Senso s: F om Biosenso s o he Elec onic Nose
By Diego L. Ga cía-González and Ramón Apa icio
Ins i u o de la G asa
A da. Pad e Ga cía Teje o, 4, 41012 Se illa Spain
E-mail: [email protected]
CONTENTS
1. In oduc ion.
2. Senso y quali y.
3. Senso s.
3.1. Me al oxide semiconduc o senso s (MOS).
3.2. Me al oxide semiconduc o ield-e ec ansis o
(MOSFET) senso s.
3.3. Conduc ing polyme senso s.
3.4. Acous ic senso s.
3.5. Biosenso s.
4. The elec onic nose.
5. The elec onic ongue.
6. Da a analysis.
6.1. P e-p ocessing da a analysis.
6.2. Mul i a ia e da a analysis.
6.2.1. Clus e analysis.
6.2.2. Fac o ial analysis.
6.2.3. Disc iminan analysis.
6.2.4. Reg ession analysis.
6.3. A i icial neu al ne wo ks.
Re e ences.
RESUMEN
Senso es: De los biosenso es a la na iz elec ónica.
Los ecien es a ances en los sis emas de senso es han pe -
mi ido el desa ollo de nue as aplicaciones en muchos campos
ecnológicos. Es e a ículo de e isión desc ibe el es ado ac ual
de es a nue a ecnología, con especial én asis en las aplicacio-
nes alimen a ias. El diseño, la ecnología y el mecanismo senso-
ial de cada ipo de senso son analizados en el a ículo. También
se desc iben las p incipales ca ac e ís icas de la na iz y la lengua
elec ónica (senso es de sabo ). Finalmen e, se desc ibe b e e-
men e el uso de algunos p ocedimien os es adís icos en sis emas
de senso es.
PALABRAS-CLAVE:
A he nálisis senso ial – Biosenso es -
Na iz elec ónica – Senso es.
SUMMARY
Senso s: F om Biosenso s o he Elec onic Nose.
The ecen ad ances in senso de ices ha e allowed he
de eloping o new applica ions in many echnological ields. This
e iew desc ibes he cu en s a e-o - he-a o his senso
echnology, placing special emphasis on he ood applica ions.
The design, echnology and sensing mechanism o each ype o
senso a e analysed. A desc ip ion o he main cha ac e is ics o
he elec onic nose and elec onic ongue ( as e senso s) is also
gi en. Finally, he applica ions o some s a is ical p ocedu es in
senso sys ems a e desc ibed b ie ly.
KEY-WORDS: Biosenso s – Elec onic nose – Senso s –Sen-
so y analysis.
1. INTRODUCTION
Fo ages, he e has been much in e es in he
design o de ices o he sensing o ood la ou . The
de ices, o ins umen s, a e claimed o ope a e on
p inciples simila , in many aspec s, o he human
ol ac o y sys em. Today, he me hodologies de o ed
o he senso y assessmen o oods u s a e based
on ei he he classical panels o ained human
beings o he analysis o some chemical compounds by
headspace-gas ch oma og aphy (-mass spec ome y)
o he global e alua ion o he odou in ensi y o
ola ile compounds by he eme gen senso
echnologies.
The expe s’ panel, usually he only o icial
me hod o he senso y e alua ion, allows he
senso y quali ica ion o oods u s by hei own
po en ial consume s, as well as he e alua ion o all
he senso y pe cep ions in only one analysis. I has,
howe e , he g ea disad an age o being a leng hy
and expensi e me hodology whose inal esul
depends on many ac o s as, o example, he
panellis s’ aining and he speci ic ocabula y used
in he senso y analysis.
An al e na i e me hod is he iden i ica ion and
quan i ica ion o he chemical compounds
esponsible o he senso y pe cep ions. Non- ola ile
compounds a e mainly ocused on phenolic
compounds and aglycons, esponsible o as ing
pe cep ions (as ingen , bi e and pungen ) (Mo ales
and Tsimidou 2000), while ola ile compounds a e
esponsible o he a oma o oods u s (Apa icio
e
al
. 1996). The i s se o compounds is quan i ied by
high pe o mance liquid ch oma og aphy (HPLC)
(Mon edo o
e al
. 1992) while he quan i ica ion o
ola ile compounds can be ca ied ou by a ious
analy ical echniques (Mo ales
e al
. 1992). The la e
can be summa ised as:
1. hose no in ol ing concen a ion:
• di ec injec ion.
• s a ic headspace
2. hose wi h concen a ion-dis illa ion.
• simul aneous dis illa ion-ex ac ion.
• dynamic headspace/pu ge-and- ap.
• supe c i ical luid ex ac ion.
• solid phase mic o ex ac ion
G asas y Acei es
96 Vol. 53. Fasc. 1 (2002), 96-114
The use ulness o hese echniques in senso y
analysis is based on he ac ha ola ile compounds
a e esponsible o he oods u odou s. Thus, an
in e es ing app oach has been ca ied ou in i gin
oli e oil o es ablish he ela ionships be ween
ola ile compounds and la ou (Apa icio
e al.
1996),
and be ween phenols and as e (Mo ales and
Tsimidou 2000) using he ma hema ical algo i hm o
he s a is ical senso y wheel (Apa icio
e al
. 1994).
These echniques can assess he senso y quali y
ou ine es s bu canno be used on-line because
hey a e ime-consuming and hey p e iously need o
a sample p e- ea men o a ola ile concen a ion
phase.
An al e na i e, ecen ly de eloped, is he use o
senso s ha ha e been widely used in many ields o
analy ical chemis y. Concep ually speaking, his kind
o senso is a de ice ha is able o gi e a signal
p opo ional o he physical o chemical p ope y o
which he de ice esponds (K ess-Roge s 1997); o
example, he senso s ansduce he signals
p oduced by magne ic, he mal and chemical ields.
Based on hese ac s, he mos common de ini ion is
ha a senso is a single ansduce in which, due o a
physical e ec , a comple e ansduc ion p ocess
akes place (Middelhoek 2000). The elec onic
in eg a ion o a ious senso s inside one se
cons i u es an a ay o senso s, o he so-called
elec onic nose, ha is he kind o low cos s uc u e
habi ually used o analyse he a oma o he
oods u s.
