Au Nanoparticles@UiO-66 Composite Film-Coated Carbon Cloth Substrate for High-Performance H2O2 Electrochemical Sensing.

Due to the strong oxidizability of H2O2, rapid, accurate, sensitive, and stable sensors of hydrogen peroxide (H2O2) have attracted wide attention in the chemical industry, food, medicine, household detergents, and environmental monitoring fields. Here, a high-performance H2O2 electrochemical sensing platform is proposed based on an Au nanoparticles@UiO-66 film coated on a carbon cloth (CC) electrode (Au NPs@UiO-66/CC electrode). The Au NPs@UiO-66/CC electrode was prepared through solvothermal growth of a UiO-66 film on a functionalized three-dimensional CC electrode, followed by in situ deposition of Au NPs into the UiO-66 film under a periodic galvanostatic pulse current. The in situ preparation strategy greatly improves the electrical interaction between Au NPs@UiO-66 and the CC substrate without sacrificing the electrochemical activity of the Au NPs@UiO-66/CC electrode. Meanwhile, thanks to the high specific surface area of the three-dimensional Au NPs@UiO-66/CC electrode, the optimized Au NPs@UiO-66/CC electrode illustrates excellent electrochemical detection capability for H2O2 with an extensive linear range (0.1-21 mM), high sensitivity (1048.01 µA mM-1 cm-2), and lower limit of detection [0.033 µM (S/N = 3)] and limit of quantification [0.109 µM (S/N = 3)]. At the same time, the encapsulated structure of Au NPs in the UiO-66 film also endows the composite electrode with specific sensing performance owing to the regular opening channels of the UiO-66 films that prevent large-size interferents from reacting to the electrochemically active Au NPs. Together with all these advantages, the proposed sensing platform would exhibit great potential for electrochemical sensors and bioelectronics.

Electrostatic Adsorption Behaviors of Charged Polymer-Tethered Nanoparticles on Oppositely Charged Surfaces.

Polymer-grafted hairy nanoparticles (HNPs) that combine the unique properties of inorganic nanoparticles (NPs) and polymers are attractive building blocks for the layer-by-layer assembly of functional hybrid materials, but the adsorption behaviors of HNPs on substrates remain unclear. This article describes a systematic study on the adsorption behaviors of charged polymer-grafted HNPs on oppositely charged substrates in different solvent media via a combination of experiments and simulations. It is shown in simulations that the adsorption process of HNPs is associated with the release of counterions around charged polymers on HNPs, thus resulting in a higher energy barrier of NP adsorption than bare NPs without charged polymer tethers. This energy barrier decreases with decreasing the dielectricity of solvents or ionization degree of grafted polymers or increasing ionic strength of the solution. Furthermore, the theoretical prediction is confirmed in experiments by using a model system of poly(acrylic acid)-grafted silica NPs and poly(diallyldimethylammonium chloride)-modified wafers. The work provides guidance for the electrostatic assembly of HNPs into functional hybrid composites with applications in membranes, optical devices, and biomedicines.

Polymeric Ligand-Mediated Regioselective Bonding of Plasmonic Nanoplates and Nanospheres.

Nanoparticle (NP) clusters are attractive for many applications, but controllable and regioselective assembly of clusters remains challenging. This communication reports a strategy to precisely assemble Ag nanoplates (NP-As) and Au nanospheres (NP-Bs) grafted with copolymer ligands into defined ABx clusters with controlled coordination number (x) and orientation of the NPs. The directional bonding of shaped NPs relies on the stoichiometric reaction of complementary reactive groups on copolymer ligands. The x value of NP clusters can be tuned from 1 to 4 by varying the number ratio of reactive groups on single NP-Bs to NP-As. The regioselective bonding of nanospheres to the edge or face of a central nanoplate is governed by the steric hindrance of copolymeric ligands on the nanoplate. The clusters exhibit distinctive plasmonic properties that are dependent on the bonding modes of NPs. This study paves a route to fabricating nanostructures with high precision and complexity for applications in plasmonics, catalysis, and sensing.

Shape Complementarity Modulated Self-Assembly of Nanoring and Nanosphere Hetero-nanostructures.

Shape complementarity is of paramount importance in molecular recognition, but has rarely been adopted in the self-assembly of colloidal particles, especially in the case of nanoparticles of different shapes. Here, we demonstrated a simple, yet powerful strategy for fabricating gold nanoring-based heterogeneous nanostructures (AuNR-HNs) with well-defined geometries and high yield. The assembly of various geometries of AuNR-HNs is modulated by the shape complementarity of plasmonic nanorings and nanospheres. We also present experimental evidence of dark quadrupolar ring mode excitation in AuNR-HNs through single-particle optical measurements. Our strategy will be beneficial in the study of nanoparticle assembly, photonic element interaction, and the development of plasmon-based optical devices.

