Electrophoretic separation of gold nanoparticles according to bifunctional molecules-induced charge and size

Microfluidic SERS devices: brightening the future of bioanalysis

A new avenue has opened up for applications of surface-enhanced Raman spectroscopy (SERS) in the biomedical field, mainly due to the striking advantages offered by SERS tags. SERS tags provide indirect identification of analytes with rich and highly specific spectral fingerprint information, high sensitivity, and outstanding multiplexing potential, making them very useful in in vitro and in vivo assays. The recent and innovative advances in nanomaterial science, novel Raman reporters, and emerging bioconjugation protocols have helped develop ultra-bright SERS tags as powerful tools for multiplex SERS-based detection and diagnosis applications. Nevertheless, to translate SERS platforms to real-world problems, some challenges, especially for clinical applications, must be addressed. This review presents the current understanding of the factors influencing the quality of SERS tags and the strategies commonly employed to improve not only spectral quality but the specificity and reproducibility of the interaction of the analyte with the target ligand. It further explores some of the most common approaches which have emerged for coupling SERS with microfluidic technologies, for biomedical applications. The importance of understanding microfluidic production and characterisation to yield excellent device quality while ensuring high throughput production are emphasised and explored, after which, the challenges and approaches developed to fulfil the potential that SERS-based microfluidics have to offer are described.

Mass-determining role in the electrophoretic separation of colloidal plasmonic nanoparticle oligomers

Gel electrophoresis techniques have been commonly applied in sieving plasmonic nanoparticle oligomers, while the intrinsic role in determining their phoresis velocity differences through the gel remains debatable. In this work, we explore the components and yield in each gel band after bundling two rationally designed types of nanoparticles in a system for electrophoretic separation. All results indicate that the mass property of plasmonic oligomers plays an essential role in determining their phoresis velocity divergences during separation. Further theoretical simulations reveal that the grounds for the mass-determining role stemmed from the random inelastic collisions among the oligomers and the gel-network microchannel. Moreover, under the guidance of such a mass-determining role, it is easy to achieve the direct electrophoretic separation of hetero-structured plasmonic dimers with high purity and high yield. This work will not only facilitate the precise nano-engineering of complex plasmonic oligomers with unique optical properties, but also might remove the obstacles toward their industrial manufacture with high purity.

Low-electric-potential-assisted diffusiophoresis for continuous separation of nanoparticles on a chip

Nanoparticle separation techniques are of significant importance in nanoscience and nanotechnological applications and different concentration gradients, electric/dielectric forces, flow/pressure fields, and acoustic waves have been intensively investigated. However, precise separation of nanoparticles has many technical challenges in terms of sizes, shapes, and material properties, limiting the separation resolution, capability, applicability, throughput and so on. In this study, we present a microfluidic device for continuous separation of nanoparticles by combining diffusiophoresis (DP) and electrophoresis (EP) to achieve high separation performance. Concentration gradients formed from sodium chloride (NaCl) and potassium acetate (K-acetate) passively drive the diffusiophoretic migration of nanoparticles. Simultaneously, a low electric potential is additionally applied to impose a synergistic effect on nanoparticle migration by size and surface charge, which is called low-electric-potential-assisted DP (LEPDP). Using a LEPDP-based separation device, we demonstrate the separation of nanoparticles having different sizes (diameters of 500, 200, and 50 nm) and under different surface-charge conditions (carboxylated polystyrene, silica, and polylactide). The resulting separation performance exceeded 95%, in terms of size uniformity, which is about two times better than that obtained using DP alone. We also emphasize that the enhancement of separation performance only needs a small voltage (<1 V), thereby demonstrating that our multiphysical approach could be utilized for high-resolution and portable nanoparticle separation on a chip without the side effects associated with high electric fields. Lastly, we ensure that rapid and precise bio/chemical sensing and analysis of various nanosized particles would be envisioned by strategically combining two nonlinear but synergistic migration effects.

Design and Simple Assembly of Gold Nanostar Bioconjugates for Surface-Enhanced Raman Spectroscopy Immunoassays

Immunoassays using Surface-Enhanced Raman Spectroscopy are especially interesting on account not only of their increased sensitivity, but also due to its easy translation to point-of-care formats. The bases for these assays are bioconjugates of polyclonal antibodies and anisotropic gold nanoparticles functionalized with a Raman reporter. These bioconjugates, once loaded with the antigen analyte, can react on a sandwich format with the same antibodies immobilized on a surface. This surface can then be used for detection, on a microfluidics or immunochromatographic platform. Here, we have assembled bioconjugates of gold nanostars functionalized with 4-mercaptobenzoic acid, and anti-horseradish peroxidase antibodies. The assembly was by simple incubation, and agarose gel electrophoresis determined a high gold nanostar to antibody binding constant. The functionality of the bioconjugates is easy to determine since the respective antigen presents peroxidase enzymatic activity. Furthermore, the chosen antibody is a generic immunoglobulin G (IgG) antibody, opening the application of these principles to other antibody-antigen systems. Surface-Enhanced Raman Spectroscopy analysis of these bioconjugates indicated antigen detection down to 50 µU of peroxidase activity. All steps of conjugation were fully characterized by ultraviolet-visible spectroscopy, dynamic light scattering, ζ -Potential, scanning electron microscopy, and agarose gel electrophoresis. Based on the latter technique, a proof-of-concept was established for the proposed immunoassay.