The senso s ha e some ad an ages o e o he
ci ed echniques. They do no use eagen s o need
o any kind o p e- ea men , hey ha e good le els o
sensi i i y and speci ici y, and besides hey a e apid
supplying esul s o he analys . This is a
non-des uc i e echnique ha can be used in an on
line quali y con ol du ing he manu ac u ing p ocess.
Bu senso s ha e ob iously disad an ages mainly
ocused on he ep oducibili y, eco e y, and he
e ec ha humidi y and empe a u e ha e on he
senso esponses. The analysis o hese aspec s,
ad an ages and disad an ages o he senso
echnologies, oge he wi h he applica ions o he
di e en kinds o senso s in ood science a e he
objec i es o his e iew. Finally, he e is a schema ic
analysis o he ma hema ical p ocedu es used o
analyse he senso esul s as well as he applica ion
ields and he e e ences.
2. SENSORY QUALITY
Senso y quali y consis s on a se o quali y
a ibu es, ha can be de ec ed h ough sense
o gans, like appea ance, size and shape, colou ,
iscosi y, kines he ic sensa ions, ac ile sensa ions,
and chemes he ic sensa ions (s imuli elici ed om
some chemical i i an s), odou and as e (Ange osa
2000). Colou and la ou a e, howe e , he
sensa ions ha mainly con ibu e o ood
accep abili y by consume s (Mo ales and Tsimidou
2000) o when moni o ing ood indus y p ocesses
(Table I). Fla ou is a complex sensa ion consis ing
p ima ily o odou and as e, bu is complemen ed by
ac ile and kines he ic sensa ions (Reineccious
1993). The impo ance o la ou in he global
senso y quali y lies in he ac ha i plays a c ucial
ole in ood ecogni ion, selec ion, accep ance and
nu i ion, as i is pa ly esponsible o aiding he
diges ion o ood humans (Enso 1989).
The main human senso con ibu ing o la ou is
he ol ac ion sys em ha de ec s he odo an
compounds. The basic cha ac e is ics o odo an
molecules a e o ha e low ela i e molecula masses
(up o app oxima ely 300 Da), o be small and pola ,
and also o be hyd ophobic (C a en
e al.
1996). In
he nasal mucus, he e a e small odou an binding
p o eins (OBPs) ha accommoda e he hyd ophobic
molecules in solu ion (B ee 1997). The OBPs wo k
as binde s o he odou an compounds and s a e s
o he ansduc ion eac ion. A e c ossing he
mucus laye , he odou an s each he chemosenso y
cilia y memb ane o he ol ac o y neu on in which he
G- ecep o p o eins a e loca ed (Ga dne and
Ba le 1994). Mo e han 100 million ol ac o y cells
wi h hese ecep o p o eins a e inside he ol ac o y
epi helium. The ecep o s in e ac wi h odou an
molecules and gene a e a second se o messenge s
ha cause exci a ion in he neu ones. The signal is
p ocessed in he ol ac o y bulb and hen he
in o ma ion is ansmi ed o he limbic sys em whe e
he associa ions be ween odou in o ma ion and
ecalled memo ies ake place (Kelle
e al
. 1999). The
in o ma ion o he ecep o s is also ansmi ed o he
ce eb al co ex whe e is combined wi h he as e o
p oduce he la ou sensa ion.
On he o he hand, he as e ecep o s a e mainly
si ua ed on he ongue su ace, o e he en i e o al
Table I
Applica ion o he senso y panel in di e en
c i ical poin s o he ood indus y
Moni o ing o he ood ipening (e.g., wine, cheese)
Quali y assu ance o he selec ed aw oods
Moni o ing o he cooking p ocesses
Moni o ing o he e men a ion p ocesses
Moni o ing o he indus ial p ocesses (e.g., la ou ing,
blending, colou ing).
Moni o ing he s o age o oods and ood p oduc s
(e.g., eshness and ageing con ol)
Quali y assu ance o he manu ac u ed ood p oduc s
Moni o ing o he p oduc -packaging in e ac ions
Moni o ing o he o e all quali y o he inal ood o ood p oduc .
Vol. 53. Fasc. 1 (2002) 97
ca i y, down he oesophagus and e en on he
achea and la ynx. When an app op ia e soluble
subs ance eaches he as e cell, he in e ac ion o
he molecule wi h he G- ecep o p o ein leads o
chemical changes ha esul in a neu al impulse
(Winquis
e al.
1999). The esul is he as ing
pe cep ion ha has ou basic as es (bi e , swee ,
sal and sou ) al hough he senso y assesso s a e
able o dis inguish be ween dozens o di e en
as es, o ins ance quali ying i gin oli e oil (Apa icio
e al
. 1994).
3. SENSORS
Today, se e al comme cial senso s a e a ailable
on he ma ke besides he g ea numbe o p o o ype
senso s ha a e being designed by esea ch
ins i u ions yea by yea . All hese ypes o senso s
exhibi physical and chemical in e ac ions wi h he
chemical compounds when hey low o e , o a e in
con ac wi h, he senso s. The choice o senso s is
hence qui e la ge, so ha we ha e classi ied hem
in o b oad classes ollowing an a bo escen
s uc u e. The odou senso s, as ing senso s and
biosenso s, cons i u e he i s se . The la e wo
g oups a e he esul o a e y ecen esea ch, and
hence he e is no a axonomic s uc u e o hese
senso s ye . The senso s o he o me g oup can
howe e be clus e ed in o wo g ea g oups
acco ding o hei wo king empe a u e: ho senso s
and cold senso s. The g oup o he so-called “ho
senso s” mainly clus e s he di e en kinds o me al
oxide gas senso s (based on doped o undoped
semiconduc o s), while he g oup o he “cold
senso s” clus e s he piezoelec ic c ys al senso s
(bulk acous ic wa e senso s, su ace acous ic wa e
senso s) and he conduc ing o ganic polyme
senso s. Figu e 1 shows schema ic diag ams o he
main odou senso s while Table II shows he physical
esponses o hese senso s o he concen a ion o
ola ile compounds. When he analys s wo k wi h an
a ay o senso s o he same class (e.g., me al oxide
gas senso s made wi h doped plus undoped
semiconduc o s) o combining di e en classes (e.g.,
conduc ing polyme senso s plus piezoelec ic
c ys al senso s), he esul ing ins umen is
habi ually called “elec onic nose”. Fu he mo e,
Winquis
e al
. (1999) ha e ecen ly gone beyond
desc ibing he combina ion o an elec onic nose and
an elec onic ongue.
