Inkjet Color Printing by Interference Nanostructures

Tunable and Non-Invasive Printing of Transmissive Interference Colors with 2D Material Inks

Interference colors hold significant importance in optics and arts. Current methods for printing interference colors entail complex procedures and large-scale printing systems for the scarcity of inks that exhibit both sensitivity and tunability to external fields. The production of highly transparent inks capable of rendering transmissive colors has presented ongoing challenges. Here, a type of paramagnetic ink based on 2D materials that exhibit polychrome in one magnetic field is invented. By precisely manipulating the doping ratio of magnetic elements within titanate nanosheets, the magneto-optical sensitivity named Cotton-Mouton coefficient is engineerable from 728 to a record high value of 3272 m-1 T-2, with negligible influence on its intrinsic wide optical bandgap. Combined with the sensitive and controllable magneto-responsiveness of the ink, modulate and non-invasively print transmissive interference colors using small permanent magnets are precised. This work paves the way for preparing transmissive interference colors in an energy-saving and damage-free manner, which can expand its use in widespread areas.

Metal-organic framework photonic crystals with bidisperse particles-based brilliant structural colors and high optical transparency for elaborate anti-counterfeiting

Photonic crystals (PCs) have attracted great interest and wide applications in displays, printing, anti-counterfeiting, etc. However, two main challenges significantly hinder their applications: 1) the tradeoff between high optical transparency across the whole visible range and brilliant colors requiring a large refractive index contrast (Δn), and 2) the way of regulating structural colors by altering tens of different sizes. To address these issues, a new type of metal-organic framework (MOF)-based transparent photonic crystal (TPC) has been fabricated through self-assembling MOF particles into three-dimensional ordered structures which were then infiltrated by polydimethylsiloxane (PDMS). Compared to conventional PCs, these TPCs exhibit 1) both brilliant forward iridescent structural colors and high transmittance (>75 %) across the whole visible spectra range, and 2) conveniently adjustable colors based on bidisperse particles. The unique color-generating mechanism of the light diffraction by each plane lattice and the small Δn between MOF particles and PDMS are the keys to TPCs' characteristics. Moreover, the prepared invisible anti-counterfeit labels can reversibly hide-reveal patterns with elaborate and exchangeable color contrast in a non-destructive way, showing potential applications in anti-counterfeiting, information encryption, and optical devices.

Recent advances of additively manufactured noninvasive kinematic biosensors

The necessity of reliable measurement data assessment in the realm of human life has experienced exponential growth due to its extensive utilization in health monitoring, rehabilitation, surgery, and long-term treatment. As a result, the significance of kinematic biosensors has substantially increased across various domains, including wearable devices, human-machine interaction, and bioengineering. Traditionally, the fabrication of skin-mounted biosensors involved complex and costly processes such as lithography and deposition, which required extensive preparation. However, the advent of additive manufacturing has revolutionized biosensor production by facilitating customized manufacturing, expedited processes, and streamlined fabrication. AM technology enables the development of highly sensitive biosensors capable of measuring a wide range of kinematic signals while maintaining a low-cost aspect. This paper provides a comprehensive overview of state-of-the-art noninvasive kinematic biosensors created using diverse AM technologies. The detailed development process and the specifics of different types of kinematic biosensors are also discussed. Unlike previous review articles that primarily focused on the applications of additively manufactured sensors based on their sensing data, this article adopts a unique approach by categorizing and describing their applications according to their sensing frequencies. Although AM technology has opened new possibilities for biosensor fabrication, the field still faces several challenges that need to be addressed. Consequently, this paper also outlines these challenges and provides an overview of future applications in the field. This review article offers researchers in academia and industry a comprehensive overview of the innovative opportunities presented by kinematic biosensors fabricated through additive manufacturing technologies.

