Hybrid nanostructures for enhanced light-harvesting: plasmon induced increase in fluorescence from individual photosynthetic pigment-protein complexes

We studied the optical response from more than 13,?000 individual photosynthetic pigment-protein complexes interacting with spherical gold nanoparticles. The nanodots were arranged in a quasi-hexagonal array by diblock copolymer micellar nanolithography. Exciting the proteins within the spectral range of the nanoparticles' plasmon resonance yields a clear enhancement of the protein fluorescence intensity, whereas excitation far out of the plasmon resonance features no effect. This result indicates a strategy for the construction of efficient hybrid light-harvesting devices.     

Plasmonic enhancement of fluorescence on silver nanoparticle films

The localized plasmon controlled fluorescence has been discussed by comparing the fluorescence enhancement of dyes on different shaped silver nanoparticle self-assembled films. A trilayer structure, composed of a silver nanoparticle monolayer, a proper thickness polyelectrolyte spacing layer and a dye-adsorbed layer, was constructed to study the plasmon enhanced fluorescence properties. The effective coupling of the plasmon band with the excitation or emission of dye resulted in different enhancement factors. Moreover, the plasmon enhanced fluorescence resonance energy transfer (FRET) of two dyes was observed. The FRET efficiency of the spherical silver nanoparticle self-assembled film had a 2.8-fold increase. The improvement of FRET efficiency via localized surface plasmons would increase the sensitivity of FRET-based bioassays.     

Spatially correlated fluorescence/AFM of individual nanosized particles and biomolecules

Individual fluorescent polystyrene nanospheres (<10-100-nm diameter) and individual fluorescently labeled DNA molecules were dispersed on mica and analyzed using time-resolved fluorescence spectroscopy and atomic force microscopy (AFM). Spatial correlation of the fluorescence and AFM measurements was accomplished by (1) positioning a single fluorescent particle into the near diffraction-limited confocal excitation region of the optical microscope, (2) recording the time-resolved fluorescence emission, and (3) measuring the intensity of the excitation laser light scattered from the apex of an AFM probe tip and the AFM topography as a function of the lateral position of the tip relative to the sample substrate. The latter measurements resulted in concurrent high-resolution (approximately 10-20 nm laterally) images of the laser excitation profile of the confocal microscope and the topography of the sample. Superposition of these optical and topographical images enabled unambiguous identification of the sample topography residing within the excitation region of the optical microscope, facilitating the identification and structural characterization of the nanoparticle(s) or biomolecule(s) responsible for the fluorescence signal observed in step 2. These measurements also provided the lateral position of the particles relative to the laser excitation profile and the surrounding topography with nanometer-scale precision and the relationship between the spectroscopic and structural properties of the particles. Extension of these methods to the study of other types of nanostructured materials is discussed.     

Plasmon Enhanced Fluorescence Based on Porphyrin–Peptoid Hybridized Gold Nanoparticle Platform

A porphyrin–peptoid‐hybridized silica‐coated gold nanoparticle is developed, which is inspired by the protein–chlorophyll ensemble found in photosynthetic antenna. In the natural antenna, chlorophylls are integrated into dense assemblies that are supported by frameworks of proteins, which ensure optimal pigment arrangement for effective light harvesting. In the subject platform, porphyrins are conjugated to the peptoid helix scaffold in a structurally well‐defined alignments and subsequently immobilized on the surface of nanoparticles. This prevents intermolecular aggregation among porphyrins and allows high resolution analysis of the effect of porphyrin configuration on the optical properties of the system. Interestingly, under the influence of plasmon from the gold nanoparticle core, the fluorescence of porphyrin is enhanced up to 24‐fold at the wavelength where the plasmon resonance matches the porphyrin excitation wavelength. In addition, differences in porphyrin configuration result in spectral modification of their fluorescence emissions. Particularly, the peptoid bearing two porphyrins at a distance of 6 Å shows the most significant alteration in fluorescence. The platform can facilitate extensive studies on the relationship between porphyrin arrangement design and their photophysical interaction in antenna complexes.     

Large enhancement of fluorescence efficiency from CdSe/ZnS quantum dots induced by resonant coupling to spatially controlled surface plasmons

Nanoengineered fluorescent response is reported from semiconductor core-shell (CdSe/ZnS) quantum dots in proximity to the surface plasmon polariton field of periodic Ag nanoparticle arrays. Tuning the surface plasmon polariton resonance to the quantum dot exciton emission band results in an enhancement of up to approximately 50-fold in the overall fluorescence efficiency, in a design where each Ag nanoparticle is interconnected by a continuous Ag thin film. Propagating modes of surface plasmon resonances have a direct impact on the fluorescence enhancement.     

