Light-Controlled Particle Enrichment Patterns in Droplets.

Precision manipulation of particle-enrichment patterns in droplets is challenging but important in biochemical analysis and clinical diagnosis. Herein, a light strategy for precisely manipulating particle enrichment patterns is reported. Focused laser irradiation to the droplet induces a Marangoni flow owing to a localized photothermal effect, which carries in-droplet particles and concentrates them at the laser-spot-acted region. Owing to high flexibility of light, multiple particle-enriched sites are formed in a droplet, and the concentrated particles can be transported and reconstructed on demand. In addition to the island-like enrichment pattern, this optical particle manipulation strategy enables the formation of various particle-enriched patterns, such as the line-shape and circle-shape patterns. Further, light directly acts on the working fluid instead of target particles, considerably weakening dependence on particle properties. For particles whose density is similar to that of the working fluid, a portion of particles can still be concentrated. It is also found that only a small portion of submicron particles can be concentrated, while nanoparticles are hardly concentrated by this light strategy. Moreover, high reconfigurability of light enables in-parallel high-throughput operations, which is demonstrated using two laser beams to form two particle enrichment sites in a droplet simultaneously. Finally, this light strategy is also demonstrated by concentrating cells and nucleic acid molecules. This work paves the way for the applications of optofluidics in cell sorting, point-of-care analysis, and drug screening.

Microstructure Design Strategy for Molecularly Dispersed Cobalt Phthalocyanine and Efficient Mass Transport in CO2 Electroreduction.

Cobalt phthalocyanine (CoPc) has attracted particular interest owing to its excellent activity during the electrochemical CO2 conversion to CO. However, the efficient utilization of CoPc at industrially relevant current densities is still a challenge owing to its nonconductive property, agglomeration, and unfavorable conductive substrate design. Here, a microstructure design strategy for dispersing CoPc molecules on a carbon substrate for efficient CO2 transport during CO2 electrolysis is proposed and demonstrated. The highly dispersed CoPc is loaded on a macroporous hollow nanocarbon sheet to act as the catalyst (CoPc/CS). The unique interconnected and macroporous structure of the carbon sheet forms a large specific surface area to anchor CoPc with high dispersion and simultaneously boosts the mass transport of reactants in the catalyst layer, significantly improving the electrochemical performance. By employing a zero-gap flow cell, the designed catalyst can mediate CO2 to CO with a high full-cell energy efficiency of 57% at 200 mA cm-2 .

Droplet transportation on photosensitive lubricant-impregnated slippery surfaces in response to the light induced Marangoni effect and asymmetrical wetting ridges.

Flexible control of droplet transportation is crucial in various applications but is constrained by liquid-solid friction. The development of biomimetic lubricant-impregnated slippery surfaces provides a new solution for flexible manipulation of droplet transportation. Herein, a light strategy is reported for flexibly controlling droplet transportation on photosensitive lubricant-impregnated slippery surfaces. Owing to the localized heating effect of a focused laser beam via photothermal conversion, the resultant thermal Marangoni flow and horizontal component of the surface tension associated with the asymmetric wetting ridges are together responsible for actuating droplet transportation. It is found that the asymmetry of the wetting ridge is dominated by the thickness of the infused oil layer, which directly affects the droplet transportation. The feasibility of this light strategy is also demonstrated by uphill movement, droplet coalescence, and chemical reaction. This study provides a new design for potential applications in open droplet microfluidics, analytical chemistry, diagnosis, etc.

Spontaneous Imbibition in Paper-Based Microfluidic Devices: Experiments and Numerical Simulations.

Microfluidic paper-based analytical devices (µPADs) have quickly been an excellent choice for point-of-care diagnostic platforms ever since they appeared. Because capillary force is the main driving force for the transport of analytes in µPADs, low spontaneous imbibition rates may limit the detection sensitivity. Therefore, quantitative understanding of internal spontaneous capillary flow progress is requisite for designing sensitive and accurate µPADs. In this work, experimental and numerical studies have been performed to investigate the capillary flow in a typical filter paper. We use light-transmitting imaging technology to study wetting saturation changes in the paper. Our experimental results show an obvious transition of a saturated wetting front into an unsaturated wetting front as the imbibition proceeds. We find that the single-phase Darcy model considerably overestimates the temporal wetting penetration depths. Alternatively, we use the Richards equation together with the two-phase flow material properties that are obtained from the image-based pore-network modeling of the filter paper. Moreover, we have considered a dynamic term in the capillary pressure due to strong wetting dynamics in spontaneous imbibition. As a result, the numerical predictions of spontaneous imbibition in the paper are significantly improved. Our studies provide insights into the development of a quantitative spontaneous imbibition model for µPADs applications.