Water-alcohol-TiO2 dispersions as sustainable ink

Nanocrystalline titanium dioxide (TiO₂) is a widespread multifunctional and environmentally friendly material that has numerous applications requiring micro-/nanofabrication or thin film deposition. In most cases, the fabrication of titania films can be achieved using cost-efficient solution chemistry combined with various coating or printing techniques. The practical implementation of these methods requires the preparation of a suitable ink with properly adjusted rheological properties. Conventionally, such adjustments are achieved based on TiO₂ hydrosols containing various organic surfactants and stabilizing agents. However, the use of such additives may affect the properties of the deposited functional layer, which can be crucial for electronic and optical applications. In this work, we address a comprehensive study of simple surfactant-free TiO₂ dispersion systems based on various water-alcohol solvents and demonstrate the possibility of controlling the rheological properties of the titania ink in a wide range that is suitable for several printing applications. As a particular example, we demonstrate the application of a water-i-propanol-TiO₂ dispersion as a functional ink for the offset printing of interference images.

Conductive Gels: Properties and Applications of Nanoelectronics

Conductive gels are a special class of soft materials. They harness the 3D micro/nanostructures of gels with the electrical and optical properties of semiconductors, producing excellent novel attributes, like the formation of an intricate network of conducting micro/nanostructures that facilitates the easy movement of charge carriers. Conductive gels encompass interesting properties, like adhesion, porosity, swelling, and good mechanical properties compared to those of bulk conducting polymers. The porous structure of the gels allows the easy diffusion of ions and molecules and the swelling nature provides an effective interface between molecular chains and solution phases, whereas good mechanical properties enable their practical applications. Due to these excellent assets, conductive gels are promising candidates for applications like energy conversion and storage, sensors, medical and biodevices, actuators, superhydrophobic coatings, etc. Conductive gels offer promising applications, e.g., as soft sensors, energy storage, and wearable electronics. Hydrogels with ionic species have some potential in this area. However, they suffer from dehydration due to evaporation when exposed to the air which limits their applications and lifespan. In addition to conductive polymers and organic charge transfer complexes, there is another class of organic matter called "conductive gels" that are used in the organic nanoelectronics industry. The main features of this family of organic materials include controllable photoluminescence, use in photon upconversion technology, and storage of optical energy and its conversion into electricity. Various parameters change the electronic and optical behaviors of these materials, which can be changed by controlling some of the structural and chemical parameters of conductive gels, their electronic and optical behaviors depending on the applications. If the conjugated molecules with π bonds come together spontaneously, in a relative order, to form non-covalent bonds, they form a gel-like structure that has photoluminescence properties. The reason for this is the possibility of excitation of highest occupied molecular orbital level electrons of these molecules due to the collision of landing photons and their transfer to the lowest unoccupied molecular orbital level. This property can be used in various nanoelectronic applications such as field-effect organic transistors, organic solar cells, and sensors to detect explosives. In this paper, the general introduction of conductive or conjugated gels with π bonds is discussed and some of the physical issues surrounding electron excitation due to incident radiation and the mobility of charge carriers, the position, and role of conductive gels in each of these applications are discussed.

Structural color printing via polymer-assisted photochemical deposition

Structural color printings have broad applications due to their advantages of long-term sustainability, eco-friendly manufacturing, and ultra-high resolution. However, most of them require costly and time-consuming fabrication processes from nanolithography to vacuum deposition and etching. Here, we demonstrate a new color printing technology based on polymer-assisted photochemical metal deposition (PPD), a room temperature, ambient, and additive manufacturing process without requiring heating, vacuum deposition or etching. The PPD-printed silver films comprise densely aggregated silver nanoparticles filled with a small amount (estimated <20% volume) of polymers, producing a smooth surface (roughness 2.5 nm) even better than vacuum-deposited silver films (roughness 2.8 nm) at ~4 nm thickness. Further, the printed composite films have a much larger effective refractive index n (~1.90) and a smaller extinction coefficient k (~0.92) than PVD ones in the visible wavelength range (400 to 800 nm), therefore modulating the surface reflection and the phase accumulation. The capability of PPD in printing both ultra-thin (~5 nm) composite films and highly reflective thicker film greatly benefit the design and construction of multilayered Fabry-Perot (FP) cavity structures to exhibit vivid and saturated colors. We demonstrated programmed printing of complex pictures of different color schemes at a high spatial resolution of ~6.5 μm by three-dimensionally modulating the top composite film geometries and dielectric spacer thicknesses (75 to 200 nm). Finally, PPD-based color picture printing is demonstrated on a wide range of substrates, including glass, PDMS, and plastic, proving its broad potential in future applications from security labeling to color displays.

