Nanoparticles weaponized with built‐in functions for imaging‐guided cancer therapy

Imaging‐guided cancer therapy (IGCT) is rapidly emerging as a powerful precision cancer theranostic paradigm, as it can offer both diagnostic information and therapeutic benefits. But it relies on advanced theranostic agents that are largely constructed on top of nanomaterials. Conventionally, such theranostic agents are constructed by integrating different functional units, which will unnecessarily increase the size of the multi‐functional agents and inevitably encounter unwanted disassociation of the functional units in vivo, leading to false diagnosis and poor therapeutic efficacy. In this review, we highlight the state‐of‐the‐art progresses in biomedical applications of nanomaterials possessing built‐in functions suitable for cancer theranostic applications, particularly those who can convert near‐infrared (NIR) light into heat or radicals, as phototherapy holds remarkable promise for high‐precision cancer therapy. In addition, phototherapy can naturally be combined with photoacoustic imaging for realizing imaging‐guided therapy. In this context, the interactions between NIR and nanomaterials are classified and outlined. Then, different approaches for boosting imaging sensitivity and therapeutic efficacy, by improving the utilization of NIR light through different mechanisms, are discussed. Through examples on delineation and treatment of tumors in vivo, the opportunities and benefits of the theranostic nanoprobes for IGCT are highlighted. Moreover, the current challenges and future perspectives regarding both fundamental studies and clinical translation of the NIR‐mediated cancer theranostic agents are discussed.


INTRODUCTION
Imaging-guided cancer therapy (IGCT) holds remarkable promise for revolutionizing the cancer treatment in the future, as high-precision therapies will expectedly be enabled through imaging guidance. [1][2][3][4][5][6] For example, digital subtraction angiography (DSA) invented for blood vessel imaging is widely used for the diagnosis and treatment of arterial and venous occlusions, including carotid artery stenosis, pulmonary embolisms, acute limb ischemia, arterial stenosis, and so forth. With respect to cancer treatment, it facilitates transarterial chemoembolization as a palliative nonsurgical therapeutic option for the treatment of unresectable hepatocellular carcinoma. Although the X-ray-based DSA can provide very high-precision images, it remains difficult to clearly resolve the boundary between carcinoma and its surrounding healthy tissues, apart from its ionization radiation nature. Another example is white-light endoscopy that is also adopted for imaging-guided therapy by the endoscopic surgeon. It is impossible to noninvasively extract the pathophysiological information through white-light endoscopy either. In this context, IGCT really awaits advanced imaging techniques to precisely pinpoint the tumors, by resolving the limited malignant information, to improve the following treatment efficacy. In this respect, molecular imaging technique that largely relies on molecular recognition between exogenous probes and molecular targets associated with tumors comes into play.
In principle, IGCT can adopt different imaging techniques such as magnetic resonance imaging (MRI), X-raybased DSA and computed tomography (CT), ultrasound imaging, photoacoustic (PA) and fluorescence imaging, and so forth. 5,[7][8][9][10][11][12][13] The clinic dilemma of selecting imaging modality is that each single imaging modality has its own unique advantages and intrinsic limitations, such as high spatial resolution/unlimited penetration depth versus ionizing radiation for CT and insufficient sensitivity/high cost for MRI, and high sensitivity/high temporal resolution/low cost/nonionizing radiation versus limited penetration depth for PA and fluorescence imaging (Figure 1), 5,7,8,14 and so forth. Nevertheless, CT, MRI, and ultrasound have already been used in the clinic for IGCT, apart from DSA, for example, imaging-guided radiofrequency ablation of tumors. In comparison with these clinical imaging techniques, photoacoustic imaging (PAI) and fluorescence imaging techniques possess outstanding features for advanced IGCT, including fast acquisition speed, real-time visualization, easy operation, high spatiotemporal resolution, and high throughput. 5,7,10,15 PAI relies on PA effect of light absorption and subsequent thermoelastic expansion of tissues. [16][17][18][19] Hence, it is able to overcome the disadvantages of conventional optical imaging that is suffering from biological tissue absorption/scattering of photons, thus giving rise to deeper penetration and higher spatial resolution of soft tissues especially when contrast F I G U R E 1 Comparison of the major characteristics of imaging modalities for cancer diagnosis. PET, positron emission tomography; SPECT, single-photon emission computed tomography; CT, computed tomography; MRI, magnetic resonance imaging; OI, optical imaging; PAI, photoacoustic imaging (the data were taken from literature 5 ) agents with efficient photothermal conversion are used. Thus, PAI can naturally be combined with photothermal therapy (PTT) and even photodynamic therapy (PDT). Recent advances in fluorescence imaging probes emitting nearinfrared (NIR) light even in the second NIR window (NIR II) (1000-1700 nm) of human tissues have shown that the NIR II fluorescence imaging can also provide super-high spatiotemporal resolution and remarkable sensitivity to embrace bright future in precise imaging-guided surgery. 11,12,15,[20][21][22][23][24] Regarding the therapeutics of cancer, the majority of clinical cancer therapies rely on surgical resection, chemotherapy, and radiotherapy. As for oncological surgery, precise and complete removal of tumors remains challenging because it is extremely difficult to identify the tumor boundary. In several clinical studies, fluorescence imaging-guided surgery in patients was used to identify tumor resection margins. [24][25][26] However, the clinically available organic dyes such as methylene blue, indocyanine green (ICG), and 5-aminolevulinic acid (5-ALA) do not inherently possess higher binding affinity to cancer tissue than to healthy tissue, leading to poor contrast at the tumor margin. The chemo drugs also lack in specificity in cancer treatment apart from serious systematic side effects and drug resistance. In difference, X-ray-and -raybased radiation therapies have higher spatial selectivity, but lead to unwanted damage to health tissues.
Nanomaterials with therapeutic and diagnostic functions have been demonstrated to be particularly suitable for IGCT. 1,[61][62][63][64][65] To date, a large number of multifunctional cancer nanoprobes have been reported. But most of them were obtained by integrating different functional parts into one nanoplatform. For example, doxorubicin (DOX) has widely been used as a model chemo drug to create theranostic probes by being attached on the surface of versatile functional nanoparticles capable of tumor imaging. Such a strategy sounds superficially reasonable for IGCT, but it faces the inherent flaws: (a) tumor diagnosis and therapy are expecting different pharmacokinetics from the therapeutic drugs and diagnostic agents, respectively, and (b) these two functional components are prone to disassociate upon metabolism even during the circulation in blood stream, making it even impossible to accurately monitor the delivery of the loaded chemo drug. To address above problems, it is important to develop theranostic platforms based on single nanoparticle that intrinsically possesses physicochemical properties suitable for both cancer imaging and therapy.
Functional inorganic nanoparticles that exhibit strong light absorption in the NIR region have potentials to address above flaws. The NIR light absorption generally can originate from strong localized surface plasmon resonance (LSPR) that is typically for metallic nanostructures and anion-/cationdeficient inorganic compounds, or interband and intraband transitions typically for narrow bandgap semiconductors, or ligand-metal charge transfer normally for metal-organic complexes. The absorbed NIR light will be dissipated through radiative and nonradiative ways. The former pathway can generate NIR fluorescence for diagnosis, whereas the latter pathways can create local heat for PTT and PAI, or radicals for PDT to further combine with PAI. Different from chemo therapy, PTT and PDT only need single or very limited shots, which makes it more reasonable to be combined with imaging guidance on top of nanoplatforms.
In this review, we will be focusing on the state-of-theart progresses related to nanomaterials with built-in functions suitable for cancer theranostic applications. Different approaches for boosting imaging sensitivity and therapeutic efficacy, by improving the utilization of NIR lights, are discussed. Through examples on efficient delineation and treatment of tumors in vivo, the opportunities and benefits of the theranostic nanoprobes for imaging-guided therapy are highlighted. Moreover, the current challenges and future perspectives on both fundamental studies and clinical translation of the NIR-mediated cancer theranostic agents are discussed. Concerning organic semiconducting materials designed for theranostic applications, they are comprehensively summarized previously and would not be discussed herein. 3,66

