Anisotropic organization and microscopic manipulation of self-assembling synthetic porphyrin microrods that mimic chlorosomes: bacterial light-harvesting systems

Biomimetic light-harvesting antennas via the self-assembly of chemically programmed chlorophylls

The photosynthetic pigment "chlorophyll" possesses attractive photophysical properties, including efficient sunlight absorption, photoexcited energy transfer, and charge separation, which are advantageous for applications for photo- and electro-functional materials such as artificial photosynthesis and solar cells. However, these functions cannot be realized by individual chlorophyll molecules alone; rather, they are achieved by the formation of sophisticated supramolecules through the self-assembly of the pigments. Here, we present strategies for constructing and developing artificial light-harvesting systems by mimicking photosynthetic antenna complexes through the highly ordered supramolecular self-assembly of synthetic dyes, particularly chlorophyll derivatives.

Stereospecific Self-Assembly Processes of Porphyrin-Proline Conjugates: From the Effect of Structural Features and Bulk Solvent Properties to the Application in Stereoselective Sensor Systems

Conjugating the porphyrin ring with an amino acid via amide linkage represents a straightforward way for conferring both amphiphilicity and chirality to the macrocycle. Proline residue is a good choice in this context since its conformational rigidity allows for porphyrin assembling where molecular chirality is efficiently transferred and amplified using properly honed aqueous environments. Herein, we describe the evolution of the studies carried out by our group to achieve chiral systems from some porphyrin-proline derivatives, both in solution and in the solid state. The discussion focuses on some fundamental aspects reflecting on the final molecular architectures obtained, which are related to the nature of the appended group (stereochemistry and charge), the presence of a metal ion coordinated to the porphyrin core and the bulk solvent properties. Indeed, fine-tuning the mentioned parameters enables the achievement of stereospecific structures with distinctive chiroptical and morphological features. Solid films based on these chiral systems were also obtained and their recognition abilities in gaseous and liquid phase are here described.

Differential Polarization Imaging of Plant Cells. Mapping the Anisotropy of Cell Walls and Chloroplasts

Modern light microscopy imaging techniques have substantially advanced our knowledge about the ultrastructure of plant cells and their organelles. Laser-scanning microscopy and digital light microscopy imaging techniques, in general—in addition to their high sensitivity, fast data acquisition, and great versatility of 2D–4D image analyses—also opened the technical possibilities to combine microscopy imaging with spectroscopic measurements. In this review, we focus our attention on differential polarization (DP) imaging techniques and on their applications on plant cell walls and chloroplasts, and show how these techniques provided unique and quantitative information on the anisotropic molecular organization of plant cell constituents: (i) We briefly describe how laser-scanning microscopes (LSMs) and the enhanced-resolution Re-scan Confocal Microscope (RCM of Confocal.nl Ltd. Amsterdam, Netherlands) can be equipped with DP attachments—making them capable of measuring different polarization spectroscopy parameters, parallel with the ‘conventional’ intensity imaging. (ii) We show examples of different faces of the strong anisotropic molecular organization of chloroplast thylakoid membranes. (iii) We illustrate the use of DP imaging of cell walls from a variety of wood samples and demonstrate the use of quantitative analysis. (iv) Finally, we outline the perspectives of further technical developments of micro-spectropolarimetry imaging and its use in plant cell studies.

Investigation of Morphology-Controlled Ultrafast Relaxation Processes of Aggregated Porphyrin

Here, we have synthesized rod and flake shaped morphology of porphyrin aggregates from 5, 10, 15, 20-tetra (4-n-octyloxyphenyl) porphyrin (4-opTPP) molecule which are evident from scanning electron microscopy (SEM). The formation of J-type aggregation is evident from steady state and time-resolved fluorescence spectroscopic studies. Ultrafast transient absorption spectroscopic studies reveal that the excited state lifetime is controlled by the morphology and the time constant for S₁→S₀ relaxation changes from 3.05 ps to 744 ps with changing the shape from rod to flake, respectively. In spite of similar exciton coupling energy in both the aggregates, the flake shaped aggregates undergo a faster exciton relaxation process and the non-radiative relaxation channels are found to depend on the shape of aggregates. The fundamental understanding of morphology controlled ultrafast relaxation processes of aggregated porphyrin is important for designing efficient light harvesting devices.

