Lecture 8 - SERRS and more Raman

Question 1

What is SERRS?

Answer 1

• Surface enhanced resonance Raman scattering, combo of SERS and resonance Raman.
• Need to use a dye as analyte to get both effects
• This can have a combined effect of increased enhancement by 10E12!

Question 2

What has SERRS been used in?

Answer 2

• DNA detection
• Covid tests (gold nanoparticles which are red are covered in antibodies or virus molecule which produces a red line when positive)
  SERRS laser can be shown on the red line to detect covid levels, even at low levels, to see how much is present.
• Used in HCG pregnancy hormones

Question 3

What is the modern Raman instrumentation?

Answer 3

• Laser at desired wavelength
• A shutter which decides when and for how long a sample is exposed to laser
• Band pass filter which only one wavelength from laser to reach sample
• Notch filter which absorbs all other light frequencies from incident and Rayleigh scattering to only allow stokes through to detector.
• Spectrograph uses gratings and prisms to split Raman scattering into its different wavelengths
• CCD detector measures intensity at each wl and constructs spectra.

Question 4

What is the DISPERSIVE Raman instrumentation?

Answer 4

• Light enters prism and broke up into diff freq
• Detected by focusing onto device sensitive to light
• Uses CCD for detection when visible laser used but cant be used above 800nm so Raman nearing IR wavelengths are done by Fourier transform (usually 1064 nm)
• The detector has different parts which read diff wavelengths
• Read in terms of no of photons vs Raman shift.

Question 5

What is FOURIER TRASFORM Raman intrumentation?

Answer 5

• Light of freq’s split into 2 different paths to create inference pattern
• The total intensity id measured vs distance travelled in a time distance
• FT converts the time/distance domain to freq.
• A single detector used for all photons of all Raman shifts.
• Spectra then done by mathematical FT of detector output - interferogram

Question 6

What is an interferogram?

Answer 6

• Measures total intensity vs distance moved by mirror then converts from time/distance domain to freq domain.

Question 7

When is FT Raman used?

Answer 7

• With NIR excitation - 1064 nm
• 1064 nm high wl - low intensity low energy to peaks are weak but no fluorescence
• Uses InGaAs detector which is cooled with liquid N.

Question 8

How is mapping and imaging done?

Answer 8

• A filter is put on the laser to cut out other wl except one for a desired vibration and uses a detector (camera) allows image
  sampling.
• A microscope and stage which can be auto moved can also be used to give series of spectra and create a map.
• This can be used to look and surfaces of medicine.

Question 9

Differences between imaging and mapping?

Answer 9

• Imaging uses a monochromator. Also uses a filter only allowing one vibration to hit detector. Detector is like a camera and takes pic of surface. This gives an intensity image corresponding to the intensity of the one vibration.
• Mapping requires a monochromator. The whole spectrum is took at each spot on the surface. Needs removable stage. Take a sample, move it, then retake.

Question 10

What are the ad/disad of mapping vs imaging?

Answer 10

• Imaging is rapid but only get info on one part of the spectrum.
• Mapping can take several hours but the whole sample can be done so ID of material can be done.

Question 11

How can bands be assigned for larger molecules?

Answer 11

• Can use 3N-6/5
• Hooke’s law
• Can use a database
• Can use density function calculations

Question 12

What is a density function calculation?

Answer 12

• Can predict vibrational frequencies for large molecules
• Assumes the molecule is isolated in a gas phase.
• Use of computers make this popular as can be a lot faster

Question 13

What is the increasing strength of all Raman techniques?

Answer 13

SERRS (10E12) > SERS (10E6) > Resonance Raman (10E4) > Raman