Perspective pubs.acs.org/JPCL
Probing, Sensing, and Fluorescence Enhancement with Single Gold Nanorods Saumyakanti Khatua and Michel Orrit* Huygens-Kamerlingh Onnes Laboratory, Leiden University, P. O. 9504, Niels Bohrweg 2, 2300 RA Leiden, Netherlands ABSTRACT: Gold nanorods with dimensions around 10−100 nm present original optical properties. Their main advantages are the tunability from 600 to 1000 nm of their main absorption band, and its high intensity, stemming from the good conducting properties of gold in this spectral range. Gold nanorods have been applied to tracking, probing, sensing, and manipulation experiments. Here, we discuss experiments done with single gold nanorods with emphasis on recent results from our group.
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the particle’s shape, size, and surroundings, one thus can tune the plasmon resonance. Monitoring the plasmon resonance enables sensing, that is, detection of local changes of polarizability in the close neighborhood of the metal nanoparticle. In organic dyes and in semiconductor nanocrystals, only a handful of electrons are responsible for the optical properties, which are deeply influenced by changes in electronic states. Because metal nanoparticles involve a large number of electrons, the plasmon oscillation is largely insensitive to discrete few-electron events such as electron ejection, surface reactions, and so forth. Therefore, contrary to dyes and semiconductor nanoparticles, the plasmon resonance of metal nanoparticles is largely immune to blinking and bleaching. Strong optical interactions facilitate optical detection but also make manipulation by optical beams possible. Metal nanoparticles, in particular gold nanorods, can thus be used as probes, handles, and antennas for light or other electromagnetic waves.
etal nanoparticles present original physical properties compared to the bulk metal. For example, the chemical reactivity and catalytic activity1 or the magnetic properties2 of metal nanorods may differ profoundly from those of the bulk metals. In this short Perspective, we shall focus on the optical properties of individual gold nanorods. These comparatively simple systems can give rise to a wealth of interesting phenomena and applications. The specific and tunable optical response of metal nanoparticles is chiefly due to collective oscillation modes of their conduction electrons. All the conduction electrons of the particle (about 30 000 of them for a 10 nm gold nanosphere) collectively respond to an applied optical wave, much in the way water sloshes back and forth in a bucket. The collective optical response of a metal particle is much stronger than that of an insulator particle of the same size. These collective electronic modes of metal nanoparticles are called surface plasmon resonances. They are standing waves of surface plasmon polaritons on the metal particle’s surfaces. Oscillation of the Fermi electron gas for fixed ion background creates a restoring Coulomb field as soon as local charge neutrality is violated. For simple charge oscillation modes, this field acts as a simple restoring spring. Together with the inertia of the mobile electrons, described by an effective mass in the metal, the Coulomb field defines an oscillation frequency, the surface plasmon resonance (SPR) frequency. For noble metals such as gold, this mass-spring system resonates in the visible spectral range. The surface plasmon of gold spheres less than 50 nm in diameter lies around 520 nm, almost independently of particle size between 10 and 50 nm. In contrast to molecules whose resonances are governed by localized electronic states called molecular orbitals, the electronic states of metal nanoparticles addressed by light are collective and delocalized over the whole conducting volume of the particle. Therefore, the shape, size of particle, and its close environment do change the oscillator’s spring constant, whereas the mass, being that of electrons, does not change. By changing © 2014 American Chemical Society
Monitoring the plasmon resonance enables sensing, that is, detection of local changes of polarizability in the close neighborhood of the metal nanoparticle. The previous discussion applies in principle to any good metal, that is, to metals with relatively low losses in the optical domain. For applications in nanoscience, however, other properties are important too. Gold is our preferred metal in this review for the following reasons. First, thanks to pioneering Received: June 19, 2014 Accepted: August 14, 2014 Published: August 14, 2014 3000
