Electrodes and electrocatalysts for electrochemical hydrogen peroxide sensors: a review of design strategies

Abstract

H₂O₂ sensing is required in various biological and industrial applications, for which electrochemical sensing is a promising choice among various sensing technologies. Electrodes and electrocatalysts strongly influence the performance of electrochemical H₂O₂ sensors. Significant efforts have been devoted to electrode nanostructural designs and nanomaterial-based electrocatalysts. Here, we review the design strategies for electrodes and electrocatalysts used in electrochemical H₂O₂ sensors. We first summarize electrodes in different structures, including rotation disc electrodes, freestanding electrodes, all-in-one electrodes, and representative commercial H₂O₂ probes. Next, we discuss the design strategies used in recent studies to increase the number of active sites and intrinsic activities of electrocatalysts for H₂O₂ redox reactions, including nanoscale pore structuring, conductive supports, reducing the catalyst size, alloying, doping, and tuning the crystal facets. Finally, we provide our perspectives on the future research directions in creating nanoscale structures and nanomaterials to enable advanced electrochemical H₂O₂ sensors in practical applications.

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Muhammad Adil Riaz

Dr Muhammad Adil Riaz completed his MPhil in Chemical & Biomolecular Engineering from the Hong Kong University of Science and Technology in 2016. His research project in MPhil focused on designing novel graphene-based nanoporous sorbent materials for environmental remediation. He then pursued his doctoral study in Chemical Engineering at the University of Sydney, Australia, from 2017 to 2021. He worked with Professor Yuan Chen on designing novel metal–carbon nanocomposites for hydrogen peroxide electrochemical detection. He has 12 publications in high-impact factor scientific journals such as Sensors & Actuators B, Journal of Hazardous Materials and Carbon, etc.

Yuan Chen

Yuan Chen received a bachelor's degree from Tsinghua University and a PhD from Yale University. He is a professor at The University of Sydney. His research focuses on carbon materials and their sustainable energy and environmental applications, including batteries, supercapacitors, electrocatalysts, membranes, and antibacterial coatings. He is a fellow of the Royal Society of Chemistry and the Institution of Chemical Engineers. He is currently serving as an editor for Carbon and Journal of Alloys and Compounds.

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1. Introduction

Hydrogen peroxide (H₂O₂) can be generated in cells by the oxygen metabolism of organic molecules. It plays several essential functions in biological systems. Cells use H₂O₂ as a reactant in oxidative biosynthesis against bacterial pathogens or as a signaling molecule to regulate various physiological processes.¹ As an antioxidant defense mechanism, cells secrete enzymes to decompose H₂O₂ into H₂O and O₂ to maintain an average intracellular concentration of 1–100 nM or an intercellular concentration of 10–1000 nM. A high H₂O₂ concentration induces oxidative stress to cells, damages cell membranes and DNA, and causes errors in cell signaling, leading to various diseases (Fig. 1a and b).² The fluctuation of H₂O₂ concentration in the cellular microenvironment of specific organs/tissues has been used as an indicator for cardiovascular, neurological (Alzheimer and Parkinson) diseases, and diabetes. The average H₂O₂ concentration (1–5 μM) in the blood can increase to 30–50 μM in the case of inflammation. The urinary H₂O₂ concentration can also indicate oxidative stress.^3,4 Cancer cells have a high H₂O₂ concentration, which initiates tumor formation and helps in the progression and metastasis of tumors.⁵ H₂O₂ is often overproduced in the microenvironment of tumor cells due to aberrant metabolism, which can be used as a cancer diagnostic tool.⁶ H₂O₂ can also provide an indirect measurement of glucose because H₂O₂ is produced as a by-product during the enzymatic electrochemical sensing of glucose.⁷ H₂O₂ is a biomarker for lung diseases, such as chronic obstructive pulmonary disease, asthma, and lung cancers. It can be detected in exhaled breath vapors from patients.⁸ Furthermore, increased H₂O₂ concentration in the epidermis can also be used to monitor various skin disorders.⁹

Fig. 1 Biological roles of H₂O₂. (a) Different H₂O₂ concentrations generate varied responses at the cellular level. Reproduced with permission from ref. 2. Copyright 2017, Elsevier. (b) Four main biological functions of H₂O₂. Reproduced with permission from ref. 1. Copyright 2006, Mary Ann Liebert Inc.

Apart from the biological significance, H₂O₂ is widely used as an oxidizer, bleaching agent, and disinfectant in many fields. Vaporized H₂O₂ is a common sterilant used in industry and healthcare facilities. Thus, monitoring H₂O₂ is critical to avoid unwanted exposure to workers as higher concentrations of H₂O₂ produce various undesirable effects,¹⁰ such as H₂O₂ induced skin blanching¹¹ and portal venous gas embolism from accidental concentrated H₂O₂ ingestions.¹² Various sensors are available to monitor H₂O₂ vapors at the ppm level to prevent such accidents. The H₂O₂ workplace airborne exposure limit by OSHA is 1 ppm over an 8 hour shift. H₂O₂ is also used in hospitals for instrument disinfection,^16–18 orthopedic surgery,¹⁹ and treatment of brain ischemia as an antiseptic.¹³ Furthermore, it is used in industry to sanitize fruits/vegetables after harvest, disinfect food packaging, and as a preservative in milk to increase its shelf life.¹⁴ As a bleaching agent, H₂O₂ is used in teeth whitening products,¹⁵ cosmetics,¹⁶ and the paper/pulp industry.¹⁷ As an oxidizing agent, it is used to degrade environmental pollutants and as odor control in wastewater.¹⁸ Therefore, there is a clear need to measure H₂O₂ in stationary and flowing aqueous systems to avoid potentially harmful exposures.

Various methods have been utilized for H₂O₂ sensing based on different sensing principles. Colorimetric sensors use chemicals to react with H₂O₂ in solution and detect its presence qualitatively by observing color fading of analyte solution or quantitatively by measuring changes in the absorbance of visible light.¹⁹ Fluorescence sensors utilize luminescent materials that can absorb UV light and re-emit it as visible light where H₂O₂ induced quenching of signals provides quantitative sensing.^20,21 Fluorescence microscopic imaging has been used to visualize intracellular H₂O₂ near cellular organelles, such as mitochondria.²² Some optical sensors based on electrochemiluminescence utilize electron stimuli to generate ROS, which reacts with H₂O₂ to generate luminescent species in their excited state. The emitted light intensity from these luminescent species is proportional to the H₂O₂ concentration.^23–25 Sensors based on field-effect transistors detect H₂O₂ by measuring the current intensity changes between the source and drain electrodes induced by surface potential changes of the gate electrode upon H₂O₂ binding.²⁶ Surface-enhanced Raman spectroscopy sensors measure the change in Raman scattering of active plasmonic surfaces, such as Au or Ag, upon exposure to H₂O₂.²⁷ Surface plasmon resonance sensors measure variations in the angle of reflection of incident light of active plasmonic surfaces induced by H₂O₂ adsorption.²⁸ There are also some less common H₂O₂ sensing methods, including calorimetric,²⁹ magnetic,³⁰ and coulometric approaches,³¹ as well as gas/liquid chromatography.^32,33

