IEEE SENSORS JOURNAL, VOL. 16, NO. 21, NOVEMBER 1, 2016 7663

Markers Detection in Transformer Oil by PlasmonicChemical Sensor System Based on POF and MIPs

N. Cennamo, L. De Maria, C. Chemelli, A. Profumo, L. Zeni, and M. Pesavento

Abstract— We present a multichannel optical chemical sen-sor based on molecularly imprinted polymers (MIPs) andsurface plasmon resonance (SPR) in a D-shaped plastic opticalfiber (POF), for simultaneous determination of two importantanalytes, dibenzyl disulfide, and furfural, whose presence in thetransformer oil is an indication of underway corrosive or ageingprocesses, respectively, in power transformers. Furthermore,an investigation on the dependence of performances of this opticalplatform on the gold film thickness is reported. The low cost,highly selective, and sensitive performance of the SPR-POF-MIPplatforms and the simple and modular scheme of the opticalinterrogation layout make this system a potentially suitableon-line diagnostic tool for power transformers.

Index Terms— Dibenzyl disulfide, furfural (furan-2-carbaldehyde), molecularly imprinted polymers, plastic opticalfibers, power transformers.

I. INTRODUCTION

THE oil-filled power transformers are a key componentof an Electrical Transmission and Distribution (T&D)network. Their failure can have a relevant impact on main-tenance costs due to out-of-services. Nowadays the increasingenergy peak demand and its timing change can often exposepower transformers to irregular stresses and/or overloads thatcan compromise their long term integrity. The availabilityof reliable and potentially low-cost sensors to be used asdiagnostic tools for detecting ageing and failures of thesecomponents is of significant interest to improve managementof the electric power system assets [1]–[4]. In particular,for oil-filled transformers, the frequent control of chemicalmarkers in the insulating oil could provide an early warningof incipient failures (partial discharges, over temperature, hotspot) or of occurring accelerated aging on dielectric parts oftransformers [5]–[10].

Bio and chemical optical sensors based on SPR in opticalfibers have been shown to be able to play an importantrole in numerous relevant fields, including pharmaceutical

Manuscript received June 24, 2016; revised August 11, 2016; acceptedAugust 13, 2016. Date of publication August 25, 2016; date of current versionSeptember 28, 2016. This work was supported by the Research Fund for theItalian Electrical System under the Contract Agreement between RSE and theMinistry of Economic Development-General Directorate for Nuclear Energy,Renewable Energy and Energy Efficiency in compliance with the Decree ofMarch 8, 2006. The associate editor coordinating the review of this paper andapproving it for publication was Dr. Marco Petrovich.

N. Cennamo and L. Zeni are with the Department of Industrial andInformation Engineering, Second University of Naples, 81031 Aversa, Italy(e-mail: nunzio.cennamo@unina2.it).

L. De Maria and C. Chemelli are with Research on Energetic System S.p.A,20134 Milan, Italy.

A. Profumo and M. Pesavento are with the Department of Chemistry,University of Pavia, 27100 Pavia, Italy.

Digital Object Identifier 10.1109/JSEN.2016.2603168

researches, medical diagnostics, industrial applications, envi-ronmental monitoring, food safety and security, where fast,portable, low cost and rugged units are needed for earlydetection and identification [11]–[14]. In general, the opticalfiber is either a glass one or a plastic one (POF). POFs areespecially advantageous due to their excellent flexibility, easymanipulation, great numerical aperture, large diameter, andthe fact that plastic is able to withstand smaller bend radiithan glass [15]. The advantage of using POFs is that themain features of POFs, that have increased their popularityand competitiveness for telecommunications, are exactly thosethat are important for optical sensors based on glass opticalfibers, with the addition of simpler manufacturing and handlingprocedures.

The combination of a D-shaped plastic optical fiber and aMolecularly Imprinted Polymer (MIP) receptor is an effectiveway to obtain a highly selective and sensitive SPR opticalsensor platform. In literature several examples of applicationsof this optical chemical sensor platform are reported, as forexample the selective detection of trinitrotoluene (TNT), forsecurity applications [16], the furfural (furan-2-carbaldehyde)detection in power transformer insulating oil [17], and thedetection of L-nicotine [18] in clinical applications.

