# Nanosensor for Hydrogen Detection and High-Resolution Electrochemical Imaging

Shu Wu, Jean-Marc Noël, Michael Mirkin

► To cite this version:

Shu Wu, Jean-Marc Noël, Michael Mirkin. Nanosensor for Hydrogen Detection and High-Resolution Electrochemical Imaging. Analysis & Sensing, 2025, pp.e202500094. 10.1002/anse.202500094. hal-05381342

HAL Id: hal-05381342

https://hal.science/hal-05381342v1

Submitted on 25 Nov 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>
# Nanosensor for Hydrogen Detection and High-Resolution Electrochemical Imaging

Shu Wu,[a] Jean-Marc Noël[b] and Michael V. Mirkin*[a]

[a] S. Wu and M. V. Mirkin
Department of Chemistry and Biochemistry
Queens College-CUNY
Flushing, NY 11367, USA
mmirkin@qc.cuny.edu

[b] J.-M. Noël
Université Paris Cité
ITODYS, CNRS
F-75013, Paris, France

Supporting information for this article is given via a link at the end of the document.

**Abstract:** Electrochemical sensors for hydrogen can be useful for development and optimization of water electrolyzers and fuel cells based on hydrogen evolution and oxidation reactions (HER and HOR). A nanometer-sized hydrogen sensor used as a tip in the scanning electrochemical microscope (SECM) can probe HER and HOR electrocatalysts and photocatalysts on the nanoscale. However, Pt tips and chemically modified nanoelectrodes previously employed for hydrogen sensing suffered from surface passivation, low amperometric signal, and stability issues. Here, we report the preparation of hydrogen sensors by covalently attaching ferrocene groups to the surface of Pt or carbon nanoelectrodes through oxidation of ferroceneacetate ions. Mediated oxidation of hydrogen at surface-modified nanoelectrodes produced measurable and stable current suitable for amperometric measurements and SECM imaging.

## Introduction

Hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) are central to alternative energy and solar fuel technology.[1,2] Electrochemical mapping of reaction rates at the active sites of electrocatalyst, photocatalyst, and cocatalyst surfaces with nanoscale spatial resolution is essential for mechanistic studies and optimization of these processes.[3-5] Scanning electrochemical microscopy (SECM) has been employed for high-resolution mapping of catalytic surfaces in operando and investigating HER[6-16] and HOR.[11,12,17] To probe a catalytic surface reaction, the SECM tip electrode has to measure either the production or consumption of H₂.

Measuring hydrogen oxidation current at a small electrode is notoriously difficult.[12] A very high current density is required to attain a measurable current (i.e., ~1 pA or higher) at a nanoelectrode. For instance, the 1 pA current flowing at a 20 nm-diameter disk electrode corresponds to the current density of 0.32 A/cm². Like other inner-sphere electrochemical processes,[18,19] HOR is very sensitive to adsorption, surface passivation, and contamination, and therefore cannot support such a high current density. Bard and coworkers found that the HOR is rapid on a clean Pt surface but greatly inhibited by the formation of a surface oxide layer and the specific adsorption of some anions.[12] The smaller the tip electrode, the stronger the effects of its surface deactivation and adsorption, and the harder to measure the HOR current. Most other materials used to fabricate nanoelectrodes, including carbon, are not suitable for HOR.

To overcome these problems, we recently introduced carbon and platinum nanoelectrodes chemically modified with surface-bound redox molecules, which enable mediated ET between the electrode surface and dissolved electroactive species.[20] Immobilizing a molecular catalyst on the nanoelectrode surface can alleviate the Pt passivation problem and render CNE suitable for hydrogen sensing. For instance, carbon nanoelectrodes modified with ferrocene (Fc) or Ru(bpy)₃ can oxidize hydrogen[20] or hydrogen peroxide[21] and be used as nanotips for electrocatalytic activity mapping. However, chemically modified nanoelectrodes previously employed for hydrogen sensing suffered from low amperometric signal and stability issues. For example, to immobilize Ru(bpy)₃ species on a
carbon nanoelectrode (CNE), it was biased at a high positive potential (e.g., +2.0 V vs. Ag/AgCl).[20] This protocol often results in the loss of carbon from the apex of the CNE, which may not change the shape of steady-state voltammograms but prevents such an electrode from serving as an SECM tip. Additionally, because of the strong light absorption a carbon nanoelectrode could not be used as a lightguide for through-tip illumination in photo-SECM,[22] whereas surface modification of Pt electrodes employed in such experiments has been less successful. Here, we show that chemically modified nanoelectrodes can be prepared by the oxidation of ferroceneacetate ions ($FcCH_2COO^−$) at the Pt or CNE surface. This procedure, previously developed for immobilizing ferrocene groups on macroscopic carbon surfaces,[23,24] relies on the generation of a zwitterionic structure, ferrocenium acetate ($Fc^+CH_2COO^−$), through the electrooxidation of $FcCH_2COO^−$ (Eq. 1). The ferrocenium group in $Fc^+CH_2COO^−$ then oxidizes the carboxylate functional group, leading to the cleavage of the molecule into the free radical $FcCH_2^•$ and the release of $CO_2$ (Eq. 2):

