jean-mlchel kauffmann free university of brussels (ulb...

TRENDS M ELECTROANALYUCAL CHEMISTRY

Jean-Mlchel Kauffmann

Free University of Brussels (ULB), Pharmaceutical Institute,

Campus Plaine,. CP 205/6. Bd. du Triomphe, 1050 Brussels, Belgium

INTRODUCTION

Modem analytical instrumentation takes advantages of the

constant progresses and innovative developments originating from

various technological domains such as in electronics, microfluidics,

micromachining, in material science, etc... With respect to the increasing

demands for specific and highly sensitive devices and by considering the

marked progresses observed in spectroscopy and separation sciences it is

of interest to analyze the current status of electroanalytical

instrumentation while identifying some major new research trends in the

area.

Electroanalysis rely on several physico-chemical phenomena

occurring at the surface of an electrode (solid or liquid). As such, the

technique is distinct from spectrophotometry since in electrochemistry,

the analytical signal corresponds to an heterogeneous phenomena. This

unique situation may led to some drawbacks such as kinetic effects (slow

electron transfer rates), possible surface problems (fouling by products o f

the reaction), and continuous surface evolution with time (passivation)

which limit the applicability and accuracy of electrochemical

instrumentation. Yet this unique feature may also be advantageoulsy

exploited for highly sensitive and selective assays e.g. by sélective

accumulation of the analyte of interest at, or in, the electrode. Additional

interest is that electrodes may be remote controlled, can be machined

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1st Eagean Analytical Chemistry Days, 18-20 November 1998, Ege Üniversitesi, Izmir

into different configurations and sizes and that the electrode surface

"architecture" may be modifed by a great number of selected (bio)

chemicals (see Table 1 for a compilation of pros and cons of

voltammetric techniques).

Electroanalytical techniques monitor at the electrode-solution

interface either a modification of the ionic equilibrium, either an electron

transfer related to the analyte concentration. The former is a static

technique, it measures a potential difference at zero current and is called

potentiometry. The latter, based on monitoring an electron flux through

an electrode submitted to a perturbation of its potential, is a dynamic

technique called (volt) amperometry or simply voltammetry.

Table 1: A compilation of characteristics of analytical voltammetric

methods.

ADVANTAGES DRAWBACKS

High sensitivity,

large dynamic range

Crucial incidence of the sensing surface

Selectivity (potential) Lack of high selectivity

In situ enrichment Complex sensing process

Surface modification Oxygen interference

Microsized Limited investigation capability

j Remote control Relatively low analysis rate

No incidence of turbidity Temperature dependency

Flow rate dependency Accuracy of flow rate control

Large choice of electrodes, cells, detection modes

Surface cleaning, renewing and handling

Automation capabilities.Relatively low cost

Reference electrode instability

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IL POTENTIOMETRY

Following the discovery of the pH sensitive glass

electrode [1] and thanks to the theories developed by Nemst as early as

1889, electrometric pH measurements were successfully applied in

analytical laboratories. In the early sixthies, the development of new

solid state matrices and the discovery of new ionophores (valinomycin

[2]) permitted to considerably enlarge the application field of

potentiometric methods (ionometry). At present, a great diversity of

potentiometric ion selective electrodes are commercially available (Table

2). Yet constant efforts are produced in order to synthesize new

ionophores [3], find new sensitive ion-pairs [4] and to select other

polymeric material than PVC [6,7] e.g. showing antifouling properties

(polyurethane based membranes) [6], for electrode preparation. Novel

impulses in potentiometric sensors have been observed with the renewed

interest in biosensor technology [8] in microelectrodes conceptions for

capillary zone electrophoresis (CZE) [9] and in chemically sensitive

field effect transistors (CHEMFETS) [10]. The latter electrodes have

been recently launched on the market for pH measurements and mày

likely replace the classical pH glass electrodes in many applications

thanks to their robustness, microconception, fast response rate and

precision (± 0,02 pH). Worth to mention also is a new pH sensitive

electrode based on iridium oxide sensing tip [11]. The electrode exhibits

a Nemstian slope over pH 0-14, it can be machined in any dimension and

slope and is unbreakable. Recent instrumentations suitable for industrial

process control and for environmental monitoring use ion selective

electrodes integrated into portable devices or into automatically controlled equipments i.e. permitting sample digestion, auto-calibration

and exhaustive data treatments (e.g. on-line nitrate and ammonia

analysis, trace fluoride and organic fluoride analysis ...) In order improve

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the electrode lifetime, its stability and the reproducibility o f the

measurements in complex matrices "new clog" free ion-selective

electrode configurations have been designed as well

Table 2 Example of electrodes used in ionometry

ELECTRODES SPECIES DETECTED

Glass membrane 11*, Na+, Ag+, Li+, Cs+, Rb+, N H / , K+, T f

Polymer membrane

• Ion exchange

• Ionophore

CvT, Cl', Mg^, CeT, n o 3\ b f4\ Cl\ C104‘

Organic cations and anions (drugs, surfactants)

