Sensor and biosensor preparation, optimisation and applications of Prussian Blue modified electrodes

Three-dimensional paper-based microfluidic electrochemical integrated devices (3D-PMED) for wearable electrochemical glucose detection

Wearable electrochemical sensors have attracted tremendous attention in recent years. Here, a three-dimensional paper-based microfluidic electrochemical integrated device (3D-PMED) was demonstrated for real-time monitoring of sweat metabolites. The 3D-PMED was fabricated by wax screen-printing patterns on cellulose paper and then folding the pre-patterned paper four times to form five stacked layers: the sweat collector, vertical channel, transverse channel, electrode layer and sweat evaporator. A sweat monitoring device was realized by integrating a screen-printed glucose sensor on polyethylene terephthalate (PET) substrate with the fabricated 3D-PMED. The sweat flow process in 3D-PMED was modelled with red ink to demonstrate the capability of collecting, analyzing and evaporating sweat, due to the capillary action of filter paper and hydrophobicity of wax. The glucose sensor was designed with a high sensitivity (35.7 μA mM-1 cm-2) and low detection limit (5 μM), considering the low concentration of glucose in sweat. An on-body experiment was carried out to validate the practicability of the three-dimensional sweat monitoring device. Such a 3D-PMED can be readily expanded for the simultaneous monitoring of alternative sweat electrolytes and metabolites.

Wearable Bioelectronics: Enzyme-Based Body-Worn Electronic Devices

In this Account, we detail recent progress in wearable bioelectronic devices and discuss the future challenges and prospects of on-body noninvasive bioelectronic systems. Bioelectronics is a fast-growing interdisciplinary research field that involves interfacing biomaterials with electronics, covering an array of biodevices, encompassing biofuel cells, biosensors, ingestibles, and implantables. In particular, enzyme-based bioelectronics, built on diverse biocatalytic reactions, offers distinct advantages and represents a centerpiece of wearable biodevices. Such wearable bioelectronic devices predominately rely on oxidoreductase enzymes and have already demonstrated considerable promise for on-body applications ranging from highly selective noninvasive biomarker monitoring to epidermal energy harvesting. These systems can thus greatly increase the analytical capability of wearable devices from the ubiquitous monitoring of mobility and vital signs, toward the noninvasive analysis of important chemical biomarkers. Wearable enzyme electrodes offer exciting opportunities to a variety of areas, spanning from healthcare, sport, to the environment or defense. These include real-time noninvasive detection of biomarkers in biofluids (such as sweat, saliva, interstitial fluid and tears), and the monitoring of environmental pollutants and security threats in the immediate surrounding of the wearer. Furthermore, the interface of enzymes with conducting flexible electrode materials can be exploited for developing biofuel cells, which rely on the bioelectrocatalytic oxidation of biological fuels, such as lactate or glucose, for energy harvesting applications. Crucial for such successful application of enzymatic bioelectronics is deep knowledge of enzyme electron-transfer kinetics, enzyme stability, and enzyme immobilization strategies. Such understanding is critical for establishing efficient electrical contacting between the redox enzymes and the conducting electrode supports, which is of fundamental interest for the development of robust and efficient bioelectronic platforms. Furthermore, stretchable and flexible bioelectronic platforms, with mechanical properties similar to those of biological tissues, are essential for handling the rigors of on-body operation. As such, special attention must be given to changes in the behavior of enzymes due to the uncontrolled conditions of on-body operation (including diverse outdoor activities and different biofluids), for maintaining the attractive performance that these bioelectronics devices display in controlled laboratory settings. Therefore, a focus of this Account is on interfacing biocatalytic layers onto wearable electronic devices for creating efficient and stable on-body electrochemical biosensors and biofuel cells. With proper attention to key challenges and by leveraging the advantages of biocatalysis, electrochemistry, and flexible electronics, wearable bioelectronic devices could have a tremendous impact on diverse biomedical, fitness, and defense fields.

Chemically bound Prussian blue in sodium alginate hydrogel for enhanced removal of Cs ions

A new approach for efficient removal of radioactive ¹³⁷Cs was developed using a sodium alginate hydrogel beads-based adsorbent containing chemically bound Prussian blue (PB). Sodium alginate was crosslinked with Fe (III) ions to form hydrogel beads, in which Fe (III) had a dual function; it served as a crosslinking agent and also led to PB formation via reaction with hexacyanoferrate. Fe (III) ions, an unusual crosslinking agent for sodium alginate gel, led to stable, homogeneous distribution of PB inside the beads. The amount of embedded PB in the composite beads was more than two times larger than in the conventional sodium alginate-PB composite beads, resulting in an adsorption capacity for Cs ions that was two to five times higher, mainly because of a higher PB contents and because of the microporosity of the sodium alginate hydrogel.

