Technologies for nanofluidic systems: top-down vs. bottom-up--a review

Recent Advances in Graphene-Enabled Materials for Photovoltaic Applications: A Comprehensive Review

Graphene's two-dimensional structural arrangement has sparked a revolutionary transformation in the domain of conductive transparent devices, presenting a unique opportunity in the renewable energy sector. This comprehensive Review critically evaluates the most recent advances in graphene production and its employment in solar cells, focusing on dye-sensitized, organic, and perovskite devices for bulk heterojunction (BHJ) designs. This comprehensive investigation discovered the following captivating results: graphene integration resulted in a notable 20.3% improvement in energy conversion rates in graphene-perovskite photovoltaic cells. In comparison, BHJ cells saw a laudable 10% boost. Notably, graphene's 2D internal architecture emerges as a protector for photovoltaic devices, guaranteeing long-term stability against various environmental challenges. It acts as a transportation facilitator and charge extractor to the electrodes in photovoltaic cells. Additionally, this Review investigates current research highlighting the role of graphene derivatives and their products in solar PV systems, illuminating the way forward. The study elaborates on the complexities, challenges, and promising prospects underlying the use of graphene, revealing its reflective implications for the future of solar photovoltaic applications.

Nanoparticle Assembly: From Self-Organization to Controlled Micropatterning for Enhanced Functionalities

Nanoparticles form long-range micropatterns via self-assembly or directed self-assembly with superior mechanical, electrical, optical, magnetic, chemical, and other functional properties for broad applications, such as structural supports, thermal exchangers, optoelectronics, microelectronics, and robotics. The precisely defined particle assembly at the nanoscale with simultaneously scalable patterning at the microscale is indispensable for enabling functionality and improving the performance of devices. This article provides a comprehensive review of nanoparticle assembly formed primarily via the balance of forces at the nanoscale (e.g., van der Waals, colloidal, capillary, convection, and chemical forces) and nanoparticle-template interactions (e.g., physical confinement, chemical functionalization, additive layer-upon-layer). The review commences with a general overview of nanoparticle self-assembly, with the state-of-the-art literature review and motivation. It subsequently reviews the recent progress in nanoparticle assembly without the presence of surface templates. Manufacturing techniques for surface template fabrication and their influence on nanoparticle assembly efficiency and effectiveness are then explored. The primary focus is the spatial organization and orientational preference of nanoparticles on non-templated and pre-templated surfaces in a controlled manner. Moreover, the article discusses broad applications of micropatterned surfaces, encompassing various fields. Finally, the review concludes with a summary of manufacturing methods, their limitations, and future trends in nanoparticle assembly.

Comprehensive advances in the synthesis, fluorescence mechanism and multifunctional applications of red-emitting carbon nanomaterials

Red emitting fluorescent carbon nanomaterials have drawn significant scientific interest in recent years due to their high quantum yield, water-dispersibility, photostability, biocompatibility, ease of surface functionalization, low cost and eco-friendliness. The red emissive characteristics of fluorescent carbon nanomaterials generally depend on the carbon source, reaction time, synthetic approach/methodology, surface functional groups, average size, and other reaction environments, which directly or indirectly help to achieve red emission. The importance of several factors to achieve red fluorescent carbon nanomaterials is highlighted in this review. Numerous plausible theories have been explained in detail to understand the origin of red fluorescence and tunable emission in these carbon-based nanostructures. The above advantages and fluorescence in the red region make them a potential candidate for multifunctional applications in various current fields. Therefore, this review focused on the recent advances in the synthesis approach, mechanism of fluorescence, and electronic and optical properties of red-emitting fluorescent carbon nanomaterials. This review also explains the several innovative applications of red-emitting fluorescent carbon nanomaterials such as biomedicine, light-emitting devices, sensing, photocatalysis, energy, anticounterfeiting, fluorescent silk, artificial photosynthesis, etc. It is hoped that by choosing appropriate methods, the present review can inspire and guide future research on the design of red emissive fluorescent carbon nanomaterials for potential advancements in multifunctional applications.

Synthesis of nanoparticles via microfluidic devices and integrated applications

In recent years, nanomaterials have attracted the research intervention of experts in the fields of catalysis, energy, biomedical testing, and biomedicine with their unrivaled optical, chemical, and biological properties. From basic metal and oxide nanoparticles to complex quantum dots and MOFs, the stable preparation of various nanomaterials has always been a struggle for researchers. Microfluidics, as a paradigm of microscale control, is a remarkable platform for online stable synthesis of nanomaterials with efficient mass and heat transfer in microreactors, flexible blending of reactants, and precise control of reaction conditions. We describe the process of microfluidic preparation of nanoparticles in the last 5 years in terms of microfluidic techniques and the methods of microfluidic manipulation of fluids. Then, the ability of microfluidics to prepare different nanomaterials, such as metals, oxides, quantum dots, and biopolymer nanoparticles, is presented. The effective synthesis of some nanomaterials with complex structures and the cases of nanomaterials prepared by microfluidics under extreme conditions (high temperature and pressure), the compatibility of microfluidics as a superior platform for the preparation of nanoparticles is demonstrated. Microfluidics has a potent integration capability to combine nanoparticle synthesis with real-time monitoring and online detection, which significantly improves the quality and production efficiency of nanoparticles, and also provides a high-quality ultra-clean platform for some bioassays.

