Recent progress in the applications of gold-based nanoparticles towards tumor-targeted imaging and therapy

Cancer death rate remains high all over the world, scientists are paying increasing attention to meet the requirements for precise diagnosis and therapy. Therefore, early diagnosis and active treatment can effectively improve the five-year survival rate of patients. In recent years, gold-based nanomaterials have received increasing attention in medical fields due to their excellent biocompatibility, low toxicity and unique properties. In addition, because of the inherent nature of gold nanomaterials including for computed tomography (CT), fluorescence/optical imaging (FI/OI), surface enhanced Raman spectroscopy imaging (SERS), photoacoustic imaging (PAI) and photothermal therapy (PTT), various gold nanomaterials were developed as theranostic nanoplatforms. In this review, we summarized the latest developments of nanomaterials in imaging and combined therapy, and the prospects for the future application of gold-based theranostic nanoplatforms were also proposed.


Introduction
In 2022, the United States estimates that there will be 1 918 030 new cancer cases and 609 360 cancer deaths. The number of cancer deaths was increasing every year until 1991, aer which the mortality rate has declined year by year. By 2017, the estimated number of cancer-caused deaths was 2.9 million. 1,2 Early detection, early diagnosis and early treatment was the only way of improving the survival rate for cancer patients.
Gold nanomaterials have been widely used in biomedical research due to their unique properties and good biocompatibility. [3][4][5][6] As early as the 1970s, colloidal gold had been used in the treatment of rheumatoid arthritis patients. Thereaer, gold nanoparticles (27 nm) were used as carriers to load recombinant human tumor necrosis factor for treating advanced malignant tumor patients and then progressed to phase I clinical research. Results of clinical experiment showed that no obvious side effects were observed in the patients. 7 In another clinical study registered in Russia, a 15 nm gold shell was used for arterial porridge in the photothermal treatment group, which achieved better clinical effect. 8 These results demonstrated that gold nanomaterials have high scientic value and excellent clinical application prospects.
The synthesis and modication methods of gold nanoparticles (AuNPs) are mature, simple, and the structure is relatively easy to control (Scheme 1). Various AuNPs with different size and shape are prepared, such as spheres, 9,10 rods, 11,12 clusters, [13][14][15][16] cages, 17,18 stars 18,19 and so on. Moreover, AuNPs possess some unique properties: (1) imaging, including computed tomography (CT), uorescence/optical imaging, surface enhanced Raman spectroscopy imaging (SERS), photoacoustic imaging (PAI). (2) Therapy, including drug/gene delivery, photothermal therapy (PTT). On the basis of these properties, researchers continuously develop new methods for preparing/ modifying AuNPs so as to improve or obtain new performances. In this review, we summarized the recent development of gold-based nanoplatform including polymer-coated gold nanoplatform, gold-based hybrid nanoplatform, and membrane-coated gold nanoplatform, and their applications in tumor diagnosis and therapy were also classied and discussed. Moreover, the prospect for the development of gold-based nanoplatform is also proposed.

Preparation and properties of gold nanoparticles
AuNPs possess a variety of different sizes and shape, including sphere, rod, cage, cluster and so on. As time goes on, researchers can synthesize AuNPs with almost all sizes and shapes. There are three main synthetic methods: (1) citric acid reduction HAuCl 4 . 20,21 Classic method for preparing gold nanosphere is citric acid reduction HAuCl 4 . The diameter of the gold nanospheres can be controlled at 15-150 nm via changing the concentration of sodium citrate, and diameter greater than 25 nm possess gold photothermal effect. (2) Seed-mediated method. 22 Classic methods for preparing gold nanorod is seed-mediated method. Electrochemical methods and photochemical reduction are also used to prepare gold nanorod. Different from gold nanosphere, gold nanorods are anisotropic and have two absorption peaks of localized surface plasmon resonance (LSPR). The lateral LSPR band is xed at approximately 520 nm, while the longitudinal LSPR band is adjustable in the visible to near infrared (NIR) range.
(3) Template method. 23 Classic methods for preparing gold nanocage are template method which uses silver nanocube as a template and reduces the seed crystal to induce the growth of gold spheres around the gold template, thereby forming a mesoporous shell. The gold nanocage has a hollow and porous wall structure, which can be used as carrier.
