Solid-liquid Contact Electrification Research Articles

Free Radicals Generated in Perfluorocarbon-Water (Liquid-Liquid) Interfacial Contact Electrification and Their Application in Cancer Therapy.

Electron transfer during solid-liquid contact electrification has been demonstrated to produce reactive oxygen species (ROS) such as hydroxyl radicals (•OH) and superoxide anion radicals (•O2-). Here, we show that such a process also occurs in liquid-liquid contact electrification. By preparing perfluorocarbon nanoemulsions to construct a perfluorocarbon-water "liquid-liquid" interface, we confirmed that electrons were transferred from water to perfluorocarbon in ultrasonication-induced high-frequency liquid-liquid contact to produce •OH and •O2-. The produced ROS could be applied to ablate tumors by triggering large-scale immunogenic cell death in tumor cells, promoting dendritic cell maturation and macrophage polarization, ultimately activating T cell-mediated antitumor immune response. Importantly, the raw material for producing •OH is water, so the tumor therapy is not limited by the endogenous substances (O2, H2O2, etc.) in the tumor microenvironment. This work provides new perspectives for elucidating the mechanism of generation of free radicals in liquid-liquid contact and provides an excellent tumor therapeutic modality.

Development of label-free triboelectric nanosensors as screening platforms for anti-tumor drugs

This study introduces a general label-free drug screening platform with potential to support high-throughput drug screening for any protein target. Its innovation lies in quantifying the affinity of molecule-molecule interactions via voltage output variation, which achieves unprecedented selectivity and sensitivity. It combines computational analysis complemented by experimental substantiation with a solid-liquid triboelectric nanosensor. Owing to its high binding affinity, FKBP-rapamycin is used as a model system to demonstrate the platform's sensitivity. Subsequently, the research is extended to study drug interactions with an oncogenic protein ATG4B. The findings unveil the binding of S130 and Tioconazole to ATG4B, a critical cysteine protease involved in autophagy, while revealing that Dexamethasone does not bind ATG4B. This binding exerts a specific inhibitory effect on autophagic flux and triggers cancer cell apoptosis. The results support the previously verified inhibitory effects of these drugs and the effectiveness of a newly developed self-powered drug screening platform, leveraging the principles of solid-liquid contact electrification. This approach adeptly confronts enduring challenges in traditional drug development methods where biochemical assays need to be designed for each individual protein target or only time-consuming and concentration-demanding molecular interaction measurements were made available.

A Comprehensive Review of Advancements and Challenges from Solid–Solid to Liquid–Solid Triboelectric Nanogenerators

AbstractAs the increasing scarcity of fossil fuels and the ever‐present need to address environmental concerns are faced, the development of efficient and sustainable energy harvesting techniques gains more and more significance. The liquid–solid triboelectric nanogenerator (TENG) has emerged as a promising solution for harvesting mechanical energy in water. Although the liquid–solid contact electrification technique is studied by numerous researchers, there is still a need to enhance energy harvesting efficiency in liquid–solid TENG. This comprehensive review discusses recent advances in kinetic energy harvesting from water, focusing on the solid–solid and liquid–solid interaction mechanisms when using TENGs as an energy conversion medium. The review summarizes the recent developments in structure design and material improvements, as well as hybridization with other generators as methods of enhancing the output performance of TENG. Besides discussing advancements of TENG, the potential impact and future challenges of liquid–solid TENG are highlighted. This paper provides insight into the current state of knowledge and the future directions of research in the field of liquid–solid TENG energy harvesting.

The output improvement of droplet-based nanogenerators through enhanced coupling displacement and conducting currents

The advent of triboelectric nanogenerator (TENG) preludes the high entropy energy harvesting and is gradually applied to droplet energy harvesting. However, the mainstream research concentrates on exploring the solid-liquid contact electrification (CE) and electrostatic induction (EI) processes, ignoring the possibility of charge extraction from water. As a result, the droplet-based nanogenerator (DNG) is limited to small alternating-current (AC) output, which also asks for complex power management. This study reveals that the displacement current is not the sole driving force during the power generation of DNG, and the contribution of the conducting current is equally important. Here, the existence of conducting charge is confirmed by a top-electrode-only DNG, and the charge transfer processes in traditional DNGs are re-examined. Finally, a three-electrode total current DNG is introduced to enhance the coupling of displacement current and conducting current. The charge extraction efficiency is then improved by more than one order of magnitude (charge extraction from single droplet is increased from 1 to 27.3 nC) and retains the direct-current (DC) and high voltage (up to 2315 V) output characteristics.

