English

• 1.   Introduction

  Nanoparticles (NPs) have attracted researchers' attention due to the multifunctionality for biomedical applications [1-3]. NPs can potentially pass through different tissues or organs [4, 5], connect receptors from cell membranes [6, 7], access into cells to discharge goods [8-14], affect the intracellular pursuing pathways [15]. For example, NPs are generally internalized into cells by endocytosis, which is different from small molecules. The NPs can accumulate in tumor site with long retention time, which is named as enhanced permeability and retention (EPR) effect [16, 17]. Consequently, NPs-based materials can be applied in theranostics of diseases, such as cancers [18-22], infectious diseases [23] and so on. NPs can act as nanocarriers that encapsulate drugs and highefficiently reach lesion location. Under external or internal stimuli, such as pH [24-26], temperature [27], light [28, 29], enzyme [30], cargoes can be released in a controllable manner.

  NPs' structure is one of the most important factors that influence NPs' performance for theranostics of disease, which can be transformed in physiological/pathological conditions [31]. In order to obtain high performance in various biological applications, smart transformable NPs are developed recently. The sizes, surface charges, shapes or morphologies of NPs can be precisely modulated by pH, enzyme, and temperature and so on. In this review, we will summarize the ways for transformation of NPs, which is applied in disease diagnostics and therapy (Scheme 1). Transformable NPs are divided by following terms: ⅰ) Size: small NPs turn into big ones and big NPs turn into small ones; ⅱ) Surface charges change from negative to positive; ⅲ), Morphology transformation from NPs to nanofibers (NFs) or large aggregates. All kinds of transformation of NPs show great merits such as increased penetration, long retention, and so on, which can be utilized for high-performance nanomedicine.

  Scheme1

  

  图 Scheme1  NPs with transformable structures including size, surface charge and morphology

  Scheme1.  NPs with transformable structures including size, surface charge and morphology

  下载: 全尺寸图片 幻灯片

  2.   Size of NPs

  2.1   Small NPs into big ones

  The size of NPs was an important parameter for drug delivery, considering the circulation, penetration and efficacy of cellular uptake of NPs. Big ones had long retention time, resulting in longterm imaging or high accumulation of drugs in lesion location [32, 33]. However, small ones showed deep penetration features [34]. Researchers devoted to developing transformable NPs in size and utilizing merit of different sizes of NPs for theranostics. In this part, size changes of NPs from small ones to big ones would be discussed in terms of stimulations.

  2.1.1   pH

  It is well-known that pH (6.5-6.8) among tumor site and pH (4.0-5.0) of lysosome are lower than normal tissues of the body [35, 36]. Therefore, the pH-responsive drug delivery system based on NPs was designed and applied to enrich drugs in tumor for enhancing therapeutic efficacy.

  Grinstaff et al. [37, 38] prepared pH-responsive polymeric NPs. The NPs kept the small size (100 nm) due to the hydrophobic repeat unit, trimethoxybenzylidene in pH 7.4. However, the hydrophobic trimethoxybenzylidene would be cleaved off in pH 5.0, resulting in the increase of hydrophilicity and the size of NPs expanded from 100 nm to 1000 nm after culturing 24 h (Fig. 1A and B). The chemotherapeutic agent paclitaxel-loaded expansile NPs (Pax-eNPs), efficiently released drugs through the expansion process and inhibited peritoneal carcinomatosis. Moreover, the expansion in size increased the retention time of Pax-eNPs in tumor site for more than 7 days, realizing long-term drug release and inhibition of the establishment and growth of intraperitoneal tumor. These advantages proved that upon stimulation of pH, NPs were enlarged and contributed to long-term sustained release, inhibiting growth and metastasis of tumor effectively.

  图 1

  

  图 1  A) Expansion of eNPs at acidic environment. Up: schematic diagram; below: molecular formula. B) Scanning electron microscope (SEM) imaging of eNPs at pH of 7.4 and 5.0, indicating NPs' expansion. Reproduced with permission [38]. Copyright 2013, Royal Society of Chemistry

  Figure 1.  A) Expansion of eNPs at acidic environment. Up: schematic diagram; below: molecular formula. B) Scanning electron microscope (SEM) imaging of eNPs at pH of 7.4 and 5.0, indicating NPs' expansion. Reproduced with permission [38]. Copyright 2013, Royal Society of Chemistry

  下载: 全尺寸图片 幻灯片

  Wang and co-workers [39] demonstrated swelling pH-sensitive NPs with Förster resonance energy transfer (FRET) effects could monitor microenvironmental pH around NPs (Fig. 2A). Bis-pyrene (BP) as donor, which could form J-type nanoaggregates via π-π and hydrophobic interactions in aqueous solution [40, 41] with bright green fluorescence, was conjugated with hydrophilic poly(amino ester)s (PbAEs) (P-BP). P-BP as a pH-sensitive carrier, selfassembled into NPs and further encapsulated nile red (NR) with red emission as an acceptor to form FRET NPs. When pH was decreased, the NPs (~42 nm) with red emission fluorescence would swell into large ones with an average diameter of ~138 nm due to the protonation of polymer chain (Fig. 2B), displaying gradually increasing of blue fluorescence and decreasing of red fluorescence due to the disassembly of BP and releasing of NR. The NPs with Arg-Gly-Asp (RGD) peptide sequence as targeting unit were cultured with human primary glioblastoma (U87) cells where α_vβ₃ integrin was over-expressed. NPs entered U87 cells by receptor mediated endocytosis and the endocytic microenvironmental pH could be in situ monitored by fluorescence intensity ratios of P-BP/NR at 418 nm to that at 635 nm (Fig. 2C). Fluorescence signal of BP increased and NR decreased over time. Cultured NPs with cells revealed the acidity of endocytic microenvironmental in living cells (Fig. 2D). The same group [42] also used pHresponsive NPs for BP loading and releasing, which in situ formed nanoaggregates with green fluorescence in living cells, contributing to cell imaging.

  图 2

  

  图 2  A) Schematic illustration of self-assembly process to form NPs, followed by expansion at low pH. NR was loaded into NPs, size of NPs was increased under acidic condition, which contributed to the releasement NR. B) Dynamic light scattering indicated size distribution with pH changes from 7.4 to 5.0. C) Fluorescence intensity ratio of P-BP/NR's (418 nm to 635 nm) changes upon pH. D) In situ Confocal Laser Scanning Microscope (CLSM) image of P-BP/NR in living U87 cells at different time point. Reproduced with permission [39]. Copyright 2015, Royal Society of Chemistry

  Figure 2.  A) Schematic illustration of self-assembly process to form NPs, followed by expansion at low pH. NR was loaded into NPs, size of NPs was increased under acidic condition, which contributed to the releasement NR. B) Dynamic light scattering indicated size distribution with pH changes from 7.4 to 5.0. C) Fluorescence intensity ratio of P-BP/NR's (418 nm to 635 nm) changes upon pH. D) In situ Confocal Laser Scanning Microscope (CLSM) image of P-BP/NR in living U87 cells at different time point. Reproduced with permission [39]. Copyright 2015, Royal Society of Chemistry

  下载: 全尺寸图片 幻灯片

  Shao et al. [43] designed core-shell NPs (Ag₂S(DOX)@CS), which could form large ones by changing pH due to pH-responsive chitosan (CS). At pH 7.4, doxorubicin (DOX) was completely encapsulated in NPs by hydrophobic interactions between oleoyl groups and DOX. However, at lower lysosomal pH, amine groups were protonated and charge repulsion of oleoyl-CS chains was increased to induce expansion of NPs, which led to DOX release for tumor therapy. In addition, Ag₂S quantum dots (QDs) in Ag₂S(DOX)@CS endowed photoluminescence, effectively monitoring the nanocarrier of Ag₂S(DOX)@CS (Fig. 3).

