Covalent Linkages of Molecules and Proteins to Si–H Surfaces Formed by Disulfide Reduction

Abstract

Thiols and disulfides contacts have been, for decades, key for connecting organic molecules to surfaces and nanoclusters as they form self-assembled monolayers (SAMs) on metals such as gold (Au) under mild conditions. In contrast, they have not been similarly deployed on Si owing to the harsh conditions required for monolayers formation. Here, we show that SAMs can be simply formed by dipping Si−H surfaces into dilute solutions of organic molecules or proteins comprising disulfide bonds. We demonstrate that S−S bonds can be spontaneously reduced on Si−H, forming covalent Si−S bonds in the presence of traces of water, and that this grafting can be catalyzed by electrochemical potential. Cyclic disulfide can be spontaneously reduced to form complete monolayers in 1 hour and the reduction can be catalyzed electrochemically to form full surface coverages within 15 minutes. In contrast, the kinetics of SAM formation of the cyclic disulfide molecule on Au was found to be three folds slower than that on Si. It is also demonstrated that dilute thiol solutions can form monolayers on Si−H following oxidation to disulfides under ambient conditions; the supply of too much oxygen, however, inhibits SAM formation. The electron-transfer kinetics of the Si-S enabled SAMs on Si-H is comparable to that on Au SAMs, suggesting that Si−S contacts are electrically transmissive. We further demonstrate the prospective of this spontaneous disulfide reduction by forming a monolayer of protein azurin on a Si-H surface within 1 hour. The direct reduction of disulfides on Si electrodes opens new capabilities for a range of fields, including molecular electronics, for which highly conducting SAM-electrode contacts are necessary, and for emerging fields such as bio-molecular electronics as disulfide linkages could be exploited to wire proteins between Si electrodes, within context of the current Si-based technologies.

Spontaneous Grafting of OH-Terminated Molecules on Si−H Surfaces via Si–O–C Covalent Bonding

“…XRR analysis details and fitting parameters are discussed in supporting information (Table S1). AFM imaging for the functionalized Si-H surfaces shows smooth edges and individual flat terraces of roughness~0.2 nm (Figure 2e,f), consistent with results from typical alkyne/alkene monolayers formed on Si-H [16,34]. Further, AFM images for non-functionalized Si-H surfaces ( Figure S3) showed minimal change after functionalization demonstrating that the monolayers are formed smoothly without causing any dramatic changes to the crystalline Si surface.…”

“…Recently, we have demonstrated that spontaneous nanoscale corrosion of Si-H surfaces leads to disulfide bond breaking and enables the formation of high-quality SAMs of thiols and disulfides on Si [16,34]. In this study, we turn our attention to OH contact groups as they are commonly available functionalities that could broaden the source of the organic molecules that can form monolayers on Si.…”

Silicon – single molecule – silicon circuits

“…A key feature of the synthesis conditions recently developed 1,2 is that they require no heating, pressure, irradiation, or external catalysis. This differs from the wide variety of approaches previously used for grafting molecules onto silicon 3 using, e.g., [4][5][6] Lewis acids, 7 Grignard reagents, 8,9 electrografting, 10 and microwave 11 or UV-Visible irradiation, [12][13][14][15][16][17][18] involving perhaps ultra-high vacuum technologies, 19,20 hightemperature solution chemistry, 21,22 or high-temperature highpressure processes in supercritical CO 2 .…”

“…Recently, synthetic techniques compatible with technologies used in silicon fabrication plants were developed that can assemble molecules with either thiol 1 or disulfide 2 terminating groups between two silicon-electrode contacts. This can, in principle, pave the way for the inclusion of single-molecules, or else nanoscopic self-assembled monolayers (SAMs), to be imbedded into silicon diodes and transistors.…”

Functionalization of Oxide‐Free Silicon Surfaces for Biosensing Applications

“…Grafting of sulfhydryls on to SiH surfaces have been investigated with some success by UV illumination, [ 77 ] heat, [ 78 ] or electrochemical grafting. [ 79 ] The later recently published electrochemical grafting protocol interestingly showed monolayer formation under ambient condition that may even proceed spontaneously without applied voltage or catalyst. They suggested that the formation of the monolayer, formed within an hour is guided by water generating local nanoscale oxidation of the Si surface.…”