1. Introduction

 

The heat generation by the metallic nanoparticles are extremely high by absorption of near infrared (NIR) light and acquire attentiveness in cancer treatment, because the penetration depth of NIR light into biological tissue is relatively high. Platinum nanoparticle have astounding thermoplasmonic properties in photothermal cancer therapy, and surface temperature up to 900K are easily achieved with diameter of 50-70 nm. At this highest temperature, platinum nanoparticles remain stable and immensely promising thermoplasmonic applications in the field of biomedical engineering, life sciences and nanomedicine. Recent advances in research literature forecast aluminium nanoparticle be the next successful plasmonic material for inexpensive applications due to their natural abundance compared to noble metals like gold and silver. Mostly, it is found that the LSPR peaks were found in the UV to visible regime of the EM spectrum for precious metals such as gold, silver, and aluminium. However, the LSPR of aluminium are in the UV domain, which can be tuned beyond the visible domain (i.e., NIR) by suitable changes in the parameters of geometry.

The LSPR for a small metal nanosphere having volume V and diameter, d, which is lesser than the illumination wavelength, λ, therefore it can be considered that EM field uniformly polarizes the spherical particles. In this case, sphere nanoparticles which is in an EM field assumed to be an electromagnetic dipole with polarizability given by

                          (1)

In this expression, ?(?) and ?? are the permittivity of the nanoparticle and medium respectively, in which they depend upon frequency. Here, α is polarizability of the uniformly polarized metallic nanosphere and its corresponding extinction crossection, σ_ext., is given as [47]

                   (2)

Where k is the angular wavenumber, that is k=2πλ. Also, we know that σext=σscat+σabs, where σscat and σabs are scattering and absorption crossection of the metal nanoparticles, respectively.

3. Theoretical Background

The generation of heat in a metal nanoparticle by localized surface plasmon resonance (LSPR) is due to coupling of electromagnetic waves. Since metal nanoparticles have significant number of free conduction electrons, so the movement of these electrons has constant collisions with lattice atoms resulting the heat generation in metal nanoparticles arises from Joule effect, which is the major mechanism that causes thermoplasmonics to work. The maximum amount of electromagnetic energy is transferred into heat at the localized surface plasmon resonance (LSPR). Here, we concentrate on the absorption process, σabs, this is a function of wavelength which is used to calculate the subsequent heat generation by a nanostructure when exposed to light at a specific wavelength. Thus, absorption is most important factor in the field of thermoplasmonics. The shape of metal nanoparticle is one of the critical factor for the resonance condition of the absorbed light. The determination of σabs can be evaluate for metal nanosphere using Mie theory. The heat power, q, absorbed (and delivered) by the nanoparticle will be same and equal, when the nanoparticles having quite a distance so they can be assumed optically independent. That is

                                       (3)

In above equation,  denotes the intensity of incoming light (power per unit area).

The heat generation in metal nanoparticle can be obtained from the heat power density q(r) which is the function of the electric field complex amplitude E(r) given as:

                          (4)

Where J*(r) represents the complex amplitude of J(r) (electronic current density). Here  and , it has been observed that the heat source density is related to the square of the electric field inside the metal nanoparticle expressed as

                   (5)

This result is considered as a primary factor while designing the effective plasmonic nanosources of heat.

3.1. Temperature Profile Under Continuous Illumination

The temperature distribution and their influence in surrounding media by metal nanoparticles under illumination is governed by steady state heat equation, the heat transfer equation reads

                       (6)

In eq (6) ρ, c and k are the density, volumetric heat capacity and the thermal conductivity (of the surrounding medium) respectively and T(r) is the temperature distribution inside and outside the metal nanoparticle produced by heat source density q(r).

Inside the metal nanoparticle,

                                   (7)

Outside the metal nanoparticle,

0                                         (8)

It is noted that, there are so many factors which causes the temperature increase by the metal nanoparticle which depends upon the shape, absorption crossection, thermal conductivity of the homogenous medium and the wavelength of the illuminated light. As optical absorption is the primary factor which form the basis of thermoplsmonics, and absorption crossection defines the how much light re-radiated in the form of light. The temperature increases of metal nanosphere of radius R related to heat power density q and thermal conductivity k of the medium is simply given by

                                       (9)

However, the nanostructures are in close contact with an interface which separates the two media having different thermal conductivities. Generally, most common situation occurred when researcher performing their experiment. In this case, the eq (9) can be written as:

                             (10)

In eq (10),  is the effective conductivity.

