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Molecular-selective adsorption property of chemically surface modified nanoporous alumina membrane by di(1-naphthyl)silanediol to anthracenes.

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Full Paper
Received: 22 July 2009
Revised: 14 October 2009
Accepted: 15 October 2009
Published online in Wiley Interscience: 26 November 2009
(www.interscience.com) DOI 10.1002/aoc.1586
Molecular-selective adsorption property of
chemically surface modified nanoporous
alumina membrane by di(1-naphthyl)silanediol
to anthracenes
Kenji Kakiagea, Toru Kyomena,b , Masafumi Unnoa,b and Minoru Hanayaa,b∗
Nano-porous alumina membrane (NPAM) formed by the anodic oxidation of aluminum is an attractive composite as the base
material for a functional filter, because of its honeycombed ordered structure with large surface area per weight and also high
shape stability. In this work, we investigated the adsorption properties of the NPAM possessing π -electron systems on the
surface, which were produced through chemical surface modification by di(1-naphthyl)silanediol, to aromatic compounds using
anthracenes as typical aromatic compounds. The chemically surface-modified NPAM exhibited strong affinity to anthracene
molecules and the affinity was observed to be weakened remarkably with the introduction of methyl and phenyl substituents
c 2009 John Wiley & Sons, Ltd.
to anthracene, indicating a molecular-selective adsorption property of the NPAM. Copyright Keywords: nanoporous alumina membrane; chemical surface modification; di(1-naphthyl)silanediol; adsorption property; molecular
selectivity
Introduction
198
Organosilicon compounds having silanol moieties and/or alkoxysilyl groups possess high bonding ability to the surfaces of metal
oxides forming Si–O–metal bonds.[1 – 3] They can produce functional properties reflecting the characteristics of the organic
fragments included in them to the metal-oxide surfaces through
the chemical surface modification. Nano-porous alumina membrane (NPAM), which is a typical porous material formed by
the anodic oxidation of aluminum, has a regular honeycombed
structure with nanometer to sub-micrometer pores, and the pore
size can be controlled easily by changing the anodic oxidation
conditions.[4 – 6] Because of its characteristic ordered nanostructure, the NPAM has been used as a filter for microfiltration, as
a template to form nanotube and nanowire arrays of inorganic
materials, and as a mask in vacuum evaporation to form nanodot
arrays.[7,8] Besides those applications, the NPAM is also used directly as a substrate in sensor applications.[9] However, there have
been no studies so far attempting to add a certain function to
the NPAM based on π -electron systems of organic compounds.
NPAMs have large surface area per weight of more than 10 m2 g−1
and high shape stability as a characteristic of metal-oxide composites. Therefore, the chemical surface modification of NPAMs by
the organosilicon compounds having π -electron systems would
produce functional materials such as a molecular selective filter
for aromatic compounds with low flow resistance due to the
straight-pore structure.
Recently, we examined the chemical surface modification of
NPAMs by di(1-naphthyl)silanediol, (Nap)2 Si(OH)2 , in order to
obtain the quantitative information on the reaction conditions
for the efficient surface modification of metal oxides by silanols,
and attained to modify the surface of NPAMs by (Nap)2 Si(OH)2
with high covering ratio. [Precise information about the relation
Appl. Organometal. Chem. 2010, 24, 198–200
between the reaction conditions and the covering ratios of
the NPAM surface by (Nap)2 Si(OH)2 in the chemical surface
modification will be published elsewhere.] In this work, we
examined for the first time the adsorptive properties of the
NPAM possessing π -electron systems on the surface, which
were produced through chemical surface modification by di(1naphthyl)silanediol, to aromatic compounds using anthracene
and its derivatives, and found molecular-selective adsorption
properties of the NPAM.
Experimental
Nanoporous alumina membranes with a pore diameter
of ∼200 nm in a round shape of 21 mm o.d. and
∼60 µm thickness were purchased from Whatman. Di(1naphthyl)silanediol, (Nap)2 Si(OH)2 , was synthesized by lithiation
of 1-bromonaphthalene followed by the substitution reaction
with tetrachlorosilane and subsequent hydroxylation, as has been
reported previously.[10]
The chemically surface-modified NPAMs with (Nap)2 Si(OH)2
were prepared by immersing NPAMs to the solutions of
∗
Correspondence to: Minoru Hanaya, Gunma University, Department of
Chemistry and Chemical Biology, Graduate School of Engineering, Tenjin-cho 15-1, Kiryu, Gunma 376-8515, Japan. E-mail: hanaya@chem-bio.gunma-u.ac.jp
a Department of Chemistry and Chemical Biology, Graduate School of
Engineering, Gunma University, Tenjin-cho 1-5-1, Kiryu, Gunma 376-8515,
Japan
b International Education and Research Center for Silicon Science, Graduate
School of Engineering, Gunma University, Tenjin-cho 1-5-1, Kiryu, Gunma
376-8515, Japan
c 2009 John Wiley & Sons, Ltd.
