Article

DNA Origami Assembled Nanoantennas for Manipulating Single-Molecule Spectral Emission

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Abstract

The emission spectrum of a dye is given by the energy of all of the possible radiative transitions weighted by their probability. This spectrum can be altered with optical nanoantennas that are able to manipulate the decay rate of nearby emitters by modifying the local density of photonic states. Here, we make use of DNA origami to precisely place an individual dye at different positions around a gold nanorod and show how this affects the emission spectrum of the dye. In particular, we observe a strong suppression or enhancement of the transitions to different vibrational levels of the excitonic ground state, depending on the spectral overlap with the nanorod resonance. This reshaping can be used to experimentally extract the spectral dependence of the radiative decay rate enhancement. Furthermore, for some cases, we argue that the drastic alteration of the fluorescence spectrum could arise from the violation of Kasha's rule.

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... Previous systems employed to study spectral reshaping include lithographically fabricated arrays coated with emitters [14][15][16][17], plasmonic antennas immersed in fluorophore solutions [18][19][20][21][22] as well as nanoparticles encapsulated by quantum emitters in a core-shell geometry [23][24][25][26]. More recently, spectral reshaping was studied at room temperature on single fluorophores immobilized close to the tip of gold nanorods using DNA-origami [27]. The DNAorigami technique [28] allows for bottom-up fabrication of 3D nanostructures [28][29][30] with dimensions in the tens to hundreds of nanometers, and has found several applications in nano-photonics [31] as it enables the precise positioning of both metallic nanoparticles and single photon emitters with controlled orientation and stoichiometry [32][33][34]. ...
... Some notable examples include the enhancement of single photon emission [35][36][37][38][39], surface-enhanced Raman scattering (SERS) [40][41][42], directing light [43][44][45], ultra-fast phenomena [46], strong coupling [47,48] and super-resolution microscopy [49]. In Ref. [27] pronounced reshaping of the light emission was demonstrated when the nanorod LSPR overlaps with the free-space fluorophore spectrum. While most results could be explained by the Purcell effect in the weak coupling regime through Fermi's Golden Rule [23], some discrepancies were noticed [27] and attributed to a possible violation of Kasha's rule: ...
... In Ref. [27] pronounced reshaping of the light emission was demonstrated when the nanorod LSPR overlaps with the free-space fluorophore spectrum. While most results could be explained by the Purcell effect in the weak coupling regime through Fermi's Golden Rule [23], some discrepancies were noticed [27] and attributed to a possible violation of Kasha's rule: ...
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Controlling the light emitted by individual molecules is instrumental to a number of novel nanotechnologies ranging from super-resolution bio-imaging and molecular sensing to quantum nanophotonics. Molecular emission can be tailored by modifying the local photonic environment, for example by precisely placing a single molecule inside a plasmonic nanocavity with the help of DNA origami. Here, using this scalable approach, we show that commercial fluorophores experience giant Purcell factors and Lamb shifts, reaching values on par with those recently reported in scanning tip experiments. Engineering of plasmonic modes enables cavity-mediated fluorescence far detuned from the zero-phonon-line (ZPL) - at detunings that are up to two orders of magnitude larger than the fluorescence linewidth of the bare emitter and reach into the near-infrared. Our results evidence a regime where the emission linewidth is dominated by the excited state lifetime, as required for indistinguishable photon emission, baring relevance to the development of nanoscale, ultrafast quantum light sources and to the quest toward single-molecule cavity-QED. In the future, this approach may also allow to design efficient quantum emitters at infrared wavelengths, where standard organic sources have a reduced performance.
... The first two, spheres and rods, are nowadays relatively straightforward to fabricate through bottom-up techniques, such as colloidal chemical synthesis, with precise control over their size [127], [131], [132], [134]. Moreover, advances in self-assembly techniques allow for the arrangement of several QE units, and introduce paths towards the placement of QEs at controlled distances from the nanoparticles [43], [162]- [168]. The other two selected nanostructures, disks and crescents, can be fabricated with precise control over their shape and size through top-down methods like lithography and evaporation techniques. ...