  图 3

  

  图 3  Synthetic pathway and structural transformation of Ag₂S(DOX)@CS. Ag₂S QDs could bind N-hydroxysuccinimide and chitosan to obtain NPs. After that, DOX was loaded into NPs and formed into Ag₂S(DOX)@CS. NPs turned large to release DOX at 37 ℃ and pH 5.0. Reproduced with permission [43]. Copyright 2017, Elsevier

  Figure 3.  Synthetic pathway and structural transformation of Ag₂S(DOX)@CS. Ag₂S QDs could bind N-hydroxysuccinimide and chitosan to obtain NPs. After that, DOX was loaded into NPs and formed into Ag₂S(DOX)@CS. NPs turned large to release DOX at 37 ℃ and pH 5.0. Reproduced with permission [43]. Copyright 2017, Elsevier

  下载: 全尺寸图片 幻灯片

  2.1.2   Enzyme

  Enzyme is a kind of important material in living body which could mediate various physiological activities [44]. Enzyme as a stimulating agent could minimize damage on body comparing with other stimulation methods. Wang and co-workers [45] designed a small molecule precursor, which was composed of purpurin 18 (P18) as a photoacoustic (PA) unit, peptide Pro-LeuGly-Val-Arg-Gly (PLGVRG) as an enzyme-responsive linker, and RGD as a targeting ligand. The in situ self-assembled NFs based on P18 residues at tumor site performed assembly-induced retention (AIR) effect. As shown in Fig. 4A, small molecules could target to α_vβ₃ integrins, which were over-expressed on cancer cellular membranes. Then, tumor microenvironment over-expressed gelatinase could cut off the PLGVRG linker selectively. The hydrophobicity of molecules was enhanced and their steric hindrance was reduced, leading to the formation of NFs. The self-assembled NFs showed prolonged retention time (Fig. 4B and C). Therefore, the strategy could be utilized as long-term PA imaging. Meanwhile, the accumulation and long-term retention of therapeutic agents could dramatically improve therapeutic efficacy. Similarly, Wang's group [46] further investigated a new molecular probe with excellent PA properties for bacterial infections imaging application in vivo. The multifunctional building blocks were cut off by gelatinase, followed by selfassembly into twisted fibers, inducing improved PA signals. The in situ self-assembly method would be the basis of a highly sensitive and specific system for imaging bacterial infections.

  图 4

  

  图 4  A) Molecular formula and schematic diagram of self-assembly from small molecules to NPs (left). Enzyme induced the formation of NPs and enhanced PA imaging in vivo (right). B) Quantification of in situ PA imaging intensity at tumor site from 0.5 h to 24 h after injection, indicating AIR effect. C) In vitro amount of NPs and phosphate buffered saline (PBS) after injecting for different time. Reproduced with permission [45]. Copyright 2015, Wiley Publishing Group

  Figure 4.  A) Molecular formula and schematic diagram of self-assembly from small molecules to NPs (left). Enzyme induced the formation of NPs and enhanced PA imaging in vivo (right). B) Quantification of in situ PA imaging intensity at tumor site from 0.5 h to 24 h after injection, indicating AIR effect. C) In vitro amount of NPs and phosphate buffered saline (PBS) after injecting for different time. Reproduced with permission [45]. Copyright 2015, Wiley Publishing Group

  下载: 全尺寸图片 幻灯片

  2.1.3   Temperature

  Wang et al. [47] reported a thermo-controlled polymer-peptide for efficient proliferative inhibition by modulating HER2 receptor clustering. Functional paring motifs (HBP) contained a HER2 targeting peptide and BP as a fluorescence module, which were covalently linked with thermal responsive polymer. The temperature responsive polymer-peptide collapsed above 40 ℃ (>LCST) as a "shield", which blocked HBP aggregation without fluorescence. When the temperature was below 35 ℃, the polymer would extend to expose HBP forming aggregates and emitting green fluorescence. The increasing fluorescence may monitor the HER2 receptor clustering. This in situ phase transition induced the formation of HBP nanoaggregates further led to the receptor clustering and cytoplasmic domain phosphorylation, inhibiting cancer cell proliferation effectively.

  2.1.4   Glutathione

  Glutathione (GSH) is a kind of polypeptide as reducing agent that could decompose disulfide bond effectively [48]. Wang and co-workers [49] designed a new bio-orthogonally system that could modulate NPs' optical properties in living species. NPs which exhibited inert optical property were self-assembled from BP and chloro-substituted cysteine (Cy). Utilizing abundant GSH as activator, NPs' optical property could be modulated to possess binary fluorescence signals efficiently. GSH labeled Cy exhibited 30-fold fluorescence enhancement (Fig. 5A). Hydrophobic BP residues self-assembled into larger NPs and exhibited nearly 36-fold enhanced fluorescence signal intensity at 520 nm (Fig. 5B). The size of NPs was enlarged during the in situ self-assembly process, which could exhibit fluorescence signals in both channels (Fig. 5C and D). This strategy showed good specificity for GSH, resulting in potential application of GSH detection in vivo. Furthermore, the disease with high expressed GSH could be imaged by this transformable nanoprobe.

  图 5

  

  图 5  Fluorescence spectra of nanoemitters after adding GSH (500 equiv.) at λ_max = 820 nm (A) and λ_max = 520 nm (B), respectively. The intensity of fluorescence was increased over time. C) Ex vivo fluorescence images of tumor and different tissues after 12 h post-injection of nanoemitters or PBS (1, tumor; 2, lung; 3, heart; 4, kidney; 5, spleen; 6, liver). D) Bio-distribution of nanoemitters at different tumor or tissues 12 h post-injection from C). Reproduced with permission [49]. Copyright 2016, American Chemical Society

  Figure 5.  Fluorescence spectra of nanoemitters after adding GSH (500 equiv.) at λ_max = 820 nm (A) and λ_max = 520 nm (B), respectively. The intensity of fluorescence was increased over time. C) Ex vivo fluorescence images of tumor and different tissues after 12 h post-injection of nanoemitters or PBS (1, tumor; 2, lung; 3, heart; 4, kidney; 5, spleen; 6, liver). D) Bio-distribution of nanoemitters at different tumor or tissues 12 h post-injection from C). Reproduced with permission [49]. Copyright 2016, American Chemical Society

  下载: 全尺寸图片 幻灯片

  In order to realize the transformation of NPs from small to large, the hydrophobicity of the responsive materials should be modulated by stimuli. On the one hand, increasing hydrophobicity of small NPs could induce further aggregation to form large NPs, which could be accomplished by cleavage of hydrophilic groups. On the other hand, NPs could expand their sizes by increasing the hydrophilicity, which could be achieved by protonation of pHresponsive amino groups under acidic environment. However, the hydrophilicity should be precisely modulated in order to prevent the total disassembly of NPs.

  2.2   Big NPs into small ones

  NPs in large scale generally showed long circulation time in blood and long-term retention in lesion locations. However, the large NPs were difficult to penetrate into deep tissues of lesion locations [50, 51]. Thus, combining long circulation and retention of large NPs and deep penetration of small ones, transformable NPs from large to small were developed for nanomedicine, especially for drug delivery [52]. The large NPs were delivered to lesion locations, followed by degradation into small NPs under stimulation, including pH, ultraviolet (UV) light, enzyme etc.

