Plasmonics in Analytical Spectroscopy - ACS Symposium Series (ACS

Chapter 14

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Plasmonics in Analytical Spectroscopy Pedro H. B. Aoki,1,2 Carlos J. L. Constantino,1 Osvaldo N. Oliveira Jr,*,2 and Ricardo F. Aroca1,2 1Faculdade

de Ciências e Tecnologia, UNESP Univ Estadual Paulista, Presidente Prudente, 19060-080, SP, Brazil 2São Carlos Institute of Physics, University of São Paulo, CP 369, 13560-970 São Carlos, SP, Brazil *E-mail: [email protected].

Surface plasmon resonances (SPR) can be excited in thin metal films and in metal nanoparticles as localized surface plasmon resonances (LSPR). The surface plasmon is extremely sensitive to the refractive index of the environment surrounding the metal film or metal nanoparticle. This is why refractive index sensing has been the source for the development of an array of techniques harnessing the power of both SPR and LSPR. In addition, LSPR is at the center of plasmon enhanced spectroscopy with a myriad of analytical applications. Here we examine the basic physical model of plasmon enhancement, with the intention of facilitating the design of plasmonic nanostructures and experiments, taking advantage of these emerging techniques. In particular, we discuss the plasmon enhanced work based on shell-isolated nanoparticles (SHINs) in Raman scattering (SHINERS) and in fluorescence (SHINEF). Typical examples have been selected to illustrate the physical interpretation of observations.

Introduction The central element in plasmonic sensing is the surface-plasmon polariton at a single interface, which is today part of the rapidly expanding field of plasmonics (1, 2). As pointed out in Maier’s book (3), the surface plasmon resonances can be conveniently classified into two groups - surface plasmon polaritons (SPP) and the localized surface plasmon resonances: “However, history has shown that © 2015 American Chemical Society In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


despite the fact that the two main ingredients of plasmonics - surface plasmon polaritons and localized surface plasmons - have been clearly described as early as 1900, it is often far from trivial to appreciate the interlinked nature of many of the phenomena and applications of this field. This is compounded by the fact that throughout the 20th century, surface plasmon polaritons have been rediscovered in a variety of different contexts” (3). The fundamental difference is that SPP are nonradiative surface plasmon excitations, while LSPR are radiative, and both will give rise to analytical spectroscopic techniques (4). Undoubtedly, the present interest in surface plasmons has come from recent advances in the investigation of the electromagnetic properties of nanostructured materials. Surface plasmons are usually generated by the electric field component of the electromagnetic radiation and the optical response of the materials is the main source of interest for applications. However, it is also possible to excite a surface plasmon with high-energy electrons. In particular, unique optical properties of metal nanoparticles arise from the large density and susceptibility of their free electrons, and the particle plasmon mode strongly interacts with optical waves. Analytical spectroscopy harnesses the properties of surface plasmon resonances (collective charge density fluctuations) that can be excited optically for a broad range of applications. The framework for the discussion in this Chapter is provided by the classical electromagnetic (EM) theory (5). The central material property for SPP and LSPR is the dielectric function , which determines the relationship between the and the electric field (5, 6): electric displacement

The property of a material is a second rank tensor, although for an isotropic medium it reduces to a scalar (7). The frequency dependence or optical dispersion of .

makes the refractive index

also frequency dependent:

When the medium is represented by a negative refractive index,

it is called metamaterial (8). The bulk value of can be used not only to study the properties of surface plasmon resonances, but also to describe, with some corrections, the properties of LSPR in metallic nanoparticles (3, 9). Surface plasmons can be excited in metals throughout the visible region of the electromagnetic spectrum, being highly sensitive to changes in the refractive index at the interface. The latter feature makes them amenable to a wide scope of applications, especially in bio-science (10), and promotes development of new instrumentation (11). Similarly, LSPR can be excited in noble metal nanoparticles in the visible, and give rise to techniques based on refractive index sensing (4, 12). In addition, the coupling of LSPR with molecular spectroscopy has spurred development of linear and non-linear plasmon enhanced analytical techniques. The most prominent is the surface enhanced Raman scattering (SERS) (13, 14), a powerful vibrational identification technique that can achieve the limit of single molecule detection (SMD) (15). 270 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Surface enhanced fluorescence (SEF) (16, 17) is a successful analytical technique, also referred to as metal enhanced fluorescence (MEF) (18). The literature on surface plasmon resonances is overwhelming and a search in the Web of Science® leads to more than 30 thousand hits for SPR, as seen in Figure 1, while the most dynamic analytical techniques are SERS and SEF or MEF. Figure 1 is given to illustrate activity in the SPR field and the main analytical techniques. There are many excellent review articles, monographs, and the original research papers on the surface plasmon field, and the choice of contributions for this present chapter is obviously subjective and incomplete. We do apologize to colleagues whose work is not cited here; further information can be found in review articles and references cited therein. For example, Pitarke, Silkin, Chulkov, Echenique (19) and Wang, Plummer, Kempa (1) discussed the physics of ongoing research, Kawata (20) reviewed spectroscopic and imaging applications, while a discussion of LSPR sensing is offered by Anker, Hall, Lyandres (4), and Mayer, Hafner (12).

