We present a generalized table of extinction coefficient data for silver nanoparticles from 8 to 100 nm. This table allows for easy and quick estimation of the concentration and size of modified and mono-dispersed silver nanoparticles from their optical spectra. We obtained data by determining the silver content of citrate-stabilised silver nanoparticles using sodium cyanide to dissolve the nanoparticles, and measuring solution conductivity with a pH meter and a cyanide-ion selective electrode. The quantification of the silver ion concentration enabled the calculation of extinction coefficients. Experimentally calculated extinction coefficients, in the current work, are in good agreement with collated literature values measured by different authors with non-standardized methodology and each for a limited range of particle size. They are also in good agreement with our theoretical calculations using Mie theory. Thus, we provide a highly standardized and comprehensive tabulated reference data-set.

An important feature necessary for biological stability of gold nanoparticles is resistance to ligand exchange. Here, we design and synthesize self-assembled monolayers of mixtures of small ligands on gold nanoparticles promoting high resistance to ligand exchange. We use as ligands short thiolated peptidols, e.g. H-CVVVT-ol, and ethylene glycol terminated alkane thiols (HS-C11-EG4). We present a straightforward method to evaluate the relative stability of each ligand shell against ligand exchange with small thiolated molecules. The results show that a ligand with a ‘thin’ stem, such as HS-C11-EG4, is an important feature to build a highly packed self-assembled monolayer and provide high resistance to ligand exchange. The greatest resistance to ligand exchange was found for the mixed ligand shells of the pentapeptidols H-CAVLT-ol or H-CAVYT-ol and the ligand HS-C11-EG4 at 30:70 (mole/mole). Mixtures of ligands of very different diameters, such as the peptidol H-CFFFY-ol and the ligand HS-C11-EG4, provide only a slightly lower stability against ligand exchange. These ligand shells are thus likely to be suitable for long-term use in biological environments. The method developed here provides a rapid screening tool to identify nanoparticles likely to be suitable for use in biological and biomedical applications.