  2.2.1   pH

  Kwon et al. [53] described a pH-sensitive polymeric micelle which was prepared by Michael addition between hydrophilic methyl ether poly(ethylene glycol) (mPEG) and hydrophobic PbAEs with pH-sensitivity. The micelle could load DOX effectively [54]. At pH value of 7.4, DOX was released at a low speed which could be confirmed by fluorescence signal of DOX in the mainly cytoplasm. When pH was decreased to 6.4, the speed of releasing DOX was faster. In vivo experiment results also proved that drug loaded micelle showed higher inhibition of tumor proliferation comparing with free drug. The survival rate of DOX loaded micelle group after three weeks was higher than free DOX group and saline control group. Wang et al. [55] prepared a pH-sensitive polymer, named as platinum (Pt) prodrug conjugated poly(amidoamine)-graft-polycaprolactone (PCL-CDM-PAMAM/Pt), to form iCluster/Pt, where PCL was hydrophobic units, CDM involved acid-labile amide bond and PAMAM/Pt was small nanoparticle-prodrug (Fig. 6A). The iCluster/Pt with an average size of nearly 100 nm was accumulated in tumor site through intravenous (i.v.) injection. At tumor acidic environment, accumulated NPs could be degraded into small poly (amidoamine) (PAMAM) with an average diameter of 5 nm (Fig. 6B). NPs changed from big ones to small ones, showing great merits that large NPs increased circulation in blood and small ones was good for increasing penetration into tumor cells up to 70μm from the blood vessels at 90 min post injection (Fig. 6C). Prodrugs were released from particles and ultimately induced tumor death. pH-responsive chemical bond was also used by Ge and his coworkers [56]. Core-shell micelles and PAMAM dendrimers selfassembled into micelle clusters. Under acidic condition in tumor, PAMAM carriers were released successfully, and exhibited high depth of penetration in tumor and high-efficient cellular uptake. Anticancer drugs were further internalized into cells and eventually killed tumor cells.

  图 6

  

  图 6  A) Schematic diagram of the conjugated polymer and its transformation under acidic environment. B) TEM image of NPs treated with phosphate buffer at different time (0, 4, 24 h). Scale bar, 100 nm. C) Time and penetration depthdependent distribution of NPs. The core of NPs were labeled with rhodamine B for red fluorescence signal and prudrugs were labeled with fluorescein for red fluorescence signal, which indicating efficiency penetration of prodrugs. Reproduced with permission [55]. Copyright 2016, National Academy of Sciences

  Figure 6.  A) Schematic diagram of the conjugated polymer and its transformation under acidic environment. B) TEM image of NPs treated with phosphate buffer at different time (0, 4, 24 h). Scale bar, 100 nm. C) Time and penetration depthdependent distribution of NPs. The core of NPs were labeled with rhodamine B for red fluorescence signal and prudrugs were labeled with fluorescein for red fluorescence signal, which indicating efficiency penetration of prodrugs. Reproduced with permission [55]. Copyright 2016, National Academy of Sciences

  下载: 全尺寸图片 幻灯片

  Wang et al. [57] developed pH-sensitive cluster nanobombs (SCNs) based smart drug delivery. Polymer coated drugs and SCNs were self-assembled through dialysis and obtained nanobombs (SCNs/Pt) with an average size of 80 nm at pH 7. Pt loaded pHinsensitive cluster nanostructures (ICNs/Pt) was used as control group and could remain their initial size as SCNs/Pt when pH changed. Under acidic tumor microenvironment, SCNs/Pt was degraded and formed particles with little size (10 nm). As mentioned above, small particles owned enhanced permeable of tumor and could be substantially taken up by cells. Cy5 dyed NPs were monitored in tumor site through CLSM and higher intensity of fluorescence from SCN group indicated that more SCNs/Pt accessed into tumors. The size changeable SCNs/Pt showed enhanced efficacy of killing tumor cells comparing with ICNs/Pt group.

  Gene, including DNA and RNA, is a class of nanodrugs that can be applied in tumor therapy [58]. Cheng and co-workers [59] reported efficient DNA and small interfering RNA (siRNA) transportation by assembled NPs which could be degraded at target site under suitable pH environment. In the study, phenylboronic acid (PBA)-modified generation 2 dendrimers were selfassembled into nanoaggregates under pH 7.4. The assembled structures were disassembled and small dendrimers were released in acidic cellular vesicles. Therefore, the dendrimers could be utilized to load DNA and siRNA and deliver them.

  2.2.2   UV light

  Light responsive NPs, including UV light response, NIR light response, and so on, draw great attention in nanomedicine [60]. Researchers could prepare light-responsive NPs that could disassemble into small ones under UV irradiation. Kohane et al. [61] reported that NPs with spiropyran (SP) as responsive group could diminish from 150 nm to 40 nm under UV light irradiation (Fig. 7A and B). Therefore, the photoswitchable NPs showed high tissue penetration capability, high loading of drugs and controllable cargo release property. The same group [62] also investigated another SP-based NPs with UV light response. Under irradiation at 365 nm, the size of SP-based NPs was reduced from 103 to 49 nm. In addition, NPs could recover under dark or visible light. The NPs showed improved efficiency on killing tumor cells under light irradiation.

  图 7

  

  图 7  A) Schematic diagram of UV light induced NPs reduction. B) Images of photo (up) and fluorescence signal (below). Reproduced with permission [61]. Copyright 2012, American Chemical Society

  Figure 7.  A) Schematic diagram of UV light induced NPs reduction. B) Images of photo (up) and fluorescence signal (below). Reproduced with permission [61]. Copyright 2012, American Chemical Society

  下载: 全尺寸图片 幻灯片

  2.2.3   Enzyme

  Fukumura and co-workers [63] employed QDs to obtain long blood circulation time and accumulate drug in tumor site. Comparing with traditional fluorophores, QDs, exhibiting thin photoluminescence spectra, high resistance of degradation and two-photon absorption cross-section, can be used for better photo imaging by multiphoton microscope [64]. The multistage QDbased NPs (QDGelNPs) showed large scale of more than 100 nm. The QDGelNPs with enzyme responsive units, which was cleaved by Gelatinases A (MMP-2) in tumor, disassembled into QDs with small size. The small particles could effectively accumulate in tumor site and enter into cells for tumor imaging.

  Spherical nucleic acids could be obtained by introducing oligonucleotides into nucleic acids, and could anticipate in receptor-mediated endocytosis, further uptaken by cells effectively [65]. Zhang and co-workers [66] prepared a stable amphiphilic conjugate consisting of oligonucleotides and paclitaxel, which assembled into spherical NPs. The NPs could be substantially uptaken by cells. Under intercellular enzyme, as-synthesized NPs were degraded into small components and released the paclitaxel, which ultimately induced cellular apoptosis with high bioavailability.

  2.2.4   GSH

  Cui et al. [67] reported a stable nanostructure that could be potentially used for self-delivery. This nanostructure composed of a hydrophobic anticancer drug camptothecin (CPT) and a peptide sequence derived from Tau protein, which was connected with a reducible disulfylbutyrate linker. Nanostructure itself was unstable and could be degraded, releasing loaded drugs with a slow speed under GSH. Therefore, the nanostructure could induce apoptosis of cancer cells in a controllable way.

  Smaller NPs with deep penetration ability for entering tumor cells could be obtained by different kind of stimulations and drugs could be released effectively. The external stimulation, for example, UV light irradiation, showed low tissue penetration and could potentially damage normal organs or tissues, which was difficult for further clinical applications. The endogenous stimuli, such as enzyme, pH, etc., were promising candidates to achieve transformation of NPs with high biocompatibility and specificity, even if they were difficult to control.