Probing dynamically tunable localized surface plasmon resonances of film-coupled nanoparticles by evanescent wave excitation

The localized surface plasmon resonance (LSPR) spectrum associated with a gold nanoparticle (NP) coupled to a gold film exhibits extreme sensitivity to the nanogap region where the fields are tightly localized. The LSPR of an ensemble of film-coupled NPs can be observed using an illumination scheme similar to that used to excite the surface plasmon resonance (SPR) of a thin metallic film; however, in the present system, the light is used to probe the highly sensitive distance-dependent LSPR of the gaps between NPs and film rather than the delocalized SPR of the film. We show that the SPR and LSPR spectral contributions can be readily distinguished, and we compare the sensitivities of both modes to displacements in the average gap between a collection of NPs and the gold film. The distance by which the NPs are suspended in solution above the gold film is fixed via a thin molecular spacer layer and can be further modulated by subjecting the NPs to a quasistatic electric field. The observed LSPR spectral shifts triggered by the applied voltage can be correlated with angstrom scale displacements of the NPs, suggesting the potential for chip-scale or flow-cell plasmonic nanoruler devices with extreme sensitivity.     

Photothermal properties of gold nanoparticles under exposure to high optical energies

Photothermal (PT) efficacy and damage thresholds of gold nanoparticles (NP)-spheres, rods and silica-gold shells-were experimentally studied during their excitation with nanosecond laser pulses at the fluence levels at and above the NP damage threshold. The maxima of PT efficacy of gold NPs with near-infrared (NIR) plasmon resonances (gold rods and shells) and the minima of their damage thresholds were found to be shifted from their plasmon resonance NIR wavelengths into non-resonant visible wavelengths. This suppression of PT efficacy of NIR plasmon resonances (bleaching) was found to be up to 18 times for the rods and up to 22 times for the shells. During laser-induced deterioration the NPs maintained their PT properties at least within 40-150?ns after exposure to laser pulses. PT properties of the gold NPs can be enhanced with the pulse train mode within the above time. The PT bubbles generated around superheated NPs were used as their optical markers and allowed us to quantify PT efficacy of plasmon resonance through the bubble parameters under the conditions when other methods of NP detection are not applicable.     

Conjugation of membrane-destabilizing peptide onto gelatin–siloxane nanoparticles for efficient gene expression

One of the crucial steps in gene delivery with non-viral vectors is the escape of DNA complexes from the endosome. In order to improve gene transfection efficiency, we designed a novel gene delivery vector gelatin–siloxane nanoparticles (GS NPs) conjugated with two different membrane-destabilizing peptides, octaarginine (R8) and a subunit of influenza virus haemagglutinin HA2. Both R8-GS NPs and HA2-GS NPs had high positive surface charges. They could condense and protect DNA against serum/DNase degradation. Results from flow cytometry and confocal laser scanning microscope respectively indicated that R8-GS NPs had higher uptake efficiency than HA2-GS NPs, whereas HA2-GS had higher endosome escaping efficiency. Furthermore, in vitro transfection displayed a higher gene expression level with HA2-modified GS NPs, which suggested that endosome escaping is the crucial step for nanoparticle mediated gene therapy.     

Plasmon-enhanced fluorescence in heterochlorophyllous peridinin-chlorophyll-protein photosynthetic complex

We study the fluorescence properties of peridinin-chlorophyll-protein light-harvesting complex reconstituted with both chlorophyll a and chlorophyll b coupled to spherical gold nanoparticles. Since there is a bidirectional energy transfer between the chlorophylls, the impact of plasmon excitations upon the energy transfer dynamics can be studied in such a hybrid nanostructure. The results show that the emission of both chlorophyll a and chlorophyll b in a reconstituted PCP is strongly enhanced when the light-harvesting complexes are separated from the metallic nanoparticles by a 12-nm-thick SiO₂ layer, the average enhancement reaches a factor of 8. The analysis of fluorescence decay curves indicates that the energy transfer between chlorophyll a and chlorophyll b is also influenced by plasmon excitations in gold nanoparticles.     

Multifunctional Au@mSiO2/Rhodamine B Isothiocyanate Nanocomposites: Cell Imaging,Photocontrolled Drug Release,and Photothermal Therapy for Cancer Cells

The synthesis of Au@mesoporous SiO₂/rhodamine B isothiocyanate (Au@mSiO₂/RBITC) composite nanoparticles (NPs) is presented and their unique biofunctional properties are studied. The structure and morphology of the NPs are characterized by X‐ray powder diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy. These NPs can not only be functionalized for fluorescence imaging, but also possess well‐defined mesopore structures for drug loading and strong infrared surface plasmon absorption for light‐controlled drug release and photothermal therapy for cancer cells. In the biological experiments, one 808 nm laser is coupled to a confocal laser scanning microscopy (CLSM) system to monitor the photothermal therapy, drug release, and cell position and viability in real time by using the multichannel function of CLSM for the first time. Such novel nanomaterials offer a new chemotherapeutic route for cancer treatment by combining cell imaging and hyperthermia in a synergistic way.     

