Surface plasmon resonance (SPR) is an interesting phenomenon occurring in metal NPs, due to the excitation of electrons in the metal surface layer by photons of the incident light [24]. When interacting with light, metal nanomaterials convert light into heat if their oscillation is resonant at the practical frequency of the light [25]. Our group studied photothermal therapy using AuNPs and has been receiving a great deal of interest among researchers because of its excellent optical properties and heat-absorbing capacity [26]. Several gold nanostructures such as AuNPs, AuNRs, gold nanocrystals, gold nanostars, and gold nanoflowers have existed in prior arts. AuNPs have been shown to destruct both cancer and bacterial cells. Our group has demonstrated the potential application of AuNRs for photothermal therapy [7,22,27,28,29,30,31,32]. We have summarized the NIR-light-responsive materials for photothermal cell treatments in Table 1.

Importantly, AuNRs have two characteristic optical absorptions, i.e., the transverse and the longitudinal, corresponding to the aspect ratio (i.e., length/diameter) of the rods. Thus, by tuning the aspect ratio of AuNRs, the SPR region could be shifted to the NIR region for PTT. Upon NIR light irradiation, AuNRs absorb light which is effectively converted into heat because the excited conduction band electrons decay to the ground state by releasing their energy. The NIR light provides the maximal penetration of light, up to 10 cm (breast tissue), depending on the tissue types due to relatively lower scattering and absorption from the intrinsic tissue chromophores [40].

Reducing the cytotoxicity by coating silica NPs on the AuNRs is one of the useful studies for optothermal cancer cell lysis. Earlier, we demonstrated that silica-coated negatively charged (−24 mV) AuNRs added to the HeLa (derived from the name Henrietta Lacks) cells showed minimal toxicity [41]. Another study from our group reported that Si-AuNRs have a 36.13% greater cell growth rate for MDA-MB-231 cells (human breast cancer cells) under the NIR laser irradiation than the normal incubator condition by enhancing the activity of heat shock protein (HSP) [29]. Our recent study demonstrated the photothermal application of copper–gold (Cu-Au) tripods for CT-26 cells (colon carcinoma cells) death under the laser irradiation of 633 nm wavelength, 150 mW/cm² for 10 min [42]. Additionally, magnetic nanoparticles (MNPs)-conjugated AuNRs could generate a rapid photothermal effect and produce a bactericidal effect by enhanced magnetic separation [7].

Silver nanoparticles (AgNPs) have the potential thermal activity to control cell behavior effectively. Boca and co-workers reported that modified synthesis of chitosan-coated silver nano-triangles (Chit-AgNTs) showed effective photothermal activity against human non-small lung cancer cells (NCI-H460). In more detail, enough positively charged Chit-AgNT with the zeta-potential of +39 mV can provide enough surface charge to stablize the particles. Chitosan can be the alternative to biopolymers to make the NPs more stable and more biocompatible [33]. The efficiency of the conversion of light into heat is similar for both Au and AgNPs [43,44]. Liang and co-workers observed that spiky star-shaped Au/Ag NPs also have the potential to deal with cancer cells by using photothermal effects. They made fluorescein isothiocyanate (FITC)-labeled modified chitosan-coated Au/Ag NPs and used oral cancer cell (SAS) with 150 mW of NIR laser for a treatment period of 12 h to induce the ablation of the cancer cell compounded by the laser wavelength of 800 nm [45]. However, the antimicrobial role of AgNPs products is now a matter of interest. Properties such as biocidal, virucidal, localized surface plasmon resonance (LSPR), and anticancer activity make AgNPs more selective for modern biomedical applications [46]. Smaller particles that give higher cytotoxic effects impair large surface area. Because of the well-known shapes of AgNPs that can utilize the nanostructure in the biological field such as nanowire, nanorod, and nanoplate [47]. AgNPs, used as anti-angiogenesis in albino mice and LD50 (lethal dose 50): 3.5 µL/mL AgNP function as an anti-cancer material for MCF-7 breast cancer cells [48]. Sahu and co-workers reported that AgNPs have an anticancer effect on hepatic cells. They applied AgNPs on HepG2 (hepatocellular carcinoma) cells with a size of 20 nm and the dose was 1–20 µg/mL with an incubation time of 24 h under the NIR irradiation, finding that AgNPs affected the hepatic cells, though they had a cytotoxic effect [49].

Palladium nanoparticles (PdNPs) are widely known for their use in the green synthesis method. Ruiz and co-workers found that PdNPs synthesized by the green method with sizes of around 6 nm can accommodate the cancer cell cytoplasm. After applying the NIR laser, they showed the cell ablation effect on the cancer cells [50]. Ultrathin hexagon-shaped PdNPs with sizes ranging from 28 to 60 nm have excellent catalytic and plasmonic properties upon NIR laser [51]. In another study, Tang and co-workers suggested that high photothermal conversion of PdNPs has been found in the NIR at 808 nm wavelength. They also reported that the surface modification of the palladium nanosheets reduced the glutathione with sustained blood circulation and with a high accumulation rate in the tumor site [52]. They reported a higher conversion efficiency of PdNPs compared to typical gold nanorods and the efficacy of the photothermal conversion is 93.4% at a laser power of 808 nm, killing over 70% of cancer cells within 4 min. After modifying the surface of PdNPs with chitosan oligosaccharide, it shows improved biocompatibility [53]. PdNPs–Chitosan compounds can functionalize the RGD (arginylglycylaspartic acid) peptide, improving its accumulation in breast cancer cells and showing a therapeutic effect under an 808 nm laser [54]. Flower-shaped PdNPs embedded in chitosan/polyvinyl alcohol membrane with sizes ranging from 30 to 50 nm also have photothermal and wound-healing activity as reported in an article. Briefly, different concentrations of PdNPs were used, and 60 µg/mL of PdNPs rapidly went up to 56.5 °C and 6.25 µg/mL of PdNPs went to 33.7 °C [55]. Chaga Mushroom-derived PdNPs are useful for tri-modal anticancer therapy, controlled delivery of doxorubicin, and photothermal activity upon laser irradiation, while 40 µg/mL Chaga-PdNPs under 808 nm laser irradiation for 4 min can cause the cell ablation in HeLa cells [56].

