Transcript Tools II Voltammetric Detectors 1. 2. 3. 4. Methods: Stripping, Metals DPV LC EC Rat Brains Anodic Stripping Voltammetry 1.

Tools II Voltammetric Detectors
1.
2.
3.
4.
Methods: Stripping, Metals
DPV
LC EC
Rat Brains
Anodic Stripping Voltammetry
1. General Process
2. Hanging Mercury Drop Electrode (HMDE)
3. Mercury Film Electrode
4. Non-mercury Electrode
1. Some electrode surface that forms an alloy with metals,OR, which
catalyzes deposition
2. Preconcentrate metal by reducing from large volume into the alloy
3. Strip with an anodic sweep to oxidize from the alloy
Coulombs   N moles 
 nelectrons  
id t d    mole   moleelectron   s  t d   moles
C
*
M
id t d

 4 3
nF    ro
 3
Deposition current
And time of deposition
Concentration in the drop
HMDE, spherically shaped
ro
5
1/ 2


0
.
725
x
10
nD


M
1/ 2 *
5
3/ 2 1/ 2
i p  AD M C M  2.69 x10 n  

ro


Compare to the Randles-Sevich Equation:
i p  2.69 x105 n 3/ 2 ADox1/ 2 1/ 2 Cox*
For a Mercury Film Electrode the rate of arrival of the metal atoms at the surface
For oxidation is altered due to the geometry of the electrode
2
2
n F v AC
ip 
2.7 RT
*
M
Compare to the peak for voltammetry at a thin layer electrode:
n F  VC
i 
4 RT
2
Because the current at a thin film
Electrode is of the same class as
p
Current at a thin film, the peaks drop
To baseline, resulting in better deiscrimination of
Metals with similar potentials
2
0
Figure 11.8.6 bard and faulkner
Tool II Voltammetric Detectors
1.
2.
3.
4.
Methods: Stripping, DPV
Metals
LC EC
Rat Brains
Goal to use the electrstatic attraction for accumpulation
Similar goal in using Nafion – also anionic material easily manipulated
, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Chem. Rev. 2004, 104, 4587. (b) Haile, S. M. A
Note the selectivity in potential and the discrimination between various
metals
Makes use of the enhanced microscopic surface area and electric fields of the
JChem Ed 2007
Tool II Voltammetric Detectors
1.
2.
3.
4.
Methods: Stripping, DPV
Metals
LC EC, LC EC MS
Rat Brains
Many organics are easily oxidized
absolute value m
-3
-2.5
-2
E vs SCE
-1.5
-1
-0.5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0
0.5
1
1.5
2
Most solid metal electrodes do not oxidize in
this range and so have low background current
Many others are easily reduced
All are candidates for voltammetric analysis
Signal to Noise in Voltammetric Analysis
The limit of detection (LOD) is often defined as 3xpeak to peak signal due to noise
“Noise” in voltammetric methods is the result of
1. Competing or background electron transfer events originating in the
a) Solvent
b) Surface
2. Currents arising from charging of the electrode surface
The limit of detection is also affected by distortion of the signal due to IR error
Solutions to the IR and Capacitive Current Problems
1 C
C
i  nFAD
 nFAD 2
1
Dt
 t  2
icapacitive
E

exp
Rs
 t 


 Rs C 
Signal
Background
These are on two different time scales!
Modulate the time domain of the experiment to
discriminate against capactive currents.
Square Wave Voltammetry
1 cycle
E
tp
1
2
2Epulse
 ip 
nFACO* D O
t p
 p
Estep
 p  dim ensionless peak current
t
Kissinger
Bard
AC Voltammetry
0.3
0.2
0.1
0.2
0.1
E
-0.1
0
-0.1
Ac perturbation
-0.2
-0.3
-0.2
0
50
100
150
200
250
300
350
400
450
Time
-0.3
0
50
100
150
200
250
300
350
400
450
0.3
Time
0.2
0.1
DC sweep
E
E
0.3
0

0
-0.1
E
-0.2
-0.3
0
50
100
150
200
250
300
350
400
450
AC Voltammetry
1/ 2
 nF 



D
RT
R
2
o
 E dc   E 




4 RT cosh 
ln
1
/
2

nF  DO    

 RT 
Ip 
n 2 F 2 A 1/ 2 Do CO*  E
4 RT

w
64
49
35
25
16
I
I
n 2 F 2 A 1/ 2 Do CO*  E
-0.3
-0.2
-0.1
0
Edc
0.1
0.2
0.3

Differential Pulse Voltammetry
  exp 
 nFA Do CO*  
   ' 