K+, Li+, Ca++, Mg++, Na+, Cd++, Ba++

Solid membrane

• Monocrystal

• Polycrystal

FS~, Ag+, Hg4̂ , Cl’, Bf, cr, B r, T, CIST, SCN~,

Cd^, Pb^, Cu++

Gas-dif¥usion

membrane

ISFET

co2, n h 3, h 2s, s o 2, h c n , HCl

ff", H2S, Li+, Na+, K+, NH4+, Ca ^

IF SET = Ion Selective Field Effect Transistor

Potentiometrie stripping analysis (PSA)

The technique has been suggested in the late seventies but has

been only recently successfully applied in routine analysis thanks to the

tremendous progresses observed in computers allowing rapid control and

data recordings [12, 13], The operating principle comprises two steps, an

electrolytic step in which the analyte (trace element or ion) is

concentrated at the electrode surface, and a second step where the

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electrode is disconnected and its potential is monitored during the

stripping of the deposited metal (ion) see eqs 1 and 2. Stripping may be

achieved by oxidation or reduction

Electrolysis: M"+ + n e '------ > M (Hg) eq. 1

Stripping: M (Hg) + oxidant--------M”+ + ne' eq. 2

(oxidant = dissolved oxygen or H g^or applied current)

Modem instrumentation allow potential changes to be monitored

at a rate o f 30,000 times/sec. The dt/dV vs V curve recorded has a typical

stripping voltammetric shape (sharp peaks). PSA is a non destructive

method which permits the simultaneous determination of trace metals at

the sub ppb level (0.1 to 0.01 ppb). Mercury film glassy carbon (MFE)

and gold electrodes are generally used for traces metal analysis [12-14],

PSA offers advantages over anodic stripping voltammetry (ASV) since

oxygen removal is not necessary and PSA is less affected by surface

active compounds.

Several recent successful applications of PSA concerned the

investigation of DNA and oligonucleotide at both mercury [15,16] and

carbon based electrodes [17]. Trends are directed towards the

modification of electrodes by single strand DNA or appropriate

oligonucleotide chains for PSA detection of hybridization (complement

fixation) and interaction studies with synthetic drugs and potential toxic

(mutagenic) compounds. Further investigations are oriented towards

sensitivity improvements e.g. by using DNA dendrimers,

i.e.oligonucleotide showing numerous electroactive sites (guanine),

strongly adsorbed at carbon paste electrodes [18],

In the following parts and for clarity of the presentation it was

more appropriate to discriminate between the conventional "naked"

electrodes currently applied in electroanalysis from those which have

been subjected to modification strategies and which are still under

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intensive investigations (see table 3 for a possible classification).

Table 3. Classification and examples of solid electrodes (classical and

modified electrodes suitable for electroanalysis).

CLASSICAL (A) COMPOSITES (B) MODIFIED (C)

Platinum (Pt) Carbon black or A orB

Gold (Au) graphite +

Graphite (C) + Polymer

Glassy carbon Nujol, silicone, teflon, Polymer + catalyst

(GCE) PEG, Epoxy, Kel-F, Conducting polymer

Tin oxide Nafion, PVC, Conducting polymer +

Indium oxide Polystyrene, CA catalyst

Ruthenium oxide Chelating agent

Silver Ion exchanger

Copper Silicate

Nickel Biocatalyst

Lipid

Surfactant

(B) + Au, Pt, Ag, Ru =

Consolidated

electrodes

PEG=polyethylenglycol, PVC = polyvinylchloride, CA = cellulose

acetate.

in . VOLTAMMETRY AT CLASSICAL ELECTRODES

Voitammetry

These methods include processes in which electronic transfer

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occurs at an electrode submitted to a potential which may be constant

(amperometry) or may vary with time in a predetermined manner, and

the current is measured as a function of time or potential.

Tensammetry

Techniques where no electron transfer occurs such as in

tensammetry (measure of charge fluctuation in the electrode double

layer) offer useful mechanistic informations with regard to the strength

of analyte-electrode interaction [20] but have seen limited analytical

applications due to lack of selectivity and sensitivity [21,22]. Actually,

tensammetry is better applied to the dropping mercury electrode (DME)

unless reversible and reproducible responses are achieved with solid

electrodes. The latter has been recently reported at gold based electrodes

modified by self-assembling of thiol with attached antibodies [22,23] or

oligonucleotides for highly sensitive affinity biosensing based on

capacity changes in flow injection analysis [24],

Voltammetrv and adsorptive stripping voltammetry.

The great contribution of J. Heyrovsky and his successors in

developing and studying the DME has considerably contributed to

widespread use and development of modern electroanalysis [25]. Over

the years, polarography has taken advantage of the progresses observed

in electronics and computers. Modem instrumentation (alternating and

pulsed techniques) allows for a better discrimination between the analyte

current and the residual current (background). Meanwhile new

methodological approaches for improved sensitivy based on the "in situ"

preconcentration of the analyte at the electrode surface (adsorptive

stripping) have been successfully developed and applied [26, 27].

Presently, hanging mercury (HMDE) or mercury film electrodes allow

routinely the determination of trace metals with detection limits as low as

1 or 0.1 nmoles/1 [27], Applied to organic compounds, adsorptive

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stripping voltammetry, by judicious selection of the technique, may also

considerably improve the sensitivity of the measurements e.g. down to

10'u M as shown for the determination of the antitumors drug

mitoxantrone at the carbon paste electrdde (CPE) [28].

Trends in polarographic and voltamperometric instrumentations are

directed towards the improvement of software for better flexibility o f

potentiostat control, modulation and ease of data manipulation.

Established factories show advances in microelectrode construction and

in mercury electrode technology (better control of drop growth, new and

safer design ...). Progresses are also observed in the commercial

launching of automated and integrated systems (sample digestion +

analysis) allowing determinations with high sampling rates for trace and

ultra-trace analysis and spéciation of heavy metals and ions [29], With

respect to the latter, electroanalysis is particularly well-suited and further

innovation is expected with the development of devices for the "in situ"

remote control (no sample contamination) of trace metals in natural

waters [30], Several strategies may be considered, but rapid scan

voltammetric techniques (anodic stripping square wave voltammetry)

which are quasi insensitive to the presence of dissolved oxygen

(electrochemical reduction of the latter is a slow process) is very

promising. Microelectrodes must be used for this purpose and surface

modification strategies may be required (polymer film modified

electrode) for minimizing surface fouling during continuous monitoring

[30],

Despite some selectivity, thanks to potential adjustement and/or

applied potential wave form (AC, DPV, SWV), voltammetric analysis at

classical electrodes often suffers from poor accuracy (surface fouling,

peaks overlapping) when directly applied in complex matrices containing

mixtures of interfering electroactive compounds such as in biological

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samples (in vitro and in vivo), in waste water, drug formulations, in cell

culture media etc (Table 4). Yet, several strategies already well-

established or still under intensive development, may be applied for

improving voltammetric analysis as illustrated below.

Table 4 - Examples of some electrooxidable molecules present in serum

and urine.

URINE SERUM

Ascorbic acid Uric acid

5HIAA* Ascorbic acid

Xanthurenic acid Epinephrine

Homovanillic acid Glutathione

Uric acid Tocopherol

Serotonin Lipoic acid

Oxalic acid Ubiquinone

Tryptophan Uric acid

Tyrosine Iron(II), Cu(H)

* 5-hydroxy-indole-3-acetic acic

Chemometrv [31]

The voltammogram of a mixture of compounds i.e. curve

showing peaks overlapping, may be treated mathematically using

multivariation calibration methods. Recent reports have shown the

successful! use of the Partial Least Squares Regression (PLS)

methodology for analyzing a mixture of inorganic [32,33] and organic

compounds [34,35] in real samples [35]. PLS is a method based on factor

analysis and modeling. The optimum number of factors depends on the

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number of independently varying chemical species. Other sources of

systematic signal variations must also be considered such as noise

interaction between components (mutual influence on peak shapes), etc.

The advantage of the PLS is that the method can be used to describe

linear and non-linear systems. Actually many organic compounds exhibit

electrochemical irreversible voltammetric behavior leading to peak

potential changes with concentration.