Electrochemical sensors utilize electron-driven chemical reactions of H₂O₂ for sensing, such as the oxidation/reduction of H₂O₂ by electrocatalysts on the surface of electrodes, which generate electrical currents based on H₂O₂ concentration. The oxidation of H₂O₂ (H₂O₂ → O₂ + 2H⁺ + 2e⁻; E⁰ = 0.68 V vs. normal hydrogen electrode (NHE)) releases electrons at anodes. Reducing H₂O₂ (H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O; E₀ = 1.77 V vs. NHE) requires receiving electrons from cathodes. They are further categorized into amperometric, potentiometric, or impedance sensors based on their measured concentration-dependent variables, i.e., current, voltage, or resistance, respectively. Electrochemical sensors are most widely used for H₂O₂ sensing due to their high sensitivity, low cost, and simple operation. They can also be combined with other methods to form multiple sensing platforms, for example, photoelectrochemical sensors,^34–37 which utilize visible light to drive the electrochemical reaction, plasmon-assisted electrochemical sensors,^38,39 organic electrochemical transistors,⁴⁰ and spectroelectrochemical sensors.⁴¹

Although significant efforts have been devoted to developing electrochemical H₂O₂ sensors, to the best of our knowledge, no comprehensive review has described both recent state-of-the-art studies of electrocatalyst materials used in electrochemical H₂O₂ sensors and the design of electrodes to achieve ultrasensitive H₂O₂ detection. Here, we first describe several widely used electrode structures, including rotation disc electrodes, freestanding electrodes, all-in-one electrodes, and representative commercial H₂O₂ probes. Next, we discuss the latest electrocatalyst design strategies in two areas: increasing the number of active sites and improving their intrinsic catalytic activities for H₂O₂ redox reactions. Finally, we propose several research directions that we expect to play critical roles in enabling practical applications.

2. Electrode structures of electrochemical H₂O₂ sensors

A key component of H₂O₂ electrochemical sensors is the electrodes, and electrochemical reactions occur on the electrode surfaces. Several types of electrodes have been used in research labs or in field applications, including glassy carbon electrodes (GCEs) and freestanding electrodes used in 3-electrode cells and integrated all-in-one electrodes, such as screen-printed electrodes (SPEs) (see Fig. 2) or integrated probes in commercial products (Table 1).

Fig. 2 Common electrochemical H₂O₂ sensing platforms used in research labs. (a) A 3-electrode cell controlled by a potentiostat. Adapted with permission from ref. 42. Copyright 2015, American Chemical Society. (b) A portable sensing platform using an all-in-one screen-printed electrode (SPE) and a hand-held commercial potentiostat (PalmSens).

Table 1 Comparison of different types of electrodes used in H₂O₂ electrochemical sensors

Electrodes	Advantages	Drawbacks
Glassy carbon electrodes (in 3-electrode cells)	High sensitivity and reproducibility	No intrinsic catalytic activity (requires the deposition of catalysts with binders)
	Low noise in the signal
	Fast detection	High cost
	High conductivity	Limited to research labs
Freestanding electrodes (in 3 electrode cells)	Wide detection ranges	High noise in the signal
	High intrinsic activity	Low reproducibility
	No binders required	Long response time
Integrated all-in-one electrodes (e.g., screen-printed electrodes or all-in-one probes)	Suitable for portable detections	Low sensitivity
	Low cost	Low conductivity substrates (mostly made of plastics or ceramics)
	Low sample volume
	Can be flexible/wearable	Low or no catalytic activity
	Many commercial products	Additional surface modifications may be required

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Table 2 Comparison of freestanding macroscopic 3D working electrodes

Freestanding electrodes	Advantages	Drawbacks
Carbon-based	High conductivity	Low intrinsic catalytic activity (require surface modifications and the deposition of active catalysts)
	Low cost
	Biocompatibility
Metal-based	High intrinsic catalytic activity	Low surface area
	High conductivity	High cost
Unconventional materials	Low cost	Low intrinsic catalytic activity (require surface modifications and the deposition of active catalysts)
Glass, Si, ITO, FTO, PET, and ceramics	Robust in many practical applications
Electrode arrays (micro and nano)	Multiplexing	High fabrication cost
	Minimized mass transfer resistance	Low utilization of surfaces

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A typical configuration of electrochemical H₂O₂ sensors involves a 3-electrode cell, including a working electrode, a reference, and a counter electrode. The electrodes are immersed in H₂O₂ containing analytes and controlled by a potentiostat. A bulky potentiostat would limit sensors’ portability. The reference and counter electrodes are typically silver/silver chloride (Ag/AgCl) and platinum (Pt) wire/mesh, respectively. Several types of working electrodes have been used with significantly different sensing performances, including rotation disc electrodes (RDEs), freestanding electrodes in compact 2D films, 3D foams, or 1D wires, and microelectrode arrays. Another electrode configuration combines all three electrodes on a single substrate suitable for portable on-site detection. We describe these working electrodes and their characteristics below.

2.1. Rotation disc electrodes

RDEs rotate themselves during operation, effectively preventing mass transfer limits in solutions. They offer high sensitivity, reproducibility, and signal/noise ratios. GCEs are the most commonly used in H₂O₂ sensing among various available carbon or metal-based RDEs. Glassy carbon has high electrical conductivity, is resistant to chemical corrosion, and is inert to H₂O₂ reduction or oxidation. Electrocatalysts are deposited on GCEs to catalyze reactions involving H₂O₂. However, the cost of GCEs is relatively high. Their small surface area also constrains the mass loading of electrocatalysts, which restricts the lower detection limits of H₂O₂ sensors. Furthermore, it is also tedious to prepare electrocatalyst ink and deposit the ink on GCEs. Thus, the usage of GCEs is usually restricted to research labs.

2.2. Freestanding electrodes

Freestanding electrodes use mechanically stable and electrically conductive substrates to host electrocatalysts. They have much larger surface areas than GCEs to accommodate a high mass loading of electrocatalysts. Sometimes, electrocatalysts are directly synthesized on substrates of freestanding electrodes, which avoids electrocatalyst ink preparation and deposition and the use of nonconductive polymer binders in ink, improving the electrical conductivity of electrodes. Freestanding electrodes have different structures, as illustrated in Fig. 3a, including compact 2D films, 3D foams, and 1D wires.

