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## Creator

[Chandra Wulandari](https://orcid.org/0000-0002-9990-2746), Atqiya Muslihati, Ni Luh Wulan Septiani, Ahmad Nuruddin, Brian Yuliarto, [Yusuke Yamauchi](https://orcid.org/0000-0001-7854-927X), [Joel Henzie](https://orcid.org/0000-0002-9190-2645), [Erwin Peiner](https://orcid.org/0000-0001-5801-813X), [Hutomo Suryo Wasisto](https://orcid.org/0000-0002-4522-3625), Nugraha Nugraha

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[Carbon-black-coupled bimetallic CoCu-ZIF composites for monohydroxylated polycyclic aromatic hydrocarbon metabolite sensing](https://mdr.nims.go.jp/datasets/8e42e92b-cdb0-40a3-a508-69062fa4c515)

## Fulltext

Carbon-black-coupled bimetallic CoCu-ZIF composites for monohydroxylated polycyclic aromatic hydrocarbon metabolite sensingcommunicationsmaterials ArticleA Nature Portfolio journalhttps://doi.org/10.1038/s43246-026-01150-9Carbon-black-coupled bimetallic CoCu-ZIF composites for monohydroxylatedpolycyclic aromatic hydrocarbonmetabolite sensingCheck for updatesChandra Wulandari 1,2,3,4,5, Atqiya Muslihati1,2,5, Ni Luh Wulan Septiani6, Ahmad Nuruddin2,Brian Yuliarto2,7, Yusuke Yamauchi 8,9, Joel Henzie 3 , Erwin Peiner 10 ,Hutomo Suryo Wasisto 5 & Nugraha Nugraha2,7Polycyclic aromatic hydrocarbons are hazardous, carcinogenic pollutants produced by theincomplete combustion of organic materials, and their detection remains a technological challenge.Here, we develop bimetallic cobalt-and-copper-based zeolitic imidazolate framework (CoCu-ZIF)particles coupled with carbon black (CB) for highly sensitive and selective electrochemical sensing of2-Naphthol, a monohydroxylated polycyclic aromatic hydrocarbon. The CoCu-ZIF with dual redox-active sites of Co3+/Co2+ and Cu2+/Cu+ promotes efficient electron transfer during the oxidationprocess, thus effectively catalyzing the oxidation of 2-Naphthol. Carbon black provides a conductivenetwork facilitating efficient electron transport between CoCu-ZIF and electrode surface. Thefabricated sensors exhibit a sensitivity and limit of detection of 0.98 μA μM⁻¹ and 0.46 μM, respectively,during exposure to 2-Naphthol within a detection range of 1–500 μM. Moreover, due to their excellentsensing performance, selectivity, reproducibility, and recovery rate in non-biological urine, thedevelopedcomposites offer strongpotential for real-timepolycyclic aromatic hydrocarbonmonitoringand future use in environmental sensor technologies.Polycyclic aromatic hydrocarbons (PAHs) arepollutants produced from theincomplete combustion of carbon-containing organic compounds at bothindustrial and household levels, consisting of two to six aromatic rings1.PAHs are classified as priority pollutants because their exposure to thehuman body through the formation of reactive oxygen species can lead tocellular dysfunction due to oxidative damage to macromolecules (e.g.,deoxyribonucleic acid (DNA), proteins, and lipids)2.Onceentering thebodythrough exhalation or skin contact, PAHs are metabolized by the liver intomonohydroxylated compounds (OH-PAHs), which aremore harmful thantheir parent compounds3,4. Various studies have shown that PAH exposurecan be detected by measuring OH-PAH levels in urine or feces5,6. AmongOH-PAHs, 2-Naphthol is themost dominant biomarker for PAHexposure,accounting for 60–90% of the total detected OH-PAHs7. Therefore, highlysensitive and selective sensors for 2-Naphthol are essential to improve theaccuracy and reliability of PAH monitoring in biological samples.The standard analytical methods for OH-PAHs are gas chromato-graphy, mass spectrometry, and isotope dilution8–10. However, they requireexpensive instrumentation, complex operation, and extensive sample1Doctoral Program of Engineering Physics, Faculty of Industrial Technology, Institut Teknologi Bandung, Bandung, Indonesia. 2Advanced Functional MaterialLaboratory, Faculty of Industrial Technology, Institut Teknologi Bandung, Bandung, Indonesia. 3Research Center for Materials Nanoarchitectonics (MANA),National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, Japan. 4Physics Study Program, Universitas Pendidikan Indonesia,Bandung, Indonesia. 5PT Biostark Analitika Inovasi, Bandung, Indonesia. 6Research Center for Electronics, National Research and Innovation Agency (BRIN), KSTSamaun Samadikun, Bandung, Indonesia. 7Research Center for Nanosciences and Nanotechnology (RCNN), Institut Teknologi Bandung, Bandung, Indonesia.8Department of Materials Process Engineering, Graduate School of Engineering, Nagoya University, Nagoya, Japan. 9Australian Institute for Bioengineering andNanotechnology (AIBN) and School of Chemical Engineering, The University of Queensland, Brisbane, QLD, Australia. 10Institute of Semiconductor Technology(IHT), Laboratory for Emerging Nanometrology (LENA), Technische Universität Braunschweig, Braunschweig, Germany. e-mail: henzie.joeladam@nims.go.jp;e.peiner@tu-braunschweig.de; h.wasisto@biostark-ai.com; nugraha@itb.ac.idCommunications Materials |           (2026) 7:195 11234567890():,;1234567890():,;http://crossmark.crossref.org/dialog/?doi=10.1038/s43246-026-01150-9&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s43246-026-01150-9&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s43246-026-01150-9&domain=pdfhttp://orcid.org/0000-0002-9990-2746http://orcid.org/0000-0002-9990-2746http://orcid.org/0000-0002-9990-2746http://orcid.org/0000-0002-9990-2746http://orcid.org/0000-0002-9990-2746http://orcid.org/0000-0001-7854-927Xhttp://orcid.org/0000-0001-7854-927Xhttp://orcid.org/0000-0001-7854-927Xhttp://orcid.org/0000-0001-7854-927Xhttp://orcid.org/0000-0001-7854-927Xhttp://orcid.org/0000-0002-9190-2645http://orcid.org/0000-0002-9190-2645http://orcid.org/0000-0002-9190-2645http://orcid.org/0000-0002-9190-2645http://orcid.org/0000-0002-9190-2645http://orcid.org/0000-0001-5801-813Xhttp://orcid.org/0000-0001-5801-813Xhttp://orcid.org/0000-0001-5801-813Xhttp://orcid.org/0000-0001-5801-813Xhttp://orcid.org/0000-0001-5801-813Xhttp://orcid.org/0000-0002-4522-3625http://orcid.org/0000-0002-4522-3625http://orcid.org/0000-0002-4522-3625http://orcid.org/0000-0002-4522-3625http://orcid.org/0000-0002-4522-3625mailto:henzie.joeladam@nims.go.jpmailto:e.peiner@tu-braunschweig.demailto:h.wasisto@biostark-ai.commailto:nugraha@itb.ac.idwww.nature.com/commsmatpreparation. Electrochemical sensors offer a promising alternative fordetecting OH-PAHs, as their hydroxyl groups undergo redox reactions atspecific potentials, producing measurable electrochemical signals11. Zhanget al. measured the amperometric response to 2-Naphthol using a meso-NiCo2O4 nanosphere-modified electrode12. Using linear sweep voltam-metry, Li et al. detected 2-Naphthol with an electrode based on poly(N,N-dimethylacrylamide) and multi-walled carbon nanotubes13. Noble metalsand two-dimensional materials have also been used to produce strongelectrochemical responses with OH-PAH14–16. Several studies have suc-cessfully determined OH-PAH levels in urine with an average recovery rateof over 90%11,17. In electrochemical sensing, high density of active sites andefficient mass transport are required, yet these are often limited in con-ventional materials such as carbon-based materials, metal oxides, andconductive polymers. Carbon materials often rely on defect/dopant engi-neering because their pristine surfaces typically exhibit low catalytic activityand limited oxidation resistance18. In parallel, metal oxidesmay suffer fromlimited accessibility of active sites because of their non-porouscharacteristics19, while polymeric films can impose additional mass- andcharge-transfer resistance when thicker coatings are required20. In contrast,metal organic frameworks (MOFs) can effectively overcome these limita-tions due to their high specific surface area and intrinsic porosity. The targetanalytes can rapidly diffuse to the active sites, where they undergo redoxreactions catalyzed by abundant active-metal centers21,22. Therefore, MOFsare among the most widely explored materials to enhance the performanceof electrochemical devices in various applications23–25.Various strategies have been reported to improve the electrochemicalproperties of MOFs, including the introduction of functional groups, metaldoping (or bimetallic systems), guest molecule incorporation, or defectengineering26. Among those approaches, having bimetallic structures cantransform theMOF properties due to their multi-metal active sites, internalsynergistic interactions, and morphological changes26,27. Moreover, mod-ification of the physicochemical properties of MOFs can be done by con-trolling the proportions of metal precursors. Feng