SPR-POF-MIP sensors are good candidates for in situdetection of different diagnostic markers directly in the mineraloil matrix. This methodology potentially allows to overcomeproblems, foreseen by current practices, of periodical col-lection of oil samples from the transformer and applicationof more expensive and time-consuming standard analyticalmethods (by gas chromatography for instance).

In this work the Authors present a sensor array, based onmultichannel chemical SPR-MIP sensor in POF, for simul-taneous determination of dibenzyl disulfide (DBDS) andfurfural (2-FAL) in the oil transformer. Moreover, the effectof the gold layer thickness on SPR-POF-MIP sensor isinvestigated.

II. PROBLEMS IN POWER TRANSFORMERS

The power transformer is a key component of the ElectricTransmission and Distribution system. Its integrity assessmentis very complex but essential to avoid irreversible damageswith consequent heavy impacts on maintenance costs andon T&D network services, due to outages. Among causeswhich can lead to a transformer failure (i.e., hot spots,partial discharges), the accelerated degradation of its solidinsulating system, i.e., oil impregnated cellulosic insulationmaterials, strongly depends on the operating condition of thetransformer [19], [20].

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7664 IEEE SENSORS JOURNAL, VOL. 16, NO. 21, NOVEMBER 1, 2016

In the power transformers the degradation of its dielectricparts begins much earlier than the predicted end-of-life ofthe transformer (foreseen as 30 years), due to an acceleratedthermal aging of both the insulating oil and the paper. Whilethe regeneration of a degraded insulating oil is possible byappropriate treatments or even by its replacement with anew compatible oil [21], the refurbishment of degraded paperrequires invasive and costly operations that must be necessarilyperformed by the manufacturer, because it can involve thetotal replacement of transformer windings. For this reason itis well established [22], [23] that the end of useful life of atransformer is mainly determined by thermal deterioration ofpapers and that a careful monitoring of parameters linked tothis process is of fundamental importance for utilities to checkthe “health” of the in-service transformers.

One of the main components of the insulation paper (i.e.,Kraft paper) is cellulose, which consists of a long linearchain of β-D glucose molecules [23], [24]. As the paperages, its mechanical strength changes significantly becauseof degradation (depolymerization) of the cellulose chains; ina domino effect, the decrease of mechanical strength alsoreduces the ability of the transformer to withstand short circuitstress, confirming that these two properties are not mutuallydisjoint but are in synergy. For this reason, the mechanicalstrength of the paper is considered an important diagnosticparameter in transformers and its reduction down to 50%is assumed as an indication of the end-of-life of a trans-former [22]. The viscometric test, according to the IEC60450 [25], provides a reliable and direct measure of thedegree of polymerization (DP) value but actually this methodis not practical, because it requires a sample of paper fromthe transformer insulation system, which is impossible toaccess during regular operation. The indirect measure ofthe insulating paper decomposition products dissolved in oil,such as carbon dioxide and carbon monoxide (CO2 and CO)[26], [27], methanol [22], [28], furfural (furan-2-carbaldehyde,2-FAL) and related furans is generally preferred. WhileCO and CO2 can be generated by decomposition of the oilduring long term oxidation too, methanol, furfural and furansare only formed by the processes of thermal degradation of thecellulose [22], [26], [29]. Their concentration in transformeroil is strictly correlated to the degree of polymerization of thepaper [22], [26], [27], as for instance in the case of Kraft insu-lating paper. Although methanol has been widely demonstratedto be very promising as a marker of ageing [22], [28], [29],currently the 2-FAL concentration is used worldwide as oneof the main indicators to estimate the ageing of the paper in atransformer [22], [26]. As the presence of furan compounds inoil is not related to the degradation of the oil itself, they canbe used as chemical markers in transformer insulating oil toassess the overall degree of polymerization with a high degreeof confidence [26], [30]. Currently, common practice consistsof periodic oil sampling from in-service transformers and inanalysis usually performed in a laboratory remote from thesampling site. It is carried out usually by chromatographicmethods, such as for instance by high pressure liquid chro-matography (HPLC) according to the IEC 61198 method [31].This technique is quite complex, requiring the extraction of the

Fig. 1. Steps to realize the optical sensor platform.

substances of interest from the sample before the injection onthe chromatographic column, the use of expensive equipmentand specialized operators both for acquisition and for dataanalysis, and a long time period for obtaining the results.

