Supramolecular behavior of fluorous polyglycerol dendrons and polyglycerol dendrimers with perfluorinated shells in water

Article abstract of DOI:10.1039/c1nj20741e

In this article, we describe the synthesis of a perfluoro-tagged polyglycerol dendron and its aggregation behavior in the presence of polyglycerol dendrimers with perfluorinated shells in water. The perfluoro-alkyl-perfluoro-alkyl interactions between the perfluorinated shells of the dendrimers and the perfluorinated tags of the dendrons lead to highly stable supramolecular architectures, due to self-assembly of the perfluorinated moieties. Furthermore, we show that the size of the resulting supramolecular complexes can be tuned by simple variation of the dendrimer-dendron ratio. Complexes at various ratios are characterized by optical microscopy, DLS, and TEM. In general, the results presented herein demonstrate that perfluoro-alkyl-perfluoro-alkyl interactions are applicable for the formation of stable supramolecular structures in water and thus provide a new tool for the design of supramolecular architectures in addition to traditional non-covalent interactions.

Full text of DOI:10.1039/c1nj20741e

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PAPER
Supramolecular behavior of fluorous polyglycerol dendrons and
polyglycerol dendrimers with perfluorinated shells in waterwz
Maximilian Zieringer,a Monika Wyszogrodzka,b Karina Biskupa and
Rainer Haag*a
Received (in Montpellier, France) 25th August 2011, Accepted 26th October 2011
DOI: 10.1039/c1nj20741e
In this article, we describe the synthesis of a perfluoro-tagged polyglycerol dendron and its
aggregation behavior in the presence of polyglycerol dendrimers with perfluorinated shells in
water. The perfluoro-alkyl–perfluoro-alkyl interactions between the perfluorinated shells of the
dendrimers and the perfluorinated tags of the dendrons lead to highly stable supramolecular
architectures, due to self-assembly of the perfluorinated moieties. Furthermore, we show that the
size of the resulting supramolecular complexes can be tuned by simple variation of the
dendrimer–dendron ratio. Complexes at various ratios are characterized by optical microscopy,
DLS, and TEM. In general, the results presented herein demonstrate that perfluoro-
alkyl–perfluoro-alkyl interactions are applicable for the formation of stable supramolecular
structures in water and thus provide a new tool for the design of supramolecular architectures in
addition to traditional non-covalent interactions.
solid-phase extraction of light perfluoro-tagged compounds.3
Introduction
In addition, these methods have been effectively used in the fields
of proteomics, microarray analysis, and catalyst immobilization
on solid supports.4 In a similar manner dendritic macromolecules
with perfluorinated shells can serve as fluorous phase and thus
have been used as a homogeneous support for perfluoro-tagged
catalysts in cross-coupling reactions.5 However, although the
self-assembly of perfluoro-tagged compounds has been well-
described, interactions between perfluoro alkyl chains (fluorous
interactions) of different compounds have been scarcely used as a
tool to design supramolecular architectures.6 Recently, we have
reported size-tunable supramolecular complexes that consist of
polyglycerol (PG) dendrimers with perfluorinated shells and
perfluoro-tagged molecules.7 These supramolecular complexes
are soluble even in polar organic solvents such as DMF or
DMSO, although the PG dendrimers with perfluorinated shells
are only soluble in fluorinated solvents.7
One of the major challenges in supramolecular chemistry is the
prediction and control of molecule aggregation used to develop
new molecular architectures and materials. In recent years,
fluorous interactions have been investigated as alternatives to
traditional noncovalent interactions, such as dipole inter-
actions, hydrogen bonding, metal coordination, and electrostatic
interactions. Among noncovalent interactions, fluorous interactions
are unique because of the physicochemical properties of
fluorocarbons, including weak intermolecular dispersion inter-
actions, high gas solubilities, combined hydro- and lipophobicity,
and the large miscibility gaps that occur in solvent systems
composed of fluorocarbons and hydrocarbons.1 As a consequence
of the physicochemical properties of fluorocarbons, compounds
with heavy perfluoro-tags (ponytails) preferentially partition
into the fluorous phase of a biphasic solvent system. In 1994
Horvath and Rabai generalized this partitioning in their
´
In this article, we present water-soluble supramolecular
architectures based on fluorous interactions. For this purpose,
we have used an amphiphilic PG dendron that consists of two
perfluoro-alkyl chains that are coupled by a bridging unit to
the core of a [G3] PG dendron 1 (Fig. 1). The perfluoro-tagged
PG dendron has been obtained by synthesis of a precursor and
consecutive addition of the perfluorinated chains to its bridging
unit. The perfluoro-tagged PG dendron is well soluble in protic
solvents such as methanol and water. To investigate formation
of supramolecular complexes in water, [G3.5] PG dendrimers
with perfluorinated shells 2 are chosen as the fluorous counterpart
to the PG dendrons (Fig. 2). These dendrimers are insoluble in
water and only soluble in fluorinated solvents, because of the
‘‘fluorous biphase concept’’ (FBC).2 This partitioning of
perfluoro-tagged compounds into a fluorous phase was further
exploited in the so-called ‘‘light fluorous technology’’ where
fluorous silica gels provide stationary fluorous phases for
a Institut fur Chemie und Biochemie, Freie Universitat Berlin, Takustr. 3,
14195 Berlin, Germany. E-mail: haag@chemie.fu-berlin.de
¨
¨
b Organische Chemie, Technische Universitat Dortmund,
Otto-Hahn-Str. 6, 44227 Dortmund, Germany
¨
w This article is part of the themed issue Dendrimers II, guest-edited
by Jean-Pierre Majoral.
