Inherently electroactive graphene oxide nanoplatelets as labels for specific protein-target recognition

Immobilization-Free Electrochemical Sensor Coupled with a Graphene-Oxide-Based Aptasensor for Glycated Albumin Detection

An immobilization-free electrochemical sensor coupled with a graphene oxide (GO)-based aptasensor was developed for glycated human serum albumin (GHSA) detection. The concentration of GHSA was monitored by measuring the electrochemical response of free GO and aptamer-bound GO in the presence of glycated albumin; their currents served as the analytical signals. The electrochemical aptasensor exhibited good performance with a base-10 logarithmic scale. The calibration curve was achieved in the range of 0.01-50 µg/mL. The limit of detection (LOD) was 8.70 ng/mL. The developed method was considered a one-drop measurement process because a fabrication step and the probe-immobilization process were not required. This simple sensor offers a cost-effective, rapid, and sensitive detection method, and could be an alternative approach for determination of GHSA levels.

Applications of Graphene and Its Derivatives in Chemical Analysis

In this review, the applications of graphene and its derivatives in the chemical analysis have been described. The properties of graphene materials which are essential for their use in chemical and biochemical analysis are characterized. The materials are used in sensors and biosensors, in electrochemistry, in chromatography and in the sample preparation techniques. Chemical and electrochemical sensors containing graphene materials are useful devices for detecting some chemical and biochemical compounds. Chromatographic columns for HPLC with graphene containing stationary phases may be used for separation of polar and nonpolar components of some specific mixtures. Graphene materials could be successfully employed during sample preparation for analysis with SPE, magnetic SPE, and SPME techniques. HighlightsThe review of the applications of graphene (G) and its derivatives, graphene oxide (GO) and reduced graphene oxide (rGO), in chemical and biochemical analysis is proposed.The electron donor-acceptor and proton donor-acceptor interactions for the graphene based materials - analytes systems and their impact on the analysis results are discussed, particularly:    i) in electrochemistry,ii) in chromatography,iii) in modern sample preparation techniquesiv) in sensors of different types.The essence of the thermal stability and the nomenclature of the graphene based materials in their different applications in chemical systems of different classes was discussed (and suggested).The benefits of using SPME fibers with immobilized graphene materials have been presented in detail.

Physical Structure and Electrochemical Response of Diamond-Graphite Nanoplatelets: From CVD Synthesis to Label-Free Biosensors

Hybrid diamond-graphite nanoplatelet (DGNP) thin films are produced and applied to label-free impedimetric biosensors for the first time, using avidin detection as a proof of concept. The DGNPs are synthesized by microwave plasma chemical vapor deposition through H₂/CH₄/N₂ gas mixtures in a reproducible and rapid single-step process. The material building unit consists of an inner two-dimensional-like nanodiamond with preferential vertical alignment covered by and covalently bound to nanocrystalline graphite grains, exhibiting {111}_diamond||{0002}_graphite epitaxy. The DGNP films' morphostructural aspects are of interest for electrochemical transduction, in general, and for Faradaic impedimetric biosensors, in particular, combining enhanced surface area for biorecognition element loading and facile Faradaic charge transfer. Charge transfer rate constants in phosphate buffer saline/[Fe(CN)₆]^4- solution are shown to increase up to 5.6 × 10^-3 cm s^-1 upon N₂ addition to DGNP synthesis. For the impedimetric detection of avidin, biotin molecules are covalently bound as avidin specific recognition elements on (3-aminopropyl)triethoxysilane-functionalized DGNP surfaces. Avidin quantification is attained within the 10-1000 μg mL^-1 range following a logarithmic dependency. The limits of detection and of quantitation are 1.3 and 6.4 μg mL^-1 (19 and 93 nM), respectively, and 2.3 and 13.8 μg mL^-1 (33 and 200 nM) when considering the nonspecific response of the sensors.