... We consider three different arrangements of these four canonical nanostructures: single nanoantennas (with the QE in proximity to one isolated nanostructure) [13], [42], [51], [80], [84], [85], [88], [168], [171], gap nanoantennas (or dimers, where two similar nanostructures are separated by a gap with the QE in between) [43], [53], [172], [173], and nanoparticle-on-mirror constructs (where the QE is positioned in the gap between a metallic nanostructure and a metallic substrate that acts as a mirror [22], [40], [113], [133], [174], [175]). All of these configurations have been shown to Figure 2: Nanoantennas considered in this work to enhance the photoluminescence of a nearby QE, represented by an electric dipole (red arrow). ...
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In the realm of nanotechnology, the integration of quantum emitters with plasmonic nanostructures has emerged as an innovative pathway for applications in quantum technologies, sensing, and imaging. This research paper provides a comprehensive exploration of the photoluminescence enhancement induced by the interaction between quantum emitters and tailored nanostructure configurations. Four canonical nanoantennas (spheres, rods, disks, and crescents) are systematically investigated theoretically in three distinct configurations (single, gap, and nanoparticle-on-mirror nanoantennas), as a representative selection of the most fundamental and commonly studied structures and arrangements. A detailed analysis reveals that the rod gap nanoantenna configuration achieves the largest photoluminescence enhancement factor, of up to three orders of magnitude. The study presented here provides insights for the strategic design of plasmonic nanoantennas in the visible and near-IR spectral range, offering a roadmap for these structures to meet specific requirements in plasmon-enhanced fluorescence. Key properties such as the excitation rate, the quantum yield, the enhanced emitted power, or the directionality of the emission are thoroughly reviewed. The results of this overview contribute not only to the fundamental understanding of plasmon-enhanced emission of quantum emitters but also set the basis for the development of advanced nanophotonic devices with enhanced functionalities.
... [7][8][9][10][11] Especially, modular DNA origami objects 12,13 have been increasingly utilized as templates in developing plasmonic nanoparticle assemblies, optically active substrates, single-molecule tracking and sensing devices, nanoantennas, metasurfaces, and photonic crystals. [14][15][16][17][18][19][20][21][22][23] Versatile DNA nanostructures have also been used in lithographic settings as masks in pattern transfer 24 and in fabricating accurate inorganic [25][26][27][28][29][30][31] and metallic nanoshapes 27,32,33 with intriguing optical properties. [33][34][35] In this article we use a DNA-assisted lithography (DALI)based strategy 33,36 to create optically active substrates with aligned bowtie-shaped silver particle-aperture pairs in a layered configuration of materials (Fig. 1a). ...
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Varying the spacing between layers of aligned metal nanoparticles and apertures enables control over the coupling strength of their plasmon resonances and therefore the overall field enhancement.
... These results are in agreement with the earlier reports on plasmon-emitter interactions which resulted in the modulation of the uorescence spectral prole of the organic emitters. 16,22,78 Here, we demonstrate strategies to modify the PL landscape of QDs through selective plasmonic resonance coupling. Specically, the plasmonic eld enhances the band edge emission of QDs through on-resonance coupling more effectively than the trap-state emission which is in off-resonance. ...
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The photoluminescence properties of quantum dots (QDs) are often enhanced by eliminating surface trap states through chemical methods. Alternatively, a physical approach is presented here for improving photoluminescence purity in QDs by employing frequency-specific plasmon resonance coupling. Emitter-bound plasmonic hybrids are designed by electrostatically binding negatively charged QDs in water to positively charged gold nanoparticles having a thin polymer coating. Herein, two types of QDs are used: (i) bare CdSe, which exhibits both band edge and trap state emission, and (ii) CdSe overcoated with a ZnS shell (CdSe/ZnS) devoid of trap state emission. Tuning the extinction spectrum of the plasmonic system to match the band edge emission of CdSe enables the selective enhancement of band edge emission over trap state emission. Excellent match in the extinction spectrum of the gold nanoparticle with both, experimentally calculated photoluminescence enhancement factor and theoretically calculated radiative rate enhancement signifies the role of frequency-specific plasmon resonance coupling. Plasmon-coupled photoluminescence of CdSe/ZnS is further investigated by varying the number density of emitter on the surface of plasmonic nanoparticle. An enhancement in the photoluminescence is observed at a lower emitter density of CdSe/ZnS and the photoluminescence enhancement factor closely follows the plasmon resonance. However, photoluminescence quenching occurs with an increase in CdSe/ZnS due to plasmon-assisted nonradiative energy transfer between nearby QDs, as indicated by a red shift in the PL maximum. These studies establish that resonance plasmonic coupling is a convenient physical strategy for tuning the intrinsic photoluminescence properties of QDs for various optoelectronic applications.
