Does Size Matter? — Week 10 at Stanford Chemical Engineering

Apr 29, 2019

Hello all,

Week 10 was a very exciting week, particularly because I was able to obtain some incredible data, which I am extremely happy about! Let me tell you about it:

If you’ll recall, during week 9, working with 4 nm platinum nanoparticles, I tried to establish the UV-Vis calibration curve, but unfortunately, I underestimated the concentration of my platinum stock solution and was thus outside of the ideal absorbance range in my measurements. Therefore, as soon as week 10 began, I got right back to attempting this task once again. I repeated the exact same process as I did previously, only this time, I did not dilute my samples as much as to not fall into sub-optimally low absorbance ranges. 15 UV-Vis samples later, I was delighted to see that my data was absolutely beautiful. 15 data points all within the ideal absorbance range that formed a perfect line — rejoice! Now, all that was left to do was to measure the concentration of the stock solution in order to complete my calibration curve. I prepared an ICP sample of my 4 nm particles for concentration measurement using the scarily powerful acids that I have discussed in previous posts, but unfortunately, I was informed that the ICP machine is currently down for maintenance! Sad… it will be back up by the beginning of week 12, and I can’t wait to obtain a final product once my measurements are complete!

By that time, it was Wednesday already. With the time remaining in the week, I decided to push forward and continue on to the next portion of my UV-Vis side-project — if you remember, when I started this tangent, one of my primary questions was whether the size of the nanoparticles plays a role in their spectroscopical properties (if particle size matters, then it would be very hard to make a UV-Vis measurement system applicable, since you would need to have a separate calibration curve for every single particle size). Thus, now that I had done as much as I could in regards to the 4 nm particles, I decided to keep going and to create the calibration curve for 2 nm particles. After I had both, I would be able to compare them and come to a conclusion.

Thus, I set forth: on Wednesday, I spent the day synthesizing a brand new batch of 2 nm nanoparticles using the same procedure which I know so so well that I could basically recite it in my sleep by now. Then, for the remainder of the week, I went through a process identical to that which I did for the 4 nm particles: use the ultra-precise micropipette to make many samples at different dilutions and measure their absorbances. Upon seeing the data I obtained from this run, I was once again delighted — all the data points perfectly formed a straight line with little to no deviation from the trend at all. Two successful calibration curves in one week? This couldn’t get much better! Just like I had done with the 4 nm particles, I prepared an ICP sample to measure the concentration of the 2 nm platinum stock solution, and this measurement will arrive back within a week or so.

However, just because I was waiting for the exact concentration measurements, doesn’t mean I couldn’t mess around with the data a bit to gauge the situation and come to a preliminary conclusion. I sat down at my laptop and looked at my Excel spreadsheet, upon which I made a couple of interesting observations. First, the slope of the line for the small particles is just a tad bit larger than that of the big particles (14.6 and 13.4, respectively). Yet at the same time, with samples of the 2 nm and 4 nm particles at the same level of dilution, I noticed that the absorbance values for the 2 nm particles were consistently just slightly larger than those of the 4 nm ones (for example, at a factor of dilution of 25, the small particles had an absorbance of ~0.58, whereas it was ~0.53 for the large ones). If you’ll recall from AP Chemistry class, absorbance correlates linearly to concentration, so this slightly larger absorbance for the small particles means that the 2 nm platinum stock solution was just slightly more concentrated than the 4 nm one. It seems somewhat suspicious that the small particles are just slightly more concentrated but also have a slightly larger slope — and that this difference is by nearly the same factor (14.6/13.4 and 0.58/0.53 are not too far from equal). Could the difference in concentration account for the difference in slope? Well, indeed, I won’t go too far into the details, but essentially, once you do some math and normalize the slopes accounting for the difference in absorbances, the slopes turn out to be nearly the same! Different particle sizes but little to no difference in slope of the calibration curve — what does this mean? Size does not matter!

At least for the time being, that is my conclusion. Whether this holds true once my ICP measurements come back remains to be seen, but this seems to be a pretty reliable preliminary approximation of the final result. When I saw this, I was extremely happy. The conclusion that particle size does not matter has tremendous implications and increases the impact of this kind of work on the scientific community immensely. If particles size did matter, we would need to create a separate calibration curve for every single size out there, meaning a single curve wouldn’t be too meaningful… but since it looks like size doesn’t matter, the single curve that I have created may be applicable to anyone who works with platinum nanoparticles and could leave a large, lasting impact. That is something that excites me.

~Ethan Y. Feng

One Reply to “Does Size Matter? — Week 10 at Stanford Chemical Engineering”

  1. Ivana B says:

    This is so exciting! I can’t wait for ICP to be up and running, so that you could finally put numbers to your work!

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