Near-infrared light-responsive inorganic nanomaterials for photothermal therapy

School of Pharmacy,Shenyang Pharmaceutical University,No.103,Wenhua Road,Shenyang 110016,China

Near-infrared light-responsive inorganic nanomaterials for photothermal therapy

Zhihong Bao*,Xuerong Liu,Yangdi Liu,Hongzhuo Liu,Kun Zhao

School of Pharmacy,Shenyang Pharmaceutical University,No.103,Wenhua Road,Shenyang 110016,China

A R T I C L EI N F O

Article history:

Received 9 September 2015

Received in revised form 29 October 2015

Accepted 4 November 2015

Available online 26 January 2016

Inorganic nanomaterials

Novel nanomaterials and advanced nanotechnologies prompt the fast development of new protocols for biomedical application.The unique light-to-heat conversion property of nanoscale materials can be utilized to produce novel and effective therapeutics for cancer treatment.In particular,near-infrared(NIR)photothermal therapy(PTT)has gained popularity and very quickly developed in recent years due to minimally invasive treatments for patients.This review summarizes the current state-of-the-art in the development of inorganic nanocomposites for photothermal cancer therapy.The current states of the design, synthesis,the cellular uptake behavior,the cellular cytotoxicity and bothin vivoandin vitro

nanoparticle assisted photothermal treatments of inorganic photothermal therapy agents (PTA)are described.Finally,the perspective and challenges of PTT development are presented and some proposals are suggested for its further development and exploration.This summary should provide improved understanding of cancer treatment with photothermal nanomaterials and push nanoscience and nanotechnology one step at a time toward clinical applications.

?2016 The Authors.Production and hosting by Elsevier B.V.on behalf of Shenyang Pharmaceutical University.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

Directly or indirectly,cancer affects most people’s lives.Cancer is one of the leading causes of death and accounted for 8.2 million deaths worldwide in 2012[1]and the incidence rate is increasing year by year.The main reason for this dismal picture is that even with the current state of the art of cancer diagnosis,usually this disease is detected in an advanced stage, when the primary tumor has metastasized and invaded other organs,which is beyond surgical intervention.In addition, current chemo-and radiation therapies have many wellknown disadvantages,including relatively poor specifcity toward malignant tissues,systemic side effects,low effcacy and drug resistance[2].Therefore,to advance cancer therapy, therapeutic methods that should selectively eliminate only diseased cells/tissues without causing collateral damage will be expected.

As a promising alternative or supplement to conventional cancer treating approaches,photothermal therapy(PTT)hascaused considerable attention because of its advantages including minimal invasion,few complications,and rapid recovery.PTT,also known as photothermal ablation or optical hyperthermia,employing photo-absorbers and near-infrared light energy sources,provides a precise and minimally invasive alternative for cancer treatment[3,4].It is a procedure based on localized heating due to light absorption for selective destruction of abnormal cells.To enhance anti-cancer effcacy and optimize therapy,integration of multiple treatment strategies with synergistic effects is highly expected[5].Among these treatment strategies,the combination of PTT with chemotherapy,termed chemo-photothermal therapy,as a minimally invasive,controllable,and highly effcient treatment method, has drawn widespread attention[6,7].In PTT,near-infrared(NIR) light(650–900 nm)is preferred for such an application because of its easy operation,its ability to be locally focused on a specifc region,and its minimal absorbance by skin and tissues to allow for noninvasive penetration of reasonably deep tissues [8].The key component of this technique is a photothermal transducer that can effciently absorb and convert NIR light into heat through a non-radiative mechanism.

Over the past decade,many different types of photothermal therapy agents(PTA)have been reported,including organic compounds or materials(e.g.,indocyanine green[9,10]and polyaniline[11])and inorganic nanomaterials(e.g.,noble metal nanoparticles[12,13],metal chalcogenide[14,15],and carbonbased materials[16–19]).When combined with NIR light,all of them are able to generate suffcient heat to raise the local temperature and thus kill cancer cells.Organic photothermal therapy has good biocompatibility and biodegradability,and therefore can be used for nanobiotechnology.However,the low photothermal conversion effciency,poor photothermal stability and complicated synthesis process of these materials limit their application for PTT.As an alternative,inorganic PTA have received great interest in recent years,because of their high photothermal conversion effciency and the ease of synthesis and modifcation;for example,the inorganic nanoparticle size,shape and surface properties can be facilely controlled.

In the past decade,near-infrared light-responsive inorganic nanomaterials,such as gold nanoparticles,carbon nanotubes,and copper sulfde nanoparticles(Fig.1)effciently convert optical energy into thermal energy and enhance the effcacy of photothermal ablation therapy.Some applications are under clinical trials.In this review,we summarize the recent advances in the structural and functional evolution of inorganic nanomaterials employed in PTT.These recent progresses in materials design will lead to deeper insight of the chemistry and photonic as well as to promote the development of PTT into practical applications.The aim of this review is to arouse more attention toward inorganic photothermal nanomaterials used in cancer therapy and to encourage future work to push forward the advancement of this biomedical area.

2.Various inorganic nanomaterials for PTT

For biomedical applications,inorganic nanomaterials,including Au-based nanomaterials,Pd nanoparticles,CuS nanoparticles, graphene,and carbon nanotubes etc.,have attracted much attention in PTT.This article summarizes recent progress on various inorganic photothermal nanomaterials,including the background,synthesis,modifcation,cytotoxicity as well as their applications in biomedicine.

Fig.1–Various types of inorganic nanocomposite materials for photothermal therapy for cancer.

2.1.Colloidal noble metal nanoparticles

Noble metal nanoparticles,especially for Au and Pd nanoparticles,have been proven to show strong scattering and absorption of light in visible and near-infrared region owing to their localized surface plasmon resonances.The absorbed light is then turned into thermal energy.With pulsed light irradiation,transient thermal power generated in nanoparticles introduces abundant thermodynamic effects,such as ablation,ultrafast heating,thermal expansion,surface melting,and reshaping.

2.1.1.Gold nanoparticles

Localized surface plasmon resonance of gold nanocrystals endows them the capability to strongly absorb and/or scatter light at synthetically controllable resonance wavelengths,which is the underlying reason for their many applications[20–23]. A wide variety of Au nanostructures,including aggregates of colloidal particles,nanoshells,nanocages,nanorods and nanocrosses have been demonstrated for cancer photothermal therapy with NIR light.In the case of spherical gold nanoparticles,the absorption maximum exists between 400 and 600 nm.Therefore,inin vivoapplications,very low light penetration and thus ineffcient photothermal heating is generated [24].In contrast,gold nanorods(GNRs)have attracted much interest because the absorption range of light can be fnely tuned by adjusting the aspect ratio,so the heating effciency can be maximized by using～800 nm absorption maximum.Also,theyhave the advantages of effcient large-scale synthesis,easy functionalization,and colloidal stability[25,26].However the clinical application of GNRs was limited due to a slight cytotoxicity caused by the remaining excess cetyltrimethylammonium bromide(CTAB),which is used as a template during synthesis and envelops the surfaces of the GNRs[27,28].To distinguish between the toxicity of the GNRs core and the exterior ligands,Zhu et al.[29]systematically evaluated the cellular uptake behavior and cytotoxicity of Au nanorods with various surface coatings,including organic cetyltrimethylammonium bromide(CTAB),poly(sodium 4-styrenesulfonate)(PSS),and poly(ethylene glycol)(PEG),and inorganic mesoporous silica (m-SiO2),dense silica(d-SiO2),and titanium dioxide(TiO2).The cellular behavior of Au nanorods was found to be highly dependent on the surface coating.CTAB-,PSS-,and m-SiO2-coated Au nanorods exhibit notable cytotoxicity,while PEG-, d-SiO2-,andTiO2-coated Au nanorods do not induce cell injury. Thus,the surface modifcations of GNRs shall reduce the cytotoxic effect.For example,phosphatidylcholine(PC)-modifed nanorods[30],poly(4-styrenesulfonic acid)(PSS)-coated nanorods[31],GNR-embedded polymeric nanoparticles[32],and PEG-modifed nanorods[33]have shown cytotoxicities lower than that of the CTAB-capped nanorods themselves and good photothermal effects.

