Science Buddies: "Ask an Expert"

Hello,

As part of my science fair project, I am preparing a series of silver nanoparticle aggregate sizes suspended in a colloidal solution, then testing the different sizes' effect against bacteria. I do not have access to an electron microscope, so I can not determine the aggregate sizes by that method, so instead I would like to use a spectrophotometer.
I have never used a spectrophotometer before but I know you must set a wavelength (how do I find this?) and choose a mode (which mode), then the results will show on the device.
However, can anybody help me formulate an ideal method that would allow me to determine the overall aggregation state? What data should be tested/calculated, which will indicate my various aggregate sizes?
Any help is valuable to me. Thanks very much!
Ryan

p.s here's a picture of the spectrophotometer I have: https://drive.google.com/file/d/0B0CRqr ... sp=sharing

Hi Ryan,

I don’t have any experience with evaluating nanoparticles but after some searching I did find a few references about estimating nanoparticle size with UV-visible spectra. Links to the publications I found are listed below. Unfortunately only two of these are open access. Based on reading the abstracts from these articles it appears that spectral data (absorbance for wavelengths from 350 to 700 nm) can be used to estimate particle size. I think you’d need to find publications that describe the mathematical procedures used make the particle size estimates.

http://link.springer.com/chapter/10.100 ... -27594-4_4

http://citeseerx.ist.psu.edu/viewdoc/do ... 1&type=pdf

http://pubs.acs.org/doi/abs/10.1021/ac0702084

http://pubs.acs.org/doi/abs/10.1021/jp8 ... ode=jpccck

http://50.87.149.212/sites/default/file ... alysis.pdf

You might be able to use the spectrophotometer you showed in your post. This instrument can be used to record spectra over a range of wavelengths but the process is somewhat labor intensive. You use the “wavelength” knob to set a wavelength, then read and record the absorbance (you want the instrument set for the absorbance mode) for your sample. You repeat this for each wavelength of interest. At the end you’ll have a data table with a list of wavelengths and corresponding absorbance values. These are the data that you would use to estimate particle size with the appropriate mathematical procedure.

I hope this helps. Please post again if you have more questions.

A. Norman

Hi Norman,

I have a question about using the spectrophotometer. I would like to find results for a small volume of liquid about 1 mL. If the cuvette is about 6 mL in size, does it need to be filled all the way for the spectrophotometer to obtain/calculate data or can the spectrophotometer read quantities of only 1 mL in a 6 mL cuvette? I'll be testing this tomorrow Thanks!

Ryan

Hi Ryan,

The cuvette should be about 75% full. If you don’t have enough liquid in there the light from the spectrophotometer might partly bypass your solution. You don’t want to overfill either or you’ll risk spilling liquid into the instrument.

I hope this helps. Please post again if you have more questions.

A. Norman

Oh no, this may be a significant problem then. I do not have enough of my solution to fill that much. Is there any way around this? The max I can use is 2 mL even so that may not be enough.

Actually, the cuvette I'm using is test tube shape like. But I do recall seeing rectangular ones, but I believe they are too small for the hole and I'm afraid the cuvette would fall in and be stuck. However, those are about 3-4 mL in volume, so I could fill it enough for 75%.

Hi Ryan,

I’ve only used the test tube shaped cuvettes with the Spectronic 20 instrument. I think you need an accessory adapter to use the square cuvettes. See the link below.

http://www.thomassci.com/Instruments/Sp ... NIC-20D?q=*

You might be able to find special, low volume cuvettes that fit the accessory adapter.

I hope this helps. Please post again if you have more questions.

A. Norman