Self-assembly of highly ordered micro- and nanoparticle deposits

The role of superlattice phases and interparticle distance in the magnetic behaviour of SPION thin films

Superparamagnetic iron oxide nanoparticles (SPIONs) with tailored surface modifications were employed to fabricate ordered thin films through a drop-casting technique. By systematically varying the ligand chain length using stearic acid, decanoic acid, and hexanoic acid, we precisely controlled the interparticle distances within the films. Comprehensive investigations utilizing superconducting quantum interference device (SQUID) magnetometry elucidated the films' superparamagnetic behaviour at room temperature, as well as notable exchange interactions at lower temperatures. Notably, these exchange characteristics exhibit a correlation with the blocking temperatures of the thin films. We postulate that these characteristics can be explained by different superlattice phases formed in the thin films, as indicated in previous studies, highlighting the profound influence of self-assembly and particle packing on the magnetic properties. To validate our hypothesis regarding the internal structure, we conducted grazing-incidence small-angle X-ray scattering (GISAXS) and scanning transmission electron microscopy (STEM) measurements, enabling us to assess the quality of internal ordering without compromising the integrity of the films. With this study we demonstrated how the use of simple building blocks, guided by the intrinsic driving force of self-assembly, can lead to remarkable magnetic properties in the resulting films.

Cost-Efficient Deterministic Engineering of Single Photon Emitters in Two-Dimensional Materials

Two-dimensional materials have recently emerged as promising candidates for quantum light emission. Their tunable bandgaps, layer-dependent excitonic properties, and strong confinement of charge carriers provide a versatile platform for manipulating and controlling quantum states. Several approaches─such as strain engineering, defect engineering and surface functionalization─have been explored to induce single-photon emitters in these materials. In this work, we present a practical and cost-efficient methodology for deterministic strain engineering of single-photon emitters within thin flakes of GaSe. Our approach utilizes optically active microparticles with a distinctive bipyramidal shape, whose emission does not interfere optically with that of GaSe. The results show strong agreement with previous studies on strain-induced single-photon sources in multilayer GaSe, demonstrating that the proposed technique is a promising platform for generating nonclassical light emission in layered materials. Compared to other local strain engineering techniques for single-photon sources in two-dimensional materials, our method offers greater accessibility and lower cost, making it feasible for implementation in most laboratories performing the experimental research in the field. This increased accessibility can help advance the understanding of two-dimensional semiconductor systems and their potential applications in nanophotonics and quantum light technologies.

Orientation of Nonspherical Nanoparticles in Ordered Block Copolymer for Functional Materials

In the fields of controllable catalysis, electromagnetic field manipulation, and nanoscience, mediated self-assembly has become a key method for controlling the orientation of nonspherical nanoparticles. The ordered structures formed by block copolymer self-assembly can provide an orientation matrix for nonspherical nanoparticles. Based on self-consistent field theory, this study investigates the orientation effects of monaxially symmetric cylindrical nanoparticles in the lamellar phases formed by block copolymers. Using cylindrical and pore-containing ring nanoparticles as models for nonspherical particles, we successfully describe the particles' anisotropy and nonconvex surface properties. Numerical results show that the orientation effect of the lamellar ordered structure exhibits a nontrivial dependence on the geometric and topological properties of nonspherical particles. In addition to interfacial tension effects, the orientation mechanism of small-sized nanoparticles mainly arises from the stretching effect of the polymer, manifested in two main effects: (1) the particle deforms the polymer chain, reducing its conformational entropy, thus tending to align in a specific orientation; (2) the orientation field at the polymer chain ends is discontinuous, and the nanoparticles can embed and adopt a specific orientation. For nonconvex nanoparticles, the geometric size of the pore structure adjusts the polymer's free volume, influencing the orientation effect. This study not only deepens the understanding of the orientation mechanism in block copolymer-mediated nanoparticle self-assembly, but also provides potential theoretical insights for the design and application of energy catalysis, biomedical materials, and functional nanostructures.

