## 1 Introduction
For typical power transformers containing Kraft insulating paper, it is generally assumed that a furan content in the range between 0.1 and 1.0 ppm infers a DP decrease between 700 and 450, equivalent to a moderate deterioration of these electrical equipment and thus an early warning of degradation of paper insulation. Currently, common practice consists of periodic oil sampling from in-service transformers and in analysis usually performed in a laboratory remote from the sampling site. It is carried out usually by chromatographic methods, such as for instance by high pressure liquid chromatography (HPLC) according to the IEC 61198 method [13]. This technique is quite complex, requiring the extraction of the substances of interest from the sample before the injection on the chromatographic column, the use of expensive equipment and specialized operators both for acquisition and for data analysis, and a long time period for obtaining the results. Normally, these measurements are scheduled on an annual basis and only in case of critical situations is an intensification of the oil sampling required. Recently methods or devices alternative to HLPC have been reported in the literature. These are mainly based on electrochemical techniques, but they are not very sensitive and not highly selective [14, 15]. In particular, those based on amperometric techniques are potentially subject to noise and to electromagnetic interferences and therefore they are not suitable for an on-line application. Moreover, the electrochemical measurements require the presence of electrolytes to improve the electrical conduction of the system, so that the original sample must be modified before measurement, most often by complex extraction procedures [15]. To overcome these problems a new approach, exploiting surface plasmon resonance (SPR) on plastic optical fiber and molecularly imprinted polymers (MIP), is proposed in this work. Recently, SPR biosensors in optical fibers have been shown to be able to play an important role in numerous important fields, including pharmaceutical research, medical diagnostics, environmental monitoring, food safety and security, where fast, portable, low cost and rugged units are needed for early detection and identification [16, 17]. In general, the optical fiber employed is either a glass one or a plastic one (POF). For low-cost SPR sensing systems, POFs are especially advantageous due to their excellent flexibility, easy manipulation, great numerical aperture, large diameter, and the fact that plastic is able to withstand smaller bend radii than glass. The advantage of using POFs is that the main features of POFs, that have increased their popularity and competitiveness for telecommunications, are exactly those that are important for optical sensors based on glass optical fibers, with the addition of simpler manufacturing and handling procedures [18, 19]. The required selectivity can be obtained in different ways by using a receptor layer very specific for the considered analyte in contact with the SPR active surface. Most often it consists of a thin layer or even a monolayer of biological receptor molecules such as for example antibodies. Molecularly imprinted polymers (MIPs) are synthetic receptors obtained by molecular imprinting methods [20, 21], presenting a number of favorable aspects for sensing in comparison to bioreceptors, including a better stability out of the native environment, reproducibility and low cost. A further reason for using chemical instead of biological receptors, in the particular application examined herein, is that the measurements should be done directly in the sample, _i.e._, in a mineral oil phase, in which the most usual biological receptors do not perform at their best. On the contrary, MIPs should present higher affinity and selectivity for the substrate in non-aqueous media [22, 23]. This combined sensing approach, based on a POF-MIP sensor, could be extremely suitable for on-line application.