Performance of layer-by-layer deposited low dimensional building blocks of graphene-prussian blue onto graphite screen-printed electrodes as sensors for hydrogen peroxide

Chemiluminescence "Add-and-Measure" Sensing Paper Based on the Prussian Blue/Metal-Organic Framework MIL-101 Nanozyme for Rapid Hydrogen Peroxide Detection

In this work, a chemiluminescent sensing paper has been developed using a peroxidase biomimetic metal-organic framework as a versatile host platform. For the first time, we have explored the use of in situ growth of Prussian Blue nanoparticles (PB-NPs) onto the MIL-101(Fe) structure for the assembly of a ready-to-use sensing paper. In situ growth of PB-NPs has been performed on the surface of the MIL-101(n) family. This novel composite, named PB-NPs@MIL-101(Fe), has been successfully used to develop a sensing paper for one-step detection of H2O2 in real samples (commercial disinfectant solutions and tap water samples). The as-prepared material was fully characterized, including X-ray analysis, Fourier transform infrared, scanning and transmission electron microscopies, nitrogen isotherms, and elemental analysis. After the characterization, the analytical performance of the PB-NPs@MIL-101(Fe) sensing paper was evaluated. The low-cost sensor (0.15 euro per unit) was able to detect down to 8.2 μM (corresponding to 8.2 × 10-11 mol) H2O2 using only 10 μL of sample with satisfactory reproducibility (relative standard deviation 17%).

Structure-Function Studies of Glucose Oxidase in the Presence of Carbon Nanotubes and Bio-Graphene for the Development of Electrochemical Glucose Biosensors

In this work, we investigated the effect of multi-walled carbon nanotubes (MWCNTs) and bio-graphene (bG) on the structure and activity of glucose oxidase (GOx), as well as on the performance of the respective electrochemical glucose biosensors. Various spectroscopic techniques were applied to evaluate conformational changes in GOx molecules induced by the presence of MWCNTs and bG. The results showed that MWCNTs induced changes in the flavin adenine dinucleotide (FAD) prosthetic group of GOx, and the tryptophan residues were exposed to a more hydrophobic environment. Moreover, MWCNTs caused protein unfolding and conversion of α-helix to β-sheet structure, whereas bG did not affect the secondary and tertiary structure of GOx. The effect of the structural changes was mirrored by a decrease in the activity of GOx (7%) in the presence of MWCNTs, whereas the enzyme preserved its activity in the presence of bG. The beneficial properties of bG over MWCNTs on GOx activity were further supported by electrochemical data at two glucose biosensors based on GOx entrapped in chitosan gel in the presence of bG or MWCNTs. bG-based biosensors exhibited a 1.33-fold increased sensitivity and improved reproducibility for determining glucose over the sweat-relevant concentration range of glucose.

Potassium cobalt hexacyanoferrate as a peroxidase mimic for electrochemical immunosensing of Lactobacillus rhamnosus GG

In this paper, the potassium cobalt hexacyanoferrate (II), K₂CoFe(CN)₆, with peroxidase-like activity was used for the fabrication of a novel label-free Lactobacillus rhamnosus GG (LGG) electrochemical immunosensor. The K₂CoFe(CN)₆ nanocubes were made by a simple hydrothermal method and followed by low-temperature calcination. In addition to structural characterization, the peroxidase-mimicking catalytic property of the material was confirmed by a chromogenic reaction. It is known that H₂O₂ can oxidize electroactive thionine molecules under the catalysis of horseradish peroxidase (HRP). In this nanozyme-based electrochemical immunoassay, due to the steric hindrance, the formation of immune-complex of LGG and LGG antibody on the modified GCE inhibits the catalytic activity of the peroxidase mimics of K₂CoFe(CN)₆ and thus reduced the current signal. Therefore, the developed electrochemical immunosensor achieved quantitative detection of LGG. Under optimal conditions, the linear range of the sensor was obtained from 10¹ to 10⁶ CFU mL^-1 with a minimum detection limit (LOD) of 12 CFU mL^-1. Furthermore, the immunosensor was successfully applied in the quantitative detection of LGG in dairy product samples with recoveries ranging from 93.2% to 106.8%. This protocol presents a novel immunoassay method, which provides an alternative implementation pathway for the quantitative detection of microorganisms.

One-Step Interfacial Integration of Graphene Oxide and Organic Chromophores into Multicomponent Nanohybrids with Photoelectric Properties

