Preparation of Janus-type catalysts and their catalytic performance at emulsion interface

TEMPO-Grafted Polystyrene/Polymethacrylate Organosiloxane Janus Nanohybrids as Efficient Pickering Interfacial Catalyst for Selective Aerobic Oxidation of Cinnamyl Alcohol

The novel 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) groups immobilized on functional polymers or nanoparticles emerged as potential Pickering interfacial catalysts (PICs) for effective catalysis in biphasic systems. In this study, a snowman-shaped Janus-structured polymer with TEMPO-anchored nanohybrid particles (SM-JPP-TEMPO) was prepared and employed as a potential PIC in the Anelli-Montanari system for the selective oxidation of alcohol. The amphiphilic character of SM-JPP-TEMPO particles plays a dual role as an emulsifier and catalyst in the Pickering emulsion. As a result, it enables smaller droplets (102 μm) at the water-in-oil (W/O) interface and reduces the interfacial tension from 26.58 to 17.38 mN/m, which improves the stability of the Pickering emulsion system. This constructed Pickering emulsion microreactor offers a larger interface contact area and shortens the mass transfer distance of the substrate of cinnamyl alcohol, which significantly enhances the catalytic conversion at the Anelli-Montanari oxidation system, thus achieving remarkable conversion efficiency of (92.3%) with excellent selectivity (99%) in static (stirring-free) condition. It was found that the Janus nanohybrid catalyst (SM-JPP-TEMPO) enhanced 1.29-fold catalytic efficiency compared to the TEMPO grafted spherical polystyrene nanoparticle (PS-NPs-TEMPO) catalyst (72%). Moreover, after seven consecutive cycles, the Janus nanocatalyst (SM-JPP-TEMPO) maintained the conversion significantly. Hence, these results collectively highlight that the amphiphilic SM-JPP-TEMPO catalyst provides an efficient and eco-friendly strategy for the intensification of liquid-liquid biphasic reaction systems for potential applications in industries.

Temperature-Sensitive Janus Particles PEG/SiO2/PNIPAM-PEA: Applications in Foam Stabilization and Defoaming

The current study presents a scalable approach for the preparation of temperature-responsive PEG/SiO2/PNIPAM-PEA Janus particles and, for the first time, investigates their potential applications in stabilizing foam and defoaming by adjusting the temperature. The method utilizes a (W1 + O)/W2 emulsion system, which incorporates appropriate surfactants to stabilize the emulsion and prevent rapid dissolution of the hydrophilic triblock polymer PEG-b-PTEPM-b-PNIPAM in water. The PEG/SiO2/PNIPAM-PEA Janus particles with temperature-responsive characteristics were synthesized in a single step that combined the sol-gel reaction and photoinduced free radical polymerization. The contact angle of the hydrophilic PEG/SiO2/PNIPAM surface was measured to be 54.7 ± 0.1°, while the contact angle of the hydrophobic PEA surface was found to be 122.4 ± 0.1°. By incorporating PEG/SiO2/PNIPAM-PEA Janus particles at a temperature of 25 °C, the foam's half-life is significantly prolonged from 42 s to nearly 30 min. However, with an increase in temperature to 50 °C, the foam's half-life rapidly diminished to only 44 s. This innovative application effectively enhances foam stabilization at low temperatures and facilitates the rapid dissipation of foam at high temperatures.

Fabrication of Versatile Janus Microparticles through Geometry and Surface Chemistry Control

Amphiphilic Janus particles typically comprise two distinct hemispheres with spatially dispersed physicochemical properties. The anisotropic structure and physicochemical properties of Janus particles can be exploited for various applications. However, their preparation typically requires complex and sophisticated processes and expensive equipment to control the formation of different structures and chemical compositions. Herein, a simple synthetic approach for the facile fabrication of versatile Janus particles with efficient control of the Janus ratio and wettability based on particle fixation at a three-phase interface and photopolymerization is reported. Agarose gel and surfactant are used to control the surface-coated boundaries of the Janus particles through the equilibrium of a floating microparticle at the fluid interface. poly(propylene glycol) diacrylate or poly(N-isopropylacrylamide) coating on polystyrene-based microparticles allows easy control of the chemical functionality of the particle surfaces. Depending on the particle morphology and wettability, the interfacial behavior between two immiscible liquids can be adjusted, which allows the stabilization of Pickering emulsions that encapsulate independent oil droplets in water or vice versa. This facile approach has the potential to enable more efficient mass production of Janus particles and their use in various applications, such as biomedical and environmental engineering.