Concentration cell-based potentiometric analysis for point-of-care testing with minimum background

One of the most critical problems of point-of-care testing is how to reduce the interference of background, especially under resource-limited conditions when sample pretreatment is not available. In this work we report a potentiometric method for point-of-care testing with minimum background. The method is based on the principles of a concentration cell which is a type of galvanic cells. It is an electrochemical cell having two carbon electrodes. The potential of each electrode is determined by ratio of a redox couple (i.e. Fe(CN)₆^4-/3-) on the electrode surface. On one electrode, the adsorbed enzyme catalyzes the oxidation of analyte by Fe(CN)₆^3- which produces Fe(CN)₆^4-. The shift of the potential was because of the analyte as well as the background. In the other channel, no enzyme was present so that the shift of the potential, if any, is owing to the background. By measuring the potential difference between the two electrodes (i.e. voltage of the concentration cell), analyte can be quantitatively determined with most of the background eliminated. As the proof-of-concept analyte, blood glucose is quantitatively detected using a voltammeter with acceptable selectivity and accuracy. Noble metal electrodes that are indispensable for conventional electrochemical sensing are not required. All these features simplify the fabrication procedure and reduce the cost for the detection. Therefore, we believe it is promising for electrochemical point-of-care testing.

Enhanced cesium removal from real matrices by nickel-hexacyanoferrate modified activated carbons

After nuclear disasters, radioactive cesium partitions to soils and surface water, where it decays slowly. Hexacyanoferrates (HCFs) have excellent cesium removal properties but their structure is typically powdery. Many carrier materials, such as biomass or magnetic particles, have been used to provide a suitable substrate for HCFs that can be used in filters. This research uses the sorption properties of activated carbon (AC) to incorporate Ni-HCF, resulting in good structural properties of the hybrid material. These HCF-modified ACs show drastically improved sorption properties towards Cs after one, two and three HCF impregnation cycles. The activated carbon from brewer's spent grain with one modification cycle removes more than 80% of 1 mg L^-1 Cs in a sea water solution and more than 98% of 1 mg L^-1 Cs from surface water at a low AC dosage (0.5 g L^-1). Iron and nickel leaching is studied and found to be dependent on the type of modified AC used and the leaching solution. Iron leaching can be problematic in surface and seawater, whereas nickel leaching is especially pronounced in seawater.

Exploring the Confinement Effect of Carbon Nanotubes on the Electrochemical Properties of Prussian Blue Nanoparticles

A novel and efficient photochemical method has been proposed for the encapsulation of Prussian blue nanoparticles (PBNPs) inside the channels of carbon nanotubes (PB-in-CNTs) in an acidic ferrocyanide solution under UV/vis illumination, and the confinement effect of CNTs on the electrochemical properties of PBNPs is systematically explored. PB-in-CNTs show a faster electron-transfer process, an enhanced electrocatalytic activity toward the reduction of H₂O₂, and an increased anti-base ability compared to PBNPs loaded outside of CNTs (PB-out-CNTs). In addition, PB-in-CNTs show an increased electrochemical reversibility and an unexpected diameter-independent catalytic activity with the decrease of CNT diameters. The improved electrochemical properties of PB-in-CNTs are attributed to the modified electronic properties and dimensions of PBNPs induced by the confinement effect of CNTs. This work provides further insights into the confinement effect on the properties of nanomaterials and will inspire extensive relevant investigations in the development of novel composites or excellent catalysts.

