Q&A with Prof Karen Faulds - MSG Online Seminar Series - August 2020

Post date: Aug 30, 2020 9:7:15 AM

For our August blog entry we have dedicated a section to the questions that were not answered during our August 2020 Online Seminar.

Prof Karen Faulds has kindly provided the answers to the questions directed to them concerning their talk on

‘Multiplexed and Sensitive Bioanalysis using SERS and SESORS’

Question 1:

What is the difference between SORS and SESORS?

Prof Karen Faulds:

It is the combination of SORS and SERS. In SORS the intrinsic Raman scattering is being measured at depth. In SESORS nanoparticles are used to enhance the Raman scattering giving a much stronger signal. These nanoparticles could then (one day!) perhaps be used to target e.g. cancer cells at depth in tissue. The SESORS and nanoparticle enhancement allows signals to be detected at deeper depths than can be achieved by Raman alone.

--

Question 2:

The dye with an absorbance max of 676 nm also has a pretty high intensity when excited with 830 nm laser - that looks a bit too far away for pre-resonance (?), so why the strong signal?

Prof Karen Faulds:

Good spot! These chalcogen dyes are really strong Raman reporters even without the resonance contribution. They bind really well to the metal surface and are very symmetrical and well orientated for SERS. There is perhaps also some pre-resonance contribution in there and the gold nanoparticles used will be closer in resonance to the 676 nm than the 830 nm so this may also contribute to the enhancement seen for the lower absorbing dye. That is my hand waving answer as I think there are a few potential effects at play!

--

Question 3:

In the bacteria work you have a strong non linear response to the NP. Was this due to simply saturation of the NPs or some other fundamental effect?

Prof Karen Faulds:

Again, good question! It is very likely saturation, If you use even higher CFU the signal starts to plateau. It is unlikely to be a linear trend because it is not a 1 to 1 binding i.e. 1 (or even x) nanoparticles binding to each bacteria. Also, at high bacteria concentrations then there will be a lot of nanoparticles binding to the bacteria so a lot of nanoparticles in the sampling volume. If there are too many nanoparticles in the sampling volume then you get self absorbance of the Raman scattered light by the nanoparticles so a dampening of the scattered light which leads to a plateauing (or saturation) of the signal. Basically a reduced amount of the scattered light is getting back to the detector as it has been absorbed by the high number of nanoparticles in the system bound to the high number of bacteria. This is a good point to note for nanoparticle based SERS in general, too many nanoparticles in the sample volume can often dampen the signal due to self absorbance of the scattered light, so sometimes diluting them will increase the SERS signal so nanoparticle concentrations in measurements and assays should be optimised.