The resolution of a mixture of two, three or four pyrazine

derivatives has been realized by applying PLS and PCR (Partial

Component Regression) methods to the voltammetric data [36],

Multicomponent polarography has been shown to be suitable for

organochlorine pesticides [32] for binary and ternary mixtures of

nitrofurane derivatives [35], Successfull, HPLC validated, simultaneous

analysis of these drugs have been performed in veterinary formulations

using the PLS methodology [35], A multivariate curve resolution has

been recently described in studies concerning zinc glutathione binding by

differential pulse polarography (DDP) [37].

Voltammetric measurements in complex matrices require

frequent surface renewing and/or cleaning which impair the selectivity of

the technique. Taking this into account a methodology using the least

median of squares (LMS) has been shown to be a robust regression

procedure for constructing a calibration curve and determining its linear

range and detection limit [38].

Worth to mention here is the approach which uses an ensemble of

microelectrodes for multi-analyte real time measurements assisted by

computer treatment of the data (see refs. 31 and 39 for reviews on

potentiometric and amperometric sensor arrays and on chemometric

techniques in electrochemistry)..

Validation o f a voltammetric method.

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One aspect which has been been often overlooked in the past by

electrochemists and which is currently mandatory for usefulness and

approval of any analytical methodology is the validation of the developed

method. In other words, electroanalytical methods must be thoroughly

described in in terms of the analytical figures of merit. For instance, with

fespect to voltammetric techniques devoted to drug analysis the

following items must be specified :

Peak identity : the technique must allow discrimination against

struturally related molecules .

Peak purity : it must allow discrimination against impurity(ies) at

around the level of interest (following official limits).

For raw materials, investigation should follow requirements of

pharmacopoeia monography limits. For formulations, data must be given

on the influence of the matrix on peak current (Ip) and potential (Ep), the

influence of side drugs on Ep and Ip must be studied and selectivity

towards degradation products must also be demonstrated.

For physiological samples, in addition to influence of matrix and side

drugs on Ep and Ip, selectivity towards metabolites must be

demonstrated.

Optical purity : the method should be able to discriminate enantiomers.

Accuracy and rudgeness : recovery tests should be realized on doped

samples. Accuracy should be confirmed by comparing with an official

independent method. Repeatability, reproducibility, linear range and

detection limit (less important for formulations) must be provided.

Hyphenated techniques

Amperometric detection is among the most sensitive measuring

technique in electroanalysis. The electrode, being polarized at a constant

potential, allows the capacitive current to reach its lowest value. As

already mentioned, selectivity (despite possible potential adjustement)

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may be insufficient. Actually, when the analyzes require a high level of

confidence in peak purity or the simultaneous determination of a large

number of compounds, alternative strategies must often be employed.

In front of the requirements mentioned above, the combined use

of electrochemical detection and a pre-separation step is highly

recommended. Electrochemical detectors may ideally be coupled on-line

with chromatography, already they have been suggested by Kemula in

1952 (chromatopolarography) [see in 25] but have emerged with the

development of reversed phase high performance liquid chromatography

(HPLC). Following the first successfull determination of

neurotransmittors at trace levels (pg) and since the successfull launching

of P.T. Kissinger's type thin-layer cell [40], many examples have shown

the applicability of EC detection in HPLC and its complementarity with

HPLC-UV detection [41], Other commercial EC detectors contain a

serial array (8 or 16) of porous electrodes (100% electrolysis) set at

incrementally higher voltages [42], Eluted compounds are detected at a

given electrode depending on their individual oxidation potential. There,

the voltage rather than wavelength (diode array) is the third dimension.

The use of an electrode array detector increases the confidence of peaks

confirmation, provided the co-eluting compounds have different

oxidation potentials. The multiple information obtained with this type of

detector requires data storage, use of peak detection and ratio algorithms

as well as appropriate signal conversion [42],

In the early eighties, a second marked impulse in the applicability

of electrochemical detection was observed with the launching of ECD

allowing sequences of potential pulses to be applied for cleaning and

activation of gold (Au) and platinum (Pt) for carbohydrates determination

[43], Actually the latter are not electroactive at carbon electrodes but at

Au and Pt. Once oxidized, however, they remain very strongly adsorbed

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at the latter precluding any further detection. Pulsed amperometric

detection in liquid chromatography has already shown many interesting

applications such as in the analysis of carbohydrates, alcohols,

aminoacids, sulfur containing pesticides etc., with a sensitivity generally

better than UV detection [44,45],

Other possible improvement in confidence of peaks purity may

arise from the use of microelectrodes and rapid voltammetric scans in a

specific voltage range [46,47] and with the use of new detectors with

quasi no dead volume and high signal/noise ratio (thin-layer or radial

flow) for microbore LC .

