ING. CARLOS SUAREZ VALDEZ RUBRICA

Dopamine (DA) is an important biomolecule. It is formed by the decarboxylation of DOPA and is a precursor of two other neurotransmitters; adrenaline and noradrenalin. DA is a well-known catecholamine neurotransmitter of the human central nervous system and brain. It controls the brain's reward and pleasure centers as well as helps to regulate movement and emotional responses. In addition, DA plays a vital role in the functioning of the central nervous, cardiovascular, renal and hormonal systems (Lévesque et al., 2007). An abnormal DA level in the brain causes several disease conditions, such as pleasurable, rewarding feelings and sometimes even euphoria. Meanwhile, a deficiency of DA in the brain may lead to neurological disorders, such as schizophrenia and Parkinson’s disease (Cederfjäll et al., 2013). Selvaraju et al. (Selvaraju et al., 2003a, 2003b; Selvaraju et al., 2005; Selvaraju et al., 2014) and Abraham John and his coworkers (Kalimuthu et al., 2009; Raj et al., 2013; Revin et al., 2012) extensively investigated the electrochemical detection of DA in the presence of many interference such as ascorbic acid (AA), uric acid (UA), tyrosine (Tyr), methionine (Met) and serotonin (5-HT).

Where else, nitric oxide (NO) is a hydrophobic, highly labile free radical that is naturally produced within the human body and plays a vital role in a wide range of biological and cellular functions. NO is used for communication in between cells and

involve in the regulation of blood pressure, the immune response, platelet aggregation and clotting, neurotransmission and possibly respiration (Bredt et al., 1992; Burnett, 1997). Abnormal NO production and bioavailability may cause several diseases such as obesity, diabetes (both type I and II), atherosclerosis, hypertension and heart failure (Napoli et al., 2001; Petros et al., 1991). Thus, the development of sensor for the precise and selective measurement of NO that appears to be in low level within living systems can make a great contribution to disease diagnosis.

Several methods have been reported to detect DA and NO, including chemiluminescence (Beckman et al., 1995), paramagnetic resonance spectrometry (Wennmalm et al., 1990), paramagnetic resonance imaging spectrophotometry (Kuppusamy et al., 1996) and bioassay (Wallace et al., 1995). Among these methods, the electrochemical detection of DA and NO is the only available technique sensitive enough to detect relevant concentrations of DA and NO in real time and in vivo. Electrochemical biosensor has a series of advantages such as high sensitivity towards electroactive species, rapid and accurate response and most importantly it is portable and inexpensive compared to other existing biosensor. Moreover, this technique is also highly selective toward DA and NO in the presence of interfering species such as nitrite, nitrate, AA, UA and L-arginine.

A variety of materials have been reported to have potential as electrochemical sensors, including organic conjugated polymer (Mulchandani et al., 1995; Mulchandani

et al., 1996), metal and semiconductor nanoparticles (Yin et al., 2011) and carbon

nanomaterials (Shah et al., 2013). Among the carbon nanomaterials, graphene has been widely explored in the fabrication of electrochemical sensors, and especially biosensor, because of its fascinating two dimensional conjugated structures and unique properties

such as a high conductivity (Li et al., 2009), high electrocatalytic activity (Wang et al., 2010) and large surface area (Reina et al., 2008). These properties give graphene an advantage as a sensor. For example, Zhou et al. has successfully fabricated the chemically reduced graphene oxide-modified GCE (CR-GO/GC) for the detection of inorganic and organic electroactive compounds (H2O2, NADH, DA, AA, UA, glucose) (Zhou et al., 2009). Their research has proved that CR-GO demonstrated favourable electrochemical activity which extremely attractive for a wide range of electrochemical sensing and biosensing applications. Moreover, the potential sensor application of graphene cooperated with metal such as platinum (Pt) were also been studied by some researchers. In 2011, Sun et al. have studied the simultaneous electrochemical detection of AA, DA and UA at the glassy carbon electrode modified by graphene-Pt nanocomposite (Sun et al., 2011). The observed results show that this new nanocomposite posses a great potential as electrochemical biosensors.

Up to date, the hybridization of graphene with a polymer has attracted wide attention because this graphene-based polymer composite has remarkable electrocatalytic, mechanical, electrical and thermal properties. Many polymers have been used to synthesize graphene/polymer composites. For example, Liu and co-workers (Liu et al., 2014) developing a poly(o-phenylenediamine) (PoPD)/graphene hybrid composite using electropolymerization technique which performed on the graphene/GCE for the detection of DA in human urine samples. This modified electrode has a detection limit of 7.5 mM. Meanwhile, Han and his co-workers (Han et al., 2010) has synthesize chitosan-graphene composite by a together-blending and in situ chemical reduction method to modify a GCE for DA sensor. The simultaneous detection of DA in the presence of AA and UA shows a detection limit of 1 µM. Thus, this study focused on the synthesis of a reduced graphene oxide-based polymer composite with Nf. Nf is a

conductive polymer, which is important in enhancing the properties of the hybrid material. It also acts as a dispersant for graphene because of it perfluoroalkyl backbone, which has a higher hydrophobicity and leads to a stronger interaction with graphene. In addition, Nf has advantages as an electrode modifier because it possess an excellent antifouling capacity, high permeability to cations and strong adsorption ability (Nigović

et al., 2014b).

Herein, we report a selective and sensitive electrochemical sensing platform based on hydrothermally prepared rGO-Nf nanohybrid modified electrode toward the detection of DA in the presence of AA and UA. The influences of experimental parameters such as hydrothermal process time, scan rate and pH of the electrolyte on electrocatalytic performance were also investigated. To the extent of our knowledge, there is little research focused on the rGO-Nf nanohybrid materials for the electrochemical detection of NO with a lowest detection limit. In this present work, the novel rGO-Nf nanohybrid was also employed as a sensing electrode material in an electrochemical sensor to study its sensitivity and selectivity toward NO. The electrochemical signal obtained from the NO sensor could be optimized by controlling the loading volume of the material on the electrode surface. The interference of AA and DA during the determination of NO was also studied. The high sensitivity and selectivity of the rGO-Nf nanohybrid modified electrode could make them a suitable candidate for detection of a wide range of biomolecules in biosensor.