Weekly report

2009/09/28

nian

Urea impedimetric biosensor based on polymer degradation onto interdigitated electrodes

M. Cortina, M.J. Esplandiu, S. Alegret, M. del Valle ∗Sensors & Biosensors Group

Department of Chemistry, Autonomous University of Barcelona, Edifici Cn, 08193 Bellaterra, Barcelona, Catalonia, Spain Available online 19 May 2006

Abstract

The measurement of urea was accomplished with an interdigitated electrode by using ac impedance spectroscopy to follow capacitance changes, which were produced during enzyme-catalyzed dissolution of a polymer coating.

The employed coating was the enteric polymer Eudragit S-100, on which urease enzyme was immobilized by carbodiimide coupling.

Urea was determined in the 0.02–2 M range, proving that the combination of the biofunctionalized electrode and the impedance technique results in an elegant approach for urea biosensing.

1. Introduction

EIS measures the impedance (Z) of a system. The response is a current that differs in amplitude and phase with the applied voltage.

Impedance measurements have resulted in a useful tool forstudying the anodic behaviour of metals, corrosion processesand surfaces of polymer-modified electrodes

EIS （ Electrochemical impedance spectroscopy ）

The ac impedance biosensors can be applied to any biological or chemical process that implies a variation in the electrochemical characteristics of the sample under test.

R¨ohm Pharma Eudragit S-100 is an enteric polymer, a copolymer of methyl methacrylate and methacrylic acid that undergoes a breakdown process at pH values higher than 7

Eudragit S-100

The feasibility of an urea biosensor based on the measurement of the change in electrode impedance with the degradation of the Eudragit S-100 coating as a result of a urea–urease interaction.

Improvements are accomplished through the covalent enzyme immobilization to the degradable Eudragit S-100, resulting in a more close and intimate contact ofenzyme and polymer.

2.1. Apparatus ：R¨ohm Pharma Eudragit S-100 ， urease ， Urea

2.2. Apparatus ：IM6-e impedance analyzer

frequency range from 200 kHz to 50 Hz zero dc potential ac amplitude of 100 mV

2. Experimental

(1) The transducer was constructed using thick-film technology or PCB fabrication procedures(2) The enzyme was coupled to Eudragit S-100 and it was spray-coated over the working area of the electrodes.

2.3. Biosensor construction

Materials :(1) carbon and silver screen-printed (2) copper tracks

Design ：14 digits (length 6 mm, width 0.2 mm, electrode spacing 0.5 mm)

6 mm

0.2 mm

0.5 mm

0.5 cm2

2.3.1. Polymer deposition

Eudragit S-100 solid

Eudragit S-100 solution (1%)

0.1 M NaCl 3 M NaOH

3 M HCl

pH 11

pH 7.2

50 ml stored at 4 ◦C

spray

To ensure effective drying of the polymer film, the electrodes were left 24 h at 4 ◦C before use.

2.3.2. Enzyme deposition

This enzyme catalyzes the hydrolysis of urea to produce ammo-nium and bicarbonate ions

0.1 M phosphate buffer

Eudragit S-100 solution (1%)

pH 7.2

Carbodiimide (12 mg)

Centrifuged after its acidfication to pH 4.5

Enzyme(1ml)

pH 7.8 (containing 1 M NaCl)

3. Results and discussion

The drawbacks of screen-printed ：(1)High impedance contributions

(2)Poor reproducibility

Silver ： A deficient electrochemical stability

Carbon ： A low adhesion 、 A high lateral diffusion

3.1. Preliminary studies

Re ： electrolyte resistance Rct ： charge-transfer resistance Cdl ： double-layer capacitance Cg ： the geometric capacitance of the polymer Rb ： bulk resistance

3.2. Polymer dissolution

Spray over the

electrodes

An hour exposition in

the phosphate buffer solution

Be measuredsolution (pH7.2)

Eudragit

hydration process

degradation

(1)Eudragit degradation at pH greater than 7 took place by pore formation on the surface of the film

(2)The Cg of intact polymer layer increased gradually as the polymer degradation progresses

(3)This process allowed the electrolyte molecules to penetrate into the polymer and eventually reach the electrode surface, then spread over it

+ +- -

3.3. Urea measurements

Standards were prepared in a 140mM NaCl and 0.1mM (pH 6) saline solution and containing urea 0.002, 0.02, 0.2, 0.5, 1 and 2M

3.4. Real samples

Eudragit

urease

carbodiimide

electrodes

urea

urine

•Copper interdigitated electrodes had shown to be the best evaluated devices in order to monitor polymer degradationthrough impedance measurement, since a great polymer adhesion onto this material mtransducers was observed. Further-more, they provided the more reproducible and better performance.

•The polymer degradation has been successfully coupled to an enzymatic reaction with covalently immobilized biological material, achieving a biosensor very simple in concept.

• EIS measurement through interdigitated electrodes makes possible an easy monitoring of urea concentration after calibration.

•The degradation of the biosensor in the measurement step forces this design to be used for disposable devices, although the reduced cost and simplicity of fabrication makes them ideal for this purpose.

4. Conclusions

end