Bimetallic nanoparticles of noble metals are of high interest in imaging, biomedical devices, including nanomedicine, and heterogeneous catalysis. Synthesis, properties, characterization, biological properties, and practical applicability of nanoparticles on the basis of platinum group metals and the coin metals Ag and Au are discussed, also in comparison with the corresponding monometallic nanoparticles. In addition to the parameters that are required to characterize monometallic nanoparticles (mainly size, size distribution, shape, crystallographic nature, surface functionalization, charge), further information is required for a full characterization of bimetallic nanoparticles. This concerns the overall elemental composition of a bimetallic nanoparticle population (ratio of the two metals) and the internal distribution of the elements in individual nanoparticles (e.g., the presence of homogeneous alloys, core-shell systems, and possible intermediate stages). It is also important to ensure that all particles are identical in terms of elemental composition, that is, that the homogeneity of the particle population is given. Macroscopic properties like light absorption, antibacterial effects, and catalytic activity depend on these properties. The currently available methods for a full characterization of bimetallic nanoparticles are discussed, and future developments in this field are outlined.

The azidation of glutathione (GSH)‐functionalized ultrasmall gold nanoparticles (2 nm) by the azide transfer reagent imidazole‐1‐sulfonyl azide hydrogen sulfate leads to azide‐terminated nanoparticles with high yield. A subsequent copper‐catalyzed azide‐alkyne cycloaddition (CuAAC), i. e. a click reaction, leads to covalently functionalized nanoparticles. This was demonstrated with two alkyne‐functionalized dyes, i. e. FAM‐alkyne and AlexaFluor‐647‐alkyne, that were covalently coupled to the nanoparticles. The integrity of the glutathione ligand and the successful surface azidation were demonstrated by one‐dimensional and two‐dimensional NMR spectroscopy. The surface composition of the nanoparticles was determined by quantitative NMR spectroscopy and UV/vis spectroscopy. Each nanoparticle carries 125 glutathione molecules of which 118 were substituted by an azide group. After dye conjugation, either 6 FAM molecules or 11 AlexaFluor‐647 molecules were present on each nanoparticle, respectively. The dye‐clicked nanoparticles were highly fluorescent due to the absence of surface plasmon resonance. The post‐functionalization of GSH avoids a chemical reaction of a functional ligand during the reduction reaction, gives a high yield (up to 50 mg nanoparticles per batch), is based on water as solvent, and is applicable for metallic nanoparticles in general.

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Ultrasmall gold nanoparticles with a metallic core diameter of 2 nm were surface-conjugated with peptides that selectively target epitopes on the surface of the WW domain of the model protein hPin1 (hPin1-WW). The binding to the gold surface was accomplished via the thiol group of a terminal cysteine. The particles were analyzed by NMR spectroscopy, highresolution transmission electron microscopy, and differential centrifugal sedimentation. The surface loading was determined by conjugating a FAMlabeled peptide, followed by UV−vis spectroscopy, and by quantitative 1 H NMR spectroscopy, showing about 150 peptide molecules conjugated to each nanoparticle. The interaction between the peptide-decorated nanoparticles with hPin1-WW was probed by 1 H− 15 N-HSQC NMR titration, fluorescence polarization spectroscopy (FP), and isothermal titration calorimetry (ITC). The particles showed a similar binding (K D = 10−20 μM) compared to the dissolved peptides (K D = 10−30 μM). Small-angle Xray scattering (SAXS) showed that the particles were well dispersed and did not agglomerate after the addition of hPin1-WW (no cross-linking by the protein). Each nanoparticle was able to bind about 20 hPin1-WW protein molecules. An unspecific interaction with hPin1 was excluded by the attachment of a nonbinding peptide to the nanoparticle surface. The uptake by cells was studied by confocal laser scanning microscopy. The peptide-functionalized nanoparticles penetrated the cell membrane and were located in the cytosol. In contrast, the dissolved peptide did not cross the cell membrane. Peptide-functionalized nanoparticles are promising agents to target proteins inside cells.