Plasma potential measurements using an emissive probe made of oxide cathode

A novel emissive probe consisting of an oxide cathode coating is developed to achieve a low operating temperature and long service life.The properties of the novel emissive probe are investigated in detail,in comparison with a traditional tungsten emissive probe,including the operating temperature,the electron emission capability and the plasma potential measurement.Studies of the operating temperature and electron emission capability show that the tungsten emissive probe usually works at a temperature of 1800 K-2200 K while the oxide cathode emissive probe can function at about 1200 K-1400 K.In addition,plasma potential measurements using the oxide cathode emissive probe with different techniques have been accomplished in microwave electron cyclotron resonance plasmas with different discharge powers.It is found that a reliable plasma potential can be obtained using the improved inflection point method and the hot probe with zero emission limit method,while the floating point method is invalid for the oxide cathode emissive probe.

Automatic emissive probe apparatus for accurate plasma and vacuum space potential measurements

We have developed an automatic emissive probe apparatus based on the improved inflection point method of the emissive probe for accurate measurements of both plasma potential and vacuum space potential.The apparatus consists of a computer controlled data acquisition card,a working circuit composed by a biasing unit and a heating unit,as well as an emissive probe.With the set parameters of the probe scanning bias,the probe heating current and the fitting range,the apparatus can automatically execute the improved inflection point method and give the measured result.The validity of the automatic emissive probe apparatus is demonstrated in a test measurement of vacuum potential distribution between two parallel plates,showing an excellent accuracy of 0.1 V.Plasma potential was also measured,exhibiting high efficiency and convenient use of the apparatus for space potential measurements.

Steady and oscillatory plasma properties in the near-field plume of a hollow cathode

Hollow cathodes serve as electron sources in Hall thrusters,ion thrusters and other electric propulsion systems.One of the vital problems in their application is the cathode erosion.However,the basic erosion mechanism and the source of high-energy ions cause of erosion are not fully understood.In this paper,both potential measurements and simulation analyses were performed to explain the formation of high-energy ions.A high-speed camera,a single Langmuir probe and a floating emissive probe were used to determine the steady and oscillatory plasma properties in the near-field plume of a hollow cathode.The temporal structure,electron temperature,electron density,and both static and oscillation of plasma potentials of the plume have been obtained by the diagnostics mentioned above.The experimental results show that there exists a potential hill（about 30 V） and also severe potential oscillations in the near-plume region.Moreover,a simple 2 D particle-in-cell model was used to analyze the energy transition between the potential hill and/or its oscillations and the ions.The simulation results show that the energy of ions gained from the static potential background is about 20 e V,but it could reach to 60 e V when the plasma oscillates.

Experimental studies of plasma sheath near meshes of different transmissivity

The measurements of the potential distributions in the boundary layer near meshes with different mesh spacing were conducted in weakly collisional plasmas using a fine-structured emissive probe and the results of the sheath thickness and electric field at the sheath-presheath edge were compared with theoretical models of collisional presheath and collisionless sheath. It was shown that, because the meshes are partially transparent to ions, the sheath is thinner and the electric field is stronger for the mesh of higher transmissivity, owing to the increased ion density in the sheath contributed from the ions transmitted from the other side of the mesh. However, the potential profiles in the presheath remain almost the same for different meshes except for the shift of the sheath-presheath edge. The thickness of the sheath decreases while the electric field at the edge increases with the increase of the neutral gas pressure. Furthermore, depending on the pressure, the measured electric fields at the edge are close to that from the models of a transition region.

Diagnosis of a low pressure capacitively coupled argon plasma by using a simple collisional-radiative model

This paper proposes a simple collisional-radiative model to characterise capacitively coupled argon plasmas driven by conventional radio frequency in combination with optical emission spectroscopy and Langmuir probe measurements. Two major processes are considered in this model, electron-impact excitation and the spontaneous radiative decay. The diffusion loss term, which is found to be important for the two metastable states （4s[3/2]2, 4s＇[1/2]0）, is also taken into account. Behaviours of representative metastable and radiative states are discussed. Two emission lines （located at 696.5 nm and 750.4 nm） are selected and intensities are measured to obtain populated densities of the corresponding radiative states in the argon plasma. The calculated results agree well with that measured by Langmuir probe, indicating that the current model combined with optical emission spectroscopy is a candidate tool for electron density and temperature measurement in radio frequency capacitively coupled discharges.