Since, the eq (9) cannot hold when the nanoparticle is non spherical, such as ellipsoidal, rods, torus, and disc. However, the temperature increase of the non-spherical nanoparticle can be evaluate by introducing by a correction factor β in eq (9), it yields

                             (11)

In above eq, Reff is effective radius of the nanoparticle, and β = 1 in the case of spherical nanoparticle.

3.2. Temperature Profile Under Pulsed Wave Illumination

As most of the experiments in the field of thermoplasmonics are employed by continuous wave (CW) illumination to give rise of temperature by the metal nanoparticles. Recent advances shows when the metal nanoparticle is under pulsed illumination give rise to various phenomenon at the nanoscale (femtosecond to nanosecond range), which leads to numerous potential of applications. The heat generation by metal nanoparticles under the pulsed illumination of the incoming incident light followed by some process at different time scale enumerated as:

1. The metal nanoparticle under pulse illumination produces the excitation at plasmon resonance resulting the non-equilibrium in electronic gas. The increase in the temperature through electron-electron collision due to energy absorbed by free electrons takes place in interval of ≅ 10 to 100 fs (femtosecond).
2. Then, these hot electron cools down and transfer the energy to the lattice and relax through scattering with electrons, phonons, and impurities through electron-phonon collision up to ≅ 10 ps (picosecond).
3. Finally, the hot carriers diffuse the energy into the surrounding medium and cause the overall temperature to rise by the process of thermal diffusion. This last process happens for a time interval of ≅ 100ps to a few ns (nanosecond).

Since, the expression for the temperature increase of the metal nanoparticle under pulsed illumination can be expressed as:

                             (12)

Where F is energy density of light, V is volume of the nanoparticle and cn represents the thermal capacity of the nanoparticle. The eq (12) does not include the morphology of nanoparticles, so it is valid for moderate temperature increase under the pulsed laser.

3.3. Numerical Simulations in Thermoplasmonics

Numerical methods in thermoplasmonics have been developed to find out the temperature distribution of the plasmonic nanostructure in its surrounding medium. The numerical simulations played an important role in designing the effective plasmonic nanostructures.. As for theoretical investigation of heat generation by plasmonic nanostructure numerical set-up coupling optics and thermodynamics is essentially required, because of the dynamics of light absorption and transportation of heat in surrounding medium in plasmonic nanostructures. This section is dedicated to render a brief description for basic understanding of some numerical simulation techniques.

The optical properties such as extinction spectra which is the combination of scattering and absorption of EM wave by plasmonic nanoparticle were calculated using Mie theory, and this is incredibly famous theory for figuring out the exact crossection of the spherical nanoparticle. The calculation of optical properties of plasmonic nanoparticle based on the solutions of the Maxwell’s equation in the form of infinite series expansion. Extended Mie theory compute the results for plasmonic nanoparticle such as sphere, hollow sphere, coated sphere, and infinite cylinder. The optical spectra of spherical plasmonic nanoparticle and their LSPR characteristics can be evaluated from Mie theory simulations using the Matlab program.

The discrete dipole approximation (DDA) and boundary element method (BEM) are extensively used numerical techniques to investigate the distribution of electric field and optical properties of plasmonic nanostructure of an arbitrary shapes. DDA and BEM are referred as discretization methods to solve Maxwell’s equations of targeted arbitrary geometry in the presence of dielectric medium under specific boundary conditions. However, there is MNPBEM well known Matlab toolbox to solve Maxwell’s equations for plasmonic nanoparticles of arbitrary shape with changing parameters surrounded by complex refractive index and segregated by abrupt interfaces. MNPBEM toolbox performed well to calculate steady state temperature distribution with high accuracy and less simulation time to display output. A Fortran version of the DDA has been created, known as DDSCAT, for the computation of scattering and absorption of light by the arbitrary nanogeometries surrounded by the dielectric environment. DDSCAT is an open-source software which supports calculations for various plasmonic nanogeometries in which they may be anisotropic or heterogeneous. DDA and BEM are widely used simulation techniques to investigate the optical spectra of the colloidal particle and plasmonic nanodimer, which helps in the development of effective plasmonic devices.

