Biomedical Engineering Reference
In-Depth Information
Silver and gold plasmons oscillate at frequencies in the visible region and
therefore, they are suitable for use with the visible and NIR laser systems
commonly used in Raman scattering.
For analytical applications, it is important to distinguish two types
of SERS signals when using colloidal nanoparticles [59]. First, the “aver-
age SERS” enhanced spectra, [60] i.e. the SERS spectrum of a given ana-
lyte obtained from an ensemble of colloidal particles and aggregates and
characterized by a stable intensity pattern, with well-dei ned and reproduc-
ible frequencies and bandwidths. Second, SERS intensities obtained from
silver or gold nanostructures sustaining a “hot spot” (large enhancement
factors), which permits the detection of a few molecules with l uctuating
spectral characteristics [61]. Although large enhancement factors have been
reported, attempts are still underway to improve the SERS signals by using
metal nanoparticles, aligned carbon nanotubes etc. along with the theoreti-
cal ef orts to understand the huge enhancements. SERS can mainly take
place either due to chemical, electromagnetic or combined ef ect [62-63].
Speckled SiO
2
@Au core-shell particles with speckled particle shells i.e. shells
constituting the nanoparticles should be ideal for enhancing the Raman
signal as there would be natural surface roughness due to small particles,
increasing the area for molecular adsorption and the nanogaps between the
particles to enhance the electric i eld between the particles.
Here the Raman enhancement due to speckled nanoparticle shells
in case of Crystal Violet (CV) molecules as well as Single Wall Carbon
Nanotubes (SWNT) is demonstrated. CV molecules also known as methyl
violet are known since nineteenth century. h ey are water soluble and their
18.0k
1590
λ
Laser
= 532 nm
16.0k
14.0k
12.0k
10.0k
8.0k
6.0k
4.0k
2.0k
0.0
500
1350
B
A
1000
1500
2000
Raman shift (cm
-1
)
Figure 4.13
Raman spectra of Single Wall Carbon Nanotubes (SWNT) without (A) and
with (B) SiO
2
@Au particles.