Biothiol-specific fluorescent probes with aggregation-induced emission characteristics

Biothiols are important species in physiological processes such as regulating protein structures, redox homeostasis and cell signalling. Alternation in the biothiol levels is associated with various pathological processes, therefore non-invasive fluorescent probes with high specificity to biothiols are highly desirable research utilities. Meanwhile, fluorescent probes with aggregationinduced emission properties(AIEgens) possess unique photophysical properties that allow modulation of the sensing process through controlling the aggregation-disaggregation or the intramolecular rotational motions of the fluorophores. Herein we review the recent progress in the development of biothiol-specific AIEgens. In particular, the molecular design principles to target different types of biothiols and the corresponding sensing mechanisms are discussed, along with the potential of the future design and development of multi-functional bioprobes.

A Label-free Fluorescent Aptasensor for Turn-on Monitoring Ochratoxin A Based on AIE-active Probe and Graphene Oxide

A label-free fluorescent aptasensor for specific and ultrasensitive monitoring ochratoxin A（OTA） was developed using the specific aptamer of OTA（OSA） as recognition dement, an aggregation-induced emission（AIE） molecule（a 9,10-distyrylanthracene with two ammonium groups, DSAI） as a fluorescent probe, and graphene oxide（GO） as a quencher. In the absence of OTA, the AIE probe DSAI and OSA complex（DSAI/OSA） is adsorbed on the GO surface, and the fluorescence of DSAI will be quenched efficiently via the fluorescence resonance energy transfer（FRET） from DSAI to GO. Upon the OTA addition, a more stable complex（OSA-OTA） is formed and released from GO. Meanwhile, DSAI and OSA-OTA can form a new complex（DSAI/OSA-OTA）, then the fluorescent signal of DSAI recovers gradually. Therefore, by introducing GO and DSAI, the fluorescence signal of DSAI can be easily turned from ＂off＂ to ＂on＂ after the addition of OTA, and the ultrasensitive detection of OTA by monitoring the change of the fluorescence signal of DSAI can be readily realized. The detection limit of the assay can reach 0.324 nmol/L with a linear detection range of 10-200 nmol/L. And the aptasensor exhibits high selectivity for OTA against other analogues. Moreover, it has been successfully applied for the detection of OTA in red wine samples.

A near-infrared fluorescent probe for monitoring fluvastatin-stimulated endogenous H_2S production

Most reported fluorescent probes have limitations in practical applications in living systems due to the strong autofluorescence background,construction of probes with near-infrared（NIR） fluorescence emission is an accessible approach for addressing this challenge.We here designed a NIR fluorescent probe for monitoring the endogenous production of H2S in living cells.The designed probe showed significant NIR fluorescence turn-on response to H2S with high selectivity,enabling the sensitive detection H2S.Importantly,the probe could be applied in monitoring the endogenous production of H2S in raw 264.7 macrophages.This study showed that fluvastatin can promote the activity of cystathionineγ-lyase（CSE） for generation H2S.

AIEgen based light-up probes for live cell imaging

Fluorescent light-up probes comprising a tetraphenylethene unit with aggregation-induced emission(AIE)characteristics and a water-soluble peptide have been designed and synthesized which provide cell membrane and nuclear permeability to live cells.This strategy has offered new opportunities for the development of probes with light-up ability and good signal-to-noise ratio.The selectivity or targeting specificity is determined by the peptide sequence,i.e.a nuclear localization signal that leads to nucleus imaging and a cell biomarker targeting peptide that offers specific light-up imaging of HT-29 cells.