The Role of Entropy in Nanoparticle Agglomeration.

Agglomeration processes in non-interacting particle systems can be understood from a thermodynamic point of view. If the enthalpy of agglomeration is negligible, the distribution of agglomeration states adopts the state of highest entropy. Herein, we provide the exact analytical solution to the mole fractions of agglomerates comprising i monomers, xi =2-i .

Tracking of photochemical Ostwald ripening of nanoparticles through voltammetric atom counting.

We report the tracking of atom count in individual nanoparticles during photochemical Ostwald ripening. The nano-impact technique, in conjunction with UV-Vis and TEM analysis, is used to follow the photochemical formation of silver nano-prisms from spherical seed particles. A mechanism of photochemical Ostwald ripening is deduced and key growth stages are identified.

Femtomolar Detection of Silver Nanoparticles by Flow-Enhanced Direct-Impact Voltammetry at a Microelectrode Array.

We report the femtomolar detection of silver (Ag) nanoparticles by direct-impact voltammetry. This is achieved through the use of a random array of microelectrodes (RAM) integrated into a purpose-built flow cell, allowing combined diffusion and convection to the electrode surface. A coupled RAM-flow cell system is implemented and is shown to give reproducible wall-jet type flow characteristics, using potassium ferrocyanide as a molecular redox species. The calibrated flow system is then used to detect and quantitatively size Ag nanoparticles at femtomolar concentrations. Under flow conditions, it is found the nanoparticle impact frequency increases linearly with the volumetric flow rate. The resulting limit of detection is more than 2 orders of magnitude smaller than the previous detection limit for direct-impact voltammetry (900 fM) [J. Ellison et al. Sens. Actuators, B 2014, 200, 47], and is more than 30 times smaller than the previous detection limit for mediated-impact voltammetry (83 fM) [T. M. Alligrant et al. Langmuir 2014, 30, 13462].

Bi2 O3 Nanoparticle Clusters: Reversible Agglomeration Revealed by Imaging and Nano-Impact Experiments.

Colloidal suspensions of Bi2 O3 nanoparticles were studied in aqueous solution using imaging and electrochemical techniques. Nanoparticle tracking analysis revealed the particles to be agglomerated. In contrast, electrochemical detection via the nano-impacts technique showed almost exclusive detection of monomeric nanoparticles. Comparison of the two techniques allows the conclusion to be drawn that the agglomeration/deagglomeration of the nanoparticles is reversible. A minimum rate constant for the deagglomeration process was estimated.

Electrochemical Nanoparticle Sizing Via Nano-Impacts: How Large a Nanoparticle Can be Measured?

The field of nanoparticle (NP) sizing encompasses a wide array of techniques, with electron microscopy and dynamic light scattering (DLS) having become the established methods for NP quantification; however, these techniques are not always applicable. A new and rapidly developing method that addresses the limitations of these techniques is the electrochemical detection of NPs in solution. The 'nano-impacts' technique is an excellent and qualitative in situ method for nanoparticle characterization. Two complementary studies on silver and silver bromide nanoparticles (NPs) were used to assess the large radius limit of the nano-impact method for NP sizing. Noting that by definition a NP cannot be larger than 100 nm in diameter, we have shown that the method quantitatively sizes at the largest limit, the lower limit having been previously reported as ∼6 nm.1.