Functional liquid droplets for analyte sensing and energy harvesting

Over the past century, rapid miniaturization of technologies has helped in the development of efficient, flexible, portable, robust, and compact applications with minimal wastage of materials. In this direction, of late, the usage of mesoscale liquid droplets has emerged as an alternative platform because of the following advantages: (i) a droplet is incompressible and at the same time deformable, (ii) interfacial area of a spherical droplet is minimum for a given amount of mass; and (iii) a droplet interface allows facile mass, momentum, and energy transfer. Subsequently, such attributes have aided towards the design of diverse droplet-based microfluidic technologies. For example, the microdroplets have been utilized as micro-reactors, colorimetric or electrochemical (EC) sensors, drug-delivery vehicles, and energy harvesters. Further, a number of recently reported lab-on-a-chip technologies exploit the motility, storage, and mixing capacities of the microdroplets. In view of this background, the review initiates discussion by highlighting the different attributes of the microdroplets such as size, shape, surface to volume ratio, wettability, and contact line. Thereafter, the effects of the surface or body forces on the properties of the droplets have been elaborated. Finally, the different aspects of such liquid droplet systems towards technological adaptations in health care, sensing, and energy harvesting have been presented. The review concludes with a tight summary on the potential avenues for further developments.

Hygro-responsive, Photo-decomposed Superoleophobic/Superhydrophilic Coating for On-Demand Oil-Water Separation

The superoleophobic/superhydrophilic material has attracted considerable interest due to the incomparable property of it for the oil-water separation. However, it is a challenge to make the prepared surface superoleophobic and superhydrophilic at the same time since the oleophobic surface tends to repel water. Herein, a hygro-responsive superoleophobic/superhydrophilic coating was fabricated by liquid-phase deposition of TiO₂ with perfluorooctanoic acid. The wettability of the coating could complete the transformation from superoleophobicity/superhydrophilicity to superhydrophobicity/superoleophilicity, both of which exhibit excellent selective superwettability under the air, underwater, salt, alkali, and acid conditions. The hygro-responsive coating can separate different types of oil-water mixtures, and the separation efficiency could be over 99% using different capillary forces acting on the oil and water phases before and after wettability transformation. Last but not least, long-chain perfluoroalkyl substances on the coating could be decomposed by UV irradiation, which could reduce the harm to the environment and human beings. It is anticipated that the developed superoleophobic/superhydrophilic coating provides a feasible solution for the application of oil-water separation.

Directional anchoring patterned liquid-infused superamphiphobic surfaces for high-throughput droplet manipulation

High-throughput experiments involving isolated droplets based on patterned superwettable surfaces are important for various applications related to biology, chemistry, and medicine, and they have attracted a large amount of interest. This paper provides a directional anchoring liquid-infused superamphiphobic surface (DAS), via combining concepts based on the droplet-anchoring behavior of beetle backs with patterned wettability, the directional adhesion of butterfly wings, and the slippery liquid-infused surfaces (SLISs) of pitcher plants. Regularly arranged ">"-shaped SLIS patterns were created on a superamphiphobic (SAM) background through ultrafast-laser-based technology. Improved directional anchoring abilities with a sliding angle difference of 77° were achieved; this is the largest sliding angle difference in a one-dimensional direction achieved using an artificial surface, to the best of the authors' knowledge. Thanks to the directional anchoring abilities, the DAS coupled droplet 'anchoring' and 'releasing' abilities. Furthermore, a high-throughput droplet manipulation device was designed, on which a micro-droplet array with a large number of droplets can be 'captured', 'transferred', or 'released' in a single step. With the addition of lubricant, the DAS can work continuously for even more than 30 cycles without cross-contamination between different droplets. The DAS also shows good stability under an ambient atmosphere and can maintain its functionality when manipulating corrosive droplets. The DAS and corresponding high-throughput droplet manipulation method are excellent candidates for practical applications.

Bioinspired wettable-nonwettable micropatterns for emerging applications

Superhydrophilic and superhydrophobic surfaces are prevalent in nature and have received tremendous attention due to their importance in both fundamental research and practical applications. With the high interdisciplinary research and great development of microfabrication techniques, artificial wettable-nonwettable micropatterns inspired by the water-collection behavior of desert beetles have been successfully fabricated. A combination of the two extreme states of superhydrophilicity and superhydrophobicity on the same surface precisely, wettable-nonwettable micropatterns possess unique functionalities, such as controllable superwetting, anisotropic wetting, oriented adhesion, and other properties. In this review, we briefly describe the methods for fabricating wettable-nonwettable patterns, including self-assembly, electrodeposition, inkjet printing, and photolithography. We also highlight some of the emerging applications such as water collection, controllable bioadhesion, cell arrays, microreactors, printing techniques, and biosensors combined with various detection methods. Finally, the current challenges and prospects of this renascent and rapidly developing field are proposed and discussed.