Facile Fabrication of Monodisperse Micron-Sized Dual Janus Silica Particles with Asymmetric Morphology and Chemical Environment

Janus particles are a kind of materials with asymmetric morphology or surface chemical environment. But so far, the preparation of particles with dual asymmetry is still a challenging problem. Hence the cation surfactant hexadecyl trimethyl ammonium bromide and co-surfactant octadecylamine are applied to improve the Pickering emulsion stability, and the micron-sized silica particles are arranged in a single layer at the toluene-water interface through electrostatic interaction. Furthermore, organosilane reagents are added in the preparation process, resulting in the construction of asymmetric hydrophilic or hydrophobic mesoporous precisely onto the micron-sized silica particles surface. The cation surfactant-assisted Pickering emulsion method is simple, effective, and convenience, which can be applied in the synthesis of various dual Janus silica particles for specific applications.

Application of Janus Particles in Point-of-Care Testing

Janus particles (JPs), named after the two-faced Roman god, are asymmetric particles with different chemical properties or polarities. JPs have been widely used in the biomedical field in recent years, including as drug carriers for targeted controlled drug release and as biosensors for biological imaging and biomarker detection, which is crucial in the early detection and treatment of diseases. In this review, we highlight the most recent advancements made with regard to Janus particles in point-of-care testing (POCT). Firstly, we introduce several commonly used methods for preparing Janus particles. Secondly, we present biomarker detection using JPs based on various detection methods to achieve the goal of POCT. Finally, we discuss the challenges and opportunities for developing Janus particles in POCT. This review will facilitate the development of POCT biosensing devices based on the unique properties of Janus particles.

Pickering Emulsion Catalysis: Interfacial Chemistry, Catalyst Design, Challenges, and Perspectives

Pickering emulsions are particle-stabilized surfactant-free dispersions composed of two immiscible liquid phases, and emerge as attractive catalysis platform to surpass traditional technique barrier in some cases. In this review, we have comprehensively summarized the development and the catalysis applications of Pickering emulsions since the pioneering work in 2010. The explicit mechanism for Pickering emulsions will be initially discussed and clarified. Then, summarization is given to the design strategy of amphiphilic emulsion catalysts in two categories of intrinsic and extrinsic amphiphilicity. The progress of the unconventional catalytic reactions in Pickering emulsion is further described, especially for the polarity/solubility difference-driven phase segregation, "smart" emulsion reaction system, continuous flow catalysis, and Pickering interfacial biocatalysis. Challenges and future trends for the development of Pickering emulsion catalysis are finally outlined.

Compositionally Anisotropic Colloidal Surfactant Decorated with Dual Metallic Nanoparticles as a Pickering Emulsion Stabilizer and Their Application in Catalysis

We aim to introduce compositionally anisotropic Janus particles, hemispheres of which was modified by hydrophilic poly(2-dimethyl amino ethyl methacrylate) [poly(DMAEMA)] brushes to display amphiphilic surfactant-type characteristics. Acquired by the electrohydrodynamic co-jetting technique, these colloidal surfactants were employed to stabilize octanol/water-based Pickering emulsion, which shows prolonged stability for more than 4 months. To explore their potential as the interfacial catalyst, iron(0) nanoparticles were incorporated in one hemisphere during electrojetting, whereas gold nanoparticles (GNPs) were patched onto the surface of the other hemisphere, which was previously modified by the poly(DMAEMA) brush. Ultimately, simultaneous rapid reduction (100% conversion in 1 min) of p-nitrophenol or methyl orange (MO) by GNPs in the aqueous phase and dechlorination of trichloroethylene (a hazardous chlorinated solvent waste) present in the octanol phase were accomplished at the organic-water interface stabilized by the Janus particles decorated by dual metallic nanoparticles. In addition, facile recovery and recyclability of the catalyst were also achieved. The novel colloidal surfactant demonstrated in this study may open up a new avenue to accomplish catalysis of several organic reactions occurring at the water-oil interface.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Multiphase Microreactors Based on Liquid-Liquid and Gas-Liquid Dispersions Stabilized by Colloidal Catalytic Particles

Pickering emulsions, foams, bubbles, and marbles are dispersions of two immiscible liquids or of a liquid and a gas stabilized by surface-active colloidal particles. These systems can be used for engineering liquid-liquid-solid and gas-liquid-solid microreactors for multiphase reactions. They constitute original platforms for reengineering multiphase reactors towards a higher degree of sustainability. This Review provides a systematic overview on the recent progress of liquid-liquid and gas-liquid dispersions stabilized by solid particles as microreactors for engineering eco-efficient reactions, with emphasis on biobased reagents. Physicochemical driving parameters, challenges, and strategies to (de)stabilize dispersions for product recovery/catalyst recycling are discussed. Advanced concepts such as cascade and continuous flow reactions, compartmentalization of incompatible reagents, and multiscale computational methods for accelerating particle discovery are also addressed.

