A microfluidic system incorporated with peptide/Pd nanowires for heterogeneous catalytic reactions

Article abstract of DOI:10.1039/c0lc00448k

Highly stable diphenylalanine peptide nanowires were selectively self-assembled in the reaction zone of a microfluidic system and applied for microchemical hydrogenation and Suzuki coupling reactions through the hybridization of Pd nanoparticles.

Full text of DOI:10.1039/c0lc00448k

LabonaChip
Micro- & nano- fluidic research for chemistry, physics, biology, & bioengineering
www.rsc.org/loc
Volume 11 | Number 3 | 7 February 2011 | Pages 361–556
ISSN 1473-0197
COMMUNICATION
Park and Kim et al.
A microfluidic system incorporated with peptide/Pd nanowires for
heterogeneous catalytic reactions

COMMUNICATION
www.rsc.org/loc | Lab on a Chip
A microfluidic system incorporated with peptide/Pd nanowires for
heterogeneous catalytic reactions†
a
b
b
ac
Hyang-Im Ryoo,‡ Joon Seok Lee,‡ Chan Beum Park* and Dong-Pyo Kim*
Received 27th September 2010, Accepted 11th November 2010
DOI: 10.1039/c0lc00448k
6
Highly stable diphenylalanine peptide nanowires were selectively
self-assembled in the reaction zone of a microfluidic system and
applied for microchemical hydrogenation and Suzuki coupling
reactions through the hybridization of Pd nanoparticles.
through imprint lithography technique, followed by in situ
growth of peptide/Pd NW within microchannels (Fig. 1 and
Scheme S1†). Firstly, a polymer microchannel with high optical
transparency and strong organic solvent resistance was fabricated
from polyvinylsilazane, which demonstrated reliable micro-
chemical characteristics and stabilities for running heterogeneous
Microfluidics has received much attention over the past decade in
diverse fields including chemical synthesis, biological sensing, medical
1
diagnostics, and environmental analysis. For example, a micro-
6
catalytic synthesis reactions. In the reaction zone of a micro-
channel, the vertically well-aligned diphenylalanine (Phe–Phe, FF)
peptide NW was grown on the polymer surface at comparable
conditions in a site-selective manner by covering the unwanted
modified surface with aluminium tape.
reactor that consists of a network of microscale flow channels can
2
serve as a miniaturized system. The miniaturized system reduces the
time and cost of bulk chemical processes with improved safety and
selectivity compared to those of conventional vessel reactors. In
particular, an increased surface-to-volume ratio in a microfluidic
reactor can dramatically enhance the mass transport of reactants and
The uncovered microchannel surface was coated with a mussel-
inspired polydopamine (PDA) as an adhesion promoter by simply
7
immersing in an alkaline dopamine solution. Subsequently, an
amorphous FF peptide film prepared by drying the FF solution
ꢀ
3
products with higher heat transfer coefficients. Such improvements
under anhydrous conditions was treated with aniline vapor at 100 C,
allow one to utilize the full potential of catalysts under isothermal
operation. In this regard, the immobilization of catalyst within the
microchannel is highly desirable; it can avoid the dispersion problem
of catalysts in organic solvents as well as the filtration after chemical
reaction in the microchannel. Attempts were made to create nano-
then the FF peptide NWs were vertically grown in a self-assembled
8
manner. The morphology of FF peptide NWs could be controlled
by adjusting aging time, temperature, and film thickness (data not
shown). In this work, the FF peptide NWs typically grew to have 10
ꢀ
mm height and 300 nm diameter, and further annealed at 100 C for
12 h to enhance the dimensional stability. Fig. 2 shows SEM images
4,5
structures that hold catalysts within the microchannel. Up to date,
the nanostructures such as carbon nanotubes (CNTs), Al anodic
oxide nanotubes and Si nanowires were grown by gas phase tech-
of FF peptide NWs formed in high density on the bottom and
sidewalls of the polyvinylsilazane-derived microchannel.
ꢀ
niques at extremely high temperatures over 500 C or high-cost
Note that PDA exhibited a universal adhesive property owing to
the presence of catechol moieties for facile covalent and coordination
7,9
MEMS technology with time consuming skills. These approaches
required the use of durable ceramic or metal substrates or extra steps
for removing the template, and encountered poor adhesion between
nanostructures and microfluidic surface.
bonding to a variety of substrates. Therefore, the selected bare
surface of the polyvinylsilazane microchannel was initially coated
with the PDA layer to strengthen the interfacial adhesion of the
grown NWs to the underlying surface. Hence, the FF peptide NWs
on the PDA layer were firmly adhered to the surface even under
vigorous stirring or ultrasonication, while FF peptide NWs on the
bare surface were easily detached at the same agitation (Fig. S1 of the
ESI†). Prior to Pd nanoparticle (NP) deposition step, the FF peptide
NWs were immersed in an aqueous dopamine solution for 16 h to
coat ca. 200–400 nm thick PDA ad-layer. In addition, the hydro-
phobic peptide NW film exhibited a high static water contact angle
Herein, we report on the microfluidic reactor system incorporated
with vertically aligned peptide/Pd hybrid nanowires (NWs) having
excellent stability applicable to soft polymeric substrates under mild
conditions. Microchannels with a dimension of 500 mm (width) ꢁ 50
mm (height) ꢁ 3 cm (length) were fabricated from polyvinylsilazane
a
School of Applied Chemistry and Biological Engineering, Chungnam
National University, 220 Kung-dong, Yusung-gu, Daejeon, 305-764,
Korea. E-mail: dpkim@cnu.ac.kr; Fax: +82-42-823-6665; Tel: +82-42-
ꢀ
ꢀ
10
(
q ¼ 124 ), as reported previously. But it became very hydrophilic
8
21-6695
b
(q # 10 ) after the PDA coating due to polar catechol moieties
Department of Materials Science and Engineering, Korea Advanced
Institute of Science and Technology (KAIST), 335 Science Road,
Daejeon, 305-701, Korea. E-mail: parkcb@kaist.ac.kr; Fax: +82-42-350-
3
310; Tel: +82-42-350-3340
c
Graduate School of Analytical Science and Technology, Chungnam
National University, 220 Kung-dong, Yusung-gu, Daejeon, 305-764,
Korea. E-mail: dpkim@cnu.ac.kr; Fax: +82-42-823-6665; Tel: +82-42-
821-6695
†
Electronic supplementary information (ESI) available: Materials,
fabrication of microfluidic system incorporated with peptide/Pd hybrid
NWs and explanation on microchemical heterogeneous catalytic
reactions. See DOI: 10.1039/c0lc00448k
Fig. 1 Scheme for peptide/Pd nanowires built-in microfluidic system for
‡
These authors contributed equally to this work.
heterogeneous catalytic hydrogenation and Suzuki coupling reactions.
3
78 | Lab Chip, 2011, 11, 378–380
This journal is ª The Royal Society of Chemistry 2011