Recent Advances in Patterning Natural Polymers: From Nanofabrication Techniques to Applications

The development of a flexible and efficient strategy to precisely fabricate polymer patterns is increasingly significant for many research areas, especially for cell biology, pharmaceutical science, tissue engineering, soft photonics, and bioelectronics. Recent advances of patterning natural polymers using various nanofabrication techniques, including photolithography, electron-beam lithography, dip-pen nanolithography, inkjet printing, soft lithography, and nanoimprint lithography are discussed here. Integrating nanofabrication techniques with naturally derived macromolecules provides a feasible route for transforming these polymer materials into versatile and sophisticated devices while maintaining their intrinsic and excellent properties. Furthermore, the corresponding applications of these natural polymer patterns generated by the above techniques are elaborated. In the end, a summary of this promising research field is offered and an outlook for the future is given. It is expected that advances in precise spatial patterns of natural polymers would provide new avenues for various applications, such as tissue engineering, flexible electronics, biomedical diagnosis, and soft photonics.

On-demand mixing and dispersion in mini-pillar based microdroplets

The analysis and detection of ultra-trace biomarkers are often carried out in microliter droplets. Common stirring approaches have some difficulties in precise and contactless mixing and dispersion in microdroplets. In this work, an open mini-pillar-based platform that integrates with ultrasound units is developed to achieve contactless mixing and dispersion in microliter samples. On such a platform, mini-pillars can anchor microdroplets as individual microreactors, and each ultrasound unit can be remotely controlled to achieve on-demand contactless micro-stirring, which is also confirmed by mixing and dispersing of Fe3O4 nanoparticles (1 μm) in microdroplets (10 μL). Such on-demand high-throughput mixing and dispersion that integrates ultrasound mixing with microdroplet technology provides a potential robot-based platform for achieving high-throughput and ultra-trace biosensing in microliter droplets.

Customizing oil-wettability in air-without affecting extreme water repellency

Lotus leaf inspired superhydrophobic interfaces strongly repel the aqueous phase-but inherently display super-oil-affinity in air. However, superamphiphobic interfaces repel both the aqueous phase and the oil/oily phase strongly, due to their contact angles of above 150°. The fundamental criteria for optimizing such distinct super liquid wettabilities are different. Thus, in the past, distinct synthetic approaches were adopted to achieve these two different types of liquid-repellent interfaces for different prospective and relevant applications. Here, in this communication, a rapid and scalable spray deposition process is introduced for tailoring different oil-wettabilities in air, without perturbing the superhydrophobicity. An appropriate dilution of a reaction mixture of strategically selected two small molecules that readily reacted through the 1,4 conjugate addition reaction provided a facile basis for customizing oil wettability-starting from superoleophilicity to superoleophobicity, keeping intact the super water repellence. The synthesized superhydrophobic and superamphiphobic interfaces remained efficient for sustaining exposures of various practically relevant physical manipulations and abrasions and chemically complex aqueous phases. Furthermore, both the superhydrophobic and superamphiphobic interfaces were successfully extended for comparing the oil/water separation, anti-fouling and self-cleaning performances. Such a simple and common synthetic approach for preparing extremely water repellent interfaces that have differences in oil-wettability in air would be useful for practically relevant outdoor applications.

Bioinspired surfaces with wettability: biomolecule adhesion behaviors

Surface wettability plays an important role in regulating biomolecule adhesion behaviors. The biomolecule adhesion behaviors of superwettable surfaces have become an important topic as an important part of the interactions between materials and organisms. In addition to general research on the moderate wettability of surfaces, the studies of biomolecule adhesion behaviors extend to extreme wettability ranges such as superhydrophobic, superhydrophilic and slippery surfaces and attract both fundamental and practical interest. In this review, we summarize the recent studies on biomolecule adhesion behaviors on superwettable surfaces, especially superhydrophobic, superhydrophilic and slippery surfaces. The first part will focus on the influence of extreme wettability on cell adhesion behaviors. The second part will concentrate on the adhesion behaviors of biomacromolecules on superwettable surfaces including proteins and nucleic acids. Finally, the influences of wettability on small molecule adhesion behaviors on material surfaces have also been investigated. The mechanism of superwettable surfaces and their influences on biomolecule adhesion behaviors have been studied and highlighted.