z Electronic supplementary information (ESI) available: Experimental
details, synthesis and characterization of compound 3, and supporting
figures. See DOI: 10.1039/c1nj20741e
c
402 New J. Chem., 2012, 36, 402–406
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Fig. 1 Perfluoro-tagged [G3] glycerol dendron 1.
Scheme 1 Synthesis of perfluoro-tagged [G3] PG dendron 1:
(i) DIPEA, sodium ascorbate, CuSO4 (15 mol%), THF/H2O.
(ii) Rf8CH2CH2SH, AIBN (cat.), 80 1C, 24 h. (iii) Dowex-50W resin,
MeOH/CH2Cl2, reflux, 18 h.
hydrophilic moieties in the amphiphiles are necessary to obtain
water soluble dendron–dendrimer complexes even at low ratios.
Therefore, we choose a [G3] PG dendron as a hydrophilic
moiety. Two perfluoro-octyl chains are selected to enhance the
probability of the amphiphiles to form supramolecular complexes
with the dendrimers via fluorous–fluorous interactions.
To generate the perfluoro-tagged [G3] PG dendron 1, precursor
5 is synthesized by ‘‘click’’ reactions of the diallyl azide compound
3 and the protected propargyl [G3] PG dendron 4 (Scheme 1).
Compound 3 forms the bridging unit in the perfluoro-tagged
dendron 1. The synthesis of 3 is described in the ESI.z The
synthesis of the propargyl [G3] dendron 4 has been previously
described.9 For the ‘‘click’’ reaction between the compounds 3 and
4, we use 15 mol% of CuSO4, 30 mol% of sodium ascorbate, and
15 mol% of N,N-diisopropylethylamine (DIPEA) to generate the
desired product in very good yields. These reaction conditions
have already been optimized to avoid partial cleavage of the acetal
groups.9 Radical addition of 1H,1H,2H,2H-heptadecafluorode-
can-1-thiol to the allyl moieties of the precursor 5 and consecutive
deprotection by the use of an acidic ion exchange resin afforded
the perfluoro-tagged [G3] PG dendron 1 in good yields. The
synthesis of [G3.5] dendrimers with perfluorinated shells 2 has
been reported previously.8
Fig. 2 [G3.5] polyglycerol dendrimer with perfluorinated shell 2.
fully functionalized periphery of the PG core.7 To obtain
structural information of supramolecular complex formation,
various ratios between compounds 1 and 2 are studied in water
by dynamic light scattering (DLS), optical microscopy, and
TEM. We find that supramolecular complexes are formed
by these compounds in water; moreover we find that the
dendron–dendrimer ratio has a significant influence on the
structure of the resulting complexes.
Results and discussion
Design and synthesis of perfluoro-tagged [G3] polyglycerol
dendron
We have designed the perfluoro-tagged [G3] PG dendron 1 to
form supramolecular complexes with [G3.5] PG dendrimers
with perfluorinated shells 2 in water.
