Tuesday, February 2, 2010

ELECTRONIC TONGUN

an 5, 2010

ELECTRONIC TONGUE / ELECTRONIC NOSE

This post has been taken from http://csrg.ch.pw.edu.pl/tutorials/electronicT_N/

ELECTRONIC TONGUE / ELECTRONIC NOSE (ETongue, ENose) are systems for automatic analysis and recognition (classification) of liquids or gases, including arrays of non-specific sensors, data collectors and data analysis tools. Electronic tongues are used for liquid samples analysis, whereas electronic noses - for gases. The result of Etongue/Enose can be the identification of the sample, an estimation of its concentration or its characteristic properties. This new technology has many advantages. Problems associated with human senses, like individual variability, impossibility of on-line monitoring, subjectivity, adaptation, infections, harmful exposure to hazardous compounds, mental state, are no concern of it.
Synonyms of an electronic tongue: artificial tongue, taste sensor
Synonyms of an electronic nose: artificial nose, olfactory system

APPLICATIONS OF E-TONGUES/E-NOSES:

Foodstuffs Industry
  • food quality control during processing and storage (water, wine, coffee, milk, juice...)
  • optimalization of bioreactors
  • control of ageing process of cheese, whiskey
  • automatic control of taste
Medicine
  • non-invasive diagnostics (patient's breath, analysis of urine, sweat, skin odour)
  • clinical monitoring in vivo
  • identification of unpleasant odour of pharmaceuticals
Safety
  • searching for chemical/biological weapon
  • searching for drugs, explosives
  • friend-or-foe identification
Environmental pollution monitoring
  • monitoring of agricultural and industrial pollution of air and water
  • identification of toxic substances
  • leak detection
Quality control of air in buildings, closed accommodation (i.e. space station, control of ventilation systems)
Chemical Industry
  • products purity
  • in the future - detection of functional groups, chiral distinction
Legal protection of inventions - digital "fingerprints" of taste and odours

SENSING METHODS APPLIED

ETongue ENose
Potentiometric sensors

Measurements of conductivity

Voltamperommetry

Optical sensors

Biosensors 		Conductivity sensors:
  • MOSFET (Metal-oxide-silicon field-effect-transistor)
  • CP (Conducting Polymer)
Piezoelectric sensors
  • QMB (Quartz Crystal Microbalance)
  • SAW (Surface Acoustic Wave)
Optical sensors

PATTERN RECOGNITION

The electronic tongue or nose system performance is dependent on the quality of functioning of its pattern recognition block. Various techniques and methods can be used separately or together to perform the recognition of the samples. After measurement procedure the signals are transformed by a preprocessing block. The results obtained are inputs for Principal Components Analysis, Cluster Analysis or Artificial Neural Network.
Measurement
Sensors arrays' outputs are arranged in data matrix (Fig. 1).

Fig. 1. Data matrix
Each sample is characterized by unique and typical set of data, forming "fingerprint" of an analyte in m-dimensional pattern space.
Preprocessing
Preprocessing is the phase in which linear transformation on the data matrix is performed (without changing the dimensionality of the problem) in order to enhance qualitative information. Typical techniques include manipulation of sensor baseline, normalization, standarization and scaling of response for all the sensors in an array.
Principal Component and Cluster Analysis
A multi-sensor system produces data of high dimensionality - hard to handle and visualize. Principal Component Analysis (PCA) and Cluster Analysis (CA) are multivariate pattern analysis techniques reducing dimensionality of the problem and reducing high degree of redundancy.
PCA is a linear feature-extraction technique finding most influential, new directions in the pattern space, explaining as much of the variance in the data set as possible. This new directions - called principal components - are the base for a new data matrix. Usually 2 or 3 of them are sufficient to transfer more than 90% of the variation of the samples.
The base principle of Cluster Analysis is the assumption of close position of similar samples in multidimensional pattern space. Similarity between each 2 samples is calculated as a function of the distance between them - usually in Euclidean sense - and displayed on a dendrogram (Fig. 2).

Fig. 2. Cluster Analysis: a), b) different types of dendrograms
Artificial Neural Networks (ANN)
Neural Networks are information processing structures imitating behavior of human brain. Their main advantages, such as: adaptive structure, complex interaction between input and output data, ability to generalize, parallel data processing and handling incomplete or high noise level data make them useful pattern recognition tools. There are many possible architectures and algorithms available in the literature, but the most common in measurement applications is feed-forward network (multilayer perceptron MLP) and back-propagation learning algorithm.
The base units of artificial neural networks are neurons and synapses. Neurons are organized in layers and connected by synapses. Their task is to sum up their inputs and non-linear transfer of the result, which is then transmitted via synapsis with modification by means of the synapsis weights - this signal, in turn, is the input for the next layer of the network (Fig. 3).

Fig. 3. Neural Networks: a) single neuron, b) feed-forward network
The use of ANN involves 3 phases:
  • The learning phase - after establishing number of neurons, layers, type of architecture, transfer function and algorithm, network is forced to provide desired outputs corresponding to a determined input. It is made by adjusting the synapses' weights in order to minimize the difference between desired and current output.
  • The validation phase - verification of the generalization capability of network by means of data different (but with similar characteristics) from data used in the learning phase.
  • The production phase - in which the network is capable of providing outputs corresponding to any input.