 
I  


t

  1   1   ' 
Want
 i max
nf E  E o
 nFA Do C
 
     '
  exp
  1  
 


 1  
 '  exp 

nf E   E  E o
i
 i  i  i '
*
O
DO
DR
E  E
fE
2
i '
t

Summary
0.3
0.2
AC voltammetry
Ip 
2
Do C  E
1/ 2
*
O
4 RT
0.1
E
n F A
2
0
-0.1
-0.2
-0.3
0
50
100
150
200
250
Time
DP Voltammetry
SW Voltammetry
 i max
 ip 
 nFA Do CO*   1   
 
 



1


     ' 
nFACO* D O
t p
 p
300
350
400
450
GCE
LC EC
Control xD at a fixed distance:
Rotating Disk electrode, RDE
Wall Jet Electrode, WJE
RDE
WJE
 D 13  16 

x D  161
. 
1 

 2 
1
5
1 5


3 D 3  12 a 2 x 4

x D  0.986 4 
3


4
V


  kinematic vis cos ity
  angular velocity electrode   2f
(1 / s)
f  rotation rate of electrode
r  electrode radius (cm)
x  dim ension along electrode surface from jet
a  jet orifice diameter (cm)
V  velocity of propelled solution (ml / s)
Glassy Carbon Electrode
CFME= Carbon Fiber Microelectrode
Poly(3-methylthiophene) (P3MT)
Bare=dotted lines
Set as Eapp
Point is that the peak is lost under flow
conditions, so that the selectivity is lost
Set as Eapp
Point is that the peak is lost under flow
conditions, so that the selectivity is lost
LC EC MS
Some analytes are poorly suited to MS because of the difficulty in the ionization
Pathway.
P-chloroaniline (CPA) is one example
Anticarcinogen
sulofenur
Another way to get
Ions for the mass
Spec is to allow
Electrochemical
Reactions with donors
And or acceptors
TMPD can be used as
A donor for PAH, while
Dicyanodichloroquinone
DDQ is used as an
acceptor
Nice exam question,
Why?
Tool II Voltammetric Detectors
1.
2.
3.
4.
Methods: Stripping, DPV
Metals
LC EC
Rat Brains
An example of a unique system of electrochemistry which invokes
1. Concepts of microelectrodes
2. Suppression of capacitance
3. Blocking of surfaces to enhance selectivity
4. And the diquinone functionality just examined is
The In vivo detection of dopamine in rat brains for nuerochemistry studies
Diffusion in more than one dimension (Non-linear)
C
 2C
 D 2
t
x
Planar Electrode
C
 2C
 2C
 2C
 D 2 D 2 D 2
t
y
x
z
Planar microelectrode
nanorod
Diffusion in more than one dimension (Non-linear)
C
 2C
 2C
 2C
 D 2 D 2 D 2
t
y
x
z
C
 2C
 2C
 2C
 D 2 D 2 D 2
t
y
x
z
This equation solved for Cottrell conditions
0.7823


