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Common interfering speciesAscorbic acid
Irreversible
Common interfering speciesUric acid
Irreversible
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How do we detect all 3 species at once?
Next week
Detecting dopamine contd..Modified electrodes
Carbon paste
Carbon nanotubes -Carbon paste
Ag nanoparticles Carbon paste
Ag nanoparticlesCarbon nanotubes Carbon paste
Synergistic effect
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Can we improve resolution between signals?
• Differential pulse voltammetry can be considered as a derivative of linear sweep voltammetry
• This technique involves a series of regular voltage pulses superimposed on the potential linear sweep.
• The current is sampled immediately prior to the application of the pulse and at the end of the pulse and the difference is recorded. This gives rise to a differential current response.
• This eliminates the effect of the charging current (remember chronoamperometry)
It is used for electroanalytical chemistry – can detect quantities of solute of the order of 10-8 M
In these measurements, the effect of the charging current can be minimized, so high sensitivity is achieved.
Faradaic current is extracted, so electrode reactions can be analysed more precisely.
Reversible reactions show symmetrical peaks, and irreversible reactions show asymmetrical peaks.
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Binary mixture
Keep AA concentration the same and vary DA concentration
Keep DA concentration the same and vary AA concentration
Nanostructured surfaces – electrochemically made
AFM images of a) electrodeposited pyramidal, b) rodlike,c) spherical, and (d) a sputtered gold surfaces.
Pyramidal, rodlike, and spherical gold nanostructures wereelectrodeposited from aqueous solutions of 0.1 M HClO4
containing 40, 4, and 40mM HAuCl4, respectively, at 0.08,0.08, and 0.2 V vs. Ag/ AgCl, respectively, for 2 min.Shape is controlled by applied electrode potential and concentration of HAuCl4.
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Separation of AA and UA
We observe here electrocatalytic effects –each electrochemical reaction is catalysed to a different extent – rates of electron transfer are different as well as the peak positions – allows us to resolve the processes
Keep AA concentration the same and vary UA concentration
Vary the concentration of both AA and UA
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Chronoamperometric detection under stirring conditions – AA detection at 10 μM additions - electrode held at 0.0 V vs Ag/AgCl
Pyramids > Rods > Spheres : shape dependent electrochemistry
Use electrostatics• It is known that ascorbic acid exists in its anionic form (pKa 4.1) and dopamine in its cationic form (pKa 8.9) at physiological pH 7.4
• We can take advantage of this by modifying the electrode surface with a charged species e.g. Nafion®
• Nafion® is one of the most commonly studied cation exchange polymers
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Upon hydration, Nafion absorbs relatively large amounts of water and can increase its dry weight by as much as 50 percent or more depending upon equivalent weight, counter ion, and temperature
Therefore Nafion with all this negative charge would repel anionic ascorbic acid and attract the cationic dopamine, therefore an electrode modified with Nafion in a binary mixture only shows a signal due to dopamine
DA signal only
In this case we do not necessarily need a nanostructured electrode – a smooth one will suffice