LOCALIZED SURFACE PLASMON RESONANCE FOR IGCT
Localized surface plasmon resonance arises from the collective oscillation of free charge carriers, that is, electrons or holes, in nanoparticles in resonance with the electromagnetic field of incident light. 67-69 LSPR has been observed in metallic nanostructures 67,[70][71][72] and anion-/cation-deficient inorganic compounds. 40,46,68,[73][74][75] The LSPR of noble metal nanoparticles (eg, Au, Ag, and Pd), originating from the electronic collective motion, typically locates in the visible range. But it can be tuned into the NIR region by increasing the particle size or varying the particle morphology. Oxygen-deficient transition metal oxides such as WO 3-x and MoO 3-x exhibit NIR LSPR due to a large number of electron carriers endowed by oxygen vacancy, whereas cation-deficient compounds such as copper chalcogenides (Cu 2-x E, E = S, Se, Te) present NIR LSPR owing to free holes enabled by the cation-deficient structure. 68,73,74,76 It is deserved to mention that the LSPR frequency of the anion-/cation-deficient compounds is not sensitive to particle size, which provides valuable opportunities to shorten the biological half-lives of the resulting theranostic probes by largely reducing their size.

Noble metallic nanoparticles
Noble metal nanomaterials especially gold nanoparticles present remarkable LSPR effects for biomedical applications. 70,71,77 Owing to their excellent biocompatibility and chemical stability as well as strong morphologydependent plasmonic properties, Au nanoparticles are promising photothermal therapeutic agents for PAI-guided PTT of tumors. However, for spherical solid Au nanoparticles, only those larger than 50 nm present strong NIR absorption. But smaller particles are typically more preferable for tumor theranostic applications owing to their longer blood residence time and shorter biological half-life. 78 Different approaches are recently developed to solve this contradiction by inducing Au nanoparticles to aggregate within tumors, 72,79-81 because the LSPR absorption can remarkably be shifted to NIR region owing to a strong coupling of plasmonic resonance among particles. 70 Different types of stimulus have been used to trigger the aggregation of Au nanoparticles in vivo. For example, Cheng et al reported light-responsive Au particles for sensitive tumor PAI and PTT. 72 In this proof-of-concept Reproduced with permission. 80 Copyright 2017, The Royal Society of Chemistry study, PEGylated Au particles were prepared and then covalently modified with photo-labile diazirine moieties that can cross-link the particles upon 405 nm laser irradiation. The light-induced aggregation of Au particles can be realized in vivo after the Au particles are trapped in tumor through blood circulation. In consequence, the LSPR is dramatically shifted to NIR, which gives rise to remarkably enhanced PAI contrast and PTT efficacy (Figure 2A-C). 72 Apart from light, tumor microenvironmental abnormal factors such as mild acidic pH and overexpressed tumor-associated enzymes can also be used as stimuli. [79][80][81] For instance, Yu et al designed a smart pH-sensitive assembly formed by single-stranded DNA-Au (ssDNA-Au) conjugate and -CD (alpha-cyclodextrin) through noncovalent bonding interactions. 80 In weak acidity environment, the pyridine-2-imine moiety at the end of single-strand DNA will be protonated. In consequence, -CD falls off and Au nanoparticles modified with complementary ssDNA sequences, respectively, will form aggregates through interparticle interactions via complementary base pairing in Yang et al reported a slightly complicated matrix metalloproteinase (MMP) responsive system similar to the above one. But the two Au samples were also modified with DOX and PEGylated MMP-2 cleavable peptide substrate (PEG 5000 -GPLGVRGC-SH), apart from the complimentary ssDNA sequences. 81 Both of these two Au nanoparticles have high physiological stability, but will form aggregates after the PEG 5000 -GPLGVRGC-SH ligand is cleaved by MMP-2 in tumor matrix through the base pairing of the complementary ssDNA.
An alternative approach to effectively shift LSPR of Au particles to NIR is to change the morphology of the particles from spherical to elongated, 82,83 branched, [84][85][86] hollow, 87 and so forth. For example, the maximum LSPR absorption can be tuned even into NIR II region by creating highly branched structures such as the so-called nanoechinus and nanostars in literature. Vijayaraghavan et al reported 350 nm Au nanoechinus that exhibited photodynamic and photothermal therapeutic effects in both the NIRI and NIRII windows. 84 The Au nanoechinus exhibited remarkably enhanced extinction coefficient in NIR regions, 3-4 orders higher than those of gold nanoparticles reported in literature, owing to the hierarchical structures formed by nanorods and tips. Very recently, Deng et al reported ∼100 nm Au nanostars coated with DOX-loaded metal organic framework. Upon 1064 nm light irradiation, Au nanostars exhibited outstanding photothermal effects to promote the drug release for synergistic chemo-photothermal therapy, meanwhile enabling the PAI. 86 In comparison with Au nanoparticles, Pd nanoparticles exhibit a stronger photothermal conversion ability owing to their higher NIR absorption. Xiao et al reported porous Pd nanoparticles of 23 nm with a photothermal conversion efficiency (PCE) up to 93.4% under 808-nm laser irradiation. 88 Even for 4.4-nm Pd nanosheets, the PCE was also observed as high as 52% under 808-nm excitation. In addition, the small nanosheets can partly be cleared through renal clearance pathway. 89 Most importantly, Pd nanosheets can also catalyze endogenous H 2 O 2 to supply O 2 , favorable for simultaneously overcoming hypoxia and enhancing PDT activity. 90,91 Apart from Au and Pd, other plasmonic metal nanoparticles such as Pt, 92 Ag, 93 Ru, 94 Bi, [95][96][97] and Re 98 have also been explored as tumor theranostic agents, which will not be discussed in further detail.

Anion-deficient metal oxide nanoparticles
Oxygen-deficient transition metal oxides have been widely studied for their LSPR effect and tunable NIR absorption. 69,74 Because each oxygen vacancy is compensated by two free electrons to maintain charge neutrality, the free electrons render anion-deficient metal oxide nanocrystals plasmonic. 69 As the free electron density is generally 3 or 4 orders of magnitude lower than that in Au nanoparticles, these metal oxides present LSPR directly in NIR spectra region irrespective of particle size. 69 This offers unique advantages over metal counterparts as small theranostic agents are preferred to be eliminated through renal clearance. Normally by manipulating the oxygen vacancy degree, the LSPR intensities and frequencies can also easily be tuned to meet different requirements.
Tungsten suboxide (usually denoted as WO 3-x ) belongs to such a category. 99, 100 Wen et al reported 1.1-nm biocom-patible WO 3-x nanodots showing strong LSPR absorption in the range of 600-1100 nm. 75 Moreover, the absorbance of the resulting particles can be enhanced by lowering both pH and oxygen level, which is aligned with the mild acidic and hypoxic microenvironment of tumors. Apart from the outstanding performance for PAI and PTT, the nanodots also showed strong radiosensitizing effect for enhanced radiotherapy, and CT contrast enhancement effect due to the strong Xray attenuation ability of tungsten ( Figure 3). 75 In addition to aforementioned functions, W 18 O 49 can also act as a photosensitizer for generating singlet oxygen for PDT. 101