The dendrimers have a molecular weight of 15 648 g molꢀ1
and are highly hydrophobic due to the 32 perfluoro-hexyl
chains that form the shell.8 We assume that relatively large
Formation of supramolecular complexes in water
The [G3] bi-perfluoro-tagged glycerol dendron 1 is highly soluble
in protic solvents, such as methanol and water. It has a critical
c
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Fig. 3 Microscope picture of micron sized bubbles formed by
self-assembly of perfluoro-tagged [G3] PG dendron 1 and [G3.5] PG
dendrimer with perfluorinated shell 2: (left) optical micrograph of a
dendron–dendrimer ratio of 2 : 1 in water; (right) proposed structure
of dendrimer stabilized micron-bubbles.
micellar concentration (CMC) of 10ꢀ4 M in water. PG dendrimers
with perfluorinated shells 2 that are usually insoluble in polar
solvents can be fully dissolved in aqueous solutions of the
perfluoro-tagged dendron 1 upon ultrasonication; this is an
indication of the formation of supramolecular complexes.
Similar to our previous studies of supramolecular structures
in organic solvents, the dendron–dendrimer ratios in this study
are 1 : 1, 2 : 1, and 4 : 1 based on a dendrimer concentration of
1 mM.7 DLS measurements of these solutions indicate the
presence of particles larger than 1 mm. Optical microscopy reveals
that the supramolecular complexes self-assemble around residual
air in water in a similar manner as it was observed in organic
solvents (Fig. 3).7 We assume that the structure of the observed
micron-bubbles corresponds to the structures that have been
observed in organic solvents. The dendrimers form a first layer
around air. A second layer around the dendrimer layer is formed
by the dendrons with the perfluoro alkyl chains of the dendron
inserted into the perfluorinated shells of the dendrimers and the
PG domain oriented towards the water. There is a clear trend in
the average diameters ([M]) of the micron-bubbles assembled in
water; they decrease with increasing dendron–dendrimer ratio.
At a 1 : 1 ratio, the average diameter is 4.57 mm (coefficient of
variation (CV) = 24.07).10 The average diameter is decreased
about 40% at the ratio of 2 : 1 ([M] = 2.80 mm, CV = 55.78). In
the sample containing the highest dendron–dendrimer ratio
(4 : 1), no micron-bubbles are detected by optical microscopy.
Although micron-bubbles are observed at low dendron–
dendrimer ratios in water, they cannot be found at increased
ratios (44 : 1) in water. DLS measurements point to aggregates
around 1 mm. The same phenomenon has been found in
organic solvents.7 These results indicate that other supramolecular
structures are favored at increased guest–host ratios. To investi-
gate the supramolecular behavior at increased guest–host ratios,
we use the complex system consisting of a 16 : 1 ratio of [G3]
bi-perfluoro-tagged glycerol dendron 1 and [G3.5] dendrimer
with perfluorinated shell 2 in water. As a control, an aqueous
solution that contained only the dendron 1 is also prepared. It is
noteworthy that the dendron concentration (1.6 ꢁ 10ꢀ2 M) in both
samples is above its CMC of 10ꢀ4 M. Dynamic light scattering
(DLS) measurements reveal aggregates of 9.8 nm (CV = 4.08)
corresponding to pure surfactant micelles containing only dendron,
and aggregates of 11.5 nm (CV = 6.09) corresponding to
Fig. 4 TEM images of the dendron–dendrimer mixture (16 : 1) (A)
and perfluoro-tagged [G3] PG dendron 1 (B) in water upon 50-fold
dilution negatively stained with 4.5% aqueous phosphotungstic acid/
0.1% trehalose.
complexes containing the dendron–dendrimer mixture. TEM
measurements confirm this (Fig. 4). The PDI value of the samples
is 0.5 because of the presence of few large aggregates. In both
samples, the small aggregates are the dominant species and there
are only few large spherical aggregates; TEM measurements show
sizes for the large aggregates from 50 to 100 nm. Upon 50-fold
dilution, these larger aggregates are no longer present in the sample
containing a pure amphiphile. However, the number and size of
aggregates decrease in the sample containing the dendron–dendrimer
mixture. Sizes of residual aggregates range from 20 to 30 nm in
diameter. This implies that the formation of large aggregates is
concentration dependent, since there are no effects on the sizes
of the smaller complexes.
