Synthesis and Photodynamics of 9-Mesitylacridinium Ion-Modified Gold Nanoclusters

Optical control of mitochondrial reductive reactions in living cells using an electron donor-acceptor linked molecule

It has been known for decades that intracellular redox reactions control various vital functions in living systems, which include the synthesis of biomolecules, the modulation of protein functions, and cell signaling. Although there have been several reports on the control of such functions using DNA and RNA, the non-invasive optical control of biological functions is an important ongoing challenge. In this study, a hybrid of an electron donor-acceptor linked molecule based on a ferrocene(Fc)-porphyrin(ZnP)-fullerene(C₆₀) analogue and an elaborately designed nano-carrier, referred to herein as a MITO-Porter, resulted in a successful photoinduced intermolecular electron transfer reaction via the long-lived intramolecular charge separation, leading to site-specific reductive reactions in the mitochondria of living HeLa cells. A Fc-ZnP-C₆₀ linked molecule, 1-Oct, was designed and prepared for taking advantage of the unique photophysical properties with excellent efficiency (i.e. a long lifetime and a high quantum yield) for photoinduced charge separation. The targeted delivery of 1-Oct to mitochondria was accomplished by using a combination of the Fc-ZnP-C₆₀ molecule and a drug delivery nano-carrier, MITO-Porter, that was recently established by our group for intracellular cargo delivery. The successful delivery of 1-Oct by the MITO-Porter permitted the optically-controlled generation of O₂^- in the mitochondria of HeLa cells and the following induction of apoptosis as a cell signalling response was observed in confocal laser microscopy experiments. The obtained results indicate the use of an electron donor-acceptor system such as this can be a promising tool for the non-invasive triggering of redox-coupled cellular activities in living systems.

Photoinduced electron transfer reaction in mitochondria for spatiotemporal selective photo-oxidation of lipids by donor/acceptor linked molecules

Donor-acceptor-linked molecules have been synthesized and utilized to induce the rapid and site-selective lipid-oxidation in mitochondria by utilizing a photoinduced intermolecular electron transfer reaction. Two water-soluble donor-acceptor molecules (1 and 2) were designed and synthesized for this purpose. 2 was prepared to modulate its affinity to cell membrane in mitochondria. Confocal laser microscopy experiments revealed that 1 and 2 possess high localization abilities in mitochondria. By the photoinduced electron transfer, 2 exhibited the remarkable oxidation ability of lipids, mainly cardiolipin. In HeLa cells, 2 triggered mitochondrial lipid oxidation, which was followed by apoptotic cell death, under illumination within a few seconds. These results show that the present molecular system is highly promising to utilize the photoinduced intermolecular electron transfer reaction in a precise spatiotemporal manner in a cell by using light.

Optical control of neuronal firing via photoinduced electron transfer in donor-acceptor conjugates

A rationally designed donor–acceptor conjugate efficiently generates a photoinduced charge-separated state in a cellular environment, achieving photoinduction of neuronal firing.

A series of porphyrin–fullerene linked molecules has been synthesized to evaluate the effects of substituents and molecular structures on their charge-separation yield and the lifetime of a final charge-separated state in various hydrophilic environments. The selected high-performance molecule effectively achieved depolarization in a plasma cell membrane by visible light as well as two-photon excitation using a near-infrared light laser. Moreover, it was revealed that the depolarization can trigger neuronal firing in rat hippocampal neurons, demonstrating the potential and versatility for controlling cell functions using light.

Neurotoxin-directed synthesis and in vitro evaluation of Au nanoclusters

A glioma-specific theranostic agent is prepared by using Chlorotoxin fusion protein to direct the synthesis of Au nanoclusters, which exhibit bright fluorescence and high specificity to target and treat glioma cells.

Amplified and selective detection of manganese peroxidase genes based on enzyme-scaffolded-gold nanoclusters and mesoporous carbon nitride

This work has demonstrated an amplified and selective detection platform using enzyme-scaffolded-gold nanoclusters as signal label, coupling with mesoporous carbon nitride (MCN) and gold nanoparticles (GNPs) modified glassy carbon electrode (GCE). Streptavidin-horseradish peroxidase (SA-HRP) has been integrated with gold nanoclusters (GNCs) as scaffold using a simple, fast and non-toxic method. The mechanisms of enzymatic amplification, redox cycling and signal amplification by this biosensor were discussed in detail. GNCs might perform important roles as electrocatalyst as well as electron transducer in these processes. The concentrations of reagents and the reaction times of these reagents were optimized to improve the analytical performances. Under the optimized condition, the signal response to enzyme-scaffolded-gold nanoclusters catalyzed reaction was linearly related to the natural logarithm of the target nucleic acid concentration in the range from 10(-17)M to 10(-9)M with a correlation coefficient of 0.9946, and the detection limit was 8.0×10(-18)M (S/N=3). Besides, synthesized oligonucleotide as well as Phanerochaete chrysosporium MnP fragments amplified using polymerase chain reaction and digested by restriction endonucleases were tested. Furthermore, this biosensor exhibited good precision, stability, sensitivity, and selectivity, and discriminated satisfactorily against mismatched nucleic acid samples of similar lengths.

