Silver nanoparticles in plant health: Physiological response to phytotoxicity and oxidative stress

Silver nanoparticles (AgNPs) have gained significant attention in various fields due to their unique properties, but their release into the environment has raised concerns about their environmental and biological impacts. Silver nanoparticles can enter plants following their exposure to roots or via stomata following foliar exposure. Upon penetrating the plant cells, AgNPs interact with cellular components and alter physiological and biochemical processes. One of the key concerns associated with plant exposure to AgNPs is the potential of these materials to induce oxidative stress. Silver nanoparticles can also suppress plant growth and development by disrupting essential plant physiological processes, such as photosynthesis, nutrient uptake, water transport, and hormonal regulation. In crop plants, these disruptions may, in turn, affect the productivity and quality of the harvested components and therefore represent a potential threat to agricultural productivity and ecosystem stability. Understanding the phytotoxic effects of AgNPs is crucial for assessing their environmental implications and guiding the development of safe nanomaterials. By delving into the phytotoxic effects of AgNPs, this review contributes to the existing knowledge regarding their environmental risks and promotes the advancement of sustainable nanotechnological practices.

Stimuli-responsive nanoparticle self-assembly at complex fluid interfaces: a new insight into dynamic surface chemistry

The self-assembly of core/shell nanoparticles (NPs) at fluid interfaces is a rapidly evolving area with tremendous potential in various fields, including biomedicine, display devices, catalysts, and sensors. This review provides an in-depth exploration of the current state-of-the-art in the programmed design of stimuli-responsive NP assemblies, with a specific focus on inorganic core/organic shell NPs below 100 nm for their responsive adsorption properties at fluid and polymer interfaces. The interface properties, such as ligands, charge, and surface chemistry, play a significant role in dictating the forces and energies governing both NP-NP and NP-hosting matrix interactions. We highlight the fundamental principles governing the reversible surface chemistry of NPs and present detailed experimental examples in the following three key aspects of stimuli-responsive NP assembly: (i) stimuli-driven assembly of NPs at the air/liquid interface, (ii) reversible NP assembly at the liquid/liquid interface, including films and Pickering emulsions, and (iii) hybrid NP assemblies at the polymer/polymer and polymer/water interfaces that exhibit stimuli-responsive behaviors. Finally, we address current challenges in existing approaches and offer a new perspective on the advances in this field.

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.

Interfacial Colloidal Self-Assembly for Functional Materials

ConspectusSelf-assembly bridges nanoscale and microscale colloidal particles into macroscale functional materials. In particular, self-assembly processes occurring at the liquid/liquid or solid/liquid/air interfaces hold great promise in constructing large-scale two- or three-dimensional (2D or 3D) architectures. Interaction of colloidal particles in the assemblies leads to emergent collective properties not found in individual building blocks, offering a much larger parameter space to tune the material properties. Interfacial self-assembly methods are rapid, cost-effective, scalable, and compatible with existing fabrication technologies, thus promoting widespread interest in a broad range of research fields.Surface chemistry of nanoparticles plays a predominant role in driving the self-assembly of nanoparticles at water/oil interfaces. Amphiphilic nanoparticles coated with mixed polymer brushes or mussel-inspired polydopamine were demonstrated to self-assemble into closely packed thin films, enabling diverse applications from electrochemical sensors and catalysis to surface-enhanced optical properties. Interfacial assemblies of amphiphilic gold nanoparticles were integrated with graphene paper to obtain flexible electrodes in a modular approach. The robust, biocompatible electrodes with exceptional electrocatalytic activities showed excellent sensitivity and reproducibility in biosensing. Recyclable catalysts were prepared by transferring monolayer assemblies of polydopamine-coated nanocatalysts to both hydrophilic and hydrophobic substrates. The immobilized catalysts were easily recovered and recycled without loss of catalytic activity. Plasmonic nanoparticles were self-assembled into a plasmonic substrate for surface-enhanced Raman scattering, metal-enhanced fluorescence, and modulated fluorescence resonance energy transfer (FRET). Strong Raman enhancement was accomplished by rationally directing the Raman probes to the electromagnetic hotspots. Optimal enhancement of fluorescence and FRET was realized by precisely controlling the spacing between the metal surface and the fluorophores and tuning the surface plasmon resonance wavelength of the self-assembled substrate to match the optical properties of the fluorescent dye.At liquid/solid interfaces, infiltration-assisted (IFAST) colloidal self-assembly introduces liquid infiltration in the substrate as a new factor to control the degree of order of the colloidal assemblies. The strong infiltration flow leads to the formation of amorphous colloidal arrays that display noniridescent structural colors. This method is compatible with a broad range of colloidal particle inks, and any solid substrate that is permeable to dispersing liquids but particle-excluding is suitable for IFAST colloidal assembly. Therefore, the IFAST technology offers rapid, scalable fabrication of structural color patterns of diverse colloidal particles with full-spectrum coverage and unprecedented flexibility. Metal-organic framework particles with either spherical or polyhedral morphology were used as ink particles in the Mayer rod coating on wettability patterned photopapers, leading to amorphous photonic structures with vapor-responsive colors. Anticounterfeiting labels have also been developed based on the complex optical features encoded in the photonic structures.Interfacial colloidal self-assembly at the water/oil interface and IFAST assembly at the solid/liquid/air interface have proven to be versatile fabrication platforms to produce functional materials with well-defined properties for diverse applications. These platform technologies are promising in the manufacturing of value-added functional materials.