Highly efficient light-emitting diodes via self-assembled InP quantum dots

Heavy-metal-free quantum dot light-emitting diodes (QLEDs) face commercialization challenges due to low efficiency and poor stability. Spin-coated quantum dot films often create charge leakage areas, limiting device performance. Here, we develop an evaporative-driven self-assembly strategy that enables the preparation of uniform and dense InP-based quantum dot films. During device operation, these films effectively suppress performance degradation caused by charge leakage. QLEDs with uniform and dense InP-based quantum dot films achieve high external quantum efficiency (26.6%) and luminance (1.4 × 105 cd m-2), along with considerable stability (extrapolated T50 lifetime of 4026 hours at 1000 cd m-2). For a 2 × 3 cm2 InP-based device, the peak external quantum efficiency reaches 21.1%. By combining high-performance QLEDs with lithography technology, we fabricate miniaturized QLEDs with a minimum pixel size of 3 μm, achieving a resolution as high as 5080 pixels per inch and a peak external quantum efficiency of 22.6%.

Crack patterns induced by auto-stratification in drying sessile droplets of dairy proteins

Exploring the interfacial mechanisms involved in the evaporation of colloidal solutions is currently an open question with potential biomedical and industrial applications. In biological systems, unraveling evidence of self-arrangement is even more challenging due to the different size, structure and charge of their multi-components. In this work, we study the evaporation dynamics in mixes of dairy proteins, i.e., whey proteins and casein micelles. Combining the observation of crack formation in drying droplets and the evaluation of the elastic response in films during the evaporation, we highlight interfacial stratification resulting in the external accumulation of whey proteins. We also relate such preferential segregation to the mechanical properties of the dry matrices, showing that whey protein overrepresentation confers a brittle behavior to the mixes. These experimental results enable the development of a first predictive model of the elastic modulus for biocolloid binary mixes, taking into account their deformability and different ratio in the samples. Unraveling the link between colloid self-arrangement and the evolution of skin properties during the evaporation is a first step towards controlling structure and use properties of the dry particles composing dairy powders.

Self-assembled colloidal glass with 100% lanthanide nanocrystal loading for high-resolution X-ray imaging

Solution-processable colloidal scintillators are emerging as a promising alternative to traditional bulky scintillators, addressing critical limitations in X-ray imaging technologies. Existing X-ray screens fabricated with colloidal powders in polymer matrices suffer from low spatial resolution at elevated particle concentrations due to severe optical losses induced by nanoparticle aggregation, fundamentally constraining high-resolution imaging capabilities. To resolve these challenges, we developed a novel class of bright, transparent colloidal glasses achieving 100% particle loading through self-assembly of sub-5 nm lanthanide-doped CaMoO4 nanocrystals. By modulating solvent surface tension and volatility during the evaporation process, we successfully produced crack-free, densely packed transparent colloidal scintillator films. The self-assembled colloidal glass demonstrates an impressive 80% photoluminescence quantum yield and >80% transparency across the visible spectrum. Moreover, the developed screen exhibits remarkable sensitivity, detecting radiation doses as low as 186 nGy s-1 with an outstanding X-ray imaging resolution of 27.1-line pairs per millimeter, outperforming most conventional organic and inorganic scintillators. These findings illuminate a compelling pathway for utilizing nanomaterials to replace traditional single-crystal scintillators in high-resolution X-ray imaging, potentially revolutionizing medical imaging and radiation detection technologies.

Facile Fabrication of Robust Supraparticles for Spatially Orthogonal Cascade Catalysis

Macroscopically sized supraparticles (SPs) are emerging as cutting-edge materials for industrial applications because of their unique properties unachievable for their nano-building blocks, but their effective methods are lacking. Here, a conceptually novel strategy is developed to assemble binary or ternary nanoparticles (NPs) within compartments of droplets through electrostatic interactions, making it possible to facilely fabricate millimeter-sized multicomponent ionic supraparticles (ISPs). The assembled ISPs possess unexpectedly high mechanical strength (50 N per bead), being amenable to practical applications. The key factors governing the assembly behavior of nano-building blocks within water droplet compartments are identified through regulating the size and charge density of NPs or ionic strength, providing key insights into the multileveled assembly of NPs beyond the conventional assembly. The strategy is demonstrated to be versatile since a library of tailor-made ISPs containing multicomponent, diversely shaped, and differently sized NPs can be facilely fabricated. As proof of this concept, it is showcased that this method enables the preparation of spatially orthogonal cascade catalysts by co-assembling acidic, basic, and metal sites in single millimeter-scaled particles. The catalysts exhibit significantly enhanced catalytic efficiency in a one-pot cascade synthesis of α-alkylated nitriles and high operational stability (200 h) in industrially preferred fixed-bed reactors.