Based on these results, we concluded that increasing
guest–host ratios led to smaller aggregates (Scheme 2). This
can only be explained by favored fluorous–fluorous interactions
between the perfluoro alkyl chains of the guest molecules and
the perfluorinated shell of the dendrimers compared to
Scheme 2 Structures of supramolecular aggregates based on fluorous–
fluorous interactions between dendrimers with perfluorinated shell
and perfluoro-tagged guest molecules in dependence of the guest–host
ratio.
c
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Table 1 Effective sizes of supramolecular aggregates in water
amphiphiles in water was studied over the concentration range
of 4 ꢁ 10ꢀ7 to 1 ꢁ 10ꢀ3 M. Aqueous solutions were prepared 24 h
before measurement. The surface tension was determined twice per
minute and the measurement was stopped when the value did not
change by more than 0.1 mN mꢀ1 over three minutes. Equili-
bration time was generally between 50–80 min below the CMC and
25–50 min at higher concentrations.
Average diameter
of aggregates
Coefficient
of variation
System
Ratio
Dendron : dendrimera,b
Dendron : dendrimera,b
Dendron : dendrimera
Dendron : dendrimera,c
Dendrond
1 : 1
2 : 1
4 : 1
16 : 1
—
4.75 ꢂ 0.16 mm
2.80 ꢂ 0.22 mm
B1 mm
24.07
55.78
—
6.09
4.08
11.5 ꢂ 0.10 nm
9.8 ꢂ 0.06 nm
a
Measured at a constant dendrimer concentration of 1.06 ꢁ 10ꢀ3 M;
Optical microscopy. The images of the micron-sized bubbles
formed in water were recorded on a Zeiss Axioskop 40 Pol.
The images were recorded at rt and were evaluated with
AxioVisions 3.1. Objectives used were LD EC-Epiplan-
NEOFLUAR 20x/0.22DIC and EC-Epiplan-NEOFLUAR
50x/0.55DIC. Samples of the various dendron–dendrimer
ratios were prepared by dissolving a certain amount of poly-
glycerol dendrimer with perfluorinated shell (1.06 ꢁ 10ꢀ3 M)
in a solution of the [G3] bi-perfluoro-tagged glycerol dendron
in water. To ensure proper mixing and dissolution of the
compounds all solutions were homogenized by ultrasonication
for half an hour. As solvents, distilled water was used. 20 mL of the
samples were placed on a microscope slide (76 ꢁ 26 mm) and
covered with an additional thinner slide for visualization.
b
size determination by optical microscopy. Errors represent the
c
standard deviation of the mean. Measured at a constant dendrimer
concentration of 1.06 ꢁ 10ꢀ3 M; size determination by DLS and TEM.
d
Measured at a dendron concentration of 1.06 ꢁ 10ꢀ3 M; size
determination by DLS and TEM.
interactions only between dendrons. Since smaller aggregates
of the dendrimers offer a larger fluorous surface and thus more
binding sites for perfluoro-tagged guest molecules such as
amphiphile 1, this finding is reasonable. These results are
summarized in Table 1.
Conclusions
We have shown that supramolecular complexes, which consist
of perfluoro-tagged PG dendrons 1 and PG dendrimers with
perfluorinated shells 2, can be formed in water. In addition the
size of the resulting complex structures can be tuned by variation
of the dendron–dendrimer ratio. The size-tunability of the
supramolecular particles formed by fluorous–fluorous inter-
actions between dendrimers and dendrons is highly attractive
and can be controlled by the dendron–dendrimer ratio. Studies
on the use of these systems as pore forming agents for polymeric
materials are currently in progress. In general, the results
observed also demonstrated that fluorous–fluorous interactions
are applicable for the formation of stable supramolecular struc-
tures in water. Thus, they provide a new tool for the design
of supramolecular architectures in addition to charge neutral
non-covalent interactions, such as hydrogen bonding, dipole
interactions, and arene–arene interactions. In contrast to other
non-covalent interactions, fluorous interactions are highly
stable in aqueous environments. Therefore, it is reasonable to
assume that greater structural complexity and functionality in
supramolecules can be achieved by the hierarchical use of these
interactions.
Dynamic Light Scattering (DLS). Dynamic Light Scattering
measurements were conducted in Millipore filtered water using
a Zetasizer Nano (Malvern Instruments Ltd.). Aqueous solutions
were prepared 24 h before measurement. Samples of dendron–
dendrimer ratios were prepared by dissolving a certain amount of
polyglycerol dendrimer with perfluorinated shell (1.06 ꢁ 10ꢀ3 M)
in a solution of the [G3] bi-perfluoro-tagged glycerol dendron
in water.
