A Universal Approach for the Self-Assembly of Hydrophilic Nanoparticles into Ordered Monolayer Films at a Toluene/Water Interface

Tunable Approach to Induce the Formation of Flexible Nanofilms from Small (3 nm) Gold Nanoparticles at Oil-Water Interfaces

The self-assembly of gold nanoparticles (AuNPs) into thin films at the liquid-liquid interface has promising applications in industries such as catalysis, optics, and sensors. However, precise control over their formation is complex, influenced by several factors which scale differently with core size. Due to their small free energy of adsorption, there are few examples of AuNPs with core diameters <10 nm. The present research evaluates the adsorption of ∼3 nm AuNPs from either side of the oil-aqueous interface with variations in ligand shell composition, the oil phase composition, and the structure of alcohol additives to best drive thin-film formation. Film formation and quality are evaluated, and a recent thermodynamic model is used to gain insight into the primary forces promoting this adsorption. Results demonstrate that longer-chain alcohol additives (namely, n-butanol and n-hexanol) induced adsorption more efficiently than shorter-chain alcohols (ethanol). The volume of alcohol additive needed to induce adsorption was dependent upon the ligand composition, suggesting that the mechanism for induced interfacial adsorption is via interaction with the AuNP ligand shell. Comparison with the thermodynamic model indicates that the driving force for this induced adsorption is the alteration of the three-phase contact angle. Additionally, the use of various oils demonstrates that as oil-water interfacial tension increases, more AuNPs adsorb to the interface. This relationship is also supported by the model. Insight gained for favorable conditions of adsorption for AuNPs < 10 nm as well as the underlying thermodynamic mechanism is important in working toward the ability to fine-tune such films for industrial applications.

Interface-confined precise processing of Ag nanowire into AgPd-nanoparticle-sealed AgAu nanotroughs for boosting ethanol electrooxidation

The functions of nanomaterials are closely linked with their fine structures and compositions. Precisely processing nanoparticles into morphology- and composition-varied nanostructures can a cutting-edge technology for producing complex nanostructures. Herein, we develop an interface-confined precise processing strategy towards toluene/water-interfacial Ag nanowires. Interfacial Ag nanowires are transformed into AgPd-nanoparticle-sealed AgAu nanotroughs with abundant AgPd/AgAu hetero-junctions (i.e., AgPdAu hetero-junction nanostructures). By adjusting the reaction conditions, composition-varied AgPdAu hetero-junction nanostructures can be obtained. The formation of AgPdAu hetero-junction nanostructures can be attributed to interface-confined precise etching towards Ag nanowires separately from the two subphases of the water and the toluene. Composition-optimized Ag13Pd67Au20 hetero-junction nanostructure shows satisfactory catalytic performance towards ethanol electrooxidation: ∼4 and 2 times in electrochemical-activity-surface-area-normalized activities; ∼6 and 5 times in mass-normalized activities higher than commercial Pd/C and Pt/C, respectively. The outstanding catalytic capability of Ag13Pd67Au20 may be attributed to optimized composition, porous nanostructures as well as abundant AgPd/AgAu hetero-junctions. This work demonstrates the feasibility of precisely processing interfacial nanoparticles, opening the way for creating morphology-well-defined composition-varied complex nanostructures.

2D superlattices via interfacial self-assembly of polymer-grafted Au nanoparticles

Nanoparticle (NP) superlattices are periodic arrays of nanoscale building blocks. Because of the collective effect between functional NPs, NP superlattices can exhibit exciting new properties that are distinct from those of individual NPs or corresponding bulk materials. In particular, two-dimensional (2D) NP superlattices have attracted increasing attention due to their emerging applications in micro/opto-electronics, catalysis, sensing, and other fields. Among various preparation methods, evaporation-induced interfacial self-assembly has become the most popular method for preparing 2D NP superlattices because it is a simple, low-cost, and scalable process that can be widely applied to various NPs. Introducing soft ligands, such as polymers, can not only provide convenience in controlling the self-assembly process and tuning superlattice structures but also improve the properties of 2D NP superlattices. This feature article focuses on the methods of evaporation-induced self-assembly of polymer-grafted Au NPs into free-standing 2D NP superlattice films at air/liquid interfaces and 2D NP superlattice coatings on substrates, followed by studies on in situ tracking of the self-assembly evolution process through small-angle X-ray scattering. Their application in nano-floating gate memory devices is also included. Finally, the challenges and perspectives of this direction are discussed.

A Review on Self-Assembly of Colloidal Nanoparticles into Clusters, Patterns, and Films: Emerging Synthesis Techniques and Applications

The colloidal synthesis of functional nanoparticles has gained tremendous scientific attention in the last decades. In parallel to these advancements, another rapidly growing area is the self-assembly or self-organization of these colloidal nanoparticles. First, the organization of nanoparticles into ordered structures is important for obtaining functional interfaces that extend or even amplify the intrinsic properties of the constituting nanoparticles at a larger scale. The synthesis of large-scale interfaces using complex or intricately designed nanostructures as building blocks, requires highly controllable self-assembly techniques down to the nanoscale. In certain cases, for example, when dealing with plasmonic nanoparticles, the assembly of the nanoparticles further enhances their properties by coupling phenomena. In other cases, the process of self-assembly itself is useful in the final application such as in sensing and drug delivery, amongst others. In view of the growing importance of this field, this review provides a comprehensive overview of the recent developments in the field of nanoparticle self-assembly and their applications. For clarity, the self-assembled nanostructures are classified into two broad categories: finite clusters/patterns, and infinite films. Different state-of-the-art techniques to obtain these nanostructures are discussed in detail, before discussing the applications where the self-assembly significantly enhances the performance of the process.