... The advent of the DNA origami technique 11 enabled the bottom-up self-assembly of colloidal metallic 12 and recently dielectric 13,14 NPs together with organic dyes and quantum dots with nanometric positional precision and stoichiometric control [15][16][17] . In this way, the DNA origami technique was exploited to fabricate diverse OAs with single emitters placed at specific locations to enhance the fluorescence intensity 18,19 and photostability 20 , and to direct or tune the emission [21][22][23][24] . These DNA origami based OAs can very precisely manipulate the photophysical properties of single photon emitters located at the hotspot as demonstrated for example by maximum values of fluorescence enhancement (FE) reaching three orders of magnitude 25 and forward to backward directivities over 10 dB 22 . ...
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Optical antennas have been extensively employed to manipulate the photophysical properties of single photon emitters. Coupling between an emitter and a given resonant mode of an optical antenna depends mainly on three parameters: spectral overlap, relative distance, and relative orientation between the emitter's transition dipole moment and the antenna. While the first two have been already extensively demonstrated, achieving full coupling control remains unexplored due to the challenges in manipulating at the same time both the position and orientation of single molecules. Here, we use the DNA origami technique to assemble a dimer optical antenna and position a single fluorescent molecule at the antenna gap with controlled orientation, predominately parallel or perpendicular to the antenna's main axis. We study the coupling for both conditions through fluorescence measurements correlated with scanning electron microscopy images, revealing a 5-fold higher average fluorescence intensity when the emitter is aligned with the antenna's main axis and a maximum fluorescence enhancement of ~ 1400-fold. A comparison to realistic numerical simulations suggests that the observed distribution of fluorescence enhancement arises from small variations in emitter orientation and gap size. This work establishes DNA origami as a versatile platform to fully control the coupling between emitters and optical antennas, trailblazing the way for self-assembled nanophotonic devices with optimized and more homogenous performance.
... (7)(8)(9)(10)(11) Especially, modular DNA origami objects (12,13) have been increasingly utilized as templates in developing plasmonic nanoparticle assemblies, optically active substrates, single-molecule tracking and sensing devices, nanoantennas, metasurfaces, and photonic crystals. (14)(15)(16)(17)(18)(19)(20)(21)(22)(23) Versatile DNA nanostructures have also been used in lithographic settings as masks in pattern transfer (24) and in fabricating accurate inorganic (25)(26)(27)(28)(29)(30)(31) and metallic nanoshapes (27,32,33) with intriguing optical properties. (33)(34)(35) In this article we use a DNA-assisted lithography (DALI) -based strategy (33,36) to create optically active substrates with aligned bowtie-shaped silver particle-aperture pairs in a layered configuration of materials (Fig. 1a). ...
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Here we show how surface-enhanced Raman spectroscopy (SERS) features can be fine-tuned in optically active substrates made of layered materials. To demonstrate this, we used DNA-assisted lithography (DALI) to create substrates with silver bowtie nanoparticle-aperture pairs and then coated the samples with rhodamine 6G (R6G) molecules. By varying the spacing between the aperture and particle layer, we were able to control the strength of the interlayer coupling between the plasmon resonances of the apertures and those of the underlying bowtie particles. The changes in the resulting field enhancements were confirmed by recording the Raman spectra of R6G from the substrates, and the experimental findings were supported with finite difference time domain (FDTD) simulations including reflection/extinction and near-field profiles.
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An ideal nanofabrication method should allow the organization of nanoparticles and molecules with nanometric positional precision, stoichiometric control, and well-defined orientation. The DNA origami technique has evolved into a highly versatile bottom-up nanofabrication methodology that fulfils almost all of these features. It enables the nanometric positioning of molecules and nanoparticles with stoichiometric control, and even the orientation of asymmetrical nanoparticles along predefined directions. However, orienting individual molecules has been a standing challenge. Here, we show how single molecules, namely, Cy5 and Cy3 fluorophores, can be incorporated in a DNA origami with controlled orientation by doubly linking them to oligonucleotide strands that are hybridized while leaving unpaired bases in the scaffold. Increasing the number of bases unpaired induces a stretching of the fluorophore linkers, reducing its mobility freedom, and leaves more space for the fluorophore to accommodate and find different sites for interaction with the DNA. Particularly, we explore the effects of leaving 0, 2, 4, 6, and 8 bases unpaired and find extreme orientations for 0 and 8 unpaired bases, corresponding to the molecules being perpendicular and parallel to the DNA double-helix, respectively. We foresee that these results will expand the application field of DNA origami toward the fabrication of nanodevices involving a wide range of orientation-dependent molecular interactions, such as energy transfer, intermolecular electron transport, catalysis, exciton delocalization, or the electromagnetic coupling of a molecule to specific resonant nanoantenna modes.