Tuning the Intensity of Metal‐Enhanced Fluorescence by Engineering Silver Nanoparticle Arrays

It is demonstrated that silver nanoparticle (SNP) arrays fabricated by combining nanoimprint lithography and electrochemical deposition methods can be used as substrates for metal‐enhanced fluorescence, which is widely used in optics, sensitive detection, and bioimaging. The method presented here is simple and efficient at controlling the nanoparticle density and interparticle distance within one array. Furthermore, it is found that the fluorescence intensity can be tuned by engineering the feature size of the SNP arrays. This is due to the different coupling efficiency between the emission of the fluorophores and surface plasmon resonance band of the metallic nanostructures.     

Confocal total-internal-reflection fluorescence microscopy with a high-aperture parabolic mirror lens

We present a theoretical study of a new total-internal-reflection fluorescence microscope for the detection of fluorescence at a water-glass interface. The system is designed for confocal imaging and spectroscopy of nanoparticles and single molecules. Focusing and fluorescence collection through standard glass coverslips is accomplished by a parabolic mirror lens. The large aperture of the element is used to excite fluorescence within the evanescent field of a diffraction-limited focus and to collect focal emission with high efficiency. Tight focusing and supercritical excitation reduce the detection volume for fluorescent analyte molecules well below that of an attoliter (10(-18) L), which can be advantageous for monitoring surface binding of single molecules without interference from fluorescence of the unbound bulk. Calculations of the electric fields in the focus region and simulated confocal imaging demonstrate the performance of the system.     

Actively controllable EIT-like resonance between localized and propagating surface plasmons for optical switching

We theoretically investigate the plasmonic interaction between radiative localized surface plasmon resonances and subradiative propagating surface plasmon modes in a nanostructure consisting of a periodic array of gold nanobars and an optically thick gold film, separated by a silica dielectric spacer layer. A controllable transparency window within the broad dipole resonance profile is observed clearly in the reflectance spectra via tailoring the length of the bar, the periodicity of the nanoparticle array, or the incident angle of applied field, respectively, a classic analog of electromagnetically induced transparency (EIT). We believe that the last excitation configuration is particularly beneficial for the realization of active manipulation of plasmonic optical switching without using coupling/control fields required in the conventional EIT scheme.     

Plasmonic enhancement of molecular fluorescence

Metallic nanoparticles are known to dramatically modify the spontaneous emission of nearby fluorescent molecules and materials. Here we examine the role of the nanoparticle plasmon resonance energy and nanoparticle scattering cross section on the fluorescence enhancement of adjacent indocyanine green (ICG) dye molecules. We find that enhancement of the molecular fluorescence by more than a factor of 50 can be achieved for ICG next to a nanoparticle with a large scattering cross section and a plasmon resonance frequency corresponding to the emission frequency of the molecule.     

New method based on capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) to monitor interaction between nanoparticles and the amyloid-β peptide

A novel application of capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) was proposed to efficiently detect and monitor the interaction between polymeric nanoparticles and the β-Amyloid peptide (Aβ(1-42)), a biomarker for Alzheimer's Disease (AD), at concentrations close to physiological conditions. The CE-LIF method allowed the interaction between PEGylated poly(alkyl cyanoacrylate) nanoparticles (NPs) and the soluble Aβ(1-42) peptide monomers to be highlighted. These results were confirmed by surface plasmon resonance (SPR) and confocal laser scanning microscopy (CLSM). Whereas SPR showed an interaction between the NPs and the Aβ(1-42) peptide, CLSM allowed the formation of large aggregates/assemblies at high NP and peptide concentrations to be visualized. All these results suggested that these nanoparticles could bind the Aβ(1-42) peptide and influence its aggregation kinetics. Interestingly, the non-PEGylated poly(alkyl cyanoacrylate) NPs did not alter the aggregation kinetics of the Aβ(1-42) peptide, thus emphasizing the high level of discrimination of the CE-LIF method with respect to NPs.     