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work by Murphy’s and El-Sayed’s groups,3,4 the synthesis of gold nanoparticles is very well developed. Gold is chemically inert under normal conditions but can bind covalently to sulfur compounds. Gold nanoparticles can, thus, be conjugated to biomolecules5 and resist oxidative conditions without releasing poisonous ions. The plasmon resonance of gold covers the spectral domain extending from green to infrared,6 which is very convenient for many laser systems. Finally, gold is an excellent conductor in the red to infrared spectral range, particularly at wavelengths longer than 700 nm. We now consider more specifically the surface plasmon resonance for the geometry of an elongated gold nanoparticle, or gold nanorod. To a good approximation, a nanorod’s shape is a spherically capped cylinder (Figure 1a). Among the many
transient charges. In an electrostatic approximation and for frequencies well below the plasmon resonance, the elongated shape of the rod produces a lightning rod effect, with concentration of the field close to the rod’s tips. However, when in addition the oscillation frequency hits the surface plasmon resonance (SPR), the field close to the tips is further enhanced and reaches a value up to 40 times that of the incoming field, as is apparent in the field map (Figure 1d). This nanorod geometry has many advantages. The synthesis conditions may be adapted easily to obtain different sizes and aspect ratios, by tuning micelle surfactants, catalysts, concentrations, and growth conditions.4,6 Various aspect ratios correspond to different SPR positions. The spread of SPR wavelengths makes it relatively quick and easy to distinguish isolated single nanorods from clusters of two or more nanorods: a simple scattering spectrum of a single rod shows a single narrow Lorentzian SPR, whereas clusters of nanorods present broadened or split SPR’s.8,9 Because of the elongated geometry of a nanorod, its near field is significantly increased compared to that of a nanosphere. Finally, the anisotropy of nanorod enables measurements of rotational diffusion and the application of a mechanical torque by a polarized light beam.10,11 Although other shapes of nanoparticles can present these advantages separately, rods are the only ones having them all. This explains the spectacular flourishing of experiments with (single) gold nanorods in the past 10 years. In this short Perspective, we cover a small number of experiments done on single gold nanorods in our group over the last five years. We first consider sensing applications, in which a nanorod can detect binding of polarizable molecules at its surface,12 or changes in oscillation frequencies due to metal deposition.13 The field enhancement close to the rod tips gives rise to antenna effects and to better coupling of incoming and emitted light waves with nanoemitters (molecules, quantum dots, etc.) in the rod’s near field. This effect can enhance the fluorescence of weak emitters up to 1000 times.14 Fluorescence enhancement hoists the signal of single molecules well above the background of unenhanced, weakly emitting molecules. This method generalizes single-molecule spectroscopy to a broader class of weak emitters and, by the reduced near-field volume, generalizes fluorescence correlation spectroscopy to much higher concentrations.15 Finally the photoluminescence of gold nanorods can be excited with one or two photons. This signal, which can be characterized by its quantum yield,16 is intense enough for tracking single nanorods. As optical labels, nanorods are about as bright as single dye molecules, but they are very photostable (no blinking, no bleaching). Moreover, their anisotropy enables monitoring of their rotational diffusion by the polarization of photoluminescence.17 Sensing with Gold Nanorods. The change of any property of a nanoparticle upon interaction with the analytes to be detected can be used for sensing. The plasmon resonance is particularly convenient for sensing, and film plasmons are already in use in commercial devices to measure protein binding at surfaces. The plasmon resonance of metal nanoparticles is also sensitive to a change of local refractive index because of the change of Coulomb field caused by analyte’s polarization charges. The ensuing change in spring constant shifts the SPR. Single gold nanorods offer the potential of single-molecule sensing through their narrow and intense plasmon resonance. The local polarization change is of course most effective where the plasmon field is strongest, that is, in the vicinity of the tips of the rod for the longitudinal plasmon mode. Hereafter, we discuss a recent
Figure 1. (a) Electron micrograph of gold nanorods synthesized by seeded growth; (b) scheme of transient charges developed in the surface plasmon oscillation for longitudinal and transverse modes. The longitudinal resonance is the narrowest and most intense one; (c) calculated scattering spectra of gold nanorods for a few aspect ratios. The spectra are normalized to their maximum value; (d) calculated intensity map (squared electric field) around a gold nanorod for light frequency at the longitudinal surface plasmon resonance. Note the large enhancement close to the tips. The scale bar is 10 nm long.
