Peter N. Ciesielski

Functionalized nanoporous gold leaf electrode films for the immobilization of photosystem I.

Plants and some types of bacteria demonstrate an elegant means to capitalize on the superabundance of solar energy that reaches our planet with their energy conversion process called photosynthesis. Seeking to harness Natures optimization of this process, we have devised a biomimetic photonic energy conversion system that makes use of the photoactive protein complex Photosystem I, immobilized on the surface of nanoporous gold leaf (NPGL) electrodes, to drive a photoinduced electric current through an electrochemical cell. The intent of this study is to further the understanding of how the useful functionality of these naturally mass-produced, biological light-harvesting complexes can be integrated with nonbiological materials. Here, we show that the protein complexes retain their photonic energy conversion functionality after attachment to the nanoporous electrode surface and, further, that the additional PSI/electrode interfacial area provided by the NPGL allows for an increase in PSI-mediated electron transfer with respect to an analogous 2D system if the pores are sufficiently enlarged by dealloying. This increase of interfacial area is pertinent for other applications involving electron transfer between phases; thus, we also report on the widely accessible and scalable method by which the NPGL electrode films used in this study are fabricated and attached to glass and Au/Si supports and demonstrate their adaptability by modification with various self-assembled monolayers. Finally, we demonstrate that the magnitude of the PSI-catalyzed photocurrents provided by the NPGL electrode films is dependent upon the intensity of the light used to irradiate the electrodes.

Photosystem I – Based biohybrid photoelectrochemical cells

Photosynthesis is the process by which Nature coordinates a tandem of protein complexes of impressive complexity that function to harness staggering amounts of solar energy on a global scale. Advances in biochemistry and nanotechnology have provided tools to isolate and manipulate the individual components of this process, thus opening a door to a new class of highly functional and vastly abundant biological resources. Here we show how one of these components, Photosystem I (PSI), is incorporated into an electrochemical system to yield a stand-alone biohybrid photoelectrochemical cell that converts light energy into electrical energy. The cells make use of a dense multilayer of PSI complexes assembled on the surface of the cathode to produce a photocatalytic effect that generates photocurrent densities of approximately 2 microA/cm(2) at moderate light intensities. We describe the relationship between the current and voltage production of the cells and the photoinduced interactions of PSI complexes with electrochemical mediators, and show that the performance of the present device is limited by diffusional transport of the electrochemical mediators through the electrolyte. These biohybrid devices display remarkable stability, as they remain active in ambient conditions for at least 280 days. Even at bench-scale production, the materials required to fabricate the cells described in this manuscript cost approximately 10 cents per cm(2) of active electrode area.

Rapid assembly of photosystem I monolayers on gold electrodes.

Photosystem I (PSI) has drawn widespread interest for use in biomimetically inspired energy conversion devices upon extracting it from plants or cyanobacteria and assembling it at surfaces. Here, we demonstrate that a critically dense monolayer of spinach-derived PSI must be formed on an electrode surface to achieve optimal photocurrents, and we introduce a new method for preparing these dense PSI monolayers that reduces the time required for assembly by approximately 80-fold in comparison to that for adsorption from solution. This method consists of applying a vacuum above the aqueous PSI solution during assembly to concentrate PSI and precipitate it into a thick layer onto the surface of various self-assembled monolayers or directly onto the electrode surface. Rinsing with water yields a dense monolayer of PSI that draws approximately 100 nA/cm2 of light-induced current from the gold electrode in the presence of appropriate mediators.

Patterned nanoporous gold as an effective SERS template

We demonstrate large area two-dimensional arrays of patterned nanoporous gold for use as easy-to-fabricate, cost-effective, and stable surface enhanced Raman scattering (SERS) templates. Using a simple one-step direct imprinting process, subwavelength nanoporous gold (NPG) gratings are defined by densifying appropriate regions of a NPG film. Both the densified NPG and the two-dimensional grating pattern are shown to contribute to the SERS enhancement. The resulting substrates exhibit uniform SERS enhancement factors of at least 10(7) for a monolayer of adsorbed benzenethiol molecules.

3D Electron Tomography of Pretreated Biomass Informs Atomic Modeling of Cellulose Microfibrils

Fundamental insights into the macromolecular architecture of plant cell walls will elucidate new structure-property relationships and facilitate optimization of catalytic processes that produce fuels and chemicals from biomass. Here we introduce computational methodology to extract nanoscale geometry of cellulose microfibrils within thermochemically treated biomass directly from electron tomographic data sets. We quantitatively compare the cell wall nanostructure in corn stover following two leading pretreatment strategies: dilute acid with iron sulfate co-catalyst and ammonia fiber expansion (AFEX). Computational analysis of the tomographic data is used to extract mathematical descriptions for longitudinal axes of cellulose microfibrils from which we calculate their nanoscale curvature. These nanostructural measurements are used to inform the construction of atomistic models that exhibit features of cellulose within real, process-relevant biomass. By computational evaluation of these atomic models, we propose relationships between the crystal structure of cellulose Iβ and the nanoscale geometry of cellulose microfibrils.

