Palladium beats platinum for peroxide sensing in carbon nanotube hybrid electrodes

Electrochemical sensors promise cheap, portable, and disposable platforms for everything from glucose monitoring to drinking-water safety, but their performance ultimately hinges on the electrode material at the heart of the device. A new study published in Results in Chemistry by Fernando Perez, Chloe Miller, and Bhavik Anil Patel at the University of Sussex has now systematically compared three precious-metal blacks—platinum, palladium, and ruthenium—embedded within multi-wall carbon nanotube (MWCNT) composite electrodes, revealing a striking analyte-specific pattern: palladium black outperforms platinum for hydrogen peroxide detection, while platinum black is the superior catalyst for nitrite. The finding challenges the assumption that a single universal catalyst can serve all sensing needs and offers a rational design framework for next-generation multi-analyte sensors.

Hydrogen peroxide and nitrite are two of the most biologically and environmentally relevant small molecules an electrochemical sensor can target. Hydrogen peroxide is a key reactive oxygen species generated in metabolic and inflammatory processes, and it is also the signal-generating product of first-generation oxidase-based biosensors, including the classic glucose oxidase platform. Nitrite, meanwhile, is a major intermediate in nitric oxide metabolism, contributing to vascular regulation and immune function, with altered concentrations indicating endothelial dysfunction or inflammatory states. Beyond biology, nitrite concentrations in drinking water are regulated due to potential health impacts. Yet direct electrochemical detection of both analytes on conventional carbon-based electrodes is inherently difficult: they exhibit slow electron-transfer kinetics and low electrocatalytic activity on carbon surfaces, often requiring high overpotentials that reduce sensitivity and increase susceptibility to interference.

To overcome these limitations, the Sussex team fabricated bulk composite electrodes consisting of 15 wt% MWCNTs, 10 wt% of either platinum black (PtB), palladium black (PdB), or ruthenium black (RuB), and 75 wt% epoxy resin. The 15 wt% carbon loading was selected based on previous optimisation studies from the group, which demonstrated that this composition provides the maximum carbon loading compatible with reproducible composite fabrication and favourable electrochemical performance. The precious-metal loading was fixed at 10 wt%, representing the highest practical loading that yielded mechanically stable and homogeneous composites. The paste was packed into 3D-printed PLA cylindrical casings with a 2 mm internal diameter recess, cured for 48 hours, sanded with silicon carbide, and polished in alumina slurry to produce a flat disc electrode. Unlike surface-modified films or deposited coatings, the precious-metal blacks are incorporated directly within the bulk composite matrix, providing a mechanically robust architecture that anchors the catalytic particles and preserves efficient electron-transfer pathways during prolonged operation.

The intrinsic electrochemical properties of each composite were first assessed using the outer-sphere redox probe hexaammineruthenium(III) chloride in cyclic voltammetry. Surprisingly, the addition of precious-metal blacks significantly reduced the normalised probe current, with the reduction being greatest for the RuB composite. The authors interpret this as evidence that the particulate distribution of RuB particles within the composite significantly impedes the conductive pathways and surface access of the MWCNT network. However, electrodes containing PdB and PtB showed a significant reduction in the peak-to-peak potential separation (∆E), suggesting that these particles actually reduce the contact resistance of the electrode. The team caution that the outer-sphere probe response reflects changes in composite accessibility and electron-transfer pathways, whereas the hydrogen peroxide and nitrite responses are dominated by electrocatalytic activity—so a reduced probe current does not necessarily indicate poorer analytical performance.

Capacitance measurements and electrochemical impedance spectroscopy (EIS) provided further insight. All precious-metal black composites exhibited significantly increased capacitance compared with the unmodified MWCNT electrode, consistent with the incorporation of metal-black particles increasing the electrochemically accessible surface area. The PtB composite had the highest capacitance, followed by PdB and then RuB. Meanwhile, the unmodified MWCNT electrode exhibited the highest charge-transfer resistance, indicating slower interfacial electron-transfer kinetics compared with all the precious-metal black modified electrodes, with no significant differences in Rct observed between the different precious-metal black composites. The simultaneous increase in capacitance and decrease in charge-transfer resistance following precious-metal black incorporation indicate improved electrochemically active surface area and interfacial electron transfer across all three materials.

The analyte-specific results are the centrepiece of the study. For hydrogen peroxide detection at +0.85 V vs Ag|AgCl in pH 7.4 phosphate-buffered saline, the PdB composite performed best with a sensitivity of 0.013 μA μM⁻¹, followed by PtB (0.008 μA μM⁻¹) and RuB (0.003 μA μM⁻¹), with limits of detection of 24, 20, and 17 μM respectively. For nitrite, the ranking flipped: the PtB composite exhibited the greatest sensitivity at 0.0022 μA μM⁻¹, followed by PdB (0.0012 μA μM⁻¹) and RuB (0.0003 μA μM⁻¹), with limits of detection of 30, 56, and 37 μM respectively. Crucially, normalising the current to electrode capacitance rather than conductive area fraction produced the same trends, indicating that the superior performance of PdB for peroxide and PtB for nitrite cannot be explained solely by differences in electrochemically accessible surface area. The authors suggest that differences in the adsorption energies and reaction kinetics of analyte-derived intermediates on Pd and Pt surfaces—which are influenced by the electronic structure of the respective metals—may contribute to the observed analyte-specific behaviour.

Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX) revealed distinct variations in catalyst architecture between the composites. The PdB electrodes showed a highly homogeneous distribution of particles with relatively consistent sizes, with EDX indicating 9.64 ± 0.34% Pd on the surface. The PtB electrodes displayed a mixture of smaller particles together with occasional larger clusters but an overall reasonably uniform spatial distribution, with 8.59 ± 0.16% Pt detected. The RuB electrodes exhibited the greatest heterogeneity, with clear regions of little or no Ru deposition evident despite 8.95 ± 2.76% Ru being detected. Quantitative image analysis showed that RuB composites contained a significantly higher particle density than both PdB and PtB composites despite exhibiting lower electrochemical performance, while PtB composites displayed the lowest nearest-neighbour distance, indicating a more closely packed particle arrangement. These findings suggest that electrocatalytic performance is governed not only by the intrinsic catalytic properties of the precious-metal blacks but also by their spatial organisation within the composite electrode.

Because hydrogen peroxide is the enzymatically generated reporter molecule in first-generation glucose biosensors, the two best peroxide-detecting composites—PdB and PtB—were subsequently evaluated as transducer platforms for glucose oxidase-based glucose sensing. Glucose oxidase was entrapped on the electrode surface via crosslinking with bovine serum albumin and glutaraldehyde. Both biosensors exhibited Michaelis–Menten kinetics with comparable K_M values of approximately 8 mM, consistent with established glucose oxidase performance and indicating that the enzyme–substrate interaction remained unchanged between the two platforms. However, the PdB biosensor produced a significantly greater normalised current response, a higher V_max of 2.51 ± 0.14 μA compared with 2.01 ± 0.10 μA for PtB, and a markedly faster response time of 16.1 ± 6.2 s versus 28.9 ± 3.0 s. The improved performance of the PdB biosensor is therefore attributable to the transducer rather than the enzyme layer, reflecting more efficient electrochemical detection of the enzymatically generated hydrogen peroxide.

Batch-to-batch reproducibility of the PdB glucose biosensors was evaluated across five independent batches, each containing six biosensors, yielding a relative standard deviation of 9.7%—indicating good reproducibility, with the observed variation likely attributable to minor differences in manual composite packing, surface finishing, and enzyme layer deposition. Operational and shelf-life stability were assessed by performing glucose calibrations on the day of fabrication and subsequently every ten days over a 30-day period. No significant changes in V_max were observed throughout the study, indicating that the overall current-generating capability of the biosensor was maintained. A transient decrease in K_M was observed on day 20, but the authors attribute the overall stability to a combination of physical enzyme entrapment within the composite architecture and the provision of a conductive microenvironment that facilitates efficient electron transfer while limiting enzyme loss from the electrode surface.

The study demonstrates that incorporating precious-metal blacks into MWCNT composite electrodes enables tuneable and analyte-specific electrocatalytic performance, with PdB and PtB offering the most advantageous profiles. While several previously reported sensors achieve lower limits of detection, many utilise surface-modified films, nanoparticle coatings, or deposited metal-black layers that suffer from limited mechanical and structural stability. In contrast, the bulk composite approach provides a mechanically robust electrode architecture, and the systematic comparison of three precious metals within an identical platform enables direct evaluation of how precious-metal identity influences electrochemical properties, particle architecture, and analyte-specific sensing behaviour. The authors note that operation at +0.85 V may increase susceptibility to oxidation of co-existing electroactive species in complex matrices, and that future studies should evaluate lower operating potentials, interference effects, and performance in real samples. Nevertheless, the work establishes a versatile materials-design framework in which precious-metal composition and catalyst architecture can be tailored to the target analyte—palladium for peroxide, platinum for nitrite—providing guidance for the development of next-generation electrochemical sensors and biosensors.

Subject of Research: Nanotechnology

Article Title: Precious-metal black carbon nanotube hybrid nanomaterial for sensitive detection of hydrogen peroxide and nitrite

Article References: Perez, F., Miller, C., & Patel, B. A. (2026). Precious-metal black carbon nanotube hybrid nanomaterial for sensitive detection of hydrogen peroxide and nitrite. Results in Chemistry, 31, Article 103981. https://doi.org/10.1016/j.rechem.2026.103981

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103981

Keywords: electrochemical sensors, carbon nanotubes, palladium black, platinum black, hydrogen peroxide, nitrite, glucose biosensor, nanomaterials, Precious-metal, black, carbon, nanotube