Please cite this article in press as: S.K. Srivastava, B.D. Gupta, Inﬂuence of ions on the surface plasmon resonance spectrum of a ﬁber optic refractive index sensor, Sens. Actuators B: Chem. (2011), doi:10.1016/j.snb.2011.01.068 ARTICLE IN PRESS G Model SNB-12870; No. of Pages 4 Sensors and Actuators B xxx (2011) xxx–xxx Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Inﬂuence of ions on the surface plasmon resonance spectrum of a ﬁber optic refractive index sensor Sachin K. Srivastava, Banshi D. Gupta ∗ Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India article info Article history: Received 31 October 2010 Received in revised form 28 January 2011 Accepted 30 January 2011 Available online xxx Keywords: Surface plasmon resonance Fiber optic sensor Ion concentration Refractive index abstract Surface plasmon resonance (SPR) spectra of a ﬁber optic SPR probe coated with thin gold layer have been studied experimentally for four ionic (NaCl, KCl, Na 2 SO 4 and MgCl 2 ) and two non-ionic (sucrose and urea) liquid samples of different refractive indices. For the same value of refractive index, the response curves for ionic and non-ionic liquid samples have been found to differ, implying the inﬂuence of ions on the resonance wavelength of the SPR spectrum. It is observed that the resonance wavelength, for a given refractive index of the liquid, is higher for ionic sample than that in the case of non-ionic one. This extra increase in resonance wavelength is attributed to the interaction of ions with the free electrons of the metal ﬁlm. The study ﬁnds importance in situations where ions get generated during the processes. In such studies, the effect of ions on SPR spectrum must be incorporated otherwise the SPR spectrum may get wrongly interpreted. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In recent years, surface plasmon resonance technique and its usefulness in various applications have got wide attention of many researchers [1–3]. Surface plasmons are the coherent oscillations of conduction electrons at a metal–dielectric interface [4]. These are transverse magnetic (TM) surface waves and the ﬁeld associ- ated to them decays exponentially in the metal as well as in the dielectric medium. Due to their TM character, surface plasmons can be excited by p-polarized light. Kretschmann conﬁguration is generally employed to excite surface plasmons. In this conﬁgura- tion, the excitation of surface plasmons is carried out by means of evanescent ﬁeld of p-polarized light incident through a high index prism on the metal–dielectric interface at the base of the prism. When the angle of incidence of the beam is greater than or equal to the critical angle it gets total internally reﬂected (TIR) from the base of the prism resulting in the production of evanes- cent wave which propagates along the interface as does the surface plasmon wave. The surface plasmons are excited when the wave vectors of these two waves become equal. For a given refractive index of the dielectric medium, the two become equal for a par- ticular value of the angle of incidence called resonance angle. The resonance angle changes with the change in the refractive index of the dielectric medium in contact of the metal ﬁlm. This prop- ∗ Corresponding author. E-mail addresses: banshigupta@yahoo.co.in, bdgupta@physics.iitd.ernet.in (B.D. Gupta). erty of surface plasmons is widely used in sensing applications [1,5]. The prism based SPR sensing device has a number of short- comings which can be removed by replacing the prism with an optical ﬁber [6–8]. In a SPR based ﬁber optic sensor, a small por- tion of the cladding is removed from the middle portion of the ﬁber and is coated with a thin layer of metal. Light from a poly- chromatic source is launched in the ﬁber probe. The evanescent wave at the core–metal layer excites the surface plasmons at the metal–dielectric interface. The resonance occurs at a particular wavelength which is determined from the spectrum recorded at the other end of the ﬁber. The resonance wavelength ( res ) depends on the refractive index of the medium around the metal layer. Numerous efforts have been made to increase the sensitivity of these sensors [9]. In sensing, the use of SPR technique is limited to only one type of sensor speciﬁc species, either ionic or non- ionic [6,10–14] and hence no study has reported the effect of ions on surface plasmons and hence on the response curve of the sen- sor. In the present study, we report the effect of ionic and non- ionic solutions on the transmission spectrum of a ﬁber optic SPR probe. It is observed that, for solutions of the same refractive index,  res for ionic solution is larger than that for non-ionic one. Thus the calibration curve of the sensor for non-ionic sam- ples cannot be used for ionic samples. Further, it differs from one ionic to another ionic sample. Thus, the study becomes more important when the ions get generated in the sample during chemical reactions. In such studies, the effect of ions must be incorporated as a correction term otherwise the results may get misinterpreted. 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.068