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was cut to a size of 10 mm ꢁ40 mm and cleaned with acetone,
ethanol, 2-propanol, and piranha solutions for 10 min each. The
cleaned silicon wafer was rinsed with deionized water several
times and immersed in a 5% HF solution for 3 min. Then, the
wafer was placed into a solution of AgNO3 (5 mm) and HF (4.8m)
for 1 min to deposit silver nanoparticles onto the wafer. Silver
nanoparticles etched down the Si wafer vertically in a solution of
H2O2 (0.4m) and HF (4.8m). To control the length of SiNWs, we
varied the etching time from 5 to 45 min. After sufficient etching,
the silver nanoparticles were removed by bathing the Si wafer in
a diluted nitric acid solution. The as-synthesized silicon nanowires
were immersed in a 5% HF solution for hydrogen termination and
decorated with platinum nanoparticles through the immersion of
SiNWs into H2PtCl6 (1 mm) and HF (4.8m) solution for 10 min. In
order to create the working electrode, SiNWs were cut into 10ꢁ
10 mm sections and connected to a copper wire using carbon
paste. Next, SiNWs and carbon paste were covered with insulating
epoxy resin with the exception of the specific area to be exposed.
Characterization: The morphology of silicon nanowires was ana-
lyzed using an S-4800 field emission scanning electron microscope
(Hitachi High-Technologies Co., Japan) and 300 keV field-emission
transmission electron microscope (FEI, Model Tecnai G2 F30 S-Twin,
Netherlands). UV/Vis absorption spectra were measured using a V-
650 spectrophotometer (JASCO Inc., Japan). The photocurrent was
measured in a three-electrode configuration using Ag/AgCl in 3m
NaCl (0.197 V vs. standard hydrogen electrode) as a reference elec-
trode, a platinum wire as a counter electrode, and an SiNW elec-
trode as a working electrode in a solution of phosphate buffer
(50 mm, pH 7.5). The system was connected to a potentiostat/gal-
vanostat (WonATech, Model WMPG1000, Korea) under a 450 W Xe
lamp illumination with a 420 nm cut-off filter. Cyclic voltammetry
was scanned at a rate of 50 mV sÀ1 and linear sweep voltammetry
routes, donating a proton to Mred1 and shuttling electrons
from SiNWs to Mox.
To confirm the role of PtNPs on SiNWs, we tested visible
light-driven NADH regeneration by Pt-SiNWs with the applied
voltage of À0.8 V. As shown in Figure 4c, the catalytic activity
of PtNPs significantly enhanced both turnover number and
turnover frequency of M from 0.96 and 0.035 sÀ1 to 1.44 and
0.065 sÀ1, respectively. To achieve biocatalytic artificial photo-
synthesis, we coupled photoelectrochemical NADH regenera-
tion with a redox enzymatic reaction using L-glutamate dehy-
drogenase (GDH) as a model enzyme that can produce L-gluta-
mate from a-ketoglutarate in the presence of enzymatically
active NADH. Under À0.8 V (vs. Ag/AgCl) of applied potential
after 5 h of illumination, the concentration of synthesized L-
glutamate with Pt-SiNWs was measured as 1.54-fold higher
than that with bare SiNWs (Figure 4d). The persistent photo-
electroenzymatic reaction with Pt-SiNW in comparison to SiNW
further indicates enhanced stability of SiNW photocathode by
the incorporation of PtNPs, as reported previously.[6]
In summary, we successfully demonstrate visible light-en-
hanced electroenzymatic synthesis using silicon nanowire
(SiNW) photocathodes. The photoelectrochemical approach
does not need any sacrificial electron donor, though such is
donor is a critical requirement for traditional photochemical
cofactor regeneration. Furthermore, visible-light absorption by
SiNWs allows to lower the applied potential more than in the
case of electrochemical NADH regeneration. Based on multiple
electrochemical analyses of different types of SiNWs, we reveal
that p-type SiNWs can transfer photoexcited electrons efficient-
ly to NAD+ through M, an electron mediator. Under visible-
light illumination, the NADH regeneration rate is six times
higher than that under dark conditions, which is partly due to
the photocatalytic effect of SiNWs. While an optimum length
of SiNWs exists for their best performance, platinum nanoparti-
cles (PtNPs) deposited at the surface of SiNWs as an electroca-
talyst significantly enhance the rate of NADH regeneration and
the yield of redox enzymatic synthesis. Our work shows that
Pt–SiNWs is a promising photocathode material for redox enzy-
matic reactions coupled with photoelectrochemical NADH re-
generation to synthesize valuable chemicals by diverse cofac-
tor-dependent biocatalysts without sacrificial electron donor
and high overpotential.
was scanned at a rate of 10 mVsÀ1
.
NADH regeneration and enzymatic synthesis of L-glutamate:
Photoelectrochemical NADH regeneration of SiNW photocathode
was conducted in a three-electrode system under a 450 W Xe lamp
illumination with a 420 nm cut-off filter for visible-light excitation
of photons. SiNW electrodes were immersed in a phosphate buffer
solution (50 mm, pH 7.5) containing 1 mm NAD+ and 250 mm M.
During NADH regeneration, the concentrations of NAD+ and
NADH were measured from peak intensities of absorption at
260 nm and 340 nm, respectively. The enzyme reaction with 40 U
of GDH was carried out in a phosphate buffer (50 mm, pH 7.5) with
1 mm NAD+, 250 mmM, 5 mm a-ketoglutarate, and 0.1m (NH4)2SO4.
Acknowledgements
This study was supported by grants from the National Research
Foundation (NRF) via the National Leading Research Laboratory
(NRF-2013R1A2A1A05005468) and the Intelligent Synthetic Biol-
ogy Center of Global Frontier R&D Project (2011–0031957), Re-
public of Korea.
Experimental Section
Materials: Unless noted otherwise, all chemicals including hydro-
fluoric acid (HF), silver nitrate (AgNO3), hydrogen peroxide (H2O2),
nitric acid (HNO3), chloroplatinic acid hexahydrate (H2PtCl6·6H2O),
b-nicotinamide adenine dinucleotide hydrate (NAD+), ammonium
sulfate [(NH4)2SO4], l-glutamic dehydrogenase from bovine liver
(GDH), and a-ketoglutaric acid disodium salt dehydrate were pur-
chased from Sigma–Aldrich (USA). Lightly doped (1–10 Wcm for p-
type, 1–30 Wcm for n-type) silicon wafers (100) were obtained
from Tasco (Korea). The rhodium complex {[Cp*Rh(bpy)(H2O)]2+},
M, was synthesized according to a literature report.[18b]
Keywords: artificial photosynthesis · biocatalysis · cofactors ·
photochemistry · silicon
Silicon nanowire electrode preparation: Silicon nanowires
(SiNWs) were synthesized by the metal-assisted chemical etching
method according to a literature report.[10] Briefly, a silicon wafer
3584–3588; c) M. Lee, J. U. Kim, J. S. Lee, B. I. Lee, J. Shin, C. B. Park,
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2014, 7, 3007 – 3011 3010