Recent Progress on Asymmetric Carbon- and Silica-Based Nanomaterials: From Synthetic Strategies to Their Applications

HIGHLIGHTS

The synthetic strategies and fundamental mechanisms of various asymmetric carbon- and silica-based nanomaterials were systematically summarized. The advantages of asymmetric structure on their related applications were clarified by some representative applications of asymmetric carbon- and silica-based nanomaterials. The future development prospects and challenges of asymmetric carbon- and silica-based nanomaterials were proposed.

ABSTRACT

Carbon- and silica-based nanomaterials possess a set of merits including large surface area, good structural stability, diversified morphology, adjustable structure, and biocompatibility. These outstanding features make them widely applied in different fields. However, limited by the surface free energy effect, the current studies mainly focus on the symmetric structures, such as nanospheres, nanoflowers, nanowires, nanosheets, and core–shell structured composites. By comparison, the asymmetric structure with ingenious adjustability not only exhibits a larger effective surface area accompanied with more active sites, but also enables each component to work independently or corporately to harness their own merits, thus showing the unusual performances in some specific applications. The current review mainly focuses on the recent progress of design principles and synthesis methods of asymmetric carbon- and silica-based nanomaterials, and their applications in energy storage, catalysis, and biomedicine. Particularly, we provide some deep insights into their unique advantages in related fields from the perspective of materials’ structure–performance relationship. Furthermore, the challenges and development prospects on the synthesis and applications of asymmetric carbon- and silica-based nanomaterials are also presented and highlighted. [Image: see text]

Bifunctional Janus Silica Spheres for Pickering Interfacial Tandem Catalysis

Nature provides much inspiration for the design of multistep conversion processes, with numerous reactions running simultaneously and without interference in cells, for example. A key challenge in mimicking nature's strategies is to compartmentalize incompatible reagents and catalysts, for example, for tandem catalysis. Here, we present a new strategy for antagonistic catalyst compartmentalization. The synthesis of bifunctional Janus catalyst particles carrying acid and base groups on the particle's opposite patches is reported as is their application as acid-base catalysts in oil/water emulsions. The synthesis strategy involved the use of monodisperse, hydrophobic and amine-functionalized silica particles (SiO2 -NH2 -OSi(CH3 )3 ) to prepare an oil-in-water Pickering emulsion (PE) with molten paraffin wax. After solidification, the exposed patch of the silica particles was selectively etched and refunctionalized with acid groups to yield acid-base Janus particles (Janus A-B). These materials were successfully applied in biphasic Pickering interfacial catalysis for the tandem dehydration-Knoevenagel condensation of fructose to 5-(hydroxymethyl)furfural-2-diethylmalonate (5-HMF-DEM) in a water/4-propylguaiacol PE. The results demonstrate the advantage of rapid extraction of 5-hydroxymethylfurfural (5-HMF), a prominent platform molecule prone to side product formation in acidic media. A simple strategy to tune the acid/base balance using PE with both Janus A-B and monofunctional SiO2 -NH2 -OSi(CH3 )3 base catalysts proved effective for antagonistic tandem catalysis.

Synthesis of Shape-Controllable Anisotropic Microparticles and "Walnut-like" Microparticles via Emulsion Interfacial Polymerization

Anisotropic microparticles have plenty of applications for their asymmetric structure and precisely modified surface. In our research, the uniform anisotropic microparticles with benzyl chloride group were synthesized successfully via emulsion interfacial polymerization. By varying the degree of cross-linking and the concentration of slightly hydrophilic monomer 4-vinyl benzyl chloride (VBC), several types of microparticles with different concavities and different shapes of microparticles (hemisphere, bowl-like, egg-like, etc.) were obtained. Nanoporous microparticles with a walnut-like heterostructure were achieved with modified hydrophilic seeds with the same strategy. The potential applications of shape-controllable fluorescent microparticles and surface modification of microparticles by thiol-click reaction were explored. The modified microparticles achieved in this study are very useful in labeling, tracing, protein separation, and other biomedical fields.

