It should be possible yes, but there is potentially an easier route by reacting APTES with the NHS ester of PEG folate. I think this reagent is commercially available.

Is there free COOH available?!

Thank you Nick Gee .

Dear Riaz A. Khan Yes there is free -COOH available in COOH-PEG-Folate.

Thank you.

Nick Gee How to react APTES with NHS? Could you tell me the protocol of it including reaction time, PH and buffer? I take the experiment within a microfluidic chip made up of PDMS and glass. Thanks!

I think you mean APTES and the NHS ester of the other molecule? If so then it’s a simple amine to NHS ester coupling and the required conditions will depend on the solubilities of the two molecules. Probably an aqueous/organic mixture would be okay. Whatever the specifics, you need the amine to be deprotonated.

Nick Gee Could you tell me the way of amine deprotonating? I can't couple APTES with the NHS ester of the other molecule now. The NHS ester molecule is NHS-PEG-biotin in PH=8.3 sodium bicarbonate solution. Thanks!

If you have an aqueous buffer at pH 8.3 then your amine will probably be sufficiently deprotonated to react. I assume that you mean your intention is to use a pH 8.3 bicarbonate buffer, you cannot actually store nhs ligands in that buffer.

Nick Gee You mean pH 8.3 bicarbonate can not as the solvent of NHS ester?

It can be the solvent, but only add the NHS ligand immediately before you are ready to initiate the reaction. NHS ligands are unstable at pH 8.3, but it is entirely appropriate to do the reaction at that pH. The ligand will be reactive for an hour or so.

Nick Gee Thank a lot. If the NHS ligand reaction time timeout (such as 2 hours or overnight ), will the amide bonds be affected?

No, the other molecule has a single amine and there is no need to stop the reaction. Any surplus active groups will just decay overnight. Amide bonds that have been formed are completely stable. To get high efficiency of conversion to amide the nhs ester will need to be in molar excess (because of the unwanted hydrolysis that will be happening in parallel).

Nick Gee Your answer has helped me a lot！Is there a catalyst that can catalyze this reaction？How much is the dosage?

No you don’t need a catalyst. You just need the amine to be -NH2 rather than NH3+. This is because the lone pair on the nitrogen reacts. If the lone pair has picked up a proton, the reaction with NHS ester cannot happen. In reality, you will normally have some of the amines as NH2 and some as NH3+, but at pH8.3 you’ll have enough NH2 to get a fast reaction.