possible excitation modes of the electron gas, two will retain our attention. In the longitudinal mode, electrons are shifted along the rod axis and accumulate at the surface of the rod, mostly at one end. A concomitant depletion of electrons appears on the surface at the other end (Figure 1b). The oscillation frequency of this mode depends critically on the aspect ratio of the rod (Figure 1c). Whereas Ohmic dissipation in gold is still significant for a gold nanosphere’s plasmon at 520 nm, at wavelengths longer than 600 nm, it becomes very small. An enormous advantage of the rod shape is, therefore, that it shifts the plasmon resonance to a spectral region of weak dissipation (Figure 1c),7 where plasmon oscillations can reach large amplitudes. In addition to the longitudinal mode, we find two degenerate transverse modes in which the oscillations are perpendicular to the rod axis, at about 520 nm and nearly independent of the rod’s aspect ratio (see Figure 1c). As Ohmic losses are still strong for those transverse modes, they are rather weak and broad and often can be neglected against the longitudinal mode. Let us discuss the field created by the rod’s 3001
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mode, where the length of the rod oscillates, and the breathing mode, where the diameter of the rod oscillates. The frequencies of these modes are directly related to the dimensions of the rod (length, width) and to the metal’s mechanical properties, mass density, and elastic moduli. The extensional mode mostly gives access to the Young modulus (corresponding to the stretching of a wire) and the breathing mode mostly provides a coefficient close to the compression modulus. The sensitivity of the frequency of these modes to mass changes could possibly be used for mass sensing. However, the quality factor of acoustic vibrations of gold nanoparticles is limited by several damping effects.20 As long as the mechanical quality factors are not significantly larger than the plasmonic ones, it is much easier to detect molecular adsorption by the shift of plasmon frequency than by a shift of mechanical vibration frequency. The higher absolute frequency and the higher signal-to-noise ratio yield much higher measurement bandwidths for the optical plasmon measurement than for the mechanical one. Nonetheless, we studied the change of acoustic mode frequencies of gold nanorods covered with increasing silver deposits13 monitoring the plasmon resonance. The sensitivity of both measurements reached a single monolayer of silver atoms deposited uniformly on the gold nanorod. Field Enhancement by a Gold Nanorod. The concentration of the electric field of an incoming wave by the charges of a gold nanorod can be exploited to enhance the interactions of light with all kinds of nano-objects. A well-known example is the surface-enhanced Raman scattering (SERS) effect, which can reach single-molecule sensitivity,21,22 corresponding to enhancement of intensities by up to 12 orders of magnitude. Such huge factors are obtained in very few particular “hot spots”, which are believed to correspond to small gaps in extended conducting metal structures or aggregates. Isolated gold nanorods cannot offer such giant enhancement factors, but they have the immense advantage that their moderate enhancement is well understood and well controlled by the reproducible shape of the rods. In the following, we discuss the enhancement of the fluorescence of organic dyes by gold nanorods.
experiment where binding events of single protein molecules were detected with a single gold nanorod.12 Binding was detected by the slight shift of the plasmon resonance due to the adsorption of single molecules at the tip of the nanorod. For this, the optical absorption of the rod was measured with high sensitivity by photothermal contrast.18 To achieve maximum sensitivity, the heating wavelength in the photothermal measurement was chosen at half-maximum of the rod’s absorption band, on the red wing of this band. Any plasmon shift gave rise to a concomitant change of absorption, which was detected as a signal change. The photothermal probing beam was chosen well outside of the absorption range of the rod, so that high probing intensities and low shot noise were achieved. Figure 2 shows examples of time traces of a
Figure 2. Steps due to single proteins adsorbing or desorbing from a single gold nanorod. The plasmon shifts are detected through changes of the photothermal signal (reproduced with permission from ref 12).
