Advances in sensors based on conducting polymers
Neetika Gupta, Shalini Sharma, Irfan Ahmad Mir and D Kumar*
Department of Applied Chemistry, Delhi College of Engineering, University of Delhi, Delhi 110 042 Received 28 September 2005; accepted 13 April 2006
Conducting polymers with a blend of interesting optical and mechanical properties can be used in situations where inorganic materials are not suitable. Synthetic capabilities as well as fabrication techniques have been developed to such an extent that molecular electronic devices based on conducting polymers can be designed, marking evolution of a sophisticated technology in the field of microelectronics. These electro-active conducting polymers cover a broad spectrum of applications from solid-state technology to biotechnology and sensor technology. Present paper addresses the various issues on sensors for chemical and biochemical species.
Keywords: Conducting polymers Enzymes, Immobilization, Sensors, Transducer
Introduction
Polymers used in sensor devices either participate in sensing mechanism or immobilize the component responsible for sensing the analyte. A new class of polymers known as intrinsically conducting polymers (CPs) or electroactive conjugated polymers exhibit interesting electrical and optical properties, which were found only in inorganic systems. Electrically conducting polymers differ from all the familiar inorganic semiconductors (silicon and germanium) in two important features that polymers are molecular in nature and lack long-range order1. CPs contain π-electron backbone which is responsible for their unusual electronic properties such as electrical conductivity, low energy optical transitions, low ionization potential and high electron affinity, and are used to enhance speed, sensitivity and versatility of sensors. Properties of CPs depend strongly on doping level, ion size of the dopant, protonation level and water content. CPs finding ever-increasing use in diagnostic medical reagents2 and with a distinguishable chemical memory are prominent new materials for the fabrication of industrial sensors. A number of reviews3-14 on the use of conducting polymers in the fabrication of sensors have been published. Sensors may be classified depending on the mode of transduction and application
Sensors Based on Transduction
A) Potentiometric Sensors
Potentiometric sensors may be either symmetric or asymmetric. In symmetric potentiometric sensors, the selective membrane is symmetrically bathed by two- electrolyte solutions and an external sample solution.
In asymmetric potentiometric sensors, there is no internal filling solution. Hence, the sensor membrane is only in contact with one aqueous phase, i.e., the sample, while the internal contact is with a solid-state ionic or electronic conductor. In potentiometric sensors, cell potential is monitored in zero current condition (equilibrium)15.
Potentiometric biosensors incorporating enzymes and antibodies consume or produce species, which can be monitored by an ion-selective electrode directly (commonly H+ or NH4
+), or it is coupled with a second reaction. Examples include a sensor for glucose, which utilizes the coupling of the oxidation of glucose by glucose oxidase with the activity of fluoride ions through the action of a second catalytic reaction on an organofluorine compound, and similar sensors for urea, penicillin, malate and sucrose.
B) Amperometric Sensors
In amperometric sensors, signal is proportional to the concentration of analyte species. Suitable target species are electroactive species that are capable of being oxidized or reduced, with the oxidation or reduction potential being zero. Cells can be either two electrode (working and reference) or three electrode (working, counter and reference) systems. The best-
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*Author for correspondence E-mail: drdkumar@yahoo.co.uk
known amperometric sensor is the Clark Oxygen Cell, developed by Leland C Clark in 1956. In a biosensor, concentration of an enzyme substrate is measured indirectly through the consumption of oxygen by oxidase enzyme catalyzed reactions, or by the generation of hydrogen peroxide (H2O2). Oxygen and H2O2 being the co-substrates and the product of several enzyme reactions are detected for amperometric estimation. Electrochemical biosensors (Fig. 1) are based on mediated or unmediated electrochemistry for electron transfer16. Umana &
Waller17 reported a novel approach to electrode immobilization of an enzyme, glucose oxidase, by electropolymerization of pyrrole in the presence of the enzyme. Under optimum conditions, glucose oxidase can be incorporated into the polypyrrole films during their formation and such electrode can be used in the determination of glucose in aqueous solutions for a period of up to 7 days.
