Nanotechnology in Biological Engineering II

Transcription

1 Nanotechnology in Biological Engineering II THE EFFECT OF ELECTRICAL DOUBLE LAYER ON THE ELECTROCHEMICAL PROCESSES OF NANOMETER INTERDIGITATED ELECTRODES Xiaoling Yang 1 and Guigen Zhang 1,2,3 1 Micro/Nano Bioengineering Laboratory, 2 Nanoscale Science and Engineering Center, 3 Faculty of Engineering The University of Georgia, Athens, GA

2 Motivation Electrochemical sensor plays an important role in clinical diagnosis: Features:High Sensitivity, Real time detection, Simple operation Micro/nano interdigitated electrodes (Micro/Nano IDEs) based electrochemical biosensors are getting more and more popular: Features: small dimension, low sample volume, low cost Application: Glucose sensor, immune sensor, gas sensor 2

3 Motivation Problem: For sensing purpose, IDEs measure faradic current When the electrodes get to nanometer scale, the faradic response at electrode may be affected by electrical double layer (EDL). 3

4 EDL Effect on Single Electrodes EDL formation: A charged electrode can attract oppositely charged species in the solution and forms EDL (compact layer and diffuse layer) EDL effect Diffuse Layer Diffusion Layer OHP Compact Layer Diffuse Layer Diffuse layer Gouy Plane Diffusion Layer x Bulk Solution reactant Concentration reactant Concentration Potential Potential Micro electrodes: diffusion only Nano electrodes: diffusion and electromigration 4

5 EDL Effect on Nano IDEs y Diffuse Layer Diffusion Layer reactant Concentration Symm µ Diffuse Layer r1 r2 Diffuse Layer Symm Potential r 0 IHP OHP OHP IHP x 1 x Diffuse layer and diffusion layer overlap Diffuse layers overlap at two electrodes of nanoides For a widespread application of nanoides, it is imperative to elucidate the effect of the EDL on the faradic reactions of nanoides. But this problem is too complicated to be solved by analytical means. 5

6 Objective To investigate the EDL effect on the faradic reaction of nano IDEs by a fully numerical method developed using COMSOL Multiphysics 6

7 Model Development Previously our group developed a fully numerical method by COMSOL Multiphysics using Nernst Plank Poisson equation to simulate EDL affected faradic reaction at single nanometer electrodes.* In this study we expand previous method to further address the issue at nano IDEs. Electrolyte G C G C G C r =1nm *Yang, X.; Zhang, G. Nanotechnology 2007, 18, y G Electrolyte IHP OHP OHP IHP C 7 x 1 x

8 Model Development Initial Concentrations O z e R z 1 Oxidized species (OS) and Counter ion : 5mM Reduced species: 0mM f b Supporting electrolyte (SE): SE is excess C SE = 500mM Boundary Conditions Potential at electrode and bulk solution E G = 0.3V to 0.4V at y 20mV/s; E b E C = 0.3V E b = 0V Current Flux IHPOHP OHPIHP Butler Volmer Equation E G V VE C x 1 x J f k = J 0 b = k 0 exp[ αf ( E V E exp[( 1 α) F( E V E V: potential at OHP, i.e. the Position of Electron Transfer 8 0' 0' ) / RT] c ) / RT] c R O

9 Model Development Governing Equations Potential distribution Poisson equation B a b c ε 2 IHP Electrode surface ε OHP PET ( εε V ) 0 ε 1 = 6 ε 2 = 78 = ρ ε 1 r 0 r 0 l 1 Radial Distance (r) r 0 l 1 l 2 Electrolyte Mass transport Nernst Plank equation c Assume: perfectly smooth electrode surface; no specific adsorption t j = z j F ( D j c j D j c j V RT ) y ρ = 0 ρ IHP OHP = j z j c j OHP IHP ρ = 0 x 1 x 9

10 EDL Effect and the Charge Valences of Redox Species 60 Current Density (ka/m 2 ) Generator Collector Diffusion z = 1 z = 1 IHP OHP 4nm OHP IHP Potential (V) EDL affected voltammetric curve deviated from diffusion controlled case, when inter electrode spacing w gap = 4nm 10

11 EDL Effect with Changing Gap Spacing Potential (V) B IHP OHP OHP IHP wgap =4nm wgap = 16nm EG =0.3V, EC =0.3V EG = 0.4V, EC = 0.3V Distance/(w gap 2µ) Limiting Current (ka/cm 2 ) A Generator Collector w gap (nm) Diffusion z = 1 z = 1 Potential Distribution at w gap = 4nm & 16nm limiting current and w gap More diffuse layer overlap have more EDL effect! 11

12 EDL Effect and Supporting Electrolyte Current (ka/cm 2 ) A Generator Collector z = 1 Diffusion 500 mm 0 mm z = 1 Current (ka/cm 2 ) B z = 1 Generator Collector Diffusion 500mM 0 mm z = Potential (V) Potential (V) The voltammetric curve deviate significantly from diffusion controlled case when supporting electrolyte is absent in the solution. 12

13 Conclusion The effect of EDL on the voltammetric performance of nano IDEs is dependent on The charge valence of redox species The gap spacing between electrodes The concentration of supporting electrolyte This work demonstrates that a complete computer modeling approach is well suited for elucidating the electrochemical processes of electrodes with complex geometries when faradic reactions and the EDL effect are of concerns. 13

14 Acknowledgement National Science Foundation The Faculty of Engineering at The University of Georgia. My advisor Dr. Guigen Zhang All my group members Thank you! 14