I am preparing gold nanoparticles on Nitrocellulose in order to perform some tests (the tests themselves are unfortunately private so I can not share those details). In preparation for the tests, I am depositing gold nanoparticles (20nm) on to different nitrocellulose brands. One is an unbacked nitrocellulose from cytiva, the other is a high flow brand (backed). On our end, we are not blocking or spraying test lines, simply pipeting the particles on to the nitrocellulos (it is a requirement of the next test(s) to be done). The high-flow gave a nice red color when the nanoparticles were deposited. They sat in the fridge ”drying” for 1-2 days and remained red. When done again with a new section of the same nitrocelluose sheet, they turned a dull purple over the same dry time and conditions.
The prep was redone on the cytiva unbacked nitrocellulos. this also went from red to purple. The outlier seems to be the initial High flow test (red remains red) and the other successive preps seem to be the expected outcome. I am more curious which is the “usable” material and which has possibly gone bad.
could they be getting less red as more of the nitrocellulose absorbs and “binds” to the gold? If the initial prep was red and remained red, does that indicate that the particles did not bind? In all three preps, I took a control sample and pipetted it with PBS to ensure the particles did not disperse from the initial location (indicating some loose or unbound particles). Withou an SEM, visially, it seems that all three stayed localized to the initial pipet location (no drifting or unbinding as far as I can tell).
TLDR: the gold particles are going from a nice bright red to a dull purple when pipetted to nitrocllulose, it this supposed to happen?
The color change (i.e. plasmon shift) commonly stems from two processes - ligand exchange and\or aggregation. The nitro group is very electronegative and can function as a strong ligand. If the gold NPs are capped with a weak ligand (e.g. citrate), a ligand exchange may have occurred, thus causing a plasmon shift.
On the other hand, perhaps the nanoparticles aggregated due to weak ligands and low temperature. Do the sheets behave the same under room temperature while kept in the dark? This may be worth examining. Also, high ionic strength solvents (such as PBS) are detrimental to gold NP stability and may cause aggregation, especially at higher concentrations. The drying process itself could also lead to aggregation and plasmon shift.
I agree with Mr Cohen. I would also add, you may want to try a different pH for conjugation. If your gold allows it, a higher or lower ph may protect your particles from aggregation or color change. Also,, consider mixing some more protein with your conjugate before adding it (BSA is popular)
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