Fig. 3 Freestanding working electrodes used for H₂O₂ sensing. (a) Freestanding electrodes in different shapes: compact 2D film, porous 3D foam, and 1D wire. (b) An rGO–CNT paper electrode demonstrated good mechanical strength and flexibility. Reproduced with permission from ref. 52. Copyright 2015, Elsevier. (c) Bendable 3D polyimide foam electrode coated with rGO nanosheets and Pd nanoparticles. Reproduced with permission from ref. 59. Copyright 2019, American Chemical Society. (d) Vertical Au nanowire-based stretchable electrodes on a polydimethylsiloxane (PDMS) substrate. Reproduced with permission from ref. 66. Copyright 2018, American Chemical Society. (e) A steel microneedle electrode for H₂O₂ sensing in living cells. Reproduced with permission from ref. 67. Copyright 2017, Royal Society of Chemistry. (f) A graphene microfiber-based flexible electrode. Reproduced with permission from ref. 68. Copyright 2018, Elsevier. (g) A 3D printed Ag microelectrode array. Reproduced with permission from ref. 69. Copyright 2016, Elsevier. (h) A CNT nanoelectrode array supported on a Si substrate. Reproduced with permission from ref. 72. Copyright 2017, Elsevier.

2.2.1 2D film electrodes. Compact 2D film-shaped freestanding electrodes use various 2D compact films as substrates, including paper and films made of different carbon materials.^43–45 For example, photo paper coated with Ag nanoparticles was used as freestanding electrodes for H₂O₂ sensing.⁴³ Carbon materials offer high electrical conductivity. For example, carbon fiber films⁴⁴ and electrospun carbon nanofiber films⁴⁵ contain highly conductive 1D electron transport channels. Carbon cloth made up of micrometer-sized carbon fiber bundles has also been used.⁴⁶ Furthermore, graphene films offer high electrical conductivity and large surface areas, obtained by either assembling reduced graphene oxide (rGO) nanosheets^47–49 or direct synthesis by chemical vapor deposition (CVD).^50,51 Carbon nanotubes (CNTs) have been incorporated in rGO films to prevent the stacking of graphene nanosheets, resulting in freestanding rGO–CNT paper electrodes with larger surface areas and good mechanical strength and flexibility (Fig. 3b).⁵² On the contrary, although graphite paper also has high electrical conductivity, it is seldom used to fabricate freestanding electrodes for H₂O₂ sensing because of its low surface area.

2.2.2 3D foam electrodes. 3D metal foams provide a large surface area to host electrocatalysts, and they also have interconnected channels to enable efficient mass transfer. They are often used as substrates in freestanding electrodes. For example, Cu foam-based freestanding electrodes have delivered ultra-high sensitivity (up to 6349 μA mM⁻¹ cm⁻²) in H₂O₂ sensing.^53,54 Similarly, Ni foams loaded with electrocatalysts have also acted as freestanding electrodes.⁵⁵ Alternatively, 3D graphene foams with hierarchically porous structures and stacked graphene nanosheets can also serve as freestanding electrodes. For example, Cao et al. affixed a CVD-grown 3D graphene foam on a glass wafer, coating one side of the graphene foam with Ag paste as an electrode for amperometric sensing of H₂O₂.⁵⁶ Loading catalytically active metallic nanoparticles (e.g., Pt, Cu₂O and Prussian blue) can further boost the H₂O₂ sensing performances of graphene foam electrodes.^49,57,58 Furthermore, Yang et al. coated rGO nanosheets on a 3D polyimide foam, followed by electrodeposition of Pd nanoparticles (Fig. 3c). The resulting Pd/3D rGO@PI foam showed high catalytic activity for H₂O₂ reduction.⁵⁹

2.2.3 1D wire electrodes. 1D wire-shaped freestanding electrodes are attractive for H₂O₂ sensing, and in particular micrometer size fiber electrodes can be used to monitor neurotransmitters. For example, microelectrodes made of commercial carbon fibers (7 μm in diameter) were used in an in vitro system for H₂O₂ sensing in 2010, demonstrating a sensing limit of 2 μM in rat brain tissues.⁶⁰ Carbon fiber-based microelectrodes also enabled highly selective amperometric sensing of dynamic H₂O₂ concentration changes in a rat brain induced by microinfusion of artificial cerebrospinal fluid.⁶¹ The surface of carbon fiber electrodes was often further functionalized to introduce electroactive species, which boosted their sensing performances.^62,63 Selective H₂O₂ sensing in the brain is often challenging because other molecules, such as adenosine and histamine, can also be oxidized under a similar voltage as H₂O₂. A size-exclusion membrane (1,3-phenylenediamine) was electrodeposited on carbon fiber-based microelectrodes, blocking other neurochemicals and enabling selective H₂O₂ sensing.⁶⁴ Furthermore, the volume of analytes in biochemical applications is sometimes tiny. Zhang et al. combined micropipette tips with carbon microfiber electrodes to achieve efficient H₂O₂ sensing in 20 μL samples where H₂O₂ was generated as an intermediate reaction product of glucose electro-oxidation.⁶⁵ Metal wires and nanowires have also been used as 1D wire electrodes. Zhai et al. used vertical Au nanowires (v-AuNWs) with nanoparticle heads on one side and nanowire tails on the opposite side to seamlessly integrate with soft and curvilinear biological tissues (Fig. 3d).⁶⁶ The resulting nanowire electrodes achieved an H₂O₂ detection limit of 80 μM with a linear range of 0.2–10.4 mM at 20% strain. Furthermore, a stainless-steel microneedle with its tip coated with electroactive nanomaterials was used as a 1D wire electrode for H₂O₂ sensing in living cells (Fig. 3e).⁶⁷ Carbon nanomaterials, such as CNTs and graphene, can also be integrated into fibers as 1D wire freestanding electrodes, offering a high surface area and excellent electrical conductivity. For example, Peng et al. synthesized graphene fibers of 50 μm using a hydrothermal method. A graphene fiber was sealed in a tube with epoxy resin with an exposed section of 5 mm in length (Fig. 3f). The graphene fiber electrode demonstrated good H₂O₂ sensitivity and biocompatibility with living cells.⁶⁸

2.2.4 Microelectrode/nanoelectrode arrays. Besides relying on a single electrode, microelectrode arrays (MEAs) or nanoelectrode arrays (NEAs) have been fast-developing directions for electrochemical H₂O₂ sensors. In general, electrode arrays can increase the sensing sensitivity and provide the opportunity to obtain multiplexed data. Yang et al. 3D printed an MEA (6 × 2 mm) made of Ag microelectrodes (30 × 15 μm), in which individual microelectrodes were spaced 30, 100, or 180 μm apart from each other.⁶⁹ The authors studied the effects of microelectrode density in MEAs on the overall H₂O₂ sensing performance of MEAs. It was revealed that a high microelectrode density (30 μm apart) had the highest sensitivity. However, the overlap of the diffusion layer from neighboring microelectrodes increased the noise level (12 nA) and sensing limit (2.7 μM). In contrast, the MEA with a low electrode density (180 μm apart) showed a reduced noise level (1.2 nA) and higher sensing limit (1.7 μM) but suffered from low sensitivity. Hence, the authors concluded that an intermediate microelectrode density (100 μm apart) would be optimum for H₂O₂ sensing. NEAs are expected to further improve the sensing performance with a higher nanoelectrode density and minimized diffusion layer overlapping. When NEAs are supported on a macroscopic conductive substrate, they can also be used as a freestanding electrode. Several studies have explored NEAs for H₂O₂ sensing, such as arrays of Au nanospheres supported on a Si substrate⁷⁰ and Fe oxide nanopatterns supported on an indium tin oxide substrate.⁷¹Fig. 3h shows vertically aligned CNT nanoarrays grown on a Si wafer as an NEA. The authors studied the effect of flow patterns on its electrochemical performance and concluded that through-flow sensing produced much higher signals than stirred sensing.⁷²

