Catalytic Pollutant Upgrading to Dual-Asymmetric MnO2 @polymer Nanotubes as Self-Propelled and Controlled Micromotors for H2 O2 Decomposition.

Industrial and disinfection wastewater typically contains high levels of organic pollutants and residue hydrogen peroxide, which have caused environmental concerns. In this work, dual-asymmetric MnO2 @polymer microreactors are synthesized via pollutant polymerization for self-driven and controlled H2 O2 decomposition. A hollow and asymmetric MnO2 nanotube is derived from MnO2 nanorods by selective acid etching and then coated by a polymeric layer from an aqueous phenolic pollutant via catalytic peroxymonosulfate (PMS)-induced polymerization. The evolution of particle-like polymers is controlled by solution pH, molar ratios of PMS/phenol, and reaction duration. The polymer-covered MnO2 tubing-structured micromotors presented a controlled motion velocity, due to the reverse torque driven by the O2 bubbles from H2 O2 decomposition in the inner tunnels. In addition, the partially coated polymeric layer can regulate the exposure and population of Mn active sites to control the H2 O2 decomposition rate, thus avoiding violent motions and massive heat caused by vigorous H2 O2 decomposition. The microreactors can maintain the function of mobility in an ultra-low H2 O2 environment (<0.31 wt.%). This work provides a new strategy for the transformation of micropollutants to functional polymer-based microreactors for safe and controlled hydrogen peroxide decomposition for environmental remediation.

Confined Tri-Functional FeOx @MnO2 @SiO2 Flask Micromotors for Long-Lasting Motion and Catalytic Reactions.

H2 O2 -fueled micromotors are state-of-the-art mobile microreactors in environmental remediation. In this work, a magnetic FeOx @MnO2 @SiO2 micromotor with multi-functions is designed and demonstrated its catalytic performance in H2 O2 /peroxymonosulfate (PMS) activation for simultaneously sustained motion and organic degradation. Moreover, this work reveals the correlations between catalytic efficiency and motion behavior/mechanism. The inner magnetic FeOx nanoellipsoids primarily trigger radical species (• OH and O2 •- ) to attack organics via Fenton-like reactions. The coated MnO2 layers on FeOx surface are responsible for decomposing H2 O2 into O2 bubbles to provide a propelling torque in the solution and generating SO4 •- and • OH for organic degradation. The outer SiO2 microcapsules with a hollow head and tail result in an asymmetrical Janus structure for the motion, driven by O2 bubbles ejecting from the inner cavity via the opening tail. Intriguingly, PMS adjusts the local environment to control over-violent O2 formation from H2 O2 decomposition by occupying the Mn sites via inter-sphere interactions and enhances organic removal due to the strengthened contacts and Fenton-like reactions between inner FeOx and peroxides within the microreactor. The findings will advance the design of functional micromotors and the knowledge of micromotor-based remediation with controlled motion and high-efficiency oxidation using multiple peroxides.

Emerging microplastics in the environment: Properties, distributions, and impacts.

Microplastics (MPs) are emerging and recalcitrant micropollutants in the environment, which have attracted soaring interests from a wide range of research disciplines. To this end, numerous technologies have been devised to understand the properties, environmental behaviors, and potential impacts/hazards of MPs. Herein, we present a review on the properties, environmental distribution and possible impacts. In this review, a comprehensive introduction of the most universal types of MPs, their shapes and characters will be first presented. Then the distributions of MPs in the environment and the impacts on microbe, plants, and human will be reported. Finally, major challenges and directions will be discussed to provide some clues to the better understanding, control and migration of MPs pollution in future studies.

Manganese-Based Micro/Nanomotors: Synthesis, Motion, and Applications.

As emerging micro/nano-scale devices, micro/nanomotors have been innovatively applied in the environmental and biomedical applications. In this paper, the recent advances of Mn-based micro/nanomotors (Mn-micro/nanomotors) in catalytic oxidation of organic contaminants and the mechanisms in decomposition of H2 O2 (e.g., the generation of O2 bubbles and reactive oxygen species) are reviewed. The intrinsic characteristics and synthetic strategies of Mn-based materials are discussed, aiming to gain comprehensive understandings on the asymmetric design of micro/nanomotors. Mn-micro/nanomotors have many advantages such as flexible structures, biocompatibility, powerful motion, long lifetime, and low-cost as compared to noble-metal micro/nanomotors. These merits fulfil Mn-micro/nanomotors great promises from proof-of-concept studies to realistic applications, including pollutant decomposition, trace detection of heavy metal ions, oil removal, drug delivery, isolation of biological targets, and killing bacteria and cancer cells. The great flexibility in fabrication enables diverse and innovative strategies to address challenges for Mn-micro/nanomotors, including high consumption of H2 O2 and non-directional motion. Meanwhile, a perspective of Mn-micro/nanomotors in water remediation by coupling the motors with other Fenton/Fenton-like systems to enhance the catalytic activity and to yield more reactive oxygen species is presented. Directions to the design of on-demand H2 O2 -fueled Mn-micro/nanomotors for advanced purification of organic contaminants in aquatic systems are also proposed.

