A. Wisitsoraat

Electrochemical cholesterol sensing system with electropolymerized carbon nanotube electrode

In this work, a practical cholesterol electrochemical sensing system is developed based on electropolymerized CNTs electrode and low cost potentiostat. Cholesterol sensor is fabricated by two-step electrochemical polymerized enzyme immobilization on a vertically aligned CNT electrode. Vertically aligned CNTs are selectively grown on a 1 mm2 window of gold coated SiO2/Si substrate by thermal chemical vapor deposition (CVD) with water-assisted etching. CNTs are then simultaneously functionalized and enzyme immobilized by electrochemical polymerization of polyaniline and cholesterol enzymes. Subsequently, ineffective enzymes are removed and new enzymes are electrochemically recharged. Scanning electron microscopic characterization indicates polymer-enzyme nanoparticle coating on CNT surface. Cyclic voltammogram (CV) measurements in cholesterol solution show the oxidation and reduction peaks centered around 450 and −220 mV, respectively. An approximately linear relationship between the cholesterol concentration and the response current could be observed in the concentration range of 50–300 mg/dl with a sensitivity of approximately 0.22 µA/mg·dl−1, which is considerably higher compared to previously reported CNT bioprobe. Therefore, the electropolymerized CNT bioprobe is promising for cholesterol detection in normal cholesterol concentration in human blood.

Low cost hot embossing process for plastics microfluidic chips fabrication

In this work, we develop plastic microfluidic chips based on low cost hot embossing process with metal micromold. Metal micromold was formed on aluminum substrates by CNC milling machine and high precision micromachine. The hot embossing system is in-house made with computer aid design by Solid Work program. The system consists of four main parts, structural body, heating system, compressive system and control electronics. The compressive system consists of two top hydraulic single-stage pistons and four middle-stage pistons. Polymethyl methacrylate (PMMA) microfluidic chips were then produced by hot embossing under different applied temperatures and time. It was found that optimum temperature and time for minimum contraction and depth error were 80 degree C and 2–5 minutes. The developed technique offer advantages for microfluidic chip fabrication in term of quality, complexity and cost.

Fabrication of QCM sensor array and PDMS micro chamber for biosensor applications

In this work, an array of quartz crystal microbalance (QCM) sensors is fabricated on a single quartz crystal substrate and attached with poly(dimethylsiloxane) (PDMS) micro chamber for using in flow-injection system. The Cr/Au electrode array is deposited on both side of quartz substrate by sputtering through electroplated microshadow mask. PDMS micro chamber was fabricated by mold casting technique with SU-8 mold. Fabricated PDMS micro chamber and QCM sensor array were attached together for using in flow-injection system. In our experiment, the QCM electrodes were coated with the carboxylic poly(vinyl chloride) (PVC-COOH) and then the carboxylic group was activated with carbodiimide hydrochloride (EDC) and N-hydroxylsuccinimide (NHS) for protein binding. The scanning electron micrograph shown the trapped protein on the modified sensing layer, it confirmed that our QCM sensor and PDMS micro chamber can be used as QCM biosensor array. This new QCM sensor array provide possibility of multiple detection in small amount of sample within a single quartz crystal substrate that avoid the error signal from the different properties of each sensor in an array.

Rapid cholesterol detection by functionalized carbon nanotube based electrochemical sensor on flow injection microfluidic chip

In this work, CNTs electrochemical electrode has been integrated on a flow injection based microfluidic chip for detection of cholesterol (CHO). Metal electrodes were deposited and CNTs were grown on a glass substrate by sputtering and chemical vapor deposition at low temperature. Polydimethylsiloxane microfluidic componet was then bonded on substrate. Cholesterol oxidase/potassium hexacyanoferrate were immobilized by polyvinyl alcohol using in-channel flow processing. Cholesterol detections were made by flow injection analysis and fast response time and high sensitivity have been achieved with high throughput of more than 100 samples per hour.

Symmetrical Wheatstone Microcantilever Sensor with On-chip Temperature Sensors

This work has focused on the design, finite element modeling and testing of a symmetrical Wheatstone MEMS cantilever beam with on-chip temperature sensors. The stress induced on gold surface with polysilicon piezoresistive sensing is demonstrated. In principle, adsorption of biochemical species on a functionalized surface of the microfabricated cantilever will cause a surface stress and consequently the cantilever bending. The sensing mechanism relies on the piezoresistive properties of the polysilicon wires encapsulated inside the beam. The beam is fabricated and bending analysis is performed so that the beam tip deflection could be predicted. The piezoresistor designs on the beams were varied, within certain constraints, so that external read-out circuit could measure the sensitivity of the sensing technique. The mass detection of 0.0058-0.0110 g is measured by the beam resistor series as a balanced Wheatstone bridge configuration. The voltage output of the bridge is directly proportional to the amount of bending in the MEMS cantilever. The temperature dependency and sensor performance have been characterized in experiments. Compensation by resisters on the substrate, which act as temperature sensors significantly reduces the temperature dependence effect.