Dibenzyl disulfide (DBDS) too is an important analyte inthe control of transformer oil since it is commonly addedto the transformer oils as an antioxidant. At the same timeit is responsible for the corrosive properties of the oil,even at relatively low concentration [32]–[35]. Similarly to2-FAL, the determination of its level in transformer oils is ofparamount importance for diagnostic purposes to monitor the“health status” of the transformer, and a number of analyticalmethods for its determination are described in the litera-ture [34], but not any sensor has been previously proposed,despite of the fact that this would be of interest for in situdetermination.

The cases of 2-FAL and DBDS determination are here usedas a proof of principle for the realization of a multichanneloptical chemical sensor for the on-line control of the trans-former oil, with the required detection limits and dynamicdetection range.

III. THE SENSORS SYSTEM

A. SPR Sensors in a D-Shaped POF

We used a plastic optical fiber with a PMMA coreof 980 μm and a fluorinated polymer cladding of 20μm.On the basis of experimental results, the configuration basedon a large diameter fiber is better in terms of sensitivity andresolution but not in terms of signal-to-noise ratio, SNR [36].

The optical sensor manufacturing implies the followingthree simple steps (see Fig. 1):

a) removal of the cladding of a plastic optical fiber alonghalf the circumference of a short portion of the optical fiberembedded in a resin block;

b) spin coating an “optical buffer” (Microposit S1813 pho-toresist) on the exposed core of the fiber;

c) sputtering a thin gold film.The plastic optical fiber without jacket was firstly embedded

in a resin block for polishing procedure. This procedure wascarried out with a 5μm polishing paper in order to remove thecladding and part of the core of the fiber. After 20 complete

CENNAMO et al.: MARKERS DETECTION IN TRANSFORMER OIL BY PLASMONIC CHEMICAL SENSOR SYSTEM 7665

strokes with an “8-shaped” pattern in order to completelyexpose the core, a 1 μm polishing paper was used for another20 complete strokes with an “8-shaped” pattern. The finallength of sensing region was about 10 mm.

Successively the optical buffer (Microposit S1813 photore-sist) was spin coated on the sensing region. The samplewas spun at 6,000 rpm for 60 seconds to obtain a 1.5 μmfinal thickness of the photoresist buffer. It has been alreadyverified [37] that the POF-SPR sensor with a photoresist bufferlayer (with high refractive index) between the core of thePOF and the gold film exhibits better performance in termsof detectable refractive index range and SNR. The refractiveindex, in the visible range of interest, is about 1.49 forPMMA, 1.41 for fluorinated polymer and 1.61 for MicropositS1813 photoresist.

Finally, a thin gold film was sputtered by a sputteringmachine (Bal-Tec SCD 500), with a current of 60 mA.The sputtering process was repeated for different sensors,obtaining layers of different thickness characterized by a goodadhesion to the substrate, also verified by the sample resistanceto rinsing in de-ionized water.

Transduction methods based on SPR are common tools forsurface interaction analysis and chemical sensing, widely usedas a detection principle for sensors that operate in differentareas of chemical sensing as reported in several recent reviewpapers [16]–[18]. In these cases on the gold surface there is achemical layer (MIP) for the selective detection and analysisof the analyte.

In this work we have used two specific MIPslayers as receptors for the detection of 2-FAL and DBDS.The refractive index of the MIP, synthesized as previouslydescribed [16]–[18], is higher than 1.4 RIU, in the visiblerange of interest.