Ordered Micro/Nanostructures with Geometric Gradient: From Integrated Wettability "Library" to Anisotropic Wetting Surface.

Geometric gradients within ordered micro/nanostructures exhibit unique wetting properties. Well-defined and ordered microsphere arrays with geometric gradient (OMAGG) are successfully fabricated through combining colloidal lithography and inclined reactive ion etching (RIE). During the inclined RIE, the graded etching rates in vertical direction of etcher chamber are the key to generating a geometric gradient. The OMAGG can be used as an effective mask for the preparation of micro/nanostructure arrays with geometric gradient by selective RIE. Through this strategy, a well-defined wettability "library" with graded silicon cone arrays is fabricated, and the possibility of screening one desired "book" from the designated wettability "library" is demonstrated. Meanwhile, the silicon cone arrays with geometric gradient (SCAGG) can be applied to control the wetting behavior of water after being modified by hydrophilic or hydrophobic chemical groups. Based on this result, a temperature-responsive wetting substrate is fabricated by modifying poly n-isopropyl acrylamide (PNIPAM) on the SCAGG. These wettability gradients have great potential in tissue engineering, microfluidic devices, and integrated sensors.

Unidirectional Wetting of Liquids on "Janus" Nanostructure Arrays under Various Media.

We report the unidirectional wetting behavior of liquids (water and oil) on Janus silicon cylinder arrays (Si-CAs) under various media (air, water, and oil). The Janus cylinders were prepared by chemical modification of nanocylinders with different molecules on two sides. Through adjusting surface energies of the modified molecules, the as-prepared surfaces could control the wetting behavior of different types of liquids under various media. We discuss the regularity systematically and propose a strategy for preparing anisotropic wetting surfaces under arbitrary media. That is, to find two surface modification molecules with different surface energies, one of the molecules is easy to be wetted by the liquid under the corresponding media, while the other one is difficult. Additionally, by introducing thermal-responsive polymer brushes onto one part of Janus Si-CAs, the surfaces show thermal-responsive anisotropic wetting property under various media. We believe that due to the excellent unidirectional wettability under various media, the Janus surfaces could be applied in water/oil transportation, oil-repellent and self-cleaning coatings, water/oil separation, microfluidics, and so on.

Thermal-Responsive Anisotropic Wetting Microstructures for Manipulation of Fluids in Microfluidics.

We show morphology-patterned stripes modified by thermal-responsive polymer for smartly guiding flow motion of fluid in chips. With a two-step modification process, we fabricated PNIPAAm-modified Si stripes on silicon slides, which were employed as substrates for fluid manipulation in microchannels. When the system temperature switches between above and below the lower critical solution temperature (LCST) of PNIPAAm, the wettability of the substrates also switches between strong anisotropy and weak anisotropy, which resulted in anisotropic (even unidirectional) flow and isotropic flow behavior of liquid in microchannels. The thermal-responsive flow motion of fluid in the chip is influenced by the applied pressure, the thickness of PNIPAAm, and dimension of the microchannels. Moreover, we measured the feasible applied pressure scopes under different structure factors. Because of the excellent reversibility and quick switching speed, the chip could be used as a thermal-responsive microvalve. Through tuning the system temperature and adding the assistant gas, we realized successive "valve" function. We believe that the practical and simple chip could be widely utilized in medical detection, immunodetection, protein analysis, and cell cultures.

Controlling flow behavior of water in microfluidics with a chemically patterned anisotropic wetting surface.

We report the flow behavior of water in microfluidic systems based on a chemically patterned anisotropic wetting surface. When water flows inside a microchannel on top of a micropatterned surface with alternating hydrophilic/hydrophobic stripes, it exhibits an anisotropic flowing characteristic owing to the anisotropic wettability; thus, the patterned surface acts as a microvalve for the microfluidic system. The anisotropic flow of water is influenced by the microscale features of the patterns and the dimensions of the microchannels. Furthermore, by reasonably combining the patterned surface and microchannel together, we realize the transportation of water in a microchannel along a "virtual" wall, which is the boundary of the hydrophilic and hydrophobic area. We believe that the chemically patterned surfaces could be an alternative strategy to control the flow behavior of water in microfluidic channels.

