Catalytic Pathway of Nanozyme "Artificial Peroxidase" with 100-Fold Greater Bimolecular Rate Constants Compared to Those of the Enzyme

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.

CoOx/CeO2@C nanopetals derived Cobalt-Cerium Prussian blue with enhanced Dual-Enzyme mimetic activity for detection of ascorbic acid in rat brain during calm/ischemic processes

In this study, we demonstrate that a highly efficient colorimetric sensor prepared from carbon-shielded Co-Ce Prussian blue analog (PBA) nanopetals (CoOx/CeO2@C) by green chemical deposition method and thermal annealing processes for detection of ascorbic acid (AA) in cerebral microdialysis fluids. The synthesized CoOx/CeO2@C showed high dual-mimetic activity, i.e., peroxidase- and catalase-like activity, and great catalytic stability. The combination of carbon film and Co-Ce PBA nanopetals (1) greatly enhances the interfacial electron transfer rate of the nanopetals due to excellent electrical conductivity of carbon, and (2) protects nanopetals from acidic chemical environments during the catalytic process, which greatly reduces loss of the catalytic activity of the cobalt-cerium (hydroxide) oxides. Based on the peroxidase-like property of CoOx/CeO2@C nanopetals, this sensor has a good linear range from 0.1 to 150 μM with a low detection limit of 0.04 μM, i.e., improved sensitivity for AA colorimetric measurement. The developed colorimetric strategy with a green synthetic pathway, catalytic stability and wide linear range confirms the monitoring of AA in brain systems during calm/ischemic processes.

Fast Reaction of the Prussian Blue Based Nanozyme "Artificial Peroxidase" with the Substrates: Pre-Steady-State Kinetic Approach

This letter introduces the pre-steady-state kinetic approach, which is traditional for evaluation of elementary constants in molecular (enzyme) catalysis, for nanozymes. Apparently, the most active peroxidase-mimicking nanozyme based on catalytically synthesized Prussian Blue nanoparticles has been chosen. The elementary constants (k1) for the nanozymes' reduction by an electron-donor substrate (being the fastest stage according to steady-state kinetic data) have been determined by means of stopped-flow spectroscopy. These constants have been found to be dependent on both the size of the nanozyme and the reducing substrate redox potential. For the smallest nanozymes (32 nm in diameter), log(k1) linearly decays with an increase of the substrate redox potential (cotangent value ≈125 mV). On the contrary, for the largest nanozymes with a diameter above 150 nm, k1 is almost independent of it. Moreover, for the substrate with the lowest redox potential (K4[Fe(CN)6]), the rate constant under discussion (k1) is almost independent of the nanozymes' size. Perhaps, the rate of the intrananozyme electron transfer causing bleaching becomes comparative or even lower than that of the nanoparticle interaction with the fastest substrate. Anyway, the elementary constant of nanozyme reduction with potassium ferrocyanide (k1) reaches the value of 1 × 1010 M-1 s-1, which is 3-4 orders of magnitude faster than for enzymes peroxidases. The obtained results obviously demonstrate that the pre-steady-state kinetic approach is able to discover novel advantages of nanozymes from both fundamental and practical points of view.

Elucidating the catalytic mechanism of Prussian blue nanozymes with self-increasing catalytic activity

Although Prussian blue nanozymes (PBNZ) are widely applied in various fields, their catalytic mechanisms remain elusive. Here, we investigate the long-term catalytic performance of PBNZ as peroxidase (POD) and catalase (CAT) mimetics to elucidate their lifespan and underlying mechanisms. Unlike our previously reported Fe3O4 nanozymes, which exhibit depletable POD-like activity, the POD and CAT-like activities of PBNZ not only persist but slightly enhance over prolonged catalysis. We demonstrate that the irreversible oxidation of PBNZ significantly promotes catalysis, leading to self-increasing catalytic activities. The catalytic process of the pre-oxidized PBNZ can be initiated through either the conduction band pathway or the valence band pathway. In summary, we reveal that PBNZ follows a dual-path electron transfer mechanism during the POD and CAT-like catalysis, offering the advantage of a long service life.

