[Nemanja Radovanovic]

We’ve tried to make the core particles but the problem we ran into was that we weren’t able to reach the pH value close to 7, after the dripping step and the addition of ammoniac even after 5 ddH20 washes pH was still around 11 (with every subsequent wash the supernatant was murkier and the precipitate was thinner) should we adjust pH by any further means or is the problem related to something else? We did heat up  NaOH solution to recommended temperature without degassing it.

Thank you in advance and have a good day.

[admin]

Dear Nemanja,

to start off very cheesy. You are not alone.

With an increasing number of people facing issues trying the BOMB 1.1 MNP synthesis, I have spent the last couple of weeks looking into what could be the source of these issues and sadly I have not figured it out completely. But I got a couple of time-saving remarks for anyone who is struggling.

All the factors that Tomek mentioned, from correct Fe(II):Fe(III) ratios to stirring speed to chemical quality, temperature and temperature homogeneity, NaOH concentration and/or pH and many more can influence this procedure. It might even be something really hard to put in to numbers such as a stir-bar that incorporates more or less air into the reaction solution. What that means is that while it should be very reproducible it only is if you can actually control all of the influential factors, which is a bit tricky.

I am currently in the process of developing an alternative that seems slightly more forgiving. This “new” method performs very reliable for me but I have yet to adjust it to produce slightly smaller NPs (I think). What I am currently getting is magnificently black (very important), very magnetic, but has an average size of ~60nm.

At this size range the NPs are like single domain but not super-paramagnetic.

However, one important question that I am struggling to answer is, what is the ideal NP size to use as a core?

If anyone can tell me that, I’ll try to post my modified protocol ASAP.

In the meantime the time-saving advice.

If it isn’t raven black don’t bother.

While a light microscope does not offer the magnification/resolution to see the individual particles it can give you very valuable information on the colour.

The ideal result of the Fe(II):Fe(III), 1:2 molar ratio is magnetite. Magnetite will look absolutely perfectly black under the light microscope (aperture diaphragm fully open). From my rather extensive experience, if it looks even remotely brown, it will not withstand washing. This will than show as a brown “bloom”, increase in pH with every wash step and lead to unstable beads.

The washing can lead to a slow-ish oxidation turning the raven black Magnetite into dark brown Maghemite which is perfectly OK as a core material. However right after the synthesis the NPs have to be black as can be. Macroscopically it is very hard to distinguish between very dark brown and perfect black so I strongly recommend using a microscope until you develop an eye for it. One way that works quite well, but is a bit trickier until you have seen the ideal blackness once, is simply placing a drop of the suspended NPs on blotting paper (or a tissue), I cannot stress this enough: it has to be black, or rather grey because of the paper, but without a hint of brown.

Unfortunately that is all I have to offer aside from reiterating the advice that is already posted somewhere in this thread.

 

Cheers

Tim M

[Malgorzata Cebrat]

Hi,

I have successfully synthesized the NPs once but I was never lucky enough to repeat the procedure. I have the same problem as reported in this thread – the NPs during washing with water start to desintegrate, the supernatant is getting brown (or even reddish) and the leftover NPs are becoming less paramagnetic. It seems that, at least in my hands, the whole problem is the washing step so I thought that maybe it can be ommited? Because I coat the beeds soon after the synthesis with TEOS and this procedure uses NH4OH, I thought that maybe extensive washing of the beeds and bringing them to pH7.0 is not so important.

So what I did: I’ve settled the NPs after the synthesis with the magnet and washed few times in ethanol (not large volumes – ~10 ml per wash, but please note that my synthesis reaction is 20 times smaller than the original procedure) and added the beads immediately after that to the warm ethanol/NH4OH solution (according to the procedure of TEOS coating). The NPs are still black and the solution did not change  color. The NPs are still paramagnetic. I’ll let you know about the results of the coating and DNA binding but so far it looks promising 🙂

Cheers – Gosia

[Malgorzata Cebrat]

An update – I have tested the silica-coated NPs in DNA isolation and they worked very well. So to summarize my experiment: to avoid the desintegration of MNPs during washing with water after the synthesis, I’ve pelleted them with magnet, washed three times in small volume of ethanol (instead of water) and then immediately proceeded to coating with TEOS.

Hope that helps, it would be great if someone could confirm that this is repeatable 😉

Gosia

[Author]

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