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Surface-Enhanced Raman Scattering


Raman scattering is a weak effect. If analyte molecules are adsorbed on a plasmonic metal particle surface, the Raman scattering signal can be enormously enhanced. The effect is then referred to as surface-enhanced Raman scattering (SERS). In extreme cases the enhancement is so strong that even single molecule detection becomes possible.

The SERS enhancement mechanism is actually quite complicated. From a theoretical point of view, the total SERS enhancement involves (i) chemical enhancement, (ii) resonance Raman enhancement, (iii) charge-transfer resonance enhancement, and (iv) plasmon resonance enhancement processes [1]. The last contribution is believed to be the largest to the observed SERS signal, and it is often referred to as the electromagnetic (EM) enhancement mechanism.

The EM enhancement

The largest EM enhancement occurs near interacting metal nanoparticles, for example silver or gold. If metal particles are illuminated with light that is in resonance with the particle LSPR frequency, the EM enhancement is particularly strong. The nanoparticle LSPR is generally a function of the particle size, shape and surrounding medium. If there are other nanoparticles nearby, inter-particle coupling effects influence tremendously the LSPR position (“hot spots”).

Consider a simplified electromagnetic enhancement model for SERS in Figure 1. Using the dipolar picture, the incident field can simultaneously excite dipole fields in both particle and molecule. Hence we have two dipole field sources in the system. Since a molecule is in a close proximity to a metal particle, we should expect some kind of coupling between the two dipole fields. It is easier to view this as two processes happening simultaneously.

Electromagnetic Enhancement Model
Figure 1. Schematic illustration of the electromagnetic SERS enhancement using the two process mechanism. Both metal particle and a molecule are represented as point dipoles with different polarizabilities. For a more detailed description please refer to the text.

First, we start with a metal particle. The incident electromagnetic field, Equation 2 , excites the particle LSP inducing oscillating dipole Equation 3 , where Equation 4  is the particle polarizability that generally is a tensor. The induced particle polarization generates large local fields Equation 5 , i.e. the incident field is enhanced yielding Equation 6 . The total field excites the target molecule that emits Raman scattering. The Raman intensity is then Equation 7 .

Second, we start with a molecule. The emitted field from a molecule, Equation 8 , is also enhanced by a metal particle yielding Equation 9 . In other words, the emitted field from the molecule at a certain frequency Equation 10  is again enhanced by the particle LSP. The Raman intensity is then Equation 11 . Hence, the Raman signal is enhanced by Equation 12  in each step.

There is another way to derive the SERS electromagnetic enhancement factor, Equation 13 , using the coupled dipole approach.

One can define the dipole moments of a particle and a molecule as follows:

Equation 14
Equation 15  [Eq. 1]

where Equation 16  is the incident field, Equation 4  and Equation 17  are the particle and molecule polarizabilities, respectively. Since the molecule is at the vicinity of the particle surface, we add the particle dipole field (Equation 18 ) to the incident field when calculating the dipole moment of the molecule (Equation 19  is a constant). In the same way we add the dipole field from the molecule (Equation 20 ) to the incident field when defining the dipole moment for the particle.

The coupled dipole system becomes:

Equation 21
Equation 22  [Eq. 2]

Solving the system yields

Equation 23
Equation 24  [Eq. 3]


Near the particle LSPR Equation 25  and the effective particle polarizability Equation 26  will dominate the system. The polarizability is modified and is Raman active because there is a contribution from Equation 17 . Remembering that the Raman scattering intensity scales as Equation 27 , the SERS intensity enhancement becomes

Equation 1 [Eq. 4]

Therefore, the SERS intensity enhancement scales as the 4th power of the ratio between the field in the vicinity of the nanoparticle and the incident field.