Supramolecular self-assembly of bacteriochlorophyll c molecules in aerosolized droplets to synthesize biomimetic chlorosomes.

The unique properties of chlorosomes, arising out of the self-assembled bateriochlorophyll (BChl) c structure, have made them attractive for use in solar cells. In this work, we have demonstrated the self-assembly of BChl c in aerosolized droplets to mimic naturally occurring chlorosomes. We compare two different methods for self-assembly of BChl c, one using a single-solvent and the other using two-solvents, and demonstrate the superiority of the two-solvent method. Results show that the self-assembled BChl c sprayed at different concentrations resulted in a varying red shift of 69-75 nm in absorption spectrum compared to the solution, which has peak at 668 nm corresponding to the monomeric BChl c. The sample fluoresces at 780 nm indicating a quality of self-assembly comparable to that observed in naturally occurring chlorosomes. In order to mimic chlorosomes, solution containing BChl c, BChl a, lipids and carotenes in same proportion as in chlorosomes is sprayed. The resulting self-assembly has an absorption peak at 750 nm, shifted by 82 nm compared to that of monomers and the fluorescence peak at 790 nm. Thus in presence of lipids and carotenes, both the absorption and fluorescence peaks are red shifted. Further, using grazing incidence small angle X-ray scattering (GISAXS), we characterized the deposited films, and the 2D X-ray scattering patterns of sample clearly indicate the distinct lamellar structure as present in chlorosomes. The results of this work provide new insights into self-assembly in aerosolized droplets, which can be used for assembling a wide range of molecules.

Directed assembly of the thylakoid membrane on nanostructured TiO2 for a photo-electrochemical cell.

The thylakoid membrane mainly consists of photosystem I (PSI), photosystem II (PSII) and the cytochrome b6f embedded in a lipid bilayer. PSI and PSII have the ability to capture sunlight and create an electron-hole pair. The study aims at utilizing these properties by using the thylakoid membrane to construct a photo-electrochemical cell. A controlled aerosol technique, electrohydrodynamic atomization, allows a systematic study by the fabrication of different cell configurations based on the surfactant concentration without any linker, sacrificial electron donor and mediator. The maximum photocurrent density observed is 6.7 mA cm(-2) under UV and visible light, and 12 µA cm(-2) under visible light illumination. The electron transfer occurs from PSII to PSI via cytochrome b6f and the electron at PSII is regenerated by water oxidation, similar to the z-scheme of photosynthesis. This work shows that re-engineering the natural photosynthesis circuit by the novel technique of electrospray deposition can result in an environmentally friendly method of harvesting sunlight.

Linker-free deposition and adhesion of Photosystem I onto nanostructured TiO2 for biohybrid photoelectrochemical cells.

Photosystem I (PSI) from oxygenic photosynthetic organisms is an attractive sensitizer for nano-biohybrid solar cells as it has a combined light-harvesting and reaction center in one protein complex and operates at a quantum yield close to one in biological systems. Using a linker-free deposition technique enabled by an electrospray system, PSI was coupled to 1-D nanostructured titanium dioxide thin films to fabricate an electrode for a photoelectrochemical cell. After deposition, the surfactant in the PSI aggregate was dissolved in the surfactant-free electrolyte, ensuring that partly hydrophobic PSI was not resuspended and stayed in contact with titanium dioxide. A maximum current density of 4.15 mA cm(-2) was measured after 10 min of electrospray deposition, and this is the highest current density reported so far for PSI-based photoelectrochemical cells. The high current is attributed to 1D nanostructure of titanium dioxide and orientation of the PSI onto the surface, which allows easy transfer of electrons.

Aerosolized droplet mediated self-assembly of photosynthetic pigment analogues and deposition onto substrates.

Self-assembled photosynthetic molecules have a high extinction coefficient and a broad absorption in the infrared region, and these properties can be used to improve the efficiency of solar cells. We have developed a single-step method for the self-assembly of synthetic chlorin molecules (analogues of native bacteriochlorophylls) in aerosolized droplets, containing a single solvent and two solvents, to synthesize biomimetic light-harvesting structures. In the single-solvent approach, assembly is promoted by a concentration-driven process due to evaporation of the solvent. The peak absorbance of Zn(II) 3-(1-hydroxyethyl)-10-phenyl-13(1)-oxophorbine (1) in methanol shifted from 646 nm to 725 nm (∼ 80 nm shift) after assembly, which is comparable to the shift observed in the naturally occurring assembly of bacteriochlorophyll c. Although assembly is thermodynamically favorable, the kinetics of self-assembly play an important role, and this was demonstrated by varying the initial concentration of the pigment monomer. To overcome kinetic limitations, a two-solvent approach using a volatile solvent (tetrahydrofuran) in which the dye is soluble and a less volatile solvent (ethanol) in which the dye is sparingly soluble was demonstrated to be effective. The effect of molecular structure is demonstrated by spraying the sterically hindered Zn(II) 3-(1-hydroxyethyl)-10-mesityl-13(1)-oxophorbine (2), which is an analogue of 1, under similar conditions. The results illustrate a valuable and facile aerosol-based method for the formation of films of supramolecular assemblies.

Characterization and deposition of various light-harvesting antenna complexes by electrospray atomization.

Photosynthetic organisms have light-harvesting complexes that absorb and transfer energy efficiently to reaction centers. Light-harvesting complexes (LHCs) have received increased attention in order to understand the natural photosynthetic process and also to utilize their unique properties in fabricating efficient artificial and bio-hybrid devices to capture solar energy. In this work, LHCs with different architectures, sizes, and absorption spectra, such as chlorosomes, Fenna-Matthews-Olson (FMO) protein, LH2 complex, and phycobilisome have been characterized by an electrospray-scanning mobility particle-sizer system (ES-SMPS). The size measured by ES-SMPS for FMO, chlorosomes, LH2, and phycobilisome were 6.4, 23.3, 9.5, and 33.4 nm, respectively. These size measurements were compared with values measured by dynamic light scattering and those reported in the literature. These complexes were deposited onto a transparent substrate by electrospray deposition. Absorption and fluorescence spectra of the deposited LHCs were measured. It was observed that the LHCs have light absorption and fluorescence spectra similar to that in solution, demonstrating the viability of the process.