$$FcCH_2COO^− \rightarrow Fc^+CH_2COO^− + 1e^− \quad (1)$$

$$Fc^+CH_2COO^− \rightarrow FcCH_2^• + CO_2 + 1e^− \quad (2)$$

The $FcCH_2^•$radical subsequently undergoes spontaneous grafting onto the electrode surface. This strategy, initially employed for grafting carbon electrodes, is extended here to the modification of Pt nanoelectrodes. The robust film obtained is then used to achieve a more stable response toward hydrogen detection.

## Experimental

**Chemicals and Materials.** Tetrabutylammonium hexafluorophosphate (n-Bu₄NPF₆), tetrabutylammonium hydroxide (1 M in methanol), sodium perchlorate (anhydrous, ≥99.9%, trace metals basis), triethanolamine (≥99.9%), perchloric acid (70%, 99.999% trace metals basis), and ferrocenemethanol (Fc) purchased from Sigma-Aldrich, ferroceneacetic acid (98%) purchased from Tokyo chemical industry, and potassium phosphate dibasic (purified) purchased from Reagents were used as received. 0.1 M phosphate buffer (PB; pH 7.0) solution was prepared from sodium phosphate dibasic (Sigma-Aldrich, 99.95% trace metals basis) and sodium phosphate monobasic (Sigma-Aldrich, anhydrous, 99.998% trace metals basis). Deionized water from the Milli-Q Advantage A10 system (Millipore Corp.) equipped with Q-Gard T2 Pak, a Quantum TEX cartridge and a VOC Pak was used to prepare all aqueous solution. Organic solutions were prepared from acetonitrile (Sigma-Aldrich, ≥99.9%, HPLC).

**Fabrication of platinum and carbon nanoelectrodes.** Pt nanoelectrodes were fabricated by pulling 25-µm-diameter annealed Pt wires (Goodfellow) into borosilicate glass capillaries (1.0 mm o.d., 0.2 mm i.d.; Sutter Instrument Company) under vacuum with a P-2000 laser pipette puller (Sutter Instrument Co.) as described previously.[25] The nanoelectrodes were polished on a 50-nm lapping tape (Precision Surfaces International) under video microscopic control. The radii of Pt nanoelectrodes were estimated from steady-state voltammetry and TEM images (Fig. S1).

Nanopipettes were fabricated by pulling quartz capillaries (1.0 mm o.d., 0.5 mm i.d.; Sutter Instrument Company) with a P-2000 laser pipette puller. Carbon was deposited on the inner wall of the nanopipette through the chemical vapor deposition (CVD) at 950 °C, using methane as carbon source and argon as protective gas (methane/argon ratio: 5/3) as described previously.[26] A 1-hour deposition time was sufficient to nearly completely fill the nanopipettes with carbon. The electrostatic discharge (ESD) protection was used to avoid CNE damage during all steps of nanoelectrode preparation.[27] The radii of carbon nanoelectrodes were evaluated from steady-state voltammetry and TEM images (Fig. S2). The surface area of C and Pt tips used for hydrogen detection in acidic and basic solutions varied from $2.8 \times 10^{-11} \text{ cm}^2$ to $3.1 \times 10^{-10} \text{ cm}^2$, corresponding to the 30 nm to 100 nm radii.