Space- and time-resolved small angle X-ray scattering to probe assembly of silver nanocrystal superlattices

The structure of nanocrystal superlattices has been extensively studied and well documented, however, their assembly process is poorly understood. In this work, we demonstrate an in situ space- and time-resolved small angle X-ray scattering measurement that we use to probe the assembly of silver nanocrystal superlattices driven by electric fields. The electric field creates a nanocrystal flux to the surface, providing a systematic means to vary the nanocrystal concentration near the electrode and thereby to initiate nucleation and growth of superlattices in several minutes. Using this approach, we measure the space- and time-resolved concentration and polydispersity gradients during deposition and show how they affect the superlattice constant and degree of order. We find that the field induces a size-selection effect that can reduce the polydispersity near the substrate by 21% leading to better quality crystals and resulting in field strength-dependent superlattice lattice constants.

Sorting Metal Nanoparticles with Dynamic and Tunable Optical Driven Forces

Precise sorting of colloidal nanoparticles is a challenging yet necessary task for size-specific applications of nanoparticles in nanophotonics and biochemistry. Here we present a new strategy for all-optical sorting of metal nanoparticles with dynamic and tunable optical driven forces generated by phase gradients of light. Size-dependent optical forces arising from the phase gradients of optical line traps can drive nanoparticles of different sizes with different velocities in solution, leading to their separation along the line traps. By using a sequential combination of optical lines to create differential trapping potentials, we realize precise sorting of silver and gold nanoparticles in the diameter range of 70-150 nm with a resolution down to 10 nm. Separation of the nanoparticles agrees with the analysis of optical forces acting on them and with simulations of their kinetic motions. The results provide new insights into all-optical nanoparticle manipulation and separation and reveal that there is still room to sort smaller nanoparticle with nanometer precision using dynamic phase-gradient forces.

Gelatin-modified gold nanoparticles for direct detection of urinary total gelatinase activity: Diagnostic value in bladder cancer

Matrix metalloproteinases (MMPs), in particularly gelatinases (MMP-2 and MMP-9) were reported as urinary markers of bladder cancer. In this work, we developed a simple colorimetric gold nanoparticle (AuNP) assay for rapid and sensitive detection of urinary total gelatinase activity based on the surface plasmon resonance (SPR) property of AuNPs. Gelatin-modified AuNPs were stably suspended in solution even upon addition of an aggregation inducer as 6-mercaptohexan-1-ol (6-MCH). Gelatinases digest gelatin capping. Subsequently, addition of 6-MCH leads to AuNPs aggregation with red to blue color shift. In a pilot study, results of the developed AuNP assay were consistent with zymography for qualitative detection of urinary total gelatinase activity. The sensitivity and specificity of both assays were 80% and 90.9% respectively. The absorption ratios, A₆₂₅/A₅₃₀ of the reacted AuNP solutions were used to quantify the total gelatinase concentration. The best cut off value was 0.01895ng/μg protein, at which the sensitivity was 87.5% and the specificity was 86.4%. The developed AuNP assay is simple, low-cost and can aid non-invasive diagnosis of bladder cancer.

Gel electrophoretic analysis of differently shaped interacting and non-interacting bioconjugated nanoparticles

Gel electrophoresis is demonstrated for monitoring bioaffinity interactions between protein-functionalized nanoparticles featuring different shapes as well as for particle separation.

Tunable loading of single-stranded DNA on gold nanorods through the displacement of polyvinylpyrrolidone

A quantitative and tunable loading of single-stranded (ss-DNA) molecules onto gold nanorods was achieved through a new method of surfactant exchange. This new method involves the exchange of cetyltrimethylammonium bromide surfactants for an intermediate stabilizing layer of polyvinylpyrrolidone and sodium dodecylsulfate. The intermediate layer of surfactants on the anisotropic gold particles was easily displaced by thiolated ss-DNA, forming a tunable density of single-stranded DNA molecules on the surfaces of the gold nanorods. The success of this ligand exchange process was monitored in part through the combination of extinction, X-ray photoelectron, and infrared absorption spectroscopies. The number of ss-DNA molecules per nanorod for nanorods with a high density of ss-DNA molecules was quantified through a combination of fluorescence measurements and elemental analysis, and the functionality of the nanorods capped with dense monolayers of DNA was assessed using a hybridization assay. Core-satellite assemblies were successfully prepared from spherical particles containing a probe DNA molecule and a nanorod core capped with complementary ss-DNA molecules. The methods demonstrated herein for quantitatively fine tuning and maximizing, or otherwise optimizing, the loading of ss-DNA in monolayers on gold nanorods could be a useful methodology for decorating gold nanoparticles with multiple types of biofunctional molecules.