Synthesis of amphiphilic polyhedral oligomeric silsesquioxane having a hydrophobic fluorescent dye group and its formation of fluorescent nanoparticles in water

Article abstract of DOI:10.1246/cl.2010.1045

In this study, we synthesized an amphiphilic polyhedral oligomeric silsesquioxane (POSS) having controlled numbers of hydrophilic and fluorescent hydrophobic groups (seven and one, respectively) by the successive reactions of a starting compound, octa(3-aminopropyl)-POSS, with gluconolactone and with dansyl chloride under appropriate conditions. The product formed fluorescent nanoparticles with relatively welldefined morphology in water by self-aggregation, which was confirmed by SEM, DLS, and fluorescence measurements.

Full text of DOI:10.1246/cl.2010.1045

doi:10.1246/cl.2010.1045
Published on the web August 21, 2010
1045
Synthesis of Amphiphilic Polyhedral Oligomeric Silsesquioxane Having a Hydrophobic
Fluorescent Dye Group and Its Formation of Fluorescent Nanoparticles in Water
Shin-ya Kuwahara, Kazuya Yamamoto, and Jun-ichi Kadokawa*
Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065
(
Received June 30, 2010; CL-100599; E-mail: kadokawa@eng.kagoshima-u.ac.jp)
In this study, we synthesized an amphiphilic polyhedral
oligomeric silsesquioxane (POSS) having controlled numbers
of hydrophilic and fluorescent hydrophobic groups (seven and
one, respectively) by the successive reactions of a starting
compound, octa(3-aminopropyl)-POSS, with gluconolactone
and with dansyl chloride under appropriate conditions. The
product formed fluorescent nanoparticles with relatively well-
defined morphology in water by self-aggregation, which was
confirmed by SEM, DLS, and fluorescence measurements.
Amphiphilic molecules, of which soap is a typical example,
possess antagonistic hydrophilic and hydrophobic moieties in
the same molecule. In aqueous media, such molecules self-
assemble into diverse aggregate morphologies, depending on the
molecular shape and solution conditions.1 Recently, we have
reported synthesis of organic­inorganic amphiphilic hybrids
­3
4­6
based on the poly(2-aminopropylsiloxane) backbone.
The
hybrids were synthesized by the successive reactions of amino
groups in the polysiloxane with fatty acid chlorides and with
sugar lactones. In the products, the former acted as hydrophobic
groups, whereas the latter functioned as hydrophilic groups for
the amphiphilic property. The further functionalization of the
remaining free amino groups in the products with fluorescent
dye moieties was conducted by reaction with fluorescein
thioisocyanate to give the fluorescent-dye-labeled amphiphilic
6
polysiloxanes. We also confirmed the formation of nano-
particles from dispersion of the products in water, which
exhibited fluorescence emission. Furthermore, cellular uptake of
the amphiphilic polysiloxane-based nanoparticles in human
aortic endothelial cells was observed by fluorescence microsco-
py. These results suggest that such organic­inorganic amphi-
philic hybrids have a possible potential to use as biomaterials in
biological and medical applications. Moreover, silicon-contain-
ing materials such as polydimethylsiloxane have various
interesting properties conferring advantages as biomaterials,
such as hemocompatibility, biocompatibility, and anti-inflam-
Scheme 1. Synthesis of amphiphilic POSS 3.
amino groups in OapPOSS with appropriate hydrophilic and
hydrophobic substrates to synthesize amphiphilic fluorescent
POSS derivatives. It has been considered that the functionaliza-
tion of the desired numbers of amino groups to respective
hydrophilic and hydrophobic groups by controlling the reaction
conditions possibly yields a POSS derivative with definite
numbers of hydrophilic and hydrophobic groups. As sources for
the hydrophilic and hydrophobic moieties, gluconolactone and
dansyl chloride were employed due to the presence of the
reactive sugar lactone and sulfonyl chloride groups, respectively,
for the amino groups (Scheme 1). Because the dansyl group is
a fluorescent dye, it can be expected to act not only as the
hydrophobic group, but also as the fluorescent moiety for the
formation of the fluorescent nanoparticles.
7
matory activity. On the basis of the above viewpoints and
backgrounds, in this study, we noted polyhedral oligomeric
8
silsesquioxanes (POSSs) in place of the polysiloxanes as the
silicon-containing inorganic framework for the synthesis of new
organic­inorganic amphiphilic hybrids. Because POSS deriva-
tives do not exhibit polydispersity, which is different from
polysiloxanes, the former would have an advantage over the
latter in the formation of fluorescent nanoparticles with well-
defined morphologies by self-aggregation. Indeed, we found that
the amphiphilic POSS prepared in this study formed more
morphologically controlled nanoparticles in water compared
with the previously reported amphiphilic polysiloxanes.
Accordingly, the reaction of OapPOSS with gluconolactone
was first performed in varying feed molar ratios in DMSO
1
2
solvent. The MALDI-TOF MS measurements of the crude
products were conducted to evaluate how many amino groups
participated in the reaction with gluconolactone under each
condition. Consequently, when the reaction was carried out in a
64:1 feed molar ratio of gluconolactone to OapPOSS in DMSO
9­11
We employed an octa(3-aminopropyl)-POSS (OapPOSS)
as a starting POSS derivative and attempted reactions of eight
Chem. Lett. 2010, 39, 1045­1047
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1
046
Figure 1. MALDI-TOF MS of the products obtained by the
reaction of OapPOSS with gluconolactone in DMSO at 80 °C
for 24 h (feed molar ratio of gluconolactone to OapPOSS =
1
Figure 2. H NMR spectrum of 3 in D O.
2
64:1).
at 80 °C for 24 h, the MALDI-TOF MS of the crude products,
which were obtained as an acetone-insoluble fraction, showed
