Polyethylenimine mediated synthesis of copper-iron and nickel-iron hexacyanoferrate nanoparticles and their electroanalytical applications

Peroxidase (POD) Mimicking Activity of Different Types of Poly(ethyleneimine)-Mediated Prussian Blue Nanoparticles

Prussian blue nanoparticles (PBNPs) have been identified as a promising candidate for biomimetic peroxidase (POD)-like activity, specifically due to the metal centres (Fe3+/Fe2+) of Prussian blue (PB), which have the potential to function as catalytically active centres. The decoration of PBNPs with desired functional polymers (such as amino- or carboxylate-based) primarily facilitates the subsequent linkage of biomolecules to the nanoparticles for their use in biosensor applications. Thus, the elucidation of the catalytic POD mimicry of these systems is of significant scientific interest but has not been investigated in depth yet. In this report, we studied a series of poly(ethyleneimine) (PEI)-mediated PBNPs (PB/PEI NPs) prepared using various synthesis protocols. The resulting range of particles with varying size (~19-92 nm) and shape combinations were characterised in order to gain insights into their physicochemical properties. The POD-like nanozyme activity of these nanoparticles was then investigated by utilising a 3,3',5,5'-tetramethylbenzidine (TMB)/H2O2 system, with the catalytic performance of the natural enzyme horseradish peroxidase (HRP) serving as a point of comparison. It was shown that most PB/PEI NPs displayed higher catalytic activity than the PBNPs, with higher activity observed in particles of smaller size, higher Fe content, and higher Fe2+/Fe3+ ratio. Furthermore, the nanoparticles demonstrated enhanced chemical stability in the presence of acid, sodium azide, or high concentrations of H2O2 when compared to HRP, confirming the viability of PB/PEI NPs as a promising nanozymatic material. This study disseminates fundamental knowledge on PB/PEI NPs and their POD-like activities, which will facilitate the selection of an appropriate particle type for future biosensor applications.

Polyethylenimine as a Non-Innocent Ligand for Hexacyanoferrates Immobilization

To understand how polyethyleneimine (PEI), as a ligand, affects structure and properties of the transition metals hexacyanoferrates (HCFs) immobilized in cross-linked PEI matrix, we have synthesized Cu(II), Zn(II), and Fe(III) HCFs via successive ion-exchange reactions with metal salts and K₄[Fe^II(CN)₆] or K₃[Fe^III(CN)₆]. The structure and properties of the obtained materials in comparison with the crystalline HCF analogs were investigated with FT-IR, Mössbauer, and UV-Vis spectroscopy. Complete reduction of Fe(III) to Fe(II) by PEI in HCF(III) was confirmed. When synthesis was performed at pH favoring binding of precursor metal ions by PEI, cyano-bridged hybrids rather than polymer-HCFs composites were formed. Although the obtained hybrids did not demonstrate sorption activity toward cesium ions, known for crystalline HCFs, they are of interest for the other applications. SQUID measurements revealed a significant difference in magnetic properties of PEI-HCFs hybrids in comparison with crystalline HCFs. Due to the Fe(III) to Fe(II) reduction in HCF ions, Cu(II) and Fe(III) HCFs(III) lost the molecular magnets properties in PEI matrix, but magnetic ordering, including ferromagnet-antiferromagnet interactions, was observed in all hybrids over the broad temperature range.

Synthesis and Applications of Prussian Blue and Its Analogues as Electrochemical Sensors

Prussian blue (PB) and its analogue (PBA) are a kind of representative cyanide-based coordination polymer. They have received enormous research interest and have shown promising applications in the electrochemical sensing field due to their excellent electrochemical activity and unique structural characteristics including open framework structure, high specific surface area, and adjustable metal active sites. In this review, we summarize the latest research progress of PB/PBA as an electrochemical sensor in detail from three aspects: fabrication strategy, synthesis method and electrochemical sensor application. For the fabrication strategy, we discussed different fabrication methods containing the combination of PBA and carbon materials, metal nanoparticles, polymers, etc., respectively, as well as their corresponding sensing mechanism for improving performance. We also presented the synthesis methods of PB/PBA materials in detail, such as: coprecipitation, hydrothermal and electrodeposition. In addition, the effects of different methods on the morphology, particle size and productivity of PB/PBA materials are also concluded. For the application of electrochemical sensors, the latest progress of such materials as electrochemical sensors for glucose, H2O2, toxic compounds, and biomolecules have been summarized. Finally, we conclude remaining challenges of PB/PBA-based materials as electrochemical sensors, and provide personal perspectives for future research in this field.

