A high yield, one-pot dialysis-based process for self-assembly of near infrared absorbing gold nanoparticles

DNA-mediated hierarchical organization of gold nanoprisms into 3D aggregates and their application in surface-enhanced Raman scattering

Colloidal crystallization using DNA provides a robust method for fabricating highly programmable nanoparticle superstructures with collective plasmonic properties. Here, we report on the DNA-guided fabrication of 3D plasmonic aggregates from polydisperse gold nanoprisms. We first construct 1D crystals via DNA-induced and shape-directed face-to-face assembly of anisotropic gold nanoprisms. Using the near-T_m thermal annealing approach that promotes long-range DNA-induced interaction and ordering, we then assemble 1D nanoprism crystals into a 3D nanoprism aggregate that exhibits a polycrystalline morphology with nanoscale ordering and microscale dimensions. The presence of closely packed nanoprism arrays over a large area gives rise to strong near-field plasmonic coupling and generates a high density of plasmonic hot spots within the 3D nanoprism aggregates that exhibit excellent surface-enhanced Raman scattering performance. The plasmonic 3D nanoprism aggregates demonstrate significant SERS enhancement (<10⁶), and low detection limits (10^-9M) with good sample-to-sample reproducibility (CV ∼ only 5.6%) for SERS analysis of the probe molecule, methylene blue. These findings highlight the potential of 3D anisotropic nanoparticle aggregates as functional plasmonic nanoarchitectures that could find applications in sensing, photonics, optoelectronics and lasing.

DNA-induced assembly of gold nanoprisms and polystyrene beads into 3D plasmonic SERS substrates

The utilization of nanoparticle-polymer bead hybrid nanostructures as a SERS substrate depends on the control of the deposition, density, and distribution of nanoparticles on the bead surface. Here we demonstrate the fabrication of a large area SERS substate via a two- step DNA mediated assembly of gold nanoprisms and polystyrene (PS) beads into a large ensemble of beads that are densely coated with nanoprisms. First, nanoprisms are loaded on PS beads through DNA hybridization. The close packed arrangement of anisotropic nanoprisms in different orientations on a bead surface results in a plasmonic substrate with a variable nanogap size ranging 1-20 nm. Nanoprisms-coated beads are then assembled into a large stack or aggregate of beads using a DNA-induced crystallization approach. Each aggregate consists of 20-50 nanoprisms-coated beads, leading to the formation a large area of three-dimensional SERS substrate with a high-density of hot spots for SERS enhancement. An excellent enhancement factor (EF) of [Formula: see text] and a very high detection sensitivity (up to 10^-10 M) are observed for the analysis of a probe molecule (Methylene blue) using the SERS substrate.

Surfactant-Free Synthesis and Scalable Purification of Triangular Gold Nanoprisms with Low Non-Specific Cellular Uptake

Gold nanoprisms possess remarkable optical properties that make them useful for medical biotechnology applications such as diagnosis and photothermal therapy. However, shape-selective synthesis of gold nanoprisms is not trivial and typically requires either toxic surfactants or time-consuming purification protocols, which can limit their applicability. Here, we show how triangular gold nanoprisms of different sizes can be purified by precipitation using the non-toxic glutathione ligand, thereby removing the need for toxic surfactants and bottleneck purification techniques. The protocol is amenable for direct scaling up as no instrumentation is required in the critical purification step. The new purification method provides a two-fold increased yield in gold nanoprisms compared to electrophoretic filtration, while providing nanoprisms of similar localized surface plasmon resonance wavelength. Crucially, the gold nanoprisms isolated using this methodology show fewer non-specific interactions with cells and lower cellular internalization, which paves the way for a higher selectivity in therapeutic applications.

Characterization of Conventional One-Step Sodium Thiosulfate Facilitated Gold Nanoparticle Synthesis

Gold-gold sulfide nanoparticles are of interest for drug delivery, biomedical imaging, and photothermal therapy applications due to a facile synthesis method resulting in small particles with high near-infrared (NIR) absorption efficiency. Previous studies suggest that the NIR sensitivity of these nanoparticles was due to hexagonally shaped metal-coated dielectric nanoparticles that consist of a gold sulfide core and gold shell. Here, we illustrate that the conventional synthesis procedure results in the formation of polydisperse samples of icosahedral gold particles, gold nanoplates, and small gold spheres. Importantly, through compositional analysis, via UV/vis absorption spectrophotometry, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS), we show that all of the nanoparticles exhibit identical face center cubic (FCC) gold crystalline structures, thus suggesting that sulfide is not present in the final fabricated nanoparticles. We show that icosahedrally shaped nanoparticles result in a blue-shifted absorbance, with a peak in the visible range. Alternatively, the nanoplate nanoparticles result in the characteristic NIR absorbance peak. Thus, we report that the NIR-contributing species in conventional gold-gold sulfide formulations are nanoplates that are comprised entirely of gold. Furthermore, polydisperse gold nanoparticle samples produced by the traditional one-step reduction of HAuCl4 by sodium thiosulfate show increased in vitro toxicity, compared to isolated and more homogeneous constituent samples. This result exemplifies the importance of developing monodisperse nanoparticle formulations that are well characterized in order to expedite the development of clinically beneficial nanomaterials.

Directed Self-Assembly of Micron-Sized Gold Nanoplatelets into Oriented Flexible Stacks with Tunable Interplate Distance

A growing demand for control over the interparticle spacing and the orientation of anisotropic metallic particles into self-assembled structures is fuelled by their use in potential applications such as in plasmonics, catalysis, sensing, and optoelectronics. Here, we present an improved high yield synthesis method to fabricate micron- and submicron-sized gold nanoplatelets with a thickness less than 20 nm using silver nanoplatelets as seeds. By tuning the depth of the secondary minimum in the DLVO interaction potential between these particles, we are able to assemble the platelets into dynamic and flexible stacks containing thousands of platelets arranged face-to-face with well-defined spacing. Moreover, we demonstrate that the length of the stacks, and the interplate distance can be controlled between tens and hundreds of nm with the ionic strength. We use a high frequency external electric field to control the orientation of the stacks and direct the stacks into highly organized 2D and 3D assemblies that strongly polarize light.

A high yield, controllable process for producing tunable near infrared-absorbing gold nanoplates

This study optimizes a new synthesis technique, DiaSynth, to produce near-infrared absorbing gold nanoplates with prescribed localized surface plasmon resonance wavelengths in high yield without the need for additional laborious purification steps.