**Chemical modification of nanoelectrode surface.** Ferrocene group was immobilized on the Pt and carbon nanoelectrode surfaces via oxidation of tetrabutylammonium ferroceneacetate, using the protocol similar to that previously developed of functionalization of macroscopic carbon surfaces.[23,24] Briefly, 1 mM tetrabutylammonium ferroceneacetate was prepared by stoichiometric neutralization of ferroceneacetic acid with tetrabutylammonium hydroxide, and dissolved in acetonitrile containing 0.1 M n-Bu₄NPF₆. The prepared solution was degassed with argon. A nanoelectrode was biased at a positive potential (+1.0 V vs. Ag/AgCl) for either 1000 seconds (Pt nanoelectrodes) or 200 seconds (CNEs) to oxidize the precursor species $FcCH_2COO^−$ to ferrocenium and form zeitterionic $Fc^+CH_2COO^−$.
Then, carboxylate was oxidized, producing ferrocenylmethyl radicals ($FcCH_2$') and $CO_2$. Finally, $FcCH_2$ was covalently attached to the carbon surface presumably through the reaction of ferrocenylmethyl radicals with the $sp^2$ carbon atoms.[23] All modified nanoelectrodes were rinsed with acetonitrile and characterized by cyclic voltammetry and chronoamperometry ($i-t$ curves).

**Hydrogen detection in solution.** $H_2$ was produced by a hydrogen generator (Park, H2F-100NA) and bubbled through the basic solution for 10 mins. The electrochemical experiments were performed in this solution using a CHI 760E bipotentiostat (CH Instruments) inside a Faraday cage. Voltammograms and $i-t$ curves were obtained in an airtight vial. A three-electrode setup included a home-made Pt or carbon nanoelectrode used as a working electrode, a Pt wire or a graphite rod counter electrode, and a commercial Ag/AgCl electrode serving as a reference.

**SECM setup and procedures.** Substrate generation/tip collection (SG/TC) mode of SECM was employed for hydrogen detection in an acidic solution. The experiments were carried out using a previously described home-built instrument.[28] A 5-µm-diameter Pt disk ultramicroelectrode (UME) serving as a substrate was biased at -1 V vs Ag/AgCl reference to generate hydrogen, which was oxidized at the $FcCH_2$-modified nanoelectrode tip. The Pt UME (CH Instruments, model CHI 107) was polished before the experiment. A nanoelectrode tip was initially positioned a few tens of micrometers above Pt UME substrate with the help of a long-distance optical microscope. A digital angle gauge (DWL-80Pro, Digi-Pas) was used to ensure the horizontal orientation of the substrate plane and the correct tip/substrate alignment. A current vs. distance curve was obtained during the subsequent fine approach. To map HER at the Pt UME, the tip was positioned about one radius above the substrate electrode and scanned laterally in the x-y plane. All experiments were carried out in a Faraday cage at room temperature (23 ± 2 °C). The concentration of perchloric acid in the solution was 1 mM, and no hydrogen bubble formation was detected in SECM experiments at the selected potentials and under given conditions.

# Results and Discussion

## Surface Modification and Hydrogen Detection with Pt-$CH_2Fc$ and $C-CH_2Fc$ Nanoelectrodes

Since the immobilization of $CH_2Fc$ on Pt was attempted herein for the first time, we investigated the electrooxidation of $FcCH_2COO^-$ at the macroscopic Pt electrodes and compared it to the results obtained previously at carbon electrodes. Typical cyclic voltammograms (CVs) recorded in acetonitrile solution containing 1 mM $FcCH_2COO^-$ at both electrode materials (Fig. S3A,B) exhibited similar irreversible peaks, followed by a reversible process, in agreement with previously reported results.[23,24] The former process is characteristic of $FcCH_2\cdot$ formation, while the latter presumably corresponds to $FcCH_2\cdot$ dimerization.[24] The similarity of electrochemical responses observed at Pt and C electrodes after one cycle of grafting within the range -0.4 V/0.8 V at 50 mV/s attests to the versatility of this electrografting strategy. However, since covalent grafting on platinum is generally considered less favorable than on carbon, this result might also suggest an electropolymerization process rather than covalent electrografting. Nevertheless, the grafting of radical species on platinum (e.g., as those involved in the electrografting of aryldiazonium salts, has previously been demonstrated.[29,30]

