BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Principle of a gold electrochemical cell in the presence of aqueous chloride solution O ON BOARD Au cathode Au anode Au+4 CI 4 CI+ Au [AuCl4 [AuCl4]:AucI4 H2o, Na*cl In reality, multiple reactions occur simultaneously at the anode under an applied voltage in the passive and transpassive' regime Au+4C|-->[AuC4}+3e o Au+ mH2O-> Au(H20)m+3e- o 2 AU+ 3H2O-> Au2O3+ 6H+ 6e- o2Cr→>Cl2+2e o Au2O3+ 8CI 6H-> 2(AuCl4] 3H2O · Design of anode o Need a material that is stable in the presence of chloride ions in the absence of a potential many metals corrode with 0 applied potential in vivo many metals spontaneously form an oxide layer by reaction with water/O2 in physiological conditions in presence of potential, reacts to form a biocompatible soluble compound Pourbaix diagram: shows thermodynamically favored species under applied potential at varying pH Evans diagram: shows current produced due to electrochemical dissolution of the anode; the current is a measure of the rate of electrons being produced and thus measures the kinetics of the reaction Lecture 5- Programmed Drug Delivery 4 of 12BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 • Principle of a gold electrochemical cell in the presence of aqueous chloride solution: o ON BOARD: H2O, Na+Cl￾Au cathode Au anode 4 Cl- + Au [AuCl4]- 3 e- [AuCl4]- Au + 4 Cl- [AuCl4]- e- 3 e￾reduction oxidation • In reality, multiple reactions occur simultaneously at the anode under an applied voltage in the ‘passive and transpassive’ regime5 : o Au + 4Cl- -> [AuCl4]- + 3 e￾o Au + mH2O -> Au(H2O)m 3+ + 3 e￾o 2 Au + 3H2O -> Au2O3 + 6H+ + 6e￾o 2Cl- -> Cl2 + 2e￾o Au2O3 + 8Cl- + 6H+ -> 2[AuCl4] - + 3H2O • Design of anode: o Need a material that is: ƒ stable in the presence of chloride ions in the absence of a potential • many metals corrode with 0 applied potential in vivo • many metals spontaneously form an oxide layer by reaction with water/O2 in physiological conditions • in presence of potential, reacts to form a biocompatible soluble compound • Pourbaix diagram: shows thermodynamically favored species under applied potential at varying pH • Evans diagram: shows current produced due to electrochemical dissolution of the anode; the current is a measure of the rate of electrons being produced and thus measures the kinetics of the reaction Lecture 5 – Programmed Drug Delivery 4 of 12