Thermophoresis-Induced Polymer-Driven Destabilization of Gold Nanoparticles for Optically Directed Assembly at Interfaces.

The limitations of conventional template-based methods for the deposition of nanoparticle assemblies into defined patterns on solid substrates call for the development of techniques that do not require templates or lithographic masks. The use of optically-induced thermal gradients to drive the migration of colloids toward or away from a laser spot, known as opto-thermophoresis, has shown promise for the low-power trapping and optical manipulation of a variety of colloidal species. However, the printing of colloids using this technique has so far not been established. Herein, a method for the optically directed printing of noble metal nanoparticles, specifically gold nanospheres is reported. The thermophoresis of the polymer polyvinylpyrrolidone and gold nanospheres toward a laser spot led to the deposition of nanoparticle aggregates, capable of serving as surface-enhanced Raman scattering substrates. The influence of heating laser power and the concentrations of polymer, salt, and surfactant on the nanoparticle deposition rate and structure of the printed pattern are studied, showing that a variety of conditions can permit printing, suggesting facile generalization to different nanoparticle compositions, sizes, and shapes. These findings will greatly benefit future efforts for directed nanoparticle assembly, and drive applications in sensing, photothermal heating, and relevant applications in biomedicine and devices.

Bubble Printing of Layered Silicates: Surface Chemistry Effects and Picomolar Förster Resonance Energy Transfer Sensing.

The assembly of nanoparticles on surfaces in defined patterns has long been achieved via template-assisted methods that involve long deposition and drying steps and the need for molds or masks to obtain the desired patterns. Control over deposition of materials on surfaces via laser-directed microbubbles is a nascent technique that holds promise for rapid fabrication of devices down to the micrometer scale. However, the influence of surface chemistry on the resulting assembly using such approaches has so far not been studied. Herein, the printing of layered silicate nanoclays using a laser-directed microbubble was established. Significant differences in the macroscale structure of the printed patterns were observed for hydrophilic, pristine layered silicates compared to hydrophobic, modified layered silicates, which provided the first example of how the surface chemistry of such nanoscale objects results in changes in assembly with this approach. Furthermore, the ability of layered silicates to adsorb molecules at the interface was retained, which allowed the fabrication of proof-of-concept sensors based on Förster resonance energy transfer (FRET) from quantum dots embedded in the assemblies to bound dye molecules. The detection limit for Rhodamine 800 sensing via FRET was found to be on the order of 10-12 M, suggesting signal enhancement due to favorable interactions between the dye and nanoclay. This work sets the stage for future advances in the control of hierarchical assembly of nanoparticles by modification of surface chemistry while also demonstrating a quick and versatile approach to achieve ultrasensitive molecular sensors.

Investigating Solvent-Induced Aggregation in Edge-Functionalized Layered Silicates via All-Atom Molecular Dynamics Simulations.

Molecular dynamics simulations can provide the means to visualize and understand the role of intermolecular interactions in the mechanisms involved in molecular aggregation. Along these lines, simulations can allow the study of how surface chemical modifications can influence nanomaterial assembly at the molecular level. Layered silicate clays have been of significant interest for some time, particularly with regard to their use in organic/inorganic nanocomposites. However, despite numerous reports on the covalent linkage of organic moieties via silanol condensation, the theoretical understanding of these systems has heretofore been limited to noncovalent interactions, specifically ionic interactions at the charged basal surfaces. Herein, a model for edge-functionalized layered aluminosilicate clay, based on the siloxane linkage, is presented. In addition to reproducing experimentally observed degrees of molecular aggregation of clay-linked perylene diimide derivatives with different terminal functional groups as a function of solvent composition, a molecular-level understanding of the role of van der Waals interactions and hydrogen bonding of the different end-groups on the aggregation state in different water/N,N-dimethylformamide mixtures is obtained. The reported model provides a means to simulate organic moieties covalently bound to the layered silicate edge, which will enable future simulations of nanocomposites and organic/inorganic hybrids based on this system.

