Force Measurement for the Interaction between Cucurbit[7]uril and Mica and Self-Assembled Monolayer in the Presence of Zn2+ Studied with Atomic Force Microscopy

Force spectroscopy with atomic force microscopy (AFM) revealed that cucurbit[7]uril (CB[7]) strongly binds to a mica surface in the presence of cations. Indeed, Zn²⁺ was observed to facilitate the self-assembly of CB[7] on the mica surface, whereas monocations, such as Na⁺, were less effective. The progression of the process and the cation-mediated self-assembled monolayer were characterized using AFM, and the observed height of the layer agrees well with the calculated CB[7] value (9.1 Å). We utilized force-based AFM to further study the interaction of CB[7] with guest molecules. To this end, CB[7] was immobilized on a glass substrate, and aminomethylferrocene (am-Fc) was conjugated onto an AFM tip. The single-molecule interaction between CB[7] and am-Fc was monitored by collecting the unbinding force curves. The force histogram showed single ruptures and a unimodal distribution, and the most probable unbinding force value was 101 pN in deionized water and 86 pN in phosphate-buffered saline buffer. The results indicate that the unbinding force was larger than that of streptavidin-biotin measured under the same conditions, whereas the dissociation constant was smaller by 1 order of magnitude (0.012 s^-1 vs 0.13 s^-1). Furthermore, a high-resolution adhesion force map showed a part of the CB[7] cavities on the surface.

Tuning Surface Charge and Morphology for the Efficient Detection of Dopamine under the Interferences of Uric Acid, Ascorbic Acid, and Protein Adsorption

In this research, we aimed to evaluate the impact of the surface charges and morphologies of electrodes on electrochemically detecting dopamine (DA) in the presence of protein adsorption, uric acid (UA), and ascorbic acid (AA). Through the electropolymerization of functionalized 3,4-ethylenedioxythiophenes (EDOT) directly on Au electrodes, we successfully created PEDOT-coated electrodes with three different functional groups and nanostructures. Negatively charged carboxylic acid groups attracted DA while reducing the interferences of UA and AA due to electrostatic effect. We used charge-free tetra(ethylene glycol) and zwitterionic phosphocholine groups are used to evaluate the interference of protein adsorption on DA sensing because they both can effectively prevent the nonspecific adsorption of proteins. These two electrodes can avoid protein adsorption, yet proved ineffective for DA sensing: both tetra(ethylene glycol) and the phosphocholine groups are electroneutral and have minimal electrostatic interactions with DA. We also used three proteins of different isoelectric points - bovine serum albumin, lysozyme, and fibrinogen - to evaluate the influence of protein adsorption on DA detection. We found that for an electrode coated with carboxylic acid-functionalized PEDOT, the adsorption of positively charged lysozyme can promote the detection sensitivity of AA and UA, and that all protein adsorption lowers the sensitivity of DA. In contrast, nanostructures promote the detection sensitivity of all three molecules. All of our tested functionalized PEDOT-coated electrodes demonstrated good stability and functionality in buffers.