1. Surface-Enhanced Raman Scattering (SERS)
m = aE
Enhancement
Factor (EF) =
sSERS
sRaman
Chemical
Electromagnetic
Raman Scattering
(w0 –wvib)
Laser Excitation
(w0)
Haynes, McFarland, and Van Duyne, Anal. Chem.,77, 338A-346A (2005).

2. SERS Enhancement Mechanisms
Chemical Mechanism:
Laser excites (a) new electronic states arising from
chemisorption or (b) shifted or broadened adsorbate
electronic states yielding a resonance condition.
Short range (1-5 Å)
No roughness requirement
Contributes EF ~ 102 – 104
Electromagnetic Mechanism:
LSPR induces large electromagnetic fields at roughened
metal surface where molecules are adsorbed.
Long range (2-4 nm)
Affected by all factors determining LSPR
Contributes EF > 104

3. Localized Surface Plasmon Resonance
Non-resonant
Resonant
Resonant l is absorbed
EM fields localized at nanoparticle surface

4. Noble Metal Nanoparticles
Metal Salt
Reducing Agent
Capping Agent
Metal Colloids
coated with
Reducing Agent/Capping Agent
heat or
sonication hn
Aged

5. Noble Metal Nanoparticles
D = 24.8 ± 4.1 nm
50 nm

6. Nanostructured Substrates

7. Localized Surface Plasmon Resonance
The resonance results in (1) wavelength-selective extinction and (2) enhanced EM fields at the surface.
Spectral location of the LSPR is dependent upon particle size, shape, composition, and dielectric environment.

8. Field Enhancement by Metallic Spheres #1
When a metallic sphere is subject to an external electric field, E0, the field outside the sphere is given by the following equation:
where the polarizability of the sphere is
where a is the sphere radius and
The condition for achieving the greatest external field strength occurs when the denominator in the above equation is large.

9. Beyond Metallic Spheres
For spheroidal particles, the expression for g becomes:
where is a variable that is calculated based on the aspect ratio of the particle. The table below lists for a few different aspect ratios.
Aspect Ratio
2:1
4.76
3:1
8.20
4:1
12.26
5:1
16.91

10. Spectroscopic Consequences of Field Enhancement
The SERS enhancement is determined by g in the following manner:
The magnitude of the extinction and scattering cross sections are also related to g :

11. Resonant Rayleigh Scattering
Light that is elastically scattered due to the LSPR can be used as an in situ monitor of nanoparticle optical properties.
20 mm
The primary benefit of scattering spectroscopy is that the single-to-noise ratio is much higher than extinction spectroscopy when examining single nanoparticles.

12. LSPR Dielectric Response
450
500
550
600
650
700 20 40 60 80
100
Wavelength (nm) I n t e s i y A
510.2
574.2
588.0
600.8
611.9 1
1.2
1.4
1.6
480
520
560
640
Refractive Index l m a x ( ) B
max
= 203.1*RI
Blue = N2 (1.000), Green = Methanol (1.329), Red = 1-Propanol (1.385), Purple = Chloroform (1.446), Orange = Benzene (1.501)

13. Single Nanoparticle Sensing – Proof of Concept
Wide-field Image

14. Biosensing with Single Nanoparticles
Streptavidin:
60kDa Tetrameric Protein
~4 nm x 4 nm x 5nm
Biotin:
Vitamin H
Ka = 1014 M-1

15. Biosensing with Single Nanoparticles
Monitoring Biotin-Streptavidin Binding with Single Nanoparticles
1200
508.0
520.7
800 y t i s n e
Dlmax = nm
Conc = 10 nM SA
~700 SA molecules t n I
400
450
500
550
600
Wavelength (nm)

16. Comparison of Unenhanced and Enhanced Raman Spectra

17. Relationship between the LSPR and Laser Wavelength
lmax = 690 nm
1575 cm-1 band
N = 13 points
Range = 475 – 700 nm
High = 662 nm
EF = 1.9 x 107

18. Locating Landmines with SERS
Roughened Au
lex = 785 nm
Sylvia, J. M. et al. Anal. Chem. 2000, 72,5834.

19. Locating Landmines with SERS
115 mW
30 sec
TNT
DNT
DNB
Sylvia, J. M. et al. Anal. Chem. 2000, 72,5834.

20. Are you getting the concept?
Why hasn’t surface-enhanced Raman replaced normal
Raman completely? In other words, why would someone
do a normal Raman scattering experiment?

21. Partition Layers to Detect Non-Traditional Analytes with SERS – Kyle Bantz

22. Partitioning PCBs

23. Are you getting the concept?
Kyle fabricates a new Ag SERS substrate and wants to calculate the EF. She doses the substrate with benzenethiol (packing density = 6.8 x 1014 molecules/cm2), and measures the spectrum with a laser spot size of 1.26 mm2. The intensity of the 1000 cm-1 shift ring breathing stretch is
35793 adus. For her standard, she puts undiluted benzenethiol (r = g/cm3) into a cuvette with a known probe volume of mL and measures a 1000 cm-1 shift band intensity of adus. What is the EF (assuming that the laser power and collection time were the
same for both measurements)?

24. Single Molecule SERS EF= 1014-1015 !
S. Nie, et. al. Science 1997, 275,
K. Kneipp, et. al. Phys. Rev. Lett. 1997, 78,

25. Hyper Raman and Surface-Enhanced Hyper Raman Spectroscopy
With focused, pulsed laser, you can induce a non-
linear interaction:
Incident: n0
Scattered: 2n0, 2n0 ± n1
Selection Rules:
All IR active modes are also hyper-Raman active
Some hyper-Raman active modes are neither IR
or Raman active

26. Coherent Anti-Stokes Raman
Phase-match wp and ws in a four wave mixing process:
Incident: np, ns
Scattered: 2np - ns
ICARS ~ Ip2Is
Large signal when:
np-ns = D D
CARS Advantage:
The nAS signal beam is spatially and temporally removed
from the fluorescence signal.