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

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We demonstrate the utility of electrospray gas-phase ion-mobility analysis as a new method to investigate nanoparticle flocculation, or aggregation. Au nanoparticle (Au-NP) solutions were sampled via electrospray (ES), followed by differential ion-mobility analysis (DMA) to determine the particle mobility distribution. Multimodal size distributions obtained with ES-DMA indicated the presence of single Au-NPs (monomer) as well as larger Au-NP clusters such as dimers, trimers, and tetramers under specific solution conditions. The fraction of each aggregate species as a function of time was quantitatively characterized, from which the degree of aggregation, aggregation rate, and stability ratio at different ionic strengths were determined. The latter enabled the extraction of a surface potential (or surface charge density) of 64 +/- 2 mV for 10 nm Au-NPs, which is in good agreement with values obtained from other methods, thus validating our approach. Our results show that ES-DMA is a valuable tool for quantitatively probing the early stages of colloidal aggregation or as a preparatory tool for the size election of aggregates.