Hybrid Nanoparticles at Fluid-Fluid Interfaces: Insight from Theory and Simulation

Hybrid nanoparticles that combine special properties of their different parts have numerous applications in electronics, optics, catalysis, medicine, and many others. Of the currently produced particles, Janus particles and ligand-tethered (hairy) particles are of particular interest both from a practical and purely cognitive point of view. Understanding their behavior at fluid interfaces is important to many fields because particle-laden interfaces are ubiquitous in nature and industry. We provide a review of the literature, focusing on theoretical studies of hybrid particles at fluid-fluid interfaces. Our goal is to give a link between simple phenomenological models and advanced molecular simulations. We analyze the adsorption of individual Janus particles and hairy particles at the interfaces. Then, their interfacial assembly is also discussed. The simple equations for the attachment energy of various Janus particles are presented. We discuss how such parameters as the particle size, the particle shape, the relative sizes of different patches, and the amphiphilicity affect particle adsorption. This is essential for taking advantage of the particle capacity to stabilize interfaces. Representative examples of molecular simulations were presented. We show that the simple models surprisingly well reproduce experimental and simulation data. In the case of hairy particles, we concentrate on the effects of reconfiguration of the polymer brushes at the interface. This review is expected to provide a general perspective on the subject and may be helpful to many researchers and technologists working with particle-laden layers.

Solvent-driven biotoxin into nano-units as a versatile and sensitive SERS strategy

In recent years, marine biotoxins have posed a great threat to fishermen, human security and military prevention and control due to their diverse, complex, toxic and widespread nature, and the development of rapid and sensitive methods is essential. Surface-enhanced Raman spectroscopy (SERS) is a promising technique for the rapid and sensitive in situ detection of marine biotoxins due to its advantages of rapid, high sensitivity, and fingerprinting information. However, the complex structure of toxin molecules, small Raman scattering cross-section and low affinity to conventional substrates make it difficult to achieve direct and sensitive SERS detection. Here, we generate a large number of active hotspot structures by constructing monolayer nanoparticle films with high density hotspots, which have good target molecules that can actively access the hotspot structures using nanocapillaries. In addition, the efficient and stable signal can be achieved during dynamic detection, increasing the practicality and operability of the method. This versatile SERS method achieves highly sensitive detection of marine biotoxins GTX and NOD, providing good prospects for convenient, rapid and sensitive SERS detection of marine biotoxins.

Long-Range SERS Detection of the SARS-CoV-2 Antigen on a Well-Ordered Gold Hexagonal Nanoplate Film

The development of an effective method for identifying severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) via direct viral protein detection is significant but challenging in combatting the COVID-19 epidemic. As a promising approach for direct detection, viral protein detection using surface-enhanced Raman scattering (SERS) is limited by the larger viral protein size compared to the effective electromagnetic field (E-field) range because only the analyte remaining within the E-field can achieve high detection sensitivity. In this study, we designed and fabricated a novel long-range SERS (LR-SERS) substrate with an Au nanoplate film/MgF2/Au mirror/glass configuration to boost the LR-SERS resulting from the extended E-field. On applying the LR-SERS to detect the SARS-CoV-2 spike protein (S protein), reagent-free detection achieved a low detection limit of 9.8 × 10-11 g mL-1 and clear discrimination from the SARS-CoV S protein. The developed technique also allows testing of the S protein in saliva with 98% sensitivity and 100% specificity.

Self-assembled plasmonic nanoarrays for enhanced bacterial identification and discrimination

The rapid and accurate bacterial testing is a critical step for the management of infectious diseases, but challenges remain largely due to a lack of advanced sensing tools. Here we report the development of highly plasmon-active, biofunctional nanoparticle arrays for simultaneous capture, identification, and differentiation of bacteria by surface-enhanced Raman scattering (SERS). The nanoarrays were facilely prepared through an electrostatic mechanism-controlled self-assembly of metallic nanoparticles at liquid-liquid interfaces, and exhibited high SERS sensitivity beyond femtomole, good reproducibility (relative standard deviation of 2.7%) and stability. Modification of the nanoarrays with concanavalin A allowed to effective capture of both Gram-positive and Gram-negative bacteria (bacterial-capture efficiency maintained beyond 50%) at bacterial concentrations ranging from 50 to 2000 CFU mL^-1, as determined by the plate-counting method. Moreover, single-cell Raman fingerprinting and discrimination of eight different bacteria species with high signal-to-noise ratio, excellent spectral reproducibility, and a total assay time of 1.5 h was achieved under fairly mild conditions (24 μW, acquisition time: 1 s). Collectively, we believe that our biofunctionalized, SERS-based self-assembled nanoarrays have great potential to help in rapid and label-free bacterial diagnosis and phenotyping study.

Instant interfacial self-assembly for homogeneous nanoparticle monolayer enabled conformal "lift-on" thin film technology

Thin film fabrication is of great importance in modern engineering. Here, we propose a universal and conformal thin film technique enabled by the wetting empowered interfacial self-assembly. By tailoring the contact angle of nanoparticle (NP), a NP monolayer can be assembled instantly (within 5 seconds) with an excellent harvesting efficiency (up to 97.5 weight %). This self-assembly strategy presents a universal applicability on various materials, e.g., nonmetal, metal, and core-shell structures, and can achieve a monolayer with same in-plane area as a 95 cm² wafer in a single process, indicating great potential for scale-up manufacturing. Through a template transfer, we coat the surface of different substrates (plastic, paper, etc.) with the assembled film in a conformal and nondestructive “lift-on” manner and subsequently demonstrate fluorescent micropatterns. This self-assembly strategy has great implications in advancing thin film technology in a user-friendly and cost-effective fashion for applications in anti-counterfeiting, actuators, and wearable/flexible electronics.