Enhancing electricity generation during water evaporation through a symmetric double Schottky-junction design

Extensive efforts have been made to investigate energy conversion process based on metal-semiconductor Schottky junctions. In this study we report the exploration of electricity generation through water evaporation on the surface with double Schottky junctions of symmetric design. By using such symmetric Schottky junctions at both side of a silicon wafer, the electricity generation is enhanced over the system with just a single Schottky junction. In the symmetric-junction system, water evaporation on the surface of one Schottky junction breaks the symmetry, leading to the generation of a continuous and direct current. The evaporation-induced cooling and continuous liquid-solid contact electrification at the Schottky junction on one side of the system enable the fast flow of the hot electrons across the Schottky junction on the other side. This work not only demonstrates a continuous and direct-current generation system, which has the potential of harvesting a broad range of waste heat and enabling many promising applications, but also offers a general strategy using the symmetric design of Schottky junctions to enhance electricity generation in diverse energy conversion systems.

A Microfluidic Flow Sensor Based on CuO Nws Enhanced Solid-Liquid Contact Electrification for Trapping Platelets on Collagen and Its Quantification

Platelet number plays an important role in several kinds of diseases such as leukemia, hepatitis C, diabetes, COVID-19, microbial infections, etc. These diseases can result in thrombocytopenia, a condition where patients suffer from reduced platelets, thus requiring the need to frequently monitor the platelet count. Currently existing techniques are complex, costly, and time consuming. The target diseases continue to evolve urging the need to develop rapid, facile point-of-care devices with high precision. In this regard, the current study is focused on the development of a platelet quantification technique based on trapping of platelets in a microfluidic channel adhered with collagen layer, and CuO NWs grown on Cu tube integrated on both the sides of the channel. This trapping of platelets results in a change in the triboelectric output performance from solid-liquid contact electrification due to change in flow rate of platelet content in the plasma. These changes in triboelectric output are enhanced by CuO NWs due to its high surface-area-to-volume ratio. The study of flow sensing by integrating the microfluidic device with solid-liquid contact electrification has been reported for the first time. The flow sensing signal was optimized by variation in viscosity of the flowing liquid, channel geometry, flow rate, and triboelectric voltage measuring methods. The platelet rich plasma (PRP) and platelet poor plasma (PPP) were prepared and mixed in different ratios to prepare the solutions with different concentration of platelets (0% - 100%). The calibration curve for platelet quantification has shown a variation of 9 V for change in platelet concentration from 0% to 100%. The proposed device exhibited a linear response with the variation in platelet counts. The obtained results prove its potential and feasibility as a point-of-care device.

Liquid-solid contact electrification through the lens of surface and interface science

Triboelectric nanogenerators (TENGs) are cutting-edge technology that converts mechanical energy into electrical energy through contact electrification (CE). The CE process involves the chemical, physical, and mechanical properties of material surfaces and interfaces. Studying CE from the perspective of interfacial science can provide a comprehensive understanding of the charging mechanism and improve the performance of TENGs through structural design and technical optimization. Therefore, this paper reviews the research progress in CE from the perspective of surface and interface science. Specifically, we introduce the static and dynamic behavior of the liquid-solid (L-S) interaction during CE separately. Then, the interface phenomenon involved in gas-liquid-solid CE, such as surface wettability and ionic effect on liquid film drainage between the bubble and solid surface in water is discussed. This new interdisciplinary area of surface science of TENGs provides many research opportunities, while it is essential to explain critical phenomena in the CE process.

Dual-sensory fusion self-powered triboelectric taste-sensing system towards effective and low-cost liquid identification.

Infusing human taste perception into smart sensing devices to mimic the processing ability of gustatory organs to perceive liquid substances remains challenging. Here we developed a self-powered droplet-tasting sensor system based on the dynamic morphological changes of droplets and liquid-solid contact electrification. The sensor system has achieved accuracies of liquid recognition higher than 90% in five different applications by combining triboelectric fingerprint signals and deep learning. Furthermore, an image sensor is integrated to extract the visual features of liquids, and the recognition capability of the liquid-sensing system is improved to up to 96.0%. The design of this dual-sensory fusion self-powered liquid-sensing system, along with the droplet-tasting sensor that can autonomously generate triboelectric signals, provides a promising technological approach for the development of effective and low-cost liquid sensing for liquid food safety identification and management.

