Control Of Ion Flux Research Articles

Spatially-resolved spectroscopic investigation of the inhomogeneous magnetic field effects on a low-pressure capacitively-coupled nitrogen plasma

Although magnetized plasmas have been frequently used to enhance the process rate or improve the film quality via the control of ion flux as well as energy and plasma density in semiconductor processes, the inhomogeneous magnetic field—which leads to plasma non-uniformity—remains as a problem to be solved. To address this problem, it is essential to conduct a comprehensive assessment of the magnetic effect throughout the entire discharge. Therefore, in the present study, we investigated the magnetic field effects (B < 100 G) on a capacitively-coupled nitrogen plasma based on spectroscopic analyses. The spatially-resolved emission spectra were measured along the radial direction at various vertical positions under the pressures of 10 mTorr and 250 mTorr both with and without magnetic field. By analyzing emission spectra such as N2 FPS, N2 SPS, N2+ FNS, and N I, we were able to obtain the radial distributions of reactive species density, vibrational temperature, and excitation temperature. In low-pressure plasma, with the application of a magnetic field, maximum increases in vibrational temperature and excitation temperature of 462 K and 491 K, respectively, were observed within the bulk region beneath the magnet. This magnetic effect resulted in a significant increase in reactive species density along the radial direction. It was also found that the local enhancement of ion density by magnetic field was strongly related to the increase in excitation temperature and the density of the N2+(B) state. From this result, it is suggested that introducing an asymmetric magnetic field could modulate the spatial distributions of the physical and chemical properties of the plasma.

Label-Free Detection of Waterborne Pathogens Using an All-Solid-State Laser-Induced Graphene Potentiometric Ion Flux Immunosensor.

Waterborne pathogens are harmful microorganisms transmitted through water sources. Early and rapid pathogen detection is important for preventing illnesses and implementing stringent water safety measures to minimize the risk of contamination. This work introduces a miniaturized all-solid-state potentiometric ion flux immunosensor for the rapid and label-free detection of waterborne pathogens. A screen-printed silver/silver chloride electrode coated with a reference electrode membrane and polyurethane as an all-solid-state reference electrode was combined with a solid-state contact ion-selective electrode (ISE). An all-solid-state ISE was constructed on laser-induced graphene by coating it with a cationic marker and a carboxylated poly(vinyl chloride)-based membrane for immobilizing antibodies and controlling ion fluxes through the membrane. Proof-of-concept was achieved by detecting Escherichia coli and Salmonella enterica serovar Typhimurium using the assembled immunosensors within 10 min. The potentiometric response shift attributed to the blocking effect in the ion flux caused by pathogen-antibody interaction corresponded to pathogen concentration, indicating detection limits of 0.1 CFU/mL and working ranges of 0.1-105 CFU/mL. Furthermore, the developed sensors revealed high selectivity and were directly applied in groundwater and tap water without any sample preparation, demonstrating high recovery percentages. The simple operation and elimination of sample preparation are key benefits to further usability of the developed immunosensors for efficient pathogen detection.

Potassium dependent structural changes in the selectivity filter of HERG potassium channels

The fine tuning of biological electrical signaling is mediated by variations in the rates of opening and closing of gates that control ion flux through different ion channels. Human ether-a-go-go related gene (HERG) potassium channels have uniquely rapid inactivation kinetics which are critical to the role they play in regulating cardiac electrical activity. Here, we exploit the K+ sensitivity of HERG inactivation to determine structures of both a conductive and non-conductive selectivity filter structure of HERG. The conductive state has a canonical cylindrical shaped selectivity filter. The non-conductive state is characterized by flipping of the selectivity filter valine backbone carbonyls to point away from the central axis. The side chain of S620 on the pore helix plays a central role in this process, by coordinating distinct sets of interactions in the conductive, non-conductive, and transition states. Our model represents a distinct mechanism by which ion channels fine tune their activity and could explain the uniquely rapid inactivation kinetics of HERG.