  3.   Surface charge of NPs

  Through electrostatic interactions, cationic polymers can access into cells efficiently comparing with anionic polymers. Thus, positively charged polymers, such as poly(ethleneimine) and poly-(_L-lysine) (PLL) can potentially use as nanocarriers to deliver drugs especially genes to the nucleus [68-70]. Nevertheless, for in vivo applications, the cationic polymer carrier would be rapidly cleared out because of their non-specific cellular uptake which could trigger grievous serum inhibition. However, the anionic polymers possessed a long blood circulation time and could highly accumulate at tumor site compared with cationic polymers. Therefore, using either cationic polymers or anionic polymers directly as nanocarriers are less effective [71]. Polymeric NPs need to be designed to combine the advantages given by cationic and anionic polymer. It is ideal for smart NPs with desirable transformation in surface charges for effective drug delivery.

  Radosz and co-workers [72] reported a charge-reversal NPs that can apply in nuclear drug delivery. Primary amines from PLL NPs were used to accomplish the charge conversion through amidizion. The interaction between PLL and cells were inhibited evidently through the charge change. Kataoka et al. [73] designed a pH sensitive siRNA conjugate with following advantages: easily to possess reduced immunogenicity, enhanced reversible polyion complex stability, endosomal cleavability and mono-siRNA releasability. Maleic acid amide (MAA) was utilized to link siRNA because of its consistency at neutral pH and rapidly hydrolysis at acidic atmosphere [74]. As a consequence, MAA moieties would be degraded, causing surface charge change in endosomal pH, which led to the instability of the conjugate. Meantime, mono-siRNA was released and inhibited tumor proliferation efficiently.

  Wang et al. [75] described dual pH responsive NPs. On the one hand, the NPs changed surface charge from negative into positive in tumor acidity, leading to high-efficient internalization. On the other hand, the loaded DOX in NPs was released in lysosomal acidic pH. NPs were synthesized from multi-stage reaction through cysteamine modified mPEG-b-poly-(allyl ethylene phosphate), 2-iminothiolane and sulfhydryl-reactive derivative of DOX. The surface of NPs with negatively charged would transfer into positively charged under extracellular pH of 6.5-6.8, as amide bond between dimethyl maleic acid and amino group was destroyed, which induced NPs easily to access in cells. NPs were further degraded and triggered drug release in lysosomal acidic pH. Proliferation of cancer cells was successfully inhibited through dual pH response. Wang and co-workers [76] reported surface charge transformable NPs encapsulated Chlorin-e6 (Ce6), which was synthesized from 3-(dibutylamino)-1-propylamine, 1, 6-hexanediol diacrylate and mPEG-NH2 through Michael addition. NPs could convert into positively charged, effectively release reactive oxygen species under acidic environment, which led to high efficiency on killing bacteria.

  Tsien et al. [77] developed activatable cell penetrating peptides, abbreviated as ACPPs, which conjugated with dendrimer NPs for fluorescence and MR imaging in vitro and in vivo. On the NPs surfaces, polycations ACPPs were further conjugated with polyanions through protease-cleavable linkers and reached electric neutrality. This firm connection between two polymers would maintain until protease cut it down. Drug-loaded polycations would be released. The NPs covered with polycations ACPPs were easier to be taken into cells than neutral NPs. Furthermore, other payloads, such as Gd chelate and fluorescent dye, could be linked to the polycation for dual modality imaging.

  NPs with transformable surfaces in charge generally responded to tumor acidic environment or lysosomal acidic pH, which contributed to controlled drug release and enhancement of cellular uptake. Besides, the specific physiological/pathological conditions, such as acidic infectious region, also provided the developing room of charge transformable NPs for bacterial killing.

  4.   Morphology of NPs

  In recent years, more and more evidences have proven that morphology of NPs showed important influence on NPs' circulation time, penetration and cellular uptake [78]. As a result, NPs were designed and prepared to show controllable and transformable morphologies to obtain new biological effects of NPs and enhanced treatment efficacy of diseases. The morphology transformation would be focused and reviewed including NPs to NFs and NPs to large aggregates.

  4.1   NPs to NFs

  4.1.1   pH

  Wang et al. [79] reported a morphology transformable structure (BP-KLVFFG-PEG, BKP), modulated by hydrogen bonds, π-π interactions and hydrophilic-lipophilic balance. BKP was selfassembled into NPs through hydrophobic and π-π interactions, leading to the formation of BP J-type aggregates under aqueous solution. NPs could be transferred into NFs by H-bonding interactions of peptide KLVFF in 12 days, in which the transformation rate could be adjusted by the length of hydrophilic chains. This study provided one rule to design structural transformation of NPs within expected time intervals. Therefore, the same group [80] further reported an in situ self-assembled nest-like NFs for homing theranostic agents (Fig. 8A). NPs was injected into mice by i.v. injection and transformed into NFs at the tumor microenvironment acidity to construct host carrier, which showed a long retention time for 96h confirmed by ex vivo fluorescence images of tumor (Fig. 8B and C). Small model molecules including NR and DOX, could bind to NFs through hydrophobic interaction and eventually accumulate in tumor. BioTEM images revealed that NFs formed in the extracellular matrixof tumor site under tumor acidic microenvironment (Fig. 8D). The formation of NFs was further confirmed by Energy-dispersive Xray spectroscopy (EDS) through detecting iodine elements labeled Lys of NFs. The slice of tumor had strong green fluorescence, further indicating the existence of NFs in tumor tissue (Fig. 8E).

  图 8

  

  图 8  A) Schematic illustration of NPs transformation at acidic pH. B) Timedependent ex vivo fluorescence images of tumor and organs (heart, liver, spleen. lung and kidney) 96h post injecting PBS and NPs. C) Average fluorescence signal of tumor and liver at different time point. D) TEM images of tumor tissue slices from MCF-7 xenografted mice after injection of NPs. Blue arrows indicated the formation of NFs. E) 3D reconstructed green channel of CLSM image of NFs in tumor tissue slice. Reproduced with permission [80]. Copyright 2017, Wiley Publishing Group

  Figure 8.  A) Schematic illustration of NPs transformation at acidic pH. B) Timedependent ex vivo fluorescence images of tumor and organs (heart, liver, spleen. lung and kidney) 96h post injecting PBS and NPs. C) Average fluorescence signal of tumor and liver at different time point. D) TEM images of tumor tissue slices from MCF-7 xenografted mice after injection of NPs. Blue arrows indicated the formation of NFs. E) 3D reconstructed green channel of CLSM image of NFs in tumor tissue slice. Reproduced with permission [80]. Copyright 2017, Wiley Publishing Group

  下载: 全尺寸图片 幻灯片

  4.1.2   Over-expressed receptors

  Wang and co-workers [81] reported self-assembled peptide NPsincluding BP unit, KLVFFpeptidemotifandpeptideRGD, which specific binding activity to integrin α_vβ₃, driven by the coordination interactions between RGD and the metal ions Ca²⁺. The peptide formed NPs in water would transformed into NFs induced by metal ions Ca²⁺. The self-assembled NPs were verified to transform into NFs on the surface of U87 cells by Ca²⁺ via ligandreceptor binding, which led to cell death. They [82] further constructed transformable NPs which could in situ transformed into NFs and inhibit tumor invasion and metastasis in vivo. NPs (1-NPs) are peptide based nanostructures including BP unit, KLVFF peptide motif and Y-type RGD-YIGSR motif as target motifs. NPs could target and accumulate in tumor site through morphology transformation. 1-NPs transformed into 1-NFs network with the help of Ca²⁺ via ligand-receptor binding as artificial extracellular matrix (AECM) (Fig. 9A). The AECM could act as long-term barriers and competed with natural ECM, resulting in high efficient tumor metastasis and growth inhibition of tumor. The results of in vivo fluorescence imaging revealed that 1-NPs could stay at tumor site for more than 72h, much longer than control groups (nontransformable 2-NPs, 24h) (Fig. 9B).

