First Week - Creating Nanoparticles

This week I started making gold nanoparticles (NPs) using two different methods, using citrate and CTAB surface coatings. The first is very easy to make, simply by heating up a gold solution until it boils and injecting trisodium citrate into it. This makes a red colored solution which contains the nanoparticles. The second method is much harder. The first step is preparing a "seed" solution that contains very small nanoparticles and then scaling these up in order to become bigger and easier to work with. Once the seeds have been made, they must sit for a few hours to grow and then the particles can be made by adding the seeds to a mixture of a gold solution, silver nitrate, and ascorbic acid. This is a very sensitive process, especially when adding the silver nitrate and ascorbic acid and can be messed up easily, which is what makes it difficult. However, if made correctly, the particles will be coated in CTAB and should be a red/pink color. The ones I made this week (two batches of 8 samples each) were relatively red/pink, but a few samples were purple/blue, which indicates aggregation (some of the particles have clumped together). This isn't what I want, as I want the particles to be as separate as possible in order to continue with my work wrapping DNA around each particle. If the particles are clumped, the DNA won't be long enough to wrap around and will cause problems with my experiments. I also made two batches of citrate-coated particles, which look a little redder than the CTAB ones.

Citrate-coated NPs. The middle is a nice red color, but the others are an ugly dark purple which indicates aggregation of particles.

When I ran my samples through the UV-Vis machine, which measures the absorption spectrum of the particles (how much of each color in the visible light spectrum is absorbed by the solution), most seemed decent, with a good solid peak of about 530 nanometers. This means that the particles are mostly absorbing green light / reflecting red light, which explains why the solution looks red. I had a few that absorbed way too much of other colors, so I'm not going to be using them to continue my research.

Absorption Spectrum from the first batch of CTAB-coated NPs.

I also ran my samples through a DLS machine which measures the size of the particles. Most of them were around 40 nanometers in diameter, which is a little on the large side, but it'll still work - I'll just have to correct for this when measuring how long the DNA should be. I had a few that were way too large, so I won't use these either.

The last analysis that I did is called zeta potential, and this basically measures the charge on the surface on the particles. Almost all of my particles performed well in this test and had the correct charge on their surface (about 30 millivolts). This is all well, however, the most important test is the absorption spectrum measurement, so all of the ones that did not do well in that test I will most likely be trashing. Next week, I'll start wrapping DNA around my particles and hope that all goes well - if not, I may need to create new particles.