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[Makoto Sasaki](https://orcid.org/0000-0002-7696-6405), [Mitsuhiro Ebara](https://orcid.org/0000-0002-7906-0350)

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[Effect of nanosizing on sodium cobalt(ii) hexacyanoferrate(ii) nanoparticles in nanofibers for enhanced ammonium adsorption capacity](https://mdr.nims.go.jp/datasets/6eaa72c9-ee57-4791-a575-0e1671845241)

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Effect of nanosizing on sodium cobalt(ii) hexacyanoferrate(ii) nanoparticles in nanofibers for enhanced ammonium adsorption capacity6126 |  Chem. Commun., 2025, 61, 6126–6129 This journal is © The Royal Society of Chemistry 2025Cite this: Chem. Commun., 2025,61, 6126Effect of nanosizing on sodium cobalt(II)hexacyanoferrate(II) nanoparticles in nanofibersfor enhanced ammonium adsorption capacity†Makoto Sasaki ab and Mitsuhiro Ebara *abIn this study, sodium cobalt(II) hexacyanoferrate(II) (NaCoHCF)nanoparticles were synthesized and incorporated into electrospunnanofibers to enhance their ammonium adsorption capacity. Wesuccessfully synthesized nanosized NaCoHCF using microfluidicsand incorporated it into the nanofibers. This resulted in an approxi-mately three-fold improvement in adsorption performance com-pared to that of micro-sized NaCoHCF.Ammonium (NH4+) is a crucial compound in the nitrogen cycleand is widely present in the environment owing to both naturalprocesses and human activities.1,2 However, excessive ammoniumcan cause severe toxicity in both the environment and livingorganisms. For example, in many vertebrates, an increase inammonium concentration in the body can lead to acute ammoniapoisoning, followed by convulsions and ultimately death.3 Thisphenomenon is believed to result from its effects on the centralnervous system.4,5 Additionally, whereas ammonium serves as anessential nitrogen source for plants, excessive uptake has alsobeen reported to induce ammonium toxicity in them.6,7 Toprevent toxicity associated with ammonium accumulation, tech-nologies for removing it from wastewater, industrial effluent, soil,and even blood are of considerable importance.Among various methods, adsorption is known to be a highlyeffective method of removing ammonium, and several ammoniumadsorbents—including zeolites and activated carbon—have beendeveloped.8–13 Among these, sodium cobalt(II) hexacyanoferrate(II)(NaCoHCF), as reported by Y. Jiang et al. in 2018, has attractedconsiderable attention owing to its exceptionally high ammoniumadsorption capacity via ion exchange reactions, as well as its highselectivity for ammonium, making it a promising material forvarious applications.14 Rather than using NaCoHCF in powderform, incorporating it into a substrate to create a filter offersseveral advantages, including the prevention of powder leakage,enhanced reusability, and multifunctional capabilities. However,when powdered adsorbents are used in liquid media, aggregationcan occur, potentially reducing the adsorption efficiency. None-theless, incorporating NaCoHCF into a filter can help preventaggregation and maintain the adsorption performance.In this study, electrospun nanofibers were selected as sub-strates for incorporating NaCoHCF. Electrospinning is a techniquefor fabricating nanofibers through the application of a high voltagewhile extruding a polymer solution through a needle from asyringe. This method not only allows for the simple fabricationof nanofibers but also has the advantage of being able to producenanofibers from various polymers by optimizing multiple para-meters—such as the solution concentration, applied voltage, injec-tion distance, and flow rate.15 Additionally, nanofibers producedby electrospinning exhibit extremely fine fiber diameters and highspecific surface areas, which are considered to be beneficial forammonium adsorption.16–18 In our previous study, nanofiberscontaining micro-sized NaCoHCF were fabricated via electrospin-ning and their ability to adsorb ammonium from solution