Although ligand-conjugated GNRs are effective for photothermal killing of cancer cellsin vitro,desirable photothermal therapeutic effects in anin vivoanimal model are limited due to a high liver uptake during circulation[34].A high-level localization in the liver of CTAB-stabilized GNRs at 0.5 h after intravenous injection,which might be associated with the hard and rigid characteristics of GNRs,was reported[35].To overcome the limited effect of GNRs onin vivocancer photothermal therapy,PEGylation modifed GNRs attempted to lower the cytotoxicity and the liver accumulation of GNRs.However,complete suppression of tumor growth when using a hyperthermiabased treatment was not achieved,probably due to the very fast excretion of the PEGylated GNRs from the body(half-life of～1 h).Thus,a new biocompatible vehicle for the effcient delivery of GNRs into tumor sites is still an unmet need for safe and effective cancer therapy based on GNRs.In addition, specifc targeting therapy of GNRs also is another key issue for effcient cancer photothermal therapy.The biological modifcation of Au nanoparticles can be achieved on their surfaces, which is benefcial to improve biological activity and provide targeting property.For example,Aptamer-conjugated nanorods [36],folate-conjugated nanorods[37],and RGD-conjugated dendrimer modifed nanorods[38]have demonstrated selective and effcient photothermal killing of targeted tumor cells. Choi et al.[39]developed photo-cross-linked,Pluronic-based, temperature-sensitive nanocarriers that possessed excellent reservoir characteristics and a simple loading method with high loading capacity for large molecules(e.g.,proteins and gold nanoparticles).Importantly,these nanocarriers showed a long circulation time,a good tumor accumulation,and low liver uptake,which were associated with the fexible and soft characteristics as well as the hydrophilic surface from the PEG part of Pluronic.Furthermore,the tumor targeting and prolonged circulation(up to 72 h)were signifcantly improved and could be optimized by chitosan conjugation.The GNR-loaded,Pluronicbased nanocarriers as a hyperthermia agent were applied for enhanced cancer photothermal therapy.The GNR-loaded nanocarriers showed serum stability and photothermolysis of cancer cellsin vitro.The GNR concentration and the laser power density required for photodestruction of cancer cells were also signifcantly reduced,compared to other formulations,by using the nanocarrier system.Most of all,the optimized GNR-loaded nanocarriers resulted in a very impressive therapeutic effectin vivoin nude mice bearing tumors,and complete resorption of the tumor was achieved(Fig.2).As a theranostic platform,GNRs bear two disadvantages:(1)a relatively low specifc surface area limits the loading amount of drugs,and due to the often-observed clustering and aggregating of the GNRs within cells;(2)when GNRs were exposed to NIR laser,the desirable NIR window shifts to the visible spectral region and the advantage of deep tissue penetration is lost.To overcome these two drawbacks of GNRs,Zhang et al.[40]developed mesoporous silica-coated gold nanorods(Au@SiO2)as a novel cancer theranostic platform.The large specifc surface area of mesoporous silica guarantees a high drug payload.More interestingly,Au@SiO2as a drug carrier,under laser irradiation the drugrelease rate becomes much faster for any pH because the laserconverted heat dissociates the strong interactions between doxorubicin(DOX)and silica,thus more DOX molecules are released.The incorporation of the two nanomaterials created a new functionality:NIR laser-controlled drug release.For the goal of on-demand therapy and personalized medicine,the therapeutic modes of Au@SiO2-DOX,either chemotherapy or hyperthermia,can be manipulated simply by changing laser power density.

Fig.2–(a)Schematic illustration of the preparation of the pluronic-based nanocarriers and GNR loading into the nanocarriers.(b)Change in tumor volumes(an enlarged graph at initial time)and(c)the tumor images after NIR laser irradiations at 24 and 48 h after single i.v.injection of the nanomaterials.Reproduced with permission from Reference[39].

PTT has been demonstrated with certain types of Au nanostructures in early clinical trials.As an example,pilot clinical studies with Au nanoshell(Au nanoshells about 150 nm in diameter with a coating of polyethylene glycol 5000)have been approved by the Food and Drug Administration(FDA),wherein the nanoshells are given intravenously to patients for the treatment of head and neck cancer,as well as primary and/or metastatic lung tumors[41,42].For practical application,location and size of cancer shall be confrmed frst before therapy. Second,the treatment procedure needs to be monitored in real time during therapy.Finally,the effectiveness has to be assessed after therapy.Based on aforementioned claims,the design and synthesis of new PTT agents with imaging are of great importance.Ke et al.[43]developed a novel multifunctional theranostic agent for ultrasound contrast imaging and PTT.The gold-nanoshell microcapsules(GNS-MCs)were obtained by electrostatic adsorption of gold nanoparticles as seeds onto the polymeric microcapsule surfaces,followed by the formation of gold nanoshells by using a surface seeding method. Poly(lactic acid)(PLA),which is biodegradable and possesses an ultrasound signaling capability,was used as polymeric microcapsule in this study.NH2OH·HCl was used to reduce the gold precursor(HAuCl4)to bulk metal without the nucleation of new particles.Subsequently,all the added gold precursor was reduced and incorporated into larger particles that were deposited on the surface of the capsules.Finally,the microcapsules were freeze-dried.The encapsulated water in the inner aqueous phase of the microcapsules was sublimated to leave a small hollow space that could provide a basis for the ultrasound-responsive properties.In thein vivoultrasound imaging process,GNS-MCs were intravenously injected into rabbits,pulse inversion harmonic imaging(PIHI)contrast mode (with mechanical index,MI=0.42)and conventional B-mode sonograms before and after administration of GNS-MCs,as shown in Fig.3a and b.Excellent enhancements of rabbit kidney images suggested that GNS-MCs were able to traverse pulmonary capillaries to achieve systemic enhancement.The contrast enhancement can last more than 5 minutes,which is long enough to satisfy the clinical requirements.To evaluate the localized tumor photohyperthermic effect of GNS-MCs,HeLa cells (human cervical carcinoma cell lines)cultured in six-well plates were incubated with the GNS-MCs for 1 h,followed by illumination with an NIR laser(808 nm and 8 W/cm2for 10 min). Under an inverted fuorescence microscope(Fig.3c–f),a dark region was observed in the presence of both the agent and the laser(Fig.3f)that arises from the NIR laser-induced hyperthermic effect on cancer cells.In contrast,exposure of cancer cells to either GNS-MCs or a high-intensity NIR laser alone did not compromise cell viability(Fig.3c–e),thus indicating that GNS-MCs will cause cancer cells to die through photothermal effect only under NIR laser irradiation.

At the next step in the development of Au nanoshells,a novel multifunctional drug-delivery platform is developed based on cholesteryl succinyl silane(CSS)nanomicelles loaded with doxorubicin,Fe3O4magnetic nanoparticles,and gold nanoshells (CSS-DOX-Fe3O4-Au-shell nanomicelles),which can combine magnetic resonance(MR)imaging,magnetic-targeted drug delivery,light-triggered drug release,and PTT into one nanomaterial[44].The synthesis of the CSS-DOX-Fe3O4-Aushell nanomicelles was a multistep procedure.Especially,an enhancement for T2-weighted MR imaging is observed for the CSS-DOX-Fe3O4-Au-shell nanomicelles compared with that of sodium citrate modifed Fe3O4nanoparticles.In addition,the samples were irradiated repeatedly over a period of 10 min,followed by 1 h intervals with the laser turned off.A rapid release was observed upon NIR irradiation and the DOX release rate slowed down when the NIR irradiation was switched off.After the frst NIR exposure for 10 min,the percentage of released DOX increased from 7.1%to 18.4%.The percentage increased to 19.7%over the whole period without NIR laser irradiation, signifcantly lower than that with NIR laser irradiation(39.5%). This research achieved a synergistic effect in killing cancer cells by combined photothermal therapy and the magnetic-feldguided drug delivery.

Recently,the branched or star-shaped Au nanostructures consisting of a core and protruding arms have received particular interest due to their unique morphology and optical properties[45,46].Owing to the presence of sharp tips as well as their high surface to volume ratios,branched Au nanostructures could be more effective in photothermal conversion and drug loading relative to those with smooth surfaces [47].Wang et al.[48]prepared the Au nanohexapods,consisting of an octahedral core and six arms grown on its six vertices, by reducing HAuCl4with DMF in an aqueous solution containing Au octahedral seeds(Fig.4a).By controlling the length of the arms,their localized surface plasmon resonance(SPR)peaks could be tuned from the visible to the near-infrared region for deep penetration of light into soft tissues.When compared with the PEGylated nanorods(53.0±0.5°C)and nanocages (48.7±3.5°C),PEGylated nanohexapods showed the highest (55.7±2.4°C)photothermal conversion effciencyin vivo,owing to their highest tumor uptake and photothermal conversion effciency per Au atom.The different result using Au nanohexapods,nanorods,and nanocages indicates that all these Au nanostructures could absorb and convert NIR light into heat (Fig.4b).Au nanohexapods exhibited the highest cellular uptake and the lowest cytotoxicityin vitrofor both the as-prepared and the PEGylated samples.Combined together,Au nanohexapods are promising candidates for cancer theranostics in terms of both photothermal destruction and contrast-enhanced diagnosis.

2.1.2.Palladium nanoparticles

A wide variety of anisotropic gold nanostructures,including aggregates of colloidal particles,nanorods,nanoshells, nanocrosses,have been demonstrated for cancer photothermal therapy with NIR light.However,studies have shown that many anisotropic gold nanostructures exhibiting NIR SPR lack good photothermal stability upon irradiation with highpower NIR lasers.The heat generated by NIR irradiation can melt the anisotropic gold nanostructures into solid particles,leading to the loss of their NIR SPR[49,50].To overcome this limit,other noble nanoparticles with tunable localized surface plasmon resonance in NIR region have been designed and synthesized.Especially,palladium,because of its signifcantly higher bulk melting point(MPPd=1.828 KversusMPAu=1.337 K),should show enhanced photothermal stability.Using this point,Huang et al.[13]prepared ultrathin hexagonal palladium nanosheets with tunable(826–1068 nm)and strong SPR absorption(extinction coeffcient,4.1×109M?1cm?1)in the NIR region using a general CO-confned growth method.The nanosheet edge length is synthetically controllable from 20 to 160 nm,leading to tunable NIR SPR.Unlike anisotropic gold nanorods,the two dimensional structure of the palladium nanosheets appears to be highly stable upon NIR irradiation.Upon irradiation for 30 min by a NIR laser(808 nm,2 W),the sheet-like structure of the palladium nanosheets was retained well leading to a good SPR response in the NIR region,whereas gold nanorods were severely distorted under a similar irradiation power.Furthermore,the as-prepared palladium nanosheets appear to belargely biocompatible.The viable cell count for healthy liver cells was reduced by only 20%after 48 h of exposure to a 600 mg/mL solution of palladium nanosheets by incubating liver cancer cells with polyethyleneimine-exchanged palladium nanosheets.After 5 min of irradiation of an 808 nm laser with a power of 1.4 W/cm2,～100%of the cells were killed.This work frst suggests that Pd nanosheets have great potential in cancer photothermal therapy.

