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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. [ABSTRACT FROM AUTHOR]

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 (kcatapp = 6 x 104 s-1). Being reagentless, reproducible, simple and scalable, the proposed approach blazes new trails for the electrosynthesis of functional conductive and electroactive nanomaterials. [ABSTRACT FROM AUTHOR]

We report the single‐step fabrication of advanced electrochemical biosensors upon drop‐casting of "artificial peroxidase", that is, oxidase‐containing mixtures. Compared to bienzyme biosensors, we substitute peroxidase with more active and stable nanozymes based on catalytically synthesized Prussian Blue. The electroactivity of nanozymes in the sensing membrane was facilitated with carbon black nanoparticles, which resulted in record sensitivity of the H2O2 transducer exceeding 1.1 A M−1 ⋅ cm−2. Both glucose and lactate oxidases were co‐immobilized with nanozymes in alkoxysilane or perfluorosulfonated ionomer membranes. The proposed drop‐casting approach results in biosensors that are advantageous over those conventionally produced upon layer‐by‐layer immobilization in terms of i) almost an order of magnitude increased sensitivity, ii) unsurpassed biosensor‐to‐transducer sensitivity ratio reaching the value of 0.3, and iii) three times extended operational lifetime. [ABSTRACT FROM AUTHOR]

• Advantageous sensitivity of the flow-through (bio)sensors upon power generation. • Up to 3 times increased signal-to-noise ratio compared to three-electrode setup. • Potentiostat-free operation, simplicity of read-out. We report on advanced Prussian Blue (PB) based flow-through (bio)sensors operated in power generation mode. The performance of such (bio)sensors in wall-jet cell is significantly improved with respect to three-electrode regime in terms of up to 1.5 times higher sensitivity probably due to the enhanced mass transport compared to it in batch cell upon stirring. Most importantly, potentiostat-free operation upon short-circuiting PB modified electrode with Ag|AgCl reference through an ammeter drastically decreases noises, which results in up to 3 times increased signal-to-noise ratio compared to conventional three-electrode flow-through systems. These findings, as well as the simplicity of the proposed approach, are encouraging for the high throughput robust sensing and open new horizons for elaboration of electrochemical analyzers. [ABSTRACT FROM AUTHOR]

Abstract: We report on Prussian Blue based (bio)sensors displaying advantageous analytical performance characteristics when operated both in conventional three‐electrode regime powered by potentiostat and in power generation mode. ‘Self‐powered’ sensors are commonly aimed to produce sufficient energy for signalling and, thus, being energetically autonomous. In order to generate maximum energy output, the electrodes with high potential difference are preferable. In contrast, we propose the systems with the lowest potential difference between the working and the counter electrodes. Short‐circuiting the Prussian Blue modified working and the Ag|AgCl reference electrode, working electrode potential is adjusted to 0.00 V without potentiostat. The efficiency of the (bio)sensors in power generation mode is achieved due to high catalytic activity of Prussian Blue at 0.00 V versus Ag|AgCl reference. Here we discuss the benefits of the Prussian Blue based (bio)sensors operated through power generation in a short‐circuit regime without potentiostat, including an order of magnitude decreased noise and simplified readout. The multimodality of the sensors would provide their versatile functionality for the electroanalysis and elaboration of miniature highly sensitive wearable devices. [ABSTRACT FROM AUTHOR]

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 H 2 O 2 reduction can be measured, which is proportional to the concentration of the hybridized labeled sequences. The current of H 2 O 2 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. [Display omitted] • Catalytically synthesized functionalized nanozymes possess high catalytic activity. • Nanozyme functionalization with azide groups enables 'click' conjugation. • Efficient direct electrocatalysis with the labels provides mediator-free operation. • Electrocatalytic signal amplification allows detecting down to 0.01 nM of DNA. [ABSTRACT FROM AUTHOR]