Bubble Printing of Ti3C2TX MXene for Patterning Conductive and Plasmonic Nanostructures.

MXenes represent a novel class of 2D materials with unique properties and have great potential for diverse applications in sensing and electronics; however, their directed assembly at interfaces has not yet been achieved. Herein, the plasmonic heating of MXenes was exploited to achieve the controlled deposition of MXene assemblies via a laser-directed microbubble. The influence of various factors such as solvent composition, substrate surface chemistry, MXene concentration, and laser fluence was investigated, establishing the optimal conditions for rapid patterning with good fidelity. Printed MXene assemblies showed good electrical conductivity and plasmonic sensing capabilities and were able to meet or exceed the state of the art without additional postprocessing steps. This represents the first study of a directed approach for microfabrication using MXenes and lays the foundation for future work in optically directed assembly of MXenes and MXene-based nanocomposites at interfaces toward sensors and devices.

Layered silicate edge-linked perylene diimides: Synthesis, self-assembly and energy transfer.

The control over intermolecular interactions between chromophores at nanomaterial interfaces is important for sensing and light-harvesting applications. To that aim, inorganic nanoparticles with anisotropic shape and surface chemistry can serve as useful supports for organic modification. Herein, novel asymmetric perylene diimides with aspartic acid and oleyl terminal groups were grafted to the edges of the layered silicate clay Laponite, a water-dispersible discoidal nanoparticle. The photophysical properties and solvent-dependent self-assembly of the nanoclay-grafted perylenes were investigated, revealing that the polarity of the terminating ligand dictates the aggregation behavior in aqueous solution, where increased water content generally led to the formation of perylene H-aggregates. The anionic basal surface of the nanoclay provided a binding site for a cationic fluorophore, leading to energy transfer from the face-bound donor to the edge-bound perylene acceptor. This study encourages further research on the use of functional ligands for the formation of organic-inorganic hybrids, particularly where inorganic template particles with specific surface chemistry can be exploited to study intermolecular interactions. Overall, these findings should advance further design and implementation of novel semiconducting ligands towards inorganic-organic hybrids, with potential applications in sensing and energy harvesting.

Layered Double Hydroxide-Bismuth Molybdate Hybrids toward Water Remediation via Selective Adsorption of Anionic Species.

The steady release of anthropogenic toxins into the biosphere is compromising water security globally. Herein, CoAl layered double hydroxide, a clay-like layered material with a cationic surface charge, was organically modified and used to template the growth of Bi2MoO6. The resulting nanohybrid selectively removed the anionic dye methyl orange from aqueous solution and showed an enhancement of greater than 300% in the maximum adsorptivity (1.95 mmol/g) compared to modified CoAl layered double hydroxide (0.42 mmol/g). Interestingly, the observed improvement in adsorption occurs without any significant increase in the surface area of the hybrids. Furthermore, these hybrids exhibit increased broadband visible light absorption, and their photoactivity is slightly improved compared to CoAl layered double hydroxide. This study demonstrates that composites of clay-like materials with Aurivillius oxides are promising sorbent materials for water decontamination and photocatalytic antifouling membranes and shows that the synthetic strategy that was first established with an anionic layered silicate nanoclay can be generalized to other ionic layered materials.

Surface-Encapsulated Bismuth Molybdate-Layered Silicate Hybrids as Sorbents for Photocatalytic Filtration Membranes.

Groundwater is being depleted globally at an average rate of more than one meter per year, during a period when more than a quarter of the human population has no access to potable water. Aside from overexploitation, freshwater security is also threatened by climate change and chemical pollution. The contamination of surface and groundwater by industrial substances is also undermining the vitality of ecosystems. It was previously shown that {100}-faceted Bi2MoO6-Laponite hybrids effectively bind and photodegrade molecular species, aiding in the decontamination of water. In this study, the encapsulation of Bi2MoO6-Laponite particles with the polymers butyl acrylate and styrene further enhanced adsorption of methylene blue by 31.4%, with a specific adsorption capacity of 192 µmol/g. The polymer-particle composites were deposited to form membranes and their efficacies in water filtration and photodegradation were examined. Among the different surface modifications examined, the highest dye sorption was obtained by butyl acrylate and styrene (3:2) with a 5 mol % cross-linker. This study provides a method for enhancing the molecular adsorption of composite particles used in membranes capable of multiple cycles of adsorption and photodegradation, advancing the application of such systems to water filtration.

