.
Introduction
hydrophilic headgroups connected by a hydrophobic spacer,
appear to be excellent candidates for the production of organic
The formation and properties of nanotubes are currently the
focus of intense studies because of their potential use in
chemistry, biology, and materials science.1-3 In particular,
organic nanostructures that possess electrochemical/photo-
chemical properties, and stimuli responsiveness to external
environmental changes are a main goal.4,5 Applications for such
structures are increasing. They include and span fields from
membrane mimetic systems to biosensors, from drug and/or gene
delivery systems to metallic nanorods for electronics.
1
6,11
supramolecular structures such as nanotubes, nanofibers, and
7
,9,15
helical ribbons.
In the search for the production of redox
active nanostructures, we have focused our attention on one of
the most powerful natural antioxidants, i.e., Vitamin C. This
inhibits free radical-initiated lipid peroxidation, a process
presumably implicated in a variety of chronic health problems
16,17
such as aging, cancer, and cardiovascular diseases.
Ascorbic
6
-14
acid is poorly soluble in hydrophobic media. Thus, amphiphilic
ascorbyl derivatives must be synthesized to extend their reducing
properties to encompass hydrophobic environments. In previous
papers, we reported the synthesis and properties of alkanoyl-
Bolaamphiphiles, i.e. surfactant molecules that contain two
†
University of Florence.
Lund University.
Australian National University.
‡
6
-O-ascorbic acid esters, (ASCn with n the hydrophobic chain
§
1
8-26
(
(
(
1) Matsui, H.; Douberly, G. E. J. Langmuir 2001, 17, 7918-7922.
2) Ai, S.; Lu, G.; He, Q.; Li, J. J. Am. Chem. Soc. 2003, 125, 11140-11141.
3) Chupin, V.; Killian, J. A.; de Kruijff, B. Biophys. J. 2003, 84, 2373-
length, 8 e n e 18).
In these surfactants only the vitamin
(15) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812-2818.
(16) Nihro, Y.; Miyataka, H.; Sudo, T.; Matsumoto, H.; Sathoh, T. J. Med. Chem.
1991, 34, 2152-2157.
2
381.
(
(
(
(
(
(
4) Hong, B. H.; Lee, J. Y.; Lee, C.-H.; Kim, J. C.; Bae, S. C.; Kim, K. S. J.
Am. Chem. Soc. 2001, 123, 10748-10749.
(17) Bisby, R. H.; Parker, A. W. J. Am. Chem. Soc. 1995, 117, 5664-5670.
(18) Capuzzi, G.; Lo Nostro, P.; Kulkarni, K.; Fernandez, J. E.; Vincieri, F. F.
Langmuir 1996, 12, 5413-5418.
5) Hou, S.; Wang, J.; Martin, C. R. J. Am. Chem. Soc. 2005, 127, 8586-
8
587.
6) Shimizu, T.; Iwaura, R.; Masuda, M.; Hanada, T.; Yase, K. J. Am. Chem.
Soc. 2001, 123, 5947-5955.
(19) Capuzzi, G.; Lo Nostro, P.; Kulkarni, K.; Fernandez, J. E. Langmuir 1996,
12, 3957-3963.
7) Song, J.; Cheng, Q.; Kopta, S.; Stevens, R. C. J. Am. Chem. Soc. 2001,
(20) Capuzzi, G.; Kulkarni, K.; Fernandez, J. E.; Vincieri, F. F.; Lo Nostro, P.
J. Colloid Interface Sci. 1997, 186, 271-279.
1
23, 3205-3213.
8) Djalali, R.; Chen, Y.-f.; Matsui, H. J. Am. Chem. Soc. 2002, 124, 13660-
3661.
9) Song, J.; Cheng, Q.; Stevens, R. C. Chem. Phys. Lipids 2002, 114, 203-
(21) Lo Nostro, P.; Capuzzi, G.; Pinelli, P.; Mulinacci, N.; Romani, A.; Vincieri,
F. F. Colloids Surf. A 2000, 167, 83-93.
1
(22) Lo Nostro, P.; Capuzzi, G.; Romani, A.; Mulinacci, N. Langmuir 2000,
16, 1744-1750.