Fig.3–In vivoultrasonograms in the rabbit right kidney(a)pre-and(b)post administration of GNS-MCs.Both PIHI (MI=0.42)and conventional B-mode images are shown.(c–f)Fluorescence microscopy images of HeLa cells with different treatments stained with calcein AM;(c)no agent and no laser irradiation;(d)laser irradiation only;(e)agent only;(f)with both agent and laser irradiation.Note that the dark area represents the region of killed HeLa cells.(Scale bars:500 mm; GNS-MCs agent concentration:0.3 mg/ml;NIR laser:808 nm,8 W/cm2,10 min.)Reproduced with permission from Reference[43].

Fig.4–(a)TEM images of the Au nanohexapods. (b)18F-FDG PET/CT co-registered images of mice intravenously administrated with aqueous suspensions of PEGylated nanohexapods.Tumors were treated either with (solid circle+left arrow)or without(solid circle)laser irradiation.Reproduced with permission from Reference [48].

In addition,the sizes of mentioned Au nanorods and nanoshells are considerably large.For example,the size of Au nanorods is typically of～10 nm in diameter and～50 nm in length and Au nanoshells are more than 100 nm in diameter [27,51,52].A lot of investigations have demonstrated that the optimum intravenously administered nanoparticles should be between 10 and 50 nm in diameter to increase bloodstream circulation time[53,54]because larger nanoparticles are removed by the reticuloendothelial system(expressed as RES;e.g.liver, spleen),and smaller particles are removed by the renal system [55,56].To overcome slow renal clearance and high,nonspecifc accumulation in the reticuloendothelial system after systematic administration in the applications of those nanomaterials.Tang et al.[57]further successfully synthesized the ultrasmall Pd nanosheets(SPNS)with an average diameter of～4.4 nm(Fig.5),which is below the glomerular fltration-size threshold(10 nm)and thus particularly interesting for renal clearance studies.In addition,these SPNS were surface functionalized with reduced glutathione(SPNS-GSH). GSH(a tripeptide)can not only serve as capping agent to render the nanoparticles with relatively low affnities to serum proteins and lead to the desired stealthiness to the RES organs, but also contribute to effcient renal clearance of small-sized nanoparticles out of the body[58].In Fig.5d,the smaller sized Pd nanosheets modifed by GSH were both helpful in prolonging their circulation and half-life in the blood.The circulation half-lives was remarkably increased from 0.08 h for large Pd nanosheets(LPNS)to 1.25 h for SPNS-GSH.Additionally,a higher tumor accumulation was observed.Importantly,the total amount of Pd in SPNS-GSH formulation was signifcantly low in major organs,indicating that plenty of the SPNS-GSH were rapidly excreted from the body within the frst 24 h(Fig.5e). To minimize toxicity risks,an ideal nanomaterial-based therapy agent should be effectively cleared out of the body after treatment.The renal excretion has been recognized as a desirable pathway for nanoparticle clearance.It was found that more than 6.6%of the SPNS-GSH were excreted out of the body within 1 day p.i.and up to 30.9%after 15 day p.i.(Fig.5f).These observations confrm that SPNS-GSH could be cleared out from the body through the renal excretion route into the urine.Subsequently,they also developed a versatile system combining chemotherapy with PTT for cancer therapy[59].The system is based on ultrasmall Pd nanosheets(SPNS)functionalized with the anticancer drug doxorubicin hydrochloride(DOX)mainly through Pd–N coordination bonding.SPNS have an average diameter of～4.4 nm.After the SPNS-DOX,hybrid nanoparticles are surface-functionalized with reduced glutathione(GSH),the obtained SPNS-DOX-GSH composite exhibits the following synergistic properties for cancer therapy:(1)The coordinative loading of DOX on SPNS enhances its accumulation in tumor tissue,which signifcantly reduces the laser power required to achieve effective tumor ablation;(2)The DOX was released fromSPNS-DOX in a pH-responsive manner.The release rate of DOX increased signifcantly with decreasing pH of buffer solutions.The cancer cells usually have a lower pH than normal cells.Thus SPNS-DOX should release DOX faster in cancer cells than in normal cells;(3)The size of SPNS-DOX-GSH is below the renal fltration size,allowing the therapeutic agents to be effectively cleared from the body.

Fig.5–(a)TEM image of ultra-small palladium nanosheets(SPNS).Inset:diameter distribution of the SPNS.(b)Absorption spectrum of the SPNS.(c)Photothermal effect of SPNS.The temperatureversustime plots was recorded for various concentrations of SPNS upon irradiation by a 1 W laser.(d)SPNS-GSH showed prolonged blood circulation compared with LPNS and SPNS.(e)Biodistribution of LPNS,SPNS and SPNS-GSH in various organs at 24 h p.i.Samples were measured using ICP-MS.Error bars were based on the standard error of the mean of triplicate samples.(f)Urinary cumulative excretion of SPNS-GSH in rats(n=3)following i.v.administration at a single dose of 10 mg/kg.The amount of Pd in urinary samples were measured by ICP-MS.Reproduced with permission from Reference[57].

Besides the 2-dimensional Pd sheets,designed Pd nanoparticles also show very broad absorption through the UV–Vis-NIR region.This broad absorption nature would help to extend our choices on the wavelengths of NIR lasers in PTT application.Recently,Xiao et al.prepared small Pd NPs with a special porous architecture,which exhibit superior performance in PTT compared with solid Pd nanocubes due to the enhanced NIR absorption[60].The photothermal conversion effciency of the porous Pd is as high as 93.4%,comparable to typical gold nanorods.In the presence of porous Pd NPs(with concentration as low as 23 mg/mL),almost 100%HeLa cells were destroyed after 4 min illumination with an 808 nm laser at a power density of 8 W/cm2.Seventy percent of cells were killed after 4 min irradiation with a 730 nm laser at power density 6 W/cm2.These results demonstrated the high effcacy of porous Pd nanoparticles for PTT.As the size of porous Pd NPs is relatively small,it would prolong the blood circulation time when it is administered to a live animal’s body.In particular,the porous structure of Pd NPs may allow us to deliver the drug into the cancer cell.This porous Pd structure holds great potential in PTT and drug delivery.

2.2.Metal chalcogenide

Although the metallic nanoparticles(Au and Pd nanoparticles) with bioinert properties show great promise for clinical applications,they are non-biodegradable,and their long-term metabolism has raised concerns[61–63].Based on reported studies,semiconductor metal chalcogenide nanoparticles are a class of inorganic photo absorbers that provide an alternative to noble metal nanoparticles.For clinical requirement, before PTT application,cellular uptake and cytotoxicity properties of metal chalcogenide nanoparticles(MCNPs)have to be investigated.Cellular uptake of CuS andWS and their good biocompatibility were confrmed with both healthy and cancer cell lines.Several research groups have demonstrated that cell uptake and cellular toxicity of MCNPs depend on the particle size,shape,surface charge and functional groups[64,65].No cytotoxicity is observed up to 100 mg/mL for non-modifed 100 nm MCNPs[66–69],which is far beyond the concentration required for most therapeutic treatments.

In contrast to exogenous gold,copper is essential for human health[70].In adults,the highest safe intake level of Cu is 10 mg daily[71],indicating that CuS nanoparticles(CuS NPs)may be metabolized by humans.CuS NPs with particle sizes of 35 and 11 nm[14,72],fower-like CuS superstructures(1 μm in diameter)[73],and Cu9S5plate-like nanocrystals(70 nm×13 nm)[74] have intense optical absorption at NIR region.However,critical pharmacological parameters such as body disposition and long-term metabolism of these CuS nanostructures remain unknown.Moreover,data regarding the cytotoxicity profle of the CuS nanostructures are lacking.This knowledge is essential for clinical applications of CuS nanomaterials.Recently,Guo et al.[75]compared degradability and toxicity between two types of photothermal nanoparticles,i.e.,hollow gold nanospheres (HAuNSs)and hollow CuS nanoparticles(HCuSNPs),in mice following systemic administration.The two nanoparticles were formulated with similar particle size and morphology.They were both surface-modifed with polyethylene glycol(PEG)to evade uptake by monophagocytic systems.The injected PEGylated HCuSNPs(PEG-HCuSNPs)are eliminated through both hepatobiliary(67 percentage of injected dose,%ID)and renal (23%ID)excretion within one month post injection.By contrast,3.98%ID of Au is excreted from liver and kidney within one month after i.v.injection of PEGylated HAuNSs(PEGHAuNSs).Comparatively,PEG-HAuNSs are almost nonmetabolizable,while PEG-HCuSNPs are considered biodegradable nanoparticles.PEG-HCuSNPs do not show signifcant toxicity by histological or blood chemistry analysis.