Microplastics remediation in aqueous systems: Strategies and technologies.

In recent years, the ubiquitous detection and accumulation of microplastics (MPs) in the aquatic environment have raised significant concerns on water security and long-term ecological impacts all around the world. Nevertheless, critical reviews on strategic control and effective remediation of MPs in the aqueous phase are still lacking. In this work, we summarise the origins and types of MPs, and then introduce the methodologies for extraction, identification and quantification. More importantly, we for the first time provide a comprehensive overview of the recent advances in the emerging MPs removal and transformation technologies. Except for biodegradation, this review presents new applications of advanced oxidation processes (AOPs) for MPs degradation and utilisation, including photocatalysis, photoreforming and Fenton-like reactions. Physical or catalytic thermal treatment can transform plastics into value-added nanocarbons or hydrocarbons. These transformation technologies demonstrate great potentials in dealing with MPs. The review will guide researchers to further explore the feasible approaches and develop new strategies for advanced control and remediation of MPs in the future.

Mechanical agitation accelerated ultrasonication for wastewater treatment: Sustainable production of hydroxyl radicals.

Low efficiency in energy conversion has long been the bottleneck in sonochemistry-based water treatment technologies. In this work, we reported a simple and efficient strategy by introducing mechanical agitation into a low powered ultrasonic system to facilitate the production of cavitation bubbles. The coupled system remarkably intensifies the evolution of reactive oxygen species (ROS) for degradation of refractory organic pollutants. We in-situ monitored the generation of hydroxyl radicals (•OH) by selective scavenging tests and chemical trapping experiments. The operational factors such as rotation speed, gas atmosphere, solution temperature and pH were carefully evaluated for their impacts on the degradation of a plastic microcontaminant, diethyl phthalate (DEP). It was found that the degradation efficiency is closely related to the population of cavitation bubbles in the solution, which was collaboratively governed by the aforementioned factors. A high mechanical agitation speed (600 rpm), great solubility of inert gas atmosphere (Argon), and low reaction temperature (15 ºC) are beneficial to the generation of cavitation bubbles and the associated production of ROS. This work shows a facile strategy to intensify the mechanical energy-to-chemical conversion and provides new mechanistic insights into the ultrasound-based advanced oxidation without external chemical inputs.

[Spectrum analysis of blue to purple membrane transition induced by diverse valent cations].

The visible difference spectra, M412 yield and M412 decay lifetime in blue membrane (bM) to purple membrane (pM) transition induced by Na+ , Mg2+ and Tb3+ metal ions were characterized. The transition ability from bM to pM induced by Tb3+ , Mg2+ and Na+ has distinguished difference, their concentration ratio at the midpoint of ion-induced absorbance changes is 1:2.5:650. Meanwhile, the curve of absorbance changes at 540 nm is similar to that of M412 yield changes in bM to pM transition. The M412 decay lifetime of regenerative pM induced by Tb3+ was prolonged remarkably when more Tb3+ was added. However, for the other two ions, additional ions have no effects on its lifetime. These results suggest that diverse valence metal ions exist in different binding ways from pM.

Melittin-regenerated purple membrane.

We have investigated the character of melittin-regenerated purple membrane. Adding melittin to blue membrane causes the color transition and partial regeneration of the photocycle and the proton pump. The reconstitution of bacteriorhodopsin by melittin is proved to be charge-dependent. In studying the location of melittin binding on the blue membrane, we suggest that melittin anchors on the membrane through both hydrophobic and electrostatic interactions. The electrostatic interaction is dominant. The binding sites for the electrostatic interaction should be on the surface of the membrane.

Glycolipid biotinylation on purple membrane with maintained bioactivity.

In this study, labeled purple membrane (PM, Halobacterium salinarium) patches with maintained bioactivity were prepared by biotinylation of the glycolipids on the extracellular (EC) surface of the PM. After in situ streptavidin incubation, the extracellular surface could be directly visualized because it was completely covered with bright dots. The height of a single biotin/streptavidin interaction layer was approximately 2.5 nm. By using calcium thioglycolate-modified tips and an atomic-force microscope (AFM), two distinct topographical features of PMs placed in a buffer with low salt concentration were recognized: the flat EC surface and the domelike cytoplasmic (CP) surface. Biotinylation and AFM with modified tips were simultaneously used with consistent results. Those observations are useful for future studies on PM bioconjugation and oriented assembly as well as the design of bacteriorhodopsin-based photoelectric devices.

Asymmetric distribution of biotin labeling on the purple membrane.

This work examined the biotin modification of bacteriorhodopsin (BR) in the purple membrane (PM). The results of flash kinetic absorption measurements showed that photocycle was maintained in biotinylated BR. Biotinylated BR also maintained its photoelectric activity, as indicated by the photoelectric response of the bilayer lipid membrane (BLM). Atomic force microscopy (AFM) of stretavidiin-bound biotin revealed that biotin molecules covered both surfaces of the, but the amount of biotinylated BR on the extracellular (EC) surface was markedly higher than on the cytoplasmic (CP) surface. Further studies showed that, after reaction with fluorescamine (FL), biotin labeling occurred only on the CP surface. These results are informative for future work on bioconjugation of BR as well as work on oriented assembly and the design of BR-based photoelectric devices.

Different interactions between the two sides of purple membrane with atomic force microscope tip.

Atomic force microscopy (AFM) is known to be capable of measuring local surface charge density based on the DLVO model. However, it has failed to distinguish charge density difference between the extracellular and cytoplasmic sides of purple membrane (PM) in previous studies. In this paper, tapping-mode AFM with thioglycolate-modified tips was used to image PM in buffers of different salt concentrations. When imaged in 25 mM KCl buffer, the topography of membranes appeared to be of two different types, one flat and the other domelike. Such a difference was not observed in buffers of high salt concentrations. This suggests that the topography variation results from differences in electrostatic interaction between the AFM tip and the different membrane surfaces. With images of papain-digested PM and high-resolution images of membrane surface structure, we proved that the membrane surfaces with flat topography were on the extracellular side while the surfaces with domelike topography were on the cytoplasmic side. Hence, this provides a straightforward method to distinguish the two sides of PM without the requirement of high-resolution imaging. Force-distance curves clearly demonstrated the different tip-sample interactions. The force curves recorded on the extracellular side of PM were consistent with the DLVO model, so its surface charge density can be estimated well. However, the curves recorded on the cytoplasmic side had a much longer decay length, which is supposed to be relevant to the flexibility of the C-terminus of bacteriorhodopsin (bR).

Studies on the temperature effect on bacteriorhodopsin of purple and blue membrane by fluorescence and absorption spectroscopy.

Fluorescence and absorption spectra were used to study the temperature effect on the conformation of bacteriorhodopsin (bR) in the blue and purple membranes (termed as bRb and bRp respectively). The maximum emission wavelengths of tryptophan fluorescence in both proteins at room temperature are 340 nm, and the fluorescence quantum yield of bRb is about 1.4 fold higher than that of bRp. As temperature increases, the tryptophan fluorescence of bRb decreases, while the tryptophan fluorescence of bRp increases. The binding study of extrinsic fluorescent probe bis-ANS indicated that the probe can bind only to bRb, but not to bRp. These results suggest that significant structural difference existed between bRb and bRp. It was also found that both kinds of bR are highly thermal stable. The maximum wavelength of the protein fluorescence emission only shifted from 340 nm to 346 nm at 100 degrees C. More interestingly, as temperature increased, the characteristic absorption peak of bRb at 605 nm decreased and a new absorption peak at 380 nm formed. The transition occurred at a narrow temperature range (65 degrees C-70 degrees C). These facts indicated that an intermediate can be induced by high temperature. This phenomenon has not been reported before.

Structural changes of purple membrane and bacteriorhodopsin during its denaturation induced by high pH.

Bacteriorhodopsin (bR) trimers naturally form two-dimensional hexagonal crystals in purple membrane (PM), which make it very stable. However, the dnaturation of bR was found to occur during a very narrow pH range when the pH was increased above 12.0, as indicated by inactivation of the photochemical cycle observed by flash photolysis kinetic spectra. Here, atomic force microscopy was used to study the surface structural changes of PM during the denaturation process induced by high pH. Together with the absorption and fluorescence spectra, it was found that the structural changes could be divided into three steps. First, some hydrophobic amino acids of bR become exposed to the aqueous environment and PM loses its 2D crystalline structure, transforming into the so-called "nonisland" structure. Second, bR molecules are extracted out of membrane and form protrusions on the surface like islands in the sea; therefore, the "nonisland" structure transforms into the "island" structure. Finally, most bRs break off from the membrane and form large depositions.