Facet Engineering of Bismuth Molybdate via Confined Growth in a Nanoscale Template toward Water Remediation.

Certain nanomaterials can filter and alter unwanted compounds due to a high surface area, surface reactivity, and microporous structure. Herein, γ-Bi2MoO6 particles are synthesized via a colloidal hydrothermal approach using organically modified Laponite as a template. This organically modified Laponite interlayer serves as a template promoting the growth of the bismuth molybdate crystals in the [010] direction to result in hybrid Laponite-Bi2MoO6 particles terminating predominantly in the {100} crystal facets. This resulted in an increase in particle size from lateral dimensions of <100 nm to micron scale and superior adsorption capacity compared to bismuth molybdate nanoparticles. These {100}-facet terminated particles can load both cationic and anionic dyes on their surfaces near-spontaneously and retain the photocatalytic properties of Bi2MoO6. Furthermore, dye-laden hybrid particles quickly sediment, rendering the task of particle recovery trivial. The adsorption of dyes is completed within minutes, and near-complete photocatalytic degradation of the adsorbed dye in visible light allowed recycling of these particles for multiple cycles of water decontamination. Their adsorption capacity, facile synthesis, good recycling performance, and increased product yield compared to pure bismuth molybdate make them promising materials for environmental remediation. Furthermore, this synthetic approach could be exploited for facet engineering in other Aurivillius-type perovskites and potentially other materials.

Few-Layer ZnIn2S4/Laponite Heterostructures: Role of Mg2+ Leaching in Zn Defect Formation.

Designing nanostructures with extended light absorption via defect engineering is a useful approach for the synthesis of efficient photocatalysts. Herein, ZnIn2S4 was grown hydrothermally in the modified interlayer space of Laponite, resulting in lamellae consisting of Zn-defective ZnIn2S4 several unit cells thick. In the process it was found that Mg2+ leached from Laponite during synthesis led to the formation of Zn defects in ZnIn2S4. This resulted in nanohybrids with light absorption extended across the visible spectrum and in improved charge transfer due to the layered structure formed via confined growth. Compared with pure ZnIn2S4, Zn-defective ZnIn2S4-Laponite hybrids have increased photocurrent generation and photocatalytic performance. The leaching of Mg2+ and the resulting formation of Zn defects was attenuated by addition of 4 mM Mg2+ to the reaction, due to a combination of shifting of the equilibrium of Mg2+ leaching toward stability, and increased ionic strength. In summary, this work demonstrates the growth of ∼1 nm thick lamellae of ZnIn2S4, presents a unique strategy to generate cation defects in nanomaterials and the mechanism behind it, and also provides an approach to mitigate Mg2+ leaching in such syntheses.

Heterostructured Monolayer MoS2 Nanoparticles toward Water-Dispersible Catalysts.

MoS2 is a 2D semiconductor where exfoliation to a single layer results in improved catalytic properties. However, its high surface energy can lead to extensive aggregation, resulting in degraded catalytic performance and stability. Combined with a lack of dispersibility in water, this represents a pitfall for catalysis in the aqueous phase. Herein, we present the use of nanoscopic layered silicates pillared with a cationic surfactant to template the growth of MoS2 in the interlayer space. This provides heterostructured layered nanoparticles ∼25 nm wide by 3-8 nm thick containing isolated MoS2 layers. The resulting nanohybrids retain the disc-like morphology and surface chemistry of the clays, providing good aqueous stability, while also providing access to the catalytic edge-sites of the MoS2 layer. In addition to significant enhancement of catalytic dye degradation, molecular aggregation on the highly charged clay interface is comparable to unmodified clays. These particles are ideal for studies of charge-transport properties in confined semiconductor layers, as well as hierarchical self-assembly into functional materials. This study paves the way to colloidal synthesis of nanoparticulate heterostructures with other functional layered materials, particularly where particle exfoliation, covalent modification, and aqueous stability are concerns.