Uniform Deposition of Particles in Large Scale by Drying of Binary Droplets

The evaporation of liquid droplets often results in a ring-like deposition pattern of particles, presenting challenges for applications requiring highly uniform patterns. Despite extensive efforts to suppress the coffee ring effect, achieving a uniform particle distribution remains a great challenge due to the complex and non-equilibrium nature of the evaporation process. In this work, a one-step drying method is introduced and demonstrated for binary droplets (water and 2-methoxyethanol) that produces uniform deposition of nano- and micro-particles. By adjusting the initial water volume fraction, we effectively control the interplay between capillary and Marangoni flows, resulting in deposition patterns that vary from coffee ring to uniform and to volcano-like. Through both theoretical and experimental analyses, we determine the conditions necessary for achieving such high uniformity. This approach requires no special substrate treatment, particle modification, or controlled environments, and works for various particles, including silica and polystyrene. This method provides a robust solution for fabricating uniform patterns that are crucial for many practical applications, ranging from printing to microelectronics to bio-pharmacy.

Rapid Drying Principle for High-speed, Pinhole-Less, Uniform Wet Deposition Protocols of Water-Dispersed 2D Materials

Inexpensive, high-speed deposition techniques that ensure uniformity, scalability, wide applicability, and tunable thickness are crucial for the practical application of 2D materials. In this work, rapid drying is identified as a key mechanism for pioneering two high-speed wet deposition methods: hot dipping and air knife sweeping (AKS). Both techniques allow thickness control proportional to flake concentration, achieving tiled monolayers and pinhole-free coverage across the entire substrate, as long as evaporation outpaces flake diffusion. AKS prevents non-uniformity along substrate edges by eliminating contact line pinning. The achieved deposition speed of 0.21 m2 min-1 with AKS significantly surpasses traditional methods, enabling the equipment for large substrates > 1 m2. Combined with the ultralow debonding force for mechanically susceptible flexible display production and short-circuit-proof nanometer-thin capacitors with capacitance comparable to commercial multilayer ceramic capacitors (MLCCs), these new protocols showcase simple and swift solutions for manufacturing 2D materials-based nanodevices.

Supra-particle formation by evaporation of aerosol droplets containing binary mixtures of colloidal particles: Controlling the final morphology

HYPOTHESIS

Supra-particle formation by evaporation of an aqueous aerosol droplet containing nano-colloidal particles is challenging to investigate but has significant applications. We hypothesise that the Peclet number, Pe, which compares the effectiveness of evaporation-induced advection to that of colloidal diffusion, is critical in determining supra-particle morphology and can be used to predict the dried morphology for droplet containing polydisperse nanoparticles.

EXPERIMENTS

Sterically-stabilized diblock copolymer nanoparticles were prepared via polymerization-induced self-assembly (PISA). The systematic study was performed for evaporation rates by levitating an aqueous aerosol droplet and collecting dry supra-particles using electrodynamic balance (EDB) and falling droplet column (FDC), respectively for single-size particles and binary mixtures particles. The supra-particle morphology was characterized using scanning electron microscopy (SEM).

FINDINGS

We validate the hypothesis of a higher Pe increases the degree of buckling for both unimodal and bimodal nanoparticle size distributions by employing a higher evaporation rate (K) to increase Pe. However, if Pe is increased by lowering the mean diffusion coefficient (Davg) at a fixed K, the degree of buckling is reduced. For the binary mixture of nanoparticles of differing size, this can be achieved by reducing the concentration of smaller nanoparticles relative to that of larger nanoparticles. Hence consideration of Pe alone is insufficient to reliably predict the final supra-particle morphology.