Microfiber based detectors (Pt, Au, C, Cu) although not yet

commercially available(because of lack of rudgeness of the sensor

design) are under extensive investigation for microflow system in LC,

FI A or in capillary zone electrophoresis (CZE) [48-52], Currently,

research in hyphenated techniques combining a separation step with

subsequent EC are mainly concerned with microelectrode based detectors

and with a better control o f the cleaning/activation step at these

electrodes.

Microelectrodes

The remarkable progresses observed in electronics permit

measurement of currents as low as picoamperes (10'12 A) allowing the

use of microelectrodes (diameter generally comprised between 1 and 25

im). Microelectrodes exhibit a high mass transfer per unit ratio, low

residual current and fast response rates.

In addition to the already above mentioned applications,

microelectrodes may be successfully used to monitor neurotransmitters

"in vivo" [53,54] or biological substrates (insuline, NO, O2, serotonine,

hydrogen peroxide) in cell cultures or on isolated cells [55-59], Particular

emphasis is currently made on the development of catalyst modified

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microelectrodes for NO [56,57] and insuline [56] monitoring on single

cells and enzyme modified microelectrodes (see below) for hydrogen

peroxide and choline detection at the low micromolar level in the

extracellular fluid of brain tissue [59],

IV. VOLTAMMETRY AT MODIFIED ELECTRODES.

Electrodes modified with a chemical component.Since the beginning of the eighties and with the pionnering work

of R.M. Murray [60] the research in modified electrodes has literally

exploded due to the number of possible applications and to the quasi

unlimited modification strategies. The surface modification consists to

elaborate a new architecture onto the electrode surface in order to confer

to the electrode the additional properties of the modifier i.e., catalysis,

selectivity and antifouling characteristics (see Table 3 for examples of

modifiers). Note that with respect to this approach, mercury film

electrodes may also be considered as modified electrodes. Yet, most of

the modifiers currently used are synthetic catalysts (organometallics,

metal oxides [61], organic conducting salts ...), polymers (polypyrrole,

polythiophene, Nafion, ...) or complexing agents for trace metal analysis

[62,63],

Polymer membranes (Teflon) have been stretched over electrode

surfaces to allow permselective diffusion and amperometric monitoring

of gases such as oxygen (Clark electrode) or, as shown with the recently

commercialized device, for the sensitive (Ippm) determination of the

endothelial relaxing factor i.e., nitric oxide (NO) [64],

Polymer membranes may help to entrap physically or chemically the

catalyst and additionally they may act as permselective barrier or as

antifouling substrates for selective analyte measurements in complex

matrices [62, 64], Several electrode substrates may be employed, the

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most frequently used, however, are carbon based. The carbon paste

electrode (CPE) has seen almost exponential use [63] since its first

description by R.N. Adams in 1969 [65], This electrode offers several

interesting features such as ease of use, and preparation, low residual

current within a large potential range, ease of renewal, no memory

effect... It is readily modified by a variety of (bio) chemicals. The CPE,

however suffers from its ease of preparation which may vary from one

laboratory to the other i.e., by the nature of the graphite particles

(differences in size, in pretreatment) and by the ratio graphite/Nujol [63].

In table 5 are listed some reagents used for preparing modified CPEs and

their possible application. Modified electrodes, once optimized, may be

used in an electrochemical detector coupled to hydrodynamic process (

e.g. in HPLC, CZE, FIA) [66, 67] or applied to the analysis of biological

components released "in vivo" [53] or released from single cells [57] and

for "in situ" remote pollution control (see above) as well as in portable

devices for metal analysis [68, 69]. It is out of scope here to present an

overview of the tremendous research activity in modified electrodes (see

Table 5 and refs 70-77]. Worth to mention, however, is the presently

limited (if no) commercial availability of modified electrodes, except

biosensors. Actually, like biosensors, modified electrodes have to face

many problems limiting their mass production [78] such as

irreproducibility of surface modification, irreversibility of the response,

instability of the modifier (catalyst, membrane), etc.

Electrodes modified with a biological component

Electrochemical biosensors are sensing devices which intimately

combine a biological element (enzyme, antibody, nucleic acid,

microorganism, ce ll...) and a physical transducer (electrode, fiber optic,

piezoelectric quartz crystal ...) which are currently extensively

investigated and described in the literature [8,78,79] and which have

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reached successful commercialisation since the pioneering work of

Clark and Lyons in 1962. The attractiveness of such a hybrid device is its

selectivity and sensitivity allowing direct and reagentless determinations

in complex matrices.