  图 9

  

  图 9  A) SEM images of surface of cell treated with 1-NPs, 2-NPs and PBS (control). B) Fluorescence images in vivo of mice injected by transformable 1-NPs, nontransformable2-NPs and PBS wereacquiredat different time point. Reproduced with permission [82]. Copyright 2017, American Chemical Society

  Figure 9.  A) SEM images of surface of cell treated with 1-NPs, 2-NPs and PBS (control). B) Fluorescence images in vivo of mice injected by transformable 1-NPs, nontransformable2-NPs and PBS wereacquiredat different time point. Reproduced with permission [82]. Copyright 2017, American Chemical Society

  下载: 全尺寸图片 幻灯片

  The in situ morphology change from NPs to NFs in tumor site could be accomplished through different stimulations. NFs with long-term retention could be monitored by fluorescence imaging. Moreover, other morphology transformation of NPs should be paid much attention. The new biological effects were expected with the morphology transformation of NPs.

  4.2   NPs to nanoaggregates

  4.2.1   pH

  Gu and co-workers [83] reported a liquid metal nanodrug with a core-shell nanospheres structure while liquid-phase galliumindium alloy as core and thiolated polymer as shell (Fig. 10A). NPs could be easily synthesized and tailored through ligand-mediated self-assembly by using ultrasonication and could accumulate in tumor site through binding tumor-target ligand. The DOX loaded nanospheres with initial average diameter of 107nm could fuse to large nanoaggregates and decompose at mild acidic environment in 72h after incubation (Fig. 10B). The endocytosis and fuse process of NPs in cells were observed by TEM imaging (Fig. 10C). Fusion of NPs in the endosomes was clearlyobserved 1h post incubation and obviously increased after 4h. The fusion of NPs promoted drug release and also displayed an enhancement of contrast when using X-ray for imaging.

  图 10

  

  图 10  A) Schematic illustration of NPs' fusion and degradation. B) TEM image of NPs incubated for different time at pH 5.0. Scale bar: 100nm (5min, 1h and 4h); 400nm (72h). C) TEM images of HeLa cells cultured by NPs for 1h and 4h. Red arrows referred to fused nanospheres; green arrows referred to dispersion of single nanosphere in the cytosol. Scale bar: 2 μm. Reproduced with permission [83]. Copyright 2015, Institute of Physics Publishing

  Figure 10.  A) Schematic illustration of NPs' fusion and degradation. B) TEM image of NPs incubated for different time at pH 5.0. Scale bar: 100nm (5min, 1h and 4h); 400nm (72h). C) TEM images of HeLa cells cultured by NPs for 1h and 4h. Red arrows referred to fused nanospheres; green arrows referred to dispersion of single nanosphere in the cytosol. Scale bar: 2 μm. Reproduced with permission [83]. Copyright 2015, Institute of Physics Publishing

  下载: 全尺寸图片 幻灯片

  4.2.2   Enzyme

  Xing and co-workers [84] reported tumor microenvironment induced aggregation of upconversion nanocrystals (UCNs), leading to the enhanced upconversion emission and singlet oxygen generation from the photosensitizers attached on UCNs for photodynamic tumor treatment. They prepared polyacrylic acid and polyethylenimine coated Nd³⁺-doped UCNs. Furthermore, an enzyme-responsive peptide, Ac-FKC(StBu)AC(SH)-CBT containing a side-protected cysteine and 2-cyanobenzothiazole (CBT) was modified on the UCN surfaces to form the cross-linking of rareearth UCN (CRUN), which can specifically react with cathepsin B (CtsB), one important lysosomal cysteine protease overexpressed in various malignant tumors [85]. The in vivo experimental results revealed that the UCNs could selectively accumulate into tumor due to the CtsB activated covalent cross-linking. The cross-linked aggregates of CRUN showed enhanced upconverting emission upon irradiated by 808 nm laser, resulting in amplifying signals reactive singlet oxygen (for example, ¹O₂) from photosensitizer for enhanced PDT (Fig. 11A). Moreover, in order to increase the uptake of cancer cells, the surface of CRUNs was conjugated with folic acid to recognize the folate receptors over-expressed in many tumors. FA-PEG@CRUN group showed the higher fluorescence and PA signals via post injection than the control group (Fig. 11B). In addition, using FA-PEG@CRUN with irradiated at 808 nm showed improved inhibition of tumor proliferation comparing with the control group (Fig. 11C). Gao et al. [86] proposed a kind of similar strategy to form aggregates of gold NPs (AuNPs) with enhanced accumulation and retention in tumor, realizing long-term drug delivery. The Ala-Ala-Asn-Cys-Lys was linked to 2-cyano-6-aminobenzothiazole modified AuNPs by a click cycloaddition. The AuNPs aggregates were obtained through legumain, which showed higher accumulation in glioma site compared with PEGylated NPs. The coupling of cysteine and 2-cyanobenzothiazole could be a general strategy for the in vivo cross-linking. The cysteine was caged by a responsive unit, which could be cleaved under specific pathological conditions. The in vivo aggregation could induce the enhancement of the imaging signals, increasing of retention time.

  图 11

  

  图 11  A) NPs' motion pathway during the whole microenvironment. Cross-linking was induced by UV light. B) PA images of corresponding quantitative results of NPs at control group (up) and tumor site (down) in different time interval (0 h and 1 h) after injection. C) In vivo tumor volume changing by time after injecting NPs and with stimulation. Reproduced with permission [84]. Copyright 2016, Nature

  Figure 11.  A) NPs' motion pathway during the whole microenvironment. Cross-linking was induced by UV light. B) PA images of corresponding quantitative results of NPs at control group (up) and tumor site (down) in different time interval (0 h and 1 h) after injection. C) In vivo tumor volume changing by time after injecting NPs and with stimulation. Reproduced with permission [84]. Copyright 2016, Nature

  下载: 全尺寸图片 幻灯片

  Gianneschi and co-workers [87] designed MMP-responsive NPs with a surface comprised of MMP-substrates peptide and a hydrophobic paclitaxel core. The peptide sequence GPLGLAGGERDG shell was cleaved under MMP enzymes, the morphology transformation from NPs of 20 nm to large aggregates with micrometer scale was observed, releasing the paclitaxel. Gu et al. [88] focused on targeting delivery and constructed "transformable" core-shell based nanogel (CS-NG) loaded with tumor necrosis factor-related apoptosis inducing ligand and cilengitide. CS-NG could assemble into micro-sized extracellular large aggregates under overexpressed hyaluronidase (HAase) in tumor microenvironment, further release drugs to inhibit tumor proliferation in vitro and in vivo. Gianneschi and Christman's group [89] also investigated fluorescent MMP-2/9 responsive NPs owning a polynorbornebe backbone and peptide sequences that could target to myocardial infarction. NPs achieved long retention time after i.v. injection and could change their structures from micellar to network-like scaffolds under the stimulation of MMP-2/9 with average diameter expansion from 115 nm to about 1000 nm (Fig. 12A and B). The long retention in tissue for more than 7 days indicated the high efficient therapeutic was achieved (Fig. 12C and D). Internal stimulation like pH and enzyme could contribute to the aggregate of NPs. Even the structure of the aggregates was not well controlled, the aggregation induced enhancement of imaging signals and/or release of drug, leading to improved efficacy of theranostics. Nevertheless, morphology change from NPs to nanoaggregate provided a new strategy for theranostics.