et al. reported that themixed metal (Zn, Zr)-UiO-66 produced a missing cluster defect, which istunable by the ratio of Zn and Zr precursors. At specific ratios, (Zn, Zr)-UiO-66 owns twomissing linkers that contribute to the formation of clusterdefects, resulting in significantly higher surface area and catalytic conversionthanpristineUiO-6628.Hou et al. produced a vacancy-richMOFbypartiallyreplacing the Fe3+ with Cu2+ through a cation-exchange strategy to detectammonia gas29. Despite all those success stories on metal precursor engi-neering strategies to either modify the material properties or enhance theelectrochemical sensor performance, the potential of such metal-modifiedMOF structures, especially based on bimetallic zeolitic imidazolate frame-work (ZIF), for the detection of a low-molecular-weight OH-PAH (i.e., 2-Naphthol) has not been proven.In this work, we developed bimetallic cobalt-and-copper-based ZIF(CoCu-ZIF) particles coupled with carbon black (CB) for highly sensitiveand selective electrochemical sensing of 2-Naphthol, a low-molecular-weight OH-PAH.Here, ZIF-67 was used as the parentmaterial, comprisingCo2+ cations coordinated with imidazole anions to form a tetrahedral fra-mework. In addition, ZIF-67 was selected as the main sensing material dueto its high surface area, abundant porosity, stronger chemical stability, andenhanced redox activity compared to other ZIF variants. ZIF-67 is generallymore effective in electron transfer and catalytic processes, compared to ZIF-8 (Zn2+) which is more beneficial for adsorption purposes. Its large surfacearea offers numerous active sites for surface reactions, while the porousstructure facilitates analyte diffusion and provides channels for efficientredox interactions30,31. Our previous studies have also demonstrated thesuccessful use of bimetallic ZIF-67 for electrochemical sensors32. Theintroduction of Cu into ZIF-67 was intended to generate a variation inredox-active sites and modulate the local electronic environment. Theseproperties lead to dynamic Co and Cu redox-couples interconversion,which facilitates electron transfer between the metal centers, therebyenhancing catalytic activity toward an electro-oxidation of 2-Naphthol33–35.Furthermore, the inherently low electrical conductivity of CoCu-ZIF wasaddressed by integrating it with carbon-based materials to improve theoverall detection response36–39.To ensure the desired morphology and structure of CoCu-ZIF par-ticles, their metal precursor ratios were carefully controlled in one-potsynthesis via chemical co-precipitation. Various material characteriza-tions (i.e., X-ray diffraction (XRD), Fourier-transform infrared(FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), nitrogen adsorption/desorption analysis, and X-ray photoelectronspectroscopy (XPS)) methods were conducted to investigate the effects ofCu incorporation andCo/Cu ratio onmorphology, surface, and electronicstates of the bimetallicCoCu-ZIF. Furthermore, to enhance their electricalconductivity, the CoCu-ZIF particles were combined with CB by ultra-sonication, resulting in CoCu-ZIF@CB composites. To prove theirfunction as OH-PAH electrochemical sensors, these composites weredeposited on a glassy carbon electrode (GCE) and subsequently used tomonitor various 2-Naphthol concentrations in an amperometric sensingfashion. In addition to sensitivity measurements for 2-Naphthol, repro-ducibility tests usingmultiple electrodes and cross-sensitivity assessmentswith various interfering agents were conducted to demonstrate thepractical applicability of CoCu-ZIF@CB composites as 2-Naphtholsensors.Results and discussionMaterial and structural characteristics of CoCu-ZIF particlesThe CoCu-ZIF particles were successfully fabricated through a co-precipitation method involving Co and Cu precursors, and2-methylimidazole (2-MeIm) in methanol solution (see Fig. 1a). We pro-duced CoxCuy-ZIF particles with different Co:Cu precursor molar ratios,where x and y are the molar fractions of Co and Cu, respectively. Foursample variants of ZIF-67, Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIFparticles were initially prepared for investigation in terms of their materialand structural characteristics prior to combination with CB and depositionon a GCE for detecting target 2-Naphthol (see Fig. 1b).After 24 h of aging, the precipitates of all CoCu-ZIF variants wereharvested. Figure 2a shows the particles resulting from a synthesis with60mL of total precursors, producing masses of 66.5mg, 43.5mg, 36.0 mg,and 7.0mg for ZIF-67, Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIF sam-ples, respectively. The particle amounts decreased as the concentration ofCu precursors increased. Furthermore, a higher Cu fraction (i.e., Co4Cu3-ZIF) did not yield harvestable precipitates, as shown in SupplementaryFig. 1. The recipes used to synthesize bimetallic CoCu-ZIF particles couldnot be directly adopted to produce their single-metal Cu-ZIF counterparts.Since the formation enthalpy of Cu-(2-MeIm) is less exothermic(–14.03 kJ mol–1) compared to their Co-based counterparts(–46.01 kJ mol–1), Cu in ZIF frameworks is more difficult to form andthermodynamically less stable40. The synthesis of Cu-ZIF particles needs ahigher metal composition compared to the ligand, as opposed to thecomposition used in this bimetallic study41. The crystallographic structuresof the obtained CoCu-ZIF particles were analyzed by XRD, in which theirdiffraction peaks match well with those of simulated ZIF-67 in the CODreference card: 7222297 (see Fig. 2b)42. The XRD patterns show the majorpeaks of ZIF-67 located at 2θ (hkl) of 12.81° (112), 14.79° (022), 16.53° (013),18.11° (222), 22.21° (114), 24.57° (233), 25.68° (224), 26.74° (134), 29.71°(334), 30.64° (044), 31.56° (244), and 32.44° (235). Here, θ denotes the angleof X-ray incidence relative to the crystal plane defined by Miller indices(hkl). Generally, the incorporation of Cu2+ metal ions does not cause asignificant change to the crystallographic structure of ZIF, indicating amaintained framework. A slight shifting to lower diffraction angles wasobserved along with an increasing fraction of Cu, indicating a largerinterplanar spacing (d-spacing) in bimetallic ZIF. This finding can beexplained by the larger ionic radius of Cu2+ (0.73 Å), which substitutes thesmaller Co2+ (0.70 Å) in the crystal lattice, causing d-spacing expansion43,44,as reported in apreviousCoCu-MOFstudy45–48. TheBraggequationanalysisof the (112) peakwas used to calculate the corresponding d(112)-spacing (thedistance between the (112) planes) for all samples. The d(112)-spacing valueshttps://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 2www.nature.com/commsmatof ZIF-67, Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIF are 6.903 Å,6.932 Å, 6.935 Å, and 6.944 Å, respectively.Supplementary Fig. 2 shows the FTIR spectra of all four differentCoCu-ZIF samples to clarify the presence of the functional groups in thosematerials. The fingerprint of 2-MeIm is identified by the existence of peaksat 600–1500 cm–1 reflecting the in-plane andout-of-plane bendingmodes ofthe imidazole ring. A peak at 1583 cm–1 and 1689 cm–1 results from theC =N stretching and C =C band in the imidazole rings49,50. The presence ofthese characteristic peaks of the imidazolate linker has confirmed the suc-cessful fabrication of ZIF materials. Additional peaks at 2924, 3130, and3431 cm–1 correspond to aliphatic C–H stretching, aromatic C–H stretch-ing, and O–H stretching, respectively. The sharp peak at 422 cm–1 comesfrom the M–N bonds, where M represents the metal (Co or Cu). The onlyslight shift of theM–N bond vibration indicates that Cu2+ coordinates withthe deprotonated imidazolate linker in similar energy ranges as theCo2+ 51–53.The materials synthesized by bimetallic precursors inherit the dode-cahedron morphology of ZIF-67, which is favored in many ZIF-basedelectrochemical studies because of its high number of exposed active sites toachieve strong catalytic activity54. By defining the ZIF-67 in Fig. 3a as areference (no Cu incorporation), various alterations were observed for thedifferent precursor ratios. Particle size becomes the first parameter to benoticed, where the higher Cu fraction results in a larger particle size. TheZIF-67, Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIF samples possessaverage particle sizes of 678.45 nm, 1972.69 nm, 1941.12 nm, and5237.80 nm, respectively. Referring to LaMer’smodel, the small and narrowsize distribution obtained for ZIF-67 indicates a promoted nucleation overparticle growth due to anunhindered coordination ofCo2+ and 2-MeIm55,56.In contrast, the presence of Cu2+ introduces a disturbance to the overallmetal–ligand coordination, thereby slowing down nucleation and favoringcrystal growth, resulting in larger particle sizes in bimetallic CoCu-ZIF27,40.For Co4Cu0.5-ZIF (see Fig. 3b), the Cu2+ content might not be sufficient toaccommodate coordination across the solution, leading to non-uniformparticle size. At the intermediate fraction of Co4Cu1-ZIF (see Fig. 3c), theparticle exhibits amore uniform sizedistribution.Notably, Co4Cu2-ZIF (seeFig. 