Crack control in dried ferro-colloidal droplets: effect of particle aspect-ratio and magnetic field orientations

Crack formation in dried colloidal films is a common phenomenon encountered in diverse fields, from coatings and materials science to biological and environmental applications. Understanding the mechanisms behind crack patterns and their dependency on external factors is crucial for tailoring deposit structures. In this study, we investigate the impact of an externally directed magnetic field on the crack morphology and self-assembly in dried deposits composed of anisotropically shaped ferro-colloidal particles of varying sizes. Employing a sessile drop configuration, distinct crack patterns are observed in ring-like deposits as the magnetic field is applied in parallel, perpendicular, and oblique orientations. Notably, crack propagation in the oblique field direction transitions from wavy to helical-shaped patterns depending on the size of the nanoparticles, in contrast to the patterns seen in parallel and perpendicular fields. Our findings demonstrate that ferro-colloids align with the magnetic moment along the tensile stress direction, particularly at the edges of the deposits where cracks propagate. The particle orientation and self-assembly in the deposits were controlled by the interaction of hydrodynamic and magnetic forces, with force calculations revealing that this interaction strongly depends on particle size and field angle. This interaction leads to crack alignment along the particle's long axes, emphasizing the influence of the magnetic field on the deposit's structural integrity. Additionally, ferro-colloid concentration significantly impacts crack density, with higher concentrations promoting the development of prominent cracks at the rim edges of the deposits. By leveraging the interplay between magnetic interactions and evaporation dynamics, we can develop novel strategies for manipulating nanoscale structures for advanced technology.

Droplet-Pen Writing of Ultra-Uniform Graphene Pattern for Multi-Spectral Applications

Artificial optical patterns bring wide benefits in applications like structural color display, photonic camouflage, and electromagnetic cloak. Their scalable coating on large-scale objects will greatly enrich the multimodal-interactive society. Here, a droplet-pen writing (DPW) method to directly write multi-spectral patterns of thin-film graphene is reported. By amphiphilicity regulations of 2D graphene nanosheets, ultra-uniform and ultrathin films can spontaneously form on droplet caps and pave to the substrate, thus inducing optical interference. This allows the on-surface patterning by pen writing of droplets. Specifically, drop-on-demand thin films are achieved with millimeter lateral size and uniformity up to 97% in subwavelength thickness (<100 nm), corresponding to an aspect ratio of over 30 000. The pixelated thin-film patterns of disks and lines in an 8-inch wafer scale are demonstrated, which enable low-emittance structural color paintings. Furthermore, the applications of these patterns for dual-band camouflage and infrared-to-visible encryption are investigated. This study highlights the potential of 2D material self-assembly in the large-scale preparation and multi-spectral application of thin film-based optical patterns.

Magnetic Nanoparticle Aggregation and Complete De-encapsulation of Such Aggregates from a Liquid Drop Interior

Magnetic nanoparticles (MNPs) have been extensively used for drug delivery, on-demand material deposition, etc. In this study, we demonstrate the capability to extract such MNPs on-demand from a magnetic nanoparticle-laden drop (MNLD) (i.e., a drop of a stable aqueous dispersion of MNPs), suspended inside a highly viscous polymer (poly(dimethylsiloxane) or PDMS) medium in the presence of an externally applied magnetic field. The phenomena involve the aggregation of the MNPs inside the drop and the consequent extraction of the MNP aggregate out of the drop, with the drop retaining its original shape after the MNP aggregate extraction. We define this latter phenomenon as de-encapsulation. This is the first study that, to the best of our knowledge, demonstrates such precise, controlled, and on-demand removal of the NPs from the interior of a drop (where the NPs, which were originally inside the drop, breach the drop interface and get completely separated from the drop as an aggregate) without any permanent deformation of the drop. We quantify the effect of the changes in the MNP concentration and the drop volume in determining the de-encapsulation distance, which refers to the distance between the drop and the location of the magnet at the time instant when the MNP aggregate leaves the drop. We further identify the volume of the aggregates extracted from the drop and the mechanisms causing such de-encapsulation. We propose a theory to describe the process, and our theoretical predictions capture the experimental trends well. In addition, we also demonstrate multiple, back-to-back MNP aggregate extractions from a single MNLD at different sites, indicating the possibility that the MNLD can be used as an on-demand carrier and depositor of materials. Overall, our results, in addition to demonstrating the first-of-its-kind de-encapsulation of NPs (in the form of an aggregate) from the drop interior, demonstrate a method to control the dynamics, extraction, and targeted deposition of the MNPs.