Optimized Design and Preparation of Ag Nanoparticle Multilayer SERS Substrates with Excellent Sensing Performance

Nanoparticle multilayer substrates usually exhibit excellent SERS activity due to multi-dimensional plasmon coupling. However, simply increasing the layers will lead to several problems, such as complex manufacturing procedures, reduced uniformity and poor reproducibility. In this paper, the local electric field (LEF) characteristics of a Ag nanoparticle (AgNP) multilayer were systematically studied through finite element simulations. We found that, on the glass support, the LEF intensity improved with the increase in the layers of AgNPs. However, the maximum LEF could be obtained with only two layers of AgNPs on the Au film support, and it was much stronger than the optimal value of the former. To verify the simulation results, we have successfully prepared one to four layers of AgNPs on both supports with a liquid-liquid interface self-assembly method, and carried out a series of SERS measurements. The experimental results were in good agreement with the simulations. Finally, the optimized SERS substrate, the 2-AgNP@Au film, showed an ultra-high SERS sensitivity, along with an excellent signal uniformity, which had a detection ability of 1 × 10-15 M for the Rhodamine 6G (R6G) and a relative standard deviation (RSD) of 11% for the signal intensity. Our study provides important theoretical guidance and a technical basis for the optimized design and application of high-performance SERS substrates.

Self-assembled nano-Ag/Au@Au film composite SERS substrates show high uniformity and high enhancement factor for creatinine detection

Serum creatinine is a key biomarker for the diagnosis and monitoring of kidney disease. Rapid and sensitive creatinine detection is thus important. Here, we propose a high-performance nano-Ag/Au@Au film composite SERS substrate for the rapid detection of creatinine in human serum. Au nanoparticles (AuNPs) and Ag nanoparticles (AgNPs) with uniform particle size were synthesized by a chemical reduction method, and the nano-Ag/Au@Au film composite SERS substrate was successfully prepared via a consecutive layer-on-layer deposition using an optimized liquid-liquid interface self-assembly method. The finite element simulation analysis showed that due to the multi-dimensional plasmonic coupling effect formed between the AuNPs, AgNPs, and the Au film, the intensity of the local electromagnetic field was greatly improved, and a very high enhancement factor (EF) was obtained. Experimental results showed that the limit of detection (LOD) of this composite SERS substrate for rhodamine 6G (R6G) molecules was as low as 1 × 10^-13M, and the Raman EF was 15.7 and 2.9 times that of the AuNP and AgNP monolayer substrates respectively. The results of different batch tests and SERS mapping showed that the relative standard deviations of the Raman intensity of R6G at 612 cm^-1were 12.5% and 11.7%, respectively. Finally, we used the SERS substrate for the label-free detection of human serum creatinine. The results showed that the LOD of this SERS substrate for serum creatinine was 5 × 10^-6M with a linear correlation coefficient of 0.96. In conclusion, the SERS substrate has high sensitivity, good uniformity, simple preparation, and has important developmental potential for the rapid detection and application of disease biomarkers.