Figure 1. Retrieved results from the Web of Science for the following topics: Surface plasmon resonance (SPR); Surface plasmon polaritons (SPP); Localized surface plasmon resonance (LSPR); Surface plasmon resonance sensing (SPR sensing); Localized surface plasmon resonance sensing (LSPR sensing); Surface enhanced Raman scattering (SERS); Surface enhanced fluorescence (SEF); Metal enhanced fluorescence (MEF); Plasmonics. 271 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Surface Plasmon Resonances The Bulk Plasmon


In metals, the electron gas can be treated as a continuous fluid with electron in Amperes/m2. density , velocity and electrical current density The dielectric function represents the linear response of the electron gas, and the electric field is the driving force, which can be assumed to obey

where the term introduces friction or damping. Equation [2] can be rewritten in terms of the current:

The plasma frequency for the electron gas is defined as Assuming an harmonic time dependence current, the solution for the equation of motion is

.

for E and for the induced

with the current density being

Using Ampere’s law, i.e. , neglecting polarization and taking the time derivative and eliminating H, the wave equation is:

272 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Using a plane wave solution

,

conducting medium is

. Now we write the expression for

for the free electrons in a metal using

the dielectric function the plasma frequency of the metal: Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on December 10, 2015 | http://pubs.acs.org Publication Date (Web): December 8, 2015 | doi: 10.1021/bk-2015-1215.ch014

the dispersion for the

The dielectric function without damping is simply the Drude formula:

, and the common limiting forms are:

In addition, since the propagation equation for the transverse mode is , then , or , which is the dispersion relation for the transverse bulk plasmon shown in Figure 2. The following asymptotic forms are derived from the propagation equation:

. Most importantly, the surface mode can be found in the frequency region . The surface plasmon resonance is localized at a planar interface between two media with frequency dependent dielectric functions . Surface plasmons from planar surfaces as well as bulk plasmons and plasmon modes of metal nanoparticles and metal films can be probed using photons or electrons. Electron energy loss spectroscopy (EELS) is used to experimentally distinguish between bulk volume plasmons, surface plasmons on flat metal films, and localized surface plasmon modes in metal nanoparticles (21). Taking advantage of the atomic-resolution imaging through scanning transmission electron microscopy (STEM) coupled with high resolution EELS, one may now obtain simultaneous morphological and spectral analysis of individual metal nanoparticles (22, 23). Probing with electrons surpasses the commonly used 273 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


method of scanning near-field optical microscope (SNOM or NSOM) to achieve spatially resolved surface plasmon detection with detailed mapping of the near field properties in individual or array of nanostructures (24).

Figure 2. Diagram for the dispersion of the transverse bulk plasmon. The straight line is the photon line in vacuum.

The complete derivation of surface plasmons (25) leads to the following important points: -

for the surface plasmon to exist, signs:

and

must have opposite

.

In particular, the frequency of the nonradiative surface plasmon at the metalvacuum interface of smooth metal surfaces is related to the wave vector by (26):

Since the right hand side of (8) must be positive, then:


The relations (8) and (9) determine the interval in which the surface plasmon resonance occurs. For large wave vectors, the surface plasmon approaches asymptotically a constant frequency , which for simple metal-vacuum interface . This relationship has been confirmed experimentally by measuring is the characteristic electron energy loss spectrum of aluminium (27). The peak corresponding to the excitation of the bulk plasmon was found at the energy Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on December 10, 2015 | http://pubs.acs.org Publication Date (Web): December 8, 2015 | doi: 10.1021/bk-2015-1215.ch014

. The lower peak, corresponding to the excited surface plasmon, . However, the optical excitation and surface was detected at plasmon resonance have a different wave vector k, and in order to transform the metal from a reflector into a reservoir of photons, an external structure is needed to provide the wave vector matching. In other words, SPR cannot be excited directly by light if the surface is perfectly smooth. “However, any surface roughness permits the surface to impart some additional momentum to the SPR so that it can couple to the radiating electromagnetic field”, as was demonstrated by Teng and Stern in their report on plasma radiation from metal grating surfaces (28). In addition, prism coupling can be used to match the momenta, as demonstrated by Otto (29) and by Kretchmann and Raether (30). Since then, this so-called attenuated total reflection (ATR) method is the basis for the traditional SPR spectroscopy, an analytical technique used in biosensing, which measures the absorption of light at resonance via total internal reflection (TIR) excitation of SPR. Typically, it uses a monochromatic laser source for illumination of a metallic gold film on a glass substrate at a specific angle of incidence, where the momentum of incident light matches that of the SPR. A broadband/white light source can also be used (11). SPR spectroscopy has a broad range of applications to monitor biological interactions (2), in sensing heavy metal ions (31), in SPR imaging (32) and detecting virus (33). Although SPR cannot be excited by direct illumination on optically flat metal surfaces, a minor enhancement could be observed for light scattering of molecules located on it, and a clear physical model for this phenomenon can be found in the work of Efrima and Metiu (34). Moreover, without surface roughness there is no SERS enhancement (14, 35). Notwithstanding the fact that metal-molecule interactions may lead to substantial spectral changes in the vibrational spectrum. However, there are genuine attempts to use SPR, excited with the Kretschmann configuration, to measure plasmon enhanced spectra. A brief review of these efforts can be found in the work of Meyer, Le Ru, and Etchegoin (36).

Properties of Localized Surface Plasmon Resonances Plasmons in Nanoparticles A clear picture of the properties of LSPR can provide a guide for the synthesis of specific nanostructures (37–39), or development of nanostructured substrates for analytical applications (40–42). It is now possible to carry detailed 275 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


studies of isolated single nanoparticles (43), though most commonly ensemble measurements are obtained with averaging over a distribution of sizes and shapes. The optical response of the nanoparticle (NP) is a function of its size, shape (geometry) and the environment (9). The same geometrical parameters govern the optical response of NP aggregates and of NP ensemble: size, shape, environment, and, at this time, the gap between NP becomes extremely important. The spectral response of LSPR can be characterized in the near-field with properties, such as the intensity and spatial distribution of the electromagnetic field enhancement, closely related to the sensing capabilities of a nanostructure, and with far-field quantities such as absorption, scattering, and extinction (44). The light interaction with arrays of nanoparticles has also been investigated (45), as new coupling resonances might make it possible to design plasmonic NPs with unexpected optical properties (46). In addition, the excitation wavelength and associated electric field polarization also play a determining role. At the origin of LSPR is the problem of a metal sphere interacting with an oscillating electromagnetic field. Gustav Mie in 1908 reported the optical by solving the Maxwell response of a sphere with dielectric function equations, whose complete development is found in textbooks (47). A power series expansion of the absorption and scattering coefficients is obtained, and their efficiencies are characterized by cross-sections, commonly given in cm2. , is the sum of the absorption and scattering The extinction cross section, cross sections: . In the limit of small particles compared to the , simple expressions are obtained wavelength, i.e., when the sphere radius (5, 47). for

where is the dielectric function of the material and is the dielectric constant of the medium. The quasi-static results (9) and (10) are valid for sub-wavelength spheres, and in the power series expansion (Mie theory) of the absorption and scattering coefficients they are set by considering only the first term. The quasi-static relationships have far reaching implications for analytical spectroscopic applications: The scattering scales with , and grows very fast with increasing particle size. The absorption cross-section varies as , and consequently absorption is more important than scattering for very small particles. The latter is illustrated in Figure 3 with Mie computations of absorption and scattering cross sections for Ag and Au spheres of . Absorption dominates for nanoparticles below 100 nm in size, and scattering overcomes absorption for larger NPs (43). 276 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 3. Absorption and scattering cross sections for Ag and Au spheres. Mie computations carry out with programs developed in reference (48).