2.3. All-in-one electrodes

All-in-one electrodes have working, counter, and reference electrodes fabricated together on one substrate as a single chip or packaged into one container as a single probe. Their integrated structure and compact design are most suitable for portable sensing applications. We describe both these types of all-in-one electrodes and some representative commercial products in the subsections below.

2.3.1. Single chips with multiple electrodes. Ink containing various active materials can be printed on inert substrates, such as ceramics, plastics, or paper, to fabricate multiple electrodes as sensing chips. These chips are often designed for low-cost disposable one-time use. However, because of their less conductive substrates, their electrochemical performance is usually inferior to that obtained using GCEs. Different printing or patterning methods have been used to fabricate electrodes. Screen printing is one of the most widely used methods because of its excellent versatility. Commercial companies, such as Pine Research, Dropsens, and Lanprintech, provide various SPEs. Many research studies have utilized these commercial SPEs for H₂O₂ sensing with working electrodes further modified by additional electrocatalysts to enhance the H₂O₂ sensing performance. For example, N-doped graphene nanoribbons,⁷³ chitosan-decorated pristine graphene,⁷⁴ 3D graphene/Co oxides,⁷⁵ and Au nanoparticles supported on carbon black⁷⁶ have been used to modify SPEs. These electrocatalysts were deposited on SPEs by drop-casting,^73,76 or electrodeposition.⁷⁴ Commercial companies also provide SPEs modified with electrocatalysts specifically designed for H₂O₂ sensing. For example, Dropsens and Rusens sell SPEs modified by ferrocyanide and Prussian blue, respectively. Zimmer & Peacock sell SPEs modified by some enzymes.

Furthermore, sensing chips on flexible substrates can be used as wearable sensors in contact with human skin for monitoring various health and environmental conditions.^77–80 However, few studies have explored such chips for H₂O₂ sensing, which may be because continuous H₂O₂ monitoring in human body fluids is not in high demand, unlike glucose monitoring which is required in diabetes patients. Also, H₂O₂ is usually not present in interstitial fluids (sweat or tears) from the skin and body, unlike some other analytes (e.g., lactate, glucose, Ca). Nonetheless, Santhiago et al. fabricated an H₂O₂ sensing chip by printing electrode patterns on cellulosic paper using ink containing carbon black and redox-active Prussian blue nanoparticles.⁸¹ Apart from its H₂O₂ sensing capability, the chip showed excellent mechanical stability with excellent folding stability (>20 000 cycles), and its electronic circuit can be crumpled without significant sensing performance loss. Maier et al. also fabricated H₂O₂ sensing chips by printing carbon materials and Prussian blue on a paper substrate.⁸ The chips were used to monitor the concentration of H₂O₂ in artificial breaths. They were integrated into commercial masks for H₂O₂ sensing of the exhaled breath of patients with respiratory diseases, reducing the high cost typically required for breath collection and analysis.

2.3.2. Single probes with packaged electrodes. Using separated reference electrodes and working electrodes often brings various inconveniences in practical applications. Thus, various commercial products have integrated a reference electrode with an H₂O₂ sensing working electrode into a single probe (see the commercial products illustrated in Fig. 4a and b). There are also many research studies on developing single probes for challenging measurement conditions. For example, Fig. 4c shows a probe with microelectrodes sealed in an insulated container, used to monitor H₂O₂ related ROS generated in cold atmospheric plasma under ultrahigh voltages.⁸² It is desirable to evaluate the defense potential of biofilms against electrochemically produced H₂O₂ in bioelectrochemical systems. However, it is impossible to measure H₂O₂ concentrations under the interference of a strong electric field using common electrodes.⁸³Fig. 4d shows an all-in-one probe used in a bioelectrochemical system to monitor H₂O₂ concentration. The probe was also used to measure H₂O₂ decomposition in filamentous fungus to understand the microorganism's antioxidant defense mechanism. The fungus was alive after the electrochemical test, indicating the benefit of using an integrated probe.⁸⁴

Fig. 4 All-in-one probe sensors for H₂O₂ sensing. (a and b) Structures of commercial H₂O₂ probes for biological applications (WPI HPO sensors). (c and d) H₂O₂ probes for H₂O₂ measurement under the interference of high voltages and currents, respectively. Reproduced with permission from ref. 82 and 83. Copyright 2019, American Chemical Society & Copyright 2015 Elsevier respectively.

2.3.3. Representative commercial H₂O₂ probes. Commercial electrochemical H₂O₂ sensors usually utilize designs of a single probe or a single chamber, which encapsulates all electrodes. They are convenient to be used in practical scenarios. For example, an HPO macro-sensor from World Precision Instruments (WPI) encapsulates an H₂O₂ sensing electrode and a reference electrode in a single 2 mm diameter stainless steel probe. The probe also contains internally refillable electrolytes and a gas permeable membrane to prevent unwanted species from entering the probe tip. An H₂O₂ microsensor from WPI uses a 100 μm diameter carbon fiber working electrode tip, enclosed in a hypodermic needle to insert into tissues beneath the skin. An L-shape working electrode design allows tissue bath studies. A list of representative commercial all-in-one H₂O₂ sensors and their key features are provided in Table 3.