Table 5.

MODIFIER APPLICATION Ref.

Lipids Functionalization, membrane barrier 19

Surfactants Functionalization, membrane barrier 70,71

Silica Increasing adsorption, sites 72

Hydrophobic

microparticlesImproving adsorption - extraction 73

Resins Improving adsorption - extraction 74

Inorganics Catalysis 61

Organometallics Catalysis, complexation 61

Metals Catalysis 75

Alkylamines Chemical preconcentration (Shiff

bases)

76

Complexing

agents

Selective preconcentration 63 ,77

At present, biosensors are gathering a number of researchers from

almost all the scientific areas: microbiologists, organic chemists,

biochemists, electrochemists, analytical chemists, physicians, specialists

in surface science, in electronics, etc. This renewed interest can be

explained by considering the recent progresses observed in a number of

various biotechnological related areas:

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- better availability of enzymes and antibodies.

- progresses in micromachining, in membrane technology.

- use of new biosensor concepts redox mediators, catalysis, micro

devices [80],

Currently, much efforts are concerned with the development of

configurations allowing direct electron transfer between the enzyme and

the electrode surface [81, 82] and with the development of affinity

sensors with immobilized antibody and receptor [83] as well as DNA

sensors [17,18]. Following the very successfull launching of the pen

sized amperometric biosensor for glucose [80], new electrochemical

microbiosensors are under investigation for the determination of

cholesterol [84], for drugs analysis (theophylline, paracetamol,

salicylates,...) [85-87] pesticides [88], etc...

Recently, a more elaborate instrumentation comprising an

electrooptical biosensor has been launched on the market (Perkin-Elmer

[89], The apparatus is useful in molecular biology for DNA and

oligonucleotides (antigens) determination at the attomole level. The

entire system uses very sophisticated technology and the principle of

operation dates back to A.J. Bard electrochemiluminescent discovery in

the early sixthiesl. The equipment is automated and is designed for the

sensitive (through electrogenerated luminescense) and selective

(sandwich immunoassay) determination of the product (nucleic acid) of

the polymerase chain reaction (PCR). Briefly, the assay is based on the

photodetection of light emitted by electrooxidation of a labelled PCR

product. The label (tris [2,2'rbipyridine) ruthenium(II) chelate in its

oxidized form will react with tripropylamine radical (also generated at

the electrode) in the sample with emission of light at 620 nm [89,90],

Progresses in biosensor technology are oriented towards the

immobilization of enzymes in electropolymerized films for accurate

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control of the polymer film thickness and hence of the amount of enzyme

entrapped [91].

With regard to electrode modification by biological components,

new investigations are dealing with molecules mimicking the cell

membrane structure. The conventional electrodes mentioned in Table 3

have been modified by lipids [19, 92] surfactants [70, 71, 93-95] or

alkylthiols, by dip coating [96], by mixing into the CPE [70, 71] by self-

assembly [97-99], or by the Langmuir-Blodgett technique [100]. Such

devices are designed to observe parallelism with regards to drug-

membrane interaction [19] and behave as a channel mimetic sensing

membrane [100], Applications in organic and inorganic compound

analysis at trace levels have been reported [92, 93, 97, 101].

V. MISCELLANEOUSFow Injection Analysis (FIA) is well suited for electrochemical

detectors [102-104], FIA systems with amperometric detection and for

accomodating on-line solid interfaces (reactors, membranes, ...) are seing

growing interest in electroanalysis due to the high sampling rate,

automation capabilities, flexibility and reproducibility achieved [103],

Amperometric monitoring of an enzyme immunoassay (detection of the

product of enzymatic reaction) e.g. using interdigitated array electrodes

in a microelectrode cell allow rapid, accurate and sensitive determination

of theophylline throughout its therapeutic range [105,106], Other

prospects in electroanalysis concern the use of electrochemical

biosensors in organic media [107] and in microemulsion systems [108],

Use of the latter brings less pratical complication than with organic

solvents, (evaporation, toxicity). Voltammetric measurements [95,109]

may advantageously be applied in such media after organic solvent

extraction of the investigated analyte, e.g. organochloride pesticides

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[106], Mixtures of the latter have been studied in emulsified media by

differential pulse polarography based on differences in hydrolysis rates

[110].

Finally, we may point out promising trends usings cyclodextrins

and cyclodextrin inclusion complexes [111] in solution for increasing

bioelectrocatalytic efficacy [112] or immobilized at the electrode surface

[100,113] for selectivity improvements.

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