Electrochemical sensors based on conducting polymer—polypyrrole

Electrochimica Acta Reviewarticle Cite this article as: A. Ramanavicius, A. Ramanaviciene, A.

Malinauskas, Electrochemical sensors based on conducting

polymer – polypyrrole (Review) Electrochimica Acta 2006, 51, 6025–

6037.

Journal version is available at:

http://www.sciencedirect.com/science/article/pii/S0013468606003252

RESEARCH ARTICLE

(Advanced authors version)

6025 A. Ramanavicius, A. Ramanaviciene, A. Malinauskas, Electrochemical sensors based on conducting

polymer – polypyrrole (Review) Electrochimica Acta 2006, 51, 6025–6037.

Electrochemical sensors based on

conducting polymer – polypyrrole (Review)

A. Ramanavicius1,2∗, A. Ramanaviciene

1,2, A. Malinauskas

3

1.Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, 03225 Vilnius, Lithuania

2.Laboratory of Immunoanalysis and Nanotechnology, Institute of Immunology of Vilnius University, Mol˙et˛u pl. 29,

08409 Vilnius, Lithuaniac

3. Department of Organic Chemistry, Institute of Chemistry, Goˇstauto 9, 01108 Vilnius, Lithuania

∗ Corresponding author. Tel.: +370 5 2330987; fax: +370 5 2330987. E-mail address: [email protected] (A. Ramanavičius).

DOI:

Abstract

Conducting polymers can be exploited as an excellent tool for the preparation of nanocomposites with

nano-scaled biomolecules. Polypyrrole(Ppy) is one of the most extensively used conducting polymers

in design of bioanalytical sensors. In this review article significant attention ispaid to immobilization

of biologically active molecules within Ppy during electrochemical deposition of this polymer. Such

unique properties ofthis polymer as prevention of some undesirable electrochemical interactions and

facilitation of electron transfer from some redox enzymes are discussed. Recent advances in

application of polypyrrole in immunosensors and DNA sensors are presented. Some new

electrochemical targetDNA and target protein detection methods based on changes of semiconducting

properties of electrochemically generated Ppy doped by affinityagents are introduced. Recent progress

and problems in development of molecularly imprinted polypyrrole are considered.

© 2006 Elsevier Ltd. All rights reserved.

Keywords:Conducting polymers; Polypyrrole; Biosensor; DNA sensor; Immunosensor; Molecularly

imprinted polymers; Bioelectrochemistry; Nanotechnology;Nanobiotechnology

1. Introduction

Nanotechnology is rapidly evolving to open new

materialsuseful in solving challenging bioanalytical problems,

includingspecificity, stability and sensitivity. Here conducting

polymers can be exploited as an excellent tool for the

preparationof nanocomposites with entrapped nano-scaled

biomolecules,mainly proteins and single stranded DNA

oligomers. Some conductingpolymers doped and/or covalently

or not covalentlymodified by bionanomaterials mentioned

exhibit unique catalytic[1] or affinity [2] properties that can be

easily appliedin the design of bioanalytical sensors

(biosensors).

Polypyrrole is one of the most extensively used conducting

polymers in design of bioanalytical sensors [3] as well as for

A. Ramanavicius, A. Ramanaviciene, A. Malinauskas, Electrochemical sensors based on conducting


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other purposes. Since 1990 up to June 2005 period solelyin the

Journal ElectrochimicaActa over 300 papers appearedon

various properties and applications of polypyrrole.

Versatilityof this polymer is determined by a number of

properties: redox activity [4], ability to form nanowires with

room temperature conductivity ranging from 10−4 to 10−2 S

cm−1 [5], ion-exchange and ion discrimination capacities [6,7],

electrochromic effect depending onelectrochemical

polymerization conditions and charge/discharge processes [8],

strong absorptiveproperties towards gases [9],proteins [10],

DNA [11], catalytic activity [12,13,14], corrosion protection

properties [15], etc. Most of these properties are depending on

the synthesis procedure as well as on the dopant nature

[2].Polypyrrole might be electrochemically generated and

deposited on the conducting surfaces. This technique is

successfully exploited for development of various types of

electrochemical sensors and biosensors. Here several major

directions are straightforward: (i) catalytic sensors based on

immobilized enzymes [1,16,17]; (ii) immunosensors based on

immobilized affinity exhibiting proteins [18]; (iii) DNA

sensors based on covalently immobilized and/or entrapped

ssDNA[19,20,21]; (iv) affinity sensors based on molecularly

imprinted polymers [22].Versatility of this polymer is

determined by the following: its biocompatibility; capability to

transduce energy arising from interaction of analyte and

analyte-recognizing-site into electrical signals that are easily

monitored; capability to protect electrodes from interfering

materials; easy ways for electrochemical deposition on the

surface of any type of electrodes. Nowadays this polymer

becomes one of the major tools for nanobiotechnological

applications [23].

The aim of this study is to review major advances and

applications of this polymer in design of catalytical

biosensors,immunosensors, DNA sensors and molecularly

imprinted polymerbased sensors.

2. Experimental

2.1.Electrochemical polymerization of polypyrrole versus

chemical polymerization

Polypyrrole was firstly synthesized in 1912 [24]. Polypyrrole

synthesized by conventional chemical methods is insoluble in

common solvents because of strong inter-chain interactions

[25]. Two major ways are applied for polypyrrolesynthesis

which are based on induction of polymerization by different

factors: (i) chemical initiation by oxidative agents [24,26]; (ii)

photo induced synthesis [27]; (iii) electrochemical activation

by anodic current [28]. All polymerization initiation methods

mentioned have particular application, e.g. chemical initiation

by oxidative agents might be successfully applied if a great

amount of polypyrrole is needed for application in the design

of chromatography columns [29] or for some other

purposes.By using chemical [26] or even biochemical

[30]methods itis easy to prepare Ppy particles of different

and/or controlled size ranging from several nanometers up to

several micrometers and/or containing various inclusions.

Moreover, by chemical methods it is possible to uniformly

perform overoxidation of this polymer, what is on special

interest of affinity chromatography since molecularly imprinted

Ppy might be produced, which might exhibit selectivity to

molecules ranging from the small organics [31,32,33] to high

molecular weight biomolecules [22]. Photo induced Ppy

synthesis is attractive in photolithographicapplication of this

polymer, since it allows alterations in synthesized Ppy

morphology by change of excitation light wave length [34] and

theoretically it might be applied for the design of electronic

chips. However, because of slow light induced polymerization

rate this polymerization type is still not very often applied if

compared with chemical or electrochemical polymerization.

