Controlling the Interaction and Non-Close-Packed Arrangement of Nanoparticles on Large Areas

Branched Au Nanoparticles on Nanofibers for Surface-Enhanced Raman Scattering Sensing of Intracellular pH and Extracellular pH Gradients

The development of plasmonic-active nanosensors for surface-enhanced Raman scattering (SERS) sensing is important for gaining knowledge on intracellular and extracellular chemical processes, hypoxia detection, and label-free detection of neurotransmitters and metabolites, among other applications in cell biology. The fabrication of SERS nanosensors for optophysiology measurements using substrates such as nanofibers with a uniform distribution of plasmonic nanoparticles (NPs) remains a critical hurdle. We report here on a strategy using block copolymer brush-layer templating and ligand exchange for fabricating highly reproducible and stable SERS-active nanofibers with tip diameters down to 60 nm and covered with well-dispersed and uniformly distributed branched AuNPs, which have intrinsic hotspots favoring inherently high plasmonic sensitivity. Among the SERS sensors investigated, those with Au nanostars with short branches [AuNS(S)s] exhibit the greatest SERS sensitivity, as verified also by COMSOL Multiphysics simulations. Functionalization of the AuNS(S)s with the pH-sensitive molecule, 4-mercaptobenzoic acid, led to SERS nanosensors capable of quantifying pH over a linear range of 6.5-9.5, covering the physiological range. These pH nanosensors were shown to be able to detect the intracellular pH as well as extracellular pH gradients of in vitro breast cancer cells with minimal invasiveness and improved SERS sensitivity, along with a high spatial resolution capability.

Controlling the Interparticular Distances of Extended Non-Close-Packed Colloidal Monolayers

A versatile method for the preparation extended, well-ordered, non-close-packed monolayers of silica nanoparticles (137 ± 4 nm diameter) with adjustable interparticle distances is presented, which is based on a simple self-assembly procedure using aqueous dispersion with different ionic strengths. It is shown that these structures can be successfully transferred to air without suffering from aggregation. Scanning electron microscopy (SEM) is used to characterize the structures after transfer into the atmosphere. These investigations were combined with a quartz crystal microbalance with dissipation (QCM-D) experiments to follow the self-assembly process in solution. The nearest-neighbor distance distribution reveals a monotonous decrease of the average nearest-neighbor distance from 290 to 200 nm with increasing ionic strength from 0.05 to 1 mM, which indicates an increased shielding of the electrostatic interaction with increasing ionic strength. The observed saturation coverages for all studied ionic strengths are well explained with an effective hard-sphere model in which the saturation coverage is limited by Coulomb repulsion. However, at ionic strengths above 1 mM, significant amounts of aggregates are found in the dried samples, suggesting that the observed aggregates at high ionic strengths are formed during the drying process caused by capillary forces between the particles. Tuning the barrier for lateral diffusion, e.g., by changing the surface morphology or functionalization of the particles will offer a route to further extend the range of particle distances. The present approach can be easily expanded to a broad range of colloidal materials on surfaces, while it only requires low-cost laboratory equipment.

Nanoparticle processing: Understanding and controlling aggregation

Nanoparticles (NPs) are commonly defined as particles with size <100 nm and are currently of considerable technological and academic interest, since they are often the starting materials for nanotechnology. Novel properties develop as a bulk material is reduced to nanodimensions and is reflected in new chemistry, physics and biology. With reduction in size, a greater function of the atoms is at the surface, and promote different interaction with its environment, as compared to the bulk material. In addition, the reduction in size alters the electronic structure of the material, resulting in novel quantum effects. Size also influences mobility, primarily controlled by Brownian motion for NPs, and relevant in biological and environmental processes. However, the small size also leads to high surface energy, and NPs tend to aggregate, thereby lowering the surface energy. In all applications, the uncontrolled aggregation of NPs can have negative effects and needs to be avoided. There are however examples of controlled aggregation of NPs which give rise to novel effects. This review article is focused on the NP features that influences aggregation. Common strategies for synthesis of NPs from the gas and liquid phases are discussed with emphasis on aggregation during and after synthesis. The theory involving Van der Waals attractive force and electrical repulsive force as the controlling features of the stability of NPs is discussed, followed by examples of how repulsive and attractive forces can be manipulated experimentally to control NP aggregation. In some applications, NPs prepared by liquid methods need to be isolated for further applications. The process of solvent removal introduces new forces such as capillary forces that promote aggregation, in many cases, irreversibly. Strategies for controlling aggregation upon drying are discussed. There are also many methods for redispersing aggregated NPs, which involve mechanical forces, as well as manipulating capillary forces and surface characteristics. We conclude this review with a discussion of aggregation relevant real-world applications of NPs. This review should be relevant for scientists and technologists interested in NPs, since emphasis has been on the practical aspects of NP-based technology, and especially, strategies relevant to controlling NP aggregation.