2
14.
(
(
10) Zhang, S. Biotechnol. AdV. 2002, 20, 321-339.
11) Claussen, R. C.; Rabatic, B. M.; Stupp, S. I. J. Am. Chem. Soc. 2003, 125,
(23) Palma, S.; Manzo, R. H.; Allemandi, D.; Fratoni, L.; Lo Nostro, P. Langmuir
2002, 18, 9219-9224.
1
2680-12681.
(24) Lo Nostro, P.; Ninham, B. W.; Fratoni, L.; Palma, S.; Manzo, R. H.;
Allemandi, D.; Baglioni, P. Langmuir 2003, 19, 3222-3228.
(25) Lo Nostro, P.; Ninham, B. W.; Ambrosi, M.; Fratoni, L.; Palma, S.;
Allemandi, D.; Baglioni, P. Langmuir 2003, 19, 9583-9591.
(26) Ambrosi, M.; Lo Nostro, P.; Fratoni, L.; Dei, L.; Ninham, B. W.; Palma,
S.; Manzo, R. H.; Allemandi, D.; Baglioni, P. Phys. Chem. Chem. Phys.
2004, 6, 1401-1407.
(
(
(
12) Pakhomov, S.; Hammer, R. P.; Mishra, B. K.; Thomas, B. N. Proc. Natl.
Acad. Sci. U.S.A. 2003, 100, 3040-3042.
13) Bianco, A.; Kostarelos, K.; Prato, M. Curr. Opin. Chem. Biol. 2005, 9,
1
-6.
14) Yang, M.; Yang, Y.; Yang, H.; Shen, G.; Yu, R. Biomaterials 2005, 27,
46-255.
2
10.1021/ja057730x CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 7209-7214
9
7209

A R T I C L E S
Ambrosi et al.
Figure 1. (a) Conductivity as a function of temperature for 4% w/w sample.
The Krafft point determined was 33.6 °C. (b) Conductivity as a function of
BOLA12 concentration (P% w/w). The critical aggregation concentration
was about 0.55% ≈ 10 mM.
Figure 2. 5% w/w BOLA12/water sample (in the condensed phase). The
white arrow points at a group of tubules. The black arrow indicates the
carbon support film.
C primary -OH group is involved in the ester bond, and the
other hydrophilic residues are still available for establishing
hydrogen bonds and for performing redox and complexing
functions.2 Hydrated ASCn surfactants form lamellar phases
1,22
(
coagels), micelles or gel-like dispersions depending on com-
23-25
position, solvent nature, and temperature.
Due to their
peculiar phase behavior and to their redox functionalities, these
nanoassemblies represent an excellent tool for the delivery of
hydrophobic and sensitive drugs.2
7,28
Here we report the synthesis of a new decamethylene-group-
spanned bolaamphiphile, BOLA12, with two vitamin C rings
as headgroups (Figure S1, Supporting Information). Nanotubes
are found in aqueous dispersions of BOLA12 at room temper-
ature above 0.5% w/w. X-ray diffraction (XRD), small-angle
X-ray-scattering (SAXS), cryo-transmission electron microscopy
Figure 3. 5% w/w BOLA12/water sample above the phase transition
temperature, plunged into the cryogene after 1-2 min to let the solvent
evaporate.
(
(
cryo-TEM), conductivity, and differential scanning calorimetry
DSC) measurements were used to explore the aggregate
nanostructures. Reduction of Pd(II) on the outer surface of the
BOLA12 nanotubes resulted in a homogeneous coating with
palladium as shown by SEM experiments.
Results and Discussion
On dispersion in water at room temperature and above 0.5%
w/w, BOLA12 produces opaque, mostly semitransparent con-
densed phases. Upon heating, these systems turn into a
transparent homogeneous liquid.
Conductivity. The phase transition temperature (Ttrans) and
the critical aggregation concentration (CAC) were determined
by conductivity (Figure 1,a and b, respectively).
Figure 4. 10% w/w BOLA12/water placed in the CEVS at 34 °C.
sample was placed on the grid above the phase transition. After
blotting the excess solution with filter paper, the sample was
allowed to form the condensed phase and then plunged into
the cryogene. In Figure 3, the sample was kept on the grid for
A 4% w/w sample was used for the evaluation of Ttrans. The
4
% concentration was chosen to guarantee the presence of
aggregates in the liquid phase after the transition, and to obtain
a significant conductivity gradient. Ttrans was assigned to the
midpoint between the two intersection points as shown in Figure
1
-2 min above the phase transition temperature, before being
plunged into the cryogene.
The average outer width of the tubes is about 28 nm, the
mean inner diameter is about 15 nm, and the length ranges
roughly between 0.2 and 1 µm. Figure 3 shows a more detailed
image, as the solvent was evaporated to avoid the background
from the vitreous ice. It is evident that the tubes are hollow, as
shown by the nearly electron-transparent central core.5
A micrograph recorded from the condensed phase formed
by the 10% w/w sample is reported in Figure 4. In this case,
the sample was left at 34 °C on the grid for approximately 10
s to form the condensed phase. Two coexisting phases, sheetlike
1
a, and comes out to be about 33.6 °C. The critical aggregation
concentration (CAC) was established at 48 °C, in the concentra-
tion range between 0.05 and 5.5% w/w. The CAC was
determined as the intersection point between the two straight
lines that give a linear fit to the experimental conductivity data
in the low and high concentration domains as shown in Figure
1
b and results in about 0.55% w/w (∼10 mM).
Cryo-TEM. Figures 2 and 3 show the structures obtained
from the 5% w/w sample in the condensed phase. Tubular
structures are clearly seen in the micrographs. In Figure 2, the
(
white arrow) and elongated structures (black arrows), are
shown.
When the 10% and 40% w/w samples were prepared at higher
temperature in the CEVS (48 °C), no aggregated structures were
(
27) Palma, S.; Manzo, R. H.; Allemandi, D.; Fratoni, L.; Lo Nostro, P. J. Pharm.
Sci. 2002, 91, 1810-1816.
(
28) Bilia, A. R.; Bergonzi, M. C.; Vincieri, F. F.; Lo Nostro, P.; Morris, G. A.
J. Pharm. Sci. 2002, 91, 2265-2270.
7210 J. AM. CHEM. SOC.
9
VOL. 128, NO. 22, 2006

Nanotubes from a Vitamin C-Based Bolaamphiphile
A R T I C L E S
Figure 6. SAXS intensity distribution for anhydrous BOLA12 and its
aqueous dispersions at room temperature. The three different ordered
structures have been referred to as A (anhydrous surfactant molecule), B
(hydrated surfactant molecule), and C (supramolecular assembly).
Figure 5. X-ray profiles for pure anhydrous BOLA12 and its aqueous
dispersions at room temperature.
-
1
order appears at about 0.35 Å . The peak broadening is due
to the polydispersity of the aggregated structure. The extra four
found. Below the phase transition temperature, the 40% w/w
sample was too thick to be imaged.
-
1
peaks in the range 0.022-0.115 Å indicate the presence of
XRD. X-ray diffraction measurements were carried out either
on the pure anhydrous BOLA12 or on BOLA12/water samples
between 10% and 40% w/w. The diffractograms are shown in
Figure 5. The low-angle region gives information on the long-
range ordering of the system. For the anhydrous sample the
main peak corresponds to a spacing (d) of 25 Å, while for all
the hydrated samples the spacing increases up to about 40 Å.
Corey-Pauling-Koltun (CPK) model calculations2 gave a
value of 26 Å for the length of BOLA12 with the aliphatic chain
in its fully stretched conformation. Therefore, in the dry state,
BOLA12 molecules produced compact monolayers with the
hydrophobic tails fully extended and perpendicular to the planes
formed by the headgroups.
very large ordered structures that are formed upon hydration.