Self-assembled micropillar arrays via near-field electrospinning

Self-assembly in near-field electrospinning is reported for the first time in this paper, which realized the conversion from two-dimensional planar printing to three-dimensional (3D) structures. Repeatedly stacked fibres formed a micropillar array structure (MPAS) with intervals on the deposition paths by adding carbonyl iron powder particles to a polyethylene oxide (PEO) solution. The growth process of the self-assembled MPAS is documented, and the mechanism of the self-assembled MPAS is proposed. In addition, the effects of substrate speed and injection speed on self-assembly were investigated. Electric field distribution simulations show that the electric field strength around the MPAS is enhanced by nearly ten times so that the micropillar can attract the jet for further deposition. Self-assembly can obtain MPASs with arbitrary paths on different substrates, and the interval of the MPAS can be controlled by using bulging substrates. Furthermore, a self-assembled MPAS has been successfully used to prepare mold cavities, which can be used to prepare MPASs of other materials. Due to their small feature size, large surface area and structural periodicity, micropillar arrays will have promising applications, such as hydrophobicity of surfaces and electrochemical detection. Self-assembly in near-field electrospinning can significantly reduce the preparation cost of an MPAS and provide new processes and ideas.

Integrated Glass Microfluidics with Electrochemical Nanogap Electrodes

We present a framework for the fabrication of chip-based electrochemical nanogap sensors integrated with microfluidics. Instead of polydimethylsiloxane (PDMS), SU-8 aided adhesive bonding of silicon and glass wafers is used to implement parallel flow control. The fabrication process permits wafer-scale production with high throughput and reproducibility. Additionally, the monolithic structures allow simple electrical and fluidic connections, alleviating the need for specialized equipment. We demonstrate the utility of these flow-incorporated nanogap sensors by performing redox cycling measurements under laminar flow conditions.

Towards atom manufacturing with framework nucleic acids

Atom manufacturing has become a blooming frontier direction in the field of material and chemical science in recent years, focusing on the fabrication of functional materials and devices with individual atoms or with atomic precision. Framework nucleic acids (FNAs) refer to nanoscale nucleic acid framework structures with novel properties distinct from those of conventional nucleic acids. Due to their ability to be precisely positioned and assembled at the nanometer or even atomic scale, FNAs are ideal materials for atom manufacturing. They hold great promise for the bottom-up construction of electronic devices by precisely arranging and integrating building blocks with atomic or near-atomic precision. In this review, we summarize the progress of atom manufacturing based on FNAs. We begin by introducing the atomic-precision construction of FNAs and the intrinsic electrical properties of DNA molecules. Then, we describe various approaches for the fabrication of FNAs templated materials and devices, which are classified as conducting, insulating, or semiconducting based on their electrical properties. We highlight the role of FNAs in the fabrication of functional electronic devices with atomic precision, as well as the challenges and opportunities for atom manufacturing with FNAs.

Translational Nanomedicines Across Human Reproductive Organs Modeling on Microfluidic Chips: State-of-the-Art and Future Prospects

Forecasting the consequence of nanoparticles (NPs) and therapeutically significant molecules before materializing for human clinical trials is a mainstay for drug delivery and screening processes. One of the noteworthy obstacles that has prevented the clinical translation of NP-based drug delivery systems and novel drugs is the lack of effective preclinical platforms. As a revolutionary technology, the organ-on-a-chip (OOC), a coalition of microfluidics and tissue engineering, has surfaced as an alternative to orthodox screening platforms. OOC technology recapitulates the structural and physiological features of human organs along with intercommunications between tissues on a chip. The current review discusses the concept of microfluidics and confers cutting-edge fabrication processes for chip designing. We also outlined the advantages of microfluidics in analyzing NPs in terms of characterization, transport, and degradation in biological systems. The review further elaborates the scope and research on translational nanomedicines in human reproductive organs (testis, placenta, uterus, and menstrual cycle) by taking the advantages offered by microfluidics and shedding light on their potential future implications. Finally, we accentuate the existing challenges for clinical translation and scale-up dynamics for microfluidics chips and emphasize its future perspectives.

A Review on Low-Dimensional Nanomaterials: Nanofabrication, Characterization and Applications

The development of modern cutting-edge technology relies heavily on the huge success and advancement of nanotechnology, in which nanomaterials and nanostructures provide the indispensable material cornerstone. Owing to their nanoscale dimensions with possible quantum limit, nanomaterials and nanostructures possess a high surface-to-volume ratio, rich surface/interface effects, and distinct physical and chemical properties compared with their bulk counterparts, leading to the remarkably expanded horizons of their applications. Depending on their degree of spatial quantization, low-dimensional nanomaterials are generally categorized into nanoparticles (0D); nanorods, nanowires, and nanobelts (1D); and atomically thin layered materials (2D). This review article provides a comprehensive guide to low-dimensional nanomaterials and nanostructures. It begins with the classification of nanomaterials, followed by an inclusive account of nanofabrication and characterization. Both top-down and bottom-up fabrication approaches are discussed in detail. Next, various significant applications of low-dimensional nanomaterials are discussed, such as photonics, sensors, catalysis, energy storage, diverse coatings, and various bioapplications. This article would serve as a quick and facile guide for scientists and engineers working in the field of nanotechnology and nanomaterials.

Directed Assembly of Nanomaterials for Making Nanoscale Devices and Structures: Mechanisms and Applications

Nanofabrication has been utilized to manufacture one-, two-, and three-dimensional functional nanostructures for applications such as electronics, sensors, and photonic devices. Although conventional silicon-based nanofabrication (top-down approach) has developed into a technique with extremely high precision and integration density, nanofabrication based on directed assembly (bottom-up approach) is attracting more interest recently owing to its low cost and the advantages of additive manufacturing. Directed assembly is a process that utilizes external fields to directly interact with nanoelements (nanoparticles, 2D nanomaterials, nanotubes, nanowires, etc.) and drive the nanoelements to site-selectively assemble in patterned areas on substrates to form functional structures. Directed assembly processes can be divided into four different categories depending on the external fields: electric field-directed assembly, fluidic flow-directed assembly, magnetic field-directed assembly, and optical field-directed assembly. In this review, we summarize recent progress utilizing these four processes and address how these directed assembly processes harness the external fields, the underlying mechanism of how the external fields interact with the nanoelements, and the advantages and drawbacks of utilizing each method. Finally, we discuss applications made using directed assembly and provide a perspective on the future developments and challenges.