Distorted colloidal arrays as designed template.

In this paper, a novel type of colloidal template with broken symmetry was generated using commercial, inductively coupled plasma reactive ion etching (ICP-RIE). With proper but simple treatment, the traditional symmetric non-close-packed colloidal template evolves into an elliptical profile with high uniformity. This unique feature can add flexibility to colloidal lithography and/or other lithography techniques using colloidal particles as building blocks to fabricate nano-/micro-structures with broken symmetry. Beyond that the novel colloidal template we developed possesses on-site tunability, i.e. the transformability from a symmetric into an asymmetric template. Sandwich-type particles with eccentric features were fabricated utilizing this tunable template. This distinguishing feature will provide the possibility to fabricate structures with unique asymmetric features using one set of colloidal template, providing flexibility and broad tunability to enable nano-/micro-structure fabrication with colloidal templates.

pH-Responsive, Wide Color Gamut Dynamic Color Display Enabled by PDMAEMA Brush-Based Fabry-Perot Resonant Cavity.

Dynamic color-changing materials have attracted broad interest due to their widespread applications in visual sensing, dynamic color display, anticounterfeiting, and image encryption/decryption. In this work, we demonstrate a novel pH-responsive dynamic color-changing material based on a metal-insulator-metal (MIM) Fabry-Perot (FP) cavity with a pH-responsive poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) brush layer as the responsive insulating layer. The pH-responsive PDMAEMA brush undergoes protonation at a low pH value (pH < 6), which induces different swelling degrees in response to pH and thus refractive index and thickness change of the insulator layer of the MIM FP cavity. This leads to significant optical property changes in transmission and a distinguishable color change spanning the whole visible region by adjusting the pH value of the external environment. Due to the reversible conformational change of the PDMAEMA and the formation of covalent bonds between the PDMAEMA molecular chain and the Ag substrate, the MIM FP cavity exhibits stable performance and good reproducibility. This pH-responsive MIM FP cavity establishes a new way to modulate transmission color in the full visible region and exhibits a broad prospect of applications in dynamic color display, real-time environment monitoring, and information encryption and decryption.

The fabrication of long-range ordered nanocrescent structures based on colloidal lithography and parallel imprinting.

A method for fabricating nanocrescent structures is presented based on a combination of colloidal lithography and parallel imprinting. In this process, non-close-packed colloidal spheres were prepared by a simple lift-up soft lithography technique, and subsequently the individual particles were used as shadow masks to angle deposit a layer of silver on the silicon substrates. Then, the silver-coated samples were etched to get silicon crescent nanohole arrays, which served as templates to mold patterned photocurable resin membranes. The patterned photocurable resin membranes were used to print gold nanocrescent nanostructures onto glass substrates. The size of the opening and the width of the gold nanostructures could be freely adjusted by changing the azimuth angle and tilt angle. Very importantly, the central angle of the nanocrescents could be adjusted in the range of 0°-360°. This method provides a low-cost and highly reproducible way to prepare complex nanostructure arrays for applications related to near field enhancement materials, optical sensors and surface-enhanced Raman spectroscopy, etc.

Halogen Bonding-Driven Reversible Self-Assembly of Plasmonic Colloidal Molecules.

Colloidal molecules (CMs) assembled from plasmonic nanoparticles are an emerging class of building blocks for creating plasmonic materials and devices, but precise yet reversible assembly of plasmonic CMs remains a challenge. This communication describes the reversible self-assembly of binary plasmonic nanoparticles capped with complementary copolymer ligands into different CMs via halogen bonding interactions at high yield. The coordination number of the CMs is governed by the number ratio of complementary halogen donor and acceptor groups on the interacting nanoparticles. The reversibility of the halogen bonds allows for controlling the repeated formation and disassociation of the plasmonic CMs and hence their optical properties. Furthermore, the CMs can be designed to further self-assemble into complex structures in selective solvents. The precisely engineered reversible nanostructures may find applications in sensing, catalysis, and smart optoelectronic devices.

Fabrication of Centimeter-Scale Plasmonic Nanoparticle Arrays with Ultranarrow Surface Lattice Resonances.

Plasmonic surface lattice resonances (SLRs) supported by metallic nanoparticle (NP) arrays show diverse applications including nanolasers, sensors, photocatalysis, and nonlinear optics. However, to rationally fabricate high-quality plasmonic NP arrays with ultranarrow SLR line widths over large areas remains challenging. This article describes a general approach for the efficient fabrication of centimeter-scale inorganic NP arrays with precisely controlled NP size, composition, position, and lattice geometry. This method combines the processes of solvent-assisted soft lithography and in situ site-specific NP growth to reproducibly create many replicates of NP arrays without utilizing cleanroom and specialized equipment. For demonstration, we show that Au NP arrays exhibit ultranarrow SLRs with a line width of 4 nm and a quality factor of 218 toward the theoretical limit.