Different kinds of AuNPs have various size and shape, however, these AuNPs usually possess some common properties. For example, due to the high atomic number and electron density, AuNPs can effectively absorb X-ray irradiation, which is superior to the iodine-based contrast agents used in clinic. Next, the absorption on AuNPs or other metal surfaces can enhance the intensity of the vibration spectra of Raman active molecules, which then developed into a new technology-surface enhanced Raman spectroscopy imaging (SERS) for detecting the margin of tumor. In addition, the irradiation of AuNPs with laser of certain wavelength induces localized temperature increases. On the one hand, the increased temperature will result in photothermal ablation, which is then widely used in tumor therapy; on the other hand, the heating will also cause a sound wave, which can be captured and transferred as photoacoustic imaging. AuNPs have many other properties, this review will then summarize and discuss in the following text.

Polymer-coated gold nanoparticles for imaging and therapy
When a variety of NPs enter the human body, the stability of these NPs is the key problem that needs to be addressed.
Polymer coating is a common technique. Meade group reported a strategy for early detection of pancreatic adenocarcinoma through conjugating dithiolane-Gd(III) complex to AuNPs (Fig. 1A). 24 In this work, Gd-DO3A-C6 amine and Gd-DTPA-C4 amine were conjugated with lipoic acid followed by anchoring onto the surface of AuNPs. The results demonstrated that these conjugates could acquire MR images at both low and high magnetic eld strengths due to the high Gd(III) payload, and could be used as potential candidates for diagnosis and treatment of pancreatic disease. Similarly, Zhang group fabricated c(RGDyC), Gd-and 99 mTc-labeled AuNPs (RGD@AuNPs-Gd99 mTc) probe with different sizes of AuNPs (29, 51 and 80 nm), and their potential radiosensitization therapy guided by MRI/ SPECT both in vitro and in vivo were also evaluated (Fig. 1B). 25 The results demonstrated that the RGD@AuNPs-Gd99 mTc with 29 nm Au NPs were clearly the most efficient in vivo.
Undoubtedly, there simple decorations greatly improve the stability in vivo and endow them with excellent targeting ability. Along with going deep gradually into the research, these simple decorations cannot meet the demand. More and more multifunctional polymers are decorated onto the surface of AuNPs to endow them with more and better functions. Zhang group fabricated a multifunctional nanocomposite AuNP@CD-AD-DOX/RGD via host-guest interaction for targeted cancer therapy. 26 In the work, AuNP@CD was rstly synthesized as the core to offer binding sites for adamantane-modied PEG ligands. Aerward, anticancer drug doxorubicin (DOX) was conjugated onto adamantane via hydrazine bond that could be broken under acid tumor environment (AD-Hyd-DOX), while RGD was conjugated onto adamantane via PEG ligand (AD-PEG8-GRGDS). Thereaer, AD-Hyd-DOX and AD-PEG8-GRGDS were both anchored onto AuNP@CD via host-guest interaction. The results illustrated that AuNP@CD-AD-DOX/RGD could be uptaken by cancer cells and released DOX rapidly in the acid environment and thereby inducing cell apoptosis. A similar structure was also reported by Xia group. 27 They reported a three-in-one nanoplatform for sensing, self-assembly and cascade catalysis via cyclodextrin modied AuNPs (CD@AuNPs). AuNPs were employed as uorescent sensing based on guest replacement reaction, while the cyclodextrin on the surface of AuNPs not only endowed the AuNPs with the ability of self-assembly to form one-and two-dimensional architectures using tetrakis(4-carboxyphenyl) porphyrin as Scheme 1 Illustration of various shape of gold nanoparticles and their applications.
mediator, but also endowed the AuNPs with the ability of catalytic activity.
Dendrimer is a category of nano-sized polymer with a well-dened composition and branched tree-like structure. The uniform size and globular shape make dendrimer a nanoplatform to encapsulate or stabilize various inorganic nanoparticles, including metal or metal derivatives. Shi group reported a facile approach to fabricate low generation poly(amidoamine) (PAMAM) dendrimer stabilized gold nanoparticles (Au DSNPs) for in vivo computed tomography (CT) imaging applications. 28 In this work, PAMAM dendrimers (G2) were employed as stabilizers to form Au DSNPs through a simple hydrothermal method, followed by neutralizing with acetic anhydride. The as-prepared Au DSNPs with an Au core size of 5.6 nm performed much better in CT imaging of some major organs of rat than that of Omnipaque (a clinical contrast agent). Kono group modied PEG-PAMAM dendrons onto the surface of a gold nanorod (GNR) by adding PEG-PAMAM dendrimers (G2-G4) with a cystamine core during the GNR growing reaction. 29 The dendrimer modied GNR exhibited excellent biocompatibility against HeLa cells, however, the GNR-cored PEG-PAMAM dendrimers killed the HeLa cells effectively under near-infrared laser irradiation. In vivo experiments also demonstrated that the tumor volume obviously decreased with the irradiation of near infrared laser. Thereaer, Kono group further fabricated pH-sensitive drug-dendrimer conjugatedhybridized gold nanorod for combined cancer photothermalchemotherapy (Fig. 2). 30 Briey, poly(ethylene glycol)-modied PAMAM G4 dendrimer (PEG-PAMAM) were covalently linked on the surface of mercaptohexadecanoic acid-functionalized gold nanorod (MHA-AuNR) followed by conjugation of anticancer drug doxorubicin (DOX) through acid-labile-hydrazone bond. The obtained PEG-DOX-PAMAM-AuNR particles could realize controlled release at different pH value. In addition, PEG-DOX-PAMAM-AuNR with irradiation groups showed signicant tumor growth suppression or delay in comparison with the control groups, which illustrated that the combined photothermal-chemotherapy exhibited higher therapeutic efficacy then the single treatment.
