Electrochemical determination of estradiol using a thin film containing reduced graphene oxide and dihexadecylphosphate Bruno C. Janegitz a, ⁎, Fabrício A. dos Santos a , Ronaldo C. Faria b , Valtencir Zucolotto a a Nanomedicine and Nanotoxicology Group, Instituto de Física de São Carlos, Universidade de São Paulo, 13566-390 São Carlos, SP, Brazil b Departamento de Química, Universidade Federal de São Carlos, 13565-970 São Carlos, SP, Brazil abstract article info Article history: Received 28 July 2013 Received in revised form 9 November 2013 Accepted 17 December 2013 Available online 27 December 2013 Keywords: Reduced graphene oxide (RGO) Dihexadecylphosphate film (DHP) Electrochemical determination Estradiol determination Graphene is a material that has attracted attention with regard to sensing and biosensing applications in recent years. Here, we report a novel treatment (using ultrasonic bath and ultrasonic tip) to obtain graphene oxide (GO) and a new stable conducting film using reduced graphene oxide (RGO) and dihexadecylphosphate film (DHP). The GO was obtained by chemical exfoliation and it was reduced using NaBH 4 . Subsequently, RGO–DHP disper- sion was prepared and it was dropped onto a glassy carbon electrode by casting technique. The electrode was characterized by cyclic voltammetry and electrochemical spectroscopy impedance. The voltammetric behavior of the RGO–DHP/GC electrode in the presence of estradiol was studied, and the results reported an irreversible oxidation peak current at 0.6 V. Under the optimal experimental conditions, using linear sweep adsorptive stripping voltammetry, the detection limit obtained for this hormone was 7.7 × 10 -8 mol L -1 . The proposed electrode can be attractive for applications as electrochemical sensors and biosensors. © 2013 Published by Elsevier B.V. 1. Introduction Carbon nanostructured materials have been extensively used in sensing and biosensing applications in the last years [1–5]. For example, electrochemical sensors for pharmaceuticals/biological analysis [6–15] using pristine or modified nanomaterials have been proposed, which the modification of electrode can provides, e.g., increase in the analytical signal or improves the selectivity. Ensafi et al. have proposed a SiO 2 –Al 2 O 3 mixed-oxide electrode modified with Mn nanoparticles for oxidation of captopril. They have obtained a detection limit of 0.095 mmol L -1 and have determined this compound in samples such as pharmaceutical and human urine [11]. Karimi-Maleh et al. have constructed a multi- walled carbon paste electrode based on NiO–carbon nanotubes nano- composite and an anthracene-diol modifier for simultaneous determina- tion of cysteamine, nicotinamide adenine dinucleotide, and folic acid in biological and pharmaceutical samples, which have presented a detection limits of 0.007, 0.6, and 0.9 mmol L -1 , respectively [6]. Moradi et al. proposed a sensor using FePt particles, multi-walled carbon nanotubes and an amide ligand as a mediator for simultaneous determination of three organic compounds in biological samples [10]. Detection limits of 0.05, 0.8 and 1.0 μmol L -1 were achieved for glutathione, nicotinamide adenine dinucleotide and tryptophan, respectively. Graphene-based materials have been used in several different types of applications, such as hydrogen storage [16], solar cells [17], sensors [18], and biosensors [19]. The applications of this material are related to their interesting properties, which include a high surface area [20], excellent electric conductivity [20], and strong mechanical strength [21]. The synthesis of graphene sheets remains a challenge, and several different methods have been proposed to prepare this material [22–26]. Novoselov et al. were the first to prepare graphene sheets by the exfoli- ation of pyrolytic graphite, which is also known as the scotch-tape method [27]. Other methods to obtain graphene include thermal decomposition under ultra-high vacuum (UHV) conditions [28], chem- ical vapor deposition (CVD) growth on metal substrates, substrate-free CVD [29], epitaxial growth in SiC [25] and the chemical exfoliation [30]. The chemical exfoliation of graphite utilizes oxidizing reagents (e.g., sulfuric acid, potassium permanganate and hydrogen peroxide) to obtain graphene oxide (GO). GO is a form of graphene that has emerged as an important derivative of graphene and can be reduced in presence of a reducing agent [30], including hydrazine, ascorbic acid or sodium borohydride. In this regard, reduced graphene oxide (RGO) has been used in electroanalysis [31]. The dispersion of graphene, GO or RGO in water is an important issue for the fabrication of many graphene-based devices [32], including electrochemical biosensors. To maintain the properties of individual graphene sheets, it is necessary to maintain stable suspensions of RGO in aqueous solutions. In this context, there is a need to develop procedures for directly dispersing rel- atively pure graphene sheets in aqueous solutions [33–35]. Some com- pounds that have been used to prepare graphene dispersions include poly(diallyldimethylammonium chloride) (PDDA), poly(ethylenimine) (PEI), poly(sodium styrenesulfonate) (PSS), poly(allylamine hydrochlo- ride) (PAH), poly(acrylic acid) (PAA), and sodium dodecyl sulfate (SDS). Tummala et al. studied the effects of the structural properties of SDS on graphene [36]. Tang et al. proposed the preparation of graphene nano- sheets from natural graphite modified with the cationic surfactant Materials Science and Engineering C 37 (2014) 14–19 ⁎ Corresponding author. Tel.: +55 163373 9875, +55 163371 5381. E-mail address: brunocj@ymail.com (B.C. Janegitz). 0928-4931/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.msec.2013.12.026 Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec