Highly Tunable Growth and Etching of Silica Shells on Surfactant‐Free Gold Nanostars

Abstract

Herein, we report an efficient method to coat gold nanostars with a silica shell of variable thickness and then tunably etch it by using a mild silica etching reagent, NaBH 4 . This method generates a novel type of nanostar coating in which silica only covers the core, leaving the tips variably exposed.Gold nanostars are a subtype of colloidal nanomaterial that consists of a central core from which multiple sharp branches protrude. [1][2][3][4] They exhibit intense and tunable localized surface plasmon resonances (LSPRs) in the near-infrared (NIR), that have been leveraged in biological applications, especially for multimodal imaging and photothermal therapy. [5][6][7][8] Owing to their resonances in the NIR and intense near-field enhancements, nanostars can afford intense imaging response and photothermal heating without substantial tissue damage, which is important in view of clinical implementations. To better understand how to best leverage these particles, it is first necessary to characterize their physical and optical properties, and to do so quantitatively, while also trying to design predictive tools that could aid us in tuning their synthesis to achieve the desired behavior. As a first step toward this goal, we have recently implemented an experimental-computational approach to calculate the extinction coefficient of gold nanostars employing finite element (FEM) calculations, obtaining very similar results to what achieved by Hamad-Schifferli and co-workers, who employed discrete dipole approximation (DDA) calculations. [9][10] The evaluation of extinction coefficients for plasmonic nanoparticles is the gateway to quantification and real life applicability; nonetheless, it is extremely complex for nanoparticles of complex morphology such as nanostars, and often not practical. Therefore, we hypothesized that by going back to the drawing board, and cleverly designing SiO 2 coating protocols, which would leave only the nanostar spikes exposed from the silica shell, we could still leverage the intense near-field enhancements at the tips while having to deal with a much more simplified morphology. Moreover, the partial silica coating would increase colloidal stability and biocompatibility, leading to improved applicability in vitro. As we previously observed, [11] the optical properties of tunable silica shells can be reflected in the overall dielectric function of the core-shell colloid, leading to surface-enhanced Raman scattering (SERS) enhancement factors that depend both on the extent of tip exposure and the thickness of the silica shell, providing SERS tags of variable brightness. Furthermore, in view of the potential use of nanostars as hot electron-based photocatalysts, [12][13] the possibility to isolate the tips from the remainder of the nanostar, thus enabling us to prove their role as source of hot electrons, could lead to interesting fundamental studies on the generation and fate of these hot carriers. While protocols exist to coat nanostars with silica, and silica coated nanostars have found wi...

Ultra-high SERS detection of consumable coloring agents using plasmonic gold nanostars with high aspect-ratio spikes

“…First, the morphology of surfactant-free nanostars can be easily tuned by changing the synthesis parameters including gold salts, seeds, ascorbic acid, AgNO 3 , and HCl concentration. 33,48,51 On the other hand, for surfactantbased nanostars synthesis, the surfactant molecules bind on highly energetic crystal facets of intermediated seeds to stabilize the seeds, thereby generating of a smaller number of nucleation centers to grow the spikes which result in a lower number of spikes. 41 Moreover, the plasmon peak of surfactantfree stars can be easily tuned to the visible to NIR region so that we can utilize the 785 lasers to achieve the highest SERS enhancement.…”

SERS Nanoprobe for Intracellular Monitoring of Viral Mutations

“…To create SANSPs, we first conjugated silica-coated gold nanostars (SiO 2 @AuNS) with Cy3-tagged beacon DNA via Au-thiol covalent bonding. The SiO 2 @AuNS synthesis is depicted in Figure S1; the detailed protocol for silica shell growth and etching was reported elsewhere by our group . SiO 2 @AuNSs possess excellent plasmonic response in the near-infrared (NIR), good biocompatibility in cells, as well as robust surface functionalization .…”

“…The SiO 2 @AuNS synthesis is depicted in Figure S1; the detailed protocol for silica shell growth and etching was reported elsewhere by our group. 22 SiO 2 @AuNSs possess excellent plasmonic response in the near-infrared (NIR), good biocompatibility in cells, as well as robust surface functionalization. 23 In addition, SiO 2 @AuNSs can be partially etched to leave only the tips of nanostars exposed and amenable to the covalent conjugation of the DNA beacons, designed in particular to avoid binding the DNA at the basis of the AuNS spikes, where nonspecific adsorption and steric hindrance could compromise efficient target detection.…”

Gold Nanostars in Biology and Medicine: Understanding Physicochemical Properties to Broaden Applicability

“…One approach we developed leverages the use of silica shells. In this approach, a spherical shell can be grown to completely cover the nanostars and then be tunably etched using NaBH 4 as a mild reducing agent . This functionalization scheme not only stabilizes the nanostars rendering them more biocompatible but also forces the functionalized aptamers to selectively bind only to the tips of the spikes, thus avoiding possible conformational issues, such as adsorption of the strands to the side facets of the spikes, where they may lay down flat due to the high affinity of nucleobases for gold.…”

“…In this approach, a spherical shell can be grown to completely cover the nanostars and then be tunably etched using NaBH 4 as a mild reducing agent. 76 This functionalization scheme not only stabilizes the nanostars rendering them more biocompatible but also forces the functionalized aptamers to selectively bind only to the tips of the spikes, thus avoiding possible conformational issues, such as adsorption of the strands to the side facets of the spikes, where they may lay down flat due to the high affinity of nucleobases for gold. Added bonuses would be the availability of a 3D space that can be explored by the oligos at the tips of the spikes compared to the valley and the likelihood of rendering stoichiometric ratios between nanoparticles and bound molecules determined exclusively by the number of tips exposed, being thus more manageable.…”