However,with further studies,the researchers found that nanoparticle-mediated photothermal ablation is employed primarily as a local cancer treatment at the primary site.Thus, it is less effective in controlling metastatic cancer.An ideal cancer PTT should not only eradicate the treated primary tumors,but also induce a systemic antitumor immunity,control metastatic tumors and achieve the goal of long-term tumor resistance.For this reason,one promising strategy is to combine photothermal therapy with immunotherapy[76,77].Laserinduced tumor cell death,on the other hand,can release tumor antigens into the surrounding milieu.Concomitantly,immunoadjuvants for cancer immunotherapy promote antigen uptake and presentation by professional antigen-presenting cells,thus triggering specifc antitumor immunity[76].Therefore,PTT may act synergistically with immunotherapy to enhance immune responses,rendering the tumor residues and metastases more susceptible to immune-mediated killing. Recently,Lu et al.[78]developed hollow copper sulfde nanoparticles with photothermal immunotherapy(Fig.6).Theysynthesized immunoadjuvants,oligodeoxynucleotides containing cytosine-quanine(CpG)coated hollow copper sulfde nanoparticles(HCuSNPs-CpG)for“photothermal immunotherapy”in a mouse breast cancer model.Success of this technique relies on photothermally triggered disintegration of HCuSNPs,allowing the HCuSNPs-CpG conjugates to reassemble and transform into chitosan–CpG nanocomplexes.The chitosan–CpG nanocomplexes increase their tumor retention and promote CpG uptake by plasmacytoid dendritic cells.The HCuSNPs-CpG-mediated photothermal immunotherapy elicits more effective systemic immune responses than immunotherapy or PTT alone,resulting in combined anticancer effects against primary treated as well as distant untreated tumors. Strong antitumor effectiveness,combined with quick elimination,would seem to justify further development of this HCuSNPs conjugate-based photothermal immunotherapy.

Fig.6–Diagram of HCuSNPs-CpG-mediated photothermal immunotherapy of both primary treated and distant untreated tumors.Reproduced with permission from Reference[78].

Besides aforementioned CuS nanostructures,novel class of metal chalcogenide as photothermal therapy agent is twodimensional(2D)transition-metal dichalcogenides(TMDCs). For example,MoS2,MoSe2,WSe2and WS2,all consist of a hexagonal layer of metal atoms(M)sandwiched between two layers of chalcogen atoms(X)within stoichiometry MX2.The common feature of these materials is the layered structure with strong covalent bonding within each layer and weak van der Waals forces between different MX2sheets.For their special characteristics,TMDCs have become the rising star in recent years, offering great opportunities in physics,chemistry and materials science.However,the exploration of this new class of TMDCs nanomaterials in the area of biomedicine is still at its infant stage.Currently,Chou et al.demonstrated the possibility of using as-made MoS2nanosheets as a new NIR absorbing agent forin vitro-photothermal-killing of cancer cells for the frst time[79].Chen and co-workers presented the fabrication of a two-dimensional MoS2/Bi2S3 composite theranostic nanosystem for multimodality tumor CT and PA imaging and photothermal therapy[80].Li et al.demonstratedin vivophotothermal-ablation of tumors by local injection of Bi2Se3nanosheets directly into tumors[81].In recent years,Cheng et al.[82]used the Morrison method to fabricate singlelayered WS2nanosheets with high-yield.Subsequently,using the thiol chemistry method,the surface of WS2nanosheets is coated with polyethylene glycol(PEG),which greatly improves the physiological stability and biocompatibility of those nanosheets(Fig.7a).It is well-known that X-ray computed tomography(CT)imaging is one of the most commonly used imaging tools for clinic diagnosis and medical research.Based on a lot of studies,CT contrast agents often absorb and weaken the incident X-rays to produce tissue contrasts in the diagnosticregime.Thus the attenuation of CT contrast depends on the interaction between X-ray and the inner shell electrons of atoms with high atomic numbers.Fig.7b–d presents the CT images and Hounsfeld unit(HU)values of different concentrations of WS2–PEG in water,which show a sharp signal enhancement as the increase of WS2–PEG concentrations.The slope of the HU value for WS2–PEG is about 22.01 HU L/g,which appeared to be much higher than that of iopromide(15.9 HU L/g),a commercial iodine-based CT contrast agent used in the clinic. Utilizing the strong absorbance in the NIR region and strong X-ray attenuation ability of WS2,Liu et al.successfully demonstrate thein vivoenhanced X-ray CT and photoacoustic tomography(PAT)bimodal imaging of tumors,respectively.In animal experiments,after either intratumoral injection with a low dose of WS2–PEG or intravenous injection with a moderate dose of this nanoagent,realizing 100%of tumor elimination after NIR laser irradiation at a relatively low power density(Fig.7e and f).These works encourage further indepth investigations of this novel type of nanomaterials for biomedical applications.

Fig.7–(a)A scheme showing the exfoliation and PEGylation of WS2nanosheets.(b)CT images of WS2–PEG solutions with different concentrations.In vivodual-model imaging in 4T1-tumor bearing mice.(c)CT images of mice before and after i.t. injection with WS2–PEG(5 mg/ml,20 μl).(d)CT images of mice before and after i.v.injection with WS2–PEG(5 mg/ml, 200 μL).The CT contrast was obviously enhanced in the mouse liver(green dashed circle)and tumor(red dashed circle). (f)Survival curves of mice after various treatments as indicated in(e).Reproduced with permission from Reference[82].

2.3.Carbon-based nanomaterials(e.g.,graphene oxide and carbon nanotubes)

2.3.1.Graphene oxide

Graphene oxide(GO)is a two-dimensional material obtained from the oxidative exfoliation of graphite.Graphene and GO have become one of the most attractive materials for the following reasons:(1)large surface area;(2)lightweight;(3)high strength and electrical conductivity;(4)the capacity of optical property-expressing plasmon,fuorescence,and nonlinear emission.The absorbance of GO extends from the ultraviolet(UV) wavelength to the NIR region.Thus,the absorbance at 808 nm was used to express the PTT.This photothermal property of GO was applied inin vivophotothermal ablation of tumors[83]. However,this GO dispersion was not easily achieved in bioapplications because of the aggregation that is caused by the high degree of the binding between GO and proteins or with other salts in serum.Therefore,the carboxyl groups in the asprepared GO were functionalized covalently using amineterminated PEG(PEG–GO)to increase the level of dispersion and decrease cytotoxicity.Robinson et al.[84]developed nanosized,reduced GO sheets(nano-rGO)(～20 nm in average lateral dimension)with noncovalent PEGylation(PEG-rGO).The nano-rGO was aggregated in the solution after reduction due to the removal of functional groups from the GO sheets.The increased hydrophobicity of the nano-rGO sheets caused aggregation even with the remaining PEG chains attached to GO through the reduction.To restore the dispersion,the PEG-rGO was PEGylated functioned again using sonication with a polymer(two methoxy-terminated PEG and one C17 chain attached to the poly maleic anhydride)to form polymer coated PEG-rGO(expressed as polymer-2PEG-rGO).The polymer-2PEG-rGO regained stability as a homogeneous suspension in buffers and other biological solutions without aggregation even under harsh centrifugation conditions.In addition,it is worth noting that the polymer-2PEG-rGO resulted in a signifcant～6.8 fold increase in the NIR absorption at 808 nm than non-reduced nano-GO and covalently PEGylation nano-GO.This enhance was ascribed to the increase of the degree of the π conjugation in GO after chemical reduction.Subsequently,the high NIR absorbance of polymer-2PEG-rGO allowed for effective photothermal heating of solutions at a low concentration of polymer-2PEG-rGO.At a concentration of～20 mg/L,rapid photothermal heating occurred upon irradiation of a low power 808 nm laser at 0.6 W/cm2.Temperatures above the photoablation limit of 50°C were readily reached within 5 min of irradiation.This work shall lead to systematicin vivoinvestigations of nano-rGO for photothermal treatment of tumor models in mice using low doses of nano-rGO at low laser powers.

To further enhance the photothermal effect of nanomaterials,the plasmon-richAu nanoparticles[85]and quantum dots(QDs)[86]were combined with GO.For example,Lim et al. [85]synthesized reduced GO-coated gold nanoparticles(gold nanoshells and nanorods)by electrostatic interactionin situchemical reduction.The new hybrid material generated welldefned r-GO-AuNS and r-GO-AuNR.The r-GO as shell and Au nanoshell/Au nanorod as core existed in the hybrid nanostructures.The r-GO-AuNS and r-GO-AuNR colloidal solutions exhibit good stability at room temperature,because the carboxylic acid and hydroxyl groups still exist in incomplete reduced r-GO.The photothermal performance of r-GO-AuNS or r-GO-AuNR was studied in dry and solution state under NIR illumination(808 nm,continuous wave,power density:3.0 W/ cm2).For the dry state,r-GO-AuNS/r-GO-AuNR led to a 2.9 fold increase in ΔT upon irradiation compared withAu nanoparticles and non-reduced GO-AuNS/GO-AuNR.For the solution state, solutions with the same optical density and sample volume were illuminated for 5 min at 3.0 W/cm2,continuous wave(CW): 808 nm.The heating rates of r-GO-AuNS/r-GO-AuNR solution were greater than Au nanoparticles and non-reduced GO-AuNS/GO-AuNR.These independent measurements demonstrate the greater photothermal effect of particles coated with r-GO,which could be attributable to the interactions between the r-GO and the gold plasmons.The therapeutic effect of the photothermal rGO-AuNS/r-GO-AuNR was further demonstrated on human umbilical vein endothelial cells(HUVECs). HUVECs were incubated with non-reduced GO-or r-GO-coated and uncoated Au nanoparticles for 24 h followed by irradiation(3.0 W/cm2,CW:808 nm)for 1 min.The cell viability in r-GO-AuNS and r-GO-AuNR were 23%and 33%,respectively,whereas 41–43%for Au NS or GO-Au NS,and 53–57%for Au NR and GO-Au NR,respectively.These results showed that r-GO coating on plasmonic nanoparticles accelerated cell killing. The main reason for increased killing of cells is that r-GO-Au nanoparticles showed very powerful phototoxicity for cancer cells.Showed excellent photothermal properties,which may be useful in improving biomedical applications based on the photothermal effect,by increasing their effcacy and/or decreasing the duration of therapy.