A one-step protocol for interfacial self-assembly of graphene oxide (GO), glutamine-substituted perylene diimide (PDI-glu), 10,12-pentacosadiynoic acid (PCDA), and zinc acetate into three- and four-component hybrid nanofilms through hydrogen and coordination bonding was developed. The hybrids deposited onto solid supports were studied after polymerization of PCDA by UV-vis absorption, fluorescence, and Raman spectroscopies, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results of spectroscopic studies suggest that the hybrids assembled through H-bonds can maintain the light-induced Förster energy transfer from the PDI-glu chromophore to the conjugated polymer and then to GO leading to fluorescence quenching. In the hybrids assembled through coordination bonding with zinc clusters, the energy transfer proceeds from PDI-glu to the PDA polymer, whereas the transfer from PDA to GO is quenched completely. Another important characteristic of these ultrathin hybrids is their stability with respect to photobleaching of chromophores due to the acceptor properties of GO. The as-assembled hybrid nanofilms were integrated with conventional photovoltaic planar architectures to study their photoelectric properties. The zinc-containing hybrids integrated with a hole transport layer exhibited photovoltaic properties. The cell with the integrated four-component hybrid comprising both PDI-glu and PDA showed a photocurrent/dark current ratio almost an order higher than that of the three-component hybrid assembled with PDA only. The supramolecular method based on the interfacial self-assembly can be extended to a wide variety of organic chromophores and polymerizable surfactants for integrating them into multicomponent functional GO-based nanohybrids with targeted properties for organic electronics.

Graphene Oxide-Cytochrome c Multilayered Structures for Biocatalytic Applications: Decrypting the Role of Surfactant in Langmuir-Schaefer Layer Deposition

Graphene, a two-dimensional single-layer carbon allotrope, has attracted tremendous scientific interest due to its outstanding physicochemical properties. Its monatomic thickness, high specific surface area, and chemical stability render it an ideal building block for the development of well-ordered layered nanostructures with tailored properties. Herein, biohybrid graphene-based layer-by-layer structures are prepared by means of conventional and surfactant-assisted Langmuir-Schaefer layer deposition techniques, whereby cytochrome c molecules are accommodated within ordered layers of graphene oxide. The biocatalytic activity of the as-developed nanobio-architectures toward the enzymatic oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt and decolorization of pinacyanol chloride is tested. The results show that the multilayer structures exhibit high biocatalytic activity and stability in the absence of surfactant molecules during the deposition of the monolayers.

Magnetic zirconium-based Prussian blue analog nanozyme: enhanced peroxidase-mimicking activity and colorimetric sensing of phosphate ion

A magnetic zirconium hexacyanoferrate-based Prussian blue analog (MB@ZrHCF) nanozyme was synthesized using dopamine (DA) reduction-assisted method and employed for colorimetric PO₄^3- sensing. The MB@ZrHCF exhibits enhanced peroxidase-mimicking activity and ultrafast catalytic rate via the color reaction of 3,3',5,5'-tetramethylbenzidine (TMB) oxidized by hydrogen peroxide (H₂O₂). The catalytic reaction mechanism of MB@ZrHCF catalyzing H₂O₂ to produce hydroxyl radical (∙OH) was studied. Then, MB@ZrHCF was successfully applied to the detection of H₂O₂. Additionally, the catalytic activity of the nanocomposite is inhibited due to the steric hindrance effect from the coordination of PO₄^3- and Zr(IV) node. Based on this, the MB@ZrHCF nanozyme can be used to detect PO₄^3- in two linear ranges (10-100 µM and 100-200 µM) with a limit of detection of 2.25 µM. The proposed colorimetric sensor possesses excellent selectivity and reliability for PO₄^3- sensing, which can be successfully applied to detect PO₄^3- in sea and tap water samples.

Nitrogen-rich core-shell structured particles consisting of carbonized zeolitic imidazolate frameworks and reduced graphene oxide for amperometric determination of hydrogen peroxide

Core-shell structured particles were prepared from carbonized zeolitic imidazolate frameworks (ZIFs) and reduced graphene oxide (rGO). The particles possess a nitrogen content of up to 10.6%. The loss of nitrogen from the ZIF is avoided by utilizing the reduction and agglomeration of graphene oxide with suitable size (>2 μm) during pyrolysis. The resulting carbonized ZIF@rGO particles were deposited on a glassy carbon electrode to give an amperometric sensor for H₂O₂, typically operated at a voltage of -0.4 V (vs. Ag/AgCl). The sensor has a wide detection range (from 5 × 10^-6 to 2 × 10^-2 M), a 3.3 μM (S/N = 3) detection limit and a 0.272 μA·μM^-1·cm^-2 sensitivity, much higher than that of directly carbonized ZIFs. The sensor material was also deposited on a screen-printed electrode to explore the possibility of application. Graphical abstract Nitrogen doped carbon (NC) derived from carbonized zeolitic imidazolate frameworks is limited because of low nitrogen content. Here, nitrogen-rich NC@reduced graphene oxide (rGO) core-shell structured particles are described. The NC@rGO particles show distinctly better H₂O₂ detection performance than NC.

The interlocked in situ fabrication of graphene@prussian blue nanocomposite as high-performance supercapacitor

High-quality graphene@prussian blue (G@PB) nanocomposite sheets fabricated via the one-step in situ hydrothermal method show great promise for energy-storage hybrid electrodes with excellent electrochemical performance.

