Imaging nanometer metallocatalysts formed by photosynthetic deposition of water-soluble transition-metal compounds

Article abstract of DOI:10.1021/jp044115r

A method of imaging nanometer metallocatalysts formed by photosynthetic precipitation of the water-soluble transition-metal compounds [PtCl ₆]^2- and [RuCl₆]^2- is reported. Hexachloroplatinate and hexachlororuthenate can accept up to four electrons from Photosystem I (PSI) reaction centers in photosynthetic thylakoid membranes, thereby converting [PtCl₆]^2- and [RuCl₆] ^2- anions to either metallic platinum (Pt) and ruthenium (Ru) and/or partially oxidized nanometer catalysts at the reducing sides of PSI molecules. Use of this method can potentially create nanometer-sized Pt and/or bimetallic catalysts (such as Pt-Ru) on biomembranes and molecules at pH 7 and room temperature with preservation of the biological function of the molecules.

Full text of DOI:10.1021/jp044115r

5
409
2005, 109, 5409-5413
Published on Web 03/08/2005
Imaging Nanometer Metallocatalysts Formed by Photosynthetic Deposition of
Water-Soluble Transition-Metal Compounds
†
‡
,†
James W. Lee, Ida Lee, and Elias Greenbaum*
Oak Ridge National Laboratory, Chemical Sciences DiVision, P.O. Box 2008,
Oak Ridge, Tennessee 37831-6194, and Department of Electrical Engineering,
UniVersity of Tennessee, KnoxVille, Tennessee 37991
ReceiVed: December 26, 2004; In Final Form: February 21, 2005
A method of imaging nanometer metallocatalysts formed by photosynthetic precipitation of the water-soluble
2-
2-
transition-metal compounds [PtCl
6
]
and [RuCl
6
]
is reported. Hexachloroplatinate and hexachlororuthenate
can accept up to four electrons from Photosystem I (PSI) reaction centers in photosynthetic thylakoid
2-
2-
membranes, thereby converting [PtCl
6
]
and [RuCl
6
]
anions to either metallic platinum (Pt) and ruthenium
(Ru) and/or partially oxidized nanometer catalysts at the reducing sides of PSI molecules. Use of this method
can potentially create nanometer-sized Pt and/or bimetallic catalysts (such as Pt-Ru) on biomembranes and
molecules at pH 7 and room temperature with preservation of the biological function of the molecules.
Oxygenic photosynthesis is the key biological process that
converts the electromagnetic energy of sunlight to chemical
energy via the reduction of atmospheric carbon dioxide (CO2)
to carbon fixation compounds using electrons from water
oxidation. Photosynthesis supports virtually all life on Earth.
However, it is also possible to use electrons from the photo-
synthetic process to create nanometer-sized catalysts at certain
biomolecular interfaces, such as the reducing side of a Photo-
system I (PSI) reaction center using water-soluble transition-
2
-
metal compounds including hexachloroplatinate [PtCl6] and
2-
hexachlororuthenate [RuCl6] . Figure 1A presents a schematic
illustration of the photosynthetic thylakoid membrane, which
can be isolated from green plants such as spinach leaves. The
key components in the thylakoid membrane are the two
photosynthetic reaction centers, Photosystems I (PSI) and II
(PSII), both of which are well-organized chlorophyll-protein
complexes ∼10 nm in size. Upon actinic illumination, water is
+
oxidized to protons (H ) and molecular oxygen (O2) at PSII,
which is driven by the P680 photochemical charge separation.
Electrons acquired from water splitting are transferred vectorially
from P680 to the QB side of PSII, where they are accepted by
the plastoquinone (PQ) pool in the thylakoid membrane and
then transferred through a series of electron carriers, including
the cytochrome (Cyt) b/f complex and plastocyanin (PC), to
PSI, where the electrons are energized again by the PSI
photochemical charge separation reaction. In normal photosyn-
thesis, electrons emergent from PSI are used to reduce carbon
dioxide to produce sugars through a collection of enzymatic
reactions known as the Calvin cycle. It is also known that the
electrons emergent from PSI can be used to reduce the com-
mon Hill reagent ferricyanide ([Fe(CN)6]3 ) to ferrocyanide
Figure 1. Photosynthetic deposition of nanometer catalysts at the
reducing (FAB) side of Photosystem I (PSI) reaction centers in the
thylakoid membrane. Panel A shows the chemistry of the nanometer-
scale catalyst deposition by photosynthetic reduction of water-soluble
transition-metal compounds, whereas panel B depicts the catalytic
activity demonstrated by the observed simultaneous photoevolution of
O
2
and H
2
from water in the photosynthetically deposited catalyst
thylakoid membrane.
4
-
(
[Fe(CN)6] ) as first demonstrated by Robert Hill over 60 years
1
ago. Our previous work indicated that the Hill reaction could
be extended to reduce certain water-soluble transition-metal
compounds (such as [PtCl6] or [RuCl6] ) to generate H2
photoevolution, as demonstrated by the observation that il-
lumination of the thylakoid membranes in the presence of
-
2
-
2-
*
Author to whom correspondence should be addressed. Telephone: 865-
5
74-6835. Fax: 865-574-1275. E-mail address: greenbaum@ornl.gov.
†
‡
Oak Ridge National Laboratory.
University of Tennessee.
2-
2-
2 mM [PtCl6] or [RuCl6] can lead to simultaneous photo-
1
0.1021/jp044115r CCC: $30.25 © 2005 American Chemical Society

