Elsevier

Sensors and Actuators B: Chemical

Volume 188, November 2013, Pages 221-226
Sensors and Actuators B: Chemical

Sensors based on surface plasmon resonance in a plastic optical fiber for the detection of trinitrotoluene

https://doi.org/10.1016/j.snb.2013.07.005Get rights and content

Abstract

The objective of this study is to present an optical chemical sensor based on surface plasmon resonance (SPR) in a plastic optical fiber (POF) for the selective detection and analysis of trinitrotoluene (TNT) in aqueous solution.

The fabricated optical chemical sensor was realized removing the cladding of a plastic optical fiber along half the circumference, spin coating on the exposed core a buffer of Microposit S1813 photoresist, and finally sputtering a thin gold film. A molecularly imprinted polymer (MIP) film was, then, deposited on the thin gold film to allow the selective detection of TNT. The film was a methacrylic acid-divinyl benzene copolymer obtained by radicalic polymerization at high temperature.

The experimental results show that the use of MIP layer is suitable for highly selective detection of TNT, down to about 50 μM (i.e. ∼11 μg/ml). This shows that the direct detection of a low molecular mass substance is possible by SPR transduction, even at low concentration. Furthermore, the proposed sensing head is low cost and relatively easy to realize, and may be very attractive for the detection and analysis of trinitrotoluene (TNT), in particular considering that it does not involve the use of any electrical devices and it can be used for remote/online monitoring exploiting, f.i., a fiber optic link.

Introduction

The detection of explosives in aqueous samples is highly required for toxicological and environmental reasons. Besides classical approaches, typically based on chromatographic techniques, sensor methods have been proposed for the detection of such substances [1], [2], [3], [4] mainly because of the need of on site and remote detection. For example, electrochemical transduction has been widely used [13], even if for different reasons it can be not completely suited for remote sensing. To this aim, useful transduction methods can be those based on surface plasmon resonance (SPR) on optical fiber [5], [6], [7].

SPR is a very sensitive technique for determining small refractive index changes at the interface between a metallic layer with a fixed receptor and a dielectric medium (aqueous medium with analyte). Thus SPR is a common tool for surface interaction analysis and biosensing, widely used as a detection principle for sensors that operate in different areas of bio and chemical sensing as reported in several recent review papers [8], [9], [10]. In the present investigation, SPR is applied as the transduction optical method to obtain a sensor for explosives with characteristics suitable for on line and remote sensing. As a proof of principle, 2,3,6-trinitrotoluene (TNT), an aromatic nitroderivative with low molecular mass, has been considered. SPR biosensors are commonly regarded as suitable for the analysis of medium or large molecular weight analytes, however several approaches have been proposed for the detection of low molecular mass substances [11], [12] too, mainly based on reactions which give reaction products inducing measurable refractive index changes upon binding on the surface of the optical device. Another aspect to be considered is that the use of an optical fiber, differently from the commercially available devices, makes the remote sensing straightforward, and may reduce the cost and dimension of the device, with the possibility of integration of SPR sensing platform with optoelectronic devices, eventually leading to “Lab-on-a-chip”.

In this regard, several configurations based on SPR in silica optical fiber for measuring refractive indexes ranging from 1.33 to 1.43 (refractive index of aqueous medium), can be found in the recent literature [10], [13]. Furthermore, investigations have also been devoted to plastic optical fibers (POF) as they represent an easier to handle platform with mechanical properties making it more resilient, cheaper and of safer use f.i. in in vivo applications. For example, we developed a new geometry for a low cost POF sensor system [14], [15] for bio-applications [16].

In the present work, the above configuration, is combined with a molecular imprinted polymer (MIP) as the recognition element. MIPs are synthetical receptors obtained by the molecular imprinting methods [17], [18], presenting a number of favorable aspects for sensing in comparison to bioreceptors such as, for example, antibodies [19], including a better stability out of the native environment, the reproducibility and the low cost. They are porous solids containing specific sites interacting with the molecule of interest according to a “key and lock” model. For this reason, a distinctive feature of MIPs, in comparison with other receptors, is the selectivity. In the case of the MIP for TNT here considered, it has been found to be very good, even for molecules of similar chemical structure [4], [20], [21]. However only a relatively little number of sensors have been found in the recent literature, in which MIPs are used in connection with SPR transduction for the detection of small molecules of biological and toxicological interest [13], [22], [23]. In no cases, explosives detection has been reported based on sensors of this kind.