C) Piezoelectric Sensors
In these sensors, an acoustic wave is propagated by an externally applied alternating current between two electrodes or interdigited electrode fingers deposited on a piezoelectric substrate such as quartz. The subclasses of piezoelectric transducers are based on the way of acoustic wave propagated between the electrodes. Most applications of these devices have been found for gas phase monitoring, where hydrogen sulphide, carbon dioxide, oxygen, nitrogen dioxide, molecular hydrogen, mercury, toluene, and acetone sensors have been fabricated15.
D) Calorimetric/Thermal Sensors
Thermistors, whose resistance changes markedly with temperature, are often employed as cheap, sensitive temperature sensors. The most commonly used approach in the thermal enzyme probes18 is related to the enzyme directly attached to the thermistor. It is observed that the heat evolved in the
enzymatic reactions is lost to the surrounding solution without being detected by thermistor resulting in the decrease in sensitivity of the biosensor.
E) Optical Sensors
Optical sensors are based on the measurements of light absorbed or emitted as a consequence of a biochemical reaction. Light waves are guided by means of optical fibers to suitable detectors. Such sensors can be used for measurement of pH, O2, CO2
etc. A commercial optical sensor, a hybrid electrochemical/optical LAPS (light addressable potentiometric sensor), was developed by M/s Molecular Devices in Palo Alto, USA. Development of cheap, efficient, fiber optic cable for applications in telecommunications, and technological advances in the design of small monochromators and laser diodes has stimulated research into optical sensors.
Typically, these devices incorporate a material at the tip or on the side of the cable, which can generate an optical signal related to the concentration of target species in the sample. Both fluorescence and absorbance have been used for this purpose via direct and indirect mechanism.
Sensors Based on Application Mode
A) Industrial/Chemical Sensors
Chemical sensors convert a chemical state into an electric signal. In such sensors, a sensitive layer is in chemical contact with the analyte. A change in the chemistry of the sensitive layer (a reaction) is produced after the exposure to analyte. The sensitive layer is on a platform that allows transduction of the change to electric signals. Such sensors are small, economical and operate in real time. Every chemical sensor is divided into two domains, the physical transducer and the chemical interface; the analyte interacts chemically with the surface, producing a change in physical or chemical properties (Fig. 2).
Chemical sensors can further be classified as follows:
a) Gas Sensors
CPs show promising applications for sensing gases with acid, base or oxidizing characteristics. Sensors were designed by the electrochemical deposition of appropriate polymer across a gap of 12 µm between two gold microband electrodes. The polymer films were initially oxidized to a known extent potentiostatically. Upon exposure to the vapors, the polymers show conductivity changes that are rapid and in general reversible at room temperature. To detect HCl in sub ppm levels, composites of alkoxy
Fig. 1Mediated and unmediated electron transfer
substituted tetra-phenyl-porphyrin polymer composite films were developed19. The sensor response and recovery behavior improved, if the matrix has a glass transition temperature below the sensing temperature.
Alkoxy group imparts basicity to the materials and hence increases sensitivity to HCl.
Nylander et al20 investigated the gas sensing property of polypyrrole by exposing polypyrrole- impregnated filter paper to ammonia vapor while Dhawan et al21 investigated the gas sensing property of polyaniline by exposing it to ammonia vapors or solution. The performance of the sensor was found linear at room temperature with higher concentration (0.5-5%) responding in minutes. Nucleophilic gases (ammonia and methanol, ethanol vapors) cause a decrease in conductivity, with electrophilic gases (NO2, PCl3, SO2) having the adverse effect22. Most of the widely studied CPs in gas sensing applications are polythiophene and its derivatives23,24, polypyrrole25,26 polyaniline and composites27-29 of these polymers.
b) pH Sensors
Polyaniline has been found most suitable organic material for pH sensing in aqueous medium30-33. Conductivity decreases rapidly with increase in pH at a given potential. Similarly, at a given pH, conductivity changes with respect to the change in potential. Using this property, Demarcos &
Wolfdeis34 developed an optical pH sensor based on polypyrrole. Other studies on optical pH sensors35-37, based on polyaniline for measurement of pH (2-12), reported that polyaniline films synthesized within 30 min are very stable in water. Jin et al38 prepared polyaniline film by chemical oxidation at room temperature and improved stability of the film significantly by increasing reaction time up to 12 h.