Electron and energy transfer in a porphyrin-oxoporphyrinogen-fullerene triad, ZnP-OxP-C60

A multichromophoric triad, ZnP-OxP-C60 containing porphyrin (ZnTPP hereafter ZnP), oxoporphyrinogen (OxP) and fullerene (C60) has been synthesized to probe the intramolecular dynamics of its electron and energy transfer in relation to the presence of the closely linked electron deficient OxP-C60 'special pair', constructed as a mimic of the naturally occurring photosynthetic antenna-reaction center. The DFT optimized structure of the triad reveals the relative spatial remoteness of the ZnP entity with proximal OxP/C60 entities. Free-energetics of different energy and electron transfer events were estimated using spectral, computational and electrochemical studies, according to the Rehm-Weller approach. Femtosecond transient absorption spectral studies revealed energy transfer from 1ZnP* to OxP to yield ZnP-1OxP*-C60, and electron transfer to yield ZnP˙+-OxP-C60˙- and/or ZnP-OxP˙+-C60˙- charge seperated states. That is, the ZnP entity in the triad operates as both antenna and electron donor to generate relatively long-lived charge separated states thus mimicking the early photoevents of natural photosynthesis.

The Assembly of Porphyrin Systems in Well-Defined Nanostructures: An Update

The interest in assembling porphyrin derivatives is widespread and is accounted by the impressive impact of these suprastructures of controlled size and shapes in many applications from nanomedicine and sensors to photocatalysis and optoelectronics. The massive use of porphyrin dyes as molecular building blocks of functional materials at different length scales relies on the interdependent pair properties, consisting of their chemical stability/synthetic versatility and their quite unique physicochemical properties. Remarkably, the driven spatial arrangement of these platforms in well-defined suprastructures can synergically amplify the already excellent properties of the individual monomers, improving conjugation and enlarging the intensity of the absorption range of visible light, or forming an internal electric field exploitable in light-harvesting and charge-and energy-transport processes. The countless potentialities offered by these systems means that self-assembly concepts and tools are constantly explored, as confirmed by the significant number of published articles related to porphyrin assemblies in the 2015-2019 period, which is the focus of this review.

Covalently Linked Bis(Amido-Corroles): Inter- and Intramolecular Hydrogen-Bond-Driven Supramolecular Assembly

Four bis-corroles linked by diamide bridges were synthesized through peptide-type coupling of a trans-A₂ B-corrole acid with aliphatic and aromatic diamines. In the solid state, the hydrogen-bond pattern in these bis-corroles is strongly affected by the type of solvent used in the crystallization process. Although intramolecular hydrogen bonds play a decisive role, they are supported by intermolecular hydrogen bonds and weak N-H⋅⋅⋅π interactions between molecules of toluene and the corrole cores. In an analogy to mono(amido-corroles), both in crystalline state and in solutions, the aliphatic or aromatic bridge is located directly above the corrole ring. When either ethylenediamine or 2,3-diaminonaphthalene are used as linkers, incorporation of polar solvents into the crystalline lattice causes a roughly parallel orientation of the corrole rings. At the same time, both NHCO⋅⋅⋅NH corrole hydrogen bonds are intramolecular. In contrast, solvation in toluene causes a distortion with one of the hydrogen bonds being intermolecular. Interestingly, intramolecular hydrogen bonds are always formed between the -NHCO- functionality located further from the benzene ring present at the position 10-meso. In solution, the hydrogen-bonds pattern of the bis(amido-corroles) is strongly affected by the type of the solvent. Compared with toluene (strongly high-field shifted signals), DMSO and pyridine disrupt self-assembly, whereas hexafluoroisopropanol strengthens intramolecular hydrogen bonds.