Absorption-based detection methods are very sensitive, and large absorption cross sections permit development of new techniques, such as photothermal microscopy (49) for the detection of single absorbing nano-objects. Large absorption cross sections may be used directly to create local nanometric heating for biomedical applications (33). In the quasi-static limit, the theory of scattering and absorption of radiation by a small sphere predicts a resonant field enhancement due to a resonance in the g

factor (14, 40);

, if the Frölich condition

is satisfied, with the caveat that is a complex number, and the imaginary part plays an important role. Under these circumstances, the nanoparticle acts as an electric dipole, resonantly absorbing and scattering electromagnetic fields, placing it at the center of plasmon enhanced optical signals. Therefore, the optical , determines the region of the electromagnetic property of the material, spectrum where the effect would be observed, with distinctions induced by the environment, , the size and the shape of the nanoparticle (9). The final objective is a fundamental understanding that allows the optimization of these radiative (scattering) and nonradiative (absorption) properties for applications. Since the scattering scales with and grows very fast with increasing particle size, one should be expected to use larger nanoparticles for enhanced spectroscopy, as “the most intense SERS is really frequency-shifted elastic scattering by the metal. Under appropriate circumstances the field enhancement will scale as E4, where E is the local optical field”(Moskovits, see (51)). 277 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Field enhancement is an extraordinary phenomenon associated with metal nanoparticles excited by photons or electrons, at optical wavelengths, where local optical fields in metal nanostructures can achieve strengths that are orders of magnitude greater than that of the incident field (52). The spectral response of localized surface plasmon resonances both in the near-field and far-zone regimes is well understood in terms of classical electrodynamics (53, 54). Commonly, far-field quantities such as absorption, scattering, and extinction are measured to qualify the plasmonic response of a metallic nanoparticle; however, they do not provide information about the strength of the electromagnetic fields at the surface or at a distance from the particle surface. Therefore, near-field properties, such as the intensity and spatial distribution of the electromagnetic field enhancements, are also being measured and calculated (55). Although the exact relationship between the energy-loss map (ELM) for a given plasmon resonance and the optical near-field map is still an object of discussion, we have chosen to illustrate the near field with an energy-loss map of a plasmon resonance. Theory and experiments seem to indicate that ELM represents the electric field strength (56). The intensity of a plasmon, excited by electrons, has been mapped with high spatial resolution using EELS. This is illustrated for two gold spheres (35 nm and 25 nm) in Figure 4, adapted from reference (22). The EELS results are in full agreement with theory and optical measurements. The dependence of both near- and far-field measures on particle size has been comprehensively studied for spherical gold and silver nanoparticles, beyond the quasi-static approximation (44). It has been shown that for the peak wavelength of the primary resonance, the surface average of electric field intensity and scattering increase monotonically with particle size, while absorption does not. This size dependence of the primary resonance translates into a wavelength-dependent local field enhancement factor (EF) defined as a ratio of the normal component of the electric field on the surface of the Au nanosphere to the electric field of the incident electromagnetic wave at a point of observation. The profiles of the magnitude of the field EF as a function of incident wavelength for Au , have been reported by and Ag nanospheres, with Geshev, Klein, Witting, Dickmann and Hietschold (57), according to which even for spherical nanoparticles the field EF varies substantially with the excitation wavelength. For practical definitions of EF see also reference (58). Notably, the dipolar mode is the dominant excitation for sizes below 120 nm (59), while for larger diameters the multipolar excitations become important (47). Therefore, the most commonly used NPs for SERS have been of diameter below 100 nm, probably because for small sizes of nanospheres (20–50 nm) the electromagnetic enhancement effect can be successfully explained in terms of a simple electrostatic model. However, larger gold NPs (~100 nm) have been synthesized by various groups and their SERS activity thoroughly investigated (59–62). In fact, gold NPs as large as 120 nm are the most effective enhancers in shell–isolated nanoparticle enhanced Raman scattering (SHINERS) experiments (35). Similarly, large spherical silver NPs (~100 nm) have been tested for plasmon enhancement (63). In an early report on single molecule detection (SMD), enhanced vibrational Raman spectra from single hemoglobin molecules 278 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


attached to 100-nm-sized immobilized Ag particles were used (64). However, the extremely high field enhancement needed for SMD came from the dimer system, not from the single Ag sphere. Under appropriate illumination the spatial location between the two spheres becomes a “hotspot” for electromagnetic enhancement (41). Plasmon enhanced spectroscopic experimental results indicate that Ag or Au nanoparticle aggregation is a necessary condition for the hotspot effect to be observed. The most important part of this enhancement is due to an increase of the local electric field between Ag or Au nanoparticles (58), which is the next topic of discussion of plasmon properties.

Figure 4. A. STEM images of Au spheres. B. The corresponding EELS map of the intensity of the localized surface plasmon resonance at λ = 506 nm. Adapted with permission from reference (22). Copyright 2007 IOP Publishing.