As we all know,when early studies on GO-assisted cancer therapeutics,GO was limited to serving as the drug delivery vehicle,as GO-assisted chemotherapy.Generally,the hexagonal arrangement of carbon in GO favors the noncovalent loading of anticancer drug cargo using π–π stacking.The GO exhibits a radical increase in drug loading of approximately 200%by weight,and this is the frst drug carrier to achieve over 100% loading consistently.In addition,the GO was able to unload the cargo under highly acidic and basic conditions because of the compromise in the hydrogen bonds between the–COOHand the–OH groups of GO,and between the–OH and the–NH2groups of DOX[87,88].Following the studies on the aforementioned spontaneous release,the photothermal characteristics of GO were subsequently introduced to determine the combination of PTT and chemotherapy.Wang et al.[89]used mesoporous silica-coated GO(expressed as GS)to administer chemotherapy and PTT.In this study,the GO was coated with mesoporous silica to form a sandwich structure.The GS was then coated with PEG(expressed as GSP)to achieve solubility and IL31 peptides for glioma cell targeting(expressed as GSPI). Finally,GSPI loaded the chemotherapeutic drug DOX,yielding GSPID(Fig.8a),the photothermal effect of GSPI could promote the release of DOX.The NIR irradiation apparently enhanced the cumulative release of DOX at different time and pH values due to heat stimulative dissociation of the strong interactions between DOX and GSPI including π–π stacking and pore adsorption(Fig.8b).The result means the photothermal effect of GSPI could signifcantly increase the sensitivity of chemotherapy(Fig.8c).Regarding the targeting property of IL31 peptides,we confrmed that GSPID exhibited signifcantly higher cellular uptake and cytotoxicity in glioma cells,and no apparent effect on normal cells,compared to GSPD.These fndings provided an excellent drug delivery system for combined therapy of glioma due to the advanced chemo-photothermal synergistic targeted therapy and good drug release properties of GSPID,which could effectively avoid frequent and invasive dosing and improve patient compliance.

Fig.8–(a)Design of GSPID as a multifunctional drug delivery system for combined dhemo-photothermal targeted therapy of Glioma.(b)Cumulative release profles of DOX from GSPID at different pH values with 6 W/cm2NIR irradiation.Data are expressed as mean±SEM(n=3).(c)Cell viability profles of glioma cells.Reproduced with permission from Reference[89].

2.3.2.Carbon nanotubes

Carbon nanotubes(CNTs)are carbon nanomaterials,including both single-walled nanotubes(SWNTs)and multi-walled nanotubes(MWNTs).The strong optical absorption and high photon-to-thermal energy conversion effciency of CNTs in the NIR region combined with a high-absorption cross-section make CNTs a suitable candidate for PTT[90,91].The proper surface functionalization of CNTs renders them biocompatible and enables them serve as effcient cancer drug delivery vehicles. Based on a lot of studies,the loading of aromatic drugs(e.g. DOX)using CNTs by employing noncovalent π–π stacking is a simple process.In particular,the surface of the CNTs can be occupied approximately 70–80%by DOX molecules[92].Based on this,Liu et al.[93,94]demonstrated DOX loading,delivery and chemotherapy using the composite.The SWNTs were coated with mesoporous silica(MS)to load drug and thenfurther functionalized with polyethylene glycol(PEG)to acquire enhanced solubility and stability in physiological environments,expressed as SWNT@MS-PEG.Interestingly,SWNT@ MS-PEG with drug molecules(DOX),photothermal heating of the core SWNT under NIR laser would trigger release of drug molecules(DOX)loaded inside the mesoporous silica shell,resulting in the enhanced cancer cell killing.This enhancement of therapeutic effect was ascribed so that DOX molecules are encapsulated inside mesoporous structure of the MS shell.There was a weaker interaction between DOX and nanotube surface in the SWNT@MS-PEG/DOX complex.The drug release in the SWNT-PEG/DOX appears to be less sensitive to temperature, while in the SWNT@MS-PEG/DOX photothermal heating could instantly trigger DOX release from pores as also seen in many other MS coated nanostructures.Next,4T1 cells were incubated with SWNT@MS-PEG/DOX,SWNT@MS-PEG,and free DOX for 1 h,followed by irradiation with the 808 nm laser at different power densities for 20 min(Fig.9a).It was found that SWNT@MS-PEG/DOX treated cells showed remarkably reduced viabilities as laser power intensities increase.In comparison, the free DOX induced cancer cell killing was not signifcantly affected by laser irradiation.Photothermal heating induced by SWNT@MS-PEG without chemotherapy,on the other hand,appeared to be much less effective compared to the combination therapy,especially under lower laser powers(Fig.9b).Therefore,it is concluded that NIR-light triggered intracellular drug release in such combined photothermal and chemotherapy could offer an obvious synergistic effect to destruct cancer cells.In vivoBald/c mice were developed by cancer 4T1 cells and i.v. injected with SWNT@MS-PEG/DOX,SWNT@MS-PEG,DOX,and PBS,respectively.After 24 h,the tumors were irradiated by the 808 nm laser at a moderate power density of 0.5 W/cm2for 20 min.It was found that the tumor surface temperatures of mice treated with SWNT@MS-PEG/DOX and SWNT@MS-PEG were increased and maintained at～48°C during laser irradiation.In contrast,the mice treated with PBS and DOX showed no apparently temperature increase in the tumor region after being irradiated by the laser.Remarkably,the tumor growths on mice with injection of SWNT@MS-PEG/DOX were effectively inhibited after NIR laser irradiation as a result of the combined chemo-photothermal therapy.In addition,SWNT was loaded with docetaxel(DTX)using π–π accumulation,and was subsequently subjected to surface modifcation conducted using poly-N-vinylpyrrolidone(PVP)and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000]-maleimide[95].Maleimide was further conjugated with the targeting NGR peptide(Asn-Gly-Arg)to form NGR-SWNT/ DTX.The NGR-SWNT/DTX was administered intravenously to the mice bearing S180 tumor xenografts followed by irradiation with an 808 nm laser at a power density of 1.4 W/cm2(NGRSWNT/DTX with laser)for 13 days.The tumor volume was inhibited at the early stages(7th day)of combined chemotherapy and PTT.The other groups that were treated by chemotherapy only(NGR-SWNT/DTX)or PTT only(NGRSWNT with laser)exhibited a continual increase in tumor volume.Further surface engineering of those nanostructures may allow active tumor targeting and more precisely controlled drug release under other stimuli in addition to NIR light, to achieve cancer therapy with even better specifcity.

The combination of PTT nanomedicine-treatment together with antibody-based immunotherapy may be a novel cancer therapeutic strategy,which not only is able to destroy the primary tumor,but also able to inhibit cancer metastasis at distant organs in the body[96].However,whether and how CNT-based photothermal therapy would trigger any immunological response and play any effect in inhibiting tumor metastasis remain largely unknown.Recently,Wang et al.[97] reported that photothermal ablation of primary tumors with single-walled carbon nanotubes(SWNTs)in combination with anti-CTLA-4 antibody therapy is able to prevent the development of tumor metastasis in mice.It is found that polymercoated SWNTs could not only be used for photothermal tumor destruction,which releases tumor-associated antigens,but also can act as an immunological adjuvant to greatly promote maturation of dendritic cells(DCs)and production of anti-tumor cytokines.The mice bearing subcutaneous 4T1 murine breast tumors were intratumorally injected with SWNTs (dose=0.33 mg/kg).After irradiation with an 808 nm NIR laser at 0.5 W/cm2for 10 min,the tumor temperature jumped to 53°C, which is high enough to effectively ablate tumor cells.After SWNT-induced photothermal treatment,all tumors on micewere completely eliminated,without showing a single case of tumor relapse at their original sites.In addition,both SWNTs alone and SWNT-based PTT were able to increase the secretion of pro-infammatory cytokines IL-1β,IL-12p70,IL-6 andTNF-α.Particularly,the serum level of TNF-α,which plays an important role in anti-tumor immune responses,was dramatically enhanced after SWNT-treated PTT.It is likely that PTT with CNTs is not just burning of tumors,but also to inhibit cancer metastasis.Thus,immunological responses triggered by PTT may offer clinically valuable therapeutic advantages over surgery in cancer treatment.

Fig.9–(a)A scheme showing NIR-triggered drug release from SWNT@MS-PEG/DOXin vitro.The uncovered DOX fuorescence from its quenched state inside SWNT@MS-PEG/DOX could be an indicator of drug release. (b)Relative viabilities of 4T1 cells after various treatments. In this experiment,4T1 cells were incubated with SWNT@ MS-PEG/DOX(L+),SWNT@MS-PEG(L+),SWNT@MS-PEG/DOX, DOX(L+)and free DOX([DOX]=25 μM),for 1 h.Then,the cells were washed with fresh cell culture and irradiated with the 808 nm laser at different power densities for 20 min.Afterwards,those cells were re-incubated for additional 24 h before the MTT assay.Pvalues were calculated by Tukey’s post-test(***P＜0.001,**P＜0.01,or *P＜0.05).Reproduced with permission from Reference[95].