Cation-deficient metal chalcogenide nanoparticles
Copper chalcogenides (Cu 2-x E, with E = S, Se, Te, 0 ≤ x ≤ 1) are typical cation-deficient compounds exhibiting strong NIR The inherent low free carrier density and high effective mass of free holes from Cu 2-x E materials give rise to lower energy LSPR covering the entire first NIR window and even the NIR II window of human tissues, 68,73,74,106,107,109-113 depending on copper vacancy degree, which makes Cu 2-x E nanoparticles very attractive for biomedical applications because the tissue absorption and scattering of light are extremely low in the NIR II region. In many cases, the PCE of Cu 2-x E nanoparticles were reported to be above 50% under NIR excitation, which is highly essential for sensitive PAI-guided PTT. 46,114,115 To explore the size-dependent contrast enhancement effects for PAI, Gao and co-workers reported PEGylated copper sulfide nanoparticles with controllable size between 3 and 7 nm via aqueous synthesis. 112 Although the contrast enhancement effect of the PEGylated nanoparticles is positively correlated with the particle size, the in vivo studies revealed that copper sulfide nanoparticles smaller than 5 nm presented higher tumor imaging performance, especially at the tumor boundary site (Figure 4). Ultrasmall Cu 2-x S nanoparticles of 2 nm showing strong LSPR in the range 600-1100 nm were reported by Shi and co-workers. 113 These ultrasmall particles presented excellent performance for PAI-guided PTT of tumor in vivo ( Figure 5). Up to 3month biosafety studies revealed that these particles have negligible side effects according to hemolysis/thrombosis tests and histological analysis on tissues of heart, liver, spleen, lung, and kidney. Moreover, it was found that 99% of the Cu 2-x S nanoparticles intravenously delivered were cleared through feces within 5 days. In another study, glutathione was used as a ligand to directly prepare 4.8-nm water-soluble CuS nanoparticles via aqueous synthesis for PAI and PTT application. 116 The quantitative analysis of the copper content in urine samples revealed that the accumulative urinary clearance of the nanoparticles reaches 54.6% within 24 h postinjection.
As an analog of Cu 2-x S nanoparticles, the Cu 2-x Se counterparts have gained intensive attentions owing to their stronger NIR LSPR. 42,46,74,115,[117][118][119][120] For example, Zhang et al reported 3.6-nm PEGylated Cu 2-x Se nanoparticles directly synthesized in aqueous system with PCE up to 64.8%, which directly led to sensitive PAI for guiding the PTT (Figure 6). 115 In addition, the biocompatible surface modification enabled long blood circulate time up to 8 h, which ended up with a tumor uptake around 4.4%ID/g 12 h after the intravenous injection of PEGylated particles. Moreover, experimental results suggested that the nanoparticles can be partially excreted through renal clearance.
Apart from the photothermal effects, cation-deficient Cu 2-x E nanoparticles can also be used to generate high level ROS, because Cu 2-x E under NIR irradiation can catalyze H 2 O 2 to produce hydroxyl radical (•OH), 28,40,42,46,121 which is also known as photo-Fenton reaction as shown in  Figure 7A. 46 This catalysis reaction can generate not only toxic •OH by decomposing endogenous H 2 O 2 within tumor, but also O 2 to relieve the tumor hypoxia. However, even in tumor microenvironment, the endogenous O 2 remains limited. To address this issue, an innovative system was proposed to remarkably increase the intratumor H 2 O 2 level by attaching glucose oxidase (GOD) on the surface of 4.8 nm Cu 2-x Se nanoparticles. Upon oxidation of endogenous glucose within tumors, the release of H 2 O 2 was monitored through oxyhemoglobin/hemoglobin with highly sensitive PAI ( Figure 7B-D). When the H 2 O 2 concentration reached its maximum, NIRII irradiation was applied to trigger the photo-Fenton reactions that generated vast amounts of ROS within a short time, giving rise to excellent therapeutic efficacy ( Figure 7E-F). 46 In fact, apart from the radicals generated through the photo-Fenton reactions, singlet state oxygen ( 1 O 2 ) that also has therapeutic effects can also be produced via energy transfer directly from excited state Cu 2-x E nanoparticles to ground state molecular oxygen (O 2 ), as shown in Figure 8A-C. 40,46 Therefore, Cu 2-x Se particles offer great benefits integrating deep tissue penetration of NIR II light with multiple photogenerated toxic species to facilitate highly efficacious cancer phototherapies ( Figure 8D-G). 40,42,46 In addition to inducing strong LSPR, Cu 2-x E also provide vacancies for metal guests to achieve doped materials with additional functions for biomedical applications. For example, CuS:Gd 122,123 and Cu 2-x Se:Fe 124 for PAI/MRI and PTT, Cu 2-x S:Pt for CT imaging and chemo-photothermal synergistic therapy, 125 and CuS: 64 Cu for positron emission tomography (PET)/CT imaging and PTT 126 have been reported. It was further demonstrated that doping Cu 2-x Se nanoparticles with Fe 3+ can tune the position and enhance the intensity of the NIR LSPR. The magnetism in the resulting particles can also be finely tuned by the doping level to enable dual-modal PAI/MRI. 124 Toward biomedical applications, noble metal particles were also combined with Cu 2-x E nanoparticles to achieve dual plasmonic hybrid nanostructures with hopefully rich photothermal behaviors. [127][128][129] It has been demonstrated that noble metal particles can enhance the plasmonic absorption of copper chalcogenides by changing the local electronic field. 29,130 For example, Au-Cu 9 S 5 , 130 Cu 7 S 4 -Au, 131 Pt-CuS, 48 Au@Cu 2-x S, 132-134 Au@Cu 2-x Se, 133,135 and CuS@Cu 2 S@Au 29 nanoparticles showed enhanced photothermal conversion efficiencies than the initial Cu 2-x E counterparts.

BANDGAP TRANSITION IN NIR FOR IGCT
Apart from abovementioned LSPR, the bandgap transition of semiconducting materials in NIR can also be utilized to convert NIR light into heat. However, there is a limited

Ternary chalcogenide nanoparticles
In addition to forming doping structures showing LSPR, the cation-deficient Cu 2-x E also allows the formation of cationalloyed ternary metal chalcogenides, in which the NIR LSPR is greatly suppressed. But strong absorption involving 3d orbitals of magnetic transition metal ion is generated in NIR region. Particularly, magnetically engineering of Cu 2-x E can introduce intermediate band (IB) associated with 3d orbitals of the magnetic cation into the fundamental gap. 136,137 The electronic transitions from valence band (VB) to the empty IB of 3d orbitals of magnetic cation cover almost entire visible and NIR regions. 136 The following nonradiative relaxation within the broad IB owing to indirect VB-IB gap gives rise to high photothermal conversion efficiencies that were observed from magnetic ternary nanoparticles including CuFeS 2 nanoparticles, 136 137,143 and so forth. One of the unique advantages is that through these particles MRI can be well combined with PAI for dual-modality imaging, further with PTT and even photo-enhanced CDT for imaging-guided therapy. 138,144 For example, ultrasmall magnetic CuFeSe 2 ternary nanocrystals of ∼5.0 nm were prepared by aqueous method under ambient conditions. 144 The resulting particles presented broad band absorption covering 500-1100 nm and remarkably high PCE up to 82% under 808-nm laser irradiation. Benefiting from these excellent properties, the ultrasmall particles can serve as multimodality imaging agent for MRI, PAI, and CT, apart from PTT. 144