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Optical antennas are nanostructures designed to manipulate light-matter interactions by interfacing propagating light with localized optical fields. In recent years, numerous devices have been realized to efficiently tailor the absorption and/or emission rates of fluorophores. By contrast, modifying the spatial characteristics of their radiation fields remains challenging. Successful phased array nanoantenna designs have required the organization of several elements over a footprint comparable to the operating wavelength. Here, we report unidirectional emission of a single fluorophore using an ultracompact optical antenna. The design consists of two side-by-side gold nanorods self-assembled via DNA origami, which also controls the positioning of the single-fluorophore. Our results show that when a single fluorescent molecule is positioned at the tip of one nanorod and emits at a frequency capable of driving the antenna in the antiphase mode, unidirectional emission with a forward to backward ratio of up to 9.9 dB can be achieved.
Article
Plasmonic nanostructures dramatically alter the radiative and non-radiative properties of single molecules in their vicinity. This coupling induced change in decay channels selectively enhances specific vibronic transitions, which can enable plasmonic control of molecular reactivity. Here, we report coupling dependent spectral emission shaping of single Rhodamine 800 molecules in the vicinity of plasmonic gold nanorods. We show that the relative vibronic transition rates of the first two vibronic transitions of the spontaneous emission spectrum can be tuned in the weak coupling regime, by approximately 25-fold, on the single molecule level.
Article
Most measurements of fluorescence lifetimes on the single-molecule level are carried out using avalanche photon diodes (APDs). These single-photon counters are inherently slow and their response shows a strong dependence on photon energy, which can make deconvolution of the instrument response function (IRF) challenging. An ultrafast time resolution in single-molecule fluorescence is crucial, e.g., in determining donor lifetimes in donor-acceptor couples which undergo energy transfer, or in plasmonic antenna structures, where the radiative rate is enhanced. We introduce a femtosecond double-excitation (FeDEx) photon correlation technique, which measures the degree of photon antibunching as a function of time delay between two excitation pulses. In this boxcar integration, the time resolution of fluorescence transients is limited solely by the laser pulse length and is independent of the detector IRF. The versatility of the technique is demonstrated with a custom-made donor acceptor complex with one donor and two acceptors; and with single dye molecules positioned accurately between two gold nanoparticles using DNA origami. The latter structures show ~75-fold radiative-rate enhancements and fluorescence lifetimes of down to 17 ps, which is measured without the need of any deconvolution. With the potential of measuring sub-picosecond fluorescence lifetimes plasmonic antenna structures can now be optimized further.
Article
Fluorescent molecules are highly susceptible to their local environment. Thus, a fluorescent molecule near a plasmonic nanoparticle can experience changes in local electric field and local density of states that reshape its intrinsic emission spectrum. By avoiding ensemble averaging while simultaneously measuring the super-resolved position of the fluorophore and its emission spectrum, single-molecule hyperspectral imaging is uniquely suited to differentiate changes in spectrum from heterogeneous ensemble effects. Thus, we uncover for the first time single-molecule fluorescence emission spectrum reshaping upon near-field coupling to individual gold nanoparticles using hyperspectral super-resolution fluorescence imaging, and we resolve this spectral reshaping as a function of the nanoparticle/dye spectral overlap and separation distance. We find dyes bluer than the plasmon resonance maximum are red-shifted and redder dyes are blue-shifted. The primary vibronic peak transition probabilities shift to favor secondary vibronic peaks, leading to effective emission maxima shifts in excess of 50 nm, and we understand these light-matter interactions by combining super-resolution hyperspectral imaging and full-field electromagnetic simulations.