Tension gradient-driven oil/water interface rapid particle self-assembly and its application in microdroplet motion control

HYPOTHESIS

A binary mixture was used during injection with one water-miscible component and the other water-immiscible, which can help particles to migrate toward and then self-assemble at the interface.

EXPERIMENTS

The ethanol-tetrachloromethane binary mixture was used to verify the self-assembly method, with the diameter of droplets being about 1 mm. As the ethanol diffused into the colloidal solution, the colloidal particles efficiently moved towards and self-assembled on the oil/water interface, while a colloidal particle film with high-coverage was able to rapidly form on the droplet surface even in an ultra-low concentration colloidal solution. The effects of ethanol concentration and particle concentration on self-assembly were investigated.

FINDINGS

The driving force for self-assembly originated from the tension gradient generated by ethanol's concentration gradient at the particle/liquid interfaces, where the concentrations of ethanol and the colloidal solution had significant effects on self-assembly. The simulation and calculations results aligned well with experiments, providing the theoretical basis for this self-assembly method. Further, as-prepared magnetic particle-coated droplets transformed into a non-wetting soft solid, which had long lifetimes and could be precisely moved, coalesced, and transferred in various two-dimensional and three-dimensional liquid environments. Thus, wider applications are facilitated, such as droplet transfer, microreactor and other potential fields.

A cosolvent surfactant mechanism affects polymer collapse in miscible good solvents

The coil-globule transition of aqueous polymers is of profound significance in understanding the structure and function of responsive soft matter. In particular, the remarkable effect of amphiphilic cosolvents (e.g., alcohols) that leads to both swelling and collapse of stimuli-responsive polymers has been hotly debated in the literature, often with contradictory mechanisms proposed. Using molecular dynamics simulations, we herein demonstrate that alcohols reduce the free energy cost of creating a repulsive polymer-solvent interface via a surfactant-like mechanism which surprisingly drives polymer collapse at low alcohol concentrations. This hitherto neglected role of interfacial solvation thermodynamics is common to all coil-globule transitions, and rationalizes the experimentally observed effects of higher alcohols and polymer molecular weight on the coil-to-globule transition of thermoresponsive polymers. Polymer-(co)solvent attractive interactions reinforce or compensate this mechanism and it is this interplay which drives polymer swelling or collapse.

Macroscopic two-dimensional monolayer films of gold nanoparticles: fabrication strategies, surface engineering and functional applications

In the last few decades, two-dimensional monolayer films of gold nanoparticles (2D MFGS) have attracted increasing attention in various fields, due to their superior attributes of macroscopic size and accessible fabrication, controllable electromagnetic enhancement, distinctive optical harvesting and electron transport capabilities. This review will focus on the recent progress of 2D monolayer films of gold nanoparticles in construction approaches, surface engineering strategies and functional applications in the optical and electric fields. The research challenges and prospective directions of 2D MFGS are also discussed. This review would promote a better understanding of 2D MFGS and establish a necessary bridge among the multidisciplinary research fields.