  图 12

  

  图 12  A) TEM images of NPs (left) and larger aggregates (right) after treatment by enzyme. B) Size distribution of NPs from DLS before and after enzyme activation. C) Image of healthy myocardium, scale bar: 1 mm. D) Time-dependent fluorescence signals of tissue after injection enzyme responsive NPs (top) and non-responsive NPs (Bottom), scale bar: 50 μm. Reproduced with permission [89]. Copyright 2015, Wiley Publishing Group

  Figure 12.  A) TEM images of NPs (left) and larger aggregates (right) after treatment by enzyme. B) Size distribution of NPs from DLS before and after enzyme activation. C) Image of healthy myocardium, scale bar: 1 mm. D) Time-dependent fluorescence signals of tissue after injection enzyme responsive NPs (top) and non-responsive NPs (Bottom), scale bar: 50 μm. Reproduced with permission [89]. Copyright 2015, Wiley Publishing Group

  下载: 全尺寸图片 幻灯片

  4.3   Others

  Chan and co-workers [90] reported a DNA-functionalized gold NPs system. They constructed a "core-satellite" nanocomposite system in which big NPs as core and medium, small NPs as satellites were connected through DNA strand, while medium and small NPs were linked to big NPs separately (Fig. 13A). When one DNA strand was introduced into this system, the separated small and medium NPs linked with each other to form an intermediate state. Ultimately, the linkage between big and small NPs was broken, and medium and small NPs were linked (Fig. 13B). This achievement could modulate the targeting capability during the transformation, which was a promising smart system for drug delivery.

  图 13

  

  图 13  Under the mediation of DNA, shape change of NPs was shown as schematic diagram and TEM images. A) Basic building block such as different scales of DNAfunctionalized gold NPs and linker DNA strands. Structure model of DNA-linked NPs and its shift through adding different DNA chains. B) Corresponding TEM images of morphology changes. Reproduced with permission [90]. Copyright 2015, Science

  Figure 13.  Under the mediation of DNA, shape change of NPs was shown as schematic diagram and TEM images. A) Basic building block such as different scales of DNAfunctionalized gold NPs and linker DNA strands. Structure model of DNA-linked NPs and its shift through adding different DNA chains. B) Corresponding TEM images of morphology changes. Reproduced with permission [90]. Copyright 2015, Science

  下载: 全尺寸图片 幻灯片

  5.   Conclusion and outlook

  NPs with transformable structure and unique properties have won great attentions for biological applications and translational medicine. The modulation of nanostructures including size, surface charge and morphology according to the need of biomedical imaging and therapy implied that extended studies should be focused on as bellows. The structures of NPs should be more precisely modulated including size, surface charges and shape under specific physiological/pathological conditions to meet the imaging and therapy requirement such as enhanced blood circulation, high tissue-penetration and long retention etc. In order to fully investigate the transformable NPs in vivo, the techniques for structural characterization of NPs in vivo should be developed.

  Many approaches have been attempted for clinic application based on traditional NPs. However, little of them came into commercial use because of unexpected toxicity in clinic settings, no statistically significant difference between the experimental and the control group, etc. The transformable NPs showed highperformance theranostics outcomes due to long-term retention, optimized biodistribution and so on. Therefore, the application of transformable NPs on clinic settings should be carefully investigated. We believe that by collaborations with researchers from the field of material science, biology and chemistry, etc., the transformable nanosystems would substantially contribute to public healthcare in the future.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 51573031, 21373726, 21303723, 21603028 and 21573036), Science Fund for Creative Research Groups of the National Natural Science Foundation of China (No.11621505) and CAS Key Research Program for Frontier Sciences (No. QYZDJ-SSW-SLH022). Key Project of Chinese Academy of Sciences in Cooperation with Foreign Enterprises (No. GJHZ1541).

• 1. [1]

     J.W. Hickey, J.L. Santos, J.M. Williford, H.Q. Mao, J. Control. Release 219(2015) 536-547. doi: 10.1016/j.jconrel.2015.10.006

  2. [2]

     Y. Yang, Q. Guo, J. Peng, et al., J. Biomed. Nanotechnol. 12(2016) 1963-1974. doi: 10.1166/jbn.2016.2298

  3. [3]

     C.A. Schütz, L. Juillerat-Jeanneret, H. Mueller, I. Lynch, M. Riediker, Nanomedicine 8(2013) 449-467. doi: 10.2217/nnm.13.8

  4. [4]

     T.T. Goodman, J. Chen, K. Matveev, S.H. Pun, Biotechnol. Bioeng. 101(2008) 388-399. doi: 10.1002/bit.v101:2

  5. [5]

     L. Miao, C.M. Lin, L. Huang, J. Control. Release 219(2015) 192-204. doi: 10.1016/j.jconrel.2015.08.017

  6. [6]

     T. Dai, N. Li, F. Han, et al., Biomaterials 83(2016) 37-50. doi: 10.1016/j.biomaterials.2016.01.011

  7. [7]

     P. Ruenraroengsak, J.M. Cook, A.T. Florence, J. Control. Release 141(2010) 265-276. doi: 10.1016/j.jconrel.2009.10.032

  8. [8]

     G. Wang, J. Dong, T. Yuan, et al., Macromol. Biosci. 16(2016) 990-994. doi: 10.1002/mabi.v16.7

  9. [9]

     M. Kanapathipillai, A. Brock, D.E. Ingber, Adv. Drug Deliv. Rev. 80(2014) 107-118.

  10. [10]

      L. Wang, L.L. Li, H.L. Ma, H. Wang, Chin. Chem. Lett. 24(2013) 351-358. doi: 10.1016/j.cclet.2013.03.018

  11. [11]

      J.C. Chen, J.Z. Li, J.H. Liu, L.Q. Xu, Chin. Chem. Lett. 26(2015) 1319-1321. doi: 10.1016/j.cclet.2015.05.050

  12. [12]

      Y. Chen, Z. Huang, H. Zhao, et al., ACS Appl. Mater. Interfaces 9(2017) 8602-8608. doi: 10.1021/acsami.7b01157

  13. [13]

      X. Wu, Y.M. Zhang, Y. Liu, RSC Adv. 6(2016) 99729-99734. doi: 10.1039/C6RA21900D

  14. [14]

      X.Q. Quan, L. Kang, X.Z. Yin, Z.H. Jin, Z.G. Gao, Chin. Chem. Lett. 26(2015) 695-699. doi: 10.1016/j.cclet.2015.04.024

  15. [15]

      Y. Wang, Y.X. Lin, Z.Y. Qiao, et al., Adv. Mater. 27(2015) 2627-2634. doi: 10.1002/adma.v27.16

  16. [16]

      H. Kobayashi, R. Watanabe, P.L. Choyke, Theranostics 4(2013) 81-89.

  17. [17]

      H. Maeda, K. Tsukigawa, J. Fang, Microcirculation 23(2016) 173-182. doi: 10.1111/micc.2016.23.issue-3

  18. [18]

      Y. Park, Y.M. Ryu, Y. Jung, et al., ACS Nano 8(2014) 8896-8910. doi: 10.1021/nn5009269

  19. [19]

      L.L. Rui, H.L. Cao, Y.D. Xue, et al., Chin. Chem. Lett. 27(2016) 1412-1420. doi: 10.1016/j.cclet.2016.07.011

  20. [20]

      W.Q. Lim, S.Z.F. Phua, H.V. Xu, S. Sreejith, Y. Zhao, Nanoscale 8(2016) 12510-12519. doi: 10.1039/C5NR07853A

  21. [21]

      Q.L. Jiang, L. Hai, L. Chen, et al., Chin. Chem. Lett. 19(2008) 127-129. doi: 10.1016/j.cclet.2007.12.002

  22. [22]

      Y. Wang, H. Zhao, J. Peng, et al., J. Biomed. Nanotechnol. 12(2016) 2097-2111. doi: 10.1166/jbn.2016.2319

  23. [23]

      W. Kim, J.C. Lee, J.H. Shin, et al., Anal. Chem. 88(2016) 5531-5537. doi: 10.1021/acs.analchem.6b01123

  24. [24]

