Ernö Pretsch – Symposium

Development and Application of Chemical Sensors The Swiss Chemical Society (Division of Analytical Chemistry)

June 28 – 29, 2007 CH-8093 ETH Zürich, Switzerland

CHN building, Room C14

Thursday, 28 June 07 Bernhard Wehrli 10:00 - 10:30 Welcome and introduction

Róbert E. Gyurcsányi 10:30 - 11:00 Hybridization-modulated ion fluxes through peptide-nucleic-acid-functionalized gold nanotubes. A new approach to quantitative label-free DNA analysis

Philippe Bühlmann 11:00 - 11:30 Ion-selective electrodes with three-dimensionally ordered macroporous (3DOM) carbon as solid contact

Ernö Pretsch Climbing the gate 11:30 - 12:30

lunch break 12:30 - 14:00 Milena Koudelka-Hep 14:00 - 14:30 Microelectrode arrays: technology and applications

Marylou Tercier-Waeber 14:30 - 15:00 Voltammetric microsensors and submersible probes for reliable efficient environmental trace metal analysis and speciation

George W. Luther III 15:00 - 15:30 Voltammetric solid state (micro)electrodes as chemical sensors to understand redox processes: from sediments and microbial mats to hydrothermal vents

coffee break, Posters 15:30 - 16:30 Mark E. Meyerhoff 16:30 - 17:30 Electrochemical sensors for nitrosothiol species based on immobilized chemical/biochemical catalysts: design and biomedical applications

Ernö Lindner 17:30 - 18:00 Mathematical model of current polarized ionophore-based ion-selective membranes

Conference Dinner 20:00 - 22:00

Friday, 29 June 07 Magdalena Maj-Zurawska 9:00 - 9:30 Screen printed electrochemical DNA biosensors in dsDNA testing. Interactions with various chemical compounds

Joseph Wang 9:30 - 10:00 Nanomaterials for monitoring and controlling biomolecular interactions

coffee break, Posters 10:00 - 11:00 Peter C. Hauser 11:00 - 11:30 Contactless conductivity detection for microseparation techniques

Niels Peter Revsbech 11:30 - 12:00 In situ analysis of marine environments by amperometric gas microsensors and microscale biosensors

lunch break 12:00 - 13:30 William Davison 13:30 - 14:00 New developments in aquatic and sediment chemistry stimulated by the use of in situ dynamic measurements using DGT

Eric Bakker 14:00 - 15:00 Good times with ions and membranes: a friendship and a legacy

end of conference 15:00

Hybridization-modulated ion fluxes through peptide-nucleic-acid-functionalized gold nanotubes. A new approach to quantitative label-free DNA analysis

Róbert E. Gyurcsányi,† # Gyula Jágerszki,† Lajos Höfler,† Ernő Pretsch‡ † Department of Inorganic and Analytical Chemistry, #Research Group for Technical Analytical Chemistry of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, Szt. Gellért tér, H-1111 Budapest, Hungary (e-mail: robertgy@mail.bme.hu), and ‡Laboratorium für Organische Chemie, ETH Zürich, HCI E 313, CH-8093 Zürich, Switzerland

The extremely low detection limits required for bioanalysis are usually achieved with some

chemical or physical amplification mechanism. This lecture will present the use and synthesis of

chemically modified nanotubes/nanopores as ion-channel mimetic sensing platforms for label-free

detection of proteins1 and nucleic acids2. A special emphasis will be given to gold nanotubes

chemically modified with peptide nucleic acid (PNA) and to their use for label-free quantification of

complementary DNA sequences. Selective binding of DNA to the PNA modified nanotubes are

shown to decrease the flux of the detected ionic markers through the nanotubes in a

concentration-dependent manner. The strong dependence of the biorecognition-modulated ion

transport through the nanopores on the ionic strength suggests a dominantly electrostatic

exclusion mechanism of the ion flux decrease as a result of DNA binding to the PNA-modified

nanopores.

References (1) Gyurcsányi, R. E.; Vigassy, T.; Pretsch, E. Chemical Communications 2003, 2560-2561. (2) Jágerszki, G.; Gyurcsányi, R. E.; Höfler, L.; Pretsch, E. Nano Lett. 2007, 10.1021/nl0705438.