Solvent-Assisted Self-Assembly of Gold Nanorods into Hierarchically Organized Plasmonic Mesostructures.

Plasmonic supercrystals and periodically structured arrays comprise a class of materials with unique optical properties that result from the interplay of plasmon resonances, as well as near- and far-field coupling. Controlled synthesis of such hierarchical structures remains a fundamental challenge, as it demands strict control over the assembly morphology, array size, lateral spacing, and macroscale homogeneity. Current fabrication approaches involve complicated multistep procedures lacking scalability and reproducibility, which has hindered the practical application of plasmonic supercrystal arrays. Herein, these challenges are addressed by adding an organic solvent to achieve kinetic control over the template-assisted colloidal assembly of nanoparticles from aqueous dispersion. This method yields highly regular periodic arrays, with feature sizes ranging from less than 200 nm up to tens of microns. A combined experimental/computational approach reveals that the underlying mechanism is a combination of the removal of interfacial surfactant micelles from the particle interface and altered capillary flows. Assessing the efficacy of such square arrays for surface-enhanced Raman scattering spectroscopy, we find that a decrease of the lattice periodicity from 750 nm down to 400 nm boosts the signal by more than an order of magnitude, thereby enabling sensitive detection of analytes, such as the bacterial quorum sensing molecule pyocyanin, even in complex biological media.

Optical Nanoprinting of Colloidal Particles and Functional Structures.

Recent advances in chemical sciences have enabled the tailorable synthesis of colloidal particles with variable composition, size, shape, and properties. Building superstructures with colloidal particles as building blocks is appealing for the fabrication of functional metamaterials and nanodevices. Optical nanoprinting provides a versatile platform to print various particles into arbitrary configurations with nanometric precision. In this review, we summarize recent progress in optical nanoprinting of colloidal particles and its related applications. Diverse techniques based on different physical mechanisms, including optical forces, light-controlled electric fields, optothermal effects, laser-directed thermocapillary flows, and photochemical reactions, are discussed in detail. With its flexible and versatile capabilities, optical nanoprinting will find promising applications in numerous fields such as nanophotonics, energy, microelectronics, and nanomedicine.

A Retrospective: 10 Years of Oligo(phenylene-ethynylene) Electrolytes: Demystifying Nanomaterials.

In this retrospective, we first reviewed the synthesis of the oligo(phenylene-ethynylene) electrolytes (OPEs) we created in the past 10 years. Since the general antimicrobial activity of these OPEs had been reported in our previous account in Langmuir, we are focusing only on the unusual spectroscopic and photophysical properties of these OPEs and their complexes with anionic scaffolds and detergents in this Feature Article. We applied classical all-atom MD simulations to study the hydrogen bonding environment in the water surrounding the OPEs with and without detergents present. Our finding is that OPEs could form a unit cluster or unit aggregate with a few oppositely charged detergent molecules, indicating that the photostability and photoreactivity of these OPEs might be considerably altered with important consequences to their activity as antimicrobials and fluorescence-based sensors. Thus, in the following sections, we showed that OPE complexes with detergents exhibit enhanced light-activated biocidal activity compared to either OPE or detergent individually. We also found that similar complexes between certain OPEs and biolipids could be used to construct sensors for the enzyme activity. Finally, the OPEs could covalently bind to microsphere surfaces to make a bactericidal surface, which is simpler and more ordered than the surface grafted from microspheres with polyelectrolytes. In the Conclusions and Prospects section, we briefly summarize the properties of OPEs developed so far and future areas for investigation.