B. MIP Receptor For 2-FAL

The gold planar surface over the D-shaped POF (SPRactive surface) was washed with ethanol and then dried in athermostatic oven at 60 °C prior to deposition of the sensinglayer, a specific molecularly imprinted polymer (MIP) layer.

The prepolymeric mixture for MIP was prepared accordingto the classical procedure reported in [17] with only slightmodifications. Divinylbenzene (DVB), the cross-linker, wasalso the solvent in which the functional monomer (that is,methacrylic acid, MAA), and the template, furfural (2-FAL)are dissolved. The reagents were at molar ratio 1 (2-FAL):4 (MAA):40 (DVB). For example, a typical prepolymer mix-ture for the MIP specific for 2-FAL is composed of 20 μLof furfural, 80 μL of MAA and 1.4 mL of DVB. Notice thatDVB is at the same time the cross linker and the solvent.The mixture was uniformly dispersed by sonication (visu-ally homogeneous solution) and de-aerated with nitrogen for10 min. Then the radical initiator AIBN (16 mg in theexample described) was added to the mixture. Fifty μL ofthe prepolymeric mixture were dropped over the sensingregion of the optical fiber and spun for 45 s at 700 rpm.Thermal polymerization was then carried out for 16 h at70 °C. The template was extracted by repeated washings with96% ethanol.

Fig. 2. Cross-section view of the chemical sensor.

Fig. 3. Experimental setup.

A schematic cross section view of the sensor with thereceptor layer is shown in Figure 2.

In particular, the template molecule was extracted by wash-ing with ethanol 10 times, leaving the imprinted sites free forsuccessive template rebinding.

C. MIP Receptor for DBDS

The same method previously proposed for 2-FAl was usedfor the DBDS sensor. Briefly, the polymeric film was preparedby dropping of prepolymeric mixture over the gold layer ofthe optical sensor platform (washed with ethanol beforehand).It was then spun and polymerized and the template moleculewas extracted by washing with ethanol.

The prepolymeric mixture for this MIP had the followingcomposition: DBDS as template (20 mg), MAA as functionalmonomer (30 μl), DVB as cross-linker (465 μl) and AIBN asthe radicalic initiator (15 mg). The reagents were at molar ratio1 (DBDS): 4 (MAA): 40 (DVB). The mixture was uniformlydispersed by sonication (visually homogeneous solution) andde-aerated with nitrogen for 10 min.

D. Experimental Setup

The experimental measurements for the characterizationof the SPR-POF sensors have been previously carried out indifferent ways, i.e. spectral and amplitude mode. In this workwe have used a particular setup based on the spectral modeconfiguration. It consists of a halogen lamp, a beamsplitter (50/50) illuminating simultaneously the twooptical chemical sensors (with different MIP), and twoidentical spectrometers (see Fig. 3). The here used halogenlamp (HL–2000–LL, Ocean Optics) exhibits a wavelengthemission range from 360 nm to 1700 nm, while the spectrum

7666 IEEE SENSORS JOURNAL, VOL. 16, NO. 21, NOVEMBER 1, 2016

analyzer detection range was from 330 nm to 1100 nm. Thespectral resolution of the spectrometer (USB2000+UV–VISspectrometer, Ocean Optics) was 1.5 nm (δλDR). Twospectrometers were finally connected to a computer.

The SPR spectra along with data values were displayedonline on the computer screen and saved with the help ofadvanced software provided by Ocean Optics. The transmis-sion spectra have been normalized to the spectra obtained in airbefore MIP deposition, in which not any plasmon resonanceis excited, due to the refractive index range for which theplatform here described is suitable [16]–[18].

As we are just interested in the resonance wavelengthand the FWHM (full width at half maximum) whatever theintensity, the normalized transmission spectra are convenientlyanalyzed exploiting Matlab’s built-in functions to extract therequired values.