3.1. Me al oxide semiconduc o senso s
(MOS)
These senso s a e he mos comme cially
ex ended in he sensing sys ems. They consis o a
ce amic o me ( ubula o la ) hea ed by wi e and
coa ed wi h a me al oxide semiconduc ing ilm. Fo
his eason, hey a e usually called me al oxide, o
ce amic, gas senso s (Schalle
e al
. 1998) al hough
hey can also be called Taguchi and Figa o because
o he in en o and he i s company espec i ely.
The oxide coa ing may be ei he n- ype o p- ype
semiconduc o s. The n- ype semiconduc o (SnO2,
ZnO, In2O3, WO3, Fe2O3, Ga2O3, TiO2) esponds o
oxidising compounds and i s he mal o pho oly ic
exci a ion esul s in an excess o elec ons ha
inc eases he eac i i y wi h oxidising molecules. The
p- ype semiconduc o (oxides o nickel o cobal )
esponds o educing compounds and i s exci a ion
p omo es eac ions wi h educing compounds due o
an elec on de iciency in i s alence band. These
me al oxides (mos ly SnO2, Ga2O3 and TiO2) can also
be doped wi h me als such as Cu (Zhang and Liu
2000), P plus Nb (Comini
e al
. 2000) and Sb, Bi, Fe,
Au, Cd (Mo imi su
e al
. 2000), o me al oxides o Co,
C , La, Ce (Fukui and Ka suki 2000),
Mg, Al (Xu
e al
.
2000), Ru, Rh and I (Lang
e al.
2000) among
o he s
.
These doping me als help o diminish he
humidi y and empe a u e dependence (Fukui and
Ka suki 2000; Mo imi su
e al.
2000), and o imp o e
he sensi i i y and he selec i i y (Comini
e al.
2000;
Mielle 1996).
Fla MOS senso Tubula MOS senso
MOSFET senso CP senso
BAW senso SAW senso
Figu e 1
Schema ic diag ams o he main odou senso s: Fla and ubula
me al oxide semiconduc o (MOS), me al oxide semiconduc o
ield e ec ansis o (MOSFET), conduc ing polyme (CP), bulk
acous ic wa e (BAW) and su ace acous ic wa e (SAW) senso s.
Table II
Physical p ope ies o he senso s desc ibed
in he e iew
Senso Physical p ope y
Me al-oxide senso s (MOS, MOSFET)
Conduc ing polyme senso s
Elec ochemical senso s
Acous ic senso s (SAW, BAW, QMB,
Can ile e )
Calo ime ic senso s
Op ical senso s
Resis ance and impedance
Resis ance and impedance
Conduc ance, in ensi y and ol age
Mass and equency shi
Tempe a u e
Op ical p ope ies
98 G asas y Acei es
The me al ilm hickness u he classi ies senso s
in o hin ilm (6-1000 nm) and hick ilm (10-300 µm)
senso s. The hickness is only ela ed o he
echnique used (physical o chemical apou
deposi ion o hin ilms
e sus
sc een p in ing o
pain ing deposi ions o hick ilms) bu also wi h he
esponse ( as e o hin ilms), sensi i i y (highe o
hin ilms), and ep oducibili y (highe o hick ilms)
and cos (much mo e low o hick ilms). The sc een-
p in ing echniques, based on a plana in eg a ed
mic oelec onic echnology, allow ob aining low size
senso s wi h high pe o mance in a ious applica ion
ields. Ne e heless, sc een-p in ed ce amic gas
senso s need imp o emen s as lowing he powe
consump ion, ge ing bes selec i i y and acili a ing
moun ing echnology. These aspec s a e eached
wi h mic omachined me al oxide gas senso s, whe e
sensi i e laye is deposi ed in a hin dielec ic ilm o
low he mal conduc i i y, ge ing isola ion be ween
subs a e and gas sensi i i y hea ed a ea (Simon
e
al.
2001).
The mechanism o sensing o hese senso s is
ela ed o he semiconduc o beha iou when
exchanging oxygen be ween he ola iles and he
me al ilm. In non-ope a ing, when only he ca ie
gas (e.g., syn he ic ai ) lows in he senso chambe ,
he oxygen is adso bed on he su ace and inside he
me al coa ing, illing he senso la ice acancies, and
la e oxygen a ac s ee elec ons om he
semiconduc o ma e ial wi h which he senso
conduc ance inc eases. In ope a ion, when he
in e ac ions be ween adso bed ola ile molecules
and me al ilm ake place, he conduc ance
dec eases due o he eac ion wi h he adso bed
oxygen. Two mechanisms, ha can occu
simul aneously, ha e been desc ibed in his phase
(Schalle
e al
. 1998). A ela i ely low empe a u es,
when educing ola iles compounds a e adso bed on
he su ace, he cha ges a e ans e ed be ween he
ola iles and adso bed oxygen. A high empe a u e,
howe e , he loaded oxygen eac s wi h he
adso bed ola ile compounds, and conduc ance
dec ease in a non-linea manne . Zhang and Liu
(2000) ha e s udied he pa icula cases o NO2 and
CO2 o e a ious ansi ion me als. Fu he mo e, he
au ho s ha e sugges ed some mechanisms ha
p oduce mo e oxygen acancies in he ma e ial,
leading o a dec ease in he ee elec on
concen a ion and an inc ease in sensi i i y o he
ma e ial (e.g., he subs i u ion o in ions o coppe
ions).
Recen ly o he measu emen pa ame e s ha e
been s udied, in addi ion o he elec ical esis ance
change. The empe a u e d op du ing he sensing
eac ion has been p oposed (Liu
e al
. 2000; Takada
2000). The empe a u e dec ease is caused by a
change in he he mal conduc i i y o he senso s
and i is ela ed o he eac ion be ween educing
gases and su ace oxygen ad-ions. They ha e a
wo king empe a u e be ween 200oC and 400oC
(Simon
e al.
2001), and he o ganic ola iles a e
comple ely combus ed o wa e and ca bon dioxide
when se les he senso su ace.