Opto-Thermophoretic Attraction, Trapping, and Dynamic Manipulation of Lipid Vesicles.

Lipid vesicles are important biological assemblies, which are critical to biological transport processes, and vesicles prepared in the lab are a workhorse for studies of drug delivery, protein unfolding, biomolecular interactions, compartmentalized chemistry, and stimuli-responsive sensing. The current method of using optical tweezers for holding lipid vesicles in place for single-vesicle studies suffers from limitations such as high optical power, rigorous optics, and small difference in the refractive indices of vesicles and water. Herein, we report the use of plasmonic heating to trap vesicles in a temperature gradient, allowing long-range attraction, parallel trapping, and dynamic manipulation. The capabilities and limitations with respect to thermal effects on vesicle structure and optical spectroscopy are discussed. This simple approach allows vesicle manipulation using down to 3 orders of magnitude lower optical power and at least an order of magnitude higher trapping stiffness per unit power than traditional optical tweezers while using a simple optical setup. In addition to the benefit provided by the relaxation of these technical constraints, this technique can complement optical tweezers to allow detailed studies on thermophoresis of optically trapped vesicles and effects of locally generated thermal gradients on the physical properties of lipid vesicles. Finally, the technique itself and the large-scale collection of vesicles have huge potential for future studies of vesicles relevant to detection of exosomes, lipid-raft formation, and other areas relevant to the life sciences.

Chiral metamaterials via Moiré stacking.

Chiral metamaterials have attracted strong interest due to their versatile capabilities in spin-dependent light manipulation. Benefiting from advancements in nanofabrication and mechanistic understanding of chiroptical effects, chiral metamaterials have shown potential in a variety of applications including circular polarizers, chiral sensors, and chiroptical detectors. Recently, chiral metamaterials made by moiré stacking, superimposing two or more periodic patterns with different lattice constants or relative spatial displacement, have shown promise for chiroptical applications. The moiré chiral metamaterials (MCMs) take advantage of lattice-dependent chirality, giving cost-effective fabrication, flexible tunability, and reconfigurability superior to conventional chiral metamaterials. This feature article focuses on recent progress of MCMs. We discuss optical mechanisms, structural design, fabrication, and applications of the MCMs. We conclude with our perspectives on the future opportunities for the MCMs.

Optothermal Manipulations of Colloidal Particles and Living Cells.

Optical manipulation techniques are important in many fields. For instance, they enable bottom-up assembly of nanomaterials and high-resolution and in situ analysis of biological cells and molecules, providing opportunities for discovery of new materials, medical diagnostics, and nanomedicines. Traditional optical tweezers have their applications limited due to the use of rigorous optics and high optical power. New strategies have been established for low-power optical manipulation techniques. Optothermal manipulation, which exploits photon-phonon conversion and matter migration under a light-controlled temperature gradient, is one such emerging technique. Elucidation of the underlying physics of optothermo-matter interaction and rational engineering of optical environments are required to realize diverse optothermal manipulation functionalities. This Account covers the working principles, design concepts, and applications of a series of newly developed optothermal manipulation techniques, including bubble-pen lithography, opto-thermophoretic tweezers, opto-thermoelectric tweezers, optothermal assembly, and opto-thermoelectric printing. In bubble-pen lithography, optical heating of a plasmonic substrate generates microbubbles at the solid-liquid interface to print diverse colloidal particles on the substrates. Programmable bubble printing of semiconductor quantum dots on different substrates and haptic control of printing have also been achieved. The key to optothermal tweezers is the ability to deliver colloidal particles from cold to hot regions of a temperature gradient or a negative Soret effect. We explore different driving forces for the two types of optothermal tweezers. Opto-thermophoretic tweezers rely on an abnormal permittivity gradient built by structured solvent molecules in the electric double layer of colloidal particles and living cells in response to heat-induced entropy, and opto-thermoelectric tweezers exploit a thermophoresis-induced thermoelectric field for the low-power manipulation of small nanoparticles with minimum diameter around 20 nm. Furthermore, by incorporating depletion attraction into the optothermal tweezers system as particle-particle or particle-substrate binding force, we have achieved bottom-up assembly and reconfigurable optical printing of artificial colloidal matter. Beyond optothermal manipulation techniques in liquid environments, we also review recent progress of gas-phase optothermal manipulation based on photophoresis. Photophoretic trapping and transport of light-absorbing materials have been achieved through optical engineering to tune particle-molecule interactions during optical heating, and a novel optical trap display has been demonstrated. An improved understanding of the colloidal response to temperature gradients will surely facilitate further innovations in optothermal manipulation. With their low-power operation, simple optics, and diverse functionalities, optothermal manipulation techniques will find a wide range of applications in life sciences, colloidal science, materials science, and nanoscience, as well as in the developments of colloidal functional devices and nanomedicine.