The film showed rapid reversible colour change upon pH change. The pH of solution could be determined by monitoring either absorption at a fixed wavelength or maximum absorption wavelength of the film.
Effect of pH on the change in electronic spectrum of polyaniline polymer was explained on the basis of different degree of protonation of the imine nitrogen atoms in the polymer chain39. The optical pH sensor could be kept exposed in the air for over one month without any deterioration in sensor performance.
c) Ion-selective Sensors
Ion-selective sensors have been developed using either the polymer as the conductive system/component, or as a matrix for the conductive system. When such system comes in contact with the analytes to be sensed, some ionic exchange occurs, which gets transmitted as an electronic signal for display. Ion-selective sensors find wide applications in medical, environmental and industrial analysis, and also in measuring hardness of water. In ion-selective sensors, polymers have been utilized to entrap sensing elements. A Ca++ selective polyaniline (PANI) based membrane has been developed for all solid-state sensor applications40. The membrane is made of electrically conducting polyaniline bis [4-(1,1,3,3- tetramethyl butyl) phenyl] phosphoric acid (DTMBP- PO4H), dioctylphenyl phosphonate (DOPP) and cationic (tridodecylmethylammonium chloride TDMACl) or anionic potassium tetrakis (4- chlorophenyl) borate as lipophilic additives. PANI is used as membrane matrix, which transforms ionic response to an electronic signal.
d) Alcohol Sensors
CPs gained popularity as their use in sensor for alcohol vapors, such as methanol, ethanol and propanol11,41. PANI doped with camphor sulphonic acid (CSA) also showed a good response for alcohol vapors42-45. PANI and its substituted derivatives46 such as poly(o-toluidine), poly(o-anisidine), poly(N-methyl aniline), poly(N-ethyl aniline), poly(2,3-dimethyl aniline), poly(2,5-dimethyl aniline) and poly(diphenyl amine) were found sensitive to methanol, ethanol, propanol, butanol and haptanol vapors. All polymers respond to the saturated alcohol vapors by undergoing a change in resistance; resistance decreases in the presence of small chain alcohols (methanol, ethanol and propanol), while an opposite trend was observed with butanol and haptanol vapors. Such change in resistance of the polymers on exposure to different
Fig. 2Configuration of a chemical sensor
alcohol vapors was attributed to their chemical structure, chain length and dielectric nature. All the polymers showed measurable responses for short chain alcohols, at concentration up to 3000 ppm, but none of them is suitable for long chain alcohols.
Polypyrrole47 doped with dodecyl benzene sulphonic acid (DBSA) and ammonium persulphate (APS) showed a linear change in resistance when exposed to methanol vapors in the range 87-5000 ppm. Barlett &
Ling-Chung48 also detected methanol vapors by the change in resistance of a polypyrrole film.
e) Humidity Sensors
Humidity sensors are useful for the detection of the relative humidity (RH) in various environments.
Polymeric electrolyte systems have been used in humidity sensor devices based on variation of the electrical conductivity with water vapor. Polymer electrolytes containing polymer cation/ anion with its counter ions and mixtures or complexes of inorganic salts with polymer are the major class of materials for fabrication of humidity sensor49. Although polymer electrolytes containing hydrophilic groups [-COOH, - SO3H, -N+(R)3Cl, etc.] are potentially excellent materials for sensing low humidity because of their solubility in water.
B) Biosensors
A biosensor is an analytical device incorporating a biological or biologically derived material, either intimately associated or integrated within a physico- chemical transducer. The change in electronic conductivity of conducting polymers in response to change in pH has been made use of in fabricating sensors for biomolecules. Specificity to the desired molecule can be achieved by immobilizing the appropriate enzyme into the polymer matrix.
