Tailoring the DNA SAM surface density on different surface crystallographic features using potential assisted thiol exchange

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

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

* Corresponding author. Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1, Canada.

E-mail address:   (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.

https://doi.org/10.1016/j.electacta.2017.12.114

0013-4686/© 2017 Elsevier Ltd. All rights reserved.

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 MgCl2. 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.