Evaporative self-assembly in colloidal droplets: Emergence of ordered structures from complex fluids

Colloidal droplet evaporation is an intriguing and intricate phenomenon that has captured the interest of scientists across diverse disciplines, including physical chemistry, fluid dynamics, and soft matter science, over the past two decades. Despite being a non-equilibrium system with inherent challenges posed by coffee ring formation and Marangoni effects, which hinder the precise control of deposition patterns, evaporative self-assembly presents a convenient and cost-effective approach for generating arrays of well-ordered structures and functional patterns with wide-ranging applications in inkjet printing, photonic crystals, and biochemical assays. In the realm of printed electronics and photonics, effectively mitigating coffee rings while achieving uniformity and orderliness has emerged as a critical factor in realising the next generation of large-area, low-cost, flexible devices that are exceptionally sensitive and high-performance. This review highlights the evaporative self-assembly process in colloidal droplets with a focus on the intricate mechanical environment, self-assembly at diverse interfaces, and potential applications of these assembling ordered structures.

An innovative strategy for Gefitinib quantification in pharmaceutical and plasma samples using a graphene quantum dots-combined gold nanoparticles composite electrochemical sensor

An innovative methodology is proposed for quantifying Gefitinib (GFT) using an electrochemical sensor constructed from a composite of graphene quantum dots (GQDs) and gold nanoparticles (AuNPs). GQDs were synthesized from graphite, preserving graphene's large surface area and excellent electron transfer capabilities while enhancing dispersibility. The combination of GQDs with AuNPs resulted in an AuNPs@GQDs composite, which was used to construct the sensor. The synthesized nanomaterials were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the electrochemical performance of the sensor was evaluated via cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Under optimized conditions, the sensor displayed a linear calibration curve for GFT detection within the range 0.01 to 10.0 µM, with a limit of detection (LOD) of 0.005 µM (S/N = 3). The sensor demonstrated excellent anti-interference properties and stability in tests using pharmaceutical formulations and plasma samples. Compared to chromatographic methods, the sensor exhibited similar accuracy and recovery. Its easy fabrication and high sensitivity make it a promising tool for pharmaceutical analysis and clinical therapeutic drug monitoring.

Interfacial Self-Assembly of Oriented Semiconductor Monolayer for Chemiresistive Sensing

Semiconductor nanofilm fabrication with advanced technology is of great importance for next-generation electronics/optoelectronics. Fabrication of high-quality and perfectly oriented semiconductor thin films and integration into high-performance electronic devices with low cost and high efficiency are huge challenges. Here we exquisitely utilized the Marangoni effect to perfectly guide tin disulfide (SnS2) nanocoins into an ordered assembly in milliseconds, resulting in an uniaxial-oriented monolayer semiconductor film. Further exploration revealed that the formed "crumple zone" at the interface caused by the Marangoni force endows the nanofilm with a rapid healable capability, which can be easily transferred to arbitrary substrates. As a proof of concept, the nanocoin-monolayer was transferred onto a micro-interdigitated electrode substrate to form a high-performance chemiresistive sensor that can effectively monitor the trace amounts of toxic gases. In addition, the assembled monolayer nanofilms can be conformally printed on freeform surfaces: both flat and nonflat substrates. This efficient and low-cost Marangoni force-assisted surface self-assembly (MFA-SSA) strategy is promising for advanced microelectronics and real industrial applications.