Top-down and bottom-up approaches to transparent, flexible and luminescent nitrogen-doped carbon nanodot-clay hybrid films

Two easy approaches are successfully employed for the preparation of nitrogen-doped carbon nanodot (NCND)-clay hybrids (bulk solids and thin films). Fluorescent and small NCNDs are intercalated within the interlayer space of LAPONITE® clay with a simple ion exchange reaction in bulk or embedded between functionalized LAPONITE® sheets by combining a layer-by-layer approach with a self-assembly process. In both cases, homogeneous hybrids with 2D-ordered NCNDs (accounting for >20 wt%) are produced, with the NCND optoelectronic properties preserved. Drop casting of suspensions or self-assembly on flexible substrates allows the fabrication of luminescent flexible films. The transparency of the films is found to be adjustable either by controlling the concentration of the drop-cast suspensions or by the number of layers in the self-assembly procedure. The prepared films are stable over time: the inert LAPONITE® platelets not only guide the highly ordered 2D assemblies of NCNDs in the interlayer space but also protect them from external agents, which could affect their surfaces and thus alter their optoelectronic properties.

Graphene/Carbon Dot Hybrid Thin Films Prepared by a Modified Langmuir-Schaefer Method

The special electronic, optical, thermal, and mechanical properties of graphene resulting from its 2D nature, as well as the ease of functionalizing it through a simple acid treatment, make graphene an ideal building block for the development of new hybrid nanostructures with well-defined dimensions and behavior. Such hybrids have great potential as active materials in applications such as gas storage, gas/liquid separation, photocatalysis, bioimaging, optoelectronics, and nanosensing. In this study, luminescent carbon dots (C-dots) were sandwiched between oxidized graphene sheets to form novel hybrid multilayer films. Our thin-film preparation approach combines self-assembly with the Langmuir-Schaefer deposition and uses graphene oxide nanosheets as template for grafting C-dots in a bidimensional array. Repeating the cycle results in a facile and low-cost layer-by-layer procedure for the formation of highly ordered hybrid multilayers, which were characterized by photoluminescence, UV-visible, X-ray photoelectron, and Raman spectroscopies, as well as X-ray diffraction and atomic force microscopy.

Glucose sensing on graphite screen-printed electrode modified by sparking of copper nickel alloys

Electric spark discharge was employed as a green, fast and extremely facile method to modify disposable graphite screen-printed electrodes (SPEs) with copper, nickel and mixed copper/nickel nanoparticles (NPs) in order to be used as nonenzymatic glucose sensors. Direct SPEs-to-metal (copper, nickel or copper/nickel alloys with 25/75, 50/50 and 75/25wt% compositions) sparking at 1.2kV was conducted in the absence of any solutions under ambient conditions. Morphological characterization of the sparked surfaces was performed by scanning electron microscopy, while the chemical composition of the sparked NPs was evaluated with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The performance of the various sparked SPEs towards the electro oxidation of glucose in alkaline media and the critical role of hydroxyl ions were evaluated with cyclic voltammetry and kinetic studies. Results indicated a mixed charge transfer- and hyroxyl ion transport-limited process. Best performing sensors fabricated by Cu/Ni 50/50wt% alloy showed linear response over the concentration range 2-400μM glucose and they were successfully applied to the amperometric determination of glucose in blood. The detection limit (S/N 3) and the relative standard deviation of the method were 0.6µM and <6% (n=5, 2µM glucose), respectively. Newly devised sparked Cu/Ni graphite SPEs enable glucose sensing with distinct advantages over existing glucose chemical sensors in terms of cost, fabrication simplicity, disposability, and adaptation of green methods in sensor's development.

Graphene-based screen-printed electrochemical (bio)sensors and their applications: Efforts and criticisms

K.S. Novoselov in his Nobel lecture (December 8, 2010), described graphene as "more than just a flat crystal" and summarized the best possible impression of graphene with (i) it is the first example of 2D atomic crystals, (ii) it demonstrated unique electronic properties, thanks to charge carriers which mimic massless relativistic particles, and (iii) it has promise for a number of applications. The fascinating and unusual properties of this 2D material were indeed recently investigated and exploited in several disciplines including physics, medicine, and chemistry, indicating the extremely versatile and polyedric aspect of this nanomaterial. The utilization of nanomaterials, printed technology, and microfluidics in electroanalysis has resulted in a period that can be called the "Electroanalysis Renaissance" (Escarpa, 2012) in which graphene is without any doubt a forefront nanomaterial. The rise in affordable fabrication processes, along with the great dispersing attitude in a plenty of matrices, have made graphene powerful in large-scale production of electrochemical platforms. Herein, we overview the employment of graphene to customize and/or fabricate printable based (bio)sensors over the past 5 years, including several modification approaches such as drop casting, screen- and inkjet-printing, different strategies of graphene-based sensing, and applications as well. The objective of this review is to provide a critical perspective related to advantages and disadvantages of using graphene in biosensing tools, based on screen-printed sensors.

Prussian blue nanoparticles encapsulated inside a metal–organic framework via in situ growth as promising peroxidase mimetics for enzyme inhibitor screening

Metal–organic framework-based peroxidase mimetics for enzyme-inhibitor screening.

An electrochemical biosensing platform based on 1-formylpyrene functionalized reduced graphene oxide for sensitive determination of phenol

A novel electrochemical biosensing platform is proposed. New tyrosinase-based biosensor can be used to detect phenols.