Growth and branching of gold nanoparticles through mesoporous silica thin films

Recent advances in gold Janus nanomaterials: Preparation and application

Gold Janus nanomaterials have a tremendous significance for the novel bifunctional materials, significantly expanding the application scope of gold nanomaterials, especially Janus gold-thiol coordination polymer due to their exceptional biological characteristics, stability, plasmon effect, etc. The recent research on Janus gold nanoparticles and monolayer films of preparation and application has been summarized and in this review. To begin, we briefly introduce overview of Janus nanomaterials which received intense attention, outline current research trends, and detail the preparation and application of gold nanomaterials. Subsequently, we present comprehensively detailing fabrication strategies and applications of Janus gold nanoparticles. Additionally, we survey recent studies on the Janus gold nano-thickness films and point out the outstanding advantage of application on the tunable surface plasmon resonance, high sensitivity of surface-enhanced Raman scattering and electrical analysis fields. Finally, we discuss the emerging trends in Janus gold nanomaterials and address the associated challenges, thereby providing a comprehensive overview of this area of research.

Simultaneous Nanolocal Polymer and In Situ Readout Unit Placement in Mesoporous Separation Layers

Bioinspired solid-state nanopores and nanochannels have attracted interest in the last two decades, as they are envisioned to advance future sensing, energy conversion, and separation concepts. Although much effort has been made regarding functionalization of these materials, multifunctionality and accurate positioning of functionalities with nanoscale precision still remain challenging. However, this precision is necessary to meet transport performance and complexity of natural pores in living systems, which are often based on nonequilibrium states and compartmentalization. In this work, a nanolocal functionalization and simultaneous localized sensing strategy inside a filtering mesoporous film using precisely placed plasmonic metal nanoparticles inside mesoporous films with pore accessibility control is demonstrated. A single layer of gold nanoparticles is incorporated into mesoporous thin films with precise spatial control along the nanoscale layer thickness. The local surface plasmon resonance is applied to induce a photopolymerization leading to a nanoscopic polymer shell around the particles and thus nanolocal polymer placement inside the mesoporous material. As near-field modes are sensitive to the dielectric properties of their surrounding, the in situ sensing capability is demonstrated using UV-vis spectroscopy. It is demonstrated that the sensing sensitivity only slightly decreases upon functionalization. The presented nanolocal placement of responsive functional polymers into nanopores offers a simultaneous filtering and nanoscopic readout function. Such a nanoscale local control is envisioned to have a strong impact onto the development of new transport and sensor concepts, especially as the system can be developed into higher complexity using different metal nanoparticles and additional design of mesoporous film filtering properties.

Metal-incorporated mesoporous oxides: Synthesis and applications

Mesoporous oxides are outstanding metal nanoparticle catalyst supports owing to their well-defined porous structures. Such mesoporous architectures not only prevent the aggregation of metal nanoparticles but also enhance their catalytic performance. Metal/metal oxide heterojunctions exhibit unique chemical and physical properties because of the surface reconstruction around the junction and electron transfer/interaction across the interface. This article reviews the methods used for synthesizing metal-supported hybrid nanostructures and their applications as catalysts for environmental remediation and sensors for detecting hazardous materials.

TiO2 mesoporous thin film architecture as a tool to control Au nanoparticles growth and sensing capabilities

In this paper, a systematic study regarding the effect of the mesoporous structure over Au nanoparticles (NPs) growth inside and through the pores of mesoporous TiO2 thin films (MTTFs) is presented, and the effect of such characteristics over the composites' sensing capabilities is evaluated. Highly stable MTTFs with different pore diameters (range: 4-8 nm) and pore arrangements (body- and face-centered cubic) were synthesized and characterized. Au NPs were grown inside the pores, and it was demonstrated-through a careful physicochemical characterization-that the amount of incorporated Au and NP size depends on the pore array; being higher for bigger pore diameters and face-centered cubic structures. The same structure allows the growth of more and longer tips over Au NPs deposited at the thin film-substrate interface. Finally, to confirm the effect of the structural characteristics of the composites over their possible applications, the materials were tested as surface-enhanced Raman scattering (SERS)-based substrates. The composites with a higher amount of Au and more ramified NPs were the ones that presented better sensitivity in the detection of a probe molecule (4-nitrothiophenol). Overall, this work demonstrates that the pore size and ordering in MTTFs determine the materials' accessibility and connectivity, and therefore, have a clear impact on their potential applications.