single gold nanorod preferentially tip-functionalized with biotin residues. The rod was placed in a flow cell, where a solution of the complementary binding protein streptavidin could be flushed. The traces show binding and unbinding events of single streptavidin molecules, and the number of events scales with the concentration (the streptavidin molecules were bound to a heavier protein, 300 kDa in mass, to magnify the steps and make them more obvious). The amplitude of the steps is consistent with plasmon shifts calculated from a single protein’s volume. However, although unbinding events between biotin and streptavidin would be very unlikely in solution, we do observe them in Figure 2. This is because we had to use short linkers to maximize the overlap of the protein molecule with the near-field of the rod. The biotin ligand, thus, was not optimally enclosed in streptavidin’s binding pocket. Therefore, the binding constant of the couple was strongly reduced in those experiments. The remarkable advantage of this method is that the binding molecule does not need to interact directly with the heating laser. As the protein is detected through its refractive index only, no absorbing or fluorescent label is needed in this experiment. Of course, the recognition of the binding molecule has to be very efficient and specific, to exclude any nonspecific binding event. This was easy in the experiment of Figure 2, where only a protein solution in buffer was introduced in the flow cell but would be much more difficult with real biological solutions such as blood plasma. Gold nanorods may also be used for sensing through properties other than optical absorption, but that are accessible optically. An example of such a property is the acoustic vibration frequency, which can be measured easily in optical pump−probe experiments.19 The main vibration modes detected in response to a short pump pulse are the extension
Intensity fluctuations due to enhanced fluorescence bursts can be observed at dye concentrations up to 105 times higher than those used in standard fluorescence correlation spectroscopy. In fluorescence enhancement, a metal nanostructure acts as an antenna by better matching molecular currents in the emitter to the light wavelength. This process is at work for emission of light as well as for absorption of light, just as an antenna improves both emission and reception of radio waves by small electronic devices.23 Fluorescence enhancement, thus, essentially results from these two factors. Compared to molecules far away, the molecule in the vicinity of the nanostructure feels a higher local optical field and is excited at a higher rate. In addition to this excitation enhancement, spontaneous emission by the molecule can also be helped, or enhanced, by currents in the metal nanostructure. Fluorescence enhancement is the ratio of the fluorescence rate measured for an enhanced molecule to that of an unenhanced one. In the most favorable conditions, fluorescence enhancement is the 3002
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Figure 3. (a) Enhanced fluorescence spots in a solution of crystal violet (CV) around isolated gold nanorods immobilized on a glass surface; (b) fluorescence time trace of one of the spots of (a) showing the very short bursts of fluorescence; (c) resonance of the enhancement factor when the SPR of the rod is matched to the fluorescence spectrum of the dye (here, CV, fluorescence maximum at 640 nm).
product of excitation and emission enhancements. As the field enhancement close to the tip of a nanorod can easily reach a factor of 40 at resonance24 (by combining the lightning rod effect with resonant enhancement), fluorescence enhancement could in principle exceed 106 for properly matched excitation and emission wavelengths. In practice, however, fluorescence enhancement is limited by molecular properties25 such as the mismatch of the broad fluorescence spectrum with the surface plasmon spectrum. The largest enhancement factor by gold nanorods measured so far was only about 1000.14 It was measured on a weakly emitting dye, crystal violet, with a quantum yield of about 2%. Such high enhancements make it possible to detect the presence of a single weakly emitting dye molecule in the near field of the nanorod. Fluorescence enhancement, therefore, enables generalizing single-molecule microscopy to dyes with low quantum yield. It is interesting to note that even dyes with very low fluorescence yields can be made visible by fluorescence enhancement if sufficiently high excitation intensities are used. Indeed, a lower yield not only reduces the background of unenhanced molecules. The reduction of fluorescence by internal conversion is partially offset by a larger enhancement factor. Indeed, a dark molecule with fast internal conversion is able to come closer to the metal surface before being totally quenched by the metal, thereby benefiting from a larger field enhancement. Ultimately, only the background from gold photoluminescence limits the visibility of single-molecule bursts. Another important application of fluorescence enhancement stems from the very small volume where this process is active. The near-field area around the tip of a nanorod is typically a few nanometers in dimension, or about 1 zeptoliter (1 zL =1000 nm3). This volume is about 105 times smaller than the focus area of a confocal microscope, 0.1 femtoliter (300 nm in diameter, 1000 nm in length). Therefore, intensity fluctuations due to enhanced fluorescence bursts can be observed at dye concentrations up to 105 times higher than those used in standard fluorescence correlation spectroscopy (FCS). If the rods are freely diffusing in the solution, the bursts duration is related only to the diffusion time of the molecules through the near-field region. Molecules sticking to the surface