Label-Free Protein Analysis by Pyro-Electrohydrodynamic Jet Printing of Gold Nanoparticles

The pyro-electrohydrodynamic jet (p-jet) printing technology has been used for the fabrication of confined assemblies of gold nanoparticles with a round shape and a diameter ranging between 100 and 200 μm. The surface-enhanced Raman spectroscopy (SERS) performance of the p-jet substrate was evaluated by using Rhodamine 6G (R6G) as a reference. The results demonstrate that this kind of SERS substrate exhibits strong plasmonic effects and a significant reproducibility of the signal with a coefficient of variation below 15%. We tested the signal behavior also in case of the bovine serum albumin (BSA) as a model analyte, to demonstrate the affinity with biomolecules. Strong SERS activity was measured also for BSA across the whole spot area. The spectral patterns collected in different locations of the sensing area were highly reproducible. This observation was substantiated by multivariate analysis of the imaging datasets and opens the route towards a potential application of this kind of SERS substrate in biosensing.

Novel Insights into Inkjet Printed Silver Nanowires Flexible Transparent Conductive Films

Silver nanowire (AgNWs) inks for inkjet printing were prepared and the effects of the solvent system, wetting agent, AgNWs suspension on the viscosity, surface tension, contact angle between ink droplet and poly(ethylene) terephthalate (PET) surface, and pH value of AgNWs ink were discussed. Further, AgNWs flexible transparent conductive films were fabricated by using inkjet printing process on the PET substrate, and the effects of the number printing layer, heat treatment temperature, drop frequency, and number of nozzle on the microstructures and photoelectric properties of AgNWs films were investigated in detail. The experimental results demonstrated that the 14-layer AgNWs printed film heated at 60 °C and 70 °C had an average sheet resistance of 13 Ω∙sq⁻¹ and 23 Ω∙sq⁻¹ and average transparency of 81.9% and 83.1%, respectively, and displayed good photoelectric performance when the inkjet printing parameters were set to the voltage of 20 V, number of nozzles of 16, drop frequency of 7000 Hz, droplet spacing of 15 μm, PET substrate temperatures of 40 °C and nozzles of 35 °C during printing, and heat treatment at 60 °C for 20 min. The accumulation and overflow of AgNWs at the edges of the linear pattern were observed, which resulted in a decrease in printing accuracy. We successfully printed the heart-shaped pattern and then demonstrated that it could work well. This showed that the well-defined pattern with good photoelectric properties can be obtained by using an inkjet printing process with silver nanowires ink as inkjet material.

Periodic planar Fabry-Perot nanocavities with tunable interference colors based on filling density effects

Structural colors of high performance and economically feasible fabrication are desired in various applications. Herein, we demonstrate that reflective full-color filters based on the interference effect can be realized in periodic Fabry-Perot (F-P) nanocavity arrays of the same thickness. Enabled by simply adjusting the nanocavity size and array period, the resonant wavelengths can be successively tuned in the whole visible light range, which is mainly attributed to the varied effective refractive index introduced by the different filling density of the F-P nanocavity. Compared to the plasmonic colors utilizing the similar nanostructures, the proposed interference colors offer unique advantages of higher color contrast, wider gamut, and lower fabrication requirements. Besides, these color filters do not involve modulating the vertical dimensions of the F-P nanocavities, which is conducive to the monolithic integration of multicolor optical cavities and their large-area applications in consumable products combined with replica patterning techniques, such as nanoimprinting and soft lithography.

All-Dielectric Silicon Nanoring Metasurface for Full-Color Printing

Structural color has been particularly attractive as it provides a highly promising approach for next-generation color printing. Plasmonic nanostructures have been intensively investigated for color printing, while suffering from intrinsic loss that degrades the quality of the coloration. Dielectric materials have emerged as an alternative because of their high refractive index that enables highly confined optical modes within the nanostructure at the diffraction limit. Here, we demonstrate an all-dielectric nanoring metasurface for coloration. By harnessing the intrinsic nanoring structure design, vivid structural color has been achieved in the visible range. The color gamut is expected to occupy 115% of the standard color space (sRGB) on the chromaticity diagram of the International Commission on Illumination (CIE) 1931 in theory. Our structure can be applied to various complex devices and materials and find potential applications such as displays, information, and art works.

