Small noble metal clusters can serve as optical probes for their biomolecular environment. In particular small silver clusters in the size regime in which "each atom counts" exhibit remarkable optical properties. This is due to the molecular-like transitions in these systems which allow for narrow absorption and discrete electronically excited states as well as fluorescence. This is complementary to large-size metal nanoparticles with properties adressed to plasmonic behaviour.
Moreover the small clusters have been shown to be biocompatible enabling the possibility of biosensing applications, even in living cells.
The biomolecular environment can serve as template for the formation of the clusters, stabilizing the structure and charge state and generates an optical fingerprint through the structure specific interaction of excited states of biomoleclue and cluster-chromophore. We study isolated hybrid systems consisting of small noble metal clusters and biomolecules in the gas phase and at support. to gain fundamental insight into the mechanisms for absorption and emission enhancement and to propose building blocks for applications. Our ultimate goal is to design a probe that produces a binding site-specific signal with an enhanced intensity which allows for a more sensitive detection in future biochips suitable for label free-detection.
In our Letter on the TrpGlyAg3+, we explored experimentally and theoretically the optical properties of an isolated silver trimer-dipeptide complex and we rationalize the enhancement of absorption that is induced by the metallic particle. Furthermore, we show that its binding to a dipeptide or tripeptide can profoundly affect the conformational landscape.
In another joint experimental and theoretical study, we have demonstrated the synthesis of the smallest possible precursor for bio-gold hybrids consisting of a single gold cation bound to tryptophan and report on its unique optical properties: a strong absorption band in the visible spectral region attributed to charge transfer excitations.
In brief, we have developed a new route for producing small gold biomolecule complexes. The complexation with a metal cation drastically changes the optical properties of tryptophan. An intense absorption band in the visible range is observed and theoretically confirmed for gold. The present work opens the way for using gold atoms as efficient protein labeling agents for absorption spectroscopy in the visible range.
Optical photofragmentation spectroscopy was carried out in an ion trap. Theoretical calculations employed the correlated wavefunction-based approximate coupled cluster method CC2 within the linear response formalism.
We show in our work comparing copper-, silver- and gold-cluster-biochromophore hybrid systems that the silver-based representatives exhibit markedly different optical properties than analogue copper and gold hybrids. This is reflected in an energetically localized resonance at approx. 320 nm enhancing strongly the absorption of biomolecules. In contrast, copper and gold hybrid systems exhibit very different behaviour due to a small s-d energy gap leading to a high density of states with low intensities spread in a broad energy interval. Our results provide insight into the factors governing the proper choice of metallic labels for bioanalytical applications based on localized enhancement of absorption and fluorescence.
In order to elucidate how noble metal clusters can be utilized as biosensing materials we have performed a theoretical investigation of structural and optical properties of silver and gold cluster-dipeptide hybrids bound to the FS defect of the MgO (100) surface. We use DFT and its TDDFT variant combined with the polarizable embedded cluster model for the description of the extended MgO environment. As model peptide we have chosen CysTrp since the cysteine residue interacts strongly with metal particles through the sulfur atom and tryptophan is the most important chromophoric aminoacid. Our results show that in the case of CysTrp bound to the supported Ag4 cluster an intense optical signal arises at 400 nm. In contrast, in the case of gold, no strongly localized absorption is present since the optical response of supported gold-peptide hybrids is dominated by a large number of low-intensity d-electron excitations that are spread over a broad energy range. For the silver species, a drastic change of the absorption energy is triggered by the binding event of the cluster to the biomolecule. The resulting change in the optical signal of supported silver clusters can therefore be exploited for the optical detection of peptides in future cluster-based biochips.