Intelligent Polymer Systems.pdf

INTELLIGENT POLYMER

SYSTEMS

Introduction

Intelligent polymer systems possess the ability to sense, process information, and

actuate responses. Energy is usually required to implement these functions and

so energy conversion/storage capabilities are desirable if the system is to be truly

autonomous. Ideally, these functions would be integrated at the molecular level.

It has been clearly demonstrated over the past 20 years that inherently

conducting polymers (ICPs) are capable of providing all of the above functions

and as such they have a critical role to play in the development of intelligent

polymer systems (see E LECTRICALLY A CTIVE P OLYMERS ).

This article brieﬂy reviews the properties of ICPs and their ability to function

as sensors, processors, actuators, and energy conversion/storage systems.

To illustrate the ability of ICPs to provide the range of functions required for

intelligent polymer systems, we will draw on examples that utilize polypyrroles

( 1 ) or polythiophenes ( 2 ).

231

232 INTELLIGENT POLYMER SYSTEMS

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3–4 for optimal conduc-

tivity; ie, there is a positive charge on every third or fourth pyrrole or thiophene

along the polymer chain, near which the dopant anion A − is electrostatically

attached.

Polyanilines are also of interest but they are less amenable to use in a wide

range of environments because of the need to retain protonation to ensure con-

ductivity. Consequently, most of the examples here will focus on polypyrrole and

polythiophene.

Each of these materials may be produced via either chemical or electrochemi-

cal oxidation of the appropriate monomer (1). For polypyrrole the electrodeposition

process is described simplistically in equation 1.

∼

(1)

A dopant counterion (A − ) is incorporated during electrosynthesis to balance

the charge on the polymer backbone. A wide range of dopants can be incorporated

using this approach. Common chemical oxidants such as FeCl 3 and (NH 4 ) 2 S 2 O 8

may also be used and these provide the anion from the oxidant, as the dopant

anion A − . In general, chemical oxidation provides ICPs as powders, while elec-

trochemical synthesis leads to ﬁlms.

An important feature of ICPs is that they are amenable to oxida-

tion/reduction processes that can be initiated at moderate potentials. For

polypyrroles and polythiophenes, two oxidation states can be reversibly switched,

as shown in equation 2 (Z

NH or S). The oxidized forms exhibit good electrical

conductivity (

σ =

1–100 S

cm − 1 ), while the reduced forms have very low conduc-

tivity (

σ ∼

10 − 8 S

cm − 1 ).

(2)

If the dopant anion A − is small and mobile (eg, Cl − ) and the polymer has a

high surface-area-to-volume ratio, then upon reduction the anion will be efﬁciently

ejected from the polymer. However, if the dopant is large and immobile (eg, if A −

is a polyelectrolyte such as polystyrene sulfonate) then an electrically induced

cation-exchange process occurs, according to equation 3:

(3)

where the cation (X + ) is incorporated from the supporting electrolyte solution (2).

The exact nature of the reduction process has a dramatic effect on the physical and

For polypyrroles and polythiophenes, n is usually

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INTELLIGENT POLYMER SYSTEMS 233

chemical properties of the polymer. These changes are important in determining

the sensing, information processing, and actuation capabilities of the systems as

discussed below.

Sensing

Nature has developed recognition systems that are able to discriminate on the ba-

sis of highly speciﬁc molecule–molecule interactions generating a unique signal.

Alternatively, nature utilizes arrays of less speciﬁc sensors to collect information

that is deciphered using pattern recognition processes carried out in the brain.

Both approaches have also been pursued using ICPs in the development of syn-

thetic sensors, as both chemical and physical sensors.

As chemical sensors, speciﬁcity can be induced by using molecular recogni-

tion components from nature. For example, the ICP may be used as an immobi-

lization platform for enzymes (3–5), antibodies (6–10), oligonucelotides (11–13),

or even whole living cells (14,15). The bioactive component may be incorporated

during the polymerization process and the ICP provides signal transduction and

transmission capabilities (Figure 1 a ). The majority of enzyme-containing ICP sen-

sors generate a signal because of the enzymatic generation of an electroactive

product (eg, H 2 O 2 ) or the consumption of an electroactive product (eg, O 2 ). Al-

ternatively, the bioevent may trigger a change in pH of the analyte solution that

alters the resistance of the polymer (16). The mechanism of signal generation

with antibody-containing conducting polymer sensors appears to be associated

Fig. 1. ( a ) Polymeriztaion of polypyrrole onto an electrode using an oligonucleotide (A)

as the dopant. The oligonucleotide is physically entrapped within the polymer during syn-

thesis. ( b ) As a target oligonucleotide (T) passes across the surface of the oligonucleotide

(A) doped polypyrrole electrode, a hybridization reaction occurs (indicated by the dashed

lines). This results in a change in the current/potential response observed at the electrode.

234 INTELLIGENT POLYMER SYSTEMS

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with the modiﬁcation of cation movement into and out of the polymer upon oxida-

tion/reduction in the presence of the antigen (17,18). In the case of complementary

oligonucleotide binding, only limited examples use direct electrochemical signal

transduction (19,20), and here the mechanism of signal generation is not clear

(Figure 1 b ).

The selective detection of metal ions has also been achieved by the cova-

lent attachment of molecular recognition moieties to the ICP backbone. The usual

approach has been to synthesize a monomer or dimer containing the appropriate

recognition group and this is subsequently polymerized (21). Swager (22) have pre-

pared polythiophenes containing crown ethers and calixarenes covalently bound

to the bithiophene repeat units, which exhibit controllable selectivity toward Li + ,

Na + , and K + ions.

