Surface Enhanced Raman Spectroscopy

Analytical, Biophysical and Life Science Applications

Sebastian Schlücker

• Publisher: Wiley-VCH
• Year: 2011
• Print ISBN:
  9783527325672
• eBook ISBN:
  9783527633067
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Covering everything from the basic theoretical and practical knowledge to new exciting developments in the field with a focus on analytical
and life science applications, this monograph shows how to apply surface-enhanced Raman scattering (SERS) for solving real world problems.

From the contents:

* Theory and practice of SERS
* Analytical applications
* SERS combined with other analytical techniques
* Biophysical applications
* Life science applications including various microscopies

Aimed at analytical, surface and medicinal chemists, spectroscopists, biophysicists and materials scientists. Includes a Foreword by the renowned Raman spectroscopist Professor Wolfgang Kiefer, the former Editor-in-Chief of the Journal of Raman Spectroscopy.

1

Basic Electromagnetic Theory of SERS

Pablo G. Etchegoin and Eric C. Le Ru

1.1 Introduction

This chapter is aimed at introducing the newcomer to the field of surface-enhanced Raman spectroscopy (SERS), and is not intended to supplant the already available exhaustive literature in the field either in the form of review articles [1, 2] or books [3, 4]. As a technique, SERS is relatively exposed to the dangers of specialization due to its (intrinsic) multidisciplinary nature. The technique is becoming widespread and is finding new and exciting horizons in analytical chemistry [5–7], biology and biotechnology [8–12], forensic science [13, 14] and in the study of artistic objects [15–17]. While this is in many ways an advantage, it is also a handicap in the sense that scientists approaching the technique from a more ‘biological’ or ‘applied’ aspect might not have the appropriate background (or predisposition) to venture into the depths of electromagnetic theory and to understand the basic concepts of the theory of plasmon resonances in metallic nanostructures. This could be particularly true for students in the biotechnology field, who might find it desirable to have access to the elementary concepts (with a bare minimum of mathematics) but with enough insight to understand what they are actually doing in the lab. We believe that the success and use of the technique — in an environment which is by nature multidisciplinary — will be more effective if accessible presentations of the basic principles aimed at broader audiences are available at all times (and reviewed over prudent periods of time). This chapter (hopefully) fulfils part of that requirement.

This chapter is organized as follows: in Section 1.2, we introduce the basic principles of plasmon resonances and their associated field enhancements. Section 1.3, on the other hand, looks at the field enhancement distribution and localization produced by these plasmon resonances, while Sections 1.4 and 1.5 study the origin of the enhancement factor (EF) and its characteristic magnitude. Finally, Section 1.6 presents some conclusions and summarizes several main concepts.

1.2 Plasmon Resonances and Field Enhancements

1.2.1 Optical Properties of Simple Metals

None of the modern optical techniques such as surface-enhanced fluorescence (SEF) [18–20], surface plasmon resonance spectroscopy [21–23] or SERS itself [1, 4] would exist without the particular optical properties of coinage metals (with silver (Ag) and gold (Au) standing out as the most useful ones). The first obvious question is then what is it that makes the optical properties of metals so interesting? Hence, it is worth spending a few paragraphs on the topic of the optical properties of bulk metals such as Ag and Au to understand why they are so interesting, and why we use them in the aforementioned techniques.

The optical properties of bulk materials are characterized by their dielectric function

(ω). Most students from scientific disciplines would have come across the related index of refraction n(ω), which is linked to the former by

. Both n(ω) and

(ω) depend on the frequency (ω) of the light (with ω = 2πc/λ, where c is the speed of light and λ the wavelength), due to the fact that most materials respond differently to electromagnetic waves at different frequencies (wavelengths). The dielectric function can therefore be considered indistinctly as either a function of ω (

(ω)) or λ (

(λ)). We shall use one or the other according to convenience. In the most elementary treatments of the optics of material objects (lenses, prisms, etc.) [24], both the dielectric function and the index of refraction are positive real numbers (more precisely

, n ≥ 1). More often than not, however, the dielectric function of materials at a given wavelength will be a complex (rather than real) number, and the material will not be transparent. In fact, this is more the rule than the exception, since the list of transparent materials constitutes a really small fraction of the materials we see around us. Metals are amongst the list of materials in which

(ω) is complex. The ultimate reason for the optical properties of materials is their electronic structure, and this is a canonical topic in solid-state theory [25, 26]. We shall not dwell too much on the details of the connection between the dielectric function of metals and their electronic structure (see Appendix D of Ref. [4] for a slightly more in-depth discussion), but rather take the properties of

(ω) of metals as given.

