Polish Patent:

Dybko A., Wróblewski W., Maciejewski J., Brzózka Z., Romaniuk R.

Method of preparation of optomembrane for fiber optic chemical sensors

No. PL 174935 B1, issued on 30.10.1998

Fiber optic chemical sensors (FOCS) are based on immobilization of an appropriate reagent in an optomembrane. Development of the immobilization procedure is the basic problem in the design of an optrode. The immobilization procedure should meet several requirements. The reagent used should not be washed out during the measurements. Its optical properties should not be changed and the indicator should still work properly after immobilization. The optomembrane should be compatible with fiber optics and a sample under tests.

Many different types of chemical sensing layers for FOCS have been developed. A few types of dye immobilization techniques are possible: the covalent chemical binding (to the surface or to the matrix of the membrane), physicochemical adsorption and physical entrapment of the reagent (for example, in the sol-gel matrix). Frequently several phenomena occur simultaneously.

Our work combines two types of membrane preparation using polymer track membrane (PTM). PTM can be made of various kind of polymers: polypropylene, polycarbonate, polyethyleneterephthalate, polyvinylidene fluoride, cellulose esters (nitrate or acetate) etc. The polymer foil is irradiated with high energy ions and tracks after passing ion beams are etched out. PTM has high porosity and the diameter of the pores can be controlled during their preparation. Such membranes are commonly used for microfiltration in medicine, biotechnology, electronics. The application of PTM causes the increase of the active surface of the membrane for reagent immobilization and thus increase the amount of incorporated indicator.

Due to high porosity PTM has greater surface than a plain membrane (i.e. without pores). A rough approximation of the gain in the active surface of PTM can be calculated with an assumption that micropores have cylindrical shape. Although the shape of the pores is quite regular the walls of the pores are not smooth and the total surface is greater than calculated. The gain in the surface is dependent on the number of the pores per square centimeter, the diameter of the pores and the thickness of the membrane. The total active surface can be expressed as:

SPTM = SPL - SH + SC

where : SPTM - total active surface of PTM,

SPL - surface of plain membrane,

SH - surface of the holes,

SC - lateral surface of cylindrical pores.

It can be calculated that total active surface of PTM is greater than plain membrane if the thickness of the membrane is greater than the radius of the pores. This condition is fulfilled for the most of the membranes because the predominantly PTM have a thickness greater than 10 um and pores diameters are in the range of 1 um. The calculated ratio SPTM / SPL for our membrane is 235. Larger active surface allows to immobilize the large amount of the indicator which leads to the high dynamic range of the changes of the output signal.

A microporous polyethyleneterephthalate (PETP) membrane was chosen as a polymeric support. The PETP membrane of the thickness of 30 um, with micropores with the diameter of 0.2 um was used in the experiments. A piece of PETP membrane was immersed into tetrahydrofurane (THF). Then the pressure in the beaker was reduced by means of a water pump in order to remove the air from the micropores and to allow the solvent to fill them. After 5 minutes, the membrane was dipped into a saturated solution of an indicator in THF containing 25 mg of poly(vinyl chloride) (PVC), (high molecular weight, Fluka). PVC is a well-known polymer used for the preparation of hydrophillic membranes and its good permeability for many ions has been proved in many cases. The beaker was sealed and the membrane was left standing for 2 hours immersed in this solution. The membrane was then dried at the room temperature. Before the use, the membrane was kept overnight in distilled water.

Microphotograph of PETP membrane covered with PVC.