The ion channels of endomembranes.

The endomembrane system consists of organellar membranes in the biosynthetic pathway [endoplasmic reticulum (ER), Golgi apparatus, and secretory vesicles] as well as those in the degradative pathway (early endosomes, macropinosomes, phagosomes, autophagosomes, late endosomes, and lysosomes). These endomembrane organelles/vesicles work together to synthesize, modify, package, transport, and degrade proteins, carbohydrates, and lipids, regulating the balance between cellular anabolism and catabolism. Large ion concentration gradients exist across endomembranes: Ca2+ gradients for most endomembrane organelles and H+ gradients for the acidic compartments. Ion (Na+, K+, H+, Ca2+, and Cl-) channels on the organellar membranes control ion flux in response to cellular cues, allowing rapid informational exchange between the cytosol and organelle lumen. Recent advances in organelle proteomics, organellar electrophysiology, and luminal and juxtaorganellar ion imaging have led to molecular identification and functional characterization of about two dozen endomembrane ion channels. For example, whereas IP3R1-3 channels mediate Ca2+ release from the ER in response to neurotransmitter and hormone stimulation, TRPML1-3 and TMEM175 channels mediate lysosomal Ca2+ and H+ release, respectively, in response to nutritional and trafficking cues. This review aims to summarize the current understanding of these endomembrane channels, with a focus on their subcellular localizations, ion permeation properties, gating mechanisms, cell biological functions, and disease relevance.

Label-free optical detection of calcium ion influx in cell-derived nanovesicles using a conical Au/PDMS biosensor.

Ion channels, which are key to physiological regulation and drug discovery, control ion flux across membranes, and their dysregulation leads to various diseases. Ca2+ monitoring is crucial for cellular signaling when performing Ca-based assays in ion channel research; these assays are widely utilized in both academic and pharmaceutical contexts for drug screening and pharmacological profiling. However, existing detection methods are limited by slow detection speeds, low throughput, complex processes, and low analyte viability. In this study, we developed a label-free optical biosensing method using a conical Au/polydimethylsiloxane platform tailored to detect Ca2+ influx in A549-originated nanovesicles facilitated by the transient receptor potential ankyrin 1 (TRPA1) channel. Nanovesicles expressing cellular signaling components mimic TRPA1 signal transduction in cell membranes and improve analyte viability. The conical Au/polydimethylsiloxane sensor converted Ca2+ influx events induced by specific agonist exposure into noticeable changes in relative transmittance under visible light. The optical transmittance change accompanying Ca2+ influx resulted in an enhanced sensing response, high accuracy and reliability, and rapid detection (∼5 s) without immobilization or ligand treatments. In the underlying sensing mechanism, morphological variations in nanovesicles, which depend on Ca2+ influx, induce a considerable light scattering change at an interface between the nanovesicle and Au, revealed by optical simulation. This study provides a foundation for developing biosensors based on light-matter interactions. These sensors are simple and cost-effective with superior performance and diverse functionality.

Air-Stable Protective Layers for Lithium Anode Achieving Safe Lithium Metal Batteries.

With markedly expansive demand in energy storage devices, rechargeable batteries will concentrate on achieving the high energy density and adequate security, especially under harsh operating conditions. Considering the high capacity (3860mA h g-1 ) and low electrochemical potential (-3.04V vs the standard hydrogen electrode), lithium metal is identified as one of the most promising anode materials, which has sparked a research boom. However, the intrinsically high reactivity triggers a repeating fracture/reconstruction process of the solid electrolyte interphase, side reactions with electrolyte and lithium dendrites, detrimental to the electrochemical performance of lithium metalbatteries (LMBs). Even worse, when exposed to air, lithium metal will suffer severe atmospheric corrosion, especially the reaction with moisture, leading to grievous safety hazards. To settle these troubles, constructing air-stable protective layers (ASPLs) is an effective solution. In this review, besides the necessity of ASPLs is highlighted, the modified design criteria, focusing on enhancing chemical/mechanical stability and controlling ion flux, are proposed. Correspondingly, current research progress is comprehensively summarized and discussed. Finally, the perspectives of developing applicable lithium metal anodes(LMAs) are put forward. This review guides the direction for the practical use of LMAs, further pushing the evolution of safe and stable LMBs.

