Photoresist Microelectrochemistry Nanoliter Droplet Method

2 General Aspects It is beyond the scope of this treatment to discuss the current and potential distribution for photoresist microelectrodes in detail. Therefore, only a few comments will be made on the influence of the geometry and the resist barrier on electrochemical measurements. Photoresist microelectrodes can be treated as a recessed microdisk electrode. The diffusion field and the current distribution are 

Owing to the finite electrolyte volume ofthe nanoliter droplet, the diffusion boundary layer reaches the dimensions of the droplet within seconds, assuming a diffusion coefficient of for example D = 110-5 cm2 s 1. A constant diffusion gradient can not be established because the bulk concentration is reduced. Thus, measurements of diffusion limited currents are not stable over time. The electron transfer reactions discussed below were carried out on anodic oxide layers with maximum current densities 3 to 4 orders of magnitude lower than the diffusion limited current densities, thus ensuring a stable support of consumables over the time of the measurement. In order to prevent evaporation of electrolyte, the ambient was saturated with water vapor. 

The use of a small electrolyte covered resist area around the microelectrode is essential when capacitance measurements are performed, as the resist capacitance is parallel to the electrode capacitance. With a specific resistance of a = 1012 LI cm and a dielectric constant of e = 1.5 for the resist, and assuming a typical electrode capacitance of 10pFcm 2 with a 50 tm electrode, an error of 5% is obtained, if the electrolyte covered surface is 10 3cm2 (/=1000Hz) [88]. Thus, for capacitance measurements, the use of nanoliter droplets is essential. 

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