By using chemically induced polymerization the Ppy ismainly

produced in the bulk solution and just some amount

ofsynthesized polypyrrole is covering the surface of

introducedmaterials. It means that chemically induced

polymerization isnot very efficient with respect to deposition of

Ppy over somesurfaces. Moreover, Ppy is almost insoluble in

usual solvents,except some cases where it is doped with proper

agents increasingsolubility of this polymer [35] and it means

that deposition(e.g. by solvent evaporation) of this polymer

from the solutioncontaining dissolved polymer is possible at

the stage where thepolymer is still in the form of colloid

particles, before its precipitation [30].However, the major

obstacle for use of this deposition method for designing of Ppy

based sensors is a poor adherence of this deposit to the surface,

contrary to the film obtained by electrochemical

polymerization. But all these disadvantages might be avoided if

electrochemical polymerization is applied. It allows deposition

of Ppy over electrodes deposited in the electrochemical cell.

That is the reason why electrochemical polymerization has

found an application as a general deposition method if thin Ppy

layers are requested. By using this method thickness and

morphology of deposited layer might be controlled by

application of well-defined potential and known current

passing through the electrochemical cell [36]. Electrochemical

deposition of Ppy might be performed from various solvents

(e.g. acetonitrile, water, etc.). From the point of view of

nanostructuring of this polymer it is really very important that

Ppy synthesis might be performed from water solution at

neutral pH, since it opens the ways for entrapment and/or

doping of polypyrrole by various biomaterials like small

organic molecules, proteins, DNA and even living cells.

However, if buffers with low bufferingcapacitance are used as

polymerization solution, a potentialproblem is the local

production of a great amount of protonsin the course of the

polymerization which may affect the properties of the

biomolecules to be entrapped inside the Ppy film. In particular

cases overoxidizedPpy might be synthesized and entrapped

molecules and/or dopants might be extracted from the Ppy

structure. In such cases so called molecularly imprinted

polymers might be designed. Moreover, electrochemical

polymerization is applied for deposition of polypyrrole layers

inside geometrically complicated electrochemical cells [37]

and there is almost no doubt that this polymerization method

might be extremely useful for deposition of Ppy layers inside

microfluidic devices. Furthermore, electrochemically

synthesized Ppy has some attractive features, such as good

conductivity and very high adherence of these films to the

mostly for biosensor design used substrates leading towards

sufficient stability of biosensors, even in a neutral pH region.

On the other hand, the electrochemical properties of

Ppystrongly depend on the redox state of this polymer. At

positive potentials an overoxidation of Ppy is occurring what is

leading towards lowering of Ppy conductivity and makes easier

leakage of anionic molecules if they were included into

polymeric backbone. Overoxidation of Ppy appears at lover

positive potentials in water and/or oxygen-containing

environment

https://www.researchgate.net/publication/46149321_Conducting_polymer-based_nanostructurized_materials_Electrochemical_aspects?el=1_x_8&enrichId=rgreq-0465731305baf26ead4b45ec21e94d66-XXX&enrichSource=Y292ZXJQYWdlOzIyMzIyOTAwMDtBUzo5OTQ4OTIxMzg0NTUzMUAxNDAwNzMxNDg1NzQx



6027

and in this case it is leading towards partial destruction of

polymeric backbone and generation of oxygen-containing

(carboxyl, carbonyl and hydroxyl) groups. OveroxidizedPpy

has been used in many electroanalytical applications that utilize

its permselectivity and is often used as discrimination

membrane which significantly increases selectivity of

electrochemical biosensors [38,39]. The capability of

electrochemical polypyrrole synthesis is significantly extended,

since some different electrochemical techniques might be

applied for deposition of Ppy over the electrodes: constant

potential electrodeposition, galvanostatic deposition, cyclic

voltammetry, and potential pulse techniques [40].

According to our experience based on application of

conducting polymers in biosensor design the potential pulse

technique is the most suitable for nanostructuring of Ppy by

entrapment of biologically active materials within backbone of

this polymer. Potential pulse techniques enable to increase

concentration of entrapped biologically active material within

nano-thin layers of polypyrrole[40] because various potentials

might be applied in step manner. Higher potential steps are

applied for initiation of Ppy polymerization and lower or

negative potential steps are used for attraction of higher

amount of biomaterial, which is entrapped into polymeric

backbone during polymerization step that is initiated by

potential in the range of +0.6 to +1.2V versus Ag/AgCl. The

number of potential steps, the profile of potential applied and

duration of each step might be set up individually depending on

the application of determined requirements. All factors

mentioned enable to prepare large variation of nanostructured

polymeric layers with different analytical characteristics even

if the same bulk solution is used for polymerization. In general,

electrochemical polymerization is more versatile if compared

with chemical one in terms of possible variations and control of

polymerizationconditions. Moreover, combination of

electrochemical techniqueswith some chemical surface

modification techniquesopens new opportunities for

development of new nanobiostructuralassemblies based on this

polymer. More in detail, it wasdemonstrated that the surface of

electrochemically depositedPpy after some additional

electrochemical/chemical functionalizationmight be covalently

modified by enzymes [41]. Thosestructures were applied in

bio-catalytic biosensor design, andit was demonstrated that Ppy

layers modified by the sameenzyme exhibit significantly

different selectivity towards varioussubstrates if different Ppy

modification approaches areapplied.

However, there are some particular cases where chemical

methods have some advantages if compared to electrochemical

methods. Chemical methods are still mostly used if large extent

of Ppy or appropriate Ppy structures e.g. nanoparticles orPpy

coated nanoparticles of other materials are needed.

Nanocomposites of chemically synthesized polypyrrole are

mainly applied for affinity chromatography purposes [33], but

electrochemical methods are mainly used for construction of

chemical sensors, biosensors and actuators. So, in general, both

Ppy synthesis methods are finding particular application areas

for various technological purposes.

2.2. Polypyrrole as versatile immobilization matrix in

design of biosensors

Most important considerations during the creation of any type

of electrochemical biosensors are: (i) the immobilization of the

bio-catalyst; (ii) application of appropriate electrochemical

technique (e.g. potentiometric, amperometric and impedimetric

techniques are mainly applied for analytical signal registration

in design of electrochemical sensors and biosensors); (iii)

establishment of efficient electron transfer if amperometric

detection is applied. Consequently, immobilization of

biologically active material is of pivotal importance in the

creation of biosensors [42], since it allows application of the

same biologically active material for a number of analysis

cycles. The requirements for successful biomaterial

immobilization are: (i) biological recognition properties and/or

catalytic properties of biomaterial should remain after

immobilization; (ii) the biomaterial should be well fixed

on/within the substrate, otherwise the biosensor will lose its

activity; (iii) improve or at least minimally decrease selectivity

of constructed biosensor or bioanalytical system; (iv) improve

electron transfer if amperometric measurements are applied as

signal transduction system. To solve the majority and

sometimes all these tasks, conducting polymers can be

considered as a very effective substrate for biomaterial

immobilization. Among other conducting polymers,

polyaniline is often used as immobilizing substrate for

biomolecules[39] and sometimes as efficient electrocatalysts.