Liquid–Solid Triboelectric Probes for Real‐Time Monitoring of Sucrose Fluid Status

AbstractTriboelectric probes have rapidly developed as an efficient tool for understanding contact electrification at liquid–solid interfaces. However, the liquid–solid electrification process is susceptible to interference from chemical components in mixed solutions, severely limiting the potential applications of triboelectric probes in various liquid environments. This study for the first time reports a triboelectric probe capable of sucrose solution concentration sensing, finding that the dissolution of sucrose destroys the hydrogen bond network between water molecules and forms sucrose–water hydrogen bonds, which alters the fluid mechanics characteristics of the solution and enhances its conductivity, thereby reducing the droplet size and causing an ion charge shielding effect that significantly affects the electron transfer in liquid–solid contact electrification. Owing to the feedback of the triboelectric probe on the sucrose concentration gradient‐type sensing electrical signals, efficient sensing of sucrose solution was achieved (sensitivity of −0.0038%−1, response time of 90 ms). The triboelectric probe is also used as a wireless smart terminal to enable real‐time detection of sucrose solution. This work contributes to the understanding of the structure–function relationship between micro hydrogen bonding and macro performance, and provides a promising solution for building sustainable intelligent sensors.

The Effect of a Magnetic Field on Solid–Liquid Contact Electrification for Streaming Flow Energy Harvesting

In recent years, the triboelectric nanogenerator (TENG) has been recognized as a promising method for energy harvesting and self-powered devices. However, in order to improve the output efficiency of the TENG, it is necessary to change the types of dielectric materials, which requires advanced technology and a high cost to implement. To address this issue, we developed a parallel electrode magnetic-TENG (Mag-TENG) based on contact electrification of a liquid–solid interface under the effect of the magnetic field, which enhances the output performance of the TENG without having to develop the dielectric material. Our experimental results achieved a higher output of the TENG under the influence of a magnetic field when an increase of the magnetic field strength went from 0 to 360 mT, and the flow rate of unsteady seawater was variable from 390 to 690 mL/min. Specifically, compared to the without-magnetic field case, the output current increased by approximately 6.5 times and the output voltage by 2.7 times. These findings suggested that using a magnetic field to enhance the TENG’s efficiency has significant potential for energy harvesting from seawater and self-powered flow sensors.

Peptide Oxidation Induced by Liquid-Solid Contact Electrification as Revealed in Liquid Microjunction-Surface Sampling Probe Mass Spectrometry.

Reproducible peptide oxidation was observed using a homebuilt liquid microjunction-surface sampling probe (LMJ-SSP) platform for analyzing peptide standards. Although electrochemical oxidation and corona discharges have previously been associated with analyte oxidation in electrospray ionization (ESI) and ESI-related ambient ionization mass spectrometry (MS) methods, they were unlikely the causes for the peptide oxidation observed in the LMJ-SSP studies. A systematic investigation demonstrated that analyte oxidation was induced during the droplet drying on a solid surface through liquid-solid electrification processes. To minimize unwanted analyte oxidation, the water content in the sample solution should be decreased and the use of hydroxyl-functionalized substrates, such as glass slides, should be avoided. In addition, if water is an essential solvent component, adding an antioxidant, such as ascorbic acid, to the sample solution before droplet evaporation on the solid surface could lower the percentage of analyte oxidation. The present findings apply to all the MS methods that involve drying microliters of sample solution onto a suitable substrate in their sample preparation protocols.

Triboelectric charge-separable probes for quantificationally charge investigating at the liquid-solid interface

Thoroughly researching liquid-solid contact electrification can significantly enhance our understanding of liquid energy harvesting, chemical processes, and geological mechanisms. However, this research is challenging because liquid changes dynamically, the interface chemistry is complex, and there are few rapid and simple measurement methods available. In this study, a liquid-solid triboelectric nanogenerator was developed as a probe to successfully quantify the charges with varying polarities at the liquid-solid interface. Owing to the design of two spatially arranged electrodes, this proposed probe was able to quantify the interfacial charges within a mere 50 ms propelled by the significant electrical potential difference between the liquid and the electrodes. By concomitantly analyzing the charge signal and short-circuit current signal, the probe is utilized for detecting the flow state of the liquid. Finally, the probe has been demonstrated to be applicable for liquid energy harvesting. This study presents a promising approach to address the challenges of liquid-solid contact electrification research and provides a novel perspective for enhancing our understanding of the chemical processes at liquid-solid contact electrification.

A Liquid Triboelectric Series.