Fluorescence-detected linear dichroism imaging in a re-scan confocal microscope equipped with differential polarization attachment

Confocal laser scanning microscopy is probably the most widely used and one of the most powerful techniques in basic biology, medicine and material sciences that is employed to elucidate the architecture of complex cellular structures and molecular macro-assemblies. It has recently been shown that the information content, signal-to-noise ratio and resolution of such microscopes (LSMs) can be improved significantly by adding different attachments or modifying their design, while retaining their user-friendly features and relatively moderate costs. Differential polarization (DP) attachments, using high-frequency modulation/demodulation circuits, have made LSMs capable of high-precision 2D and 3D mapping of the anisotropy of microscopic samples—without interfering with their ‘conventional’ fluorescence or transmission imaging (Steinbach et al. in Methods Appl Fluoresc 2:015005, 2014). The resolution and the quality of fluorescence imaging have been enhanced in the recently constructed Re-scan confocal microscopy (RCM) (De Luca et al. in Biomed Opt Express 4:2644–2656, 2013). In this work, we developed the RCM technique further, by adding a DP-attachment modulating the exciting laser beam via a liquid crystal (LC) retarder synchronized with the data acquisition system; by this means, and with the aid of a software, fluorescence-detected linear dichroism (FDLD), characteristic of the anisotropic molecular organization of the sample, could be recorded in parallel with the confocal fluorescence imaging. For demonstration, we show FDLD images of a plant cell wall (Ginkgo biloba) stained with Etzold’s staining solution.

Synthesis and Self-Aggregation of π-Expanded Chlorophyll Derivatives to Construct Light-Harvesting Antenna Models

Chlorosomes are one of the elegant light-harvesting antenna systems in anoxygenic photosynthetic bacteria, whose core is constructed from J-type self-aggregation of bacteriochlorophyll- c, bacteriochlorophyll- d, bacteriochlorophyll- e, and bacteriochlorophyll- f molecules without the influence of polypeptides. Chlorosomal supramolecular models were built up using synthetic porphyrin-type bacteriochlorophyll- d analogues with a methoxycarbonylethenyl, formyl, vinyl, or ethyl group at the 8-position. Their chlorosomal self-aggregates in an aqueous micelle solution showed relatively intense absorption bands around 500-600 nm where antennas of natural oxygenic phototrophs, as well as green sulfur bacteria possessing bacteriochlorophylls- c/ d, absorb light less efficiently; this observation is called the "green gap". Furthermore, the functional chlorosomal models were constructed by simple addition of a small amount of an energy acceptor model bearing a bacteriochlorin moiety to the pigment self-assemblies in an aqueous micelle. The resulting excited energy donor-acceptor supramolecules played the roles of chlorosomal light-harvesting and energy-transfer antenna systems and were efficient at light absorption in the "green gap" region.

Polymer-Chlorosome Nanocomposites Consisting of Non-Native Combinations of Self-Assembling Bacteriochlorophylls

Chlorosomes are one of the characteristic light-harvesting antennas from green sulfur bacteria. These complexes represent a unique paradigm: self-assembly of bacteriochlorophyll pigments within a lipid monolayer without the influence of protein. Because of their large size and reduced complexity, they have been targeted as models for the development of bioinspired light-harvesting arrays. We report the production of biohybrid light-harvesting nanocomposites mimicking chlorosomes, composed of amphiphilic diblock copolymer membrane bodies that incorporate thousands of natural self-assembling bacteriochlorophyll molecules derived from green sulfur bacteria. The driving force behind the assembly of these polymer-chlorosome nanocomposites is the transfer of the mixed raw materials from the organic to the aqueous phase. We incorporated up to five different self-assembling pigment types into single nanocomposites that mimic chlorosome morphology. We establish that the copolymer-BChl self-assembly process works smoothly even when non-native combinations of BChl homologues are included. Spectroscopic characterization revealed that the different types of self-assembling pigments participate in ultrafast energy transfer, expanding beyond single chromophore constraints of the natural chlorosome system. This study further demonstrates the utility of flexible short-chain, diblock copolymers for building scalable, tunable light-harvesting arrays for technological use and allows for an in vitro analysis of the flexibility of natural self-assembling chromophores in unique and controlled combinations.

Biofest: Bioinspired Chemistry, Biomaterials and Bioelectrochemistry

Bio, bio, bio! This issue features the latest advances in bioinspired chemistry, biomaterials and bioelectrochemistry. Containing both original research and informative review articles, it is a must read for those committed to a multidisciplinary approach to these bio-orientated fields/topics.