Constructing Colorful Surfaces with Mechanical Robustness for Magnesium Alloys via a Reagent-Free Method

The colorful and mechanically robust surfaces of metallic materials are important for their applications in electronics, automobile, aerospace, and so on, but it is challenging to prepare them at a reasonable cost. Herein, we propose a simple, environment-friendly, and cost-effective method, reagent-free hydrothermal treatment, to prepare colorful surfaces of magnesium alloys with mechanical robustness. The as-treated surface mainly composes of magnesium (hydr)oxides with a biomimetic microstructure, whose thickness and roughness increase linearly with the hydrothermal time. By adjusting the hydrothermal time, a series of surface colors of magnesium alloys are obtained because of light interference. The as-treated surface with film thickness more than 1.1 μm exhibits high Vickers hardness (∼500 HV), low friction coefficient (∼0.26), and low wear rate (1.06 × 10^-5 mm³·N^-1·m^-1), which is superior to most magnesium alloys after surface treatments. The microstructure of the as-treated surface can be retained after harsh tribological tests, demonstrating attractive mechanical robustness. Furthermore, the method proposed here was extended to the surface treatments of other series of magnesium alloys, which verifies the great potential of this method for large-scale industrial application of colorful metallic materials with mechanical robustness.

Printing of Colorful Cellulose Nanocrystalline Patterns Visible in Linearly Polarized Light

This study addresses the inkjet printing approach for fabrication of cellulose nanocrystalline (CNC) patterns with tunable optical properties varied by the thickness of deposited layers. In particular, forming functional patterns visible only in linearly polarized light is of the primary interest. The possibility of controlling the bright iridescent color response associated with the birefringence in the chiral anisotropic structure of inkjet-printed layers of CNC with sulfo-groups (s-CNC) has been thoroughly investigated. In this connection, we have elaborated an appropriate synthesis sequence for deriving printable inks in the form of sedimentation-stable s-CNC colloids with various concentrations of solid phase and experimentally determined the optimal regimes of their inkjet printing. For this purpose, the rheological parameters and s-CNC particle concentration have also been optimized. The study is accomplished with a comprehensive optical characterization of the deposited s-CNC layers with variable thickness, drying conditions, and the polarization state. The experimental results demonstrate the feasibility of inkjet printing technology to perform the precise fabrication of optically active s-CNC patterns with variable optical properties. These results are particularly relevant for applications requiring special conditions of color demonstration in security printing for such as anticounterfeiting applications, polygraphy decoration printing, and color photo filters.

Low-Temperature Preparation of SiO2/Nb2O5/TiO2–SiO2 Broadband Antireflective Coating for the Visible via Acid-Catalyzed Sol–Gel Method

Multilayer broadband antireflective (AR) coatings consisting of porous layers usually suffers poor functional durability. Based on a quarter-half-quarter multilayer structure, AR coatings with dense SiO2 film as the top layer are designed, and refractive index for each layer is optimized. After heat-treated at only 150 °C, refractive index of Nb2O5 film reaches to 2.072 (at 550 nm), which can meet design requirements of the middle layer. TiO2–SiO2 composites with controllable refractive indices are selected to be used as the bottom layer. The obtained triple-layer AR coating presents excellent performance, and the average transmittance at 400–800 nm attains 98.41%. Dense layers endow the multilayer structure good abrasion-resistance, and hexamethyldisilazane is further used to modify the surface of the AR coating, which can greatly improve the hydrophobicity of the coating. The proposed triple-layer broadband AR coating has potential value in practical applications of sol–gel deposition.

Ionic Conductive Gels for Optically Manipulatable Microwave Stealth Structures

Smart structures with manipulatable properties are highly demanded in many fields. However, there is a critical challenge in the pursuit of transparent windows that allow optical waves (wavelength of µm-nm) for transmitting while blocking microwave (wavelength of cm) in terms of absorbing electromagnetic energy, specifically for meeting the frequency requirement for the 5th generation (5G) mobile networks. For fundamentally establishing novel manipulatable microwave absorbing structures, here, new polymeric aqueous gels as both optically transparent materials and microwave absorbing materials are demonstrated, in which polar networks play significant roles in attenuating electromagnetic energy. By manipulating the hydrogen bonding networks, the resulting optically transparent solid-state gels are able to offer the capabilities for absorbing microwaves. Interestingly, such gels can be switched into an optically opaque state via converting the amorphous state into a polycrystal state when the temperature is decreased. Such ionic conductive gels can endow the assembled sandwich windows with effective microwave absorbing capability in the range of 15-40 GHz, covering a branch of 5G frequency bands. The results highlight a new strategy for using ionic conductive gels to design and fabricate manipulatable microwave stealth structures for various applications.

Surface Topography Effects of Globular Biomolecules on Hydration Water

Hydration water serves as a microscopic manifestation of structural stability and functions of biomolecules. To develop bio-nanomaterials in applications, it is important to study how the surface topography and heterogeneity of biomolecules result in their diversity of the hydration dynamics and energetics. We here performed molecular dynamics simulations combined with the steered molecular dynamics and umbrella sampling to investigate the dynamics and escape process associated with the free energy change of water molecules close to a globular biomolecule, i.e., hemoglobin (Hb) and G-quadruplex DNA (GDNA). The residence time, power of long-time tail, and dipole relaxation time were found to display drastic changes within the averaged hydration shell of 3.0-5.0 Å. Compared with bulk water, in the inner hydration shell, the water dipole moment displays a slower relaxation process and is more oriented toward GDNA than toward Hb, forming a hedgehog-like structure when it surrounds GDNA. In particular, a spine water structure is observed in the GDNA narrow groove. The water isotope effect not only prolongs the dynamic time scales of libration motion in the inner hydration shell and the dipole relaxation processes in the bulk but also strengthens the DNA spine water structure. The potential of the mean force profile reflects the integrity of the hydration shell structure and enables us to obtain detailed insights into the structures formed by water, such as the caged H-bond network and the edge bridge structures; it also reveals that local hydration shell free energy (LHSFE) depends on H-bond rupture processes and ranges from 0.2 to 4.2 kcal/mol. Our results demonstrate that the surface topography of a biomolecule influences the integrity of the hydration shell structure and LHSFE. Our studies are able to identify various further applications in the areas of microfluid devices and nano-dewetting on bioinspired surfaces.

Direct Visualization of the Behavior and Shapes of the Nanoscale Menisci of an Evaporating Water Droplet on a Hydrophilic Nanotextured Surface via High-Resolution Synchrotron X-ray Imaging

Despite considerable research interest due to omnipresent and practical importance of interfacial phenomena (e.g., wetting and dewetting) on nanotextured surfaces in the academic and industrial fields, direct visualization of the behavior and shapes of liquid-vapor interfaces between nanoscale structures remains an arduous task because of the resolution limitations of visualization techniques. In this study, we succeeded in a first-hand visualization of the behavior and shapes of the liquid-vapor interfaces of a water droplet between nanometer-scale pillar during evaporation by introducing a synchrotron X-ray imaging technique with spatially high resolution (40 nm/a pixel). On the basis of the visualization data, we intensively analyzed and discussed the spreading and evaporation phenomena of a liquid droplet on hydrophilic nanotextured surfaces.

Micro-/Nanofluidics for Liquid-Mediated Patterning of Hybrid-Scale Material Structures

Various materials are fabricated to form specific structures/patterns at the micro-/nanoscale, which exhibit additional functions and performance. Recent liquid-mediated fabrication methods utilizing bottom-up approaches benefit from micro-/nanofluidic technologies that provide a high controllability for manipulating fluids containing various solutes, suspensions, and building blocks at the microscale and/or nanoscale. Here, the state-of-the-art micro-/nanofluidic approaches are discussed, which facilitate the liquid-mediated patterning of various hybrid-scale material structures, thereby showing many additional advantages in cost, labor, resolution, and throughput. Such systems are categorized here according to three representative forms defined by the degree of the free-fluid-fluid interface: free, semiconfined, and fully confined forms. The micro-/nanofluidic methods for each form are discussed, followed by recent examples of their applications. To close, the remaining issues and potential applications are summarized.

Optical interference-based sensors for the visual detection of nano-scale objects

In this study, we present a new concept for the simple visual detection of nano-scale objects in solutions. To achieve this goal, we developed chromogen-free interference-based sensors that provided a color visible reaction directly after the interaction of the analyte with the substrate. The effect is based on the strong optical interference occurring at the interface between the inkjet printed sol-gel titania film (a layer with high refractive index) and the adsorbed nano-sized objects (layer with low refractive index), which can be detected even with the naked eye. Herein, we have developed a synthetic strategy for the inkjet printing of interference sensors with controllable color change through thickness adjustment.