Studies in the near-far-field (44, 53, 54) have revealed properties with important implications for analytical spectroscopy: a) The plasmon bandwidth (FWHM-full width at half maximum) increases with particle size, providing a 279 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


wider window for detection. b) The optimal wavelength for surface-enhanced spectroscopy, which depends on the near field, can be significantly red shifted compared to the commonly detected far-field extinction. c) The field strength decays exponentially in the direction perpendicular to the surface and reaches out further with increasing size of the metal core. d) The local field enhancement factor has strong wavelength dependence. In summary, for SERS and SEF, larger nanoparticles scatter strongly, dampening the LSPR and making it broader. Plasmon coupling “To develop the right substrate for efficient sensing of a specific adsorbate, it is therefore crucial to understand which microscopic properties determine the substrate’s plasmon resonances” (65). The Ag or Au spheres forming dimers (66) provide the simplest nanostructures leading to new resonances arising from the strong electromagnetic coupling between closely spaced particles, and these modes could be described using a plasmon hybridization model (67). A bonding interaction may be excited by incoming light polarized parallel to the interparticle axis giving rise to an attractive interaction, while the antibonding repulsive mode is excited when the electric field of the incoming light is polarized perpendicular to the interparticle axis. The modeling has continued to advance including two and three dimensional nanoparticle clusters with unique electrical, magnetic and Fano-like resonances (68). Coupled plasmon resonances are also predicted which cannot couple to plane wave incident radiation, because they do not have a net dipole, and hence, are called “dark” modes. By analogy, in the Mie series of a single nanoparticle, there is the dipole mode that couples most strongly to the incident light field, and quadrupoles, octupoles and so on, that are also localized resonances of the nanoparticle. The fabrication and characterization of metallic nanostructures exhibiting dark plasmon modes is now widely investigated (24, 68, 69). Dark modes may arise from the interaction of the bright modes of coupled nanoparticles, where the resulting collective plasmon mode has a net zero dipole moment. Dark modes can store electromagnetic energy more efficiently than bright modes, due to an inhibition of radiative losses. Correspondingly, dark plasmons have narrower spectral line widths that could lead to their use in sensors based on changes of refractive index. For practical analytical applications, when the coupled mode retains a dipole moment and can couple strongly to light, we have “bright” modes. In enhancing nanostructures, such as colloid clusters (70), the plasmonic response of closely-spaced nanoparticles produces a highly inhomogeneous local field that deviates significantly from that of the constituent nanoparticles; which is the result of strong near-field interactions, with field-intensity concentrated in hotspots. Given the high enhancement of optical signal that can be achieved with hotspots, there are extensive efforts towards engineering local field enhancements (71). The vast computational work on isolated nanoparticles (spheres and other shapes) showed EFs, which are not high enough for single-molecule detection using SERS. Very high EFs are predicted and experimentally observed only in highly localized regions, in the junctions between two particles, i.e. hotspots with EF that could be greater than ten orders of magnitude (15, 66). A table with the EF 280 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


range for non-hotspot and hotspot substrates for SERS can be found in reference (72). In any given plasmon enhancing substrate, hotspots are highly spatially localized regions exhibiting extreme field enhancements, and they are a sensitive function of the geometry and wavelength of excitation. Therefore, a substrate contains hotspots and many spatial locations with moderate enhancement factors, i.e., there exists a probability distribution of enhancement factors for a given substrate (58, 72). In practice, it is possible to define an average value of EF for a large area of the substrate (commonly the area illuminated by the microscope objective), and the site specific EF or hotspot. Hotspot computations have been carried out for several geometries. The EFs estimated are similar to those determined for the dimers. An excellent discussion and summary of results on hotspots can be found in a review by Schlucker (72). Strong EM fields may also be found at sharp edges and tips. Such hotspots can also provide single molecule sensitivity, which is the case of tip-enhanced Raman scattering (TERS) where the enhancement is achieved through the use of a sharpened metallic tip (73). So, it seems that in SERS, single molecule detection and hotspots go hand in hand. In fact, the technique developed for characterization of hotspots is based on SMD:“super-resolution imaging approach is inherently single molecule in nature, because it requires that only a single emitter be active at a given time, such that its position can be uniquely determined” (74). Experimentally, the approach requires that only a single species is active and detected within a diffraction limited spot at a given time. Spatial characterization of hotspots has been performed with super-resolution fluorescence imaging (75), and with SERS imaging at the single molecule limit (74). These techniques provide an indirect way of estimating the size of the hotspot (probably less than 15 nm (75)) based on molecular spectroscopic data at the molecule-plasmonic interface.

Surface Enhanced Raman Scattering (SERS) and Shell–Isolated Nanoparticle Enhanced Raman Scattering (SHINERS) Raman scattering is an established spectroscopic technique providing fingerprint information about molecular structure and functional groups in all classes of materials (76, 77). Despite enormous advances in technology (77), due to the low cross section in inelastic Raman scattering (as low as 10-30 cm2/molecule), it may be challenging to obtain good quality spectra in highly diluted systems. This limitation can be circumvented by plasmonic enhancement of the optical signal, as in surface-enhanced Raman scattering (SERS) or surface-enhanced resonant Raman scattering (SERRS) (51, 58). The effect is achieved placing the probe molecule close to an appropriate metallic nanostructure (mainly silver and gold). The plasmonic origin of SERS is accepted (13, 14, 78); however, when the molecule is directly adsorbed onto the metal nanoparticle, there are nuances (79) in the enhanced spectra discussed in several review articles (80, 81). The SERS phenomenon has been exploited in various analytical techniques, all of them based on nanostructures. In fact, nanostructure fabrication is a driving force in analytical applications of SERS (40, 82), and in practice, 281 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


