6 research outputs found
Large Photocurrent Response and External Quantum Efficiency in Biophotoelectrochemical Cells Incorporating Reaction Center Plus Light Harvesting Complexes
Bacterial photosynthetic reaction
centers (RCs) are promising materials
for solar energy harvesting, due to their high ratio of photogenerated
electrons to absorbed photons and long recombination time of generated
charges. In this work, photoactive electrodes were prepared from a
bacterial RC-light-harvesting 1 (LH1) core complex, where the RC is
encircled by the LH1 antenna, to increase light capture. A simple
immobilization method was used to prepare RC-LH1 photoactive layer.
Herein, we demonstrate that the combination of pretreatment of the
RC-LH1 protein complexes with quinone and the immobilization method
results in biophotoelectrochemical cells with a large peak transient
photocurrent density and photocurrent response of 7.1 and 3.5 μA
cm<sup>–2</sup>, respectively. The current study with monochromatic
excitation showed maximum external quantum efficiency (EQE) and photocurrent
density of 0.21% and 2 μA cm<sup>–2</sup>, respectively,
with illumination power of ∼6 mW cm<sup>–2</sup> at
∼875 nm, under ambient conditions. This work provides new directions
to higher performance biophotoelectrochemical cells as well as possibly
other applications of this broadly functional photoactive material
Hybrid Wiring of the Rhodobacter sphaeroides Reaction Center for Applications in Bio-photoelectrochemical Solar Cells
The
growing demand for nonfossil fuel-based energy production has drawn
attention to the utilization of natural proteins such as photosynthetic
reaction center (RC) protein complexes to harvest solar energy. The
current study reports on an immobilization method to bind the wild
type Rhodobacter sphaeroides RC from
the primary donor side onto a Au electrode using an immobilized cytochrome <i>c</i> (cyt <i>c</i>) protein via a docking mechanism.
The new structure has been assembled on a Au electrode by layer-by-layer
deposition of a carboxylic acid-terminated alkanethiol (HOOC (CH<sub>2</sub>)<sub>5</sub>S) self-assembled monolayer (SAM), and layers
of cyt <i>c</i> and RC. The Au|SAM|cyt <i>c</i>|RC working electrode was applied in a three-probe electrochemical
cell where a peak cathodic photocurrent density of 0.5 μA cm<sup>–2</sup> was achieved. Further electrochemical study of the
Au|SAM|cyt <i>c</i>|RC structure demonstrated ∼70%
RC surface coverage. To understand the limitations in the electron
transfer through the linker structure, a detailed energy study of
the SAM and cyt <i>c</i> was performed using photochronoamperometry,
ellipsometry, photoemission spectroscopy, and cyclic voltammetry (CV).
Using a simple rectangle energy barrier model, it was found that the
electrode work function and the large barrier of the SAM are accountable
for the low conductance in the devised linker structure
The Role of Gold-Adsorbed Photosynthetic Reaction Centers and Redox Mediators in the Charge Transfer and Photocurrent Generation in a Bio-Photoelectrochemical Cell
Bacterial photosynthetic reaction centers (RCs) are promising
materials
for solar energy harvesting, due to their high quantum efficiency.
A simple approach for making a photovoltaic device is to apply solubilized
RCs and charge carrier mediators to the electrolyte of an electrochemical
cell. However, the adsorption of analytes on the electrodes can affect
the charge transfer from RCs to the electrodes. In this work, photovoltaic
devices were fabricated incorporating RCs from purple bacteria, ubiquinone-10
(Q2), and cytochrome c (Cyt c) (the latter two species acting as redox
mediators). The adsorption of each of these three species on the gold
working electrode was investigated, and the roles of adsorbed species
in the photocurrent generation and the cycle of charge transfer were
studied by a series of photochronoamperometric, X-ray photoelectron
spectroscopy (XPS), atomic force microscopy (AFM), and cyclic voltammetry
(CV) tests. It was shown that both redox mediators were required for
photocurrent generation; hence, the RC itself is likely unable to
inject electrons into the gold electrode directly. The reverse redox
reactions of mediators at the electrodes generates electrical current.