In addition,SWNTs decorated with noble metals were used to conduct effcient PTT and surface-enhanced Raman spectroscopy(SERS)imaging.The DNA-functionalized SWNTs are modifed with noble metal(Ag or Au)nanoparticlesviaanin situsolution phase synthesis method comprised of seed attachment,seeded growth,and surface modifcation with PEG, yielding SWNT-Ag-PEG and SWNT-Au-PEG nanocomposites stable in physiological environments[98].Subsequently,utilizing folic acid(FA)conjugated SWNT-Au-PEG-FA,selective cancer cell labeling and Raman imaging is realized.Owing to the strongly enhanced Raman signals of SWNT-Au-PEG-FA,the cancer cells showed remarkably shortened imaging time compared to that when using a non-enhanced SWNT-nanoprobe (Fig.10).Moreover,the SWNT-Au-PEG-FA nanocomposite also exhibits dramatically improved photothermal cancer cell killing effcacy.The enhancement is attributable to the strong surface plasmon resonance absorption by the gold shell grown on the nanotube surface.The photostability of SWNT-Au-PEG-FA was compared with that of Au NR by exposing them for 1 h to an 808 nm laser with a power density of 1 W/cm2.Au NR exhibited a complete loss of NIR absorbance,whereas SWNT-Au-PEG-FA retained nearly 87%of the absorbance intensity.Taking the intrinsic properties of both SWNTs and gold nanoparticles together,the SWNT-Au nanocomposite developed here may be an interesting and promising nano-platform in biosensing, optical imaging,and phototherapy.

Carbon-based nanomaterials,i.e.graphene oxide and carbon nanotubes,are fabricated and utilized as a multifunctional platform for chemotherapy and photothermal therapy.Carbonbased nanomaterials have demonstrated large heating effciency and high drug loading amount.However potential clinical implementations of carbon-based nanomaterials are still hampered by distinctive barriers such as poor bioavailability and intrinsic toxicity,which cause diffculties in tumor targeting and penetration as well as in improving therapeutic outcome.For sure,this will be one of the main working areas in the feld of carbon-based nanomaterials during the next years.

Fig.10–Schematic illustration of the synthetic procedure used for the SWNT@Au nanocomposite.Reproduced with permission from Reference[98].

3.Conclusions and perspectives

In summary,we have presented a detailed review of the inorganicnanocompositematerialsforPTT.Inorganic nanocomposites such as gold nanoparticles,palladium nanoparticles,metal chalcogenide nanoparticles,carbon nanotubes and graphene oxide have been extensively explored as photothermal therapy agents(PTA)for cancer therapy. We summarized for each kind of inorganic PTA,fundamental light-to-heat conversion property,synthesis method,the effciency ofin vitroandin vivothermal therapies,and multifunctional synergy therapies(already possible by the combination of many different techniques such and PTT,CT,and PAT).Based on studies reviewed,inorganic PTA-assisted cancer therapy can effectively induce site-specifc cell death in bothin vitroandin vivotreatments.The tremendous development of nanotechnology brings us closer to the dream of clinical application of nanoparticles in photothermal therapies of tumors. However,the following disadvantages of inorganic PTA are required to note for the clinical execution of these cancer therapies in the future:

(1)Photothermal conversion effciency and stability:Accordingtoclinicalrequirement,theinorganic nanoparticles with high energy conversion effciency and good stability should be synthesized.The studies show that high photothermal conversion effciency requires large absorption cross sections of nanoparticles for optical wavelengths.This would ensure an effcient absorption of optical radiation,thus achieving the PTT with lowpower laser sources.For example,the optical-response band and the photothermal effciency ofAu nanoparticles can be tuned and improved by exploring plasmon hybridization by the introduction of the dielectric gap in the form of core–shell structures[99].In addition,owing to the presence of sharp tips as well as their high surfaceto-volume ratios,the absorption of branched Au nanostructures could be more effective in photothermal conversion.

Besides high photothermal conversion,good photothermal stability also is necessary in PTT application.From the shapes of the molecules,the anisotropic nanostructures lack good thermal stability.For example,gold nanorods(GNRs)have the tendency to transform into nanospheres when exposed to NIR laser,accompanied with the disappearance of the NIR absorption band[50].The photothermal stability of GNRs also can be improved by design of core–shell structures.Thus,it is believed that the performance of PTA can be further improved with the reasonable design and synthesis.

(2)Toxicity:Generally,the clearance of PTA and their acute and long-term toxicity need to be thoroughly examinedbefore use.In addition,toxicity of PTA should only be activated in the presence of optical radiation.PTA should be non-toxic to both healthy cells and cancer cells without NIR radiation.This is required to achieve a selective treatment with minimum side effects.Thus, these nanoparticles should meet the requirement of the safety,effectiveness,and quality control standards of new drug development.For GNRs,the cetyltrimethylammonium bromide(CTAB)used as a surfactant stabilizer during the synthesis process could cause cytotoxicity and thus needs to be replaced prior to anyin vitroorin vivoapplication.To overcome the limitation of GNRs inin vivocancer PTT,PEGylation of GNRs was attempted to lower the cytotoxicity and the liver accumulation of GNRs.Additionally,the CNTs were reported to show genotoxicity,as they can pierce the cells and enter the nucleus easily.CNTs with appropriate surface coatings have been found to be not obviously toxic to animals and could be gradually excreted from mice over time[100,101].Based on aforementioned studies,to reduce the toxicity of PTA,the surface of the PTA is often modifed by biological molecules(PEG, FA,GSH,etc.).Thus,it is required to have charge,or functional groups,or hydrogen bonds,etc.on the surface of the photothermal materials.

(3)Visual-guided therapies:The unique physicochemical properties of nanomaterials have offered an opportunity to integrate different theranostic modalities into a single nanoplatform for combined cancer treatments with real-time diagnosis.Before these new nanomaterials are tested in cancer patients,detailed preclinical studies should be conducted,especially to investigate the pharmacokinetics,in vivotumor targeting,and therapeutic effects of these nanoparticles.The PEGylated PTA of gold nanoparticles and carbon-based materials have demonstrated large heating effciency and outstanding biocompatibility.However,they have the main drawback of showing a weak fuorescence,which makes it hard to track them in realin vivotreatments.This implies the incorporation of luminescent nanothermometers in the volume to be treated simultaneously with the photothermal agents.Therefore,fuorescence imaging and realtime control can be realized and adjusted using various dopants such as quantum dots,organic dye,etc.We believe that multifunctional nanoparticles will be developed offering heating,tracking and sensing in a single structure in the near future.

On the basis of the current research,we think that the ultimate challenge for cancer treatment is to be able to diagnose and cure cancer without surgical intervention and avoiding the occurrence of side effects.New technology should aim to develop nanomaterials that allow for effcient,specifcin vivodelivery of therapeutic agents without systemic toxicity,and the dose delivered as well as the therapeutic effcacy can be accurately monitored non-invasively over time.Therefore,the research on new inorganic nanomaterials as PTA still is an attractive feld that should be highly improved.This will be one of the main working areas in the feld of nanotechnology and nanomedication in the next years.

Acknowledgments

This study was fnancially supported by the National Natural Science Foundation of China(Grant No.21401132),Research Project of Science and Technology of Department of Education of Liaoning Province(Grant No.L2014377)and the Key Laboratory Program of Functional Inorganic Material Chemistry(Heilongjiang University),Ministry of Education.

R E F E R E N C E S

[1]Ferlay J,Soerjomataram I,Ervik M,et al.GLOBOCAN 2012 v1.0,Cancer incidence and mortality worldwide:IARC CancerBase No.11[Internet].Lyon,France:International Agency for Research on Cancer,＜http://globocan.iarc.fr＞; 2013[accessed 08.15].

[2]Zhang ZJ,Wang J,Chen CY.Near-infrared light-mediated nanoplatforms for cancer thermo-chemotherapy and optical imaging.Adv Mater 2013;25:3869–3880.

[3]Paiva MB,Blackwell KE,Saxton RE,et al.Nd:YAG laser therapy for palliation of recurrent squamous cell carcinomas in the oral cavity.Lasers Surg Med 2002;31:64–69.

[4]Paiva MB,Blackwell KE,Saxton RE,et al.Palliative laser therapy for recurrent head and neck cancer:a phase II clinical study.Laryngoscope 1998;108:1277–1283.

[5]Gao L,Fei JB,Zhao J,et al.Hypocrellin-loaded gold nanocages with high two-photon effciency for photothermal/photodynamic cancer therapy in vitro.ACS Nano 2012;6:8030–8040.

[6]Wang Y,Wang KY,Zhao JF,et al.Multifunctional mesoporous silica-coated graphene nanosheet used for chemo-photothermal synergistic targeted therapy of glioma.J Am Chem Soc 2013;135:4799–4804.

[7]Liu SH,Guo YB,Huang RQ,et al.Gene and doxorubicin codelivery system for targeting therapy of glioma. Biomaterials 2012;33:4907–4916.

[8]Weissleder R.Receptor-targeted optical imaging of tumors with near-infrared fuorescent ligands.Nat Biotechnol 2001;19:316–317.

[9]Zheng XH,Zhou FF,Wu BY,et al.Enhanced tumor treatment using biofunctional indocyanine greencontaining nanostructure by intratumoral or intravenous injection.Mol Pharm 2012;9:514–522.

[10]Yu J,Javier D,Yaseen MA,et al.Self-assembly synthesis, tumor cell targeting,and photothermal capabilities of antibody-coated indocyanine green nanocapsules.J Am Chem Soc 2010;132:1929–1938.

[11]Li D,Huang J,Kaner RB.Nanofbers:a unique polymer nanostructure for versatile applications.Acc Chem Res 2009;42:135–145.

[12]Hu M,Chen J,Li ZY,et al.Gold nanostructures:engineering their plasmonic properties for biomedical applications. Chem Soc Rev 2006;35:1084–1094.

[13]Huang XQ,Tang SH,Mu XL,et al.Freestanding palladium nanosheets with plasmonic and catalytic properties.Nat Nanotechnol 2011;6:28–32.

[14]Zhou M,Zhang R,Huang M,et al.A chelator-free multifunctional[64Cu]CuS nanoparticle platform for simultaneous micro-PET/CT imaging and photothermal ablation therapy.J Am Chem Soc 2010;132:15351–15358.

[15]Hessel CM,Pattani VP,Rasch M,et al.Copper selenide nanocrystals for photothermal therapy.Nano Lett 2011;11:2560–2566.

[16]Moon HK,Lee SH,Choi HC.In vivo near-infrared mediated tumor destruction by photothermal effect of carbon nanotubes.ACS Nano 2009;3:3707–3713.

[17]Kim J,Galanzha EI,Shashkov EV,et al.Golden carbon nanotubes as multimodal photoacoustic and photothermal high-contrast molecular agents.Nat Nanotechnol 2009;4:688–694.

[18]Li M,Yang X,Ren J,et al.Using graphene oxide high nearinfrared absorbance for photothermal treatment of Alzheimer’s disease.Adv Mater 2012;24:1722–1728.

[19]Yang K,Zhang SA,Zhang GX,et al.Graphene in mice: ultrahigh in vivo tumor uptake and effcient photothermal therapy.Nano Lett 2010;10:3318–3323.

[20]Mayer KM,Hafner JH.Localized surface plasmon resonance sensors.Chem Rev 2011;111:3828–3857.

[21]Alkilany AM,Thompson LB,Boulos SP,et al.Gold nanorods: their potential for photothermal therapeutics and drug delivery,tempered by the complexity of their biological interactions.Adv Drug Delivery Rev 2012;64:190–199.

[22]Chen HJ,Shao L,Li Q,et al.Gold nanorods and their plasmonic properties.Chem Soc Rev 2013;42:2679–2724.

[23]Li JJ,Gupta S,Li C,et al.Gold nanoparticles in cancer theranostics.Imaging Med Surg 2013;3:284–291.

[24]Huang X,Jain PK,El-Sayed IH,et al.Plasmonic photothermal therapy(PPTT)using gold nanoparticles. Lasers Med Sci 2008;23:217–228.

[25]Zhou W,Shao J,Jin Q,et al.Zwitterionic phosphorylcholine as a better ligand for gold nanorods cell uptake and selective photothermal ablation of cancer cells.Chem Commun 2010;46:1479–1481.

[26]Huang HC,Barua S,Kay DB,et al.Simultaneous enhancement of photothermal stability and gene delivery effcacy of gold nanorods using polyelectrolytes.ACS Nano 2009;3:2941–2952.

[27]Alkilany AM,Nagaria PK,Hexel CR,et al.Cellular uptake and cytotoxicity of gold nanorods:molecular origin of cytotoxicity and surface effects.Small 2009;5:701–708.

[28]Connor EE,Mwamuka J,Gole A,et al.Gold nanoparticles are taken up by human cells but do not cause acute cytotoxicity.Small 2005;1:325–327.

[29]Zhu XM,Fang CH,Jia HL,et al.Cellular uptake behaviour, photothermal therapy performance,and cytotoxicity of gold nanorods with various coatings.Nanoscale 2014;6:11462–11472.

[30]Takahashi H,Niidome Y,Niidome T,et al.Modifcation of gold nanorods using phosphatidylcholine to reduce cytotoxicity.Langmuir 2006;22:2–5.

[31]Hauck TS,Ghazani AA,Chan WC.Assessing the effect of surface chemistry on gold nanorod uptake,toxicity,and gene expression in mammalian cells.Small 2008;4:153–159.

[32]Kim E,Yang J,Choi J,et al.Synthesis of gold nanorodembedded polymeric nanoparticles by a nanoprecipitation method for use as photothermal agents.Nanotechnology 2009;20:365602.

[33]von Maltzahn G,Park JH,Agrawal A,et al.Computationally guided photothermal tumor therapy using long-circulating gold nanorod antennas.Cancer Res 2009;69:3892–3900.

[34]Dickerson EB,Dreaden EC,Huang X,et al.Gold nanorod assisted near-infrared plasmonic photothermal therapy (PPTT)of squamous cell carcinoma in mice.Cancer Lett 2008;269:57–66.

[35]Niidome T,Akiyama Y,Yamagata M,et al.Poly(ethylene glycol)-modifed gold nanorods as a photothermal nanodevice for hyperthermia.J Biomater Sci Polym Ed 2009;20:1203–1215.

[36]Huang YF,Sefah K,Bamrungsap S,et al.Selective photothermal therapy for mixed cancer cells using aptamer-conjugated nanorods.Langmuir 2008;24:11860–11865.

[37]Huff TB,Tong L,Zhao Y,et al.Hyperthermic effects of gold nanorods on tumor cells.Nanomedicine 2007;2:125–132.

[38]Li Z,Huang P,Zhang X,et al.RGD-conjugated dendrimermodifed gold nanorods for in vivo tumor targeting and photothermal therapy.Mol Pharm 2010;7:94–104.

[39]Choi W,Kim JY,Kang C,et al.Tumor regression in vivo by photothermal therapy based on gold-nanorod-loaded, functional nanocarriers.ACS Nano 2011;5:1995–2003.

[40]Zhang ZJ,Wang LM,Wang J,et al.Mesoporous silica-coated gold nanorods as a light-mediated multifunctional theranostic platform for cancer treatment.Adv Mater 2012;24:1418–1423.

[41]Nanospectra Biosciences,Inc.Pilot study of aurolase therapy in refractory and/or recurrent tumors of the head and neck.＜http://clinicaltrials.gov/ct2/show/ NCT00848042＞2009[accessed 08.15].

[42]Gad SC,Sharp KL,Montgomery C,et al.Evaluation of the toxicity of intravenous delivery of auroshell particles(goldsilica nanoshells).Int J Toxicol 2013;31:584–594.

[43]Ke HT,Wang JR,Dai ZF,et al.Gold-nanoshelled microcapsules:a theranostic agent for ultrasound contrast imaging and photothermal therapy.Angew Chem Int Ed 2011;50:3017–3021.

[44]Ma Y,Liang XL,Tong S,et al.Gold nanoshell nanomicelles for potential magnetic resonance imaging,light-triggered drug release,and photothermal therapy.Adv Funct Mater 2013;23:815–822.

[45]Goodrich GP,Bao L,Gill-Sharp K,et al.Photothermal therapy in a murine colon cancer model using nearinfrared absorbing gold nanorods.J Biomed Opt 2010;15:doi:10.1117/1.3290817.

[46]Van de Broek B,Devoogdt N,D’Hollander A,et al.Specifc cell targeting with nanobody conjugated branched gold nanoparticles for photothermal therapy.ACS Nano 2011;5:4319–4328.

[47]Hasan W,Stender CL,Lee MH,et al.Tailoring the structure of nanopyramids for optimal heat generation.Nano Lett 2009;9:1555–1558.

[48]Wang YC,Black KCL,Luehmann H,et al.Comparison study of gold nanohexapods,nanorods,and nanocages for photothermal cancer treatment.ACS Nano 2013;7:2068–2077.

[49]Huang XH,El-Sayed IH,Qian W,et al.Cancer cell imaging and photothermal therapy in the near-infrared region by using gold nanorods.J Am Chem Soc 2006;128:2115–2120.

[50]Huang HC,Rege K,Heys JJ.Spatiotemporal temperature distribution and cancer cell death in response to extracellular hyperthermia induced by gold nanorods.ACS Nano 2010;4:2892–2900.

[51]Loo C,Lin A,Hirsch L,et al.Nanoshell-enabled photonicsbased imaging and therapy of cancer.Technol Cancer Res Treat 2004;3:33–40.

[52]Rasch MR,Sokolov KV,Korgel BA.Limitations on the optical tunability of small diameter gold nanoshells.Langmuir 2009;25:11777–11785.

[53]Choi HS,Liu W,Misra P,et al.Renal clearance of quantum dots.Nat Biotechnol 2007;25:1165–1170.

[54]Jiang W,Kim BYS,Rutka JT,et al.Nanoparticle-mediated cellular response is size dependent.Nat Nanotechnol 2008;3:145–150.

[55]Sadauskas E,Wallin H,Stoltenberg M,et al.Kupffer cells are central in the removal of nanoparticles from the organism.Part Fibre Toxicol 2007;4:10–17.

[56]De Jong WH,Hagens WI,Krystek P,et al.Particle sizedependent organ distribution of gold nanoparticles afterintravenous administration.Biomaterials 2008;29:1912–1919.

[57]Tang SH,Chen M,Zheng NF.Sub-10-nm Pd nanosheets with renal clearance for effcient near-infrared photothermal cancer therapy.Small 2014;10:3139–3144.

[58]Zhou C,Long M,Qin YP,et al.Luminescent gold nanoparticles with effcient renal clearance.Angew Chem Int Ed 2011;50:3168–3172.