Exploiting Additives for Directing the Adsorption and Organization of Colloid Particles at Fluid Interfaces

The self-assembly of colloids at fluid interfaces is a well-studied research field both for gaining fundamental insights and for material fabrication. The fluid interface allows the confinement of particles in two dimensions and may act as a template for guiding their organization into soft and reconfigurable structures. Additives (e.g., surfactants, salts, and polymers) in the colloidal suspension are routinely used as a practical and effective tool to drive particle adsorption and tune their interfacial organization. However, some phenomena lying at the heart of the accumulation and self-assembly of particles at fluid interfaces remain poorly understood. This Feature Article aims to critically analyze the mechanisms involved in the adsorption and self-organization of micro- and nanoparticles at various fluid interfaces. In particular, we address the role of additives in both promoting the adsorption of particles from the bulk suspension to the fluid interface and in mediating the interactions between interfacial particles. We emphasize how different types of additives play a crucial role in controlling the interactions between suspended particles and the fluid interface as well as the interactions between adsorbed particles, thus dictating the final self-assembled structure. We also critically summarize the main experimental protocols developed for the complete adsorption of particles initially suspended in the bulk. Furthermore, we highlight some special properties (e.g., reconfigurability upon external stimulation and dissipative self-assembly) and the application potential of structures formed by colloid self-organization at fluid interfaces mediated/promoted by additives. We believe our contribution serves both as a practical roadmap to scientists coming from other fields and as a valuable information resource for all researchers interested in this exciting research field.

Integrating Pt16 Te Nanotroughs and Nanopillars into a 3D "Self-Supported" Hierarchical Nanostructure for Boosting Methanol Electrooxidation

To develop durable and low-price catalysts of methanol oxidation to commercialize direct methanol fuel cell, many attempts have been made at fabricating Pt-based hybrids by designing component-, morphology-, facet-, integration-pattern-varied nanostructures, and have achieved considerable successes. However, most of present catalysts still lack robust catalytic durability especially owing to the corrosion of mixed carbon and the poor mechanical stability of catalyst layer. Herein, Te nanowire array is transformed at an air/water interface into a 3D Pt₁₆ Te hierarchical nanostructure via an interface-confined galvanic replacement reaction. As-formed Pt₁₆ Te nanostructure has an asymmetrical architecture composed of nanotroughs and nanopillars, and nanopillars are perpendicular to nanotroughs with a loose arrangement. Pt₁₆ Te hierarchical nanostructure has a "self-supported" feature and, when directly used as the catalyst of methanol electrooxidation, exhibits superior catalytic activity (>four times larger in mass activity than state-of-the-art Pt/C in either acidic or basic solution) and long-term durability (after 500 cycles of cyclic voltammetric measurement, more than 55% of the initial specific activity remains whereas Pt/C only remains 22.2% in acidic solution and almost loses all activity in basic solution). This study fully demonstrates that designing "self-supported" catalyst film may be the next promising step for improving the catalytic performance of Pt-based hybrids.

Hierarchical Assembly of Two-Dimensional Polymers into Colloidosomes and Microcapsules

Hierarchical assembly of two-dimensional (2D) polymers to 3D microstructures provides new means of creating functional materials with exotic properties for extensive applications. Herein, we report an approach of assembling 2D covalent organic framework (COF) colloidosomes or microcapsules from small molecules. We polymerized monomers to produce narrowly distributed COF particles with average particle sizes greater than 490 nm, which were further used as stabilizers to prepare various water-in-oil Pickering emulsions with droplet sizes of 10-120 μm on average. The emulsion droplets were subsequently applied as templates for interfacial polymerization of the same monomers. The COF microcapsules with varied diameters and shell thicknesses of 0.2-3.1 μm were thus obtained, which possessed good stability, high crystallinity, and surface areas no less than 540 m²/g. The approach also permits facile loading of water-soluble substances such as salts, dyes, or proteins. The loaded molecules demonstrated different permeability against the shell, in which 98% of the encapsulated salts could be released in 1 h while only 18% of dye molecules and almost none of the fluorescent proteins diffused out from the microcapsules. Such an assembling approach may greatly extend the applications of 2D polymers and their microcapsules.

Self-assembly of colloidal nanoparticles into 2D arrays at water-oil interfaces: rational construction of stable SERS substrates with accessible enhancing surfaces and tailored plasmonic response

Self-assembly at water-oil interfaces has been shown to be a cheap, convenient and efficient route to obtain densely packed layers of plasmonic nanoparticles which have small interparticle distances. This creates highly plasmonically active materials that can be used to give strong SERS enhancement and whose structure means that they are well suited to creating the highly stable, reproducible and uniform substrates that are needed to allow routine and accurate quantitative SERS measurements. A variety of methods have been developed to induce nanoparticle self-assembly at water-oil interfaces, fine tune the surface chemistry and adjust the position of the nanoparticles at the interface but only some of these are compatible with eventual use in SERS, where it is important that target molecules can access the active surface unimpeded. Similarly, it is useful to transform liquid plasmonic arrays into easy-to-handle free-standing solid films but these can only be used as solid SERS substrates if the process leaves the surface nanoparticles exposed. Here, we review the progress made in these research areas and discuss how these developments may lead towards achieving rational construction of tailored SERS substrates for sensitive and quantitative SERS analysis.

