Review
Applications of electronic nos and tongues
in food analysis
Anil K.Deisingh,*†David C.Stone&Michael Thompson
Department of Chemistry,University of Toronto,80St George Street,Toronto,Ontario M5S3H6,Canada
(Received23June2003;Accepted in revid form25February2004)
Summary This reviewexamines the applications of electronic nos and tongues in food analysis.A brief history of the development of nsors is included and this is illustrated by
descriptions of the different types of nsors utilized in the devices.As pattern
recognition techniques are widely ud to analy the data obtained from the
multinsor arrays,a discussion of principal components analysis and artificial neural
networks is esntial.An introduction to the integration of electronic tongues and nos is
also incorporated and the strengths and weakness of both are described.Applications
described include identification and classification offlavour and aroma and other
measurements of quality using the electronic no.The us of the electronic tongue in
model analys and other food,beverage and water monitoring applications are
岩烧
discusd.
Keywords Beverage analysis,bionsor,food analysis,pattern recognition,nsor arrays.
Introduction
In the past decade,many papers have appeared in the literature describing the us of electronic nos and,more recently,electronic tongues. The devices are typically array of nsors ud to characterize complex samples.Arrays of gas nsors are termedÔelectronic nosÕwhile arrays of liquid nsors are referred to asÔelectronic tonguesÕ(Stetter&Penro,2002).The former group are ud in quality control and process operations in the food industry while the latter are widely ud in taste studies.In this review,we will discuss the principles behind the design of elec-tronic nos and t
ongues,describe the ns of taste and smell and evaluate the various us of the devices.Development of nsors
The principles of nsor design and technology will be described in this ction.This is necessary to both fully understand the subject and to also appreciate their impact upon the development of the electronic nos and tongues.
A chemical nsor is a device which responds to a particular analyte in a lective way by means of a reversible chemical interaction and can be ud for the quantitative or qualitative determination of the analyte(Cattrall,1997).All nsors are com-pod of two main regions:thefirst is where the lective chemistry occurs and the cond is the transducer.The transducer allows the conversion of one form of energy to another.The chemical reaction produces a signal such as a colour change,fluorescence,production of heat or a change in the oscillator frequency of a crystal(Cattrall,1997). Other parts of a nsor include the signal process-ing electronics and a signal display unit.The major regions of a typical nsor are shown in Fig.1.
*Correspondent:Fax:(868)662-7177;
e-mail:
屏障的近义词
注意英文
†Prent address:Caribbean Industrial Rearch Institute,
University of the West Indies Campus,St Augustine,
Trinidad and Tobago,West Indies
International Journal of Food Science and Technology2004,39,587–604587
doi:10.1111/j.1365-2621.2004.00821.x
Ó2004Blackwell Publishing Ltd
Several categories of transducers are available and the include:
1Electrochemical,such as ion-lective electrodes (ISE),ion-lective field effect transistors (FET),solid electrolyte gas nsors and miconductor-bad gas nsors.
surface acoustic wave (SAW)nsors.Piezoelectric materials are nsitive to changes in mass,density or viscosity and,there-fore,frequency can be ud as a nsitive trans-duction parameter (Hall,1990).Quartz is the most widely ud piezoelectric material becau it can act as a mass-to-frequency transducer.
3Optical,such as optical fibres,as well as the more traditional absorbance,reflectance,luminescence and surface plasmon resonance (SPR)techniques.4Thermal systems,in which the heat of a chemical reaction involving the analyte is monitored with a transducer such as a thermistor.
A subdivision of the nsor grouping is the bionsors.The incorporate a biological nsing element positioned clo to the transducer to give a reagentless nsing system specific for the target analyte (Hall,1990).This ensures specificity of the biological molecules for target species.