However, thenecessity to detect bio-analytes at neutral pH

range leads to electro-inactivity of the deposited films,

discouraging the use of polyaniline and polythiophene as

biosensing materials. As opposed to polyaniline, polypyrrole

might be easily deposited from neutral pH aqueous solutions

containing pyrrole monomer. It makes this polymer very

attractive and at present it is one of the most extensively

studied materials useful for immobilization of different

biomolecules and even living cells. On the other hand, Ppy is

often used in catalytic and affinity biosensors because of good

biocompatibility and the easy ways for immobilization of

biomolecules [43]. Biomaterials might be immobilized by

various

methods: (i) adsorption on electrochemically or chemically

formed Ppy surface [44]; (ii) entrapment during

electrochemical deposition of polypyrrole[17,28,36,37,40]; (iii)

self entrapped if biomaterial is able to initiate polypyrrole

synthesis [30]. Asit was mentioned in previous paragraph, by

chemical initiationit is easy to produce large extent of

polypyrrole modified withbiomaterials, however, this method

is not well suitable for theformation of well defined layered

structures such as usually areneeded for sensor design. On the

contrary, the electrochemicalPpy polymerization allows the

formation of uniform films, thethickness and morphology of

such films might be controlled by regulation of passing current

and/or potential applied. Here the application of pulsed

potential techniques allows preconcentration of biologically

active molecules (e.g. DNA, enzymes, etc.) by applying of

proper potential between the pulses initiating polymerization of

polypyrrole[45]. In majority of cases the pulse technique

allows at least to avoid a strong diminution of biologically

active compound concentration near the electrode surface

which takes place at the steady-state diffusion regime and it

strongly enhances amount of inside the film incorporated

biologically active compound if compared to the steady-state

polymerization. Moreover, it was shown that Ppy is able very

effectively discriminate cations and anions, since permeability

and permselectivity of Ppy depends on the counter ion

incorporated during polymerization as well as on the ions

present in the sample [7]. In particular cases anions (e.g.

phosphate) doped electrochemically deposited Ppy if properly

doped by some anions might be not permeable for anions what



6028

is very useful for electrochemical biosensors since the majority

of electrochemically interfering materials present in biological

samples are anions [38,39]. It was also demonstrated that Ppy

protects electrodes from fouling by proteins and another

biological substances present in the real samples as blood

serum and urine [46]. It was shown that various

nanocomposites that might be designed using combinatorial

methods by polymerization of a number of electrochemically

polymerizable compounds are useful

for biosensor design [47].

The stability of Ppy based biosensors is sufficient and mainly

determined by degradation of Ppy in the water surrounding if

biosensor is applied for continuous measurements. In

conclusion, from the point of view of electrochemical

biosensingPpy has a number of very attractive characteristics:

(i) it might be synthesized electrochemically and modified by

enzymes in several different ways that gives different

analytical characteristics for constructed biosensors; (ii) it

protects electrodes from fouling and interfering materials such

as electroactive anions; (iii) it is biocompatible and, hence,

causes minimal and reversible disturbance to the working

environment; (iv) in some particular cases it might be exploited

as redox mediator able to transfer electrons from the redox

enzymes towards electrodes.

2.3. Catalytic biosensors based on polypyrrole

Catalytic biosensors are described as compact

analyticaldevices, incorporating a bio-catalytic element or

integratedwithin a transducer system [48]. The detection of

analyte inthis kind of biosensors is based on specific catalytic

conversionof the analyte of interest by a bio-catalyst

immobilized onthe suitable signal transducer. The specific

interaction of analytewith bio-recognition element results in a

change of one ormore physicochemical properties (e.g. electron

transfer, capacity,optical properties) which can be detected and

measured viasignal transduction and registered by registration

devices.Electrochemicalcatalytic biosensors are on special

interest since theycan be applied for detection of analytes in

non-transparent samplesand the majority of electrochemical

catalytic sensors arerelatively cheap and easy in application.

Fig. 1. Generalized scheme of electrochemical biosensors

based on glucoseoxidase. Mox, oxidized form of redox

mediator; Mred, reduced form of redoxmediator.

Thus, as it was mentioned previously, the redox enzymes

aremainly used in the design of catalytic biosensors. This class

ofenzymes can be divided into several major subtypes that

differwith respect to cofactors involved into catalytic reaction.

Cofactorsmainly effect the signal transduction process and

should betaken into account during catalytic biosensor

construction and allthese biosensors have some specific

properties mainly related tothe used enzymes. If enzymes are

applied in the design of amperometric biosensors efficient

electron transfer route is crucial for registration of analytical

signal and the most successful sensorsare so called “reagent

less” biosensors that are operating withoutaddition of any other

soluble materials essential for analyticalsignal registration.

Successful application of polypyrrole modified by enzymesin

the design of catalytic biosensors started by entrapmentof

glucose oxidase from Aspergilusnigerwithin

polypyrrole[49,50], later polyaniline was also successfully

modified withthis enzyme [51,52] and employed for glucose

sensing. Now,FAD-dependent oxidases [53,54,55,56,57],

NAD+-dependent dehydrogenases [58,59,60,61,62], PQQ-

dependent dehydrogenases [16,63,64,65,66,67], peroxidases

[68,69,70,71] and some multicofactor enzymes [72,73,74]are

mostly used in the design of catalytic biosensors. The

embedment of enzymes within a conducting polymer film

prevent the enzyme from being leached out, while at the same

time maintaining accessibility of the catalytic sites due to the

permeability of the film to analytes[75]. Pulse technique for the

electrochemical deposition of polymer films on electrode

surfaces enabled the increase in the concentration of entrapped

enzyme within thin layers of Ppy[40]. Further, the enzyme

activity is usually detected by characterization of final reaction

products or redox mediators using amperometric or

potentiometric methods.

