plasmon enhanced spectroscopy

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Submitted to: Physical Chemistry Chemical Physics

Plasmon enhanced spectroscopy

Ricardo F. Aroca

Materials and Surface Science Group, Department of Chemistry and Biochemistry, University of

Windsor. Windsor, Ontario, N9B 3P4 (Canada)

E-mail: [email protected]

Abstract

Surface enhanced spectroscopy encompasses a broad field of linear and nonlinear optical

techniques that arose with the discovery of surface-enhanced Raman scattering (SERS) effect.

SERS enabled ultrasensitive and single molecule detection with molecular fingerprint specificity,

opening the door for a large variety of chemical sensing applications. Basically, from the

beginning it was realized, that the necessary condition for SERS to be observed was the presence

of a metallic nanostructure, and with this condition, the optical enhancement found a home in the

field of plasmonics. Although plasmonic practitioners claim that SERS is “the most spectacular

application of plasmonics”, perhaps is more appropriate to say that the spectacular development

of plasmonics is due to SERS. Here is a brief recollection from surface enhanced spectroscopy to

plasmon enhanced spectroscopy.

1. Introduction

The inelastic scattering of light, excited far from molecular electronic resonances, is a very

inefficient process, and the production of a single Raman photon may require 108 excitation

photons. This small cross-section for spontaneous Raman scattering limits its sensitivity and

applications. In the sixties and seventies of the last century, and thanks to new continuous wave

laser sources, there was increasing activity to develop techniques to detect minute molecular

concentrations1, 2

. In fact, the first report commonly cited on the anomalously enhanced Raman

scattering intensity of pyridine adsorbed at a silver electrode3, was indeed a work aimed to

improve sensitivity (1974): “In order to study the behavior of species adsorbed at about a

monolayer coverage with Raman techniques it has been found necessary therefore to prepare

solid metal electrodes with high surface areas”3. The remarkable sensitivity of the Raman signal

observed for pyridine and other amines adsorbed on a silver surface (1977) was confirmed for a

number of amines and reported by Jeanmarie and Van Duyne4. The authors clearly pointed out in

that report to a new component in the observed Raman intensity: “Given that the experimentally

Page 1 of 18 Physical Chemistry Chemical Physics

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mailto:[email protected]


observed intensities of NR scattering from adsorbed pyridine in our laboratory are 5-6 orders of

magnitude greater than expected, we felt that some property of the electrode surface or the

electrode/solution interface is acting to enhance the effective Raman scattering cross section for

these adsorbed amines”. Further they rule out the resonant Raman scattering (RRS) as a source

of the enhancement: “A study of the wavelength dependence of the surface pyridine signal

showed, within the experimental error, a fourth power wavelength dependence over the range

4600 Å to 6300 Å. This immediately ruled out the possibility that the observed signal

enhancement was caused by coupling of laser field with an electronic transition in the adsorbed

species on the electrode surface”. The anomalously intense Raman-spectra of pyridine at a

silver electrode was also reported by Albretch and Creighton5. The discovery opened the door to

numerous experimental and theoretical efforts with the common objective of elucidating the

physical nature of the new phenomenon. Since the experiments were conducted on surfaces, the

new name coined for the effect was; surface-enhanced Raman scattering (SERS) and also the

general term surface-enhanced spectroscopy (SES) was also used early on6. In view of the fact

that spectroelectrochemistry was central to the developments, it was suggested that an electric

field enhancement would be one source that will help to explain the observed spectra. Today,

there is overwhelming evidence pointing to the central role of plasmonics7 in the enhancement

mechanism of SERS, in particular, and SES in general8-10

. The connection to plasmons was

proposed at the very beginning of the SERS mechanism discussion. Moskovits (1978) wrote: “I

propose that the anomalous intensity arises from preresonant or resonant excitation of

conduction electrons resonances in adsorbate covered metal bumps on the surface”11