These small size senso s a e obus , hey ha e a
qui e good sensi i i y, hei signal p ocessing is
simple, and hey ha e a e y low cos (Simon
e al.
2001). Thei sensi i i y is, howe e , a ec ed by
humidi y and poisoning ma e ials. Al hough he
senso s a e made o ma e ials ela i ely esis an o
humidi y and ageing, he humidi y is s ill he main
p oblem as he senso sensi i i y dec eases when
humidi y inc eases. Ano he d awback is he
possibili y o he senso poisoning by sulphu
compounds (e.g. H2S o SOx) ha p oduces he o al
o pa ial i e e sible des uc ion o he senso .
Sulphu is known as an inhibi o o me al and oxide
ca alys s. On me als, sulphu chemiso bs an
unoccupied o bi al o he me al ia i s occupied lone
pai o bi al, while, on oxides, sulphu is subs i u ed
o la ice oxygen a oms. This i e e sible p ocess
can be caused by he p esence o sulphu
compounds o some acids (e.g. ace ic) in he
samples, e.g. cheeses, wines, inega s, c uci e ous
ege ables o lampan i gin oli e oils. E hanol also
a ec s o senso bu only p oducing a empo a y
blinding e ec o any o he analy es p esen in he
sample. O he undesi able cha ac e is ics o hese
senso s a e: hei slow baseline eco e y when
compounds wi h high molecula weigh a e
analysed, hei high wo king empe a u e ha makes
hem inapp op ia e in en i onmen s con aining
lammable chemicals, hei poo speci ici y and
selec i i y, and he senso d i mainly caused by
sensi i i y loss.
Mos o he me al-oxide senso s show a
non-linea esponse o a gi en chemical compound
o odou . Se e al a emp s o ind a sa is ac o y
Figu e 2
Responses o a MOS senso (Alpha MOS 4000) o i gin oli e
oils cha ac e ised by di e en le els o oxida ion. Oxidized oli e
oil samples we e p epa ed placing i gin oli e oil samples in he
beam o an ul a iole ligh o di e en days (0, 3, 8, 12 days).
Vol. 53. Fasc. 1 (2002) 99
ela ion be ween he senso esponses and he
analy e concen a ion o he odou in ensi y ha e
been done. The o mula R=1/ACα (whe e, R is he
esponse in ensi y, A and α a e cons an , and C is he
concen a ion) has been p oposed o explain he
senso esponse a mode a e concen a ion and
below he sa u a ion le el o he senso (Wilson
e al.
2000). Khol
e al
. (2000) ha e also exp essed he
senso esponse as R-R0/R0=K(C)1/2 (whe e K is a
senso -dependen cons an ). Mielle
e al
. (2000)
ha e ound ano he ela ion be ween he compound
concen a ion and he senso esponse:
R-R0/R0=10(a +b)logC, whe e “a” and “b” a e cons an s
o each compound.
The applica ion o hese senso s has been mainly
ocused on he de ec ion o gas leakage, combus ible
and oxic gases (Shu me and Ga dne 1992; Xu
e
al.
2000; Ménil
e al.
2000; Simon
e al.
2001) bu
hei applica ions on ood echnology ha e always
been o ele ance. Thus, hese senso s ha e been
applied o de ec ion o ola ile compounds o
di e en oods u s as co ee (Ga dne
e al.
1992),
milk (Sbe eglie i
e al.
1998), s awbe y
(Hi sch elde
e al.
1998), juices (Bazemo e
e al.
1997), mea (B aggins
e al.
1999) and edible oils
(Bazzo
e al.
1998). Figu e 2 shows he esponses o
a MOS senso (AlphaMOS 4000) o i gin oli e oils
cha ac e ised by di e en le els o oxida ion.
3.2. Me al oxide semiconduc o ield-e ec
ansis o (MOSFET) senso s
These senso s comp ise h ee laye s, a silicon
semiconduc o , a silicon oxide insula o and a
ca aly ic me al (Pd, P , I , Rh) called he ga e
(Schalle
e al.
1998). This design co esponds o he
ansis o s designed by Lunds öm in 1975
(Middelhoek 2000). This kind o ansis o s ope a es
by means o h ee con ac s, wo allow he cu en in
(sou ce) and ou (d ain), and hi d ac s as he ga e
and egula es he cu en h ough he ansis o .
When a ol age is applied on he ga e, an elec ic
ield is gene a ed which a ec s he ansis o
conduc i i y. I apou (pola compounds) in e ac s
wi h he ga e, hen he cu en lowing h ough he
ansis o (senso ) changes and a shi o he
conduc ance is p oduced. The in e ac ion be ween
he apou and he ga e depends on he ga e
s uc u e; i can be hin, made o a po ous me al ilm
(6-20 nm), o hick, buil wi h a dense me al ilm
(100-200 nm). The la e kind o ga e ( ansis o )
wo ks well wi h apou s con aining molecules ha
can dissocia e hyd ogen, because hese a oms lead
o a po en ial change in his kind o ansis o . Almos
all kind o compounds, howe e , espond well when
he ga e is hin, due o he p obable mechanisms o
ol age shi s desc ibed by Lunds öm
e al
. (1975;
1990) and Spe z
e al
. (1992).
MOSFET a e obus senso s wi h low sensi i i y
o humidi y bu whose selec i i y and sensi i i y can
be a ec ed by he wo king empe a u e (75-200 oC),
kind o me al ga e (as desc ibed) and mic os uc u e
o he ca aly ic me al. Thei obus cha ac e is ic
makes hem pa icula ly use ul in en i onmen al
applica ions (Spe z
e al.
2000). They show good
sensi i i y o oxic and lammable subs ances (Zhao
e al.
2000) and because o hei sensi i i y o
e hanol hey a e also used in ood cooking,
e men ing, and wine-making p ocesses (Zhao
e al.
2000). Finally, hese senso s ha e been used o he
e hylene measu emen du ing ui ipening p ocess
(Winquis
e al.
1990).
3.3. Conduc ing polyme senso s
Since Diaz
e al
. (Pe saud and T a e s 1997)
designed he i s polypy ole senso , he in e es in
conduc ing polyme s (CPs) has inc eased yea by
yea . These senso s a e made o a b oad ange o
monome s ha a e polyme ised by chemical o
elec ochemical me hods. A hin ilm o polyme is
deposi ed on o a sensing subs a e o med by wo
pa allel pla inum o gold pla ed elec odes wi h a
insula ing base such as oxidized silicon (Pa idge
e al.
