In the last few years of research, it has been observed that noble metal nanoparticles due to their outstanding optical properties exhibit exotic applications in the field of nanoscience. Scientists perceive that plasmonic nanoparticles can be used as a nanoheaters and opens a new path in the area of nanoscale research. The plasmonic nanoparticles function as nanoheaters which can be remotely controlled by light leading to the development of an emerging branch known as thermoplasmonics. Heat generation induced by light absorption is considered as a big advantage to realize different plasmonic applications. The photothermal effect induced by plasmonic nanoparticles of different nano geometry, which act as nano sources of heat are described in the present paper. Along with optical properties of plasmonic nanoparticles as a major point of interest, heat generation induced due to light absorption by nanoparticles is also considered a valuable resource in various plasmonic applications e.g. photothermal treatment, thermoelectric power generation etc. and this consequence of plasmonic nanoparticles put up the basis for the exciting field of thermoplasmonics. In this review, basic understanding and some simulation techniques for measuring the heat unleashed by plasmonic nanoparticles at the nanoscale order is summarized along with the recent advances in the application of thermoplasmonics specially to biomedical applications using photothermal effect, particularly for photothermal cancer therapy, drug and gene delivery, photothermal imaging, and nano-surgery.

Keywords: Nanoparticles, Plasmonics, Thermoplasmonics, Biomedical Applications

In the past two decades, it has been observed that noble metal nanoparticles encounter so much intrigue in nanoscience due to their astounding optical properties. The plasmonic nanoparticles exhibit the physical phenomenon named localized surface plasmon resonance (LSPR) that can be tuned in a broader range of the electromagnetic spectrum. The localized surface plasmon resonance is an optical phenomenon responsible for enrichment in absorption and scattering of light. When optical properties of metallic nanoparticles (NPs) were major point of interest, heat generation induced by light absorption is considered as side effect in plasmonic applications. In the beginning of 21^st century scientists perceive that plasmonic nanoparticles can be used as a nanoheaters and opens a new path in the area of nanoscale research. These plasmonic nanoparticles function as a nanoheaters can be remotely controlled by light, develops novel branch in plasmonic emerging as thermoplasmonics. Plasmonic materials as a nanosources of heat opening doors for thermal based modern application. In 1999, first application of thermoplasmonics induced and study the denaturation of protein by the nanometer size particles. The photothermal properties of plasmonic nanoparticles are widely used in biomedical field cancerous cells and tumors, hyperthermia therapy, cell biology, also several potential applications in nonbiomedical field, such as nanofabrication, nanofluids, solar and thermal energy harvesting, nanochemistry, heat-assisted magnetic recording (HAMR), photonics and optoelectronics.

This article intended to give the recent advances in the field of thermoplasmonics, investigation based on plasmonic material nanogeometries as source of heat. Herein, we report on the fundamental physics which governs the heat generation and optical absorption mechanism by plasmonic nanoparticles from both theoretical and experimental outlook. We then examine how theoretical prediction can be done for temperature increase under continuous and pulsed illumination. In the last few years photothermal properties of nanostructures compels scientist from theoretical and experimental point of view on account of multitude potentially useful applications. Plasmonic nanomaterials characteristics are based on the geometry (size and shape) and the material from which they composed. Most of the research extensively done for gold nanoparticles (AuNPs) because AuNPs are distinctive nanomaterials with inherent property to produce light confinement at nanoscale which set off thermal effect. Under illumination, part of light scattered in surrounding from the metal nanoparticle, while the other part gets absorbed and dissipated in the form of heat. Scattering and absorption can be significantly varied, determined by shape and size of plasmonic nanomaterials. Here main purpose is to use plasmonic nanomaterials as a controlled nanosource of heat subjected to external illumination. Surface plasmon resonance (SPR) in plasmonic nanoparticle are ideally discovered in the visible and infrared regions of the electromagnetic spectrum, and they can be tuned by changes in shape, size, and composition. Noble metals are widely used as plasmonic nanostructures due to their strong resistant to oxidation. Recently, researchers attracted attention towards abundant and economical nonnoble metals (Cu, Al, Mg, In, Ga, Pb, Ni, Fe, Co and their hybrids) used in area such as nanoantenna, photocatalysis, sensing, metamaterials and magnetoplasmonics with novel composition, morphology and properties. In literature, it is forecasted that aluminum to be the succeeding best plasmonic material for economical applications. In particular, aluminum nanoparticles feature SPR in the UV range and researchers have in view to bringing SPR of aluminum tuned into the visible or IR range of the electromagnetic spectrum. Preceding years most applications reported on nanoholes in metal layers, and thermal effects are very high for such type of system, owing to the great amount of metal under optical illumination (nanoheaters). Also, tunning of SPR from visible to IR domain can be done by using a distinctive arrangement i.e., sphere nanodimer.