Despite non-invasive instant monitoring of sweat metabolites is becoming a general trend in early diagnostics and screening, the reliability and accuracy of the on-skin electrochemical biosensors in real-life scenarios still remain questionable. As a rule, mass transport effects in scantily excreted liquids are ignored, when considering the design of such wearable setups. Here we provide a comprehensive investigation of the disruption factors for commonly used Prussian Blue based (bio)sensors under different hydrodynamic conditions (2 × 10−5 – 5 × 100 mm s−1 electrolyte velocity). A huge effect of flow on the (bio)sensors response has been revealed and explained with transport limitations for both analyte influx and reaction product outflux. It suggests no need for improving the sensor sensitivity, while minimizing analyte consumption and enhancing product withdrawal. Some strategies concerning measurement schemes and sensor design ensuring reliable sweat analysis have been discussed and illustrated for lactate and glucose on-skin monitoring. [Display omitted] [ABSTRACT FROM AUTHOR]

In contrast to "self-powered" (bio)sensors aiming to generate maximum energy output, we propose the systems with the lowest potential difference between the working and the counter electrodes, which in galvanic mode would provide achievement of the best analytical performance characteristics. Prussian Blue based (bio)sensors known to operate at 0.00 V versus Ag|AgCl reference, in the short-circuit regime generate the current proportional to analyte concentration. Sensitivity and dynamic range of Prussian Blue based (bio)sensors in power generation mode are, respectively, even slightly higher and wider compared to the same (bio)sensors operated in the conventional three-electrode regime powered by a potentiostat. Selectivity of the (bio)sensors in power generation mode is similarly high relative to both oxygen, allowing H2O2 detection by its reduction, and reductants. Among the most important advantages of the proposed power generation mode is an order of magnitude decreased noise compared to performance in a conventional three-electrode setup powered by a potentiostat. Noiseless performances of Prussian Blue based (bio)sensors would open new horizons for electrochemical analysis. [ABSTRACT FROM AUTHOR]

We report on the microscopy-free, pure electrochemical approach for estimation of the continuity of electroactive inorganic films. Impedance spectra of the transition metal hexacyanoferrate films, electrochemically deposited under the optimized conditions, have been fitted to the Randles-type equivalent circuit, complicated by diffusion impedance with reflective boundary conditions, connected in series to the charge transfer resistance. An apparent anti-Ohmic trend is revealed in the sharp decrease of the latter with an increase in amount of the deposited inorganic film commonly presuming its thickness. Assigning charge transfer resistance to the resistance of the electrode|film interface, its observed decrease with subsequent saturation is explained in terms of an increase of the interface area until the entire electrode is covered with the inorganic film completely. Scanning electron microscopy (SEM) confirms the “anti-Ohmic” and saturation regions correspond to the partially covered and fully covered electrode surfaces, respectively. The dependence of charge transfer resistance on the amount of the deposited film thus provides an estimation of the continuity of inorganic films, which is highly important in a view of their practical applications. Except for cost efficiency, microscopy-free tools are attractive because in some cases, for instance for recessed electrodes, microscopy is hardly applicable. [ABSTRACT FROM AUTHOR]

We report on the simultaneous monitoring of sweat lactate concentration and sweat secretion rate. For this aim lactate oxidase-Prussian Blue enzyme-nanozyme type lactate biosensors were elaborated. The use of siloxane-perfluorosulfonated ionomer composite membrane for enzyme-nanozyme immobilization results in the biosensor displaying flux independence in the whole range of physiological sweat secretion rates (0.025–2 μl cm-2 min-1). On the contrary, current response of the biosensor based on solely siloxane membranes becomes saturated at physiological sweat lactate concentration, depending mostly on the flow rate. Accordingly, for simultaneous monitoring of sweat lactate concentration and its secretion rate both flow-through biosensors were integrated with high-accuracy wearable electronic devices allowing real-time remote monitoring. As found, during exhaustive physical exercise sweat secretion rate and lactate content are independent of each other, thus, confirming that this excretory liquid is suitable for non-invasive diagnostics. [Display omitted] • Flow-independent enzyme-nanozyme lactate biosensors elaborated. • Wearable high-accuracy device for real-time remote metabolite monitoring designed. • The independence of sweat secretion rate and lactate content shown. [ABSTRACT FROM AUTHOR]