The finite difference time domain (FDTD) and finite element method (FEM) are used to solve Maxwell’s curl equations in a finite space by discretizing partial differential form, both of these techniques can meet the high order accuracy with high discretization. FDTD and FEM both have their own importance like FDTD mostly preferable for nonlinear materials while amorphous griding is advantageous to FEM. Both of these techniques are the part of domain discretization methods and do not have intricate mathematical background and are relatively easy to put into practices. The use of basis function in FDTD allows for a high level of accuracy when accounting for the geometry of nanostructure which is an important aspect when studying the optical properties by altering the shape of plasmonic nanostructure. The confinement of EM field in nanogeometry at optical near field distribution makes FEM an appropriate technique for the computation of light localization and their respective applications. The FDTD method’s main idea is to stagger the vector components of an EM wave (i.e., electric, and magnetic field). Nowadays, most of the FEM simulations are conducted by COMSOL Multiphysics software. With the use of COMSOL Multiphysics researchers are capable to design the effective nanostructure model that helps in improving their physical properties .

4. Selected Applications in the Biomedical

Thermoplasmonics has shown great promises in biomedical applications such as photothermal cancer therapy, drug and gene delivery, photothermal imaging and nano-surgery. In the treatment of Cancer, plasmonic nanoparticle are used for photo-damage of cancer cell with less side effects by heating the nanoparticle under the appropriate illumination of laser beam. The major benefit of plasmonic nanomaterial is the controlling of heat at the nanoscale which played an important role to develop the potential application of thermoplasmonics.

4.1. Photothermal Cancer Therapy

Plasmonic nanoparticles are deposited in tumor cells targeted for the treatment of the disease by hyperthermia (temperature produced cell death) in photothermal therapy. Although, the delivery of plasmonic nanoparticles at the tumor cell is a big issue for photothermal therapy targeting cancer. The aim of these particles here is to maintain a uniform distribution within the tumor cell. Ultimately, the particles can be delivered by two different techniques: 1) Local injection, which is not an easy approach because it requires precise tumor location and makes uniform delivery bit challenging. 2) Delivery via the EPR effect (enhanced permeability and retention). In photothermal therapy it is an important aspect to select right external laser to stimulate the plasmonic nanoparticle accumulated at targeted tumor for better efficacy of treatment. As a result, an NIR laser with minimum optical absorption is commonly used for photothermal therapy because it can reach at targeted tumor without affecting healthy tissues.

Doughty et al. conveyed a study of the optical properties for different nanostructures of gold in targeting cancer treatment with photothermal effect to enhance therapeutic efficacy. The nanoparticles with homogeneous distribution at targeted tumor under appropriate illumination causes temperature increase and commences photo-damage when it reaches 41°C. In photothermal therapy needed temperature is more than 50°C is required for destruction of each and every cell of targeted tumor. The fine tuning in the absorption spectra of gold nanoparticle by changing the shape, size and synthesis condition of colloidal gold nanoparticles lead to absorb light of specific wavelength makes best choice for photothermal therapy. Instead of gold, another plasmonic nanoparticle such as platinum promises a suitable choice for photothermal cancer therapy. Samadi et al. given a study based on platinum nanoparticle of different sizes such as 50 nm and 70 nm absorb light at NIR hence it can penetrate deep into biological tissue and causes the destruction of cancer cell by photothermal effect. The main benefit of both particles (gold and platinum) is that they are not cytotoxic for living cell. The main advantage of both particles (gold and platinum) is that they are not cytotoxic to the living cell.

4.2. Photothermal Imaging

When an EM wave hits the metal nanoparticle, so it produces LSP and consequently causes temperature changes in the surrounding medium, resulting in its refractive index varying. In 2015, Cui et al. investigated an efficacious approach for latent fingerprints imaging by using Cu₇S₄ nanocomposite with diverse backdrop colours, which is very significant in forensic science for criminal investigation through high resolution photothermal imaging. In 2017, the Li group reported the spatial distribution of microparticles in tablets with spatial resolution of 0.65 µm by epi-MIP (mid infrared photothermal) microscope for active pharmaceutical ingredient (API) in drug visualization. Photothermal microscopy is an effective technique to detect small absorbing nanoparticles within surrounding medium by nanolens effect. In 2020, the Shi group demonstrated photothermal imaging of Ag nanowire in the presence of glycerol local environment with high scattering cross sections for analysis of photothermal signal via nano objects. Recently, researcher found that graphene, iron oxide nanoparticle, carbon dot, and their hybrid structure are suitable agent for photothermal imaging, but these nonmetal nanomaterials are bit challenging to accurately control optical characteristic to have maximum absorption in the NIR range in the view of metal nanoparticles. The photothermal imaging strongly depends upon the optical absorption efficiency and the intensity of external light source.