Laminar regenerated cellulose membrane employed for high-performance photothermal-gating osmotic power harvesting

Inspired by acute temperature sensors in the mammalian sensory system to seek comfortable living environment, we construct temperature sensing nanochannels that are tightly linked to photo gating to form photothermal controlled nanochannels membrane. This membrane arises from the composite of laminar regenerated cellulose (RC) membrane formed by dissolution and regeneration of cellulose in the novel superbase-derived ionic liquid and poly-l-lysine (PLL)-modified conical nanochannels polyethylene terephthalate (PET) substrate. Fe3O4 nanoparticles are introduced into RC membrane as gating modifiers, controlling ion flux and osmotic energy conversion. Nanochannels are activated upon photo, inducing temperature changes result in PLL molecule structure shifted from α-helix to β-sheet. The difference in the rectification ratio at different temperatures is associated with photothermal-dependent opening and closing, its maximum is 116.76. This system could deliver an output power of approximately 4.9 W/m2 in osmotic energy harvesting. Our results suggest a simple photothermal-gating ion transport principle in laminar RC membrane.

Interaction of Small Hydrocarbons with Fusion Relevant Beryllium Thin Films

Ion-surface collision studies are carried out with small deuterated hydrocarbon cations i.e. CDx + with x = 2-4 colliding with fusion relevant Beryllium (Be) thin films with ions incident energy as low as 0 eV and as high as Ein = 100 eV. Be films are coated on stainless steel surface by the technique of Thermionic Vacuum Arc (TVA); a novel thin film deposition method with primary as well distinguished characteristic of control of ion flux and respective dose towards the substrate. Prior to scattering, methane-d4 99 atom % D is ionized by electron impact and ions are mass and energy analyzed. Ionization and collisions are performed in ultra high vacuum conditions. In these kinds of collision experiments, we have recorded secondary ion mass spectra and plotted respective incident energy resolved abundances of secondary product ions. Relative abundances in percentage of total secondary ions are plotted and it is observed that such beryllium films can accumulate charged hydrocarbon layers as surface adsorbates. These self assembled layers play a primary role in surface-scattering of primary ions. Moreover, it is seen that bond dissociation energy in lighter hydrocarbons is higher than that for heavier species and shows primarily that the deuterium atoms are loosely bounded to carbon atoms in heavier hydrocarbons than in lighter ones.

Soft conducting polymer polypyrrole actuation based on poly(N-isopropylacrylamide) hydrogels

The conducting polymer, polypyrrole (PPy), has been widely studied as electrochemical actuators due to their electrochemical stability, fast actuation and high strains. However, conducting polymers films are rigid and brittle, which limit their applications. Conducting polymer hydrogel composites take advantage of hydrogels’ elastic properties and extend their use to biological applications, implants and flexible sensor applications. We systematically investigated the out-of-plane actuation of poly(N-isopropylacrylamide) (pNIPAM) hydrogel by utilising the redox properties of a PPy film. We assess the PPy film growth with different sized dopants and find that PPy-dodecylbenzene sulfonate (DBS) films exhibited the largest strain (∼20 %) with a cation-driven actuation. We show that the Young's Modulus of the composite hydrogel was ∼10 kPa, which remained constant regardless of the redox state of PPy. Furthermore, we show that the PPy-DBS film grown in the pNIPAM hydrogel exhibits more than x2 the actuation of the PPy-DBS film alone by electrochemical switching of the redox state. This system has potential applications for soft actuators, controlling ion flux, drug delivery or applying electrical stimuli for cell culture studies.