Recent advances in biomimetic soft robotics: fabrication approaches, driven strategies and applications

Compared to traditional rigid-bodied robots, soft robots are constructed using physically flexible/elastic bodies and electronics to mimic nature and enable novel applications in industry, healthcare, aviation, military, etc. Recently, the fabrication of robots on soft matter with great flexibility and compliance has enabled smooth and sophisticated 'multi-degree-of-freedom' 3D actuation to seamlessly interact with humans, other organisms and non-idealized environments in a highly complex and controllable manner. Herein, we summarize the fabrication approaches, driving strategies, novel applications, and future trends of soft robots. Firstly, we introduce the different fabrication approaches to prepare soft robots and compare and systematically discuss their advantages and disadvantages. Then, we present the actuator-based and material-based driving strategies of soft robotics and their characteristics. The representative applications of soft robotics in artificial intelligence, medicine, sensors, and engineering are summarized. Also, some remaining challenges and future perspectives in soft robotics are provided. This work highlights the recent advances of soft robotics in terms of functional material selection, structure design, control strategies and biomimicry, providing useful insights into the development of next-generation functional soft robotics.

Adhesive lift method for patterning arbitrary-shaped thin ion-selective films in micro/nanofluidic device

Micro/nanofluidic platforms with nanoporous films have been utilized as research tools for studying electrokinetic phenomena occurring not only in macro-scale systems such as electro-desalination but also in micro-scale systems such as bio-molecular preconcentrators. However, due to the limitations of fabrication techniques, studies with nanoporous films are mainly limited to vary the physicochemical properties of the films such as surface charge and pore size, despite the enormous effect of the membrane morphology on the phenomena that is to be expected. Therefore, we propose an economic and feasible nanofabrication method called the "adhesive lift method" for patterning thin arbitrarily-shaped nanoporous film to integrate it into micro/nanofluidic platforms. The conformal patterning of the nanoporous films (Nafion or poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS) in this work) was accomplished with spin coating, oxygen plasma treatment and the "adhesive lift technique". Using the fabricated platforms, the initiation of ion concentration polarization along the film with various shapes was demonstrated. In particular, various electrokinetic characteristics of overlimiting conductance depending on the length scale of the microchannels were successfully demonstrated. Therefore, the presented adhesive lift method would provide platforms which can nearly mimic practical macro-scale fluidic systems so that the method would be very useful for studying various electrokinetic phenomena inside it.

Defect-mediated ripening of core-shell nanostructures

Understanding nanostructure ripening mechanisms is desirable for gaining insight on the growth and potential applications of nanoscale materials. However, the atomic pathways of nanostructure ripening in solution have rarely been observed directly. Here, we report defect-mediated ripening of Cd-CdCl₂ core-shell nanoparticles (CSN) revealed by in-situ atomic resolution imaging with liquid cell transmission electron microscopy. We find that ripening is initiated by dissolution of the nanoparticle with an incomplete CdCl₂ shell, and that the areas of the Cd core that are exposed to the solution are etched first. The growth of the other nanoparticles is achieved by generating crack defects in the shell, followed by ion diffusion through the cracks. Subsequent healing of crack defects leads to a highly crystalline CSN. The formation and annihilation of crack defects in the CdCl₂ shell, accompanied by disordering and crystallization of the shell structure, mediate the ripening of Cd-CdCl₂ CSN in the solution.

Understanding the ripening of core-shell nanostructures is challenging. Here, the authors use liquid cell transmission electron microscopy to show that the atomic ripening pathway for Cd-CdCl₂ core-shell nanoparticles is mediated by crack defects.

Edge-Topological Regulation for in Situ Fabrication of Bridging Nanosensors

In situ fabrication of well-defined bridging nanostructures is an interesting and unique approach to three-dimensionally design nanosensor structures, which are hardly attainable by other methods. Here, we demonstrate the significant effect of edge-topological regulation on in situ fabrication of ZnO bridging nanosensors. When employing seed layers with a sharp edge, which is a well-defined structure in conventional lithography, the bridging angles and electrical resistances between two opposing electrodes were randomly distributed. The stochastic nature of bridging growth direction at the sharp edges inherently causes such unintentional variation of structural and electrical properties. We propose an edgeless seed layer structure using a two-layers resist method to solve the above uncontrollability of bridging nanosensors. Such bridging nanosensors not only substantially improved the uniformity of structural and electrical properties between two opposing electrodes but also significantly enhanced the sensing responses for NO₂ with the smaller variance and the lower limit of detection via in situ controlled electrical contacts.

Microfluidics and surface-enhanced Raman spectroscopy, a win-win combination?

With the continuous development in nanoscience and nanotechnology, analytical techniques like surface-enhanced Raman spectroscopy (SERS) render structural and chemical information of a variety of analyte molecules in ultra-low concentration. Although this technique is making significant progress in various fields, the reproducibility of SERS measurements and sensitivity towards small molecules are still daunting challenges. In this regard, microfluidic surface-enhanced Raman spectroscopy (MF-SERS) is well on its way to join the toolbox of analytical chemists. This review article explains how MF-SERS is becoming a powerful tool in analytical chemistry. We critically present the developments in SERS substrates for microfluidic devices and how these substrates in microfluidic channels can improve the SERS sensitivity, reproducibility, and detection limit. We then introduce the building materials for microfluidic platforms and their types such as droplet, centrifugal, and digital microfluidics. Finally, we enumerate some challenges and future directions in microfluidic SERS. Overall, this article showcases the potential and versatility of microfluidic SERS in overcoming the inherent issues in the SERS technique and also discusses the advantage of adding SERS to the arsenal of microfluidics.

Recent progress and perspectives in applications of 2-methacryloyloxyethyl phosphorylcholine polymers in biodevices at small scales

Bioinspired materials have attracted attention in a wide range of fields. Among these materials, a polymer family containing 2-methacryloyloxyethyl phosphorylcholine (MPC), which has a zwitterionic phosphorylcholine headgroup inspired by the...

Controlled Growth of Organosilane Micropatterns on Hydrophilic and Hydrophobic Surfaces Templated by Vacuum Ultraviolet Photolithography

In this report, micropatterns of (3-aminopropyl)trimethoxysilane (APTMS) were developed on hydrophilic and hydrophobic surfaces after patterning using 172 nm vacuum ultraviolet (VUV) photolithography. Self-assembled monolayers (SAMs) formed on Si substrates through UV hydrosilylation of 1-hexadecene (HD) and 10-undecenoic acid (UDA) were used as hydrophilic and hydrophobic surfaces, respectively. For templating the HD- and UDA-SAMs, the VUV light was exposed to HD- and UDA-SAMs from the slits of photomasks in atmospheric and evacuated environments, respectively. Various oxygenated groups were generated at the exposed domains of HD-SAM, while the COOH groups were trimmed from the irradiated domains of UDA-SAM. The APTMS molecules were immobilized on the domains that were terminated by oxygenated groups after chemical vapor deposition (CVD). The thicknesses of the developed APTMS micropatterns increased significantly by raising the CVD temperature and in the presence of ambient air in the CVD Teflon container as well. The increase in thicknesses was ascribed to the formation of APTMS multilayers, which were mediated by H₃N⁺ ions. Also, the developed APTMS micropatterns on the UDA-SAM patterned by VUV light irradiation in a high-vacuum environment (HV-VUV) were thicker than those on the VUV/(O) patterned HD-SAM due to the presence of inactive oxygenated groups at the surface of VUV/(O)-terminated domains of HD-SAM such as COO-C and C-O-C groups. The presence of water or ambient air facilitated the silane coupling between the silyl groups with the oxygenated and amino groups The combination of VUV photolithography and the CVD method with control of the conditions would enable us to control the thicknesses and shapes of the developed APTMS micropatterns. These findings illustrate the applicability of VUV photolithography for templating hydrophobic and hydrophobic surfaces toward the development of organosilane architectures, which can be feasible for several applications.

Toward the Development of Rapid, Specific, and Sensitive Microfluidic Sensors: A Comprehensive Device Blueprint

Recent advances in nano/microfluidics have led to the miniaturization of surface-based chemical and biochemical sensors, with applications ranging from environmental monitoring to disease diagnostics. These systems rely on the detection of analytes flowing in a liquid sample, by exploiting their innate nature to react with specific receptors immobilized on the microchannel walls. The efficiency of these systems is defined by the cumulative effect of analyte detection speed, sensitivity, and specificity. In this perspective, we provide a fresh outlook on the use of important parameters obtained from well-characterized analytical models, by connecting the mass transport and reaction limits with the experimentally attainable limits of analyte detection efficiency. Specifically, we breakdown when and how the operational (e.g., flow rates, channel geometries, mode of detection, etc.) and molecular (e.g., receptor affinity and functionality) variables can be tailored to enhance the analyte detection time, analytical specificity, and sensitivity of the system (i.e., limit of detection). Finally, we present a simple yet cohesive blueprint for the development of high-efficiency surface-based microfluidic sensors for rapid, sensitive, and specific detection of chemical and biochemical analytes, pertinent to a variety of applications.

Micro/nanotechnology-inspired rapid diagnosis of respiratory infectious diseases

Humans have suffered from a variety of infectious diseases since a long time ago, and now a new infectious disease called COVID-19 is prevalent worldwide. The ongoing COVID-19 pandemic has led to research of the effective methods of diagnosing respiratory infectious diseases, which are important to reduce infection rate and help the spread of diseases be controlled. The onset of COVID-19 has led to the further development of existing diagnostic methods such as polymerase chain reaction, reverse transcription polymerase chain reaction, and loop-mediated isothermal amplification. Furthermore, this has contributed to the further development of micro/nanotechnology-based diagnostic methods, which have advantages of high-throughput testing, effectiveness in terms of cost and space, and portability compared to conventional diagnosis methods. Micro/nanotechnology-based diagnostic methods can be largely classified into (1) nanomaterials-based, (2) micromaterials-based, and (3) micro/nanodevice-based. This review paper describes how micro/nanotechnologies have been exploited to diagnose respiratory infectious diseases in each section. The research and development of micro/nanotechnology-based diagnostics should be further explored and advanced as new infectious diseases continue to emerge. Only a handful of micro/nanotechnology-based diagnostic methods has been commercialized so far and there still are opportunities to explore.

A Microfabrication Method of PCL Scaffolds for Tissue Engineering by Simultaneous Two PDMS Molds Replication

Not very far away, "tissue engineering" will become one of the most important branches of medical science for curing many types of diseases. This branch needs the cooperation of a wide range of sciences like medicine, chemistry, cellular biology, and genetic and mechanical engineering. Different parameters affect the final produced tissue, but the most important one is the quality and biocompatibility of the scaffold with the desired tissue which can provide the functionality of "native ECM" as well. The quality of the scaffold is directly dependent on its materials, design, and method of fabrication. As to the design and fabrication, there are two main categories: (a) random microporosity such as phase separation, electrospinning, and fused deposition modeling (3D printing) and (b) designed microporosity mostly achievable by stereo lithography and soft lithography. The method of fabrication implemented in this research is a novel method in soft lithography employing a type of "replica molding" with one pair of polydimethylsiloxane (PDMS) molds in contrast to traditional replica molding with just one single mold. In this operation, the solution of polycaprolactone in chloroform is initially prepared, and one droplet of the solution is placed between the molds while a preset pressure is applied to maintain the molds tightly together during the solidification of the polymer layer and vaporization of the solvent. Thus, a perfect warp and woof pattern is created. In this research, it has been approved that this is a feasible method for creating complex patterns and simple straight fiber patterns with different spacings and pore sizes. Cell attachment and migration was studied to find the optimum pore size. It was shown that the small pore size improves the cells' adhesion while reducing cell migration capability within the scaffold.

Metal-Free Fabrication of Fused Silica Extended Nanofluidic Channel to Remove Artifacts in Chemical Analysis

In microfluidics, especially in nanofluidics, nanochannels with functionalized surfaces have recently attracted attention for use as a new tool for the investigation of chemical reaction fields. Molecules handled in the reaction field can reach the single-molecule level due to the small size of the nanochannel. In such surroundings, contamination of the channel surface should be removed at the single-molecule level. In this study, it was assumed that metal materials could contaminate the nanochannels during the fabrication processes; therefore, we aimed to develop metal-free fabrication processes. Fused silica channels 1000 nm-deep were conventionally fabricated using a chromium mask. Instead of chromium, electron beam resists more than 1000 nm thick were used and the lithography conditions were optimized. From the results of optimization, channels with 1000 nm scale width and depth were fabricated on fused silica substrates without the use of a chromium mask. In nanofluidic experiments, an oxidation reaction was observed in a device fabricated by conventional fabrication processes using a chromium mask. It was found that Cr⁶⁺ remained on the channel surfaces and reacted with chemicals in the liquid phase in the extended nanochannels; this effect occurred at least to the micromolar level. In contrast, the device fabricated with metal-free processes was free of artifacts induced by the presence of chromium. The developed fabrication processes and results of this study will be a significant contribution to the fundamental technologies employed in the fields of microfluidics and nanofluidics.

Low-Dimensional Nanostructures Based on Cobalt Oxide (Co3O4) in Chemical-Gas Sensing

Highly sensitive, stable, low production costs, together with easy maintenance and portability, sensors are ever most demanded nowadays for monitoring and quantification of hazardous chemicals/gases in the environment. The utilization of one dimensional (1D) metal oxide nano structured chemical/gas sensors for environmental monitoring is vastly investigated because of their superior surface to volume ratio, stability, and low production costs, to provide information on the presence of chemical species. Several outstanding attempts have been pursued investigating 1D nano structures of Co3O4 over the past decades as an active material for chemical analytes detection owing to its superior catalytic effect together with its excellent stability. This article reviews the state-of-the-art of growth and characterization of Co3O4 1D nano structures and their functional characterization as chemical/gas sensors. Moreover, fundamental concepts and characteristic features, that enhance the key performances of chemical/gas sensors, are discussed. Finally, challenges and prospective for growth and fabrication of 1D Co3O4 chemical/gas sensors are discussed.

Lithographic Processes for the Scalable Fabrication of Micro- and Nanostructures for Biochips and Biosensors

Since the early 2000s, extensive research has been performed to address numerous challenges in biochip and biosensor fabrication in order to use them for various biomedical applications. These biochips and biosensor devices either integrate biological elements (e.g., DNA, proteins or cells) in the fabrication processes or experience post fabrication of biofunctionalization for different downstream applications, including sensing, diagnostics, drug screening, and therapy. Scalable lithographic techniques that are well established in the semiconductor industry are now being harnessed for large-scale production of such devices, with additional development to meet the demand of precise deposition of various biological elements on device substrates with retained biological activities and precisely specified topography. In this review, the lithographic methods that are capable of large-scale and mass fabrication of biochips and biosensors will be discussed. In particular, those allowing patterning of large areas from 10 cm² to m², maintaining cost effectiveness, high throughput (>100 cm² h^-1), high resolution (from micrometer down to nanometer scale), accuracy, and reproducibility. This review will compare various fabrication technologies and comment on their resolution limit and throughput, and how they can be related to the device performance, including sensitivity, detection limit, reproducibility, and robustness.

Vapor-phase production of nanomaterials

The synthesis of nanomaterials, with characteristic dimensions of 1 to 100 nm, is a key component of nanotechnology. Vapor-phase synthesis of nanomaterials has numerous advantages such as high product purity, high-throughput continuous operation, and scalability that have made it the dominant approach for the commercial synthesis of nanomaterials. At the same time, this class of methods has great potential for expanded use in research and development. Here, we present a broad review of progress in vapor-phase nanomaterial synthesis. We describe physically-based vapor-phase synthesis methods including inert gas condensation, spark discharge generation, and pulsed laser ablation; plasma processing methods including thermal- and non-thermal plasma processing; and chemically-based vapor-phase synthesis methods including chemical vapor condensation, flame-based aerosol synthesis, spray pyrolysis, and laser pyrolysis. In addition, we summarize the nanomaterials produced by each method, along with representative applications, and describe the synthesis of the most important materials produced by each method in greater detail.

Tunable Nanochannels Connected in Series for Dynamic Control of Multiple Concentration-Polarization Layers and Preconcentrated Molecule Plugs

Integration of ionic permselective medium (e.g., nanochannels, membranes) within microfluidic channels has been shown to enable on-chip desalination, sample purification, bioparticle sorting, and biomolecule concentration for enhanced detection sensitivity. However, the ion-permselective mediums are generally of fixed properties and cannot be dynamically tuned. Here we study a microfluidic device consisting of an array of individually addressable elastic membranes connected in series on top of a single microfluidic channel that can be deformed to locally reduce the channel cross-section into a nanochannel. Dynamic tunability of the ion-permselective medium, as well as controllability of its location and ionic permselectivity, introduces a new functionality to microfluidics-based lab-on-a-chip devices, for example, dynamic localization of preconcentrated biomolecule plugs at different sensing regions for multiplex detection. Moreover, the ability to simultaneously form a series of preconcentrated plugs at desired locations increases parallelization of the system and the trapping efficiency of target analytes.

Effects of the Interfacial Modeling Approach on Equilibrium Calculations of Slip Length for Nanoconfined Water in Carbon Slits

In this investigation, equilibrium molecular dynamics simulations were conducted to assess the influence of the interface modeling approach on the calculation of hydrodynamic slip in carbon nanochannels. A Green-Kubo formalism was implemented for the calculation of the slip length in water confined by graphite layers. The nonbonded interactions between solid and liquid atoms (interface models) were modeled using parameters optimized to represent the wetting behavior and adsorption energy curves from electronic structure calculations. Conventional carbon-oxygen-only interaction models were compared against comprehensive models able to represent the molecular-orientation-dependent energy of interaction. Quasi-universal relationships built under the premise of the slip length dependence on the water-graphite affinity and characterized by macroscopic wettability were critically assessed. It was found that the wetting behavior cannot fully characterize the hydrodynamic slip because interface models that produced the same surface wettability yielded different values of the friction coefficient. Alternatively, the density depletion length, used to characterize the interfacial liquid structuring and the availability of momentum carriers (interfacial water molecules), was able to accurately represent the slip length trends independently of the interface model. These findings reassert the importance of physically sound interface models to study interfacial transport properties and the need of reliable parameters and characterization procedures to support theoretical models that seek to unveil the inconsistencies in hydrodynamic slip calculations.

Advanced Top-Down Fabrication for a Fused Silica Nanofluidic Device

Nanofluidics have recently attracted significant attention with regard to the development of new functionalities and applications, and producing new functional devices utilizing nanofluidics will require the fabrication of nanochannels. Fused silica nanofluidic devices fabricated by top-down methods are a promising approach to realizing this goal. Our group previously demonstrated the analysis of a living single cell using such a device, incorporating nanochannels having different sizes (102-103 nm) and with branched and confluent structures and surface patterning. However, fabrication of geometrically-controlled nanochannels on the 101 nm size scale by top-down methods on a fused silica substrate, and the fabrication of micro-nano interfaces on a single substrate, remain challenging. In the present study, the smallest-ever square nanochannels (with a size of 50 nm) were fabricated on fused silica substrates by optimizing the electron beam exposure time, and the absence of channel breaks was confirmed by streaming current measurements. In addition, micro-nano interfaces between 103 nm nanochannels and 101 μm microchannels were fabricated on a single substrate by controlling the hydrophobicity of the nanochannel surfaces. A micro-nano interface for a single cell analysis device, in which a nanochannel was connected to a 101 μm single cell chamber, was also fabricated. These new fabrication procedures are expected to advance the basic technologies employed in the field of nanofluidics.

Seeding, Plating and Electrical Characterization of Gold Nanowires Formed on Self-Assembled DNA Nanotubes

Self-assembly nanofabrication is increasingly appealing in complex nanostructures, as it requires fewer materials and has potential to reduce feature sizes. The use of DNA to control nanoscale and microscale features is promising but not fully developed. In this work, we study self-assembled DNA nanotubes to fabricate gold nanowires for use as interconnects in future nanoelectronic devices. We evaluate two approaches for seeding, gold and palladium, both using gold electroless plating to connect the seeds. These gold nanowires are characterized electrically utilizing electron beam induced deposition of tungsten and four-point probe techniques. Measured resistivity values for 15 successfully studied wires are between 9.3 × 10-6 and 1.2 × 10-3 Ωm. Our work yields new insights into reproducible formation and characterization of metal nanowires on DNA nanotubes, making them promising templates for future nanowires in complex electronic circuitry.

A Peptide-Based Method for the Fabrication of 1D Rail-Like Nanoparticle Chains and 2D Nanoparticle Membranes: Higher-Order Self-Assembly

Functionalized histidine-rich peptide sequences were designed for the site-directed assembly of nanoparticles. TEM and AFM images shown that the peptides self-assembled into well-ordered nanofibrils at pH 7.2. The nanofibrils could lie parallel to one another and form membranes when the solution was acidic (pH 3.8) resulting from the hierarchical assembly of the nanofibrils in the direction of the peptide backbone. These peptide structures served as a template for nucleation and growth of Au nanocrystals. Further characterization showed that the Au nanocrystals grew on both sides of the nanofibrils, and a 1D system with a rail-like structure and a 2D membrane were synthesized after reduction with hydrazine hydrate at neutral and acidic pH values, respectively. The size and packing density of the Au nanocrystals were positively correlated with the incubation time of the Au ions. This approach can be extended further to the controlled synthesis of 1D and 2D architectures formed from metals, metal sulfides, and metal oxides in a low-cost and simple manner. Finally, the nanostructures could catalyze the reduction of p-nitrophenol with rate constants of 0.83±0.14 and 0.69±0.09 min^-1 for the 1D and 2D structures, respectively.

Microfluidic Formation of Honeycomb-Patterned Droplets Bounded by Interface Bilayers via Bimodal Molecular Adsorption

Assembled water-in-oil droplets bounded by lipid bilayers are used in synthetic biology as minimal models of cell tissue. Microfluidic devices successfully generate monodispersed droplets and assemble them via droplet interface bilayesr (DIB) formation. However, a honeycomb pattern of DIB-bounded droplets, similar to epithelial tissues, remains unrealized because the rapid DIB formation between the droplets hinders their ability to form the honeycomb pattern. In this paper, we demonstrate the microfluidic formation of a honeycomb pattern of DIB-bounded droplets using two surfactants with different adsorption rates on the droplet surface. A non-DIB forming surfactant (sorbitan monooleate, Span 80) was mixed with a lipid (1,2-dioleoyl-sn-glycero-3-phosphocholine, PC), whose adsorption rate on the droplet surface and saturated interfacial tension were lower than those of Span 80. By changing the surfactant composition, we established the conditions under which the droplets initially form a honeycomb pattern and subsequently adhere to each other via DIB formation to minimize the interfacial energy. In addition, the reconstituted membrane protein nanopores at the DIBs were able to transport molecules. This new method, using the difference in the adsorption rates of two surfactants, allows the formation of a honeycomb pattern of DIB-bounded droplets in a single step, and thus facilitates research using DIB-bounded droplet assemblies.