Hydrogen-Induced Aggregation of Au@Pd Nanoparticles for Eye-Readable Plasmonic Hydrogen Sensors.

Plasmonic materials provide a promising platform for optical hydrogen detection, but their sensitivities remain limited. Herein, a new type of eye-readable H2 sensor based on Au@Pd core-shell nanoparticle arrays (NAs) is reported. After exposed to 2% H2, Au@Pd (16/2) NAs demonstrate a dramatic decrease in the optical extinction intensity, along with an obvious color change from turquoise to gray. Experimental results and theoretical calculations prove that the huge optical change resulted from the H2-induced aggregation of Au@Pd nanoparticles (NPs), which remarkably alters the plasmon coupling effect between NPs. Moreover, we optimize the sensing behavior from two aspects. The first is selecting appropriate substrates (either rigid glass substrate or flexible polyethylene terephthalate substrate) to offer moderate adhesion force to NAs, ensuring an efficient aggregation of Au@Pd NPs upon H2 exposure. The second is adjusting the Pd shell thickness to control the extent of NP aggregation and thus the detection range of the as-prepared sensors. This work highlights the advantage of designing eye-readable plasmonic H2 sensors from the aspect of tuning the interparticle plasmonic coupling in NP assemblies. Au@Pd NAs presented here have several advantages in terms of simple fabrication method, eye-readability in air background at room temperature, tunable detection range, and high cost-effectiveness.

Centimeter-Scale Superlattices of Three-Dimensionally Orientated Plasmonic Dimers with Highly Tunable Collective Properties.

The precise organization and orientation of plasmonic molecules on substrates is crucial to their application in functional devices but still remains a grand challenge. This article describes a bottom-up strategy to efficiently fabricate centimeter-scale superlattices of three-dimensionally oriented plasmonic dimers with highly tunable collective optical properties on substrates. The in-plane (i.e., X-Y plane) and out-of-plane (i.e., along Z-axis) orientation of the constituent plasmonic dimers can be precisely controlled by a combination of directional capillary force and supporting polymer film. Our experimental measurements and numerical simulations show that the macroscopic dimer superlattices exhibit polarization-dependent plasmon Fano resonances in air and multimodal surface lattice resonances with high quality factors in a homogeneous medium, owing to the high positional and orientational ordering of the subunits. Our strategy enables the fabrication of complex plasmonic nanostructures with precise configurations for advanced plasmonic devices such as plasmon nanolasing and metamaterials.

Site-Selective Assembly of Centimeter-Scale Arrays of Precisely Oriented Magnetic Nanoellipsoids.

The precise organization and orientation of anisotropic nanoparticles (NPs) on substrates over a large area is key to the application of NP assemblies in functional optical, electronic, and magnetic devices, but achieving such high-precision NP assembly still remains challenging. Here, we demonstrate the site-selective assembly of magnetic nanoellipsoids into large-area precisely positioned, orientationally controlled arrays via a combination of chemical patterning and magnetic manipulation. Magnetic ellipsoidal NPs are selectively positioned on predetermined chemical patterns with high fidelity through electrostatic interactions and aligned uniformly in line with an applied magnetic field. The position, orientation, and interparticle spacing of the ellipsoids can be precisely tuned by controlling the chemical patterns and magnetic field. This approach is simple to implement and can generate centimeter-scale arrays in high yield (up to 99%). The arrays exhibit collective magnetic responses that are dependent on the orientation of the ellipsoids. This work offers a tool for the fabrication of precisely engineered arrays of anisotropic NPs for applications such as metasurface and artificial spin ice.

Laser-Scanning-Guided Assembly of Quasi-3D Patterned Arrays of Plasmonic Dimers for Information Encryption.