What follows is a brief review of the evolution of nsors as this illustrates howdevelopments in the field aro.The first nsor was the glass pH electrode,which appeared in 1930.Initial devel-opments were slow and it was not until 1956that a significant invention was reported.This was the
Clark oxygen electrode,which spurred rearch in biomedical areas (Thompson &Stone,1997).The piezoelectric mass deposition nsor (quartz crystal microbalance,QCM)was produced in 1959.In 1961,a solid electrolyte nsor was reported while in 1962,the first bionsor (an enzyme electrode)was described by Clark &Lyons (1962).In this ca,gluco oxida was held between membranes and the concentration of oxygen in an internal solution was measured.The platinum electrode responded to the peroxide produced by the reaction of the enzyme with its substrate:
gluco þoxygen !gluconic acid þhydrogen peroxide
This investigation led to the production of the first gluco analyzer for measuring gluco in blood.Also in 1962,a metal oxide miconductor gas nsor (the Taguchi nsor)was reported and this was followed,in 1964,by the piezoelectric bulk acoustic wave (BAW)chemical vapour nsor.In 1966,the first gluco nsor and a fluoride ISE were reported in the literature.Many ISEs are available and they are very popular as the basis of electronic tongues.The advantages of ISEs include the linear respon that they showand the ability to obtain direct potentiometric measurements.They also measure ion activity of the ions rather than the total content.
The 1970s and 1980s sawmany developments including the ion-lective FET (1970),a fibre optic gas nsor (1970),a palladium gate FET hydrogen nsor (a metal oxide miconductor FET or MOSFET,1975),an enzyme FET bio-nsor (1977)and the SAW vapour/thin film nsor (1979).In 1980,a liquid-pha BAW operation was described while in 1982the SPR nsor made its debut.An evanescent wave fibre optic nsor was developed in 1984and in 1986there was the production of a BAW liquid-pha immunonsor.Further details of the systems are given in the succeeding ctions.
Since then,there has been the refinement of the various device technologies leading to the produc-tion of arrays of nsors (as ud in electronic nos and tongues),microarrays and the development of micro-Total Analytical Systems (l m-TAS).The last category is also known as lab-on-a-chip or
事业单位抚恤金
as
Food analysis using arrays of bionsors A.K.Deisingh et al.
588International Journal of Food Science and Technology 2004,39,587–604
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integrated systems as they incorporate all analyt-ical operations on a single chip.
Sensor arrays
Four major categories have been involved in the development of electronic nos and each will be briefly described.
1Catalytic or tin oxide nsor:A commercially available Taguchi Gas Sensor(TGS)can be and is widely ud as the core-nsing element in array-bad odour detectors.This consists of an elec-trically heated ceramic pellet upon which a thin film of tin(II)oxide doped with precious metals is deposited(Persaud&Travers,1997).Tin(II) oxide is an n-type miconductor and when oxygen adsorbs on the surface,one of the negat-ively charged oxygen species is generated depend-ing on the temperature.This results in the surface potential becoming increasingly negative and the electron donors within the material become pos-itively charged.When an oxidizable material comes into contact with the nsor surfaces the adsorbed oxygen is consumed in the resulting chemical reactio
n.This reduces the surface poten-tial and increas the conductivity of thefilm. Several recent developments with tin oxide detec-tors have led to further advantages over the Taguchi nsor,which generally requires high power consumption and high temperatures.The include the fabrication of thin-film tin(II)oxide arrays using planar microelectronic technology leading to reduced size and lower power u, the production of thin-film nsors by chemical vapour deposition and the u of screen printing to make thick-film nsors(Persaud&Travers, 1997).
2Conducting polymer nsors:Many other materials are conducting(or miconducting) and showa variation in conductivity w ith sorption of different gas and vapours.Conducting poly-mers are very popular in the development of gas-and liquid-pha nsors with polypyrrole and polyaniline being the favoured choices.Materials ud to make conducting polymers tend to have some common features,including the ability to form them through either chemical or electrochemical polymerization and the ability to change their conductivity through oxidation or reduction.Conducting polymers are widely ud
as odour-nsing devices,the major reasons for
this being(Persaud&Travers,1997):
(a)the nsors display rapid adsorption and
desorption phenomena at room temperature;
(b)power consumption is low;
(c)specificity can be achieved by modifying the
structure of the polymer;
(d)they are not easily inactivated by contami-
nants;
(e)they are very nsitive to humidity.