Engineering one-dimensional trough-like Au-Ag2S nano-hybrids for plasmon-enhanced photoelectrodetection of human α-thrombin

One-dimensional (1D) morphology-unique Au-Ag₂S nano-hybrids are achieved by combining the interfacial self-assembly of Ag nanowires, interface-oriented site-specific etching of Ag nanowires with AuCl₄^-, and the sulfurization of S^2-. The as-formed Au-Ag₂S nano-hybrid has a trough-like morphology. The wall of the Au-Ag₂S nanotrough is a Ag₂S/Au/Ag₂S trilayer wall, but the Ag₂S layer is a Ag₂S-rich mixture of Ag₂S and Au rather than pure Ag₂S because of the diffusion of Au atoms towards Ag₂S. The Au-Ag₂S nanotrough shows strong absorption in the visible region (400-800 nm) and exhibits a favorable photoelectrochemical (PEC) response, the photocurrent of which is ∼8.5 times larger than that of pure Ag₂S. This enhanced PEC response originates from the localized plasmonic resonance effect of Au. Moreover, the PEC biosensor based on the Au-Ag₂S nanotroughs shows high sensitivity and selectivity, satisfactory reproducibility, and good stability towards human α-thrombin (TB) detection: a sensitive linear response ranging from 1.00 to 10.00 pmol L^-1 and a low detection limit of 0.67 pmol L^-1. This study provides a new model for studying the PEC behavior of plasmonic metal/semiconductor materials, and this Au-Ag₂S nanotrough may also be useful in the fields of photocatalysis and photovoltaics.

Synergistic plasmon resonance coupling and light capture in ordered nanoarrays as ultrasensitive and reproducible SERS substrates

An effective SERS substrate for on-field detection needs to satisfy high sensitivity to analyte and signal reproducibility even in the special case of tilting or bending of substrates. Herein, we transferred monolayer AuNPs into a nanocavity to construct a Au particle-in-hemispherical honeycomb nanoarray (PIHHN) as an ultrasensitive and spatially reproducible SERS substrate. The capacity of detection for R6G in an optimal PIHHN substrate is as low as a concentration of 10^-15 M, and the RSD of signal deviation is no more than 5.6%. FDTD simulations explain that placing AuNPs into a metallic nanocavity can capture and focus the light field to improve the interaction between the light and the substrate and provide the collective effect of multiple plasmon coupling, which can induce a stronger electromagnetic field. In addition, the system can generate more hot spots between AuNPs and between AuNPs and the metallic nanocavity. In particular, when the substrate is tilted or bent at an angle from 0° to 60°, the SERS performance remains stable due to the rotational symmetry of the PIHHN structure, which demonstrates the capability of on-field detection. Furthermore, the PIHHN substrate is employed as a highly sensitive multiplex sensor in on-field analysis for contaminated soil, achieving the detection of analytes down to 0.5 ppb.

Polarization- and Wavelength-Dependent Shell-Isolated-Nanoparticle-Enhanced Sum-Frequency Generation with High Sensitivity

Sum-frequency generation (SFG) spectroscopy is a highly versatile tool for surface analysis. Improving the SFG intensity per molecule is important for observing low concentrations of surface species and intermediates in dynamic systems. Herein, Shell-Isolated-Nanoparticle-Enhanced SFG (SHINE-SFG) was used to probe a model substrate. The model substrate, p-mercaptobenzonitrile adsorbed on a Au film with SHINs deposited on top, provided an enhancement factor of up to 10^{5}. Through wavelength- and polarization-dependent SHINE-SFG spectroscopy, the majority of the signal enhancement was found to come from both plasmon enhanced emission and chemical enhancement mechanisms. A new enhancement regime, i.e., the nonlinear coupling of SHINE-SFG with difference frequency generation, was also identified. This novel mechanism provides insight into the enhancement of nonlinear coherent spectroscopies and a possible strategy for the rational design of enhancing substrates utilizing coupling processes.

Preparation of Monolayer Photonic Crystals from Ag Nanobulge-Deposited SiO2 Particles as Substrates for Reproducible SERS Assay of Trace Thiol Pesticide

Surface-enhanced Raman scattering (SERS) greatly increases the detection sensitivity of Raman scattering. However, its real applications are often degraded due to the unrepeatable preparation of SERS substrates. Herein presented is a very facile and cost-effective method to reproducibly produce a novel type of SERS substrate, a monolayer photonic crystal (PC). With a building block of laboratory-prepared monodisperse SiO₂ particles deposited with space-tunable silver nanobulges (SiO₂@nAg), a PC substrate was first assembled at the air-water interface through needle tip flowing, then transferred onto a silicon slide by a pulling technique. The transferred monolayer PCs were characterized by SEM and AFM to have a hexagonal close-packed lattice. They could increase Raman scattering intensity by up to 2.2 × 10⁷-fold, as tested with p-aminothiophenol. The relative standard deviations were all below 5% among different substrates or among different locations on the same substrate. The excellent reproducibility was ascribed to the highly ordered structure of PCs, while the very high sensitivity was attributed to the strong hotspot effect caused by the appropriately high density of nanobulges deposited on SiO₂ particles and by a closed lattice. The PC substrates were validated to be applicable to the SERS assay of trace thiol pesticides. Thiram pesticide is an example determined in apple juice samples at a concentration 10²-fold lower than the food safety standard of China. This method is extendable to the analysis of other Raman-active thiol chemicals in different samples, and the substrate preparation approach can be modified for the fabrication of more PC substrates from other metallic nanobulge-deposited particles rather than silica only.