The objective of this work is to exploit the POF based SPR sensor with an MIP layer, containing a relatively high density of recognition elements, for the detection of TNT. The optical properties of the polymer layer and their influence on the plasmonic resonance effect will be discussed as well.

Section snippets

Reagents

Divynilbenzene [1321-74-0] (DVB), 2,2′-azobisisobutyronitrile [78-67-1] (AIBN), 2,2-dichloro-N-[1,3-dihydroxy-1-4(4-nitrophenyl)propan-2-yl]acetamide [56-75-7] (CAP, chloramphenicol) were obtained from Sigma–Aldrich and used without any further purification. Methacrylic acid (MAA) [79-41-4] (Sigma–Aldrich cod. M0782) and divinylbenzene (DVB) [1321-74-0] (Sigma–Aldrich cod. 414565) were purified with molecular sieves (Sigma–Aldrich cod 208604) prior to use in order to remove stabilizers.

Results

In the present work a sensor set up much simpler than in previous investigations was tested for its applicability to the detection of an analyte of relatively low mass as TNT (MW = 227.13), and using a direct measure, instead of a competition method.

Sensor selectivity

A ‘control experiment’ was also carried out, in which the response of a sensor based on SPR in POF without MIP layer has been studied. In Fig. 5 the experimentally obtained SPR transmission spectra, normalized to the spectrum with air as the surrounding medium, for the sensor without MIP film, at different concentrations of TNT, are presented. It is evident that when the MIP film is not present, there is no shift of the resonance wavelength showing that there is not any adsorption of TNT on

Discussion and conclusions

The experimental procedures and the results, as described in the previous sections, are influenced by the optical properties of the MIP layer, in particular by its refractive index.

The refractive index of MIP layer spin coated on the gold layer can be estimated as the average of the bulk material one, i.e. 1.62 [27], since it is practically all composed of DVB with only a 10% in volume of MAA, and the air. From the measurement of the plasmon resonance wavelength when the air is the surrounding

Acknowledgments

The authors would like to thank the COST Action TD1001 Novel and Reliable Optical Fibre Sensor Systems for Future Security and Safety Applications (OFSeSa). The work was partially supported by POR Campania FSE 2007/2013 -“Embedded System”- and by the project “Development and support of multidisciplinary postdoctoral programmes in major technical areas of national strategy of Research-Development-Innovation” 4D-POSTDOC, contract no. POSDRU/89/1.5/S/52603, co-funded by the European Social Fund

N. Cennamo is a post-doc at the Department of Industrial and Information Engineering at Second University of Naples, Italy. His main interest deals with the design of advanced optical sensors.

References (27)

  • M. Iga et al.

    Gold thickness dependence of spr-based hetero-core structured optical fiber sensor

    Sensors and Actuators B: Chemical

    (2005)
  • R.G. Smith et al.

    A review of biosensors and biologically inspired systems for explosives detection

    Analyst

    (2008)
  • M. Cerruti et al.

    Polymer-oligopeptide composite coating for selective detection of explosives in water

    Analytical Chemistry

    (2009)
  • Cited by (0)

    N. Cennamo is a post-doc at the Department of Industrial and Information Engineering at Second University of Naples, Italy. His main interest deals with the design of advanced optical sensors.

    G. D’Agostino is a post-doc at the Department of Chemistry at University of Pavia, Italy. His main interest deals with specific synthetic receptors and sensors.

    R. Galatus is lecturer of Optical Integrated Circuits at the Technical University of Cluj-Napoca and post-doc in the 4D-POSTDOC/POSDRU/89/1.5/S/52603, European Social Fund project. Her research interests include the design and fabrication of optical biosensors and medical signal processing.

    L. Bibbò is a Ph.D. student at Second University of Naples, Naples, Italy. His main interests are focused on the optical sensors.

    M. Pesavento is full professor of analytical chemistry at the University of Pavia. Her research interests are in the field of environmental analytical chemistry, in particular in the development of specific sensors.

    L. Zeni is full professor of electronics at the Second University of Naples and president of the Research Consortium on Advanced Remote Sensing Systems – CO.RI.S.T.A., Naples, Italy. His research interests include the design and fabrication of optical fiber sensors and optoelectronic devices.

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