The triboelectric series is a generally accepted method for describing the triboelectric effect. It provides a way to control the double face of the ubiquitous triboelectric effect: causes of unpredictable accidents and the resultant surface charge as energy sources. However, previous studies have been biased in solids despite being observed in liquids (liquid-solid contact electrification). Therefore, a liquid triboelectric series is necessary to be established to manipulate the liquid triboelectric effect according to the appropriate goal. In this study, a liquid triboelectric series is first established to describe the triboelectric properties of each liquid when contact electrification occurs with a solid surface. The liquid triboelectric series covers electrolytes, organic solvents, oxidants, and higher sugar alcohols. Common chemical groups can be derived from the liquid triboelectric series that hydroxyl groups enhance, and benzene groups suppress the liquid triboelectric effect. The results are demonstrated by the amplified efficiency of an energy harvester and particle contamination after surface washing. This study will play a pivotal role in understanding the liquid-solid contact electrification phenomenon and providing new perspectives on the applications of the liquid triboelectric effect.

Liquid-solid contact electrification and its effect on the formation of electric double layer: An atomic-level investigation

Contact electrification at liquid-solid interface has aroused many research interests. Some recent researches suggest that the formation of electric double layer may be dominated by the charge transfer from liquid-solid contact electrification, which challenges the traditional electric double layer models. However, how the liquid-solid contact electrification affects the electrical double layer is still lacking of atomic-level mechanism details. In this work, an atomic-level investigation on the effect of contact electrification on the liquid-solid electric double layer are performed via first-principles and molecular dynamics simulations. The results suggest that the liquid-solid contact electrification can influence the ions distribution in the solution, thus influencing the formation of electric double layer and can even reversing its polarity under specific surface charge density. The mechanism is that the contact charges on the solid surface can affect the adsorption energy of anion/cation at the liquid-solid interface. The conclusion of this work can provide a more comprehensive understanding of the effect of contact electrification to the electric double layer.

Triboelectric Mechanism of Oil-Solid Interface Adopted for Self-Powered Insulating Oil Condition Monitoring.

The liquid-solid contact electrification mechanism has been explored in the aqueous solution system, but there are few systematic studies on oil-solid triboelectrification. Herein, an oil droplet triboelectric nanogenerator (Oil-droplet TENG) is designed as the probe to investigate the charge transfer properties at oil-solid interface. The charge transfer kinetics process is disclosed by the electrical signals produced, showing that the electron species initially predominated the oil-solid triboelectrification. The molecular structure and electronic properties of oil also affect triboelectric performance. Further, the charge transfer principle in multi-component liquid mixture during the electric double layer (EDL) development process is proposed to explain the component competition effect. As a proof of concept, a tubular-TENG is designed as a self-powered sensor for transformer oil trace water detection. The device demonstrates high water sensitivity with a detection limit of 10µL L-1 and a response range of 10-100µL L-1 . This work not only reveals the oil-solid triboelectric and charge transfer competition mechanism in EDL, but also open up a new channel for real-time online monitoring of trace water in transformer oil, which holds promise for information perception and intelligent operation of transformers in the power industry.

Triboelectric Nanosensor Integrated with Robotic Platform for Self-Powered Detection of Chemical Analytes

Rapid on-site detection of hazardous chemicals is imperative for remote security and environmental monitoring applications. However, the implementation of current sensing technologies in real environments is limited due to an external high-power requirement, poor selectivity and sensitivity. Recent progress in triboelectric nanosensors and nanogenerators presents tremendous opportunities to address these issues. Here, we report an innovative self-powered triboelectric nanosensor for detection of Hg2+ ions, a harmful chemical pollutant, in a rapid single step on-site detection mechanism. Based on the mechanism of solid-liquid contact electrification, tellurium nanowire (Te NW) arrays serving as a solid triboelectric material as well as the sensing probe underwent periodic contact and separation with the Hg2+ solution, leading to the in situ formation of mercury telluride nanowire (HgTe NWs) owing to the selective binding affinity of Te NWs toward Hg2+ ions. To realize the on-site sensing potential, Te NW arrays were mounted onto the robotic hands equipped with additional wireless transmission functionality for rapid detection of Hg2+ ions in resource-limited settings by employing a simple "touch and sense" mechanism. Such a demonstration of direct integration of self-powered sensors with robotics would lead to the development of low-cost, automated chemical sensing machinery for the on-field detection of harmful analytes.

