Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters. Over the last decade, different nanomaterials have been exploited to design highly efficient biosensors for the detection of analyte biomolecules. The discovery of graphene has spectacularly accelerated research on fabricating low-cost electrode materials because of its unique physical properties, including high specific surface area, high carrier mobility, high electrical conductivity, flexibility, and optical transparency. Graphene and its oxygenated derivatives, including graphene oxide (GO) and reduced graphene oxide (rGO), are becoming an important class of nanomaterials in the field of biosensors. The presence of oxygenated functional groups makes GO nanosheets strongly hydrophilic, facilitating chemical functionalization. Graphene, GO and rGO nanosheets can be easily combined with various types of inorganic nanoparticles, including metals, metal oxides, semiconducting nanoparticles, quantum dots, organic polymers and biomolecules, to create a diverse range of graphene-based nanocomposites with enhanced sensitivity for biosensor applications. This review summarizes the advances in two-dimensional (2D) and three-dimensional (3D) graphene-based nanocomposites as emerging electrochemical and fluorescent biosensing platforms for the detection of a wide range of biomolecules with enhanced sensitivity, selectivity and a low limit of detection. The biofunctionalization and nanocomposite formation processes of graphene-based materials and their unique properties, surface functionalization, enzyme immobilization strategies, covalent immobilization, physical adsorption, biointeractions and direct electron transfer (DET) processes are discussed in connection with the design and fabrication of biosensors. The enzymatic and nonenzymatic reactions on graphene-based nanocomposite surfaces for glucose- and cholesterol-related electrochemical biosensors are analyzed. This review covers a very broad range of graphene-based electrochemical and fluorescent biosensors for the detection of glucose, cholesterol, hydrogen peroxide (H₂O₂), nucleic acids (DNA/RNA), genes, enzymes, cofactors nicotinamide adenine dinucleotide (NADH) and adenosine triphosphate (ATP), dopamine (DA), ascorbic acid (AA), uric acid (UA), cancer biomarkers, pathogenic microorganisms, food toxins, toxic heavy metal ions, mycotoxins, and pesticides. The sensitivity and selectivity of graphene-based electrochemical and fluorescent biosensors are also examined with respect to interfering analytes present in biological systems. Finally, the future outlook for the development of graphene based biosensing technology is outlined.

Improving enzyme immobilization with high loading capacity and achieving direct electron transfer (DET) between the enzyme and the electrode surface is key to designing highly sensitive enzymatic electrochemical biosensors. Herein, we report a novel approach based on the selective modification of the outer surface of halloysite nanotubes (HNTs) that supports silver nanoparticles (AgNPs) to obtain a hybrid nanocomposite. AgNPs of about 10 nm average size could be uniformly supported on silane-modified HNTs through in situ reduction of Ag⁺ ions. The resultant nanocomposite shows an excellent support capability for the effective immobilization and electrical wiring of redox enzyme glucose oxidase (GOx). The GOx immobilized HNT/AgNPs were deposited on the glassy carbon electrode (GCE) and utilized for the bioelectrocatalyzed electrochemical detection of glucose. The GOx modified composite electrodes show glucose sensitivity as high as 5.1 μA mM⁻¹ cm⁻², which is higher than for the electrodes prepared without surface functionalization.

Development of biosensors with high sensitivity, high spatial resolution, and low cost has received significant attention for their applications in medical diagnosis, diabetes management, and environment-monitoring. However, achieving a direct electrical contact between redox enzymes and electrode surfaces and enhancing the operational stability still remain as challenges. Inorganic metal nanocrystals (NCs) with precisely controlled shape and surface structure engineered with an appropriate organic coating can help overcome the challenges associated with their stability and aggregation for practical biosensor applications. Herein, we describe a facile, room-temperature, seed-mediated solution-phase route to synthesize monodisperse Pd@Pt core–shell nanocubes with subnanometer-thick platinum (Pt) shells. The enzyme electrode consisting of Pd@Pt core–shell NCs was first covered with a chitosan (CS) polymer and then glucose oxidase (GOx) immobilized by a covalent linkage to the CS. This polymer permits covalent immobilization through active amino (−NH) side groups to improve the stability and preserve the biocatalytic functions while the Pd@Pt NCs facilitate enhanced direct electron transfer (DET) in the biosensor. The resultant biosensor promotes DET and exhibits excellent performance for the detection of glucose, with a sensitivity of 6.82 μA cm^–2 mM^–1 and a wide linear range of 1–6 mM. Our results demonstrate that sensitive electrochemical glucose detection based on Pd@Pt core–shell NCs provides remarkable opportunities for designing low-cost and sensitive biosensors.

Green fabrication of TiO ₂ nanorods supported PtNPs for durable electrocatalysts toward oxygen reduction reaction (ORR).