1996). The mos common polyme s used in he
syn hesis o CPs a e polypy oles, polyanilines,
poly iophenes, bu ane o decane sulphonic acids,
pa a
- oluenesulphonic acid and i s e a-e hylammonium
sal , sodium monohyd ogenosulpha e and
e ae hylammonium e a luo obo a e (Mielle 1996)
al hough new polyme s a e being syn hesized o
open hese senso s o new applica ions. Thus,
polyme s wi h a conjuga ed bonding sys em a e
being now used due o he excellen elec ical
cha ac e is ics o hese ma e ials o mic oelec onic
de ice manu ac u ing (Rella
e al.
2000). Senso s
based on aniline and de i a es can be doped a
speci ic a ios wi h o he compounds, as sulphonic
acids (Koul
e al.
2001) o HCl (Campos
e al.
2000),
so enhancing hei elec ical-op ical p ope ies and
solubili y in o ganic sol en s. The dopan s p o ide
ex a cha ges in he chain o he polyme wi h he
o ma ion o cha ged de ec s, such as spinless
bipola ons (Rella
e al.
2000) o small egions o
posi i e cha ge in he polyme chain ha p o ide
mobile holes o elec on anspo .
The CP sensing cha ac e is ics a e based in a
measu able change in he elec ical conduc i i y
when CPs a e exposed o ola ile compounds. A
ixed ol age among he elec odes p oduces a
cons an cu en ha passes h ough he conduc ing
polyme . When gas lows on he senso , he ola ile
compounds adhe e o i s su ace, and an adso p ion-
deso p ion p ocess occu s on he polyme laye
which al e s he elec on low in he sys em and
hence he senso conduc i i y. The e o e hese
100 G asas y Acei es

changes in conduc i i y a e based in he mobili y o
cha ges along he conjuga ion o he π-backbone o
he polyme . Coun e ions (s abilised ions ha a e pu
inside he polyme by he manu ac u e in o de o
balance o he cha ges inside he mic os uc u e o
polyme laye ) can p oduce some modi ica ions such
as changes in mo phology o polyme -dopan
complex, in he concen a ion o chemical de ec s
(pola ons o bipola ons) o he oxida ion s a e o he
polyme (Tan and Blackwood 2000) al hough he
ope a ing mechanism is no ye well unde s ood.
Spec al s udies, as UV-VIS spec oscopy, FTIR
spec oscopy, and elec on spin esonance
spec oscopy (ESR), a e being ca ied ou o know
mo e abou he in e ac ions occu ing du ing he
sensing p ocess (Tan and Blackwood 2000; Koul
e
al.
2001).
The senso cha ac e is ics explain why hey a e
ones o he mos used in senso a ays. They a e
easily ab ica ed, hei powe consump ion is low,
and hey a e obus and can wo k a oom
empe a u e. As i is said abo e, he e a e many ypes
o polyme s, including a b oad ange o de i a es,
and mo e ypes o polyme s appea each yea . The
na u e o he dopan s also modi ies hei ini ial
p ope ies (Koul
e al.
2001), and i means he
possibili y o designing senso s ailo ed o speci ic
se ies o ola iles (e.g. ke ons, aldehydes o
alcohols) o owa ds a pa icula applica ion (e.g.
cha ac e isa ion o au hen ica ion). The oxida ion
s a e o he polyme can also be changed a e
deposi ion o ailo he sensing cha ac e is ics
(Pa idge
e al.
1996). CPs a e esilien o poisoning
by he compounds ha inac i a e he desc ibed
ino ganic semiconduc o senso s bu su p isingly
hei li e ime, only abou 9-18 mon hs, is sho e .
The andom na u e o he deposi ion p ocess is,
howe e , a p oblem o he p oduc ion o
ep oducible senso s, al hough he use o ou
elec odes, ins ead o wo, seems o imp o e he
esul s (Pa idge
e al.
1996). The senso s ha e also
o he d awbacks as hei oo long esponse ime
(20-40 s), in compa ison wi h MOS (Mielle 1996),
and hei inhe en d i o e ime o wi h changes in
empe a u e. The main p oblem is, howe e , ha hey
a e ex emely sensi i e o mois u e.
Because CPs a e sensi i e o many ypes o
ola iles, many applica ions a e desc ibed in he
li e a u e. They a e sensi i e o alcohols, ace one,
benzene and o he pola ola ile chemicals (Ha ield
e al.
1994). Re e ences show applica ions o CPs in
oods u s like bee and o he alcoholic be e ages
(Pea ce
e al.
1993; Sla e
e al.
1993), oli e oil
(Apa icio
e al.
2000), o ange juices (Bazemo e
e al.
1998), ishes (Luzu iaga
e al.
1999), milk (Magan
e
al.
2001). Figu e 3 shows he esponses o polyme
senso s (A omaScan A20S) o i gin oli e oils
cha ac e ised by di e en le els o oxida ion.
Conduc ing polyme s a e o en associa ed o
o he ansduce mechanisms as sensing ma e ials.
This is he case o he op ical senso s based on CPs,
since hese polyme s show spec al changes in he
UV- isible egion when he doping/dedoping p ocess
happens (Kond a owicz
e al.
2001). Op ical senso s
inco po a ing CPs sensi i e o ammonia, humidi y,
pH and ace ic acid ha e been ecen ly de eloped
(Kond a owicz
e al.
2001; de Ma cos
e al.
2000).
Senso s based on ield e ec ansis o (FET)
inco po a ing a CP as he ga e ma e ial (Ha ield
e
al.
2000) o qua z-c ys al mic obalance associa ed
o CP (Sla e
e al.
1992) a e o he examples. Finally,
CPs p o ide a su ace o immobilise biological
elemen s like enzymes o an ibodies, in o de o
p epa e biosenso s. Recen ly, a dielec ic moni o ing
has been p oposed o s udy he adso p ion p ocess
o biomolecules on o CPs, such as DNA on o
polypy oles (Saoudi
e al.
2000).
