Accessing the spatiotemporal heterogeneities of single nanocatalysts by optically imaging gas nanobubbles

Catalysts that catalyze the generation of products in the gas phase, especially those involved in the hydrogen evolution reaction (HER), hold great promise for ecofriendly and sustainable energy development. In general, gas chromatography is widely used to measure catalytic activity. Unfortunately, it gives an averaged output that washes out the heterogeneities among individuals. To assess the unique catalytic properties at the single nanoparticle level, various methods based on single particle catalysis have been proposed. Over the past fifteen years, tremendous breakthroughs have been achieved, which uncovered hidden spatial and temporal heterogeneities. Although powerful, effectively quantifying the activities of single HER nanocatalysts remains challenging because of the fast diffusion of hydrogen (H₂). In 2017, a novel approach based on a nanobubble indicator was proposed to correlate the kinetics of gas bubble evolution with the catalytic activities of individual nanoentities during the HER process. Since then, a plethora of optical microscopy techniques have been utilized to monitor dynamically evolved nanobubbles and to measure the catalytic activities of single HER catalysts. In this minireview, we summarized state-of-the-art optical microscopy for in operando imaging of dynamic nanobubbles involved in gas-generating reactions while highlighting some important discoveries, including the blinking photocatalytic activity and heterogeneous distribution of active sites. Finally, challenges and future perspectives in this promising field were identified.     

Generation and stability of bulk nanobubbles: A review and perspective

Bulk nanobubbles (BNBs) are submicron gaseous domains dispersed in solutions, which are supposed to survive for several hours or even days. In recent years, there has been a rapid growth in the research and extraordinary applications of BNBs. Conventional theories based on gas diffusion and Laplace pressure, however, predicted that nanoscale gas bubbles in water should dissolve within microseconds, presenting a modern-day paradox in current nanobubbles researches. Also, it is still challenging to efficiently produce BNBs and determine their gaseous nature with the available techniques. In this review, we start from a general introduction and brief history of nanobubbles researches and revisit the current progress on the generation methods and detection techniques. Two possible formation mechanisms are suggested, and the plausibility of the proposed theories on BNBs stability is discussed with some suggestions for future studies on bulk nanobubbles.     

Nanobubbles and nanoparticles

Gas saturated solutions have attracted great attention in the past two decades with reports of stable nanobubbles in solutions. The fundamental interest focus arises from the surprising stability which opens up a wide range of potential applications where the interactions between particles and nanobubbles are important. Here, we review the current state of knowledge on systems involving both nanobubbles and nanoparticles. As nanoparticles and nanobubbles are found together in many circumstances, particularly those involving applications of nanobubbles, knowledge of these systems is important. This includes examining the formation of nanoparticles from nanobubbles, the nucleation of nanobubbles from nanoparticles, and the interactions between nanobubbles and nanoparticles. It is clear that further work is required to more fully understand these systems, in particular on the problem of nanobubble nucleation and nanobubble–nanoparticle interactions at the submicron scale.     

Transient behavior of the hydrophobic surface/water interface: from nanobubbles to organic layer

We report the formation and subsequent change of the water-depleted layer at a hydrophobic surface/water interface. With water as the solvent, surface plasmon resonance measurements indicate time dependent evolution of two separate states. The first state is the water-depleted layer, and it is characterized by a layer of nanobubbles on the surface and is short-lived in time (order of 10 min). The second state is a final equilibrium state, which occurs in approximately 30 h, where a layer is formed with organic characteristics. If, instead of water, an aqueous solution is exposed to the hydrophobic surface, the evolution from nanobubbles to an organic like layer shows dependency on the surface energy of the liquid media.     

固液界面纳米气泡研究

固液界面纳米气泡是近十年来表面科学的重要发现之一。从利用原子力显微镜(AFM)在固液界面上观察到纳米气泡以来,科学工作者们已经证实了纳米气泡在固液界面上存在。由于其在微机电系统(MEMS)、微生化系统、表面科学、流体动力学等领域潜在的应用价值,各国学者们对纳米气泡的自身性质及影响因素已经开展了多方面的研究。但纳米气泡稳定性(反常的长寿)的原因仍然是未解决的问题之一。本文综述了纳米气泡的形成及影响因素,重点评述了纳米气泡稳定性理论,包括线张力理论、动态平衡理论、杂质理论和克努森气体理论等。同时,介绍了固液界面纳米气泡的应用,并展望了未来研究的重点和方向。     