3Acoustic wave nsors:AT-cut quartz crystals
(+35°15¢orientation of the plate with respect to
the crystal plane)are favoured as piezoelectric nsors becau of their excellent temperature
coefficients.The type of acoustic wave generated
in piezoelectric materials is determined by the
crystal cut,thickness of the material ud and by
the geometry and configuration of the metal electrodes employed to produce the electricfield (Thompson&Stone,1997).One of thefirst nsors
to be introduced was the thickness-shear mode (TSM)nsor,which,if the substrate is quartz,may commonly be termed the QCM or BAW nsor.A
typical TSM nsor is shown in Fig.2.The nsor consists of overlapping metal electrodes at the top
and bottom and the device is normally1.56mm
thick and12.5mm in diameter.This type can be
ud with up to10MHz fundamental resonance frequency with a standing resonant wave being generated where the wavelengths are related to the thickness.As the thickness increas(for example,
儿童画鱼due to added mass by deposition on the surface),
the wavelength increas and the frequency decrea-
s.Thus,the TSM can act as a mass-nsitive device.A major advance on the TSM nsor was
the SAW version consisting of interdigitated elec-
trodes fabricated on to quartz containing a thin
film of material(Fig.3).When a potential is applied across the two halves of the interdigital transducer(IDT),a surface Rayleigh wave is launched in both directions across the surface from
the IDT.Adsorption of odours to the coating
results in a change in mass and the acoustic wave is perturbed leading to a frequency shift(Persaud& Travers,1997).SAW nsors can be operated at热心公益事迹材料
higher frequencies than QCM nsors thereby leading to better nsitivities.Acoustic wave n-
sors can also be operated in the liquid-pha.
4MOSFET technology:In the1970s,improve-
ments in miconductor technology led to the devel-
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Ó2004Blackwell Publishing Ltd International Journal of Food Science and Technology2004,39,587–604
opment of a FET.This is a very high impedance transistor and the most nsitive measurements of small potentials requiring very lowcurrent flow s are made using this technology.In the FET,current flows along a miconductor path called the chan-nel,at one end of which is a source electrode.At the opposite end is the drain electrode.The effective electrical diameter of the channel can be varied by application of a voltage to a control or gate electrode.The conductivity of the FET depends on the electrical diameter of the channel.A small change in gate voltage leads to a large variation in current from the source to the drain.This allows the signal to be amplified.For the MOSFET,the thermal oxidation process ud to form the silicon dioxide layer on the silicon surface of the device also forms a double layer,which can induce a conducting channel in the silicon substrate.In the MOSFET,the conducting channel is insulated from the gate terminal by a layer of oxide.Thus,there is
no conduction even if a rever voltage is applied to the gate.FET nsors can be operated both with and without a reference electrode.Other chemical principles are being applied to vapour-nsing devices and the include:
5Ion mobility spectrometry (IMS)which has the ability to parate ionic species at atmospheric
pressure.However,there is also rearch under-way to develop low-pressure IMS systems.This latter technique can be ud to detect and char-acterize organic vapours in air.This involves the ionization of molecules and their subquent drift through an electric field.Analysis is bad on analyte parations resulting from ionic mobilities rather than ionic mass.A major advantage of operation at atmospheric pressure is that it is possible to have smaller analytical units,lower power requirements,lighter weight and easier u (Graby Ionics,2002).
6Other mass spectrometric (MS)techniques that are in commercial development.Two recent developments in MS are atomic pressure ioniza-tion (API)and proton transfer reaction (PTR).Both are rapid,nsitive and specific and allow measurements in real-time.Additionally,they do not suffer the drift or calibration problems cur-rently experienced by electronic nos.With API-MS,ionization takes place at atmospheric pres-sure,which allows nebulization and ionization to be independent of
each other.The solute and the solvent elute from a capillary,which is surrounded by the nebulizing gas,usually nitrogen.The capillary and gas are contained in a probe which can be heated up to 700°C depending on the analyte being investigated.The combination of nebulizer gas and heat convert the solvent flow into an aerosol,which evaporates rapidly.Inside the heated source is a corona discharge needle,which ionizes the solvent molecules.In the atmo-spheric region around the corona pin,a combina-tion of collisions and charge transfer reaction produces a chemical ionization reagent known as gas plasma.Sample molecules which elute and pass through this region can be ionized by the transfer of a proton to produce (M +H)+
or
Food analysis using arrays of bionsors A.K.Deisingh et al.