The application of plasmonic dimeric nanostructures in color displays, data storage, and especially metamaterials necessitates the patterning of dimers into ordered arrays, but controllable assembly of plasmonic nanoparticles into patterned dimer arrays on substrates still remains a challenge. Here, a facile laser-scanning-based strategy to fabricate quasi-3D patterned arrays of plasmonic nanoparticle dimers with controlled orientation for plasmonic information encryption is reported. Laser scanning of polymer-covered plasmonic nanoparticle (e.g., gold) arrays selectively exposes the surface of irradiated nanoparticle via localized photothermal heating, guiding the assembly of another type of nanoparticles onto the exposure nanoparticle surface to form dimers on substrates. This combined top-down/bottom-up approach is highly flexible in forming high-resolution patterns of plasmonic dimers from nanoparticles of different sizes and shapes. The z-axis orientation, interparticle spacing, and nanoparticle size and shape of plasmonic dimers can be precisely tuned, enabling the modulation of the coupled resonances of the dimer arrays. Moreover, it is demonstrated that the patterned dimer arrays can be used in information encryption where their plasmonic color can be repeatedly displayed and erased. This work provides an important addition to tools for the fabrication of patterned complex plasmonic nanostructures from as-synthesized nanoparticles with broad applications.

Ultrahigh-Sensitivity Sandwiched Plasmon Ruler for Label-Free Clinical Diagnosis.

Optical biosensors, especially those based on plasmonic structures, have emerged recently as a potential tool for disease diagnostics. Plasmonic biosensors have demonstrated impressive benefits for the label-free detection of trace biomarkers in human serum. However, widespread applications of these technologies are hindered because of their insufficient sensitivity, their relatively complex chemical immobilization processes, and the use of prism couplers. Accordingly, a sandwiched plasmon ruler (SW-PR) based on a Au nanohole array with ultrahigh sensitivity arising from the plasmonic coupling effect is developed. Highly confined surface charges caused by Bloch wave surface plasmon polarizations substantially increase the coupling efficiency. This platform exhibits thickness sensitivity as high as 61 nm nm-1 and can detect at least 200 000-fold lower analyte concentrations than a nanowell sensing platform with the same wavelength shift. Additionally, the sandwiched plasmonic biosensor allows precise and label-free testing of clinical biomarkers, namely C-reactive protein and procalcitonin, in patient serum samples without requiring a sophisticated prism coupler, extra antibodies, or a chemical immobilization technique. This study yields new insight into the structural design of plasmon rulers and will open exciting avenues for disease diagnosis and therapy follow-up at the point-of-care.

Colloidal lithography-based fabrication of highly-ordered nanofluidic channels with an ultra-high surface-to-volume ratio.

This article shows a new strategy for the fabrication of nanofluidics based on nanoscale gaps in nanopillar arrays. Silicon nanopillar arrays are prepared in a designed position by combining conventional photolithography with colloidal lithography. The nanogaps between the pillars are used as nanochannels for the connection of two polydimethylsiloxane-based microchannels in microfluidics. The gap between neighbouring nanopillars can be accurately controlled by changing the size of initial colloidal spheres and by an etching process, which further determines the dimensions of the nanochannels. At a low ionic strength, the surface charge-governed ion transportation shows that the nanochannels possess the same electrokinetic properties as typical nanofluidics. Benefiting from the advantage of photolithography, large-area nanochannel arrays can be prepared in a parallel manner. Due to the perm-selectivity of the nanochannels, the nanofluidic chips can be used to preconcentrate low concentration samples. The large-area ordered nanostructures preserve their high-throughput property and large surface-to-volume ratio, which shows their great potential in the development of nanofluidics and their applications, such as in the separation of small molecules, energy conversion, etc.

Graded nanowell arrays: a fine plasmonic "library" with an adjustable spectral range.

We present an effective approach for fabricating graded plasmonic arrays based on ordered micro-/nanostructures with a geometric gradient. Ag nanowell arrays with graded geometric parameters were fabricated and systematically investigated. The order of the graded plasmonic arrays is generated by colloidal lithography, while the geometric gradient is the result of inclined reactive ion etching. The surface plasmon resonance (SPR) peaks were measured at different positions, which move gradually along the Ag nanowell arrays with a geometric gradient. Such micro-/nanostructure arrays with graded and integrated SPR peaks can work as a fine plasmonic "library" (FPL), and the spectral range can be controlled using a "coarse adjustment knob" (lattice constant) and a "fine adjustment knob" (pore diameter). Additionally, the spectral resolution of the FPL is high, which benefits from the high value of the full height/full width at half-maximum and the small step size of the wavelength shift (0.5 nm). Meanwhile, the FPL could be effectively applied as a well-defined model to verify the plasmonic enhancement in surface enhanced Raman scattering. As the FPL is an integrated optical material with graded individual SPR peaks, it can not only be a theoretical model for fundamental research, but also has great potential in high-throughput screening of optical materials, multiplex sensors, etc.