Stereoselective Self-Aggregation of 31 -Epimerically Pure Amino Analogs of Zinc Bacteriochlorophyll-d in an Aqueous Micelle Solution

Zinc bacteriochlorophyll-d analogs possessing an amino group instead of the original hydroxy group at the C3¹ position were prepared by chemical modification of naturally occurring chlorophyll-a. The synthetic 3¹ -epimers were successfully separated by reverse phase HPLC to give diastereomerically pure samples. The stereochemistry of the chiral C3¹ -center in the separated amines was determined by NMR analysis of their diastereomeric amides as well as by their asymmetric synthesis from authentic stereoisomers. Both the epimers were monomeric in tetrahydrofuran to give sharp electronic absorption bands, while they self-aggregated to form chlorosomal oligomers with the redshifted bands in an aqueous Triton X-100 micelle solution (pH = 6.9). The resulting oligomers deaggregated by addition of p-toluenesulfonic acid to give monomeric N-protonated ammonium species. The aggregation and deaggregation were dependent on the 3¹ -stereochemistry, indicating that each epimer produced supramolecularly different self-aggregates.

Self-assembly and structural-functional flexibility of oxygenic photosynthetic machineries: personal perspectives

This short review, with a bit of historical aspect and a strong personal bias and emphases on open questions, is focusing on the (macro-)organization and structural-functional flexibilities of the photosynthetic apparatus of oxygenic photosynthetic organisms at different levels of the structural complexity-selected problems that have attracted most my attention in the past years and decades. These include (i) the anisotropic organization of the pigment-protein complexes and photosynthetic membranes-a basic organizing principle of living matter, which can, and probably should be adopted to intelligent materials; (ii) the organization of protein complexes into chiral macrodomains, large self-assembling highly organized but structurally flexible entities with unique spectroscopic fingerprints-structures, where, important, high-level regulatory functions appear to 'reside'; (iii) a novel, dissipation-assisted mechanism of structural changes, based on a thermo-optic effect: ultrafast thermal transients in the close vicinity of dissipation of unused excitation energy, which is capable of inducing elementary structural changes; it makes plants capable of responding to excess excitation with reaction rates proportional to the overexcitation above the light-saturation of photosynthesis; (iv) the 3D ultrastructure of the granum-stroma thylakoid membrane assembly and other multilamellar membrane systems, and their remodelings-associated with regulatory mechanisms; (v) the molecular organization and structural-functional plasticity of the main light-harvesting complex of plants, in relation to their crystal structure and different in vivo and in vitro states; and (vi) the enigmatic role of non-bilayer lipids and lipid phases in the bilayer thylakoid membrane-warranting its high protein content and contributing to its structural flexibility.

Structural and Functional Hierarchy in Photosynthetic Energy Conversion-from Molecules to Nanostructures

Basic principles of structural and functional requirements of photosynthetic energy conversion in hierarchically organized machineries are reviewed. Blueprints of photosynthesis, the energetic basis of virtually all life on Earth, can serve the basis for constructing artificial light energy-converting molecular devices. In photosynthetic organisms, the conversion of light energy into chemical energy takes places in highly organized fine-tunable systems with structural and functional hierarchy. The incident photons are absorbed by light-harvesting complexes, which funnel the excitation energy into reaction centre (RC) protein complexes containing redox-active chlorophyll molecules; the primary charge separations in the RCs are followed by vectorial transport of charges (electrons and protons) in the photosynthetic membrane. RCs possess properties that make their use in solar energy-converting and integrated optoelectronic systems feasible. Therefore, there is a large interest in many laboratories and in the industry toward their use in molecular devices. RCs have been bound to different carrier matrices, with their photophysical and photochemical activities largely retained in the nano-systems and with electronic connection to conducting surfaces. We show examples of RCs bound to carbon-based materials (functionalized and non-functionalized single- and multiwalled carbon nanotubes), transitional metal oxides (ITO) and conducting polymers and porous silicon and characterize their photochemical activities. Recently, we adapted several physical and chemical methods for binding RCs to different nanomaterials. It is generally found that the P(+)(QAQB)(-) charge pair, which is formed after single saturating light excitation is stabilized after the attachment of the RCs to the nanostructures, which is followed by slow reorganization of the protein structure. Measuring the electric conductivity in a direct contact mode or in electrochemical cell indicates that there is an electronic interaction between the protein and the inorganic carrier matrices. This can be a basis of sensing element of bio-hybrid device for biosensor and/or optoelectronic applications.