Monitoring nanobubble nucleation at early-stage within a sub-9 nm solid-state nanopore

Nanobubble nucleation study is important for understanding the dynamic behavior of nanobubble growth, which is instructive for the nanobubble applications. Benefiting from nanopore fabrication, herein, we fabricated a sub-9 nm SiN_X nanopore with the comparable size to nanobubbles at early-stage. The confined nanopore interface serves as a generator for producing nanobubbles by the chemical reaction between NaBH₄ and H₂O and as an ultra-sensitive sensor for monitoring the H₂ nanobubble nucleation process. By carrying out the NaBH₄ concentration-dependent experiments, we found the life-time of nanobubbles decreased 250 times and the frequency of nanobubble generation increased 38 times with the NaBH₄ concentration increasing from 6 to 100 mM. The long-time equilibrium between gas molecules inward flux and outward flux could prolong the life-time of nanobubbles to hundreds of milliseconds at low NaBH₄ concentration. The raw current trace depicted that the transient accumulation and dissolution of cavity occurred during all the life-time of nanobubbles. Therefore, the sub-9 nm SiN_X nanopore shows a strong ability for real-time monitoring the nanobubble nucleation at early-stage with high temporal and spatial resolution. This work provides a guide to study the dynamic and stochastic characteristics of nanobubbles.     

Hydrophilic Domains Enhance Nanobubble Stability

Highly stable nanoscale gas states at solid/liquid interfaces, referred to as nanobubbles, have been widely studied for over a decade. In this study, nanobubbles generated on a hydrophobic Teflon amorphous fluoroplastic thin film in the presence and absence of hydrophilic carbon domains are investigated by peak force quantitative nanomechanics. On the hydrophobic surface without hydrophilic domains, a small number of nanobubbles are generated and then rapidly decrease in size. On the hydrophobic surface with hydrophilic domains, the hydrophilic domains have a significant effect on the generation and stability of nanobubbles, with bubbles remaining on the surface for up to three days.     

固液界面纳米气泡的研究进展

根据经典热力学理论,在水中纳米级的气泡难以长期稳定存在.近年来却有大量的实验结果表明固液界面存在纳米气泡,原子力显微镜也直接观察到了纳米气泡.有关纳米气泡的研究具有巨大的理论和实际意义,它对表面科学、流体动力学、生物科学以及一些应用领域都有深远的影响.纳米气泡会引起流体在界面的滑移,减少流动阻力,并与表面粘附、胶体分散、矿石浮选、废渣处理等方面密切相关.目前关于纳米气泡的研究才刚刚开始,对于它的基本物化性质的了解还不多,但其重要性已经引起相关领域的极大关注.本文综述了从提出纳米气泡存在一直到实验证明的过程、纳米气泡的形成机制和形貌、分布特征等基本性质以及纳米气泡的存在对疏水长程作用和流体滑移的影响,并阐述了生物学中一些与纳米气泡存在有关的问题.     

Aptamer-conjugated nanobubbles for targeted ultrasound molecular imaging

Targeted ultrasound contrast agents can be prepared by some specific bioconjugation techniques. The biotin-avidin complex is an extremely useful noncovalent binding system, but the system might induce immunogenic side effects in human bodies. Previous proposed covalently conjugated systems suffered from low conjugation efficiency and complex procedures. In this study, we propose a covalently conjugated nanobubble coupling with nucleic acid ligands, aptamers, for providing a higher specific affinity for ultrasound targeting studies. The sgc8c aptamer was linked with nanobubbles through thiol-maleimide coupling chemistry for specific targeting to CCRF-CEM cells. Further improvements to reduce the required time and avoid the degradation of nanobubbles during conjugation procedures were also made. Several investigations were used to discuss the performance and consistency of the prepared nanobubbles, such as size distribution, conjugation efficiency analysis, and flow cytometry assay. Further, we applied our conjugated nanobubbles to ex vivo ultrasound targeted imaging and compared the resulting images with optical images. The results indicated the availability of aptamer-conjugated nanobubbles in targeted ultrasound imaging and the practicability of using a highly sensitive ultrasound system in noninvasive biological research.     

Stable Encapsulated Air Nanobubbles in Water

The dispersion into water of nanocapsules bearing a highly hydrophobic fluorinated internal lining yielded encapsulated air nanobubbles. These bubbles, like their micrometer‐sized counterparts (microbubbles), effectively reflected ultrasound. More importantly, the nanobubbles survived under ultrasonication 100‐times longer than a commercial microbubble sample that is currently in clinical use. We justify this unprecedented stability theoretically. These nanobubbles, owing to their small size and potential ability to permeate the capillary networks of tissues, may expand the applications of microbubbles in diagnostic ultrasonography and find new applications in ultrasound‐regulated drug delivery.     