wassuccessfully demonstrated.19 However, the micro-sized NaCoHCFparticles were entangled with multiple nanofibers due to their size.Therefore, it was hypothesized that if nanosized NaCoHCF couldbe incorporated within a single nanofiber, more efficient adsorp-tion could be realized. To achieve this, it would be necessary tosynthesize NaCoHCF with particle sizes smaller than the fiberdiameter.In this study, we focused on the synthesis of NaCoHCFnanoparticles and their incorporation into nanofibers to improvetheir ammonium adsorption performance. Two different synth-esis methods were employed to compare the NaCoHCF micro-particles (micro-NaCoHCF) and NaCoHCF nanoparticles (nano-NaCoHCF). Micro-NaCoHCF was synthesized by mixing aqueoussolutions of Na4[Fe(CN)6]�10H2O and Co(NO3)2�6H2O in a beakerunder stirring, followed by freeze-drying of the resulting particledispersion. By contrast, nano-NaCoHCF was obtained by mixingthe two aqueous solutions within a Y-shaped microchannel, witha Research Center for Macromolecules and Biomaterials, National Institute forMaterials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.E-mail: EBARA.Mitsuhiro@nims.go.jpb Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1Tennodai, Tsukuba, Ibaraki, 305-8577, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5cc00625bReceived 4th February 2025,Accepted 18th March 2025DOI: 10.1039/d5cc00625brsc.li/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 31 March 2025. Downloaded on 4/22/2025 4:42:19 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttps://orcid.org/0000-0002-7696-6405https://orcid.org/0000-0002-7906-0350http://crossmark.crossref.org/dialog/?doi=10.1039/d5cc00625b&domain=pdf&date_stamp=2025-03-29https://doi.org/10.1039/d5cc00625bhttps://doi.org/10.1039/d5cc00625bhttps://rsc.li/chemcommhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5cc00625bhttps://pubs.rsc.org/en/journals/journal/CChttps://pubs.rsc.org/en/journals/journal/CC?issueid=CC061033This journal is © The Royal Society of Chemistry 2025 Chem. Commun., 2025, 61, 6126–6129 |  6127the particles forming during the process, followed by freeze-dryingof the obtained dispersion (Fig. 1).In this synthesis method, turbulence can occur when the twosolutions collide, promoting nucleation and suppressing particlegrowth, thereby enabling the synthesis of nanoparticles. Thesynthesized micro- and nano-NaCoHCFs can then be incorpo-rated into poly(ethylene-co-vinyl alcohol) (EVOH) nanofibers viaelectrospinning. The incorporation state can be observed usingscanning electron microscopy (SEM) and transmission electronmicroscopy (TEM), followed by an ammonium adsorption test toevaluate the adsorption performance.In this study, the particle sizes of the synthesized micro- andnano-NaCoHCF were measured using dynamic light scattering(DLS), the results of which are presented in Fig. 2(a) and (b).The average particle sizes calculated using the cumulantmethod were 4473.2 and 45.9 nm, respectively, confirming thatthe nano-NaCoHCF had a particle size approximately 1/100th thatof the micro-NaCoHCF. Additionally, the nano-NaCoHCF exhib-ited a lower polydispersity index (PDI), suggesting that the parti-cles were monodispersed. This was likely owing to turbulenceduring the mixing of the two solutions in the microchannel,leading to the formation of multiple locally supersaturatedregions that enhanced nucleation. Consequently, a large portionof the solute was consumed during nucleation, suppressingparticle growth and resulting in smaller particle sizes and lowerPDI values.20 Moreover, synthesis was conducted with variousflow rates of the mixed solutions; however, particles of approxi-mately 50 nm were obtained regardless of the flow rate, indicatingno major change owing to flow rate variations (Fig. S1, ESI†). Theminimum flow rate used in this study was 5 mL min�1. However,even under these conditions, sufficient turbulence was achievedto facilitate nanoparticle synthesis. A previous study by Yamamotoet al. also reported that the particle size of