One-Dimensional Nanostructured Oxide Chemoresistive Sensors

Day by day, the demand for portable, low cost, and efficient chemical/gas-sensing devices is increasing due to worldwide industrial growth for various purposes such as environmental monitoring and health care. To fulfill this demand, nanostructured metal oxides can be used as active materials for chemical/gas sensors due to their high crystallinity, remarkable physical/chemical properties, ease of synthesis, and low cost. In particular, (1D) one-dimensional metal oxides nanostructures, such as nanowires, exhibit a fast response, selectivity, and stability due to their high surface-to-volume ratio, well-defined crystal orientations, controlled unidirectional electrical properties, and self-heating phenomenon. Moreover, with the availability of large-scale production methods for nanowire growth such as thermal oxidation and evaporation-condensation growth, the development of highly efficient, low cost, portable, and stable chemical sensing devices is possible. In the last two decades, tremendous advances have been achieved in 1D nanostructured gas sensors ever since the pioneering work by Comini on the development of a SnO₂ nanobelt for gas sensor applications in 2002, which is one such example from which many researchers began to explore the field of 1D-nanostructure-based chemical/gas sensors. The Sensor Laboratory (University of Brescia) has made major contributions to the field of metal oxide nanowire chemical/gas-sensing devices. Over the years, different metal oxides such as SnO₂, ZnO, WO₃, NiO, CuO, and their heterostructures have been grown for their nanowire morphology and successfully integrated into chemoresistive gas-sensing devices. Hence in this invited feature article, Sensor Laboratory research on the synthesis of metal oxide nanowires and novel heterostructures and their characterization and gas-sensing performance during exposure to different gas analytes has been presented. Moreover, some new strategies such as branched-like nanowire heterostructures and core-shell nanowire structures adopted to enhance the performance of nanowire-based chemical sensor are presented in detail.

Direct Fabrication of Freestanding and Patterned Nanoporous Junctions in a 3D Micro-Nanofluidic Device for Ion-Selective Transport

In the field of micro-nanofluidics, a freestanding configuration of a nanoporous junction is highly demanded to increase the design flexibility of the microscale device and the interfacial area between the nanoporous junction and microchannels, thereby improving the functionality and performance. This work first reports direct fabrication and incorporation of a freestanding nanoporous junction in a microfluidic device by performing an electrolyte-assisted electrospinning process to fabricate a freestanding nanofiber membrane and subsequently impregnating the nanofiber membrane with a nanoporous precursor material followed by a solidification process. This process also enables to readily control the geometry of the nanoporous junction depending on its application. By these advantages, vertically stacked 3D micro-nanofluidic devices with complex configurations are easily achieved. To demonstrate the broad applicability of this process in various research fields, a reverse electrodialysis-based energy harvester and an ion concentration polarization-based preconcentrator are produced. The freestanding Nafion-polyvinylidene fluoride nanofiber membrane (F-NPNM) energy harvester generates a high power (59.87 nW) owing to the enlarged interfacial area. Besides, 3D multiplexed and multi-stacked F-NPNM preconcentrators accumulate multiple preconcentrated plugs that can increase the operating sample volume and the degree of freedom of handling. Hence, the proposed process is expected to contribute to numerous research fields related to micro-nanofluidics in the future.

Wetting-Controlled Localized Placement of Surface Functionalities within Nanopores

In the context of sensing and transport control, nanopores play an essential role. Designing multifunctional nanopores and placing multiple surface functionalities with nanoscale precision remains challenging. Interface effects together with a combination of different materials are used to obtain local multifunctionalization of nanoscale pores within a model pore system prepared by colloidal templating. Silica inverse colloidal monolayers are first functionalized with a gold layer to create a hybrid porous architecture with two distinct gold nanostructures on the top surface as well as at the pore bottom. Using orthogonal silane- and thiol-based chemistry together with a control of the wetting state allows individual addressing of the different locations within each pore resulting in nanoscale localized functional placement of three different functional units. Ring-opening metathesis polymerization is used for inner silica-pore wall functionalization. The hydrophobized pores create a Cassie-Baxter wetting state with aqueous solutions of thiols, which enables an exclusive functionalization of the outer gold structures. In a third step, an ethanolic solution able to wet the pores is used to self-assemble a thiol-containing initiator at the pore bottom. Subsequent controlled radical polymerization provides functionalization of the pore bottom. It is demonstrated that the combination of orthogonal surface chemistry and controlled wetting states can be used for the localized functionalization of porous materials.

Pressure-Driven Micro-Casting for Electrode Fabrication and Its Applications in Wear Grain Detections

The microelectrode is an essential and vital part in microsensors that are largely used in industrial, chemical, and biological applications. To obtain desired microelectrodes in great quality, it is also of great necessity and significance to develop a robust method to fabricate the microelectrode pattern. This work developed a four-terminal differential microelectrode that aims at recognizing microparticles in fluids. This microelectrode pair consisted of a high height–width ratio microelectrode array fabricated using a pre-designed microelectrode pattern (a micro-scale channel) and melted liquid metal. The surface treatment of microelectrodes was also investigated to reveal its impacts on the continuality of melting metal and the quality of the fabricated microelectrode patterns. To evaluate the performance of micro-casting fabricated electrodes, a microfluidic device was packaged using a microelectrode layer and a flow layer. Then impedance cytometer experiments were performed using sample fluids with polymer particles in two different sizes in diameter (5 μm and 10 μm). In addition, engine oil was tested on the microelectrodes as complex samples. The number of abrasive particles in the engine oil can be collected from the developed microfluidic device for further analysis.