Silica-Coated Plasmonic Metal Nanoparticles in Action

Hybrid colloids consisting of noble metal cores and metal oxide shells have been under intense investigation for over two decades and have driven progress in diverse research lines including sensing, medicine, catalysis, and photovoltaics. Consequently, plasmonic core-shell particles have come to play a vital role in a plethora of applications. Here, an overview is provided of recent developments in the design and utilization of the most successful class of such hybrid materials, silica-coated plasmonic metal nanoparticles. Besides summarizing common simple approaches to silica shell growth, special emphasis is put on advanced synthesis routes that either overcome typical limitations of classical methods, such as stability issues and undefined silica porosity, or grant access to particularly sophisticated nanostructures. Hereby, a description is given, how different types of silica can be used to provide noble metal particles with specific functionalities. Finally, applications of such nanocomposites in ultrasensitive analyte detection, theranostics, catalysts, and thin-film solar cells are reviewed.

Hierarchical organization and molecular diffusion in gold nanorod/silica supercrystal nanocomposites

Hierarchical organization of gold nanorods was previously obtained on a substrate, allowing precise control over the morphology of the assemblies and macroscale spatial arrangement. Herein, a thorough description of these gold nanorod assemblies and their orientation within supercrystals is presented together with a sol-gel technique to protect the supercrystals with mesoporous silica films. The internal organization of the nanorods in the supercrystals was characterized by combining focused ion beam ablation and scanning electron microscopy. A mesoporous silica layer is grown both over the supercrystals and between the individual lamellae of gold nanorods inside the structure. This not only prevented the detachment of the supercrystal from the substrate in water, but also allowed small molecule analytes to infiltrate the structure. These nanocomposite substrates show superior Raman enhancement in comparison with gold supercrystals without silica owing to improved accessibility of the plasmonic hot spots to analytes. The patterned supercrystal arrays with enhanced optical and mechanical properties obtained in this work show potential for the practical implementation of nanostructured devices in spatially resolved ultradetection of biomarkers and other analytes.

Plasmonic Surfaces for Cell Growth and Retrieval Triggered by Near-Infrared Light

Methods for efficient detachment of cells avoiding damage are required in tissue engineering and regenerative medicine. We introduce a bottom-up approach to build plasmonic substrates using micellar block copolymer nanolithography to generate a 2D array of Au seeds, followed by chemical growth leading to anisotropic nanoparticles. The resulting plasmonic substrates show a broad plasmon band covering a wide part of the visible and near-infrared (NIR) spectral ranges. Both human and murine cells were successfully grown on the substrates. A simple functionalization step of the plasmonic substrates with the cyclic arginylglycylaspartic acid (c-RGD) peptide allowed us to tune the morphology of integrin-rich human umbilical vein endothelial cells (HUVEC). Subsequent irradiation with a NIR laser led to highly efficient detachment of the cells with cell viability confirmed using the MTT assay. We thus propose the use of such plasmonic substrates for cell growth and controlled detachment using remote near-IR irradiation, as a general method for cell culture in biomedical applications.

High viscosity to highly dispersed PtPd bimetallic nanocrystals for enhanced catalytic activity and stability

A facile high-viscosity-solvent method is developed to synthesize highly dispersed bimetallic PtPd nanocrystals with high activity, stability and durability.

Preparation of Anisotropic and Oriented Particles on a Flexible Substrate

Elongated plasmonic nanoparticles show superior optical properties when compared to spherical ones. Facile, versatile and cost-effective bottom-up approaches for fabrication of anisotropic nanoparticles in solution have been developed. However, fabrication of 2-D plasmonic templates from elongated nanoparticles with spatial arrangement at the surface is still a challenge. We used controlled seed-mediated growth in the presence of porous and functionalized surface of flexible polydimethylsiloxane (PDMS) templates to provide directional growth and formation of elongated gold nanoparticles (AuNPs). Atomic force microscopy (AFM) and spectroscopy revealed embedding of the particles within the functionalized porous surface of PDMS. Nanoparticles shapes were observed with transmission electron microscope (TEM), UV-Vis spectroscopy, and X-ray powder diffraction (XRPD) measurements, which revealed an overall orientation of particles at the surface. Anisotropic and oriented particles on a flexible substrate are of interest for sensing applications.