Open microfluidics: droplet microarrays as next generation multiwell plates for high throughput screening

Multiwell plates are prominent in the biological and chemical sciences; however, they face limitations in terms of throughput and deployment in emerging bioengineering fields. Droplet microarrays, as an open microfluidic technology, organise tiny droplets typically in the order of thousands, on an accessible plate. In this perspective, we summarise current approaches for generating droplets, fluid handling on them, and analysis within droplet microarrays. By enabling unique plate engineering opportunities, demonstrating the necessary experimental procedures required for manipulating and interacting with biological cells, and integrating with label-free analytical techniques, droplet microarrays can be deployed across a more extensive experimental domain than what is currently covered by multiwell plates. Droplet microarrays thus offer a solution to the bottlenecks associated with multiwell plates, particularly in the areas of biological cultivation and high-throughput compound screening.

High-throughput fabrication of monodisperse spherical supraparticles through a reliable thin oil film and rapid water diffusion

A supraparticle is a spherical superstructure composed of fine building blocks, typically synthesized through colloidal assembly from evaporating and contracting suspension droplets. Microfluidic emulsification is known to be effective in producing large amounts of water-in-oil droplets. However, the process of supraparticle self-assembly has been limited by the evaporation of the oil that supports it and the sluggish shrinkage of water droplets. These are caused by the high volatility and low diffusion rates of water in the bulk oil layer, making the process last hours or even days. To address these challenges, we introduce a new system in this paper: the supraparticle reliable fabrication (SURF) system. This microfluidic-based system can quickly and reliably assemble spherical supraparticles in 20 min. The SURF system combines a conventional flow focusing device with a thinly layered low-volatile/water-soluble oil, and an open-microfluidic droplet evaporator. This setup facilitates the creation of uniform supraparticles with various materials and diameters (coefficient of variation: <3.5%). As a proof-of-concept for potential biochemical applications, we demonstrate a sensitive chemical reaction on the fabricated supraparticles, emphasizing the effectiveness of the SURF system as an alternative to traditional supraparticle synthesis and particle-based applications.

Evaporation-induced fractal patterns: A bridge between uniform pattern and coffee ring

HYPOTHESIS

The rich variety of patterns induced by evaporating drops containing particles has significant guidance for coating processes, inkjet printing, and nanosemiconductors. However, most existing works construct a uniform pattern by suppressing the coffee ring effect, and establishing the connection between them is still an academic challenge.

EXPERIMENTS

We report uniform, polygonal, and coffee ring patterns obtained by adjusting the solute concentration that sets in when an ethanol drop with dissolved ibuprofen is deposited on a silicon wafer.

FINDINGS

Pattern formation involves rich hydrodynamic events: spreading, evaporative instability, dewetting, film formation, and particle deposition. Based on the distinct multiscale properties, this series of patterns is directly connected from the perspective of fractal geometry, which allows us to name them "fractal deposition patterns". A theoretical model considering film stability is established to explain the mechanism behind pattern formation, which is well verified by experiments. This work has presented a unique strategy that can directly connect uniform, polygonal, and coffee ring patterns under the same physics, hoping to provide instructive guidance for practical applications.

Ordered stripes to crack patterns in dried particulates of DNA-coated gold colloids via modulating nanoparticle-substrate interactions

The surface pattern in dried droplets of nanoparticle suspension possesses direct correlation with the evaporation profile, which apart from the bulk parameters, can also be altered by tuning the nanoscale interactions. Here, we show that, for sessile drops of DNA-coated gold nanoparticle (DNA-AuNP) solution, the alteration in evaporation pathway of TPCL (three-phase contact line) from stick-slip to mixed mode leads to a surface morphological transition from concentric rings with stripes to radial crack formation within the coffee ring deposit. A freshly cleaned silicon substrate offers hydrophilic/favorable substrate-nanoparticle interaction and produces multiple ordered stripes due to stick-slip motion of the TPCL. Using a SiO₂/Si substrate with ∼200 nm of oxide layer leads to an increase in the initial water contact angle θ_i-w by ∼40°, due to increased hydrophobicity of the substrate. Three distinct modes of evaporation are observed - constant contact radius (CCR), constant contact angle (CCA) and mixed mode, resulting in the formation of radial cracks on a thick coffee ring structure. The critical thickness (h_c), beyond which the cracks start to appear, was measured to be ∼600 nm and is in close agreement with the theoretical estimate of ∼510 nm. Through in situ contact angle and ex situ SEM measurements, we provide an understanding of the observed surface morphological transition in the dried particulate at various nanoparticle densities. Further analysis of the coffee ring width (d), linear crack density (σ) and crack spacing (λ) provides insight into the mechanism of crack formation for droplets dried on oxide-coated substrates.