3d) shows markedly increased average particle size and broadened sizedistribution (~3–6 μm), indicating contributions from both enlarged pri-mary particles and aggregation/coalescence. This suggests that at high Cufraction, growth and aggregation become more pronounced, whilecoordination-related defects or lattice distortions may contribute to theincreased particle size57,58. Supporting evidence from X-ray photoelectronspectroscopy (XPS) is provided later in this section.Following FE-SEM analysis, energy-dispersive X-ray (EDX) spectro-scopymappingwas performed to screen elemental composition, specificallyto confirm the incorporation of Cu into the ZIF matrix. Figure 4a–d showsthe superimposedmaps of elemental distributions consisting of carbon (C),nitrogen (N), oxygen (O), cobalt (Co), and copper (Cu) for singleCoCu-ZIFparticles. Cu is represented by light blue in the mapping experiments. Cuincreased in intensity in the superimposed images, reflecting the higher Cucontent as a greater amount of Cu precursor was used in the synthesis.Individual mappings for each element and EDX spectra are included inSupplementary Fig. 3. The EDX spectra provide the atomic percentages foreach element in the bulk composition. As shown in Supplementary Table 1,the atomic concentration ratios of N/CoCu for each CoCu-ZIF are close tothe theoretical CoN4 coordination in ZIF-67 of 459,60. Meanwhile, the highN/Co ratio inZIF-67 indicates ahighportionof unreacted2-MeIm,which isformed by rapid nucleation due to the abundant availability of the 2-MeImligand. This supports the discussion of Fig. 3a.Furthermore, quantification of the element composition was con-ducted using XPS, the corresponding low-resolution survey spectra areshown in Supplementary Fig. 4. A near-surface composition in atomicpercentage is obtained from the CoCu-ZIF main peaks (i.e., C 1 s, N 1 s, O1 s, Co 2p, and Cu 2p) as listed in Supplementary Table 1. A similar findingon the N/CoCu ratio has been observed with both EDX and XPS. Theincorporation ofCu into the bimetallic ZIFs results in a lowerN/CoCu ratio,suggesting partial loss or reduced coordination of 2-MeIm ligands. Thiscondition is commonly reported in carbonized ZIF and is related to morecoordinatively unsaturated metal sites, which can improve the material’scatalytic reactivity61,62. The metal contents of the CoCu-ZIF samples werequantified using atomic absorption spectroscopy (AAS), as shown inTable 1. According to AAS, the Co:Cu metal fractions obtained in the finalproducts follow the ratios prepared for synthesis. With increasing Cucontent in the precursor, theCo concentration in thefinal product graduallydecreases, while theCu concentration increases correspondingly. This trendconfirms the successful incorporation of Cu into the ZIF frameworkthrough partial substitution of Co sites. Supplementary Table 2 comparesthe Co:Cu atomic fractions obtained from EDX and XPS analyses. In gen-eral, the incorporated Cu content in the final CoCu-ZIF samples was lowerthan expected from the initial precursor ratio, likely due to theweakerCu–NFig. 2 | Material characteristics of the as-prepared CoCu-ZIF particles with dif-ferent metal precursor molar ratios. a Photographs of all four CoCu-ZIF particlevariants resulting from the same synthesis volume. Increasing Cu precursor con-centration results in a lower number of synthesis end-products (particles), as indi-cated by the yellow arrow in those images. b X-ray diffraction (XRD) patternsshowing the crystalline phases present in the samples, with characteristic diffractionpeaks corresponding to ZIF-67 in the COD ref. card: 7222297. A left-shifted (112)peak is observed with an increase of the Cu fraction (see the middle panel).Fig. 1 | Bimetallic cobalt-and-copper-based ZIF (CoCu-ZIF) particles coupledwith carbon black (CB) for electrochemical sensing of 2-Naphthol. a Synthesis ofCoCu-ZIF particles using co-precipitation followed by a mixing process with CB toresult in CoCu-ZIF@CB composites. b Illustration of CoCu-ZIF@CB compositesdeposited on a glassy carbon electrode (GCE) and their expected output sensingsignal to 2-Naphthol. The response signal is shown by the appearance of square wavevoltammetry (SWV) peaks at the redox potential of 2-Naphthol after undergoing theredox reaction.https://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 3www.nature.com/commsmatcoordination bonds compared to Co–N bonds. This observation is con-sistent with a lower exothermic formation enthalpy of Cu–(2-MeIm) rela-tive to Co–(2-MeIm)40.Figure 4e presents the high-resolution XPS spectra of Co 2p for eachbimetallic CoCu-ZIF and exhibits the two spin-orbit doublets 2p3/2 and 2p1/2 at around 781 eV and 796 eV, respectively. Peak fitting reveals that eachspin-orbit doublet possesses three distinct peaks identified asCo3+ (~780 eVand~796 eV) andCo2+ (~782 eVand~797 eV), followed by their respectiveshake-up satellites at a higher binding energy61,62. In Fig. 4f, the XPS spectraof Cu 2p also reveal 2p3/2 and 2p1/2 doublets as a result of spin-orbit cou-pling, located at around 933 eV and 953 eV, respectively. The 2p3/2 regioncomprises Cu+ and Cu2+, with a different binding energy of Cu2+ for eachsample. Meanwhile, the 2p1/2 region exhibits only Cu+ at a similar bindingenergy for each sample63,64. Based on the quantitative analysis in Supple-mentary Table 3, the calculated Co3+/Co2+ ratios from 2p3/2 for Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIF are 1.07, 1.98, and 1.82, respectively.Meanwhile, the Cu+/Cu2+ ratios from 2p3/2 are 2.05, 3.05, and 4.31,respectively. The elevated mixed-valence contributions at higher Cu frac-tions indicate changes in the metal–ligand coordination environment,consistent with the decreasing N/CoCu ratio depicted in SupplementaryTable 161,62. This interpretation is further supported by high-resolutionN1 sspectra (Supplementary Fig. 5a), where theM–Ncomponent systematicallyshifts from 398.63 eV (Co4Cu0.5-ZIF) to 398.30 (Co4Cu2-ZIF), indicating amodified electronic environment of coordinated nitrogen upon Cu incor-poration. The converging evidence of the perturbed coordination ispotentially associatedwithpartial ligand loss,whichmaydisrupt the originalZIF framework and promotes structural defect formation62,65. These defectsmay contribute to the increased particle size observed in bimetallic CoCu-ZIF (see Fig. 3) by altering nucleation and growth kinetics57.Besides the intensity analysis, the binding energy shifts can provideinformation regarding the electronic environment of the metal centers.Generally, the Co 2p and Cu 2p binding energies shift as the Cu content inthe bimetallic CoCu-ZIF increases. As shown in Fig. 4e, the Co3+ bindingenergy decreases, while the Co2+ binding energy increases with higher Cucontent. InFig. 4f,Cu2+peaks also shifts slightly to a lowerbinding energy, asthe Cu+ peak shifts dramatically to a higher binding energy. The observedshifts in the Co and Cu binding energies correlate with an electron transferbetween the metal centers, where Cu2+ is reduced to Cu+ and promotesreduction of Co3+ to Co2+. A similar electronic interaction was observed inother bimetallic systems and has been confirmed by density functionaltheory (DFT) studies33–35. DFT analysis quantified core-level binding energyshifts and electron transport between the two metal centers. Furthermore,Fig. 3 | Morphologies and size distributions of CoCu-ZIF particles with differentmetal precursor molar ratios. The field emission scanning electron microscopy(FE-SEM) images in low and high magnification, and size distribution of a ZIF-67,b Co4Cu0.5-ZIF, c Co4Cu1-ZIF, and d Co4Cu2-ZIF particles, respectively. Theirrespective size distributionswere generated from low-magnification FE-SEM imagesat two different positions.https://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 4www.nature.com/commsmatthese studies also demonstrated that strong electronic coupling between CoandCuenhances charge transfer and catalytic activity.Moreover, themetal-ligand system reflected by the N 1 s and C 1 s spectra shifting to lowerbinding energy (see Supplementary Fig. 5) indicates electron delocalizationaround the ligandatoms.These