4nFADO CO* 
0.8862

 0.7854 

i ss 
 0.214e
rO



4 DO t
 2
ro
As t goes to infinity:
4nFADO CO*
 0.9994
iss 
rO
Planar microelectrode
C = 1 mM; r = 250 nm, n = 2, D = 1x10-5 cm2/s
55
E
Normalized current
current
Normalized
44
Eo
33
t
t=0
22
Cottrell
Cottrell
The difference in
Shape can be used
To simultaneously
Determine n and D
Microelectrode
11
00
00
55
10
10
15
15
Time
Time (s)
(s)
20
20
25
25
Fitting of the potential step chronoamperometry (inset) leads to:
n= 2
D = 2.7x10-10 m2/s
Paddon, Christopher A., Debbie S. Silvester, Farrah L. Bhatti, Timothy J. Donohoe, Richard C. Compton,
Coulometry on the Voltammetric Timescale: Microdisk Potential-Step Chronoamperometry in Aprotic Solvents
Reliably Measures the Number of Electrons Transferred in an Electrode Process Simultaneously with the Diffusion
Carbon Nanotubes
Two types of carbon surfaces
nanorod
Nanotube surface
Contains the “basal”
Plane of layered carbon
And the broken edges
59 1/s
170 1/s
The two surfaces
Have different e.t.
Rate constants
170 1/s
S. Tsujimura, T. Nakagawa,
K. Kano, and T. Ikeda
Electrochemistry 2004 72
437-439
59 1/s
http://www.nanoscienceworks.org/nanopedia/557px-eight_allotropes_of_carbon.png
Desirable as electrode surface because of enhanced flux
(3D solution to Fick’s Laws)
Campbell Sun Crooks JACS 1000 121 3779
Jun Li, et al JPChem B, 2002, 106, 9299-9305
Multiple walled nanotube
Dopamine is a potent neurotransmitter and hormone in the brain.
Can be supplied as medication that acts on the sympathetic nervous
system producing effects such as increased heart rate and blood pressure.
Deficits in dopamine are linked to behavioral diseases such as attentiondeficit hyperactivity disorder (ADHD).
Abnormally high dopamine levels have been linked to psychosis,
schizophrenia and cocaine addiction
Typical cylindrical carbon-fiber microelectrodes are 5-30
mircometers in diameter and 25-400 micrometers in length.
Due to the small size of the probe, minimal tissue damage
occurs during insertion into the brain
B. Jill Venton and R. Mark Wightman, Anal. Chem., 2003, 414A, Psychoanalytical Electrochemistry,
Dopamine and Behavior
Voltammetric detection of dopamine. When sufficient potential is applied
to the electrode, dopamine is oxidized to dopamine-o-quinone, donating two
electrons that are detected as current. When the potential is returned, any
dopamine-o-quinone remaining at the electrode surface is reduced back to
dopamine by accepting electrons, producing current in the opposite direction.
In the example shown, the potential is applied by fast-scan cyclic
voltammetry. With this technique, the resultant current comprises timeresolved peaks that aid analyte identification. These measurements are
typically repeated several times per second.
InVivo Monitoring of Dopamine Release in Rat Brain with Differential Normal Pulse Voltammetry
Analytical Chemistry 1984, 58, 3, 573, F. G. Gonan, Florence Navarre, and M .J. Buda
Measurement of Dopamine (see structure) is in a background of other
Easily oxidizable materials such as ascorbic acid (AA) 3,4dihydroxyphenylacetic acid (DOPAC). Could not really solve the problem by
Instrumental methods so instead inhibited the ability of the rat to produce
DOPAC by lesion to the brain.
DOPAC
Dopamine
Ascorbic Acid
InVivo Monitoring of Dopamine Release in Rat Brain with Differential Normal Pulse Voltammetry
Analytical Chemistry 1984, 58, 3, 573, F. G. Gonan, Florence Navarre, and M .J. Buda
B. Jill Venton and R. Mark Wightman, Anal. Chem., 2003, 414A, Psychoanalytical Electrochemistry,
Dopamine and Behavior
B. Back ground current
attributed to the charging of
the double layer
(rearrangement of charged
species around the electrode)
and is proportional to the scan
rate and capacitance of the
electrode.
C. Resulting backgroundsubtracted cyclic
voltammogram. Indicates that
change in current is
attributable to oxidation of
dopamine and reduction of the
electron form quinone back to
dopamine. Dopamine has an
oxidation peak of +.6V
Shows the response as a carbonfiber microelectrode is lowered
through the nucleus accumbens in
150 micrometer increments, while
dopamine fibers in the medial
forebrain bundle were electrically
stimulated.
In the release from a single
recording site in the nucleus
accumbens varied while the
stimulating electrode was
incrementally lowered through the
dopamine fiber pathway.
With these methods, an area w/
high release can be targeted for
measurements during an
experiment
CVs can be compared based on peak height, the relative ratio of oxidative and
reductive peaks, as well as peak location and shape
Although using CV are a powerful tool for
identifying electroactive compounds, some
species such as norepinephrine and
dopamine have nearly identical CV and
cannot be differentiated by CV analysis
alone. This is due to the structure similarity.
To Identify dopamine changes in vivo,
electrodes are placed in a known
dopaminergic regions w/ low norepinephrine
content
Reduce ascorbic acid background by coating the electrode with
A blocking membrane which exlcudes anions
Information about Nafion here
Drug effect on Dopamine
Represents examples of various drugs on electrically
stimulated dopamine release in freely moving rates.
Haloperidol a dopamine receptor antagonist, increases
dopamine release by blocking D2 receptor on the
dopamine terminals.
Nomifensine a dompamine transporter antagonist
causes an increase in dopaminergic signal by blocking
uptake.
Ethanol a general depressant that affects multiple ligandgated ion channels in the brain, decreases electrically
evoked dopamine releases
Electrically evoked vs
naturally occurring dopamine
No signals were detected when the rats were sitting quietly in the test cage.
However, when a barrier was lifted and the rats entered a novel environment,
dopamine transiently increased in the nucleus accumbens. It is clear that
dopamine concentrations increased only at the initial contact w/ the male and not
before or afterward.
Tools II Voltammetric Detectors
1.
2.
3.
4.
Methods: Stripping, DPV
Metals
LC EC and LC EC MS
Rat Brains
END