An alternative route to appropriately functionalized ICPs is the use of sul-

fonated species containing the desired molecular recognition/receptor site as

the dopant anion for the conducting polymer chains. For example, calixarene-

containing polypyrroles (23) and polyanilines (24) have recently been prepared

via the use of sulfonated calixarenes as dopant anions. Similarly, the incorpora-

tion of metal complexing agents such as sulfonated 8-hydroxyquinoline as dopants

in polypyrroles provides a simple route to metal ion-selective ICPs (25).

ICPs have also been assembled as microsensing arrays with a view to col-

lecting less speciﬁc data subsequently deciphered using pattern recognition soft-

ware. The so-called electronic noses are based on this principle (26–30). A range

of conducting polymers with differing molecular selectivity respond (by changes

in electronic resistance of the polymer) to a complex mixture to produce a unique

pattern of responses. This approach has been used to differentiate beers (26), to

detect microorganisms (28,29), for wastewater management (31,32), and for wine

characterization (33). Stuetz and co-workers (31) demonstrated the ability of a

nonspeciﬁc sensor array (consisting of 12 conducting polymers) to detect changes

in the organic content of a wastewater sample. These researchers were able to

draw a comparison between the odor proﬁles of sewage liquids and corresponding

BOD, GOD, and TOC measurements using this sensor array. Using a nonspe-

ciﬁc conducting polymer sensor array, Bourgeois and co-workers (32) developed

an on-line continuous measurement system that monitored changes in water and

wastewater quality. By analyzing the headspace gas from sparged samples, which

ﬂowed over the sensor array, they were able to successfully monitor samples both

in a laboratory and in ﬁeld setting. Guadarrama and co-workers (33) used the

response of a conducting polymer sensor array to characterize varieties of Span-

ish wine. Using static and dynamic headspace sampling combined with statistical

analysis utilizing pattern recognition techniques, they were able to characterize

these wines by considering the sensitivity of the polymer ﬁlms to moisture and

ethanol content.

The array approach has also been developed for amperometric sensing when

used in solution. Change in amperometric responses in the presence of different

ions is used as the signal transduction method. This has been used by us to dis-

criminate between simple ions (34,35) and even proteins (36). The approach used

is similar to the “electronic nose” in that none of the sensing elements is speciﬁc;

however, each polymer has a different selectivity series, giving rise to a unique

pattern of responses for any given protein.

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INTELLIGENT POLYMER SYSTEMS 235

Actuating

ICPs are capable of undergoing transformations at the molecular level through to

the macro-molecular level. The former can be used to build responsive molecule

release systems, while the latter forms the basis of artiﬁcial muscles. A change in

oxidation state of the polymer (see equations 2 and 3) is accompanied by dramatic

changes in chemical and physical properties. Chemical changes have been studied

at the molecular level using a technique known as inverse chromatography both

in solution (37,38) and in gas phase (39,40). These studies revealed that, at least

when the dopant A − is small and mobile, reduction decreases the anion-exchange

capacity of the polymer and increases the hydrophobicity of the polymer back-

bone. The dopants can be chosen so that the release process has the desired effect

on the chemical composition of the immediate environment. For example, Miller

described the triggered release of glutamate (41) and salicylate (42) among other

compounds.

The movement of ions has also been tracked using electrochemical quartz

crystal microbalance techniques (43–45). As ions move in and out of the polymer

matrix, upon oxidation and reduction, a resonant frequency shift is observed for

the polymer coated quartz crystal. The magnitude of the frequency shift is indica-

tive of the amount of ions moving either in or out of the polymer. The direction of ion

ﬂow has been shown to be dependent upon the nature of the dopant bound within

the polymer. Small spherical-like anions are known to move easily in and out of

the polymer matrix. When the polymer is oxidized the backbone becomes cationic

(p-type) and anions move into the polymer, effectively doping the polymer. Upon

reduction of the polymer backbone, the cationic characteristics of the polymer are

lost and the anions are expelled under the negative electric ﬁeld of the reduction

potential. Ion ﬂux can be reversed by the incorporation of large anion species into

the polymer during the growth phase, such as dodecylbenzene sulfonate (46,47)

or polyelectrolytes such as polystyrene sulfonate (48). Upon reduction of the poly-

mer the bulky anion cannot be expelled while the polymer backbone looses its net

cationic charge, resulting in the bulk material having an anionic character. The

result of this process is that cation species are incorporated into the polymer to

achieve electrostatic balance.

The incorporation of either anions or cations during redox cycling results

in polymer swelling. This principle has been exploited in applications such as

polymer actuators. These incorporation/exclusion events at the molecular level

also result in changes in the overall volume (dimensions) of the polymer (49). It

was these volume changes that led Baughman and colleagues to the concept of

electromechanical actuators (artiﬁcial muscles) based on conducting polymers.

Massoumi and Entezami (50,51) utilized conducting polymer bilayers to

demonstrate that active cations and electrolyte cations (ie, sulfosalicylic acid and

2-ethylhexyl phosphate) can be transported in to and out (controlled release) of

the bilayer outer ﬁlm.

Pyo and Reynolds (52,53) reported the selective releasing of ions, such as

adenosine 5 -triphosphate and heparin respectively, from conducting polymer

bilayer structures. Piro and co-workers (54) successfully incorporated oligonu-

cleotides into conducting polymer ﬁlms using a two-step polymerization process.

The release of these trapped oligonucleotides was also investigated, where the

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