Schematics of a chemically modified nanopore Scanning electron micrograph of gold nanotubes

60 nm

Ion-selective electrodes with three-dimensionally ordered macroporous (3DOM) carbon as solid contact

Chun-Ze Lai, Melissa A. Fierke, Andreas Stein, Philippe Bühlmann University of Minnesota, Dept. of Chemistry, 3 Smith Hall, 207 Pleasant St., SE, MN 55455, Minneapolis, USA. Phone: 001 612-624-1431, e-mail: buhlmann@chem.umn.edu, http://www.chem.umn.edu/groups/buhlmann/CVBuhlmann.html

Electrodes with three-dimensionally ordered macroporous (3DOM) carbon as the intermediate

layer between an ionophore-doped solvent polymeric membrane and a metal contact are

presented as a novel approach to solid-contact ion-selective electrodes (SC-ISEs). Due to the

well-interconnected pore and wall structure of 3DOM carbon, filling of the 3DOM pores with an

electrolyte solution results in a nanostructured material that exhibits high ionic and electric

conductivity. The long-term drift of freshly prepared SC-ISEs with 3DOM carbon contacts is only

11.7 µV/h, and does not increase when in contact with solution for one month, making this the

most stable SC-ISE reported so far. The electrodes show good resistance to the interference from

oxygen. Moreover, in contrast to previously reported SC-ISEs with conducting polymers as the

intermediate layer, 3DOM carbon is an electron conductor rather than a semiconductor,

eliminating any light interference.

Besides reporting on this new type of solid contacts, this presentation will also discuss some the

challenges and ambiguities that arise in the quantification of long-term drifts. It will also address

the heretic question whether a good reference electrode really requires the presence of a redox

couple at the conductor interface.

Climbing the gate

Ernö Pretsch Laboratorium für Organische Chemie, ETH Zürich, CH-8093 Zürich, Switzerland. E-mail: pretsche@ethz.ch.

At that time the gates of my Oxford college were locked at twenty minutes past midnight. Any undergraduate who wanted to come back into the place after that time did so by climbing in. ...

The first wall was rather difficult. I got over it and went forward until I came to the second wall which was about the same height as the first wall. I climbed this second wall, only to find myself outside again. My double effort had involved my climbing in and out across a corner.

I started again and with more careful direction came up to the proper second wall. There was an iron gate in this second wall, as the gate was lower than the rest of the wall and also offered better footholds, I climbed the gate. As I was sitting astride the top of the gate it swung open. It had never been closed.

Edward de Bono, The Mechanism of Mind. Cape, London, 1969.

In the first part of this talk, recent results in the field of potentiometric sensors will be reviewed.

They involve spectacular improvements in their performance, both in terms of lower detection limit

and selectivity behavior. Low detection limits achieved in small analyte samples allow for the

detection of subfemtomole amounts of ions. This progress is, among others, relevant for

potentiometric immunoassays based on nanoparticle labels including quantum dots.

Various approaches in non-steady-state methods open up a variety of novel applications. This

brings potentiometry closer to traditional electrochemistry and, besides potentiometric bioanalysis,

currently is the most exciting field.

Much of the novel developments would have been possible decades ago. However, they only

emerged along with a change of view in potentiometry with ion-selective electrodes. This will be

illustrated in the second part of the talk, which focuses on some historical developments.

Microelectrode arrays: technology and applications

L. Berdondini, O. Frey, S. Generelli, O. Guenat*, K. Imfeld, P. van der Wal, M. Koudelka-Hep Institute of Microtechnology, University of Neuchâtel, Switzerland. Phone: 0041 (0)32 720 53 05, e-mail: milena.koudelka@unine.ch, http://www-samlab.unine.ch/people/perso.asp?ID=2. *Ecole Polytechnique de Montréal, Eng. Physics, Canada.

Over recent years, the use of microelectrode arrays (MEAs) has steadily expanded in many

(bio)electroanalytical areas, including applications in environmental and biomedical analysis, life

science R&D, chemical and bioprocess control.

To illustrate the advances in this field, three examples of our current research will be presented.