REFERENCES:

  • Craven M. A., Gardner J. W., Electronic noses - development and future prospects, Trends in Analytical Chemistry, vol. 15 (1996), 486
  • D'Amico A., Di Natale C., Paolesse R., Portraits of gasses and liquids by arrays of nonspecific chemical sensors: trends and perspectives, Sensors and Actuators B, 68 (2000), 324
  • Nagle H. T., Schiffman S. S., Gutierrez-Osuna R., The how and why of electronic noses, IEEE Spectrum, September 1998, 22
  • Di Natale C., Davide F., D'Amico A., Pattern recognition in gas sensing: well-stated techniques and advances, Sensors and Actuators B, 23 (1995), 111
  • Gardner J. W., Detection of vapours and odours from a multisensor array using pattern recognition Part I. Principal Component and Cluster Analysis, Sensors and Actuators B, 4 (1991), 109
  • Gardner J. W., Hines E. L., Tang H. C., Detection of vapours and odours from a multisensor array using pattern recognition Part II. Artificial Neural Networks, Sensors and Actuators B, 9 (1992), 9
  • Toko K., Taste sensors with global selectivity, Materials Science and Engineering, C4 (1996), 69
  • Vlasov Y., Legin A., Non-selective chemical sensors in analytical chemistry: from "electronic nose" to "electronic tongue", Journal of Analytical Chemistry, 361 (1998), 255
  • Krantz-Ruckler C., Stenberg M., Winquist F., Lundstrom I., Electronic tongues for environmental monitoring based on sensor arrays and pattern recognition: a review, Analytica Chimica Acta, 426 (2001), 217
  • Winquist F., Holmin S., Krantz-Ruckler C., Wide P., Lundstrom I., A hybrid electronic tongue, Analytica Chimica Acta, 406 (2000), 147

LINKS:

Commercially available e-noses/e-tongues:
Chemometrics:
Posted by Binoy at 1:50 PM 0 comments

Fiber optic chemical sensors

This post is taken from http://csrg.ch.pw.edu.pl/tutorials/fiber/
INTRODUCTION
Fiber optic chemical sensors (FOCS) can offer several advantages over traditional sensors. The light weight and small size of fiber optic sensors are strongly complemented by their strong immunity to electromagnetic interference. Since the fiber sensors are made of glass they are environmentally rugged and can tolerate high temperatures, vibration, shock and they can operate in extremely harsh conditions. There are many excellent reviews on FOCS published in recent years [1,2,3].
There are many types of fiber optic chemical sensors which can measure concentration of neutral or charged species. The principle of operation of FOCS will be explained on the example of pH sensor because it is one of the most important sensor in analytical chemistry.

Fig.1 Schematic experimental set-up of FOCS.
The sensor consists of three main parts: light source, optrode and detector. The main part of the sensor, so-called optrode, contains an appropriate indicator which changes its optical properties in dependence on the analyte. In most cases, it is necessary to use an indicator because the analyte does not give or exhibit changes of optical properties. The indicator can change, for example, absorbance or fluorescence intensity. The light source is matched to the so-called analytical wavelength of the indicator then the best sensitivity of the sensor can be obtained. Detector, usually photodiode or PMT, converts optical signal into electric one which is next electronically processed.

OPERATING PRINCIPLE


Fig.2 shows operating principle of a pH sensor based on absorbance indicator.
The pulse of light from a light emitting diode (LED) is coupled into optical fiber and transmitted to a pH sensitive membrane. The membrane changes its absorbance (colour) in dependence on pH of the sample. If the absorbance is quite low the light is only slightly absorbed which is depicted by almost the same pulse of the light returning to the photodiode. When the pH of the sample is changed the absorbance of the membrane increases so the returning pulse of the light is smaller (see Fig.3).

Fig.3 Sensor based on absorbance indicator at solution of different pH.
Light is usually modulated to a square wave because the measurements are not influenced by an ambient light and in order to increase the signal to noise ratio.
Sensors can be also based on the use of fluorescence indicator immobilized in the membrane.

Fig.4 Sensor based on fluorescence indicator.
In this case, the light excites molecules of the indicator which emits light at different wavelength. The analyte can, for example, influence on the intensity of the fluorescence radiation which is depicted by different amplitude of the returning pulses (see Fig.4).

Fig.5 Sensor based on fluorescence indicator at solution of different pH.

SIGNAL CONVERSION IN THE SENSOR

Typical conversion process of the signal in the FOCS is presented in Fig.6.

Fig.6 Conversion of the signal in fiber optic chemical sensor.
Chemical signal caused by the analyte is converted into optical one in chemooptical interface. The chemooptical interface consists of membrane with an appropriate indicator. The indicator changes its optical properties (e.g. absorbance, fluorescence) in dependence on analyte. Such an optical signal with information about sample under test is converted into electric signal in an optoelectronic interface. The main part of this interface is the photodetector connected to an electronic circuit. The electric signal can be acquired and processed by PC-lab card. In the case of a pH sensor signal is processed according to procedure shown in Fig.7.