Optimizing the Composition of the Substrate Enhances the Performance of Peroxidase-like Nanozymes in Colorimetric Assays: A Case Study of Prussian Blue and 3,3'-Diaminobenzidine

One of the emerging trends in modern analytical and bioanalytical chemistry involves the substitution of enzyme labels (such as horseradish peroxidase) with nanozymes (nanoparticles possessing enzyme-like catalytic activity). Since enzymes and nanozymes typically operate through different catalytic mechanisms, it is expected that optimal reaction conditions will also differ. The optimization of substrates for nanozymes usually focuses on determining the ideal pH and temperature. However, in some cases, even this step is overlooked, and commercial substrate formulations designed for enzymes are utilized. This paper demonstrates that not only the pH but also the composition of the substrate buffer, including the buffer species and additives, significantly impact the analytical signal generated by nanozymes. The presence of enhancers such as imidazole in commercial substrates diminishes the catalytic activity of nanozymes, which is demonstrated herein through the use of 3,3'-diaminobenzidine (DAB) and Prussian Blue as a model chromogenic substrate and nanozyme. Conversely, a simple modification to the substrate buffer greatly enhances the performance of nanozymes. Specifically, in this paper, it is demonstrated that buffers such as citrate, MES, HEPES, and TRIS, containing 1.5-2 M NaCl or NH4Cl, substantially increase DAB oxidation by Prussian Blue and yield a higher signal compared to commercial DAB formulations. The central message of this paper is that the optimization of substrate composition should be an integral step in the development of nanozyme-based assays. Herein, a step-by-step optimization of the DAB substrate composition for Prussian Blue nanozymes is presented. The optimized substrate outperforms commercial formulations in terms of efficiency. The effectiveness of the optimized DAB substrate is affirmed through its application in several commonly used immunostaining techniques, including tissue staining, Western blotting assays of immunoglobulins, and dot blot assays of antibodies against SARS-CoV-2.

Catalytically synthesized Prussian Blue nanozymes as labels for electrochemical DNA/RNA sensors

We propose catalytically synthesized nanozymes based on Prussian Blue (PB) and azidomethyl-substituted poly (3,4-ethylenedioxythiophene) (azidomethyl-PEDOT) as novel electrocatalytic labels for DNA/RNA sensors. Catalytic approach allowed to synthesize highly redox and electrocatalytically active Prussian Blue nanoparticles functionalized with azide groups that enable 'click' conjugation with alkyne-modified oligonucleotides. Both competitive and sandwich-type schemes were realized. As the sensor response the direct (mediator-free) electrocatalytic current of H₂O₂ reduction can be measured, which is proportional to the concentration of the hybridized labeled sequences. The current of H₂O₂ electrocatalytic reduction is only 3-8 times increased in the presence of the freely diffusing mediator catechol, which indicates high efficiency of direct electrocatalysis with the elaborated labels. Electrocatalytic amplification of the signal allows robust detection of (63-70)-base target sequences with concentrations below 0.2 nM in blood serum within an hour. We believe, the use of advanced Prussian Blue based electrocatalytic labels sets new avenues for point-of-care DNA/RNA sensing.

Single Printing Step Prussian Blue Bulk-Modified Transducers for Oxidase-Based Biosensors

We report on hydrogen peroxide sensors made through a single printing step with carbon ink containing catalytically synthesized Prussian blue nanoparticles. Despite their reduced sensitivity, the resulting bulk-modified sensors displayed both a wider linear calibration range (5 × 10^-7-1 × 10^-3 M) and an approximately four times lower detection limit versus the surface-modified sensors due to the dramatically decreased noise resulting in, on average, a six times higher signal-to-noise ratio. The corresponding glucose and lactate biosensors demonstrated similar and even higher sensitivities compared to those of biosensors based on surface-modified transducers. The biosensors have been validated through analysis of human serum. The decreased time and cost for production of single printing step bulk-modified transducers, as well as their analytical performance characteristics, which are advantageous over conventional surface-modified ones, would be expected to enable their wide use in (bio)sensorics.

