The choice of nanoparticles is important. Avoid non-specific interactions and look for specific interactions e.g. Gold nanoparticles for Thiol groups (cysteine) etc. pH of the solution is also important, it decides the charge on the amino acid.

• i work on silver nanoparticles my nanoparticles sense methionine,histidine,Arginine ,Arginine Hcl,Glutamic Acid and Cystein the result reproduce 2,3 times but after a week the result was not same as earlier . any suggestion for improving my work

Are your nanoparticles stabilized?

my nanoparticles stable about 5 month i m working on freshly synthesize nanoparticles. I prepare  amino acid solutions in distilled water but i used amino acid stock solution. not every day fresh solution

What is the stabilizer you are using? In other words are your nanoparticles coated with citrate, tartarate, PEG, BSA etc

i m not use any other aditional stabilizing agent my ligand itself used as stabilizing and reducing agent