Optothermophoretic Manipulation of Colloidal Particles in Nonionic Liquids.

The response of colloidal particles to a light-controlled external temperature field can be harnessed for opto-thermophoretic manipulation of the particles. The thermoelectric effect is regarded as the driving force for thermophoretic trapping of particles at the light-irradiated hot region, which is thus limited to ionic liquids. Herein, we achieve opto-thermophoretic manipulation of colloidal particles in various non-ionic liquids, including water, ethanol, isopropyl alcohol and 1-butanol, and establish the physical mechanism of the manipulation at the molecular level. We reveal that the non-ionic driving force originates from a layered structure of solvent molecules at the particle-solvent interface, which is supported by molecular dynamics simulations. Furthermore, the effects of hydrophilicity, solvent type, and ionic strength on the layered interfacial structures and thus the trapping stability of particles are investigated, providing molecular-level insight into thermophoresis and guidance on interfacial engineering for optothermal manipulation.

Metal Nanoparticle Growth within Clay-Polymer Nacre-Inspired Materials for Improved Catalysis and Plasmonic Detection in Complex Biofluids.

Recent studies have shown that layered silicate clays can be used to form a nacre-like bioinspired layered structure with various polymer fillers, leading to composite films with good material strength, gas-barrier properties, and high loading capacity. We go one step further by in situ growing metal nanoparticles in nacre-like layered films based on layered silicate clays, which can be used for applications in plasmonic sensing and catalysis. The degree of anisotropy of the nanoparticles grown in the film can be controlled by adjusting the ratio of clay to polymer or gold to clay and reducing agent concentration, as well as silver overgrowth, which greatly enhances the surface enhanced Raman scattering activity of the composite. We show the performance of the films for SERS detection of bacterial quorum sensing molecules in culture medium, and catalytic properties are demonstrated through the reduction of 4-nitroaniline. These films serve as the first example of seedless, in situ nanoparticle growth within nacre-mimetic materials, and open the path to basic research on the influence of different building blocks and polymeric mortars on nanoparticle morphology and distribution, as well as applications in catalysis, sensing, and antimicrobial surfaces using such materials.

Detection and imaging of quorum sensing in Pseudomonas aeruginosa biofilm communities by surface-enhanced resonance Raman scattering.

Most bacteria in nature exist as biofilms, which support intercellular signalling processes such as quorum sensing (QS), a cell-to-cell communication mechanism that allows bacteria to monitor and respond to cell density and changes in the environment. As QS and biofilms are involved in the ability of bacteria to cause disease, there is a need for the development of methods for the non-invasive analysis of QS in natural bacterial populations. Here, by using surface-enhanced resonance Raman scattering spectroscopy, we report rationally designed nanostructured plasmonic substrates for the in situ, label-free detection of a QS signalling metabolite in growing Pseudomonas aeruginosa biofilms and microcolonies. The in situ, non-invasive plasmonic imaging of QS in biofilms provides a powerful analytical approach for studying intercellular communication on the basis of secreted molecules as signals.