# Functionalization of nanoparticles

I want to functionalize silver nanoparticles with thiol group. My question is "how can I find that the thiolated compound is attached to the surface of nanoparticles or not?"

If the thiolate bounds to the Au nanoparticle, both the electronic, as well as the mechanical properties of the thiolate and the Au nanoparticle change. So you are going to monitor these changes with a spectroscopy susceptible to this, comparing the spectra of the pure starting materials with the one(s) of the decorated NP.

One of the easier available techniques is to rely on FT-IR spectroscopy, as by the established bond between the two partners, some absorption bands newly appear, and others change their position. The former are the ones more easily taken as a quantitative reference, too (because yes, IR spectroscopy allows both qualitative, as well as quantitative characterisations, too).

In the instance of Au NP, to check for a bonding of the NPs to the mercapto decoration is possible both by IR spectroscopy, as well as by (surface enhanced) Raman spectroscopy. In a work deploying $\omega$-mercaptoundecanoic acid (MUA) by Tripathy and Yu, the later was the preferred check to monitor the S-Ag interaction:

(source, doi 10.1016/j.saa.2008.12.004, Spectrochimica A, 72, 2009, 841-844).

Similar as in the case of the Au NP (vide infra), the key feature is the disappearance of the S-H vibration. To quote from the section about their characterisation by Raman spectroscopy:

The most interesting feature was the disappearance of S–H stretching ($\pu{2547 cm^{−1}}$) peak. The disappearance of S–H stretching indicated the covalent interaction of sulphur and silver, which is in good agreement with the FT-IR data [16]. However some of the expected peaks such as C–Sgauche stretching could not be identified which may be due to their low intensity values.

As an example, were gold nanoparticles were decorated by dodecanthiol (DT) chains:

(source, Sharma et. al. in Indian Journal of Pure & Applied Physics 52, 2014, 93-100.)

Other than the absorption band at $694$ and $\pu{728 cm^{-1}}$, the authors equally draw attention that

the "S-H vibration band at 2576 cm−1 for pure DT is observed while it is almost absent for AuNPs. This suggests that as the thiol gets bound onto the surface of gold, the S-H bond of thiol breaks in order to form the S-Au bond." (loc. cit., p. 97).

It may be helpful to mention that short wavelengths of light ($\pu{< 400 nm}$) may break the $\ce{Au-S}$ bond between nanoparticle, and thiol, hence if you doubt the presence of thiolate decoration on the NP, select an absorption band and monitor its attenuation while shining the short wavelength light on the sample.

And complementary to IR spectroscopy, is Raman spectroscopy.

• Nice answer. My only comment is that the OP specifically refers to Ag nanoparticles, whereas your answer exclusively discusses Au nanoparticles. How do you think the FTIR spectra in particular would compare between and Ag and Au analogs? – airhuff Jun 18 '17 at 22:25
• I find it kind of ironic that if there is a question explicitly asking about silver nanoparticles, the answer that exclusively talks about gold, gets upvoted, while the one that tells silver is different from gold, gets the downvotes. Nice answer by the way. – Greg Jun 19 '17 at 1:33
• @airhuff Sorry, this was an error by mine. I found a work including spectra of the two worlds (IR and Raman) of Ag NPs and mercaptans and added this ahead of the section of Au NPs. Of course the best were to have an example "same mercaptan, once to decorate AgNP, the other on AuNP" that however I did not find (yet). On the other hand, attenuation of the SH-stretch appears to be (again) the indicator. – Buttonwood Jun 19 '17 at 13:57
• @Greg The error to display Au instead of Ag is on my side; by an edit I rectify for the situation. The later surfacing answer by #Vic Lineal offers a sound additional insight, appreciated by a tip to the hat (+1), too. – Buttonwood Jun 19 '17 at 14:03

It's complicated, because there is no general pattern of reactivity for thiol compounds and silver nanoparticles (unlike, say, gold nanoparticles, for which the reaction with thiols is widely studied and understood). The chemistry and local structural changes are varied; cysteine for instance is known to form well-defined layers around silver nanoparticles, but unfortunately it also tends to aggregate them. Different thiols vary in the local surface chemistry they produce - some generate $\ce{Ag2S}$-like layers, and that can compromise the stability of the nanoparticles.

I'd suggest reducing the list of possibilities to the reagents you can use and then looking for literature to see how they characterise the reaction.