. A detailed

theoretical treatment of this model was reported in 1980 by Gersten and Nitzan12

and, in the

same year, a similar treatment for molecules adsorbed at the surface of spherical particles was

given by Kerker, Wang and Chew13

. The evidence for plasmonic-SERS was also emerging as

part of the many experimental communications. For example, in 1979 Tsang, Kirtley and

Bradley14

reported their findings on: Surface-enhanced Raman-spectroscopy and surface-

plasmons. They concluded that “...molecular Raman scattering is strongly enhanced under

conditions which permit the direct excitation of the surface plasmon modes of Ag.” During the

first decade the field of SERS saw a large number of results from different groups that, at times,

seem confusing and even contradictory, a fertile ground to put forward a variety of theoretical

models. The first edited book on SERS, and, in particular, the survey by Horia Metiu15

(pages 1-

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34) is a good reading to capture the flavor of the times. There was an excellent attempt to

organize the theoretical discussion and focus the SERS efforts in the review by Moskovits

(1985)16

that also convey the message of the impact that SERS research was having in the

parallel development of plasmon research: “Besides producing enormous interest in Raman

spectroscopy of surfaces, the discovery of SERS has stimulated and resurrected activity in

classical electrostatics and electromagnetic theory, especially as applied to small particles; in

the problem of radiating multipoles near metal surfaces, in optics of small particles and in the

generation of surface plasmons”. The new developments were in the field of localized surface

plasmon resonances (LSPR), or the electromagnetics of metallic nanostructures, i.e., the field of

Plasmonics7. It was becoming more and more evident that the central player in SERS and SES

was the nanostructure used as isolated nanoparticles (metal colloidal solutions), metallic tips, and

aggregation of nanoparticles or two dimensional nanostructured surfaces such as metal island

films. The nanostructures can directly absorb the light to stimulate the LSPR, excitations with a

large scattering power[chapter 5 in 7]. These radiative nanostructures have cross sections several

orders of magnitude better than the quantum emitters (molecules)17

. In addition, it had been

demonstrated experimentally that the enhancement has distance dependence (as predicted for an

electromagnetic enhancement13

), and can be observed at a distance from the plasmon excited

nanostructure, i.e., chemical adsorption is not a pre-requisite for SERS. The distance dependence

is a unique near-field property proved elegantly in tip-enhanced Raman scattering (TERS)18

by

recording TERS spectra exhibiting a strong signal increase when the tip–sample distance is

lowered below 30 nm. The non-radiative surface plasmons that can be excited on flat metallic

surfaces had been extensively studied long before the SERS discovery19

and the excitation of

surface plasmons can be done (with the help of a prism) using the Otto20

or Kretschmann21

configurations, providing the basis for the very sensitive technique known as Surface Plasmon

Resonance Spectroscopy22

. When resonance is achieved, energy is transferred from photons to

plasmon excitations, and the SPR spectrum is defined by a sharp minimum of the reflectance

with the variation of the angle of incidence. These are non-radiative excitations and strictly

speaking there is no SERS on flat metallic surfaces. Fresnel equations provide an understanding

and explanation for the local field on illuminated flat surfaces that could enhance the

spectroscopic intensity by as much as 16 times23

. The changes in Raman intensities due to

molecule-surface interactions have also been investigated on single metal crystals 24

.


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Nonetheless, there is a stream of research developments that propose to use the non-radiative

surface plasmon on the surface of metallic films (mainly gold and silver) to extract the

amplification of the Raman signal for a molecule adsorbed onto the metal film. Recently, an

instrumental development has been proposed combining surface plasmon resonance (SPR) and