Molecular Iron Oxide Clusters Boost the Oxygen Reduction Reaction of Platinum Electrocatalysts at Near-Neutral pH.

The oxygen reduction reaction (ORR) is a key energy conversion process, which is critical for the efficient operation of fuel cells and metal-air batteries. Here, we report the significant enhancement of the ORR-performance of commercial platinum-on-carbon electrocatalysts when operated in aqueous electrolyte solutions (pH 5.6), containing the polyoxoanion [Fe28 (µ3 -O)8 (L-(-)-tart)16 (CH3 COO)24 ]20- . Mechanistic studies provide initial insights into the performance-improving role of the iron oxide cluster during ORR. Technological deployment of the system is demonstrated by incorporation into a direct formate microfluidic fuel cell (DFMFC), where major performance increases are observed when compared with reference electrolytes. The study provides the first examples of iron oxide clusters in electrochemical energy conversion and storage.

Upper Limit of Light-Levitated Droplet Motion.

The light-enabled droplet levitation shows promising potential in applications in biotechnology, clinical medicine, and nanomaterials. In particular, light-levitated droplets have good followability with a moving laser beam, resulting in flexibility in manipulating their motion. However, it is still unclear whether there exists an upper limit to the light-levitated droplet motion with a moving laser beam. Therefore, the motion of light-levitated droplets above the free interface is studied to determine the upper limit of motions of the droplets with a moving laser beam. We demonstrate that an inefficient interface temperature response because of a very high moving speed of the laser beam and the resultant small upward vertical component of vapor flow are responsible for the existence of an upper-limit velocity. Above the upper limit, the light-levitated droplets are unable to stably move with the laser beam and finally disappear. By contrast, the droplets can stably move with the laser beam in a wide range at or below this upper limit. In addition, an almost linear relationship between the upper-limit velocity of the light-levitated droplets and the input laser power is presented. The findings of the present study are informative for the implementation of this light levitation technology.

Light-Fueled Beating Coffee-Ring Deposition for Droplet Evaporative Crystallization.

Condensed deposition favors biochemical analysis, bioassays, and clinical diagnosis, but the existing strategies may suffer from low resolution, inaccurate control, cross-contamination, or miscellaneous apparatus. Herein, we propose a noncontact light strategy to enable the condensed deposition for droplet evaporative crystallization, in which the photothermal effect of a focused infrared laser is employed to induce intense evaporation. Due to the localized heating effect, not only can the droplet evaporative crystallization on the hydrophobic substrate be promoted, but also the resultant intensified Marangoni flow enables the movement of the early-formed crystals, preventing the pinning of the triple-phase contact line. Synergy of the Marangoni flow and nonuniform evaporation makes the solutes tend to accumulate near the focused light beam region, which facilitates the condensed deposition. More importantly, this light strategy not only enables condensed deposition on the hydrophobic surface with low hysteresis, but also works successfully on the hydrophilic substrate with high hysteresis via adjusting input laser power. It is demonstrated that the light strategy proposed in the present study has great potential for relevant applications.

Controllable light-induced droplet evaporative crystallization.

Droplet evaporative crystallization is one of the practical tools for clinical diagnosis, environmental monitoring, and pharmaceutical synthesis. Herein, we proposed a controllable and flexible light strategy to manipulate the droplet evaporative crystallization, in which the photothermal effect of a focused infrared laser actuated intense evaporation to attain the droplet evaporative crystallization. Due to the localized heating effect, not only the droplet evaporative crystallization could be promoted, but also the resultant Marangoni-flow enabled the crystals to be concentrated, exhibiting excellent controllability. Besides, a relationship between the crystallization starting time and the solution concentration/laser power was achieved, which benefited the manipulation of the droplet evaporative crystallization. The light strategy proposed in the present study possesses promising potential for future applications.

Light fueled mixing in open surface droplet microfluidics for rapid probe preparation.