E. Experimental Procedure

The SPR transmission spectra are obtained by dropping20 μl of the appropriate solution (oil with analytes) on theplatforms with MIPs. The solvent was a mineral oil for elec-trical transformer (Nytro Libra), containing or not containingthe analytes. All the solutions are stored at a temperatureof 25°C. Steady state wavelength shift was obtained after5 minutes incubation. The platform was rapidly washed withhexane between successive measurements. Each experimentalvalue is the average of 5 subsequent measurements (saved after5 minutes incubation) with the respective standard deviations.

The possibility of performing measurement in a particu-larly convenient way for practical application, i.e. in a fewμl drop, is offered by the shape of the optical platformhere proposed, which presents a flat surface and can beeasily maintained in a horizontal position. In addition, thisSPR-POF sensor platform could be, if necessary, integratedinto a thermo-stabilized flow cell developed for bio-chemicalsensing applications [38]. The temperature sensitivity ofthe device is particularly important when the platform isexploited to monitor bio-chemical receptors undergoing bind-ing processes because the kinetic of the binding processes hasto be taken into account to perform reliable measurements andit is a function of the temperature.

IV. SPR SENSOR PARAMETERS

In SPR sensors with spectral interrogation, the resonancewavelength (λres) is determined by the refractive index of thesensing layer (ns). If the refractive index of the sensing layeris altered by δns, the resonance wavelength shifts by δλres.The sensitivity (Sn) of an SPR sensor with spectral interroga-tion is defined as [39]–[41]:

Sns =δλres

δns

[ nmRIU

](1)

For particular bio-chemical optical sensor, the sensitivity ismore conveniently defined as [11]–[18], [42]:

S = δλresδCanalyte

[nmM

](2)

In other words, this sensitivity can be defined by calculatingthe shift in resonance wavelength per unit change in analyteconcentration.

Owing to the fact that the vast majority of the field of theSurface Plasmon Wave (SPW) is concentrated in the dielectric,the propagation constant of the SPW is extremely sensitiveto changes in the refractive index of the dielectric itself. Thisproperty of SPW is the underlying physical principle of affinitySPR bio/chemical sensors. In the case of artificial receptors,as molecular imprinted polymers (MIPs), the polymeric filmon the surface of metal selectively recognizes and captures theanalyte present in a liquid sample so producing a local increasein the refractive index at the metal surface. The refractive indexincrease gives rise to an increase in the propagation constantof SPW propagating along the metal surface which can beaccurately measured by optical means. The magnitude of thechange in the propagation constant of an SPW depends on therefractive index change and the overlap with the SPW field.In the MIP layer, the SPW propagation is directly influencedby the refractive index change induced by the analyte binding.

The resolution (�n) of the SPR-based optical sensor can bedefined as the minimum amount of change in refractive indexdetectable by the sensor.

The bio-chemical sensing applications require an SPRsensor’s resolution value (�n) of around (10−4 – 10−3)RIU [11]–[15], [42].

With spectral interrogation, this parameter definitelydepends on the spectral resolution (δλDR) of the spectrom-eter used to measure the resonance wavelength in a sensorscheme. Therefore, if there is a shift of δλres in the resonancewavelength corresponding to a refractive index change of δns,then resolution can be defined as [39]–[41]:

�n = δnsδλres

δλD R [RIU ] (3)

The signal-to-noise ratio (SNR) of the SPR sensor withspectral interrogation is defined as [39]–[41]:

SN R(ns ) =(

δλres

δλSW

)

ns

(4)

where δλSW is calculated as the variation of the full width athalf maximum (FWHM) of the SPR curve for the same wave-length variation. SNR is a dimensionless parameter stronglydependent on the refractive index (ns).

V. EXPERIMENTAL RESULTS

A. Detection of the Markers 2-FAL and DBDS in Oil

Measurements were simultaneously carried out on twoSPR-POF-MIP optical chemical sensors based on differentMIP receptors, one for dibenzyl disulfide (DBDS) and one spe-cific for furfural (2-FAL), however with the same composition(see Fig. 3). The two sensors were prepared in the same way,with equal apparent thickness of Au layer (60 nm) and MIPlayer (1.5 μm). Figure 4 and Figure 6 show typical experi-mental spectra of 2-FAL and DBDS detection in transformeroil samples (Nytro Libre), respectively.