The immobilized Fc moieties were further identified by CVs recorded in acetonitrile at both types of electrodes (Figs. S3C,D). Based on the evolution of the cathodic peak current (produced by the reduction of immobilized ferrocenium), the surface concentration was estimated to be ~0.8 x $10^{-10}$ mol/cm² on carbon and ~1 x $10^{-10}$ mol/cm² on platinum. These values, which indicate comparable grafting efficiency, correspond to approximately 20-25% of the theoretical full coverage ($5 \times 10^{-10}$ mol/cm² assuming the formation of a close-packed monolayer of Fc.[31]" This procedure was then used to modify nanoelectrodes (Fig. 1A). A typical CV of $FcCH_2COO^-$ oxidation at a Pt nanoelectrode (Fig. 1B) shows an irreversible peak corresponding to $FcCH_2\cdot$ formation, whereas the second reversible redox process is less pronounced than at macroscopic electrodes. Similar CVs were obtained at CNEs, (data not shown). To ensure efficient grafting, the reaction was carried out at a constant potential of +1 V vs Ag/AgCl for 1000s, allowing for effective $FcCH_2\cdot$ generation, as detailed in the experimental section. The electrochemical characterization of the Fc immobilized at a nanoelectrode is, however, not straightforward due to the small amount of grafted moieties, however, the presence of a multilayer Fc film on the surface is evidenced by blocking the oxidation of ferrocenemethanol (FcMeOH; Fig. 1C). The magnitude of this effect varies from nearly
complete (Fig. 1C) to partial blocking (Fig. 2A) corresponding to either complete or partial coverage of the electrode surface. Similar responses were observed on modified CNEs (Fig. S4).

The modified nanoelectrodes were then employed to investigate the hydrogen oxidation reaction (HOR) in a hydrogen-saturated solution. The voltammograms of FcMeOH oxidation (Fig. 2A) and chronoamperograms of HOR (Fig. 2B) obtained at the same 60 nm Pt nanoelectrode before (1) and after (2) surface modification with CH₂Fc point to efficient electrooxidation of hydrogen mediated by surface-bound Fc, as shown in Eqs (3) and (4) and illustrated in Fig. 2C

$$FcCH_2 \rightarrow Fc^+CH_2 + 1e^- \quad (3)$$

$$2Fc^+CH_2 + H_2 \rightarrow 2FcCH_2 + 2H^+ \quad (4)$$

The formal potential of adsorbed FcCH₂ species (E°' ≈ 0.4 V vs. Ag/AgCl; Fig. S3C) is sufficiently positive to oxidize hydorogen (E°' ≈ -0.2 V vs. Ag/AgCl) in aqueous solutions.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** (A) Scheme of the FcCOO⁻ electrooxidation at a Pt nanoelectrode to generate FcCH₂• and (B) a corresponding CV recorded in 1 mM FcCH₂COO⁻ in acetonitrile solution containing 0.1 M n-Bu₄NPF₆. (C) Voltammograms of FcMeOH oxidation at a Pt nanoelectrode before (orange curve) and after (blue curve) surface modification with CH₂Fc. The electrode radius, a = 100 nm.</figcaption></figure>

A similar behaviour was observed with carbon electrodes (Fig. S5).
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Voltammograms of FcMeOH (A) and chronoamperograms of HOR (B) recorded at the same Pt nanoelectrode before (1) and after (2) surface modification with CH₂Fc. *a* = 60 nm. (A) Solution contained 1 mM FcMeOH in 0.1 M PB (pH 7). (B) Hydrogen was continuously bubbled through 0.1 M K₂HPO₄ solution. *E* = 1.2 V vs. Ag/AgCl. (C) Scheme of the mediated oxidation of hydrogen by immobilized ferrrocenium.</figcaption></figure>