Article
We demonstrate the capability of DNA self-assembled optical antennas to direct the emission of an individual fluorophore, which is free to rotate. DNA origami is used to fabricate optical antennas composed of two colloidal gold nanoparticles separated by a predefined gap and to place a single Cy5 fluorophore near the gap center. Although the fluorophore is able to rotate, its excitation and far-field emission is mediated by the antenna, with the emission directionality following a dipolar pattern according to the antenna main resonant mode. This work is intended to set out the basis for manipulating the emission pattern of single molecules with self-assembled optical antennas based on colloidal nanoparticles.
Article
One of the major difficulties hindering the widespread application of colloidal anisotropic plasmonic nanoparticles is the limited robustness and reproducibility of multistep synthetic methods. We demonstrate herein that the reproducibility and reliability of colloidal gold nanorod (AuNR) synthesis can be greatly improved by disconnecting the symmetry breaking event from the seeded growth process. We have used a modified silver-assisted seeded growth method in the presence of the surfactant hexadecyltrimethylammonium bromide and n-decanol as a co-surfactant, to prepare small AuNRs in high yield, which were then used as seeds for the growth of high quality AuNR colloids. Whereas the use of n-decanol provides a more rigid micellar system, the growth on anisotropic seeds avoids sources of irreproducibility during the symmetry breaking step, yielding uniform AuNR colloids with narrow plasmon bands, ranging from 600 to 1250 nm, and allowing fine tuning of the final dimensions. This method provides a robust route for the preparation of highly-quality AuNR colloids with tunable morphology, size and optical response, in a reproducible and scalable manner.
Article
We demonstrate, experimentally and theoretically, that the photon emission from gold nanorods can be viewed as a Purcell effect enhanced radiative recombination of hot carriers. By correlating the single-particle photoluminescence spectra and quantum yields of gold nanorods measured for five different excitation wavelengths and varied excitation powers, we illustrate the effects of hot carrier distributions evolving through interband and intraband transitions and the photonic density of states on the nanorod photoluminescence. Our model, using only one fixed input parameter, describes quantitatively both emission from interband recombination and the main photoluminescence peak coinciding with the longitudinal surface plasmon resonance.
Article
DNA nanotechnology has developed into a state where the design and assembly of complex nanoscale structures is fast, reliable, cost effective, and accessible to nonexperts. Nanometer-precise positioning of organic (e.g., dyes and biomolecules) and inorganic (e.g., metal nanoparticles and colloidal quantum dots) components on DNA nanostructures is straightforward and modular. In this article, we identify the opportunities and challenges that DNA-assembled devices and materials face for optical antennas, metamaterials, and sensing applications. With the ability to arrange hybrid components in defined geometries, plasmonic effects will, for example, amplify molecular recognition transduction such that single-molecule events will be measureable with simple devices. On a larger scale, DNA nanotechnology has the potential to break the symmetry of common self-assembled functional materials, creating predefined optical properties such as refractive-index tuning and topological insulation.
Article
Single-photon nanoantennas are broadband strongly scattering nanostructures placed in the near field of a single quantum emitter, with the goal to enhance the coupling between the emitter and far-field radiation channels. Recently, great strides have been made in the use of nanoantennas to realize fluorescence brightness enhancements, and Purcell enhancements, of several orders of magnitude. This perspective reviews the key figures of merit by which single photon nanoantenna performance is quantified, and discusses the recent advances in measuring these metrics unambiguously. Next, this perspective discusses what the state of the art is in terms of fluoresent brightness enhancements, Purcell factors, and directivity control on the level of single photons. Finally, I discuss future challenges for single-photon nanoantennas.
Article
Optical nanoantennas are known to focus freely propagating light and reversely to mediate the emission of a light source located at the nanoantenna hotspot. These effects were previously exploited for fluorescence enhancement and single-molecule detection at elevated concentrations. We present a new generation of self-assembled DNA origami based optical nanoantennas with improved robustness, reduced interparticle distance, and optimized quantum-yield improvement to achieve more than 5000fold fluorescence enhancement and single-molecule detection at 25 µM background fluorophore concentration. Besides outperforming lithographic optical antennas, DNA origami nanoantennas are additionally capable of incorporating single emitters or biomolecular assays at the antenna hotspot.