A chemical/molecular 4-input/2-output keypad lock with easy resettability based on red-emission carbon dots-Prussian blue composite film electrodes

In this work, a resettable 4-input/2-output keypad lock system based on red-emission carbon dots (rCDs) and Prussian blue (PB) modified electrodes was developed. Electrochromic PB layers were first electrochemically deposited on the indium tin oxide (ITO) electrode surface. An admixture of rCDs and chitosan (Chi) was then cast on the surface of PB layers, forming rCDs-Chi/PB film electrodes. UV-vis absorption of the films was sensitive to the applied potential since the blue PB constituent of the films would be transformed to nearly colorless Prussian white (PW) at the reduction potential of -0.2 V and then from PW back to PB at the oxidation potential of 0.4 V, and the transformation between PB and PW would also influence the fluorescence emission of the rCD constituent in the films. The addition of cysteine (Cys) in the testing solution could reduce the PB in the films into PW and generate an amperometric electrocatalytic current at 0.4 V. Meanwhile, the addition of Fe3+ in solution could greatly quench the fluorescence from the rCD component in the films. Thus, the responses of UV-vis absorbance, fluorescence emission and amperometric current of the rCDs-Chi/PB film electrode system exhibited potential-, Cys- and Fe3+-responsive switching properties. Based on the aforementioned work, a combinational logic gate circuit with 3 inputs and 3 outputs was established. In particular, on the same platform, a novel chemical/molecular 4-input/2-output keypad lock with easy resettability was elaborately designed with amperometric current and fluorescence peak intensity as two different types of outputs, so that a higher security level could be achieved.

Photoanode for Aqueous Dye-Sensitized Solar Cells based on a Novel Multicomponent Thin Film

Most of the dye-sensitized solar cells (DSSCs) developed so far use organic electrolytes and water-sensible sensitizers. The search for aqueous DSSCs, a promising technology for solar-energy conversion, implies finding materials that are stable in aqueous solution. In this study, Prussian blue (PB) was utilized as an innovative sensitizer in a photoanode for DSSCs and a novel synthetic approach to a carbon nanotubes/TiO₂ /PB nanocomposite thin film was developed. The photoresponse was evaluated in a total aqueous electrolyte, and photocurrents of 600 μA cm^-2 were achieved.

Application of pyrite and chalcopyrite as sensor electrode for amperometric detection and measurement of hydrogen peroxide

The sensing performance of solid-state amperometric sensors based on natural sulfide minerals, i.e., pyrite and chalcopyrite, has been characterized for the detection and measurement of hydrogen peroxide (H₂O₂) in aqueous medium.

Development of Amine-Oxidase-Based Biosensors for Spermine and Spermidine Analysis

In this work a detailed description of the development of amine oxidase-based electrochemical biosensors for the selective determination of the biogenic amines is presented. The enzymes required for this operation are Polyamine Oxidase (PAO) and Spermine Oxidase (SMO) which are physically entrapped in poly(vinyl alcohol) bearing styrylpyridinium groups (PVA-SbQ), a photo-cross-linkable gel, onto screen printed electrode (SPE) surface. The developed biosensors are deeply characterized in the analysis of biogenic amines by using flow injection amperometric (FIA) technique. The enzymatic electrodes are characterized by good sensitivity, long-term stability, and reproducibility. To test the feasibility of the developed biosensors in the analysis of real matrices, they are used for the analysis of blood samples. The results obtained are in good agreement with those obtained with the GC-MS reference method.

A new enzyme immunoassay for alpha-fetoprotein in a separate setup coupling an aluminium/Prussian blue-based self-powered electrochromic display with a digital multimeter readout

A new immunoassay was designed for the detection of disease-related biomarkers (alpha-fetoprotein, AFP, as a model), coupling an aluminium (Al)/Prussian blue-based electrochromic display with a digital multimeter readout.

Multilayer sensing platform: gold nanoparticles/prussian blue decorated graphite paper for NADH and H2O2 detection

An exfoliated graphite paper based multilayer sensing platform was fabricated and applied for sensitive detection of NADH and H₂O₂.

Construction of a sensitive non-enzymatic fructose carbon paste electrode – CuO nanoflower: designing the experiments by response surface methodology

CuO nano-flowers were synthesized and used to modify a carbon paste electrode (CPE) for voltammetric determination of fructose.