Stem cell therapy is to transplant healthy stem cells into patients, thereby repairing damaged tissues, so as to achieve the purpose of treatment. How to effectively trace the trajectories of transplanted cells in vivo and understand the migration, homing and survival of transplanted cells remains a great challenge. Modo group reported an AuNP conjugate to track the transplanted stem cells (Fig. 3A). 31 Briey, Gd chelates were conjugated onto Cy3 labelled deoxythymidine oligonucleotides via click chemistry (Cy3-labeled Gd-DNA). Aerward, Cy3labeled Gd-DNA was anchored onto the surface of AuNPs through gold-thiol chemistry (DNA-Gd@AuNP). The diameter of the NPs increased from 15.3 AE 1.4 nm (size of AuNPs observed by TEM) to 35 AE 0.7 nm (hydrodynamic radius of DNA-Gd@AuNPs). The experiment results showed that DNA-Gd@AuNP exhibited remarkably T 1 relaxivity and could be used for in vivo detection of transplanted cells. The postmortem histological verication of DNA-Gd@AuNP in the transplanted cells further corroborated the results with that of in vivo MR images. DNA-Gd@AuNP therefore offer the opportunity to visualize the transplanted cells through MRI technique as well as reect the information of the survival status and location of the transplanted cells, which help us understand the therapeutic mechanism. In addition, Drezek group fabricated AuNP-PAMAM conjugates as non-viral transfection agents, which was prepared by crosslinking PAMAM dendrimers to carboxylic-terminated AuNP through EDC chemistry. 32 There-aer, the optimal vector was obtained through adjusting the ratio of amine and carboxyl groups, and thereby yielding high transfection efficiency and low cytotoxicity. Zhang group modied AuNPs with red-emitting rey luciferase (RfLuc)based bioluminescence (BL) tags for CT/BL bimodal imaging to track the transplanted cells in a murine model of idiopathic pulmonary brosis (IPF) (Fig. 3B). 33 This technique combined CT and BL imaging enables in situ visualization of the location, distribution and survival of the transplanted cells.
The above-mentioned literature summarized the examples that various polymer/DNA chains were directly anchored onto the surface of AuNPs, which construct a relatively simple and easy system for diagnosis and therapy. Thereaer, some response mechanisms were also introduced into the modication of AuNPs to further improve the imaging or therapeutic effects. Shuai group loaded DOX into a pH-sensitive core and acted as a template to introduce a gold nanocage followed by conjugation of polyethylene glycol (PEG). 34 This DOXloaded pH-sensitive gold nanocage (D-PGNC) was relative stable due to the cross-linking and gold decoration of interlayer in the blood stream, but turned on in the tumor region due to the acid tumor microenvironment, high concentration of GSH and nearinfrared (NIR) irradiation. Aerward, the results of synergistic anticancer therapy in vivo illustrated that D-PGNC plus NIR groups showed the longest survival time (a 45 days survival rate of 83.3%) than that of control groups.