Surface-enhanced fluorescence from silver fractallike nanostructures decorated with silver nanoparticles

Fluorescence emission of fluorophore molecules in the close vicinity of a nanostructured metal surface can be enhanced through a local electromagnetic field with the help of surface plasmon resonance. The fluorescence enhancement effect is very sensitive to the topography and dielectric property of the metal substrate. In the current work, metal substrates with complex structures, which are made of silver fractallike structures and nanoparticles (NPs), are prepared through electrochemical reduction followed by physical deposition. The surface-enhanced fluorescence of Rhodamine 6G monolayer molecules deposited on the prepared complex substrates are investigated with the laser spectroscopic technique. The experimental results show that the fractallike structure decorated with silver NPs presents stronger fluorescence enhancement, compared with silver NPs or pure silver fractallike structures.     

In situ Raman scattering study on a controllable plasmon-driven surface catalysis reaction on Ag nanoparticle arrays

Control of the plasmon-driven chemical reaction for the transformation of 4-nitrobenzenethiol to p,p'-dimercaptoazobenzene by Ag nanoparticle arrays was studied. The Ag nanoparticle arrays were fabricated by means of nanosphere lithography. By changing the PS particle size, the localized surface plasmon resonance (LSPR) peaks of the Ag nanoparticle arrays can be tailored from 460 to 560 nm. The controlled reaction process was monitored by in situ surface-enhanced Raman scattering. The reaction can be dramatically influenced by varying the duration of laser exposure, Ag nanoparticle size, laser power and laser excitation wavelength. The maximum reaction speed was achieved when the LSPR wavelength of the Ag nanoparticle arrays matched the laser excitation wavelength. The experimental results reveal that the strong LSPR can effectively drive the transfer of the 'hot' electrons that decay from the plasmon to the reactants. The experimental results were confirmed by theoretical calculations.     

Plasmon-enhanced resonance Raman scattering and fluorescence in Langmuir-Blodgett monolayers

Localized surface plasmon resonances in silver and gold nanostructures are engaged to enhance the inelastic Raman scattering and the fluorescence of a phopholipid containing a sulforhodamine 101 acid chloride dye known as Texas Red. The most efficient coupling and enhancement are attained when the excitation laser line is in resonance with both the chromophore and the plasmon absorption of the nanostructure, the case of surface-enhanced resonance Raman scattering, allowing single-molecule detection. The tagged phospholipid was incorporated into a single fatty acid Langmuir monolayer at varying concentrations and transferred onto an evaporated Ag nanoparticle film. Surface-enhanced fluorescence is achieved using shell-isolated silica-coated gold nanoparticles, an emission enhancement named SHINEF.     

Nanoparticle-induced enhancement and suppression of photocurrent in a silicon photodiode

Nanoparticles are capable of both enhancing and suppressing the photocurrent in a silicon diode when deposited on the active face of the device. Photocurrent imaging of the individual nanoparticles and nanoparticle aggregates responsible for this effect reveals that Au nanospheres, nanoshells, and nanoshell dimers each exhibit unique wavelength-dependent suppression-enhancement characteristics. In contrast, silica nanospheres provide a sizable and relatively uniform photocurrent enhancement across the same spectral range (532-980 nm). Unusual light-harvesting behavior observed correlates with a highly complex energy flow (optical "vortexing") for the forward scattered light of plasmon resonant nanoparticles into the device.     

Modelling photosystem I as a complex interacting network

In this paper, we model the excitation energy transfer (EET) of photosystem I (PSI) of the common pea plant Pisum sativum as a complex interacting network. The magnitude of the link energy transfer between nodes/chromophores is computed by Forster resonant energy transfer (FRET) using the pairwise physical distances between chromophores from the PDB 5L8R (Protein Data Bank). We measure the global PSI network EET efficiency adopting well-known network theory indicators: the network efficiency (Eff) and the largest connected component (LCC). We also account the number of connected nodes/chromophores to P700 (CN), a new ad hoc measure we introduce here to indicate how many nodes in the network can actually transfer energy to the P700 reaction centre. We find that when progressively removing the weak links of lower EET, the Eff decreases, while the EET paths integrity (LCC and CN) is still preserved. This finding would show that the PSI is a resilient system owning a large window of functioning feasibility and it is completely impaired only when removing most of the network links. From the study of different types of chromophore, we propose different primary functions within the PSI system: chlorophyll a (CLA) molecules are the central nodes in the EET process, while other chromophore types have different primary functions. Furthermore, we perform nodes removal simulations to understand how the nodes/chromophores malfunctioning may affect PSI functioning. We discover that the removal of the CLA triggers the fastest decrease in the Eff, confirming that CAL is the main contributors to the high EET efficiency. Our outcomes open new perspectives of research, such comparing the PSI energy transfer efficiency of different natural and agricultural plant species and investigating the light-harvesting mechanisms of artificial photosynthesis both in plant agriculture and in the field of solar energy applications.