Binary chalcogenide nanoparticles
Bismuth chalcogenide (Bi 2 E 3 ) as direct bandgap semiconductors showing very narrow bandgap energies, for example, 1.3 eV for Bi 2 S 3 , 0.16 eV for Bi 2 Se 3 , and 0.13 eV for Bi 2 Te 3 , therefore they are inherently suitable NIR absorbers. [145][146][147] As one of the representative examples, Liu et al reported Tween 20-coated 10-nm wide and 50-nm long Bi 2 S 3 nanorods showing PCE of 28%. 145 Although the PCE is lower than those for abovementioned materials, Bi 2 E 3 remain unique for medical applications owing to the large X-ray attenuation coefficient of Bi, that is, 5.74 cm 2 /g at 100 keV. 95,145,148-151 Therefore Bi 2 E 3 nanoparticles are expected for combining CT with PTT apart from PAI for imaging-guided therapy (Figure 9). The same group further loaded hydrophobic bis-N-nitroso compounds with Tween 20-coated nanorods to thermally trigger the release of NO for realizing NO-enhanced PTT. 152 In fact, the PTT can be resisted by the heat shock protein 70 overexpressed by tumor cells upon heat treatment. To address this issue, the same group further combined LY294002 (a strong inhibitor of phosphoinositide 3-kinases) with Tween 20-coated Bi 2 S 3 nanoparticles (8 nm) to inhibit the expression of the heat shock protein HSP70. The experimental results revealed that efficacious antitumor performance can be achieved under low-power 808-nm light irradiation. They also found that the theranostic probe can activate the mitochondrial apoptosis pathway. 153 In order to directly obtain the biocompatible Bi 2 E 3 nanoparticles, Wang et al developed an aqueous synthetic approach for obtaining bovine serum albumin (BSA)-capped Bi 2 S 3 nanoparticles directly through a biomineralization process. 146 The Bi 2 S 3 nanoparticles of 6.1 nm obtained under ambient conditions presented an outstanding blood circulation behavior with blood half-life up to 15 h. Moreover, the tumor uptake reached 8.3% ID/g 24 h after the intravenous injection of the BSA-capped particles. Owing to the high PCE (51%) and large X-ray attenuation coefficient, sensitive tumor PA/CT imaging was enabled. Most importantly, the tumors were eradicated owing to the synergistic PTT and RT with the survival rate of tumor-bearing mice up to 100% over 40 days observation after the treatments ( Figure 10). By similar approaches, 2.7-nm Bi 2 Se 3 nanoparticles were also reported through the BSA-mediated synthesis. 147 Despite the ultrasmall size, they exhibited PCE close to 51% under 808-nm laser irradiation, rendering them suitable for PA/CT imagingguided PTT and RT applications as well ( Figure 11).
Apart from Bi 2 E 3 , transition-metal binary chalcogenides such as molybdenum disulfide (MoS 2 ) 154,155 and tungsten disulfide (WS 2 ) 156,157 were also reported as photothermal agents. Nevertheless, their performance for utilizing NIR is not comparable with that of Bi 2 E 3 as the latter has more suitable bandgap.

Single-element semiconductor nanoparticles
Black phosphorus (BP) as a layer-structured semiconductor possess a unique layer thickness-dependent direct bandgap varying from 0.3 eV (bulk) to 2.0 eV (monolayer) as shown in Figure 12A, [158][159][160][161][162] it is therefore potentially suitable for biomedical applications. [163][164][165][166][167][168][169][170] For example, Sun et al reported 3.2-nm PEGylated BP nanoparticles prepared via an innovative one-pot solventless high-energy mechanical milling technique. The ultrasmall BP particles exhibited PCE close to 37% under 808-nm light irradiation, 169 leading to excellent performance for targeting tumors through enhanced permeability and retention (EPR) effect to simultaneously realize PAI and PTT of tumors in vivo ( Figure 12B-D). In addition, the BP particles were found excreted via both liver and kidney. In another study, it is found that more than 65% of 3.3-nm PEGylated BP nanoparticles were cleared into urine within 8 h. 171 Apart from the photothermal conversion ability, BP nanosheets are able to efficiently generate singlet oxygen under NIR irradiation, rendering them useful for PDT applications as well, owing to effective energy transfer between BP particles and triplet oxygen. 160 Recently, it was for the first time found that BP nanoparticles can also show NIR II fluorescence. 172 Via the abovementioned solventless high-energy mechanical milling approach, 4.6-nm thick and 20-nm wide two-dimensional BP particles showing a very broad emission covering 900-1650 nm were prepared. After being encapsulated within nanospheres with the aid of a PEGylated lipid, they were used for NIRII fluorescence and PA imaging in vivo. 172 Owing to the extremely  147 Copyright 2016, American Chemical Society high signal-to-noise ratio for emission beyond 1400 nm, blood vessels, liver, and spleen can clearly be observed, meanwhile the corresponding PAI can also be carried out for semiquantifying the biodistribution and pharmacokinetics of the BP probes.