of the gold nanorods will be quenched by efficient energy transfer to the metal and will not contribute to the signal. On the other hand, if the rod is immobilized on a solid surface (Figure 3), sticking of the molecules to the surface within the near-field area of the rod followed by bleaching in the high enhanced intensity will determine the duration of the bursts.26 Fluorescence enhancement by gold nanorods may be applied to emitters with very low quantum yield or to emitters with long lifetimes. Fluorescence correlation spectroscopy at high concentrations is very attractive in cells where the concentration of the biomolecules cannot be adjusted and usually lies in the micromolar rather than picomolar range. Photoluminescence of Gold Nanorods. The photoluminescence of gold nanoparticles can be excited with one or two photons.27,28 We mostly discuss one-photon excited photoluminescence in what follows. In semiconductors, electrons and holes created by photoexcitation relax, respectively, to the bottom of the conduction band and the top of the valence band and usually remain there for nanoseconds, long enough for radiative recombination to take place. In metals, no such gap stops the fast relaxation of electron and holes toward the Fermi level. Gold, therefore, emits luminescence with a very weak quantum yield, of the order of 10−10.29 Nevertheless, there is a nonvanishing probability for electron and hole to recombine radiatively before they reach the Fermi level. In gold nanorods, this weak emission is further enhanced by plasmonic antenna effects, and the yield can reach 10−7 for nanospheres30 and 10−6 for nanrods.16 Despite its rather low yield, because the absorption cross section of gold nanoparticles is so large, their photoluminescence is intense enough to be easily detected. A single
A single nanorod’s photoluminescence is comparable to a single molecule’s fluorescence in intensity, but the rod’s signal does not bleach or blink. 3003
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Figure 4. Rotational diffusion of single gold nanorods in glycerol at low temperatures, (a) Time traces of the two polarized components of a single nanorod’s photoluminescence, showing the fluctuations due to tumbling of the rod in glycerol; (b) linear dichroism trace deduced from (a); (c) autocorrelation function of the linear dichroism trace, showing the characteristic diffusion time (about 30 s); (d) histograms of diffusion times for a small ensemble of nanorods at different temperatures. Notice the broadening of the histograms at temperatures lower than 230 K (reproduced from ref 17).
strong interaction with light. In the electrostatic approximation, the polarizability of a nanosphere scales as the ratio
nanorod’s photoluminescence is comparable to a single molecule’s fluorescence in intensity, but the rod’s signal does not bleach or blink. The only precaution to take with gold nanorods is that they should not be overheated more than some tens of Kelvin, to avoid reshaping. This surface diffusion process leads to more rounded shapes of gold nanorods when they are heated around the boiling point of water or above10,31 and irreversibly shifts the SPR to the blue. Photoluminescence can be used as an optical signal to gain information about the position and orientation of nanorods in any situation where scattering is difficult to measure, that is, in scattering media, in environments with difficult optical access, or when high numerical apertures are difficult to achieve. As an example, we briefly describe local viscosity measurements in glycerol, monitored through the rotational diffusion of individual nanorods.17 Figure 4 presents rotational diffusion measurements on single gold nanorods dispersed in glycerol between 220 and 240 K. The tumbling times of nanorods in this temperature range are on the order of some seconds. Because different rods have different hydrodynamic volumes, their tumbling times should be normalized to their volumes. This is done here by normalizing the times to the time at the highest temperature measured (238 K), where glycerol is assumed to be homogeneous. The data of Figure 4d show that different nanorods experience different local viscosities when the temperature is reduced below 230 K. This surprising finding indicates the presence of dynamical heterogeneity on a large length scale (the size of the nanoparticles, a few tens of nanometers) and on a large time scale (their rotational diffusion time, seconds to minutes). Optical Trapping and Manipulation of Single Gold Nanorods. The metallic character of gold nanorods is responsible for their
εp − εm εp + 2εm
where εp and εm are the (complex) dielectric permittivities of the particle and of the surrounding medium, respectively. Because the permittivity of gold around 600 nm is about −10, the above ratio is about 2 in water. The corresponding ratio for a dielectric particle such as a silica bead (permittivity 2.5) is only about 0.14, 14 times weaker. For a given laser intensity, an optical trap will thus hold a metal nanosphere in water with a force 14 times larger than a silica sphere with the same volume. Optical tweezers can trap gold nanospheres as small as 10 nm in diameter32 and gold nanorods as small as 8 × 40 nm.33 The disadvantage of metal particles, however, is the imaginary part of their dielectric permittivity. Strong optical forces are thus always associated with optical heating, which can degrade the particle’s surroundings, or even the nanoparticles themselves in the case of reshaping gold nanorods. One of the uses of optical tweezers is to hold nanoparticles suspended in a liquid, avoiding any contact with a solid surface. The damping of acoustic oscillations of gold nanospheres and nanorods has been studied in this way.20 Thanks to the anisotropic plasmonic polarizability of gold nanorods, optical torques can be applied in a polarized light beam also to comparatively small rods.10 These torques are comparable to those applied with much larger anisotropic crystalline dielectric particles. Outlook and Conclusion. Gold nanorods are fascinating systems that present a wealth of original physical, in particular 3004
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thereby extend the dwell time of the dyes in the near field without occupying these sensitive spots for ever. Assembling two or more nanorods in a controlled way, much as was done by Acuna et al. with DNA origami and gold nanospheres,5 would give access to dipolar antennas and therefore to enhancements many orders of magnitude larger than the ones available from isolated rods. Eventually, all the experiments listed above should be transported from the in vitro conditions of current demonstrations to complex environments such as those found in living cells, at membranes, in the cytosol, or even inside the nucleus. To reach this ambitious goal, we need to make full use of the small size and strong optical interactions of gold nanorods.