5410 J. Phys. Chem. B, Vol. 109, No. 12, 2005
Letters
]2
6
-
Figure 2. Scanning electron microscopy (SEM) images of [PtCl
photoprecipitated and dried thylakoid materials at magnifications of
3
0× (top) and 5000× (bottom), using an electron beam with an
excitation energy of 20 kV.
evolution of H2 and O2, whereas the control sample (thylakoid
2
-
2-
membranes without the addition of 2 mM [PtCl6] or [RuCl6] )
2-5
could not. This is a significant result, because it demonstrated
a transformation of the photosynthetic molecular system to
achieve a new type of photosynthesis-photoevolution of H2
and O2 from water (Figure 1B), instead of the usual CO2 fixation
activity. The observed photoevolution of H2 represents a
catalytic reduction of protons by the electrons from the oxygenic
photosynthetic water-splitting process. Noble metals such as
platinum are known to have low overpotentials for catalytic
activity for electrolytic H2 production. Therefore, the observed
Figure 3. SEM analysis of nanometer-scale metallocatalysts formed
2-
by the photosynthetic precipitation of [PtCl
6
]
with thylakoids. The
SEM imaging (top) of the sample on the silicon substrate revealed
individual nanocatalysts. The Pt peak in the SEM X-ray fluorescence
(XRF) analysis (bottom) of the nanoparticles verified the identity of
the platinum nanocatalyst. The Si and O peaks were from the silicon
substrate. In each experiment, the SEM beam and the X-ray detector
might be at slightly different locations with respect to the specimen.
The detection of Al (base) and/or Cu (clips) peaks is dependent on the
location of these components with respect to the SEM beam and the
X-ray detector. These results represent the first time that photosyntheti-
cally precipitated platinum nanocatalysts were isolated and imaged via
SEM.
H2-producing catalytic activity could be explained if one
assumes that the Hill oxidants [PtCl6]2- and [RuCl6]2- could
accept four electrons consecutively from PSI, thereby converting
-
2-
[
PtCl6]2 and [RuCl6] anions to lower-valence platinum (Pt)
and ruthenium (Ru) at the reducing (FAB) side of PSI, according
to the following reactions:
2
-
-
[
PtCl6] - + 4e f PtV + 6Cl
(1)
easily imaged at high (nanometer-scale) resolution using scan-
6
,7
ning electron microscopy (SEM). As shown in Figure 2, the
thylakoid sample (dried) can be visualized by SEM only at very
low resolution (with a magnification of ∼30×). When the
magnification is increased to 5000× with a 20-kV electron
beam, significant electronic charge accumulates in the sample,
typically resulting in a ring-like image (see bottom of Figure
2) that is reminiscent of the nature of the nonconductive
materials that prevent visualization at the nanometer scale using
SEM.
The previous inability to visualize any of the postulated
formations of nanometer catalysts in the thylakoid samples
directly poses a challenge to the aforementioned model of
photosynthetic deposition of transition-metal compounds il-
lustrated in Figure 1. Here, we report experiments that enable
the first SEM visualization of the nanometer-sized metallocata-
lysts from the treated thylakoid samples.
and
RuCl6] - + 4e f RuV + 6Cl
2
-
-
(2)
[
Alternatively, photochemical reduction may not necessarily
proceed completely to metal formation. In that case, the
catalytically active species could be a partially oxidized insoluble
nanometer structure. In either case, because of the complexity
of the biological sample, the formation of the nanometer
catalysts has never been physically confirmed by electron
microscopy imaging. The specific difficulties in visualizing such
nanometer catalysts in the biological samples are (i) the amount
of catalyst is relatively small, in comparison with the complex
matrix of the thylakoid material, and (ii) the thylakoid mem-
branes are a soft and nonconductive material that cannot be