Luminescent nanoparticles
Apart from nonradiative recombination to promote heat or radials, the bandgap transition in NIR can also lead to NIR fluorescence that has been receiving intensive investigations for biomedical imaging, [173][174][175] particularly for realtime imaging-guided resection of tumors. 176 Owing to the ultrahigh sensitivity, the NIR fluorescence imaging offers opportunity to clearly identify the tumor margin for pre-cisely guiding the surgery, which is essential for minimizing recurrence and improving prognosis. In comparison with NIR I luminescence imaging, NIR II luminescence imaging is characterized by improved sensitivity and increased spatiotemporal resolution and penetration depth, owing to reduced tissue autofluorescence and incident light attenuation from tissues. 11,22,23,[177][178][179][180] Great efforts have therefore been dedicated to developing NIR II imaging probes based on quantum dots (QDs), 10,15,22,173,174,181 single-walled carbon nanotubes, 182 small molecular dyes, 11 conjugated polymers, 18 lanthanide-based nanoparticles, 13,21,177,183,184 and aggregation-induced emission luminogens. 185 Fluorescence imaging-guided surgery requires high sensitivity and specificity with respect to tumor imaging. Therefore, high photoluminescence quantum efficiency is utmost important. Among all fluorescent nanocrystals reported so  175,188 InAs, 15 and so forth generally meet the requirement for generating tunable NIR I/NIR II emissions owing to the quantum confinement effect. 10,174 For example, Wang reported DOTA-Gd-modified Ag 2 S nanoprobes showing strong NIRII emission at 1200 nm with a fluorescence quantum yield up to 15%. 176 Apart from assisting T 1 MRI of a brain tumor (U87MG) in a mouse model, the NIR II fluorescence enabled successful intraoperative resection of the tumor.
Apart from the high sensitivity correlated with the fluorescence quantum yield, fluorescence imaging-guided surgery also requires clear delineation of tumors, which is more associated with specificity of the probes. To address this issue, Zhang reported in vivo assembly of NIRII emitting NaGdF 4 :5%Nd@NaGdF 4 nanoparticles functionalized with complementary DNA and targeting peptides to improve the imaging-guided surgery for metastatic ovarian cancer. 13 The enhanced tumor-to-normal tissue ratio of 12.5 was achieved owing to the assembled nanoparticles with long tumor retention, facilitating the abdominal ovarian metastases surgical delineation. Moreover, metastases of less than 1 mm were clearly visualized and completely excised under NIRII bioimaging guidance. 13 In fact, the fluorescence imaging-guided surgery is a very important frontier subject as tumor resection remains the major clinic measure for cancer therapy. But the biomedical applications of fluorescent QDs 174,175,189 and rare-earth nanoparticles 78, 177,190 have been summarized very comprehensively elsewhere, and these luminescent materials do not belong to the categories of materials that exhibit multiple intrinsic properties or can be used for multiple therapeutic applications, we therefore would not discuss these NIR II luminescent materials and their biomedical applications in further detail.

CHARGE TRANSFER TRANSITION IN NIR FOR IGCT
According to the charge transfer direction in transition metal complexes, the charge transfer transitions (CTT) can be divided into ligand-to-metal charge transfer (LMCT), metal-to-ligand charge transfer, and metal-to-metal charge transfer. 191,192 The CTT is an orbital-and spin-allowed transition and becomes preferable in comparison with d-d electronic transition as the latter is parity forbidden and therefore it exhibits large extinction coefficients. The weak nature of the attraction in transition metal complexes also makes the latter suitable as NIR absorbers and the absorption properties can be manipulated by the interactions between the ligands and the metal ions.

