1 Abstract—The addition of PEG of different molecular weights has important effects on the physical, electrical and electrochemical properties of iron(III)-tosylate doped PEDOT. This particular polymer can be easily spin coated over plastic discs, optimizing thickness and uniformity of the PEDOT-PEG films. The conductivity and morphological analysis of the hybrid PEDOT-PEG polymer by 4-point probe (4PP), 12-point probe (12PP), and conductive AFM (C- AFM) show strong effects of the PEG doping. Moreover, the conductive films kinetics at the nanoscale, in response to different bias voltages, change radically depending on the PEG molecular weight. The hybrid conductive films show also interesting electrochemical properties, making the PEDOT PEG doping appealing for biosensing applications both for EIS-based and amperometric affinity/catalytic biosensors. Keywords—Atomic force microscopy, biosensors, four-point probe, nano-films, PEDOT. I. INTRODUCTION OR several years, the impact of conducting polymers (CPs) has increased in all kinds of technological applications. These materials are very interesting within the fields of organic electronics and bioengineering due to their low-cost, flexibility and ease of manufacturing on conductive and non-conductive substrates [1]. The CPs are versatile materials since their properties can be easily modulated by the most common surface functionalization techniques, and a wide range of molecules can be incorporated to the structure of the films to further functionalize them or act as dopants improving the conductivity of the polymer itself [2]. Among many CPs, poly(3,4 ethylene dioxythiophene) (PEDOT) has been one of the most studied and used due to its long term stability in air, low cost for mass production, high conductivity and desirable electro-optical properties. This polymer has several applications in the development of OLEDS [3], [4], lithium batteries [5], [6], supercapacitors [7]- [9], and biosensors [10]-[13]. Giulio Rosati (Dr) is with the Department of Information Engineering, University of Padova Via G. Gradenigo 6/b, 35131, Padova, Italy (corresponding author, e-mail: rosatigiulio@gmail.com). Noemi Rozlòsnik (Prof. Dr.) is with Department of Micro- and Nano- technology, Danish Technical University, Kgs. Lyngby 2800, Copenhagen, Denmark. Luciano Sappia and Rossana Madrid 2 are with the Laboratorio de Medios e Interfases, Departamento de Bioingeniería, Fac. de Cs. Exactas y Tecnología, Universidad Nacional de Tucumán, Av. Independencia 1800, 4000 San Miguel de Tucumán, Argentina, and the Instituto Superior de Investigaciones Biológicas, CONICET, Chacabuco 461, T4000ILI San Miguel de Tucumán, Argentina. PEDOT films can be deposited by three main pathways, i.e., electrochemical polymerization (EP) [14]-[16], chemical oxidation polymerization (COP) [17]-[19], and Vapor Phase Polymerization (VPP) [20], [21]. In this work, we focus on the COP method, which allows the use of nonconductive substrates. In order to balance charges in the polymer’s backbone and improve conductivity, counter-anions are used to dope the PEDOT. The most common dopants found in the literature are polystyrene sulphonate (PSS) [22]-[25] and iron/sodium tosylate salts [26], [31]. In order to enhance the conductivity of PEDOT films, different strategies were explored, including the addition of nonconductive co-polymers and post-deposition treatments. In the case of PEDOT:PSS films, copolymerization was done adding ethylene glycol (EG) and diethylene glycol (DEG). Post-deposition treatments were performed by immersion of the films in EG [22] or formic acid [23] baths. Conversely, PEDOT:Tos films were copolymerized with polyethylene glycol (PEG) and PEG ran polypropylene glycol (PPG), as described in Table I. The incorporation of PEG-ran-PPG copolymer to PEDOT:Tos films deposited by spin-coating does not seem to affect the crystallinity of the polymer, but changes in the morphology were observed by SEM [32]. In this work, we investigate the effects of PEG as co- polymer of PEDOT:Tos films deposited by spin-coating. Electrical, morphological, and nanoscale conductivity properties of PEDOT/PEG composites are compared with pristine PEDOT films. Differently from the previous works on this topic, we chose to change only the PEG molecular weight (MW), from 20 kDa (the maximum MW reported in literature) down to 1.5 kDa, keeping constant the molarity of added PEG. As suggested by a previous work [26], a PEG molar ratio of 1.62 µM in 1 ml of oxidant solution was used. The film deposition protocol was optimized in terms of uniformity on the plastic substrates. Then, we investigated the films electrical/morphological properties by 4PP, 12PP, AFM, conductive-AFM, and cyclic voltammetry. A detailed study of the rectification effect of the composites was carried out applying bias voltage sweeps in order to study the influence of different PEG molecular weights. Our results suggest that 1.5 kDa PEG copolymer produces the smoothest films, with a homogeneous conductivity at the nanometer scale, and the highest rectification effect, with an applied bias voltage between -1 and +1 V. Giulio Rosati, Luciano Sappia, Rossana Madrid, Noemi Rozlòsnik Iron(III)-Tosylate Doped PEDOT and PEG: A Nanoscale Conductivity Study of an Electrochemical System with Biosensing Applications F World Academy of Science, Engineering and Technology International Journal of Biomedical and Biological Engineering Vol:11, No:11, 2017 586 International Scholarly and Scientific Research & Innovation 11(11) 2017 scholar.waset.org/1307-6892/10008149 International Science Index, Biomedical and Biological Engineering Vol:11, No:11, 2017 waset.org/Publication/10008149