Fig.7 Signal processing in a fiber optic pH sensor.
The signal from the sensor is acquired by the card, then it is processed according to the calibration procedure and displayed on the monitor. The acquired data can be saved on the disk.

EXAMPLE OF FIBER OPTIC pH SENSOR

The fiber optic system can be controlled by a software developed in Lab Windows environment. The main idea of the software is to design user friendly and interactive application which can be applied in many sensor configurations. The user can set up its own application based on pre-design modules. There are many modules possible to use in each phase of signal processing. Some typical ones designed for signal acquisition are presented in fig.8.

Fig.8 Examples of modules for signal aquisition.
The user has to chose a module which is the most suitable for its application and place it on the screen. Similar modules are designed for each part of signal processing. It is possible to use as many modules as the user needs. Every module has its own name and in this way it can be recognized by any module. The modules shown in fig.8 allow to set the gain of the acquisition card, sampling rate, channel of the card etc. The output signal from the card is transmitted to further processing.
Fig.9 shows application software designed for a pH sensor with the use of different modules.

Fig.9 Fiber optic pH sensor controlled by LabWindows application.
The signal from the sensor is amplified by proper settings of control module of Lab-PC+ card. Next module is used for signal processing where calibration formula is applied. The calibration data can be saved and retrieved for each sensor. Last module gives graph with dependence of the signal versus time. When appropriate settings of the card and calibration procedure were done it is possible to switch off any display which is not important during the measurements without any influence on work of the system. The modules left will be rescaled automatically. For example, only pH value can be displayed on the monitor.
Main features of the application:
  • software is user friendly and is based on typical Windows commands
  • designed process can be saved
  • display is rescaled automatically
  • user can chose modules for configuration
  • user can design his own project
Example of sensor configuration
The most basic version of the experimental set-up is shown in Fig.10.

Fig.10 Configuration of fiber optic pH sensors (GEN-square wave generator, LED-light emitting diode, PD-photodiode, AMP-transimpedance amplifier, AF-active filter).
Modulated light from a light emitting diode (LED) is transmitted to the optrode by one arm of the fiber optic bundle. The light matched to the maximum of the molar absorbance of neutral red (560 nm) is reflected in dependence on pH variations and then it is transmitted to a photodiode by the second arm of the bundle. The photodiode is connected with a transimpedance amplifier and active filter. The electrical signal obtained is acquired and processed by a pc-lab card with 12-bit A/D converter. The optrode was built on the common end of the bundle by the use of removable Teflon tube, which holds an optomembrane.

REFERENCES

  1. W.R.Seitz Chemical sensors with fiber optics, Crit. Rev. in Anal. Chem., 19, 1988, 15
  2. D.L.Wise Biosensors with fiber optics, Humana Press, New York, 1991
  3. O.S.Wolfbeis Fiber optic chemical sensors and biosensors, Boca Raton, 1991
  4. K. Seiler and W. Simon, Principles and mechanisms of ion-selective optodes, Sensors and Actuators B, 6 (1992) 295-298.
  5. A. Dybko, W. Wróblewski, J. Maciejewski, Z. Brzózka, R. Romaniuk, Novel matrix for fibre optic chemical sensors made of particle track polymer, Proc. SPIE, 2508 (1995) 351-357.
  6. A. Dybko, W. Wróblewski, J. Maciejewski, Z. Brzózka, R. Romaniuk, J. Kie^(3)kiewicz, Polymer track membranes as a trap support for reagent in fibre optic sensors, J. of Apll. Polym. Sci., 59 (1996) 719-723.
  7. A.Dybko, W.Wróblewski, J.Maciejewski, R.Romaniuk, Z.Brzózka Efficient reagent immobilization procedure for ion-sensitive optomembranes, Sensors and Actuators B - in press
  8. E. Bishop, Indicators, Pergamon, Oxford, 1972.

OPTICAL FIBER TECHNOLOGY

All optical fibers consist of a core having the refractive index higher than of surrounding cladding. They can be made of just glass or polymer, or combination of both. They also have protective polymer layers called buffer or jacket.

Fig.1 Cross-section of optical fiber
There are two methods to manufacture optical glass fiber: either directly drawing the fiber from molten glasses, which are placed in two concentric crucibles (Double Crucible method) or from a glass rod called preform. Nowadays most optical fibers are made from the preform. There are three steps in this method:
  1. Fabrication of the preform
  2. Drawing the fiber from the preform
  3. Coating and jacketing process
The preforms are fabricated using chemical vapor deposition methods [1]:
  1. Modified Chemical Vapor Deposition (MCVD)
  2. Plasma Modified Chemical Vapor Deposition (PMCVD)
  3. Plasma Chemical Vapor Deposition (PCVD)
  4. Outside Vapor Deposition (OVD)
  5. Vapor-phase Axial Deposition (AVD)
All these methods are based on thermal chemical vapor reaction that forms oxides, which are deposited as layers of glass particles called soot, outer rotating rod or inside glass tube. The same chemical reactions occur in both methods. Starting materials are solutions of SiCl4, GeCl4, POCl3, and gaseous BCl3. These liquids are evaporated within oxygen stream and form silica and other oxides. Chemical reactions proceed as follows:
SiCl4 + O2 → SiO2 + 2 Cl2
GeCl4 + O2 → GeO2 + 2 Cl2
4 POCl3 + 3 O2 → 2 P2O5 + 6 Cl2
4 BCl3 + 3 O2 → 2 B2O3 + 6 Cl2
Germanium dioxide and phosphorus pentoxide increase the refractive index of glass, whilst boron oxide - decreases. These oxides are known as dopands. Changing composition of the mixture during the process influences refractive index profile of the preform.