In the case of application of FAD-dependent oxidases

oxidation of enzymatically produced H2O2 is only possible at

high electrode potentials (Fig. 1). Some suitable redox

mediatorsable to facilitate the electron transfer between the

active site of the enzyme and electrode can be applied in the

design of electrochemical catalytic biosensors. So-far described

oxidasesentrapped within conducting polymer-based

biosensors are requiring soluble redox mediators or conducting

polymer backbone should be enhanced by redox species [76].

The most successful biosensors based on oxidases

entrapped within polypyrrole were reported when redox

polymers were constructed on the basis of pyrrole which was

copolymerized with pyrrole substituted by redox mediators.

Thus, for fixing of redox mediators several main strategies

might be applied, e.g.: (i) osmium bipyridine complex based

redox species were attached to pyrrole and after

copolymerization of this compound with pyrrole several

different redox polymers able to transfer electrons from redox

center of glucose oxidase were designed [64,77]; (ii) PQQ-

dependent glucosedehydrogenase was wired by ferrocene

derivatives [78]; (iii) modification of polymeric layers with

some soluble mediators that are almost freely diffusing within

the film [67,63,65,74].



6029


based on NAD+- dependent glucose dehydrogenase.Mox,

oxidized form of redox mediator;Mred,reduced form of redox

mediator

In the case of application of oxidases the sensor response was

dependent on the availability of molecular oxygen; it is a

significant drawback because this concentration is not constant

and might significantly differ from sample to sample. This

drawback might be avoided by application of dehydrogenases

since these enzymes do not require any oxygen as electron

acceptor. Dehydrogenases that are suitable for application in

amperometric biosensors might be divided into two major

subclasses: NAD+-dependent dehydrogenases, and PQQ-

dependent dehydrogenases. In the case of theNAD+-dependent

dehydrogenases (Fig. 2) one may successfully circumvent the

problems imposed by molecular oxygen [79].


based on PQQdependent glucose dehydrogenase.Mox,

oxidized form of redox mediator;Mred, reduced form of redox

mediator.

However, in the case of NAD+-dependent

dehydrogenaseapplication the coenzyme should be added to the

analyte solutionwhich is only possible in specifically designed

flow-injectionsystems [80], or should be entrapped within

conducting polymerbackbone [81] or graphite paste electrode

matrix [82]. Therefore,intensive attention was focused on

finding new enzymes withimproved characteristics, such as

exhibit the PQQ-dependentdehydrogenases (Fig. 3) [83]. The

use of PQQ-dependentenzymes is more promising, since these

enzymes are oxygen

independent, and their PQQ-cofactor in some cases is tightly

bound within the enzyme’s active site [84]. Significant

advantagewas achieved when polypyrrole based on osmium-

complexmodified redox polymer with entrapped PQQ-

dependent glucosedehydrogenase was applied for glucose

sensing independenton oxygen concentration fluctuations [64].

Some multicofactor enzymes, like PQQ and heme-c

based alcohol dehydrogenase were found to be able to transfer

electrons directly to some conducting surfaces including

polypyrrole (Fig. 4). Such enzymes might be easily applied in

the design of amperometric biosensors based on polypyrrole

because application of any redox mediators is not essential in

this case. It was shown that PQQ-dependent alcohol

dehydrogenase (QH-ADH) covalently attached to the backbone

of polypyrrole retains its catalytic activity [41]. Moreover, it

was demonstrated that QHADH entrapped within polypyrrole

exhibit direct electron transfer to this polymer [72]. It might be

predicted that Ppy and hemec- containing dehydrogenases

based polymeric configurations might be promising in the

design of biofuel cells [85] and other bioelectronic devices

[86]. However, there is just a very limited number of such

enzymes able to directly transfer electrons toward backbone of

conducting polymers. On the other hand, the highest currency

densities in biosensors based on PQQ and

heme-c based alcohol dehyrogenase were achieved when they

were deposited over electropolymerized 4-ferrocenylphenol, N-

(4-hydroxybenzylidene)-4-ferrocenylaniline, and 2-ferrocenyl-

4-nitrophenol [78].

Biosensors with different selectivity and activity

characteristicsmight be designed, since several different

conceptions might be used for immobilization of the same

enzymes: (i) adsorption of enzymes over electrochemically

deposited Ppylayer [44,87,88]; (ii) covalent attachment of

enzymes after introduction of amino groups into Ppy backbone

andformation of amide bounding due to interaction


based on direct electron transfer able PQQ-dependent alcohol

dehydrogenase. H, heme-c moiety.

with carbodiimide activated carboxylic groups of enzyme

[46,41,89]; here two major different Ppy functionalization

ways might be applied one based on application of amino

group modified pyrrole monomers forcopolymerization with

unmodified pyrrole monomers, next onintroduction of

functional groups following after preparationof Ppy film; (iii)

entrapment within backbone of polypyrrole[17,40,59,64,89].

Entrapment of enzymes within polypyrroleseems the most

promising for construction of catalytic biosensors,since this

method allows to entrap significant amount ofredox enzymes

that are able to convert high amount of analyteinto the products



6030

and this process causes high changes ofelectrochemical signals.

In some cases after enzyme entrapmentduring electrochemical

deposition of Ppy over electrodes it wasdetected that enzyme

reactivation phase is very actual before sensormight be applied

for exact analyte determination. In severalcases this phase was

15–30 h long depending on the thickness ofPpy layer formed

[17,74]. The existence of this effect is determinedby swelling

of Ppy because there are some evidencesthat immediately after

the electrochemical deposition enzyme istrapped within very

dense polymeric structure which increasessteric hindrances for

the substrate and deforms native structureof the enzyme. After

some swelling period water permeablecavities become larger

and substrate has more possibilities fordiffusion, as well as

enzyme has more space to become nativeconformation which

possesses highest activity if compared withother—not natural

conformations. It was also demonstrated thateven after

swelling period Ppy is able to accept electrons fromredox

centers of some redox enzymes and transfer those electrons to

the metal electrodes [17]. It seems that polypyrrole is an

inherently biocompatible material. The sufficient water content

ensures that the surface energy of this material is such that it

causes minimum disturbance to the biologically active

compound. In recent years, many catalytic biosensor

configurations and transducers have been designed allowing us

to detect the analyte in very lowconcentrations and with high

precision.However, for practical applications, a few main

problems remain to be solved: (i) for one-way biosensors the

production must be so reproducible that calibration-free

measurements can be performed; (ii) long-term stability in

water containing environment should be increased.