Determination of prostate cancer marker Zn2+ with a highly selective surface-enhanced Raman scattering probe on liquid-liquid self-assembled Au nanoarrays

As the concentration of Zn²⁺ in patients with prostate cancer is much less than that in healthy persons, Zn²⁺ concentration can be used as a marker to expediently screen prostate cancer. In this study, a sensitive and highly selective surface-enhanced Raman scattering (SERS) method to detect Zn²⁺ concentration in human prostatic fluids by utilizing water-insoluble 2-carboxyl-2'-hydroxyl-5'-sulfoformazylbenze (Zincon) as a SERS probe based on self-assembled Au nanoarrays at a liquid-liquid interface between n-hexane and Au colloids was proposed. Zincon showed remarkably different SERS bands before and after coordinating Zn²⁺ in the controlled conditions (70 μL of ethanol, 500 μL of n-hexane, pH value of 7.1 and 10 s of vortex mixing time), which can be used in quantifying Zn²⁺ with characteristic peaks. The proposed SERS method presented a good linear relationship ranging from 0.5 to 10 μmol/L and a satisfactory detection limit of 0.1 μmol/L as well as low interference with other metal ions. Moreover, the detection results are close to those of the conventional standard atomic absorption spectroscopy (AAS) method.

Biofriendly and Regenerable Emotional Monitor from Interfacial Ultrathin 2D PDA/AuNPs Cross-linking Films

Well-designed 2D materials with ultrathin structures show great potential for humidity-sensing performance owing to their high surface-volume ratio and a great number of exposed atoms on the surface. However, some sensing elements employed for healthcare applications may be considered as potentially risky, such as inflammation, granuloma formation, and carcinogenesis. Herein, we explored biofriendly humidity-sensing characteristics inspired by the great biocompatibility and conductivity of hyperbranched polyethyleneimine-capped gold nanoparticles and cross-linked with polydopamine from the adhesive proteins in mussels. It was successfully employed into two kinds of wearable devices, sports watches and breathing masks, for real-time recording humidity's fluctuation in expiration and sweat with changes of individual's crying, laughing, nervous, sleeping, training, and cold states. The wearable devices allow us to monitor individual's physical activities and emotional states well, suggesting a promising prospect in safe, reusable, long term, and noncontact human health monitoring applications.

A functional Au array SERS chip for the fast inspection of pesticides in conjunction with surface extraction and coordination transferring

The fast inspection of the pesticide residues on fruits and vegetables requires the development of facile, sensitive and accurate methods. Surface enhanced Raman scattering (SERS) is a promising way to provide a fast inspection method, which requires significant improvements in the reproducibility and feasibility. In the present work, an SERS method was developed for the fast inspection of pesticides on fruit peels in conjunction with surface extraction and coordination transferring. In this new method, the residual pesticides were directly extracted from fruit peels with an appropriate extraction solution and then quantitatively transferred onto an organic solvent-compatible Au array SERS chip through the strong chemical interactions between the heteroatoms in the pesticides and the gold surface. The functional SERS chip was fabricated by the interfacial assembly of an Au array on a membrane, which produced dense and uniformly distributed SERS hot spots and enabled compatibility with random curvature surfaces and handheld Raman spectrometers. As a proof of concept, sulfur atoms containing thiram on apples were detected at a concentration as low as 5 ng cm-2 with good reproducibility (relative standard deviation lower than 10%). The strong interactions between the sulfur atoms and gold surface during the coordination transferring process were confirmed by the enhanced vibrations of the specified bands occurring in both the Raman and IR spectra. This surface extraction-coordination transferring-based method holds wide applicability for heteroatom-containing pesticides, as demonstrated by the detection of various S- and P-containing pesticides.

Plasmon-promoted electrocatalytic water splitting on metal-semiconductor nanocomposites: the interfacial charge transfer and the real catalytic sites

The interfacial charge transfer and real catalytic sites on metal–semiconductor hybrid catalysts for photoelectrocatalytic water splitting are studied.

Plasmonic metal nanoparticles (NPs) have emerged as promising visible light harvesters to facilitate solar-to-chemical energy conversion via the generation of hot electrons by non-radiative decay of plasmons. As one of the most promising renewable energy production methods for the future, electrocatalytic water splitting is an ideal chemical reaction in which plasmonic NPs can be utilized for direct solar-to-fuel conversion. Due to the rapid carrier recombination on plasmonic NPs, hybrid photocatalysts integrating metals and semiconductors are usually employed to separate the hot electrons and holes. However, an understanding of the catalytic mechanism, which is critical for rational design of plasmonic electrocatalysts, including the interfacial charge transfer pathway and real reactive sites, has been lacking. Herein, we report on the combination of plasmonic Au NPs and semiconductors (Ni and/or Co hydroxides) for plasmon-promoted electrocatalytic water splitting. By using surface-enhanced Raman spectroscopy (SERS), we find a strong spontaneous interfacial charge transfer between Au and NiCo layered double hydroxide (LDH), which facilitates both the oxygen and hydrogen evolution reactions. The real catalytic sites on the hybrid material are confirmed by selective blocking of the metal surface with a thiol molecular monolayer. It is found that the plasmon-promoted oxygen evolution occurs on the LDH semiconductor but surprisingly, the hydrogen evolution sites are mainly located on the Au NP surface. This work demonstrates the critical role of interfacial charge transfer in hot electron-driven water splitting and paves the way for rational design of high-performance plasmonic electrocatalysts.