-1
The first-order peak at about 0.022-0.025 Å (C) corresponds
to a repetition dimension ranging between 230 and 250 Å. These
supramolecular structures are poorly affected by the BOLA12
concentration. Considering that a single layer of hydrated
BOLA12 monomers has a thickness of 33 Å and on the basis
of the inner diameter obtained from the cryo-TEM experiment,
the structures that produce the “C” peak comprise hollow
cylinders with a wall consisting of about 2-3 layers of hydrated
surfactant molecules (Figure 7).
9
The nanotubes formed by BOLA12 in water dispersions
produce SAXS profiles very similar to those given by lanreotide
3
1
below 20%. This octapeptide produces very long, hollow
columns with multilayered walls, and the presence of strongly
bound and interfacial water is confirmed by DSC experiments.31
By heating above 40 °C, the ordered condensed structures
disappear, and small micellar aggregates are formed. SAXS
profiles change dramatically, as illustrated in Figure 8 for the
10% w/w sample. After cooling to 20 °C the original structures
are reformed. Similar results were obtained also for the 5%,
20%, and 40% w/w samples.
In the presence of water, and on the assumption that the single
surfactant molecules lay perpendicularly to the interface, they
form monolayers separated by a thin aqueous compartment of
about 15 Å. It is interesting to note that the water interlayer
has the same thickness as that found for the single headed ASCn
derivatives,26 and for phospholipid derivatives.
30
SAXS. Small-angle X-ray-scattering experiments were carried
out on the condensed phase as well as on the liquid sample.
Figure 6 shows the SAXS profiles for pure anhydrous BOLA12
Figure 9 shows the SAXS profile for the 5% w/w liquid
sample equilibrated at 50 °C.
(black curve) and its aqueous dispersions at 5%, 10%, 20%,
2
and 40% w/w at 20 °C.
The peak at Q ) 0.243 Å-1 (A) for the powder corresponds
The Guinier plot, i.e. ln[I(Q)] vs Q , provides a first
approximation to the radius of gyration of the scattering
3
2
objects:
to 25.5 Å. This is the length of a monomer in its fully stretched
conformation, in agreement with the main signal found with
the XRD experiments, and with the calculated CPK model. In
the case of the hydrated condensed phases, the peak broadens
2
Rg
2
ln[I(Q)] ) ln[I(0)] -
Q
(1)
(
)
3
-
1
and shifts to a lower Q, 0.195 Å (B), and this corresponds to
a lamellar structure with a repetition spacing of about 33-34
Å. This feature agrees with the presence of lamellae of hydrated
solid, with a thin layer (about 8-9 Å) of hydrating water
molecules that surround the headgroups of the bolaamphiphiles.
Similar values were found from XRD experiments. The second
2
This approximation holds for RgQ < 1, in our case for Q <
.00259 Å , about 40 points are in the fitting range (see inset
-
2
0
of Figure 9). The extracted radius of gyration Rg ) 19.9(1.5
Å is associated with a spherical micelle having an effective
(
31) Val e´ ry, C.; Artzner, F.; Robert, B.; Gulick, T.; Keller, G.; Grabielle-
Madelmont, C.; Torres, M.-L.; Cherif-Cheikh, R.; Paternostre, M. Biophys.
J. 2004, 86, 2484-2501.
(
29) Koltun, W. L. Biopolymers 1965, 3, 665-679.
(
30) Caffrey, M.; Hogan, J.; Rudolph, A. S. Biochemistry 1991, 30, 2134-
(32) Guinier, A.; Fournet, G. Small Angle Scattering of X-rays; John Wiley:
2
146.
New York, 1955.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 22, 2006 7211

A R T I C L E S
Ambrosi et al.
Figure 7. Schematic structure of a nanotube produced by BOLA12.