590International Journal of Food Science and Technology 2004,39,587–604
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(M)H))ions(Ashcroft,1997).The impact of volatile organic compounds(VOCs)on the envi-ronment has led to a growing demand for devices to detect the compounds.One of the most promising us the proton transfer reaction (PTR)-MS,with thefirst instruments just appear-ing on the market(Ellis,2003).As a result of the advantages listed above,the may play an incread role in the future development of elec-tronic nos and tongues.However,a limitation is the u of quadrupole MS,which has a modest mass resolution and can only monitor a single mass channel at
any moment(Ellis,2003). Rearch is currently underway to replace the quadrupole system by a time-of-flight MS,which will allow more rapid data acquisition.
7Optical/spectroscopic techniques are being currently employed,the most popular being the u offibre optics andfluorescence.
For the electronic tongue,classical electro-chemical principles such as potentiometry,am-perometry and voltammetry have been utilized. When designing potentiometric devices,ion-lective and redox electrodes are commonly ud.Additionally,ISFET technology has been incorporated into commercially available elec-tronic tongues.Many of the available electronic tongues are bad on ISEs and a brief discussion of the principles behind this technique will now be given.
Otto&Thomas(1985)reported thefirst appli-cation of ISE arrays for multicomponent analysis. Eight nsors were ud for the simultaneous determination of sodium,potassium,calcium and magnesium at concentrations typical of biological fluids.However,there was insufficient lectivity for magnesium and sodium.Since then,multin-sor arrays have become much improved with greater lectivities being reported. Potentiometry generally assumes a linear dependence between an ISE output and the logarithm of activity of the primary ion in a solution.The electrode respon should obey the Nernst equation:
E¼E0þðRT=Z i FÞln a i;ð1Þwhere E is the potential difference of the electro-chemical cell comprising an ion-lective and a reference electrode,E0is the standard potential,R is the gas constant,T is the absolute temperature,
F is the Faraday constant,Z i is the electrical
charge of the primary ion and a i is the activity of
the primary ion.The term RT/Z i F is the nsi-
tivity of the ISE(Legin et al.,2002a).
If there are interfering ions on the ISE respon,
the Nikolsky eqn2is ud:
E¼E0þðRT=Z i FÞln½a iþR K ijða jÞZ i=Z j ;ð2Þ
where K ij is the lectivity coefficient of the ISE to
the primary ion i in the prence of an interfering
辞职报告的格式
ion,j,and Z i and Z j are the charges of the primary
and interfering ions,respectively(Legin et al.,
2002a).
Sensations of taste and smell
Taste
The taste buds,of which there are around10000,
are found mainly on the tongue with a few on the
soft palate,inner surface of the cheek,pharynx
and epiglottis of the larynx(Marieb,1998).Taste nsations can be classified intofive basic categ-
ories:sweet,sour,salty,bitter and umami.Table1
gives examples of each of the.
A single taste bud contains50–100taste cells reprenting allfive taste nsations.Each taste
cell has receptors which bind to the molecules and
ions which result in the different taste nsations (Kimball,2002).Also,some materials change in
flavour as they move through the mouth, e.g. saccharin is initially sweet but has a bitter after-
taste at the back of the tongue(Marieb,1998).
Each of the basic taste nsations has a different threshold level with bitter substances having the lowest.This is probably a protective function as
many poisonous substances are bitter(Tortora& Grabowski,1996).Sour substances have an intermediate threshold limit while sweet and salty
三生三世步生莲
Table1Taste nsations(Marieb,1998)
Sensation Elicited by the compounds
Sweet Sugars,amino acids,alcohols
Sour acetic,citric
Salty Table salt
Bitter Quinine,caffeine,aspirin,nicotine
Umami Monosodium glutamate(MSG),
disodium inositate in meat andfish
disodium guanylate in mushrooms
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