“发夹”结构DNA用于生物分析传感器研究进展

由于链内碱基互补配对作用形成的"发夹"结构DNA分子被广泛用于生物分子传感分析。双链或多链"发夹"结构DNA分子参与的杂交链式反应信号记录方式多样,主要有荧光法、比色法、电化学方法等。基于杂交链式反应的检测方法具有快速、方便、成本低、准确度高、灵敏度高、特异性强的优点,在分析传感研究中的应用尤其受到人们的关注,近些年发展迅速。本文综述了"发夹"结构DNA与杂交链式反应应用于生物传感分析的原理、信号记录方式及其在蛋白质、重金属离子、小分子、疾病标志物、DNA等检测中的研究进展。     

Spatiotemporal Heterogeneity of Reactions in Solution Observed with High‐Speed Single‐Nanorod Rotational Sensing

A high‐speed darkfield microscope has been developed to monitor the rapid rotation of single gold nanorods (AuNRs) and used to study the spatiotemporal heterogeneity of chemical reactions in free solution. A wide range of viscosities from 237 cP to 0.8 cP could be detected conveniently. We studied H₂O₂ decomposition reactions that were catalyzed by AuNRs coated with Pt nanodots (AuNR@PtNDs) and observed two different rotational states. The two states and their transitions are related to the production and the amalgamation of O₂ nanobubbles on the nanorod surface depending on H₂O₂ concentration. In addition, the local fluidic environment of pure water was found to be non‐uniform in time and space. This technique could be applied to study other chemical and biochemical reactions in solution.     

Thermodynamic stability of interfacial gaseous states

We studied the thermodynamic stability of interfacial gaseous states on atomically smooth highly ordered pyrolytic graphite (HOPG) in water using atomic force microscopy. Quasi-two-dimensional gas layers (micropancakes) required a higher supersaturation of gas than spherical-cap-shaped nanobubbles. The two forms of gas coexisted at a sufficiently high supersaturation of gas where one or more of the nanobubbles may sit on top of a micropancake. The micropancakes spontaneously coalesced with each other over time. After the coalescence of two neighboring micropancakes which each had had a nanobubble on top, one nanobubble grew at the expense of the other. We analyzed these results assuming temporal and local quasi-equilibrium conditions.     

Functional micro/nanobubbles for ultrasound medicine and visualizable guidance

Chemically functionalized gas-filled bubbles with a versatile micro/nano-sized scale have witnessed a long history of developments and emerging applications in disease diagnosis and treatments. In combination with ultrasound and image-guidance,micro/nanobubbles have been endowed with the capabilities of biomedical imaging, drug delivery, gene transfection and diseaseoriented therapy. As an external stimulus, ultrasound(US)-mediated targeting treatments have been achieving unprecedented efficiency. Nowadays, US is playing a crucial role in visualizing biological/pathological changes in lives as a reliable imaging technique and a powerful therapeutic tool. This review retrospects the history of ultrasound, the chemistry of functionalized agents and summarizes recent advancements of functional micro/nanobubbles as US contrast agents in preclinical and transclinical research. Latest ultrasound-based treatment modalities in association with functional micro/nanobubbles have been highlighted as their great potentials for disease precision therapy. It is believed that these state-of-the-art micro/nanobubbles will become a booster for ultrasound medicine and visualizable guidance to serve future human healthcare in a more comprehensive and practical manner.     

Single molecule study of perylene orange photobleaching in thin sol-gel films

The paper reports on photobleaching mechanisms of perylene orange embedded in thin sol-gel films, derived from single molecule studies. The experimental configuration uses wide-field illumination and one photon excitation of the molecules. Measurements have been performed both at ambient conditions and under vacuum in order to get information on the influence of oxygen on photobleaching in such porous samples. We have also recorded the evolution of photobleaching with respect to the excitation intensity. The results demonstrate that photobleaching from excited states higher than the first singlet and triplet states has a nonnegligible contribution as soon as the excitation energy exceeds a few hundred W/cm2 and that this process is favored in the presence of air. The study also demonstrates that perylene orange in sol-gel films is not a very efficient emitter but that photobleaching can be slow, which explains the interest for perylene orange as a good candidate to produce long lifetime solid-state lasers when embedded in monoliths of sol-gel.     