In Figure 4 are reported the SPR spectra obtained withdifferent concentrations of 2-FAL in oil. Two main SPR bands

CENNAMO et al.: MARKERS DETECTION IN TRANSFORMER OIL BY PLASMONIC CHEMICAL SENSOR SYSTEM 7667

Fig. 4. SPR transmission spectra for different 2-FAL concentrations (ppm)in oil transformer. Optical platform with gold layer 60 nm thick. Inset: zoomof resonance wavelengths.

Fig. 5. Plasmon resonance wavelength variation vs 2-FAL concentra-tion (ppm). Optical platform with gold layer 60 nm thick.

at around 520 nm and around 710 nm are evidenced, respec-tively, whose position depends on the analyte concentration.The first resonance (around 520 nm) is blue-shifted, while thesecond one (around 710 nm) is red-shifted with the concen-tration of analyte.

In fact, they clearly show that the resonance wavelength isshifted to higher values (red shifted) or lower values (blueshifted) when the 2-FAL concentration increases, indicatingthat 2-FAL effectively combines with MIP from the oil matrix.

The reason for the presence of the blue-shifted resonancecan be ascribed to the excitation of a localized surface plasmonresonance (LSPR) due to the gold nanostructures present atthe gold layer surface. In fact, the photoresist layer, apartfrom improving the adhesion of the gold film, produces somenanostructures on the surface.

In Figure 5 the wavelength shifts at increasing concentrationof 2-FAL are reported for the two dips, obtained in cases ofred and blue shifted resonance, evaluated by Matlab software.

Each experimental value is the average of 5 subsequentmeasurements and the respective standard deviations (errorbars) are also shown.

The transmission spectra of MIP for DBDS on anSPR sensor with 60 nm thick gold layer, obtained atdifferent concentrations of DBDS (ppm), are reported inFigure 6. These spectra are similar to the spectra of MIPfor 2-FAL (see Fig. 4) if the resonance wavelengths areconsidered.

In particular, Figure 6 shows the absorption spectra of

Fig. 6. SPR transmission spectra for different DBDS concentrations (ppm)in oil transformer. Optical platform with 60 nm of gold film. Inset: zoom ofresonance wavelengths.

Fig. 7. Plasmon resonance wavelength variation vs log of concentration ofDBDS (ppm). Optical platform with gold layer 60 nm thick.

DBDS sensor in the presence of different concentrationsof DBDS (ppm). Two insets of Figure 6 are enlargementsof the corresponding spectral regions. The first resonance(around 520 nm) is blue-shifted, while the second one (around710 nm) is red-shifted with the concentration of analyte.

This phenomenon is the same observed in similarPOF-MIP sensors used for 2-FAL. Figure 7 shows the dose-response curves for DBDS, obtained with the red and blueshift resonances. The blue-shifted resonance is the samein both the platforms, because it is ascribable to LSPRphenomenon (nanostructures on the surface), whereas the red-shifted resonance is a function of the gold layer thickness.When the gold film thickness changes, the red-shifted reso-nance characteristics (FWHM and resonance wavelength shift)change, too.

Table 1 summarizes some experimental results obtained,considering the red and blue shift.

Definitely, for the detection of 2-FAL and DBDSmarkers, both the resonances (blue and red shift) can be used,as reported in Table 1.

B. Selectivity Test

For both the MIPs used in this work, we have used differentanalytes for the testing of selectivity. There isn’t any aspecificadsorption of the considered substances on the MIP, becausenot any shift of resonance wavelengths is seen, althoughrelatively high concentrations are present.

For example, in the detection of DBDS, furfural (2-FAL)and diphenyl disulfide (DPDS) are impurities likely present inused transformer oils, and for this reason they are considered

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TABLE I

DETECTION OF DIAGNOSTIC MARKERS IN OIL TRANSFORMER

as possible interfering substances. The two considered inter-fering substances did not give any significant wavelength shiftwhen tested with the MIP-POF sensor developed for DBDS,thus indicating that neither of these interferents were adsorbedat the imprinted sites.

