A Nanoscale Optical Biosensor:  The Long Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles

Abstract

The elucidation of the long range distance dependence of the localized surface plasmon resonance (LSPR) of surface-confined noble metal nanoparticles is the aim of this work. It was suspected that the linear distance dependence found in CH3(CH2) x SH self-assembled monolayer (SAM) formation was the thin shell limit of a longer range, nonlinear dependence. To verify this, multilayer SAM shells based on the interaction of HOOC(CH2)10SH and Cu2+ were assembled onto surface-confined noble metal nanoparticles and were monitored using UV−visible spectroscopy. Measurement of the LSPR extinction peak shift versus number of layers and adsorbate thickness is nonlinear and has a sensing range that is dependent on the composition, shape, in-plane width, and out-of-plane height of the nanoparticles. Theoretical calculations based on an accurate electrodynamics description of the metal nanoparticle plus surrounding layered material indicate plasmon resonance wavelength shifts that are in excellent agreement with the measurements. The calculations show that the sensing range is determined by falloff of the average induced electric field around the nanoparticle. This detailed set of experiments coupled with an excellent theory versus experiment comparison prove that the sensing capabilities of noble metal nanoparticles can be size tuned to match the dimensions of biological and chemical analytes by adjusting the aforementioned properties. The optimization of the LSPR nanosensor for a specific analyte will significantly improve an already sensitive nanoparticle-based sensor.

Localized Surface Plasmon Resonance Nanosensor: A High-Resolution Distance-Dependence Study Using Atomic Layer Deposition

“…Several interesting characteristics of the long-range behavior were found including (1) the LSPR shift vs SAM thickness is nonlinear; (2) Ag nanoparticles are more sensitive than Au nanoparticles; (3) nanotriangles have larger sensing distances than nanohemispheroids; (4) increasing the nanoparticle in-plane width results in larger sensing distances; (5) decreasing nanoparticle out-ofplane height results in larger sensing distances. 7 Semiquantitative theoretical calculations revealed that the plasmon resonance shift is controlled by the average electromagnetic field over the nanoparticle surface. 7 Similarly, the short-range distance dependence (0-3 nm) of the LSPR nanosensor has been studied using alkanethiol, CH 3 -(CH 2 ) x SH (x ) 2-11, 13-15, and 17), monolayers.…”

“…7 Semiquantitative theoretical calculations revealed that the plasmon resonance shift is controlled by the average electromagnetic field over the nanoparticle surface. 7 Similarly, the short-range distance dependence (0-3 nm) of the LSPR nanosensor has been studied using alkanethiol, CH 3 -(CH 2 ) x SH (x ) 2-11, 13-15, and 17), monolayers. 8,9 It was found that eq 1 does a remarkably good job of accounting for the short-range LSPR response if one assumes a value l d ) 5-6 nm.…”

“…Furthermore, while the refractive index of the bulk SAM molecule is known, the refractive index of the SAM is not known making an accurate theoretical model of the experiment difficult. 7 Atomic layer deposition (ALD) is just such a fabrication method that produces highly uniform and controlled thin films. Precursor gases are alternately pulsed through the reactor and purged away resulting in a self-limiting growth process that constructs a film one monolayer at a time.…”

“…11,12 Depositing Al 2 O 3 multilayers onto the noble metal nanosensor should allow the long-range distance dependence of the LSPR nanosensor to be probed with 10x spatial resolution compared with previous work. 7 In this paper, we have utilized ALD of Al 2 O 3 to probe both the long-and short-range LSPR distance dependences of Ag nanoparticles in one integrated experiment. The following results are presented: (1) A detailed study of the long-range distance dependence of the LSPR sensor with 10x increased spatial resolution afforded by ALD in comparison with earlier experiments with SAMs; (2) LSPR nanosensors are shown to have single Al 2 O 3 layer detection capabilities; (3) at short range, triangular nanoparticles with fixed widths and smaller out-ofplane heights have larger LSPR sensing distances.…”

Solution-Phase, Triangular Ag Nanotriangles Fabricated by Nanosphere Lithography

“…From a practical perspective, the tunable optical properties of nanostructures make it possible to use these materials in surfaceenhanced spectroscopy, [10][11][12][13][14] optical filters, 15,16 plasmonic devices, [17][18][19][20] and sensors. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Until recently, there were only limited methods for the synthesis of monodisperse, nonspherical noble metal nanoparticles, which meant that the practical applications of these particles were limited. Recent advances in controlled syntheses of spheres, 36,37 rods, [38][39][40] triangular prisms, [41][42][43] disks, [44][45][46] cubes, 47 and branched nanocrystals 48 have improved both the fundamental understanding and practical use of these nanoparticles for sensing and spectroscopic devices.…”