The ability of Prussian Blue, ferric hexacyanoferrate (FeHCF), to sensitively and selectively detect hydrogen peroxide by its reduction in the presence of oxygen is of high importance for analytical chemistry. Success with Prussian Blue (PB) provided an appearance of contradictory reports concerning electrocatalysis of the other transition metal hexacyanoferrates (HCFs) in H2O2 reduction. Investigating thermodynamics of the catalyzed reactions as well as electrochemical properties of the hexacyanoferrates, we are able to conclude that the noniron hexacyanoferrates themselves are completely electrocataryrically inactive, except for a minor electrocatalysis in the opposite reaction, hydrogen peroxide oxidation, registered for NiHCF. Concerning the most important reaction, H2O2 reduction, the observed electrocatalytic activity (by the way, 100 times decreased compared to PB) is due to the presence of FeHCF (Prussian Blue) as defects in the structure of noniron hexacyanoferrates. This finding, considering other unique properties of transition metal HCFs, will provide a systematic search for sensing materials with improved analytical performance characteristics. [ABSTRACT FROM AUTHOR]

• Prussian Blue based nanozymes catalyze reduction of organic peroxides. • Nanozymes are significantly more active than horseradish peroxidase. • Selectivity of nanozymes to H 2 O 2 is higher as compared to peroxidase. Prussian Blue based nanozymes are able to catalyze reduction of organic peroxides similarly to the enzyme peroxidase. For the whole range of peroxides under study the catalytic rate constant for 3,3′,5,5′-tetramethylbenzidine in case of nanozymes is significantly higher as compared to horseradish peroxidase. Moreover, the selectivity of nanozymes towards hydrogen peroxide relatively organic peroxides reduction is higher than that for the enzyme, which can be explained in terms of different ability of peroxides to penetrate to the bulk of catalytically synthesized Prussian Blue nanoparticles. On the contrary, selectivity of Prussian Blue film modified electrodes is much lower and even negligible among hydrogen, urea and methyl ethyl ketone peroxides. An ability to tune the selectivity would allow individual detection of peroxides in their complex mixtures with Prussian Blue nanostructures. [ABSTRACT FROM AUTHOR]

We report on a novel electroanalytical system for hydrogen peroxide (H2O2) detection in humidity or droplets of aerosol, formed by air bubbling through a washing chamber; the resulting flow mimics the exhaled human breath. The system is based on a planar three-electrode structure (with a Prussian Blue based H2O2 transducer modified working electrode) bridged by a solid salt-saturated filament material (filter paper, cotton textile). Respective to the hydrogen peroxide content in the washing valve, the response of the aerosol-sensing system is linear in the concentration range of 0.1-10 μM, which overlaps the generally accepted H2O2 content in exhaled breath condensate (EBC), with the sensitivity of 8 A M-1 cm-2. The response to the upper limit of the calibration range is stable for more than 50 injection cycles recorded within 3 days. Both the stability and the suitable calibration range allow one to consider the reported aerosol-sensing system as a prototype for a simple (avoiding intermediate EBC collection) noninvasive diagnostic tool for pulmonary patients. [ABSTRACT FROM AUTHOR]

Simple drop-casting of the catalytically synthesized Prussian Blue nanoparticles with subsequent annealing results in modified electrodes with high electrochemical and electrocatalytic activity. The electroactive fraction of the deposited Prussian Blue being independent of the surface concentration, is decreased with an increase of the nanoparticles size reaching its lower saturation at approximately 25%. Sensitivity of the corresonding hydrogen peroxide sensor is increased linearly with the amount of Prussian Blue reaching its upper saturation together with formation of continuative film, indicating most probably, a nanoparticles monolayer. Due to their performance characteristics the Prussian Blue nanozymes modified electrodes would be highly attractive for corresponding sensors and biosensors. [ABSTRACT FROM AUTHOR]