each successful analytical application requires a finely tuned nanostructure or “SERS substrate” and optimal experimental conditions. Such techniques include Tip-Enhanced Raman Scattering (TERS) (73), electrochemical SERS, colloidal SERS (83), SHINERS (84), and many more. The introduction of shell-isolated nanoparticles (SHIN) in SHINERS by Li et al. (84) eliminates the issues related to metal-molecule interactions, particularly those due to chemical adsorption and formation of metal complexes giving rise to “first layer effects” (85). Therefore, SHINERS is a “clean” plasmonic enhancement and it is one of the main analytical techniques selected for this review, while we refer the reader to reviews on established SERS-based techniques. Furthermore, controlling the shell thickness permits to use SHINs in surface enhanced fluorescence (SHINEF) (86), which is a complementary plasmon enhanced technique. In both cases, SHINs nanoparticles may be spread as ‘smart dust’ over the probed surface, or work in the liquid phase. Literature in SHINERS has been expanding rapidly with applications to different systems, such as semiconductor materials (87), electrochemistry (88, 89), biological and food sciences (84, 90) and cancer detection (90). The key for this method is the synthesis of the core shell nanoparticles (SHINs), which may have variable core, shell, size and shape. In addition, one may design functionalized ultrathin shells capable of capturing target molecules, or fabricate two or three dimensional SHIN structures tailored for challenging applications (91). Figure 5 shows transmission electron microscope (TEM) images of three types of SHINs (nanospheres, nanorods and nanocubes).

Figure 5. TEM images recorded for SHINs of (a) spherical shape with 2 nm of silica shell, (b) nanorod with 4 nm of silica shell and (c) nanocube with 2 nm of silica shell. Adapted with permission from reference (92). Copyright 2012 Nature Publishing Group.

SHIN coatings have been designed (93, 94) to overcome the lack of compatibility of silica shells with high pH environments (87). For example, Qian, Liu, Yang and Liu (95) reported on tunable poly-(2-aminothiophenol) (PAT) shells on gold nanoparticles, which are claimed to display prominent advantages, including uniformity, chemical stability, being free of pinholes. These polymer shells were suitable for strong acid and alkaline environments with 282 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


pH ranging from 2.02 to 12.95. The goal of the work was to develop a platform for identification of trinitrotoluene (TNT) explosive, based on the formation of Meisenheimer complexes between TNT and amino groups coming from the shells. Figure 6 shows the three-step sequence of the procedure: (1) PAT assembly onto AuNP via electrostatic interactions, (2) formation of a Meisenheimer complex in the presence of TNT and (3) TNT detection via SHINERS. A clean SHINERS spectrum requires a “pinhole free” SHIN. However, a SHIN with pinholes could also be used, harnessing the high enhancement factor at the pinhole, and the chemical stability of the coated nanoparticle. This pinhole SHINERS-based approach has been implemented (96), and it may provide further opportunities for SERS applications.

Figure 6. Three-step sequence toward TNT identification: (1) PAT assemble onto AuNP via electrostatic interactions, (2) formation of a Meisenheimer complex in the presence of TNT and (3) TNT detection via SHINERS. Reproduced with permission from reference (95). Copyright 2012 The Royal Society of Chemistry.

SHINERS have also been used to characterize biological structures such as living cells. For instance, SHINs were incubated into Yeast cells and SHINERS spectra were taken directly from the cells, as shown in Figure 7a. The latter features spectra collected in different places of the same cell (I – III), the spectrum recorded from a substrate coated with SHIN (IV) and a normal Raman spectrum for yeast cells (V). A schematic representation of the SHINERS experiment on living yeast cells is depicted in Figure 7b. Taken together, these results demonstrate the use of SHINERS for in situ detection of cell components and as possible probes for the dynamics of living systems. 283 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 7. (a) SHINERS spectra (I – III) collected in different places of the same cell, spectrum recorded from a substrate coated with SHIN (IV) and a normal Raman spectrum for yeast cells (V). (b) Schematic representation of the SHINERS experiment on living yeast cells. Reproduced with permission from reference (84). Copyright 2010 Macmillan Publishers Limited.

In summary, in a typical SERS experiment the target molecule is brought to the metal surface in order to achieve the enhancement. The SERS-active substrate can be thermally evaporated thin films (97), freestanding 3D nanostructures (98), electrodeposited structures (99) or colloids with different geometries (100–102). SHINERS, similar to tip-enhanced Raman scattering (TERS), brings the SHIN (or the tip) to the target molecule to get SERS (103, 104). In both cases, one can control the distance between the target molecule and metal core (or metal tip) that acts as the Raman signal amplifier. In principle, TERS can be achieved from any surface with high spatial resolution (103). For instance, Boehme, Cialla, Richter, Roesch, Popp and Deckert (105) reported TERS on the nanoscale discrimination of protein-labeled supported lipid bilayer (SLB) structures. TERS information is based on the tip position and is characterized by lipid (orange circle) and/or protein marker modes. Therefore, TERS spectra could be attributed to different probed materials. The latter experiments are crucial for understanding lateral organization on the cell surface (106) and distribution of lipid domains (105). The total intensity of the Raman signal from the tip area is rather weak, so TERS is limited to molecules with large Raman cross-section. Besides, the 284 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


instrument is highly sophisticated and expensive, which may hamper practical applications. SHINERS is a simple technique based on plasmon enhanced electromagnetic field generated from the isolated SHINs or from nanoscale gaps (94) found among the SHIN aggregates to provide high-quality plasmon enhanced Raman spectra. One should distinguish two clearly different detection regimes in SERS spectroscopy: average SERS and Single Molecule regime of SERS detection (51). For the average SERS, it is possible to fabricate SERS substrates that will operate in a well-defined spectral region with an average enhancement factor (EF), typically 103-106, that may be used for analytical applications, including quantitative analysis (107). The EF provided by classical electrodynamics support the values in that range, and the computed magnitudes are smaller for the surface average of the nanostructure. On the other hand the experimental confirmation of the hotspot leads to Single Molecule Detection (SMD) regime of SERS (108).