Inkjet Printing of Sc-Doped TiO2 with Enhanced Photoactivity

Here we report the methodology for nanocomposite fabrication based on the inkjet printing technique. Doped TiO2 nanoparticles with Sc contents up to 10 wt.% were synthesized and adapted towards a facile fabrication of microscale structures and thin film printing. Implementation of the state-of-the-art low-temperature synthesis allowed to us successfully incorporate high concentrations of Sc3+ ions into the TiO2 lattice and improve the light absorption characteristics of the resulting materials. Without affecting the anatase structure substantially, Sc doping gave rise to an intensified absorbance capacity and provided the means for the efficient fabrication of Sc-TiO2 microarchitectures via the inkjet printing technique. The changes in the spectral and structural characteristics of the Sc-TiO2 composites were observed by Energy Dispersive X-Ray spectroscopy (EDX), X-ray diffraction (XRD), and UV-vis methods. The rheological parameters of the colloidal suspension based on the synthesized Sc-TiO2 nanoparticles were adapted for inkjet printing in terms of the optimal viscosity, morphology, and surface tension. The developed individual ink characteristics allowed us to produce a close coherence between the enhanced optical properties of the Sc-TiO2 prepared the sol–gel method and the inkjet-printed films. The introduced methodology features the possibility to inkjet-print doped and pure TiO2 robust films for potential large-scale fabrication.

Inkjet assisted fabrication of planar biocompatible memristors

In this study we address a novel design of a planar memristor and investigate its biocompatibility. An experimental prototype of the proposed memristor assembly has been manufactured using a hybrid nanofabrication method, combining sputtering of electrodes, patterning the insulating trenches, and filling them with a memristive substance. To pattern the insulating trenches, we have examined two nanofabrication techniques employing either a focused ion beam or a cantilever tip of an atomic force microscope. Inkjet printing has been used to fill the trenches with the functional titania ink. The experimental prototypes have qualitatively demonstrated memristive current–voltage behavior, as well as high biocompatibility.

A planar memristor was fabricated by a hybrid method combining AFM patterning and inkjet printing.

Scalable Inkjet-Based Structural Color Printing by Molding Transparent Gratings on Multilayer Nanostructured Surfaces

To enable customized manufacturing of structural colors for commercial applications, up-scalable, low-cost, rapid, and versatile printing techniques are highly demanded. In this paper, we introduce a viable strategy for scaling up production of custom-input images by patterning individual structural colors on separate layers, which are then vertically stacked and recombined into full-color images. By applying this strategy on molded-ink-on-nanostructured-surface printing, we present an industry-applicable inkjet structural color printing technique termed multilayer molded-ink-on-nanostructured-surface (M-MIONS) printing, in which structural color pixels are molded on multiple layers of nanostructured surfaces. Transparent colorless titanium dioxide nanoparticles were inkjet-printed onto three separate transparent polymer substrates, and each substrate surface has one specific subwavelength grating pattern for molding the deposited nanoparticles into structural color pixels of red, green, or blue primary color. After index-matching lamination, the three layers were vertically stacked and bonded to display a color image. Each primary color can be printed into a range of different shades controlled through a half-tone process, and full colors were achieved by mixing primary colors from three layers. In our experiments, an image size as big as 10 cm by 10 cm was effortlessly achieved, and even larger images can potentially be printed on recombined grating surfaces. In one application example, the M-MIONS technique was used for printing customizable transparent color optical variable devices for protecting personalized security documents. In another example, a transparent diffractive color image printed with the M-MIONS technique was pasted onto a transparent panel for overlaying colorful information onto one's view of reality.

