1,721,023 research outputs found
Sviluppo di dipositivi micro e nanometrici e loro impiego in microscopia elettrochimica a scansione (SECM)
No full textThis thesis is focused on the use of electrochemical techniques and microscopy such as scanning electrochemical microscopy and the recently developed Intermitted Contact SECM (IC-SECM). With the using of these techniques, the chemical and electrochemical properties of micro and nanostructured multilayer thin films of Pt/Al2O3 and borod doped diamond (BDD) were studied. Furthermore, a simulated model that describes the mass transport phenomena in particular SECM experiments was developed. In particular, the model takes into account of the recession degree of an active substrate with different dimensions. The model was experimentally verified. Finally, protocols for the preparation of H+ ion-selective microelectrodes for IC-SECM were developed.Il lavoro svolto in questa tesi di dottorato è stato focalizzato nell’impiego di tecniche elettrochimiche e, in particolare, di tecniche di microscopia elettrochimica quali la microscopia elettrochimica a scansione (SECM) e la più recente Intermitted Contact SECM (IC-SECM). Mediante l’impiego di queste tecniche elettrochimiche, sono state studiate le proprietà chimiche e di reattività elettrochimica di materiali micro e nano strutturati quali film multistrato di Pt/Al2O3 e di diamante drogato con boro (BDD). Parallelamente, sono stati presi in considerazioni aspetti teorici riguardanti la SECM e sono stati simulati i fenomeni di trasporto di massa nel caso di substrati costituiti da regioni attive di dimensione variabile, caratterizzati da un certo grado di recessione. Questo studio ha consentito la formulazione di un modello che è stato verificato sperimentalmente. Infine, sono stati messi a punto dei protocolli per la preparazione di sonde potenziometriche iono-selettive per H+ per IC-SECM
Determination of Trace Bismuth by Under-Potential Deposition-Stripping Voltammetry at Mesoporous Platinum Microelectrodes: Application to Pharmaceutical Products
No full textA methodology for the determination of bismuth,
based on under-potential deposition-stripping voltammetry
(UPD-SV), was investigated. It makes use of mesoporous
platinum microelectrodes (Pt-MEs) prepared by a liquid
crystal templating technique. The mesoporous microelectrodes,
which are characterised by a very high surface area,
allowed the accumulation of relatively large amounts of
bismuth at under-potential without saturation of the electrode
surface. Calibration plots for quantification of bismuth
at micromolar levels were constructed by using the charge
involved in either the anodic or cathodic peak recoded by
cyclic voltammetry that ensued the accumulation of bismuth
at the electrode surface. During the anodic scan, the oxidation
of metallic bismuth occurred; the cathodic scan involved
irreversibly adsorbed bismuth species, which are
retained on to the electrode surface. The reproducibility of
the proposed UPD-SV procedure (which was within 5 %)
was assured by the application to the Pt-MEs of a suitable
potential waveform, properly designed to avoid memory
effect due to the irreversibly deposited bismuth. The latter
phenomenon along with UPD allowed to overcome interference
due to copper, which is normally observed when
quantification of bismuth is performed by anodic stripping
voltammetry at solid electrodes involving bulk metal deposits.