Next, Klajn group developed a new concept to assemble the NPs mediated by light, which did not require the modication of light-responsive ligands. 35 In their work, a photoswitchable medium was introduced, whose acidity could be modulated reversibly under visible light and then affected the NPs interaction. Therefore, these NPs could quantitatively assemble or disperse. Gao group developed smart responsive AuNP that   could covalently crosslinking in the tumor region under 405 nm laser irradiation, via a diazirine terminal group of PEG 5000 ligands on the surface of AuNPs (20.5 nm). 36 As shown in Fig. 4, NH 2 groups were introduced to the surface of AuNPs via PEG ligands followed by conjugating with NHS-diazirine (succinimidyl 4,4-azipentanoate) to obtain diazirine-decorated AuNPs (denoted as dAuNPs). Thereaer, the average size of the asprepared dAuNPs (20.5 AE 1.9 nm) changed into largely forming particle aggregates (700 nm aer 15 min of irradiation), and the size of aggregates gradually increased with the increase of the exposure time to 405 nm laser irradiation. In addition, in vivo experiment illustrated that PA signal of the tumor sites (Fig. 4c) obviously enhanced aer 405 nm laser irradiation, and the photothermal images (Fig. 4d) of the tumor bearing mice further illustrated the in vivo crosslinking effect of dAuNPs in the tumor region aer irradiation both at 405 and 808 nm laser. Meanwhile, the tumors holding cross-linked dAuNPs groups were signicantly reduced in size with the 808 nm irradiation compared to the control groups, which also exhibited improved survival rate. In general, AuNPs with small size have longer blood retention times, but the small ones also possess low imaging capability and photothermal therapy. The mechanism of light-induced aggregation just makes up the limitation.
However, how to distinguish the tumor margins faced great challenges in surgery. Li group developed a pair of gold nanoprobes that could cross the blood-brain barrier (BBB) with the help of angiopep2 peptides (Fig. 5A). 37 Thereaer, these gold nanoprobes could further assemble into aggregations via bioorthogonal reactions under the acidic tumor environment. As time went on, the aggregates continuously trapped into the tumor region and thereby enhancing both magnetic resonance (MR) signal and surface-enhanced resonance Raman spectroscopy (SERRS) signals. Therefore, the MR signal could be used for diagnosis, while SERRS signal could be used for intraoperatively guide tumor excision with the assistance of a handheld Raman scanner. These promising candidates could signicantly improve the accuracy of brain-tumor surgery as well as the possible extension of other tumor resections. Gao group developed an aggregation mechanism triggered by legumain between Ala-Ala-Asn-Cys-Lys modied AuNPs (AuNPs-AK) and 2-cyano-6aminobenzothiazole modied AuNPs (AuNPs-CABT) (Fig. 5B). 38 The obtained AuNPs-A&C could realize aggregation under certain condition both in vitro and in vivo. In vivo uorescence imaging and PA imaging demonstrated that AuNPs-A&C could selectively accumulate in the glioma site. Aer loading doxorubicin (DOX), the median survival time increased to 288% in comparison to the saline group. Therefore, the AuNPs-A&C system could realize precisely diagnosis and therapy of glioma. Compared to the aggregation mediated by photo mentioned above, chemical reactions triggered aggregation could form more stable gold aggregates.
In this section, various kinds of polymer-coated AuNPs were reviewed and discussed. On the one hand, these decorations signicantly improve the stability of AuNPs in vivo, thereby increasing the accumulation of AuNPs in the tumor region. On the other hand, these decorations also endow AuNPs with some new characters, including targeting ability, pH sensitivity, light response, which further expand the applications of AuNPs.

Gold-based hybrid NPs for imaging and therapy
Polymer coating can endow the AuNPs with lots of new properties, however, hybrid AuNPs with inorganic/organic/metal materials will open a new world and endow AuNPs with some characters, which polymer-coating cannot offer. Silica, a common inorganic material, is oen used as surface coating, which can increase the inertness of NPs in vivo as well as endow the NPs with the ability of loading drug. Prabaharan group fabricated silica@gold NPs followed by conjugation of guar gum succinate (GGS) and DOX (FITCSiO 2 @Au-DOX-GGS-FA NPs), which was used of CT/uorescence dual imaging and chem/ phototherapy. 39 The obtained NPs exhibited uorescence and Xray attenuation properties due to the SiO 2 @Au hybrid nanostructure. In addition, DOX was conjugated onto the SiO 2 @Au NPs via hydrazone bond that could be cleaved under acidic condition. Next, many other elements are developed.
