Achieving high-purity colloidal gold nanoprisms and their application as biosensing platforms

Customizable ligand exchange on the surface of gold nanotriangles enables their application in LSPR-based sensing

Nanomaterials made of noble metals have been actively utilized in sensorics and bioanalytics. Nanoparticles of anisotropic shapes are promising for increasing sensitivity due to the generated hotspots of electron density. Such structures can be effectively manufactured by a relatively accessible colloidal synthesis. However, the shape control requires the attachment of a surfactant on specific crystal facets during their growth. Commonly used cetrimonium halides form a closely packed bilayer, lowering the surface accessibility for subsequent (bio)functionalization steps. While there are numerous studies on functionalizing gold nanospheres, novel materials, such as nanotriangles (AuNTs), often require thorough studies to adapt the existing procedures. This is mainly caused by the incomplete characterization of initial nanoparticle colloids in empirically developed protocols. Herein, we report a rational approach utilizing the surface area of AuNTs as a function of both their dimensions and concentration, determined with an express UV-VIS analysis. We demonstrate its efficiency for the exchange of cetyltrimethylammonium chloride (CTAC) with polystyrene sulfonate (PSS) and with biocompatible citrate using direct and indirect methods, respectively. Fourier-transform infrared spectroscopy unequivocally proves the ligand exchange. Such functionalization allows evaluating the bulk refractive index sensitivity of AuNTs as a measure of their potential in LSPR-based sensing.

Seed-Mediated Synthesis of Thin Gold Nanoplates with Tunable Edge Lengths and Optical Properties

Thin Au nanoplates show intriguing localized surface plasmon resonance (LSPR) properties with potential applications in various fields. The conventional synthesis of Au nanoplates usually involves the formation of spherical nanoparticles or produces nanoplates with large thicknesses. Herein, we demonstrate a synthesis of uniform thin Au nanoplates by using Au-Ag alloy nanoframes obtained by the galvanic replacement of Ag nanoplates with HAuCl₄ as the seeds and a sulfite (SO₃^2-) as a ligand. The SO₃^2- ligand not only complexes with the Au salt for the controlled reduction kinetics but also strongly adsorbs on Au {111} facets for effectively constraining the crystal growth on both basal sides of the Au nanoplates for controlled shape and reduced thicknesses. This seed-mediated synthesis affords Au nanoplates with a thickness of only 7.5 nm, although the thickness increases with the edge length. The edge length can be customizable in a range of 48-167 nm, leading to tunable LSPR bands in the range of 600-1000 nm. These thin Au nanoplates are applicable not only to surface-enhanced Raman spectroscopy with enhanced sensitivity and reliability but also to a broader range of LSPR-based applications.

Shape-Dependent Catalytic Activity of Gold and Bimetallic Nanoparticles in the Reduction of Methylene Blue by Sodium Borohydride

In this study the catalytic activity of different gold and bimetallic nanoparticle solutions towards the reduction of methylene blue by sodium borohydride as a model reaction is investigated. By utilizing differently shaped gold nanoparticles, i.e., spheres, cubes, prisms and rods as well as bimetallic gold–palladium and gold–platinum core-shell nanorods, we evaluate the effect of the catalyst surface area as available gold surface area, the shape of the nanoparticles and the impact of added secondary metals in case of bimetallic nanorods. We track the reaction by UV/Vis measurements in the range of 190–850 nm every 60 s. It is assumed that the gold nanoparticles do not only act as a unit transferring electrons from sodium borohydride towards methylene blue but can promote the electron transfer upon plasmonic excitation. By testing different particle shapes, we could indeed demonstrate an effect of the particle shape by excluding the impact of surface area and/or surface ligands. All nanoparticle solutions showed a higher methylene blue turnover than their reference, whereby gold nanoprisms exhibited 100% turnover as no further methylene blue absorption peak was detected. The reaction rate constant k was also determined and revealed overall quicker reactions when gold or bimetallic nanoparticles were added as a catalyst, and again these were highest for nanoprisms. Furthermore, when comparing gold and bimetallic nanorods, it could be shown that through the addition of the catalytically active second metal platinum or palladium, the dye turnover was accelerated and degradation rate constants were higher compared to those of pure gold nanorods. The results explore the catalytic activity of nanoparticles, and assist in exploring further catalytic applications.