2.4. Immunosensors based on polypyrrole

Immunosensors are the subject of increasing interestmainly

because of their potential application as an

alternativeimmunoassay technique in areas such as clinical

diagnosticsand environmental control. Enzyme-linked

immunosorbentassay (ELISA) is one of the most frequently

used methodsfor immunoassay, because of its good sensitivity,

selectivity,and ease in use. Although spectrometric methods are

widelyused for the detection of enzymatic products resulting

fromthe Ag–Ab reactions in ELISA, the electrochemical

methodscan provide capabilities of monitoring, free from color

and turbid interferences and which are relatively inexpensive,

that the spectrophotometric methods cannot compete with them

[90].Electrochemical affinity sensors acting on the principles

similarto ELISA usually are cheaper and faster in use if

compared totraditional ELISA. Moreover by immunosensors

samples without any analyte enrichment can be analyzed. In

many cases the purification and/or sample pretreatment step is

not needed, which is normally essential for standard analytical

methods such as mass spectrometry, gas chromatography and

high performance liquid chromatography. This factor is

important formany applications, especially in clinical

diagnostics, where differentanalytes in whole blood, serum or

urine containing a lotof different substances, such as proteins,

amino acids, sugars,hormones, etc., are analyzed. Also

immunosensors have considerable advantages over standard

methods with respect to timeand sensitivity [33].

Major indispensable condition during the development

ofaffinity sensors is immobilization of analyte binding

reagent.This task is often solved by application of conducting

polymer,polypyrrole, since Ppy is the mostly used conducting

polymerin affinity sensors because of the best biocompatibility,

due to

efficient polymerization at neutral pH and very easy ways for

immobilization of various biologically active compounds. On

the other hand, it seems that this polymer is capable to transfer

energy as electrochemical transducer [20]. Affinity sensors

mainly rely on immobilized biomolecules or artificially

formedstructures able to interact non-covalently with analyte as

interactionpartner and to form multimolecular complexes.

Amongsuch non-covalently interacting materials are DNA,

antibodies,various proteins and molecularly imprinted

polymers. Thedevelopment of immunosensors would lead to

alternatives orat least improvement in the existing

immunoassay techniques.Most related methods to

immunoassay are immunosensors; theyare mainly applied in

areas where both high selectivity andhigh sensitivity are

required [91]. Immunosensor is a devicethat is able to detect

the interaction between an antibody (Ab)and an antigen (Ag)

[92]. One of the binding able materials inimmunosensors is

usually immobilized and at least one must befound in sample

as analyte. The conversion of the binding eventinto a

measurable signal, the regenerability and the reusabilityare

among major topics and challenges in

immunosensordevelopment research. The conducting polymers

and especiallypolypyrrole can be considered as effective

material for immobilization of biomaterials and for

transducing/amplification ofanalytical signal in design of

immunosensing devices [72,93].Electrochemical modification

of electrodes by conducting polymersdoped with biologically

active compound included withinpolymeric backbone is a

simple step that is often used for creationof different

immunosensors.

In the design of immunosensors antibodies [94,95],

ligands/receptors [96] and antigens [130] are applied as

biologicalmaterials able non-covalently to bind analyte.

Antibodies areconsidered to be well-suited and mostly used

recognition elementsfor construction of immunosensors. The

high specificityand affinity of an antibody for corresponding

antigen allowsa selective binding of the analyte which is

present in nanotopico-molar range in the presence of hundreds

of other substances,even if they exceed the analyte

concentration by 2–3 orders of magnitude [97]. At the time,

antibodies can be generatedagainst almost all analytes, even if

the analyte is nonimmunogenic.Moreover, recombinant

antibody technology has now been developed to the level that

allows the expression of single chain fragments in E. coli in

large quantities [98].In many designs of electrochemical

immunosensors secondaryantibodies able to recognize analyte

complex with immobilizedreceptor were applied. Since

antibodies are usuallynot electrochemically active within the

desired potential rangeredox-active compounds and/or

enzymes (mainly horseradishperoxidase) [99,100,101,102] that

are able to generate electrochemicalsignal can be applied as

labels for indication. Labeledimmunosensor format belongs to

indirect analytical signal detectionmethods. In indirect

electrochemical immunoassays thebinding reaction is

visualized indirectly via an auxiliary reactionby a labeling

compound. Amperometric transducers in

indirectelectrochemical immunoassay are used much more

frequentlythan others. For amperometric immunoassay the

labeling redoxcompound should have the following properties:

it should bereversibly electroactive; it should not cause

electrode fouling;chemical groups for coupling should be

available [103].Species such as nitrophenol, H2O2, and NH3



6031

that can be determinedelectrochemically are the substrates or

enzymatic reactionproducts of alkaline phosphatase,

horseradish peroxidase, andurease, generally labeled on

immunoreagents. Among these,because amonia is

electrochemically inactive at low potentialsand can only be

detected by an ammonia gas-sensing electrode,potentiometry is

the only choice for the urease-labeledimmunoassay [104].

Indirect electrochemical immunoassay canbe divided into two

major types: non-amplified and amplified.In non-amplified

redox-labeled electrochemical immunoassaysthe indication of

one antigen or antibody molecule will generateone signal

equivalent. Since the sensitivity of an amperometricsensor for

the redox compound is in the lower micromolar range,this kind

of assay makes sense only if the concentration of theanalyte to

be determined is also in that range [105]. For more

sensitiveimmunoassays amplification principles are necessary.

Oneway to amplify the amperometrical signal is

preconcentrationstep. During this step concentration of redox-

active compoundis increasing many times and only after some

time (1–5 min) themeasurement starts. However, there are

often some difficultiesto regenerate the sensor before each

measurement.

The major disadvantage of indirect immunosensors is the

necessity to apply additional immunochemicals labeled by

electrochemical labels; it makes this method more expensive,

time consuming since additional procedures mainly based on

incubation with labeled antibodies are essential for indirect

detection of analyte of interest. On the one hand, such

procedures increase sensitivity, but on the other hand they

often decrease selectivity since usually broad-range-selectivity

exhibiting secondary antibodies are applied. In this respect so

called ‘label-free’ immunosensors are more attractive, since

such sensors allow measurement without any additional

hazardous reagents evenin real-time [48]. Label-free

conversion of the binding event into a measurable signal in

particular at a low concentration of the analyte, is one of the

major challenges in biosensorics[48]. This topic is quite well

solved in surface plasmon resonance sensors what was

reviewed previously [18]. Electrochemical label-free analyte

detection methods are developing not so fast but they are very

useful if colored and/or not transparent samples are under

investigation or detection of analyte should be performed in the

body of patient. The majority of label-free electrochemical

immunoassays are based on changes in charge densities or

conductivities for transduction and do not need any auxiliary

electrochemical reaction. If conducting polymers are applied

for immobilization of affinity towards analyte exhibiting

reagents after formation of immobilized receptor and analyte

complex changes in capacitance/resistance are registered [106].