Raman microscopy in the Kretschmann configuration to extract SERS from the SPR interacting

with molecules on the surface of the metallic film25

. Since direct coupling to flat metal surfaces

is impossible due to the momentum mismatch between the wave vectors of a free-space photon

and a plasmon confined to a surface, a high refractive index (prism) material coupling light into

the metal surface is needed. Recently, experimental evidence has been presented suggesting that

such coupling may be achieved using metal oxide nanoparticles of some materials, providing

enhancement for the scattering of molecules located in between the gold film surface and the

oxide nanoparticles26

. The observations seem to correlate higher refractive index of oxide

nanoparticles with more efficient scattering and proposing the possibility of localized surface

plasmons excitation of the substrate taking place when metal oxide nanoparticles are close to the

surface. The isolated oxide nanoparticles have no enhancement effect. In summary, if we replace

surface enhanced spectroscopy for plasmon enhanced spectroscopy (making the plasmon a

necessary condition for enhancement), the plasmonic origin and the framework of the field are

well demarcated.

2. Plasmon enhanced scattering. SERS (SERRS), TERS (TERRS) and SHINERS

The role of the nanostructure took central stage with the reports on single molecule detection

(SMD)27, 28

. It is now evident that the direct plasmon excitation enhances the local field at the

surface of the nanostructure, and this local field enhancement is a sensitive function of the

optical properties of the metal nanostructure, its shape and size, and, most importantly, a function

of the plasmon coupling arising at the junction of nanoparticle aggregates and organized arrays29-

31. The fabrication and characterization of metallic nanostructures is the territory of Plasmonics.

Plasmonics can be understood as the electromagnetics of metallic nanostructures, a cross

disciplinary field that exploits the unique optical properties of metallic nanostructures to enable

routing and manipulation of light at the nanoscale. We have seen in the last decade a convergent

development of plasmonics and plasmon enhanced spectroscopy. Some metal nanoparticles

exhibit LSPR based on collective oscillations of their conduction electrons and it is the


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interaction of this effect with molecules which is the origin of plasmon enhanced spectroscopy.

Since the nanostructure is the antenna transmitting the information on the molecular systems, we

tune our signal detection devices to the electromagnetic region of the spectrum where the

plasmon excitation takes place. Furthermore, when light illuminates metallic nanostructures with

nanoscale openings or gaps, light is concentrated on the nanometer scale producing an intense

electromagnetic field earmarked as ‘hotspot’. In a recent report, Cang et al.32

were able to image

the fluorescence enhancement profile of single hotspots as small as 15nm. It has become

apparent that hotspots are the source of enhancement required for SMD. It is a necessary

condition for the molecule to be located in the hotspot for the signal to be enhanced above the

threshold to see the spectrum of the single molecule33-36

. The hotspot-molecule coincidence is

statistically a rare event, and in practice one could work with higher concentrations; but low

enough to extract the SM spectra37

. In our laboratory, the Langmuir-Blodgett technique is used to

coat the enhancing surface with a single monolayer with variable concentration of the target

molecule and scanning the surface to detect hotspot-SM concurrence38

. The experimental

confirmation of the hotspot permits one to distinguish two different regimes in plasmon

enhanced spectroscopy. For instance: the average SERS and the SM regime SERS. 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), commonly in the area of 103-10

6, that

may be used for analytical applications including quantitative analysis39

. The EF provided by

classical electrodynamics support the values in that range40

, and the computed magnitudes are

smaller for the surface average of the nanostructure41

. The magnitude of the enhancement is

further damped by increasing surface coverage, and this effect has been shown experimentally

using Langmuir-Blodgett monolayers on silver nanostructures42

. Therefore, the average SERS

EF for concentrations of the analyte that warrant a complete surface coverage of the metal

nanostructure could be modest in magnitude. For instance, for a single neat monolayer of an

analyte deposited onto a silver island film, we obtain an EF of 103-10

4, reproducibly in every

section of the film for a particular laser line in resonance with the nanoparticle plasmon. In

practice, reproducible substrates for average SERS, with a reliable enhancement factor in a

particular spectral region are offered commercially by several companies. For an extensive

discussion on SERS substrate fabrication see a recent review by Brolo et al.43

.


