Highly sensitive quantification of hydrogen-transmitting coenzyme in physiological pH using silver nanoparticles dispersed on nitrogen doped graphene quantum dots

Graphene Quantum Dots-Based Nanocomposites Applied in Electrochemical Sensors: A Recent Survey

Graphene quantum dots (GQDs) have been widely investigated in recent years due to their outstanding physicochemical properties. Their remarkable characteristics allied to their capability of being easily synthesized and combined with other materials have allowed their use as electrochemical sensing platforms. In this work, we survey recent applications of GQDs-based nanocomposites in electrochemical sensors and biosensors. Firstly, the main characteristics and synthesis methods of GQDs are addressed. Next, the strategies generally used to obtain the GQDs nanocomposites are discussed. Emphasis is given on the applications of GQDs combined with distinct 0D, 1D, 2D nanomaterials, metal-organic frameworks (MOFs), molecularly imprinted polymers (MIPs), ionic liquids, as well as other types of materials, in varied electrochemical sensors and biosensors for detecting analytes of environmental, medical, and agricultural interest. We also discuss the current trends and challenges towards real applications of GQDs in electrochemical sensors.

Unusual Reactivity of Graphene Quantum Dot-Wrapped Silver Nanoparticles with Hg(II): Spontaneous Growth of Hg Flowers and Their Electrocatalytic Activity

The galvanic reaction (GR) between a graphene quantum dot (GQD)-stabilized AgNP (Ag-GQD)-modified glassy carbon (GC) surface and Hg(II) leads to complete dissolution of AgNPs within 15 min and subsequent growth of Hg(0) as a "flower" on the GQD surface. This is unusual because generally the GR of bulk Ag/AgNPs with Hg(II) leads to the formation of a Hg-Ag amalgam/core shell structure. The appearance of peaks at 99.9 and 103.9 eV in X-ray photoelectron spectroscopy confirms Hg(0) on GQDs, whereas the disappearance of a peak at 370 eV indicates complete dissolution of Ag(0). When 200 ppm Hg(II) interacts with Ag-GQDs for 10 min, coalescence of AgNPs takes place along with the formation of Hg(0) petals separately. However, Hg(0) is grown as a flower with 2 μm size, and complete dissolution of AgNPs occurs subsequently after 15 min. The reason for anti-amalgamation is the direct deposition of Hg(0) by the available oxygen functional groups, followed by its strong adsorption on the graphene surface of GQDs. The subsequent growth of Hg(0) as a flower is due to the GR between AgNPs and Hg(II). Interestingly, the Hg flower-GQD-modified GC electrode acts as a good electrocatalyst toward H₂O₂ reduction by decreasing its overpotential by 150 mV in contrast to GC/Ag-GQDs.

Bioassay of saliva proteins: The best alternative for conventional methods in non-invasive diagnosis of cancer

Non-invasive diagnosis of cancer is often the key to effective treatment and patient survival. Saliva as a multi-constituent oral fluid comprises various disease signaling biomarkers, holds great potential for early-stage cancer diagnostics with cost-effective and easy collection, storage, transport and processing. Therefore, detection of biomarkers and proteins in the saliva samples is highly demand. The current review was performed using reliable internet database (mainly PubMed) to provide an overview of the most recent developments on non-invasive diagnosis of cancers in saliva and highlights main challenges and future prospects in sensing of the salivary biomarkers. The conventional detection methods of cancer biomarkers in saliva is discussed in the paper, however, the main focus is on non-invasive diagnosis of cancers in saliva using immunosensing (electrochemical, optical, piezoelectric), DNA based sensors, aptasensors and peptide based bio-assays The reviewed literature revealed that non-invasive cancer detection methods using the mentioned biosensors and without any processing of saliva sample offers a quick, sensitive, specific and cost effective analytical tool. Besides, salivary based detection methods can be used for simultaneous detection of panels of disease specific biomarkers in a real time manner or as home testing kits in near future.

Cross-linked chitosan/thiolated graphene quantum dots as a biocompatible polysaccharide towards aptamer immobilization

Chitosan has a number of commercial and possible biomedical uses. Chitosan as a polysaccharide is a bioactive polymer with a variety of applications due to its functional properties such as antibacterial activity, non-toxicity, ease of modification, and biodegradability. In this work, cross-linked chitosan/thiolated graphene quantum dot as a biocompatible polysaccharide was modified by gold nanoparticle and used for immobilization of ractopamine (RAC) aptamer. A highly specific DNA-aptamer (5'-SH-AAAAAGTGCGGGC-3'), selected to RAC was immobilized onto thiolated graphene quantum dots (GQDs)-chitosan (CS) nanocomposite modified by gold nanostructures (Au NSs) and used for quantification of RAC. Different shapes of gold nanostructures with various sizes from zero-dimensional nanoparticles to spherical structures were prepared by one-step template-assistant green electrodeposition method. Fully electrochemical methodology was used to prepare a new transducer on a glassy carbon surface which provided a high surface area to immobilize a high amount of the aptamer. Therefore, a label free electrochemical (EC) apta-assay for ultrasensitive detection of RAC was developed. A special immobilization media consisting of Au NSs/GQDs-CS/Cysteamine (CysA) was utilized to improve conductivity and performance of the biosensor. The RAC aptamer was attached on the Au NSs of the composite membrane via AuS bond. The fabrication process of the EC aptamer based assay was characterized by some electrochemical techniques. The peak currents obtained by differential pulse voltammetry decreased linearly with the increasing of RAC concentrations and the apta-assay responds approximately over a wide dynamic range of RAC concentration from 0.0044 fM to 19.55 μM. The low limit of quantification was 0.0044 fM.