Smart membranes by electron beam cross-linking of copolymer microgels.

Poly(N-isopropylacrylamide) (pNIPAM) based copolymer microgels were used to create free-standing, transferable, thermoresponsive membranes. The microgels were synthesized by copolymerization of NIPAM with N-benzylhydrylacrylamide (NBHAM). Monolayers of these colloidal gels were subsequently cross-linked using an electron gun leading to the formation of a connected monolayer. Furthermore, the cross-linked microgel layer is detached from the supporting material by dissolving the substrate. These unique systems can be used as transferable, thermoresponsive coatings and as thermoresponsive membranes. As a proof of principle for the use of such membranes we studied the ion transport through them at different temperatures revealing drastic changes when the lower critical solution temperature of the copolymer microgels is reached.

Efficient Gating of Ion Transport in Three-Dimensional Metal-Organic Framework Sub-Nanochannels with Confined Light-Responsive Azobenzene Molecules.

1D nanochannels modified with responsive molecules are fabricated to replicate gating functionalities of biological ion channels, but gating effects are usually weak because small molecular gates cannot efficiently block the large channels in the closed states. Now, 3D metal-organic framework (MOF) sub-nanochannels (SNCs) confined with azobenzene (AZO) molecules achieve efficient light-gating functionalities. The 3D MOFSNCs consisting of a MOF UiO66 with ca. 9-12 Å cavities connected by ca. 6 Å triangular windows work as angstrom-scale ion channels, while confined AZO within the MOF cavities function as light-driven molecular gates to efficiently regulate the ion flux. The AZO-MOFSNCs show good cyclic gating performance and high on-off ratios up to 17.8, an order of magnitude higher than ratios observed in conventional 1D AZO-modified nanochannels (1.3-1.5). This work provides a strategy to develop highly efficient switchable ion channels based on 3D porous MOFs and small responsive molecules.

Control of ion-flux and ion-energy in direct inductively coupled plasma reactor for interfacial-mixing plasma-enhanced atomic layer deposition

The effects of low-energy (&amp;lt;15 eV) high-flux O2+ ion bombardment on the properties of Al2O3 films deposited on 3D nanostructures by plasma-enhanced atomic layer deposition (PE-ALD) were investigated. High-dose O2+ ion bombardment (&amp;gt;1017 cm−2 cycle−1) during the oxidation steps caused interfacial mixing, and AlSiOx films with abrupt interfaces were formed on Si surfaces. Interfacially mixed AlSiOx films were selectively formed on single-crystal Si, amorphous Si, and degraded SiO2 surfaces, whereas normal ALD Al2O3 films were formed on thermally grown SiO2 surfaces. At the same time, the interfacially mixed AlSiOx films were selectively formed on the horizontal top and bottom faces of the 3D nanostructures, whereas normal ALD Al2O3 films were formed on the vertical sidewalls. The morphology and thickness of the film deposited on the amorphous Si surface were the same as those on the single-crystal Si surface. The interfacially mixed AlSiOx film possessed rough surface morphology and a layered structure of Al-/Si-/Al-rich AlSiOx layers. The low-energy high-flux O2+ ion bombardment condition required for the interfacial-mixing ALD was realized in a direct inductively coupled plasma (ICP) reactor with a self-resonant planar coil, in which high-density plasma was excited near the substrate. The O2+ ion flux was found to be controllable over a wide range through variation in the O2 pressure. The ratio of O2+ ion flux at 0.01 Torr to that at 1 Torr was 289. The steep decrease of the ion flux with increasing pressure was attributed to the decrease of electron density in the upstream plasma for intensifying electron energy loss and the decrease of the ambipolar diffusion coefficient in the downstream plasma. A comparison of electron densities near the substrate and those at the presheath edge calculated from measured positive ion fluxes using the Bohm criterion revealed that negative ions, which significantly affect the positive ion flux, scarcely exist near the substrate. The interfacial-mixing PE-ALD has the potential to realize area-selective and topographically selective depositions, which are key technologies for fabricating next-generation electronic devices with 3D nanostructures. The direct ICP reactor is suitable for realizing selective deposition using the interfacial-mixing ALD.