Table 3 Representative commercial H₂O₂ electrochemical sensors and their key features

Probe name	Electrode materials	Housing materials	Key features
World Precision Instruments (WPI) ISO-HPO-X sensors	Platinum wire sensing electrode (HPO-2)	Stainless-steel	HPO-2 macrosensor: 100 nM–100 μM range, 8 pA μM^−1 sensitivity, for uses in cell cultures and related applications.
	Activated carbon fiber sensing electrode (HPO-100)		HPO-100 microsensor: 1 nM–1 mM range, sensitivity ≥ 1 pA nM^−1, for use in tissues and microvessels.
Seko Kontrol 500 Probe	Platinum and copper electrodes	ABS plastic material IP65	0–500 ppm, for uses in swimming pools and wastewater with a flow rate up to 30 L h^−1 and pressure up to 5 bar.
ChemDAQ's Steri-Trac H2O2 area monitor	A precious metal electrode on a porous PTFE membrane	Plastic material	0–10 ppm, 0.1 ppm resolution, for uses to monitor workplace exposure of sterilant H2O2 vapors, and wireless sensing with audible and visual alarms.
Gazdetect B12 H2O2 detector	Sensor material: Noryl	NEMA 4X polystyrene	0–10 ppm with 4–20 mA output display powered with 24 V for uses in vapor sensing, high corrosion resistance, and humidity tolerance (up to 100%).
Edaphic Scientific (ES) H2O2 micro-sensor	Silicone (membrane), glass (sensor), epoxy resin	Titanium	0.02–10%, for uses to in situ measuring and monitoring aqueous H2O2 concentrations in wastewater (colored and turbid as well). It can be used in water depths of up to 100 m, 100 bar.
ProMinent DULCOTEST H2O2 sensors (PER1 and PEROX)	Diaphragm-covered electrodes	Shaft/diaphragm material: PVC/silicone or stainless-steel/PVDF	0.5–2000 mg L^−1, PER1 for uses to measure H2O2 concentration in chemically contaminated and polluted flowing water. PEROX for uses in measuring H2O2 concentration without cross-sensitivity to chlorine.
Analytical Technologies Inc. (ATI) H2O2 Q46	Platinum electrode covered with a membrane	NEMA 4X (IP-66) and polycarbonate enclosure	0–2000 ppm, 0.05 ppm sensitivity, for uses to continuously measure the concentration of H2O2 in cooling water, aquarium, and wastewater plants.
Dräger XS EC H2O2 Sensor	Porous polymer membrane coated electrodes	Plastic	0–20 ppm, 0.1 ppm resolution, for uses to measure vaporized H2O2 in ambient air

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3. Electrocatalysts for H₂O₂ sensing

Electrochemical H₂O₂ sensing usually depends on 2-electron oxidation or reduction reactions of H₂O₂ on the surface of electrocatalysts. Thus, the performance of H₂O₂ sensors is strongly affected by the catalytic activity of electrocatalysts. In general, the activity of electrocatalysts is determined by the intrinsic activity of their catalytic sites and the total number of accessible catalytic sites. In recent years, various approaches have been explored to improve the activity of electrocatalysts used in electrochemical H₂O₂ sensors. We review these advances in the following two subsections, and the performance of representative electrocatalysts used for ultrasensitive H₂O₂ sensing is compared in Table 4.

Table 4 Design strategies and performance comparison of representative electrocatalysts used for ultrasensitive H₂O₂ sensing with a nanomolar level detection limit

Electrocatalysts	Design strategy	Potential (V)	Sensitivity μA mM^−1 cm^−2	Detection limit	Ref.
Electrocatalysts deposited on glassy carbon electrodes
1% Cu–N/C	N-doping, pore structuring	−0.7	2992	40	135
Pt-skin Pt3Co z-NWs/C	More active facets, alloying, conductive support	−0.05	3030	0.4	166
MoS2 NPs	Ultra-small nanoparticles	−0.25	2580	2.5	129
Au67Cu33 NWs	Ultrathin nanowires, alloying	+0.48	2710	2.0	131
PtFe Clusters/rGO	Alloying, pore structuring, conductive support	+0.1	4450	2.0	173
Electrocatalysts used in freestanding electrodes
ZnCo2O4/Co3O4/Cu foam	Alloying, conductive support, metal oxides	−0.1	3260	1.0	148
Fe3N–Co2N nanowire arrays on carbon cloth	Metal nitrides, alloying, conductive support	−0.2	2273.8	59	174
Ni(OH)2/rGO/Cu2O on Cu foil	Metal oxides, conductive support (nano and macro)	+0.6	1706.3	200	175
CuO nanowire array on Cu foam	Metal oxide, conductive support	−0.4	5750	560	53

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3.1. Increasing the number of active sites

3.1.1. Pore structuring. A porous structure is often desirable for electrocatalysts, which facilitates the mass transfer of electrolytes/analytes to reach catalytically active sites and provides a large surface area to host more catalytically active sites. Porous structures can be created by either top-down and bottom-up methods. The calcination of precursors with well-defined porous structures, such as metal–organic frameworks, is one of the most widely used top-down methods. Furthermore, polymers and various organic compounds are often used as sacrificial materials to create pores. They would degrade and release gases during the calcination. Another common top-down method is to use chemical or physical etching to create pores.⁸⁵ Alternatively, using various templates is one the most common bottom-up methods, where nanostructures are formed on templates mimicking their shape. Hard templates, such as silica or other metal oxide nanoparticles, need to be removed using concentrated acids or base solutions. Soft templates, such as block polymers or surfactants, can be removed more easily by calcination in air or dissolution in mild solvents. Hard templates are more often used to synthesize crystalline nanostructures because the required high-temperature annealing conditions may destroy the porous structures formed by soft templates.⁸⁶ 2D materials, such as graphene and inorganic nanosheets, can self-assemble into 3D materials with hierarchal porous structures. However, less control of the pore size and shape is currently achieved using 2D materials.

The methods described above have been explored to create various porous electrocatalysts for H₂O₂ sensing. For example, Yao et al. used a soft micelle template to prepare Pd nanostructures with 2 nm lamellar pores (Fig. 5a), which demonstrated a high sensitivity of 201 μA mM⁻¹ cm² in H₂O₂ sensing.⁸⁷ 3D hierarchal porous N-doped carbon electrocatalysts were synthesized for H₂O₂ sensing. They were obtained by carbonizing a mixture of glucose (acting as the carbon source) and melamine (acting as an N dopant and pore-forming agent) in two steps at 600 and 900 °C, respectively. The resulting electrocatalyst has a high surface area of 431.4 m² g⁻¹ with an average pore size of 10–23 nm.⁸⁸ Das et al. inkjet-printed graphene ink into 3D graphene electrocatalysts and then irradiated them with a UV light laser.⁸⁹ When the laser power increased from 40 to 85 mJ cm⁻², the catalyst surface morphology changed dramatically (Fig. 5b), and the sensitivity of H₂O₂ sensing increased from 0.54 to 3.32 μA mM⁻¹. The authors proposed that the light irradiation led to the formation of a unique 3D petal-like porous structure and a reduction in graphene sheet electrical resistance.⁸⁹ Several other studies also synthesized porous graphene foam electrocatalysts for H₂O₂ sensing by bottom-up assembling graphene oxide nanosheets.^49,57,58

Fig. 5 Electrocatalysts with porous structures for H₂O₂ sensing. (a) A bottom-up method to synthesize Pd nanostructures with lamellar pores. Reproduced with permission from ref. 87. Copyright 2017, American Chemical Society. (b) Top-down laser etching approach for H₂O₂ sensing. Reproduced with permission from ref. 89. Copyright 2016, Royal Society of Chemistry.