Fluorescence polarization measures energy funneling in single light-harvesting antennas--LH2 vs conjugated polymers

Numerous approaches have been proposed to mimic natural photosynthesis using artificial antenna systems, such as conjugated polymers (CPs), dendrimers, and J-aggregates. As a result, there is a need to characterize and compare the excitation energy transfer (EET) properties of various natural and artificial antennas. Here we experimentally show that EET in single antennas can be characterized by 2D polarization imaging using the single funnel approximation. This methodology addresses the ability of an individual antenna to transfer its absorbed energy towards a single pool of emissive states, using a single parameter called energy funneling efficiency (ε). We studied individual peripheral antennas of purple bacteria (LH2) and single CP chains of 20 nm length. As expected from a perfect antenna, LH2s showed funneling efficiencies close to unity. In contrast, CPs showed lower average funneling efficiencies, greatly varying from molecule to molecule. Cyclodextrin insulation of the conjugated backbone improves EET, increasing the fraction of CPs possessing ε = 1. Comparison between LH2s and CPs shows the importance of the protection systems and the protein scaffold of LH2, which keep the chromophores in functional form and at such geometrical arrangement that ensures excellent EET.

Self-assembling corroles

Amide-corroles form self-assembled structures via interaction of the core-NH with CO.

A nanoscale bio-inspired light-harvesting system developed from self-assembled alkyl-functionalized metallochlorin nano-aggregates

Self-assembled supramolecular organization of nano-structured biomimetic light-harvesting modules inside solid-state nano-templates can be exploited to develop excellent light-harvesting materials for artificial photosynthetic devices. We present here a hybrid light-harvesting system mimicking the chlorosomal structures of the natural photosynthetic system using synthetic zinc chlorin units (ZnChl-C6, ZnChl-C12 and ZnChl-C18) that are self-aggregated inside the anodic aluminum oxide (AAO) nano-channel membranes. AAO nano-templates were modified with a TiO2 matrix and functionalized with long hydrophobic chains to facilitate the formation of supramolecular Zn-chlorin aggregates. The transparent Zn-chlorin nano-aggregates inside the alkyl-TiO2 modified AAO nano-channels have a diameter of ∼120 nm in a 60 μm length channel. UV-Vis studies and fluorescence emission spectra further confirm the formation of the supramolecular ZnChl aggregates from monomer molecules inside the alkyl-functionalized nano-channels. Our results prove that the novel and unique method can be used to produce efficient and stable light-harvesting assemblies for effective solar energy capture through transparent and stable nano-channel ceramic materials modified with bio-mimetic molecular self-assembled nano-aggregates.

Self-assembly: from amphiphiles to chromophores and beyond

Self-assembly has been recognised as a ubiquitous aspect of modern chemistry. Our understanding and applications of self-assembly are substantially based on what has been learned from biochemical systems. In this review, we describe various aspects of self-assembly commencing with an account of the soft structures that are available by assembly of surfactant amphiphiles, which are important scientific and industrial materials. Variation of molecular design using rules defined by surfactant self-assembly permits synthesis of functional nanostructures in solution and at surfaces while increasing the strength of intermolecular interactions through π-π stacking, metal cation coordination and/or hydrogen bonding leads to formation of highly complex bespoke nanostructured materials exemplified by DNA assemblies. We describe the origins of self-assembly involving aggregation of lipid amphiphiles and how this subject has been expanded to include other highly advanced chemical systems.

Bacteriochlorophyll aggregates self-assembled on functionalized gold nanorod cores as mimics of photosynthetic chlorosomal antennae: a single molecule study

We prepare artificial aggregates that mimic the structure and function of natural chlorosomal light harvesting complexes of green photosynthetic bacteria. Gold nanorods functionalized with hydroxyl groups and immobilized on a substrate serve as cores for the growth of bacteriochlorophyll (BChl) aggregates from a buffer solution. The BChl pigments form large self-assembled aggregate particles with sizes more than twice that of natural chlorosomes. The size is controllable by the aggregation time. The aggregates are characterized on a single-particle level by atomic force microscopy, electron microscopy, and single-molecule spectroscopy. The absorption and fluorescence spectral properties which reflect the molecular level arrangement of the BChl aggregates closely resemble those of the natural chlorosomes of the photosynthetic bacterium Chlorobaculum tepidum. On the other hand, the results of linear dichroism and circular dichroism are different from those of the chlorosomes and indicate a different mesoscopic structure for the artificial aggregates. These results emphasize the structural role played by the baseplate pigment-protein complex in natural chlorosomes.

