These functional groups commonly terminate SAMs, side chains on a polymer backbone, or are present as ligands on nanostructures. N-hydroxysuccinimide (NHS) can activate a carboxylic acid functional via the formation of NHS-ester. Polymers can also be modified to include this NHS-ester in its structure such as pyrrole-NHS, which will directly cross-link with the bioreceptor. 178 However, NHS-esters are vulnerable to hydrolysis, thus EDC/NHS (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) coupling is generally used instead. As seen in Figure the EDC reacts with the carboxylic acid to form an O-acylisourea intermediate. Though this intermediate can react directly with a primary amine, it is unstable and prone to hydrolysis. As hydrolysis is largely dependent on pH, an acidic buffer is used for this step. 179 NHS or its water-soluble derivative (Sulfo-NHS) couples to the carboxylate to form a more stable ester which can react with a primary amine at physiological pH. EDC also be used to covalently link phosphates to amines. 179 Electrode functionalisation may yield a surface with amines available for cross-linking. Aldehyde functional groups form imine with an amine, which can be reduced to a more stable secondary amine using NaBH 3 CN form an irreversible bond. Glutaraldehyde is a dialdehyde used to cross-link two primary amines. One aldehyde end reacts with the first amine forming and imine, leaving the other aldehyde end free to form an imine with the second amine, linking an amine on the surface to an amine on the bioreceptor. 186