Self-assembled monolayers on Au nanoparticles terminating with TACN·Zn(II) head groups are attractive scaffolds for the formation of multivalent supramolecular structures at submicromolar concentrations in water.

A cascade of two catalytic events was used to detect enzyme activity: When a peptide substrate acting as an inhibitor for a catalytic gold nanoparticle was hydrolyzed by an enzyme, the catalytic activity of the nanoparticle was restored, and a large amount of a yellow reporter molecule was produced (see picture; S=substrate, P=product). The assay can be made selective for a particular enzyme by changing the inhibitory peptide.

In this review we describe the use of monolayer protected gold nanoparticles (Au NPs) for chemosensing applications. The attention is focused on a special subclass of Au NPs, namely those that contain binding sites for metal ions in the monolayer. It will be shown that these systems are very well-equipped for metal ion sensing as the complexation of the metal ions can affect the properties of the system in many ways leading to detectable output signals even at very low analyte concentrations. In addition, the presence of metal ions in the monolayer themselves can serve as recognition units for the highly selective interaction with small organic molecules or biomacromolecules. Key examples will be discussed that underscore the attractive properties and potential of this class of Au NPs as components of chemosensing assays.

The distribution of thiols in mixed SAMs can be determined in a straightforward manner from spectrophotometric titrations using a fluorescent probe. A plot of saturation concentration as a function of mole fraction provides information on the number of headgroups involved in binding.

The Keggin-type polyoxometalate [γ-SiW10O36]8− was covalently modified to obtain a bis-biotinylated conjugate able to bind avidin. Spectroscopic studies such as UV-vis, fluorimetry, circular dichroism, coupled to surface plasmon resonance technique were used to highlight the unique interplay of supramolecular interactions between the homotetrameric protein and the bis-functionalized polyanion. In particular, the dual recognition mechanism of the avidin encompasses (i) a complementary electrostatic association between the anionic surface of the polyoxotungstate and each positively charged avidin subunit and (ii) specific host-guest interactions between each biotinylated arm and a corresponding pocket on the tetramer subunits. The assembly exhibits peroxidase-like reactivity and it was used in aqueous solution for L-methionine methyl ester oxidation by H2O2. The recognition phenomenon was then exploited for the preparation of layer-by-layer films, whose structural evolution was monitored in situ by ATR-FTIR spectroscopy. Finally, cell tracking studies were performed by exploiting the specific interactions with a labeled streptavidin.