3.1.2. Conductive supports. Electrocatalysts containing metallic nanoparticles often suffer from poor stability due to the fast agglomeration of nanoparticles at high temperatures or under working conditions, which can be alleviated by anchoring metallic nanoparticles on conductive supports. Different methods have been used, including co-precipitation, chemical reduction, hydrothermal reduction, electrodeposition, dip coating, and sputtering. Conductive supports made of carbon materials are the most popular choice because of their low cost, high surface area, and good biocompatibility. They can be categorized based on their graphitic degrees. Partially graphitic carbon materials, such as carbon black, are widely used in commercial Pt/C electrocatalysts because of their low cost and high electrical conductivity.⁹⁰ Carbon nanofibers prepared by carbonization of polymer precursors are also widely used as conductive supports for metal nanoparticles in the electrocatalysts used for H₂O₂ sensing.^45,91–95 Fully graphitic carbons, such as CNTs^63,96–101 and graphene,^{36,47,52,102–112} are frequently used as conductive supports due to their large surface area and high electrical conductivity. Furthermore, graphene quantum dots have also been used as conductive supports to anchor metallic nanoparticles for H₂O₂ sensing.^113,114 One key advantage of nanocarbon supports such as CNTs is their excellent biocompatibility, due to which natural enzyme-based redox catalysts can be immobilized on them for biosensing. Zhang et al.¹¹⁵ loaded horseradish peroxidase enzymes on poly-L-lysine functionalized single-walled CNTs for amplified amperometric H₂O₂ sensing with a detection range of 1 μM to 10 mM. In another study,¹¹⁶ glucose oxidase was immobilized on CNT via a bridge of ionic liquid-like units for electrochemical biosensing. Beyond carbon supports, some atomically thin metal or non-metal-based layered materials have also been explored as conductive supports in electrocatalysts used for H₂O₂ sensing, including layered double hydroxides,¹¹⁷ metal dichalcogenides,^118,119 MXene,¹²⁰ or phosphorene.¹²¹

Macroscopic porous foams or aerogels are also popular as conductive supports, especially in fabricating freestanding electrodes. Porous carbon foams with hierarchal pore structures can effectively facilitate the mass transfer of electrolytes/analytes.¹²² Some metal foams have high electrical conductivity and show intrinsic catalytic activity for H₂O₂ sensing. Nanostructured electrocatalysts directly synthesized on metal foams have demonstrated some of the best performances in H₂O₂ sensing reported so far. For example, Huang et al. fabricated copper oxide (CuO) nanowire arrays on Cu foam using a facile two-step method involving alkaline oxidation of the Cu foam and subsequent low-temperature Cu(OH)₂ thermolysis in the air at 190 °C, as illustrated in Fig. 6a.⁵³ This electrocatalyst showed a high H₂O₂ sensitivity of 5750 μA mM⁻¹ cm⁻² when used as a freestanding electrode. The electrochemical impedance spectroscopy results revealed that its unique porous nanowire array structure enabled faster electron and mass transfers than planar CuO grown on the Cu foam. In another work, Cu(OH)₂ nanowires were first grown by electrochemically anodizing Cu foam and then phosphidizing in an inert atmosphere at 300 °C to form pipet-like Cu₃P nanowires with root and tip diameters of 300 and 100 nm, respectively (Fig. 6b).⁵⁴ These Cu₃P nanowires were intimately connected with the Cu foam support, achieving a sensing limit of 2 nM H₂O₂.

Fig. 6 Nanostructured electrocatalysts synthesized on metal foams. (a) CuO nanowires grown on Cu foam. Reproduced with permission from ref. 53. Copyright 2015, Royal Society of Chemistry. (b) Cu₃P nanowires grown on Cu foam. Reproduced with permission from ref. 54. Copyright 2016, American Chemical Society.

3.1.3. Reducing catalyst size. Reducing the size of metal or metal composite nanoparticles/nanosheets/nanowires may expose more catalytically active catalyst sites, resulting in improved H₂O₂ sensing activity.¹²³ Various methods, such as impregnation, co-precipitation, atomic layer deposition, or space confinement, have been utilized for this purpose.^124,125 Depositing a low mass loading of metal nanoparticles on a large surface area support can alleviate the aggregation of single atoms or metal clusters into large particles. For example, Riaz et al. loaded 4.8 wt% Pt on a high surface area nanocarbon support, which achieved an H₂O₂ sensitivity higher than that of 20 wt% Pt/C.¹²⁶ Li et al. anchored Co nanoparticles (10–50 nm) and atomic Co–N_x moieties on N doped CNT arrays by the pyrolysis of a sandwich-like urea@ZIF-67 complex.¹²⁷ The electrocatalysts were deposited on SPEs and demonstrated high sensitivity and a low detection limit of 32.4 nM. Gao et al. synthesized electrocatalysts containing thiol-stabilized nanoclusters with sub-nanometer cores of 6 Cu atoms (0.37 nm in length and 0.27 nm in width). Due to their small size, the electrocatalyst showed a lower detection limit and a broader linear H₂O₂ sensing range than Cu₂O nanoparticle-based electrocatalysts.¹²⁸ Wang et al. obtained ultra-small MoS₂ nanoparticles with a narrow size distribution (1–2 nm) by sonication of bulk MoS₂ and density gradient centrifugation at 12000 rpm. Due to their large surface area and abundant edge sites, these nanoparticles showed a high H₂O₂ sensitivity of 2580 μA mM⁻¹ cm⁻² with a low detection limit of 2.5 nM.¹²⁹ Shu et al. expanded the interlayer distance of MoS₂ from 6.15 to 9.40 Å using excessive thiourea and a shorter hydrothermal reaction time, resulting in interlayer expanded MoS₂ with exposed monolayer nanosheets. The interlayer expanded MoS₂ showed much higher H₂O₂ sensing sensitivity (1706.0 μA mM⁻¹ cm⁻²) and a low detection limit (0.2 μM) compared to non-expanded MoS₂.¹³⁰ Wang et al. synthesized ultrathin Au–Cu nanowires (∼4 nm diameter) (Fig. 7a), which achieved a low detection limit of 2.0 nM.¹³¹ Sun et al. synthesized atomic thick PtNi nanowires with an average diameter of 0.9 nm and an average length of 120 nm, and then assembled them on rGO nanosheets (Fig. 7b).¹³² The electrocatalyst showed high sensitivity in H₂O₂ sensing with a detection limit of 0.3 nM. Similarly, ultrathin Ag nanosheets (<10 nm) were also utilized for H₂O₂ sensing.¹³³ A fast-growing strategy is to reduce the size of catalytic sites to single atoms, forming single atom catalysts. Metal–nitrogen–carbon (M–N–C) active sites are active for H₂O₂ oxidation and reduction. Some studies have synthesized M–N–C electrocatalysts by carbonizing zeolitic imidazolate frameworks for H₂O₂ sensing.^134,135 They demonstrated excellent performances with low metal mass loadings.

