Editing Magnetic beads Preparation Methods (section)

== Coating ferrite MNPs with silica oxide ==

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The silica-coated magnetic beads are synthesised by a modified protocol including the hydrolysis of tetraethyl orthosilicate (TEOS) on the surface of ferrite magnetic core particles according to Stöber et al. 1968 [1]

Chemicals<br/>Ethanol (C2H6O, 99.9&nbsp;%) Riedel-de Haёn 34963<br/>Tetraethyl orthosilicate (≥99%) (GC) Aldrich 86578&nbsp;<br/>Ammonia solution (NH4OH, 25%) EMD Millipore 1.05432

Equipment and setup<br/>Fume hood<br/>Heated magnetic stirrer (e.g. IKAMAG REO)<br/>Strong neodymium permanent magnet (e.g. NdFeB N45 40x40x20 mm)<br/>Sterile plastic bottles&nbsp;

All procedures can be performed under inert N2 atmosphere or standard conditions<br/>1 Mix 45 ml of TEOS in 2 L ethanol in a large bottle (2.5 litre) using a magnetic stirrer&nbsp;and mix for 15 minutes at 300-400 rpm&nbsp;15 min&nbsp;<br/>2 Add 22.5 g (wet mass ~1.2 g dry) of magnetic core particles prewashed with ethanol and stir for 15-30 min 30 min<br/>3 Add 400 ml of pure water and 50 ml of 25% ammonia solution and stir for another 30 min 30 min&nbsp;<br/>4 Switch on the heating and allow the solution to heat up to ~80 °C with constant stirring, allow the reaction to proceed for >4 h (ideally o/n) 4 hours to o/n&nbsp;<br/>5 Cool down the solution 15 min&nbsp;<br/>6 Separate the coated MNPs using a strong neodymium magnet 10 min&nbsp;<br/>7 Wash twice with pure water 15 min&nbsp;<br/>8 Wash twice with pure ethanol 15 min&nbsp;<br/>9 Wash with pure water until the pH of the solution becomes neutral (3-4 times) 30 min&nbsp;<br/>Check the yield by weighing the wet mass of the beads ~12 h (2 h hands-on)

=== <br/>Modifications ===

By using different ratios of core particles and TEOS one can control the thickness of the glass layer and&nbsp;consequently the size of the particles. The standard ratio used in the above protocol is: 1 g of magnetic core&nbsp;particles, 2 ml TEOS, 20 ml of ethanol, 0.5 ml of 25&nbsp;% ammonia solution, 4 ml of water, which results in particles&nbsp;with an average size of ~400 nm. Increasing or decreasing the TEOS to beads ratio results in formation of other&nbsp;sizes of coated particles

=== Troubleshooting ===

'''Problem&nbsp;'''<br/>Brown colour of the reaction, low yield of beads retained<br/>'''Solution&nbsp;'''<br/>The uncoated magnetic core particles get slowly oxidized during storage, prepare fresh core particles and redo the reaction

'''Problem&nbsp;'''<br/>Large amounts of white precipitate<br/>'''Solution'''

TEOS hydrolyses spontaneously in water solution, thus forming silica&nbsp;nanoparticles. Use fresh TEOS for the reaction