2
a surface area ꢂ10 times larger than that of a flat surface, highly
increasing the amount of immobilized Pd nanoparticles in the
microchannel.
Previously reported methods for creating the nanostructures
within microfluidic channels required critical conditions that limited
the selection of device materials; for example, the metal catalyst-
supported carbon nanotubes in the reaction zone were grown on
ꢀ
metal-deposited alumina or stainless steel at over 500 C causing high
4
,5
manufacturing costs. In contrast, our peptide self-assembly-based
approach using mussel-inspired PDA is highly versatile and simple
with reliable adhesion to the substrates. Moreover, this approach is
applicable to soft polymeric substrates because of its mild fabrication
conditions.
We further investigated the heterogeneous catalytic performance
of FF peptide/Pd hybrid NWs built-in microreactors by carrying out
hydrogenation and Suzuki coupling reactions in comparison with
plain microreactors without NWs. The results of hydrogenation
conversions of 1-phenyl-1-cyclohexene (as a simple olefin) and cin-
namaldehyde (as a bi-functioned olefin with additional C]O),
respectively, are presented in Table 1. In general, the peptide NWs
built-in microreactor exhibited much higher yields of conversion at
both hydrogenation reactions. The conversion yields of two unsatu-
rated olefins were dependent on the flow rate of reactants. In
particular, when the flow rate became higher (i.e., shorter reaction
time), the catalytic performance in the FF peptide NWs built-in
microreactor was dramatically enhanced by the increased surface
area and catalyst loading which promote the efficient contact of the
Fig. 2 SEM images of vertically aligned FF peptide NWs within the
microchannel. The microchannel (500 mm width ꢁ 50 mm height ꢁ 3 cm
length) was fabricated using polyvinylsilazane through soft-lithographic
process, and the peptide/Pd NWs were subsequently grown on both
bottom and sidewalls of the channels by self-assembly.
7
abundant in PDA. Note that the hydrophilic modification of the FF
peptide NW film with PDA facilitates efficient access of aqueous
2
PdCl solution to the NW surface. Finally, the FF peptide/Pd hybrid
NWs were prepared by incubating the PDA-coated FF peptide NWs
in Pd precursor solution that grew Pd NPs along the FF peptide
NWs (Fig. 3c and d). At this step, the PDA coating not only acted as
an adhesive that was strongly binding to both organic and inorganic
surfaces but also as a reducing agent for the growth of Pd metal NPs
with the size range 4–20 nm that was presumably attributed to the
4
ꢃ1
reactant with the catalyst. For example, at the flow rate of 5 ml min
7
of 1-phenyl-1-cyclohexene, the FF peptide NWs built-in microreactor
resulted in a 61.7% yield (Table 1). In contrast, no conversions were
observed in the plain microreactor at the same flow rate. A similar
result was observed in the case of alternative hydrogenation of
unsaturated aldehyde (i.e., cinnamaldehyde) to produce 3-phenyl-
propionaldehyde and additional by-products (i.e., cinnamyl alcohol
reductive capacity of the hydroxyl groups in PDA. For comparison
with the FF peptide NWs built-in microchannels, we immobilized Pd
nanoparticles on the surface of PDA-coated, bare microchannels
without FF peptide NWs (Fig. 3a and b). An aligned array of FF
peptide NWs (of 300 nm in diameter and 10 mm long) would give
11
and 3-phenyl-1-propan-1-ol). Similarly, the incorporated Pd cata-
lyst on NWs greatly promoted the yields in comparison to the plain
microreactor. For instance, total conversion and selective yield of 3-
phenylpropionaldehyde (as a main product in the NW built-in
Table 1 Heterogeneous catalytic performance of FF peptide/Pd hybrid
NWs built-in microfluidic reactor for hydrogenation reactions at
different flow rates in comparison with the plain reactor without NWs
Flow rate/
ꢃ1
Yields (%)
ml min
(residence
time/s)
Substrate
Product
Plain
Nanowire
0
1
3
5
0
1
3
.5 (90)
.0 (45)
.0 (15)
.0 (9)
.5 (90)
.0 (45)
.0 (15)
23.4
5.8
0.5
0.0
32.5(46.8)
25.9(42.7)
18.2(29.8)
11.4(17.5)
97.0
95.1
81.4
61.7
76.0(100)
69.0(94.5)
50.7(91.5)
30.7(61.4)
a
Fig. 3 (a and b) SEM images of Pd nanoparticles on PDA-coated Si
substrate with different magnifications, (c) SEM and (d) TEM images of
Pd nanoparticles on the PDA-coated FF peptide nanowire film. The
loaded amount of Pd nanoparticles on the FF peptide NWs is much
larger than that of the flat substrate due to highly increased surface area.
The SAED pattern on (d) shows polycrystalline diffraction rings corre-
sponding to fcc Pd.
5.0 (9)
a
Conversion yield including 3-phenylpropionaldehyde product and
additional by-products (i.e., cinnamyl alcohol and 3-phenyl-1-propan-1-
ol). All reactions were carried out at room temperature (25 C).
ꢀ
This journal is ª The Royal Society of Chemistry 2011
Lab Chip, 2011, 11, 378–380 | 379