Modified Chemical Vapor Deposition (MCVD)

This method was developed by Bell Laboratories [2]. The gaseous mixture of reactants described above is fed at the end of a rotating silica tube. This tube is heated by a traversing oxygen-hydrogen burner (Fig.2). As a result of chemical reactions glass particles, called soot, are formed. These particles are deposited on internal wall of the tube. The soot is then vitrified by the traversing burner to provide a thin glass layer. The process is repeated many times as the cladding layers and core layers are formed. When the deposition is finished, the temperature of the burner is increased to collapse the tube into a solid preform. The entire process is highly automated and all process parameters are precisely controlled.

Fig. 2 Deposition by MCVD process

Plasma Modified Chemical Vapor Deposition (PMCVD)

A modification of MCVD method is a process known as PMCVD. In addition to the normal MCVD technique the radio-frequency coil around the tube generates an internal high temperature plasma.

Fig.3 Deposition by PMCVD process

Plasma Chemical Vapor Deposition (PCVD)

The PCVD method is similar to PMCVD. The radio-frequency coil is replaced by a microwave cavity resonator (Fig. 4). In this method reactions lead directly to of glass layer without forming the soot.

Fig. 4 Deposition by PCVD process

Outside Vapor Deposition (OVD)

This process is also called the "soot process". It was exclusively used by Corning since the 1970s, and the patent of such a technology has expired since July 2000.
Halogens and O2 react in a hot flame to form hot glass soot, which is deposited layer by layer on an aluminium oxide or graphite mandrel. The central mandrel is removed after deposition. In the last step, called sintering, a hollow porous preform is dehydrated and collapsed in controlled atmosphere, (e. g. helium) to form desired preform.

Fig. 5 Preform fabrication by OVD process

Vapor-phase Axial Deposition VAD

In VAD method (in contrary to above methods) the perform can be fabricated continuously. Starting chemicals are carried from the bottom into oxygen-hydrogen burner flame to produce glass soot which is deposited on the end of a rotating silica rot. A porous preform is then grown in the axial direction. The starting rod is pulled upward and rotated in the same way as that used to grow single crystals. Finally the preform is dehydrated and vitrified in ring heaters (Fig. 6). This process is preferred for the mass production.

Fig. 6 Preform fabrication by VAD process

Fiber Drawing

Optical fibers are obtained by drawing from the preform at high temperature. The drawing process must be integrated with the coating process to avoid contamination of fiber surface. These processes are shown schematically in Fig.7.

Fig. 7 Schematic of fiber drawing and draw tower solution
The tip of the perform is heated in a furnace to a molten state. Formed molten gob falls down under the force of gravity while shrinking in diameter into a proper diameter strand. It is controlled continuously during the drawing process. Diameter drift cannot exceed 0.1%. The strand is threaded through a series coating applicators immediately after drawing. Liquid prepolymer coatings are cured by thermal or ultraviolet apparatus. Dual coating, soft inner and hard outer, is needed to avoid microbending and protect against impact and crushing forces in either manufacturing process or installation. silicone coating and acrylate, Tefzel (ETFE), Teflon (PFA), nylon buffers are applied during the fiber drawing, while additional materials such as Hyrtel and PVC can be extruded after the draw process. The fiber with coatings is pulled down and wound on a winding drum. The drawing process must take place in air conditioned room, because air pollution influences fiber attenuation.

References

  1. Fibre optics: theory and applications, Serge Ungar, Wiley, New York, 1990.
  2. Handbook of fiber optics: theory and applications, Chai Yeh, Academic Press, San Diego, 1990.
Posted by Binoy at 1:45 PM 0 comments

Field effect transistors (FETs) as transducers in electrochemical sensors

This post has been taken from http://csrg.ch.pw.edu.pl/tutorials/isfet/

INTRODUCTION Chemical sensors are microdevices that connect the chemical and electrical domains (i.e. transduction of the chemical information into electric signal). The response of the sensors should be fast and selective for the analyte. Moreover, these devices should have a lifetime in the order of months. The construction of chemical sensors requires the integration of a sensing receptor and a transducing element into a defined chemical system. Field effect transistors (FETs) are very interesting because they can be made very small with current planar IC technology and have the advantage of a fast response time.

FROM MOSFET TO ISFET

The FETs are able to measure the conductance of a semiconductor as a function of an electrical field perpendicular to the gate oxide surface. In the most simple version, (i.e. a metal oxide semiconductor field effect transistor, n-channel MOSFET), a p-type silicon substrate (bulk) contains two n-type diffusion regions (source and drain). The structure is covered with a silicon dioxide insulating layer on top of which a metal gate electrode is deposited (figure 1a).