Surface Enhanced Fluorescence (SEF) and Shell–Isolated Nanoparticle Enhanced Fluorescence (SHINEF) Fluorescence is based on the absorption of one photon by a fluorophore in the UV-visible range, followed by an emission of a second photon at lower energy. This highly efficient phenomenon has been widely used in optical devices, microscopy imaging, biology, medical research and diagnostics (109–111). Following light absorption, a fluorophore is usually excited to some vibrational level of a singlet excited state S1. Within a picosecond time internal conversion takes place and molecules in condensed phases relax to the lowest vibrational level of the singlet. Since fluorescence lifetimes are typically in the nanosecond range, internal conversion is usually complete prior to emission. Therefore, the fluorescence spectrum would be the mirror image of the S0 → S1 absorption, not necessarily the entire absorption spectrum. The two central properties of and the quantum yield (Q). the fluorophore are the fluorescence lifetime The quantum yield is defined in terms of the radiative decay rate , and the decay rate of non-radiative processes . Far from saturation is proportional to the quantum of the excited state, the fluorescence power yield and the absorption power . Using the absorption cross section , where is the irradiance. Statistically, the lifetime represents the life expectation of the excited molecule, i.e., the average time the molecule spends in the excited state; . Fluorescence emission is sensitive to external parameters and the local environment of the molecule. The concept was first presented in the Proceedings of the American Physical Society (1946) by Purcell (112), where the spontaneous emission was shown not to be an intrinsic property of the emitter, which could be modified by resonant coupling to the external electromagnetic environment (the Purcell 285 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


effect). Of particular interest here is the case when molecules are near metal surfaces. It requires understanding the behavior of a fluorophore (dipole) near a metal surface, as given in a detailed theoretical account of the dipole emission near interfaces (113). The latter study was prompted by experimental findings where for molecules located at “large distances from the metal surface the fluorescence lifetime oscillated as a function of the distance, while for small distances the lifetime went monotonically towards zero” (113). The changes in lifetime area due to modifications of both the radiative and non-radiative decay rates, and could result in strong quenching of fluorescence emission at the metal surface. In other words, nonradiative energy transfer to the metal surface may be an effective channel for the excited fluorophore, which can be produced by electronic coupling of the molecular orbitals with the extended band structure of the metallic electrons in the metal substrate. The latter leads to a connection between the frequency dependence of the energy transfer and the surface plasmon modes. Here it is appropriate to separate the surface plasmon resonance excited on flat metallic surfaces (usually using Otto and Kretschmann configurations[109]) and the corresponding analytical techniques (114), from the LSPR on nanometallic structures that supports collective electron excitations producing high local-field intensities (3, 115). Plasmon enhanced fluorescence (PEF), based on LSPR, was born under the tree of surface enhanced Raman scattering, and was termed surface enhanced fluorescence (SEF) (16, 17). In 2002 the physical phenomenon was renamed as metal enhanced fluorescence (MEF) (18). The strong local-fields near metallic nanostructures can induce changes in the light emission properties of emitters in close proximity. Assuming that the emitter quantum structure is preserved, the plasmonic structure can increase the optical absorption rate; it may alter the radiative and nonradiative decay rates and emission directionality. is proportional to , where p is the The fluorescence power dipole moment, and E is the enhanced optical electric field vector at the emitter position, leading to an increased intensity. The electromagnetic field surrounding the metallic nanostructures enhances the molecule absorption and, therefore, increases the quantum yield. However, the benefits of enhanced fluorescence are only observed for emitters located at a finite distance from the metal nanostructure (17). There are, thus, two competing effects: enhancement due to the local electromagnetic environment and nonradiative decay due to radiationless energy transfer that produces quenching. When excited molecules located near plasmonic nanostructures (nanoparticles or aggregates) can have efficient nearfield coupling to localized surface plasmons, large enhancements are obtained, especially if the molecular emission frequency matches the LSPR. Notably, once LSPR is excited, it can either decay nonradiatively or, most importantly, reradiate into free space. The latter is the analog of an optical antenna and is the main source of plasmon enhancement (4). In summary, the task for analytical applications is to design plasmonic nanostructures that can reradiate, increasing the optical signal, via non-radiative coupling to an excited molecule (placed at an optimum distance) with the localized surface plasmon of the metal (116). Yet, it is necessary to review the existing data 286 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

to appreciate the effect of contributing factors to the observed enhanced signal and help the experimental planning. To help the discussion, a detailed collection of reported PEF data is given in the Appendix.


Distance Dependence A key parameter to get the most out of PEF is the thickness of the spacer layer between the fluorophore and the metal nanostructure to provide a critical distance that avoid quenching and maximize the enhancement. This metal-molecule distance dependence has been experimentally demonstrated in independent studies (117–123). Consequently, the design of a particular plasmonic substrate for SEF must include the optimization of the spacer layer, determining the nanostructure–molecule separation (124). SHINs originally developed for SHINERS (84) can be tuned for PEF, as an increase in shell thickness would provide the fluorophore with a continuous transition from fluorescence quenching (SHINERS) to fluorescence enhancement (86). This was accomplished by Guerrero and Aroca (86) in SHINEF for Langmuir–Blodgett monolayers, as depicted in Figure 8. SHINs constitute only one of the many avenues to preparation of plasmonic nanoparticles found in the literature, and enthusiastic research is ongoing in this direction (125).

Figure 8. Plasmon-enhanced fluorescence activated with Au nanoparticles coated with silica (SHINEF) over the analyzed surface (LB monolayer in this case). Reproduced with permission from reference (86). Copyright 2010 Wiley-VCH Verlag GmbH & Co.


Hotspots and the Enhancement Factor (EF) PEF enhancement factors typically range from 2 to 100, which is modest compared to SERS, as indicated in the data provided in the Appendix. This is a consequence of PEF only benefiting from the enhanced local field, leading to a


square dependence, , whereas SERS is based on the amplification of both incident and scattered field with an enhancement proportional to

, where E0 is the incident field and Eloc is the enhanced local field (14, 126, 127). The advent of SHINs allows one to tune the experimental conditions to simultaneously record SHINERS and SHINEF for a low quantum yield molecule, as illustrated in Figure 9. Experimental results for the same molecule confirm that PEF is proportional to |E|2 while SERS is given by |E|4.

Figure 9. Combined SHINERS and SHINEF spectra for an aqueous solution of crystal violet (CV) and comparison with normal Raman and fluorescence spectra. Reproduced with permission from reference (126). Copyright 2012 Wiley-VCH Verlag GmbH & Co.