Binary Thiol-Capped Gold Nanoparticle Monolayer Films for Quantitative Surface-Enhanced Raman Scattering Analysis

Surface-enhanced Raman scattering (SERS) can provide fingerprint information of analyte molecules with unparalleled sensitivity. However, quantitative analysis using SERS has remained one of the major challenges owing to the difficulty of obtaining reproducible SERS substrates with high-density hotspots. Here, we report the rational design and fabrication of a binary thiol-capped gold nanoparticle (AuNP) monolayer film (MLF) as a substrate for highly sensitive and quantitative SERS analysis. The two thiol ligands chemically bonded to the AuNPs play different roles: dodecanethiol with a long alkyl chain controls the interparticle gaps and electromagnetic coupling among AuNPs and 4-mercaptopyridine works as a Raman internal standard (IS). The binary thiol-capped AuNPs can self-assemble into an ordered MLF with high-density hotspots and uniformly distributed IS. The as-prepared MLF has been demonstrated as a reliable SERS substrate for quantitative detection of fungicide malachite green in aqueous solution, with a high enhancement factor (up to 3.3 × 10⁷) and a low detection limit (100 pM). Moreover, the MLF SERS substrate is flexible and transparent, which has enabled in situ detection of trace fungicide residues in a shrimp tissue.

Surface-Enhanced Raman Spectroscopy on Self-Assembled Au Nanoparticles Arrays for Pesticides Residues Multiplex Detection under Complex Environment

The high reproducibility of trace detection in complex systems is very hard but crucial to analytical technology and science. Here, we present a surface-enhanced Raman scattering (SERS) platform made by large-scale self-assembly of Au nanoparticle (NP) arrays at the cyclohexane/water interface and its use for pesticides residues trace detection. The analyte molecules spontaneously localize into the Au NPs’ nanogaps during the self-assembly process, yielding excellent Raman signal enhancement by surface effects, and possibly both by the concentration of the analytes into the array and by plasmonic hot-spot formation. Transmission electron microscopy (TEM) images demonstrate a good uniformity of interparticle distances (2–3 nm) in the Au NP arrays. SERS experiments on crystal violet (CV) molecules demonstrated that the relative standard deviations (RSD) of the band intensities at 1173, 1376, and 1618 cm⁻¹ were 6.3%, 6.4%, and 6.9%, respectively, indicating high reproducibility of the substrate. Furthermore, we demonstrate that two pesticides dissolved in organic and aqueous phases could be simultaneously detected, suggesting an excellent selectivity and universality of this method for multiplex detection. Our SERS platform opens vast possibilities for repeatability and sensitivity detection of targets in various complex fields.

Nanoparticle Assembly at Liquid-Liquid Interfaces: From the Nanoscale to Mesoscale

In the past few decades, novel syntheses of a wide range of nanoparticles (NPs) with well-defined chemical composition and structure have opened tremendous opportunities in areas ranging from optical and electronic devices to biomedical markers. Controlling the assembly of such well-defined NPs is important to effectively harness their unique properties. The assembly of NPs at liquid-liquid interfaces is becoming a central topic both in surface and colloid science. Hierarchical structures, including 2D films, 3D capsules, and structured liquids, have been generating significant interest and are showing promise for physical, chemical, and biological applications. Here, a brief overview of the development of the self-assembly of NPs at liquid-liquid interfaces is provided, from theory to experiment, from synthetic NPs to bio-nanoparticles, from water-oil to water-water, and from "liquid-like" to "solid-like" assemblies.

Facile On-Site Aqueous Pollutant Monitoring Using a Flexible, Ultralight, and Robust Surface-Enhanced Raman Spectroscopy Substrate: Interface Self-Assembly of Au@Ag Nanocubes on a Polyvinyl Chloride Template

Aquatic ecosystems and human health have been seriously threatened by illegal discharge of wastewater, while simple and effective monitoring methods are still sparse. Here, we propose a facile method for on-site pollutant monitoring by surface-enhanced Raman spectroscopy with a novel substrate. This substrate is fabricated by interface self-assembly of Au@Ag nanocubes (NCs) on a simultaneously formed polyvinyl chloride (PVC) template, followed by coating with a thin Au film. The Au@Ag@Au-NCs/PVC film is flexible, ultralight, and robust and could float on the surface of water and firmly contact with water even under harsh environmental conditions. Moreover, the Au@Ag@Au-NCs/PVC film is translucent, allowing penetration of laser beams and enhancement of Raman signals. When thiram was used as a model contaminant in aqueous solution, a good linear relationship ( R² = 0.972) was obtained over the range of 0.1-50 ppb with a detection limit of 0.1 ppb. Raman signals of thiram can be instantly and consecutively detected with the enhancement of the film in the simulated experiments, suggesting its possible use in the long run. Furthermore, the film can be easily regenerated by NaBH₄ solution washing, which could reduce the operating cost. In summary, the Au@Ag@Au-NCs/PVC film has great potential in on-site pollutant monitoring in aqueous environments with a portable Raman spectrometer.

Asymmetrical etching of Ag nanoparticles into symmetry-reduced bi-metallic nanocups at the single-nanoparticle level

By employing the water/air interface-confinement effect, local nanomasking and thus selective etching were achieved on single interfacial nanoparticles, developing a series of symmetry-reduced cubic, spherical, trough-like nanocups.