Transmission Electron Microscopy (TEM). Transmission
Electron Microscopy samples were prepared on copper grids
(400 mesh) coated with 0.1% collodion/C-film. Samples were
blotted (drop of 5 mL) after 60 s and stained with 4.5%
aqueous phosphotungstic acid/0.1% trehalose after 30 or
60 s and visualized using a FEI CM12 Electron Microscope
at 100 kV high tension.
Syntheses
Diallyl [G3] polyglycerol dendron (5). To a solution of
1.0 equiv. protected propargyl [G3] PG dendron 4 (2.21 g,
1.48 mmol) and compound 3 (0.32 g, 1.63 mmol, 1.1 equiv.)
dissolved in THF 15 mol% of DIPEA (diisopropylethyl
amine) (36.8 mL, 0.22 mmol) was added. After stirring for
5 min 30 mol% of sodium ascorbate (0.09 g, 0.44 mmol) was
added, followed by the addition of 15 mol% CuSO4ꢃ5H2O
(0.056 g, 0.22 mmol). A THF/H2O mixture has to be in a
1 : 1 ratio (v/v). The heterogeneous mixture was stirred
vigorously up to the point, at which TLC analysis indicated
complete consumption of the starting material. The reaction
mixture was diluted with water and extracted three times
with CH2Cl2. The combined organic layers were washed
with a small amount of saturated solution of EDTA, dried
with Na2SO4 and concentrated in vacuo. Purification by
column chromatography with 20% isopropanol in n-hexane
followed by 3% MeOH in CH2Cl2 gives the desired product
5 (2.31 g, 92%). (A stock solution of sodium ascorbate
Experimental
Methods
CMC determination. The critical micelle concentration
(CMC) of the [G3] bi-perfluoro-tagged glycerol dendron was
determined by measuring the surface tension of the amphiphiles
in deionized water (Millipore system Milli-Q plus) by the pendant
drop method. The contact angle tensiometer OCA20 (DataPhysics
Instruments GmbH, Filderstadt, Germany) was used for these
measurements and the temperature was set at 25 ꢂ 0.5 1C.
Calculation of the surface tension was done by using the
Young–Laplace-equation. The accuracy of measurements, checked
by one replicate experiment and by control of the surface tension
for pure water, was ꢂ0.4 mN mꢀ1. The behaviour of the
c
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and CuSO4ꢃ5H2O in water was prepared in a concentration of
100 mg mLꢀ1.)
Acknowledgements
1H NMR (500 MHz, CDCl3, 25 1C): d = 7.73 (m, 1 H, CH-
aromatic), 5.86–5.76 (m, 2 H, OCH2CHQCH2), 5.25–5.14
(m, 4 H, OCH2CHQCH2), 4.86 (m, 1 H, N–CH–(CH2O)2),
4.78 (s, 2 H, O–CH2–Ar), 4.21 (m, 8 H, –CHCH2–O–C(CH3)2),
4.05–3.94 (m, 12 H, –CHCH2–O–C(CH3)2, OCH2CHQCH2),
3.84 (m, 4 H, N–CH–(CH2O)2), 3.75–3.40 (m, 59 H, CH2CHO
backbone), 1.38 (2 s, 24 H, –CH3), 1.32 (2 s, 24 H, –CH3) ppm;
13C NMR (100 MHz, CDCl3, 25 1C): d = 144.98, 133.89, 122.81,
117.42, 109.17, 78.49, 78.23, 74.62, 74.48, 72.36, 72.11,
71.81–71.13, 70.50, 69.97, 68.66, 66.64, 63.87, 60.52, 26.68,
25.31 ppm; ESI-TOF MS: calculated for C81H141N3O33
(1683.9447); found 864.9635 [M+Na]2+, 1684.9552 [M+H]+,
1706.9375 [M+Na]+.
R. H. and M. Z. are indebted to the graduate school ‘‘Fluorine
as a Key Element’’ (GRK 1582) funded by the German
Science Foundation (DFG). Furthermore, we thank A. Schulz
and A. Wiedekind for their help with TEM measurements and
CMC determination. M. W. and K. B. are thankful to
W. Munch and M. Selent for their help with HPLC purification.
¨
Notes and references
1 C. P. Jariwala and L. J. Mathias, Macromolecules, 2002, 26,
5129–5136; P. Kirsch, Modern Fluoroorganic Chemistry,
Wiley-VCH, Weinheim, 2004, pp. 8–9; R. L. Scott, J. Am. Chem.