Gold adatoms modulate sulfur adsorption on gold

Sulfur adsorption on Au(111) at high coverage has been studied by density functional calculations. In this case S species organize into rectangular structures containing 8 S atoms irrespective of the S source, which have been alternatively assigned to adsorbed monomeric S, adsorbed S₂, adsorbed monomeric plus S₂ species, and gold sulfide. We found that monomeric S at the high coverage organizes into S₂ species that are stabilized into the 8-S structures by Au adatoms, forming gold disulfide complexes (Au-(S₂)₄). The Au atoms could be provided by decomposition of more diluted AuS₃ containing phases, as recently proposed, and direct removal from terraces and step edges, both explaining the surface coverage of vacancy islands coexisting with the 8-S structures. The gold-disulfide complexes capture the disorder shown in the experimental STM images, explain the intrigued features of XPS, and also, give a smooth pathway to gold sulfide formation at higher temperatures. More importantly, the gold-disulfide complexes allow a unified picture of the gold-sulfur surface chemistry at high coverage for thiols and adsorbed sulfur species where the surface chemistry remains under discussion.

Magnetic-Plasmonic Heterodimer Nanoparticles: Designing Contemporarily Features for Emerging Biomedical Diagnosis and Treatments

Magnetic-plasmonic heterodimer nanostructures synergistically present excellent magnetic and plasmonic characteristics in a unique platform as a multipurpose medium for recently invented biomedical applications, such as magnetic hyperthermia, photothermal therapy, drug delivery, bioimaging, and biosensing. In this review, we briefly outline the less-known aspects of heterodimers, including electronic composition, interfacial morphology, critical properties, and present concrete examples of recent progress in synthesis and applications. With a focus on emerging features and performance of heterodimers in biomedical applications, this review provides a comprehensive perspective of novel achievements and suggests a fruitful framework for future research.

Localized surface plasmon resonance based biosensing

INTRODUCTION

Bioanalytical sensing based on the principle of localized surface plasmon resonance experiences is currently an extremely rapid development. Novel sensors with new kinds of plasmonic transducers and innovative concepts for the signal development as well as read-out principles were identified. This review will give an overview of the development of this field. Areas covered: The focus is primarily on types of transducers by preparation or dimension, factors for optimal sensing concepts and the critical view of the usability of these devices as innovative sensors for bioanalytical applications. Expert commentary: Plasmonic sensor devices offer a high potential for future biosensing given that limiting factors such as long-time stability of the transducers, the required high sensitivity and the cost-efficient production are addressed. For higher sensitivity, the design of the sensor in shape and material has to be combined with optimal enhancement strategies. Plasmonic nanoparticles from bottom-up synthesis with a post-synthetic processing show a high potential for cost-efficient sensor production. Regarding the measurement principle, LSPRi offers a large potential for multiplex sensors and can provide a high-throughput as well as highly paralleled sensing. The main trends are expected towards optimal LSPR concepts which represent cost-efficient and robust point-of-care solutions, and the use of multiplexed devices for clinical applications.

Anisotropic noble metal nanoparticles: Synthesis, surface functionalization and applications in biosensing, bioimaging, drug delivery and theranostics

Anisotropic nanoparticles have fascinated scientists and engineering communities for over a century because of their unique physical and chemical properties. In recent years, continuous advances in design and fabrication of anisotropic nanoparticles have opened new avenues for application in various areas of biology, chemistry and physics. Anisotropic nanoparticles have the plasmon absorption in the visible as well as near-infrared (NIR) region, which enables them to be used for crucial applications such as biological imaging, medical diagnostics and therapy ("theranostics"). Here, we describe the progress in anisotropic nanoparticles achieved since the millennium in the area of preparation including various shapes and modification of the particle surface, and in areas of application by providing examples of applications in biosensing, bio-imaging, drug delivery and theranostics. Furthermore, we also explain various mechanisms involved in cellular uptake of anisotropic nanoparticles, and conclude with our opinion on various obstacles that limit their applications in biomedical field.