In this study, a contactless, flexible, and interference-free light fueled method has been developed to enhance the mixing between the ssDNA and dynabeads in a droplet, which enables rapid probe preparation for promoting the probe technology based on open surface droplet microfluidics. In this light fueled method, the use of the photothermal effect of a focused infrared laser can easily create non-uniform temperature distribution and accordingly the surface tension gradient over the interface as a result of the localized heating effect, which thereby initiates the Marangoni flow in a droplet. Experimental results confirm that the light-induced Marangoni flow greatly enhances the mixing, ensuring rapid and efficient binding between the ssDNA and dynabeads. Moreover, the mixing intensity and degree can be simply tuned by controlling the laser intensity and laser heating time. The light fueled rapid mixing method developed in the present study paves the way for rapid bio-chemical detection.

Light-Caused Droplet Bouncing from a Cavity Trap-Assisted Superhydrophobic Surface.

Actuating droplet bouncing from a rigid surface is of considerable interest for potential applications, ranging from novel droplet microfluidics to self-cleaning and anti-icing. The photothermal effect and the accompanying phase change initiate a route for manipulating the tiny amount of liquid. In this work, we present a concept of droplet bouncing from a cavity trap-assisted superhydrophobic platform actuated by the photothermal effect-induced intense evaporation, which enables the purposeful manipulation of the droplet bouncing. It is demonstrated that such a design limits the vapor transport so that the vapor pressure under the droplet is considerably improved to overcome the gravity and liquid-solid adhesion force, leading to the droplet bouncing. Moreover, experimental results indicate that droplet bouncing behaviors can be easily tuned by simply adjusting the cavity dimension and the input laser power. This work provides a new method for the manipulation of droplet bouncing, presenting promising perspectives for future possible applications.

A Dual-Functional MXene-Based Bioanode for Wearable Self-Charging Biosupercapacitors.

As a reliable energy-supply platform for wearable electronics, biosupercapacitors combine the characteristics of biofuel cells and supercapacitors to harvest and store the energy from human's sweat. However, the bulky preparation process and deep embedding of enzyme active sites in bioelectrodes usually limit the energy-harvesting process, retarding the practical power-supply sceneries especially during the complicated in vivo motion. Herein, a MXene/single-walled carbon nanotube/lactate oxidase hierarchical structure as the dual-functional bioanode is designed, which can not only provide a superior 3D catalytic microenvironment for enzyme accommodation to harvest energy from sweat, but also offers sufficient capacitance to store energy via the electrical double-layer capacitor. A wearable biosupercapacitor is fabricated in the "island-bridge" structure with a composite bioanode, active carbon/Pt cathode, polyacrylamide hydrogel substrate, and liquid metal conductor. The device exhibits an open-circuit voltage of 0.48 V and the high power density of 220.9 µW cm-2 at 0.5 mA cm-2 . The compact conformal adhesion with skin is successfully maintained under stretching/bending conditions. After repeatedly stretching the devices, there is no significant power attenuation in pulsed output. The unique bioelectrode structure and attractive energy harvesting/storing properties demonstrate the promising potential of this biosupercapacitor as a micro self-powered platform of wearable electronics.

Photothermal-Driven Droplet Manipulation: A Perspective.

Optofluidics, which utilizes the interactions between light and fluids to realize various functions, has garnered increasing attention owing to the advantages of operational simplicity, exceptional flexibility, rapid response, etc. As one of the typical light-fluid interactions, the localized photothermal effect serving as a stimulus has been widely used for fluid manipulation. Particularly, significant progress on photothermal-driven droplet manipulation has been made. In this perspective, recent advancements in localized photothermal effect driven droplet manipulation are summarized. First, the photothermal manipulation of droplets on open surfaces is outlined. An attractive droplet manipulation of light droplet levitation above the gas-liquid interface via localized photothermal effect is then discussed. Besides, the photothermal-driven manipulation of droplets in an immiscible liquid phase is also discussed. Although promising, further development of photothermal-driven droplet manipulation is still needed. The challenges and perspectives of this light droplet manipulation strategy for broad implementation are summarized, which will help future studies and applications.

Plasmonic Pt-PPCN/TiN Ohmic Junction for Photo-Thermo Catalytic Hydrogen Production.

