Tunable plasmonic properties of silver nanorods for nanosensing applications Jagmeet Singh Sekhon • S. S. Verma Received: 9 June 2011 / Accepted: 20 September 2011 / Published online: 5 October 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Localized surface plasmon resonance (LSPR) sensitivity to the surrounding medium refractive index has been studied for silver nanorods using Gans theory including the effect of retardation and surface scattering. The simulation results show the refractive index sensitivity (eV/RIU) maxima positions at width of 9, 6, and 4 nm for aspect ratios of 2, 3, and 4, respectively. Based on the sensing figure of merit (FOM), 9 nm is found to be a sig- nificant nanorod width, where the FOM dependence on width with respect to aspect ratio inverts. However, the optimal nanorod width for both the FOM and the modified figure of merit (MFOM) is about 6 nm for aspect ratios of 2, 3, and 4. A comparison with gold shows that silver nanorods exhibit relatively higher FOM and MFOM and thus, making them potential candidates for biochemical nanosensing applications. Introduction Silver and gold nanoparticles have attracted significant attention for their fascinating plasmonic properties, which originate from the coherent oscillation of their conduction electrons in response to the irradiated light [1]. The excited oscillations thus confined by the nanoparticle are referred to as localized surface plasmon resonance (LSPR). Plas- monic nanorods in this regard are of particular interest due to their easy fabrication methods [2–8], strong scattering efficiency [9], low plasmon damping [10], and physically powerful local medium refractive index sensitivity [11–19]. Easy synthesis allows one to prepare nanorods of different widths and aspect ratios in different surrounding mediums. These nanorods often have LSPR that can be tuned from visible to near IR region [8, 11, 20, 21]. Thus, the LSPR sensitivity of nanoparticles to the local medium refractive index offers the possibility of nanoscale chemical and bio- logical sensors [13, 19, 22–26]. Much of the recent attention with metal nanoparticles has been concerned with their use as small volume ultrasensitive sensors. A number of efforts have been made to describe the optimum refractive index sensitivity of noble metal nano- particles of various shapes and sizes [11–19, 22–25]. How- ever, it has been noticed that not only the sensitivity of LSPR matters for sensing applications but also the resonant line width (full width at half maxima, FWHM), which critically depends on the nanoparticle shape and size [7, 13, 18]. FWHM affects the ease of detection and for that very reason, the sensing figure of merit (FOM), i.e., the ratio of refractive index sensitivity to resonant line width, has been proposed to com- pare the overall performance of nanosensors [7]. The resonant line width is also a function of local medium refractive index [7, 19]; hence, a modified figure of merit (MFOM), defined as ratio of refractive index sensitivity to the slope of variation in resonant line width with respect to local medium refractive index, has been proposed [19]. The sensing parameters, viz. refractive index sensitivity, FOM, and MFOM, are thus the key factors to determine the detection efficiency of metal nanoparticles LSPR-based sensors and to realize their prac- tical applications. Further improvement towards the refractive index sensitivity of plasmonic nanoparticles requires the identification of decisive nanostructure and type of material. Such a fundamental understanding may allow for judicious design and selection of noble metal nanoparticles with supe- rior refractive index sensitivity. J. S. Sekhon (&)  S. S. Verma Department of Physics, Sant Longowal Institute of Engineering and Technology (SLIET), Longowal, Sangrur, Punjab 148-106, India e-mail: jagmeetsekhon@ymail.com 123 J Mater Sci (2012) 47:1930–1937 DOI 10.1007/s10853-011-5983-9