Hollow and Solid Metallic Nanoparticles in Sensing and in Nanocatalysis Mahmoud A. Mahmoud, Daniel O’Neil, and Mostafa A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ABSTRACT: When the size of a material is reduced to the nanoscale, at or below the characteristic length scale that determines their properties, the material acquires completely new properties. On this length, its characteristics become sensitive to further changes in size, shape, or whether they are hollow or solid. In this perspective article, we first discuss the different experimental techniques used in the synthesis, assembly, and handling of colloidal solid or hollow nanoparticles with single and double shells. This is then followed by comparing the experimental and theoretical (DDA and FDTD) results for solid and hollow plasmonic nanoparticles as sensors using two different methods. The first method compares the plasmonic enhancement of the radiative properties of molecules or materials (e.g., in surface enhanced Raman scattering, SERS). The second one is based on the amount of the plasmon peak wavelength shift of the nanoparticle in media with different dielectric functions. In the last section of the perspective, we present a summary of the difference between the solid and hollow nanoparticles in nanocatalysis. We present the results of a number of experiments showing that the superior catalytic properties of hollow nanoparticles are due to catalysis occurring within the cavity of the hollow nanoparticles. Finally, using a femtosecond optical technique, we show that adding a second shell of a stiff metal (like Pt or Pd) to the plasmonic hollow nanoparticles increases their mechanical stability. KEYWORDS: hollow nanoparticles, Plasmon field, nanocatalysis, nanoreactor, SERS, substrate effect, sensing, assembling I. INTRODUCTION Metallic nanoparticles have attracted attention because of their excellent optical, 1-3 catalytic, 4 and photothermal 2,5-9 proper- ties which have led to their exciting applications in such fields such as nanocatalysis, 10 nanosensing, 11-14 optical switch- ing, 15,16 magneto-plasmonic devices, 17,18 drug delivery, 19,20 and cancer diagnosis and treatment. 21-23 Many of the photonic and photothermal properties of plasmonic nanoparticles (primarily those of silver and gold) are derived from the strong plasmonic electromagnetic fields resulting from localized surface plasmon resonance (LSPR). LSPR is the coherent oscillation of the collective excitation of the nanoparticle’s electrons in the conduction band when excited by light of resonant frequency. This induces very strong surface electro- magnetic fields which are stronger than those of the exciting resonant light. Plasmonic nanoparticles can thus enhance the rates of linear optical processes like absorption, fluorescence, and Rayleigh or Raman processes as well as nonlinear processes like second harmonic generation (SESHG), 24 sum frequency generation (SFG) vibrational spectroscopy, 25 and surface enhanced fluorescence. 26 As has been observed for 40 years, plasmon fields can enhance Raman signals by up to 10 6 by the process of surface-enhanced Raman spectroscopy (SERS). The combined plasmon fields in-between the assembled plasmonic nanoparticles are very high (hot spots) due to the coupling between the electromagnetic fields resulting from the LSPR of the conduction band electrons of the individual nanoparticles in the aggregates. If the analyte molecule is located in this region the Raman scattered light can be enhanced by a factor as large as 10 14 . 27 Fluorescence can also be enhanced as long as the distance or energy level overlap minimizes electron transfer quenching processes of the fluorophore. 28 Rayleigh scattering, as used in dark-field imaging for medical diagnosis and surface imaging, is also enhanced by the plasmonic field of the silver and gold nanoparticles. 21 The plasmon field also enhances the rates of nonradiative processes such as (1) the nonradiative electronic relaxation in semiconductor-plasmonic metallic nanoparticles, 29 (2) the nonradiative exciton-exciton annihilation processes in con- jugated polymers on silver nanocubes (AgNCs) (this can be observed even with the relatively low intensity of a mercury lamp 30 ), and (3) the rate of nonradiative retinal photo- isomerization, 31 the proton pump process, 33 and the proton photocurrent produced from the photocycle of the photo- synthetic system of bacteriorhodopsin. 32 Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: June 27, 2013 Revised: August 31, 2013 Published: September 3, 2013 Perspective pubs.acs.org/cm © 2013 American Chemical Society 44 dx.doi.org/10.1021/cm4020892 | Chem. Mater. 2014, 26, 44-58