Ligand-Ligand-Interaction-Dominated Self-Assembly of Gold Nanoparticles at the Oil/Water Interface: An Atomic-Scale Simulation

The self-assembly of nanoparticles (NPs) into ordered superlattices is a powerful strategy to fabricate functional nanomaterials. Subtle variations in the interactions between NPs will influence the self-assembled superlattices. Using all-atom molecular dynamics simulations, we explore the self-assembly of 16 gold NPs, 4 nm in diameter, capped with ligands at the oil-water interface, and quantify the interactions between NPs at the atomic scale. We demonstrate that the interaction between capping ligands rather than that between NPs is dominant during the assembly process. For dodecanethiol (DDT)-capped Au NPs, the assembled superlattice is highly ordered in a close-packed configuration at a slow evaporation rate, while it is disordered at a fast evaporation rate. When replacing the capping ligands with stronger polarization than DDT molecules, the NPs form a robust ordered configuration at different evaporation rates due to the stronger electrostatic attraction between capping ligands from different NPs. Moreover, Au-Ag binary clusters exhibit similar assembly behavior with Au NPs. Our work uncovers the nonequilibrium nature of NP assembly at the atomic scale and would be helpful in rationally controlling NPs superlattice by changing passivating ligands, solvent evaporation rate, or both.

Suppression of wetting transition on evaporative fakir droplets by using slippery superhydrophobic surfaces with low depinning force

This study experimentally investigated the evaporation and wetting transition behavior of fakir droplets on five different microstructured surfaces. Diamond-like carbon was introduced as the substrate, and the influence of varying the width, height, and pitch of the micropillars was assessed. The experimental results showed that the interfacial properties of the surfaces change the evaporation behavior and the starting point of the wetting transition. An important result of this study is the demonstration of a slippery superhydrophobic surface with low depinning force that suppresses the transition from the Cassie-Baxter state to the Wenzel state for microdroplets less than 0.37 mm in diameter, without employing large pillar height or multiscale roughness. By selecting an appropriate pillar pitch and employing tapered micropillars with small pillar widths, the solid-liquid contact at the three-phase contact line was reduced and low depinning forces were obtained. The underlying mechanism by which slippery superhydrophobic surfaces suppress wetting transitions is also discussed. The accuracy of the theoretical models for predicting the critical transition parameters was assessed, and a numerical model was developed in the surface evolver to compute the penetration of the droplet bottom meniscus within the micropillars.

Capillary-Assisted Molecular Pendulum Bioanalysis

The development of robust biosensing strategies that can be easily implemented in everyday life remains a challenge for the future of modern biosensor research. While several reagentless approaches have attempted to address this challenge, they often achieve user-friendliness through sacrificing sensitivity or universality. While acceptable for certain applications, these trade-offs hinder the widespread adoption of reagentless biosensing technologies. Here, we report a novel approach to reagentless biosensing that achieves high sensitivity, rapid detection, and universality using the SARS-CoV-2 virus as a model target. Universality is achieved by using nanoscale molecular pendulums, which enables reagentless electrochemical biosensing through a variable antibody recognition element. Enhanced sensitivity and rapid detection are accomplished by incorporating the coffee-ring phenomenon into the sensing scheme, allowing for target preconcentration on a ring-shaped electrode. Using this approach, we obtained limits of detection of 1 fg/mL and 20 copies/mL for the SARS-CoV-2 nucleoproteins and viral particles, respectively. In addition, clinical sample analysis showed excellent agreement with Ct values from PCR-positive SARS-CoV-2 patients.