The rational design of photocatalysts for improving the conversion of solar energy into hydrogen is a promising route for achieving carbon neutrality. Herein, we couple plasmonic titanium nitride (TiN) with highly crystalline potassium-doped polymeric carbon nitride (PPCN) to construct a PPCN/TiN ohmic junction. Such an ohmic junction not only broadens the absorption spectrum but also inhibits the recombination of electrons and holes. In addition, Pt nanoparticles are introduced into this ohmic junction to form plasmonic Pt-PPCN/TiN, improving the capture of hot electrons generated by TiN and thereby promoting the dissociation of the O-H bond in H2O. The energy barrier decreases from 0.7 to 0.2 eV. Enhanced separation of carrier, activation of water molecules, and capture of hot electrons are jointly promoting photo-thermo catalytic hydrogen production. Therefore, under full-spectrum irradiation, the hydrogen production rate of Pt-PPCN/TiN reaches 19 085 µmol g-1 h-1. This novel plasmonic photocatalyst is promising for full-spectrum photo-thermo catalytic hydrogen production.

Light-manipulated binary droplet transport on a high-energy surface.

Flexible and precise manipulation of droplet transport is of significance for scientific and engineering applications, but real-time and on-demand droplet manipulation remains a challenge. Herein, we report a strategy using light for the outstanding manipulation of binary droplet motion on a high-energy surface and reveal the underlying mechanism. Upon irradiation to a substrate by a focused light beam, the substrate can provide a localized heating source via photothermal conversion, and a binary droplet can be flexibly transported on a high-energy surface with free contact-line pinning, exhibiting light-propelled droplet transport. We theoretically showed that the surface tension gradient across the droplet interface resulting from the localized photothermal effect is responsible for actuating droplet transport. Remarkably, the high reconfigurability and flexibility of light allowed for binary droplet transport with dynamically tunable velocity and direction as well as arbitrary trajectory assisted by 2D channels on a high-energy surface. Complex droplet transportation, controllable droplet coalescence, and anti-gravity motion were realized. The promising applicability of this light-fueled droplet platform was also demonstrated by directional transport of biosample droplets containing DNA molecules and cells, as well as successional microreactions.

Multifunctional MOF-Derived Au, Co-Doped Porous Carbon Electrode for a Wearable Sweat Energy Harvesting-Storage Hybrid System.

As an efficient alternative for harnessing the energy from human's biofluid, a wearable energy harvesting-storage hybrid supercapacitor-biofuel cell (SC-BFC) microfluidic system is established with one multifunctional electrode. The electrode integrates metal-organic framework (MOF) derived carbon nanoarrays with embedded Au, Co nanoparticles on a flexible substrate, and is used for the symmetric supercapacitor as well as the enzyme nanocarriers of the biofuel cell. The electrochemical performance of the proposed electrode is evaluated, and the corresponding working mechanism is studied in depth according to the cyclic voltammetry and density functional theory calculation. The multiplexed microfluidic system is designed to pump and store natural sweat to maintain the continuous biofuel supply in the hybrid SC-BFC system. The biofuel cell module harvests electricity from lactate in sweat, and the symmetric supercapacitor module accommodates the bioelectricity for subsequent utilization. A numerical model is developed to validate the normal operation in poor and rich sweat under variable situations for the microfluidic system. One single SC-BFC unit can be self-charged to ≈0.8 V with superior mechanical durability in on-body testing, as well as energy and power values of 7.2 mJ and 80.3 µW, respectively. It illustrates the promising scenery of energy harvesting-storage hybrid microfluidic system.

Microenvironment Regulation Strategies Facilitating High-Efficiency CO2 Electrolysis in a Zero-Gap Bipolar Membrane Electrolyzer.

In alkaline and neutral zero-gap CO2 electrolyzers, the carbon utilization efficiency of the electrocatalytic CO2 reduction to CO is less than 50% because of inherently homogeneous reactions. Utilization of the bipolar membrane (BPM) electrolyzer can effectively suppress (bi)carbonate formation and parasitic CO2 losses; however, an excessive concentration of H+ in the catalyst layer (CL) significantly hinders the activity and selectivity for CO2 reduction. Here, we report a microenvironment regulation strategy that controls the CL thickness and ionomer content to regulate local CO2 transport and the local pH within the CL. We report 80% faradaic efficiency of CO at a current density of 400 mA/cm2 without the use of a buffering layer, exceeding that of state-of-the-art catalysts with a buffering layer. A carbon utilization efficiency of 63.6% at 400 mA/cm2 is also obtained. This study demonstrates the significance of regulating the microenvironment of the CL in a BPM system.