Self-organized porphyrin arrays on surfaces: the case of hydrophilic side chains and polar surfaces

Energy collection in photosynthetic microorganisms occurs through vectorial energy transfer along organized assemblies of chromophores. This process has inspired many research groups and yielded numerous examples of self-assembled photoactive structures. Dye sensitization of solar cells usually requires covalent anchoring of dyes onto the surface of metal oxides. A new porphyrin derivative that self-assembles upon non-covalent interaction with a surface has been designed and characterized by AFM. Interaction with metal oxide surfaces is further documented by the sensitization of metal oxide surfaces and the generation of photocurrent in non-optimized dye sensitized solar cells.

Mapping microscopic order in plant and mammalian cells and tissues: novel differential polarization attachment for new generation confocal microscopes (DP-LSM)

Elucidation of the molecular architecture of complex, highly organized molecular macro-assemblies is an important, basic task for biology. Differential polarization (DP) measurements, such as linear (LD) and circular dichroism (CD) or the anisotropy of the fluorescence emission (r), which can be carried out in a dichrograph or spectrofluorimeter, respectively, carry unique, spatially averaged information about the molecular organization of the sample. For inhomogeneous samples-e.g. cells and tissues-measurements on macroscopic scale are not satisfactory, and in some cases not feasible, thus microscopic techniques must be applied. The microscopic DP-imaging technique, when based on confocal laser scanning microscope (LSM), allows the pixel by pixel mapping of anisotropy of a sample in 2D and 3D. The first DP-LSM configuration, which, in fluorescence mode, allowed confocal imaging of different DP quantities in real-time, without interfering with the 'conventional' imaging, was built on a Zeiss LSM410. It was demonstrated to be capable of determining non-confocally the linear birefringence (LB) or LD of a sample and, confocally, its FDLD (fluorescence detected LD), the degree of polarization (P) and the anisotropy of the fluorescence emission (r), following polarized and non-polarized excitation, respectively (Steinbach et al 2009 Acta Histochem.111 316-25). This DP-LSM configuration, however, cannot simply be adopted to new generation microscopes with considerably more compact structures. As shown here, for an Olympus FV500, we designed an easy-to-install DP attachment to determine LB, LD, FDLD and r, in new-generation confocal microscopes, which, in principle, can be complemented with a P-imaging unit, but specifically to the brand and type of LSM.

Chlorosome antenna complexes from green photosynthetic bacteria

Chlorosomes are the distinguishing light-harvesting antenna complexes that are found in green photosynthetic bacteria. They contain bacteriochlorophyll (BChl) c, d, e in natural organisms, and recently through mutation, BChl f, as their principal light-harvesting pigments. In chlorosomes, these pigments self-assemble into large supramolecular structures that are enclosed inside a lipid monolayer to form an ellipsoid. The pigment assembly is dictated mostly by pigment-pigment interactions as opposed to protein-pigment interactions. On the bottom face of the chlorosome, the CsmA protein aggregates into a paracrystalline baseplate with BChl a, and serves as the interface to the next energy acceptor in the system. The exceptional light-harvesting ability at very low light conditions of chlorosomes has made them an attractive subject of study for both basic and applied science. This review, incorporating recent advancements, considers several important aspects of chlorosomes: pigment biosynthesis, organization of pigments and proteins, spectroscopic properties, and applications to bio-hybrid and bio-inspired devices.

Tailoring zinc porphyrin to the Ag nanostructure substrate: an effective approach for photoelectrochemical studies in the presence of mononucleotides

The substituted porphyrin 2-cyano-3-(2'-(5',10',15',20-tetraphenyl porphyrinato zinc-(ii))yl) acrylic acid was used to modify nanostructured Ag surfaces. This porphyrin-modified surface exhibits photocurrent when exposed to a light source, which is modulated in the presence of nucleotides. The addition of the nucleotides adenosine-5'-monophosphate (AMP), guanosine-5'-monophosphate (GMP) and cytidine-5'-monophosphate (CMP) causes partial quenching of the photoelectrochemical response of the porphyrin. The quenching efficiency is 80%, 68% and 48% for AMP, CMP and GMP, respectively. This work represents a new aspect of Ag NS substrates and highlights their usefulness as transducers a for potential chemosensor systems.

