Click chemistry has been a cornerstone of modern bioconjugation since the early 2000s, prized for reactions that run cleanly, at high yield, and with minimal side products under mild aqueous conditions. The workhorse of the family is the copper(I)-catalyzed azide-alkyne cycloaddition, or CuAAC, in which an azide group and a terminal alkyne snap together into a stable triazole ring. For biosensor makers, this offers a gentle way to tether receptors, fluorophores, or nanomaterials to a surface without damaging delicate biological molecules. The catch has always been the catalyst: copper(I) is unstable in oxygenated water, so traditional protocols add chemical reducing agents such as ascorbate to generate it in bulk, which gives little control over where or when the reaction happens.

The E-click approach solves that problem elegantly. Instead of dosing the solution with a reductant, the researchers used the electrode itself to electrochemically convert copper(II) into copper(I) right at the surface, on demand. Because the catalytic species is born only where the electrode is switched on, functionalization can be turned on and off with a potentiostat, confined to selected electrodes in an array, and tuned by adjusting the applied potential. This stands in contrast to conventional modification techniques such as physisorption, entrapment, molecular imprinting, or self-assembled monolayers, which can suffer from variable reaction efficiencies, limited site control, and sensitivity to pH, ionic strength, and temperature. It also offers an alternative to other electrochemical patterning tools such as light-activated electrochemistry and scanning electrochemical microscopy.

To test the concept, the team built a model biosensor surface step by step on gold substrates. First, they cleaned the gold electrochemically in sulfuric acid and immersed it in 11-mercaptoundecanoic acid, or MUA, which self-assembles into an ordered monolayer tipped with carboxylic acid groups. Next, standard EDC/NHS coupling chemistry was used to attach propargyl-PEG2-amine, presenting terminal alkyne groups at the surface. Finally, the stage was set for the click reaction: a fluorescent azide-tagged dye, FAM azide 6-isomer, served as a stand-in for a biological receptor, so that successful coupling could be read out directly from the surface’s emission spectrum. The copper(I) catalyst was generated in situ by applying a negative potential to the gold electrode in a solution of copper sulfate and sodium chloride.

Verifying that every step had worked required a battery of complementary techniques. X-ray photoelectron spectroscopy traced the chemical evolution of the surface, revealing carbon, nitrogen, and oxygen peaks consistent with the MUA monolayer, the EDC/NHS-coupled propargyl layer, and the final FAM attachment, along with copper oxide residues from the catalytic step. Crucially, sulfur 2p signals remained intact after the full procedure, showing that the underlying self-assembled monolayer had survived the electrochemical treatment without desorbing. Cyclic voltammetry in a ferrocyanide solution tracked the blocking of electron transfer as the monolayer formed, while electrochemical impedance spectroscopy, fitted to a Randles circuit, quantified the changing charge-transfer resistance at each step. Contact angle measurements rounded out the picture, shifting with the growing polarity and then hydrophobicity of the modified surface.

With the chemistry validated, the researchers turned to optimization, and the results carry practical lessons for anyone building sensors this way. Varying the reaction duration at a fixed potential of minus 0.35 volts showed that fluorescence, and therefore the amount of bound dye, rose linearly for the first ten minutes before saturating, indicating a rapid reaction and a surface approaching full coverage by roughly thirty minutes. The team settled on ten minutes as the sweet spot balancing signal, reproducibility, and handling time. Controls incubated without any applied potential showed only weak nonspecific adsorption, confirming that the coupling truly depends on electrochemically generated catalyst.

The applied potential proved to be the trickiest variable. Cyclic voltammetry of the copper electrolyte on the modified surface showed that the organic layers slow electron transfer, pushing the copper(II) to copper(I) reduction to more negative potentials and obscuring the peaks that would normally signal a viable condition. Potentials milder than about minus 0.25 volts failed to generate enough catalyst, while pushing beyond roughly minus 0.325 volts triggered unwanted copper deposition onto the surface, visible as nucleation loops and stripping peaks in the voltammograms. The usable window is narrow, and the authors identify minus 0.35 volts as a practical compromise that generates catalyst efficiently while minimizing surface damage. The fluorescence data confirmed that even potentials showing no obvious voltammetric signature could still drive the reaction, underscoring how subtle the choice of operating conditions can be on complex functionalized surfaces.

The most eye-catching demonstration, however, moves the catalyst off the sample altogether. In a proof of concept, the researchers used a conductive atomic force microscopy cantilever as the working electrode, scanning it over a propargyl-functionalized gold surface while holding it at minus 0.35 volts relative to the substrate. The copper(I) generated near the cantilever catalyzed click reactions along the scanned region, and subsequent fluorescence spectroscopy confirmed the characteristic FAM emission on the surface. In the current setup, functionalization was not confined to the scanned area alone, because the platinum-iridium coating covers the entire front face of the cantilever and generates catalyst over a broad zone. The authors note that cantilevers insulated everywhere except at the very tip could shrink the reaction zone to the nanoscale, enabling direct-write patterning of recognition molecules.

The implications reach well beyond a fluorescent demo. Electrode arrays for multiplexed diagnostics could have each electrode functionalized with a different receptor simply by addressing them individually, leaving neighbors untouched, a capability previously shown for microelectrodes but now extended to a scanning-probe geometry. Nanoarrays, multi-analyte sensors, and nanoengineered surfaces for fundamental studies of biomolecular recognition all come within reach if the tip-localized version matures. At the same time, the study is candid about its challenges: the narrow potential window for catalyst generation on modified surfaces, the risk of copper deposition at excessive overpotentials, and the difficulty of reading the copper reduction peak through an insulating organic layer. Solving these will matter for translating E-click from the bench to robust device fabrication.

What the Aalborg and Newcastle team has delivered is a careful, quantitative map of how an electrically triggered click reaction behaves on a realistic biosensor surface, complete with the spectroscopic fingerprints, electrochemical signatures, and wetting behavior that document every step. By showing that the same chemistry works whether the catalyst is born at the flat electrode or at the tip of an AFM cantilever, they have sketched a versatile platform for surface engineering in which a potentiostat, rather than a photomask or a chemical bath, dictates exactly where molecules attach. As biosensors push toward denser arrays, smaller samples, and higher sensitivity, the ability to write functional chemistry with spatial and temporal precision, using nothing more than a carefully chosen voltage applied for ten minutes, may prove to be one of those quiet enabling technologies that reshapes how sensing devices are made.

Subject of Research: Electrochemically driven click chemistry for controlled and localized biosensor electrode surface functionalization

Article Title: Electrochemical click chemistry for controlled and localized biosensor surface functionalization

Article References: Pedersen, T., Pike, A., Cucinotta, F., Horrocks, B. R., & Gurevich, L. (2026). Electrochemical click chemistry for controlled and localized biosensor surface functionalization. Discover Electrochemistry, 3(1), Article 74. https://doi.org/10.1007/s44373-026-00160-z

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00160-z

Keywords: electrochemical click chemistry, CuAAC, biosensor functionalization, self-assembled monolayer, copper catalyst, X-ray photoelectron spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, AFM cantilever, surface patterning, multiplexed biosensors, gold electrodes