Cyclic voltammograms for the RC-exposed gold electrode revealed a
redox couple due to the adsorbed RC at ∼ +0.5 V (vs
NHE), which confirmed that the RC was still redox active, upon adsorption
to the gold. Photochronoamperometric studies also indicated that RCs
adsorb, and are strongly bound to the surface of the gold, retaining
functionality and contributing significantly to the process of photocurrent
generation. Similar experiments showed the adsorption of Q2 and Cyt
c on unmodified gold surfaces. It was indicated by the photochronoamperometric
tests that the photocurrent derives from Q2-mediated charge transfer
between the RCs and the gold electrode, while solubilized Cyt c mediates
charge transfer between the P-side of adsorbed RC and the Pt counter
electrode. Also, the stability of the adsorbed RCs and mediators was
evaluated by measuring the photocurrent response over a period of
1 week. It is found that ∼46% of the adsorbed RCs remain active
after a week under aerobic conditions. A significantly extended lifetime
is expected by removing oxygen from the electrolyte and sealing the
device
A DNA-Directed Light-Harvesting/Reaction Center System
A structurally
and compositionally well-defined and spectrally
tunable artificial light-harvesting system has been constructed in
which multiple organic dyes attached to a three-arm-DNA nanostructure
serve as an antenna conjugated to a photosynthetic reaction center
isolated from Rhodobacter sphaeroides 2.4.1. The light energy absorbed by the dye molecules is transferred
to the reaction center, where charge separation takes place. The average
number of DNA three-arm junctions per reaction center was tuned from
0.75 to 2.35. This DNA-templated multichromophore system serves as
a modular light-harvesting antenna that is capable of being optimized
for its spectral properties, energy transfer efficiency, and photostability,
allowing one to adjust both the size and spectrum of the resulting
structures. This may serve as a useful test bed for developing nanostructured
photonic systems
Reengineering the Optical Absorption Cross-Section of Photosynthetic Reaction Centers
Engineered
cysteine residues near the primary electron donor (P)
of the reaction center from the purple photosynthetic bacterium Rhodobacter sphaeroides were covalently conjugated
to each of several dye molecules in order to explore the geometric
design and spectral requirements for energy transfer between an artificial
antenna system and the reaction center. An average of 2.5 fluorescent
dye molecules were attached at specific locations near P. The enhanced
absorbance cross-section afforded by conjugation of Alexa Fluor 660
dyes resulted in a 2.2-fold increase in the formation of reaction
center charge-separated state upon intensity-limited excitation at
650 nm. The effective increase in absorbance cross-section resulting
from the conjugation of two other dyes, Alexa Fluor 647 and Alexa
Fluor 750, was also investigated. The key parameters that dictate
the efficiency of dye-to-reaction center energy transfer and subsequent
charge separation were examined using both steady-state and time-resolved
fluorescence spectroscopy as well as transient absorbance spectroscopy
techniques. An understanding of these parameters is an important first
step toward developing more complex model light-harvesting systems
integrated with reaction centers
Tacrine–Trolox Hybrids: A Novel Class of Centrally Active, Nonhepatotoxic Multi-Target-Directed Ligands Exerting Anticholinesterase and Antioxidant Activities with Low In Vivo Toxicity
Coupling
of two distinct pharmacophores, tacrine and trolox, endowed
with different biological properties, afforded 21 hybrid compounds
as novel multifunctional candidates against Alzheimer’s disease.
Several of them showed improved inhibitory properties toward acetylcholinesterase
(AChE) in relation to tacrine. These hybrids also scavenged free radicals.
Molecular modeling studies in tandem with kinetic analysis exhibited
that these hybrids target both catalytic active site as well as peripheral
anionic site of AChE. In addition, incorporation of the moiety bearing
antioxidant abilities displayed negligible toxicity on human hepatic
cells. This striking effect was explained by formation of nontoxic
metabolites after 1 h incubation in human liver microsomes system.
Finally, tacrine–trolox hybrids exhibited low in vivo toxicity
after im administration in rats and potential to penetrate across
blood–brain barrier. All of these outstanding in vitro results
in combination with promising in vivo outcomes highlighted derivative <b>7u</b> as the lead structure worthy of further investigation