Structural Color Patterns on Paper Fabricated by Inkjet Printer and Their Application in Anticounterfeiting

Inkjet-printed structural color patterns have attracted great attention in recent years because of their broadly promising applications. However, the patterns are usually fabricated on pretreated plastic substrates. Herein a convenient inkjet printing method was developed to fabricate large-scale computer-designed structural color patterns on photo paper without any treatment using inks containing monodisperse CdS spheres. By this strategy, not only were the single-color and multicolor structural color patterns on paper successfully obtained, but also invisible photonic anticounterfeiting was achieved without any external stimuli. The key point of this anticounterfeiting technique is printing patterns and the background with inks containing uniformed CdS spheres with different diameters but similar intrinsic colors, so that the invisible patterns can be observed clearly by simply changing the viewing angle. The invisible and visible can be realized without the change of intrinsic structure, and the patterns are all solids. The patterns will have long lifetime and good durability, which is beneficial for their practical usage.

Vapor sensing with color-tunable multilayered coatings of cellulose nanocrystals

Colloidal cellulose nanocrystals were LBL deposited to form firmly-stacked optical coatings in which the nanorods regulated their head-to-tail association and aligned in the axial-centrifuged direction. The periodically transition from blue to orange of reflected colors was tunable via deposition layer adjustment. While the sensing coating was exposed to vapors of NH₃^.H₂O, H₂O, HCl and HAc, respectively, the color variation in the response process was irreversible at room temperature and highly dependent on vapor diffusion and chemical interface interaction. Consequently, HAc vapor presented the longest sensing transition of wavelength, whereas the alkaline NH₃^.H₂O displays a less recovery ratio than HAc and H₂O at room temperature. Under heating at 50°C, the sensed coatings could mostly be restored to their original state except HCl-etched one. Therefore, the naked-eyed qualitative detectability of vapors by nanocellulose could be realized by the divergence in color shift which is of great importance in chemical sensors.

Resonant laser printing of structural colors on high-index dielectric metasurfaces

Man-made structural colors, which originate from resonant interactions between visible light and manufactured nanostructures, are emerging as a solution for ink-free color printing. We show that non-iridescent structural colors can be conveniently produced by nanostructures made from high-index dielectric materials. Compared to plasmonic analogs, color surfaces with high-index dielectrics, such as germanium (Ge), have a lower reflectance, yielding a superior color contrast. Taking advantage of band-to-band absorption in Ge, we laser-postprocess Ge color metasurfaces with morphology-dependent resonances. Strong on-resonance energy absorption under pulsed laser irradiation locally elevates the lattice temperature (exceeding 1200 K) in an ultrashort time scale (1 ns). This forms the basis for resonant laser printing, where rapid melting allows for surface energy-driven morphology changes with associated modification of color appearance. Laser-printable high-index dielectric color metasurfaces are scalable to a large area and open a new paradigm for printing and decoration with nonfading and vibrant colors.

Filling schemes of silver dots inkjet-printed on pixelated nanostructured surfaces

Recently, our group demonstrated an inkjet-based technique to enable high-throughput, versatile and full-colour printing of structural colours on generic pixelated nanostructures, termed as molded ink on nanostructured surfaces. The printed colours are controlled by the area of printed silver on the pixelated red, green and blue polymer nanostructure arrays. This paper investigates the behaviour of jetted silver ink droplets on nanostructured surfaces and the microscale dot patterns implemented during printing process, for achieving accurate and consistent colours in the printed images. The surface wettability and the schemes of filling silver dots inside the subpixels are crucial to the quality of printed images. Several related concepts and definitions are introduced, such as filling ratio, full dots per subpixel (DPSP), number of printable colours, colour leaking and dot merging. In our experiments, we first chemically modified the surface to control the wettability and dot size. From each type of modified surface, various filling schemes were experimented and the printed results were evaluated with comprehensive considerations on the number of printable colours and the negative effects of colour leaking and dot merging. Rational selection of the best filling scheme resulted in a 2-line filling scheme using 20 μm dot spacing and line spacing capable of printing 9261 different colours with 121 pixel per inch display resolution, on low-wettability surface. This study is of vital importance for scaling up the printing technique in industrial applications and provides meaningful insights for inkjet-printing on nanostructures.