      H.J. Li, J.Z. Du, J. Liu, et al., ACS Nano 10(2016) 6753-6761. doi: 10.1021/acsnano.6b02326

  25. [25]

      A.W. Pan, B.B. Wu, J.M. Wu, Chin. Chem. Lett. 20(2009) 79-83. doi: 10.1016/j.cclet.2008.10.012

  26. [26]

      C.Y. Ang, S.Y. Tan, C. Teh, et al., Small 13(2017) 1602379. doi: 10.1002/smll.v13.7

  27. [27]

      A.A. Manzoor, L.H. Lindner, C.D. Landon, et al., Cancer Res. 72(2012) 5566-5575. doi: 10.1158/0008-5472.CAN-12-1683

  28. [28]

      L. Paasonen, T. Laaksonen, C. Johans, et al., J. Control. Release 122(2007) 86-93. doi: 10.1016/j.jconrel.2007.06.009

  29. [29]

      L. Qin, P.F. Duan, M.H. Liu, Chin. Chem. Lett. 25(2014) 487-490. doi: 10.1016/j.cclet.2013.12.019

  30. [30]

      Y. Ding, Y. Kang, X. Zhang, Chem. Commun. 51(2015) 996-1003. doi: 10.1039/C4CC05878J

  31. [31]

      J.M. Williford, J.L. Santos, R. Shyam, H.Q. Mao, Biomater. Sci. 3(2015) 894-907. doi: 10.1039/C5BM00006H

  32. [32]

      F. Chi, Y.N. Guo, J. Liu, Y. Liu, Q. Huo, J. Phys. Chem. C 114(2010) 2519-2523. doi: 10.1021/jp910460j

  33. [33]

      H. Lee, H. Fonge, B. Hoang, R.M. Reilly, C. Allen, Mol. Pharm. 7(2010) 1195-1208. doi: 10.1021/mp100038h

  34. [34]

      L. Li, J. Sun, Z. He, Curr. Med. Chem. 20(2013) 2881-2891. doi: 10.2174/09298673113209990004

  35. [35]

      T. Volk, E. Jahde, H.P. Fortmeyer, K.H. Glusenkamp, M.F. Rajewsky, Br. J. Cancer 68(1993) 492-500. doi: 10.1038/bjc.1993.375

  36. [36]

      S. Ohkuma, B. Poole, Proc. Natl. Acad. Sci. U. S. A. 75(1978) 3327-3331. doi: 10.1073/pnas.75.7.3327

  37. [37]

      Y.L. Colson, R. Liu, E.B. Southard, et al., Biomaterials 32(2011) 832-840. doi: 10.1016/j.biomaterials.2010.09.059

  38. [38]

      A.H. Colby, Y.L. Colson, M.W. Grinstaff, Nanoscale 5(2013) 3496-3504. doi: 10.1039/c3nr00114h

  39. [39]

      Z.Y. Qiao, C.Y. Hou, W.J. Zhao, et al., Chem. Commun. 51(2015) 12609-12612. doi: 10.1039/C5CC03752B

  40. [40]

      L. Wang, W. Li, J. Lu, et al., J. Phys. Chem. C 117(2013) 26811-26820. doi: 10.1021/jp409557g

  41. [41]

      P.P. Yang, Y. Yang, Y.J. Gao, et al., Adv. Funct. Mater. 3(2015) 646-651.

  42. [42]

      Z. Duan, Y.J. Gao, Z.Y. Qiao, et al., Nanotechnology 26(2015) 355703. doi: 10.1088/0957-4484/26/35/355703

  43. [43]

      L.J. Tan, R. Huang, X.Q. Li, et al., Carbohydr. Polym. 157(2017) 325-334. doi: 10.1016/j.carbpol.2016.09.092

  44. [44]

      X.D. Zhang, N. Gao, Z. Guan, Y.H. He, Chin. Chem. Lett. 27(2016) 964-968. doi: 10.1016/j.cclet.2016.02.013

  45. [45]

      D. Zhang, G.B. Qi, Y.X. Zhao, et al., Adv. Mater. 27(2015) 6125-6130. doi: 10.1002/adma.201502598

  46. [46]

      L.L. Li, H.L. Ma, G.B. Qi, et al., Adv. Mater. 28(2016) 254-262. doi: 10.1002/adma.201503437

  47. [47]

      S.L. Qiao, Y. Wang, Y.X. Lin, et al., ACS Appl. Mater. Interfaces 8(2016) 17016-17022. doi: 10.1021/acsami.6b04580

  48. [48]

      J.M. Estrela, A. Ortega, E. Obrador, Crit. Rev. Clin. Lab. Sci. 43(2006) 143-181. doi: 10.1080/10408360500523878

  49. [49]

      H.W. An, S.L. Qiao, L.L. Li, et al., ACS Appl. Mater. Interfaces 8(2016) 19202-19207. doi: 10.1021/acsami.6b07497

  50. [50]

      S.D. Perrault, C. Walkey, T. Jennings, H.C. Fischer, W.C.W. Chan, Nano Lett. 9(2009) 1909-1915. doi: 10.1021/nl900031y

  51. [51]

      J. Wang, W. Mao, L.L. Lock, et al., ACS Nano 9(2015) 7195-7206. doi: 10.1021/acsnano.5b02017

  52. [52]

      H. Cabral, Y. Matsumoto, K. Mizuno, et al., Nat. Nano 6(2011) 815-823. doi: 10.1038/nnano.2011.166

  53. [53]

      J. Ko, K. Park, Y.S. Kim, et al., J. Control. Release 123(2007) 109-115. doi: 10.1016/j.jconrel.2007.07.012

  54. [54]

      H. Koo, M.S. Huh, I.C. Sun, et al., Acc. Chem. Res. 44(2011) 1018-1028. doi: 10.1021/ar2000138

  55. [55]

      H.J. Li, J.Z. Du, X.J. Du, et al., Proc. Natl. Acad. Sci. U. S. A. 113(2016) 4164-4169. doi: 10.1073/pnas.1522080113

  56. [56]

      J. Li, W. Ke, H. Li, et al., Adv. Healthcare Mater. 4(2015) 2206-2219. doi: 10.1002/adhm.201500379

  57. [57]

      H.J. Li, J.Z. Du, J. Liu, et al., ACS Nano 10(2016) 6753-6761. doi: 10.1021/acsnano.6b02326

  58. [58]

      W. Walther, U. Stein, Mol. Biotechnol. 13(1999) 21-28. doi: 10.1385/MB:13:1

  59. [59]

      C. Liu, N. Shao, Y. Wang, Y. Cheng, Adv. Healthcare Mater. 5(2016) 584-592. doi: 10.1002/adhm.v5.5

  60. [60]

      C. Xiao, W.Y. Zhao, D.Y. Zhou, et al., Chin. Chem. Lett. 26(2015) 817-824. doi: 10.1016/j.cclet.2015.05.013

  61. [61]

      R. Tong, H.D. Hemmati, R. Langer, D.S. Kohane, J. Am. Chem. Soc. 134(2012) 8848-8855. doi: 10.1021/ja211888a

  62. [62]

      R. Tong, H.H. Chiang, D.S. Kohane, Proc. Natl. Acad. Sci. U. S. A. 110(2013) 19048-19053. doi: 10.1073/pnas.1315336110

  63. [63]

      C. Wong, T. Stylianopoulos, J. Cui, et al., Proc. Natl. Acad. Sci. U. S. A. 108(2011) 2426-2431. doi: 10.1073/pnas.1018382108

  64. [64]

      M. Stroh, J.P. Zimmer, D.G. Duda, et al., Nat. Med. 11(2005) 678-682. doi: 10.1038/nm1247

  65. [65]

      D.S. Seferos, A.E. Prigodich, D.A. Giljohann, et al., Nano Lett. 9(2008) 308-311.