Fig. 7 Ultra-small metal nanowires for H₂O₂ sensing. (a) Au–Cu nanowires and (b) Pt–Ni nanowires. Reproduced with permission from ref. 131 and 132. Copyright 2014 & 2017, American Chemical Society.

3.2. Increasing intrinsic activity

3.2.1. Alloying. Alloys of multiple metals often show higher intrinsic activity than single metals due to the synergistic effect and optimum electronic structures in alloys. The synergistic effect means combining two metals resulting in an overall higher activity than the sum of individual metals used. In many cases, there is an excellent enhancement in their specific physical and chemical properties than in individual metals. For example, Guler et al.¹³⁶ synthesized Pd@Ag alloy nanoparticles on rGO supports which showed a higher sensitivity 1307 μA mM⁻¹ cm⁻² than sensors using individual metal nanoparticles (966 μA mM⁻¹ cm⁻² for Pd/rGO/GC and 150 μA mM⁻¹ cm⁻² for Ag/rGO/GC).

Various binary^136–139 and ternary¹⁴⁰ alloys of noble metals and transition metal alloys^{110,141–143} have been explored as electrocatalysts for H₂O₂ sensing. Some alloy particles have unique structures, such as core–shell structures^144,145 and yolk–shell structures,¹⁴⁶ which may further boost the catalytic activity. Besides alloy nanoparticles, alloy nanowires¹³² and nanosheets¹⁴⁷ of binary metals have also been explored for H₂O₂ sensing.

Nagaiah et al. alloyed Pd with a second noble metal (i.e., Au or Pt) by electrodeposition on graphite electrodes. The alloy electrocatalysts showed improved performances in H₂O₂ sensing. Visualization of local catalytic activity by scanning electrochemical microscopy revealed that the catalytic activity for H₂O₂ oxidation followed the trend: Pd₅₀Pt₅₀ > Pd₇₀Pt₃₀ > Pd > Pd₉₀Pt₁₀. A similar trend was also observed for Pd/Au alloys.¹⁴⁷ In another study, bimetallic transition metal oxide (ZnCo₂O₄) nanoflowers of 5 μm in diameters were grown on Co₃O₄ nanowires of 100 nm in diameter.¹⁴⁸ The unique morphology and synergic effects between two metal species enabled high sensitivity with a low detection limit of 1 nM.

3.2.2. Doping. Doping elements, such as O, N, and S, into metals results in metal oxides,^{75,103,149,150} metal nitrides,^151,152 and metal sulfides,^153,154 respectively. Some of them demonstrated higher catalytic activities in H₂O₂ sensing than pure metal electrocatalysts. More studies have also studied doping these elements into carbon materials, resulting in metal-free carbon electrocatalysts. Extensive efforts have been devoted to understanding the catalytic activity of N-doped carbon electrocatalysts. It is generally believed that doped N atoms induce higher positive charges at surrounding C atoms, making these C sites more favorable for H₂O₂ oxidation/reduction.^155,156 For example, DFT calculations showed that N atoms in graphene cause charge enhancement on adjacent C and H atoms. H₂O₂ physisorption on doped graphene requires less adsorption energy (−0.15 to −0.47 eV) than undoped graphene (−0.10 eV). N atoms may also provide better stabilization for H₂O₂ oxidation/reduction intermediates.¹⁵⁵ N doped CNTs also showed higher activity in H₂O₂ sensing.¹⁵⁶ For example, N doped CNTs displayed an anodic sensitivity of 830 mA M⁻¹ cm⁻² at 0.05 V, and a cathodic sensitivity of 270 mA M⁻¹ cm⁻² at −0.25 V, lowering the overpotential required for H₂O₂ oxidation and reduction.¹⁵⁷ Some studies claimed that co-doping N and S into carbon materials by pyrolyzing molecules containing N and S atoms, such as thiadiazole and thiourea, can further improve the activity of carbon electrocatalysts in H₂O₂ sensing due to the synergistic effects between N and S.^158,159

Another approach to obtain N-doped carbon electrocatalysts is to directly carbonize N-containing polymers, such as polyacrylonitrile (PAN) and polypyrrole (PPy). For example, pollack et al. synthesized an N-doped electrocatalyst by pyrolyzing electrospun PAN nanofibers at 1050 °C.¹⁶⁰ X-ray photoelectron spectroscopy results suggested that pyridinic and graphitic N have higher activity for H₂O₂ reduction than other N species. In another study, Zhang et al. synthesized N-doped carbon nanoparticles embedded in CNFs by pyrolyzing electrospun PPy/PAN nanofibers, yielding a high sensitivity of 383.9 μA mM⁻¹ cm⁻² for H₂O₂ sensing.¹⁶¹

Furthermore, graphitic carbon nitride (g-C₃N₄) also shows high activity for H₂O₂ reduction. Gomez et al. synthesized g-C₃N₄ nanosheets for H₂O₂ sensing.¹⁶² Melamine powder was first thermally annealed at 550 °C in an inert atmosphere to yield yellow color crystals and then further sonicated in 0.5 M H₂SO₄ solution for 30 min to obtain exfoliated g-C₃N₄ nanosheets as electrocatalysts. Liu et al. studied the effect of protonation of g-C₃N₄ nanosheets on H₂O₂ sensing performance.¹⁶³ g-C₃N₄ nanosheets were treated with concentrated HCl for 3 h at room temperature before they were exfoliated by ultrasonication in HNO₃. DFT calculation results revealed that the protonation changed the band structure of g-C₃N₄ from semiconducting to conducting (no bandgap), leading to better electrical conductivity. Protonated g-C₃N₄ nanosheets showed a better H₂O₂ sensing performance.

3.2.3. Tuning crystal facets. Different surface facets of nanocrystals may have different catalytic activities.¹⁶⁴ Nanocrystals in different shapes have different proportions of surface facets, which can be controlled by nucleation via adjusting their experimental synthesis parameters, such as the concentration of precursors and temperature. Nuclei growth can occur either in thermodynamic or kinetic regimes, and the interplay of these two regimes strongly influence the final structure of nanocrystals. High index surface facets usually have higher catalytic activities than low-index facets because they have more kinks, atomic steps, edges, and low coordinated atoms as active sites. However, high index facets are often unstable due to their high surface energy. High index facets can be formed by fast reduction of precursors in stabilizing agents to direct nuclei growth in the kinetic regime. They can also be formed by facet replication of seeds or selective etching.¹⁶⁵