Buckling Effect of Sole Zeolitic Imidazolate Framework-8 Nanoparticles Adsorbed at the Water/Oil Interface

The buckling phenomenon of sole zeolitic imidazolate framework-8 (ZIF-8) particles adsorbed at the water/oil interface was systematically studied. The droplet of ZIF-8 water dispersion was pended in oil for a certain time period and manually extracted to decrease the volume. With the reduction of interfacial area, the ZIF-8 particles were jammed together to form a wrinkling solid film at the water/oil interface, which could withstand the extraction of the droplet and be regenerated. The size and concentration of the particles affected the assembly kinetics. The rapidest assembly was observed for the medium-sized ZIF-8 particles (m-ZIF-8) among the three sizes tested (1.81 μm, 258 nm, and 51 nm). The droplet of 0.91 wt % m-ZIF-8 reached a nearly full surface coverage in 13 min, faster than those with the lower concentration of 0.46 or 0.28 wt %. The pH of the solution, ranging between 6 and 10.7, affected both the assembly kinetics and film stability. Cryo-scanning electron microscopy images of frozen m-ZIF-8-stabilized Picking emulsions showed a monolayer of ZIF-8 wetted by both oil and water phases. The observed buckling effect could be attributed to the stable adsorption of ZIF-8 at the water/oil interface and the interparticle interactions, related to the unique surface chemistry and polyhedral shape of the ZIF-8 crystals. This work provided some understanding on the interfacial property of ZIF-8 and the mechanism of sole ZIF-8-stabilized Pickering emulsions.

An ultrathin conformable vibration-responsive electronic skin for quantitative vocal recognition

Flexible and skin-attachable vibration sensors have been studied for use as wearable voice-recognition electronics. However, the development of vibration sensors to recognize the human voice accurately with a flat frequency response, a high sensitivity, and a flexible/conformable form factor has proved a major challenge. Here, we present an ultrathin, conformable, and vibration-responsive electronic skin that detects skin acceleration, which is highly and linearly correlated with voice pressure. This device consists of a crosslinked ultrathin polymer film and a hole-patterned diaphragm structure, and senses voices quantitatively with an outstanding sensitivity of 5.5 V Pa⁻¹ over the voice frequency range. Moreover, this ultrathin device (<5 μm) exhibits superior skin conformity, which enables exact voice recognition because it eliminates vibrational distortion on rough and curved skin surfaces. Our device is suitable for several promising voice-recognition applications, such as security authentication, remote control systems and vocal healthcare.

Though skin-attachable vibration sensors are promising for voice recognition applications, current technologies do not meet key performance requirements. Here, the authors report a flexible skin-attachable sensor with high sensitivity and flat frequency response over the vocal frequency range.

Rapid water/oil interfacial self-assembled Au monolayer nanofilm by simple vortex mixing for surface-enhanced Raman scattering

In this work, a facile surface-enhanced Raman scattering (SERS) substrate formed on a water/oil interface was prepared by adding n-hexane to Au colloids through simple vortex mixing without the involvement of any organic additives.

Automatic concentration and reformulation of PET tracers via microfluidic membrane distillation

Short-lived radiolabeled tracers for positron emission tomography (PET) must be rapidly synthesized, purified, and formulated into injectable solution just prior to imaging. Current radiosynthesizers are generally designed for clinical use, and the HPLC purification and SPE formulation processes often result in a final volume that is too large for preclinical and emerging in vitro applications. Conventional technologies and techniques for reducing this volume tend to be slow, resulting in radioactive decay of the product, and often require manual handling of the radioactive materials. We present a fully-automated microfluidic system based on sweeping gas membrane distillation to rapidly perform the concentration and formulation process. After detailed characterization of the system, we demonstrate fast and efficient concentration and formulation of several PET tracers, evaluate residual solvent content to establish the safety of the formulated tracers for injection, and show that the formulated tracer can be used for in vivo imaging.