Biosensor is a synergic combination of analytical biochemistry and microelectronics. In general, a biosensor consists of a biological component (B) in intimate contact with a suitable transducer (T) coupled through immobilization (Fig. 3). The biological component gives rise to a signal as a result of the biochemical reaction of the analyte (A), which is detected by transducer to give an electrical signal (ES). The immobilization of the biological component, though decreases its activity, imparts stability to the biological component against the environmental conditions.
Immobilization of biocatalysts in a suitable matrix is an important practice in biomedical, industrial, and
basic enzymology for repetitive and continuous processes and helps in economic utilization of the biocatalyst. The activity of the immobilized biomolecules depends on surface area, porosity, and hydrophilic character of the immobilizing matrix, reaction conditions and the methodology chosen for immobilization. The immobilization10 can be achieved through chemical bonding or physical retention (Fig. 4). Binding of biocatalyst to solid supports by the chemical bonding method can be achieved directly or through cross-linking. The bonding can be seen via ionic or covalent interactions. The cross-linking can be either through linking to itself or through co-cross- linking with a structural protein such as bovine serum albumin. Physical retention consists of entrapment in a matrix in the form of beads, fibers, or enclosing in the matrix by encapsulation or incorporating in membrane reactors.
Many biosensor fabrications are based on electrochemical transduction of the biological signal, because about 30% of the biological reactions involve consumption/liberation of protons, electrons, or ions, which are electrochemically active. There are a number of polymers, which have been used for fabrication of efficient biosensor in terms of sensitivity and stability (Table 1).
CPs are formed by a combination of donor (D) and acceptor (A) systems. These species are generally planar, having delocalized π-electron density both above and below the molecular plane.
Fig. 3Schematic representation of a biosensor [biological component (B), interface (I), transducer (T), analyte (A), out put signal (OS), recorded or displayed data (D) and processing (P)]
Fig.4Immobilization methods
Tetrathiafulvalene (TTF) and tetracyanoquinodimeth- ane (TCNQ) are typical examples of donor and acceptor systems. The donor-acceptor salts can be prepared by direct reaction of unchanged donor and acceptor in acetonitrile by mixing equal amounts of TTF and TCNQ solutions in acetonitrile to give a black precipitate. The reaction mixture is cooled overnight with stirring, filtered with suction and dried under vacuum. The precipitate is washed with acetonitrile and then with ether till the washings are colorless.
An enzyme switch11 responsive to glucose was designed by immobilizing glucose oxidase in an electropolymerised film of poly(1,2-diaminobenzene) grown on the top of a polyaniline film. Switch employed TTF as a redox mediator capable of shuttling charge between the enzyme and the conducting polymer. In the oxidized state, polyaniline, at +0.5 V versus SCE (pH 5), is insulating. On addition of glucose, polyaniline is reduced. The reduced form being conducting, there is a rapid increase in current.
Biosensors find extensive applications in medical diagnostics, environmental pollution control for measuring toxic gases in the atmosphere and toxic soluble compounds (heavy metals, nitrates, nitrites, herbicides, pesticides, polychlorinated biphenyls, polyaromatic hydrocarbons, trichloro ethylene etc.) in river water. Pollutant sensitive biocomponents have been used with a variety of detection modes for their quantitative estimation50,51. Estimation of organic compounds is very important for the control of food manufacturing process and for the evaluation of food quality. Enzyme sensors can help in the direct measurement of such compounds, including organic pollutants for environmental control. Hydrogen peroxide used in food, textile, and dye industries for bleaching and sterilization purposes, can be directly
measured by enzyme sensors as per the following equation, with the liberated oxygen detected by oxygen electrode:
Catalase
H2O2 H2O + ½ O2
The principle of the operation of a biosensor52, which starting from the analyte can provide all the information needed for its evaluation. By far the largest group of direct electron transfer biosensors is based on co-immobilization of the enzyme in a conducting polymer, namely polypyrrole53-56 and polyaniline57. Table 2 shows list of biosensors based on CPs.