Article
A colloidal quantum dot light-emitting diode (QLED) is reported with substantially enhanced electroluminescence by embedding a thin layer of Ag nanoislands into hole transport layer. The maximum external quantum efficiency (EQE) of 7.1% achieved in the present work is the highest efficiency value reported for green-emitting QLEDs with a similar structure, which corresponds to 46% enhancement compared with the reference device. The relevant mechanisms enabling the EQE enhancement are associated with the near-field enhancement via an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Ag nano­islands, which are found to lead to good agreement between the simulation results and the experimental data, providing us with a useful insight important for plasmonic QLEDs.
Article
The interaction of dyes and metallic nanostructures strongly affects the fluorescence and can lead to significant fluorescence enhancement at plasmonic hot spots, but also to quenching. Here we present a method to distinguish the individual contributions to the changes of the excitation, radiative and non-radiative rate and use this information to determine the quantum yields for single molecules. The method is validated by precisely placing single fluorescent dyes with respect to gold nanoparticles as well as with respect to the excitation polarization using DNA origami nanostructures. Following validation, measurements in zeromode waveguides reveal that suppression of the radiative rate and enhancement of the non-radiative rate lead to a reduced quantum yield. Because the method exploits the intrinsic blinking of dyes, it can generally be applied to fluorescence measurements in arbitrary nanophotonic environments.
Article
To move nanophotonic devices such as lasers and single-photon sources into the practical realm, a challenging list of requirements must be met, including directional emission(1-5), room-temperature and broadband operation(6-9), high radiative quantum efficiency(1,4) and a large spontaneous emission rate. To achieve these features simultaneously, a platform is needed for which the various decay channels of embedded emitters can be fully understood and controlled. Here, we show that all these device requirements can be satisfied by a film-coupled metal nanocube system with emitters embedded in the dielectric gap region. Fluorescence lifetime measurements on ensembles of emitters reveal spontaneous emission rate enhancements exceeding 1,000 while maintaining high quantum efficiency (>0.5) and directional emission (84% collection efficiency). Using angle-resolved fluorescence measurements, we independently determine the orientations of emission dipoles in the nanoscale gap. Incorporating this information with the three-dimensional spatial distribution of dipoles into full-wave simulations predicts time-resolved emission in excellent agreement with experiments.
Article
The amount of information obtainable from a fluorescence-based measurement is limited by photobleaching: Irreversible photochemical reactions either render the molecules non-fluorescent or shift their absorption and/or emission spectra outside the working range. Photobleaching is evidenced as a decrease of fluorescence intensity with time, or in the case of single molecule measurements, as an abrupt, single-step interruption of the fluorescence emission which determines the end of the experiment. Reducing photobleaching is central for improving fluorescence (functional) imaging, single molecule tracking and fluorescence based biosensors and assays. In this single molecule study, we use DNA self-assembly to produce hybrid nanostructures containing individual fluorophores and gold nanoparticles at a controlled separation distance of 8.5 nm. By changing the nanoparticles size we are able to systematically increase the mean number of photons emitted by the fluorophores before photobleaching.
Article
The localized plasmons of metal nanocrystals have been widely utilized to control a variety of optical signals, such as Raman, fluorescence, and circular dichroism, from proximal dye molecules. We show, on the single-particle level, that the Förster resonance energy transfer between two different fluorophores can be modulated by adjacent plasmonic nanocrystals. The donor and acceptor fluorophore molecules are embedded in a mesostructured silica shell that is uniformly coated on Au–Ag core–shell nanocrystals. The longitudinal plasmon wavelengths of the core–shell metal nanocrystals are synthetically tailored by varying the aspect ratio. Comparison of the scattering and fluorescence spectra taken from the different hybrid nanostructures indicates that the energy transfer efficiency can be controlled by the plasmon wavelength. When the plasmon peak overlaps with the emission peak of the donor, the energy transfer channel is turned off. When the plasmon peak is red-shifted to be in between the emission peak of the donor and the absorption peak of the acceptor or right at the intrinsic emission peak of the acceptor, the energy transfer channel is turned on.
Article
The enhancement of electromagnetic field on the rough metallic nanostructure has been extensively applied to obtain chemical or biological information about molecules with high sensitivity, and has received much attention due to its potential applications in new types of devices based on nanoelectronics and nanophotonics. The typical size of the field enhancement area, the so-called hotspot, is approximately one order of magnitude smaller than the optical diffraction limit. In the present study, an optical super-resolution microscopic and spectroscopic approach is introduced to explore single-molecule fluorescence within an engineered hotspot where nonhomogeneous spectral modulation is resolved beyond the optical diffraction limit for the first time. Distinct Stokes shifts from individual dyes were directly observed within single hotspots, which were found to be independent of the local electromagnetic field strength. The method reported here provides a robust tool to probe the optical properties of nanoresonantors with high temporal and spatial resolution.