Soc., 1948, 70, 4090–4093; J. H. Hildebrand and D. R. F. Cochran,
J. Am. Chem. Soc., 1949, 71, 22–25; J. G. Riess and M. LeBlanc,
Pure Appl. Chem., 1982, 54, 2383–2406.
2 I. T. Horvath and J. Rabai, Science, 1994, 266, 72–75.
´
Perfluoro-tagged [G3] polyglycerol dendron (2). To compound
5 (1.3 g, 0.77 mmol, 1.0 equiv.) four equiv. of Rf8CH2CH2SH per
allyl group (2.96 g, 6.18 mmol) were added. The solution was
stirred under reduced pressure (3 mbar) to remove oxygen from
the solution. After heating to 80 1C, a catalytic amount of AIBN
was added under an argon atmosphere and the reaction mixture
was stirred for 2 h. After further addition of the same amount of
AIBN, the mixture was stirred for another 24 h at 80 1C.
Evaporation of an excess of Rf8CH2CH2SH was followed by
flash column chromatography (petroleum ether (40–60 1C)/
CH2Cl2 (7 : 1, v/v) and then by AcOEt/n-hexane (10 : 1)). This
afforded the desired protected perfluoro-tagged [G3] PG dendron
(1.48 g, 73%). The protected perfluoro-tagged [G3] PG dendron
was subsequently dissolved in a solvent mixture of MeOH/
CH2Cl2 (25 mL gꢀ1). To this solution Dowex-50W resin
(2 g gꢀ1) was added after activation. The mixture was stirred
and heated at reflux for 18 h. The crude product was filtered,
concentrated, and dried under vacuum. The desired product 1
was obtained as a colorless wax (1.12 g, 86%).
3 D. P. Curran and Z. Luo, J. Am. Chem. Soc., 1999, 121,
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1H NMR (500 MHz, CD3OD/CDCl3, 25 1C): d = 7.67
(s,
1 H, CH-aromatic), 4.72 (p, 1 H, J = 5.8 Hz,
N–CH–(CH2O)2–), 4.60 (s, 2 H, –OCH2–Ar), 3.69 (d, 4 H,
J = 5.8 Hz, NCHCH2O), 3.62 (m, 10 H, –CHCH2–OH,
–OCH(OCH2)2), 3.50 (m, 17 H, CH2CHO backbone),
3.46–3.29 (br m, 66 H, CH2CHO backbone, –OCH2CH2CH2S–),
2.55 (m, 4 H, –SCH2CH2CF2–), 2.43 (t, 4 H, J = 7.1 Hz,
–OCH2CH2CH2S–), 2.20 (m, 4 H, –SCH2CH2CF2–), 1.67 (p, 4 H,
J = 6.5 Hz, OCH2CH2CH2S–) ppm; 13C NMR (100 MHz,
CD3OD/CDCl3, 2.5 1C): d = 144.66, 123.25, 122.00–107.00
(C-Rf8), 78.73, 78.50, 78.37, 78.00, 72.70, 71.78, 71.22, 70.97,
70.86, 70.66, 69.93, 69.60, 69.49, 69.32, 63.34, 60.54, 31.81
(t, J = 22.4 Hz), 28.92, 28.57, 22.41 ppm; 19F NMR
(470 MHz, CD3OD/CDCl3, 25 1C): d = ꢀ81.17 (t, J = 9.8
Hz, CF3CF2–), ꢀ114.59 (CF3CF2–), ꢀ121.98, ꢀ122.14
(CF3CF2–CF2CF2CF2–), ꢀ123.03 (–CF2CF2CF2–CH2), ꢀ123.60
(–CF2CF2–CH2), ꢀ126.46 (–CF2CF2–CH2) ppm; ESI MS: calcu-
lated for C77H119F34N3O33S2 (2323.6624); found 1184.8215
[M+2Na]+2, 2346.6524 [M+Na]+, 2362.6282 [M+K]+.
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10 Average diameter and coefficient of variation (CV) of all particles
were calculated from the microscopy and TEM images by measuring
the diameter of over 50 particles randomly selected from each sample.
The value of CV is defined by CV = s/[M] ꢁ 100, where s is the
standard deviation and [M] is the average diameter.
c
406 New J. Chem., 2012, 36, 402–406
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012