The first one concerns the development of thin-film MEA-based cellular interfaces for monitoring

the electrophysiological activity of neuronal networks. Such bioeletronic interfacing platforms,

providing more biologically relevant data and higher information content compared to conventional

techniques, are now largely accepted methodology in both fundamental and applied

neurophysiological research. Realization of thin-film Pt-MEAs integrated with clustering structures

and CMOS-based high-density arrays and their functional characteristics for electrophysiological

recordings of neuronal and cardiomyocyte cell cultures will be discussed.

The second example is the development of a toxicological platform for monitoring cell responses

as a function of chemical or drug exposure. The platform comprises an array of 16 ion-selective

microelectrodes (K+, Ca2+ or NH4+ - µISEs) located at the bottom of a 200 µm wide and 350 µm

deep microchannel, in which hepatocyte cells are cultured. The fabrication of the platform and the

functional characteristics of the µISEs will be presented.

The third example concerns the development of two biosensors aimed at in-vivo monitoring of

choline and L-glutamate in brain extracellular fluid. The enzymatic amperometric biosensors (Pt

electrode dimensions of 50 µm by 150 µm) are implemented on a several millimeter long Si/Si3N4

probes. Realization of the biosensors and their functional characteristics, in particular the

optimization of a poly-(o-phenylenediamine) layer anti-interference properties will be presented.

Voltammetric microsensors and submersible probes for reliable efficient environmental trace metal analysis and speciation.

Marylou Tercier-Waeber Group of Analytical and Bio-physicochemical Environmental Chemistry, Department of Inorganic and Analytical Chemistry, University of Geneva, Sciences II, 30 Quai E.-Ansermet, CH-1211 Geneva 4, Switzerland. E-mail: Marie-Louise.Tercier@cabe.unige.ch

The contamination of ecosystems (water, sediment, soil) by anthropogenic and natural inputs of

metals has lead to the need for understanding better their fate and toxicity in the environment. To

a large extent, these two parameters are related to the chemical speciation of these contaminants,

which may vary continuously in space and time. Detailed measurements of the proportion of

specific metal species or groups of homologous metal species, and their variation as a function of

the bio-physicochemical conditions of the natural media, is thus of prime importance. To make

such studies, new analytical tools capable of performing in situ, real-time monitoring with minimum

perturbation of the media are required.

The characteristics of voltammetric techniques make them particularly well suited for such

measurements provided new specific technical, analytical and conceptual developments are done.

This will be illustrated by presenting an overview of the advances/achievements made over the

last 15 years of our research in the development of rugged, reliable voltammetric (i) microsensors,

(ii) modified microelectrodes, (iii) mini- and micro-integrated analytical systems and the coupling of

these devices to submersible probes for in situ voltammetric trace metal analysis and speciation.

The significant advantages of these new tools vs traditional laboratory techniques in term of spatial

and temporal resolution of data, and thus cost effective, more efficient environmental monitoring of

the role and the fate of relevant metal species, will be illustrated by several examples of

applications in various environmental media.

References: Noël S., Tercier-Waeber M.-L.*, Lin L., Buffle J., Guenat O., Koudelka-Hep M. Integrated micro-analytical system for direct simultaneous voltammetric measurements of free metal ion concentrations in natural waters. Electroanalysis, 18 (2006) 2061-2069. Tercier-Waeber M.-L.*, Confalonieri F., Riccardi G., Sina A., Noël S., Buffle J., Graziottin F. Multi Physical-Chemical Profiler for real-time in situ monitoring of trace metal speciation and master variables: development, validation and field application. Mar. Chem., 97 (2005) 216-235. Buffle J.*, Tercier-Waeber M.-L. Voltammetric environmental trace-metal analysis and speciation : from laboratory to in situ measurements. Trends in Analytical Chemistry, 24 (2005) 172-191.

Voltammetric solid state (micro)electrodes as chemical sensors to understand redox processes: from sediments and microbial mats to hydrothermal vents.