synergistic electronic effects enable dynamicredox interconversion and facilitate charge transfer during the electro-chemical process63.The nitrogen adsorption/desorption behavior of all samples showstypical Type-1 nitrogen isotherms, where a rapid adsorption occurs atp/p0 ≈ 0.05, followed by a stagnant adsorption at p/p0 > 0.05 (Fig. 5a). Theadsorption isotherm indicates a microporous structure characteristic ofCoCu-ZIF. Brunauer–Emmett–Teller (BET) analysis reveals that the singlemetal ZIF (ZIF-67) has a large surface area. Meanwhile, a similar surfacearea ismaintained by each bimetallic ZIF. Furthermore, a density functionaltheory (DFT) analysis has confirmed the pure microporous structureknown forCoCu-ZIFwith an average pore size of around 1 nm (see Fig. 5b).From the DFT-based surface area analysis, the ZIF-67, Co4Cu0.5-ZIF,Co4Cu1-ZIF, and Co4Cu2-ZIF samples possess surface areas of 1837.35m2g–1, 1702.82m2 g–1, 1607.57m2 g–1, and 1596.37m2 g–1, respectively. Goodagreement has been reached between DFT (full lines) and adsorption/des-orption isotherm curves (filled circles, see Fig. 5b), where the bimetallic ZIFvariants (i.e., Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIF) have a slightdecrease in the porous volume compared to the singlemetallic ZIF (i.e., ZIF-67) related to their lower surface area. The Barrett-Joyner-Halenda (BJH)plots, attached as an inset in Fig. 5b, show that the material lacks meso-porosity. Generally, the incorporation of Cu into the ZIF does not lead to asignificant reduction of surface area, which otherwisemay have resulted in adecrease in electrochemical performance due to a lower number ofactive sites.Electrochemical properties of CoCu-ZIF particlesTo evaluate the electrochemical properties of the synthesized CoCu-ZIFparticles, they were first coated onto a GCE. A three-electrode system wasutilized, comprising Ag/AgCl and platinum wire as reference and counterelectrodes, respectively. The electrolyte solution of 5mM potassium ferro-cyanide [K3(FeCN)6] with 0.1M of potassium chloride (KCl) in 0.01Mphosphate-buffered saline (PBS, pH 7.4) was used in the entire series ofelectrochemical measurements of the materials. Cyclic voltammetry (CV)was performed in the potential range of –0.10 to+0.55 Vwith a scan rate of50mV s-1, producing cyclic voltammograms with distinct redox peaks,where each CoCu-ZIF variant exhibits different current levels (see Fig. 6a).The pristine ZIF-67 sample yielded the lowest currents at 0.300 V (oxida-tion) and 0.180 V (reduction) compared to its bimetallic CoCu-ZIF coun-terparts. The redox reactions in ZIF-67 were mostly attributed to theFaradaic reaction of Co (Co3+/Co2+ redox couples)66. In contrast, two redoxpairs ofCo3+/Co2+ andCu2+/Cu+ existed inCoCu-ZIF as confirmedby theirXPS spectra (see Fig. 4e, f). The coexistence of two redox pairs increases thedensity of redox-active sites, leading to higher peak currents and improvedredox reversibility. This interpretation was further supported by comparingthe electrochemical behaviors of themonometallic variants (ZIF-67 andCu-ZIF) to those of the bimetallic Co4Cu1-ZIF and the physically mixedFig. 4 | Elemental analysis of CoCu-ZIF particles in bulk and surface regions.Energy-dispersive X-ray (EDX) superimposed mappings comprising C, N, O, Co,and Cu elements for a ZIF-67, b Co4Cu0.5-ZIF, c Co4Cu1-ZIF, and d Co4Cu2-ZIF.X-ray photoelectron spectroscopy (XPS) at high resolution provides the oxidationstates of Co and Cu that are identified by the respective core level spectra of e Co 2pand f Cu 2p, respectively.Table 1 | Metal composition of CoCu-ZIF with different metalprecursor molar ratiosSample PreparedCo:Cu (at%)Metalconcentration (ppm)ObtainedCo:Cu (at%)Co CuZIF-67 4:0 5.059 – 4:0Co4Cu0.5-ZIF 4:0.5 7.326 0.252 4:0.13Co4Cu1-ZIF 4:1 6.323 0.276 4:0.16Co4Cu2-ZIF 4:2 5.135 0.295 4:0.21Atomic absorption spectroscopy (AAS) was used to measure the concentrations of Co and Cu inCoCu-ZIF. These concentrations were used to calculate the obtained Co:Cu fractions in thesynthesized product.https://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 5www.nature.com/commsmatmonometallic ZIFs (Cu-ZIF@ZIF-67). The Cu-ZIF was independentlycharacterized by FTIR spectroscopy, confirming the successful formation ofan imidazolate framework (Supplementary Fig. 6a). The Cu-ZIF@ZIF-67mixture was prepared tomatch the Co:Cu ratio of the synthesized Co4Cu1-ZIF, as described in detail in Supplementary Note 1. As shown in Supple-mentary Fig. 6b, c, Cu-ZIF@ZIF-67 exhibits neither well-defined redoxpeaks nor a semi-circle inNyquist plot. It has lower current compared to theCo4Cu1-ZIF, indicating that simple coexistence of twomonometallic phasesdoes not reproduce the enhanced performance67. Furthermore, the weakcurrent responses fromZIF-67 andCu-ZIF suggest that the improvement inCoCu-ZIF is not driven by standalone properties of either monometallicphase. These findings emphasize that synergistic effects arise when the twometals are integrated within the same framework environment67,68. Con-sistently, incorporating Cu into a Co-based ZIF framework (CoCu-ZIF)notably enhances redox response and interfacial charge-transfer kineticsrelative to the monometallic ZIFs33–35. To further elucidate the electronicinteraction between Co and Cu within the bimetallic framework, thecomposition dependence of the electrochemical properties was analyzed.The redox peaks of Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIFshifted to 0.310, 0.312, 0.330 V for oxidation and to 0.181, 0.179, 0.167 V forreduction, respectively. These potential shifts aremost likely attributed to aninternal redox-coupling mechanism between Cu+ and Co3+ centers. Theelectron-richCu+ species act as local electron donors, while theCo3+ speciesserve as electron acceptors, facilitating internal charge redistribution69,70.This internal electron transfer partially oxidizes the active sites prior toexternal electrochemical oxidation, leading to a more stabilized reducedstate71. Consequently, the oxidation peak shifts positively, while the reduc-tion peak shifts slightly negatively, reflecting the redox coupling equilibriumbetween Cu2+/Cu+ and Co3+/Co2+ pairs. In addition, these electronicbehaviors of Co and Cu metal centers also result in higher currents due tothe enhanced electron transfer. From electrical impedance spectroscopy(EIS) analysis shown in Fig. 6b, the Nyquist plots of Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIF have smaller semi-circle EIS curves than that of ZIF-67, indicating that the Cu addition can lower the charge-transfer resistance(Rct) in the material system. Nonetheless, in our experiments, the highestcurrent peaks are obtained by Co4Cu1-ZIF, followed by Co4Cu2-ZIF andCo4Cu0.5-ZIF. This could be correlated with the lack of uniformity andaggregation provided by the Co4Cu0.5-ZIF and Co4Cu2-ZIF particles inFig. 3c, d.Moreover, these electrochemical trends can be rationalized by a bal-ance between the population of surface-exposed metal sites andmorphology-dependent accessibility. SupplementaryTable 1 shows that theN/CoCu ratio of Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIF decreasesaccording to 2.92, 2.86, and 2.74, respectively. A lower N/CoCu ratioindicates a more metal-rich surface relative to the coordinating nitrogen,indicating a higher population of surface-exposedmetal centers as potentialelectroactive sites62,65. Although Co4Cu2-ZIF exhibits the lowest N/CoCuratio, its markedly enlarged particle size and aggregation can reduce theaccessibility of the external surface area and thereby hinder mass transport.In contrast, the Co4Cu1-ZIF exhibits a relatively metal-rich surface alongwith a uniform particle size distribution, which provides more accessibleactive sites and more efficient mass transport. Therefore, Co4Cu1-ZIFexhibits themost favorable characteristics and accordingly has produced thehighest peak current and lowest charge-transfer resistance in CV and EIS,respectively, rationalizing its selection for 2-Naphthol sensing.Since the Co4Cu1-ZIF had demonstrated the optimum characteristicsamong other samples (i.e., produced the highest peak current and lowestcharge-transfer resistance in the first CV and EIS assessments shown inFig. 6a, b), we decided to further explore and use this bimetallic materialvariant for the next experiments (i.e., evaluation of scan rate effects, com-posite creation involving CB, and sensing performance evaluation towards2-Naphthol). Here, the electrochemical properties of Co4Cu1-ZIF werestudied in detail by varying theCV scan rate in the range of 10 to 100mV s–1with a step interval of 10mV s–1 in potential windows of –0.10 to +0.55 V(see Fig. 6c). The faster the scan rates, thehigher the produced current due tothe decrease of diffusion layer thickness. Since a quasi-reversible electrontransfer is indicated by the redox couple peaks, theRandles–Sevcik equationcan be used to explain the electrochemical reaction through the relationshipbetween the peak current (Ip) and the square root of scan rate (v1/2)72.Figure 6d shows that the current changes linearly to the square root of thescan rate. This implies that the analyte is freely diffusing in the solution,without any adsorption to the surface of the material, which can alter thereactivity of electrodes72.Carbon-black-coupled CoCu-ZIF compositesAfter the optimal CoCu-ZIF variant had been found (i.e., Co4Cu1-ZIF), thismaterial was then coupled with CB to enhance its electrical transportproperties. Here, a simple ultrasonication method was used to form ahomogenous Co4Cu1-ZIF@CB composite for GCE modification. Prior toelectrochemical applications, the electrical properties of the materials wereevaluatedusing a PicotestM3500Amultimeter and interdigitated electrodescoatedwith the correspondingmaterials ink. Themeasured resistance listedin Supplementary Table 4 was then used to calculate conductance. Themeasurements indicated that Co4Cu1-ZIF exhibited very low conductance( < 0.01 μS), whereas CB showed a high conductance (448.4 μS). After thesetwomaterials are combined into Co4Cu1-ZIF@CB composite, the electricaltransport in Co4Cu1-ZIF@CB was significantly improved as a conductanceof 71.4 μS was reached. These results confirm that the incorporation of CBsuccessfully enhances the electrical conductivity of Co4Cu1-ZIF material.Consequently, compared to its pristine Co4Cu1-ZIF and CB counterparts,the Co4Cu1-ZIF@CB composite has demonstrated a superior CV response(see Supplementary Fig. 7a). These results are consistent with the Nyquistplots analysis in Supplementary Fig. 7b, that exhibited a relatively low Rctvalue of Co4Cu1-ZIF@CB (57.64 Ω) compared to CB (128.20 Ω) andCo4Cu1-ZIF (414.30 Ω). While CB provides excellent electrical transport,the better CV and EIS performance of the composite demonstrates thatintegrating CB with CoCu-ZIF effectively couples fast charge transfer withthe number of metal-centered redox sites.Fig. 5 | Surface analysis of CoCu-ZIF particles bynitrogen adsorption/desorption measurement.a Adsorption/desorption isotherm curves of fourCoCu-ZIF particles having different precursormolar ratios and their respectiveBrunauer–Emmett–Teller (BET) surface area(SBET). bThe differential pore volume (dVp/dDp) vs.pore size obtained from density functional theory(DFT) analysis, supported by Barrett-Joyner-Halenda (BJH) plots as inset.https://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 6www.nature.com/commsmatFigure 7a shows the FE-SEM image of the Co4Cu1-ZIF@CBcompositeconstructed by the close-packed reticular structure of CB with distributedCo4Cu1-ZIF particles. TheEDXmapping clearly identifies theCoCu-ZIF bydense carbon and cobalt mapping dots at the locations of polyhedra-shapedparticles. The carbon mapping (red dots) also displays a strong, uniformsignal across the entiremapped area, indicating even distribution of the CB.The XRD pattern of the Co4Cu1-ZIF@CB composite exhibits diffractionfeatures from both Co4Cu1-ZIF and CB (see Supplementary Fig. 7c). Thepresence of CB is evidenced by broad peak ((002) and (100) marked withblack arrows). However, its incorporation does not significantly alter thecrystallinity of the ZIF framework73. The high surface area of CoCu-ZIF is akey benefit for electrochemical reactions. Therefore, BET analysis of thecomposites (CB:Co4Cu1-ZIF ratio, 4:2) was performed to asses the effect ofCB on their surface accessibility. Supplementary Fig. 7d shows nitrogenadsorption/desorption isothermcurves,withBET surface area of 300.79 and920.98m2/g forCBandCo4Cu1-ZIF@CB, respectively.Anotable incrementwas obtained despite the compositemainly consisting of CB, indicating thatthe high surface area of Co4Cu1-ZIF (1607.57m2/g, see Fig. 5) significantlycontributes to the final surface area. These results confirm that the surfaceaccessibility ofCoCu-ZIF iswell preserved andnot hinderedby the presenceof surrounding CB.To find the optimumCo4Cu1-ZIF@CB composite composition and toinvestigate the effect of CB on thematerial’s electrochemical properties, fivesamples having different CB:Co4Cu1-ZIF ratios (i.e., 1:4, 2:4, 4:4, 4:2, and4:1) were prepared and subsequently electrochemically evaluated in CVmeasurements using 5mM [K3(FeCN)6] and 0.1MKCl in 0.01MPBS (pH7.4) (see Supplementary Fig. 8a). The composite sample having aCB:Co4Cu1-ZIF ratio of (4:2) demonstrated the highest redox current peaksof 40.50 µA, followedby theother samplespossessingCB:Co4Cu1-ZIF ratiosof (4:1), (4:4), (2:4), and (1:4), respectively. These results demonstrate theeffectiveness of CB in addressing the low conductivity issue of pristine ZIF.For the (4:2) and (4:1) variants, their obtained current levels differ onlyslightly, which indicates a possible electrochemical performance saturationwhen integrating too high CB concentrations into the composite materialsystem. This saturation can be attributed to a conductivity–active-sitestrade-off. Increasing the CB up to 4:2 forms an effective conductive networkthat connects more ZIF to the electrode, thereby enhancing the electrontransfer compared to the lower CB fractions. Further increasing the CBcontent from 4:2 to 4:1 yields diminishing improvements in conductivitywhile decreasing the fraction of electroactive ZIF relative to CB in thecomposite, impairing overall performance. In addition, excess CB maypartially cover the ZIF-particle surface, reducing electrolyte access to CoCu-ZIF active sites. A similar behavior has been reported in numerous studiesinvolving carbon-based composites for electrochemical sensing74–76.After their electrochemical properties had been evaluated, theCo4Cu1-ZIF@CB composite samples having different CB:Co4Cu1-ZIF ratios werethenassessed in the detection scenario towards 500 µM2-Naphthol (0.01MPBS, pH 7.4), as displayed in Fig. 7b. Here, two additional samples of theFig. 6 | Electrochemical properties of CoCu-ZIF particles. a Cyclic voltammetry(CV) and b electrochemical impedance spectroscopy (EIS) analyses of four differentCoCu-ZIF particles (ZIF-67, Co4Cu0.5-ZIF, Co4Cu1-ZIF, and Co4Cu2-ZIF) coatedon glassy carbon electrodes (GCEs) at a scan rate of 50 mV s–1. c Cyclic voltam-mograms of Co4Cu1-ZIF-coated GCE at different scan rates ranging from 10 to100 mV s–1, and d their corresponding plots of the square root of scan rate vs.oxidation (blue dots) and reduction (red dots) current peaks with linear fitting (fulllines). All these electrochemical properties were measured in a mixture of 5 mM[K3(FeCN)6] and 0.1 M KCl in 0.01M PBS (pH 7.4).https://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 7www.nature.com/commsmatpristine ZIF and CB denoted as (0:4) and (4:0), respectively, were alsoprepared and tested. A potential windows of –0.60 to+0.40 V was selectedfor the square wave voltammetry (SWV)measurements, as the redox peaksin the CV response to 500 µM 2-Naphthol appeared around –0.20 V. Theabsence of redoxpeak in the SWVsignal frompristineCBdemonstrates its alack of catalytic redox active-sites. Meanwhile, the presence of Co4Cu1-ZIFin the composite provides Co3+/Co2+ and Cu2+/Cu+ redox pairs that cancatalyze oxidation of 2-Naphthol. The results indicate that at specificCB:Co4Cu1-ZIF ratios of 4:2, an optimal balance between high conductivityof CB and increased surface area and accessible metal centers provided bythe ZIF was achieved. This improvement led to a more efficient redoxreaction of 2-Naphthol, shifting the peaks tomore favorable potentials. Thetrend in SWV responses is consistent with that in CV results shown inSupplementary Fig. 8b, where the Co4Cu1-ZIF@CB composite, having aCB:Co4Cu1-ZIF ratio of (4:2) outperformed the other prepared samples.Since the pH has been considered one of the most important para-meters in the test platformbecause of its strong influence on theprotonationstate of the target molecules and materials, determining the responsiblemolecule-material interaction77–79, we investigate the pH effect on the sen-sing performance of Co4Cu1-ZIF@CB/GCE. Here, the composite samplehaving a CB:Co4Cu1-ZIF ratio of (4:2) was tested by SWV using 500 µM2-Naphthol (0.01M PBS) at different pH levels (pH 2–7), as shown inFig. 7 | Carbon-black-coupled CoCu-ZIF composites for 2-Naphthol detection.a Field emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray (EDX) images of carbon-black-coupled CoCu-ZIF (Co4Cu1-ZIF@CB) compo-site for glassy carbon electrode (GCE) modification. The EDX images containsuperimposed and individual elemental mappings of C, N, O, Co, and Cu. b Squarewave voltammetry (SWV) responses of Co4Cu1-ZIF@CB/GCE having differentratios of CB:Co4Cu1-ZIF towards 500 µM 2-Naphthol in 0.01 M PBS (pH 7.4).c SWV responses of Co4Cu1-ZIF@CB/GCE having CB:Co4Cu1-ZIF ratio of (4:2)towards 500 µM2-Naphthol in 0.01MPBSwith different pH levels. d Schematic of apossible sensing mechanism of 2-Naphthol catalyzed by Co and Cumetal centers inCoCu-ZIF. e Schematic of a possible interaction mechanism between CoCu-ZIF@CB and 2-Naphthol through π–π stacking.https://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 8www.nature.com/commsmatFig. 7c. In 2-Naphthol detection, amore acidic solution could lead to a peakshift to higher potential from –0.16 to +0.21 V for pH 7 to 2, respectively,indicating the involvement of protons in the reaction12. SupplementaryFig. 9a shows a linear relationship between pH and peak potential. At pHlevels of 7 to 5, the slope of a linear regression is –60mV pH-1, which followsthe theoretical value from theNernst equation for the condition of electron-proton balance in the reaction process (–59.1 mV pH-1)12,80. However, at lowpH levels of 4 to 2, the peak potential shifted further, changing the slope to–30mV pH–1, suggesting an additional protonation effect of the 2-Naphtholand the Co4Cu1-ZIF. Since the dissociation constant (pKa) of 2-Naphtholmolecules is 9.5, they will be protonated at pH < 9.5 (neutral to acidicconditions), making themmore stable in their neutral form. Therefore, themore acidic the conditions, 2-Naphthol is expected to form stronger π-πstacking interactions with the Co4Cu1-ZIF@CB, thereby reinforcing theredox reaction. The protonation of Co4Cu1-ZIF also occurred at low pH,weakening the coordination between the metal center and the imidazoleligands79, thus promoting accessible metal sites for the redox reaction with2-Naphthol and enhancing the current peaks (see Supplementary Fig. 9b).Therefore, the Co4Cu1-ZIF@CB/GCE produced the highest peak current of311 µAwhen tested at pH 2 (see Fig. 7c). Long-term electrochemical cyclingmeasurements were carried out under these optimized conditions, exhi-biting a stable redox peak around +0.20 V over 50 cycles (see Supple-mentary Fig. 10a and 10b).The sensingmechanismwas schematically illustrated in Fig. 7d. In theelectrochemical sensor for 2-Naphthol, metal centers play a crucial role inthe sensing mechanism, acting as catalysts that facilitate the oxidation of2-Naphthol. In the case of CoCu-ZIF, the incorporation of the secondarymetal (Cu) enriches the varietyof accessible oxidation states, as evidencedbythe XPS analysis. The coherent shifts of Co 2p andCu 2p indicate electroniccoupling between Co3+/Co2+ and Cu2+/Cu+ centers. This leads to electrontransfer between the metal centers, which enhances charge-transfer effi-ciency during oxidation, thereby improving the catalytic activity of thesensor. The detailedoxidationprocess of 2-Naphtholwas analyzed based onthe obtained linear relationship of peak potential (Ep) vs. pH (see Supple-mentary Fig. 9a) according to the Nernst equation shown in Eq. 112,80.dEpdpH¼ �59:1mnmVpH�1ðT ¼ 298KÞ ð1Þwhere m and n are numbers of protons and electrons participating in theoxidation, respectively.At the operating pHof 2, themeasured slope of –30mV pH-1 results inm/n of ~0.5. This m/n ratio suggests that the oxidation of 2-Naphtholinvolves a two-electron transfer process with approximately one proton,consistent with a metal-mediated oxidation mechanism. The net stoichio-metry of 1H+ + 2e- indicates that oxidation of 2-Naphthol in this systemmost likely produces its oxidized form of 1,2-Naphthoquinone. The oxi-dation peak reflects the quantity of 2-Naphthol species oxidized into 1,2-Naphthoquinone, thereby determining the detection signal intensity.Furthermore, the specific interaction of CoCu-ZIF@CB with2-Naphthol is confirmed by the Fourier-transform infrared (FTIR) spec-troscopy, where the obtained spectra are shown in Supplementary Fig. 11.2-Naphthol is identified by four main peaks of C–H out-of-plane bending,phenol C–O stretching, C = C stretching, and O–H stretching at 844, 1217,1595, and 3553 cm–1, respectively. TheCoCu-ZIF@CB spectramainly showthe characteristic peaks of the imidazole linker at 700-1500 cm–1, with C =Cband, aliphatic and aromatic C–H stretching, and O–H stretching at 1646,2917, 3135, and 3447 cm–1, respectively49,50. The influence of carbon black isshown by the peak at 1517 cm–1, identified as C =C stretching of aromaticcompounds of carbon black overlapping with C =N stretching in CoCu-ZIF. After CoCu-ZIF@CB was treated to adsorb 2-Napthol, new peaksappear at 809 and 1226 cm–1 due to C–H out-of-plane bending and phenolC–O stretching from 2-Naphthol, with reduced intensity suggesting aninteraction with the material surface. Compared to free 2-Naphthol, theC = C stretching peaks redshifted to 1566 cm–1, suggesting that the π-πinteraction likely facilitates a binding of 2-Naphthol to the materials81. 2-Naphthol, CoCu-ZIF, and carbon black are similarly featured by conjugatedπ-electron systems due to their benzene ring, imidazole rings, and graphiticstructure, respectively. Figure 7e illustrates the π–π stacking interactionsbetween these three components, which drive the adsorptionmechanismofthe target analyte to the sensing surfaces, thus enhancing the electrochemicalsignal. A similar interaction has been exploited in the development of var-ious sensors and adsorbents82–84.Sensing performance evaluation towards 2-NaphtholAfter obtaining an optimal active material configuration for the electrode(Co4Cu1-ZIF@CB/GCE), different SWV-based characterizations (i.e., sen-sitivity, reproducibility, and selectivity) were carried out to evaluate itssensing performance towards 2-Naphthol. First, in the sensitivity test, theCo4Cu1-ZIF@CB/GCE sensor was exposed to 2-Naphthol with variousconcentrations (1 to 500 µM) in 0.01MPBS (pH2)usingpotentialwindowsof –0.60 to+0.40 V and a scan rate of 50mV s–1. The higher the 2-Naphtholconcentration, the higher the peak current produced by the sensing systemaffected by the redox reaction of adsorbed 2-Naphthol (see Fig. 8a). Thecalibration plot between 2-Naphthol concentration and peak current inFig. 8b shows a linear correlation for low (1–100 µM) and high(100–500 µM) concentrations with correlation coefficient values ofR2 = 0.95 (y = 0.98x+ 56.59) and R2 = 0.95 (y = 0.37x+ 126.99), respec-tively. Two segments of linear ranges can result from the saturation of activesites at high concentrations of 2-Naphthol, thus decreasing the sensitivityindicated by the smaller slope. In addition, the adsorption activity of2-Naphthol on the sensing surface at low concentration may form amonolayer system. Meanwhile, higher concentration possibly causes mul-tilayer adsorption, which hinders additional adsorption85. This behavior hasbeen reported in numerous studies of the electrochemical analysis ofNaphthol isomers and other analyte12,85–87.The limit of detection (LOD) is determined using the standardizedequation of LOD= 3 × SD/S, where SD is the standard deviation from theaverage of repeated blankmeasurements and S is the sensitivity that is equalto the slope of the linear regression of peak current vs. analyteconcentration88. By using the SD from three blank measurements (SD =0.15), the LOD values of Co4Cu1-ZIF@CB/GCE, i.e., detectable con-centration changes in the range of low and high 2-Naphthol concentrationsare 0.45 µM and 1.20 µM with sensitivities of 0.98 µA µM–1 and0.37 µA µM–1, respectively. The suggested safety level of 2-Naphthol inurineis 2.60 µM89. Therefore, with its superior sensitivity and an LOD that isalmost six times lower than this threshold, the electrode modified withCo4Cu1@CB composite is applicable for health monitoring vs. 2-Naphtholexposure. Supplementary Table 5 summarizes the sensing performance ofthe carbon-black-coupled bimetallicCoCu-ZIF onGCE (Co4Cu1-ZIF@CB/GCE) in comparison with previously reported electrodes for 2-Naphtholdetection.The results indicate that our sensor exhibits superiorperformancerelative to the reported systems. Specifically, CoCu-ZIF@CB/GCEdemonstrateshigher sensitivity anda competitive LODacross a broad linearrange, highlighting its applicability for both trace-level and high-concentration detection. In contrast, several previous studies achievedlower LODs but only within narrow linear ranges90,91. Most of these earlierelectrode designs primarily enhanced electrical conductivity by incorpor-ating conductive polymers, carbon nanostructures, or noble metal nano-particles. In comparison, our CoCu-ZIF@CB/GCE sensor uniquelyintegrates the conductivity enhancement provided by CB with dual redox-active sites from the Co–Cu bimetallic frameworks. This distinctive struc-tural and electronic synergy effectively boosts the catalytic activity of theelectrode toward 2-Naphthol oxidation, making it a promising platform forelectrochemical sensing applications, competitive with previousapproaches.Second, to prove the process reproducibility of the electrodes, we havebuilt five Co4Cu1-ZIF@CB/GCE samples from the as-prepared compositeink and exposed them to 100 µMof 2-Naphthol (0.01MPBS, pH2). Allfiveelectrodes have demonstrated similar current responses of (148 ± 3) µA (seehttps://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 9www.nature.com/commsmatFig. 8c). The relative standard deviation (RSD) of 2.20% indicates lowermeasurement uncertainty compared to reported ZIF-based electrochemicalsensors38,39,92, demonstrating high precision in sensor response and overallsensing reliability.Third, the Co4Cu1-ZIF@CB/GCE sample was tested to detect2-Naphthol in the presence of possible background interference substances(see Fig. 8d–f). Figure 8d compares the sensor responses toward a mixedsolution in PBS (0.01M, pH 2) containing 100 µM2-Naphthol and 100 µMof potential interfering substances commonly present in urine. These sub-stances include ascorbic acid (AA), lactic acid (LA), uric acid (UA), urea(UR), dopamine (DOP), glucose (GLU), calcium chloride (CaCl2), potas-sium chloride (KCl), and sodium chloride (NaCl). The presence of theseinterferents had a negligible influence on the sensor response, as indicatedby a relative standard deviation (RSD) of 2.96%, demonstrating the highselectivity and accuracy of the sensor. Considering the linear relationshipbetween current and concentration, this deviation corresponds to anapproximate systematic error of 2.96 µM from the true 2-Naphthol con-centration, which can be further corrected through calibration. Moreover,the interfering compounds exhibit oxidation peaks at distinct potentials,resulting in only minimal interference with the 2-Naphthol responseobserved at approximately 0.21 V93,94. The selective ability of the sensor wasalso evaluated by detecting 2-Naphthol in non-biological urine diluent,which contains calcium chloride, magnesium chloride, potassium chloride,sodium chloride, sodium phosphate, sodium sulfate, urea, and creatinine,alongwith sodiumazide as a preservative. Figure 8e, f shows the comparisonof sensor response towards 2-Naphthol in PBS and urine diluent withconcentrations of 50, 100, and 500 µM. Here, no significant difference wasobservedwhen theurinediluent bufferwasused, as indicatedby comparablecurrent responses for 50 µMof 2-Naphthol in PBS and urine diluent, whichwere (111 ± 4) µA and (104 ± 4) µA, respectively. Furthermore, we calcu-lated the recovery of the sensor as the ratio of measured to added con-centration after successively adding 50, 100, and 500 µM of 2-Naphthol tothe urine diluent. From the detected recovery concentrations of 48.54, 89.83,and 466.33 µM, recovery percentage values of 97.08%, 89.83%, and 93.26%,respectively, were obtained. These high recovery values indicate the feasi-bility of Co4Cu1-ZIF@CB/GCE sensors in real sample analyses.ConclusionsBimetallic cobalt-and-copper-based zeolitic imidazolate framework (CoCu-ZIF) particles have been successfully integrated with carbon black (CB) andglassy carbon electrode (GCE) to serve as highly sensitive and selectiveelectrochemical 2-Naphthol sensors. Among the contributing factors, thesynergistic electronic interactions between Co3+/Co2+ and Cu2+/Cu+ havebeen found to play the most critical role in enhancing the electrochemicalproperties of the ZIF. A high Cu+ content increases carrier density, pro-moting internal charge redistribution and efficient electron transfer.Although CB has been required to further enhance the electrical con-ductivity of the composite, the CoCu-ZIF has been proven to play a crucialrole in elevating 2-Naphthol detection due to its accessible dual redox-activesites and high surface area that actively promote redox reactions. Theselectivity and limit of detection (LOD) of CoCu-ZIF@CB on GCE arewithin the safety threshold of 2-Naphthol, while featuring anti-interferencecapability and generating a stable detection response in challenging mea-surement conditions including synthetic urine. Moving forward, functio-nalizing the sensor surface with molecular recognition moieties likecyclodextrin or calixarene may further improve PAH selectivity. Addi-tionally, bimetallic ZIF-based sensors show strong potential for multiplexPAH detection by targeting their distinct redox signatures.Fig. 8 | Sensing performance of carbon-black-coupled CoCu-ZIF composite onglassy carbon electrode towards 2-Naphthol. a Square wave voltammetry (SWV)response of a Co4Cu1-ZIF@CB/GCE sample towards different concentrations of2-Naphthol. b Calibration plot of measured current response obtained from eachprepared 2-Naphthol concentration. The linear regression is analysed for two groupsof low (1–100 µM) and high (100–500 µM) 2-Naphthol concentrations.cReproducibility test results showing the current responses offiveCo4Cu1-ZIF@CB/GCE samples in 100 µM 2-Naphthol. d Cross-sensitivity assessment of a Co4Cu1-ZIF@CB/GCE sample in 100 µM 2-Naphthol mixed with various 100 µM inter-ference substances (i.e., ascorbic acid (AA), lactic acid (LA), uric acid (UA), urea(UR), dopamine (DOP), glucose (GLU), calcium chloride (CaCl2), potassiumchloride (KCl), and sodium chloride (NaCl)). e, f SWV response and respective barchart of maximum current of a Co4Cu1-ZIF@CB/GCE sample duringmonitoring of100 µM 2-Naphthol diluted in phosphate-buffered saline (PBS) and urine diluent,respectively. All measurements (except urine diluent shown in f) were conducted in0.01M PBS (pH 2).https://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 10www.nature.com/commsmatMethodsMaterialsThe reagents including cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O)(ACS reagent ≥98%), K3Fe(CN)6 (ACS reagent, ≥99.0%), and NafionTMperfluorinated resin solution (5 wt.%)were purchased fromSigma-Aldrich.Copper(II) nitrate trihydrate (Cu(NO3)2·3H2O) ( > 95.0%) and 2-MeIm( > 98.0%)werepurchased fromTokyoChemical Industry.KCl (99.0%)andPBS 10X (pH 7.4) were obtained from FujifilmWako and Nacalai Tesque,respectively. The used electrodes including the GCE (diameter of 3mm), areference electrode (Ag/AgCl/saturated KCl), and a platinum counterelectrode were acquired from ALS Japan. CB for GCE modification wasobtained fromCabot Corporation. Sensing evaluationwas performed using2-Naphthol ( > 99.0%) from Tokyo Chemical Industry, with additionalinterference agents including Sigmatrix urine diluent, uric acid, ascorbicacid, glucose, and dopamine hydrochloride obtained from Sigma-Aldrich.Synthesis of bimetallic CoCu-ZIF particlesCoCu-ZIFparticleswere synthesized through co-precipitationbydissolvingCo(NO3)2·6H2OandCu(NO3)2·3H2Owith a totalmole of 2mmol in 30mLofmethanol. Separately, 8mmol of 2-MeImwas dissolved in another 30mLof methanol. After both solutions were well mixed, the metal solution wastransferred slowly to the 2-MeIm solution and stirred for 1 h at roomtemperature. The ZIF was obtained after 24 h of solution ageing, then theprecipitatewas thoroughlywashedwithmethanol anddried at 60 °C. In thiswork, the same methods were used to produce CoxCuy-ZIF with differentprecursormolar ratios of Co toCu, where x and y are themolar ratios of Coand Cu, respectively. The produced sample variants include ZIF-67,Co4Cu0.5-ZIF, Co4Cu1-ZIF, Co4Cu2-ZIF, and Co4Cu3-ZIF.Material characterizationsThe phase and crystal structure of the materials were identified from XRDpatterns obtained from an X-ray diffractometer, Rigaku MiniFlex 300/600,Japan, with a Cu anode source over a range of 2θ of 10°–90° with a scan rateof 0.02°. In addition, FTIR spectroscopy, using a Prestige 21, Japan (KBrtechnique, at wavenumber range of 400 – 4000 cm–1) was conducted toconfirm the formation and chemical properties of the CuCo-ZIF particles.Field emission scanning electron microscopy (FE-SEM) analysis using aHitachi SU8000 with an acceleration voltage of 10.0 kV was carried out toinvestigate the material topography. This FE-SEM was integrated with aBruker EDX spectrometer for elemental screening. XPSusing anUlvac PHIQuantes with Al Kα source was performed to study the near-surfacecomposition and elemental distribution of the materials. The bindingenergy scale was calibrated using the C 1 s peak of adventitious carbon at284.8 eV. Surface area, pore size, and pore volume were analyzed fromnitrogen adsorption/desorption measurements using a QuantachromeAutosorb-1 in high-purity nitrogen gas (99.999%) at 77 K. Prior to nitrogenadsorption/desorption measurements, the samples were degassed by pre-heating at 150 °C under flowing argon for 16 h. The analyses were per-formed using BET, BJH, and DFT methods.Electrochemical measurementsElectrochemical measurements were conducted using a multichannelpotentiostat (BiologicVMP3)with a standard three-electrode configurationcomprising a glassy carbon working electrode (GCE, 3mm diameter), Ag/AgCl (saturated KCl) reference electrode, and a platinum wire counterelectrode. The electrolyte consisted of 5mM K3(FeCN)6 and 0.1M KCl in0.01M PBS (pH 7.4). The GCE working electrodes were prepared by drop-casting 6 µL of a 4mg/mL suspension of CoCu-ZIF (or composite) dis-persed in deionized water, 2-propanol, and 5 wt% NafionTM (15:4:1 v/v)onto the pre-polished GCE and drying at 30 °C for 20min. Prior to elec-trochemical measurements, these modified electrodes were activated with50 CV scans at 20 and 50mV/s until a stable redox signal was observed.Final CVs were recorded in the potential window of –0.10 to +0.55 V at ascan rate of 50mV/s. EIS measurements were performed over 35 fre-quencies logarithmically spaced from 1MHz to 0.01Hz, with an ACamplitudeof 1mV.For scan-rate studies,CVswere collectedat 10–100mV/s (step size: 10mV/s) using the same potential window.Sensing electrode preparationDetection of 2-Naphthol was performed using a GCE modified with theoptimized composite of carbon black-coupledCoCu-ZIF (CoCu-ZIF@CB),as illustrated in Fig. 1. A composite ink was prepared by dissolving CB andCoCu-ZIF in 1mL of a mixed solvent comprising deionized water, 2-pro-panol, and 5 wt% NafionTM (15:4:1 v/v). After sonication for homogeniza-tion, 6 µL of this ink was drop-cast onto a pre-polished GCE and dried at30 °C for 20min. The fabricated electrodes were evaluated using SWV todetect 500 µMof 2-Naphthol in 0.01MPBS (pH7.4). Prior to each SWV, 50cycles of CV scans at 50mV/s were performed in 0.01Mpure PBS (pH 7.4)to obtain a stable redox signal. Electrochemical study was performed on theelectrodes with five different mass ratios of CB (xmg) to CoCu-ZIF (ymg)denoted as x:y (i.e., 1:4, 2:4, 4:4, 4:2, and 4:1). In addition, the effect of pH ofthe analyte solution was also studied by change the pH of 0.01MPBS usingHCl as pH adjuster in the range of pH 2–7. The used potential windowdepended on the current peaks of 2-Napthol on each measurement, i.e.,–0.60 to +0.00 V for mass ratios dependent measurement, and –0.60 to+0.40 V for pH dependent measurement.Setup for polycyclic aromatic hydrocarbon detectionPAH detection was conducted using 2-Naphthol with various concentra-tions (1, 5, 10, 50, 100, 200, 300, 400, and 500 µM) in 0.01MPBS (pH2). Foreach measurement, 20 cycles at 50mV/s CV were conducted to obtain astable signal. The detection response was obtained from the SWV signalmeasured at –0.60 to+0.40 V with a scan rate of 50mV/s. The SWV signalfrom different 2-Naphthol concentrations was used to determine sensorsensitivity and LOD (see Fig. 1b). Five different CoCu-ZIF@CB/GCE sen-sors fabricated from the same batch of ink were used to detect 100 µM of2-Naphthol for evaluating reproducibility. The anti-interference ability ofthe sensors in heterogeneous solutions was assessed by measuring2-Naphthol solutions of 100 µM (in 0.01M PBS, pH 2) mixed with theinterference compounds of ascorbic acid, lactic acid, uric acid, urea, dopa-mine, glucose, calcium chloride, potassium chloride, and sodium chloride.In addition, Sigmatrix urine diluent (5-fold dilution) was used as a buffersolution of 50, 100, and 500 µM of 2-Naphthol, which was measured toevaluate the sensor’s recovery percentage.Data availabilityAll data supporting the findings of this study are included in the Article andits Supplementary Information. Additional data are available from thecorresponding authors upon reasonable request. Requests should bedirected to Dr. Joel Henzie (henzie.joeladam@nims.go.jp), Prof. Dr. ErwinPeiner (e.peiner@tu-braunschweig.de), Dr.-Ing. 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The authors acknowledge the NationalInstitute for Materials Science (NIMS), Japan, for providing the researchinternship through the International Cooperative Graduate Program (ICGP).C.W. acknowledges the Ganesha Talent Assistantship—Research GroupScholarship provided by Institut Teknologi Bandung. The authorsacknowledge the Research Center for Materials Nanoarchitectonics(MANA), NIMS, Japan, for providing research facilities, materials, and sup-port. Open Access funding is enabled and organized by Projekt DEAL viaTechnische Universität Braunschweig.Author contributionsC.W. and H.S.W. conceived the idea and concept. C.W. designed theexperiments, formulated the materials, validated the methods, conductedfabrication and characterization of CoCu-ZIF, interpreted the experimentalresults, created the figures (illustrations and graphs), wrote the initialmanuscript (main), and revised thepaper. H.S.W. andN.L.W.S. validated themethods, analyzed the obtained data, wrote the initial manuscript (support),and revised the paper. A.M., A.N., J.H., and N.N. provided input for thediscussion of the results. J.H. provided research facilities andmaterials. E.P.revised the paper. Y.Y., J.H., H.S.W., N.N., and B.Y. led the project,supervised the work, and acquired the funding. All authors discussed theresults and approved the final submitted manuscript.FundingOpen Access funding enabled and organized by Projekt DEAL.Competing interestsThe authors declare no competing interests.Inclusion and ethics statementAll collaborators of this study have fulfilled the criteria for authorship requiredby Nature Portfolio journals and have been included as authors, as theirparticipation was essential for the design and implementation of the study.Roles, responsibilities, and contributions were agreed among collaboratorsahead of the research and manuscript writing. This research was notseverely restrictedor prohibited in the setting of the researchers, and did notresult in stigmatization, incrimination, discrimination, or personal risk toresearchers. The references that are relevant and cited in this work not onlycome from our previous studies, but also include the reports from otherresearch groups.Additional informationSupplementary information The online version containssupplementary material available athttps://doi.org/10.1038/s43246-026-01150-9.Correspondence and requests for materials should be addressed toJoel Henzie, Erwin Peiner, Hutomo Suryo Wasisto or Nugraha Nugraha.Peer review information Communications Materials thanks theanonymous reviewers for their contribution to the peer review of this work.Reprints and permissions information is available athttp://www.nature.com/reprintsPublisher’s note Springer Nature remains neutral with regard tojurisdictional claims in published maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in anymedium or format, as longas you give appropriate credit to the original author(s) and the source,provide a link to the Creative Commons licence, and indicate if changeswere made. The images or other third party material in this article areincluded in the article’s Creative Commons licence, unless indicatedotherwise in a credit line to the material. If material is not included in thearticle’sCreativeCommons licence and your intended use is not permittedby statutory regulation or exceeds the permitted use, you will need toobtain permission directly from the copyright holder. To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/.© The Author(s) 2026https://doi.org/10.1038/s43246-026-01150-9 ArticleCommunications Materials |           (2026) 7:195 14https://doi.org/10.1038/s43246-026-01150-9http://www.nature.com/reprintshttp://creativecommons.org/licenses/by/4.0/www.nature.com/commsmat Carbon-black-coupled bimetallic CoCu-ZIF composites for monohydroxylated polycyclic aromatic hydrocarbon metabolite sensing Results and discussion Material and structural characteristics of CoCu-ZIF particles Electrochemical properties of CoCu-ZIF particles Carbon-black-coupled CoCu-ZIF composites Sensing performance evaluation towards 2-Naphthol Conclusions Methods Materials Synthesis of bimetallic CoCu-ZIF particles Material characterizations Electrochemical measurements Sensing electrode preparation Setup for polycyclic aromatic hydrocarbon detection Data availability References Acknowledgements Author contributions Funding Competing interests Inclusion and ethics statement Additional information