Selenium nanoparticles and selenodiazole derivatives induce cell apoptosis by generating Compton effect and photoelectric effect, which can enhance X-ray-based cancer radiotherapy. However, the imaging capabilities of seleniumcontaining radiosensitizers are oen limited by individual differences. AuNPs, as high-Z and extremely high-chemical inert materials, are widely used as radiosensitizers and contrast agents due to their large photoelectric absorption coefficient at the tumor site. Chen group developed a simple way to fabricate a ternary heterostructure radiosensitizer (SeAuFe-EpC) with core/satellite morphology to realize multimodal imagingguided cancer radiotherapy function. 40 The introduction of Se could greatly increase the conductivity and reduce the energy barrier between these three components, so that more electrons were transferred between the Se-Au interface and the heterojunction at the Au-Fe 3 O 4 NPs interface. This synergistic interaction enhanced the energy transfer and facilitated more excited excitons generated by SeAuFe-EpC NPs, and thus promoting the transformation of 3 O 2 to 1 O 2 , which ultimately led to the apoptosis of irreversible cancer cell. In addition, due to the X-ray attenuation ability and high NIR absorption ability of AuNPs as well as the superparamagnetism of Fe 3 O 4 , SeAuFe-EpC possessed a combined in vivo computer tomography, photoacoustic and magnetic resonance three-peak imaging capabilities to visually track nanoparticles. Cui group developed a multifunctional contrast agent-based gadolinium ionsmediated self-assembly of gold nanoclusters into monodisperse spherical nanoparticles (GNCNs). 41 The as-prepared GNCNs could realize near-infrared uorescence (NIRF) imaging, CT imaging and MR imaging. Liu group fabricated hpDNA-functionalized NaYF 4 @SiO 2 -Au nanoconjugates base on the coupling of small-sized AuNPs (z2 nm) and hairpin DNA (hpDNA) conjugation, which could realize simultaneous deeptissue imaging and site-specic administration of anticancer drugs (Fig. 6). 42 In this system, upconversion nanoparticles were coated with silica and the hpDNA strands were anchored onto the AuNPs through Au-thiol bond. The coupling of Au to the upconversion nanoparticles could realize the modulation of luminescence in the NIR region (z800 nm) for minimal attenuation in tissue, while the hpDNA strands allowed a high drug loading and releasing capacity. In vivo experiments showed that the tumor in the experimental group gradually decreased in comparison with the control groups. Therefore, the as-prepared nanoconjugates were promising probes for deep-tissue imaging and guided therapy. Li group synthesized a dumbbell-shaped heterogeneous copper selenide-gold nanocrystals (CSA) by simply mixing Cu 2Àx Se nanoparticles and HAuCl 4 in an aqueous solution without using reducing agents. 43 The obtained CSA composite exhibited signicant enhancement of radiosensitization compared to Cu 2Àx Se nanocrystals (CS), AuNPs and their mixtures. In addition, the CSA composite possessed multi-modal imaging capacity including PAI, CT and SPECT, as well as synergistic radiophotothermal therapy of cancer. These results illustrated that the CSA composite could realize the precise diagnosis and treatment through multimodal imaging guided radiophotothermal therapy. Zhu group synthesized a size tunable core-shell composite (Au-SiO 2 -WO 3 ). 44 Briey, a WO 3 core with a diameter of 20-50 nm was coated with and intermediated silica layer (10-60 nm) followed by coating with Au shell (5-20 nm). Aerward, 4-mercaptobenzoic acid (4-MBA) was modied onto the surface of the Au shell and served as Raman reporters. The obtained Au-SiO 2 -WO 3 composite exhibited high resolution Raman properties for single-molecule detection.
Metal-organic framework (MOF) are three-dimensional porous materials composed of inorganic metal nodes termed secondary building units linked together by rigid multitopic organic linkers. MOF possesses lots of advantages including structure diversity, large surface area, tunable pore size and functionalities, which are widely used in the area of storage, separation, sensing, catalysis and drug/gene delivery. [45][46][47][48][49][50] MOF is rstly introduced into the area of drug delivery in 2006 by Férey group, subsequently, the MOF is widely used as vector for drug/gene delivery. 51 Boyes group rstly fabricated a Gd MOF nanoparticles (GdMOF) followed by anchoring the AuNPs onto the surface of GdMOF. 52 In this structure, GdMOF serves as MRI contrast agent, which possessed a high Gd payload, adjustable size and exhibited excellent capability of MR imaging. AuNPs were dispersed on the surface of the GdMOF structure and served as CT contrast agent. Therefore, integration of GdMOF with AuNPs together endowed this nanoplatform with the ability of MRI and CT imaging, which possessed signicant attention in the early diagnosis of cancer. Lei group fabricated a core-shell structure with a peptide modied AuNPs core and a pH-sensitive MOF (ZIF-8) shell. 53 This hybrid composite showed dual-recognition and could be used for monitoring the activity of lysosomal CaB. Briey, ZIF-8 shell possesses excellent biocompatibility and exhibits highly activated response to the acidic environment. Aer disintegration of the ZIF-8 shell, the Au-peptide core exposed to the CaB and recognized by Cab, thereby realizing a stepwise-responsive strategy for CaB detection. Triple-negative breast cancer (TNBC) is one of the most dangerous types of breast cancer with poor clinical outcome. Zeng group developed well dened AuNS@MOF-ZD2 nanocomposites by encapsulating a single AuNS within MIL-101-NH 2 (Fe) to from core-shell structure to achieve specically targeting toward triple-negative breast cancer (TNBC). 54 Fig. 7B showed the morphologies of AuNS and AuNS@MOF by coating MOF with one to four cycles. The obtained core-shell AuNS@MOF-ZD2 composites exhibited remarkable T 1 -weighted MR imaging and PTT toward TNBC with a photothermal conversion efficiency of 40.5%. Meanwhile, the temperature in the tumor sites of targeted group was 5.5 C higher than that of nontargeted group at 5 min postirradiation of 808 nm laser. In addition, the mice were alive well in the targeted group, while the mice were dead in the other control groups during the period of 40 days posttreatment. Zhang group reported a gold nanorod-based nanoplatform for the treatment of primary tumor and the inhibition of metastatic tumor through PTT. 55 In these work, AuNR was coated with mesoporous silica (AuNR@MSN) followed by loading DOX. Thereaer, the AuNR@MSN-DOX was further coated with metal(Gd 3+ )-organic network (AuNR@MSN@-MON). The MON membrane could offer superior strategy for inhibiting tumor metastasis, the Gd section endowed the NPs with the ability of MR imaging, the combination of PTT and chemotherapy obviously enhanced the antitumor therapeutic effects.
Every coin has two sides. In the ancient Rome, Janus, Rome God with two face, could look back on the past events and forward to the future. Janus NPs are composed of two different surface properties, which exhibiting absolutely different chemical or physical properties. Thereaer, Janus NPs have attracted signicant attention in recent years due to its unique nature. Filice group reported a Janus nanoplatform consisted of Fe 3 O 4 NPs/mesoporous silica core @ shell surface together with a gold nanoparticle surface. 56 Aerward, cRGD and Dye were modied onto the Janus nanoplatform due to its anisotropy. Thereaer, the obtained Janus nanoplatform possessed multimodal imaging capacity including MRI (Fe 3 O 4 core), CT (AuNP face) and uorescent tracking (Dye), which could serve as a powerful nanoplatform for targeted cancer theranosis. Sonodynamic therapy (SDT) possesses high penetration, noninvasiveness and controllability, which is suitable for detection of deep-seated tumors. Hou group reported a monodisperse Au-Fe 2 C Janus NPs with a diameter of 12 nm. 57 The experimental results illustrated that Au-Fe 2 C Janus NPs could simultaneously realize MR/MSOT/CT imaging as well as photothermal therapy (30.2%) under 808 nm laser irradiation.
Vesicle has a huge cavity, which can be used for drug loading. Different from the Janus NPs mentioned above, Yang group fabricated Au-MnO Janus NPs followed by coating with PEG and a ROS-sensitive polymer, 58 aerward, the obtained Janus nanocomposites self-assemble into a vesicle which could disassembled into small Au-MnO Janus NPs, subsequently, the MnO section degraded and release Mn 2+ for chemodynamic therapy. This Janus NPs integrated photoacoustic imaging together with T 1 -weighted MR imaging and realise synergistic SDT and CDT. A similar structure was also reported by Chen group (Fig. 8). 59 They synthesized Fe 3 O 4 -Au Janus NPs followed by graing PEG on the Au surface and ROS-generating poly(lipid hydroperoxide) (PLHP) on the Fe 3 O 4 surface, respectively. Aerward, this multifunctional Fe 3 O 4 -Au Janus NPs assembled into bilayer vesicles and DOX was also loaded in the cavity of the vesicles. Therefore, this multifunctional nanoplatform could realize multimodal imaging including MRI, PA and PET. In addition, the acidic tumor environment could trigger the disassemble of vesicles into single Janus NPs, thereby enhancing local ROS generation due to intracellular biochemical reaction between PLHP and the released ferrous ion (Fe 2+ ). Combined ROS and DOX therapy, the tumors were obviously ablated in the mice treated with JNP@PLHPVP-PEG Ve-DOX compared to the control group, which was further demonstrated through MR imaging.

