You are currently browsing the monthly archive for August 2007.
Here’s a little something for your Friday – Ask a Ninja interviews Will Ferrel and Jon Heder.
Introduction
The electron shell model for the atom is relatively modern; the model wasn’t possible until the advent of quantum mechanics in the 1930’s. The purpose of this post is to provide some historical background beyond what you’ll find in your textbook. Read the rest of this entry »
One of my colloquium students sent me this. Enjoy.
Rutherford’s experiments show that atoms aren’t solid. So why don’t we just fall through chairs when we sit in them?
They’re baaack! The fall semester starts on Wednesday, so right now everyone is busy everywhere but the lab. Diane, Evan, and Rachel each did great work this summer. Evan and Rachel are going to continue during the academic year, so you’ll be hearing more from them this fall. Hopefully I’ll be able to convince some students to enroll in our research methods course, so I’ll add some more laser jocks in training soon.
If you’re a Linfield student, and think you’re interested in doing some research, be sure to contact me.
Gu, Choi and Kim have published a paper (J. Phys. Chem A 111, 8121-8125 (2007)) titled, “A Quantum Mechanical Theory for Single Molecule – Single Nanoparticle Surface Enhanced Raman Scattering.” After a first (quick) reading of the article, the most important result seems to be an expression for the SERS enhancement factor,
where 
Since my last post, I was able to gather several successful SERS spectra of DMAC at various concentrations. After an initial SERS spectrum was gathered as a function of concentration of the molecule, I prepared a new stock solution of the molecule to make sure any pattern and correlation was indeed because of DMAC, and not because the solution had become contaminated with another substance, such as Rhodamine 6G. It was at this point that I ran into a problem. I discovered that the molecule was not fully soluble in water, and thus, tiny particles were suspended in the new stock solution, as they were in the old solution, I later noticed. Any spectra gathered up to that point, as a function of concentration, was deemed invalid. Instead, I dissolved DMAC in ethanol and added distilled water to create serial dilutions. With the new solution of DMAC in ethanol with silver colloid and salt, there seemed to be a battle between fluorescence and the SERS scattering. The intensity of the peaks seemed to decrease with increasing concentration, while a broad hump seemed to be growing in intensity, as seen below.
As a result of this, I ran a sample of DMAC through the fluorimeter to see if the solution would fluoresce at 532 nm, the excitation wavelength of the laser being used. However, no fluorescence was detected at that wavelength, so I made several different solutions of varying concentrations and colloidal silver volumes and took several spectra. One such combination can be seen below.
Working with Gaussian 03W, I have also performed some tests to find the optimum geometry of DMAC. I performed a test to calculate the different vibrational frequencies of the molecule. Since this point, I have been working on tentative assignments for the peaks using these calculated vibrational frequencies.
Further research can still be done to draw a correlation between the concentration of DMAC and its SERS spectra. There seems to be promise in what I have already done, but conclusions are yet to be drawn. So, as we near the end of the summer, I will be finishing up my vibration assignments and preparing for my multiple conferences during the year.
In these past few weeks, I have found success with derivatizing slides with silane and getting a colloidal layer to covalently bond to the surface. Both silver and silver/gold alloy slides were successfully made and characterized by their extinction spectra.
I was also able to gather SERS spectra for PNBA in colloidal alloy solutions. This was something that I had been skeptical about but found great success with.
To take it a step further, I used alloy CMF’s and successfully obtained SERS spectra of PNBA. Not all of them worked, but any success with this was seen as progress. PNBA was enhanced for CMF’s that contained a relatively small amount of silver and a larger percentage of gold which is really promising.
Now that I have seen that I can obtain PNBA spectra on alloys that contain a portion of gold (PNBA does not exhibit SERS enhancement on gold), I began to look at the effects of extended laser exposure and the photoreaction that PNBA undergoes. I have not obtained sufficient data as of yet, but it certainly gives me direction for future research.
At the beginning of this summer, Au colloid was failing to adhere to well-cleaned and silanized slides prepared using past methods that had been reliable. Many parts of the preparation scheme were tested, and the problem was identified to be the silane used.
Using freshly prepared cleaning reagents (aqua regia, piranha) and various cleaning methods (sonicating, rinsing etc.) did not increase the amount of Au that coated onto the films. Mercaptopropyltrimethoxysilane (MPTMS) was the silane being used to bind Au colloid to films at the time.
In a paper published by Michael Natan[1], mercapto silanes were noted to air oxidize, especially in humid conditions, making them less capable of binding to Au over time. It was suggested that new silanes be purchased every semester and that aminopropyltrimethoxysilane (APTMS) was most “forgiving” of silane age, purity, etc.
At that suggestion, APTMS was ordered. The new APTMS significantly improved the amount of colloid that coated onto the slides. This result has verified Natan’s observations and successfully coated Au onto the CMFs. Completed Ag-coated Au CMF’s are shown below.
[1] Keating, C.D.; Musick, M.D.; Keefe, M.H., and Natan, M.J. J. Chem. Ed. 1999, 76, 949-955.
Everyday Scientist published a top 10 list, “10 Suggestions for Becoming a More-Pretentious Graduate Student”. The first:
1) Name drop. How are people supposed to know you’re close personal friends with a variety of Nobel laureates unless you tell them? Frequently. Remember, you don’t actually have to be friends with them to claim them as friends – casual acquaintance (meaning you shook their hands once) is enough. If you are fortunate enough to correspond with one, carry a copy of the correspondence with you to impress your peers. Feel free to read it/paraphrase it to friends (by friends, I mean colleagues. If you’re truly a pretentious graduate student, you don’t have friends). They may seem like they’re ignoring you/leaving the room, but it’s just to hide their envy.
Aspiring laser jocks, and anyone else planning on a graduate school stopover should pay close attention to this list!
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