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Introduction Owing to their unique physical, chemical, and structural properties,1 graphene and its derivatives show outstanding potential in many fields, such as energy storage and conversion,2 highfrequency electronics, 3 and bioscience/biotechnologies. 1 (See the introductory article in this issue for more background on the history and properties of graphene.) Of the techniques developed to produce graphene (detailed in the articles in this issue by Nyakiti et al., Bartelt and McCarty, and Jaber-Ansari and Hersam), one of the most economical for mass production is chemical synthesis from graphite by oxidation to graphene oxide (GO) and subsequent reduction.4,5 Different functional groups and other nanomaterials can be linked to the graphene surface during the reduction procedure because GO consists of sp2-bonded carbon atoms with easily functionalized six-atom rings and is abundant in reactive carboxyl and epoxy groups. In addition to its ease of functionalization, graphene exhibits distinct properties that are very attractive for biosensing and bioimaging, such as a high electrical conductivity (up to 6000 S/cm),6 a large planar area (theoretically 2630 m2/g for single-layer graphene),7 and an excellent ability to quench fluorescence.8,9 In this article, we discuss the immobilization of enzymes, deoxyribonucleic acid (DNA), and proteins on graphene materials and review selected electrochemical and fluorescence

biosensors based on graphene and its derivatives for detecting small molecules, DNA, and cells. Graphene-based magnetic resonance and fluorescence for cell imaging are also discussed.

Graphene-based enzyme biosensors Enzymes are biological molecules (typically proteins) that catalyze chemical reactions. The reaction takes place in a small part of the enzyme called the active site, with the rest of the molecule acting as “scaffolding.” The amino acids around the active site attach to the target molecule (called the “substrate”) and hold it in position while the reaction takes place. This makes each enzyme specific for a single reaction or for a particular type of chemical bond or functional group. A key goal of modern analytical science is to monitor biomolecular content with methods that are highly sensitive, have a short turnaround time, and are inexpensive. Because enzymes often have high efficiency and target specificity, their reactions are suitable for detecting single analytes at low concentration. For example, a number of biosensors have been reported, including sensors using the enzyme horseradish peroxidase (HRP) to detect hydrogen peroxide, using glucose oxidase to detect glucose, and using acetylcholinesterase (AChE) to detect organophosphates. In the construction of an enzyme biosensor, fabrication of the sensing interface is extremely important but also extremely challenging. For instance, to improve enzyme survival (maintenance of the activity of the immobilized enzyme in the

Dan Du, College of Chemistry, Central China Normal University, China; [email protected] Yuqi Yang, College of Chemis