George W. Luther, III, Katherine M. Mullaugh, Tommy S. Moore, Shufen Ma and Robert E. Trouwborst College of Marine and Earth Studies, University of Delaware, Lewes, DE 19958, USA. Phone: 001 (302) 645-4208, e-mail: luther@udel.edu, http://www.ocean.udel.edu/cms/gluther/

We have used solid-state Au/Hg voltammetric electrodes to understand redox and biogeochemical

processes in a variety of freshwater and marine environments. These electrodes are non-specific

and have the capability of measuring a suite of chemical species including several of the principal

redox species involved in early diagenesis (O2, Mn2+, Fe2+, H2S/HS-, and I-) as well as some Fe

species [FeS and Fe(III)] and sulfur species [Sx2- and S2O3

2-]. Here we demonstrate how in situ

data obtained in complex environments can be used to study specific iron and sulfur reactions and

processes at (sub)millimeter to centimeter resolution. Examples include the oxidation of Fe2+ by

MnO2 in freshwater wetland sediments and by O2 produced by cyanobacterial mats in Yellowstone

National Park hot springs, and the formation of S2O32- in diffuse flow waters from the hydrothermal

vents at Lau Basin. In one example, we will show how kinetic data can be obtained in situ and

used to understand the interactions between biology and chemistry.

Electrochemical sensors for nitrosothiol species based on immobilized chemical/biochemical catalysts: design and biomedical applications

Mark E. Meyerhoff The University of Michigan, Ann Arbor, MI 48109-1055, USA. Phone: 001 734 763 5916, e-mail: mmeyerho@umich.edu, http://www.umich.edu/~michchem/faculty/meyerhoff/index.html

Nitrosothiols (RSNO), including nitrosoglutathione (GSNO) and nitrosocysteine (CysNO), exist in

fresh blood and are formed from the reaction of corresponding thiols with oxidation intermediates

of nitric oxide (NO) produced by endothelial (EC) and other cells. Such RSNO species are thought

to serve as stable carriers of NO in the bloodstream. RSNOs can also serve as substrates to

generate NO at the surface of new catalytic thromboresistant polymers as well as the surface of

platelets, the cells in blood that must be activated to initiate clot formation. At both type of

surfaces, generation of locally enhanced concentrations of NO serves to inhibit platelet adhesion

and activation. Detection of total RSNO levels in blood may serve as an indicator of NO production

by the EC, and also may control endogenous platelet function (i.e., higher RSNO levels serve as

natural anti-platelet agents). Hence, measurement of RSNO levels could prove to have significant

predictive value to assess the risk of thrombotic events (e.g., heart attacks, strokes, etc.) for

patients. In this presentation, the development of new amperometric RSNO sensors will be

described based on use of Cu(II) or organoselenium catalytic species as well as certain enzymes

immobilized at the surface of previously reported electrochemical NO detectors. It will be shown

that reversible sensors for monitoring RSNO levels can be prepared with detection limits in the

0.01 - 10 µM range. It will be further demonstrated that these new sensors can be used to detect

RSNO levels in whole blood samples.

Mathematical model of current polarized ionophore-based ion-selective membranes

Erno Lindner, Justin M. Zook and Richard P. Buck Department of Biomedical Engineering, 330 Engineering Technology, The University of Memphis, Memphis TN 38152. Phone: (901) 678-5641, e-mail: elindner@memphis.edu

A mathematical model is presented to describe the effects of constant current on ion-selective

membranes using theta functions. The model provides exact analytic solutions for calculating the

concentration polarization of the ionophore, the ionophore-ion complex, and the charged mobile

sites in space and time within the membrane. It also predicts the time course of the membrane

potential and the electric field inside the membrane following the application of constant current.

This analytic solution is faster to compute than the numerical simulations and provides the solution

for any given time or position directly. The simulated concentration profiles compare favorably with

concentration profiles recorded experimentally using spectro-electrochemical microscopy

(SpECM).

Screen printed electrochemical DNA biosensors in dsDNA testing. Interactions with various chemical compounds.