Reversible Partitioning of Nanoparticles at an Oil-Water Interface

The reversible adsorption of nanoparticles (NPs) to oil-water interfaces has been observed experimentally, however, models capable of interpreting and predicting the equilibrium partitioning of particles between bulk media and fluid interfaces are still lacking. Here we characterize the adsorption of 5 nm gold NPs functionalized with ion-pair ligands at the toluene-water interface. Partitioning of the NPs between the bulk aqueous phase and the interface is measured via absorbance spectroscopy for two different aqueous-phase pH values (11.0 and 11.7) and over several orders of magnitude of aqueous phase NP concentration. The surface pressure of the interfacial film in equilibrium with the bulk aqueous phase is measured using the pendant drop method. We determine the range of surface pressure where the adsorption is reversible as well as conditions under which the adsorbed NPs are irreversibly adsorbed at the oil-water interface. We analyze together the adsorption and surface pressure isotherms to obtain the two-dimensional equations of state (EOS) for the NPs in equilibrium with the bulk aqueous phase. The experimental data are then compared to the Frumkin models. We find that the adsorption isotherm and the equation of state show good agreement at low coverage with the Frumkin equations; however, both curves cannot be described with the same parameters. We also show that the low-coverage portion of the EOS can also be described by a wetting model. We hypothesize that deviations from models at higher coverage are likely due to nonequilibrium effects and possible coadsorption.

A flexible transparent Ag-NC@PE film as a cut-and-paste SERS substrate for rapid in situ detection of organic pollutants

This report presents a simple and inexpensive fabrication approach to a flexible transparent composite film as a "cut-and-paste" surface-enhanced Raman scattering (SERS) substrate for in situ detection of organic pollutants. First, a self-assembled monolayer of Ag-nanocubes (Ag-NCs) is obtained at the air/water interface. Then, the Ag-NC monolayer is retrieved onto a flexible transparent polyethylene (PE) film to achieve an Ag-NC@PE composite film as a flexible SERS substrate. As the Ag-NCs in the monolayer are closely and uniformly packed on the PE film, the Ag-NC@PE composite film shows high SERS-activity with good signal homogeneity and reproducibility. Furthermore, the flexible transparent Ag-NC@PE composite film is "cut into" small pieces and directly "pasted" onto contaminated fruits for in situ SERS detection, as a result 10 nM thiram, 1 μM 4-polychlorinated biphenyl and 10 nM methyl parathion contaminants on oranges are detected, respectively. Therefore the Ag-NC@PE composite film is an inexpensive and effective SERS substrate for rapid in situ detection of organic pollutants in aqueous solutions, on fruits and other solid objects.

Controlling nanoflake motion using stiffness gradients on hexagonal boron nitride

Molecular dynamics simulations are performed to investigate the possibility of generating motion from stiffness gradients with no external energy source.

Effects of Ionic Strength on the Colloidal Stability and Interfacial Assembly of Hydrophobic Ethyl Cellulose Nanoparticles

Nanoparticle attachment at a fluid interface is a process that often takes place concurrently with nanoparticle aggregation in the bulk of the suspension. Here we investigate systematically the coupling of these processes with reference to the adsorption of aqueous suspensions of ethyl cellulose (EC) nanoparticles at the air-water interface. The suspension stability is optimal at neutral pH and in the absence of salt, conditions under which the electrostatic repulsion among EC nanoparticles is maximized. Nonetheless, hydrophobic attraction dominates particle-interface interactions, resulting in the irreversible adsorption of EC nanoparticles at the air-water interface. The addition of salt weakens the particle-particle and particle-interface repulsive electrostatic forces. This leads to destabilization of the suspension at ionic strengths of 0.05 M or greater but does not affect nanoparticle adsorption. The energy of adsorption, the surface tension and interface coverage at steady state, and the particle contact angle at the interface all remain unchanged by the addition of salt. These findings contribute to the fundamental understanding of colloidal systems and inform the utilization of EC nanocolloids, in particular for the stabilization of foams and emulsions.

Modulation of surface bio-functionality by using gold nanostructures on protein repellent surfaces

A simple and straightforward nanofabrication method for the creation of gold nanoparticles patterns on a biologically inert plasma-deposited poly(ethyleneoxide) film for sensing and cell culture applications.

Vertical segregation in the self-assembly of nanoparticles at the liquid/air interface

Vertical segregation was induced by the size-dependent charge neutralization during the one-step interfacial self-assembly of colloidal gold nanoparticles with bimodal size distribution. This self-assembly approach also can assemble particles with tunable compositions into layered films.