Gas diffusion TiO2 photoanode for photocatalytic fuel cell towards simultaneous VOCs degradation and electricity generation.

In this work, a photocatalytic fuel cell (PFC) with a gas diffusion TiO2 photoanode is proposed to directly convert chemical energy contained in volatile organic compounds into electricity by using solar energy. The gas diffusion TiO2 photoanode is prepared by coating TiO2 nanoparticles onto Ti mesh, whose intrinsic porous structure allows for gaseous pollutants to directly transfer inside the photoanode and thereby enhances mass transport. The feasibility of the developed gas diffusion photoanode is demonstrated by degrading toluene as a model gaseous pollutant. It is shown that the newly-developed PFC yields better electricity generation and toluene removal efficiency due to the enhanced mass transport of toluene and the eliminated interference of gas bubbles. The short-circuit current density and maximum power density of the PFC with a gas diffusion TiO2 photoanode (0.1 mA/cm2 and 0.02 mW/cm2) are about 3.3 times and 4 times as those of the bubbling PFC (0.03 mA/cm2 and 0.005 mW/cm2), respectively. Both the discharging performance and toluene removal efficiency increase with increasing the light intensity and electrolyte concentration, while there exists an optimal gas flow rate leading to the best performance. The present work provides an innovative strategy for clean processing of volatile organic compounds while recycling the contained chemical energy.

Light Fueled High-Throughput Binary Droplet Splitting and Transport on High-Energy Substrate.

High-throughput droplet splitting and controllable transport of generated microdroplets on open surfaces are crucial in a broad spectrum of applications. Herein, a light strategy for controlling high-throughput splitting of binary droplets and transport of generated microdroplets on a high-energy substrate endowed by a localized photothermal effect is reported. Strong Marangoni flow as a result of the surface tension gradient and limited inward flow at the droplet bottom as a result of the significant viscous effect are together responsible for binary droplet splitting. The temperature gradients across the generated microdroplets established at the core heating zone are responsible for their transport away from the laser-acted zone. Remarkably, assisted by hydrophobic stripes on a high-energy substrate, high-throughput binary droplet splitting and controllable transport of generated microdroplets can be realized. Successful applications in biosample droplets and parallelized microreactions highlight the promising potential of this light strategy in open droplet microfluidics, biological assays and diagnosis, etc.

Light Controlled 3D Crystal Morphology for Droplet Evaporative Crystallization on Photosensitive Hydrophobic Substrate.

Controlling crystal morphology is crucial in analytical chemistry and smart materials synthesis, etc. However, flexible manipulation of 3D crystal morphology still remains challenging. Herein, we present a novel and facile light strategy for droplet evaporative crystallization to manipulate macroscopic crystal morphology on photosensitive hydrophobic substrate possessing photothermal conversion property. We demonstrate that the spherical coronal shell and alms bowl-like crystal skeletons can be achieved on smooth photosensitive hydrophobic substrate, depending on the salt concentration. Rough photosensitive hydrophobic substrate further creates a bubble-assisted light strategy, by which a cylindrical shell-like crystal skeleton with a directionally controllable cavity is achieved. Amazingly, the proper additive endows droplet evaporative crystallization to form a closed crystal skeleton with the solution wrapped inside. The present study provides new ideas for designing a novel optical droplet microfluidic platform for controlling crystal morphology.

Flexible enzymatic biofuel cell based on 1, 4-naphthoquinone/MWCNT-Modified bio-anode and polyvinyl alcohol hydrogel electrolyte.

To meet the emerging power demand of microelectronic and electronic skin based sensing platform, enzymatic fuel cells have received increasing attention due to their good human's compatibility, easy integration and cost effectiveness. Herein, we use multi-walled carbon nanotube/naphthoquinone to modify the lactate oxidase bio-anode to facilitate the electron transfer between electrocatalytic active site and electrode support. Polyvinyl alcohol hydrogel serves as the separator and lactate container. The bio-anode, Pt/C cathode and hydrogel are assembled in layer-by-layer structure, which can successfully utilize pre-stored and external lactate from human's sweat to generate the electricity. It delivers a power-density of 62.2 ± 2.4 µW cm-2 under bending/torsion conditions. Given that the broad substrate scope in sweat and easily assembled structure, it provides a plausible solution to power the miniaturized sensors and generic circuits.