The experiments depicted in Figure 9 were performed in solution, providing reproducible average values of the local field enhancement (126). Strongly confined fields between metallic nanostructures (hotspots), that can alter the light emission properties of nearby fluorophores, have been reported to yield an EF of 4.5 x106, the highest EF published so far for PEF (128). A large EF of 1340 has also been measured for single molecule fluorescence using gold bowties 288 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


(129). Similarly to SERS, reproducible average PEF enhancement factors can be attained for certain PEF substrates with quantitative analytical applications, while higher (but variable) EF can be achieved at hotspots. The contribution from hotspots to EF has also been shown in aggregation studies (130), including the use of SHINs (131). The near field response of a gold SHIN dimer embedded in water for different gap sizes was calculated. Figure 10 shows a plot of the near field intensity enhancement in the center of the gap in a dimer, formed from Au-SHINs of 50nm core diameter and 10nm SiO2 shell, illuminated at normal incidence and polarized along the dimer axis. As the gap is reduced, the interaction contributes to the enhancement of the near field intensity, in full agreement with observations. In addition, both the scattering and extinction cross sections increase and get spectrally red-shifted. This expected phenomenon is due to the coherent interaction of the plasmonic dipolar resonance excited in both SHINs.

Figure 10. SHIN dimer is illuminated at normal incidence and polarized along the dimer axis and near field intensity enhancement in the centre of the gap (point A). Reproduced with permission from reference (131). Copyright 2014 American Chemical Society.

For analytical applications, the main task may again be defined as the fabrication of a substrate, an optimized arrangement of suitable building blocks, leading to specific optical properties of the assembled nanostructure. There are various chemical methods to synthesize nanoparticles of different shapes and surface modifications, as well as approaches to assemble them and create predefined enhancing substrates (132). Therefore, surface coating of nanoparticles (mainly Ag and Au) with different sizes, shapes and materials, with different plasmon absorptions (126, 133), allows for many applications of PEF, where the core shape, the shell thickness and functionalization (134) can be tuned for specific tasks. 289 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


The Molecular Quantum Yield PEF has been measured for both high and low quantum yield molecules, as demonstrated in the data collected in the Appendix. However, some reports show a tendency to dismiss SEF or MEF for high quantum yield chromophores: “The enhancement effect is most significant for relatively weak and diluted absorbers and rather inefficient emitters that are placed in close proximity to the metal nanoparticles” (135). The experimental results show that high QY fluorophores can provide large fluorescence EFs. For instance, Gartia, Eichorst, Clegg and Liu (136) measured PEF for high quantum yield (HQY) fluorophores [R6G (Q0=0.9), and for fluorescein (Q0=0.95), together with three additional fluorophores of lower Q0. The unmodified Q0, the modified quantum yield Qmod with the estimated EFs are reproduced in Table 1, where the highest enhancement (EF=100) is found for the fluorophore with the highest Q0. From the data collected in the Appendix it can be concluded that there is no clear correlation between Q0 and EF. However, for a particular substrate such correlation has been reported (130).

Table 1. Unmodified and Modified Quantum Yields, in Addition to the Enhancement Factor for Two Fluorophores Q0

Qmod

EF

R6G

0.90

0.992

20.5

Fluorescein

0.95

0.991

100

Fluorophore

For a brief discussion of the role of the quantum yield, we can use the simplified approach to PEF enhancement presented by Bardhan, Grady and Halas (120), where the intensity ratio is given as the result of two contributions: . The local field enhancement has already been discussed. The second factor results from an increase in the radiative decay rate of the molecule leading to an enhancement of the quantum yield , with an EF equal to , which alone cannot account for the very large EF values measured experimentally. The approach neglects changes in the nonradiative decay that assumes a molecule-metal spacing greater than the critical distance. Within this approximation, a fluorophore with intrinsically high quantum yield will be mainly enhanced by . Therefore, tuning the plasmonic structure, particularly by increasing the hotspot distribution density in the PEF substrate, would further increase EF, as in aggregated nanostructures. The data collected in the Appendix support the assumption that the local field is the main source of enhancement. 290 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 11. Dual modal nanoprobe (DMNP) representation, TEM microscopy of the designed DMNP and outline of cancer marker detection using fluorescence-SERS dual DMNPs. Reproduced with permission from reference (141). Copyright 2012 The Royal Society of Chemistry.

Spectral Profile Modification An important consequence of the enhancement is the experimentally observed spectral profile modification [137] (SPM) of the fluorescence spectrum under PEF conditions. LSPR can affect not only the fluorescence intensity (enhancement or quenching) but also its spectral shape. The profile of the original fluorescence spectrum may be altered by the re-radiation of the plasmonic 291 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


nanostructure, as demonstrated by several groups (133, 137, 138), with the PEF spectrum being different from the original fluorescence spectrum. The role of nanostructure scattering (Mie scattering) is well established for both SERS and PEF (58). For example, Dragan, Mali and Geddes (139) summarize their findings stating: “the wavelength-dependent metal-enhanced fluorescence (MEF spectrum) correlates well with the plasmon specific scattering spectrum”. The latter is consistent with observation of large enhancements, particularly when the molecular emission frequency matches the LSPR. Similarly, maximum SERS enhancements are obtained when the excitation and Raman emission are within the plasmon wavelengths, as predicted by the plasmon enhancement theory (140). SPM is probably a common occurrence in SERS and PEF; however, since LSPR is very broad, the effect is more likely to be appreciated in fluorescence experiments. In summary, the fingerprints of the plasmon scattering may induce changes in PEF with respect to the original fluorescence spectral profile. We close with an example from fluorescence imaging that integrates in the plasmonic structure the basic ideas previously discussed. First, readily available fluorescent probes display relatively weak emission and are rapidly photobleached. This limitation was overcome by Lee, Chon, Yoon, Lee, Chang, Lim and Choo (141) with a highly sensitive optical imaging based on SERS and fluorescence combined in a dual modal nanoprobe (DMNP). The surface of Au nanoparticles (~40 nm diameter) was first coated with Raman reporter molecules (MGITC and Rubpy). Subsequent encapsulation with a silica shell was performed to prevent the release of the reporter molecules. The thickness of the silica shell was tuned to achieve the maximum fluorescence enhancement of the covalently attached fluorescent probes (fluorescent ITC-modified with FITC or RuITC). A final silica shell was added to minimize nanoparticle aggregation, avoid direct contact with the probed surface and to protect the fluorescent dye. Finally, the DMNP was attached to specific antibodies for targeting and imaging specific breast cancer markers in living cells, as shown by the outline in Figure 11. DMNP allows one to collect fluorescence as a fast track tool for cancer marker recognition and SERS as an accurate tool to determine the signature of specific molecular interaction. Moreover, the implementation of DMNP to early cancer diagnosis has shown to be straightforward.