## Pt-CH₂Fc and C-CH₂Fc Nanoelectrodes as SECM Tips for Hydrogen Detection

To be suitable as an SECM tip allowing for efficient sensing and mapping hydrogen fluxes, a modified nanoelectrode must exhibit measurable and stable HOR current. In substrate generation/tip collection (SG/TC) mode SECM experiments, H₂ is produced by proton reduction at the negatively biased Pt substrate and collected by oxidation at the tip surface as illustrated in Fig. 3A. We compared the responses to hydrogen of several Pt and carbon nanoelectrodes before and after surface modification with CH₂Fc. The data obtained with a relatively small (*a* = 60 nm) Pt electrode is shown in Fig. 3B. Noteworthy, the voltammograms obtained at this electrode (Figs. S6) show a significant blockage of the FcMeOH oxidation current after surface modification (cf. blue and orange curves). In the SG/TC experiments (Fig. 3B), the *i*-*t* curves of HOR were obtained at the same tip positioned at approximately 1 µm above the 1-mm-radius Pt disk substrate before (blue curve) and after (orange curve) the surface attachment of CH₂Fc and showed a much higher and more stable HOR current with the Fc groups attached to it surface. Similar data obtained with carbon tips is shown in Fig. S7.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** (A) Schematic representation of the SG/TC SECM experiment. The tip detects hydrogen generated by proton reduction at the 1-mm-radius Pt disk substrate in solution containing 1 mM HClO₄ and 0.1 M NaClO₄. (B) *i*-*t* curves of H₂ oxidation at a Pt nanoelectrode before (orange curve) and after (blue curve) surface modification with CH₂Fc. The electrode radius, *a* = 60 nm. Eₛ = -0.49 V, Eτ = 0.8 V vs. Ag/AgCl.</figcaption></figure>

The stability of the HOR current at modified nanotips can be further assessed by SECM imaging. Fig. 4B shows a SG/TC image of the hydrogen flux produced at a 5-µm-diameter Pt disk substrate immersed in the 10 mM HClO₄ solution. The image was recorded by scanning a CH₂Fc-modified 230 nm Pt tip laterally (i.e., in the x-y plane) over the substrate surface at a constant separation distance, ~150 nm. Because the separation distance was much smaller than the substrate radius, the image in Fig. 4B faithfully reproduces the size and slightly elongated shape of the Pt disk substrate (Fig. 4A) without a significant diffusion broadening effect. The lack of apparent tilt in the image indicates that the HOR current has not changed significantly during the scanning and demonstrates that stability of the modified electrode response is sufficiently high on the time scale of SECM imaging (several minutes).

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Optical micrograph of a 5-µm Pt disk electrode used as an SECM substrate (A) and SG/TC map of the flux of hydrogen generated by proton reduction at the substrate. *a* = 230 nm. Aqueous solution contained 10 mM HClO₄ and 0.1 M NaClO₄. Eₛ = 1 V; Eτ = -1 V vs. Ag/AgCl.</figcaption></figure>
**Conclusion**

High-resolution electrochemical mapping of electro- and photoelectrocatalysts of HER and HOR requires nanoelectrodes capable of efficiently oxidizing hydrogen. Bare and surface modified Pt nanoelectrodes employed previously for hydrogen measurements did not provide sufficiently high and stable amperometric response required for SECM mapping. In this article, we adapted the literature protocol used for the attachment of FeCH₂ groups to carbon surfaces to produce Fc-modified Pt and carbon nanoelectrodes. The HOR current at the fabricated electrodes is significantly enhanced and stable on the time scale of minutes, which is sufficient for hydrogen detection in the bulk solution and SECM imaging.

**Supporting Information**

Additional voltammograms, chronoamperograms, and TEM images of nanoelectrodes, including Figures S1 – S7.

**Acknowledgements**

The support of this work by the National Science Foundation grant CHE-2247262 is gratefully acknowledged.

**Conflict of Interests**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** nanoelectrode • hydrogen sensor • scanning electrochemical microscopy • modified electrode • mediated charge transfer

[1] N. S. Lewis, D. G. Nocera, "Powering the planet: chemical challenges in solar energy utilization". *Proc. Natl Acad. Sci. USA* **106**, 15729.

[2] G. Segev, J. Kibsgaard, C. Hahn, Z. J. Xu, W. H. Cheng, T. G. Deutsch, C. Xiang, J. Z. Zhang, L. Hammarström, D. G. Nocera, A. Z. Weber, P. Agbo, T. Hisatomi, F. E. Osterloh, K. Domen, F. F. Abdi, S. Haussener, D. J. Miller, S. Ardo, P. C. McIntyre, T. Hannappel, S. Hu, H. Atwater, J. M. Gregoire, M. Z. Ertem, I. D. Sharp, K. S. Choi, J. S. Lee, O. Ishitani, J. W. Ager, R. R. Prabhakar, A. T. Bell, S. W. Boettcher, K. Vincent, K. Takanabe, V. Artero, R. Napier, B. R. Cuenya, M. T. M. Koper, R. Van De Krol, F. Houle, "The 2022 Solar Fuels Roadmap". *J. Phys. D: Appl. Phys* **2022**, 55, 323003.

