Polymeric micro-channel-based functionalisation system for micro-cantilevers

Abstract

A micro-channel system intended for functionalising micro-cantilevers used for biochemical detection has been designed, realised and characterised. The chip is fabricated in the resist SU-8, which is a photosensitive polymer offering a fast, cost-efficient and easy processing. Cantilevers present a new platform for biochemical detection with a high sensitivity. This micro-channel system is used for treating such a cantilever array with probe molecules prior to detection and it can also be used as the analysis set-up. The design of the micro-channel system provides a simple way of coating separate sections of a cantilever array with different probe molecules. The use of this channel system eliminates the time-consuming step of addressing each cantilever individually to obtain an array functionalised with different probe molecules. Using this micro-channel system in combination with a cantilever array establishes a method of detecting multiple target molecules simultaneously. Fluorescence experiments have been performed to show the working principle of the micro-channel system in combination with a cantilever array.

Introduction

New methods of detecting biochemical molecules at a low concentration in a fast procedure are currently a field of high focus within the micro- and nanofabrication areas. The aim of the research is to be able to detect specific molecules at a low molecular concentration. This would be advantageous in food production, where screening for toxic agents is necessary, as well as in pharmaceutical research and medical analysis.

The idea of shrinking the size of existing detection systems to fit on a single chip was first formulated by Manz [1] and has since been named micro total analysis system (μTAS) or Lab-on-a-chip. The advantages achieved are a decrease in sample volume needed as well as in the volume of reagents required and the time needed for the analysis can be lessened. The fabrication cost of the chip can be greatly reduced, leading to the possibility of making these chips single use, which would be necessary in a hospital environment. As the chips would be very small, they can be made portable. Micrometer-sized cantilevers offer a new method for detecting biomolecules in solution at a low molecular concentration. A differential surface stress is generated from the selective binding of target molecules onto only one side of the cantilever [2]. This results in a bending of the cantilever, which can be detected by the optical leverage principle, similar to the technique used in atomic force microscopy (AFM) [3]. For example, cantilever-based sensors have been used to detect pesticides such as DDT [4]. For these measurements, cantilevers fabricated in SiN were used.

We have fabricated arrays of cantilevers in the polymeric material SU-8 and previously presented experimental results performed with these, where cystamine was immobilised onto the cantilever [5]. By using a polymeric material, one would obtain a larger cantilever deflection for a given biochemical reaction, resulting in an improved sensitivity. The cantilevers fabricated here are structured into an array, which makes simultaneous multiple detections possible by functionalising separate cantilevers with different probe molecules.

To the authors’ knowledge, no method has yet been presented of obtaining the binding of different probe molecules onto separate sections of a cantilever array simultaneously in a simple manner. Previously, each cantilever has been addressed individually with a micro-pipette [6] but this is a very time-consuming procedure. In our contribution, the different probe solutions are simply directed onto different sections of the cantilever array via separate micro-channels. The probe solution is injected into a reservoir by hand with a micro-pipette. The solution is then drawn by capillary forces into the sites where the cantilevers are positioned. Each reservoir can hold a volume of 10 μL and can easily be refilled if larger sample volumes would be required. There are three separate coating sites on each chip, which allows for individual functionalisation of separate sections of the cantilever array, as the whole chip is not flushed with the same solution. The coating sites are connected with the reservoirs via separate micro-channels. Furthermore, SU-8 is hydrophobic which eliminates the risk of cross-contamination, as only parts of the cantilevers that are submerged into the solution will actually be treated with probe molecules.