4.3. Drug and Gene Delivery

Targeted drug or gene delivery is critical for increasing therapeutic purpose while lowering dosages and reducing negative side effects. Various morphologies of plasmonic nanoparticles, such as nanospheres, nanocages, nanoshells, and liposome have been demonstrated for targeted drug and gene delivery. The plasmonic nanomaterials have been commonly used in drug delivery which behave as nanocarriers and nano sources of temperature in which drug remotely released at its location by external light regulation, and photothermal therapy can be integrated with delivery of drugs due to their tunable optical properties. The use of gold nanoparticles in drug and gene delivery provides the greater treatment effectiveness. Generally, two techniques can be used for targeted drug delivery such as capsule encapsulation by plasmonic nanoparticle and medicinal molecule within a lipid vesicle and another approach involve plasmonic heating to change the permeability of the plasmalemma, allowing molecule that would normally pass-through cell to be injected. Wang et al. reported the hydrophobic PLGA core and hydrophilic PEGylated lipid shell nanoparticles for chemotherapy to treat cancer by co-delivery of drug and gene with greater bioavailability and lower toxicity.

4.4. Plasmonic Laser Nano-Surgery

In the fields like ophthalmology and dermatology, laser nano surgery, which involves cutting tissues with laser light, has proven to be a reliable replacement for traditional sharp instruments. In comparison with traditional surgical techniques, it makes a bloodless cut in tissue an minimize the possibility to harm neighboring healthy tissues. The intensity of laser beam significantly becomes high when an ultraviolet or visible field beam is narrowly focused via high numerical aperture, and it becomes an effective approach for cell nano surgery. In 2009, the Li group used 25-50 nm gold nanoparticles amalgamated with siderophilin molecules for targeting laser photothermal therapy with nanosecond (ns) pulsed laser for therapeutic of breast cancer cell. The plasmonic induced nanobubbles in the vicinity of medium by femtosecond (fs) laser pulses is a promising therapeutic option for cell nano surgery applications. In 2014, Lachaine et al. demonstrated the dependency of the polarization for 100 nm gold nanoparticles in water medium to form different plasmonic nanobubble dimensions with linearly and circularly polarized laser pulse with duration between 45 fs to 8.8 ps at wavelength of 800 nm. The gold nanoparticle coated micro bead with dimension greater than 3 µm creates photothermal bubbles by using laser with a wavelength of 473 nm, which can be used as a portable nano-heater as well as portable micro generator described in 2017 by Sekimoto et al.. In 2021, the Nguyen group proposed an approach for immunotherapy by nanosecond pulsed laser generated nanobubbles around gold nanorods, these nanorods (38 nm long, 10 nm wide) are irradiated via laser beam of wavelength 1064 nm that destroys breast cancer cells and induces immunogenic apoptosis.

5. Conclusion

This review intends to render an insight on various progress procured in the field of thermoplasmonics. The scientific research community is pushing toward it because of the greater potential applications in this field. Plasmonic nanoparticles are used as a nano-heaters, and it generates an avenue in the area of nanoscale research. At the localized surface plasmon resonance (LSPR), the maximum amount of electromagnetic energy is converted into heat. However, Photothermal properties of plasmonic nanoparticles put up the basis for thermoplasmonics, that depends upon the shape, absorption cross-section, thermal conductivity of the homogenous medium and the wavelength of the incident light. The background of physics in photothermal effects by nanomaterials is concerned with optics and thermodynamics. Thus, theoretical investigation of heat generation requires coupling of optics and thermodynamics for simulation set-up, because of the dynamics of light absorption and transportation of heat in surrounding medium of nanomaterial. The biomedical applications of thermoplsmonics rely on photothermal effect for diagnosis and therapeutics. While precise modeling and synthesis of plasmonic nanoparticle we can achieve better photothermal response for cancer cells therapeutic. Recent literature communicates that for photothermal cancer therapy the platinum or titanium-based nanostructure is appeared as best choice despite of gold, but it is still unknown which material is perfectly suitable and holds high absorption in NIR region. So, the research should continue for finding and development in the field of thermoplasmonics since it offers various biomedical therapeutic applications.