C. Influence of the Gold Film Thicknesson the Sensor Performances

In the case of the sensor for DBDS in oil, we havetested two optical sensors platform, with different gold filmthickness (30 nm and 60 nm), for the detection of DBDSin oil.

It is well known that the thickness of the gold layer has astrong effect on the transmission spectra, since these dependon the interaction of the evanescent wave with the plasmonsat the interface with the dielectric over the gold layer [39].

The POF platform configuration with 60 nm thick gold layeris generally preferred for biosensing application in aqueousmedia to others with lower gold thickness, since it has beendemonstrated that the peak resolution is much better and thesensitivity is high [39], even if the resonance wavelength isshifted to much higher values. This could be a problem whenthe upper limit of the detectable wavelength is reached. In fact,at the high refractive index of the MIP layer (higher than1.4 RIU), some relevant SPR resonances can be shifted up tothe upper limit of the spectral detection range or, even, madeto overlap to the typical absorption band (950nm) of plasticoptical fiber in PMMA. The main challenge to obtain a widedynamic range of detection and a high signal to noise ratiois to optimize the wavelength at which the surface plasmonsare excited.

Two resonance peaks are present in the platform with 60 nmgold layer, at around 520 nm and 710 nm, when the refractiveindex of the sensing layer is high, for example greater than1.4 RIU, as it is in the cases of MIPs for 2-FAL and DBDS.

In the case of the sensor platform with gold layer 30 nmthick, only one resonance wavelength at around 610 nm isseen [43]. It is shifted to higher values (red shifted) whenthe DBDS concentration increases [43], similarly to that ataround 710 nm for the sensor with 60 nm thick gold layer.In this case the red shifted SPR resonance is located in themiddle of spectral region of detection, at about 610 nm, thusguaranteeing high signal to noise ratio (in terms of full widthat half maximum, as defined in equation 4). For this DBDS

sensor the deep absorptions at around 500 nm seem to be notwell defined.

The dose-response curve obtained with the sensor basedon 30 nm thick gold layer has a higher sensitivity [43] thanthat of the red shift at the sensor with 60 nm thick goldlayer, as defined in equation 2, but the chemical parameters ofinterest in the detection of DBDS (Kaff = 3.47 · 106 (M−1);Sens at low conc = 1.24 · 107(nm · M−1); LOD = 2.94 ·10−8(M)) are similar to those indicated in Table 1 obtainedwith 60 nm thick gold layer.

These last experimental results are very important andinstrumental to the development of a new experimental setupfor multichannel operation. In the future, we will explore thepossibility to obtain a multichannel sensor system based on acascaded configuration of devices with different thicknesses ofthe gold film. In this configuration it will be possible to useonly one spectrometer, so realizing a low cost multichanneloptical sensor system.

VI. CONCLUSIONS

Experimental results on a multichannel chemical SPR sen-sor, for the detection of two markers (DBDS and 2-FAL)directly in transformer oil, have demonstrated an attractivesensor system that has industrial applications. It has beenshown that POF-MIP sensor platform based on a gold film60 nm thick can be used to monitor the refractive indexvariation of an MIP receptor layer in relation to the amountof absorbed analyte, directly in oil power transformer, takingadvantage of the investigation of a red or a blue shiftedresonance. When the optical platform is based on a differentgold film thickness (30 nm) only one SPR resonance (red-shifted) is present at about 610 nm, i.e. in the middle ofthe wavelength range. In the case of the sensor for DBDS,actually the sensitivity is almost the double for the 30 nmthick gold layer, and the LOD is accordingly lower. In all thecases a problem is the relatively small dynamic range, whichis extended only to about 1 ppm for the two analytes.

In the future, we will develop a larger number of sensors,based on the same sensor array configuration or a cascadedconfiguration (exploiting different thicknesses of the goldfilm or different metals to achieve the SPR), for the detectionof different analytes.