Fabrication of Silica Nanoparticle Monolayer Arrays Using an Anodic Aluminum Oxide Template

Non-close-packed (NCP) silica nanoparticle monolayer arrays (SNMA) on ordered porous anodic aluminum oxide (AAO) templates were fabricated for the first time by a novel two-step spin-coating technique. The obtained NCP-SNMA-AAO was composed of silica nanoparticles (average primary particle size of 440 nm) and well-organized nanopores on the AAO substrates. NCP-SNMA-AAO with a supporting ratio of 87% silica nanoparticles showed a hydrophilic surface (water contact angle of 51.0°), while the original AAO substrate shows a hydrophobic surface (water contact angle of 107.9°). The maximum coefficient of static friction was decreased by 29% (0.327 → 0.233). The coefficient of dynamic friction was also decreased by 20% (0.281 → 0.226). We found that controlling the silica supporting ratio using the two-step spin-coating technique is an effective approach for surface modification of an AAO substrate.

A Simple Method for Patterning Nanoparticles on Planar Surfaces

This paper describes a simple method to pattern nanoparticles on planar surfaces using the antifouling property of poly(ethylene glycol) monolayers deposited from a solution on the native oxide of titanium. Atomic force microcopy was used to pattern the poly(ethylene glycol) monolayers producing protein active sites on the protein-resistant surface. Patterns with different sizes have been generated by shaving the monolayers with different repetitions. Friction force microscopy was used to image the patterns. The smallest patterns are 50 nm and the largest patterns are 500 nm at full width half maximum. The smallest pattern was produced with one shave, whereas the largest pattern was produced by shaving the monolayers 112 times. Protein-coated nanoparticles were immobilised on the shaved (protein active) part of the monolayers by dipping the patterned samples into a solution that contains 2% by volume protein-functionalized nanoparticles with a nominal diameter of 40 nm. Atomic force microscopy was used to take a topographic image of the samples. The topographic image showed that the protein-functionalized nanoparticles were attached onto the shaved part of the substrate but not on the poly(ethylene glycol)-covered part of the substrate. The level of aggregation of the nanoparticles was also investigated from the topographic image. The section analysis of the topographic image of the nanoparticle patterns showed a height of 40 nm which proved that only a monolayer of particles were deposited on the shaved part of the monolayer.

Self-assembled colloidal arrays for structural color

Structural color materials that are colloidally assembled as inspired by nature are attracting increased interest in a wide range of research fields. The assembly of colloidal particles provides a facile and cost-effective strategy for fabricating three-dimensional structural color materials. In this review, the generation mechanisms of structural colors from colloidally assembled photonic crystalline structures (PCSs) and photonic amorphous structures (PASs) are first presented, followed by the state-of-the-art and detailed technologies for their fabrication. The variable optical properties of PASs and PCSs are then discussed, focusing on their spatial long- and short-order structures and surface topography, followed by a detailed description of the modulation of structural color by refractive index and lattice distance. Finally, the current applications of structural color materials colloidally assembled in various fields including biomaterials, microfluidic chips, sensors, displays, and anticounterfeiting are reviewed, together with future applications and tasks to be accomplished.

Monolayer Arrays of Nanoparticles on Block Copolymer Brush Films

Two-dimensional arrays of nanoparticles (NPs) have widespread applications in optical coatings, plasmonic sensors, and nanocomposites. Current bottom-up approaches that use homogeneous NP templates, such as silane self-assembled monolayers or homopolymers, are typically plagued by NP aggregation, whereas patterned block copolymer (BCP) films require specific compositions for specific NP distributions. Here, we show, using polystyrene- b-poly(4-vinylpyridine) (PS- b-P4VP) and gold NPs (AuNPs) of various sizes, that a nanothin PS- b-P4VP brushlike coating (comprised of a P4VP wetting layer and a PS overlayer), which is adsorbed onto flat substrates during their immersion in very dilute PS- b-P4VP tetrahydrofuran solutions, provides an excellent template for obtaining dense and well-dispersed AuNPs with little aggregation. These non-close-packed arrays have similar characteristics regardless of immersion time in solution (about 10-120 s studied), solution concentration below a critical value (0.1 and 0.05 mg/mL studied), and AuNP diameter (10-90 nm studied). Very dilute BCP solutions are necessary to avoid deposition, during substrate withdrawal, of additional material onto the adsorbed BCP layer, which typically leads to patterned surfaces. The PS brush coverage depends on immersion time (adsorption kinetics), but full coverage does not inhibit AuNP adsorption, which is attributed to PS molecular rearrangement during exposure to the aqueous AuNP colloidal solution. The simplicity, versatility and robustness of the method will enable applications in materials science requiring dense, unaggregated NP arrays.