optical, properties. Gold particles being inert are much better suited to experiments at ambient conditions than, for example, silver nanoparticles. Nonetheless, gold forms covalent bonds with thiolated molecules, which enables conjugation with nearly any biomolecule. For biological applications, gold is much less poisonous to cells than silver. Among the large variety of gold nanoparticles (spheres, cubes, cages, shells, plates, triangles, ...) nanorods present several specific advantages. First, they are comparatively easy to synthesize in various lengths and widths, corresponding to tunable optical properties. Second, their longitudinal plasmon resonance is shifted to a spectral region where gold behaves as an excellent metal. The longitudinal surface plasmon resonance of gold nanorods is therefore narrow and intense and also enables single-molecule sensing experiments. The anisotropy of the particle is reflected in its plasmonic properties, and this opens applications such as orientational tracking and manipulation by optical torques. Compared to other structures such as dipolar antennas or bowtie antennas,34 the plasmonic enhancements and near fields of gold nanorods are rather modest. However, this disadvantage is more than compensated by their high reliability from particle to particle and the simple structure of their optical near field. A full understanding of near-field structure is cruelly lacking in the case of the hot spots enhancing Raman scattering, for example. Moreover, the open structure of nanorods keeps the metal surface free to interact with many molecules, including large proteins that would not fit in the tight hot spots of dipole antennas. For all their attractive features, however, nanorods do absorb light through their Ohmic dissipation processes, as any other gold nanostructure. Small nanorods are particularly prone to reshaping already at moderate temperatures, typically in less than an hour at the boiling temperature of water. This may be a problem in applications where intense laser beams are used to generate optical signals during long integration times or for the study of slow processes. For many applications of nanorods, improved synthesis or purification methods are needed, giving rise to smaller dispersion of parameters. Although the selection of single nanorods eventually removes all heterogeneity, the procedure of searching the right size or aspect ratio among all particles optically accessible is sometimes lengthy. Of course, distinguishing isolated nanorods from small clusters on the basis of their spectra would then become accordingly more difficult. We believe there is a big potential for optical trapping and manipulation with gold nanorods, despite the problem of unavoidable heating. This might be mitigated by isolating the particle in a layer of silica or by tethering the nanorod to a long polymer molecule to remove the hot area from the process under study. Fluorescence enhancement would benefit from a still better control of spectral overlaps between the exciting laser, the surface plasmon resonance, and the fluorescence spectrum of the species to be detected. The reduced volume of the near field opens fluorescence correlation spectroscopy to high concentrations,35 which are the natural working conditions of many biological processes. Also, much progress can be booked in the detection of single weakly fluorescent emitters, those with weak to very weak quantum yields as well as those with long fluorescence lifetimes. A fascinating prospect is the assembly of superemitters by attaching convenient dyes at the optimal distance from the gold surface, far enough to avoid quenching and close enough to exploit the largest possible near-field enhancement. To solve the problem of bleaching of the dyes, convenient binding sites for dye-labeled molecules could be attached to the tips of gold nanorods and
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies Dr. Saumyakanti Khatua is a postdoctoral researcher with Michel Orrit in Institute of physics at Leiden University. He received his Ph.D. from Rice University (2011) with Stephan Link. His current research interest is plasmon-enhanced fluorescence of weak emitters. Prof. Michel Orrit is at Leiden University since 2001. Before that, he was a CNRS researcher at Bordeaux University. The main interest of his group is the optical study of various individual nano-objects (http://www.single-molecule.nl/).
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ACKNOWLEDGMENTS Much of the work summarized here was done in collaboration with Dr. P. Zijlstra during his postdoctoral stay in our group. S.K. and M.O. acknowledge financial support from the ERC Advanced Grant SiMoSoMa, and from The Netherlands Organization for Scientific Research (ECHO fellowship).
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REFERENCES
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