27/08/2013

1

Dopamine detection(important neurotransmitter)

EC mechanism

Implantable electrodes

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2

Common interfering speciesAscorbic acid

Irreversible

Common interfering speciesUric acid

Irreversible

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3

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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4

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.

27/08/2013

5

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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6

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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7

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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8

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