Contact Electrification Behaviors of Solid–Liquid Interface: Regulation, Mechanisms, and Applications

Liquid–solid contact electrification is a universal physical phenomenon. The electrical energy produced by a drop of water during the solid–liquid triboelectric electrification (TE) process is negligible, usually at the level of nA or uA. However, by adjusting the structure and composition of materials, or by designing devices more rationally, TE of a drop of water can reach voltages in the thousands of volts and currents in the mA level, which is sufficient to power many electronic devices. Therefore, studying solid–liquid triboelectric nanogenerators is of great significance in the field of energy harvesting. To achieve the manipulation of liquid–solid triboelectrification to nurture strengths and bypass weaknesses, a thorough knowledge of the liquid–solid contact electrification process and its influencing elements is required. The concept, influencing variables, mechanisms, and current control tactics of solid–liquid triboelectrification are addressed in this review. Then, using triboelectric generator technology, solid friction layers are designed and fabricated from three perspectives: structure design, surface functionalization modification, and interface wettability control, with discussion of how the three factors interact and contribute to solid–liquid electrification behavior. How solid–liquid triboelectrification can be used to capture energy and create self‐powered sensors is discussed.

Liquid-solid contact electrification when water droplets hit living plant leaves

Contact electrification has gained interest as a mechanism for generating charges on surfaces. It has also been shown that plant leaves generate electrification by both solid-solid and liquid-solid contact. However, it is unclear how water droplets hitting a leaf causes charging and which structural features affect this phenomenon. Here, we show how the in-situ surface structures of leaves, droplet composition, and prior ionization of the droplet influence living plant liquid-solid contact electrification on several superhydrophobic and hydrophilic living plant species. We find that the hierarchical structures responsible for self-cleaning and wetting, known as the Lotus effect, also create a specific electrification pattern. Each water droplet causes a characteristic signal in the plant tissue depending on the structure and composition of the epicuticular wax layer as well as the droplet composition. These findings give insight on how contact electrification occurs in nature, suggesting the potential for deriving new bio-based materials for sensing and energy harvesting in artificial and plant-hybrid devices.

A reusable electret filter media based on water droplet charging/cleaning

Electret filter media has been considered as the most effective and economical filtration material due to its additional electrostatic filtration effect. However, the regeneration and recycling of electret filter media is a great challenge, as its stored charge would dissipate during filtration process, resulting in a significant efficiency reduction after cleaning. To address this problem, based on liquid–solid contact electrification (C-E) theory and self-cleaning property of hydrophobic surface, we successfully designed and fabricated a reusable PTFE electret filter membrane. After water droplets charging/cleaning, the surface charge of the membrane can be recovered to 100–110%, and the pressure drop can be restored to > 93%, exhibiting an excellent regeneration performance. Then the effect of particle wettability on cleaning performance was investigated. The hydrophilic PM2.5 on hydrophobic surface was found to be easier to clean by droplets rolling than the hydrophobic PM2.5 due to its stronger adhesion to the droplets. Furthermore, based on the theory of ion transfer at the solid–liquid interface, the charge evolution of the hydrophobic PTFE surface during the triboelectric electrification of water droplets was modeled. The rate of ion transfer between water droplets and hydrophobic surfaces depends linearly on the actual liquid–solid contact area. The charge saturation value of a solid surface is related to its ionic binding sites and the dielectric properties. The study promotes the development of reusable electret filters, and advances the understanding of the triboelectric properties of droplets on hydrophobic surfaces.

Spin-selected electron transfer in liquid–solid contact electrification

Electron transfer has been proven the dominant charge carrier during contact electrification at the liquid–solid interface. However, the effect of electron spin in contact electrification remains to be investigated. This study examines the charge transfer between different liquids and ferrimagnetic solids in a magnetic field, focusing on the contribution of O2 molecules to the liquid–solid contact electrification. The findings reveal that magnetic fields promote electron transfer at the O2-containing liquid–solid interfaces. Moreover, magnetic field-induced electron transfer increases at higher O2 concentrations in the liquids and decreases at elevated temperatures. The results indicate spin-selected electron transfer at liquid–solid interface. External magnetic fields can modulate the spin conversion of the radical pairs at the O2-containing liquid and ferrimagnetic solid interfaces due to the Zeeman interaction, promoting electron transfer. A spin-selected electron transfer model for liquid–solid contact electrification is further proposed based on the radical pair mechanism, in which the HO2 molecules and the free unpaired electrons from the ferrimagnetic solids are considered radical pairs. The spin conversion of the [HO2• •e−] pairs is affected by magnetic fields, rendering the electron transfer magnetic field-sensitive.