Considering that the length of the polar heagroup is ap-
2
6
proximately 6 Å and that the volume of a single -(CH2)10-
3
chain is about 296.4 Å calculated according to the Tanford’s
3
3
rule, the aggregation number (g) and the area per polar
headgroup (aP) can be estimated from the value of Reff. In
2
particular, g and aP come out to be 240 and 52.6 Å ,
respectively. The interfacial free energy gain due to the
formation of the micellar interface is given by ∆Gsurf ) 2gaPσ,
where σ is the interfacial tension of a water/n-decane interface
2
34
at 50 °C (about 50 erg/cm ). Per single monomer, ∆G1 )
-
13
∆
Gsurf/g turns out to be about 3.9 × 10
erg. The trans-to-
gauche transition for a linear alkane requires no more than 0.97
10 erg/molecule. This shows that the free energy gain due
-13
×
to the formation of the micellar interface is about 5 times larger
than the energy required to bend the aliphatic chain in the gauche
conformation. This calculation and the SAXS results suggest
the presence of nearly spherical micelles where the surfactant
chains are closely packed in a stretched or in a bent conforma-
tion.
Figure 8. SAXS intensity distribution for the 10% w/w sample as a function
of temperature, between 20 and 50 °C.
DSC. DSC thermograms were recorded for BOLA12/water
samples to study their phase behavior between 0.05% and 40%
w/w. For concentrations above 0.55%, the heat flow vs
temperature curves showed an endothermic peak that corre-
sponds to the reversible condensed-to-fluid phase transition with
no hysteresis. The values of the transition temperature, Ttrans,
and of the phase transition enthalpy change, ∆Htrans, are shown
in Table 1. Ttrans was obtained as the temperature of the
endothermic peak. ∆Htrans and ∆Hexp were determined by
integrating the heat flow curves reported in Figures S2 and S3
in the Supporting Information.
Figure 9. SAXS intensity distribution for the 5% w/w sample at 50 °C.
Guinier plot ln[I(Q)] vs Q is shown in the inset along with the resulting
linear fit.
The total condensed-to-fluid phase enthalpy change, ∆Htrans,
can be divided into three contributions: the enthalpy change
associated with the conformational and/or packing change of
the hydrocarbon tails, ∆Hchain, and the changes due to the
hydration (∆Hhydr) and to the electrostatic interactions involving
2
radius Reff ) 25.7(1.9 Å. It is important to recall that the
Guinier analysis does not provide information on the shape of
the scattering objects. However, in our case, the plots of
2
2
(33) Tanford, C. J. Phys. Chem. 1972, 76, 3020-3024.
ln[I(Q)Q] and ln[I(Q)Q ] vs Q excluded the existence of
cylinders or disks in the Q range investigated.
(
34) Zeppieri, S.; Rodr `ı guez, J.; L o` pez de Ramos, A. L. J. Chem. Eng. Data
2001, 46, 1086-1088.
7212 J. AM. CHEM. SOC.
9
VOL. 128, NO. 22, 2006

Nanotubes from a Vitamin C-Based Bolaamphiphile
A R T I C L E S
Table 1. Temperature (Ttrans ( 0.5 °C) and Enthalpy Change
supports the predominant role played by the headgroups in the
formation of the multilayered tubular nanoassemblies as shown
in Figure 7. This evidence is in agreement with previous
(
∆Htrans ( 5 J/gSURF) for the Condensed-to-Fluid Phase Transition
a
of BOLA12/Water Samples
P
T
trans
∆H
trans
∆H
exp
W
b
N
P
T
trans
∆H
trans
∆H
exp
W
b
N
3
7-39
reports.