An enzyme sensor, a combination of a transducer and a thin enzymatic layer, normally measures the concentration of an analyte. The enzymatic reaction transforms the substrate into a reaction product detectable by a transducer. The sensitive surface of the transducer remains in contact with an enzymatic layer, and it is assumed that there is no mass transfer across this interface. An external surface of the enzymatic layer is kept immersed in a solution containing the analyte under study. Analyte migrates towards the interior of the layer and is converted into the reaction products when it reacts with the immobilized enzyme52.
The glucose concentration in the blood sample can be measured directly by a biosensor (which is made specifically for glucose measurement) by simply dipping the sensor in the sample. To measure the glucose concentration49, three different transducers can be used: 1) An oxygen sensor that measures oxygen concentration; 2) A pH sensor that measures the acid (gluconic acid) production; and 3) A peroxide sensor that measures H2O2 concentration.
Urea content of blood serum depends on protein catabolism and nutritive protein intake and is regulated by renal excretion. Concentration of urea in blood is important in clinical chemistry for assessment of kidney functioning58. Komaba et al59 prepared a urea biosensor by immobilizing urease enzyme in an electropolymerised electroactive polypyrrole (PPy) on a platinum electrode. PPy electrode showed a stable potential response to urea based on the pH response of electroactive PPy film electrode. This biosensor showed a Nernstian response, with a slope of 31.8 mV decade-1 over concentration range of 1x 10-4 to 0.3 mol dm-3 urea.
Table 1Different immobilizing matrices and their suitable biological components
Matrix Biological Component
TTF. TCNQ Ascorbic acid oxidase
Glucose oxidase
Polypyrrole Glucose oxidase
Poly(ethylene-vinyl) alcohol Alcohol dehydrogenase
Polyphenol Glucose oxidase
D-amino oxidase Poly(o-phenylenediamine) Glucose oxidase
Polyurethane Glucose oxidase
Polyethylene-g-acrylic acid Glucose oxidase Viologen-acrylamide copolymer Nitrate reductase
Immunological sensors are based on the recognition involved in coupling of an antigen with an antibody, immunoagents immobilized in a polymer matrix (PVC, polyacrylamide gel, etc). Either an immobilized antibody detects an antigen or an immobilized antigen detects an antibody. Due to the interaction between an antibody and an antigen, a variation in electric charge, mass or optical properties is detected directly with a variety of transducers52.
[[
DNA biosensors have enormous applications in clinical diagnostics of inherited diseases, rapid detection of pathogenic infections, and screening of Calf thymus DNA colonies required in molecular biology. A few reports of interaction of DNA with conducting polymers are available60-62. Livache et al63 reported one step electro-deposition of PPy films functionalized by a covalently linked oligonucleotide.
Another possibility is to dope DNA probes within electropolymerised polypyrrole films and monitoring the current changes incurred by the hybridization64. Gerard et al65 have reported the results of the studies related to the characteristics of physically adsorbed DNA (Calf thymus) on conducting PPy/PVS films.
Immobilization of DNA on a conducting polymer
matrix facilitates the detection of a signal generated as a result of interaction of proteins or drugs with DNA.
