br Introduction Adenosine deaminase ADA which

Introduction Adenosine deaminase (ADA), which can catalyze the conversion of AD to inosine by removing an amino group, is a key hydrolytic enzyme of purine metabolism (Conway and Cooke, 1939), and plays an important role in an amount of diseases. For example, an increase of ADA activity in serum has been found to be closely related to pp2 tumors, liver cancer, breast cancer and colorectal cancer (Hofbrand and Janossy, 1981, Gierek et al., 1987, Gocmen et al., 2009). On the other hand, ADA deficiency is a main cause of severe combined immunodeficiency (SCID) (Resta and Thompson, 1997, Cristalli et al., 2001, Sanchez et al., 2007), autism, etc. On account of the significance of ADA, several techniques have been employed for the assay of ADA, including measuring ammonia produced (Linden, 2001), high-performance liquid chromatography (HPLC) (Paul et al., 2005), colorimetric assay (Vielh and Castellazzi, 1984), etc. Unfortunately, several limitations like time-consuming, expensive, complicated, low sensitivity and selectivity impede the broad application of these methods. A variety of strategies have been developed to overcome these shortcomings, especially the introduction of aptamers. Aptamers, which are originated from random sequence DNA or RNA libraries by SELEX (systematic evolution of ligands by exponential enrichment) (Ellington and Szostak, 1990, Tuerk and Gold, 1990, Osborne and Ellington, 1997, Breaker, 1997) are single-stranded DNA or RNA oligonucleotides and possess the ability to form defined three-dimensional structure for specific target binding (Famulok et al., 2000, Wilson and Szostak, 1999). Aptamers have received tremendous attention in the biosensor applications in recent years, because of their unprecedented advantages such as simple synthesis, easy labeling, long-term stability and excellent target recognition properties (German et al., 1998). So far, several methods based on AD aptamer for the quantitative determination of ADA activity have been developed, including colorimetric assay (Zhao et al., 2008), electrochemical detection method (Zhang et al., 2010), fluorescent sensor (Elowe et al., 2006, He et al., 2009), among which the fluorescent sensor has been extensively employed owing to the features of high sensitivity, facile operation, real-time detection. However, traditional assay based on the fluorescence aptasensor needs two single-stranded DNA labeled with a quencher (QDNA) and a fluorophore (FDNA), which makes the assay expensive and complex. In addition, some research have proved that the length of QDNA and temperature have great effects on the sensitivity (Nutiu and Li, 2004, Nutiu and Li, 2005), and side reactions such as tri-molecular complex exist in this system (Nakamura and Shi, 2009). Accordingly, there is still an urgent need for the development of new highly selective and sensitive aptamer-based sensors for ADA assay that are simple and cost-effective. Graphene, a single-atom-thick two-dimensional nanosheet, has become a hot spot because of its remarkable electronic, mechanical, and thermal properties (Geim and Novoselov, 2007, Lomeda et al., 2008, Yang et al., 2010) in recent years. Graphene oxide (GO), resulting from acid exfoliation of graphene (Kim et al., 2009), has shown great potential for biological applications because of its good water-solubility compared with graphene and versatile surface modification (Geim, 2009, Stankovich et al., 2006). Combined the above features with its superior fluorescence quenching ability, good biocompatibility, and low cytotoxicity, GO has been widely exploited, for instance the detection of nucleic acids (Lu et al., 2009, He et al., 2010, Dong et al., 2010, Liu et al., 2010), proteins (Chang et al., 2010, Jang et al., 2010), enzyme activity (Wu et al., 2011, Lin et al., 2011), virus (Jung et al., 2010), and for drug delivery (Lu et al., 2010, Liu et al., 2008). Herein, we present a sensitive and selective fluorescent aptasensor for the assay of ADA and its inhibitor based on GO using AD as the substrate. Our strategy utilizes the efficient quenching ability of GO and the different interaction intensity of aptamer, aptamer/AD complex with GO which directly induces the fluorescence intensity change. Compared with the previous report-based fluorescence signaling system (Elowe et al., 2006, He et al., 2009), in which FDNA and QDNA were used, this GO-based design which needs only one labeled single-stranded DNA, is not only convenient and cost-effective, but also possesses preeminent sensitivity resulting from the high fluorescence quenching efficiency of GO.