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SERS in the SM regime depends on the existence of hot spots, and since there is not statistically

averaged; the EF can now vary from place to place. In addition, the SM spectra may show

fluctuations, reflecting the effect of the local molecular environment. The research has moved in

the direction of nanostructure fabrication with an effort to improve the average EF and increase

the concentration of hotspots in the SERS substrate for applications in all fields of science and

engineering9, 43, 44

. In summary, the plasmonic origin of SERS provides a very clear framework

for the discussion of plasmon enhanced spectroscopy with two challenges. The first challenge is

the fabrication of substrates with well defined plasmonic enhancement properties: substrates for

average enhancement or plasmonic substrates for single molecule detection or imaging7, 45, 46

.

The second undertaking is the interpretation of the enhanced molecular spectrum. Once we

introduce the molecular system to be detected, the enhanced spectra contain a wealth of

information about the molecular system and its interactions with the environment. In addition, all

the tools of molecular spectroscopy are at the disposal of the experimenter to help and pull out

the relevant information from the SERS spectra. There are, of course, vast possibilities for tuning

the plasmonic nanostructure and capturing the frequency spectrum of a given molecular target. In

plasmonics, the enhancing ability of the substrate, or enhancement factor, is estimated

independently of the molecular system. In the average SERS regime, there is a tight relationship

between the LSPR profile and the SERS enhancement as has been established in the visible

region of the electromagnetic spectrum using wavelength-scanned surface-enhanced Raman

excitation spectroscopy (WS SERES)47

. It means that, approximately, the highest enhancement

factor is found near the maximum of the LSRP extinction, i.e., around this frequency the

strongest SERS enhancement occurs for the incident and Raman scattered photons. However,

recent results (from the same group)31

show that the scattering spectrum of a nanostructure

containing a hot spot and the corresponding SERS enhancement profile do not follow the close

relationship found in the average SERS regime. The maximum enhancement is red shifted with

respect to the LSPR maximum, reminiscent of the old situation in colloidal SERS where the

highest enhancement was observed for a small red shifted extinction band seen after colloidal

aggregation. However, dealing with individual nanoantennas brings about the role of important

factors that may contribute to the observed profile, such as polarization48

and dark plasmon

modes49

. It is possible to observe these optically dark plasmonic modes using electron


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excitation50

, an additional experimental tool (electron energy-loss spectroscopy-EELS) with a

unique resolving power to probe electromagnetic hot spots.

The computation of enhancement factor for the substrates was part of the initial theoretical

development of SERS41, 51

. Computations for spheroidal nanoparticles of Ag where carried out

by different groups, and EF of about 105 with maximum at the tips of 10

9 were found

40.

However, it was after the report on single molecule detection that the computations and the

discussion on the upper limit of the enhancement factor for certain nanostructures intensified. In

1999 Xu et al.52

, reported that the minimum aggregation number for effective SMD was the

dimer; a pair of Ag nanoparticles bridged by the target hemoglobin molecule. They found that

the electric-field strength increased by bringing the silver particles close together (from 5.5 nm to

1 nm), and the enhancement factor increases from ~ 106 to ~ 10

10, they called this spatial location

a “hot” site (hotspot). They concluded that an EF=1010

was sufficient for the single molecule

detection observed in their SERS experiments of hemoglobin on silver colloids. In addition to

the concentration of the field in the gap between silver nanoparticles, the fractal nature of

colloidal nanostructures was also investigated (1999 Markel et al.53

). It was found that in close

proximity to the surface local field intensities are heterogeneous, and the variation in field

intensity between the hotspots could lead to local SERS enhancements in excess of 1010

. The

problem has been recently (2012) revisited using FDTD (Finite-Difference Time-Domain) to

study the near-field electromagnetic intensity with similar results54

. An early discussion of the

hotspots can be found in Otto’s report55

. A broad discussion on the subject of enhancement

factors can be found in the book by Le Ru and Etchegoin56

. The theory is based on the

observation that localized surface plasmon resonances are strongly influenced by the presence of

other closely located nanoparticles leading to electromagnetic coupling, where the electric field

may be enhanced by several orders of magnitude with respect to the incident radiation. There

are now quite a few studies of the local field enhancement at hotspots, the gap between to

nanoparticles of different shapes and size. For example, lithographically fabricated Au bowtie

nanoantennas of gap sizes between 16 and 30 nm produce excellent agreement between FDTD

computations and experiment57

with intensities yielding E2 enhancement factors greater than 10

3.