Plasmon-assisted nanojet lithography

The great barrier of optical diffraction significantly limits the resolution of photolithography and the manipulation of nano-objects by light. Here, through utilizing near-field enhancement and photothermal effects, we demonstrate the nanolithography of polystyrene (PS) films and self-jetting of gold nanoparticles (Au NPs), which happen simultaneously. This nanojet lithography creates subwavelength holes via the single step of laser irradiation. We find that the laser power input strongly affects the etching of PS films, as well as the movement of Au NPs, which is a synergic effect of photoablation (including both photothermal and photochemical aspects) and gas pushing. This facile approach not only generates polymer holes with sizes below the diffraction limit, but also provides an intriguing way to detach and move particles on surfaces via thermal jetting.

Standalone interferometry-based calibration of convex lens-induced confinement microscopy with nanoscale accuracy

Strongly confined environments (confined dimensions between 1-100 nm) represent unique challenges and opportunities for understanding and manipulating molecular behavior due to the significant effects of electric double layers, high surface-area to volume ratios, and other phenomena at the nanoscale. Convex Lens-induced Confinement (CLiC) can be used to analyze the dynamics of individual molecules or particles confined in a planar slit geometry with continuously varying gap thickness. We describe an interferometry-based method for precise measurement of the slit pore geometry. Specifically, this approach permitted accurate characterization of separation distances as small as 5 nm, with 1 nm precision, without a priori knowledge or assumptions about the contact geometry, as well as a greatly simplified experimental setup that required only a lens, coverslip, and inverted microscope. The interferometry-based measurement of gap height offered a distinct advantage over conventional fluorescent dye-based methods; e.g., accurate interferometric height measurements were made at low gap heights regardless of solution conditions, while the concentration of fluorescent dye was significantly impacted by solution conditions such as ionic strength or pH. The accuracy of the interferometric measurements was demonstrated by comparing the experimentally measured concentration of a charged fluorescent dye as a function of gap thickness with dye concentration profiles calculated using Debye-Hückel theory. Accurate characterization of nanoscale gap thickness will enable researchers to study a variety of practical and biologically relevant systems within the CLiC geometry.

Softening gold for elastronics

Gold, one of the noble metals, has played a significant role in human society throughout history. Gold's excellent electrical, optical and chemical properties make the element indispensable in maintaining a prosperous modern electronics industry. In the emerging field of stretchable electronics (elastronics), the main challenge is how to utilize these excellent material properties under various mechanical deformations. This review covers the recent progress in developing "softening" gold chemistry for various applications in elastronics. We systematically present material synthesis and design principles, applications, and challenges and opportunities ahead.

Wettability and Applications of Nanochannels

Wettability in nanochannels is of great importance for understanding many challenging problems in interface chemistry and fluid mechanics, and presents versatile applications including mass transport, catalysis, chemical reaction, nanofabrication, batteries, and separation. Recently, both molecular dynamic simulations and experimental measurements have been employed to study wettability in nanochannels. Here, wettability in three types of nanochannels comprising 1D nanochannels, 2D nanochannels, and 3D nanochannels is summarized both theoretically and experimentally. The proposed concept of "quantum-confined superfluid" for ultrafast mass transport in nanochannels is first introduced, and the mostly studied 1D nanochannels are reviewed from molecular simulation to water wettability, followed by reversible switching of water wettability via external stimuli (temperature and voltage). Liquid transport and two confinement strategies in nanochannels of melt wetting and liquid wetting are also included. Then, molecular simulation, water wettability, liquid transport, and confinement in nanochannels are introduced for 2D nanochannels and 3D nanochannels, respectively. Based on the wettability in nanochannels, broad applications of various nanochannels are presented. Finally, the perspective for future challenges in the wettability and applications of nanochannels is discussed.

Two-Dimensional Nanochannel Arrays Based on Flexible Montmorillonite Membranes

Two-dimensional (2D) nanochannel arrays are constructed by bottom-up reassembly of montmorillonite monolayers that are obtained by liquid-phase exfoliation of its layered crystals, and the as-constructed interstitial space between these monolayers is uniform and provides ions with nanoscale transport channels. Surface-charge-controlled ion transport behavior is observed through these nanochannels as the electrolyte concentration reduces to 10^-4 M at room temperature. Furthermore, the nanochannel structure remains even after 400 °C heat treatment, and nanofluidic devices based on the annealed nanochannel arrays still exhibit surface-charge-governed ion transport at low electrolyte concentrations. In addition, a drift-diffusion experiment is conducted to investigate the mobility ratio of cations/anions through the nanochannels with asymmetric bulk electrolyte concentrations, and the results show that the mobility of cations is about eight to nine times that of anions, which is consistent with the fact that the montmorillonite monolayers are negatively charged and the nanochannels are permselective. Last, ionic current rectification is observed in the nanofluidic system of asymmetric geometric shape, and rectification factors of ∼2.6 and ∼3.5 can be obtained in KCl and HCl electrolytes, respectively, at a bias between -1 and +1 V because of the asymmetric electrostatic potential through the nanochannels.

Four-Point Probe Electrical Measurements on Templated Gold Nanowires Formed on Single DNA Origami Tiles

Bottom-up nanofabrication is increasingly making use of self-assembled DNA to fabricate nanowires and potential integrated circuits, although yields of such electronic nanostructures are inadequate, as is the ability to reliably make electrical measurements on them. In this paper, we report improved yields and unprecedented conductivity measurements for Au nanowires created on DNA origami tile substrates. We created several different self-assembled Au nanowire arrangements on DNA origami tiles that are approximately 70 nm × 90 nm, through anisotropic growth of Au nanorods attached to specific sites. Modifications to the tile design increased yields of the final desired nanostructures as much as 6-fold. In addition, we measured the conductivity of Au nanowires created on these DNA tiles (∼130 nm long, 10 nm diameter, and 40 nm spacing between measurement points) with a four-point measurement technique that utilized electron beam induced metal deposition to form probe electrodes. These nanowires formed on single DNA origami tiles were electrically conductive, having resistivities as low as 4.24 × 10^-5 Ω m. This work demonstrates the creation and measurement of inorganic nanowires on single DNA origami tiles as a promising path toward future bottom-up fabrication of nanoelectronics.