Figure 1. Schematic representation of a MOSFET a) and an ISFET structure b).
When a positive voltage (with respect to the silicon) is applied to the gate electrode, electrons (which are the minority carriers in the substrate) are attracted to the surface of the semiconductor. Consequently, a conducting channel is created between the source and the drain, near the silicon dioxide interface. The conductivity of this channel can be modulated by adjusting the strength of electrical field between the gate electrode and the silicon, perpendicular to the substrate surface. At the same time a voltage can be applied between the drain and the source (Vds), which results in a drain current (Id) between the n-regions.
In the case of the ISFET, the gate metal electrode of the MOSFET is replaced by an electrolyte solution which is contacted by reference electrode (then the SiO2 gate oxide is placed directly in an aqueous electrolyte solution, figure 1b) [1]. The metal part of reference electrode can be considered as the gate of the MOSFET.
In ISFET, electric current (Id) flows from the source to the drain via the channel. Like in MOSFET the channel resistance depends on the electric field perpendicular to the direction of the current. Also it depends on the potential difference over the gate oxide. Therefore, the source-drain current, Id, is influenced by the interface potential at the oxide/aqueous solution. Although the electric resistance of the channel provides a measure for the gate oxide potential, the direct measurement of this resistance gives no indication of the absolute value of this potential. However at a fixed source-drain potential (Vds), changes in the gate potential can be compensated by modulation of the Vgs. This adjustment should be carried out in such a way that the changes in Vgs applied to the reference electrode are exactly opposite to the changes in the gate oxide potential. This is automatically performed by ISFET amplifier with feedback which allow to obtain constant source-drain current. In this particular case, the gate-source potential, is determined by the surface potential at the insulator/electrolyte interface.
When SiO2 is used as the insulator, the chemical nature of the interface oxide is reflected in the measured source-drain current. The surface of the gate oxide contains OH-functionalities, which are in electrochemical equilibrium with ions in the sample solutions (H+ and OH-). The hydroxyl groups at the gate oxide surface can be protonated and deprotonated and thus, when the gate oxide contacts an aqueous solution, a change of pH will change the SiO2 surface potential. A site-dissociation model describes the signal transduction as a function of the state of ionization of the amphoteric surface SiOH groups [2,3]. Typical pH sensitivities measured with SiO2 ISFETs are 37-40 mV/ pH unit [3].
The selectivity and chemical sensitivity of the ISFET are completely controlled by the properties of the electrolyte/insulator interface. Other inorganic gate materials for pH sensors like Al2O3, Si3N4 and Ta2O5 have better than SiO2 properties in relation with pH response, hysteresis and drift. In practice, these layers are deposited on the top of the first layer of SiO2 by means of chemical vapour deposition (CVD).
ISFETs have been chosen as a transducing element because the SiO2 surface contains reactive SiOH groups which can be used for covalent attachment of organic molecules and polymers.

MEMFET AND SURFET

The ISFET can be modified with a sensing membrane, that contains an ionophore, which determines the response of the sensor [4]. If the gate oxide is covered with an ion-sensitive membrane, the device is known as a MEMFET [5]. In this case, the ion-sensing layer is penetrable for ions (unblocked); the membrane potential is generated throughout the membrane, which is detected by the FET structure. The first ISFET modified with a sensing membrane containing an ionophore, which enables the detection of the activity of an ion by its complexation, was reported by Moss [6]. A K+ - sensitive FET was obtained by solvent casting of a conventional plasticized PVC membrane, containing valinomycin on the gate oxide surface. Other approaches proposed Ca2+ sensitive MEMFET (with ion exchanger in polymeric membrane) [7] or deposition of AgBr membranes (Ag+ or Br- sensors) [8].
SURFET represents an ISFET with an ion-blocking layer, which covers the pH-sensitive sites of the gate insulator. At the surface of this layer a surface potential is established by selective association of ions. An example of a SURFET is the perylene gate ISFET with attached benzo-18-crown-6 ionophore molecules, that selectively complex potassium ions [9]. In contrast to the MEMFET, where the association coefficient of the ionophore with recognised ion in the membrane phase determines the selectivity, in SURFET the same process in the aqueous phase controls the selectivity.
It can be concluded from the literature reviews [10,11], that MEMFETs are readily fabricated by means of solvent casting of PVC membranes, with incorporated plasicizer and ionophore on the top of the ISFET gate oxide. Due to poor adhesion of the membrane to the gate oxide it can peel out easily and its electroactive components may leach out. The leaching out effect can be diminished by using extremely hydrophobic receptor or the ionophore can be covalently linked to the organic matrix at the ISFET gate oxide [12]. However, there is no thermodynamically well-defined membrane-ISFET interface and finally the pH sensitivity is not completely eliminated.