Influence of the Particle Concentration and Marangoni Flow on the Formation of Cellulose Nanocrystal Films

Cellulose nanocrystals (CNCs), ribbonlike crystalline nanoparticles, are a biobased material that can be a great alternative to obtaining films with tunable optical properties. Iridescent and light-diffracting films can be readily obtained via the drying of a suspension of these cellulose nanocrystals. The characteristics of the particle deposition process together with the self-assembly in the precluding suspension has a direct effect on the optical properties of the obtained films. Particle deposition onto a substrate is affected by the flow dynamics inside sessile droplets and usually yields a ring-shaped deposition pattern commonly referred to as the coffee-ring effect. We set out to measure and describe the drying kinetics under different conditions. We found that the Marangoni flow inside the droplet was too small to counteract the capillary flow that deposits CNCs at the edges, resulting in the coffee-ring effect, irrespective of the atmospheric humidity. By varying the amount of ethanol in the atmosphere, we were able to find a balance between (1) colloidal stability in the droplet, which is reduced by ethanol diffusion into the droplet, and (2) increasing Marangoni flow relative to capillary flow inside the droplet by changing the droplet surface tension. We could thus make iridescent films with a uniform thickness.

Sub-micron silk fibroin film with high humidity sensibility through color changing

A Sub-micron silk fibroin film with high humidity sensibility through color changing is achieved by spin-coating fibroin aqueous solution, and it can be potentially applied for low-cost and fast humidity detection, as well as anti-counterfeit labels.

Reversible sol–gel–sol medium for enzymatic optical biosensors

In this paper we for the first time report a reversible sol–gel–sol approach to obtain optical enzymatic biosensors with improved enzyme stability and good sensitivity by using desktop inkjet printing.

Molding Inkjetted Silver on Nanostructured Surfaces for High-Throughput Structural Color Printing

Inkjet printing of silver ink has been widely used to print conductive patterns in flexible electronic devices, and the printed patterns are commonly known to be colorless. We demonstrate that by printing a single type of ordinary silver nanoparticle ink on top of a substrate patterned with polymer nanostructures, the printed silver is molded by the nanostructures and gains robust structural colors. The colors are tunable by varying the geometries of nanostructures, and a broad range of visual colors can be achieved by mixing the red, green, and blue colors displayed from silver dots printed on different nanostructures. Such mechanism can enable full-color, scalable, high-throughput, versatile, and cost-effective printing of structural color images for regular publishing and displaying purposes. In experiments, we implemented a transparent polymer substrate patterned with diffractive nanostructure arrays to print full-color images. The printed images display color-shifting optically variable effects useful for security and authentication applications that demand customizable anticounterfeiting features.

Inkjet printing of TiO2/AlOOH heterostructures for the formation of interference color images with high optical visibility

This paper describes a practical approach for the fabrication of highly visible interference color images using sol-gel ink technique and a common desktop inkjet printer. We show the potential of titania-boehmite inks for the production of optical heterostructures on various surfaces, which after drying on air produce optical solid layers with low and high refractive index. The optical properties of the surface heterostructures were adjusted following the principles of antireflection coating resulting in the enhancement of the interference color optical visibility of the prints by as much as 32%. Finally, the presented technique was optimized following the insights into the mechanisms of the drop-surface interactions and the drop-on-surface coalescence to make it suitable for the production of even thickness coatings suitable for printing at a large scale. We propose that the technology described herein is a promising new green and sustainable approach for color printing.

Rapid Laser Printing of Paper-Based Multilayer Circuits

Laser printing has been widely used in daily life, and the fabricating process is highly efficient and mask-free. Here we propose a laser printing process for the rapid fabrication of paper-based multilayer circuits. It does not require wetting of the paper, which is more competitive in manufacturing paper-based circuits compared to conventional liquid printing process. In the laser printed circuits, silver nanowires (Ag-NWs) are used as conducting material for their excellent electrical and mechanical properties. By repeating the printing process, multilayer three-dimensional (3D) structured circuits can be obtained, which is quite significant for complex circuit applications. In particular, the performance of the printed circuits can be exactly controlled by varying the process parameters including Ag-NW content and laminating temperature, which offers a great opportunity for rapid prototyping of customized products with designed properties. A paper-based high-frequency radio frequency identification (RFID) label with optimized performance is successfully demonstrated. By adjusting the laminating temperature to 180 °C and the top-layer Ag-NW areal density to 0.3 mg cm(-2), the printed RFID antenna can be conjugately matched with the chip, and a big reading range of ∼12.3 cm with about 2.0 cm over that of the commercial etched Al antenna is achieved. This work provides a promising approach for fast and quality-controlled fabrication of multilayer circuits on common paper and may be enlightening for development of paper-based devices.