Here potentiometric [107], capacitive [102] and

amperometric[99,100,101] transducers have been used for

electrochemical

immunoassays that indicate the binding of analyte directly.

Amperometric techniques have been used to monitor bindingof

analyte in real-time without using a labeled compound.

Apolymer-modified antibody electrode has been used in

combinationwith pulsed amperometric detection. The current

obtainedat the immunochemical/polypyrrole based electrodes

occurs viathe following steps: diffusion of ions to the

electrode; chargetransfer at the porous polypyrrole membrane

interface; migrationthrough the polymer membrane;

adsorption–desorption ofthe analyte at the

immunochemical/polypyrrole interface withsolution. The slow

rate of adsorption–desorption process in thelast step is

considered to be the rate-determining step. Thisstep can be

controlled through the appropriate choice of electricalpotential

[108]. Pulsed amperometric detection (PAD)immunoassay

techniques are such techniques where sensor canbe used for

analyte detection in static or flow injection mode byapplying

pulsed potentials between the sensor surface (or

workingelectrode) and the reference electrode. The current

obtainedcan be directly related to the concentration of the

analyte insolution [109].

Besides amperometric transducers, capacitive transducers

have been used for the real-time and label-free measurement

ofthe Ag–Ab reaction. They are based on the principle that for

electrolytic capacitors the capacitance depends on the thickness

anddielectric behavior of a polymeric layer before and after

interactionwith analyte[110]. In some particular cases

conductivity measurements as one of transduction principle

might be appliedin the design of electrochemical

immunosensors. Conductivitymeasurements have been adapted

for immunoassay based onion concentration increased by the

action of enzymatic label.An enzyme immunoassay based on

conductivity measurementshas been reported, in which urease

was used as the secondaryantibody label. The enzyme retains

the activity under conditionsof lowionic strength, so a

lowbackground conductance could be

employed[111]. However, conductivity measurements are

difficultdue to the variable ionic background of clinical

samplesand the relatively small conductivity changes that are

observedin such high ionic strength solutions. The second

comparative‘blank’ electrode must be used, but variable drift at

two separateelectrodes poses a universal drawback [112].

The inherent speed, accuracy and precision of

electrochemicalmeasurements have stimulated efforts towards

the developmentof both competitive and non-competitive

electrochemicalimmunoassay formats [113]. Sensors

employing enzyme labelswith amperometric detection have

been frequently reported withdrugs[114], hormones

[115,116,117,118] and proteins [119] as targetanalytes, and the

detection of trace amounts in the sub-attomole(<10–18 mol/l)

range has been achieved [120]. In such detectionscheme, the

enzyme label is registered via formed/degraded

electrochemically active product [121], and the function

ofenzyme-labeled immunosensors is similar to that of catalytic

biosensors based on enzymes covalently attached to the

surfaceof polypyrrole. Glucose oxidase, horseradish

peroxidase,microperoxidase, _-galactosidase, alkaline

phosphatase andglucose-6-phosphatase dehydrogenase, all

have been employedin this mode with separation of free from

bound label [122,123,124].The NADH generated by glucose-6-

phophatase dehydrogenasereaction can be readily oxidized by

mediators such as 2,6-dichlorphenolindophenol[125] and 1,4-

benziquinone [126], theoxidation of those is followed

amperometrically

Several immunosensors were applied for continuous

measurements [106,110,127]. The major problem to use the

immunosensor for continuous measurements is stability of

Ag–Ab complexes. To overcome this problem for dissociation

of Ag–Ab complex buffers with extreme pH values, glycine

buffers or extreme salt concentrations are usually used. Flow

injection mode applied together with pulsed-amerometric

detection immunoassay techniques is among such techniques

which can be successfully applied for continuous

measurements in flow throw electrochemical cell

[106,110,127]. Electrochemical label free immunosensors

based on polypyrrole were developed [106,128]. In a novel



6032

sampling strategy, antibody-based electrodes were used for the

repeat, intermittent on-line monitoring of tissue corticosteroids

in experimental animals. Here, in a competitive assay, sample

steroid competed with enzyme-labeled corticosteroid for the

antibody immobilized on a platinum electrode surface.

Amperometric detection of the enzyme product was used to

follow the reaction, and to measure enzyme activity related to

the analyte concentration in the sample [129]. However, for

continuous or quasi-continuous measurements of an analyte the

problem of regeneration should be solved, because extreme pH

values, extreme salt concentrations or other factors which are

usually used for dissociation of Ag–Ab complexes lead to

destruction of polymeric immobilization matrix or distortion of

sequence analytical signals. The most immediate potential

applications of immunosensors are medical diagnostics

including the determination of infections [130,131],

environmental analysis, food and beverages control.

2.4. Polypyrrole in the design of DNA sensors

DNA is a unique biomolecule, which is served in known

living species as genetic information storage. Number of

genome projects is providing massive amounts of genetic

information that should revolutionize the understanding of

living nature. Because genome variations between species and

some groups of individuals are straightforward and might be

clearly distinguished it makes specific DNA sequences very

attractive as universal analyte. Nowadays the detection of

appropriated DNA sequences is important for diagnosis of

genetic or infectious diseases, environmental testing for

bacterial contamination, rapid detection of biological warfare

agents, forensic investigations and scientific explorations in

genomics and proteomics.

Moreover, DNA might be determined as important

nanomaterial, which is involved in a number of

nanotechnological and/or bioelectronic applications [132].

There were several demonstrations that DNA in combination

with conducting polymers might be applied for the formation

of unique nanostructures like polypyrrole nanotubes [133] and

polyanilinenanovires[134]. On the other hand, DNA in

combination with polypyrrole was described in a number of

DNA sensors, like in the case of immunosensors,

electrochemically labeled and label-free DNA sensors are

designed. Early works on electrochemical DNA sensors were

mainly based on the application of electrochemical labels for

indication of immobilized single-stranded DNA (ssDNA)

interaction with target DNA present in the sample [135,136].

Such approaches were mainly based on measuring changes in

the peak currents of redox labels, if the DNA duplex formed

during hybridization is exposed to the solution of the indicator

[137]. For this purpose cationic methal complexes such as

tris(2,2_-bipyridine)ruthenium (III) ([Ru(bpy)3]3+) [138] or

tris(1,10-phenanthroline)cobalt(III) ([Co(phen)3]3+) [139],

aromatic compounds such as dye Hoechst 33258 [140],

daunomycin[141] and naphthalene diimine with

covalentlyattached two fereocene moieties [142] or enzymatic

labels (e.g.horseradish peroxidase) based redox indicators

[143]might be applied.