In addition, the growing concern in thermoplasmonics, several questions raised such as: Exactly how much temperature increases for plasmonic nanostructure under optical illumination? What is the outline of temperature in the medium vicinity? Many efforts have been made in the development of experimental and theoretical methods to answer these questions. The temperature probed by nanoheaters at the nanoscale can explore by thermal microscopy techniques. Mie plot and discrete dipole approximation (DDA) are quite popular modelling for arbitrary shapes of plasmonic nanomaterials. In DDA simulation method the optical properties are more precise at higher discretization. Through these methods extinction, scattering and absorption calculations can be optimized for plasmonic nanomaterials. Simulations of plasmonic nanoparticles which are immersed in dielectric medium render the solution of Maxwell’s equations. MNPBEM is a numerical simulation Matlab toolbox for plasmonic nanoparticles, by implementing boundary element method (BEM) created by Abajo and Howie in 2002. The plasmonic nanoparticles immersed in homogeneous medium, their optical cross-section, and distribution of electric field simulations are performed using BEM. The COMSOL Multiphysics is a widely used software for study of plasmonic nanostructures and facilitates development of model for thermoplasmonic applications. By using COMSOL researchers can examine physical and geometrical characteristics of developed model and can refine it on significant design challenges.

2. Localized Plasmon Resonance in Plasmonic Materials

Plasmonic nanomaterials supports resonance when a photon of incident light strikes at metal surface. This resonance frequency can be tuned by altering size, shape, and dielectric environment of the metal nanoparticles. For example, the plasmon resonance of gold, which lies in visible region, can be tuned into the infrared region by diminishing the size of nanoparticle. Similarly, plasmon resonance of silver lies in the UV region, can be tuned into visible region of electromagnetic spectrum by making the small size of metal nanoparticles. Plasmonic nanomaterials have a sufficient number of free electrons which interacts with electromagnetic fields. Upon illumination, the free electrons interact with an external electromagnetic field (EM), so the free electrons of metal nanoparticles start oscillation in phase with applied EM field. Hence, the electron cloud of metal nanoparticle oscillates as a simple dipole parallel to the electric field as shown in figure 1.

Figure 1. Illustration of localized surface plasmon resonance for a spherical metal nanoparticle. (File:Nanoparticle lspr 2. png - Wikimedia Commons).

The calculation by finite element method (FEM) offers how absorption and corresponding temperature of plasmonic nanomaterials change with meticulous variation of size, shape, and composition. It has been found that gold-based nanostructures have received lots of attention despite of platinum or titanium, which both have a higher absorption in near infrared (NIR) and seems to be no more lethal than gold-based nanostructures. FEM provides high accuracy compared to supplementary methods and replicates the results for spherical objects on Mie’s calculations. There are significant efforts in progress to develop futures plasmonic nanoparticles, and recent developed synthesis methods have unlocked the possibility of creating a vast range of plasmonic nanoparticles with shape, size and composition customize for peculiar biological, chemical, and medical applications. However, most of the applications of noble metals are constrained by their inflated cost, substantial optical loss, and limited wavelength range of localized surface plasmon resonance (LSPR). In past years, researchers are looking into another alternative plasmonic material such as non-noble metals, which can replace the supremacy of the noble metals. Non-noble metals have received much attention due to low-cost and promising greater performance in particular applications. Non-noble metals such as Cu, Ni, Co, Fe, Al, Mg, Ga, Pb, In, and their hybrids have made significant progress in the synthesis, optical properties and specific applications based upon their plasmonic nanostructures. Metal nanoparticles contains considerable number of free electrons, they present negative real dielectric function over an extensive range of frequency, resulting in plasmonic responses in the UV to near infrared (NIR) range of the EM spectrum (Figure 2). Non noble metals, such as Mg, In, Ga and Al are appropriate to UV plasmonics because they have high density of free electrons, which cannot be attained with noble metals such as Au or Ag. Although, the practical applications of these metals are limited due to their low chemical stability.