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J. Phys. Chem. B, Vol. 109, No. 12, 2005 5411
Figure 4. SEM analysis of nonphototreated [PtCl
]2--thylakoids as
6
a control. The SEM image (top) shows that the organic materials of
the applied thylakoid materials could be completely removed by passing
the material through the sterilization flame. The Si and Al peaks in the
XRF spectrum (bottom) are from the silicon wafer and the aluminum
SEM sample holder. The absence of metallic platinum particles in this
control sample indicated that the phototreatment was required for
formation of nanometer-scale platinum particles in the photosynthetic
thylakoid membrane.
Figure 5. SEM analysis of Pt-Ru nanometer metallocatalysts formed
2-
2-
6
and [RuCl ] . The SEM
by photosynthetic coprecipitation of [PtCl
6
]
imaging result (top) of the bimetallic catalysts represents the first time
that such bimetallic catalysts were isolated and observed via SEM. The
Pt and Ru peaks in the XRF analysis (bottom) confirm the presence of
platinum and ruthenium in the catalyst material. The Al and Cu peaks
were from the sample holder. The Si and O peaks were from the silicon
substrate.
This paper reports microscopic analysis and visualization of
the nanometer-sized metallocatalysts formed by the photosyn-
thetic deposition of water-soluble transition-metal compounds
in thylakoid membranes at room temperature and neutral pH.
The thylakoid materials were isolated from spinach leaves using
2
(
100 µE m- s-1, with a wavelength of 660 nm) provided by
light-emitting diodes (QB1310CS-660, Quantum Devices, Inc.,
Barneveld, WI). Production of H2 and O2 was monitored in the
effluent gas flow as detected by the tin-oxide hydrogen sensor
and galvanic cell. After photosynthetic deposition of [PtCl6]2
with observation of H2 and O2 photoevolution for 12 h, the
8
the procedure described by Reeves and Hall, and they then
9
were suspended in Walker’s assay medium and adjusted to a
-
final chlorophyll (chl) concentration of 600 µg/mL for photo-
precipitation of catalysts for production of H2 and O2 with a
reactor-flow detection system. The chl content was determined
in 90% acetone extracts.10 Photoprecipitation of metallocatalysts
was performed in a helium atmosphere, using a reactor-flow
2
-
[
PtCl6] -phototreated thylakoid materials were washed three
times by resuspension in double-distilled water and subjected
to pelletting by centrifugation; the supernatant was then
discarded, to remove all water-soluble materials, especially
5
detection system as described previously. Ten milliliters of
2-
residual [PtCl6] . A thin layer of the washed metallocatalyst-
thylakoid membrane suspension (600 µg chl/mL) was placed
containing thylakoid material was deposited on a conductive
silicon (P111) wafer and dried at 150 °C for 10 min. The organic
in the reactor. The prospective Hill reagent (i.e., the transition-
metal compound species [PtCl6]2 and/or [RuCl6] ) was added
to the thylakoid suspension for a final concentration of 2 mM.
The reaction vessel was water-jacketed and held at 20 °C with
a temperature-controlled water bath (Lauda RM6, Brinkmann
Instruments, Germany). The reaction medium (thylakoid sus-
pension plus the transition-metal compound species) was gently
stirred by a magnetic stirring bar and purged with a helium flow
-
2-
(thylakoid) material was then removed by passing the sample
plates through a sterilization flame. The samples were then
imaged and analyzed using a Hitachi model S-4700 SEM system
with an in situ X-ray fluorescence (XRF) analysis capability.
Figure 3 presents a typical SEM image and XRF analysis
using the new method for visualizing nanometer-scale metal-
locatalysts formed by the photosynthetic precipitation of
2
-
(
50 mL/min) in the headspace above the liquid. The rate of
helium flow was controlled by a gas-flow control system (Type
60, MKS Instruments, Inc., Andover, MA). Photoprecipitation
of metallocatalysts was initiated by actinic illumination
[PtCl6] with thylakoids. The XRF spectrum (see bottom of
Figure 3) corresponded to the analysis of an individual nano-
meter-sized platinum particle, as marked in the SEM image (see
top of Figure 3). The Pt XRF peak at ∼2.1 keV is clearly above
2

