Gold nanoparticles as markers for fluorinated surfaces containing embedded amide groups

Indium tin oxide (ITO) substrates were functionalized with fluoroalkylsilanes (FAS) having formula RFC(O)N(R)(CH2)3Si(OMe)3 (1, R=H, RF = C5F11; 2, R=CH3, RF = C5F11; 3, R=H, RF = C3F7) and containingembedded amide moieties between the perfluoroalkyl chain and the syloxanic moiety. Subsequently,Au nanoparticle deposition (AuNP) onto the ITO-FAS functionalized surfaces was carried out by immersioninto a solution of citrate-stabilized AuNP. The ITO-FAS and AuNP/ITO-FAS modified systems werecharacterized by various complementary techniques and compared with AuNP/ITO modified withRF(CH2)2Si(OEt)3 (4, RF = C6F13), free from functional groups between the fluorinated tail and the syloxanicmoiety. The results showed that only ITO glasses modified with 1, 2 and 3 displayed an oleophobic, aswell as hydrophobic, behaviour and that the AuNP Surface Coverage (SC %) directly depended on the fluoroalkylsilanenature with the following trend: 60% ITO-2 > 16% ITO-3 > 9% ITO-1 > 3% ITO-4. Theobtained results revealed that, in organosilane 2, the presence of a methyl group on the amide nitrogenincreases the steric hindrance in the rotation around the NACO bond, resulting in the co-presence of twostable conformers in comparable amounts. Their co-presence in solution, combined with the lack of intermolecularNAH OCAN hydrogen bonds among the anchored molecules, has dramatic influences on thefunctionalized ITO, yielding a disorderedly packed coating able to accommodate a large quantity of AuNP.These results indicate that AuNP can act as excellent probes to evaluate the coating layer quality but, atthe same time, it is possible to tune the gold loading on electroactive surfaces depending on the chemicalstructure of the used fluorinated silane.