Further experiments and FDTD computations in 2010 58

confirmed that the EF increases with

decreasing gap size with values 2 × 1011

and 7 × 1011

at the smallest gap of ca. 8 nm. These

values seems to be in agreement with the largest enhancements reported for nanoshells and


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nanoparticle aggregates, although larger EF (1014

) have also been reported33

. Recently, the

nanogaps between the gold nanostructures have been reduced using high-resolution electron-

beam lithography fabricating bowties separated by gaps of only 0.5 nm and connected by

conductive bridges as narrow as 3 nm. The good news for computationally supported SERS

work is that the agreement between experiment and simulation demonstrates the validity of

classical electromagnetism even at the single nanometer length scales. The optical constants for

metals can be used for calculations of SERS nanostructures29

, and even the discrepancy observed

between the maximum of near-field enhancements and the maximum of the corresponding far-

field spectrum has been explained within the physics of driven and damped harmonic

oscillator59

.

3. The definition and the challenges in the SERS spectral interpretation.

To confine the discussion and dissect the properties of SERS spectra, let us introduce a working

definition of plasmon enhanced Raman scattering by Moskovits 60

:

"As it is currently understood SERS is primarily a phenomenon associated with

the enhancement of the electromagnetic field surrounding small metal (or other)

objects optically excited near an intense and sharp (high Q), dipolar resonance

such as a surface-plasmon polariton. The enhanced re-radiated dipolar fields

excite the adsorbate, and, if the resulting molecular radiation remains at or near

resonance with the enhancing object, the scattered radiation will again be

enhanced (hence 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."

Figure 1. Surface plasmon

extinction spectra for Ag and

Au nanostructures illustrating

the FWHM of the plasmon,

and the spectral region of a

full vibrational Raman

spectrum. AFM of silver

island film –SIF (inset).

First, in a nutshell, the local

field enhancement locE

caused by the incident laser

line is proportional to a g


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factor16, 61

,

m

m

ε ω -εg=

ε ω +2ε. Here is the dielectric function of the metal in a medium with

dielectric constant m . The g factor has a singularity that occurs at the plasmon resonance

condition =-2 m (the Fröhlich condition). Since, is complex, the g factor determines

which metals can be used, and the spectral region where they would be most efficient enhancers.

In summary, nanoparticles can perform as electric dipoles, resonantly absorbing and scattering

electromagnetic fields. Second, the dipolar resonance property introduces a limitation on the

size, and the effect would be most efficient for metal particles smaller than the wavelength of the

incident light (diminishing the contribution from higher multipoles). Although, the bright modes

are the dipolar LSPR modes, there is also evidence that under certain conditions of symmetry

reduction in enhancing arrays of nanostructures an increase dipole-quadrupole coupling may also

improve optical enhancement62

. The scattering cross section increases with nanoparticle size

helping the enhanced re-radiated scattering; thereby, there would be an optimum size for SERS

nanostructures. Third, the plasmon resonance peaks at 2 0LSPR m , and since the

vibrational frequency is smaller than the full width at half maximum (FWHM) of the plasmon,

both Laser vib Laserg and g can be simultaneously in resonance, as can be seen in Figure

1. Here, a full SERS spectrum recorded in the spectral window of Raman shift between 100 cm-1

to 3000 cm-1

is shown in relation to the FHHM of the plasmon absorption of a silver island film

(SIF) and gold island film. In this spectral region the full Raman spectrum spans over 94 nm,

while the FWHM of the gold plasmon is greater than 100 nm. The enhancement may then be

proportional to 2

L v Lg g or, in other words, the field enhancement will scale as E4, a

reasonably good approximation63

. The beauty of Raman spectra is that they follow the excitation

line, so when you excite with a laser line in resonance with the nanostructure, and the Raman

spectrum is under the umbrella of the plasmon, you can get your average E4 enhancement factor.