Graphene Oxide Nanoplatelets Potentiate Anticancer Effect of Cisplatin in Human Lung Cancer Cells

Graphene oxide (GO) has been widely explored by many in drug delivery strategies and toxicity assays. The toxicity of graphene oxide depends on the size of the sheets. Smaller sheets show lower toxicity, a quality which is essential for utilization in biomedical applications. However, despite vast research on GO, anticancer properties and drug carrier capabilities of graphene oxide nanoplatelets have yet to be fully explored. Herein, we have uniquely prepared graphene oxide nanoplatelets (GONPs) from well-defined stacked graphite nanofibers (SGNF) with a base of 50 × 50 nm² for toxicity and drug potentiation studies when coadministered with the chemotherapeutic drug cisplatin (CP) in human lung cancer cells, A549 cells. Results obtained from our studies have found that not only were GONPs able to act as drug carriers, but they can also significantly potentiate anticancer effect of CP in A549 cells.

Pnictogen (As, Sb, Bi) Nanosheets for Electrochemical Applications Are Produced by Shear Exfoliation Using Kitchen Blenders

Layered materials are of high importance because of their anisotropy and as a source of 2D materials. Whilst there is a plethora of multi-elemental 2D materials, the number mono-elemental 2D materials is rather limited. Herein, we demonstrate that aqueous shear exfoliation can be used to obtain As, Sb, and Bi exfoliated nanosheets. Morphological and chemical characterization of the exfoliated materials shows a decrease in thickness, sheet-to-nanosheet scale, and partial oxidation owing to a higher surface area. The electrochemical performance is tested in terms of inherent electrochemistry, electron transfer, and sensing applications as demonstrated with ascorbic acid. Potential energy-related applications are evaluated in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR), with shear-exfoliated Sb having the best electrochemical performance overall. These findings will have a profound impact on the preparation and application of 2D mono-elemental materials.

Catechol adsorption on graphene nanoplatelets: isotherm, flat to vertical phase transition and desorption kinetics

The adsorption of catechol (1,2-dihydroxybenzene) on graphene nanoplatelets (GNPs) is investigated electrochemically and spectroscopically. The reversible adsorption of catechol on GNPs is Langmuirian with an adsorption constant of (0.2 ± 0.002) mM^-1 at low adsorbate concentrations (≤100 mM). At higher concentrations (>100 mM) the adsorption of catechol on GNPs is shown to undergo a flat to vertical concentration driven phase transition. The kinetics of desorption are measured with a single particle electrochemical technique. The study of individual impacts allows the determination of the rate of catechol desorption from GNPs to be k = 0.08 ± 0.01 s^-1 with first order kinetics. The method provides a powerful and efficient generic approach to study adsorption and, importantly, desorption of molecules on nanomaterials, as well as giving insight into the modification process.

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.

Engineered Carbon-Nanomaterial-Based Electrochemical Sensors for Biomolecules

The study of electrochemical behavior of bioactive molecules has become one of the most rapidly developing scientific fields. Biotechnology and biomedical engineering fields have a vested interest in constructing more precise and accurate voltammetric/amperometric biosensors. One rapidly growing area of biosensor design involves incorporation of carbon-based nanomaterials in working electrodes, such as one-dimensional carbon nanotubes, two-dimensional graphene, and graphene oxide. In this review article, we give a brief overview describing the voltammetric techniques and how these techniques are applied in biosensing, as well as the details surrounding important biosensing concepts of sensitivity and limits of detection. Building on these important concepts, we show how the sensitivity and limit of detection can be tuned by including carbon-based nanomaterials in the fabrication of biosensors. The sensing of biomolecules including glucose, dopamine, proteins, enzymes, uric acid, DNA, RNA, and H2O2 traditionally employs enzymes in detection; however, these enzymes denature easily, and as such, enzymeless methods are highly desired. Here we draw an important distinction between enzymeless and enzyme-containing carbon-nanomaterial-based biosensors. The review ends with an outlook of future concepts that can be employed in biosensor fabrication, as well as limitations of already proposed materials and how such sensing can be enhanced. As such, this review can act as a roadmap to guide researchers toward concepts that can be employed in the design of next generation biosensors, while also highlighting the current advancements in the field.

Stochastic detection and characterisation of individual ferrocene derivative tagged graphene nanoplatelets

Graphene nanoplatelets (GNPs) are ‘tagged’ with 1-(biphen-4-yl)ferrocene, which has been studied via nano-impacts to derive the corresponding surface coverage.