[59]Tang SH,Chen M,Zheng NF.Multifunctional ultrasmall Pd nanosheets for enhanced near-infrared photothermal therapy and chemotherapy of cancer.Nano Res. 2015;8:165–174.

[60]Xiao JW,Fan SX,Wang F,et al.Porous Pd nanoparticles with high photothermal conversion effciency for effcient ablation of cancer cells.Nanoscale 2014;6:4345–4351.

[61]SharifS,Behzadi S,Laurent S,et al.Toxicity of nanomaterials.Chem Soc Rev 2012;41:2323–2343.

[62]Sadauskas E,Danscher G,Stoltenberg M,et al.Protracted elimination of gold nanoparticles from mouse liver. Nanomedicine 2009;5:162–169.

[63]Balasubramanian SK,Jittiwat J,Manikandan J,et al. Biodistribution of gold nanoparticles and gene expression changes in the liver and spleen after intravenous administration in rats.Biomaterials 2010;31:2034–2042.

[64]Vivero-Escoto JL,Slowing II,Trewyn BG,et al.Mesoporous silica nanoparticles for intracellular controlled drug delivery.Small 2010;6:1952–1967.

[65]Wu SH,Hung Y,Mou CY.Mesoporous silica nanoparticles as nanocarriers.Chem Commun 2011;47:9972–9985.

[66]Thomas CR,Ferris DP,Lee JH,et al.Noninvasive remotecontrolled release of drug molecules in vitro using magnetic actuation of mechanized nanoparticles.J Am Chem Soc 2010;132:10623–10625.

[67]Meng H,Liong M,Xia T,et al.Engineered design of mesoporous silica nanoparticles to deliver doxorubicin and p-glycoprotein siRNA to overcome drug resistance in a cancer cell line.ACS Nano 2010;4:4539–4550.

[68]Lu J,Choi E,Tamanoi F,et al.Light-activated nanoimpellercontrolled drug release in cancer cells.Small 2008;4:421–426.

[69]Lu J,Liong M,Li Z,et al.Biocompatibility,biodistribution, and drug-delivery effciency of mesoporous silica nanoparticles for cancer therapy in animals.Small 2010;6:1794–1805.

[70]Uriu-Adams JY,Keen CL.Copper,oxidative stress,and human health.Mol Aspects Med 2005;26:268–298.

[71]New Hampshire Department of Environmental Services. Copper:Health Information Summary.＜http://des.nh .gov/organization/commissioner/pip/factsheets/ard/ documents/ard-ehp-9.pdf＞2013[accessed 08.15].

[72]Ku G,Zhou M,Song S,et al.Copper sulfde nanoparticles as a new class of photoacoustic contrast agent for deep tissue imaging at 1064 nm.ACS Nano 2012;6:7489–7496.

[73]Tian Q,Tang M,Sun Y,et al.Hydrophilic fower-like CuS superstructures as an effcient 980 nm laser-driven photothermal agent for ablation of cancer cells.Adv Mater 2011;23:3542–3547.

[74]Tian Q,Jiang F,Zou R,et al.Hydrophilic Cu9S5nanocrystals: a photothermal agent with a 25.7%heat conversion effciency for photothermal ablation of cancer cells in vivo. ACS Nano 2011;5:9761–9771.

[75]Guo L,Panderi I,Yan DD,et al.A comparative study of hollow copper sulfde nanoparticles and hollow gold nanospheres on degradability and toxicity.ACS Nano 2013;7:8780–8793.

[76]Chen WR,Singhal AK,Liu H,et al.Antitumor immunity induced by laser immunotherapy and its adoptive transfer. Cancer Res 2001;61:459–461.

[77]Naylor MF,Chen WR,Teagu TK,et al.In situ photoimmunotherapy:a tumour-directed treatment for melanoma.Br J Dermatol 2006;55:1287–1292.

[78]Guo LG,Yan DD,Yang DF,et al.Combinatorial photothermal and immuno cancer therapy using chitosancoated hollow copper sulfde nanoparticles.ACS Nano 2014;8:5670–5681.

[79]Chou SS,Kaehr B,Kim J,et al.Chemically exfoliated MoS2as near-infrared photothermal agents.Angew Chem Int Ed 2013;52:4160–4164.

[80]Wang SG,Li X,Chen Y,et al.A facile one-pot synthesis of a two-dimensional MoS2/Bi2S3 composite theranostic nanosystem for multimodality tumor imaging and therapy. Adv Mater 2015;17:2775–2782.

[81]Li J,Jiang F,Yang B,et al.Topological insulator bismuth selenide as a theranostic platform for simultaneous cancer imaging and therapy.Sci Rep 2013;3:doi:10.1038/ srep01998.

[82]Cheng L,Liu J,Gu X,et al.PEGylated WS2nanosheets as a multifunctional theranostic agent for in vivo dual-modal CT/photoacoustic imaging guided photothermal therapy. Adv Mater 2014;26:1886–1893.

[83]Yang K,Zhang S,Zhang G,et al.Graphene in mice: ultrahigh in vivo tumor uptake and effcient photothermal therapy.Nano Lett 2010;10:3318–3323.

[84]Robinson JT,Tabakman SM,Liang YY,et al.Ultrasmall reduced graphene oxide with high near-infrared absorbance for photothermal therapy.J Am Chem Soc 2011;133:6825–6831.

[85]Lim DK,Barhoumi A,Wylie RG,et al.Enhanced photothermal effect of plasmonic nanoparticles coated with reduced graphene oxide.Nano Lett 2013;13:4075–4079.

[86]Hu SH,Chen YW,Hung WT,et al.Quantum-dot-tagged reduced graphene oxide nanocomposites for bright fuorescence bioimaging and photothermal therapy monitored in situ.Adv Mater 2012;24:1748–1754.

[87]Yang X,Zhang X,Liu Z,et al.High-effciency loading and controlled release of doxorubicin hydrochloride on graphene oxide.J Phys Chem C 2008;112:17554–17558.

[88]Sun X,Liu Z,Welsher K,et al.Nano-graphene oxide for cellular imaging and drug delivery.Nano Res.2008;1:203–212.

[89]Wang Y,Wang K,Zhao J,et al.Multifunctional mesoporous silica-coated graphene nanosheet used for chemophotothermal synergistic targeted therapy of glioma.J Am Chem Soc 2013;135:4799–4804.

[90]Gong H,Peng R,Liu Z,et al.Carbon nanotubes for biomedical imaging:the recent advances.Adv Drug Delivery Rev 2013;65:1951–1963.

[91]Smith BR,Ghosn EE,Rallapalli H,et al.Selective uptake of single-walled carbon nanotubes by circulating monocytes for enhanced tumour delivery.Nat Nanotechnol 2014;9:481–487.

[92]Liu Z,Sun XM,Ratchford NN,et al.Supramolecular chemistry on water-soluble carbon nanotubes for drug loading and delivery.ACS Nano 2007;1:50–56.

[93]Liu Z,Fan AC,Rakhra K,et al.Supramolecular stacking of doxorubicin on carbon nanotubes for in vivo cancer therapy.Angew Chem Int Ed 2009;48:7668–7672.

[94]Liu JJ,Wang C,Wang XJ,et al.Mesoporous silica coated single-walled carbon nanotubes as a multifunctional lightresponsive platform for cancer combination therapy.Adv Funct Mater 2015;25:384–392.

[95]Wang L,Zhang M,Zhang N,et al.Synergistic enhancement of cancer therapy using a combination of docetaxel and photothermal ablation induced by single-walled carbon nanotubes.Int J Nanomed 2011;6:2641–2652.

[96]Britten CM,Singh-Jasuja H,Flamion B,et al.The regulatory landscape for actively personalized cancer immunotherapies.Nat Biotechnol 2013;31:880–882.

[97]Wang C,Xu LG,Liang C,et al.Immunological responses triggered by photothermal therapy with carbon nanotubes in combination with anti-CTLA-4 therapy to inhibit cancer metastasis.Adv Mater 2014;26:154–8162.

[98]Wang XJ,Wang C,Cheng L,et al.Noble metal coated single-walled carbon nanotubes for applications in surface enhanced Raman scattering imaging and photothermal therapy.J Am Chem Soc 2012;134:7414–7422.

[99]Chen HJ,Shao L,Ming T,et al.Understanding the photothermal conversion effciency of gold nanocrystals. Small 2010;6:2272–2280.

[100]Liu Z,Davis C,Cai W,et al.Circulation and long-term fate of functionalized,biocompatible single-walled carbon nanotubes in mice probed by Raman spectroscopy.Proc Natl Acad Sci USA 2008;105:1410–1415.

[101]Zerda ADL,Zavaleta C,Keren S,et al.Carbon nanotubes as photoacoustic molecular imaging agents in living mice.Nat Nanotechnol 2008;3:557–562.

*< class="emphasis_italic">Corresponding author.

.School of Pharmacy,Shenyang Pharmaceutical University,No.103,Wenhua Road,Shenyang 110016,China.Tel.: +86 24 23986293fax:+86 24 23986293.

E-mail address:baozhihong@yahoo.com(Z.Bao).

Peer review under responsibility of Shenyang Pharmaceutical University.

http://dx.doi.org/10.1016/j.ajps.2015.11.123

1818-0876/?2016 The Authors.Production and hosting by Elsevier B.V.on behalf of Shenyang Pharmaceutical University.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Cancer therapy

Photothermal therapy

Multifunctional modifcation