Table 2 Comparative catalytic performance of Suzuki coupling reac-
tions in the FF peptide/Pd hybrid NWs built-in microfluidic reactor and
plain reactor with different solvents at room temperature
nanoparticles and the PDA on the nanowire strongly tethered the Pd
7,12
particles without leaching.
In summary, we demonstrated the feasibility of vertically aligned
FF peptide/Pd NWs for microfluidic systems fabricated under mild
conditions applicable to soft polymer device materials. The FF
peptide/Pd NWs within the polymer microchannel highly improved
the catalytic performance of the microreactor due to the increased
surface-to-volume ratio and catalyst loading. The strategy presented
here provides a new and facile route for fabricating the nano-
structure-embedded microchemical system that may offer new
applications in the areas of heterogeneous catalytic reactions as well
as biological sensing.
Flow rate/
ꢃ1
Yields (%)
ml min
(residence
time/s)
Substrate
Product
Solvent
EtOH/
Plain NWs
0.5 (90)
1.0 (45)
42.9 97.2
32.5 89.2
17.2 84.6
13.7 76.6
39.4 94.3
33.6 93.1
21.5 81.3
11.2 63.0
0.5 55.0
31.9 98.0
2
H O
3
5
.0 (15)
.0 (9)
This study was supported by the National Research Foundation
via National Research Laboratory (NRL) (R0A-2008-000-20041-0),
Converging Research Center (2009-0082276), Engineering Research
Center (2008-0062205) programs, and Creative Research Initiatives
THF/H
EtOH/
H O
2
2
O
O
0.5 (90)
0.5 (90)
1.0 (45)
3
5
.0 (15)
.0 (9)
(CRI) project [20100000722] administered by the Korean Ministry of
THF/H
2
0.5 (90)
Education, Science, and Technology.
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