Single graphene nanoplatelets: capacitance, potential of zero charge and diffusion coefficient

Nano-impact chronoamperometric experiments are a powerful technique for simultaneously probing both the potential of zero charge (PZC) and the diffusion coefficient (D0) of graphene nanoplatelets (GNPs). The method provides an efficient general approach to material characterisation. Using nano-impact experiments, capacitative impacts can be seen for graphene nanoplatelets of 15 μm width and 6-8 nm thickness. The current transient features seen allow the determination of the PZC of the graphene nanoplatelet in PBS buffer as -0.14 ± 0.03 V (vs. saturated calomel electrode). The diffusion coefficient in the same aqueous medium, isotonic with many biological conditions, for the graphene nanoplatelets is experimentally found to be 2 ± 0.8 × 10-13 m2 s-1. This quick characterisation technique may significantly assist the application of graphene nanoplatelets, or similar nano-materials, in electronic, sensor, and clinical medicinal technologies.

Lithium intercalation compound dramatically influences the electrochemical properties of exfoliated MoS2

MoS2 and other transition metal dichalcogenides (TMDs) have recently gained a renewed interest due to the interesting electronic, catalytic, and mechanical properties which they possess when down-sized to single or few layer sheets. Exfoliation of the bulk multilayer structure can be achieved by a preliminary chemical Li intercalation followed by the exfoliation due to the reaction of Li with water. Organolithium compounds are generally adopted for the Li intercalation with n-butyllithium (n-Bu-Li) being the most common. Here, the use of three different organolithium compounds are investigated and compared, i.e., methyllithium (Me-Li), n-butyllithium (n-Bu-Li) and tert-butyllithium (t-Bu-Li), used for the exfoliation of bulk MoS2 . Scanning transmission electron microscopy (STEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) are adopted for a comprehensive characterization of all materials under investigation. In addition, catalytic properties towards the hydrogen evolution reaction (HER) and capacitive properties are also tested. Different organolithium compounds exhibit different extent of Li intercalation resulting in different degrees of exfoliation. The inherent electrochemical behavior of MoS2 consisting of significant anodic and cathodic peaks as well as its capacitive behavior and catalytic properties towards hydrogen evolution reaction are strongly connected to the exfoliation compound used. This research significantly contributes to the development of large-scale synthesis of electrocatalytic MoS2 -based materials.

Applications of graphene and related nanomaterials in analytical chemistry

Graphene and its related materials remain a very bright and exciting prospect in analytical chemistry.

A new electrochemical aptasensor based on electrocatalytic property of graphene toward ascorbic acid oxidation

Based on the superior electrocatalytic property of graphene (GN) toward ascorbic acid (AA) oxidation, a new electrochemical aptasensor has been developed. Here, adenosine triphosphate (ATP) is used as the model to demonstrate the performance of the developed aptasensor. Briefly, GN is attached to the thiolated ATP binding aptamer (ABA) modified gold electrode through π-π stacking interaction, resulting in a significant oxidation signal of AA. In the presence of ATP, the formation of ATP-ABA complex leads to the release of GN from sensing interface, resulting in a sharp decrease of the oxidation peak current of AA and an obviously positive shift of the related peak potential. Taking both the change values of the peak current and peak potential of AA oxidation as the response signals, ATP can be detected sensitively. This is the first time to demonstrate the application of GN as the nanocatalyst in an amplified aptasensor. It can be expected that GN, as nanocatalyst, should become the very promising amplifying-elements in DNA-based electrochemical biosensors.

Unusual Fe(CN)₆³⁻/⁴⁻ capture induced by synergic effect of electropolymeric cationic surfactant and graphene: characterization and biosensing application

Herein, a special microheterogeneous system for Fe(CN)6(3-/4-) capture was constructed based on graphene (GN) and the electropolymeric cationic surfactant, an amphiphilic pyrrole derivative, (11-pyrrolyl-1-yl-undecyl) triethylammonium tetrafluoroborate (A2). The morphology of the system was characterized by scanning electron microscope. The redox properties of the entrapped Fe(CN)6(3-/4-) were investigated by cyclic voltammetry and UV-visible spectrometry. The entrapped Fe(CN)6(3-/4-) exhibited highly electroactive with stable and symmetrical cyclic voltammetric signal. A dramatic negative shift in the half wave potential can be obtained due to the unusual Fe(CN)6(3-/4-) partitioning in in this microheterogeneous system based on poly(A2+GN). Finally, the entrapped Fe(CN)6(3-/4-) was applied in the construction of the enhanced biosensors to hydrogen peroxide and sulfide.