Current interest in melanin films derived from the autoxidation of dopamine stems from their use as a universal adhesion layer. Here we report chemical and physical characterization of polydopamine films deposited on gold surfaces from stirred basic solutions at times ranging from 2 to 60 min, with a focus on times ≤10 min. Data from Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical methods suggest the presence of starting (dopamine) and intermediate (C=N-containing tautomers of quinone and indole) species in the polydopamine films at all deposition times. A uniform overlayer analysis of the XPS data indicates that film thickness increased linearly at short deposition times of ≤10 min. At deposition times ≥10 min, the films appeared largely continuous with surface roughness ≈ ≤ 2 nm, as determined by atomic force microscopy (AFM). Pinhole-free films, as determined by anionic redox probe measurements, required deposition times of 60 min or greater.

We present results of a systematic examination of functionalized gold nanoparticles (Au-NPs) by electrospray-differential mobility analysis (ES-DMA). Commercially available, citrate-stabilized Au colloid solutions (10-60 nm) were sized using ES-DMA, from which changes in particle size of less than 0.3 nm were readily discerned. It was found that the formation of salt particles and the coating of Au-NPs by salt during the electrospray process can interfere with the mobility analysis, which required the development of sample preparation and data correction protocols to extract correct values for the Au-NP size. Formation of self-assembled monolayers (SAMs) of alkanethiol molecules on the Au-NP surface was detected from a change in particle mobility, which could be modeled to extract the surface packing density of SAMs. A gas-phase temperature-programmed desorption (TPD) kinetic study of SAMs on Au-NPs found the data to be consistent with a second-order Arrhenius-based rate law, yielding an Arrhenius factor of 1.0 x 10 (11) s (-1) and an activation energy approximately 105 kJ/mol. For the size range of SAM-modified Au-NP we considered, the effect of surface curvature on the energetics of binding of carboxylic acid terminated SAMs is evidently negligible, with binding energies determined by TPD agreeing with those reported for the same SAMs on planar surfaces. This study suggests that the ES-DMA can be added to the tool set of characterization methods used to study the structure and properties of coated nanoparticles.

Commercially available monodisperse Au colloids (nominally 20 nm, 7 x10 11 particles/mL, citrate stabilized, Ted Pella Inc.) were employed herein. Commercial custom oligonucleotides were synthesized and HPLC-purified by the vendor and used as received without further purification. The 5' thiol-modified oligonucleotides were used without removing the protective S-(CH 2 ) 6 OH group from the 5' end as described in Petrovykh, D. Y., et al. 1 For brevity, these oligonucleotides are referred to as T x SH where x represents the number of thymine bases per strand, between 5 and 30.Conjugated Au colloids were prepared by adding DNA at 200 µmol/L to the particles in the ratio of 50 µL per 850 µL of gold nanoparticles and the solution was allowed to react ≥ 18 h. Salts were added in two stages. First, 20 µL of 5 mol/L NaCl and 10 µL of 1 mol/L K 2 HPO 4 were added per 850 µL of particles. Second, after ≥ 3 hours an additional 149 µL, 373 µL, or 746 µL was added to achieve the desired ionic strength, after which the reaction proceeded for