Kinetic model of the photocatalytic effect of a Photosystem I monolayer on a planar electrode surface.

Photosystem I (PSI), a photoactive protein complex that participates in the light reactions of natural photosynthesis, can exhibit photocatalytic capabilities when incorporated to electrochemical systems. Here we present a simulation for the photoelectrochemical behavior of an electrode modified with a monolayer of Photosystem I complexes during photochronoamperometric experiments in which the electrode is exposed to periods of darkness and irradiation. A kinetic model is derived from conservation statements for the various oxidation states of the reaction centers of PSI complexes and electrochemical mediators within the system. The kinetic parameters that dictate the performance of the simulation are extracted from experimental data and the resulting simulation is capable of predicting the photochronoamperometric behavior of the system over a range of overpotentials. The model is used to investigate the various contributions to the photocurrent production of the system as well as the effects of the orientation of PSI complexes adsorbed to the electrode surface.

CHAPTER 11:Simulating Biomass Fast Pyrolysis at the Single Particle Scale

Simulating fast pyrolysis at the scale of single particles allows for the investigation of the impacts of feedstock-specific parameters such as particle size, shape, and species of origin. For this reason particle-scale modeling has emerged as an important tool for understanding how variations in feedstock properties affect the outcomes of pyrolysis processes. The origins of feedstock properties are largely dictated by the composition and hierarchical structure of biomass, from the microstructural porosity to the external morphology of milled particles. These properties may be accounted for in simulations of fast pyrolysis by several different computational approaches depending on the level of structural and chemical complexity included in the model. The predictive utility of particle-scale simulations of fast pyrolysis can still be enhanced substantially by advancements in several areas. Most notably, considerable progress would be facilitated by the development of pyrolysis kinetic schemes that are decoupled from transport phenomena, predict product evolution from whole-biomass with increased chemical speciation, and are still tractable with present-day computational resources.

Direct imprinted gratings on nanoporous gold as effective SERS substrates

We demonstrate large area arrays of patterned nanoporous gold as effective surface enhanced Raman scattering (SERS) templates by using a direct imprinting process. The resulting substrates exhibit a 107 SERS enhancement factor for adsorbed benzenethiol.

Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules

Oxygenated biofuels provide a renewable, domestic source of energy that can enable adoption of advanced, high-efficiency internal combustion engines, such as those based on homogeneously charged compression ignition (HCCI). Of key importance to such engines is the cetane number (CN) of the fuel, which is determined by the autoignition of the fuel under compression at relatively low temperatures (550-800 K). For the plethora of oxygenated biofuels possible, it is desirable to know the ignition delay times and the CN of these fuels to help guide conversion strategies so as to focus efforts on the most desirable fuels. For alkanes, the chemical pathways leading to radical chain-branching reactions giving rise to low-temperature autoignition are well-known and are highly coincident with the buildup of reactive radicals such as OH. Key in the mechanisms leading to chain branching are the addition of molecular oxygen to alkyl radicals and the rearrangement and dissociation of the resulting peroxy radials. Prediction of the temperature and pressure dependence of reactions that lead to the buildup of reactive radicals requires a detailed understanding of the potential energy surfaces (PESs) of these reactions. In this study, we used quantum mechanical modeling to systematically compare the effects of oxygen functionalities on these PESs and associated kinetics so as to understand how they affect experimental trends in autoignition and CN. The molecules studied here include pentane, pentanol, pentanal, 2-heptanone, methylpentyl ether, methyl hexanoate, and pentyl acetate. All have a saturated five-carbon alkyl chain with an oxygen functional group attached to the terminal carbon atom. The results of our systematic comparison may be summarized as follows: (1) Oxygen functionalities activate C-H bonds by lowering the bond dissociation energy (BDE) relative to alkanes. (2) The R-OO bonds in peroxy radicals adjacent to carbonyl groups are weaker than corresponding alkyl systems, leading to dissociation of ROO• radicals and reducing reactivity and hence CN. (3) Hydrogen atom transfer in peroxy radicals is important in autoignition, and low barriers for ethers and aldehydes lead to high CN. (4) Peroxy radicals formed from alcohols have low barriers to form aldehydes, which reduce the reactivity of the alkyl radical. These findings for the formation and reaction of alkyl radicals with molecular oxygen explain the trend in CN for these common biofuel functional groups.