STATEMENT OF SIGNIFICANCE

Anisotropy at the molecular level has always fascinated scientists and engineering communities for over a century, however, the research on novel methods through which shape and size of nanoparticles can be precisely controlled has opened new avenues for anisotropic nanoparticles in various areas of biology, chemistry and physics. In this manuscript, we describe progress achieved since the millennium in the areas of preparation of various shapes of anisotropic nanoparticles, investigate various methods involved in modifying the surface of these NPs, and provide examples of applications in biosensing and bio-imaging, drug delivery and theranostics. We also present mechanisms involved in cellular uptake of nanoparticles, describe different methods of preparation of anisotropic nanoparticles including biomimetic and photochemical synthesis, and conclude with our opinion on various obstacles that limit their applications in biomedical field.

Greener synthesis and characterization, antimicrobial and cytotoxicity studies of gold nanoparticles of novel shapes and sizes

We hereby present a novel approach for the synthesis of gold nanoparticles (AuNPs) using water soluble, naturally-derived flavonoids.

The unusual visible photothermal response of free standing multilayered films based on plasmonic bimetallic nanocages

An unusual photothermal response in the visible region has been observed in free standing multilayered films based on the plasmonic bimetallic Au and Ag nanocages (Ag@AuNCs).

Tunable hydroxylated surfactants: an efficient toolbox towards anisotropic gold nanoparticles

Size- and shape-controlled anisotropic gold nanoparticles were reported. The organisation of tunable hydroxylated surfactants providing various nanostructured materials has been demonstrated.

Design, preparation, and evaluation of a fixed-orientation antibody/gold-nanoparticle conjugate as an immunosensing label

Herein, we describe the development of a new, highly efficient label for immunosensing comprising an antibody/PEGylated gold-nanoparticle (AuNP) conjugate in which the antibody molecules are bound to the AuNP surface in a fixed orientation. Our method exploits the high density of positive charges on the major plane of antibodies that exists when the pH of the solution is lower than the isoelectric point of the antibody; the antibody molecules interact with the negatively charged AuNP surface through their major plane, enabling the antigen binding sites to move freely and therefore to reach maximum accessibility. This directed ionic interaction is reinforced by the formation of a peptide bond between the amino group of the Lys residues in the antibodies and the carboxylic groups of the PEGylated-AuNP surface via EDC chemistry. Electrochemical analyses revealed that the fixed-orientation conjugate offers a limit of detection that is 1 order of magnitude lower than that of a randomly oriented label. The performance of the new conjugate as an immunosensing label was assessed for the quantitative detection of IgG in human serum.

Detecting secondary structure and surface orientation of helical peptide monolayers from resonant hybridization signals

Hybridization of dominant vibrational modes with meta-surface resonance allows detection of both structural changes and surface orientations of bound helical peptides. Depending on the resonance frequency of meta-molecules, a red- or blue- shift in peptide Amide-I frequency is observed. The underlying coupling mechanism is described by using a temporal coupled mode theory that is in very good agreement with the experimental results. This hybridization phenomenon constitutes the basis of many nanophotonic systems such as tunable coupled mode bio-sensors and dynamic peptide systems driven by infrared signals.

Lab-on-a-chip based immunosensor principles and technologies for the detection of cardiac biomarkers: a review

This review examines the current state of the art lab-on-a-chip and microfluidic based biosensor technologies used in the detection of cardiac biomarkers. The determination and quantification of blood based, cardiac biomarkers are crucial in the triage and management of a range of cardiac related conditions, where time delay has a major impact on short and longer-term outcomes of a patient. The design and manufacturing of biomarker detection systems are multi-disciplinary in nature and require researchers to have knowledge of both life sciences and engineering for the full potential of this field to be realised. This review will therefore provide a comprehensive overview of chip based immunosensing technology as applied to cardiac biomarker detection, while discussing the potential suitability and limitations of each configuration for incorporation within a clinical diagnostics device suitable for point-of-care applications.