For magnetite where the long range magnetic forces outweigh any charge based repulsive forces then the only hope is steric stabilization with a polymer such as 50 KDa PEG or PEI. While you may not 'appreciate' it, you have no other route to stabilize your system.

As you have found, - 35 mV or so is not sufficient to stabilize 20 nm particles. Sonication increases the collision frequency and thus the separated particles will aggregate and precipitate due to the high density of magnetite (~ 5.5 g/cm3). Your charge would need to be closer to 100 mV in absolute terms and even so the magnetic forces would overcome this repulsion. As you're working with bacteria, you must use a water-based system - practically anything else (even additives) will kill them.

I would also add a comment by Drexler:

K Eric Drexler’s Nanosystems ISBN 0-471-57518-6 (pbk.) Page xviii in the section 'Criticism of criticism'

'These observations constitute not criticisms, but rediscoveries of elementary engineering constraints'

You may find these webinars useful (or you may not). Open in IE:

Nov 11th, 2008. Dispersion and nanotechnology http://tinyurl.com/hpywsge

November 3rd, 2015 Adhesion and cohesion http://tinyurl.com/zwb2wlh

Hello Alan Sir,

Thank you very much for responding to the query.

But I have a query in respect to your view, as we are going to carry our further experiments in model study, can you provide me the valuable suggestion if we use chitosan instead of PEG/PEI.

Thanks in advance.

Regards

Rajanya Banerjee