Biomimetic dopamine derivative for selective polymer modification of halloysite nanotube lumen

Article abstract of DOI:10.1021/ja303340f

We demonstrate the use of a catecholic anchor (Dopa) for selective modification of the inner surface of an halloysite clay nanotube. Aqueous Dopa binds to alumina at the tube lumen and does not bind the silica surface under the same conditions. Selectivity of surface modification was evidenced using X-ray photoelectron spectroscopy (XPS) and ¹³C solid state NMR spectroscopy. Surface-initiated atom transfer radical polymerization (SI-ATRP) was performed through selectively adsorbed Dopa to graft a layer of polymer brush into the nanotube lumen.

Full text of DOI:10.1021/ja303340f

Article
pubs.acs.org/JACS
Biomimetic Dopamine Derivative for Selective Polymer Modification
of Halloysite Nanotube Lumen
Weng On Yah,†, ‡ Hang Xu,† Hiroe Soejima,§ Wei Ma,§ Yuri Lvov,∥ and Atsushi Takahara*,†, ‡,§
§
†Graduate School of Engineering, ‡International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), and Institute for
Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
∥Institute for Micromanufacturing, Louisiana Tech University, 911 Hergot Avenue, Ruston, Louisiana 71272, United States
S
* Supporting Information
ABSTRACT: We demonstrate the use of a catecholic anchor (Dopa)
for selective modification of the inner surface of an halloysite clay
nanotube. Aqueous Dopa binds to alumina at the tube lumen and does
not bind the silica surface under the same conditions. Selectivity of
surface modification was evidenced using X-ray photoelectron spectros-
copy (XPS) and 13C solid state NMR spectroscopy. Surface-initiated
atom transfer radical polymerization (SI-ATRP) was performed through
selectively adsorbed Dopa to graft a layer of polymer brush into the
nanotube lumen.
Among hybrid meso-materials, tubular clay minerals occupy a
special place because they are readily available at low cost while
constituting a specific nanoscale matrix for the assembly of
organic components. The environmental friendliness and
biocompatible nature make halloysite clay nanotubes
[Al2Si2O5(OH)4·nH2O] (Scheme 1) an important nanomateri-
A new tool for selective modification of silica−metal oxides
materials is catechol derivative inspired by strong wet adhesion
systems developed by nature: 3,4-dihydroxyphenylalanine
(dopamine) is found in mussel adhesive proteins that
contribute to the high sticking ability of marine adhesives.14−17
Although the binding mechanism is still under debate, the
chemisorption to metal oxide surfaces was proposed to
comprise the monodentate−bidentate bond.18−20 The binding
of catechol to silica is noncovalent; its binding energy
amounted to 14 kcal/mol, which is smaller than that for the
covalent binding with a metal oxide.21,22
Scheme 1. FE-SEM Image of Halloysite on Si-Wafer (Left)
and Schematic Illustration of Crystalline Structure of
Halloysite (Right)
Catechol or dopamine derivatives have recently been used to
anchor small functional biomolecules onto ferromagnetic Fe2O3
and semiconductor TiO2 nanoparticles for biomedical
applications.23−28 While polydopamine/melanin films can be
deposited on almost all kinds of substrate,29−34 film detachment
occurs from Si or Ge substrate, probably due to the weak
interactions of catechol with these nonmetal surfaces.35−37 In
the research for simple and versatile strategies to functionalize
tubular clay minerals and encouraged by the successful selective
modification of halloysite nanotube lumen with alkyl
phosphonic acid, we demonstrate here that the dopamine
derivative can also be covalently linked in aqueous conditions
to the alumina innermost surface but not to the silica outermost
surface of halloysite. The modified halloysite with dopamine
bearing bromo-group was further used as an initiator for the
surface initiated-ATRP (SI-ATRP) polymerization to selec-
tively graft a layer of PMMA brush in the lumen (Scheme 2).
The lumen of the halloysite tubes was immobilized with 2-
bromo-N-[2-(3,4-dihydroxyphenyl)ethyl]-isobutyryl amide
(Dopa), which also served as an ATRP initiator.38 The
al for developing new organic/inorganic composites. Halloysite,
a hydrated polymorph of kaolinite consists of silica on the outer
surface and alumina in the innermost surface.1 Therefore,
different inner and outer-compositions of these natural
nanotubes allow for different chemical reactions in these
tubes' interior and external surfaces.
Modification of the halloysite tube outer surface is usually
intended to improve clay dispersal in polymer matrix/fluidic
material,2−9 and its selective interior modification by
immobilization of functional groups via covalent bonds could
open up new applications based upon molecular recognition,
such as molecular separation, molecular storage, catalysis, and
drug delivery.10−12 However, selective modification of interiors
remains a difficult task; so far, only organophosphate has been
successfully used for stable selective modification of halloysite
inner surface.13
Received: April 6, 2012
Published: July 5, 2012
© 2012 American Chemical Society
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Scheme 2. Selective Modification of Halloysite Lumen with