Iron complex nanoparticles
The five unpaired d-electrons and empty d-orbitals endow Fe(III) ion with ideal paramagnetism for MRI and good coordination ability to combine with various ligands, respectively. Moreover, the mutual transformations between Fe(III) and Fe(II) ions are involved in many redox-related biochemical processes in vivo, including deoxyribonucleic acid synthesis, immune process, electron transport, the regulation of intracellular redox state, and so forth. [193][194][195] Therefore, Fe complexes can be applied in many fields of life sciences and pharmacy. [196][197][198] Iron-polyphenol complexes normally show a wide absorption band in the visible and NIR region due to LMCT. For example, gallic acid (GA), a kind of natural small molecule polyphenol widely existing in plants, can generate a very strong absorption in the NIR region after coordinating with Fe(III) ions. 199 Wang and co-workers reported 1.5-nm nanoparticles formed by GA and Fe(III), with polyvinylpyrrolidone (PVP) serving as a stabilizing agent. 200 The animal experiments revealed that the PVP-coated GA-Fe(III) particles are preferably accumulated in tumor sites via the EPR effect and can quickly be cleared after intravenous injection. Moreover, the particles also displayed MRI contrast enhancement effect and photothermal effects, rendering them useful for tumor MRI and PTT.
In the absence of PVP, Zeng et al found out that GA can also form particles with Fe(III) owing to the hydrophobicity of the complex. 201 They further demonstrated that the particles remain stable in weak acidic tumor sites thereby leading to excellent PAI-guided photothermal treatment of tumors, while decomposed under neutral pH, which is favorable for minimizing the side effects. 201 Following this study, they further designed a multimodal imaging probe of 2 nm by stabilizing the complex particles formed by 125 I-labeled GA and Fe(III) with PVP. In this way, this ultrasmall probe perfectly integrated the capacities for PAI, single-photon emission computed tomography (SPECT), and MRI for precise tumor diagnosis ( Figure 13A-E). 202 In spite of the small size, the resulting particles exhibited a strong photothermal effect for tumor ablation owing to the ultrahigh PCE up to 67.4%. It is deserved to mention that the systematic 125 I-based pharmacokinetic studies revealed that the above particle probe possesses excellent biodegradability. After intravenous delivery, radioactive signals from the following organs and tissues were carefully monitored including heart, lung, liver, spleen, kidney, bladder, stomach, and thyroid, apart from tumor (Figure 13F). The tumor uptake reached 5.3% ID/g 2.5 h postinjection. Different from other organs but similar to bladder, stomach presented a gradually increased signal, indicating that the decomposition products were partly excreted through bowel elimination pathway via stomach, which was confirmed by the following control experiments. Toward biomedical applications, the potential toxicity of the ultrasmall complex particles was carefully evaluated in healthy BALB/c mice through blood routine test and blood biochemical test, including white blood cell, red blood cell, hemoglobin, hematocrit, mean corpuscular volume, mean corpuscular hemoglobin, mean corpuscular hemoglobin concentration, and platelets for the former, and contents of alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, urea, and creatinine for the latter ( Figure 13G). Over 30 days observations revealed that neither acute toxicity nor noticeable inflammation or damage in any major organs was induced by the intravenously injected particles. 202 On the basis of these previous studies, Gao and co-workers also developed a coordinatively unsaturated Fe(III) complex-based activatable nanoprobe, toward sensitive tumor MRI and therapy. 203 They found that the coordinatively unsaturated complex formed by gallic acid and Fe(III) well inherits the photothermal conversion properties of the conventional coordinatively saturated complex, but becomes less stable in acidic pH range ( Figure 14A). The tumor microenvironment pH is low enough to trigger its slow decomposition to release Fe(III). In consequence, the MR signal is drastically boosted to show tumor microenvironment-enhanced MRI performance ( Figure 14B). To demonstrate the feasibility of this idea, they designed a responsive MRI probe by coating upconversion luminescence (UCL) nanoparticles with the coordinatively unsaturated GA-Fe(III) complex to monitor the release of Fe(III) ions by comparing the