However, the most advantageous are label-free

DNAsensors. Such systems are mainly based on monitoring

changesin electronic or interfacial properties accompanying

DNAhybridization [19]. It means that the key factor

concerningthe development of such electrochemical label-free

DNAsensors is the achievement of efficient interface between

thenucleic acid system and the electronic transducer.

Conductingpolymer molecular interfaces are particularly

suitable formodulating DNA interactions, for inducing

electrical signalsaccrued from such interactions, and for

localizing DNA probesonto extremely small surfaces [19].

Among other conductingpolymers polypyrrole is most

frequently used in the design ofelectrochemical DNA sensors

mainly for immobilization ofssDNA[144]. As it was

demonstrated in several studies, dopingof Ppy by ssDNA is a

very simple procedure if electrochemicaldeposition of this

polymer in the presence of short ssDNAoligonucleotides is

applied [145]. In this way the formedpolymeric layer exhibits

high affinity to complementary ssDNAstrands. Ppy/DNA

sensors are based on various ssDNA immobilization strategies

such as adsorption [11], direct covalentbinding [146],

entrapment in a polymer matrix [20,21,137,147]or indirect

binding by the use of intermediate systems likebiotine-avidine

clips [148]. The most distinct electrochemicalsignals are

generated after DNA hybridization. Two differentways of

ssDNA entrapment might be distinguished: (i) entrapment

in the presence of other counter ions [21,144,147,149];(ii)

doping of Ppy film by ssDNA if ssDNA is present assingle ion

in electrochemical-polymerization solution and isserved as

counter ion [20,153]. In this way polypyrrole bearing

[21,144,147] and doped [20,153] with single-stranded DNA

was applied for the development of DNA sensors. On the

otherhand, polypyrrole is capable to transfer energy as an

electrochemicaltransducer and direct label-free, electrical

detection ofDNA hybridization has also been accomplished by

monitoringchanges in the impedance and capacity/resistance of

conductingpolymer molecular interfaces [21]. Electrochemical

impedance technique seems most informative in label-free

DNA detectionsince this method brings the highest number of

information ontarget DNA interaction with immobilized

ssDNA[150,151].Significant differences in impedance between

electrochemical

systems containing single-stranded DNA immobilizedwithin

polypyrrole and double-stranded DNA (dsDNA) canbe

converted into electrical signals that are easily monitored[152].

However, this method can be successfully replaced byother

less sophisticated electrochemical techniques like

cyclicvotammetry[144], pulsed amperometric detection [21] or

basicamperometry[153]. Garnier et al. have reported a study in

whichnucleic acid probes were linked to the polypyrrole

surface andcyclic voltammetry investigations were performed.

This studydemonstrated a potential shift and wave broadening

in the cyclicvoltammograms registered after interaction with

target DNA[144]. Cyclic voltammetry was applied for

biosensing of DNAhybridization by polypyrrole functionalized

with ferrocenylgroups [154]. Our group has showed that pulsed

amperometricdetection might be applied for detection of target

DNA if ssDNAentrapped within Ppy is deposited over working

electrode [21].Investigations of Wang’s group illustrated that

short termcurrent peaks provoked by the hybridization event at

constantpotential might be registered if short complementary

poly-A,poly-G, poly-T and poly-C oligonucleotides are

interacting with

complementary ones. However, if not-complementary DNA

ispresent in the sample the distinctly opposite current peaks

wereobserved [153]. High stability of Ppy/DNA films allows

detectionof target DNA in flow-trough systems what is a

significantadvantage for continuous routine measurements

[155]. On theother hand, itwas demonstrated that pyrrole–

DNAmight be easilyaddressed towards appropriated electrode



6033

what is extremelyuseful for designing electrochemical-DNA

arrays [156].

In some polypyrrole based DNA sensors, quartz crystal

microbalances or fluorescence methods were successfully

applied and such sensors were used for multiple determination

of targetDNA[148]. It might be predicted that newDNAsensors

will be developed where intrinsic electron transfer properties of

DNA[157] and conducting polymers will be combined

together. Next promising direction which is offered by

nanotechnology is application of DNA in combination with

nanoparticles [158]. Here polypyrrole based nanoparticles [30]

might be applied, since previously described DNA sorption on

polypyrrole-silica nanocomposites was very efficient [159].

Carbon nanotubes modified with Ppy/ssDNA seem very

promising for electrochemical DNA sensor design

[150,151,160].

2.6. Application of polypyrrole in molecular imprinting

technology

Indispensable condition during the development of reviously

described affinity sensors is that the bind-able reagents should

be immobilized. However, given to poor chemical and physical

stability of biomolecules even if they are well immobilized,

molecularly imprinted polymers and artificial receptor based

sensors have been gaining importance as a possible alternative

to other affinity sensors (e.g. immunosensors, DNA sensors,

etc.) which are based on immobilized biomolecules [161]. The

preparation of molecularly imprinted polymers requires

polymerization around a print species using monomers that are

selected for their capacity to form specific and definable

interactions with the print species. Within polymer entrapped

molecules can be removed by solvent extraction and the

molecularly imprinted polymer is ready for use. Cavities are

formed in the polymer matrix with ‘images’ of the size and

shape of the imprinted molecules. Furthermore, chemical

functionalities of the monomer residues become spatially

positioned around the cavity in accord with complementarity to

the chemical structure of the imprint molecule. Molecularly

imprinted polymers are very promising during the development

of synthetic recognition systems and are of great interest to

workers in the field of sensor technology. Molecular imprinting

is increasingly becoming recognized as a versatile technique

for the preparation of artificial receptors based on molecularly

imprinted conducting polymers (MIPs) containing tailor-made

recognition sites. MIP is anotherclass of substances of great

interest in the field of chemical sensortechnology[162].

Moreover, these sensors are able to detect lowmolecular mass

organic molecules [31,32,33,163,164]. It is the reason why the

development of synthetic recognition systems is of great

interest to workers in the field of sensor technology [165].