A Centimeter-Scale Inorganic Nanoparticle Superlattice Monolayer with Non-Close-Packing and its High Performance in Memory Devices

Due to the near-field coupling effect, non-close-packed nanoparticle (NP) assemblies with tunable interparticle distance (d) attract great attention and show huge potential applications in various functional devices, e.g., organic nano-floating-gate memory (NFGM) devices. Unfortunately, the fabrication of device-scale non-close-packed 2D NPs material still remains a challenge, limiting its practical applications. Here, a facile yet robust "rapid liquid-liquid interface assembly" strategy is reported to generate a non-close-packed AuNP superlattice monolayer (SM) on a centimeter scale for high-performance pentacene-based NFGM. The d and hence the surface plasmon resonance spectra of SM can be tailored by adjusting the molecular weight of tethered polymers. Precise control over the d value allows the successful fabrication of photosensitive NFGM devices with highly tunable performances from short-term memory to nonvolatile data storage. The best performing nonvolatile memory device shows remarkable 8-level (3-bit) storage and a memory ratio over 10⁵ even after 10 years compared with traditional devices with a AuNP amorphous monolayer. This work provides a new opportunity to obtain large area 2D NPs materials with non-close-packed structure, which is significantly meaningful to microelectronic, photovoltaics devices, and biochemical sensors.

Conformal Nanocoatings with Uniform and Controllable Thickness on Microstructured Surfaces: A General Assembly Route

Assembling nanoparticles (NPs) on various surfaces are intensively investigated for the construction of functional nanocoatings; however, it is still a challenge to fabricate conformal nanocoatings uniformly on surfaces having micro- or nanostructures. Herein, it is demonstrated that the negatively charged SiO₂ NPs and the positively charged silicon coupling agent can be assembled layer-by-layer on the microstructures based on the combination of electrostatic interaction and condensation reaction. Conformal nanocoatings with controllable thickness are formed on the microstructured surfaces with different compositions and morphologies. The formation mechanism is confirmed by using quartz crystal microbalance with dissipation (QCM-D) to study the assembly process in real time. The universality of this method is illustrated by using other reactive building blocks with opposite charge to build up the conformal nanocoatings. Application in the preparation of antireflective nanocoatings on nonplanar optical materials is demonstrated. This simple, versatile, and scalable strategy for the preparation of conformal nanocoatings is promising for practical applications.

Structural Characterization of Ordered, Non-Close-Packed Functionalized Silica Nanoparticles on Gold Surfaces

Nanostructured surfaces play an important role in modern science and technology. In particular, ordered arrangements of nonclose-packed nanoparticles created by self-assembly offer a versatile route to prepare systems, which can be used in various applications such as sensing, plasmonic devices or antireflection coatings. Self-assembly based systems are particularly appealing as preparation is rather simple. The ability of nanoparticle systems to form nonclosed packed monolayers by self-assembly depends on the balance of various energetic contributions in particular the adsorption energy, the lateral barrier for diffusion and the repulsion between particles. Even for simple model systems such as the monodispersed silica particles adsorbed on a bare gold surface investigated here, none of these quantities is easy to determine experimentally. To this end, we will report on a detailed characterization of the adsorption in particular with respect to the structural properties of the above-mentioned model system. Based on experimental results obtained by using quartz crystal microbalance with dissipation monitoring (QCM-D) as well as scanning electron microscopy (SEM) it is possible to determine the electrostatic pair potential from the lateral arrangement of the nano particles in the limit of low coverage.

Debye ring diffraction elucidation of 2D photonic crystal self-assembly and ordering at the air–water interface

Diffraction intensities and Debye ring widths depend on the colloidal particle ordering of the 2D photonic crystals.