Pt-Decorated MWCNTs-Ionic Liquid Composite-Based Hydrogen Peroxide Sensor To Study Microbial Metabolism Using Scanning Electrochemical Microscopy

Hydrogen peroxide (H2O2) is a highly relevant metabolite in many biological processes, including the oral microbiome. To study this metabolite, we developed a 25 μm diameter, highly sensitive, nonenzymatic H2O2 sensor with a detection limit of 250 nM and a broad linear range of 250 nM to 7 mM. The sensor used the synergistic activity of the catalytically active Pt nanoparticles on a high surface area multiwalled carbon nanotube and conducting ionic liquid matrix to achieve high sensitivity (2.4 ± 0.24 mA cm-2 mM-1) for H2O2 oxidation. The unique composite allowed us to miniaturize the sensor and couple it with a Pt electrode (25 μm diameter each) for use as a dual scanning electrochemical microscopy probe. We could detect 65 ± 10 μM H2O2 produced by Streptococcus gordonii (Sg) in a simulated biofilm at 50 μm above its surface in the presence of 1 mM glucose and artificial saliva solution (pH 7.2 at 37 °C). Because of its high stability and low detection limit, the sensor showed a promising chemical image of H2O2 produced by Sg biofilms. We were also able to detect 30 μM H2O2 at 50 μm above the biofilm in the presence of the H2O2-decomposing salivary lactoperoxidase and thiocyanate, which would not otherwise be possible using an existing H2O2 assay. Thus, this sensor can potentially find applications in the study of other important biological processes in a complex matrix where circumstances demand a low detection limit in a compact space.

SERS spectroelectrochemical study of electrode processes at copper hexacyanoferrate modified electrode

Electrochemical redox processes taking place at copper hexacyanoferrate (CuHCF) modified electrode have been investigated by near-infrared laser (785nm) induced surface-enhanced Raman spectroscopy (SERS). Raman bands observed within the spectral ranges of 2200-2000cm^-1 and 500-100cm^-1, as well as their dependence on electrode potential, have been analysed. The most characteristic Raman bands, related to triple CN bond vibrations and centered at 2187 and 2127cm^-1 were assigned to the oxidised and the reduced forms of CuHCF, respectively. Time-resolved Raman spectroelectrochemical study shows that the electrochemical redox interconversions between these two forms proceed relatively slow, thus resembling the behaviour of structurally related cobalt hexacyanoferrate, and differing essentially from that of Prussian blue layer studied previously. It has been shown by the time-resolved Raman spectroelectrochemistry that the rate of some redox processes of solute species like the anodic oxidation of ascorbate or cathodic reduction of hydrogen peroxide at CuHCF modified electrode appears to be limited by the slow electrochemical redox transformations within the modifier layer itself rather than by the redox interactions of a modifier with the solute species.

A bimetallic nanocoral Au decorated with Pt nanoflowers (bio)sensor for H2O2 detection at low potential

In this work, we have developed for the first time a method to make novel gold and platinum hybrid bimetallic nanostructures differing in shape and size. Au-Pt nanostructures were prepared by electrodeposition in two simple steps. The first step consists of the electrodeposition of nanocoral Au onto a gold substrate using hydrogen as a dynamic template in an ammonium chloride solution. After that, the Pt nanostructures were deposited onto the nanocoral Au organized in pores. Using Pt (II) and Pt (IV), we realized nanocoral Au decorated with Pt nanospheres and nanocoral Au decorated with Pt nanoflowers, respectively. The bimetallic nanostructures showed better capability to electrochemically oxidize hydrogen peroxide compared with nanocoral Au. Moreover, Au-Pt nanostructures were able to lower the potential of detection and a higher performance was obtained at a low applied potential. Then, glucose oxidase was immobilized onto the bimetallic Au-Pt nanostructure using cross-linking with glutaraldehyde. The biosensor was characterized by chronoamperometry at +0.15V vs. Ag pseudo-reference electrode (PRE) and showed good analytical performances with a linear range from 0.01 to 2.00mM and a sensitivity of 33.66µA/mMcm². The good value of K_m^app (2.28mM) demonstrates that the hybrid nanostructure is a favorable environment for the enzyme. Moreover, the low working potential can minimize the interference from ascorbic acid and uric acid as well as reducing power consumption to effect sensing. The simple procedure to realize this nanostructure and to immobilize enzymes, as well as the analytical performances of the resulting devices, encourage the use of this technology for the development of biosensors for clinical analysis.

Size-tunable, highly sensitive microelectrode arrays enabled by polymer pen lithography

By combining polymer pen lithography (PPL) patterning with in situ polymerization, we report a straightforward and bottom-up approach for bench-top fabrication of microelectrode arrays (MEAs) with well-controlled dimensions. The as-fabricated MEAs can be used to electrodeposit prussian blue in situ and work as a biosensor for H₂O₂ with a detection limit as low as 5 nM at a sensitivity of 0.7 A cm^-2 M^-1.