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Local Modulation of the Redox State of p-Nitrothiophenol Self-Assembled Monolayers Using the Direct Mode of Scanning Electrochemical Microscopy

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Abstract

Local redox conversion of nitro end groups of a 4-nitrothiophenol self-assembled monolayer on gold is achieved by direct-mode scanning electrochemical microscopy (SECM). Potential pulses are applied to the modified gold surface leading to local reduction of nitro end groups to either hydroxylamine (−0.47 V, see picture) or amino groups (−0.6 V) exclusively beneath the positioned SECM tip.

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Local reduction of the terminating nitro groups of a p-nitrothiophenol self-assembled monolayer (SAM) under formation of either hydroxylamine or amino groups is invoked using the direct mode of scanning electrochemical microscopy (SECM). By choosing the appropriate potential and a potential pulse sequence, the reduction of the SAM end groups to the desired oxidation state can be achieved, locally restricted to the area of the sample surface directly underneath the positioned SECM tip. Following the “writing” of redox microstructures within the SAM end groups, the local modification of the redox states is visualized (“reading”) by using the feedback mode of SECM. The current at the Pt tip electrode is determined by the electron-transfer rate for reoxidation of the redox mediator at the sample surface. Thus, heterogeneities in the SAM surface, which are caused by local differences in the redox state of the end groups, are distinguishable due to the different electron-transfer rates governed by the redox state of the SAM end groups. To further unequivocally prove the successful local modification of the redox state of the SAM end groups during the writing process, the micropatterned surface is selectively modified with biotin at areas with reduced SAM end groups for further complementary binding of an avidin–enzyme conjugate. Selective post-functionalization with an avidin–alkaline phosphatase conjugate allows visualization of the microstructure using the generator–collector mode of SECM.

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