Portable electrochemical and plasmonic biosensors for biomedical application
Portable sensing for point-of-care (POC) has attracted a lot of attention in these years and has been developed in a wide variety for clinical and environmental applications. The idea of POC comes from the advances in portable smart electronic devices and micro-fluid systems which can be integrated and miniaturized into a lab-on-a-chip. Modern electronic devices, like smartphones, smartwatchs or even headsets, can achieve signal generating and receiving by using interfaces like camera, flashlight, audio-jack, micro-USB and wireless module. On the other side, development in the micro-fluid system enables the integration of different diagnosis techniques like separation, quenching, lysing and sensing into a single chip. Polydimethylsiloxane (PDMS) and SU-8 photoresist were commonly used in building such micro-fluid systems, which was first demonstrated by Whiteside's group. Quite a large number of applications were published by coupling such system with optical or electrical sensing techniques. In recent years, paper-based micro-fluid systems have begun to take over the traditional micro-fluid systems mainly due to paper's pump-free and cost-effective property. Some new fabrication methods like wax printing, screen printing and 3D printing were utilized to make such paper-based platforms more attractive. The paper substrate has been verified for use in different sensing stages with all of its properties, such as porous, self-pumping, flexible, anti-oxidation, cost-effective and hydrophilic capabilities. Many sensing techniques have been demonstrated to be coupled with such micro-fluid systems mentioned above for POC applications. Plasmonic and electrochemical methods are the two techniques that will be illustrated in this dissertation. The plasmonic method is new in the sensing area. Plasmon is the oscillation of free electrons at the interface of metal and dielectric material. With the incident of light, the resonance mode will be excited at some certain frequencies which can enhance the optical signal in the form of reflection or transmission peaks. The resonance frequency can be tuned by adjusting the surface refractive index, in such a way to achieve sensing. More sensitive quantification could be achieved on some molecules with a specific absorbance preference if the absorbance peak matches the resonance mode of the sensing substrate. This is the specific application for sensing some bio-targets with plasmon-induced absorbance enhancement. On the contrary, the electrochemical method is a classic method to analyze the electrical and chemical signal at the solid-liquid interface. Since it is easy to miniaturize and integrate into modern electronic devices, it is widely used in micro-fluid systems and POC applications. Most of the electrochemical sensing techniques like potentiommetry, amperometry and impedance methods create a balance of electrode kinetics and diffusion. The change of current or impedance can be sensed by applying different forms of potential at the electrode surface. Such electrical signal change indicates the variations in concentration, which can be used to quantify the ions or bio-targets. With such micro-fluid systems and sensing techniques mentioned above, many environmental or clinical applications can be transformed into point-of-care applications. For ion concentration quantification, electrochemical and colorimetric methods could be the best solution. In the area of biosensing, small molecules like glucose and larger targets like protein are what mostly interest people, since they are very useful indicators of personal health conditions. Plasmonic sensing could be used here due to refractive index change or by matching the absorbance peak with the plasmonic resonance mode. Electrochemical method could also be applied if a redox system could be built. For larger bio-target sensing, like cells and bacteria, plasmonic enhanced fluorescence or plasmonic enhancement by labeling with gold nano-particles could help with the optical colorimetric sensing. The electrochemical method is also applicable when used with optimized voltammetry method. By combining these sensing techniques and micro-fluid systems for target separation, quantification and identification, the future of POC technology is promising.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign