Theoretical Comparison of Optical Properties of Near-Infrared Colloidal Plasmonic Nanoparticles

www.nature.com/scientificreports OPEN received: 05 July 2016 accepted: 08 September 2016 Published: 26 September 2016 Theoretical Comparison of Optical Properties of Near-Infrared Colloidal Plasmonic Nanoparticles Kai Liu1, Xiaozheng Xue2 & Edward P. Furlani1,2 We study optical properties of near-infrared absorbing colloidal plasmonic nanostructures that are of interest for biomedical theranostic applications: SiO2@Au core-shell particles, Au nanocages and Au nanorods. Full-wave field analysis is used to compare the absorption spectra and field enhancement of these structures as a function of their dimensions and orientation with respect to the incident field polarization. Absorption cross-sections of structures with the same volume and LSPR wavelength are compared to quantify differential performance for imaging, sensing and photothermal applications. The analysis shows that while the LSPR of each structure can be tuned to the NIR, particles with a high degree of rotational symmetry, i.e. the SiO2@Au and nanocage particles, provide superior performance for photothermal applications because their absorption is less sensitive to their orientation, which is random in colloidal applications. The analysis also demonstrates that Au nanocages are advantaged with respect to other structures for imaging, sensing and drug delivery applications as they support abundant E field hot spots along their surface and within their open interior. The modeling approach presented here broadly applies to dilute colloidal plasmonic nanomaterials of arbitrary shapes, sizes and material constituents and is well suited for the rational design of novel plasmon-assisted theranostic applications. The interest in colloidal plasmonic nanoparticles has grown steadily in recent years as advances in particle synthesis have enabled a proliferation of applications in fields such as nanophotonics, biomedicine and analytical chemistry1,2. Many applications exploit the unique and highly tunable behavior of the particles, most notably, those that are associated with the effects of localized surface plasmon resonance (LSPR). At plasmonic resonance, there is intense absorption and scattering of incident light and highly localized field enhancement. Moreover, the LSPR wavelengths of a particle are highly dependent on its size, structure and material as well as the optical properties of the surrounding medium. The LSPR wavelength can be tuned within the ultraviolet (UV) to near-infrared (NIR) spectrum by manipulating these factors. A desired LSPR wavelength can be obtained, in principle, by controlling the dimensions and morphology of the particles during synthesis. The ability to tune the LSPR and associated behavior has proven useful for a broad range of applications involving cancer therapies3, Raman scattering4, fluorescent labeling5, nonlinear optical imaging6, biosensing7, among others. For example, recently colloidal plasmonic nanoparticles with more complex geometries (i.e. Au nanocages and nanostars) have been synthesized with excellent controllability and have been successfully demonstrated for enhanced biomedical imaging applications8–10. Of particular interest are two emerging biomedical applications that directly exploit plasmon-enhanced photothermal transduction namely thermally modulated drug delivery and photothermal cancer therapy. Most plasmon-based photothermal applications in vivo utilize Au-based nanoparticles with LSPR wavelengths in the NIR, i.e. 650–1300 nm. This is known as the near-infrared window as these are the optical wavelengths that have the deepest penetration into tissue11. In this work, we investigate and compare three distinct nanostructures with demonstrated efficacy for theranostic applications: core@Au-shell12, Au nanorod13, and Au nanocage structures14. These particles have attracted great attentions because they can be synthesized in a controllable fashion using bottom-up chemical methods, which enables tuning of their optical properties. However, they also have drawbacks. Core-shell particles with an Au shell can have limited absorption in the NIR due to a relatively thin gold shell that is required to red-shift LSPR to that range. Nanorods have a solid metallic mass, but their 1 Dept. of Electrical Engineering, University at Buffalo SUNY, NY 14260 USA. 2Dept. of Chemical and Biological Engineering, University at Buffalo SUNY, NY 14260 USA. Correspondence and requests for materials should be addressed to E.P.F. (email: ) Scientific Reports | 6:34189 | DOI: 10.1038/srep34189 1 www.nature.com/scientificreports/ Figure 1. Plasmonic nanostructures and the computational model. (a) SiO2@Au core-shell particles, (b) Au nanocages, (c) Au nanorods. (d) Computational domain showing the polarization and propagation direction of the incident field. absorption is a strong function of their orientation relative to the incident polarization, which results in less efficient heating for randomly oriented colloidal particles. Gold nanoframes are emerging as an alternative NIR nanomaterial for photothermal therapy15 and drug delivery16. However, their NIR optical behavior (e.g., sensitivity of LSPR to spatial orientation with respect to the incident polarization) is not obvious and needs to be determined using complex 3D computational modeling. Although the optical properties of various NIR plasmonic nanomaterials, such as Au spheres, nanorods, nanotori and nanoframes, have been reported, e.g. our previous work on photothermal-induced nanobubble generation17,18, these previous studies do not provide a systematic comparison of relevant optical properties that is needed to determine the optimal choice of NIR nanoparticles for photothermal applications, which is the focus of the present study. In summary, despite the growing interest and application of colloidal plasmonic particles for theranostics, rational design in this field is lacking and can be achieved using numerical multiphysics modelling. Results We used 3D full-wave computational models to study the NIR plasmonic behavior of the three nanostructures shown in Fig. 1. In our analysis, we place more emphasis on optical absorption rather than scattering as we are interested in photothermal applications in which the absorption is the dominant factor that determines the efficiency of the system. In addition, we consider subwavelength nanoparticles for which absorption dominates scattering. The comparison between intensities of absorption and scattering can be found in the Supplementary Information. The core-shell particles consist of a silica (SiO2) core with a radius Rc and a gold shell with a thickness ts as shown in Fig. 1a. The Au nanocages are cubic with twelve frame elements in the form of square Au nanowires, as shown in Fig. 1b. The nanocage geometry is defined by its length L, which defines the size of the cube, the width W that defines the cross-sectional area of the nanowire, and the aspect ratio R =  L/W. In the li (...truncated)