Figure 1.1 shows the dielectric functions of Ag and Au with their real and imaginary parts spanning from the near-UV (~300 nm) to the near-IR (NIR) range (~900 nm). These are analytical representations that interpolate rather well a collection of experimental results for

(λ) obtained with different techniques. The accuracy and limitations of these fits are discussed in more detail in Refs. [4, 27]; here, we shall take these results as the starting point of our discussion on why the optical properties of metals are interesting. The main characteristics of the real and imaginary parts of the bulk

(λ) for both metals can be summarized as follows:

• The real part of the dielectric function of both metals, for most of the visible range, is both large (in magnitude) and negative. Later, this will turn out to be one of the most important properties of these metals as far as their optical properties are concerned, and one of the main reasons for their usefulness as plasmonic materials. Furthermore, ignoring the imaginary parts of
  
  (λ) momentarily, we can claim that the real parts follow at long wavelengths one of the simplest models for the dielectric function of a (lossless) metal, which is the lossless Drude model. The latter predicts a dielectric function of the form [4, 25, 26]:

1.1

where ωp = 2πc/λp is the so-called plasma frequency1 of the metal (proportional to the square root of the density of free electrons in it). The first expression on the right-hand side in Equation (1.1) holds if we want to express the dielectric function

as a function of ω, while the last expression holds if

is expressed as a function of λ( = 2πc/ω). Figure 1.1a reveals that both Ag and Au have actually very similar electronic densities, since the real parts of their dielectric functions are not too far away from each other. This is the approximate quadratic downturn of the real part of

(λ) seen in Figure 1.1a for longer wavelengths. We can see that, to a good approximation, the simplest lossless Drude model describes already a good fraction of the experimental results for the real parts seen in Figure 1.1a.

• Real bulk metals are not lossless, and this is where the imaginary part of
  
  (λ) comes into play. Even though when the Im[
  
  (λ)] for both metals are smaller than their real counterparts for most of the visible range, their effects are important and — in some cases — crucial. The imaginary part is always related to the absorption of the material (a material with Im[
  
  (λ)] = 0 does not absorb light, and has a real index of refraction
  
  ). It turns out that the imaginary part of
  
  (λ) for Ag can be obtained by a relatively easy generalization of the lossless Drude model (Equation 1.1). For Au, the situation is slightly more complicated;
  
  (λ) has additional contributions (in addition to that from the free electrons) from other electronic transitions in its electronic band structure [27]. This is the reason for the relatively higher absorption of Au (with respect to Ag) for λ ≤ 600 nm, with a ‘double hump’ structure in the imaginary part (~400 nm), which comes from the so-called interband electronic transitions. Note, however, that for λ ≥ 600 nm, the imaginary parts of
  
  (λ) for both Ag and Au become completely comparable (Figure 1.1b) and — with their real parts being comparable too in this range — both materials are similar (from the viewpoint of their electromagnetic response). Their surface chemistries are of course different, and one material might be preferred over the other for specific chemical reasons. But, as far as the electromagnetic response is concerned, Au is comparable to Ag in the near- and far-IR range.

Figure 1...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Foreword
• Preface
• List of Contributors
• Chapter 1: Basic Electromagnetic Theory of SERS
• Chapter 2: Nanoparticle SERS Substrates
• Chapter 3: Quantitative SERS Methods
• Chapter 4: Single-Molecule- and Trace Detection by SERS
• Chapter 5: Detection of Persistent Organic Pollutants by Using SERS Sensors Based on Organically Functionalized Ag Nanoparticles
• Chapter 6: SERS and Pharmaceuticals
• Chapter 7: SERS and Separation Science
• Chapter 8: SERS and Microfluidics
• Chapter 9: Electrochemical SERS and its Application in Analytical, Biophysical and Life Science
• Chapter 10: Electron Transfer of Proteins at Membrane Models
• Chapter 11: Quantitative DNA Analysis Using Surface-Enhanced Resonance Raman Scattering
• Chapter 12: SERS Microscopy: Nanoparticle Probes and Biomedical Applications
• Chapter 13: 1-P and 2-P Excited SERS as Intracellular Probe
• Chapter 14: Surface- and Tip-Enhanced CARS
• Index