Electrocatalysts with tuned crystal facets have been used for H₂O₂ sensing. For example, Luo et al. synthesized a Pt skin of high index facets on carbon-supported Pt₃Co nanowires by annealing them in an inert atmosphere at 400 °C (Fig. 8a).¹⁶⁶ The presence of the Pt skin with nearly two atomic layers of 0.5 nm led to a low sensing limit of 0.4 nM and a current density of −1.47 mA cm⁻²_pt, which was 3 and 8 times higher than Pt₃Co nanowires without high index facets and commercial Pt/C electrocatalysts, respectively. In another work, Pt–Cu octahedra were synthesized by introducing Cl ions into Co and Pt salt precursors. Pt–Cu octahedra with (111) facets showed a better H₂O₂ sensing performance than Pt–Cu spherical nanoparticles.¹⁶⁷ Dai et al. synthesized Cu₇S₄ nanocrystals with high index facets using a solvothermal method, demonstrating a better H₂O₂ sensing performance than CuS spheres or flowers.¹⁶⁸ Similarly, the Ni₇S₆ phase with a 3D flower-like structure showed a better H₂O₂ sensing performance than other phases, such as NiS, Ni₃S₄, and Ni₉S₈.¹⁶⁹ Fe₃O₄ nanostructures with a stick shape and α-crystal phase showed better H₂O₂ sensing performances than other crystal phases and morphologies (Fig. 8b).¹⁷⁰ Au nanoparticles decorated on pyramidal Si with (111) facets showed enhanced H₂O₂ sensor performances compared to that on planar Si with (100) facets.¹⁷¹ Zhang et al. synthesized CuO–CoO core–shell nanostructures on Cu foam using a chemical bath deposition method with sequential Cu and Co hydroxide formation, followed by calcination in an inert atmosphere.¹⁷² The CoO shell changed from nanosheets to nanoneedles when the calcination time increased from 2 to 3.5 h. CuO–CoO-2.5 h achieved a sensitivity of 6349 μA mM⁻¹ cm⁻² due to its unique leaf-like morphology. DFT calculations showed that the CoO(200) surface had better electron transport properties than the CoO(111) surface, irrespective of the core surfaces (CuO(111) or CuO(200)). A longer calcination time of 3 or 3.5 h resulted in less CoO(200) surfaces.

Fig. 8 (a) Pt skin with high index facets enabled ultrasensitive H₂O₂ sensing. Reproduced with permission from ref. 166. Copyright 2018, American Chemical Society. (b) Comparison of Fe₃O₄ in different morphologies and crystal structures for H₂O₂ sensing. Reproduced with permission from ref. 170. Copyright 2018, Wiley-VCH Verlag GmbH & Co.

4. Summary and perspectives

We summarize the design strategies of electrodes and electrocatalysts for ultrasensitive electrochemical H₂O₂ sensors with detection limits down to the nanomole and sub-nanomole ranges, which is often encountered in cellular detection of H₂O₂. Both electrode structures and electrocatalysts are critical to the performance of electrochemical H₂O₂ sensors. Their designs are often intertwined together with electrocatalysts seamlessly integrated into electrode structures at the nanometer scale. Novel freestanding electrodes and SPEs are opening up more applications in portable on-site monitoring of H₂O₂. MEAs and NEAs have attracted significant interest in increasing sensitivity and providing multiplexed data. The fast development of electrocatalyst design strategies involving increasing the number of active sites and intrinsic activity at the nanoscale has improved the performance, as shown in Table 2. Based on the current research progress, we propose that the following areas should be priorities in upcoming research:

(1) Multiple electrocatalyst design strategies involving increasing the number of active sites and intrinsic activity should be seamlessly integrated to improve the electrocatalyst performance. Recent studies have demonstrated some success; for example, 3D nanoporous alloyed Pt–Fe clusters were anchored on an rGO support, which combined the strategies of nanoscale pore structuring, conducive support, reducing the catalyst size, and alloying.¹⁷³ Appropriate combinations of the design strategies discussed in this review will likely lead to novel electrocatalysts with improved performance.

(2) Single atoms anchored in high surface area porous and conductive supports can provide well-defined and high density catalytic active sites. Recent studies have reported transition metal (e.g., Fe¹⁷⁶ and Co¹⁷⁷) SACs for H₂O₂ sensing. Pt is one the most active metals as electrocatalysts for H₂O₂ oxidation/reduction. Although Pt SACs have been used in other electrocatalytic applications,^124,125 they have not been explored for H₂O₂ sensing. Pt SACs with a high Pt mass loading and good stability have excellent potential to replace currently used commercial electrocatalysts in electrochemical H₂O₂ sensors.

(3) Achieving durable sensing performances over a long-time operation requires stable electrocatalysts without fast degradation. The degradation of electrocatalysts has been studied for energy conversion processes and energy storage devices,¹²⁶ but it is rarely discussed for electrocatalysts used in electrochemical H₂O₂ sensors. There is a clear gap in current research in this respect.

(4) Most H₂O₂ sensing studies reported in the literature have tested the performance in pure solutions or simple mixtures, which are very different from complex practical samples. For example, blood, food (e.g., milk and juice), and turbid water may contain various interferents, fats, proteins, and suspended particles. Testing the H₂O₂ sensor performance in practical samples would be the first step in understanding how the sensor performance changes in complex solution environments. Next, electrode fouling is expected to be a critical issue for sensing applications in practical samples. Designing electrocatalysts and electrodes with good antifouling properties will be required for many practical sensing applications.

(5) Due to the existence of electroactive interferents at a similar potential of H₂O₂ sensing, it is challenging to develop H₂O₂ sensors with high selectivity in practical samples. The current method is to avoid the potential window of potential interferents. This approach may not work in many practical samples. Some natural enzymes have high selectivity due to their unique 3D structures and catalytic binding sites. However, the stability of natural enzymes is often poor, and their active sites are often buried in complex protein structures with limited access.¹⁷⁸ Developing artificial nano-enzymes that mimic selective catalytic active sites of natural enzymes with 3D porous nanoscale structures to improve their mass and electron transfer would be a promising research direction.^179,180

(6) Continuous H₂O₂ concentration monitoring is required in some applications, such as tracking disinfectants or environmental pollutants in wastewater. Due to different contact modes between fluids and electrodes, the fluid flow patterns would strongly influence the H₂O₂ sensor performance. Thus, designing electrodes suitable for different fluid movement conditions is required. For example, flow-injection cells are often used to assess the practical utility of sensors,⁷² which can assist the development of suitable electrodes for electrochemical H₂O₂ sensors.

(7) H₂O₂ sensing in vapors is rarely reported in the literature. Many medical and industrial settings require detecting aerosolized H₂O₂ in the vapor phase (at ppm level) after the disinfection operation of buildings or rooms. We speculate that designing suitable electrodes to combine gas adsorption and sensing may help in the development of efficient electrochemical H₂O₂ sensors used in the vapor phase.

(8) Current commercial sensors for on-site H₂O₂ monitoring in air or water would cost hundreds of dollars (e.g., Steri-Trac area monitor: $1250, Gazdetect B12 detector: $1080, and Dräger XS sensor: $810). Simplifying sensor designs and lowering their manufacturing costs through nanoscale innovations in electrodes, electrocatalysts, and other components of electrochemical H₂O₂ sensors have the potential to yield more affordable sensors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Australian Research Council under the Future Fellowships scheme (FT160100107).

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