Iwona Szpakowska, Magdalena Maj-Zurawska University of Warsaw, Faculty of Chemistry, Pasteura 1, 02 093 Warsaw, Poland

Many molecules show high affinity to nucleic acids as they interact with DNA by several

mechanisms [1]. They can join nucleic acids and inhibit basic functions of the living cells such as:

replication, transcription and translation. They can block interactions of DNA with specific, required

proteins, or can prevent the DNA from adopting conformations required for biological functions. To

analytical chemists, nucleic acids offer a powerful tool in recognition and monitoring of many

important compounds, e.g. toxic molecules and anticancer agents [2-4]. The DNA biosensor

technologies are currently under intensive investigation owing to their great applicability in rapid

and low-cost detection of specific DNA interactions. Electrochemical techniques are frequently

used for the detection of nucleic acid bases. Signals related to the oxidation of these bases are

sensitive to their interactions with various molecules. Compounds interacting with DNA change the

oxidation ability of nucleic acid bases. The interactions between DNA and tested chemical

compounds can cause chemical and conformational modifications of nucleic acids, and

consequently, the variation of the electrochemical properties of DNA. The presence of these

compounds can be measured by their effect on the oxidation of adenine and guanine bases, as

these compounds change the heights of bases oxidation peaks. Electrochemical screen printed

biosensors contain double stranded DNA immobilized on the working electrode surface. Then, the

interacting molecules are immobilized on the biosensor surface by immersing the electrode in the

sample solution. The electrochemical DNA biosensor indicates the degree of binding of molecules

to deoxyribonucleic acids. The measurement time is less than 10 minutes, the instrument needed

is inexpensive, and can be portable. The tests provided with this kind of sensors can give fast and

reliable information about affinity of chemical compounds and their behavior in the presence of

DNA. These tests can be helpful in investigation of new anticancer drugs before performing very

expensive biological tests and analyses on promising compounds.

References [1] S. Rauf et al., J. Pharm. Biomed. Anal., 2005, 37, 205. [2] G. Bagni et al., Curr. Pharm. Anal., 2005, 3 (1), 217. [3] D. Maciejewska et al., Bioelectrochemistry, 2006, 69, 1. [4] I. Szpakowska et al., Electroanalysis, 2006,18, 1422.

Nanomaterials for monitoring and controlling biomolecular interactions

Joseph Wang, Biodesign Institute, ASU, Tempe, AZ 85287, USA. E-mail: joseph.wang@asu.edu.

The unique properties of nanoscale material (nanoparticles, nanowires and nanotubes) offer

excellent prospects for the monitoring and controlling biomolecular interactions. This presentation

will describe new multi-amplification/multi-tag particle-based bioelectronic assays based on a

variety of new biomaterial-nanoparticle assemblies. We will also describe novel nanowire-based

strategies for controlling bioelectrocatalytic reactions (Figure 1).

In particular, combining the catalytic enlargement of the metal-particle tags with the effective ‘built-

in’ amplification of electrochemical stripping analysis, paved the way to remarkably low (fmol)

detection limits. New amplification platforms for carrying numerous redox tags, including carbon

nanotubes and polystyrene beads, will be discussed in connection to ultrasensitive detection of

proteins and nucleic acids. Such nanomaterial ‘carriers’ maximize the number of tags captured per

binding event. The high sensitivity of the new protocols was combined with an efficient magnetic

separation. The use of nanowires as tracers and barcodes will also be discussed. The use of

magnetic beads has been extremely useful for discriminating against unwanted constituents,

including a large excess of co-existing mismatched and non-complementary oligomers,

chromosomal DNA, RNA and proteins. TEM imaging has indicated that the DNA hybrid links the

metal nanoparticles to the magnetic beads. A new electrochemical coding bioassay, based on

different inorganic-colloid (quantum dots) nanocrystal tracers, whose metal components yield well-

resolved highly sensitive stripping voltammetric signals for the corresponding proteins, DNA and

glycans targets, will be described. The use of aptamers for nanoparticle-based detection of

multiple proteins will also be discussed. Finally, new adaptive nanowires capable of providing a

semi-analog control of bioelectrochemical processes will be illustrated. Such nanowire-based

mediated activation of bioelectrocatalytic reactions offers great promise for regulating the

operation of biofuel cells, bioreactors, and biosensing devices in response to specific needs.

Contactless conductivity detection for microseparation techniques

Peter C. Hauser Department of Chemistry, University of Basel, Spitalstrasse 51, 4004 Basel, Switzerland. Phone: ++41 (0)61 267 10 03, e-mail: Peter.Hauser@unibas.ch.

