Why electrochemical sensors

Figure 2. Nitric oxide sensor developed by the Masef research group The direct electrooxidation of NO in solutions follows a 3 step reaction. How is it made and how does it work? Why is it important?

The enzyme can be placed on the surface of the electrode, and the amount of H 2 O 2 produced can be monitored find the amount of glucose present. This method works in the presence of oxygen, because, as shown in Reaction 1, the FADH 2 cofactor is oxidized by oxygen, which is reduced to H 2 O 2.

In this method, the mediator is oxidized by the electrode and then reduced by FADH 2. Since this reaction takes place in the absence of oxygen, H 2 O 2 is not produced. Table 2. Reduction Potentials of common biological molecules. Gas phase biosensors.

Sensors,17 8 , Electrochemical Biosensors - Sensor Principles and Architectures. Sensors,8 3 , Sensors,10 9 , Sensors Basel, Switzerland , 17 2 , Contributors and Attributions Asmira Alagic admire. Impedance sensors were described for NO 2 and tobacco smoke, odor detection or determination of water in an oil-in-water emulsion. Humidity is very often a parameter for which mixed oxide conductivity sensors are used , although it can also be an unwanted interference.

A gas sensor for ethanol and acetone vapors, insensitive to humidity, can be based on sintered bismuth tungstate. In Brazil, the advantages of the use of conductometric detection coupled to flow injection analyis was described by Jardim and co-workers to measure CO 2 in the atmosphere.

Fatibello and Borges have also shown a flow injection conductometric system appropriate to evaluate acidity in industrial hydrated ethyl alcohol. In addition, an automatic rain sensors based on a conductometric sensor was proposed by Gutz and co-workers, and Krug and co-workers.

Recently, the development of electrochemical sensors based on measurement of electrochemical admittance spectroscopy were also investigated by Stradiotto and co-workers.

The interest in electrochemical sensors continues unabated, stimulated by the wide range of potential applications. There are several journals specializing in electrochemical sensors; large professional societies have electrochemical sensor divisions; and several series of international conferences on electrochemical sensors take place regularly, testifying to its importance inside chemistry and correlated areas.

Their impact is also observed in chemical education, since chemical sensors have entered the analytical curricula at many universities. During the past 20 years, electrochemical sensors have become an accepted part of analytical chemistry, because they satisfy the expanding need for rapid, simple and economic methods of determination of many analytes.

The important basis of electrochemical knowledge obtained with the evolution of voltammetric and potentiometric techniques and the later development of electrochemical sensors has Brazilian contributions. The pioneering and significant work of some research groups E.

Neves, G. Oliveira Neto, E. Gonzales, L. Avaca, O. Godinho, M. Molina, C. Melios, R. Tokoro, T. Gutz and others not cited but also important was largely responsible for popularizing the electrochemical area in Brazil and the consequent widespread use of electrochemical sensors seen nowadays.

Currently, many Brazilian workers are making significant contributions in the area and some representative works have been cited above related to the development of efficient instrumentation and automated systems, biosensors and immunosensors, screen printed electrodes, chemically modified electrodes, potentiometric sensors, microelectrodes and other advances applied for determination of inorganic, organic, pharmaceutical and biological compounds in diversified matrices.

Their significant work should be consulted to obtain a wider picture of the development of the electrochemical sensors area in Brazil.

Finally, the challenge to researchers developing these important devices has always included the practical difficulty of establishing reproducible and inexpensive methods and simple to use routine analysis equipment. With such a vast range of possibilities as described here, it is easy to realize the significance and importance of electrochemical sensors to the evolution of analytical chemistry.

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Choosing proper selective membranes and enzymes, electrochemical sensors that can probe other important biomarkers dopamine, glucose, etc. Considering the diverse demands in biomedicine, the NO sensors could be integrated with other devices like electrical stimulators and microfluidic channels, to realize close-loop, multifunctional physiological monitoring, and interrogation.

Selective poly eugenol membranes were electrochemically deposited on the WE. The conductivity of the paste was investigated with different Mo particle sizes and weight ratios. The connection area was encapsulated by adhesive Dow Coming Corp. Key modules, including the analog front-end, digital control logic and power management, have been designed based on off-the-shelf components.

A transimpedance amplifier, based on an optional amplifier, converting collected current to voltage that can be used for processing and a voltage amplifier has been included in the analog front-end. The control logic, regulating the analog front-end with the digital-to-analog converter and the successive approximation register analog-to-digital converter has been implemented in a microprogrammed control unit MCU.

The power management includes Li-battery charging and regulator circuits, based on a low dropout regulator and reference for voltage. The proposed system features a volume size of 2.

A saturated NO solution 1. The saturated solution was then diluted with PBS to obtain different NO concentrations to establish a calibration curve for a NO sensor. The NO solutions were freshly prepared for each experiment to ensure reliable NO concentrations. An amperometry method was employed for both in vitro and in vivo NO detection using the oxidation potential obtained from LSV.

NO detection was conducted in a Faraday cage to avoid electromagnetic disturbance from the surrounding environment. To acquire an accurate and stable response current signal for the NO calibration curve, especially at low concentrations, mechanical stirring was applied to achieve a uniform NO concentration before recording the response current, and then stirring was turned off during the short period of data recording.

For selectivity tests, NO and interfering chemicals glucose, sodium nitrite, sodium nitrate, ascorbic acid, and uric acid were added in sequence in PBS, and the response current was recorded with mechanical stirring. The concentrations for NO and interference chemicals were 0. The selectivity of the sensor can be evaluated by the ratio of the current response of different interfering chemicals to the current response of NO.

Continuous stirring can be applied throughout the measurement due to the relatively high concentrations of NO and interfering chemicals. The fluorescence images were obtained with fluorescence microscopy Leica Microsystems Inc.