Cleaning using nanobubbles: defouling by electrochemical generation of bubbles

Here we demonstrate that nanobubbles can be used as cleaning agents both for the prevention of surface fouling and for defouling surfaces. In particular nanobubbles can be used to remove proteins that are already adsorbed to a surface, as well as for the prevention of nonspecific adsorption of proteins. Nanobubbles were produced on highly oriented pyrolytic graphite (HOPG) surfaces electrochemically and observed by atomic force microscopy (AFM). Nanobubbles produced by electrochemical treatment for 20 s before exposure to bovine serum albumin (BSA) were found to decrease protein coverage by 26-34%. Further, pre-adsorbed protein on a HOPG surface was also removed by formation of electrochemically produced nanobubbles. In AFM images, the coverage of BSA was found to decrease from 100% to 82% after 50 s of electrochemical treatment. The defouling effect of nanobubbles was also investigated using radioactively labeled BSA. The amount of BSA remaining on a stainless steel surface decreased by approximately 20% following 3 min of electrochemical treatment and further cycles of treatment effectively removed more BSA from the surface. In situ observations indicate that the air-water interface of the nanobubble is responsible for the defouling action of nanobubbles.     

Electrochemically controlled formation and growth of hydrogen nanobubbles

Electrogenerated microscale bubbles that are confined at the electrode surface have already been extensively studied because of their significant influence on electrochemistry. In contrast, as far as we know, whether nanoscale bubbles exist on the electrode surface has not been experimentally confirmed yet. Here, we report the observation of electrochemically controlled formation and growth of hydrogen nanobubbles on bare highly oriented pyrolytic graphite (HOPG) surface via in-situ tapping mode atomic force microscopy (TMAFM). By using TMAFM imaging, we observed that electrochemically generated hydrogen gas led to the formation of nanobubbles at the HOPG surface. We then employed a combination of techniques, including phase imaging, ex-situ degassing, and tip perturbation, to confirm the gas origin of such observed nanobubbles. We further demonstrated that the formation and growth of nanobubbles could be well controlled by tuning either the applied voltage or the reaction time. Remarkably, we could also monitor the evolution process of nanobubbles, that is, formation, growth, coalescence, as well as the eventual release of merged microbubbles from the HOPG surface.     

Effects of degassing on the long-range attractive force between hydrophobic surfaces in water

The long-ranged attractions between hydrophobic amorphous fluoropolymer surfaces are measured in water with and without dissolved air. An atomic force microscope is used to obtain more than 500 measured jump-in distances, which yields statistically reliable results. It is found that the range of the attraction and its variability is generally significantly decreased in deaerated water as compared to normal, aerated water. However, the range and strength of the attraction in deaerated water remain significantly greater than the van der Waals attraction for this system. The experimental observations are consistent with (1) nanobubbles being primarily responsible for the long-ranged attraction in normal water, (2) nanobubbles not being present in deaerated water when the surfaces are not in contact, and (3) the attraction in the absence of nanobubbles being most probably due to the approach to the separation-induced spinodal cavitation of the type identified by Bérard et al. [J. Chem. Phys. 1993, 98, 7236]. It is argued that the measurements in deaerated water reveal the bare or pristine hydrophobic attraction unobscured by nanobubbles.     

Recent Advances in Atomic-scale Storage Mechanism Studies of Two-dimensional Nanomaterials for Rechargeable Batteries Beyond Li-ion

Developing new types of rechargeable batteries with high energy densities and low cost have received increasing attentions, aiming to reduce the dependence on high-priced lithium. Beyond Li-ion batteries, the potential alternatives including Na-ion batteries, Li-S batteries and Li-air batteries have been investigated recently, which are required to be viable for commercial applications. From this point of view, to understand the electrochemical reaction mechanisms and kinetics of these batteries has become the key challenge to make breakthroughs in the field of new energy storage. In this review, we present a critical overview of the two dimensional nanomaterials-based batteries (except Li-ion-based batteries) that could meet such demonds. To develop new energy storage devices with more promising performances, the microstructure evolution and atomic scale storage mechanism of these batteries are comprehensively summarized. In addition, the major challenges and opportunities of advanced characterization techniques are finally discussed. We do hope that this review will give the readers a clear and profound understanding of the electrochemical reaction mechanisms and kinetics of the as-discussed batteries, thus effectively contributing to the smart design of future-generation energy storage devices.