Quick Nanoparticle Separation Update
20nm polystrene beads in 1x PBS passed through w218. This wafer looks a little strange, like it was not fully etched (Dave do you know what happened to this one?). This was just a first stab at the benchtop diffusion with PS, so I didn’t use a “good” membrane. At any rate, the PS went through the membrane. It registered right at 20nm in the filtrate, and the fluorscent particles could be visually detected in the filtrate. This leads me to believe that the gold’s problem was indeed electrostatics.
On Thurday I talked to a student from the Krauss lab about how to get gold in a high salt solution without it aggregating. She suggested that I conjugate the gold to BSA, an idea that I had been toying with but hadn’t tried because this complicates our hard sphere approach to diffusion. On Friday I made up 5nm, 10nm, and 20nm BSA-Au in 1x PBS. All particles were 3-5nm bigger after conjugation. I set up an overnight diffusion with 5nm BSA-Au and w338. According to the ZetaSizer the BSA-Au went through!
More on these experiments and a better write up tomorrow. I will be attempting separations using all BSA-Au sizes (and potentially getting some 50nm).
One little issue that’s bugging me. The more I dilute the BSA-Au solutions, the larger they appear (growth of diameter up to 8 more nanometers). This may be a breakdown of the model since we’re now dealing with a core-shell model rather than simple sphere. Please add your own thoughts.
I think this is good for a demo experiment, but we need to find a better way to do this. Adding a 7nm protein to the surface of NPs that are <50nm, seems like a complicated situation that will make it difficult to draw reportable conclusions. Is this a covalent conjugation or just adsorption? I don't understand why the diameter would only increase by 3-5nm - maybe the surface coverage is ~50%?
Seems like there should be some off-the-shelf thiol chemistry that can be used on the gold, or is it already conjugated with something? Did any of Todd’s students suggest other alternatives? There must be a short thiol (1-2 nm) with good surface coverage that we can try.
Jess,
When you say the wafer was not fully etched, do you mean that the membrane slit did not release completely from the underlying silicon support? I’ve noticed this happens occasionally to outer samples, due to the nature of our current etch cell design…
Chris,
BSA is around 5.5nm according to DLS. My guess is that it’s adsorbing and spreading out on the surface.
Todd’s student did mention that a thiol treament might work too, but she didn’t know how to do it. That might be our best option, but at least the BSA coating worked well enough to show that electrostatics was most likely the problem before.
Dave,
The wafer has a weird radial color gradient on it, and on the outer edges looks like the surface may be rough or uneven under reflection microscopy. Everything was etched through just fine. This is something different.