Guanidine-functionalized particles and methods of making and using

A method of making a guanidine-functionalized particle, comprising:

• reacting O-methylisourea hemisulfate with a linker molecule comprising the formula
• (RO)nRa3-nSi—Sm—X
• where RO is an alkoxy group comprising one or two carbons, or is an acetoxy group,
• where n is 1, 2 or 3,
• where Ra is an unreactive group,
• where S is a spacer group comprising a backbone with m atoms,
• m is from 2-16, inclusive, and
• where X is a primary amine that is capable of reacting with the O-methylisourea to form a guanidine group;
• and,
• reacting at least one of the RO groups of the linker molecule with a hydroxyl group of the particle to form a covalent bond between the linker molecule and the particle.