The application of contactless conductivity detection in capillary electrophoresis, on-chip-

electrophoresis and chromatographic separations has been further developed. The method

generally allows the facile detection of all charged species with good sensitivity.

The separation and detection of the underivatized enantiomers of small non-UV-absorbing amines

of interest in asymmetric synthesis has been demonstrated as has been the determination of the

antibiotic tobramycin in serum. The latter application should prove useful in therapeutic drug

monitoring. A further possible clinical method is the determination of the electrically neutral

species urea after enzymatic conversion to ammonia.

A new portable and all-battery powered CE-instrument has been designed and tested in the

Tasmanian wilderness. A further instrumental advance has been made by coupling CE with

sequential injection analysis (SIA) for automated injection and capillary flushing. The approach

should be useful in process analysis. Highly rapid separations in short conventional capillaries are

also feasible with this arrangement.

Fast separations are also achieved in planar devices and the determination of biochemical species

such as underivatized amino acids, peptides and proteins is possible on this platform with an

external conductivity detector. The analysis of beverage samples for inorganic and organic

species is also feasible with such electrophoresis chips.

Contactless conductivity detection was furthermore shown to be suitable for detection in HPLC for

the quantification of non-UV-absorbing species. The method is, within limits, compatible with

gradient elution and suitable for detection when using micro-scale monolithic columns.

In situ analysis of marine environments by amperometric gas microsensors and microscale biosensors

Niels Peter Revsbech Department of Biological Sciences, Microbiology, University of Aarhus, DK-8000 Aarhus C, Denmark. Phone: 8942 3244, e-mail: revsbech@biology.au.dk, http://www.biology.au.dk/revsbech.da.htm.

Sensor analysis of natural aquatic environments such as marine sediments can be problematic

due to highly variable concentrations of interfering chemical species. It is thus as an example often

difficult to use ion-selective electrodes. Sensors based on gas-permeable membranes such as

Clark-type oxygen sensors have the advantage that ions and most large molecules are excluded

from the sensor interior, and several types of microsensors of this type have now been

successfully applied for in situ analysis of marine sediments and other types of aquatic

environments. The first microsensors to be used for environmental analysis were oxygen

microsensors, and extensive use of these sensors has significantly expanded our knowledge

about chemical and biological transformations in stratified environments such as sediments.

Recently it has been possible to make composite oxygen microsensors with a porous cathode in

front of the sensing cathode. By polarization of the front porous cathode, oxygen does not reach

the sensing cathode, and a very precise zero point calibration is obtained. By this approach

reliable oxygen readings down to 10 nM can be obtained. Such sensors have proven valuable for

in situ monitoring of very low oxygen concentrations, where signal drift by conventional oxygen

sensors or optodes generally does not allow reliable detection of concentrations below 1 µM.

Other types of highly reliable and long-term stable microsensors include a H2S microsensor and a

N2O microsensor.

During the last decade microscale biosensors for CH4, NO2-, and NO2

- + NO3- have been

developed in my laboratory. They are all based on bacteria immobilized in front of a gas sensor.

The methane biosensor has an internal reservoir of oxygen, and an internal oxygen microsensor

continuously monitors changes in the oxygen gradient within the sensor caused by bacterial

methane oxidation. This sensor thus only functions under externally anoxic conditions. The

measuring principle allows detection of methane concentrations from 2-5 µM and up to saturation.

The temperature range is, however, only from about 15-35°C. The NO2- and NO2

- + NO3-

biosensors both contain a N2O microsensor, and the bacteria in the tip of the biosensors convert

either NO2- or NO2

- + NO3- to N2O. Bacteria isolated from a Greenland freshwater spring now allow

this type of sensor to be used in a temperature range of 0 to 30°C, and with a detection limit of

down to 0.1 µM. A special electrophoretic principle allows the same sensor to be used in all

concentration ranges up to 100 mM. Contamination of the bacterial cultures within the biosensors

may lead to interferences or loss of sensitivity. Oxidation of hydrogen by such contaminating

bacteria may thus lead to hydrogen sensitivity by the methane biosensors, and contamination of

the NO2- biosensor with environmentally abundant NO3

- reducing bacteria may convert it to a NO2-

+ NO3- biosensor. Hydrogen sulfide interferes with all 3 types of biosensors, and some unknown

presumably sulfur species has been observed to cause interference on NO3- determinations in a

marine mud volcano sediment. The lifetime of the biosensors is theoretically in the order of

months, but until now problems with bacterial immobilization in the microsensor tips have resulted

in lifetimes of only a few days. Macroscale NO3- biosensors generally have lifetimes of a few

weeks.