The usefulness of the proposed method for the determination
of bismuth in real samples was demonstrated by the
analysis of a tablet of a pharmaceutical preparation, which is
used for curing ulcers
EFFECT OF THE INSULATING SHIELD THICKNESS ON THE STEADY-STATE DIFFUSION-LIMITING CURRENT OF SPHERE CAP MICROELECTRODES
No full textThe effect of the insulating shield thickness on the steady-state diffusion-limiting current of sphere caps microelectrodes is investigated. Theoretical steady-state limiting currents are obtained by using a simulation procedure, which relies on the explicit finite difference method with a fixed time grid and an exponentially spatial grid. The results obtained indicate that the current increases by decreasing the thickness of the insulating sheath or by increasing the aspect ratio of the sphere cap (h/a, where h is the height of the sphere cap and a is the electrode basal radius), similarly to other types of microelectrodes with different electrode geometry, such as disks and finite cones. The simulated data are fitted to approximate analytical expressions to describe the dependence of the limiting current on both h/a and RG (RG = b/a, where b is the overall tip radius) parameters. Theoretical currents are also compared with experimental data, which are obtained with a range of mercury coated platinum microelectrodes having different RG and h/a values. The measurements are performed by using cyclic voltammetry at 1 mVs-1, in aqueous solutions containing Ru(NH3)6Cl3 as electroactive species. A good agreement (within 3%) between theoretical and experimental steady-state currents is found. Finally, SECM operating in the feedback mode is used to asses the validity of the shape parameters found by voltammetry for sphere cap microelectrodes, whose insulating shields are of thickness comparable to the electrode radius
A Scanning Electrochemical Microscopy procedure for micropatterning Al2O3-thin films deposited on a platinum substrate.
Full text linkA scanning electrochemical microscopy (SECM) procedure for patterning alumina thin films deposited onto a platinum film (Pt/Al2O3) was investigated. The alumina surface was locally etched by hydroxide ions, which were generated at platinum microelectrodes, by exploiting the hydrogen evolution process in aqueous solutions. The local base-induced dissolution of Al2O3 to soluble AlO2- species allowed the exposure of the underlying platinum, which resulted in well-defined conducting spots embedded within an insulating matrix. Reproducibility of the spots was achieved by adding to the electrolyte solutions amounts of EDTA, which acted as a scavenger for OH- ions, while allowed the complexation of the aluminium ions formed at the sample surface during the wet etching process. The effects of electrolysis time, tip to substrate distance and overall tip dimension on the shape and size of the conducting spots were investigated in detail. Furthermore, the possibility to create complex patterns with the optimised etching procedure was also shown
Platinum/Alumina thin films prepared by r.f. magnetron sputtering as platforms in voltammetric sensing
No full textThin alumina films coated platinum substrates (Pt/Al2O3), prepared by r.f. magnetron sputtering, wereused as working electrodes for voltammetric measurements. The thickness of the alumina layers var-ied between 50 and 1000 nm. For comparison, measurements were also performed with a platinummicrodisk 12.5 µm radius (Ptdisk). Several electroactive species either charged or uncharged, dissolved inaqueous, non-aqueous and micellar media were probed by the Pt/Al2O3and Ptdiskelectrode systems. Inall cases, sigmoid well-shaped cyclic voltammograms were recoded, typical for microelectrodes work-ing under steady-state. It was established that the Pt/Al2O3samples behaved as ensembles of recessednanoelectrodes, wired in parallel, each nanoelectrode sufficiently far apart, such that no overlap of indi-vidual radial diffusion profiles occurred. The recessed nanoelectrodes were due to defects or pinholesformed during the alumina deposition. The analysis of the steady-state limiting currents, obtained atthe different electrode systems, allowed verifying that no pore size effect was evident for either chargedor uncharged species, whose average hydrodynamic radii were lower than 1 nm. Size exclusion effectswere instead observed at the thickest alumina film, using ferrocene dissolved in the hydrophobic coreof sodium dodecylsulphate micelles, whose hydrodynamic radius is about 2 nm. The Pt/Al2O3systemswere also used as substrates for electroplating. In this way, ensembles of platinum black microelectrodeswere prepared and employed for the voltammetric sensing of hydrogen peroxide
Micropipette Contact Technique as a Tool to Reveal, Characterize, and Modify Nanopore Electrodes
No full textThis paper reports on the use of a micropipette contact
method (MCM) to reveal and characterise single submicrometer-
sized pores randomly distributed within a thin insulating
alumina layer (250 nm thickness) that coats a platinum film
substrate (Pt/Al2O3). Moveable micropipette probes, with orifice
radii of 40–50 um, filled with electrolyte solutions, are brought
in contact with the sample surface to form a microcell and
allow electrochemical reactions to be confined within a limited
contact region. The aqueous solutions inside the micropipettes
contain Ru(NH3)6
3+ or Ag+ ions and cyclic voltammograms
(CVs) are obtained for several locations of 500-500 um2 above
the Pt/Al2O3 sample. The presence of nanoscopic pores is revealed
by sigmoidally shaped volatmmograms, which are recorded
when Ru(NH3)6
3+ is used as redox probe. From the
steady-state limiting current and the theoretical equation that
applies for recessed microelectrodes, the radius of each platinum
disk at the bottom of the pore is evaluated. In this way,
pore sizes spanning from about 50 nm to 350 nm are revealed.
