Bernd Küstner

SERS Labels for Red Laser Excitation: Silica‐Encapsulated SAMs on Tunable Gold/Silver Nanoshells

In a glass house: Silica-encapsulated self-assembled monolayers (SAMs) on tunable gold/silver nanoshells were used as labels for surface-enhanced Raman scattering (SERS). This concept combines the spectroscopic advantages arising from maximum surface coverage and uniform molecular orientation of the Raman reporter molecules within the complete monolayer together with the high chemical and mechanical stability of the glass shell.

Synthesis of glass-coated SERS nanoparticle probes via SAMs with terminal SiO2 precursors.

Surface-enhanced Raman scattering (SERS) combines the benefits of vibrational Raman scattering with highest sensitivity, using nanostructures that can support localized plasmon resonances. [1‐4] The main application of SERS is the label-free detection of analytes. [5‐8] An alternative and more recent approach uses SERS as a readout method in bioanalytical applications: the selective detection of proteins and oligonucleotides is achieved by employing target-specific SERS nanoparticle probes. [7‐11] The central motivation for this strategy is the option to detect numerous target molecules within a single measurement (multiplexing). The basis of spectral multiplexing in SERS arises from the small linewidth of vibrational Raman bands compared with the significantly broader emission profiles of molecular fluorophores. [12,13] Further important advantages are quantification of target concentration and the extreme sensitivity of SERS, in particular SERRS (surface-enhanced resonance Raman scattering). [14,15] Additionally, the simultaneous excitation of spectrally distinct SERS nanoparticle probes requires only a single laser wavelength. Different designs for nanoparticle-based SERS probes (SERSlabels,SERSnanotags)areavailable,whichdifferinthe plasmonic nanostructure, the Raman reporter molecule, and the optional protective shell. [9,16‐18] Using a self-assembled monolayer(SAM)ofRamanreportermoleculesonthesurface of the nanoparticle has several advantages. [18‐22] Firstly,

UV resonance Raman spectroscopic monitoring of supramolecular complex formation: peptide recognition in aqueous solution

The formation of a supramolecular complex between a tetrapeptide and an artificial receptor , is monitored at submillimolar concentrations in water by UV resonance Raman spectroscopy. Using 275 nm excitation, we selectively probe the carboxylate binding site (CBS) within the receptor, a moiety which is very efficient in binding the carboxy terminus of peptides in aqueous media. Complexation of the receptor with the tetrapeptide involves the formation of a H-bond enforced ion pair, resulting in significant changes in the corresponding UV resonance Raman spectra. Our qualitative interpretation is based on experimental reference and calculated Raman spectra on model systems. First preliminary calculations show that for a quantitative analysis, also the distinct contributions of multiple CBS conformers must be considered in addition to the H-bond induced changes upon complexation.

Reactions of Superoxide with Iron Porphyrins in the Bulk and the Near-Surface Region of Ionic Liquids

The redox reaction of superoxide (KO2) with highly charged iron porphyrins (Fe(P4+), Fe(P8+), and Fe(P8-)) has been investigated in the ionic liquids (IL) [EMIM][Tf2N] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and [EMIM][B(CN)4] (1-ethyl-3-methylimidazolium tetracyanoborate) by using time-resolved UV/vis stopped-flow, electrochemistry, cryospray mass spectrometry, EPR, and XPS measurements. Stable KO2 solutions in [EMIM][Tf2N] can be prepared up to a 15 mM concentration and are characterized by a signal in EPR spectrum at g = 2.0039 and by the 1215 cm(-1) stretching vibration in the resonance Raman spectrum. While the negatively charged iron porphyrin Fe(P8-) does not react with superoxide in IL, Fe(P4+) and Fe(P8+) do react in a two-step process (first a reduction of the Fe(III) to the Fe(II) form, followed by the binding of superoxide to Fe(II)). In the reaction with KO2, Fe(P4+) and Fe(P8+) show similar rate constants (e.g., in the case of Fe(P4+): k1 = 18.6 ± 0.5 M(-1) s(-1) for the first reaction step, and k2 = 2.8 ± 0.1 M(-1) s(-1) for the second reaction step). Notably, these rate constants are four to five orders of magnitude lower in [EMIM][Tf2N] than in conventional solvents such as DMSO. The influence of the ionic liquid is also apparent during electrochemical experiments, where the redox potentials for the corresponding Fe(III)/Fe(II) couples are much more negative in [EMIM][Tf2N] than in DMSO. This modified redox and kinetic behavior of the positively charged iron porphyrins results from their interactions with the anions of the ionic liquid, while the nucleophilicity of the superoxide is reduced by its interactions with the cations of the ionic liquid. A negligible vapor pressure of [EMIM][B(CN)4] and a sufficient enrichment of Fe(P8+) in a close proximity to the surface enabled XPS measurements as a case study for monitoring direct changes in the electronic structure of the metal centers during redox processes in solution and at liquid/solid interfaces.

Titelbild: SERS-Marker für die Anregung mit rotem Laserlicht: Glasverkapselte SAMs auf Gold/Silber-Nanoschalen (Angew. Chem. 11/2009)

Glasverkapselte SAMs (SAM: selbstorganisierte Monoschicht) auf durchstimmbaren Au/Ag-Nanoschalen werden von S. Schlucker et al. in der Zuschrift auf S. 1984 ff. vorgestellt. Diese Systeme haben ein groses Potenzial als Marker fur die oberflachenverstarkte Raman-Streuung (SERS) in bioanalytischen und biomedizinischen Anwendungen, die auf einer Anregung mit rotem Laserlicht beruhen, z. B. in Assays und der Mikroskopie. Die Methode kombiniert die spektroskopischen Vorteile von SAMs mit der Stabilitat einer Glashulle.