Copyright Adsorption property of chemically surface modified NPAM by silanol
1.2
∆Abs.
OH
(Nap)2Si(OH)2
0.4
anthracene
(b)
(a)
(c)
0
15000
(Nap)2Si(OH)2
10000
anthracene
5000
0
260
Results and Discussion
300
340
380
420
λ / nm
Figure 1. The difference spectra of (a) NPAM chemically surface-modified
by (Nap)2 Si(OH)2 ; (b) surface-modified NPAM after soaking in 1 mM
acetonitrile solution of anthracene; and (c) as-received NPAM after soaking
in the acetonitrile solution of anthracene: The spectra were obtained by
subtracting the absorption spectrum of as-received NPAM from those
of the samples (upper). The absorption spectra of (Nap)2 Si(OH)2 and
anthracene in acetonitrile solutions (lower).
(b)
∆Abs.
1.2
(c)
0.8
∆Abs. (at 393 nm)
(a)
1.6
0.2
(d)
0
(e)
0.4
0.4
0
20
40
60
80
Covering ratio / %
100
(f)
(g)
0
260
300
340
380
420
λ / nm
Figure 2. The difference spectra of NPAM chemically surface-modified by
(Nap)2 Si(OH)2 with different covering ratios from 0 to 97% after soaking
in 5 mM acetonitrile solution of anthracene; the covering ratios were 97%
(a), 78% (b), 59% (c), 47% (d), 37% (e), 24% (f) and 0% (g). The spectra were
obtained by subtracting the absorption spectrum of as-received NPAM
from those of the samples. The inset shows the relation between the
covering ratio and the absorbance at 393 nm.
In order to obtain more precise information about the
adsorption properties of the chemically surface-modified
NPAM, the adsorption of 9,10-dimethylanthracene and 9,10diphenylanthracene to the NPAM was examined by soaking the
NPAMs in 5 mM acetonitrile solutions of those anthracenes. The
results are shown in Fig. 3 as the difference spectra of the NPAMs
after soaking in the solutions of anthracenes along with the result for anthracene having no substituent. The absorption spectra
of the anthracenes in acetonitrile solutions are also shown in
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
199
Figure 1 shows the difference spectra of the chemically surfacemodified NPAM by (Nap)2 Si(OH)2 (a), the surface-modified NPAM
after soaking in 1 mM acetonitrile solution of anthracene (b), and
the as-received NPAM after soaking in the acetonitrile solution
of anthracene (c), which were obtained by subtracting the
absorption spectrum of as-received NPAM from those of the
samples. The absorption spectra of (Nap)2 Si(OH)2 and anthracene
in acetonitrile solutions are also shown in the figure. The surfacemodified NPAM exhibited absorption due to the chemically
adsorbed (Nap)2 Si(OH)2 onto the NPAM surface (Fig. 1a). After
soaking in the anthracene solution, the surface-modified NPAM
showed an additional absorption band with a vibrational structure
between 320 and 410 nm (Fig. 1b), which is assignable to the
π –π ∗ transition of anthracene,[11] indicating the adsorption of
anthracene molecules on the surface of the chemically surfacemodified NPAM. On the other hand, the as-received NPAM
exhibited almost no characteristic absorption even after soaking
in the anthracene solution (Fig. 1c).
Figure 2 shows the difference spectra of the chemically surfacemodified NPAM by (Nap)2 Si(OH)2 with different covering ratios
from 0 to 97% after soaking in 5 mM acetonitrile solution
of anthracene. The spectra were obtained by subtracting the
absorption spectrum of as-received NPAM from those of the
samples. All the samples showed almost the same spectral
features except for the intensity, and thus the magnitude of
the absorbance of the absorption due to the anthracene at the
maximum (λ = 393 nm) is considered to be the index for the
amount of anthracene molecules adsorbed on the surface of the
chemically surface-modified NPAM. The inset in Fig. 2 shows the
dependence of the absorbance at 393 nm on the covering ratio,
showing a rather linear relation between the amounts of the
adsorbed anthracene molecules and the covering ratios.