Computational characterization of electron-beam-sustained plasma

Electron-beam-sustained plasmas are of vital importance for separately controlling ion flux and ion energy. In this paper, we use an implicit particle-in-cell Monte Carlo method to study plasma kinetics in an electron-beam-sustained plasma under operating conditions relevant to the use of such plasmas for polymer processing. The results indicate that the electron and ion densities are uniformly distributed because of the uniform ionization rate and heating rate. The electron-energy distribution function is Druyvesteyn-like with an ultrahigh concentration of low-energy electrons and a high-energy tail. Low-energy electrons are beneficial for protecting the substrate in material processing and a high-energy tail is useful for the precise control of plasma-gas chemistry. For ion-energy distribution functions at the electrode surface, the low-energy (&amp;lt;5 eV) ion occupation rate increases with decreasing beam current or beam energy. The proportion of low-energy ions bombarding the electrode exceeds 99%, which indicates the superiority of electron-beam-generated plasma compared with a voltage- or current-driven discharge to obtain independent control of ion flux and ion energy. The results obtained herein are important for nondestructive etching in plasma processing because of the unique plasma characteristics provided by electron-beam injection.

Developing a high-throughput phenotyping method for oxidative stress tolerance in barley roots

BackgroundMore than 20% of the world’s agricultural land is affected by salinity, resulting in multibillion-dollar penalties and jeopardising food security. While the recent progress in molecular technologies has significantly advanced plant breeding for salinity stress tolerance, accurate plant phenotyping remains a bottleneck of many breeding programs. We have recently shown the existence of a strong causal link between salinity and oxidative stress tolerance in cereals (wheat and barley). Using the microelectrode ion flux estimation (MIFE) method, we have also found a major QTL conferring ROS control of ion flux in roots that coincided with the major QTL for the overall salinity stress tolerance. These findings open new (previously unexplored) prospects of improving salinity tolerance by pyramiding this trait alongside with other (traditional) mechanisms.ResultsIn this work, two high-throughput phenotyping methods—viability assay and root growth assay—were tested and assessed as a viable alternative to the (technically complicated) MIFE method using barley as a check species. In viability staining experiments, a dose-dependent H2O2-triggered loss of root cell viability was observed, with salt sensitive varieties showing significantly more damage to root cells. In the root growth assays, relative root length (RRL) was measured in plants grown in paper rolls under different H2O2 concentrations. The biggest difference in RRL between contrasting varieties was observed for 1 mM H2O2 treatment. Under these conditions, a significant negative correlation in the reduction in RRL and the overall salinity tolerance is reported.ConclusionsThese findings offer plant breeders a convenient high throughput method to screen germplasm for oxidative stress tolerance, targeting root-based genes regulating ion homeostasis and thus conferring salinity stress tolerance in barley (and potentially other species).

Measuring voltage and ion concentrations in live embryos.

Abstract Homeostasis of charged particles in biological systems is fundamental for life. Indeed, the efficient synthesis of ATP is predicated on an electrochemical gradient. Ion pumps and channels act as conduits that regulate membrane potential by controlling ion flux. This phenomenon is critical for the generation of action potentials in excitable cells such as neurons and muscle fibers, and for acidification of lysosomes in all cells. However, the production of action potentials or pH differentials is merely one facet of bioelectricity. Increasing evidence has shown that ion channels and pumps also play critical roles in other cellular processes: cell cycle regulation, wound healing, regeneration, and symmetry breaking during development. In recent years, the functional roles of ion channels have been explored in echinoderm development. The application of fluorescence-based ion- and voltage-sensitive dyes allows for live measurements of ion concentrations with both spatial and temporal resolution. In this chapter, we describe the use of such dyes for interrogating and visualizing ion and voltage gradients in sea urchin embryos.