"Green" Prussian Blue Analogues as Peroxidase Mimetics for Amperometric Sensing and Biosensing

Prussian blue analogs (PBAs) are well-known artificial enzymes with peroxidase (PO)-like activity. PBAs have a high potential for applications in scientific investigations, industry, ecology and medicine. Being stable and both catalytically and electrochemically active, PBAs are promising in the construction of biosensors and biofuel cells. The "green" synthesis of PO-like PBAs using oxido-reductase flavocytochrome b2 is described in this study. When immobilized on graphite electrodes (GEs), the obtained green-synthesized PBAs or hexacyanoferrates (gHCFs) of transition and noble metals produced amperometric signals in response to H2O2. HCFs of copper, iron, palladium and other metals were synthesized and characterized by structure, size, catalytic properties and electro-mediator activities. The gCuHCF, as the most effective PO mimetic with a flower-like micro/nano superstructure, was used as an H2O2-sensitive platform for the development of a glucose oxidase (GO)-based biosensor. The GO/gCuHCF/GE biosensor exhibited high sensitivity (710 A M-1m-2), a broad linear range and good selectivity when tested on real samples of fruit juices. We propose that the gCuHCF and other gHCFs synthesized via enzymes may be used as artificial POs in amperometric oxidase-based (bio)sensors.

Peroxidase-Like Metal-Based Nanozymes: Synthesis, Catalytic Properties, and Analytical Application

Nanozymes (NZs) are nanostructured artificial enzymes that mimic catalytic properties of natural enzymes. The NZs have essential advantages over natural enzymes, namely low preparation costs, stability, high surface area, self-assembling capability, size and composition-dependent activities, broad possibility for modification, and biocompatibility. NZs have wide potential practical applications as catalysts in biosensorics, fuel-cell technology, environmental biotechnology, and medicine. Most known NZs are mimetics of oxidoreductases or hydrolases. The present work aimed to obtain effective artificial peroxidase (PO)-like NZs (nanoPOs), to characterize them, and to estimate the prospects of their analytical application. NanoPOs were synthesized using a number of nanoparticles (NPs) of transition and noble metals and were screened for their catalytic activity in solution and on electrodes. The most effective nanoPOs were chosen as NZs and characterized by their catalytic activity. Kinetic parameters, size, and structure of the best nanoPOs (Cu/CeS) were determined. Cu/CeS-based sensor for H2O2 determination showed high sensitivity (1890 A·M−1·m−2) and broad linear range (1.5–20,000 µM). The possibility to apply Cu/CeS-NZ as a selective layer in an amperometric sensor for hydrogen-peroxide analysis of commercial disinfectant samples was demonstrated.

Synthesis, Catalytic Properties and Application in Biosensorics of Nanozymes and Electronanocatalysts: A Review

The current review is devoted to nanozymes, i.e., nanostructured artificial enzymes which mimic the catalytic properties of natural enzymes. Use of the term "nanozyme" in the literature as indicating an enzyme is not always justified. For example, it is used inappropriately for nanomaterials bound with electrodes that possess catalytic activity only when applying an electric potential. If the enzyme-like activity of such a material is not proven in solution (without applying the potential), such a catalyst should be named an "electronanocatalyst", not a nanozyme. This paper presents a review of the classification of the nanozymes, their advantages vs. natural enzymes, and potential practical applications. Special attention is paid to nanozyme synthesis methods (hydrothermal and solvothermal, chemical reduction, sol-gel method, co-precipitation, polymerization/polycondensation, electrochemical deposition). The catalytic performance of nanozymes is characterized, a critical point of view on catalytic parameters of nanozymes described in scientific papers is presented and typical mistakes are analyzed. The central part of the review relates to characterization of nanozymes which mimic natural enzymes with analytical importance ("nanoperoxidase", "nanooxidases", "nanolaccase") and their use in the construction of electro-chemical (bio)sensors ("nanosensors").