Tailoring the DNA SAM surface density on different surface crystallographic features using potential assisted thiol exchange Kaylyn K. Leung a, b, 1 , Andrea Diaz Gaxiola a, b, 2 , Hua-Zhong Yu c, 1 , Dan Bizzotto a, b, *, 1 a Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1, Canada b Advanced Materials and Process Engineering Laboratory, University of British Columbia, 2355, East Mall, Vancouver, BC V6T 1Z4, Canada c Department of Chemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada article info Article history: Received 23 October 2017 Received in revised form 15 December 2017 Accepted 17 December 2017 Available online 21 December 2017 Keywords: Self-assembled monolayers Electrodeposition Thiol-exchange Surface crystallography Fluorescence microscopy abstract The influence of surface crystallography and applied potential on the thiol-exchange procedure to create mixed alkylthiol DNA SAMs is detailed. A single crystal gold bead and fluorophore labeled thiol modified DNA were used to characterize the resulting surface modifications. The thiol-exchange occurs with different efficiencies on the low index planes (111,100,110) as compared to 311 and 210. Positive applied potentials (>0/SCE) result in 10  higher coverage than when compared to deposition at the open circuit potential (OCP) over the same 60 min time period. Negative potentials ( < 0/SCE) resulted in less uniform coverage with the 111 facet being significantly modified. The electrolyte used during the deposition was a 10 mM TRIS Buffer with 100 mM NaCl 500 mM MgCl 2 . The influence of [Cl  ] was studied showing it had a significant impact on the thiol-exchange at the positive potentials, where higher [Cl  ] resulted in higher DNA coverages and a more uniform coverage across the multi-crystalline surface. The local environment of the thiol-exchanged DNA SAMs were compared for different regions on the surface using potential driven DNA reorientation modulating the fluorescence intensity. These results showed a common behaviour from all surfaces suggesting that the DNA SAMs prepared by thiol-exchange were consistently prepared with a variable surface concentration controlled by potential and time. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Self-assembled monolayers(SAMs) of DNA on gold surfaces are used in the manufacturing of a variety of nucleic acid based bio- sensors (e.g., aptamer-based biosensors) [1e4]. They are conven- tionally made by exposing a clean gold surface to thiol-modified DNA molecules which chemisorb to the gold surface via a gold-thiol covalent interaction. In a consecutive step, non-specifically adsor- bed DNA, or DNA adsorbed to the gold surface via the nitrogenous bases, is displaced with exposure to a short chain alkylthiol [5,6]. Typically, this process takes place on a gold surface that is at the open circuit potential(OCP). However, the surface coverage and local environment around the DNA in a SAM are not easily controlled when manufacturing these multicomponent SAMs and are known to impact biosensor performance. Thus the ability to easily tailor the formation of a DNA SAM and control over its local environment is desirable. Control over the spacing between DNA molecules is needed to ensure the surface is easily accessible for analytes, thereby requiring a homogeneously modified surface. At the same time a sufficiently large number of adsorbed DNA mole- cules are needed to ensure high sensitivity[7 ,8]. One approach was the creation of nanostructures via electrodeposition on planar electrode surfaces which increases DNA surface coverage while maintaining accessibility [9e11]. Many strategies have been detailed for controlling the DNA surface coverage in SAMs such as increasing the immersion time or the concentration of the DNA in the immobilization buffer(IB) [5]. Increasing the ionic strength of IB can increase the DNA coverage by shielding the electrostatic repulsion between the adsorbed DNA [7]. Typically, mercaptohexanol(MCH) is used to displace non- specifically adsorbed DNA [5e7] though not completely effective [12, 13]. An alternative approach was used where thiolated DNA displaces or competes (e.g., thiol-exchange) with a preformed MCH SAM which resulted in less physisorbed (non-specifically adsorbed) * Corresponding author. Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1, Canada. E-mail address: bizzotto@chem.ubc.ca (D. Bizzotto). URL: https://www.chem.ubc.ca/dan-bizzotto 1 ISE member. 2 Current address: Bristol Centre for Functional Nanomaterials, University of Bristol, Bristol, UK, BS8 1TL. Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta https://doi.org/10.1016/j.electacta.2017.12.114 0013-4686/© 2017 Elsevier Ltd. All rights reserved. Electrochimica Acta 261 (2018) 188e197