Controllable synthesis of patchy particles with tunable geometry and orthogonal chemistry

HYPOTHESIS

Self-assembly using anisotropic colloidal building blocks may lead to superstructures similar to those found in molecular systems yet can have unique optical, electronic, and structural properties. To widen the spectrum of achievable superstructures and related properties, significant effort was devoted to the synthesis of new types of colloidal particles. Despite these efforts, the preparation of anisotropic colloids carrying chemically orthogonal anchor groups on distinct surface patches remains an elusive challenge.

EXPERIMENTS

We report a simple yet effective method for synthesizing patchy particles via seed-mediated heterogeneous nucleation. Key to this procedure is the use of 3-(trimethoxysilyl)propyl methacrylate (TPM) or 3-(trimethoxysilyl)propyl acrylate (TMSPA), which can form patches on a variety of functional polymer seeds via a nucleation and growth mechanism.

FINDINGS

A family of anisotropic colloids with tunable numbers of patches and patch arrangements were prepared. By continuously feeding TPM or TMSPA the geometry of the colloids could be adjusted accurately. Furthermore, the patches could be reshaped by selectively polymerizing and/or solvating the individual colloidal compartments. Relying on the chemically distinct properties of the TPM/TMSPA and seed-derived domains, both types of patches could be functionalized independently. Combining detailed control over the patch chemistry and geometry opens new avenues for colloidal self-assembly.

Self-assembly of Janus Au:Fe3O4 branched nanoparticles. From organized clusters to stimuli-responsive nanogel suprastructures

Janus nanoparticles offer enormous possibilities through a binary selective functionalization and dual properties. Their self-assembly has attracted strong interest due to their potential as building blocks to obtain molecular colloids, supracrystals and well-organized nanostructures that can lead to new functionalities. However, this self-assembly has been focused on relatively simple symmetrical morphologies, while for complex nanostructures this process has been unexplored. Here, we study the assembly of plasmonic-magnetic Janus nanoparticles with a branched (nanostar) - sphere morphology. The branched morphology enhances their plasmonic properties in the near-infrared region and therefore their applicability, but at the same time constrains their self-assembly capabilities to obtain more organized or functional suprastructures. We describe the self-assembly of these nanoparticles after amphiphilic functionalization. The role of the nanoparticle branching, as well as the size of the polymer-coating, is explored. We show how the use of large molecular weight stabilizing polymers can overcome the anisotropy of the nanoparticles producing a change in the morphology from small clusters to larger quasi-cylindrical nanostructures. Finally, the Janus nanoparticles are functionalized with a thermo-responsive elastin-like recombinamer. These nanoparticles undergo reversible self-assembly in the presence of free polymer giving rise to nanoparticle-stabilized nanogel-like structures with controlled size, providing the possibility to expand their applicability to multi-stimuli controlled self-assembly.

Janus particles: from concepts to environmentally friendly materials and sustainable applications

Janus particles represent a unique group of patchy particles combining two or more different physical or chemical functionalities at their opposite sides. Especially, individual Janus particles (JPs) with both chemical and geometrical anisotropy as well as their assembled layers provide considerable advantages over the conventional monofunctional particles or surfactant molecules offering (a) a high surface-to-volume ratio; (b) high interfacial activity; (c) target controlling and manipulation of their interfacial activity by external signals such as temperature, light, pH, or ionic strength and achieving switching between stable emulsions and macro-phase separation; (d) recovery and recycling; (e) controlling the mass transport across the interface between the two phases; and finally (f) tunable several functionalities in one particle allowing their use either as carrier materials for immobilized catalytically active substances or, alternatively, their site-selective attachment to substrates keeping another functionality active for further reactions. All these advantages of JPs make them exclusive materials for application in (bio-)catalysis and (bio-)sensing. Considering “green chemistry” aspects covering biogenic materials based on either natural or fully synthetic biocompatible and biodegradable polymers for the design of JPs may solve the problem of toxicity of some existing materials and open new paths for the development of more environmentally friendly and sustainable materials in the very near future. Considering the number of contributions published each year on the topic of Janus particles in general, the number of contributions regarding their environmentally friendly and sustainable applications is by far smaller. This certainly pinpoints an important challenge and is addressed in this review article. The first part of the review focuses on the synthesis of sustainable biogenic or biocompatible Janus particles, as well as strategies for their recovery, recycling, and reusability. The second part addresses recent advances in applications of biogenic/biocompatible and non-biocompatible JPs in environmental and biotechnological fields such as sensing of hazardous pollutants, water decontamination, and hydrogen production. Finally, we provide implications for the rational design of environmentally friendly and sustainable materials based on Janus particles.