Band gap tunable Zn2SnO4 nanocubes through thermal effect and their outstanding ultraviolet light photoresponse

This work presents a method for synthesis of high-yield, uniform and band gap tunable Zn₂SnO₄ nanocubes. These nanocubes can be further self-assembled into a series of novel nanofilms with tunable optical band gaps from 3.54 to 3.18 eV by simply increasing the heat treatment temperature. The Zn₂SnO₄ nanocube-nanofilm based device has been successfully fabricated and presents obviously higher photocurrent, larger photocurrent to dark current ratio than the previously reported individual nanostructure-based UV-light photodetectors, and could be used in high performance photodetectors, solar cells, and electrode materials for Li-ion battery.

Thermal Gradients on Graphene to Drive Nanoflake Motion

Thermophoresis has been emerging as a novel technique for manipulating nanoscale particles. Materials with good thermal conductivity and low surface friction, such as graphene, are best suited to serve as a platform for solid-solid transportations or manipulations. Here we employ nonequilibrium molecular dynamics simulations to explore the feasibility of utilizing a thermal gradient on a large graphene substrate to control the motion of a small graphene nanoflake on it. Attempts to systematically investigate the mechanism of graphene-graphene transportation have centered on the fundamental driving mechanism of the motion and the quantitative effect of significant parameters such as temperature gradient and geometry of graphene on the motion of the nanoflake. Simulation results have demonstrated that temperature gradient plays the pivotal role in the evolution of the motion of the nanoflake on the graphene surface. Also, the geometry of nanoflakes has presented an intriguing signature on the motion of the nanoflake, which shows the nanoflakes with a circular shape move slower but rotate faster than other shapes with the identical area. It reveals that edge effects can stabilize the angular motion of thermophoretically driven particles. An interesting relation between the effective initial driving force and temperature gradient has been quantitatively captured by employing the steered molecular dynamics. These findings will provide fundamental insights into the motion of nanodevices on a solid surface due to thermophoresis, and will offer the novel view for manipulating nanoscale particles on a solid surface in techniques such as cell separation, water purification, and chemical extraction.

Irreversible adsorption-driven assembly of nanoparticles at fluid interfaces revealed by a dynamic surface tension probe

Adsorption-driven self-assembly of nanoparticles at fluid interfaces is a promising bottom-up approach for the preparation of advanced functional materials and devices. Full realization of its potential requires quantitative understanding of the parameters controlling the self-assembly, the structure of nanoparticles at the interface, the barrier properties of the assembly, and the rate of particle attachment. We argue that models of dynamic surface or interfacial tension (DST) appropriate for molecular species break down when the adsorption energy greatly exceeds the mean energy of thermal fluctuations and validate alternative models extending the application of generalized random sequential adsorption theory to nanoparticle adsorption at fluid interfaces. Using a model colloidal system of hydrophobic, charge-stabilized ethyl cellulose nanoparticles at neutral pH, we demonstrate the potential of DST measurements to reveal information on the energy of adsorption, the adsorption rate constant, and the energy of particle-interface interaction at different degrees of nanoparticle coverage of the interface. These findings have significant implications for the quantitative description of nanoparticle adsorption at fluid interfaces.

Wetting behavior of spherical nanoparticles at a vapor-liquid interface: a density functional theory study

The wetting behavior of spherical nanoparticles at a vapor-liquid interface is investigated by using density functional theory, and the line tension calculation method is modified by analyzing the total energy of the vapor-liquid-particle equilibrium. Compared with the direct measurement data from simulation, the results reveal that the thermodynamically consistent Young's equation for planar interfaces is still applicable for high curvature surfaces in predicting a wide range of contact angles. The effect of the line tension on the contact angle is further explored, showing that the contact angles given by the original and modified Young's equations are nearly the same within the region of 60° < θ < 120°. Whereas the effect is considerable when the contact angle deviates from the region. The wetting property of nanoparticles in terms of the fluid-particle interaction strength, particle size, and temperature is also discussed. It is found that, for a certain particle, a moderate fluid-particle interaction strength would keep the particle stable at the interface in a wide temperature range.