3.4. Acous ic senso s
Elec oacous ic de ices based on he
piezoelec ical p ope ies o qua z ma e ial ha e
been success ully used o implemen senso s since
1964 (King 1964). These senso s a e based on he
p opaga ion o acous ic wa es p oduced by
piezoelec ical ma e ials (e.g. qua z o LiNbO3 o
Si-SiO2-ZnO) in a mul ilaye s uc u e (Lucklum and
Haup mann 2000). Su ace acous ic wa e (SAW)
and bulk acous ic wa e (BAW) a e he mos common
o hose acous ic senso s. The ope a ion mode o
hese senso s is based on he physical changes
p oduced on hei memb anes made o chemically
in e ac i e ma e ials (CIM). The physical changes
ha can be de ec ed a e ela ed o he piezoelec ic
ma e ials used o he acous ic ansduc ion, o he
mass densi y and ( isco-)elas ic p ope ies o CIM.
The ope a ion p inciples o SAW and BAW
senso s a e e y simila (D’Amico
e al
. 1997).
Howe e , SAW- ype senso exploi s he p opaga ion
Figu e 3
Mean o he aw esponse o conduc ing polyme senso s o
a pu e i gin oli e oil and samples spiked wi h di e en
pe cen ages o a i gin oli e oil ancid s anda d.
Vol. 53. Fasc. 1 (2002) 101
o adio equency (102-103MHz) oscilla ions (e.g.
Rayleigh wa es) along a laye consis ing a leas o a
subs a e co e ed by he CIM. The acous ic wa es
a e ansmi ed by he elec ical ield gene a ed by
elec odes deposi ed a he su ace o he solid. SAW
de ices p oduce bi-dimensional wa es a elling
h ough he su ace o he c ys al. Depending on he
geome y o he acous ic s uc u e and on he
equency scaling, SAW senso s can p oduce
di e en ypes o wa es: Rayleigh wa es, su ace
ans e se wa es, Bleus ein-Gulyae wa es, and
Lamb and Lo e wa es. The choice o one o hese
wa es o he piezoelec ic ma e ial (ZnO, AIN,
Bi12GeO20,
α-SiO2, LiTaO3, LiNbO3) is in luenced by
he cha ac e is ics o he memb ane and by he
ope a ion in gaseous o liquid en i onmen s. F om
he sensing poin o iew, he p esence o ola ile
compounds p oduces he adso p ion o molecules
on he senso and hence changes on he p ope ies
o he CIM ha a ec bo h he phase eloci y and he
p opaga ion loss o he acous ic wa e. The esul ing
is a equency shi whose magni ude is ela ed o he
amoun o ma e ial adso bed on o he laye senso .
A BAW senso is a piezoelec ic esona o , o en
made o qua z, wi h one o bo h su aces co e ed
wi h a sensi i e coa ing (memb ane) o a ew µm
(1µm-10nm) hickness (D’Amico
e al
. 1997), whose
chemical and physical ( he mal) cha ac e is ics a e
simila o he gas-ch oma og aphy s a iona y
phases. The s uc u e is connec ed o an ampli ie o
o m an oscilla o whose h ee-dimensional wa es
a el h ough he c ys al (Schalle
e al.
1998) a a
equency o 10-30MHz. In he p esence o ola ile
compounds he e is a p ocess o adso p ion o
abso p ion o he chemical species ha al e he
physical p ope ies o he memb ane and hence
a ec s he esonan equency o he s uc u e. The
changes a e de ec ed by measu ing he shi in he
esonance equency; he magni ude o he shi is
ela ed o he mass o he compound al hough he
esponse can be no-linea due o iscoelas ic
beha iou o he memb ane. Because o he
esponse is p opo ional o he mass adso bed, his
de ice (a qua z disk sandwiched be ween wo
elec odes) is also called qua z c ys al mic obalance
(QMB o QCM) (Fe a i
e al.
2000) and i was
al eady p oposed as a p omising senso o gas
de ec ion in 1964 (King 1964).
O he kinds o acous ic senso s a e he lexu al
pla e wa e (FPW) senso s and can ile e -based
senso s. FPW senso s a e simila o SAW senso s
al hough hey a e coa ed wi h a memb ane whose
hickness is much smalle han he acous ic
wa eleng h (Cai
e al.
2000) and, in consequence,
he mechanical lexu e occu s in he en i e
memb ane. Can ile e -based senso s, which de i e
om he a omic o ce mic oscopy, consis o
ee-s anding can ile e beams coa ed wi h a senso
laye ha in e ac s wi h he analy e (Ba is on
e al.
2001). The can ile e -based senso oscilla es a a
ixed esonance equency, and any change o he
mechanical p ope ies is de ec ed by measu ing he
shi o he esonance equency (dynamic mode) o
he s a ic de lec ion (s a ic mode). The mechanism o
sensing o mic ocan ile e is simila o SAW and
BAW senso s. The molecules a e abso bed in he
ac i e laye , inc easing he mass o he can ile e
(Po e
e al.
2001). This change o mass loading
esul s in a change o he ib a ion equency o
ampli ude. Thus, he esponse o a can ile e is
cha ac e ized by i s geome ical dimensions and he
ib a ion equency among o he s (Ba is on
e al.
2001).
Acous ic senso s ha e a ious ad an ages o e
o he senso s, as high sensi i i y and sho esponse
ime, low powe consump ion and size, and
obus ness (Gu ie ez
e al.
2000). The sensi i i y o
he acous ic senso is ela ed o he ope a ing
equency (SAW: 100MHz-1GHz; BAW: 10-30 MHz),
and i inc eases as he squa e o he undamen al
equency does (Ballan ine and Wohl jen 1989).
Fu he mo e, he equency shi is di ec ly
p opo ional o he sensing laye hickness o a
speci ic compound (Li and Ma 2000).
Disad an ages o acous ic senso s a e he
empe a u e and humidi y dependence, he di icul y
o eplacing senso s, he poo ep oducibili y in he
deposi ion o he coa ing ma e ial (Mielle 1996) and
ce ain le el o noise (o e all in SAW) because o
hei high equency (Schalle
e al.
1998). In he
case o QCM senso s, hei main disad an age is
ha hey need o a hin c ys al o inc ease he
sensi i i y ha p e en s hei inclusion in mul isenso
a ays (Fe a i
e al.