Article
We study the distance-dependent quenching of fluorescence due to a metallic nanoparticle in proximity of a fluorophore. In our single-molecule measurements, we achieve excellent control over structure and stoichiometry by using self-assembled DNA structures (DNA origami) as a breadboard where both the fluorophore and the 10 nm metallic nanoparticle are positioned with nanometer precision. The single-molecule spectroscopy method employed here reports on the co-localization of particle and dye, while fluorescence lifetime imaging is used to directly obtain the correlation of intensity and fluorescence lifetime for varying particle to dye distances. Our data can be well explained by exact calculations that include dipole-dipole orientation and distances. Fitting with a more practical model for nanosurface energy transfer yields 10.4 nm as the characteristic distance of 50% energy transfer. The use of DNA nanotechnology together with minimal sample usage by attaching the particles to the DNA origami directly on the microscope coverslip paves the way for more complex experiments exploiting dye-nanoparticle interactions.
Article
We show that plasmonic nanoresonators composed of two gold nanoparticles change not only the intensity but also the spectral shape of the emission of fluorescent molecules. The plasmonic resonance frequency can be tuned by varying the distance between the nanoparticles, which allows us to selectively favor transitions of a fluorescent molecule to a specific vibrational ground state. Experimental data from correlated scattering and fluorescence microscopy agree well with calculations in the framework of generalized Mie theory. Our results show that the widely used description of a dye molecule near a metal surface as a mere two-level system is inadequate.
Article
We report on the strong polarization dependence of the plasmon-enhanced fluorescence on single gold nanorods. The fluorescence from the organic fluorophores that are embedded in a mesostructured silica shell around individual gold nanorods is enhanced by the longitudinal plasmon resonance of the nanorods. Our electrostatic calculations show that under an off-resonance excitation, the electric field intensity contour around a nanorod rotates away from the length axis as the excitation polarization is varied. The polarization dependence of the plasmon-enhanced fluorescence is ascribed to the dependence of the averaged electric field intensity enhancement within the silica shell on the excitation polarization. The measured fluorescence enhancement factor is in very good agreement with that obtained from the electrostatic calculations. The fluorescence enhancement factor increases as the longitudinal plasmon wavelength is synthetically tuned close to the excitation wavelength. In addition, the polarization dependence is used to determine the orientation angle of the gold nanorods. The results are in excellent agreement with the actual measurements. Furthermore, the emission spectrum of the fluorophore is modified by the longitudinal plasmon resonance of the gold nanorods. A linear correlation between the emission peak wavelength and the longitudinal plasmon wavelength is obtained.
Article
We investigate the fluorescence from dyes coupled to individual DNA-functionalized metal nanoparticles. We use single-particle darkfield scattering and fluorescence microscopy to correlate the fluorescence intensity of the dyes with the localized surface plasmon resonance (LSPR) spectra of the individual metal nanoparticles to which they are attached. For each of three different dyes, we observe a strong correlation between the fluorescence intensity of the dye and the degree of spectral overlap with the plasmon resonance of the nanoparticle. On average, we observe the brightest fluorescence from dyes attached to metal nanoparticles that have a LSPR scattering peak approximately 40-120 meV higher in energy than the emission peak of the fluorophore. These results should prove useful for understanding and optimizing metal-enhanced fluorescence.
Article
Silver nanobars with rectangular side facets and an average aspect ratio of 2.7 have been synthesized by modifying the concentration of bromide added to a polyol synthesis. Subsequent rounding of nanobars transformed them into nanorice. Due to their anisotropy, nanobars and nanorice exhibit two plasmon resonance peaks, scattering light both in the visible and in the near-infrared regions. With a combination of discrete-dipole approximation calculations and single-nanoparticle spectroscopy, we explored the effect of nanostructure aspect ratio and corner sharpness on the frequency of plasmon resonance. Near-field calculations and surface-enhanced Raman scattering measurements on single particles were performed to show how local field enhancement changes with both the wavelength and polarization of incident light.