The Influence of Thiolate Readsorption on the Quality of Mixed Monolayers Formed through an Electrochemcial Method

Abstract

Lateral Force Microscopy (LFM) was used to probe the quality of binary mixed monolayers formed on planar polycrystalline gold through an electrochemical method. In the approach, portions of a self-assembled monolayer (SAM) composed of 2-aminoethanethiol (AET) were removed from the Au(111) surface facets by selective reductive desorption which maintained undisrupted regions of AET elsewhere on the polycrystalline surface. Monolayer voids created by this method were backfilled with 11-mercaptoundecanoic acid (MUA) and the interface characterized with ex situ LFM. This produced images with domains of high and low friction corresponding to isolated zones of MUA and AET respectively. Reverse sequence mixed monolayers were also prepared with MUA as the starting layer and rendered LFM images that mirrored the AET based layers. This demonstrates flexibility of the electrochemical method to produce heterogeneous binary SAMs, and to further probe the quality of mixed monolayers, a number of experimental conditions including desorption time, electrode configuration, and initial incubation period were studied. AET/MUA layers that produced the most enhanced LFM images were formed on a planar electrode that was vertically submerged into the electrolyte while maintaining a selective desorption potential for 5 min before backfilling with MUA. This condition allowed for the effective diffusion of AET away from the interface and created well-defined monolayer voids for backfilling. At desorption times lower than 1 min, some of the AET molecules that remained near the interface would readsorb onto the surface and interfere with the backfilling process thereby creating lower contrast LFM images. Structural features of these layers were independent of initial incubation time (10 min and 16 h); however, the contrast between domains was improved when using AET layers formed over a longer incubation period. Interestingly, the contrast was significantly reduced when mixed layers were created on electrodes set in a hanging meniscus with the electrolyte. Here, electrochemical evidence pointed to prolonged readsorption of thiolates creating less well-defined voids for backfilling, and the event was most pronounced for MUA based layers.

Quantifying the Selective Modification of Au(111) Facets via Electrochemical and Electroless Treatments for Manipulating Gold Nanorod Surface Composition

“…Control over the preparation of mixed component alkylthiolate self-assembled monolayers (SAMs) via electrochemical treatment has been demonstrated by manipulating the substrate potential during SAM deposition − and via the reductive desorption of a portion of the monolayer. − This is possible since the potential of SAM reductive desorption is sensitive to the substrate surface crystallography. Removal from Au(111) occurs at the least negative reduction potential while a more negative potential is required for more open surfaces (e.g., 210).…”

“…Electrochemical potential has also been used to remove the SAM from specific surface regions which were then backfilled with a different thiol. The resulting surface was analyzed using lateral force AFM which distinguished the short- and long-chained alkylthiolates. , In addition to reductive desorption of alkanethiolate SAMs, oxidative desorption has been demonstrated, however, with less control over the specificity of the surface feature removed . Control over the surface coverage of thiolated DNA on gold was also demonstrated by reductive desorption in the presence of a short chain alkylthiol. , …”

Molecularly-tunable nanoelectrode arrays created by harnessing intermolecular interactions

“…In situ ellipsometry study of mixed monolayers showed evidence for the formation of a low refractive index region aer desorption of the monolayer component. 14 In a lateral force microscopy study of some binary monolayers formed on planar polycrystalline gold, 15 portions of 2-aminoethanethiol were removed by selective reductive desorption and backlled with 11-mercaptoundecanoic acid. Phase formation in mixed alkanethiol monolayers 16 was also revealed by electrochemical quartz crystal nanobalance.…”

A combined top-down/bottom-up approach to structuring multi-sensing zones on a thin film and the application to SPR sensors

“…These devices should also possess surface functionalities that can effectively interact with gaseous or solution species of interest, leading to the generation of characteristic electronic and/ or optical signals [10][11][12][13]. In particular, there is a growing demand for the preparation of a multifunctional surface with a well-defined topographical structure, which would expedite the collection and analyses of complexed signals from various analytes [14][15][16] Various fabrication strategies, such as lithography [17][18][19], self-assembled monolayers (SAMs) [20][21][22][23][24], electrochemical treatment [25], and electro-polymerization [26] are available for the functionalization of an inorganic surface. Of these methods, the SAMs technique, in which organic molecules are spontaneously bound to an inorganic surface, has been the most widely used and investigated.…”