Prussian blue: from advanced electrocatalyst to nanozymes defeating natural enzyme

The pathway from the advanced electrocatalyst to nanozymes defeating natural enzyme is reviewed. Prussian blue, being the most advantageous electrocatalyst for hydrogen peroxide reduction, is obviously the best candidate for mimicking peroxidase activity. Indeed, catalytically synthesized Prussian blue nanoparticles are characterized by the catalytic rate constants, which are significantly (up to 4 orders of magnitude) higher than for enzyme peroxidase. Displaying in addition the enzymatic specificity in terms of an absence of oxidase-like activity, catalytically synthesized Prussian blue nanoparticles can be referred to as nanozymes. The latter provide the most versatile method for surface covering with the electrocatalyst, allowing to modify non-traditional materials like boron-doped diamond. For stabilization, Prussian blue core can be covered with nickel hexacyanoferrate shell; the resulting core-shell nanozymes still defeat natural enzyme in terms of activity. Discovering the catalytic pathway of nanozymes "artificial peroxidase" action, we have found the novel advantage of nanozymes over the corresponding biological catalysts: their dramatically (100 times) improved bimolecular rate constants.

Prussian Blue Nanozymes with Enhanced Catalytic Activity: Size Tuning and Application in ELISA-like Immunoassay

Prussian blue nanozymes possessing peroxidase-like activity gather significant attention as alternatives to natural enzymes in therapy, biosensing, and environmental remediation. Recently, Prussian blue nanoparticles with enhanced catalytic activity prepared by reduction of FeCl₃/K₃[Fe(CN)₆] mixture have been reported. These nanoparticles were denoted as ‘artificial peroxidase’ nanozymes. Our study provides insights into the process of their synthesis. We studied how the size of nanozymes and synthesis yield can be controlled via adjustment of the synthesis conditions. Based on these results, we developed a reproducible and scalable method for the preparation of ‘artificial peroxidase’ with tunable sizes and enhanced catalytic activity. Nanozymes modified with gelatin shell and functionalized with affine molecules were applied as labels in colorimetric immunoassays of prostate-specific antigen and tetanus antibodies, enabling detection of these analytes in the range of clinically relevant concentrations. Protein coating provides excellent colloidal stability of nanozymes in physiological conditions and stability upon long-term storage.

Development of an optical immunoassay based on peroxidase-mimicking Prussian blue nanoparticles and a label-free electrochemical immunosensor for accurate and sensitive quantification of milk species adulteration

In contrast to reported enzyme-based immunoassays, an enzyme-free immunoassay (optical and electrochemical) is presented here for the first time that can be used as point-of-need detection bioplatforms of bovine IgG as goat milk adulterant. In the first format, Prussian blue nanoparticles (PBNPs) were used as antibody catalytic labels in a competitive colorimetric microplate immunoassay. Absorbance measurement was performed photometrically at 450 nm. After in-depth optimization, excellent sensitivity was achieved (0.01% cow/goat volume ratio), which is 100 times lower than the limit allowed by the European legislation (EL) (1% v/v), thanks to the high catalytic activity of PBNPs compared with natural peroxidase. Moreover, the antibody-PBNPs bioconjugates showed excellent stability over 4 weeks (> 94% of the initial response) confirming the successful anchoring of the antibodies to the surface of the PBNPs. On the other hand, a label-free voltammetric immunoassay for the detection of bovine IgG was developed. The sensing principle was based on the hindrance of charge transfer between ferri-ferrocyanide redox couple and the screen-printed gold electrodes modified with bovine IgG antibody. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the step-by-step modification of the electrode surface. Under optimal conditions, this single-step electrochemical analysis achieved a high sensitivity of 0.1% (cow/goat) when monitoring the ferrocyanide oxidation at + 0.092 V (vs. Ag/AgCl) using differential pulse voltammetry (DPV). The selectivity of the developed immunoassays was evaluated for different species of milk of similar composition, and both immunoassays exhibited a selective response only to bovine IgG. Unlike conventional immunoassays, the developed enzyme-free immunoassays have many attractive features for the detection of milk adulteration, whether they are used in quality control laboratories for routine milk analysis (optical immunoassay) or at on-site checkpoints (electrochemical immunoassay) using wireless electrochemical detectors. The sensors provide high sensitivity (≤ 0.1%), excellent precision (RSD < 6%), low cost (no enzyme is required) and ease of operation, including handling of milk samples.