CHEMFET

ISFETs modified with plasticized PVC membranes lack a thermodynamically well-defined interface between the sensing membrane and the solid contact. Nevertheless, the PVC-modified ISFETs do not seem to suffer from the ill-defined inner contact and acceptable stabilities and drift values have been reported [7, 13, 14]. Up to now, no experimental efforts were made to improve this system because the properties of the devices are quite satisfactory. However, following studies showed, that changes of carbon dioxide concentrations in the sample solution influence strongly the measurements [15]. This was attributed to the diffusion of carbon dioxide through the membrane and the successive formation of carbonic acid at the membrane-gate oxide interface with traces of water present at the interface. Consequently, the concentration of protons, which determines potential at the membrane insulator interface, undergo large variations. This phenomenon explains why ISFETs modified with PVC membranes generally perform satisfactory (the PVC membranes usually contain reasonable amount of water and therefore H+ ions are present and control the membrane-insulator potential). Besides the CO2 interference, the need for high amount of water inside the membrane matrix was the key reason that urged to develop a thermodynamically well-defined interface.
Several approaches have been described in the literature for FET based sensors as possible solutions for these problems. In most cases, an intermediate Ag/AgCl layer is applied on the gate-insulator surface [16], which at least eliminates the CO2 interference. Various methods of deposition of a Ag/AgCl layer on a silicon substrate were reported with a conclusion, that different IC-compatible methods give satisfactory layers [17]. However, the Ag/AgCl-membrane interface becomes critical. The equilibrium state of this interface relies on the exchange of scarcely present Cl- ions in the membrane. Therefore, a better approach seems to be a deposition of an additional layer (e.g. sodium glass) between the polymer and the gate insulator or poly(vinyl alcohol) between the polymer and the Ag/AgCl layer on the top of the gate insulator [18]. In this way common ions can be provided by the intermediate layer.
Another approach, a novel architecture - chemically modified FET (CHEMFET), is designed to solve the problems (figure 2).

Figure 2. Schematic representation of a chemically-modified FET-CHEMFET.
Top: cross-section through the various layers with potential determining species.
The attachment of the membrane can be improved by mechanical [19] or chemical [20,21] anchoring to the surface of the gate oxide. For chemical attachment of polymer films the gate oxide surface is silylated with 3-(trimethoxysilyl)propyl methacrylate. The methacrylate modified surface can subsequently react with vinyl or methacryl monomers or prepolymers. The use of a UV-photopolymerizable monomers, hydroxyethyl methacrylate (HEMA), is advantageous from the point of view of the ultimately desired mass production of the CHEMFETs, which is essentially based on photolithography. The introduction of such a hydrogel layer [22,23], in which an aqueous buffered solution of salts can be absorbed, between the gate oxide and the sensing membrane eliminates the interference of CO2 on the CHEMFET response. Moreover, this stabilizes the potential developed in the sensing membrane. Plasticized PVC membranes, that contain an ionophore, are widely used as sensing membranes. Leakage of plasticizer to the contacting aqueous solution and weak adhesion of the membrane to the ISFET prompted the search for other polymer membranes like polyurethane, silicone rubber, polystyrene, polyamide and several polyacrylates [20,23,24].
The problem of the thermodynamically ill-defined membrane-gate interface was solved by an application of chemically attached poly(2-hydroxyethyl methacryalate) (polyHEMA) hydrogel between a hydrophobic membrane and the gate oxide layer. This novel architecture of FETs allows to design new chemical sensors based on polymeric membranes containing molecular receptors. CHEMFETs selective to K+ [20,25-27], Na+ [28-30], Ag+ [31], some transition metals cations (Pb2+, Cd2+) [32-34] and some anions (NO3-) [35-37] have been developed. However, the sensors exhibit limited lifetime which resultes from the leaching out of electroactive components, i.e. the ligand and the ionic sites. Electroactive components with an enhanced lipophilicity could be applied to increase durability of the sensor, but a more efficient method is based on covalent anchoring of these components to the membrane matrix. Application of membranes containing covalently bond ionophore and covalently bond ionic sites significantly improves the durability of CHEMFETs [26-29].

REFET

The use of a conventional reference electrode limites seriously the application of ISFETs with respect to the small size. Therefore the development of a miniature reference electrode made with the IC-compatible technology (reference field effect transistor - REFET) is of great interest for the wide-spread use of these sensors.
One of the approaches to solve this problem is the on-chip fabrication of an Ag/AgCl electrode with IC-compatible techniques, including a gel filled cavity and a porous silicon plug [38-42]. However, all the constructions have the disadvantage of a liquid-filled internal cavity with associated limited lifetime because of leakage of reference solution. A better approach to the problem of the reference electrode could be the application of two chemically unequally sensitive ISFETs operating in a differential mode with a common quasi-reference electrode (QRE) (e.g., a metal wire Pt), which can be easily integrated on the silicon chip [43-46]. This device have the additional advantage that external disturbances influencing both ISFETs (e.g. light and temperature) can be reduced. The accuracy of differential measurements depends on the difference in the ion sensitivity of both ISFETs, although total insensitivity of one ISFET (REFET) would be preferred. Such a reference FET should ideally case show insensitivity to all species present in the sample solution.
Originally, the oxide gate surface shows pH sensitivity, owing to the presence of hydroxyl groups, which can dissociate and can be protonated. It was reported that the total elimination of the pH-sensitive groups by chemical monolayer modification cannot be achieved [47]. However, the pH sensitivity can be suppressed by attaching to the gate surface an ion-blocking hydrophobic polymeric layer. In this modification the polymer is chemically bounded to the gate surface, which results in a long lifetime of the device. For ion-blocking layers, a stable attachment has been realized by plasma deposition [48-52]. However, potential variations with electrolyte compositions for such modified REFETs were observed [53]. Moreover, the deposition is limited to very thin polymeric layers because of diminishing electrical sensitivity (transconductance) with increasing insulator thickness [5].
In contrast to ion-blocking polymers, modification of REFETs with ion-unblocking (conductive) polymers would have the advantage of an equal transconductance of the REFET and ISFET [54,55]. Unfortunately, such ion-unblocking hydrophobic membranes result in a short lifetime of the sensor, if they are not chemically anchored to the surface. ISFETs have been modified in order to prepare REFETs with polymeric membranes which are covalently linked to the gate oxide surface [56,57].
Two types of REFET structures can be distinguished with respect to the penetration of ions into the polymeric layer, resulting in two different mechanisms of the REFET operation. In a non-ion-blocking REFET structure there is ion exchange between the solution and the polymer; consequently a thermodynamical equilibrium between ions in the solution and in the polymer is achieved and the membrane electrical potential is a membrane potential. In an ion-blocking REFET structure this ion exchange is negligible and in this case the electrical potential measured is a surface potential resulting from reversible ion-complexation reactions at the surface of the polymer.

