EDC and NHS are used to activate the amino group of chitosan, making it available and easy to react with carboxyl groups of folic acid.

EDC/NHS conjugation is a well established and popular method for conjugating a primary amine (R-NH2) with a carboxylic group (-COOH) which ultimately results in the formation of an amide bond (CONH2). So, its primarily used for conjugating peptides/proteins (containing -NH2 groups) with molecules containing -COOH or surfaces (or substrates) expressing carboxylic group (-COOH). In your case, looks like chitosan is the -NH2 containing group and folic acid is the -COOH containing group.

Having said that, the EDC (1-ethyl-3-(dimethylaminopropyl)carbodiimide) activates the carboxylic group by forming a reactive ester intermediate (o-Acylisourea ester). However, this intermediate is prone to hydrolysis and might convert back into the parent carboxylic group. Hence, NHS (N-hydroxysuccinimide) is added which replaces the previous unstable ester into a much more stable NHS-ester species. When the NHS-ester species comes in contact with a primary amine it forms an amide bond (NHS is a very good leaving group) and the coupling occurs. [NOTE: EDC/NHS is a very facile method, however a right pH condition ~5-6 is vital for the EDC/NHS coupling to occur.]

Hope that helps!

Thank you Amin Shavandi

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4966701/