Continuous Films of Self-Assembled Graphene Quantum Dots for n-Type Doping of Graphene by UV-Triggered Charge Transfer

The demands to examine components serving as one of the active layers in heterostructures of 2D materials have been recently increasing. Nanomaterials synthesized from a solution process and their self-assembly can provide a promising route to build a new type of mixed dimensional heterostructures, and several methodologies have been reported previously to construct 2D assemblies from colloidal nanostructures in solution. Graphene quantum dots (GQDs), receiving much interest due to the tunable optical band gap and the capability of chemical functionalization, are considered as emerging nanomaterials for various optoelectronic and biological applications. This study fabricates a closely packed GQDs film (GQDF) from colloidal solutions using a solvent-assisted Langmuir Blodgett method, and investigates the optical and electrical characteristics of the heterostacked graphene/GQD film (G/GQDF) structures. It is observed that the GQDF plays a role not only as a buffer layer that isolates Chemical Vapor Deposited graphene (CVD graphene) from undesired p-doping but also as a photoactive layer that triggers n-doping of the heterostacked CVD graphene film. The n-doping density of the G/GQDF device is proportional to UV irradiation time, but its carrier mobility remains constant regardless of doping densities, which are unique characteristics that have not been observed in other doping methods.

Deposition of Gold Nanotriangles in Large Scale Close-Packed Monolayers for X-ray-Based Temperature Calibration and SERS Monitoring of Plasmon-Driven Catalytic Reactions

Anisotropic plasmonic particles such as gold nanotriangles have extraordinary structural, optical, and physicochemical properties. For many applications in different fields, it is essential to prepare them in a chemically and physically stable, structurally well-defined manner, e.g., as large and uniform coverage on a substrate. We present a direct method for the large scale close-packed monolayer formation of edge-to-edge ordered, ultrathin crystalline gold nanotriangles on Si wafers or quartz glass via the transfer of these asymmetric particles to the air-liquid interface after adding ethanol-toluene mixtures without any subsequent surface functionalization. X-ray diffraction monitoring of the close-packed, large area monolayer with a mosaicity of less than 0.1° allows for calibrating the temperature of the particles during continuous laser heating. This is important for characterizing the microscopic temperature of the metal particles in the plasmon-driven dimerization process of 4-nitrothiophenol (4-NTP) into 4,4'-dimercaptoazobenzene (DMAB), monitored in real time by surface-enhanced Raman scattering (SERS). The gold nanotriangles can act as a source of hot electrons and initiate the dimerization process.

Highly Selective and Repeatable Surface-Enhanced Resonance Raman Scattering Detection for Epinephrine in Serum Based on Interface Self-Assembled 2D Nanoparticles Arrays

Target analyte detection in complex systems with high selectivity and repeatability is crucial to analytical technology and science. Here we present a two-dimensional (2D) surface-enhanced resonance Raman scattering (SERRS) platform, which takes advantages of the high selectivity of the SERRS sensor as well as the sensitivity and reproducibility of the interfacial SERS platform, for detecting trace epinephrine (EP) in the serum. To realize sensitive and selective detection of EP in a complex system, Au NPs are modified with α,β-nitriloacetic acid and Fe(NO₃)₃ to form the Au NP-(Fe-NTA) sensor, and as a consequence, EP can be rapidly captured by the sensor on the surface of Au NPs and then delivered at the cyclohexane/water interface. More importantly, we synthesized the extremely stable Au NPs (PVP-stabilized Au NPs), where the presence of PVP prevents aggregation of Au NPs during the self-assembly process and then makes a more uniform distribution of Au NPs with analytes at the cyclohexane/water interface, approximately 2 nm interparticle distance between the Au NPs, which has been proved by synchrotron radiation grazing incidence small-angle X-ray scattering (SR-GISAXS) experiments. The self-assembly method not only effectively avoids the aggregation of Au NPs and decreases the influence of the background signal but also can capture and enrich EP molecules in the cyclohexane/water interface, realizing the sensitive and selective detection of EP in complex serum sample. This strategy overcomes the difficulty of bringing nanostructures together to form efficient interparticle distance with simple fabrication and maximum uniformity and also provides a powerful nanosensor for tracing amounts of analyte molecules in a complex system with the advantages of capturing and enriching of target molecules in the liquid/liquid interface during the self-assembly process. Our SERRS platform opens vast possibilities for repeatability, sensitivity, and selectivity detection of targets in various complex fields.

Observing the dynamic “hot spots” on two-dimensional Au nanoparticles monolayer film

Interparticle spacing was controlled by evaporating water on 2D Au nanoparticles arrays.

A facile solvent mediated self-assembly silver nanoparticle mirror substrate for quantitatively improved surface enhanced Raman scattering

A simple and controllable assembly of silver nanoparticles by a mediating solvent is introduced to enhance the SERS reproducibility and sensitivity.

Gold Nanoparticle Monolayers from Sequential Interfacial Ligand Exchange and Migration in a Three-Phase System

Using a three-phase system, centimeter-scale monolayer gold nanoparticle (Au NP) films have been prepared that have long-range order and hydrophobic ligands. The system contains an interface between an aqueous phase containing Au NPs and an oil phase containing one of various types of amine ligands, and a water/air interface. As the Au NPs diffuse to the water/oil interface, ligand exchange takes place which temporarily traps them at the water/oil interface. The ligand-exchanged particles then spontaneously migrate to the air/water interface, where they self-assemble, forming a monolayer under certain conditions. The spontaneous formation of the NP film at the air/water interface was due to the minimization of the system Helmholtz free energy. However, the extent of surface functionalization was dictated by kinetics. This decouples interfacial ligand exchange from interfacial self-assembly, while maintaining the simplicity of a single system. The interparticle center-to-center distance was dictated by the amine ligand length. The Au NP monolayers exhibit tunable surface plasma resonance and excellent spatial homogeneity, which is useful for surface-enhanced Raman scattering. The "air/water/oil" self-assembly method developed here not only benefits the fundamental understanding of NP ligand conformations, but is also applicable to the manufacture of plasmonic nanoparticle devices with precisely designed optical properties.