Kinetics of picosecond laser pulse induced charge separation and proton transfer in bacteriorhodopsin.

Bacteriorhodopsin (BR) films oriented by an electrophoretic method are deposited on a transparent conductive ITO glass. A counterelectrode of copper and gelose gel is used to compose a sandwich-type photodetector with the structure of ITO/BR film/gelose gel/Cu. A single 30-ps laser pulse and a mode-locked pulse train are respectively used to excite the BR photodetector. The ultrafast falling edge and the bipolar response signal are measured by the digital oscilloscope under seven different time ranges. Marquardt nonlinear least squares fitting is used to fit all the experimental data and a good fitting equation is found to describe the kinetic process of the photoelectric signal. Data fitting resolves six exponential components that can be assigned to a seven-step BR photocycle model: BR-->K-->KL-->L-->M-->N-->O-->BR. Comparing tests of the BR photodetector with a 100-ps Si PIN photodiode demonstrates that this type of BR photodetector has at least 100-ps response time and can also serve as a fast photoelectric switch.

Mechanisms of pulse response and differential response of bacteriorhodopsin and their relations.

Bacteriorhodopsin (BR) films are oriented and deposited on indium tin oxide conductive glass by using electrophoretic sedimentation and Langmuir-Blodgett methods to construct sandwich-type photocells, respectively. The pulse response photoelectric signal of the BR photocell under pulsed laser and the differential response photoelectric signal under irradiation of interval light are measured. The origins of these two types of photoelectric responses and their correlations are analyzed. The pulse response signal initiates from the ultrafast charge separation of the retinal and the proton translocation followed by the deprotonation and reprotonation of the Schiff base and its surrounding amino acids. This is a quick response and is the preceding reaction of the differential response. The differential response signal is caused by the charging and discharging of the continuous proton current of the BR light-driven proton pump at light-on and light-off, which is a slow process. The differential response is related to not only the construction of the BR photocell but also the coupling mode of measurement. To observe the differential response signal, the BR photocell must have large enough B3 and B3' components in its pulse response as well as an alternative coupling mode to measure it.

All-trans to 13-cis retinal isomerization in light-adapted bacteriorhodopsin at acidic pH.

The flash photolysis kinetic spectra of the intermediate M(412) of bacteriorhodopsin were monitored during the process of acid titration. In the light-adapted state, the maximum peak amplitude of M(412) absorbance of bacteriorhodopsin decreased (pK(a)=3.40+/-0.05) as the pH decreased from 7.3 to 1.9. In the dark-adapted state, the maximum peak amplitude of M(412) absorbance of bacteriorhodopsin increased as the pH decreased from 6.9 to 4.1, and then decreased (pK(a)=2.85+/-0.05) as the pH dropped to 2.1. These different trends in the change in the maximum peak amplitude suggested that not only the transition of purple membrane to blue membrane had taken place in both light and dark-adapted states, but also the fraction of all-trans-bR had changed during the acid titration. The pH-dependent absorption changes at 640 nm of bacteriorhodopsin in both light- and dark-adapted states were also observed. The pK(a)-values of the purple-to-blue transition were 3.80+/-0.05 in light-adapted state and 3.40+/-0.05 in dark-adapted state, respectively. According to Balashov's method, the fraction of all-trans-bR was assayed as the pH decreased. All these results indicated that the purple-to-blue transition of light-adapted bacteriorhodopsin was accompanied by an all-trans to 13-cis retinal isomerization at acidic pH.

Characteristics and the Mechanism of Bacteriorhodopsin Photoelectric Response.

Oriented bacteriorhodopsin films were prepared on ITO conductive glass by using electrophoretic or Langmuir-Blodgett methods to construct photocells. Pulse response photovoltages under stimulation of pulsed laser and differentialresponse signals under irradiation of discontinued light were respectively measured, and the origins of the two responses and their correlation are analyzed. The pulse response photovoltage initiated from the ultrafast charge separation of the retinal and the proton translocation, followed by the deprotonation and reprotonation of the Schiff base and its surrounding amino acids. This was a quick response and was the preceding reaction of the differential response. The differential response was caused by the charging and discharging of the continuous proton current of the BR light-driven proton pump at the light-on and light-off, as well as the coupling mode of the measuring circuit, which was a slow process.