Membrane-coated gold nanoparticles for imaging and therapy
As early as 1965, bilayer phospholipid system, named liposomes, was rst established in 1965. Liposomes are nano-sized to micro-sized vesicles consisted of a phospholipid bilayer and cavity. Subsequently, liposomes can be prepared according to different method with various composition, size, charge and structure. Various liposomes were designed as important vectors, including drug/gene loading, long-circulating liposomes, triggered release liposomes. [60][61][62][63][64] In 1995, Doxil®, the rst FDA-approved nano-drug is based on PEGylated nanoliposomes, which can passively targeted to tumor region due to the EPR effect, thereby releasing doxorubicin. 65 Zhu group engineered a multifunctional nanoplatform based on gold nanostar @SiO 2 /DOX encapsulated in liposome (GSMS@lip). 66 This nanoplatform not only served as vector for delivering therapeutic drugs, but also had high photothermal efficiency. When the nanoplatform exposure to the nearinfrared (NIR) laser, the ambient temperature was raised due to the photothermal effect of gold nanostar, thereby rupturing the thermosensitive liposome and triggering the drug release. Beneting from the synergistic effect of chemotherapeutic drugs and photothermal effect, the as-prepared multifunctional nanoplatform could effectively inhibit the tumor growth. Next, De group fabricated a biodegradable plasmon resonant liposome gold nanoparticle (LiposAu NP), which was capable of killing cancer cells through PTT (Fig. 9A). 67 In this work, liposome was prepared through thin lm hydration method followed by coating gold nanoparticles on the surface of liposome. The obtained LiposAu NPs were prepared to achieve a size range of 100-120 nm. Aer exposing to 750 nm laser, the tumor xenogra was complete ablated, thereby degrading into small NPs and clearing through hepato-biliary and renal route. A similar work was also reported by Gao group, they designed and synthesized a gold nanoshell coated oleanolic acid liposome mediating by chitosan (GNOLs). 68 This liposome exhibited a slow release of oleanolic acid at pH 7.4 while a rapid release at pH 5.5, which is good for tumor targeting drug release. In addition, the gold nanoshell could activate photothermal effect and trigger drug release aer NIR irradiation.
Undoubtedly, liposomes possess many advantages, but they also face some deciencies, for example, liposomes can be cleared by reticuloendothelial system (RES) or mononuclear phagocytic system (MPS) due to surface charge or size. There-aer, K. A. Dawson rstly proposed the concept of protein corona, who found that the NPs can absorb protein on the surface when they circled in the blood, thereby changing the charge and dispersion in the biological environment. Subsequently, various proteins were anchored onto the surface of NPs to endow the NPs with new biological properties, thereby improving their performance including cellular uptake, immune response, clearance and toxicity. Li group fabricated a theranostic nanoplatform via gadolinium (Gd)-based bovine serum albumin (BSA) hybrid-coated hollow gold nanoshells (Au@BSA-Gd). 69 In addition, phototherapeutic agent [indocyanine green (ICG)] was loading into the cavity of Au NPs. Therefore, this Au NPs could be developed as theranostic nanoplatform for NIRF/PA/CT/MR quadmodal imaging as well as precise cancer therapy. Briey, hollow Au nanoshells were fabricated by the silica-template removal method. Aerward, BSA-Gd complex was anchored onto the surface of Au nanoshells through EDC chemistry followed by loading of ICG. The in vivo results demonstrated that Au@BSA-Gd could realize in vivo quadmodal NIRF/PA/CT/MR imaging capability as well as combined PTT and PDT therapy. Gold nanocages (AuNCs) and gold nanoclusters (AuClusters) are two classes of nanostructures with excellent attractive properties (Fig. 9B). 70 Wu group integrated AuNCs and AuClusters into one theranostic nanoplatform for uorescent and optoacoustic bimodal imaging as well as PTT. In this work, gold nanocages (AuNCs) were synthesized using a galvanic replacement between silver nanocubes and chloroauric acid (HAuCl 4 ). Next, Erlotinib (EB) was loading into the AuNCs followed by modication of PLL. Finally, Au cluster@BSA was anchored onto the surface of AuNCs via electrostatic interaction. The obtained hybrid gold could release EB in the presence of lysosomal proteases and low pH and induce uorescent restoration for imaging. As shown in Fig. 9B-e, no signal could be observed in the control group, while distinct signals could be observed from the AuNC in tumor, liver, spleen and lungs. In addition, the drug release rate would further accelerate under NIR irradiation, and the results demonstrated that the combined therapeutic efficacy could effectively inhibit the tumor growth.