Long-lived charge separation and applications in artificial photosynthesis

Researchers have long been interested in replicating the reactivity that occurs in photosynthetic organisms. To mimic the long-lived charge separations characteristic of the reaction center in photosynthesis, researchers have applied the Marcus theory to design synthetic multistep electron-transfer (ET) systems. In this Account, we describe our recent research on the rational design of ET control systems, based on models of the photosynthetic reaction center that rely on the Marcus theory of ET. The key to obtaining a long-lived charge separation is the careful choice of electron donors and acceptors that have small reorganization energies of ET. In these cases, the driving force of back ET is located in the Marcus inverted region, where the lifetime of the charge-separated state lengthens as the driving force of back ET increases. We chose porphyrins as electron donors and fullerenes as electron acceptors, both of which have small ET reorganization energies. By linking electron donor porphyrins and electron acceptor fullerenes at appropriate distances, we achieved charge-separated states with long lifetimes. We could further lengthen the lifetimes of charge-separated states by mixing a variety of components, such as a terminal electron donor, an electron mediator, and an electron acceptor, mimicking both the photosynthetic reaction center and the multistep photoinduced ET that occurs there. However, each step in multistep ET loses a fraction of the initial excitation energy during the long-distance charge separation. To overcome this drawback in multistep ET systems, we used designed new systems where we could finely control the redox potentials and the geometry of simple donor-acceptor dyads. These modifications resulted in a small ET reorganization energy and a high-lying triplet excited state. Our most successful example, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), can undergo a fast photoinduced ET from the mesityl (Mes) moiety to the singlet excited state of the acridinium ion moiety (Acr(+)) with extremely slow back ET. The high-energy triplet charge-separated state is located deep in the Marcus inverted region, and we have detected the structural changes during the photoinduced ET in this system using X-ray crystallography. To increase the efficiency of both the light-harvesting and photoinduced ET, we assembled the Acr(+)-Mes dyads on gold nanoparticles to bring them in closer proximity to one another. We can also incorporate Acr(+)-Mes molecules within nanosized mesoporous silica-alumina. In contrast to the densely assembled dyads on gold nanoparticles, each Acr(+)-Mes molecule in silica-alumina is isolated in the mesopore, which inhibits the bimolecular back ET and leads to longer lifetimes in solution at room temperature than the natural photosynthetic reaction center. Acr(+)-Mes and related compounds act as excellent organic photocatalysts and facilitate a variety of reactions such as oxygenation, bromination, carbon-carbon bond formation, and hydrogen evolution reactions.

Shape- and size-controlled nanomaterials for artificial photosynthesis

Nanomaterials with various shapes and sizes have been developed to mimic functions of photosynthesis in which solar energy conversion is achieved by using nanosized proteins with controlled shapes and sizes. Artificial photosynthesis consists of light-harvesting and charge-separation processes together with catalytic units of water oxidation and reduction. Nanosized mesoporous silica-alumina was utilized to encapsulate organic charge-separation molecules inside the nanospace to elongate the lifetimes of the charge-separated states, as observed in the photosynthetic reaction centers. Metal nanoparticles with controlled shapes and sizes have also been utilized as efficient catalysts for photocatalytic hydrogen evolution from water with reductants by using electron donor-acceptor organic molecules as photocatalysts. The control of the shape and size of metal nanoparticles plays a very important role in achieving high catalytic performance in catalytic hydrogen evolution in water reduction and also in catalytic oxygen evolution in water oxidation.

Improvement of durability of an organic photocatalyst in p-xylene oxygenation by addition of a Cu(II) complex

The catalytic durability of an organic photocatalyst, 9-mesityl-10-methyl acridinium ion (Acr(+)-Mes), has been dramatically improved by the addition of [{tris(2-pyridylmethyl)amine}Cu(II)](ClO(4))(2) ([(tmpa)Cu(II)](2+)) in the photocatalytic oxygenation of p-xylene by molecular oxygen in acetonitrile. Such an improvement is not observed by the addition of Cu(ClO(4))(2) in the absence of organic ligands. The addition of [(tmpa)Cu](2+) in the reaction solution resulted in more than an 11 times higher turnover number (TON) compared with the TON obtained without [(tmpa)Cu(II)](2+). In the photocatalytic oxygenation, a stoichiometric amount of H(2)O(2) formation was observed in the absence of [(tmpa)Cu(II)](2+), however, much less H(2)O(2) formation was observed in the presence of [(tmpa)Cu(II)](2+). The photocatalytic mechanism was investigated by laser flash photolysis measurements in order to detect intermediates. The reaction of O(2)˙(-) with [(tmpa)Cu(II)](2+) monitored by UV-vis spectroscopy in propionitrile at 203 K suggested formation of [{(tmpa)Cu(II)}(2)O(2)](2+), a transformation which is crucial for the overall 4-electron reduction of molecular O(2) to water, and a key in the observed improvement in the catalytic durability of Acr(+)-Mes.

Photocatalytic Hydrogen Evolution Using 9-Phenyl-10-methyl-acridinium Ion Derivatives as Efficient Electron Mediators and Ru-Based Catalysts

Photocatalytic hydrogen evolution has been performed by photoirradiation (λ > 420 nm) of a mixed solution of a phthalate buffer and acetonitrile (MeCN) (1 : 1 (v/v)) containing EDTA disodium salt (EDTA), [RuII(bpy)3]2+ (bpy = 2,2′-bipyiridine), 9-phenyl-10-methylacridinium ion (Ph–Acr+–Me), and Pt nanoparticles (PtNPs) as a sacrificial electron donor, a photosensitiser, an electron mediator, and a hydrogen-evolution catalyst, respectively. The hydrogen-evolution rate of the reaction system employing Ph–Acr+–Me as an electron mediator was more than 10 times higher than that employing a conventional electron mediator of methyl viologen. In this reaction system, ruthenium nanoparticles (RuNPs) also act as a hydrogen-evolution catalyst as well as the PtNPs. The immobilization of the efficient electron mediator on the surface of a hydrogen-evolution catalyst is expected to enhance the hydrogen-evolution rate. The methyl group of Ph–Acr+–Me was chemically modified with a carboxy group (Ph–Acr+–CH2COOH) to interact with metal oxide surfaces. In the photocatalytic hydrogen-evolution system using Ph–Acr+–CH2COOH and Pt-loaded ruthenium oxide nanoparticles (Pt/RuO2NPs) as electron donor and hydrogen-evolution catalyst, respectively, the hydrogen-evolution rate was 1.5–2 times faster than the reaction system using Ph–Acr+–Me as an electron mediator. On the other hand, no enhancement in the hydrogen-evolution rate was observed in the reaction system using Ph–Acr+–CH2COOH with PtNPs. Thus, the enhancement of hydrogen-evolution rate originated from the favourable interaction between Ph–Acr+–CH2COOH and RuO2NPs. These results suggest that the use of Ph–Acr+–Me as an electron mediator enables the photocatalytic hydrogen evolution using PtNPs and RuNPs as hydrogen-evolution catalysts, and the chemical modification of Ph–Acr+–Me with a carboxy group paves the way to utilise a supporting catalyst, Pt loaded on a metal oxide, as a hydrogen-evolution catalyst.

Electrochemical and optical characterization of triarylamine functionalized gold nanoparticles

This paper describes the synthesis, structural analysis, and investigations of the optical and electrochemical properties of some gold nanoparticles (AuNPs) which consist of a triarylamine ligand shell attached to small gold cores (Au-Tara). The triarylamine chromophores were attached to small 4-bromobenzenethiol covered gold nanoparticles (ca. 2 nm in diameter) by Sonogashira reaction. This procedure yields triarylamine redox centers attached via π-conjugated bridging units of different length to the gold core. The AuNPs were analyzed with (1)H NMR spectroscopy, diffusion ordered NMR spectroscopy (DOSY), thermogravimetric analysis (TGA), and scanning transmission electron microscopy (STEM). Cyclic voltammetry (CV) technique was used to determine the composition of the redox active particles via the Randles-Sevcik equation. The optical and electrochemical properties of the Au-Tara nanoparticles and of their corresponding unbound ligands (Ref) were investigated with UV/vis/NIR absorption spectroscopy, Osteryoung square wave voltammetry (OSWV), and spectroelectrochemistry (SEC). These data show that the assembling of triarylamines in the vicinity of a gold nanoparticle can change the optical and electrochemical properties of the triarylamine redox chromophores depending on the kind and length of the bridging unit. This is due to gold core-chromophore and chromophore-chromophore interactions.

Size- and shape-dependent activity of metal nanoparticles as hydrogen-evolution catalysts: mechanistic insights into photocatalytic hydrogen evolution

The catalytic activity of Pt nanoparticles (PtNPs) with different sizes and shapes was investigated in a photocatalytic hydrogen-evolution system composed of the 9-mesityl-10-methylacridinium ion (Acr(+)-Mes: photocatalyst) and dihydronicotinamide adenine dinucleotide (NADH: electron donor), based on rates of hydrogen evolution and electron transfer from one-electron-reduced species of Acr(+)-Mes (Acr·-Mes) to PtNPs. Cubic PtNPs with a diameter of (6.3±0.6) nm exhibited the maximum catalytic activity. The observed hydrogen-evolution rate was virtually the same as the rate of electron transfer from Acr·-Mes to PtNPs. The rate constant of electron transfer (k(et)) increased linearly with increasing proton concentration. When H(+) was replaced by D(+), the inverse kinetic isotope effect was observed for the electron-transfer rate constant (k(et)(H)/k(et)(D)=0.47). The linear dependence of k(et) on proton concentration together with the observed inverse kinetic isotope effect suggests that proton-coupled electron transfer from Acr·-Mes to PtNPs to form the Pt-H bond is the rate-determining step for catalytic hydrogen evolution. When FeNPs were used instead of PtNPs, hydrogen evolution was also observed, although the hydrogen-evolution efficiency was significantly lower than that of PtNPs because of the much slower electron transfer from Acr·-Mes to FeNPs.