Quantifying Surface Area of Nanosheet Graphene Oxide Colloid Using a Gas-Phase Electrostatic Approach.

We demonstrate a new, facile gas-phase electrostatic approach to successfully quantify equivalent surface area of graphene oxide (GO) colloid on a number basis. Mobility diameter (dp,m)-based distribution and the corresponding equivalent surface area (SA) of GO colloids (i.e., with different lateral aspect ratios) were able to be identified by electrospray-differential mobility analysis (ES-DMA) coupled to a condensation particle counter (CPC) and an aerosol surface area analyzer (ASAA). A correlation of SA ∝ dp,m2.0 was established using the ES-DMA-CPC/ASAA, which is consistent with the observation by the 2-dimensional image analysis of size-selected GOs. An ultrafast surface area measurement of GO colloid was achieved via a direct coupling of ES with a combination of ASAA and CPC (i.e., measurement time was 2 min per sample; without size classification). The measured equivalent surface area of GO was ∼202 ± 7 m2 g-1, which is comparable to Brunauer-Emmett-Teller (BET) surface area, ∼240 ± 59 m2 g-1. The gas-phase electrostatic approach proposed in this study has the superior advantages of being fast, requiring no elaborate drying process, and requiring only a very small amount of sample (i.e., <0.01 mg). To the best of our knowledge, this is the first study of using an aerosol-based electrostatic coupling technique to obtain the equivalent surface area of graphene oxide on a number basis with a high precision of measurement.

Quantitative characterization of colloidal assembly of graphene oxide-silver nanoparticle hybrids using aerosol differential mobility-coupled mass analyses.

In this work, we develop an aerosol-based, time-resolved ion mobility-coupled mass characterization method to investigate colloidal assembly of graphene oxide (GO)-silver nanoparticle (AgNP) hybrid nanostructure on a quantitative basis. Transmission electron microscopy (TEM) and zeta potential (ZP) analysis were used to provide visual information and elemental-based particle size distributions, respectively. Results clearly show a successful controlled assembly of GO-AgNP by electrostatic-directed heterogeneous aggregation between GO and bovine serum albumin (BSA)-functionalized AgNP under an acidic environment. Additionally, physical size, mass, and conformation (i.e., number of AgNP per nanohybrid) of GO-AgNP were shown to be proportional to the number concentration ratio of AgNP to GO (R) and the selected electrical mobility diameter. An analysis of colloidal stability of GO-AgNP indicates that the stability increased with its absolute ZP, which was dependent on R and environmental pH. The work presented here provides a proof of concept for systematically synthesizing hybrid colloidal nanomaterials through the tuning of surface chemistry in aqueous phase with the ability in quantitative characterization. Graphical Abstract Colloidal assembly of graphene oxide-silver nanoparticle hybrids characterized by aerosol differential mobility-coupled mass analyses.

Size-Dependent Localized Surface Plasma Resonance of Au Nanoparticles in Au/ZnO Photoanodes for Dye-Sensitized Solar Cells.

The size effect of Au nanoparticles on plasmonic ZnO dye-sensitized cells (DSSCs) was investigated. Different sized Au nanoparticles (~5 nm, 10 nm, and 20 nm) were directly deposited on ZnO nanostructures via an in situ reduction technique. The size and the loading of Au nanoparticle were controlled by varying the amount of reducing agent and the reaction time, respectively. By introducing a proper amount of Au nanoparticles into the photoanode, plasmon-enhanced light absorption, photocurrent and power conversion efficiency were demonstrated, with the enhancement increased with decreasing Au particle size. Overloading the photoanode with Au nanoparticles, however, led to a decline in photocurrent and thus the cell efficiency. Au/ZnO DSSCs with optimized film thickness and 5 nm-Au loading attained an efficiency of 3.49%, corresponding to a 59% improvement over that of pure ZnO DSSCs. The improvement in cell efficiency was ascribed to a significant increase in the photocurrent of Au/ZnO devices, as a result of enhanced light harvesting and reduced interfacial resistance in the photoanode.

Surface PEGylation of Silver Nanoparticles: Kinetics of Simultaneous Surface Dissolution and Molecular Desorption.

A quantitative study of the stability of silver nanoparticles (AgNPs) conjugated with thiolated polyethylene glycol (SH-PEG) was conducted using gas-phase ion-mobility and mass analyses. The extents of aggregation and surface dissolution of AgNPs, as well as the amount of SH-PEG adsorption and desorption, were able to be characterized simultaneously for the kinetic study. The results show that the SH-PEG with a molecular mass of 6 kg/mol (SH-PEG6K) was able to adsorb to the surface of AgNP to form PEG6K-HS-AgNP conjugates, with the maximum surface adsorbate density of ∼0.10 nm(-2). The equilibrium binding constant for SH-PEG6K on AgNPs was calculated as ∼(4.4 ± 0.9) × 10(5) L/mol, suggesting a strong affinity due to thiol bonding to the AgNP surface. The formation of SH-PEG6K corona prevented PEG6K-HS-AgNP conjugates from aggregation under the acidic environment (pH 1.5), but dissolution of core AgNPs occurred following a first-order reaction. The rate constant of Ag dissolution from PEG6K-HS-AgNP was independent of the starting surface packing density of SH-PEG6K on AgNP (σ0), indicating that the interactions of H(+) with core AgNP were not interfered by the presence of SH-PEG6K corona. The surface packing density of SH-PEG6K decreased simultaneously following a first-order reaction, and the desorption rate constant of SH-PEG6K from the conjugates was proportional to σ0. Our work presents the first quantitative study to illustrate the complex mechanism that involves simultaneous aggregation and dissolution of core AgNPs in combination with adsorption and desorption of SH-PEG. This work also provides a prototype method of coupled experimental scheme to quantify the change of particle mass versus the corresponding surface density of functional molecular species on nanoparticles.

Noble metal-titania hybrid nanoparticle clusters and the interaction to proteins for photo-catalysis in aqueous environments.

We report a systematic study of the controlled synthesis of new hybrid spherical TiO2 nanoparticle cluster (TiO2-NPC) homogeneously decorated with noble metal nanoparticles (NPs) by gas-phase evaporation-induced self-assembly. Silver NP (AgNP) was used as the representative noble metal NP. The degradation of methyl blue (MB) in the aqueous solution was chosen as the representative system for the study of photocatalysis, which were tested and evaluated with respect to irradiation conditions and the presence of bovine serum albumin (BSA). The results show that particle size and chemical composition of the hybrid nanostructure were tunable by choosing the suitable concentration of precursors. The photocatalytic activity of AgNP-decorated TiO2-NPC was strongly affected by the light irradiation and the ligand-nanoparticle interfacial interaction. The presence of BSA influenced molecular conjugation to the surface of the hybrid nanostructure. Under conditions of simultaneous competitive adsorption of MB and BSA, the combination of AgNPs improved the photocatalytic activity of the TiO2-NPC-based catalysts. Our work describes a prototype methodology to fabricate TiO2-NPC homogeneously decorated with noble metal NPs with well-controlled material properties. The mechanistic understanding developed in this study can be useful for the future optimization of material properties of hybrid nanostructures versus interfacial interactions with the surrounding molecules.