0.81 37.9 58.0
1.26 39.0 69.8
1.74 39.6 84.9
2.50 40.2 107.4
3.02 40.6 110.2
4.96 41.2 111.4
5.53 41.6 113.6 317.3 4.9 13
10.08 42.15 111.2 303.9 8.9 12
18.98 42.7 109.1 287.8 13.8
41.32 43.75 102.8 233.8 29.9
One of the most striking features of these supramolecular
nanoassemblies is the sharp monodispersity of their diameter,
as shown in Figures 2 and 3. This is a typical behavior of
tubules, as already reported in the literature and discussed by
9
7
4
0
41
Selinger and Boden.
a
Enthalpy of melting of bulk water (∆HExp ( 0.5 J/gwater), percentage of
bound water over the entire water content (Wb ( 0.2%), and number of
water molecules per polar headgroup (N ( 1) calculated from eqs 3 and 4
The formation of rodlike, helical, or planar structures is
regulated by a mechanism that involves the chemical nature of
the individual molecules and the interactions that they
4
0-42
the polar headgroups (∆Hel):23,35
produce.
In particular, chiral molecules can aggregate and
form highly packed, tilted structures. Eventually, these can
assemble and form multilayered objects with high curvature.
The balance between the twisting of the ribbons and the inter-
layer interactions limits the internal curvature of the nanotubes,
and therefore controls their size and monodispersity.41
In our case, BOLA12 possesses some crucial structural
features:
∆
Htrans ) ∆Hhydr + ∆H + ∆Hchain
(2)
el
On heating, the polar headgroups release the protons (taking
up ∆Hel). Both anionic groups and counterions then become
hydrated, with the release of ∆Hhydr. Finally ∆Hchain is taken
up by the hydrophobic tails to rearrange into a more fluid phase.
Our results indicate that all contributions increase in absolute
value with increasing surfactant concentration. Between 5% and
(1) it bears chiral headgroups
(2) the polar headgroups are quite bulky, with a five-
4
0% w/w, the exothermic ∆Hhydr contribution becomes the
membered rigid ring
3) the headgroups carry three hydroxyl residues and four
predominant contribution as shown by the decrease in ∆Htrans.
As for ASCn,26 calorimetric studies on BOLA12 show the
presence of two types of water in the condensed phase: a minor
fraction of strongly bound water molecules, and bulk water.
While the latter melts at around 0 °C, the hydration water is
essentially “frozen” and remains strongly bound to the surfactant
headgroups. Samples were first cooled quickly to -90 °C to
avoid separation of ice from the dispersion. They were then
heated to a temperature above Ttrans. Changing the cooling/
heating rates of the samples did not result in different thermal
behaviors.26 As reported previously, the percentage of strongly
bound water over the whole water content, Wb (%), can be
calculated from the decrease of the area of the endothermic
peak associated with the melting of bulk water. The number of
such strongly bound water molecules per polar headgroup, N,
can be calculated from Wb (%) according to the following
(
other oxygens that can readily establish intermolecular hydrogen
bonds
(4) the C5 and C6 carbon atoms, as well as O6 and the CdO
ester group can rotate around the σ bonds and give rise to
different conformations (flexibility)
(5) the hydrophobic chain contains 10 methylene groups, with
some degree of lateral freedom, that can partially twist and form
kinks.
These structural properties of the surfactant molecules and
the interactions that they can establish (hydrogen bonding
between headgroups and with water, and van der Waals
interactions between the alkyl chains) contribute to the formation
of the hollow, elongated, pretty monodisperse nanotubes that
are shown in the cryo-TEM pictures.
Reducing Properties. The evaluation of the reducing activity
according to the DPPH method indicates for BOLA12 a value
of 94%, comparable to that of the native ascorbic acid and of
its most common esters (between 90% and 97%).22 This large
reducing power can be exploited for the reduction of Pd2 ions
on the outer surface of the nanostructures. By performing the
Pd coating according to the procedure explained in the
Experimental Section (see Supporting Information), SEM
micrographs (see Figure 10) show the presence of several nearly
flat, elongated structures with a width ranging between 250 and
870 nm. The occurrence of such objectssthat show a regular
coating of palladium (from EDS analysis)scan be related to
the formation of bundles made up of smaller assemblies.