[3] H. S. Ahn, C. G. Zoski, A. J. Bard, in *Scanning Electrochemical Microscopy*, 3rd ed. (Eds: A. J. Bard, M. V. Mirkin), CRC Press: Boca Raton, **2022**, pp. 361–378.

[4] C. Santana Santos, B. N. Jaato, I. Sanjuán, W. Schuhmann, C. Andronescu, "Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis". *Chem. Rev.* **2023**, 123, 4972.

[5] C. L. Bentley, J. Edmondson, G. N. Meloni, D. Perry, V. Shkirskiy, P. R. Unwin, "Nanoscale Electrochemical Mapping". *Anal. Chem.* **2019**, 91, 84.

[6] H. Li, M. Du, M. J. Mleczko, A. L. Koh, Y. Nishi, E. Pop, A. J. Bard, X. Zheng, "Kinetic Study of Hydrogen Evolution Reaction over Strained MoS₂ with Sulfur Vacancies Using Scanning Electrochemical Microscopy". *J. Am. Chem. Soc.* **2016**, 138, 5123.

[7] S. S. Jamali, S. E. Moulton, D. E. Tallman, J. Weber, G. G. Wallace, "Electro-Oxidation and Reduction of H₂ on Platinum Studied by Scanning Electrochemical Microscopy for the Purpose of Local Detection of H₂ Evolution". *Surf. Interface Anal.* **2015**, 47, 1187.
[8] J. Kund, J. Romer, E. Oswald, A.-L. Gaus, M. Küllmer, A. Turchanin, M. von Delius, C. Kranz, "Pd-Modified De-alloyed Au-Ni-Microelectrodes for In Situ and Operando Mapping of Hydrogen Evolution". *ChemElectroChem* **2022**, 9, e202200071.

[9] D. Koster, R. Gutkowski, J. Masa, W. Schuhmann, "H₂ Quantification Based on Selective Pre-Concentration and Oxidative Stripping at Pd Modified Microelectrodes". *J. Electroanal. Chem.* **2018**, *812*, 207.

[10] Y. Wang, D. O. Wipf, "Visualizing Hydrogen Oxidation Reaction Activity of Polycrystalline Platinum by Scanning Electrochemical Microscopy". *J. Electrochem. Soc.* **2020**, *167*, 146502.

[11] C. G. Zoski, "Scanning Electrochemical Microscopy: Investigation of Hydrogen Oxidation at Polycrystalline Noble Metal Electrodes". *J. Phys. Chem. B* **2003**, *107*, 6401.

[12] J. Zhou, Y. Zu, A. J. Bard, "Scanning Electrochemical Microscopy: Part 39. The Proton/Hydrogen Mediator System and its Application to the Study of the Electrocatalysis of Hydrogen Oxidation". *J. Electroanal. Chem.* **2000**, *491*, 22.

[13] A. Djire, X. Wang, C. Xiao, O. C. Nwamba, M. V. Mirkin, N. R. Neale, "Basal Plane Hydrogen Evolution Activity from Mixed Metal Nitride MXenes Measured by Scanning Electrochemical Microscopy". *Adv. Funct. Mater.* **2020**, *30*, 2001136.

[14] T. Sun, H. Zhang, X. Wang, J. Liu, C. Xiao, S. U. Nanayakkara, J. L. Blackburn, M. V. Mirkin, E. M. Miller, "Nanoscale Mapping of Hydrogen Evolution on Metallic and Semiconducting MoS₂ Nanosheets". *Nanoscale Horiz.* **2019**, *4*, 619.

[15] T. Sun, Y. Yu, B. J. Zacher, M. V. Mirkin, "Scanning Electrochemical Microscopy of Individual Catalytic Nanoparticles". *Angew. Chem. Int. Ed.* **2014**, *53*, 14120.

[16] H.-J. Niu, Y. Yan, S. Jiang, T. Liu, T. Sun, W. Zhou, L. Guo, J. Li, "Interfaces Decrease the Alkaline Hydrogen-Evolution Kinetics Energy Barrier on NiCoP/Ti₃C₂Tx MXene". *ACS Nano* **2022**, *16*, 11049.

[17] J. Kim, C. Renault, N. Nioradze, N. Arroyo-Currás, K. C. Leonard, A. J. Bard, "Electrocatalytic Activity of Individual Pt Nanoparticles Studied by Nanoscale Scanning Electrochemical Microscopy". *J. Am. Chem. Soc.* **2016**, *138*, 8560.

