Membrane-based Chemomechanical Transducer System for the Detection of Molecular Interactions

Abstract

Biological and chemical events detection enabled by chemo-mechanical transduction brings forth a new era of development in genomics and proteomics. Chemo-mechanical transduction offers unique advantages such as label-free assay, real-time monitoring and the potential for a compact device. Various approaches towards chemo-mechanical transduction have been introduced depending on the structures, materials and the sensing mechanisms. Micro cantilevers have been the mainstream of chemo-mechanical transducer and detect a variety of useful reactions such as DNA hybridization, various antigen-antibody binding and even cellular binding. These approaches were, however, hardly realized into a compact device due to the bulky optical equipment and it has trouble at opaque analytes. In any of the cantilever-based approaches one of the most critical issues is the fact that the cantilever immersed in analyte, produces undesired motions and deformations due to the interaction with the fluidic flow and the nonspecific adsorption of the backside. Membrane-based approaches have emerged in order to overcome these issues and also they have several advantages. In case of membrane based transducers, the detection surface is physically isolated from the sensing surface, so it can be easily adapted to various electronic readout techniques. Secondly, membrane structures are more robust and therefore can be easily functionalized and probed using commercially available printing techniques. Finally, isolated sensing surface is capable of handling both liquid and gas samples. Although previous membrane transducer demonstrated the chemo-mechanical transduction of various biomolecular interactions but there are reliable problems associated with membrane material. The polymer materials in wet environments make different torsion due to swelling and low melting temperature and the silicon material is vulnerable to crack the surface of thin film. In this work, a highly reliable thin membrane transducer (TMT) system has been developed. The system lead to breakthrough applications in the fields of diagnostics, threat evaluation and gas sensors with the integrated platform of membrane sensor, capacitance measuring electronics and aptamer receptor. Most of all, the MEMS sensor fabricated with monolithic process bring high reliability about the system. The special feature of this membrane transducer is the first use of a silicon nitride membrane material which is highly chemical inertness, thermal stability and bio-compatibility. Sacrificial layer process, utilizing photoresist material, leads to uniform distance for the pair of electrodes and the dimple structure fabricated with one step increases the reliability of transducer. The process of double layer of photoresist for the electroplating is strictly developed to build up thick rigid electrode with low stress. Furthermore, the self-aligned proximate shadow mask is developed to deposit exquisite reaction-gold surface on the membrane, 500 μm below the contact surface. Finally, the entire fabrication uses standard low temperature processes for CMOS integration. One of the key challenges of the membrane-based chemo-mechanical transduction is effective suppression of external disturbance. This membrane transducer system utilizes common mode rejection technique to nullify the effects of major environment disturbances such as hydraulic pressure, temperature and even common chemistry. Parallel metal structures on the membrane transducer connect the mechanical sensor to the highly sensitive differential capacitive electronics. Furthermore, the printed circuit boards (PCB) are designed to realize compact membrane transducer system. This thesis demonstrates the feasibility of bio-molecular detection with the membrane-based chemo-mechanical transducer system. The specially designed molecular structure of a DNA aptamer is achieved to sensitive detection of thrombin protein. 11-mercaptoundecanoic acid (MUA) molecules are immobilized on the reaction gold and this choice of molecule is very advantageous. Not only MUA can withstand harsh packaging conditions but it also help in rejecting ground noise signal due to its high grafting density. Through peptide bonding Maleimide molecule is attached to MUA for highly selective binding with thiolated end of the receptor aptamer. The grafting density of probe DNA aptamer has been optimized with respect to the length of linker molecule to increase the surface stress on the membrane. Furthermore, through the study of molecular chemistry, molecular interaction use appropriate base buffers such as peptide bonding and aptamer buffer for epitope conformation. The membrane transducer can detect a cation in the buffer without chemo-electrical signal and recognize thrombin protein. In the result of 500 nM thrombin experiments, the largest center deflection of membrane is 24 nm calculated with integrating the infinitesimal area element of capacitance electrode. The amount of deflections is 3.2~8 times higher than that predicted with the analysis of finite element simulation (ABAQUS), which could be due to incomprehensible effects. However, this membrane-based chemo-mechanical transducer system holds a promising future in bio-chemical detection and disease diagnosis.