Droplet array for open-channel high-throughput SERS biosensing

Open-channel and high throughput are two important aspects of clinical diagnosis, correlation biochemical analysis, cell culture techniques and food safety. Here, we propose the mini-pillar based array for open-channel and high-throughput SERS detection of miRNA. The polydimethylsiloxane (PDMS) mini-pillars are used as a high-throughput platform, which have good anchoring and aggregation effects on microdroplets, greatly reducing the amount of analytical solution and facilitate the homogeneous sample distribution after evaporation. The deposited gold nanorods (Au NRs) on the pillars with optimized diameter served as SERS-active substrate, can greatly improve the sensitivity of SERS signal compared to other planar substrates. On the open-channel biological chip, sensitive, simultaneous, and specific detection of breast cancer marker miRNA-1246 can be performed. In this mini-pillar array SERS system, the limit of detection (LOD) is 10^-12 M. The mini-pillar array shows enormous potential for open channel, high-throughput biomolecular detection, providing an opportunity for biomedical point-of-care testing (POCT) and drug screening.

Hierarchically patterned self-powered sensors for multifunctional tactile sensing

Flexible sensors are highly desirable for tactile sensing and wearable devices. Previous researches of smart elements have focused on flexible pressure or temperature sensors. However, realizing material identification remains a challenge. Here, we report a multifunctional sensor composed of hydrophobic films and graphene/polydimethylsiloxane sponges. By engineering and optimizing sponges, the fabricated sensor exhibits a high-pressure sensitivity of >15.22 per kilopascal, a fast response time of <74 millisecond, and a high stability over >3000 cycles. In the case of temperature stimulus, the sensor exhibits a temperature-sensing resolution of 1 kelvin via the thermoelectric effect. The sensor can generate output voltage signals after physical contact with different flat materials based on contact-induced electrification. The corresponding signals can be, in turn, used to infer material properties. This multifunctional sensor is excellent in its low cost and material identification, which provides a design concept for meeting the challenges in functional electronics.

Functional nucleic acid-based fluorescence polarization/anisotropy biosensors for detection of biomarkers

The sensitive and selective detection of biomarkers plays a crucial role in disease diagnostics, drug discovery, and early screening of cancers. The achievement of this goal highly depends on the continuous development of biosensing technologies. Among them, fluorescence anisotropy/polarization (FA/FP) analysis receives increasing interest due to the advantage of simple operation, fast response, and no background interference. In recent decades, great progress has been achieved in FA/FP sensors thanks to the development of functional nucleic acids (FNAs) including aptamers and nucleic acid enzymes. This review focuses on FNA-based FA/FP sensors for the quantitative detection of biomarkers, such as nucleic acid, small molecules, and proteins. The design strategies, recognition elements, and practical applications are fully highlighted. The article also discusses the challenges of applying FNA-based FA/FP sensors in the next generation and the potential solutions along with future prospects. Graphical abstract.

Integrated Smart Janus Textile Bands for Self-Pumping Sweat Sampling and Analysis

Wearable sweat sensors have spearheaded the thrust toward personalized health monitoring with continuous, real-time, and molecular-level insight in a noninvasive manner. However, effective sweat sampling still remains a huge challenge. Here, we introduce an intelligent Janus textile band that bridges the gap between self-pumping sweat collection, comfortable epidemic microclimate, and sensitive electrochemical biosensing via an integrated wearable platform. The dominant sweat sampling configuration is a textile with Janus wettability, which is fabricated by electrospinning a hydrophobic polyurethane (PU) nanofiber array onto superhydrophilic gauze. Based on a contact-pumping model, the Janus textile can unidirectionally and thoroughly transport sweat from skin (hydrophobic side) to embedded electrode surface (hydrophilic side) with epidemic comfort. On-body experimentation reveals that the sensitive detection of multiple biomarkers including glucose, lactate, K⁺, and Na⁺ is achieved in the pumped sweat. Such smart Janus textile bands can effectively drain epidermal sweat to targeted assay sites via interface modifications, representing a reinforced and controlled biofluids analysis pathway with physiological comfort.

Integrated Ultrasonic Aggregation-Induced Enrichment with Raman Enhancement for Ultrasensitive and Rapid Biosensing

Enrichment and enhancement are two important aspects of ultratrace biomolecule recognition in complex biological samples. Here we integrate acoustic aggregation of modified Au nanorods with Raman enhancement for all-in-one ultratrace rapid biomolecule detection in one microliter solution. Arising from the interaction between individual nanoparticles and the acoustic field, the aggregation of Au nanorods results in rapid migration of specifically modified Au nanorods toward pressure node in a few seconds and accompanies the enrichment of specific biomolecular. As a proof concept, rapid and sensitive surface-enhanced Raman scattering (SERS) detection of nucleic acids (10^-13 M) in microliter-scale (10^-6 L) sample is achieved. Such an approach integrates ultrasonic aggregation-induced enrichment (uAIE) with Raman enhancement, holding considerable promise for efficient, sensitive, and rapid on-chip detection of ultratrace biomarkers in a clinical sample solution.

