Amperometry in
chemistry is detection of ions in a solution based on electric current or changes in electric current.
Amperometry is used in
to study
vesicle
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry)
In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form nat ...
release events using a
carbon fiber electrode. Unlike
patch clamp
The patch clamp technique is a laboratory technique in electrophysiology used to study ionic currents in individual isolated living cells, tissue sections, or patches of cell membrane. The technique is especially useful in the study of excita ...
techniques, the
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
used for amperometry is not inserted into or attached to the
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
, but brought in close proximity of the cell. The measurements from the electrode originate from an
oxidizing reaction of a vesicle cargo released into the medium. Another technique used to measure vesicle release is
capacitive
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of a c ...
measurements.
History
Electrochemical
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...
or amperometric detection as it was first used in
ion chromatography
Ion chromatography (or ion-exchange chromatography) separates ions and polar molecules based on their affinity to the ion exchanger. It works on almost any kind of charged molecule—including large proteins, small nucleotides, and amino acid ...
was single-potential or DC amperometry, useful for certain electrochemically active ions such as cyanide, sulfite, and iodide. The development of pulsed amperometric detection (PAD) for analytes that fouled
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
surfaces when detected eventually helped create a new category of ion chromatography for the determination of
carbohydrates. Another advancement, known as integrated amperometry, has increased the sensitivity for other electrochemically active species, such as
amines
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...
and many compounds that contain reduced
sulfur groups, that are sometimes weakly detected by PAD.
It was established that
neurotransmitters could be
electrochemically detected by placing a carbon electrode into
tissue and recording the
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
from oxidizing neurotransmitters. One of the first measurements was made using an
implanted carbon fiber electrode in the
neostriatum
The striatum, or corpus striatum (also called the striate nucleus), is a nucleus (a cluster of neurons) in the subcortical basal ganglia of the forebrain. The striatum is a critical component of the motor and reward systems; receives glutamat ...
of rats. Further work was done in
chromaffin cell
Chromaffin cells, also called pheochromocytes (or phaeochromocytes), are neuroendocrine cells found mostly in the medulla of the adrenal glands in mammals. These cells serve a variety of functions such as serving as a response to stress, monito ...
s to investigate
catecholamine
A catecholamine (; abbreviated CA) is a monoamine neurotransmitter, an organic compound that has a catechol (benzene with two hydroxyl side groups next to each other) and a side-chain amine.
Catechol can be either a free molecule or a su ...
release from large dense core vesicles.
Detection methods
Single-potential amperometry
Any analyte that can be oxidized or reduced is a candidate for amperometric detection. The simplest form of amperometric detection is single-potential, or
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
(DC), amperometry. A voltage (potential) is applied between two
electrodes
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials dep ...
positioned in the column
effluent. The measured current changes as an electroactive analyte is oxidized at the anode or reduced at the cathode. Single-potential amperometry has been used to detect weak acid anions, such as
cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
and
sulfide, which are problematic by conductometric methods. Another, possibly more important advantage of amperometry over other detection methods for these and other ions, such as iodide, sulfite, and
hydrazine, is specificity. The applied potential can be adjusted to maximize the response for the analyte of interest while minimizing the response for interfering analytes
Pulsed amperometry (pulsed amperometric detection, PAD)
An extension of single-potential amperometry is pulsed amperometry, most commonly used for analytes that tend to foul electrodes. Analytes that foul electrodes reduce the signal with each analysis and necessitate cleaning of the electrode. In pulsed amperometric detection (PAD), a working potential is applied for a short time (usually a few hundred milliseconds), followed by higher or lower potentials that are used for cleaning the electrode. The current is measured only while the working potential is applied, then sequential current measurements are processed by the detector to produce a smooth output. PAD is most often used for detection of carbohydrates after an anion exchange separation, but further development of related techniques show promise for amines, reduced sulfur species, and other electroactive compounds.
Principle
In order to record vesicle fusion, a carbon fiber electrode is brought close to the cell. The electrode is held at a positive potential, and when the cargo from a fused vesicle is near the electrode, oxidation of the cargo transfers electrons to the electrode. This causes a spike, the size of which can be used to estimate the number of vesicles, and the frequency gives information about the release probability.
References
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Electrophysiology