Hi all,
I'm new to SERS, but have a nice fiber optic sensor that uses visible light on gold nanopaticle monolayers to measure biomolecular interactions. I am very interested in measuring SERS signals with the setup, but must admit I'm new to the field. I have a few questions to should be easy to answer for anyone with SERS experience, but aren't so obvious to me from some of my readings. Thanks in advance.
1. Instead of using a green or red laser, would it make a difference if measured SERS from a concentrated white light source (lets say in principle, ignoring power issues).
2. In regard to power, I know that nanoparticles enhance SERS signals a great deal, so that comparatively weak light sources should still induce the scattering processes. Does anyone have any feel for the numbers at work here? A friend of mine said he only uses a mW laser. I have a 1 Watt LED that gets focused on an optical fiber endface. I'd expect that maybe 10-20% of the power interacts with the sample, but it's hard to tell. Can anyone give me a ballpark estimate here for the power ranges needed to get SERS signals measurable from, say, a benchtop Raman spectrometer.
3. I know that various nanoparticle morphologies and compositions have different Raman enhancement factors. I was thinking of looking at gold nanorods, since we're pretty sold on the gold, and I know the rods can be designed for SERS. Are there any other particles (eg silver nanoprisms) that you'd recommend over others for SERS consideration? How much better, for example, are silver nanorods?
And also, if I tried to capture raman signal through light transmitted through a 1m silica fiber, is there going to be huge loss from this? Or in other words, how much background raman signal would I expect from light going through a 1m glass fiber vs. signal from a strong collection of nanoparticles? What other fibers would you recommend?
I will try to give an answer to all of your questions:
1) SERS is the acronym of Surface Enhanced Raman Scattering. Monochromatic sources are required for the detection of frequency shifted photons (with respect to the exciting wavelength) as a consequence of scattering processes with vibrational states of the sample. Raman shifts are usually small compared to the exciting wavelength so that laser sources are employed. Even using monochromatic sources, before the introduction of notch ad super-notch filters, rejection of the elastically scattered radiation were accomplished by double monochromators. In this regards white light excitation cannot be used in SERS;
2) In SERS energy density is a better parameter than power itself. Usually micro-Raman spectrometers (i.e. Raman spectrometers coupled with a optical microscope) are employed, so that you can use different microscope objectives: 10x, 50x 100x; each will give you different energy density (and different illuminated areas). In case of organic compounds low energy density are required in order to avoid degradation of the molecules, but SERS ,usually, will be still visible;
3) Different metals and different morphologies result in different optical properties. In particular the key parameters of a SERS substrate is the plasmon resonance peak position and width. The coupling of the substrate plasmon resonance peak with the wavelength of the exciting source is critical: Ag isolated nanoparticle present a plasmon near 400 nm, while Au ones at about 560 nm , as a consequence, concerning Au,the 532nm ( Nd:YAG laser) exciting wavelength is better (or in resonance....) than the 632nm (He-Ne laser ) (out of resonance...). The enhancement factor thus will be dependent on both (plasmon peak position and exciting wavelength) rather than on the nature of the substrate alone.
I hope this will help you.
Interesting! Did your LED-SERS setup work?
When CV Raman discovered anti-Stokes shifted scattering, he also used white light passed through a filter. Back then, he had to use pure liquid samples to be able to observe the scattering. How did yours go?
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