2000).
The acous ic senso applica ions can be di ided
be ween physical and chemical applica ions
(Ballan ine and Wohl jen 1989). Some physical
p ope ies can be measu ed by acous ic senso s, as
empe a u e, p essu e, elec ic ield, displacemen
and low (Cai and G imes 2000). The common
cha ac e is ic o hese senso s is ha hey ha e
magne oelas ic hick- ilm as coa ing ma e ial. The
chemical applica ions equi e wo kinds o senso s,
liquid-phase senso s and apou -phase senso s,
acco ding o he medium in which hey ope a e
(Ballan ine and Wohl jen 1989). The apou -phase
acous ic senso s ha e been used o de ec gases
and ola iles as NO2 (Mülle
e al.
2000), NH3
(Ballan ine and Wohl jen 1989), xylene, oluene,
e achlo oe hylene (Dicke
e al.
1999), H2, oxic
gases (McGill
e al.
2000), o ganophospho ous
compounds (Zimme mann
e al.
2001) and SO2 (Qin
e al.
2000). Acous ic senso s ha e also been used
o analyse he headspace o oods u s as oli e oil
samples (Di Na ale 2001). SAW senso s wi h a
sensi i e ma e ial con aining a high deg ee o
102 G asas y Acei es
hyd oxyl g oups is used o de ec he humidi y
because i p oduces changes in he pola i y o he
coa ing ma e ial (Dicke
e al.
1999). Humidi y
senso s ha e been manu ac u ed using polyme
sensi i e o we ai , such as polyimid (Be nou
e al.
2000). Al hough SAW senso s a e no sui able o be
used in liquid-phase because o hei p opaga ion
loss inside he liquid in e ace (Tong and Zhang
1999), some au ho s ha e used hem o measu e he
low o liquid samples (Nomu a
e al
. 2001), o o
de e mine he liquid densi y and iscosi y wi h a
Lo e-mode acous ic senso (He mann
e al
. 1999).
Finally, SAW senso s a e also used in scien i ic a eas
as di e en as biochemis y, elec ochemis y
(Ballan ine and Wohl jen 1989) and immunosensing
(F eudenbe g
e al.
2001).
3.5. Biosenso s
A biosenso consis s o an immobilised biologic
molecule (enzymes, cellules o an ibodies) nex o a
ansduce , which ans o ms chemical signal in o an
elec ic signal o in o o he kind o ou pu as op ical,
acous ic and hea signal when an analy e eaches o
i . (A oyo 1998). A nold and Meye ho (K ess-
Roge s 1997) de ined biosenso s as “a sel -
con ained analy ical de ice ha esponds selec i ely
and e e sibly o he concen a ion o ac i i y o
chemical species in biological samples”. Biosenso s
hen consis o a biological sensing elemen and a
ansduc ion elemen .
The biological elemen (G i i hs and Hall 1993)
can be made o ca aly ic (enzymes, mic oo ganisms
and issues) o non-ca aly ic componen s
(an ibodies, ecep o s and nucleic acids). The
cha ac e is ics o biological elemen s allow clus e ing
he biosenso s in h ee g ea g oups: me abolism
biosenso s (enzymes, combina ions o enzymes and
co ac o s, and cells like algae and bac e ia), a ini y
biosenso s (an ibodies, “immunosenso s”) and
ecombinan biosenso s (DNA p obes).
The ansduc ion elemen allows he
ans o ma ion o he chemical signal, ob ained om a
biological p ocess, in o ano he kind o signal. Acco ding
o he kind o he ansduced signal, biosenso s can also
be g ouped in elec ochemical (ampe ome ic,
po en iome ic, conduc ime ic), op ical, calo ime ic
and acous ic biosenso s (G i i hs and Hall 1993).
The biosenso sensing mechanisms depend on
he ansduc ion echnology. In he case o
elec ochemical biosenso s o example, he
ansduc ion sys em is based on an elec ochemical
na u e p ocess, and so he ansduced signal is
elec ic. Mos o he comme cial biosenso s a e
elec ochemical; ampe ome ic biosenso s being he
mos widesp ead because he elec ochemical
ins umen a ion is ela i ely simple and inexpensi e.
The cu en lows a cons an po en ial wi h espec
o a e e ence elec ode in he ampe ome ic
senso s, and he cu en gene a ed by he oxida ion
o educ ion o elec oac i e species a he su ace o
he wo king elec ode is measu ed (G i i hs and Hall
1993). In ampe ome ic biosenso s, edox enzymes
(oxido educ ases) ha e a special impo ance, since
he ans o ma ion o he subs a e akes place wi h a
ans e o elec on be ween edox enzymes and
elec odes. These edox enzymes a e oxidases,
pe oxidases and dehyd ogenases (Re iejo and
Pinga ón 2000). The e a e ou main ca ego ies o
ampe ome ic senso s (Palleschi and Cubadda
2001): Oxygen elec odes, hyd ogen pe oxide
elec odes, ca bon-base media ed elec odes and
NADH elec ochemical senso s.
The po en iome ic biosenso s measu e he
changes o po en ial a cons an cu en (usually
ze o). Th ee kinds o ansduce a e used in he
po en iome ic biosenso s: ion selec i e elec odes
(ISEs), gas sensing elec odes, and ield e ec
ansis o s (FETs) (G i i hs and Hall 1993).
In he case o conduc ime ic biosenso s changes
o conduc i i y be ween wo elec odes a e
measu ed. This kind o ansduc ion echnology is
no widely used due o low selec i i y.
Nowadays, lo s o s udies ha e been done o
ab ica e elec on- ans e ing in e aces be ween
edox enzymes and elec odes in o de o pe o m an
enhanced elec ochemical biosenso (Shinoha a
e al.
2000). These media o s a e able o eplace oxygen as
an elec on accep o and o ope a e a a po en ial ha
g ea ly educes he e ec s o o he elec ochemically
ac i e species (Whi e and Tu ne 1997). Some o
hese media o s a e e ocene and i s de i a i es,
cobal ph alocyanine, e a hia ul alene, elec opolyme s
(Alba eda-Si en 2000), and p o eins (Shinoha a
e
al.
2000).