CONCLUSION AND REFERENCES

Conclusion
The application of field effect transistors (FETs) as transducers in electrochemical sensors was firstly described in 1970 by Bergveld. These devices can transduce an amount of charge present on the surface of the gate insulator into a corresponding drain current. The fast expantion of these transducing elements was possible due to the introduction of IC-technology in their construction, which allowed mass fabrication.
This modern technology provide a possibility to design multi-ion sensors integrated with the reference cell (REFET). These sensors are very small, longliving and use only very small amounts of ion-sensing compounds.
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Coated wire and solid state electrodes

This post has been taken from http://csrg.ch.pw.edu.pl/tutorials/solid/
In the conventional ion-selective electrodes (ISE), the ion-selective membrane is in electrical contact with the inner reference electrode through the inner reference solution. The convertion from ionic conductivity (in the membrane and the inner reference solution) to electron conductivity (in the inner reference electrode and external instrumentation) is provided by the reversible reaction of the inner reference electrode resulting in an ISE exhibiting a stable and reproducible standard potential.
The desire to miniaturise and mass fabricate sensors has led to the development of potentiometric solid-state sensors with new sensing systems, namely solid contact electrodes (SCEs) [1] such as solid crystal membranes and coated wire electrodes (CWEs) [2,3].
Coated-wire electrodes refer to a type of ISE in which an electroactive species is incorporated in a thin polymeric support film coated directly on a metallic conductor. This move to the total elimination of the internal filling solution provides new advantages. The substrate in the wire type electrodes is usually platinum wire, but silver, copper and graphite rods have also been used. CWEs are manufactured by dipping a metal wire into a solution of the membrane mixture [3]. The scheme of CWE is presented in the Fig. 1.

Fig. 1. The scheme of coated wire electrode
Sensors for Ca2+, NO3-, K+, Cl-, Li+ and ClO4- have been developed and sometimes these electrodes exhibited better selectivity than conventional type electrodes with an internal solution. Simplicity of design, lower costs, mechanical flexibility of miniaturisation and microfabrication widened the application for wire type electrodes, especially in the fields of medicine and biotechnology.
However, the configuration of CWE involves some crucial drawbacks. Commonly observed phenomena in such solid-state ion-sensors are a poor mechanical adhesion of the PVC-based sensitive layer to the transducer surface and insufficient electrochemical stability i.e. shift and drift of the EMF [4]. The standard potential of CWEs is often unstable varying for the electrode during its lifetime. These systems cannot provide very reproducible potentials due to the poorly defined charge transfer process at the interface between the ionically conducting membrane and the electrically conducting substrate. In the case of CWEs, drift characteristics were reported to be dependent on the kind of solids used [5,6] and on dissolved oxygen [6]. It has been found that usually an oxygen half-cell is set up on the metal surface so that the electrodes are susceptible to the oxygen content of the solution [7,8].
ISEs with direct contact of the membrane to a metal substrate constructed in a different manner have also been described. Some improvement has been made for this type of electrode with regard to the adhesion of the membrane by using a metal loaded epoxy as substrate [9], but these electrodes also did not possess a defined interface and were found to suffer from oxygen interface.
A more stable electrode potential can be obtained by contacting the ion-selective membrane to the solid substrate via an intermediate layer. Polymeric materials exhibiting mixed ionic and electronic conductivity, containing extended p-conjugated back-bones, such as poly(pyrrole) [10], poly(aniline) [11-13], poly(thiophene) [12] can be applied for this purpose. These materials may be prepared electrochemically by oxidation of their monomer.
Owing to the introduction of such electrically conducting polymers (CPs) t he ionic response of an ion-selective membrane (ISM) is converted to an electric signal.
To facilitate the charge transfer across the interface in solid contact ISE (SC-ISE), improve the potential stability and prevent oxygen interference the introduction SAM of a lipophilic redox-active compound was also proposed [14]. The structures of compounds used for this aim are presented in the Fig. 2.