[18] A. J. Bard, "Inner-Sphere Heterogeneous Electrode Reactions. Electrocatalysis and Photocatalysis: The Challenge". *J. Am. Chem. Soc.* **2010**, *132*, 7559.

[19] N. Nioradze, R. Chen, N. Kurapati, A. Khvataeva-Domanov, S. Mabic, S. Amemiya, "Organic Contamination of Highly Oriented Pyrolytic Graphite as Studied by Scanning Electrochemical Microscopy". *Anal. Chem.* **2015**, *87*, 4836.

[20] K. Barman, X. Wang, R. Jia, M. V. Mirkin, "Mediated Charge Transfer at Nanoelectrodes: A New Approach to Electrochemical Reactivity Mapping and Nanosensing". *J. Am. Chem. Soc.* **2021**, *143*, 8547.

[21] K. Barman, X. Wang, R. Jia, G. Askarova, G. X. Hu, M. V. Mirkin, "Voltage-Driven Molecular Catalysis of Electrochemical Reactions". *J. Am. Chem. Soc.* **2021**, *143*, 17344.

[22] T. Bo, D. Ghoshal, L. M. Wilder, E. M. Miller, M. V. Mirkin, "High-Resolution Mapping of Photocatalytic Activity by Diffusion-Based and Tunneling Modes of Photo-Scanning Electrochemical Microscopy". *ACS Nano*, **2025**, *19*, 3490.

[23] L. S. Hernández-Muñoz, F. J. González, "One-Step Modification of Carbon Surfaces with Ferrocene Groups through a Self-Mediated Oxidation of Ferroceneacetate Ions". *Electrochem. Commun.* **2011**, *13*, 701.

[24] L. S. Hernández-Muñoz, M. A. González-Fuentes, B. R. Díaz-Sánchez, R. Fragoso-Soriano, C. Vázquez-López, F. J. González, "Covalent Modification of Carbon Surfaces with Ferrocene Groups through a Self-Mediated Oxidation of Tetrabutylammonium Salts of Ferrocene-Carboxylic Acids". *Electrochim. Acta* **2012**, *63*, 287.

[25] P. Sun, M. V. Mirkin, "Kinetics of Electron Transfer Reactions at Nanoelectrodes". *Anal. Chem.* **2006**, *78*, 6526.
[26] Y. Yu, Y. Gao, K. Hu, P.-Y. Blanchard, J.-M. Noël, T. Nareshkumar, K. L. Phani, G. Friedman, Y. Gogotsi, M. V. Mirkin, "Electrochemistry and Electrocatalysis at Single Gold Nanoparticles Attached to Carbon Nanoelectrodes". *ChemElectroChem* **2015**, *2*, 58.

[27] N. Nioradze, R. Chen, J. Kim, M. Shen, P. Santhosh, S. Amemiya, "Origins of Nanoscale Damage to Glass-Sealed Platinum Electrodes with Submicrometer and Nanometer Size". *Anal. Chem.* **2013**, *85*, 6198.

[28] T. Bo, X. Wang, R. Jia, L. Han, H. L. Xin, H. Zhang, E. M. Miller, M. V. Mirkin, "Probing Activities of Individual Catalytic Nanoflakes by Tunneling Mode of Scanning Electrochemical Microscopy". *J. Phys. Chem. C* **2021**, *125*, 25525.

[29] L. Gonzalez-Macia, S. Griveau, F. d'Orlyé, A. Varenne, C. Sella, L. Thouin, F. Bedioui, "Electrografting of aryl diazonium on thin layer platinum microbands: Towards customized surface functionalization within microsystems". *Electrochem. Commun.* **2016**, *70*, 78.

[30] M. D. Raicopol, C. Andronescu, R. Atasiei, A. Hanganu, E. Vasile, A. M. Brezoiu, L. Pilan, "Organic layers via aryl diazonium electrochemistry: towards modifying platinum electrodes for interference free glucose biosensors". *Electrochim. Acta* **2016**, *206*, 226.

[31] A. Latus, J.-M. Noël, E. Volanschi, C. Lagrost, P. Hapiot, "Scanning Electrochemical Microscopy Studies of Glutathione-Modified Surfaces. An Erasable and Sensitive-to-Reactive Oxygen Species Surface". *Langmuir* **2011**, *27*, 11206 and references cited therein.