Therefore, for average SERS you can fabricate nanostructures of different shapes64, 65

, nanoshells

or nanorods66

with plasmon absorption anywhere in the UV-visible or near infrared, for specific

plasmonic applications. The situation is quite different in the SM regime; you are now dealing

with narrow plasmons of the hot spots, and for the same substrate their spatial location changes

with the frequency of the excitation laser line53

. Fourth, the plasmonic enhancement extend out


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to several nanometers, and, for SERS, is monotonically decreasing with the molecule-

nanoparticle separation d , which for a sphere of radius a is simply

12a

a d

. So the most

profitable enhancement is at 0d . However, enhancing nanoparticles can be coated with an

ultrathin silica or alumina shell where the Raman signal amplification at a distance (for instance,

2 nm) is provided by metal nanoparticles. This unique plasmonic enhancement is now called

SHINERS67

: shell-isolated nanoparticle enhanced Raman scattering. The ultrathin coating

protects the metal nanoparticles, avoiding chemical interactions or photochemical reactions at the

surface, and they can be spread as 'smart dust' over any surface. Similarly, TERS plasmon

technique exhibits not only a high sensitivity due to the strong localization of the optical near-

fields underneath the tip; but also offers a spatial resolution down to the nanometer scale68

. A

recent SM TERS work report an enhancement factor of 1013

; but the unique contribution is in the

separation of the resonance Raman contribution from the plasmonic enhancement35

.

3.1 The SERS spectra.

In the majority of cases, the spectral properties of the species adsorbed onto the metal

nanoparticles may change on account of the “chemical or physical” interactions with the

nanostructure69

, and, correspondingly, the plasmon enhanced SERS spectrum will report not only

on the target molecule; but it will contain information that shed light on these molecule-

nanostructure interactions. Since good Raman scatters are highly polarizable molecules, their

polarizability and, correspondingly, their polarizability derivatives are sensitive to the

environment (intermolecular and molecule-surface interactions) that may change the Raman

band location (wavenumber), the relative intensity, or the full width at half maximum. The SERS

spectrum may be quite different from the reference RS spectrum. The main factors that should be

considered in the interpretation of SERS spectra are.

i) For chemisorbed species, there could be new electronic states. If the excitation

frequency is close to or in-resonance with an electronic state, there is either pre-

resonance RS or resonance Raman scattering (RRS), changing the relative intensities

in the vibrational spectrum and adding absolute intensity to the plasmon

enhancement35

.


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ii) For chemisorbed species, the vibrational spectrum would be that of the complex

formed at the surface, not the original molecule60

,

iii) For a given molecular orientation at the surface, the vibrational intensities follow the

“surface selection rules”16

.

To put it briefly, it is advantageous for the practitioner to separate the plasmonic SERS

enhancement, a property of the plasmonic structure that can be fabricated and calculated

independently, based on the optical properties of the material, from the interpretation of the

SERS spectra observed for any given molecule adsorbed onto the plasmonic substrate. The

molecule-nanostructure interaction may further increase or decrease the estimated plasmonic

enhancement; but, since the source of the enhancement has been identified, it is important to

concentrate the efforts in the spectral interpretation using all the tools of molecular spectroscopy.

Understanding the role of plasmonics helps the practitioners to concentrate on the development

of nanostructures for specific applications.