Literature: Kühl, M., and N.P. Revsbech. 2000. Biogeochemical microsensors for boundary layer studies. In: The benthic boundary layer: Transport Processes and Biogeochemistry (P. Boudreau and B.B. Jørgensen, eds.), p. 180-210. Oxford University Press. Oxford. Revsbech, N.P., T. Kjær, L. Damgaard, J. Lorenzen, and L.H. Larsen. 2000. Biosensors for analysis of water, sludge, and sediments with emphasis on microscale biosensors (J. Buffle and G. Horvai, eds.) In situ monitoring of aquatic systems: Chemical analysis and speciation, pp. 195-222. Wiley, New York.

New developments in aquatic and sediment chemistry stimulated by the use of in situ dynamic measurements using DGT

William Davison Environmental Science Department, Lancaster University, Bailrigg, Lancaster, LA1 4YQ, United Kingdom. Phone: 0044 (0)1524 593935. E-mail: w.davison@lancaster.ac.uk, http://www.es.lancs.ac.uk/wdgroup/members/davison.htm

The technique of DGT (diffusive gradients in thin-films) measures directly the flux of the analyte to

a binding phase after it has passed through a well-defined physically constrained diffusion layer. It

is a dynamic sensor, as the flux depends on both the concentration in solution and its rate of

supply. A systematically developed theoretical basis for this simple device has facilitating its use

for diverse in situ measurements.

The mass of metal that accumulates in situ is determined by its transport through the diffusion

layer. Metal complexes that are sufficiently labile and mobile to dissociate in this layer contribute to

the accumulated mass, while slowly dissociating metal complexes make only a marginal

contribution. The time available for dissociation depends on the thickness of the diffusion layer.

When in situ measurements were made in a river with high DOC (15 mg L-1), using DGT with a

range of gel layer thicknesses, clear kinetic effects were apparent for some metals. The

dissociation rates of complexes of Cd, Zn and Pb were too fast to measure by DGT, while there

was appreciable kinetic limitation of the dissociation of the complexes of Fe and Al. The metals

Mn, Co, Ni and Cu represented an intermediate case, with the kinetic influence increasing

systematically from Mn to Cu. Dissociation rate constants, obtained from the DGT measurements

using an analytical solution of the system’s reaction and transport equations, were consistent with

the Eigen mechanism for complex dissociation and current understanding of the nature of metal

ion binding to fulvic substances. At the low, background concentrations of trace metals that were

measured, metals appear largely to occupy the stronger fraction of binding sites.

DGT has also been used to provide 2-dimensional images of the mobilisation of metals and

sulphides at microniches in sediments. Such images have stimulated the consideration of

diagenetic changes occurring locally in sediments and led to the development of a 3-dimensional

model of reaction and transport.

Good times with ions and membranes: a friendship and a legacy

Eric Bakker Purdue University, West Lafayette, IN 47907 USA. Phone: 001 765-494-5320, E-mail: bakkere@purdue.edu, http://www.chem.purdue.edu/people/faculty/faculty.asp?itemID=73

This talk will touch on the key results of the productive collaboration with Erno Pretsch of the past

14 years, perhaps intermixed with some anecdotes and photographs. Early in the cooperative

work with Erno, emphasis was directed to the simplification of ion-selective electrode membrane

theory by using thermodynamic principles of response. This resulted in some key works that are

still highly cited today. Soon afterwards, discrepancies between theoretical expectations and

experimental results, especially with measurements at low concentrations and with polyion

sensors, gave rise to the field of nonequilibrium potentiometry that is still the dominant direction of

research in the field.