Neither Centrifugation, Sonication, nor Filtration Fix the Aggregation in the Barcikowski Lab Samples

In early May, the Barcikowski lab shipped us four samples of gold nanoparticles for us to run separations on. They took forever to get here, and had aggregated and precipitated by the time they arrived. About a week ago, the lab sent another batch of four samples (which differed slightly from the samples they sent before), which once again took nearly two weeks in transport.

There is once again a precipitate in several of the new samples. Further, DLS measurements of the samples show all of them exclusively contain particles ~70nm in diameter. Because the particles should be ~10nm, we’ve got some kind of aggregation occurring.

To combat this aggregation, we tried several techniques, none of which had an appreciable effect on the samples.

First, we examined sample 2, which is an Au-peptide conjugate with free peptides in the supernatant. Straight from the bottle, the DLS measured an average particle size of 66.1nm. After centrifuging at 1000rpm for 3 minutes, the DLS measured an average size of 66.1 again. The sample was then centrifuged at 3000rpm for 3 minutes, and the DLS measured 44.0nm. Finally, a fresh aliquot of sample 2 was separated using a 20nm Sepcon filter. The concentration of gold was low enough that the machine had trouble getting a reading, and the average particle size of the filtrate was found to be 87.2nm.

Next, we examined sample 3, which is just unconjugated gold particles around 7nm in diameter. Straight from the bottle, the DLS measured an average size of 60.7 . After bath sonicating for a half hour, the new average size was 63.7. After probe sonicating on the lowest setting (1) for five seconds (any more and the sample risks boiling), the DLS saw particles of 62.8nm.

To sum, centrifugation cannot restore the conjugated peptides to their expected size distributions – any higher speeds than the ones we used and the gold we want will crash out. Simply filtering the aggregated sample doesn’t work either. Neither probe sonication nor bath sonication had an obvious effect on the samples. From previous experience we know that adding salt will only promote further aggregation in the samples. Either we need a new angle to disrupt the aggregation, or we need to request that the Barcikowski lab send one more batch of samples via FedEx International First mail with more than a single ice pack to keep them cool.