Sensors and Biochips

Silicon-based sensors can be classified as physical, chemical or biological, depending on the parameter to be measured. 

Sensors for measurements of physical parameters such as pressure, rotation or acceleration are commonly based on elongation or vibration of membranes, cantilevers or other proof masses. The electrochemical processes used to achieve these micromechanical structures are commonly etch-stop techniques, as discussed in Section 4.5, or sacrificial layer techniques, discussed in Section 10.7. 

For the fabrication of chemical sensors, either the unique properties of the silicon electrode itself or a variation in PS properties with absorption of molecules at the large internal surface, are exploited for sensing. Both principles are addressed below. 

The HF tester is a commercial safety tool for sensing whether an unidentified liquid contains HF [2], It shows in an exemplary way how the electrochemical properties of a silicon electrode, namely its I-V curve in HF, can be applied for sensing. The ability to dissolve an anodic oxide layer formed on silicon electrodes in aqueous electrolytes under anodic bias is a unique property of HF. HF is therefore the only electrolyte in which considerable, steady-state anodic currents are observed, as shown schematically in Fig. 3.1. This effect has been exploited to realize a simple but effective safety sensor, which allows us to check within seconds if a liquid contains HF. This is useful for safety applications, because HF constitutes a major health hazard in semiconductor manufacturing, as discussed in Section 1.2. 

The electronic circuit of the safety sensor consists of a p-type silicon electrode, an LED, a resistor, two 3 V lithium batteries, and a platinum wire as a counter electrode, connected in series, as shown in the right part of Fig. 10.7. These components are assembled in a pen-like housing, optimized to measure even thin layers of liquid on a flat surface, as shown in the left part of Fig. 10.7. This configuration is advantageous if a puddle, observed for example under a wet bench or other equipment, is to be analyzed. 

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Similarly to their natural counterparts (enzymes, antibodies, and hormone receptors), MIPs have found numerous applications in various areas. They have been used as antibody mimics in immunoassays and sensors and biochips as affinity separation materials and for chemical and bioanalysis, for directed synthesis and enzyme-like catalysis, and for biomedical applications. Concerning their commercialization, there has been great progress during the past decade, in particular in the... 

Novel electrode materials have emerged in the last decades as a consequence of new requirements for reliable, sensitive and selective biosensor devices. This paper reviewed the most important materials used for the development of biosensors, biochip and supporting matrices. A summary of the principal electrode materials and methods is presented in Table 7.2. The work focused primarily on the characterization of these materials, the most important techniques used for electrode modification and their applicability for the construction of sensors and biosensors. Cmrent and future trends in material science for biosensors have been also discussed. 

Selective electroless nickel plating of particle arrays on polyelectrolyte multilayer was investigated for the potential applications in sensors, optoelectronics, and biochips.96 This process is based on the preparation of functional colloidal arrays on surfaces. In the next step metal deposition is carried out on the surfaces of the patterned particles secured on the substrate. Samples of colloidal arrays on patterned polyelectrolyte templates are first pretreated with a Pd(II)-based catalyst. After rinsing with deionized water and drying, samples were plated with nickel using dimethylamine borane (DMAB) as a reducing agent. Based on the results of this work,96 it was shown that the selective electroless nickel deposition on 3D patterned surfaces can be successful. 

N. Pereira Rodrigues, Y. Sakai and T. Fujii, Cell-based microfluidic biochip for the electrochemical real-time monitoring of glucose and oxygen, Sensors and Actuators B Chemical, 132(2), 608-613 (2008). 

H.F. Cui, J.S. Ye, Y. Chen, S.C. Chong, X. Liu, T.M. Lim and F.S. Sheu, In situ temporal detection of dopamine exocytosis from 1-dopa-incubated nm9d cells using microelectrode array-integrated biochip, Sensors and Actuators B-Chemical, 115(2), 634-641 (2006). 

Hsieh, Y.-C. and Zahn, J. D., Glucose recovery in a microfluidic tnicrodialysis biochip. Sensors and Actuators, B Chemical, B107, 649, 2005. 

Biological sensors Diagnostic biochip Impedance biosensors Lab-on-a-Chip MEMS biosensor Nanobiosensors for nano- and microfluidics NEMS-based biosensors... 

Microelectromechanical systems (MEMSs) are integrated micromechanical and electronic parts on silicon or glass plates. They are produced by lithography technology. Various sensors, micromotors, biochips, chemical reactors, biomolecular devices, microfluid lanes, and so on, are produced by MEMSs. 

Biomaterials. Adsorbed polymers find many apphcations as surface modifiers in biomedical apphcations. By choosing a combination of hydrophobic and hydrophilic copolsrmers, surfaces can be modified to make them biocompatible (65) (see Biomolecules at Interfaces). In the area of tissue engineering (qv), adsorbed layers with specihc amino acid sequences can be used to promote cell adhesion and proliferation. The recent developments in the design of biochips to analyze specihc DNA molecules also take advantage of this technology. Polymer adsorption on patterned surfaces can be used to mimic pattern recognition. This effect can be used to develop sensors and molecular-scale separation processes (66). 

Some authors prefer to speak about a third generation of biosensors. This term is not used consistently. In some cases, it denotes a combination of sensor and its electronic evaluation unit, a so-called biochip. Also, the reagentless biosensors (sensors where all the active compoimds are immobiUzed at an electrode) are sometimes referred to as third-generation biosensors. 

R.-D. Chen, Micromolding of biochip devices designed with microchannels . Sensors and Actuators A, 128, 238-247, 2006. 

Vo-Dinh T., Development of a DNA biochip Principle and apphcations, Sensor Actuat B-Chem. 1998 51 52-59. 

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