Similar information is obtained by the use of the MCM with
the pipette filled with Ag+ ions. In this case, CVs typical for nucleation
and growth of metallic silver are recorded. The
volume of a single pore is evaluated from the charge involved
in the anodic stripping process of the metal deposited. Potential
step measurements and relevant current (I) against time (t)
transients allow establishing that only a single nucleus can be
grown in each submicometer-sized pore. Moreover, a diffusion
coefficient value of 1.41_10_5 cm2 s_1 for Ag+ is evaluated
from the experimental I vs. t1/2 linear plot. Finally, the measurements
performed at the Pt/Al2O3 sample are compared with
those obtained by using a plain platinum disk electrode 3 mm
diameter as substrate for MCM measurements
Scanning Electrochemical Microscopy and Voltammetric Investigation of Silver Nanoparticles Embedded within a Nafion Membrane
No full textIn this paper, silver nanoparticles (AgNPs), incorporated in the poly(perfluorosulfonic) acid membrane Nafion® 117, were fabricated by chemical reduction of the Ag+-loaded membranes, using HCOH in a basic medium as reducing agent. The AgNPs content within the membrane varied in proportion of the concentration of AgNO3 used in the incorporation step of Ag+ in the ion-exchange. Information on size and density of the AgNPs within the matrix was obtained by SEM analysis. The size of the AgNPs varied between 10 and 20 nm, regardless of the AgNO3 concentration used for their preparation, while AgNPs density was the higher, the greater was the AgNO3 concentration in the solution. Information on the oxidation state of Ag in the AgNPs and their distribution and aggregation on the surface of the Nafion membranes were obtained by SECM measurements. The use of [Ir(Cl)6]3- and Ru(NH3)63+ as redox mediators allowed establishing whether the AgNPs were interconnected to create nano-chain networks to provide AgNPs-loaded Nafion membranes with electronic conductivity. Moreover, combination of SECM and anodic stripping voltammetry allowed establishing the reactivity of the AgNPs-Nafion composites in terms of Ag(I) species released at the nanocomposite/solution interface. These aspects are relevant for the potential application of these AgNPs-loaded membranes for antimicrobial protection purposes
Bismuth-coated mesoporous platinum microelectrodes as sensors for formic acid detection
Full text linkMesoporous platinum microeletrodes (MPtEs) modified by sub-monolayers of irreversibly adsorbed bismuth (Bi-MPtE) were investigated for their potential use as sensors for the detection of formic acid in direct formic acid fuel cells. The mesoporous platinum films were prepared by electrodeposition of platinum on Pt microdisks substrates 25 m diameter, from hexachloroplatinic acid dissolved in the aqueous domain of the lyotropic liquid crystalline phase of octaethylene glycol monohexadecyl ether. The roughness factor (RF) of the MPtEs was about two orders of magnitude greater than those of the corresponding polished microelectrodes. Bismuth ad-atoms onto the platinum surface were deposited by under potential deposition from 1 mM Bi3+ ions in 0.5 M H2SO4 solutions. The catalytic activity of a series of Bi-MPtEs, characterized by different roughness and fractional bismuth coverage (Bi), towards the oxidation of HCOOH, was investigated by cyclic voltammetry and potential step experiments. Compared to MPtEs, Bi-MPtEs displayed enhanced electrooxidation currents at lower potentials. The stability of irreversibly adsorbed bismuth, and consequently the Bi-MPtEs catalytic activity, was found to depend on the high potential limit employed in the measurements. In general, both electrode stability and electrocatalytic performance were good, provided that the operational potential was kept 0.4 V