Controlling Catalyst-Phase Selectivity in Complex Mixtures with Amphiphilic Janus Particles

Amphiphilic Janus particles with a catalyst selectively loaded on either the hydrophobic or hydrophilic region are promising candidates for efficient and phase-selective interfacial catalysis. Here, we report the synthesis and characterization of Janus silica particles with a hydrophilic silica domain and a silane-modified hydrophobic domain produced via a wax masking technique. Palladium nanoparticles were regioselectively deposited on the hydrophobic side, and the phase selectivity of the catalytic Janus particles was established through the kinetic studies of benzyl alcohol hydrodeoxygenation (HDO). These studies indicated that the hydrophobic moiety provided nearly 100× the catalytic activity as the hydrophilic side for benzyl alcohol HDO. The reactivity was linked to the anisotropic catalyst design through microscopy of the particles. The catalysts were also used to achieve phase-specific compartmentalized hydrogenation and selective in situ catalytic degradation of a model oily pollutant in a complex oil/water mixture.

Functionalized Silicas for Metal-Free and Metal-Based Catalytic Applications: A Review in Perspective of Green Chemistry

Heterogeneous catalysis plays a key role in promoting green chemistry through many routes. The functionalizable reactive silanols highlight silica as a beguiling support for the preparation of heterogeneous catalysts. Metal active sites anchored on functionalized silica (FS) usually demonstrate the better dispersion and stability due to their firm chemical interaction with FSs. Having certain functional groups in structure, FSs can act as the useful catalysts for few organic reactions even without the need of metal active sites which are termed as the covetous reusable organocatalysts. Magnetic FSs have laid the platform where the effortless recovery of catalysts is realized just using an external magnet, resulting in the simplified reaction procedure. Using FSs of multiple functional groups, we can envisage the shortened reaction pathway and, reduced chemical uses and chemical wastes. Unstable bio-molecules like enzymes have been stabilized when they get chemically anchored on FSs. The resultant solid bio-catalysts exhibited very good reusability in many catalytic reactions. Getting provoked from the green chemistry aspects and benefits of FS-based catalysts, we confer the recent literature and progress focusing on the significance of FSs in heterogeneous catalysis. This review covers the preparative methods, types and catalytic applications of FSs. A special emphasis is given to the metal-free FS catalysts, multiple FS-based catalysts and magnetic FSs. Through this review, we presume that the contribution of FSs to green chemistry can be well understood. The future perspective of FSs and the improvements still required for implementing FS-based catalysts in practical applications have been narrated at the end of this review.

Emulsion and miniemulsion techniques in preparation of polymer nanoparticles with versatile characteristics

In recent years, polymer nanoparticles (PNPs) have found their ways into numerous applications extending from electronics to photonics, conducting materials to sensors and medicine to biotechnology. Physical properties and surface morphology of PNPs are the most important parameters that significantly affect on their exploitations and can be controlled through the synthesis process. Emulsion and miniemulsion techniques are among the most efficient and wide-spread methods for preparation of PNPs. The objective of this review is to present and highlight the recent developments in the advanced PNPs with specific properties that are produced through emulsion and miniemulsion processes.

Catalyst, Emulsion Stabilizer, and Adsorbent: Three Roles In One for Synergistically Enhancing Interfacial Catalytic Oxidative Desulfurization

A Pickering emulsion catalytic system was proposed to reduce the transfer limitation between two immiscible reactant phases for enhancing the kinetics of heterogenetic oxidative desulfurization (ODS). By loading phosphotungstic acid (HPW) nanoparticles on a novel pyridine-based porous organic polymer of P[tVPB-VP _x], the amphiphilic catalysts were produced and used as the stabilizer for Pickering emulsions. Specifically, an ultrafast ODS rate was realized in the HPW/P[tVPB-VP₁]-stabilized Pickering emulsion catalytic system, and just within 15 min, 100 ppm dibenzothiophene (DBT) was completely oxidized by H₂O₂. Because the obtained hierarchical porous HPW/P[tVPB-VP _x] catalysts showed both high adsorption capacity of DBT and excellent catalytic ODS performance, the catalysts assembling at the interface of emulsions provided this fastest reaction dynamics. Playing three roles of catalyst, emulsion stabilizer, and adsorbent, the synergistic functional catalytic emulsions can be a promising approach to significantly boost the heterogeneous catalytic ODS performance.

Hybrid Janus Particles: Challenges and Opportunities for the Design of Active Functional Interfaces and Surfaces

Janus particles are a unique class of multifunctional patchy particles combining two dissimilar chemical or physical functionalities at their opposite sides. The asymmetry characteristic for Janus particles allows them to self-assemble into sophisticated structures and materials not attainable by their homogeneous counterparts. Significant breakthroughs have recently been made in the synthesis of Janus particles and the understanding of their assembly. Nevertheless, the advancement of their applications is still a challenging field. In this Review, we highlight recent developments in the use of Janus particles as building blocks for functional materials. We provide a brief introduction into the synthetic strategies for the fabrication of JPs and their properties and assembly, outlining the existing challenges. The focus of this Review is placed on the applications of Janus particles for active interfaces and surfaces. Active functional interfaces are created owing to the stabilization efficiency of Janus particles combined with their capability for interface structuring and functionalizing. Moreover, Janus particles can be employed as building blocks to fabricate active functional surfaces with controlled chemical and topographical heterogeneity. Ultimately, we will provide implications for the rational design of multifunctional materials based on Janus particles.

Self-Assembly of Polymer Blends and Nanoparticles through Rapid Solvent Exchange

Molecular dynamics simulations were performed to study the fabrication of polymeric colloids containing inorganic nanoparticles (NPs) via the flash nanoprecipitation (FNP) technique. During this process, a binary polymer blend, initially in a good solvent for the polymers, is rapidly mixed with NPs and a poor solvent for the polymers that is miscible with the good solvent. The simulations reveal that the polymers formed Janus particles with NPs distributed either on the surface of the aggregates, throughout their interior, or aligned at the interface between the two polymer domains, depending on the NP-polymer and NP-solvent interactions. The loading and surface density of NPs can be controlled by the polymer feed concentration, the NP feed concentration, and their ratio in the feed streams. Selective localization of NPs by incorporating electrostatic interactions between polymers and NPs has also been investigated, and was shown to be an effective way to enhance NP loading and surface density as compared to the case with only van der Waals attractions. This work demonstrates that the FNP process is promising for the production of structured and hybrid nanocolloids in a continuous and scalable way, with independent control over particle properties such as size, NP location, loading, and surface density. Our results provide useful guidelines for experimental fabrication of such hybrid nanoparticles.

Facile Strategy for the Synthesis of Gold@Silica Hybrid Nanoparticles with Controlled Porosity and Janus Morphology

Hybrid materials prepared by encapsulation of plasmonic nanoparticles in porous silica systems are of increasing interest due to their high chemical stability and applications in optics, catalysis and biological sensing. Particularly promising is the possibility of obtaining gold@silica nanoparticles (Au@SiO₂ NPs) with Janus morphology, as the induced anisotropy can be further exploited to achieve selectivity and directionality in physical interactions and chemical reactivity. However, current methods to realise such systems rely on the use of complex procedures based on binary solvent mixtures and varying concentrations of precursors and reaction conditions, with reproducibility limited to specific Au@SiO₂ NP types. Here, we report a simple one-pot protocol leading to controlled crystallinity, pore order, monodispersity, and position of gold nanoparticles (AuNPs) within mesoporous silica by the simple addition of a small amount of sodium silicate. Using a fully water-based strategy and constant content of synthetic precursors, cetyl trimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS), we prepared a series of four silica systems: (A) without added silicate, (B) with added silicate, (C) with AuNPs and without added silicate, and (D) with AuNPs and with added silicate. The obtained samples were characterised by transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and UV-visible spectroscopy, and kinetic studies were carried out by monitoring the growth of the silica samples at different stages of the reaction: 1, 10, 15, 30 and 120 min. The analysis shows that the addition of sodium silicate in system B induces slower MCM-41 nanoparticle (MCM-41 NP) growth, with consequent higher crystallinity and better-defined hexagonal columnar porosity than those in system A. When the synthesis was carried out in the presence of CTAB-capped AuNPs, two different outcomes were obtained: without added silicate, isotropic mesoporous silica with AuNPs located at the centre and radial pore order (C), whereas the addition of silicate produced Janus-type Au@SiO₂ NPs (D) in the form of MCM-41 and AuNPs positioned at the silica⁻water interface. Our method was nicely reproducible with gold nanospheres of different sizes (10, 30, and 68 nm diameter) and gold nanorods (55 × 19 nm), proving to be the simplest and most versatile method to date for the realisation of Janus-type systems based on MCM-41-coated plasmonic nanoparticles.