Flow-electrochemical synthesis of Prussian Blue based nanozyme 'artificial peroxidase'

We report on fully electrochemical flow-through synthesis of Prussian Blue based nanozymes defeating peroxidase in terms of more than 200 times higher catalytic rate constant (k = 6 × 10⁴ s^-1). Being reagentless, reproducible, simple and scalable, the proposed approach blazes new trails for the electrosynthesis of functional conductive and electroactive nanomaterials.

Core-Shell Nanozymes "Artificial Peroxidase": Stability with Superior Catalytic Properties

We report on the nanoparticles composed of the catalytically synthesized Prussian Blue (PB) core stabilized with the nickel hexacyanoferrate (NiHCF) shell. Catalyzing hydrogen peroxide reduction, the resulting nanozymes (ø = 66 nm) display catalytic rate constants, which for pyrogallol or ferrocyanide are, respectively, 25 and 35 times higher than those for peroxidase enzyme. After more than half a year of storage at a room temperature, the core-shell PB-NiHCF nanozymes retain both their size and physicochemical properties; such stability is unreachable for the enzymes. Being immobilized, core-shell PB-NiHCF nanozymes (ø = 45 nm) result in a hydrogen peroxide sensor with a sensitivity similar to that of the sensor based on sole PB nanoparticles. However, whereas the latter response in hard inactivating conditions (25 min in 1 mM H₂O₂) drops down to 7.5%, the PB-NiHCF nanozymes-based sensor retains >75% of initial sensitivity. Application of the core-shell PB-NiHCF nanozymes "artificial peroxidase" would obviously open new horizons in elaboration of anti-inflammatory drugs and (bio)sensors.

Sensitive colorimetric assay for the determination of alkaline phosphatase activity utilizing nanozyme based on copper nanoparticle-modified Prussian blue

Nanozyme based on Prussian blue nanocubes (PB NCs) loaded with copper nanoparticles (Cu@PB NCs) was synthesized. The peroxidase (POD)-like activity of Cu@PB NCs was studied and utilized for detecting the activity of alkaline phosphatase (ALP). The Cu@PB NCs possess higher POD-like activity compared with PB NCs and natural horseradish peroxidase (HRP) due to the loading of copper nanoparticles. 3,3',5,5'-Tetramethylbenzidine (TMB) can be oxidized to oxTMB in the presence of Cu@PB NCs and H₂O₂, generating blue-colored compound, while introduction of pyrophosphate (PPi) leads to the POD-like activity of Cu@PB NCs decreased obviously. In the presence of ALP, PPi was hydrolyzed and then the POD-like activity of Cu@PB NCs was restored. So, according to the change of the POD-like activity of Cu@PB NCs, a sensitive colorimetric assay for ALP activity was reported. The limit of detection of the assay is 0.08 mU/mL, with linear range from 0.1 to 50 mU/mL. In addition, the assay was also applied for screening the inhibitors of ALP. Nanozyme based on Prussian blue nanocube (PB NCs) loaded with copper nanoparticles was synthesized and utilized for detecting the activity of alkaline phosphatase (ALP).