vs. Ag/AgCl. Bi-MPtEs with Bi > 0.3 provided almost sigmoidal shaped waves with low hysteresis, as those expected for microelectrodes working under steady state. The effect of concentration of HCOOH was investigated over the range 0.01 – 5 M, and linearity between current and concentration depended on both roughness factor and bismuth coverage. A Bi-MPtE characterised by RF = 210 and Bi ≥ 0.6 provided linearity up to 2 M of formic acid. Reproducibility of the sensors was within 2% (r.s.d). The same sensor, under the optimized experimental conditions, could be employed for at least two months with negligible loss of the initial performance
Characterization of metal-supported Al2O3 thin films by scanning electrochemical microscopy
No full textIn this paper, physiochemical properties of amorphous alumina thin films, grown by the metal organic chemical vapour deposition process on the surface of platinum (Pt/Al2O3) and stainless steel (SS/Al2O3), were investigated in aqueous media. The study was performed by the use of scanning electrochemical microscopy (SECM), which allowed obtaining information on uniformity, topography and chemical stability/reactivity of the alumina coatings with high spatial resolution. In particular, the effects due to local acid, base and fluoride ions attack on alumina layers of thickness of about 250 nm (in the Pt/Al2O3 sample) and 1000 nm (in the SS/Al2O3 sample) were investigated. In the acid and base attack, high concentrations of H2SO4 and KOH were electrogenerated locally by the use of a 25 m diameter platinum microelectrode. The latter was also used as SECM tip to monitor the chemical effect on the alumina layers. It was found that, regardless of the thickness of the film, alumina provided good resistance against local attack of concentrated H2SO4; instead, the film dissolved when subjected to KOH attack. The dissolution rate depended on several experimental parameters, such as SECM-tip to substrate distance, electrolysis time and alumina film thickness. The alumina layer proved also relatively poor resistance to etching in 0.1 M NaF solutions
Voltammetric Determination of Glucose at Bismuth-Modified Mesoporous Platinum Microelectrodes.
No full textMesoporous platinum microeletrodes (MPtEs) modified by a mono- or submonolayer of adsorbed bismuth (Bi-
MPtE) were investigated and employed for the detection of glucose in 0.2 M NaOH solutions. The mesoporous
platinum films were electrodeposited from hexachloroplatinic acid dissolved in the aqueous domain of the lyotropic
liquid crystalline phase of octaethylene glycol monohexadecyl ether. Bismuth ad-atoms were obtained by under
potential deposition of Bi3þ ions in 0.5 M H2SO4 solutions. The coverage of the MPtEs by irreversibly adsorbed
bismuth was checked in either a 0.5 M H2SO4 or 0.2 M NaOH solution free of Bi3þ ions, and exploiting the charge
involved in the hydrogen adsorption/desorption peaks, which decreased in proportion to the amount of platinum sites
covered by bismuth. The catalytic activity of a series of Bi-MPtEs towards the oxidation of glucose was investigated by
cyclic voltammetry, and the overall behaviour was found to be superior of that of the corresponding MPtEs. Also, at
Bi-MPtEs, the effect of interference by ascorbic acid was minimal. Calibration plots were examined over the glucose
concentration range from 0.5 mM to 300 mM. It was found that linearity, between current against glucose
concentration, and sensitivity depended on the electrode type and real surface area. At the Bi-MPtEs both linear
range and sensitivity were larger than those of the MPtEs. These results were discussed in terms of high tolerance of
the Bi-MPtEs towards the intermediate poisons originated in the electrooxidation process of glucose
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