4. Plasmon enhanced fluorescence

Plasmon enhanced luminescence would be the general chapter that study the enhancement of

emission of radiation from electronically or vibrationally excited species including

fluorescence70, 71

, phosphorescence72

, bioluminescence73

, chemiluminescence74

, electro-

luminescence75

or photoluminescence76

. We restrict the present discussion to fluorescence,

which is the spontaneous emission of radiation from an excited molecular state to a molecular

state of the same spin multiplicity. Theoretically, surface enhanced fluorescence (SEF) results

from two competing processes: the amplification due to the local field and non-radiative decay

due to radiantionless energy transfer. The same physical phenomenon was renamed (in 2002) as

metal enhanced fluorescence (MEF)77

. The theoretical prediction of maximum SEF for emitters

located at a finite distance from the metal nanostructure has been experimentally confirmed

through the years by observations with exactly the same trend in the data obtained by different

research groups using different spacer layers: Wokaun (1983)78

, Aroca (1988)79

,

Novotny(2006)80

, Lakowicz(2006,2012)81, 82

, Halas (2008)83

. After a critical distance, the trend

always mimics the surface average of the near field intensity enhancement, 2

E as calculated

directly using Mie theory. A simple example of the two competing effects in a sphere of radius a

and an emitter at a distance d is shown in Figure 2A, that gives the trend found in all


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experimental work, where the local field distance dependence is given by

6a

a d. For any

dipole emitter, at short distances from a metal surface, there is a giant enhancement of the

nonradiative decay of excitations in such emitters due to Coulomb interaction with electrons in

the metal.

Figure 2. A. Typical trend of the distance dependence observed in all experimental average

plasmon enhanced fluorescence. B. Apparent yield from 0.1 monolayer of basic fuchsin on 4 nm

SIF vs. thickness of SiOx spacer layers (adapted from reference78

). C. SEF distance dependence

function for a gold particle and a vertically oriented molecule (adapted from Novotny et al.80

).

In fact, it has been shown84

that “in a nanometer-scale proximity to metal, this enhancement is

an order of magnitude greater than in the existing theory that treat the metal as dielectric

medium”, i.e., there is a4

1

ddistance dependence. The treatment of the metal as a dielectric

medium as developed by Gersten and Nitzan70

(recently revisited85

), gives the 3

1

d dependence

for the relaxation. The trend shown in Figure 2A is the same for both3 4d and d

. It should be

pointed out that the dipole approximation fails at very short distances80

, and the x-axis in Figure

2A starts at 1. Figure 2B is from the work of Wokaun et al. (1983)78

for Ag island films coated

with ca. 2.5 nm of SiOx. Figure 2C is taken from the work of Novotny et al.80

(2006), where they

investigate SEF from a single molecule as a function of its separation from a spherical gold


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nanoparticle. Therefore, the general trend shown in Figure 2A is observed for silver and gold.

Consequently, and in contrast to SERS, average SEF cannot benefit from the plasmonic

enhancement at the metal surface, and, experimentally, the design of a particular plasmonic

substrate for SEF must include the optimization of the spacer layer, determining the

nanostructure-molecule separation. Fluorescence enhancement factors are significantly lower

than those of SERS. In SEF the enhanced incident field leads to a 2

E dependence, whereas

SERS enhancement derives from the amplification of both the incident and scattered fields with

an approximate 4

E enhancement16

. When both, the SERS and the SEF fall under the width of

the LSPR, the 4

E dependence for SERS and the 2

E for SEF can be experimentally observed86

.

The latter refers to the average or ensemble SEF. Similar to SERS the distinction between

average SEF and SEF at hot spots has been established experimentally87-89

. It means that it is

possible to attain reproducible ensemble SEF enhancement factor for practical applications, and

in addition large EF could be seen at certain locations of the substrate (hot spots). For a brief

discussion of the observations we adopt here the convenient simplified form for SEF introduced

by Halas et al.83

When the molecule is out of the zone of critical enhancement of the nonradiative

decay, the near field intensity enhancement, 2

E is simply multiplied by a factor that accounts

for the decrease in lifetime and an increase in quantum yield: 2

0 0

SEF SEFI QE

I Q , where the

concentration of the fluorophore and excitation intensity are the same for both the quantum yield

of the plasmon enhanced fluorescence SEFQ , and quantum yield of the molecule 0Q . Therefore,

the local field enhancement is multiplied by a factor corresponding to the quantum yield

enhancement (0

SEFQ

Q). The latter factor becomes important only for low quantum yield

molecules, and is close to unity for high quantum yield molecules. The data collected for

average SEF (or SHINEF) are consistent with moderate enhancement factor, as can be seen in