Self-Assembled Large-Scale Monolayer of Au Nanoparticles at the Air/Water Interface Used as a SERS Substrate

Self-assembly of metal nanoparticles has attracted considerable attention because of its unique applications in technologies such as plasmonics, surface-enhanced optics, sensors, and catalysts. However, fabrication of ordered nanoparticle structures remains a significant challenge. Thus, developing an efficient approach for the assembly of large-scale Au nanoparticles films for theoretical studies and for various applications is highly desired. In this paper, a facial approach for fabricating a monolayer film of Au nanoparticles was developed successfully. Using the surfactant polyvinylpyrrolidone (PVP), a large-scale monolayer film of well-ordered, uniform-sized Au nanoparticles was fabricated at the air/water interface. The film exhibited a two-dimensional (2D) hexagonal close-packed (HCP) structure having interparticle gaps smaller than 2 nm. These gaps generated numerous uniform "hot spots" for surface-enhanced Raman scattering (SERS) activity. The as-prepared monolayer film could be transferred to a solid substrate for use as a suitable SERS substrate with high activity, high uniformity, and high stability. The low spot-to-spot and substrate-to-substrate variations of intensity (<10%), the large surface enhancement factor (∼10(6)), and the high stability (∼45 days) make the substrate suitable for SERS measurements. Transfer of the monolayer film onto a glassy carbon electrode produced an Au electrode with clean, well-defined nanostructure suitable for electrochemical SERS measurements. The adsorption process of ionic liquids on the electrode with the monolayer film is similar to that on bulk metal electrodes. The present strategy provides an effective way for self-assembly of Au nanoparticles into well-defined nanostructures that may form optimal reproducible SERS substrates for quantitative analysis. It also provides an electrode with clean, well-defined nanostructure for electrochemical investigations.

Gold Nanoparticle Monolayers with Tunable Optical and Electrical Properties

Centimeter-scale gold nanoparticle (Au NP) monolayer films have been fabricated using a water/organic solvent self-assembly strategy. A recently developed approach, drain to deposit, is demonstrated to be most effective in transferring the Au NP films from the water/organic solvent interface to various solid substrates while maintaining their integrity. The interparticle spacing was tuned from 1.4 to 3.1 nm using alkylamine ligands of different lengths. The ordering of the films increased with increasing ligand length. The surface plasmon resonance and the in-plane electrical conductivity of the Au NP films both exhibit an exponential dependence on the interparticle spacing. These findings show great potential in scaling up the manufacturing of high-performance optical and electronic devices based on two-dimensional metallic nanoparticle superlattices.

Controlling dynamic SERS hot spots on a monolayer film of Fe3O4@Au nanoparticles by a magnetic field

A large surface-enhanced Raman scattering (SERS) effect is critically dependent on the gap distance of adjacent nanostructures, i.e., "hot spots". However, the fabrication of dynamically controllable hot spots still remains a remarkable challenge. In the present study, we employed an external magnetic field to dynamically control the interparticle spacing of a two-dimensional monolayer film of Fe3O4@Au nanoparticles at a hexane/water interface. SERS measurements were performed to monitor the expansion and shrinkage of the nanoparticles gaps, which produced an obvious effect on SERS activities. The balance between the electrostatic repulsive force, surface tension, and magnetic attractive force allowed observation of the magnetic-field-responsive SERS effect. Upon introduction of an external magnetic field, a very weak SERS signal appeared initially, indicating weak enhancement due to a monolayer film with large interparticle spacing. The SERS intensity reached maximum after 5s and thereafter remained almost unchanged. The results indicated that the observed variations in SERS intensities were fully reversible after removal of the external magnetic field. The reduction of interparticle spacing in response to a magnetic field resulted in about one order of magnitude of SERS enhancement. The combined use of the monolayer film and external magnetic field could be developed as a strategy to construct hot spots both for practical application of SERS and theoretical simulation of enhancement mechanisms.

Surface enhanced Raman scattering by graphene-nanosheet-gapped plasmonic nanoparticle arrays for multiplexed DNA detection

We have developed a new type of surface enhanced Raman scattering (SERS) substrate with thiolated graphene oxide (tGO) nanosheets sandwiched between two layers of closely packed plasmonic nanoparticles. The trilayered substrate is built up through alternative loading of interfacially assembled plasmonic nanoparticle arrays and tGO nanosheets, followed by coating the nanoparticle surfaces with poly(ethylene glycol) (PEG). Here tGO plays multifunctional roles as a 2D scaffold to immobilized interfacially assembled plasmonic nanoparticles, a nanospacer to create SERS-active nanogaps between two layers of nanoparticle arrays, and a molecule harvester to enrich molecules of interest viaπ-π interaction. In particular, the molecule harvesting capability of the tGO nanospacer and the stealth properties of PEG coating on the plasmonic nanoparticles collectively lead to preferential positioning of selective targets such as aromatic molecules and single-stranded DNA at the SERS-active nanogap hotspots. We have demonstrated that an SERS assay based on the PEGylated trilayered substrate, in combination with magnetic separation, allows for sensitive, multiplexed "signal-off" detection of DNA sequences of bacterial pathogens.