[MangoPlant]

Hi all,

I have some nanoparticles with carboxylic acid functional groups that I wanted to couple to an amine containing compound. I activated the carboxylic acid groups by adding EDC and NHS in water (pH = 5) and after 15 minutes, raised the pH to 7.5 using triethylamine (TEA). I subsequently added the amine that I hoped to couple to the carboxylic acid into the mix. Unfortunately, I cannot detect any of the desired amine on my nanoparticles, indicating that this reaction did not work. I set up a control experiment where I followed the exact same EDC NHS activation steps followed by addition of TEA, however, in this control experiment I did NOT add the amine I hoped to couple. Interestingly, the IR of the nanoparticles in both experiments is identical and shows the presence of some alkyl-group containing compound on the nanoparticles, indicating that some other amine or compound is coupling to the nanoparticles. I know that NHS esters typically hydrolyze quickly in water (and therefore should not be the source of the alkyl peaks on the IR) so what else could be coupling to the nanoparticles? Does EDC degrade to form any amines capable of reacting with activated NHS ester? The EDC I used was ~4 years old so my first thought was that it may have degraded but I wanted to get your opinions first. Any advice appreciated!

Some more info:
The nanoparticles contain no alkyl peaks before going through the above EDC/NHS and TEA reactions
All reagents and the nanoparticles are soluble/dispersible in water

[TheUnassuming]

One of the main side reactions you will see when using carbodiimides is the formation of the acyl urea.  Typically, the more polar the solvent, the faster this side reaction occurs.  You can out-compete this by using a stronger nucleophile to attack the activated acid to a certain extent.  Its possible though that your acid functionality is too hindered (especially when EDC is attached) to allow for the attack of the amine. 
Can you use a different solvent system/acid activation method?  Or perhaps can you make an analogue with the carboxylic acid extended further from the core of your particles?

[MangoPlant]

Thanks for the advice! The carboxylic acid group is very close to the nanoparticle core and unfortunately I cannot think of a way to extend it without using the EDC/NHS chemistry to couple a amine-peg-(protected COOH). I guess I will try this reaction in anhydrous DMF since the lower polarity may help avoid the competing reaction as you suggested.