Table 1. However, similar enhancement factors are observed for low and high quantum yield

molecules. Notwithstanding, recent data on hotspot SEF seem to challenge the understanding

coming from the observed ensemble SEF, or even SEF on isolated nanoparticles90

. At a hotspot

resonance, a very high enhancement of the fluorescence has been reported88

(see Table 1), which


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is almost independent of how short is the distance at which the fluorophore is positioned (~ 1-2

nm)89

. Assuming, that the understanding and control of the average SEF is ready for new

analytical developments and diverse applications, the main challenge is coming from the hot

spots and the single molecule regime of SEF (SM SEF). Experimentally, there are new

approaches such as “photoactivated localization microscopy”91, 92

, plasmonic fluorescence

applications93

, and the thrust to improve the resolution of SM SEF microscopy in order to push

the limits toward molecular scale.

Conclusions

The vast body of data accumulated in SERS and SEF and the current understanding of its

plasmonic enhancement permits to use them as a guide for the study of other plasmon

enhancement of optical signals. Perhaps the most important point when planning work in

plasmon enhanced spectroscopy is to realize the fairly different theoretical and experimental

restrictions imposed by the plasmonic nature of the enhancement. One can work with ensemble

average enhancement for qualitative or quantitative analytical applications, or carry out hot spot

dependent work in the single molecule regime. The average plasmon enhanced spectroscopy

will find many more practical applications. For instance, since Raman spectra follow the

excitation laser line, plasmonic substrates active in a certain spectral region can be fabricated for

general use. In addition, the development of specific plasmonic substrate for delicate applications

will also continue to grow, particularly in the area of substrate functionalization. The hotspot

plasmon enhancement is pushing the development of new smart instrumentation combining

optical and electronic microscopies and the nanostructure fabrication with specific shapes and

spatial arrangement. The synthesis of nanostructures is a fast growing branch of chemistry.

Acknowledgments

Financial assistance from the Natural Science and Engineering Research Council of Canada

(NSERC) is gratefully acknowledged.


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Table 1. Selected enhancement factors in plasmon enhanced fluorescence.

Molecule Quantum

yield

Average

SEF EF

Hot spot Substrate

Indocyanine green (ICG) 0.012 2970 4.5 x106 Nanoantenna array88 Indocyanine green (ICG) 0.012 50 NA Nanoshell94

IR800 : (dimethyl

{4-[1,5,5-Tris(4-dimethylaminophenyl)-2,4-

pentadienylidene]-

2, 5-cyclohexadien-1-ylidene} ammonium perchlorate)

0.03 70 NA Nanoantenna array88

TPQDI: N,N0-bis(2,6-diisopropylphenyl)-

1,6,11,16-tetra-[4-(1,1,3,3-tetramethylbutyl)phenoxy]quaterrylene- 3,4:13,14-bis(dicarboximide)

0.025 1340 Bowtie87

Terrylene ~1 20 Au nanoparticle90

Basic fuchsin 0.02 200 SIF78

CY3-labeled DNA 0.08 37 Ag colloids89

R6G-labeled DNA 0.17 17 Ag colloids89

Atto 540Q-labeled DNA 1.6x10-3

740 Ag colloids89

Cyanine 5 (Cy5) 0.28 20 Ag nanoparticle95

Cy5 Tag DNA 0.28 300-103

Ag nanoshell96

Octadecyl rhodamine B (R18) ~ 0.5 94 SHINEF86

Perylene 0.94 50 SIF97

Perylene-3,4,9,10-tetracarboxylic Acid Diimides ~ 0.98 50-400 SIF98

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