[
Sensor arrays coupled with pattern recognition are useful in the discrimination of the aromas of certain foods and beverages. Several examples are available in the literature, demonstrating the success of using polymeric arrays of sensors for the detection of food and beverages odor. Guadarrama et al66 described a sensor array based on thin films of conducting polymers with an objective to discriminate among different virgin olive oils. They designed an array of eight polymeric sensors deposited electrochemically using monomers such as 3-methylthiophene, pyrrole, aniline and using different doping agents. For the identification of wines, Guadarrama et al66 tested a set of 12 polymeric sensors as an artificial olfactory system. The set of sensors were prepared by electrochemical deposition of various conducting polymers, such as polypyrrole, poly(3- methylthiophene) and polyaniline. Persaud et al67 used organic conducting polymers derived from aromatic or heteroaromatic compounds as gas and odor sensors for perfumes. Baldacci et al68 used an odor sensor array composed of two novel conductive
Table 2Biosensors based on conducting polymers using different enzymes
Substrate or species to be determined Enzyme Polymer Detection Method
Glucose Glucose oxidase Polypyrrole Amperometry
Potentiometry Poly(N-methyl pyrrole) Amperometry
Polyaniline Amperometry
Polyindole Amperometry
Glucose dehyrogenase Polypyrrole Amperometry
D-Alanine D-Aminoacid oxidase Polypyrrole Amperometry
Atrazine Tyrosinase Polypyrrole Amperometry
Cholesterol Cholesterol oxidase &
Cholesterol esterase
Polypyrrole Amperometry
Choline Choline oxidase Substituted polypyrrole Amperometry
Glutamate Glutamate dehydrogenase Polypyrrole Amperometry
Fructose Fructose dehydrogenase Polypyrrole Amperometry
Heamoglibin Pepsin Polyaniline Conductometry
L-Lactate Lactate oxidase Polyphenylene diamine Amperometry
Lactate dehydrogenase Polyaniline Amperometry
Polypyrrole-Polyvinylsulphonate Amperometry
Lipids Lipase Polyaniline Conductometry
Phenols Tyrosinase Substituted polypyrrole Amperometry
Urea Urease Polypyrrole Amperometry
Potentiometry Conductometry
Capacitance measurement
Uric acid Uricase Polyaniline Amperometry
Triglycerides Lipase Polyaniline Conductometry
polymers, such as the polymers of 3,3/-dipentoxy-2,2/- bithiophene and 3,3/-dipentoxy-2,2/,5/,2//-terthiophene for sensing wine flavor.
Sangodkar et al69 described the fabrication of polyaniline based microsensor and microsensor arrays for the estimation of glucose, urea and triglycerides.
Polymer deposition and enzyme immobilization were done electrochemically. The enzyme was directly immobilized to the chosen microelectrodes by controlling the electrochemical potential, avoiding any contact of the enzyme solution to other microelectrodes, resulting in a sensor array for the analysis of the sample containing a mixture of glucose, urea and triolein in a single measurement using a few microlitres of the sample. This strategy was extended to other enzyme-substrate systems and intended to represent an ‘electronic tongue’. Although details are lacking, it is worth noting that a touch sensor system, including a substrate capable of propagating SAWs, and an array of reflective elements formed on the said substrate.
Conclusions
Conducting polymers have tremendous technological potential for the development of sensors. They are inexpensive, can be miniaturized and easily fabricated as compared to other miniaturized sensors. The primary output of these sensors is electrical in nature and hence easier to integrate into a single chip for further signal processing, which may lead to the development of
‘multiple sensors’ or ‘sensor arrays’. In due course of time, these sensors are bound to find their way from the laboratory to commercial markets.
Many gas sensors are found to be chemically stable and highly sensitive to even low concentrations of gases like ammonia, carbon dioxide at room temperature. These sensors exhibit stable responses up to 120 days suggesting long-term stability of the sensing material. The new sensor effectively eliminates the limitations associated with the current conducting polymer gas sensors, which are based on conductivity measurements. But an accurate, reliable, selective, sensitive, rapid, miniaturized reagent less and stable devices have only been achieved in a few cases. A survey of the sensor market identified medical applications as a major driving force for the development of the emerging sensor technologies:
fiber optic sensors, smart sensors, silicon micro machined sensors, and thin film devices.
Consequently, an assessment of the successes and
failures in these developments can provide a useful guide for further research.
The majority of sensor devices utilize many polymers with definite roles, either in the sensing mechanism or through immobilizing the species responsible for sensing of the analyte component.
This has become possible only because polymers may be tailored for particular properties, are easily processed, and may be selected to be inert in the environment containing the analyte. The collaboration of polymer scientists and technologists in research will accelerate the availability of durable and cheap artificial sensor devices for human consumption.
Acknowledgements
Authors thank Principal, Delhi College of Engineering, Delhi for encouragement.
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