Phase controlled SERS enhancement

Surface-enhanced Raman spectroscopy (SERS) has attracted increasing interest for chemical and biochemical sensing. Several studies have shown that SERS intensities are significantly increased when an optical interference substrate composed of a dielectric spacer and a reflector is used as a supporting substrate. However, the origin of this additional enhancement has not been systematically studied. In this paper, high sensitivity SERS substrates composed of self-assembled core-satellite nanostructures and silica-coated silicon interference layers have been developed. Their SERS enhancement is shown to be a function of the thickness of silica spacer on a more reflective silicon substrate. Finite difference time domain modeling is presented to show that the SERS enhancement is due to a spacer contribution via a sign change of the reflection coefficients at the interfaces. The magnitude of the local-field enhancement is defined by the interference of light reflected from the silica-air and silica-silicon interfaces, which constructively added at the hot spots providing a possibility to maximize intensity in the nanogaps between the self-assembled nanoparticles by changing the thickness of silica layer. The core-satellite assemblies on a 135\u2009nm silica-coated silicon substrate exhibit a SERS activity of approximately 13 times higher than the glass substrate

Self-assembly of vertically aligned gold nanorod arrays

The main objective of this thesis was to develop a fabrication method combining bottom-up and top-down approaches to self-assemble anisotropic building blocks into advanced nanostructures on patterned substrates. Building blocks (gold nanorods) were synthesised following a seed-mediated protocol and subsequently purified by a fractionated precipitation strategy to remove nanoparticulate byproducts formed during the chemical synthesis. Gold nanorods (GNRs) were self-assembled into discrete vertically aligned arrays based on capillary and convective forces into templates fabricated by means of lithography processes. Patterned substrates were fabricated by a series of cleanroom processes to provide different templates for the self-assembly of GNRs. The role of the patterned surface was to guide and confine the fabrication of vertically aligned GNR arrays by providing a chemical and geometrical template. Recessed gold features were produced on silica-coated silicon wafers with a variety of shapes depending on the application envisioned for the GNR arrays. Square templates were fabricated by photolithography while rectangular patterns were produced by electron beam lithography. Surface treatments were carried out to endow the patterned substrates with a wettability contrast, required for the GNR self-assembly. The hydrophilicity of the gold surface was increased by a UV-ozone treatment and the silica surface was passivated with a PEG-silane functionalisation making it hydrophobic. The seed-mediated synthesis is a well-known method to produce GNRs but it also inevitably yields nanoparticulate byproducts. In a typical synthesis of GNRs with an aspect ratio of 3, three types of impurities can be identified: (1) large spherical nanoparticles, (2) nanoplates and (3) high aspect ratio nanorods. A size- and shape-selective purification strategy was developed to remove these three nanoparticulate byproducts from GNR solutions. The purification method exploits the sharp size-dependent colloidal stability threshold exhibited by gold nanoparticles functionalised with thiol-PEG-carboxyl. The ligand provides gold nanoparticles an excellent colloidal stability due to electrostatic interparticle repulsion. These repulsions forces can be attenuated to induce nanoparticle precipitation beyond specific thresholds of ionic strength or ethanol concentrations. Based on this concept, a two-step protocol enabled the separation of GNR from nanoparticulate byproducts increasing the purity of the as-synthesised GNR solution from 88.6% to 98.1%. The fabrication of vertically aligned GNR arrays was achieved by capillary and convective assembly on patterned substrates. The wettability contrast directed the self-assembly of GNR arrays onto predefined areas with an unprecedented accuracy. Two main factors have shown to play crucial roles in the self-assembly process. The temperature controlled the confinement of GNR arrays inside the template whereas the GNR concentration influenced the quality of the hexagonal close-packed (hcp) ordering of standing GNRs. When the self-assembly was performed at 45˚C with a GNR concentration of 9 nM, the nanostructures comprised three layers of vertically aligned nanorods with an interparticle distance of 5 nm and an excellent hcp ordering over a long range (substrate scale). The surface-enhanced Raman scattering (SERS) activity of these nanostructures has exhibited a sensitivity up to 36 times, when compared to a commercial SERS substrate