only two peaks (m/z 2126.572 and 2305.439) corresponding to
the masses of the POSS derivatives 1 and 2 (Figure 1), in which
seven and eight amino groups were functionalized with hydro-
philic groups, respectively. From the integrated ratio of the
signal due to CH(OH)­CH(OH)­C=O derived from glucono-
1
lactone to the signal due to CH2Si of OapPOSS in the H NMR
spectrum of the crude products (DMSO-d6), the molar ratio of 1
to 2 was calculated as 0.215:0.785. These data indicated that one
of the two products, i.e., the derivative 1, still had an unreacted
amino group, which was further used for the reaction with
dansyl chloride. Because the two POSS products 1 and 2 could
not be separated by the simple procedure, the following
functionalization was conducted using the mixture of the two
products.
Figure 3. SEM image of the spin-coated sample prepared
from an aqueous dispersion of 3.
The reaction of the amino group in 1 with dansyl chloride
was performed by stirring a solution of the aforementioned
products, dansyl chloride, and triethylamine in a mixed solvent
of water and DMF at room temperature for 2 h. Thus, the
resulting dansyl-functionalized POSS derivative was roughly
separated from 2 by means of the difference in their solubility. In
addition to the low content of 1 in the products obtained by the
first reaction, complicated procedures including column chro-
matography on ion-exchange resin and dialysis were necessary
to obtain 3 in high purity. The improvement of the isolation
procedure of 3 for increasing the yield is now in progress.
1
Figure 2 shows the H NMR spectrum of the isolated POSS
derivative in D O. In addition to signals due to Si(CH ) N at
2
2 3
Figure 4. DLS profiles of aqueous dispersions of 3 in various
¤
0.53, 1.49, and 3.11, signals assignable to the hydrophilic
¹
4
¹3
¹3
concentrations; 1.0 © 10 (a), 1.0 © 10 (b), and 2.0 © 10
groups derived from gluconolactone at ¤ 3.45­4.25 and signals
¹1
(
c) mol L .
ascribable to the dansylamide group at ¤ 2.86 (CH ) and ¤ 7.37­
3
8.33 (aromatics) were observed. The integrated ratio of the
signals due to SiCH2, the hydrophilic, and aromatic protons
of a spin-coated sample from a dispersion of 3 in water.
Nanoparticles with relatively narrow distribution of sizes were
clearly seen. Average diameter and a standard deviation were
calculated to be 37.9 and 3.29 nm, respectively. This standard
deviation is much smaller than that calculated from the SEM
image for the amphiphilic polysiloxane nanoparticles reported in
2
9
were 16:42:6. The Si NMR spectrum of 3 in DMSO-d6 showed
four signals at ¤ ¹65.9, ¹66.0, ¹66.1, and ¹66.2, ascribable to
14
four kinds of Si atoms in 3 (Figure S1 ). Their chemical shifts
were close to those of OapPOSS (¤ ¹66.4), suggesting that the
POSS framework was maintained in the product. The NMR data
fully supported the structure of the POSS derivative 3 having
seven hydrophilic groups and a hydrophobic dansyl group.
To confirm the self-aggregation of 3, SEM and DLS
measurements were conducted. Figure 3 shows the SEM image
1
4 5
our previous paper (Figure S2 ). These results indicated that 3
formed the more morphologically controlled nanoparticles in
water compared with the amphiphilic polysiloxanes. The DLS
¹
4
¹1
measurement of a dispersion of 3 in water (1.0 © 10 mol L
,
Chem. Lett. 2010, 39, 1045­1047
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1
047
In conclusion, we reported herein the synthesis of a
fluorescent amphiphilic POSS derivative from OapPOSS as a
starting compound. Under the appropriate conditions, the
reaction of seven amino groups with gluconolactone and the
subsequent reaction of an amino group with dansyl chloride
successfully took place to give the amphiphilic OapPOSS 3 with
controlled functionalities of the hydrophilic and hydrophobic
groups. The product formed more morphologically controlled
nanoparticles in water compared with the previously reported
amphiphilic polysiloxanes, which was confirmed by the SEM
and DLS measurements. Furthermore, the product exhibited
fluorescence emission in water, because of the presence of the
dansyl group. Further studies on the synthesis of different
amphiphilic POSS having various hydrophilic and hydrophobic
groups by the same procedure as that described here and the
effect of their molecular structures on self-aggregation are now
in progress.
Figure 5. Fluorescence spectra of 3 excited at 310 nm in water
(
a) and N-butyl-5-(dimethylamino)naphthalene-1-sulfonamide
¹4
excited at 340 nm in methanol (b): concentrations; 0.90 © 10
¹1
mol L
.
References and Notes
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with increasing the concentration of the dispersion as shown in
Figures 4b and 4c; 101.0 (1.0 © 10 mol L ) and 135.4 nm
¹3
¹1
3
4
¹
3
¹1
(
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3
5
6
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¹4
¹1
(
0.90 © 10 mol L , a similar concentration as that in the DLS
measurement (Figure 4a)) exhibited an emission maximum at
03 nm by excitation at 310 nm (Figure 5a), which was a typical
fluorescence peak due to the dansyl moiety as appeared in the
fluorescence spectrum of N-butyl-5-(dimethylamino)naphtha-
lene-1-sulfonamide in methanol (Figure 5b, Ex. 340 nm, Em.
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derived from disaccharides, was previously reported. In the
study, all the amino groups in OapPOSS were functionalized
5
9
to give hydrophilic POSS derivatives.
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13 nm). The above SEM, DLS, and fluorescence analyses
suggested the formation of fluorescent nanoparticles with
relatively well-defined morphology by self-aggregation of the
amphiphilic POSS derivative 3 in water.
14 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Chem. Lett. 2010, 39, 1045­1047
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/