However, chemical crosslinking usually causes protein denaturation and greatly limit their applications. Reb blood cells (RBCs) are natural carrier in the human body, and become a hotspot in drug delivery research own to its non-immunogenic and biocompatible characters. Troyer group engineered RBCs as a carrier for loading AuNPs, thereby acting as an efficient CTcontrast agent (Fig. 10A). 71 In this work, AuNPs (5 nm) were anchored onto the surface of RBCs through the lipid thiol ligand without altering its shape, size and surface properties. The average size of RBCs is about 7 micron, which greatly inhibit their applications in vivo, but providing a new concept for construction of RBCs-based vector. Subsequently, Zhang group camouaged AuNPs with red blood cell membranes to endow these Au NPs with the surface properties of RBCs, which could be less susceptible to clearance by phagocytic cells (Fig. 10B). 72  membrane-camouaged AuNPs slightly increased (from 70.1 to 86.5) while the zeta potential increased from À42.2 to À35.1. In addition, a thiolated uorescent probe and an antibody bind assay further demonstrated that the AuNPs were successfully camouaged by RBC membrane. Similarly, Yang group fabricated RBC-membrane-coated gold nanocages (AuNCs), which exhibited excellent colloidal stability. 73 The results indicated that the fusion of RBC membrane did not change the porous and hollow structure of AuNCs, but signicantly enhanced the in vivo blood retention and circulation lifetime. In addition, van Hest group constructed a Janus polymeric motors based on gold nanoparticles and chitosan/heparin followed by coated with erythrocyte membrane, which could be used for photothermal therapy and successfully applied in thrombus ablation. 74 In this work, chitosan (a positive charge polysaccharide, CHI) and heparin (a negative glycosaminoglycan, Hep) were selected to construct biodegradable capsule with a gold layer through sputter coating, thereby obtaining a NIR-responsive Janus structure. Experiment results illustrated that the change of temperature could lead to the movement of the EM-JPMs through the self-thermophoresis effect aer exposing to laser irradiation. This biohybrid EM-JPMs exhibited excellent performance in thrombus ablation.

Outlook and challenges
Over the past two decades, various gold-based nanomaterials with different shape and size were prepared, and the development of gold-based NPs dramatically attracts scientists' attention in the medical elds such as imaging, gene/drug delivery and therapy. These multifunctional gold nanomaterials were widely developed as imaging-guided in vivo cancer therapy. Nevertheless, there are also many challenges in this eld: (1) biocompatibility, and (2) commercialization of AuNPs.

Biocompatibility
There is no doubt that gold-based nanoparticles possess lots of excellent properties, but, they still face some problems. Biocompatibility is the rst priority, especially the long-term cytotoxicity. The cytotoxicity of AuNPs depends on various parameters, including size, cell type, surface coating, tissue distribution and so on, among which size is the most important parameter. [75][76][77] The previous literature demonstrated that small AuNPs (4-5 nm) possesses higher toxicity than that of large AuNPs (about 20 nm). 78 Another literature reported that positively charged gold-dendrimer complex with a size of 5 nm were excreted aer 5 days, while negatively/neutrally charged AuNPs with a size of 5 nm or AuNPs with a size above 11 nm showed much lower excretion, only about 10% of the initial dose was excreted. 79,80 However, a simple chemical modication of AuNPs will greatly increase the cellular uptake efficiency and reduce the toxicity, such as PEG coating. 81 Jeong group found that PEG-coated AuNPs with a diameter of 13 nm induced acute inammation and apoptosis in human liver, 82 while Kreyling group found that PEG-coated AuNPs were mostly accumulated in the liver and spleen. 83

Commercialization of AuNPs
Despite AuNPs-based theranostic nanoplatforms have ushered a new period of vigorous development, few commercial opportunities have been created for their utilization in clinic. One major obstacle is the cost. For example, Xia group reported that 200 mL of PEGylated AuNPs (Au atom mass z 0.8 mg per mouse) was used for in vivo photothermal treatment of mice ($20 g), thus, the cost of PEGylated AuNPs for a normal human is very high. 84 Therefore, these problems cannot be ignored, especially in the clinical application, which have become a pressing issue placed in front of modern scientist.

Conclusion
In this review, we summarize the fabrication and application of gold-based nanoparticles in recent decades. Initially, various polymers are anchored onto the surface of gold and thereby endowing the AuNPs with excellent stability as well as various properties in vivo. Next, gold-based hybrid NPs are fabricated to further expanded their properties and applications. Finally, membrane-mediated AuNPs (liposome or RBC membrane) are developed to improve the recognition ability and circulation time in blood, and thereby improving diagnosis and treatment effects. All these techniques greatly broaden the applications of gold in vitro and in vivo. Consequently, various literature report that a great many gold-based theranostic platform are developed, which focus on the animal modes, but nearly none of them show signicant potential for cancer therapy in clinic. Therefore, scientists from various disciplines should make great efforts to cooperate in order to overcome the barriers, including chemistry, material, biology and engineering.

Conflicts of interest
There are no conicts to declare.