Self-assembly of vertically aligned gold nanorod arrays

The main objective of this thesis was to develop a fabrication method combining bottom-up and top-down approaches to self-assemble anisotropic building blocks into advanced nanostructures on patterned substrates. Building blocks (gold nanorods) were synthesised following a seed-mediated protocol and subsequently purified by a fractionated precipitation strategy to remove nanoparticulate byproducts formed during the chemical synthesis. Gold nanorods (GNRs) were self-assembled into discrete vertically aligned arrays based on capillary and convective forces into templates fabricated by means of lithography processes. Patterned substrates were fabricated by a series of cleanroom processes to provide different templates for the self-assembly of GNRs. The role of the patterned surface was to guide and confine the fabrication of vertically aligned GNR arrays by providing a chemical and geometrical template. Recessed gold features were produced on silica-coated silicon wafers with a variety of shapes depending on the application envisioned for the GNR arrays. Square templates were fabricated by photolithography while rectangular patterns were produced by electron beam lithography. Surface treatments were carried out to endow the patterned substrates with a wettability contrast, required for the GNR self-assembly. The hydrophilicity of the gold surface was increased by a UV-ozone treatment and the silica surface was passivated with a PEG-silane functionalisation making it hydrophobic. The seed-mediated synthesis is a well-known method to produce GNRs but it also inevitably yields nanoparticulate byproducts. In a typical synthesis of GNRs with an aspect ratio of 3, three types of impurities can be identified: (1) large spherical nanoparticles, (2) nanoplates and (3) high aspect ratio nanorods. A size- and shape-selective purification strategy was developed to remove these three nanoparticulate byproducts from GNR solutions. The purification method exploits the sharp size-dependent colloidal stability threshold exhibited by gold nanoparticles functionalised with thiol-PEG-carboxyl. The ligand provides gold nanoparticles an excellent colloidal stability due to electrostatic interparticle repulsion. These repulsions forces can be attenuated to induce nanoparticle precipitation beyond specific thresholds of ionic strength or ethanol concentrations. Based on this concept, a two-step protocol enabled the separation of GNR from nanoparticulate byproducts increasing the purity of the as-synthesised GNR solution from 88.6% to 98.1%. The fabrication of vertically aligned GNR arrays was achieved by capillary and convective assembly on patterned substrates. The wettability contrast directed the self-assembly of GNR arrays onto predefined areas with an unprecedented accuracy. Two main factors have shown to play crucial roles in the self-assembly process. The temperature controlled the confinement of GNR arrays inside the template whereas the GNR concentration influenced the quality of the hexagonal close-packed (hcp) ordering of standing GNRs. When the self-assembly was performed at 45˚C with a GNR concentration of 9 nM, the nanostructures comprised three layers of vertically aligned nanorods with an interparticle distance of 5 nm and an excellent hcp ordering over a long range (substrate scale). The surface-enhanced Raman scattering (SERS) activity of these nanostructures has exhibited a sensitivity up to 36 times, when compared to a commercial SERS substrate

Biocompatible gold nanorods: one-step surface functionalization, highly colloidal stability, and low cytotoxicity

The conjugation of gold nanorods (AuNRs) with polyethylene glycol (PEG) is one of the most effective ways to reduce their cytotoxicity arising from the cetyltrimethylammonium bromide (CTAB) and silver ions used in their synthesis. However, typical PEGylation occurs only at the tips of the AuNRs, producing partially modified AuNRs. To address this issue, we have developed a novel, facile, one-step surface functionalization method that involves the use of Tween 20 to stabilize AuNRs, bis(p-sulfonatophenyl)phenylphosphine (BSPP) to activate the AuNR surface for the subsequent PEGylation, and NaCl to etch silver from the AuNRs. This method allows for the complete removal of the surface-bound CTAB and the most active surface silver from the AuNRs. The produced AuNRs showed far lower toxicity than other methods to PEGylate AuNRs, with no apparent toxicity when their concentration is lower than 5 μg/mL. Even at a high concentration of 80 μg/mL, their cell viability is still four times higher than that of the tip-modified AuNRs

Supraparticles: Functionality from uniform structural motifs

Under the right process conditions, nanoparticles can cluster together to form defined particular structures, which can be termed supraparticles. Controlling the size, shape, and morphology of such entities is a central step in various fields of science and technology, ranging from colloid chemistry and soft matter physics to powder technology and pharmaceutical and food sciences. These diverse scientific communities have been investigating formation processes and structure/property relations of such supraparticles under completely different boundary conditions. On the fundamental side, the field is driven by the desire to gain maximum control of the assembly structures using very defined and tailored colloidal building-blocks, while more applied disciplines focus on optimizing the functional properties from rather ill-defined starting materials. With this review article, we aim to provide a connecting perspective by outlining fundamental principles that govern the formation and functionality of supraparticles. We discuss the formation of supraparticulates as a result of colloidal properties interplaying with external process parameters. We then outline how the structure of the supraparticles gives rise to different functional properties. They can be a result of the structure itself (emergent properties), of the colocalization of different, functional building-blocks, or of coupling between individual particles in close proximity. Taken together, we aim to establish structure-property and process-structure relationships that provide unifying guidelines for the rational design of functional supraparticles with optimized properties. Finally, we aspire to connect the different disciplines by providing a categorized overview of the existing, diverging nomenclature of seemingly similar supraparticle structures

Asymmetric gold nanodimer arrays: electrostatic self-assembly and SERS activity

A simple, scalable, low-cost and high-throughput nanofabrication method is developed to produce discrete gold nanoparticle (AuNP) dimer arrays. This method involves a two-step electrostatic self-assembly: (1) electrostatic immobilization of negatively charged AuNPs onto a positively charged surface and (2) electrostatic adsorption of a positively charged AuNP onto each pre-assembled AuNP. The latter requires a careful control of the electrostatic energy barrier originating from the interactions between the charged AuNPs and surfaces. This can readily be achieved by tuning the ionic strength of the self-assembly media. We calculate the interaction energies for immobilizing a single positively charged AuNP onto each pre-assembled NP at different ionic strengths and present successful experimental results on the synthesis of high-yield symmetric and asymmetric AuNP dimers (dimer yield: ∼85%). A theoretical and experimental investigation of their optical properties is conducted to correlate the spectral properties of these dimers with their structure. We also study the SERS activity of the as-synthesized AuNP dimers using benzenethiol as a model analyte. It is found that, with the increase of the size dissimilarity between the two NPs in the dimers, the Raman intensities of the analyte increase gradually. This trend is completely different from those of both single AuNPs and AuNP aggregates with identical particle size