I Love the Smell of Results in the Morning!

Things are looking up in p-(dimethyl)amino cinnamaldehyde (pDMAC) land. Over the last couple summers one of the students I’ve been working with (Diane, that’s you!) has collected enough SERS and Raman data to begin putting together a manuscript! I’ve been working on getting some more spectra over the last couple of weeks, as well as finishing up some calculations using Gaussian 03W.

One question that comes up frequently about our spectra is if we are seeing the cis or trans isomer of pDMAC. As we dug around looking for published spectra and frequencies, we (quite happily) discovered that there aren’t any! So this means that we can write up a nice paper about it. To strengthen it, and to help us assign the vibrations, we decided to do some Gaussian calculations on the optimized geometries (which tell us that trans is more stable than cis) and predict vibrational frequencies and the Raman spectrum. Below the fold are the experimental (normal Raman, solid pDMAC) and theoretical Raman spectra (isolated molecules in gas phase, wavenumbers scaled by 0.9806).

Lysozyme SERS – Different Spectra in Different Pubs?

Our major research goal this summer is to find out what conditions are necessary for reliable SERS of proteins. Lots of people are interested in this for many different reasons. Ours are to further collaborations with some colleagues in the biology department who are interested in finding out some structural information about proteins they are studying. Hopefully as we work on this we’ll be able to get some general information about what one needs to think about in doing SERS with proteins.

We’ve started by trying to reproduce literature results. You’d think that would be easy, but it isn’t turning out to be the case. As an example, consider lysozyme. I have four published reports of lysozyme SERS, and all four have markedly different SERS spectra. Each is reviewed in chronological order below the fold.

Formation of HCl – One Water at a Time

At some point, all students who take General Chemistry learn that acids are species that donate protons in water. For example, hydrochloric acid is formed when hydrogen chloride gas is dissolved in water:

Formation of the hydronium ion (H₃O⁺) results in a decrease of the pH of the solution. Yeah, if you’ve had General Chemistry, you definitely know that. But did you know that until recently, we didn’t know how many water molecules it took to make this happen?

What Happened in Lab: 6/15 – 6/16, 2009

Dylan is getting closer (we hope) to figuring out some of the conditions necessary for SERS of lysozyme and bovine serum albumin (BSA). On Monday we figured out that a lot of what we were seeing in our spectra were bands from the phosphate buffer [1]. That’s what happens when you do control experiments! Since then, we decided to stop using the buffer and use DI water as our solvent.

Now Dylan has several (ok, lots) of spectra with broad bands that match up with the solid state lysozyme Raman spectrum we have (which matches up exactly with all the published spectra except one!). We aren’t sure why the bands are as broad as they are. One hypothesis is that our colloids have broad size distribution and we are observing a bulk average of many different SERS active sites. We plan to make more colloids and aim for a narrower extinction spectrum, which would indicate a more monodisperse colloid. Failing that, we may try core-shell nanoparticles, starting with an Au seed.

Dylan has also tried making Ag colloids using borohydride as the reducing agent [2]. The initial results looked good, but after storing them they aggregated. With colloids, cleanliness is very important.

[1] I think this is also in some published spectra too, not necessarily assigned correctly. More on that later.
[2] This is one of the several “Lee and Meisel” preps. Moral – if a paper says “Lee and Meisel method”, it could be one of up to four different preps for silver colloids. Reviewers need to tell authors to be more specific.

Success?

We can obtain this spectrum pretty consistently.  Oddly enough, we didn’t acidify the MgSO₄ the way Han, et al. describe. We’ll see how this compares to some literature spectra (Hu, et al., Spectrochimica Acta A, vol 51, pgs. 1087-1096, 1995) obtained with borohydride reduced Ag colloids (ours are citrate reduced).

Edit – 6/11/09
It turns out that this is a pretty good spectrum of the plastic microwell plate. Doh!

So How Does This Work?

We are currently trying to replicate experimental results from a paper by Han, Huang, Zhao and Ozaki (Anal. Chem. 81 3329-3333 (2009)). This seemed like a great place to start trying to understand how to consistently obtain SERS spectra of proteins. Naturally, it isn’t as simple as one might think.

Greatest Website Ever?

Laserfest.org.  ‘Nuff said!

SERS – A Unified Approach

Last wee was Thanksgiving break, and I enjoyed a week off from teaching.  One of the things I had time to do is sit down and read an article that was published in the Journal of Physical Chemistry C last March, “A Unified Approach to Surface-Enhanced Raman Spectroscopy” (Lombardi and Birke, Volume 112, pgs 5605-5617, DOI 10.1021/jp800167v).

Lombardi and Birke’s article pulls together the many different explanations for the large Raman enhancements observed for molecules adsorbed to coinage metal surfaces with atomic scale roughness features – surface-Enhanced Raman scattering (SERS).  They argue that it is important to consider all of the resonances in such systems, which include

1. Surface plasmon resonances
2. Molecular state resonances
3. Charge transfer resonances

After laying out the theoretical expressions that govern the magnitued of the polarizability tensor in these systems, they run through a series of examples that illustrate the theory.  The theory is consistent with SERS of molecules on any type of nanoparticle or electrode surface, and for any type of excitation situation (like where the laser frequency is far from molecular and charge transfer resonances, to where the laser’s frequency is resonant).  If you’re interested in learning more, read on as I summarize.  Even better, get a copy of the original article and read it!

Great Titles

I haven’t had a chance to read this article yet, but the title is awesome:

Noninvasive Raman Spectroscopy in Living Mice for Evaluation of Tumor Targeting with Carbon Nanotubes

Lasers, mice, nanotubes…  I’ll have to read it now.