Op ical biosenso s a e based on op ical changes
while he biochemical eac ion akes place. I a ligh
beam is p opaga ed h ough a wa eguide, hen he
in e nally e lec ed ligh gene a es an
elec omagne ic “e anescen wa e” ha can be used
o exci e luo escen molecules a he su ace o he
wa eguide. The “e anescen wa e” can couple wi h
he elec on plasma o a me al when he wa eguide
is coa ed wi h a me al hin laye . I causes he
elec ons o oscilla e and hus gene a ing a su ace
plasmon wa e (G i i hs and Hall 1993) ha is
cha ac e ised by high sensi i i y and speci ici y. The
su ace plasmon is an elec omagne ic wa e
associa ed wi h he longi udinal oscilla ion o he ee
elec on gas on he in e ace o he me al and he
dielec ic (Caide and Sui 2000). Recen ly, su ace
plasmon esonance in e e ome y (SPRI) has been
es ablished as a no el me hod (Niki in
e al.
2000)
ha gains in sensi i i y and esolu ion o he
de ec ion o a mul iplici y o analy es.
Finally, he e a e calo ime ic and acous ic
biosenso s. The calo ime ic biosenso s a e based
Vol. 53. Fasc. 1 (2002) 103
on he ac ha almos all biochemical eac ions a e
exo he mic. Thei comme cial success is e y limi ed
o he da e, he mopiles and enzyme he mis o s
being he mos used. The acous ic biosenso s
measu e changes in he acous ic p ope ies o he
senso s. The e a e wo ypes: su ace acous ic wa e
o bulk wa e (G i i hs and Hall 1993).
The p incipal ad an age o enzyme biosenso s is
ela ed wi h hei high speci ici y (s e eoselec i i y
and egiospeci ici y) and ca aly ic ac i i y. Howe e ,
hey ha e he disad an ages o low s abili y and
possibili y o being eusable (A oyo 1998). These
incon eniences can be pa ially a oided by he
immobilisa ion o he enzymes, which consis s o
ixing enzymes on o a su ace by physical o
chemical me hods in o de o ob ain an insoluble
by-p oduc wi h ca aly ic ac i i y. The immobilisa ion
p o ides ad an ageous e ec s o e enzymes such
as an ac i i y inc emen , high eusabili y and
easibili y o handling and con ol. Bu he enzyme
biosenso s also ha e inhe en disad an ages such as
he e ogenei y o he sys em, high cos , and changes in
he con o ma ion o he enzymes (A oyo 1998). The
con en ional me hods o he enzyme immobilisa ion
include co alen binding, encapsula ion in a polyme
o c oss-linking o a sui able suppo ing ma ix (Lillis
e al.
2000). Table III shows di e en me hods o
immobilise he enzymes.
The applica ions o biosenso s sp ead o e a lo
o ields like clinical, ag icul u e, mili a y, indus ial
and ood a eas al hough hei comme cial se up is
being ca ied ou slowly. In medicine, biosenso s a e
used in he assessmen o d ug o biomolecule
concen a ions in blood, se um o o he co po al luid
in- i o
o
in- i o
measu emen s, and da a a e used
in diagnos ic o on-line moni o ing (Liu and Li 2000).
The ag icul u al applica ions o biosenso s a e
ocused on he de ec ion o i al, bac e ia and ungal
in ec ions, he use o e ilize s and pes icides
(G i i hs and Hall 1993), and ola iles eleased by
damaged plan s in glasshouses (Schü z
e al.
2000).
In mili a y de ence, biosenso s could be used o
de ec ne e gases and o he p oduc s used in
chemical wa a e (G i i hs and Hall 1993).
Biosenso s a e applied o he con ol p ocesses in
pha maceu ical, chemical and pe oleum indus ies,
mining and cons uc ion (G i i hs and Hall 1993).
The en i onmen al con ol o oxic compounds and
pollu an gases (Köhle
e al.
2000) and was ewa e
oxici y (Liao
e al.
2001) can also be done by
biosenso s. Finally, biosenso s ha e been also used
in Food Technology o moni o he quali y con ol o
aw ma e ials and on-line o o -line manu ac u ing
p ocesses. Thus, glucose and suc ose biosenso s
ha e been used o con ol he p ocess o alcohol
e men a ion (Fe ei a
e al.
2001), and o de ec
en e o oxins (Tempelman
e al.
1996) and bac e ia
(Su
e al.
2001) in many oods u s. Recen ly, a
biosenso based on ca alase enzyme has been
de eloped o hyd ope oxide de e mina ion in ex a
i gin oli e oil samples (Campanella
e al.
2001).
4. THE ELECTRONIC NOSE
Recen ly, senso s wi h sensi i i y o a wide ange
o ola iles compounds ha e been in eg a ed in a
sys em called elec onic nose. The name o
‘elec onic nose’ is due o he simila i ies be ween
his ins umen and he physiological sys em.
Ga dne and Ba le (1994) de ined elec onic nose
as an ins umen , which comp ises an a ay o
elec onic chemical senso s wi h pa ial speci ici y
and an app op ia e pa e n- ecogni ion sys em,
capable o ecognising simple o complex odou s.
Using his de ini ion i can be hough ha elec onic
noses a e composed o pa s such as a ays o
non-speci ic solid s a e gas senso , a a ie y o
ansduce s, da a collec o s and da a analysis ools,
all o ien ed o he classi ica ion and quan i ica ion o
chemical clus e s o ola ile compounds, including, in
pa icula , odou s (D’Amico
e al.
2000). Ne e heless,
he e m “elec onic nose” is being inco ec ly used o
Table III
Me hods o immobilising he enzymes o biosenso s
Me hod o enzyme immobilisa ion B ie desc ip ion Re e ences
En apmen
Mic oencapsula ion
Physical adso p ion
Co alen binding
C oss-linking
Immobilisa ion in ma ices like gels, polyme s o pas es.
The enzymes a e enclosed wi hin semipe meable
polyme memb anes.
an de Waals a ac ion o ces be ween an enzyme
and a solid suppo su ace.
Enzymes and ca ie s a e bound by co alen bond.
Fo ma ion o h ee-dimensional linkings be ween
he biological ma e ial and bi- o
mul i unc ional eagen s.
O ega
e al.
1998
Ramos
e al.
2001
Lü h
e al.
2000
Senillou
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Dzyade ych
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