Organo-modified hydrotalcite-quantum dot nanocomposites as a novel chemiluminescence resonance energy transfer probe

In this work, we fabricate an oriented luminescent quantum dot (QD)-layered double hydroxide (LDH) nanocomposite material by the highly orderly and alternate assembly of trace CdTe QDs in dodecylbenzene sulfonate bilayer bunches on the organo-modified LDH exterior surfaces. Interestingly, the novel QD-LDH nanocomposites can remarkably amplify chemiluminescence (CL) of the luminol-H2O2 system, which is attributed to an inhibition of QD oxidation by H2O2, an increase in the radiative decay rate, and an inhibition in the nonradiative relaxation of QDs. In addition, a novel flow-through column-based CL resonance energy transfer is fabricated using luminol as energy donors and the solid luminescent QD-LDH nanocomposites as energy acceptors for signal amplification. The applicability of this flow-through column is evaluated by determining H2O2 using luminol-H2O2 CL system. The CL intensity exhibits a stable response to H2O2 over a concentration range from 0.5 to 60 μM with a detection limit as low as 0.3 μM. Finally, the proposed method has been successfully applied to detect H2O2 in snow samples, and the results agreed with those obtained by the standard spectrophotometric method. Our findings indicate that the new luminescent QD-LDH nanocomposite material would be used for high throughput screening of complex systems with different sized QDs.

Protonation state of core nitrogens in the meso-tetra(4-carboxyphenyl)porphyrin impacts the chemical and physical properties of nanostructures formed in acid solutions

Organic self-assemblies of meso-tetra (4-carboxyphenyl)porphine (TCPP) prepared in trifluoroacetic acid (TFA) and hydrochloric acid solutions at pH < 1 were studied by X-ray photoelectron spectroscopy (XPS) in order to determine the protonation state of the porphyrin building blocks present in the solid self-assembled nanostructures. XPS measurements were conducted both at room and at elevated temperatures. Room temperature N 1s spectra showed two bands with a 3:1 intensity ratio consistent with three protonated and one unprotonated nitrogen in the structures prepared in both TFA and HCl solutions. We attribute this result to TCPP existing as a 50:50 mixture of the free-base and diacid forms of the porphyrin core in the self-assembled state. Upon heating to 150 °C the TCPP/TFA and TCPP/HCl nanomaterials exhibit loss of pyrrolic hydrogens and retain different amounts of their respective counter ions. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of the nanostructures showed that these systems have different morphologies depending on the acid employed during fabrication and the post preparation temperature treatment. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) of the TCPP nanostructures indicate that those prepared in HCl are crystalline while the nanoassemblies made in TFA are polycrystalline or amorphous.

Magneto-chiral dichroism of artificial light-harvesting antenna

We have demonstrated the presence of magneto-chiral dichroism (MChD) of chiral J-aggregates of zinc chlorins. To the best of our knowledge, this is the first observation of MChD in artificial light-harvesting antennas.

Hierarchal Light-Harvesting Aggregates and Their Potential for Solar Energy Applications

The tunable optical properties of self-assembled chromophores are exploited by photosynthetic organisms to optimize their ability to harvest a broad range of the solar spectrum. Similarly, the efficiency of solar photovoltaic and photoelectrochemical devices depends strongly on the coincidence of the absorption spectrum of the photoactive components with the spectrum of the sun. While the possibility of borrowing ideas about light-harvesting aggregates from nature in order to improve the efficiency of solar energy conversion is quite attractive, progress to date is hindered by incomplete understanding of aggregate internal structure and its relation to excitonic states. In this Perspective, we describe our recent work on the hierarchal structure of self-assembled porphyrin aggregates that are similar to light-harvesting complexes of photosynthetic bacteria. We address the question of whether aggregation can be beneficial to dye-sensitized solar energy conversion and present promising results for a solar cell based on an abundant plant pigment that displays signatures of aggregation when adsorbed on TiO2.