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Membrane-coated Optical-grating Coupler Sensors
( Eureka project no. EU 947 )

A chemical or biochemical sensor consists of a transducer (the sensing platform) coated with a membrane conferring selectivity for the species to be detected.
The first type of biosensor was based on electrochemistry: the transducer was a voltage-clamped electrode, and the selective membrane was made out of immobilized glucose oxidase. When brought into contact with a solution containing glucose, the enzyme oxidizes the sugar, producing hydrogen peroxide which is then reduced at the electrode. Although it would be no easy task to infer the dissolved glucose concentration from the measured current, using only known physico-chemical parameters of the system, the response can be calibrated with standard solutions. Despite less than optimal stability, etc., this sensor is the only one which has been fully exploited commercially to date.

Other types of transducers which have been investigated since then include the quartz crystal microbalance (QCM) and the surface plasmon resonator (SPR).
A commercial sensor based on these devices has yet to emerge. A major problem of the QCM is that the relationship between the measured output, a frequency shift of the resonant frequency of the quartz crystal, and the amount of molecules captured, is exceedingly complex when the crystal is in contact with a liquid: captured molecules may, under some circumstances, lead to an increase in resonant frequency. The SPR has some similarities to sensors based on integrated optics, but is at least an order of magnitude less sensitive (W. Lukosz, Biosensors Bioelectronics 6 (1991) 215-225), and thus may be considered to be obsolescent technology. The new generation of biosensors will undoubtedly be based on optical techniques (J.J. Ramsden, Optical Biosensors. J. molec. Recognition 10 (1997) 109--120), which offer not only unparalleled sensitivity and versatility, but can also be directly connected with optical computers, a step towards which has already been taken with the totally integrated optical sensor (TIOS) concept (R.E. Kunz, (1992) Totally integrated optical measuring sensors. Proc. SPIE 1587, 98--113).

MEMOCS is concentrating on developing sensing pads, which can either be used as stand alone devices coupled to an external reader (research intruments exist already), or as an integral part of a future TIOS device. The core of MEMOCS technology is the grating coupler sensor, comprising a diffraction grating embedded in a thin planar optical waveguide. The energies of the discrete guided modes shift according to the amount of molecules captured at the waveguide surface. From the measured shifts it is possible to precisely calculate the number of captured molecules without calibrating the system or introducing ad hoc assumptions. Millisecond time response is achievable. The current detection limit is around 10 picogram per square millimeter.

The main current topics under investigation are:

optimizing grating coupler and waveguide design in order to maximize response;

designing robust capture layers in order to enhance selectivity;

integration capture layer and grating coupler more effectively.

The partnership comprises the following:
(in alphabetical order, contact person names in parenthesis):

Artificial Sensing Instruments ASI (Dr. Tiefenthaler) integrated optical scanner manufacture and development;

ATKI, Budapest (Dr. Szabo) waveguide and grating coupler manufacture and development;

Biozentrum, Basel (Dr. Ramsden) Coordination, self-assembly, Langmuir-Blodgett technology, fundamentals of adsorption and detection;

Boehringer Mannheim, Tutzing (Dr. Sluka) Applications in clinical diagnostics;

CSEM, Neuchatel (Dr. Sigrist) Photoimmobilized protein capture layers;

ICB, Munster (Mr Orban) Covalently bound protein capture layers;

IMC, Prague (Dr. Brynda) Crosslinked protein capture layers;

MicroVacuum Ltd., Budapest (Dr. Szendro) waveguide and grating coupler manufacture and development;

Nima Technology, Coventry (Dr. Grunfeld) Manufacture and development of Langmuir-Blodgett equipment;

Zurich University (Prof. Bosshard) Antibody-antigen interactions.

We welcome enquiries from prospective new partners, both industrial and academic. The current expiration date of MEMOCS is 31 December 2001. Although initial applications are foreseen in the medical field, industrial and environmental uses are equally appropriate.


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