emissions of the underlying UCL particles at different wavelength positions in vivo ( Figure 14C-D). Through careful optical and MRI studies, it was confirmed that the release of Fe(III) ions in tumor microenvironment can drastically increase the contrast of MRI of tumors. Apart from this target-triggered feature for MRI, the released Fe(III) ions in tumor are capable of generating free radicals via Fenton reaction, known as Ferroptosis, [204][205][206] whereas the remaining GA-Fe(III) complex can still serve as heating centers for PTT ( Figure 14E). This work thus provides a novel design that integrated the PTT effect of GA-Fe(III) with activatable MRI capacity and therapeutic functions of Fe(III) ions. 203 Some other natural polyphenols showing more complicated molecular structures were also adopted to construct nanoprobes with Fe(III). [207][208][209] Chen and co-workers reported multifunctional theranostic nanoparticles composed of coordination complexes formed by Fe(III) ions and anthocyanins extracted from fruits for PAI/MRI and PTT studies. Owing to the unique structure of anthocyanin, the resulting probes can be eliminated via kidney in consequence of dynamic disassembling ( Figure 15). 210 Besides, Mao and co-workers reported a nanoprobe formed by Fe(II)-quercetin in the presence of PVP, which can not only be used for MRI and PTT of tumor but also for reducing intracellular ROS and inflammatory factor levels within tumors in vivo owing to the ROS scavenging ability of quercetin. 211 Zhang's group prepared theranostic nanoprobes by using PEGylated polydopamine to coordinate with Fe 3+ , which can induce the repolarization of macrophage from M2 to M1 phenotype. In addition, the PTTinduced tumor-associated antigens releasing can also stimulate the potential of M1-like macrophages to exhibit antigen presenting function. 212 In brief, the special biological functions of polyphenols can be used to enrich the applications of Fe-polyphenol complex nanoprobes apart from MRI and PTT.
In addition to the complexes formed with polyphenols, Fe(III) ion can also form pigments with inorganic anions, for example, the famous Prussian blue (PB) that can also form nanoparticles for tumor diagnosis and treatment. Because the charge transfer can occur between Fe 3+ and Fe 2+ via cyanide ion with an ideal formula of Fe(III) 4 [Fe(II)(CN) 6 ] 3 ⋅nH 2 O, which gives rise to strong absorption for efficiently converting NIR light into heat. Owing to the poor water solubility, the PB molecules readily agglomerate in aqueous media to form colloidal particles. 213 To control the particle size within nanometer regime, polydopamine was adopted as a surface capping agent to coat the PB nanoparticles followed by covalently attached PEG and folic acid. The resulting nanoparticles presented a good tumor targeting ability for MRI and PTT treatment. 214 Liu and co-workers reported that PB nanoparticles first modified with layer-by-layer self-assembled thin film and then with PEG on the outer surface. The resulting particles highly stable in various physiological media showed a relatively high r 1 relaxivity and strong NIR absorbance and thereby were used as a contrast agent for both MRI and PAI, apart from PTT. 215

Other metal complex nanoparticles
Apart from iron, other transition metal elements can also be adopted to construct nanoprobes composed of molecular complexes with NIR-tunable absorption. Some small molecular Ru(II) and Ru(III) complexes exhibiting strong antimetastatic activities and low-level general toxicity have been used as anticancer drugs in the clinic. However, their applications are still limited due to the poor water solubility and tumor specificity. 216 To address these issues, Zhang et al reported 6.5-nm nanodots based on Ru 3+ /phenanthroline (Ru-Phen) complex. The resulting dots presented enhanced aqueous dispersibility and tumor targeting ability through EPR effect. 217 Owing to the superior NIR absorption and high PCE of ∼60%, this complex dot was successfully used for PAI-guided cancer PTT.
Cu complex-based nanoprobes were also studied. Li et al reported ultrathin metal-organic framework of coppertetrakis(4-carboxyphenyl) porphyrin (Cu-TCPP) nanosheets with strong NIR absorption, which was applied for MR/IR thermal imaging and PTT of cancer. 218 Jiang and co-workers found out that Cu-TCPP nanosheets with an average length of 106 nm and width of 37 nm can generate 1 O 2 by reacting with H 2 O 2 to destroy tumor cells, which can potentially overcome current limitations of PDT in hypoxia tumors. 219