ZIF-8 decreases withincreasing flow velocity during mixing, but above a certain flowvelocity, there is no change in particle size.21NaCoHCF SEM images are presented in Fig. 2(c)–(f). TheSEM images confirmed that the nano-NaCoHCF had a smallerparticle size than the micro-NaCoHCF. However, the nano-NaCoHCF particle size observed in the SEM images wasapproximately 200 nm, which was larger than that from theDLS measurement results. This discrepancy was likely becauseof particle aggregation that occurred during the drying of thenano-NaCoHCF dispersion, which was necessary for SEM sam-ple preparation. Moreover, owing to the resolution limitationsof SEM, the accurate observation of the nano-NaCoHCF mor-phology can be challenging. Consequently, additional observa-tions were made using TEM (Fig. S2, ESI†), which revealed thepresence of particles of approximately 100 nm in size and cubicstructures constituting the particles themselves. BecauseNaCoHCF has been reported to have a cubic crystal structure,it could be inferred that these crystals aggregated to form theobserved particles.Nanofibers containing micro- and nano-NaCoHCF werefabricated using electrospinning, the SEM images of whichare shown in Fig. 3(a) and (b). In the EVOH/micro-NaCoHCFnanofiber, inclusions of approximately 20–50 mm diametercould be observed, which were presumed to be micro-NaCoHCF. Instead of being embedded within the nanofibers,the micro-NaCoHCF particles were present on the nanofibersheet, which was covered by a thin polymer layer. By contrast,no such inclusions were evident in the EVOH/nano-NaCoHCFnanofibers. Because the nano-NaCoHCF particle size was smal-ler than the nanofiber diameter, it could be inferred that thenano-NaCoHCF had been incorporated into the nanofibers.To verify this, TEM imaging was conducted, the results ofwhich are shown in Fig. 3(c). Inclusions were evident within thenanofibers and elemental analysis using energy-dispersive X-ray spectroscopy (EDX) confirmed the presence of Fe and CoFig. 1 Illustration of the experimental set-up for the synthesis of nano-NaCoHCF.Fig. 2 (a) and (b) Intensity-weighted size distributions of the micro-NaCoHCF and nano-NaCoHCF particles measured by dynamic lightscattering (DLS). The average diameters and polydispersion index (PDI)were calculated using the cumulant method. (c)–(f) SEM images of thesynthesized NaCoHCF particles. (c) Micro-NaCoHCF with low magnifica-tion, (d) nano-NaCoHCF with low magnification, (e) micro-NaCoHCF withhigh magnification, and (f) nano-NaCoHCF with high magnification.Communication ChemCommOpen Access Article. Published on 31 March 2025. Downloaded on 4/22/2025 4:42:19 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5cc00625b6128 |  Chem. Commun., 2025, 61, 6126–6129 This journal is © The Royal Society of Chemistry 2025atoms in these inclusions. As the EVOH contained only carbonand oxygen atoms, the detected iron and cobalt atoms evidentlyoriginated from the NaCoHCF. These findings suggest that thenano-NaCoHCF had been successfully incorporated into thenanofibers.Thermogravimetry/differential thermal analysis (TG/DTA) wasperformed to determine the NaCoHCF content of the EVOH/nano-NaCoHCF nanofibers and EVOH/micro-NaCoHCF nanofibers(Fig. S3, ESI†). In the thermogravimetric curves, both nano-NaCoHCF and micro-NaCoHCF showed similar mass loss beha-vior, suggesting that both have similar thermal tolerance. TheNaCoHCF content in the nanofibers was calculated from the finalresidual mass to be 48.6 wt% for EVOH/nano-NaCoHCF and51.5 wt% for EVOH/micro-NaCoHCF, confirming that the contentwas almost the same as the preparation ratio.To evaluate the effect of the NaCoHCF particle size on theammonium adsorption performance, adsorption tests wereconducted using EVOH/micro-NaCoHCF and EVOH/nano-NaCoHCF nanofibers, the results of which are shown in Fig. 4.The amount of ammonium adsorbed per unit mass ofNaCoHCF did not increase for the EVOH nanofibers alone,confirming that the EVOH nanofibers possessed no ammoniumadsorption capability. By