Integrated individually electrochemical array for simultaneously detecting multiple Alzheimer's biomarkers

Simultaneous detection of multiple biomarkers is benefit for reducing the detection cycles, avoiding the false-positive signals, and providing the cross validation, which provide the opportunity to understand the pathogenic mechanisms and achieve precise early diagnosis. Here, we demonstrate the mini-pillar-based individual electrochemical array for simultaneous detection of multiple biomarkers. On such platform, the mini-pillar could confine the microdroplet as individual and open-channel microreactor, which is extremely helpful for reducing reagent consumption and extracting internal information, and the electrodes array embedded in mini-pillar are integrated on one side to achieve multiple and simultaneous electrochemical sensing. The introduction of gold nanodendrites by electrodeposition has greatly enhanced sensitivity via improving probe-binding capacity and response signals. Sensitive and selective detection of multiple Alzheimer's biomarkers including Tau, ApoE4, Amyloid-β and miRNA-101 on such mini-pillar-based biosensor is also achieved. Such biosensor platform with the advantages of high-yield, high sensitivity, low-waste and multiple signals output shows great promise in sensing multiple biomolecules for disease diagnosis and health monitoring.

Fluorescent Gold Nanoclusters for Biosensor and Bioimaging Application

With the rapid development of materials technology, fluorescent gold nanoclusters (AuNCs) are emerging as novel functional materials for diagnostic applications including the detection of biomarkers and bioimaging due to the advantages of their ultra-small size, tunable emissions, size-dependent fluorescence and excellent biocompatibility. In this review, we introduced the synthetic methods, and physical and chemical properties of AuNCs. Subsequently, we described the AuNCs-based design strategies for the detection of biomarkers including small molecules, DNA and proteins. The applications of AuNCs for tumor imaging in vitro and in vivo were also presented. Finally, we discussed the challenges and potential solutions of AuNCs-based nanosensors.

Micro-/Nanostructured Interface for Liquid Manipulation and Its Applications

Understanding the relationship between liquid manipulation and micro-/nanostructured interfaces has gained much attention due to the wide potential applications in many fields, such as chemical and biomedical assays, environmental protection, industry, and even daily life. Much work has been done to construct various materials with interfacial liquid manipulation abilities, leading to a range of interesting applications. Herein, different fabrication methods from the top-down approach to the bottom-up approach and subsequent surface modifications of micro-/nanostructured interfaces are first introduced. Then, interactions between the surface and liquid, including liquid wetting, liquid transportation, and a number of corresponding models, together with the definition of hydrophilic/hydrophobic, oleophilic/olephobic, the definition and mechanism of superwetting, including superhydrophobicity, superhydrophilicity, and superoleophobicity, are presented. The micro-/nanostructured interface, with major applications in self-cleaning, antifogging, anti-icing, anticorrosion, drag-reduction, oil-water separation, water collection, droplet (micro)array, and surface-directed liquid transport, is summarized, and the mechanisms underlying each application are discussed. Finally, the remaining challenges and future perspectives in this area are included.

The role of sampling in wearable sweat sensors

Wearable sweat sensors demonstrate outstanding performance in non-invasive, real-time monitoring of vital biomarkers in sweat, which offer an opportunity for individuals to achieve dynamic monitoring their own physiology in molecular-level. As a key step in sweat analysis that impact the accuracy of results, frequently-used sweat sampling methods are introduced in this review, and the emphasis is sweat sampling in wearable sensors including absorbent materials, superhydrophobic/superhydrophilic surface, sweat guidance and epidermal microfluidic systems. In the end, we also propose the remaining challenges in the practical, large-scale application of wearable sweat sensors and provide personal prospects on the future development.

Graphene-Based Biosensors for Detection of Biomarkers

The development of biosensors with high sensitivity and low-detection limits provides a new direction for medical and personal care. Graphene and graphene derivatives have been used to prepare various types of biosensors due to their excellent sensing performance (e.g., high specific surface area, extraordinary electronic properties, electron transport capabilities and ultrahigh flexibility). This perspective review focuses on graphene-based biosensors for quantitative detection of cancer-related biomarkers such as DNA, miRNA, small molecules and proteins by integrating with different signal outputting approaches including fluorescent, electrochemistry, surface plasmon resonance, surface enhanced Raman scattering, etc. The article also discussed their challenges and potential solutions along with future prospects.