Contributions of suboolemmal acidic vesicles and microvilli to the intracellular Ca2+ increase in the sea urchin eggs at fertilization.

The onset of fertilization in echinoderms is characterized by instantaneous increase of Ca2+ in the egg cortex, which is called 'cortical flash', and the subsequent Ca2+ wave. While the cortical flash is due to the ion influx through L-type Ca2+ channels in starfish eggs, its amplitude was shown to be affected by the integrity of the egg cortex. Here, we investigated the contribution of cortical granules (CG) and yolk granules (YG) to the sperm-induced Ca2+ signals in sea urchin eggs. To this end, prior to fertilization, Paracentrotus lividus eggs were treated with agents that disrupt or relocate CG beneath the plasma membrane: namely, glycyl-L-phenylalanine 2-naphthylamide (GPN), procaine, urethane, and NH4Cl. All these pretreatments consistently suppressed the cortical flash in the fertilized eggs, and accelerated the decay kinetics of the subsiding Ca2+ wave in most cases. By contrast, centrifugation of the eggs, which stratifies organelles but not the CG, did not exhibit such changes except that the CF was much enhanced in the centrifugal pole where YG are localized. Surprisingly, we noted that pretreatment of the eggs with these CG-disrupting agents or with the inhibitors of L-type Ca2+ channels all drastically reduced the density of the microvilli and their individual shapes on the egg surface. Taken together, our results suggest that the integrity of the egg cortex ensures successful generation of the Ca2+ responses at fertilization, and that modulation of microvilli shape and density may serve as a mechanism of controlling ion flux across the plasma membrane.

An improved approach investigating epithelial ion transport in scleractinian corals

Coral epithelia control ion fluxes to the calcification site influencing biomineralization and proxy incorporation. However, data on in vivo characteristics of coral tissue such as permeability, selectivity, and active ion transport are scarce but important for calcification and proxy modeling. To investigate ion permeability and ion fluxes across coral tissues in vivo, we developed an electrophysiological approach for the assessment of active and passive epithelial transport properties. Growing Stylophora pistillata corals in a thin layer over permeable filters allowed ion exchange at the site of skeleton formation for reproducible measurements of electrophysiological properties of coral tissues in a modified Ussing chamber. Compared to former applications, electrical measurements on these coral filter units were dominated by tissue characteristics with minimal influence of skeleton or physical stress. Coral tissues were cation selective. Their overall high electrical resistance characterized them as tight epithelia indicating low paracellular permeability for passive ion diffusion. This includes ions relevant for calcification. A small short-circuit current indicates active charge transport across the entire coral tissue. The present approach is applicable to corals laterally overgrowing substrates. It allows the electrophysiological characterization of coral tissue in vivo in response to environmental conditions. This will improve our knowledge on transepithelial transport relevant for biomineralization in corals.

TiN deposition and morphology control by scalable plasma-assisted surface treatments

A method to modify the mechanical properties and morphology of thin TiN films by controlling the ion fluxes via purposefully shaped magnetic field is developed to enhance the effectiveness of plasma-enhanced deposition of TiN on a large (up to 400 mm in diameter) substrate. For this purpose, the two main schemes of the plasma control are examined. When the substrate is a part of the plasma-generating circuit, TiN is deposited in the magnetron-like arc configuration of the magnetic field. This configuration is used to control ion fluxes for cleaning, etching, and heating of the substrate, and eventually, to control the mechanical properties and morphology of the deposits. When exposing the substrate to the plasma of an external plasma source, the magnetic traps of the bottle configuration with mirrors near the plasma source and substrate surface are created. It is shown that the ion fluxes from the external plasma source can be controlled by the location and powering of the control magnetic coils, which direct nitrogen and Ti ions to the surface. The proposed method is generic and could be used for controlling various nitride materials including but not limited to BN, NbN, W2N and TaN.