5412 J. Phys. Chem. B, Vol. 109, No. 12, 2005
Letters
Figure 6. Atomic force microscopy (AFM) image of Pt-Ru particles from the sample plate of Figure 5 after washing with acetone and methanol
and drying with an argon gas stream.
the background noise. The Si and O peaks are from the substrate
silicon wafer. The SEM images clearly illustrate the nanometer-
silicon wafer. Therefore, this sample (on a silicon wafer) was
washed with organic solvents, including acetone and methanol,
and then imaged under atomic force microscopy (AFM) after
the organic solvent was removed by a gas stream of ultrapure
nitrogen (99.9997% pure). The AFM imaging (Figure 6) shows
that clean particles can be obtained with the organic solvent
treatment. The particles are in the 10-40-nm size range.
2-
11
scale platinum particles formed from the [PtCl6] -phototreated
thylakoid sample. “Spectrum 14” is automatically assigned by
the SEM machine as a label for the selected nanometer spot of
the sample. The XRF spectrum (see bottom of Figure 3) was
from this nanometer spot.
2
-
As an experimental control, a nonphototreated [PtCl6] -
We conclude that these results of microscopic analysis and
visualization provide direct evidence in support of the model
of photosynthetic metallocatalyst deposition (see Figure 1). The
results confirm that, unlike the common photosynthetic Hill
thylakoid sample was prepared in the same way as that of the
2-
[
PtCl6] -phototreated thylakoid sample for SEM analysis.
Figure 4 presents the SEM analysis result of a nonphototreated
2-
[
PtCl6] -thylakoid sample. As shown in the SEM image (see
3
-
reagent ferricyanide ([Fe(CN)6] ), hexachloroplatinate and
hexachlororuthenate are able to accept up to four electrons
top of Figure 4), the organic materials of the applied thylakoid
materials were removed by passing the material through the
sterilization flame, and no metallic platinum particles were
observed in this control sample. The absence of any metallic
platinum particles in the control sample indicated that photo-
treatment is indeed required, as expected, for formation of
nanometer-scale platinum particles in the photosynthetic
thylakoid membrane. The Si and Al peaks in the XRF spectrum
2-
consecutively from PSI, thereby converting [PtCl6]
and
2-
[RuCl6] anions to hydrogen-evolving platinum and ruthenium
nanometer metallocatalysts at the reducing side of PSI. Use of
this method can potentially create nanometer-scale metallic and/
or bimetallic (such as Pt-Ru) catalysts on biomembranes and
molecules at pH 7 and room temperature that preserve the
biological function of the molecules. Using photosynthetic
deposition of platinum (or ruthenium) at the reducing side of
PSI reaction centers, a novel biomimetic photosynthesis can be
created for simultaneous photoproduction of H2 and O2 by light-
activated water splitting and metallocatalysis. The results
reported here may be of both theoretical and practical impor-
tance.
(see bottom of Figure 4) are from the silicon wafer and the
aluminum SEM sample holder.
We are now also able to visualize Pt and/or Ru metallic
nanometer catalysts that are formed via photosynthetic copre-
2-
2-
cipitation of [PtCl6] and [RuCl6] at the reducing side of
PSI in the thylakoid membrane. Figure 5 presents the SEM
analysis of Pt and Ru nanometer metallocatalysts formed by
photosynthetic coprecipitation of [PtCl6]2 and [RuCl6] . The
SEM result (see top of Figure 5) of the two catalysts represents
the first images of coprecipitated metallocatalysts by photosyn-
thesis. The Pt and Ru peaks in the XRF analysis (see bottom of
Figure 5) confirm the presence of platinum and ruthenium in
the catalyst material. The Al and Cu peaks were from the sample
holder. The Si and O peaks were from the silicon substrate.
-
2-
Acknowledgment. The authors thank D. Lowndes and G.
Eres, for providing the conductive silicon wafers and discussion
on SEM techniques used in this study; M. K. Savage, for
editorial assistance; V. W. Pardue, for technical illustrations;
and Jennifer M. Baker, for secretarial support. This research
was supported by the Office of Basic Energy Sciences, U.S.
Department of Energy, and a DOE Office of Science Young
Scientist Award (to J. W. L.). Oak Ridge National Laboratory
is managed by UT-Battelle, LLC, for the U.S. Department of
Energy, under Contract No. DE-AC05-00OR22725.
2
-
These observations provide direct evidence that both [PtCl6]
and [RuCl6]2 can be photosynthetically reduced to microscopic
-
particles with thylakoid membranes.
According to the SEM image (see top of Figure 5), residual
organic (thylakoid) materials seem to remain on the substrate

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J. Phys. Chem. B, Vol. 109, No. 12, 2005 5413
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