Dopa Biomimetic Initiator and Subsequent Polymerization
Figure 2. Solid-state 13C NMR spectra of Dopa and halloysite-Dopa
(Dopa formula is given at the right).
which can be readily assigned; 177.0 ppm C(3), 144.3 ppm
C(9), 142.2 ppm C(10), 130.2 ppm C(6), 118.8 ppm C(8),
C(11), 116.7 ppm C(7), 62.7 ppm C(2), 60.0 ppm C(4), 39.5
ppm C(5), 34.0 ppm C(1). The spectrum of halloysite-Dopa
shows broader resonances than that of the original molecule,
suggesting slower motion of grafted Dopa inside the halloysite
lumen.41 The resonance corresponding to the catechol groups
disappears in the composite or is at least strongly decreased,
while the carbons of the aliphatic side chain are still
predominant. The 13C NMR spectra indicate that the chemical
environment of grafted Dopa is somewhat different from the
starting material due to strong catechol−alumina interaction in
the halloysite lumen.42
attachment of Dopa to the halloysite was confirmed from the
characteristic C−H and carbonyl stretching vibrations centered
at 2930 cm−1 and 1722 cm−1, respectively in the FTIR
spectrum (Figure S1, Supporting Information).20,39,40 How-
ever, the C−H and CO stretching bands are very weak. The
quantity of the Dopa attached to the halloysite was determined
by thermogravimetric analysis (TGA) to be 1.1 wt%, and the
relative amount of grafted PMMA was 7.1 wt% (Figure S3,
Supporting Information).
The presence of Dopa was confirmed by XPS measurement.
The main difference between Figure 1 (a) and (b) is the
The selective adsorption nature of Dopa on metal oxide over
silica was evidenced from solid-state 13C NMR of silica and
alumina nanoparticle before and after treatment.43 In the model
experiment, alumina nanoparticles were treated with Dopa in a
mixture solvent of THF:H2O (4:1), then sonicated and rinsed
with THF:H2O solution five times and dried under vacuum at
70 °C. For alumina−Dopa, the 13C NMR spectra of the
resulting powders (Figure 3a) displayed two resonances. The
broad resonances at 20−50 ppm (aliphatic) and 181.1 ppm
C(3) were attributed to the grafted Dopa species through
formation of alumina−catechol covalent bonds. However, when
silica nanoparticles were treated with Dopa in THF:H2O, Dopa
Figure 1. High resolution XPS of the Br 3d and N 1s region for Si (a)
and Al (b) surfaces before and after treated with Dopa.
presence of N 1s and Br 3d signal in the XPS spectra of Dopa
treated Al substrate (after vigorous rinsing and sonication) but
not detected in the Si substrate. The Al 2p and O 1s signal of Al
substrate greatly reduced to 15.6% and 47.8%, respectively,
after Dopa treatment while the Si 2p and O 1s of Si substrate
remain same concentration (Table S1, Supporting Informa-
tion). The Br 3d peak, a useful indicator because of its presence
in organic molecule but not in the substrate, showed that Dopa
was readily adsorbed to alumina but antiadsorptive to silica
substrate.
Information regarding the interaction of Dopa on halloysite
was provided by solid-state 13C NMR spectroscopy. The solid-
state 13C NMR spectrum of Dopa (Figure 2) displays sharp
resonances for all eleven carbon environments of the molecule,
Figure 3. Solid-state 13C NMR spectra of (a) alumina and (b) silica
nanoparticles before and after Dopa treatment.
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Article
species could no longer be detected as shown in Figure 3b
(broad resonances at 0−80 ppm were adventitious hydrocarbon
contamination). This behavior may be ascribed to a weak
silica−catechol bond, which can be readily removed by
sonication and THF:H2O rinses. These results are also
applicable to the alumina and silica surfaces of halloysite.
Alumina-catechol bonds are much more stable than silica−
catechol bonds, allowing selective modification of aluminol
group of halloysite lumen by catechol group under aqueous
conditions.
ACKNOWLEDGMENTS
■
The authors acknowledge the financial support of a Grant-in-
Aid for Scientific Research (A) (No. 19205031) from Japan
Society for the Promotion of Science. The present work is also
supported by a Grant-in-Aid for the GCOE Program, “Science
for Future Molecular Systems”, from the Ministry of Education,
Culture, Science, Sports and Technology of Japan. Partial
support by U.S. NSF-1029147 and EPS1003897 grants is
acknowledged. The authors thank Prof. Hiroshi Jinnai, Dr.
Tsuyoshi Higuchi (JST ERATO Takahara Soft Interfaces
Project), and Ms. Keiko Ideta of Evaluation Center of Materials
Properties and Function, Kyushu University, for their assistance
in the work and Applied Minerals, Inc. for halloysite supply.
The growth of PMMA brush from tube lumen can be
ascertained from transmission electron microscopy (TEM)
study. Figure 4 shows the TEM images of halloysite (a) and
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ASSOCIATED CONTENT
* Supporting Information
Text giving experimental details, figures, and tables showing
XPS, FTIR, and TGA. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
takahara@cstf.kyushu-u.ac.jp
■
Notes
The authors declare no competing financial interest.
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