These highly stable synthetic polymers possess molecular

recognition properties due to cavities formed in the polymer

matrix that are complementary to the analyte (ligand) both

inshape and in positioning of functional groups [166]. Some of

these polymers have shown very high selectivity and affinity

constants fully comparable to natural recognition systems

suchas antibodies [167,168,169]. In general, molecular

imprinting is atechnology for the manufacture of synthetic

polymers with predetermined molecular recognition properties

[170]. The preparation of molecularly imprinted polymers

requires polymerization around the print species using

monomers that are selected for their capacity to form specific

and definable interactions with the imprinted species [171].

Furthermore, chemical functionalities of the monomer residues

become spatially positioned around the cavity in a pattern

which is complementary to the chemical structure of the print

molecule [172]. These imprints constitute a permanent memory

for the print species and enable theimprinted polymer to rebind

the print molecule from a mixtureof closely related compounds

selectively [173]. Finally, the printmolecules are removed by

solvent extraction and the molecularlyimprinted polymer is

ready for use. Some of these polymers havebeen shown to be

useful in sensor applications, exhibiting tolerancetowards acid,

base, high temperature and organic phases[174]. It was found

that the manufacture of composites consistingof molecularly

imprinted conducting polymers resultsin obtaining materials

that exhibit both predetermined selectivemolecular recognition

and electrical conductivity [175]. Thistype of materials is of

special interest for use in the field ofsensor technology [176].

Here overoxidizedpolypyrrole is most frequently applied.

Overoxidizedpolypyrrole exhibits an improved

selectivity,which is attributed to the removal of positive

charges from Ppy films due to introduction of oxygen

functionality, such as carbonyl groups. The preparation of

molecularly imprinted polypyrrole requires polymerization

around printed species. Then within Ppy entrapped molecules

are removed by solvent and the molecularly imprinted polymer

is ready for use [29]. Such polymer possesses nano-pores and

nano-cavities that are complementary to removed dopant.

Furthermore, chemical functionalities of the monomer residues

become spatially positioned around the cavity in a pattern that

is complementary to the chemical structure of the print

molecule. Theseimprints constitute a permanent memory for

the print species and enable the imprinted polymer to

selectively rebind the print molecule from a mixture of closely

related compounds. Sensors based on molecularly imprinted

Ppy for serotonin and1-naphthalensulfonate [177], amino acids

[31,175], caffeine [32,163], atropine [178], sacharide[179],

glycoproteins [22] were reported. Both chemically and

electrochemically synthesized Ppy can be applied in the

development of molecularly imprinted polymers. The

electrochemical properties of Ppy strongly depend on their

redox state. At positive potentials overoxidation of polypyrrole

occurs. It leads to the partial degradation of polypyrrole

polymeric backbone and introduction of carboxylic, carbonilic

and hydroxilic groups into polymeric backbone that determines

semi-permeability as well as ability to recognize imprinted

molecules [38]. The best results are achieved if during

electrochemical deposition overoxidizedPpy is imprinted by

small molecular weight molecules [31,32,33,176]. Moreover,

attempts to imprint Ppy by large molecular weight rigid

structure possessing proteins were reported as well as, in this

case viral envelope proteins possessing rigid structure were

imprinted within overoxidizedpolypyrrole[22].

The conversion of the binding event into a measurable

signal in particular at a low concentration of the analyte, the

prevention or elimination of non-specific interactions, the

regenerability, and reusability among other topics are the major

challenges in molecularly imprinted polymer based biosensors.

In such sensors differences in capacitance and/or resistance

arising in electrochemical system during interaction of affinity

agents can be converted into signals that are easily monitored

[180]. Here pulsedamperometric detection can be applied as

basic electrochemical detection method [21,22].



6034

3. Conclusions and future trends

Polypyrrole is the most extensively used among other

conducting polymers for the construction of different types of

bioanalytical sensors. The background presented illustrates that

polypyrrole is a very attractive, versatile material, suitable for

preparation of various catalytic and affinity sensors and

biosensors. Both electrochemically and chemically induced

pyrrole polymerization methods has potential application in the

development of analyte-recognizing/converting layers.

In several studies it was shown that Ppy is able to transduce

analytical signal generated by some redox enzymes directly if

redox center of enzyme is deeply buried in the protein globule.

Polypyrrole modified by covalently attached redox groups

might be applied to facilitate electron transfer. From the other

side, the presented overview shows that polypyrrole might be

applied as immobilization matrix in the design of various

affinity sensors like immunosensors, DNA sensors and sensors

based on molecularly imprinted polymers. The use of

polypyrrole in conjunctionwith bioaffinity reagents has

provident to be a powerfulroute that has expanded the range of

applications of electrochemicaldetection and its future

development is expected tocontinue. The interaction between

the proteins, mainly negativelycharged at neutral pH, and the

delocalized positive chargesalong the polypyrrole chains

induces changes in capacitance ofthis nanostructured material.

Consequently, such interactions,evidenced from

electrochemical measurement are the basis ofbioaffinity signal.

The use of a wide range of counterions willprovide significant

change in affinity at the Ppy ion-exchangesites. The application

of nanoelectrode (e.g. carbon nanotubes)technologies already

established in “electronic-nose” deviceswill be beneficial to

polypyrrole based immunosensors. Furtherexploitation of this

technology to immobilize bioaffinityreagents with the polymer

matrix may enable the design ofsmaller, more compact and

portable biosensing systems.

Current achievements show that electrochemical affinity

sensor based on molecularly imprinted polypyrrole could have

a great potential for direct electrochemical sensing. It is straight

forward that in the future molecularly imprinted polymer based

sensors will require deliberate control of the molecular

structure at the surface of the electrode to exhibit higher

affinity to analyte.As the surface microstructure becomes more

complicated, more chemical methods of molecular structure

construction will be required. These methods will use

“molecular technology” instead of bulk technology mainly

used at present time. In addition, for the construction of more

complex nanostructures, somedegree of molecular self-

assembly will be needed and conductingpolymers become

more complex, versatile and will find thenew applications.

New nanotechnological approaches to overcome these

challenges are still in their infancy and application

ofconducting polymers, in particular cases polypyrrole, will

findproper place in future molecular technology.

Acknowledgments

This work was partially financially supported by Lithuanian

State Science and Studies Foundation project number C 03047

and COST program D33.

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https://www.researchgate.net/publication/223229000_Electrochemical_sensors_based_on_conducting_polymer-polypyrrole_Review?el=1_x_8&enrichId=rgreq-0465731305baf26ead4b45ec21e94d66-XXX&enrichSource=Y292ZXJQYWdlOzIyMzIyOTAwMDtBUzo5OTQ4OTIxMzg0NTUzMUAxNDAwNzMxNDg1NzQx



Electrochemical sensors based on conducting polymer—polypyrrole

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