The medium was changed after 3 days. The third or fourth passage were used for the following experiments. The corresponding response current was recorded. The NO concentration measured from the sensor and Griess methods can be acquired according to their calibration curves Fig.

SD rats and New Zealand white rabbits were sacrificed and the brain, heart, liver and kidney were partially removed for NO detection. An amperometry method was performed to detect NO released from the cells and organs. All animal procedures were completed in agreement with the institutional guidelines of the Beijing Institute of Traumatology and Orthopaedics. For NO detection in the heart region, New Zealand white rabbits were anesthetized by Nembutal, and then the chest was opened and fixed with a hemostat.

The NO sensor was inserted between the beating heart and pericarditis. The response current was recorded throughout the process and converted to NO concentration based on the calibration curve of Fig. For NO detection in the joint cavity, the sensor was implanted in the joint cavity region in New Zealand white rabbits through a surgical operation.

For HE staining, the tissues around the implantation location were cut into small pieces and fixed in formalin for 1 week. Sections were incubated with hematoxylin and eosin at room temperature and analyzed under an optical microscope.

For in vivo degradation tests, sensors were implanted into the joint cavity of New Zealand white rabbits. After 8 weeks the rabbit was sacrificed, and the tissues around the sensor were separated and observed. Moreover, the surrounding tissues of the implanted sensor, the liver, kidney, and urine of the rabbit were obtained for ICP—MS to evaluate the residual concentration of Mo and Au. Further information on research design is available in the Nature Research Reporting Summary linked to this article.

All data supporting the finding of this study are present in the article and the Supplementary Information files. All raw and processed data are available from the corresponding author on reasonable request. Calabrese, V. Nitric oxide in the central nervous system: neuroprotection versus neurotoxicity.

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Kang, S. Bioresorbable silicon electronic sensors for the brain. Nature , 71 Clementine, M. The size of the electrode plays also a very important role in the performance of amperometric sensor. The explanation of this behavior is beyond the scope of this article, can be found in standard electrochemical texts. Selectivity is as important for amperometric sensors as it is for any other type of chemical sensor.

The choice of applied potential offers surprisingly little in terms of selectivity. Much better approach is to attack the problem through the resistances. Imagine, that the sample solution contains species A, B, C, etc, and that neither is electroactive that is, it cannot be reduced or oxidized because its charge transfer resistance or the mass transport resistances are too high.

In such case, no current would flow through the electrode. This situation is represented in Figure 6. If we selectively lower the resistance in such a way that a current path opens up, the electrode becomes selective for that species.

That is shown in Figure 6 where the biocatalyst lowers the charge transfer resistance R ct l and the current corresponding to the catalyzed substance can flow. That is precisely the principle of operation of one of the most successful amperometric sensor, the glucose sensitive electrode.

There, the biocatalyst is a highly selective enzyme glucose oxidase. An example of amperometric sensor deriving its selectivity from the mass transport resistance is oxygen sensor. Here a hydrophobic Teflon membrane allows penetration of oxygen, but blocks the transport of most other chemical species. Origin of selectivity in amperometric sensors. Formulating and examining behavior of sensors in terms of equivalent electrical circuit elements is a very powerful approach.

In the above example we have represented physical phenomena such as diffusion or electron transfer by resistances. The electrode surface is commonly represented by a capacitor , etc. The equivalent electrical elements can be readily arranged relative to each other, forming an equivalent electrical circuit, such as the one shown in Figure 6.

This approach is common in electrochemistry. It makes the study and the interpretation of responses of an electrochemical experiment much easier.

The above mentioned oxygen sensor often called the Clark electrode is an important amperometric sensor, much used in water quality measurements as the dissolved oxygen content of the water is an important environmental factor affecting aquatic life. Conductometric sensors Fig. Two types of conductometric sensors with their corresponding equivalent electrical circuit diagrams: A chemiresistor configuration typical for gas sensing; B conductometric biosensor.

The last group of electrochemical sensors discussed in the article is based on modulation of resistivity of the selective material. Because the reciprocal of resistivity is conductivity , these sensors are interchangeably called conductometric sensors or chemiresistors. The two main formats in which these sensors come are shown in Figure 7.

In the first one, some material, which can change its conductivity upon interaction with chemical species is clamped between two contact electrodes and the resistance of the entire device is measured.

Such arrangement is typical for chemiresistors, used for sensing in gases. In the second version the chemically interactive layer is at the top of an electrode , which is immersed in the solution of electrolyte. A suitable counter-electrode is provided that completes the electrical circuit. This arrangement can be typically found in various biosensors , where the selectivity of the response comes from some biological interaction. Photograph of typical chemiresistors with interdigitated contact electrodes.

A common feature of all conductometric sensors is the low cost of their fabrication. By comparison with other electrochemical sensors they are really inexpensive, which makes them very popular.

However, in terms of performance, you get what you pay for. The interpretation of their signal is very difficult and often impossible. The reason for this difficulty can be seen in Figure 7 in which both types are represented by their equivalent electrical circuit diagrams.

Their resistance is interrogated by applying dc or ac voltage between the terminals 1 and 2 or 3 and 4, respectively and reporting the result as the current. If dc voltage is used and dc current is measured the conductometric sensors are formally similar to amperometric sensors. The contacts between the metal electrode and the chemically sensitive layers are represented by contact resistances.

In type A, the three equivalent resistances shown in parallel. They correspond to the surface, the bulk, and the interface resistance respectively. Thus, in the equivalent electrical circuit diagram of a common chemiresistor we have at least five resistances to account for. The problem is that any one of them, or any combination of them can be modulated by the chemical sensing interaction. That makes the task of optimization, or rational design of their selectivity exceptionally difficult.

There is also no rational relationship between their response and the concentration of species that they measure.