These results clearly show that the adsorptive property of
the NPAM to anthracene was developed by the chemical surface
modification with (Nap)2 Si(OH)2 , and also indicate that anthracene
molecules were adsorbed on the chemically surface-modified
NPAM through the interaction between anthracene and (Nap)2 Simoiety on the NPAM surface. By chemical surface modification
with (Nap)2 Si(OH)2 the surface of NPAM is covered with naphthyl
groups, and the π /π interaction between the naphthyl group and
anthracene molecule is considered as the origin of this adsorptive
property.[12]
Appl. Organometal. Chem. 2010, 24, 198–200
Si OH
0.8
ε / dm3 mol–1 cm–1
(Nap)2 Si(OH)2 at 25 ◦ C for 24 h. The solutions with different solvents and concentrations were used for the immersion to control
the covering ratio of the NPAM surface by the adsorbent of
(Nap)2 Si(OH)2 from 24 to 97%; 1 mM ethanol, 1 mM chloroform,
3 mM benzene, 1 mM benzene, 1 mM toluene and 0.3 mM cyclohexane solutions of (Nap)2 Si(OH)2 were used for preparing the NPAMs
with the covering ratios of 24, 37, 47, 59, 78 and 97%, respectively.
The adsorption of anthracenes to the surface-modified NPAMs
and also to as-received NPAMs was performed by soaking
NPAMs in 1 or 5 mM acetonitrile solutions of anthracene,
5 mM acetonitrile solution of 9,10-dimethylanthracene and 5 mM
acetonitrile solution of 9,10-diphenylanthracene at 25 ◦ C for
24 h. After the soaking, the samples were dried at ambient
temperature and used for UV–vis absorption measurements.
UV–vis absorption spectra were recorded at 25 ◦ C using a Hitachi
U-3010 spectrometer equipped with an integrating sphere.
K. Kakiage et al.
the potential of the NPAM as a functional filter for the separations of anthracenes and possibly for the separation of aromatic
compounds.
(a)
ε / dm3 mol–1 cm–1
∆Abs.
0.4
Conclusion
(b)
0.2
(c)
0
15000
10000
5000
0
320
360
400
440
λ / nm
Figure 3. The difference spectra of (a) NPAM chemically surface-modified
by (Nap)2 Si(OH)2 after soaking in 5 mM acetonitrile solution of anthracene;
(b) after soaking in the solution of 9,10-dimethylanthracene; and (c) after
soaking in the solution of 9,10-diphenylanthracene: The spectra were obtained by subtracting the absorption spectrum of as-received NPAM from
those of the samples (upper). The absorption spectra of anthracene (solid
line), 9,10-dimethylanthracene (dotted line), and 9,10-diphenylanthracene
(broken line) in acetonitrile solutions (lower).
Fig. 3. The chemically surface-modified NPAM exhibited an adsorptive property to 9,10-dimethylanthracene, but the amount of
the adsorbed molecules was much smaller than in the case of
anthracene, and only a slight adsorption was observed for 9,10diphenylanthracene. The observed difference of the adsorption
property of the NPAM to the anthracenes is considered to be
brought about by the bulkiness of the substituents introduced to
anthracene; the methyl groups of 9,10-dimethylanthracene hinder the interaction between the anthracene ring and the naphthyl
group on the NPAM, and the hindrance effect is much increased
by the phenyl groups of 9,10-diphenylanthracene in which the
phenyl rings cross to the anthracene fragment at the angle of
∼70◦ .[13] These results indicate the molecular selectivity of the
chemically surface-modified NPAM by (Nap)2 Si(OH)2 , exhibiting
In this work, we observed the chemically surface-modified NPAM
by (Nap)2 Si(OH)2 has adsorptive property to anthracene, and the
property was concluded to be brought about by the naphthyl
groups of (Nap)2 Si(OH)2 through the π /π interaction between
the naphthyl groups and anthracene molecules. The adsorptive
property was confirmed to be sensitive to the molecular structure
of anthracenes, and the surface-modified NPAM exhibited only
a slight affinity to 9,10-diphenylanthracene. The molecular
selectivity indicates a high potential of the surface-modified
NPAMs by silanols and alkoxysilanes having π -electron systems as
functional filters for separating aromatic compounds.
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[10] S. Kondo, T. Harada, R. Tanaka, M. Unno, Org. Lett. 2006, 8, 4621.
[11] J. Malkin Photophysical and Photochemical Properties of Aromatic
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200
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Copyright Appl. Organometal. Chem. 2010, 24, 198–200
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