Summary LSPR, which are resonantly driven oscillations in metal nanostructures, can be induced at specific optical frequencies producing a very strong charge displacement and local field concentrations. The characterization and assignment of these resonances is one of central tasks of plasmonics (3), and epitomize the starting point for analytical applications. The challenge for experimentalists is clear. The fabrication of nanostructures (nanoscale architectures) with increased complexity that provide the most useful optical properties for analytical techniques (142). The synthesis of nanoparticles and assembly of nanostructures with rationally designed dimensions can be realized using the “bottom-up” or the “top-down” approaches (125). 292 In Recent Progress in Colloid and Surface Chemistry with Biological Applications; Wang, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Plasmonic nanostructures, under appropriate conditions, will lower the limit of detectable concentrations for materials, opening the field of ultrasensitive analysis using plasmon enhanced spectroscopy. When the target molecule is a fluorophore, plasmonic nanostructures are able to modify its radiative and non-radiative decay rates, changing both the fluorescence lifetime and quantum yield. In addition, an increased energy transfer may affect molecular photostability that can be used to increase the signal level when imaging fluorescent molecules. Finally, the introduction of shell-isolated nanoparticles has empowered the development and rapid growth of two analytical techniques: SHINERS and enhanced fluorescence (SHINEF). In fact, many new applications will flourish by taking advantage of the complementary nature of SERS-fluorescence techniques, a dual mode approach that takes advantage of both enhanced inelastic scattering and fluorescence.

Acknowledgments This work was supported by FAPESP, CNPq, Science Without Borders Program and nBioNet network (CAPES), Brazil.

Appendix. Reported Enhancement Factors in Plasmon-Enhanced Fluorescence Table A1 Molecule

Quantum yield

λexc

λemi

Average EF

Basic fuchsin

0.02

514.5

600

200

Indocyanine green (ICG)

0.012

785

850

2970

TPQDI: N,N0bis(2,6-diisopropylphenyl)1,6,11,16-tetra-[4(1,1,3,3-tetramethylbutyl)phenoxy]quaterrylene3,4:13,14-bis(dicarboximide)

0.025

780

820

Cy5 labeled oligonucleotide

Hot spot

Metal Ag (17)

4.5 x106

Au (128)

1340

Au (129)

635

15

Ag (143)

Atto 540Q-labeled DNA

1.6x10-3

532

740

Ag (130)

CY3-labeled DNA

0.08

532

37

Ag (130)

R6G-labeled DNA

0.17

532

17

Ag (130) Continued on next page.


Table A1. (Continued)


Molecule

Quantum yield

λexc

λemi

Average EF

Hot spot

Metal

Fluo-3, 1,2-bis(2-aminophenoxyethane) N,N,N_,N_tetraacetic acid

0.15

473

518

400

Ag (144)

AzoPTCD

~ 0.98

785

813

50

Ag (145)

Rhodamine B (ethanol)

0.49 (146)

540

581

260

Ag (147)

SiC nanocrystals

0.17 (148)

360

432

176

Ag (149)

A655-DNA

0.13

650

690

170

Ag (150)

470

640

189

Au (151)

627

7-12

Lanthanide ions; Pr+3 Rhodamine B (ethanol)

0.49 (146)

554

Rhodamine 6G (R6G)

0.95

633

Chromeo 642

0.17 (154)

642

676

6-(N-(7-nitrobenz2-oxa-1,3-diazol-4yl)amino)hexanoate (NBD) (156)

0.45

476

537

100

Ag (157)

Y3N@C80 Fullerene

0.02-0.05

633

710

~100

Au (158)

Fluorescein

0.95

440

100

Ag (136)

Octadecylrhodamine B (R18)

~ 0.5

514.5

575

94

Ag (126)

785

~800

241

IRDye 800CWlabeled streptavidin

164

125

Ag (152) Au (153)

136

2530

Ag (75, 155)

Ag (159)

AF790-SA

0.04

780

820

83

Au (160)

AF750-SA

0.12

740

800

10

Au (160)

AF488-SA

0.92

480

520

7.8

Au (160)

Indocyanine green (ICG)

0.012

785

850

50

Au (161)

Perylene

0.94

~500

50

Ag (162)

Tryptophan

0.12 (163)

270

339

40

Ag (164)

Terrylene

~1

532

~600

20

Au (165) Continued on next page.


Table A1. (Continued)


Molecule

Quantum yield

λexc

λemi

Average EF

Hot spot

Metal

Fluorescein (FITC)

0.92

488

519

15

Ag (166)

Single walled carbon nanotubes (SWNTs)

0.01 – 0.03

658

1300

>10

Au (167)

Nile Blue

~1

637

661

10

Au, Ag (168)

IR800 conjugated with HSA

0.74

780

804

40

Au (169)

Crystal violet

5x10-5

514.5

740

59

Ag (131)

Eosin-Y

0.32

514.5

550

9

Au (131)

F8BT/MEH-PPV polymer

0.155

532

633

4

Au (170)

Rose Bengal

0.1

540

550

4

Au (171)

Eosin Y

0.32

540

550

2

Au (171)

CY3-labeled DNA

0.08

532

570

3

Ag (172)

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