Fig. 2. The structures used compounds for SAM preparation
In such case a lipophilic and redox-active compound is attached by self-assembly to the inner gold electrode. The redox properties of the SAM guarantee a stable potential, while their lipophilicities prevent the formation of an aqueous layer between membrane and the metal electrode.

MINIATURISED SOLID-STATE SENSORS BASED ON SILICON TECHNOLOGY

The first miniaturized potentiometric solid-state sensors based on silicon technology were developed at University of Michigan several years ago [15,16]. Silicon structures employed for this purpose possessed both: the sensing and electrical contact sites on the front side of the chip (FSC). The scheme of such a sensor is presented in the Fig. 3.

Fig. 3. The scheme of the front-side solid-state miniaturized sensor
Such sensors' design requires an additional encapsulation layer that covers the electrical contact pads. Insufficient quality of this encapsulation layer might cause a solid problem when sensors are used in an electrolyte solution. In order to eliminate this problem, potentiometric sensors with a back-side electrical contact (BSC) and front-side sensing site, fabricated with silicon technology, have been designed and applied for the fabrication of miniaturized BSC ion-sensors [17]. The structure of BSC sensor is presented in the Fig. 4.

Fig. 4.The structure of BSC sensor
The back-side contact silicon-based chips were fabricated using IC technology, by photolithography processes. The structures possess the miniaturised Ag/AgCl sensing site situated on the front of the chip. Such Ag/AgCl electrode shows the changes in EMF as a function of chloride concentration according to the Nernst equation:
E = E0 - 59.16 lg aCl-
The potentiometric response (the calibration curves) of 10 randomly chosen sensors toward changes in chloride concentration is presented in the Fig. 5.

Fig. 5. The response of 10 randomly chosen BSC sensors toward changes in chloride concentration
Based on the potentiometric characteristics of BSC chip it can be concluded that Ag/AgCl sensing site can play role of the internal reference electrode. Such sensors are useful for miniaturised ion-selective sensor preparation.
However it was found that both kinds of sensors with front and back-side electrical contact suffer from instabilities of potential values. Several approaches (e.g. the incorporation of lipophilic silver-ligand complexes within polymeric films, the intermediate pHEMA layer introduction) have been suggested to improve the stability of such sensors by establishing a reversible electron transfer pair at the membrane/solid contact interface. The scheme of miniaturised solid-state chemically modified BSC sensor with intermediate pHEMA layer and polymeric membrane is presented in the Fig. 6.

Fig. 6.The structure of BSC chemically modified sensor
Such configuration involves the charge transfer processes at both sides of the IS membrane well thermodynamically.
Ion-selective miniaturised BSC sensors with polymeric membranes are typically investigated in a galvanic cell:
Ag/AgCl/KCl(sat)/1M CH3COOLi//sample solution//liquid membrane/
/internal filing solution (pHEMA)/Ag/AgCl
It is common to divide the membrane potential (EM) into several separate potential contributions, namely the phase boundary potentials at both interfaces and the diffusion potential within the ion-selective membrane. The potential at the membrane/inner filling solution (pHEMA layer) interface can usually be assumed to be independent of the sample. The boundary potential (membrane/sample solution) depends on the ion-exchange processes between the solution and membrane phase. The diffusion potential within the membrane may become significant if considerable concentration gradients of ions with different mobilities arise in the membrane.
The selectivity of classical ISE and miniaturised sensors in the presence of the primary and interfering ions can be described by the selectivity coefficient according to the Nicolskii-Eisenman equation:


Where: E - the membrane potential; E0 - constant; aI, aJ - the activity of primary and interfering ions, respectively; ZI, ZJ - charge of primary and interfering ions, respectively; I - primary ions; J - interfering ion;
The miniaturised chemically modified back-side contact chips can be designed as anions as well as cations selective sensors. The calibration curves for NO2- - selective sensors with membranes based on linear polyurethane (Tecoflex) and containing the tetraphenyl porphyrin nitrite (CoTPPNO2) as a nitrite-selective ionophore are presented in the Fig. 7.

Fig. 7. The calibration curves for the BSC sensors based on Co(III)[TPP]NO2/TDMACl//PU/o-NPOE
The sodium selective chips based on back-side silicon sensors have also been described [18]. The potentiometric response of the BSC structures with polymeric membrane containing isodecyl acrylate/acrylinitryle and calix[4]arene as a sodium selective ionophore is presented in the Fig. 8.

Fig. 8. The calibration curves of Na+ - selective miniaturised BSC sensors with intermediate pHEMA layer and membranes based on copolymer isodecyl acrylate/acrylonitryle and calix[4]arene as a sodium selective ionophore
Plasticised PVC is commonly used as the membrane material for classical ISEs and CWEs. However it can not be applied as a polymeric matrix for miniaturised silicon-based sensors due to the lack of its adhesion to Si3N 4 surface. Insufficient adhesion may cause detachment of ion-selective membrane based on linear polymer and shortening of sensor's lifetime. The application of the photocurable polymeric matrix is recommended in order to fulfil the requirements related to membrane adhesion to sensor's support and its mechanical properties.

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