Role of inward rectifier potassium channels in salivary gland function and sugar feeding of the fruit fly, Drosophila melanogaster

The arthropod salivary gland is of critical importance for horizontal transmission of pathogens, yet a detailed understanding of the ion conductance pathways responsible for saliva production and excretion is lacking. A superfamily of potassium ion channels, known as inward rectifying potassium (Kir) channels, is overexpressed in the Drosophila salivary gland by 32-fold when compared to the whole body mRNA transcripts. Therefore, we aimed to test the hypothesis that pharmacological and genetic depletion of salivary gland specific Kir channels alters the efficiency of the gland and reduced feeding capabilities using the fruit fly Drosophila melanogaster as a model organism that could predict similar effects in arthropod disease vectors. Exposure to VU041, a selective Kir channel blocker, reduced the volume of sucrose consumption by up to 3.2-fold and was found to be concentration-dependent with an EC50 of 68μM. Importantly, the inactive analog, VU937, was shown to not influence feeding, suggesting the reduction in feeding observed with VU041 is due to Kir channel inhibition. Next, we performed a salivary gland specific knockdown of Kir1 to assess the role of these channels specifically in the salivary gland. The genetically depleted fruit flies had a reduction in total volume ingested and an increase in the time spent feeding, both suggestive of a reduction in salivary gland function. Furthermore, a compensatory mechanism appears to be present at day 1 of RNAi-treated fruit flies, and is likely to be the Na+-K+-2Cl− cotransporter and/or Na+-K+-ATPase pumps that serve to supplement the inward flow of K+ ions, which highlights the functional redundancy in control of ion flux in the salivary glands. These findings suggest that Kir channels likely provide, at least in part, a principal potassium conductance pathway in the Drosophila salivary gland that is required for sucrose feeding.

Evolution of electron energy distribution function in capacitively coupled argon plasma driven by very high frequency

Capacitively coupled plasma driven by a very high frequency power has attracted much attention due to its rather independent control of ion flux and energy. In this paper, Langmuir probe diagnostic technique is used to observe the evolution of plasma properties such as electron energy distribution function, electron temperature and density, etc. Our experiment is performed in capacitively coupled argon plasma driven by a 40.68 MHz frequency. Experimental results show that the electron energy probability function changes from bi-Maxwellian type to single-Maxwellian type and then to Druyvesteyn type with the increase of the discharge pressure. At a low gas pressure, the electron collisionless heating in bulk plasma leads to bi-Maxwellian type in electron energy possibility function (EEPF), which has a double temperatures structure in EEPF. As the gas pressure increases, the electrons with low energy are able to collide with the neutral species more frequently, thus they gain energies through collisional heating. Therefore, these electrons can overcome the dc ambipolar potential and the collisional heating becomes a main electron heating mechanism. Increasing the input power enhances the electron population with low energy. From the discharge center to the edge, electron population with low energy decreases clearly due to the dc ambipolar potential, and they are unable to reach an oscillating sheath where collisionless heating occurs. However, electron population with high energy is slightly increased. The result indicates that more uniform plasma can be achieved at a high gas pressure. Additionally, EEPFs are measured for different discharge gaps between electrodes. The change of electrode gap for the plasma leads to a transition of electron heating mode along the axial direction. In order to characterize the electron behavior further, we introduce the ratio of the cold electron density to hot electron density (α) and the ratio of cold electron temperature to hot electron temperature (β). The ratios also show the proportional distributions of the cold and hot electron populations. The electrode gap has a great influences on α while little influence on β. When the discharge gap between electrodes varies from 20 to 40 mm, α changes from 0.2 to 0.5 while β has the same trend. Spatial distributions of electron density and temperature with low and high energy are also discussed.