SUMMARY AND FUTURE PERSPECTIVES
The motivation for investigating the biomedical applications of nanoparticles with built-in functions is to develop methodologies for creating powerful probes to facilely combine advanced diagnostic and therapeutic technologies, for example, imaging-guided therapies, meanwhile pursuing theranostic probes clinically translatable in future for precision medicine. In this respect, NIR light-based theranostic technologies hold great promise, although the penetration depth of the lights currently used in the clinic is often taken as a major drawback. However, red-shifting the light into the second NIR window opens up new opportunities for novel theranostic technologies, because NIR II lights can reach internal territories due to the weaker tissue absorption and less light scattering. They also show less heating effect, which can essentially increase the contrast of PAI especially when exogenous probes are used. With respect to fluorescence imaging that will be surely very useful for guiding precise resection of tumors, NIR II fluorescence is enjoying much improved signal-to-noise ratio due to the drastically decreased background signal and remarkably increased transparency. 12,13,15,18,175 With respect to therapeutic applications, NIR II lights hold the following obvious advantages with the aid of advanced probes: (a) they offer versatile therapeutic measures, for example, PTT, PDT, photo-enhanced CDT, and so forth; (b) they can not only combine the above therapeutic approaches, but also integrate them with imaging, for example, PAI, to achieve imaging-guided therapy for precision cancer therapy; (c) they are safer than shortwavelength counterparts for all abovementioned applications; (d) they have very high spatiotemporal precision for cancer therapies; (e) it has been demonstrated at least so far through animal experiments that limited shots of NIR irradiation are enough for effectively inhibiting and even completely eliminating the tumors through PTT and PDT, which makes the light-based imaging-guided therapies very practically feasible for cancer treatments; and (f) the last but not least, there is a huge reservoir of materials for well utilizing the NIR lights toward biomedical applications; in particular, the nanomaterials with unique and remarkable built-in functions can further combine CT, MRI, and even SPECT/PET with light-based theranostics. An ideal scenario of inorganic nanoparticles with rich built-in functions would see their clinical translations if the efficacy and biosafety are rationally addressed.
Until now, a huge number of investigations based on tens of different types of nanoprobes have been published on PTT, PDT, and photo-enhanced CDT of tumors. Almost all studies have shown obvious antitumor effects, but few of them carefully deal with potential damage to surrounding tissues. Hyperthermia cancer therapy assumes that the cancer cells will be destroyed if the local temperature is elevated to 42 • C. 220,221 However, when the local temperatures is above 45 • C, damage to surrounding tissues may occur. 221 Therefore, an urgent challenge for PTT is to monitor and precisely control the temperature of the region of interest during the treatment. Although some progress have been made with respect to temperature monitoring during the photothermal treatment, 221,222 there is a clear lack of feedback loop system to stop the heating process for avoiding the damage to healthy tissues, which could hopefully be achieved through the advanced nanotechnologies. Similar problem remains with respect to PDT and photo-enhanced CDT. Therefore, developing noninvasive imaging methods to monitor the reactive species 16 is the first step for further combining its feedback with PDT, which will be an interesting subject for the future.
Apart from the efficacy, the advanced theranostic probes have to face biosafety challenges with respect to clinical translation. Intravenously administrated nanoparticles are recognized by the immune system and then largely captured by the reticuloendothelial system, for example, liver and spleen, potentially leading to long-term retention in the body and toxicity concerns, which largely hampers their clinical translation. Until now, we have very limited knowledge on the metabolism pathways of most nanoparticles that show remarkable cancer theranostic functions. It is in principle relatively easier to monitor the biodistribution of nanoparticles formed by elements that do not exist in the body, but they may have adverse effects. On the contrary, with respect to the particles formed by dietary trace elements of human being, it is then difficult to accurately trace their fate within the body. Although the toxicity studies are beyond the central topic of the current review, the long-term retention of nanoprobes may induce immune responses to produce adverse effects that are very difficult to predict at this moment. 223 To circumvent this problem, the fol-lowing strategies may deserve to follow: (a) reduce the size of the nanoprobes to enable their quick renal clearance, which is exactly the reason for nanoparticle with built-in functions to come into play; (b) design smart systems allowing to enhance the theranostic functions of small particles by assembling them within tumors upon stimuli, 224 which also may help to eventually reduce the dose of the probes; (c) further develop biomineralization process to integrate endogenous substance with inorganic minerals to increase the biocompatibility of the latter; (d) explore biodegradable materials with inherent properties for theranostic applications, which is reliable to further shorten the retention of the probes and their decomposition residues for suppressing the potential risks.
The clinical translation of the function-rich nanomaterials is warmly waited by the clinic, yet very challenging. One major challenge is that it is still difficult to compare the performance and pharmacokinetic behaviors of the nanoprobes prepared by different research labs, due to the large varieties of size, shape, and surface composition and property. Therefore, it is hard to draw reliable conclusions of single variable effects on the downstream performance for diagnostics, therapeutics, biodistribution, clearance pathway, and so forth. Apart from that, large-scale production of a given nanomaterial with high reproducibility suitable for in vivo applications remains difficult. This is because for any given nanoparticles synthesized through solution process, the particle size is not only determined by conventional synthetic parameters including the reaction temperature and time, the concentration of precursor, and its ratio to surface capping agent, but also strongly affected by the heat transduction and the mass transduction of nuclei, and particles of different sizes apart from that for the reactants. Moreover, not only different chemical processes including those for the decomposition of the precursors and the following formation of particle products are involved, but also the physical chemical processes such as Ostwald Ripening that largely determine the particle size are involved. All these aspects make the reproducibility of high-quality nanoprobes very challenging especially in industrial production scale. In spite of the safety concern and difficulties in large-scale production of multi-functional inorganic nanomaterials, cancer diagnosis and treatment would see in the near future the advanced nanoprobes that exhibit low adverse effects and remarkable therapeutic performance.

CONFLICT OF INTEREST
The authors declare no conflict of interest. CAS (2018042), and State Key Laboratory of Luminescence and Applications (SKLA-2019-01).