contrast, both the EVOH/micro-NaCoHCF and EVOH/nano-NaCoHCF nanofibers exhibited anincrease in ammonium adsorption over time—that is, after 2 hof testing, the ammonium adsorption amounts were 38.1 and126.8 mg g�1, respectively. The nano-NaCoHCF exhibited morethan three times the adsorption performance of the micro-NaCoHCF. This result could be attributed to the higher specificsurface area of the nano-NaCoHCF. In the case of the micro-NaCoHCF, ammonium was likely adsorbed only onto surfaceadsorption sites, with the internal adsorption sites not beingutilized efficiently. By contrast, the nano-NaCoHCF—with itshigher specific surface area—permitted increased utilization ofthe adsorption sites, leading to enhanced ammonium adsorptionperformance. Previous studies also observed that smaller particlesizes resulted in higher adsorption performance, consistent withthe findings of this study.22 Furthermore, in a paper published byH. Ming et al. in 2012, Prussian blue nanoparticles of differentsizes were synthesized. For nanoparticles of the same size as thosein our report, they reported a surface area of 254 m2 g�1 and apore diameter of around 8 nm.23In summary, this study successfully synthesized nano-NaCoHCF particles of approximately 50 nm size and incorpo-rated them into nanofibers. Consequently, the ammoniumadsorption performance of the NaCoHCF within the nanofiberswas considerably enhanced, exhibiting more than three timesthe adsorption capacity of the microparticles. This value sur-passed those of commonly used ammonium adsorbents—suchas zeolites (approximately 8–50 mg g�1)10 and biochar (approxi-mately 5–43 mg g�1)8—indicating that the material offeredpractical adsorption performance. Moreover, because theNaCoHCF was embedded within the nanofibers, the risk ofleakage could be minimized, making it a promising candidatefor various applications as an ammonium adsorption filter.The findings of this study are not limited to NaCoHCF butare applicable to a wide range of adsorbent materials, suggest-ing the potential for further development of electrospun nano-fiber/inorganic hybrid materials.This work was supported by JSPS KAKENHI Grant NumbersJP20H05877 and JP23KJ0260. Part of this work was supportedby ‘‘Advanced Research Infrastructure for Materials and Nano-technology in Japan (ARIM)’’ of the Ministry of Education,Culture, Sports, Science and Technology (MEXT).Data availabilityThe data supporting this article have been included as part ofthe ESI.†Fig. 3 (a) SEM image of the EVOH/micro-NaCoHCF nanofibers. (b) SEMimage of the EVOH/nano-NaCoHCF nanofibers. (c) EDX mapping of theEVOH/nano-NaCoHCF nanofibers.Fig. 4 Change of ammonium adsorption capacity of NaCoHCF in theEVOH/micro-NaCoHCF and EVOH/nano-NaCoHCF nanofibers (n = 3).ChemComm CommunicationOpen Access Article. Published on 31 March 2025. Downloaded on 4/22/2025 4:42:19 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5cc00625bThis journal is © The Royal Society of Chemistry 2025 Chem. Commun., 2025, 61, 6126–6129 |  6129Conflicts of interestThere are no conflicts to declare.References1 D. Fowler, M. Coyle, U. Skiba, M. A. Sutton, J. N. Cape, S. Reis,L. J. Sheppard, A. Jenkins, B. Grizzetti, J. N. Galloway, P. Vitousek,A. Leach, A. F. Bouwman, K. Butterbach-Bahl, F. Dentener, D. Stevenson,M. Amann and M. Voss, Philos. Trans. R. Soc., B, 2013, 368, 20130164.2 L. Y. Stein and M. G. Klotz, Curr. Biol., 2016, 26, R94–R98.3 D. Randall and T. K. Tsui, Mar. Pollut. Bull., 2002, 45, 17–23.4 M. D. Norenberg, K. V. Rama Rao and A. R. Jayakumar, Metab. BrainDis., 2009, 24, 103–117.5 M. Skowrońska and J. Albrecht, Neurochem. Int., 2013, 62, 731–737.6 T. Hachiya, J. Inaba, M. Wakazaki, M. Sato, K. Toyooka, A. Miyagi,M. Kawai-Yamada, D. Sugiura, T. Nakagawa, T. Kiba, A. Gojon andH. Sakakibara, Nat. Commun., 2021, 12, 4944.7 D. T. Britto, M. Y. Siddiqi, A. D. M. Glass and H. J. Kronzucker, Proc.Natl. Acad. Sci. U. S. A., 2001, 98, 4255–4258.8 B. Han, C. Butterly, W. Zhang, J. 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