Application of Fourier Transform Voltammetry in Micro or Nano Scale Electrochemistry

Author:Liu Zuo

Supervisor:wang li shi


Degree Year:2019





Single nanoparticle collision on ultramicroelectrode(UME)has been prevailed in microelectrochemistry in recent decade.The so called‘nano-impacts’allows us to capture one nanoparticle on the electrode to figure out the mass transfer rate,size distribution and chemical properties with respect to nanoparticles in the bulk electrolyte.Comparing with TEM,SEM or other traditional nanoparticle characterization method,it acquires just a simpler instrument with lower cost and higher repeatability,and directly revealed the electrochemical properties of individual nanoparticle.However,thoughout the previous studies,the most frequent use of characterization method to record the collision signals is chronoamperometry,which is only able to obtain a current transient reflected limited chemical properties about single nanoparticle.Here,we demonstrated Fourier Transform Fast-scan Cyclic Voltammetry(FSCV)and Sinusoidal Voltammetry(FTSV)to characterize the electrochemical transient reaction in micro or nano scale.With the application of real time background subtraction and Fourier Transform‘finger-print’phase angle characterization,FSCV and FTSV can provide more valuable informations in time domain and frequency domain with respect to the properties and kinetics of individual nanoparticle.The application is listed as follows:(1)The understanding of the physic and chemical properties of the plated mercury film on a required electrode surface is essential in mercuric based stripping voltammetric analysis but still need to be enhanced till now.In this section,we focus on a 10μm platinum ultramicroelectrode(Pt UME)to electroplate mercury by means of FTSV.The amount of deposition and the situation of the electrode surface could be reflected in both time domain and frequency domain of FTSV response.To investigate the details,the current transients were corresponding to the migration and coalescence of mercury droplets,about which the obtained background subtracted phase angle components of the fundamental harmonic were always-90°.This characteristic phase angle component could be a symbol to indicate the heterogeneous interface interaction event thoughout the mercury electroplating process.The magnitude response of current transient was in proportion of the decrement of effective surface area resulted from the coalescence of mercury droplets on the platinum surface.At the end of the experiment,FTSV was continuously applied to reveal the mercury film electrode‘collapse’event soon after the potentiostat was disconnected with the working electrode.It suggested that the presence of trace amount of Cl-would bring about serious effect to mercuric based stripping voltammetric analysis.(2)Ion transfer at single oil/water emulsion droplets via collision event is innovative but of great challenge in micro or nano scale electrochemistry.To investigate ion transfer via single emulsion toluene droplets collision,fast-scan cyclic voltammetry(FSCV)is applied.The electron transfer at the UME/oil interface is coupled with ion transfer at the oil/water interface.Thus,the obtained transient voltammograms of a myriad of ions were used to estimate thermodynamics of ion transfer at the toluene/water interface.The phase angle components of the second harmonic of the obtained FSCV responses shown the specificity to the species of transferred ion,indicated it is very sensitive to detect any micro faradic reaction.Besides,the ratio of the oxidative peak current and the reductive peak current equal to 1 indicated that the system was reversible.The relationship between the half wave potential and the transferred ion concentration shown in line but opposite of prediction.The comprehensive process of charge transfer coupled with ion transfer in droplet collision on carbon fibre UME(C UME)was acquired to be further studied.(3)Understanding the relationship between the obtained voltammetric response and the mechanism of single emulsion droplets collision on C UME is essential to build up the system of studying ion transfer at toluene/water micro emulsion droplets interface.In order to study,we applied COMSOL to figure out the situation of single toluene droplet adhering the C UME substrate by simulating the obtained voltammetric response.We ascertained the ratio of adhering height on the carbon fibre substrate and the droplet radius was f=1.4 and found out the relationship between droplets size,adhering ratio f and the obtained peak currents and potential differences of voltammetric responses.It suggested the liner relationship between the square of droplet radius and the amplitude of voltammetric peak current.Meanwhile,the electron transfer constant rate k0,ET,ET was obtained to be 0.01 cm·s–11 while the ion transfer constant rate k0,IT=0.02 cm·s–1.The current decay of the obtained signals indicated that the toluene droplets fell off or slipped to the edge of the electrode.By employing Fourier Transform Sinusoidal Voltammetry(FTSV),we inferred four possible mechanisms of droplets collision on the electrode surface and corresponding their phase angle responses.(4)Due to the intricate mechanism of electron transfer coupled with ion transfer at the emulsion droplet impact system,and difficulties in finding the suitable hydrophobic redox couple,we applied micro liquid/liquid interface system in place of ultramicroelectrode substrate as the probe to characterize emulsion droplets collision with ions transfer.Single inverse emulsion droplet(water in oil)collided and coalesced stochasticly with the tip of micropipette full of aqueous electrolyte,and FSCV/FTSV was applied to record the impact response to obtain the single fusion event voltammogram.The obtained w/o inverse emulsion could be stable for the whole detection period.With FSCV detection,the ion transfer potential of N+1111,N+2221,N+1114,N+2222,N+3333 was obtained and in line with the hydrophobicity of these ion.The decay of the current response indicated that aqueous droplet was fused with the micro liquid/liquid interface with only tens of millisecond and transferred ion was diffused off with inner aqueous electrolyte.Especially,a blank droplet with the same electrolyte as the inner pipette solution collided with the micro liquid/liquid interface could also induce a typical voltammogram.The voltammogram suggested that collision lead to the capacitative amplification and destruction of electric double layer at micro soft interface.At last,we preliminarily studied the assisted ion transfer of K+by DB18C6 with emulsion droplet collision system,the assisted ion transfer mechanism still needed to be further studied.In conclusion,the study brought up new inspritation to moniter single emulsion droplet.(5)During previous study,it suggested that liquid/liquid soft interface was an ideal electric double layer where was sensitive to catch single entity penetration response.As a result,we set up a micro water/dichloroethane(w/DCE)interface to capture platinum nanoparticle(Pt NPs)and multi-wall carbon nanotubes(MWCNTs)penetration impacts.Tha capacitative impacts shown regularly change depending on the applied potential and highly relavent to the size,properties and morphology of both nanoparticles.For example,the impact of Pt NPs penetration shown regular current pulses while the impact of MWCNTs penetration shown both pulses and staircases.The amplitude of current response of both impact species shown much stronger at the potential positive of pzc than at the potential negative of pzc,which indicated the different ability of positive and negative charge absorption.Moreover,FTSV results shown the typical‘eight’shape voltammetric response of either species of impacts.And the phase angle component of the fundamental harmonic corresponding to single entity penetration of Pt NPs and MWCNTs shown similar results,but of the second harmonic they varied from each other.This might be due to the different absorption and desorption kinetics of both kinds of nano entities.