Spherical nanoparticles of palladium, with an average diameter
of about 70-160 nm, are grouped in clusters.
2
6
equations:
3
33.79 - ∆Hexp
W (%) )
100
(3)
(4)
b
(
)
333.79
+
M
1
00 - P
n
N ) Wb
(
)(
)
2P
100Mw
where 333.79 J/g is the heat of melting of pure water as reported
3
6
in the literature, ∆Hexp (in J/gwater) is obtained from the
measured peak area normalized to the amount of water in the
sample, P is the mass percentage of BOLA12 in the sample,
Mw is the molecular weight of H2O, Mn the molecular weight
of BOLA12, and N is the number of strongly bound water
molecules per polar headgroup. The values of N show that each
headgroup in the bolaamphiphile is strongly interacting with
about 10 molecules of water. Similar findings were obtained
for the single-headed ASC12 surfactant.26 This result appears
to be reasonable for the chemical structure of BOLA12 and
(
37) Shimizu, T.; Hato, M. Biochim. Biophys. Acta 1993, 1147, 50-58.
38) Fuhrhop, J.-H.; Helfrich, W. Chem. ReV. 1993, 93, 1565-1582.
(
(39) Spector, M. S.; Easwaran, K. R. K.; Jyothi, G.; Selinger, J. V.; Singh, A.;
Schnur, J. M. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 12943-12946.
(
40) Selinger, J. V.; Spector, M. S.; Schnur, J. M. J. Phys. Chem. B 2001, 105,
7157-7169.
(
(
35) Kodama, M.; Seki, S. AdV. Colloid Interface Sci. 1991, 35, 1-30.
36) Dean, J. A. Lange’s Handbook of Chemistry; McGraw-Hill Book Com-
pany: New York, 1985.
(41) Nyrkova, I. A.; Semenov, A. N.; Aggeli, A.; Boden, N. Eur. Phys. J. B
2000, 17, 481-497.
(42) Frankel, D. A.; O’Brien, D. F. J. Am. Chem. Soc. 1994, 116, 10057-10069.
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A R T I C L E S
Ambrosi et al.
This molecule forms stable self-assembled nanotubes in water
at room temperature, that turn into clear micellar solutions upon
heating. The structure and size of the nanoassemblies have been
assessed through cryo-TEM, XRD, and SAXS experiments. The
nanotubes are made up of several layers of BOLA12 molecules
and show an outer diameter of about 25 nm. The condensed-
to-fluid phase transition has been studied by conductivity and
DSC. These measurements indicate that the phase transition is
dominated by the interactions involving vitamin C headgroups.
Each hydrophilic head is hydrated on the average by 10 strongly
bound water molecules, which contribute to the formation of
the tightly arranged surfactant layers. Because of the presence
of ascorbic acid rings in the molecular architecture, the
nanotubes perform an efficient reducing activity that leads to
the formation of a homogeneous thin layer of palladium on their
outer surfaces. This result can be exploited for the production
of nanosized metallic materials and is the subject of future
investigations.
Figure 10. SEM micrograph of a BOLA12/water (2% w/w) sample treated
with PdCl2.
This result indicates that the external surface of the nano-
assemblies can act as an efficient reducing substrate for
appropriate metal cations. The exploitation of this system as
template for the fabrication of metal nanowires and other
assemblies could be useful for the production of molecular
electronic devices and is currently under investigation.
Acknowledgment. We acknowledge the Consorzio Inter-
universitario per lo Sviluppo dei Sistemi a Grande Interfase
(CSGI, Florence), and the Ministero dell’Istruzione, dell’Universita`
e della Ricerca (MIUR, Rome) for partial financial support.
Supporting Information Available: Details for the synthesis
of BOLA12 and general procedures for sample preparation and
for experimental measurements. This material is available free
of charge via the Internet at http://pubs.acs.org
Conclusions
1
,12-Diascorbyl dodecanedioate, BOLA12, is a bolaform
surfactant that bears two redox-active vitamin C headgroups.
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