How do you know that the aromatic ring is acting as a capping agent? I mean which experiments led you to this conclusion?

Here are some points which you may consider.

1. Aromatic ring itself is hydrophobic in nature. Usually it cannot bind to silver nanoparticles which are hydrophilic in nature.

2. But when one has substituted benzene derivatives with attached functional groups (e.g. carboxyl group etc) which can bind to silver nanoparticles, then functional group bearing aromatic molecule binds to the silver nanoparticle and the aromatic ring is attached to the functional group giving an impression of a capping agent. This maynot happen in all solvents but only in certain solvents.

3. In a biomolecule, if there are groups surrounding an aromatic ring which bind to the silver nanoparticle then when these groups bind to the nanoparticle, the binding of these groups forces the aromatic ring to come close to the silver nanoparticle giving an impression/effect of capping. The aromatic ring may sometimes be so close to the silver nanoparticle that the wavefunctions may overlap.

4. It's always a play between enthalpy and entropy i.e., free energy. The overall free energy change in the reaction.

5. One can always force a non-spontaneous reaction (dG>0) to occur by coupling it to a more spontaneous reaction such that the net or combined free energy change of the process is negative. Using this concept one can make not so obvious things happen like in your case.

The aromatic molecule if have polyphenolic or amine group might act as capping agent although participation of benzene ring in direct capping is difficult to my knowwledge.

 Forall your considerations i wrote my question...because i can not imagine how aromatic rings do that.....my experiment  started from obtaining isolated pure curvularin compound (C16 H20 O6) and using it as reducing agent in synthesis of silver nanoparticles....i have a good result in my experiment and the SNPs are synthesized...but if we all agree that curvularin can act as reducing agent in the silver nanoparticles synthesis...how it can acting as a capping agent in that process??!!!.....

How do you know your compound is acting as a capping agent? May be or May be not.

May be your nanoparticles are not stable? They may be aggregating with time forming big sized nanoparticles. Some times particle size may be increasing without any visible changes in the solution upto certain time. After some time, visible changes can be seen in the solution.

i know this very well ..yeah there is a high probability that the synthesized nanopaticles  converted again to Ag+1.....but i started to work on SEM, TEM analysis after 72 hours after i had  a UV diagram confirming occurrence  the reduction of silver ( at 450 wavelength) but the question here...after 72 hours i obtained spherical silver nanopartices sized 30 nm using TEM & SEM techniques ...so who is the responsible of reserving those nanoparticles at that nanoscale through 72 hrs. after and inhibit their oxidation again.....in fact, i thinking about formation a derived compounds from curvularin (which i used as reducing agent) acting as capping agents...