Platinum nanoparticles (PtNPs) have become a scientific tool that is explored in various nanomedicine and biotechnological fields. Evidence has proved that photothermal therapies of PtNPs are highly selective for tumor ablation in both ways, such as singlet oxygen generation and the photothermal effect. Iron-conjugated PtNPs showed improved bioavailability on photothermal therapy by high NIR laser [57]. Folate functionalized 3-mercaptopropionic acid (FePtNPs) size of around 12 nm impacted the intercellular damage, which corresponded to the number of NPs causing necrosis in tumor cells in a proportional manner [58]. Trifolium-like platinum nanoparticles (TPNs) have been studied for their photothermal effects and it has been found that TPNs are effective for killing cancer cells followed by four hours of incubation and 808 nm NIR laser irradiation for 5 min. In vivo analysis of TPNs showed a clear reduction in tumor growth [59]. Peptide modification of PtNPs showed improved bioavailability and accumulation in mitochondria and PtNPs generate hypothermia in thermosensitive mitochondria, resulting in limiting tumor growth and severe damage to cancer cells. Protein-conjugated ultra-small PtNPs can specifically target mitochondria under the irradiation of 1064 nm laser by 5 min with a concentration of 32 µg/mL, 1.5 W/cm² [60]. In one study, it is shown that injectable and degradable hydrogel-based PtNPs can be used for repeatable photothermal cancer therapy. Dex-Ald and dendrimer-encapsulated platinum nanoparticles (Dex-DEPts) can raise the temperature to ~65 °C upon 808 nm NIR irradiation within 3 min. Dex-DEPts hydrogel was maintained in the tumor for one week repeatedly and it was found to cause further tumor regression [61].

Moreover, tumor-specific nanoparticles-based photothermal therapy has prominent contributions in the field of cell behavior control. Xiong and co-workers developed a hybrid biomimetic membrane (IRM), indocyanine green (ICG)-loaded magnetic nanoparticles (Fe₃O₄-ICG@IRM) for photothermal immunotherapy. This prolongs the circulation half-life, biodistribution, and response in tumor-specific immunotherapy [34]. However, nano–bio interactions and bioprocessing are aspects to be considered for the internalization of nanoparticles into the cells. Hollow copper sulfide (CuS) and rattle-like iron oxide nanoflowers@CuS core-shell hybrids (IONF@CuS NPs) are effective in the cellular metabolism of the nano-sized metals without affecting the cell viability and oxidative stress [35]. Additionally, one-dimensional polycation-coated nanohybrids Fe₃O₄@Dex-PGEA composed of polysaccharide dextran showed excellent photothermal properties, cellular uptake, and rapid clearance [36].

Carbon nanotubes (CNTs) are well-structured, hollow, graphite nanomaterials and many researchers are attracted to them for having different layers, such as SWCNT or MWCNT, and a tunable length. Because of their high mechanical strength and extended surface area and low-weight molecules, researchers are exploring their potential for biological and biomedical applications though it has cytotoxic effects [62,63]. Growing the cells for tissue regeneration and varieties of the targeted drug delivery, diagnostic and gene transfection are being studied in this field. Although CNT has specific biomedical and biological applications, it has severe toxicity toward human health and the surroundings. Dumortier et al. noted that functional carbon nanotubes have deep adverse effects on immune cells and reported few deaths [64]. For controlling the enzymatic activity, MWCNTs/SP is competent as stated by Song et al. [65]. The enhanced permeability and retention effect (EPR) and the high levels of intrinsic absorption properties make the carbon NPs captivating agents for photothermal therapy [66]. Owing to their EPR and magnificent underlying properties, the discriminatory heating of carcinogenic tissue with or without anticancer drugs is desirable to administer selectively occurring in photocoagulation accompanied by cell death, scaling down the dimensions of the carcinogenic tissue, or complete elimination of the selective tissue [67]. There are several reasons for choosing the biological treatment of carbon nanotube: First, the proper surface modification of the carbon nanomaterials makes the molecules protected from the attack of the immune system [68]. We correlated specific treatment methods by using different nanomaterials under different NIR laser conditions in Table 1. Second, carbon NPs have prominent light absorbance in the NIR, having superior tissue penetration ability [69]. SWCNTs were the first carbon NPs utilized as a photothermal agent and the administration of SWCNTs was based on intratumoral injection, and intravenous injection [37,70].

Chao et al. indicated advanced administration of carbon nanotube to the tumor metastases in sentinel lymph nodes (Figure 2) [71]. Most cancer deaths are associated with metastasis spread. So, it is decisive to destruct the cancer cell from the dominant level. In a study, researchers found that the irradiation of both primary and secondary tumors by photothermal heating prolonged the mouse survival, in contrast to the only primary tumor [72]. However, optimum renal clearance of the nanoparticles is a challenge in the way of achieving low systemic toxicity. Zeng and co-workers developed ultrasmall polypyrrole nanoparticles (Ppy NPs) of the size ~2 nm which have excellent photothermal conversion efficiency from 33.35% to 41.97% with efficient renal clearance [73]. The introduction of graphene-based nanocomposites as 4D-printed materials for on-time and position shape transformation under the NIR irradiation contributes potentially to the biomedical field [74].