We report on the boron-doped diamond (BDD) electrodes modified with Prussian Blue (PB) through electrochemical synthesis, open-circuit deposition, and immobilization of Prussian Blue based nanozymes. The later allowed attaining the utmost sensitivity of the H 2 O 2 sensor (≈0.14 A M−1 cm−2). Compared to conventional graphite electrodes modified with Prussian Blue, the use of BDD provided twenty times decreased oxygen reduction currents resulting in up to ten times increased signal-to-background ratio of the corresponding modified electrodes. The successful combination of exceptional diamond properties (including wide potential window for BDD electrodes) with advantageous performance characteristics of Prussian Blue sensing layers would open new horizons for H 2 O 2 detection as well as biosensors elaboration. • Electron transfer rate at the H-terminated B-doped diamond electrodes is sufficient. • Prussian Blue nanozymes immobilization provides the utmost sensitivity towards H 2 O 2. • O 2 reduction currents at the doped diamond.|Prussian Blue sensors are 20 times lower [ABSTRACT FROM AUTHOR]

Self-powered sensors are among promising candidates for wake-up device elaboration. Generally, such sensors are designed to generate superior energy output to the detriment of analytical performance. Here we report on power output of Prussian Blue (PB) based (bio)sensors retaining advanced analytical performance in power generation mode. The maximum power density is linearly proportional to analyte concentration: the slopes for hydrogen peroxide sensor and glucose biosensor are of 37 mW M−1 cm−2 and 3 mW M−1 cm−2, respectively. The cells consisted of power generating (bio)sensors are able to switch on a light-emitting diode (LED) at threshold concentration of 0.6 mM H 2 O 2 or 8 mM glucose that is suitable for naked-eye semiquantitative analysis. The presented prototype of signaling device is expected to have significant impact on the development of autonomous wake-up sensors for medical, industrial or environmental monitoring. • Prussian Blue (PB) based (bio)sensors power output is a function of analyte content. • The galvanic cells were made of PB based (bio)sensors. • The cells connected to a LED can be utilized as wake-up signaling devices. • Threshold level for sensor-based wake-up device is 0.6 mM of H 2 O 2. • Threshold level for biosensor-based wake-up device is 8 mM of glucose. [ABSTRACT FROM AUTHOR]

We report on a superstable hydrogen peroxide (H2O2) transducer made by sequential deposition of the iron- and nickel-hexacyanoferrate (NiHCF) layers. Both chemical and mechanical stability of the latter, as well as similarity of its structure to Prussian Blue (PB) provide a substantial stabilization of the most advantageous H2O2 transducer. The electrochemically deposited five bilayers of PB-NiHCF exhibit a complete stability under the continuous wall-jet flow of 1 mM of H2O2 during more than 2 h, maintaining current at a level of 0.2 mA cm-2, whereas common Prussian Blue loses half of its response within the first 20-25 min. Even being deposited in the open circuit regime on screen-printed electrodes, PB-NiHCF bilayers dramatically improve tolerance of the resulting transducer to alkaline solutions and iron ligands. Despite their 2-2.5 times decreased sensitivity (compared to common Prussian Blue), the sequentially deposited bilayers of PB-NiHCF provide a similar dynamic range of the transducer due to the decreased noise level. [ABSTRACT FROM AUTHOR]

In contrast to bienzyme biosensors, we propose the nanozyme-enzyme based ones substituting the enzyme peroxidase with the more active and stable nanoparticles "artificial peroxidase". The use of catalytically synthesized Prussian Blue based nanozymes simplifies assembling of hydrogen peroxide transducer providing its higher sensitivity. For immobilization of lactate oxidase the composite alkoxysilane – perfluorosulfonated ionomer (PFSI) membranes are proposed achieving the significantly improved operating stability. The resulting nanozyme-enzyme lactate biosensor displays twice higher sensitivity (>0.2 A M−1 cm−2) compared to the Prussian Blue film based one. Nanozymes "artificial peroxidase" are expected to find wide use in elaboration of oxidase based biosensors. Image 1 • The biosensor based on nanozyme substituting enzyme peroxidase. • Novel immobilization protocol for lactate oxidase providing its higher stability. • The nanozyme-enzyme lactate biosensor with twice higher sensitivity. [ABSTRACT FROM AUTHOR]