General approach to surface-accessible plasmonic Pickering emulsions for SERS sensing and interfacial catalysis.

Pickering emulsions represent an important class of functional materials with potential applications in sustainability and healthcare. Currently, the synthesis of Pickering emulsions relies heavily on the use of strongly adsorbing molecular modifiers to tune the surface chemistry of the nanoparticle constituents. This approach is inconvenient and potentially a dead-end for many applications since the adsorbed modifiers prevent interactions between the functional nanosurface and its surroundings. Here, we demonstrate a general modifier-free approach to construct Pickering emulsions by using a combination of stabilizer particles, which stabilize the emulsion droplet, and a second population of unmodified functional particles that sit alongside the stabilizers at the interface. Freeing Pickering emulsions from chemical modifiers unlocks their potential across a range of applications including plasmonic sensing and interfacial catalysis that have previously been challenging to achieve. More broadly, this strategy provides an approach to the development of surface-accessible nanomaterials with enhanced and/or additional properties from a wide range of nano-building blocks including organic nanocrystals, carbonaceous materials, metals and oxides.

Self-assembly of colloidal nanoparticles into 2D arrays at water-oil interfaces: rational construction of stable SERS substrates with accessible enhancing surfaces and tailored plasmonic response.

Self-assembly at water-oil interfaces has been shown to be a cheap, convenient and efficient route to obtain densely packed layers of plasmonic nanoparticles which have small interparticle distances. This creates highly plasmonically active materials that can be used to give strong SERS enhancement and whose structure means that they are well suited to creating the highly stable, reproducible and uniform substrates that are needed to allow routine and accurate quantitative SERS measurements. A variety of methods have been developed to induce nanoparticle self-assembly at water-oil interfaces, fine tune the surface chemistry and adjust the position of the nanoparticles at the interface but only some of these are compatible with eventual use in SERS, where it is important that target molecules can access the active surface unimpeded. Similarly, it is useful to transform liquid plasmonic arrays into easy-to-handle free-standing solid films but these can only be used as solid SERS substrates if the process leaves the surface nanoparticles exposed. Here, we review the progress made in these research areas and discuss how these developments may lead towards achieving rational construction of tailored SERS substrates for sensitive and quantitative SERS analysis.

Filter paper based SERS substrate for the direct detection of analytes in complex matrices.

Surface-enhanced Raman spectroscopy (SERS) is an emerging analytical technique for chemical analysis, which is favourable due to its combination of short measurement time, high sensitivity and molecular specificity. However, the application of SERS is still limited, largely because in real samples the analyte is often present in a complex matrix that contains micro/macro particles that block the probe laser, as well as molecular contaminants that compete for the enhancing surface. Here, we show a simple and scalable spray-deposition technique to fabricate SERS-active paper substrates which combine sample filtration and enhancement in a single material. Unlike previous spray-deposition methods, in which simple colloidal nanoparticles were sprayed onto solid surfaces, here the colloidal nanoparticles are mixed with hydroxyethyl cellulose (HEC) polymer before application. This leads to significantly improved uniformity in the distribution of enhancing particles as the film dries on the substrate surface. Importantly, the polymer matrix also protects the enhancing particles from air-oxidation during storage but releases them to provide SERS enhancement when the film is rehydrated. These SERS-paper substrates are highly active and a model analyte, crystal violet, was detected down to 4 ng in 10 µL of sample with less than 20% point-by-point signal deviation. The filter paper and HEC effectively filter out both interfering micro/macro particles and molecular (protein) contaminants, allowing the SERS-paper substrates to be used for SERS detection of thiram in mud and melamine in the presence of protein down to nanogram levels without sample pre-treatment or purification.

Ultra-Stable Plasmonic Colloidal Aggregates for Accurate and Reproducible Quantitative SE(R)RS in Protein-Rich Biomedia.

Au/Ag colloids aggregated with simple salts are amongst the most commonly used substrates in surface-enhanced (resonance) Raman spectroscopy (SE(R)RS). However, salt-induced aggregation is a dynamic process, which means that SE(R)RS enhancements vary with time and that measurements therefore need to be taken at a fixed time point, normally within a short time-window of a few minutes. Here, we present an emulsion templated method which allows formation of densely-packed quasi-spherical Au/Ag colloidal aggregates. Since the particles in the product aggregates retain their weakly adsorbed charged ligands and the ionic strength remains low these charged aggregates resist further aggregation while still providing intense SE(R)RS enhancement which remains stable for days. This eliminates a major source of irreproducibility in conventional colloidal SE(R)RS measurements and paves the way for SE(R)RS analysis in complex systems, such as protein-rich bio-solutions where conventional aggregated colloids fail.

Dataset demonstrating the working-principles of surface-exposed nanoparticle sheet enhanced Raman spectroscopy (SENSERS) for solvent-free SERS.

Here, we present surface-enhanced Raman data for the calculation of signal uniformity and enhancement factor in SENSERS (surface-exposed nanoparticle sheet enhanced Raman spectroscopy). SEM was used to characterize the microstructure of the solid sample. The interaction between the solid sample and surface-exposed nanoparticle sheet was characterized using SERS and SEM. Based on these data a "skin" versus "sheet" type calculation method was used to calculate the magnitude of Raman signal enhancement within SENSERS. The data presented in this article is related to the research article entitled "Pressing Solids Directly Into Sheets of Plasmonic Nanojunctions Enables Solvent-Free Surface-Enhanced Raman Spectroscopy" (Xu et al., 2018).

Dataset on constructing colloidal nanoparticles into dry nano-micro-particle (NMP) powders with Nanoscale Magnetic, Plasmonic and Catalytic Functionalities.

The data presented in this article is related to the research article entitled "A One-Pot Method for Building Colloidal Nanoparticles into Bulk Dry Powders with Nanoscale Magnetic, Plasmonic and Catalytic Functionalities" (Ye et al., 2019). The data shows the hydrophobicity of the nanoparticle (NP) building blocks used for constructing NMPs obtained through contact angle measurements, along with the effect of NP hydrophobicity on the stability of the parent Pickering emulsions. SEM data of the morphology of NMPs is presented. Finally, a mathematical model is presented to predict the average diameter of NMPs produced via different experimental parameters.

Dataset on constructing colloidal nanoparticles into dry nano-micro-particle (NMP) powders with nanoscale magnetic, plasmonic and catalytic functionalities.

The data presented in this article is related to the research article entitled "A One-Pot Method for Building Colloidal Nanoparticles into Bulk Dry Powders with Nanoscale Magnetic, Plasmonic and Catalytic Functionalities" (Ye et al., 2019) The data shows the hydrophobicity of the nanoparticle (NP) building blocks used for constructing NMPs obtained through contact angle measurements, along with the effect of NP hydrophobicity on the stability of the parent Pickering emulsions. SEM data of the morphology of NMPs is presented. Finally, a mathematical model is presented to predict the average diameter of NMPs produced via different experimental parameters.