Selective SERS Sensing Modulated by Functionalized Mesoporous Films

A hybrid material comprising metal nanoparticles embedded in functionalized mesoporous thin films was constructed, and its use as a selective SERS-based sensor was demonstrated. The presence of specific functional groups in the pore network allows control over the surface chemistry of the pores, tuning the selectivity for specific molecules. Amino-functionalized hybrid mesoporous thin films were used in a proof of concept experiment, to discern the presence of methylene blue (MB) in mixtures with acid blue (AB), with no need for any sample pretreatment step. Selective detection of MB was possible through entrapment of AB in the mesoporous matrix, based on its high affinity for amino groups. The sensor selectivity can be tuned by varying the solution pH, rendering a pH responsive surface and thus, selective SERS-based sensing. The developed sensors allow specific detection of molecules in complex matrixes.

Templated Growth of Surface Enhanced Raman Scattering-Active Branched Gold Nanoparticles within Radial Mesoporous Silica Shells

Noble metal nanoparticles are widely used as probes or substrates for surface enhanced Raman scattering (SERS), due to their characteristic plasmon resonances in the visible and near-IR spectral ranges. Aiming at obtaining a versatile system with high SERS performance, we developed the synthesis of quasi-monodisperse, nonaggregated gold nanoparticles protected by radial mesoporous silica shells. The radial mesoporous channels were used as templates for the growth of gold tips branching out from the cores, thereby improving the plasmonic performance of the particles while favoring the localization of analyte molecules at high electric field regions: close to the tips, inside the pores. The method, which additionally provides control over tip length, was successfully applied to gold nanoparticles with various shapes, leading to materials with highly efficient SERS performance. The obtained nanoparticles are stable in ethanol and water upon thermal consolidation and can be safely stored as a powder.

"Elastic" property of mesoporous silica shell: for dynamic surface enhanced Raman scattering ability monitoring of growing noble metal nanostructures via a simplified spatially confined growth method

The Raman enhancing ability of noble metal nanoparticles (NPs) is an important factor for surface enhanced Raman scattering (SERS) substrate screening, which is generally evaluated by simply mixing as-prepared NPs with Raman reporters for Raman signal measurements. This method usually leads to incredible results because of the NP surface coverage nonuniformity and reporter-induced NP aggregation. Moreover, it cannot realize in situ, continuous SERS characterization. Herein, we proposed a dynamic SERS monitoring strategy for NPs with precisely tuned structures based on a simplified spatially confined NP growth method. Gold nanorod (AuNR) seed NPs were coated with a mesoporous silica (mSiO2) shell. The permeability of mSiO2 for both reactive species and Raman reporters rendered the silver overcoating reaction and SERS indication of NP growth. Additionally, the mSiO2 coating ensured monodisperse NP growth in a Raman reporter-rich reaction system. Moreover, "elastic" features of mSiO2 were observed for the first time, which is crucial for holding the growing NP without breakage. This feature makes the mSiO2 coating adhere to metal NPs throughout the growing process, providing a stable Raman reporter distribution microenvironment near the NPs and ensuring that the substrate's SERS ability comparison is accurate. Three types of NPs, i.e., core-shell Au@AgNR@mSiO2, Au@AuNR@mSiO2, and yolk-shell Au@void@AuNR@mSiO2 NPs, were synthesized via core-shell overgrowth and galvanic replacement methods, showing the versatility of the approach. The living cell SERS labeling ability of Au@AgNR@mSiO2-based tags was also demonstrated. This strategy addresses the problems of multiple batch NP preparation, aggregation, and surface adsorption differentiation, which is a breakthrough for the dynamic comparison of SERS ability of metal NPs with precisely tuned structures and optical properties.

Au nanoparticle-embedded SiO2–Au@SiO2 catalysts with improved catalytic activity, enhanced stability to metal sintering and excellent recyclability

Au NP-embedded SiO₂ (SiO₂–Au@SiO₂) particles with the improved molecule accessibility, catalyst stability and catalytic performance were successfully synthesized by post-treatments such as calcination and/or etching with water or ammonia.