One-Step Covalent Immobilization of β-Cyclodextrin on sp² Carbon Surfaces for Ultrasensitive and Selective Guest Detection

The modification of solid surfaces with supramolecular hosts is a powerful method to tailor interfacial properties and confer chemical selectivity, but often involves multistep protocols that hinder facile upscaling. Here, we demonstrate the one-step covalent modification of highly oriented pyrolytic graphite (HOPG) with a β-cyclodextrin (β-CD) derivative, which efficiently forms inclusion complexes with hydrophobic guests of suitable size (e.g. ferrocene, dopamine). The modified β-CD-HOPG electrode discriminates analytes that form host-guest complexes with β-CD against a 100-fold higher background of electroactive substances that do not, even when these possess similar redox potentials. In addition, the enrichment of the analytes at the electrode surface, through complex formation with the β-CD units, leads to an enhanced electrochemical response and an improved detection limit (one order of magnitude for dopamine, compared to current cyclodextrin-based sensors). The two unique features we demonstrate for the covalently grafted β-CD-HOPG electrode (selective sensing of hydrophobic analytes and ultrasensitive dopamine detection) do not come at the expense of ease of preparation: the one-step covalent modification can be completed in minutes, using standard electrochemical cells and equipment. Also, the procedure uses robust, commercially available HOPG substrates that can be reused by mechanical exfoliation many times, lowering the operational cost and further increasing wide accessibility and attractiveness of our approach. The fundamental reasons for the high performance of the sensor may be related to its simple and well-defined structure, in which all the sensing units (covalently attached β-CD-moieties) are positioned uniformly and close to the collector surface (HOPG). This is achieved by the chemical characteristics of both reagent (a highly reactive, but sterically hindered diazonium compound, yielding monolayer modification exclusively) and substrate (low-reactive sp2 carbon surface, requiring highly reactive radical reagents for its modification), resulting in negligible side reactions despite the simplicity of the approach. These principles, and the resulting performance, can be taken as a proof of concept for entirely different sensors, e.g. using other van der Waals solids as substrates, but based on analogous chemistries.