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[43] N. Cennamo, L. De Maria, M. Pesavento, A. Profumo, C. Chemelli, andL. Zeni, “Surface plasmon resonance in a D-shaped plastic optical fibre:Influence of gold layer thickness in monitoring molecularly imprintedpolymers,” in Proc. IEEE Sensors Appl. Symp. (SAS), Catania, Italy,Apr. 2016, pp. 1–5.

N. Cennamo was born in Italy in 1975. He receivedthe degree in electronic engineering and the Ph.D.degree in electronic engineering from the SecondUniversity of Naples, Naples, Italy, in 2002 and2005, respectively.

He holds a post-doctoral position with the Depart-ment of Industrial and Information Engineering,Second University of Naples. His research inter-ests include the design and fabrication of opticalsensors, biosensors, chemical sensors, and optoelec-tronic devices.

L. De Maria received the M.S. (summa cum laude)degree in physics from the University of Milan,Italy, in 1985. She was with CISE S.p.A, Milan,where she was involved in research and developmentof coherent optical fiber sensors and spectroscopictechniques for industrial applications. she is cur-rently with RSE S.p.A, where she is involved ininnovative optical sensors for monitoring electricalequipment of the transmission and distribution net-work.

7670 IEEE SENSORS JOURNAL, VOL. 16, NO. 21, NOVEMBER 1, 2016

C. Chemelli received the degree in physics in 1982,and the Ph.D. degree in experimental physics in1988. Her principal professional experience devel-oped in the field of material science, specifically inmaterial manufacturing, processing and characteriza-tion by means of advanced spectroscopic and micro-scopic techniques. Current research interests are oninnovative materials and thin films preparation andcharacterization for energy and sensors applications.

A. Profumo received the M.S. degree in chemistryfrom the University of Pavia in 1981. She has a per-manent position with the University of Pavia since1983, where she is currently a Full Professor of Ana-lytical Chemistry. She runs scientific collaborationswith public and private foundations that refer to theDepartment for Analytical Guidance, in particular,in the sector of natural spring and drinking watersand in the environmental field as far as concernsemissions of inorganic and organic pollutants fromindustrial plants. She has co-authored 145 reviewed

papers and 200 communications at national and international conferences.Her works concentrates on development and application of selected analyticalprocedures for pollutants determinations, in particular her recent researchesconcern the trace determination of emerging pollutants in environmentalmatrices (river and ditch water, agricultural soil, compost) and developmentof procedures for efficient abatement, and application of chemically modifiedelectrodes for analytical determinations.

L. Zeni received the degree (summa cum laude)in electronic engineering and the Ph.D. degree inelectronics and computer science from the Uni-versity of Naples, in 1988 and 1992, respectively.From 2001 to 2012, he was the Vice-Director ofthe Department of Information Engineering. He waswith TU-DELFT, The Netherlands, as a VisitingScientist. He has been the National Co-Ordinatorof PRIN projects, the Scientific Co-Ordinator ofresearch contracts with public and private institu-tions and responsible for projects funded within the

7th FP of the EU. He has been a member of the Management Committeeof the COST 299 Optical Fibers for New Challenges Facing the InformationSociety and the COST TD1001 Novel and Reliable Optical Fiber SensorSystems for Future Security and Safety Applications. He is currently aFull Professor of Electronics with the Second University of Naples and thePresident of the Research Consortium on Advanced Remote Sensing Systemswith CO.RI.S.T.A. His main research interests include optical fiber sensorsfor distributed measurements of deformation and temperature, optoelectronicintegrated sensors and biosensors. He has authored 120 papers in internationaljournals, 120 publications at international conferences, and holds ten patents.He is also the Founder of the spin-off company OPTOSENSING dealing withstructural and environmental monitoring by optical fiber sensors.

M. Pesavento received the degree in chemistry in1972. Since 1994, she has been a Professor ofAnalytical Chemistry, first with Milano University,Como, Italy, and then with Pavia University, Italy.Her scientific interests are in the field of chemicalequilibria, speciation analysis, adsorption equilib-ria and separation by distribution between phases,chemosensors with biomimetical receptors. She hasauthored over 130 original scientific publications.

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