  66. [66]

      X. Tan, X. Lu, F. Jia, et al., J. Am. Chem. Soc. 138(2016) 10834-10837. doi: 10.1021/jacs.6b07554

  67. [67]

      A.G. Cheetham, P. Zhang, Y.A. Lin, L.L. Lock, H. Cui, J. Am. Chem. Soc. 135(2013) 2907-2910. doi: 10.1021/ja3115983

  68. [68]

      W. Zauner, M. Ogris, E. Wagner, Adv. Drug Deliv. Rev. 30(1998) 97-113. doi: 10.1016/S0169-409X(97)00110-5

  69. [69]

      D.T. Klink, S. Chao, M.C. Glick, T.F. Scanlin, Mol. Ther. 3(2001) 831-841. doi: 10.1006/mthe.2001.0332

  70. [70]

      S. Ogura, K. Yazaki, K. Yamaguchi, T. Kamachi, I. Okura, J. Control. Release 103(2005) 1-6. doi: 10.1016/j.jconrel.2004.11.010

  71. [71]

      H.J. Lee, W.M. Pardridge, Bioconjug. Chem. 14(2003) 546-553. doi: 10.1021/bc0256648

  72. [72]

      Z. Zhou, Y. Shen, J. Tang, et al., Adv. Funct. Mater. 19(2009) 3580-3589. doi: 10.1002/adfm.v19:22

  73. [73]

      H. Takemoto, K. Miyata, S. Hattori, et al., Angew. Chem. Int. Ed. 52(2013) 6218-6221. doi: 10.1002/anie.201300178

  74. [74]

      D.B. Rozema, K. Ekena, D.L. Lewis, A.G. Loomis, J.A. Wolff, Bioconjug. Chem. 14(2003) 51-57. doi: 10.1021/bc0255945

  75. [75]

      J.Z. Du, X.J. Du, C.Q. Mao, J. Wang, J. Am. Chem. Soc. 133(2011) 17560-17563. doi: 10.1021/ja207150n

  76. [76]

      S. Liu, S. Qiao, L. Li, et al., Nanotechnology 26(2015) 495602. doi: 10.1088/0957-4484/26/49/495602

  77. [77]

      E.S. Olson, T. Jiang, T.A. Aguilera, et al., Proc. Natl. Acad. Sci. U. S. A. 107(2010) 4311-4316. doi: 10.1073/pnas.0910283107

  78. [78]

      R. Toy, P.M. Peiris, K.B. Ghaghada, E. Karathanasis, Nanomedicine 9(2014) 121-134. doi: 10.2217/nnm.13.191

  79. [79]

      P.P. Yang, X.X. Zhao, A.P. Xu, L. Wang, H. Wang, J. Mater. Chem. B 4(2016) 2662-2668. doi: 10.1039/C6TB00097E

  80. [80]

      P. P. Yang, Q. Luo, G. B. Qi, et al. , Adv. Mater. (2017), doi: http://dx.doi.org/10.1002/adma.201605869.

  81. [81]

      A.P. Xu, P.P. Yang, C. Yang, et al., Nanoscale 8(2016) 14078-14083.

  82. [82]

      X. X. Hu, P. P. He, G. B. Qi, et al. , ACS Nano (2017), doi: http://dx.doi.org/10.1021/acsnano.1027b00781.

  83. [83]

      Y. Lu, Q. Hu, Y. Lin, et al., Nat. Commun. 6(2015) 10066. doi: 10.1038/ncomms10066

  84. [84]

      X. Ai, C.J. Ho, J. Aw, et al., Nat. Commun. 7(2016) 10432. doi: 10.1038/ncomms10432

  85. [85]

      F. Lecaille, J. Kaleta, D. Brömme, Chem. Rev. 102(2002) 4459-4488. doi: 10.1021/cr0101656

  86. [86]

      S. Ruan, C. Hu, X. Tang, et al., ACS Nano 10(2016) 10086-10098. doi: 10.1021/acsnano.6b05070

  87. [87]

      C.E. Callmann, C.V. Barback, M.P. Thompson, et al., Adv. Mater. 27(2015) 4611-4615. doi: 10.1002/adma.v27.31

  88. [88]

      Q. Hu, W. Sun, Y. Lu, et al., Nano Lett. 16(2016) 1118-1126. doi: 10.1021/acs.nanolett.5b04343

  89. [89]

      M.M. Nguyen, A.S. Carlini, M.P. Chien, et al., Adv. Mater. 27(2015) 5547-5552. doi: 10.1002/adma.201502003

  90. [90]

      S. Ohta, D. Glancy, W.C.W. Chan, Science 351(2016) 841-845. doi: 10.1126/science.aad4925

• 

  Scheme1  NPs with transformable structures including size, surface charge and morphology

  下载: 全尺寸图片 幻灯片
  

  Figure 1  A) Expansion of eNPs at acidic environment. Up: schematic diagram; below: molecular formula. B) Scanning electron microscope (SEM) imaging of eNPs at pH of 7.4 and 5.0, indicating NPs' expansion. Reproduced with permission [38]. Copyright 2013, Royal Society of Chemistry

  下载: 全尺寸图片 幻灯片
  

  Figure 2  A) Schematic illustration of self-assembly process to form NPs, followed by expansion at low pH. NR was loaded into NPs, size of NPs was increased under acidic condition, which contributed to the releasement NR. B) Dynamic light scattering indicated size distribution with pH changes from 7.4 to 5.0. C) Fluorescence intensity ratio of P-BP/NR's (418 nm to 635 nm) changes upon pH. D) In situ Confocal Laser Scanning Microscope (CLSM) image of P-BP/NR in living U87 cells at different time point. Reproduced with permission [39]. Copyright 2015, Royal Society of Chemistry

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Synthetic pathway and structural transformation of Ag₂S(DOX)@CS. Ag₂S QDs could bind N-hydroxysuccinimide and chitosan to obtain NPs. After that, DOX was loaded into NPs and formed into Ag₂S(DOX)@CS. NPs turned large to release DOX at 37 ℃ and pH 5.0. Reproduced with permission [43]. Copyright 2017, Elsevier

  下载: 全尺寸图片 幻灯片
  

  Figure 4  A) Molecular formula and schematic diagram of self-assembly from small molecules to NPs (left). Enzyme induced the formation of NPs and enhanced PA imaging in vivo (right). B) Quantification of in situ PA imaging intensity at tumor site from 0.5 h to 24 h after injection, indicating AIR effect. C) In vitro amount of NPs and phosphate buffered saline (PBS) after injecting for different time. Reproduced with permission [45]. Copyright 2015, Wiley Publishing Group

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Fluorescence spectra of nanoemitters after adding GSH (500 equiv.) at λ_max = 820 nm (A) and λ_max = 520 nm (B), respectively. The intensity of fluorescence was increased over time. C) Ex vivo fluorescence images of tumor and different tissues after 12 h post-injection of nanoemitters or PBS (1, tumor; 2, lung; 3, heart; 4, kidney; 5, spleen; 6, liver). D) Bio-distribution of nanoemitters at different tumor or tissues 12 h post-injection from C). Reproduced with permission [49]. Copyright 2016, American Chemical Society

  下载: 全尺寸图片 幻灯片
  

  Figure 6  A) Schematic diagram of the conjugated polymer and its transformation under acidic environment. B) TEM image of NPs treated with phosphate buffer at different time (0, 4, 24 h). Scale bar, 100 nm. C) Time and penetration depthdependent distribution of NPs. The core of NPs were labeled with rhodamine B for red fluorescence signal and prudrugs were labeled with fluorescein for red fluorescence signal, which indicating efficiency penetration of prodrugs. Reproduced with permission [55]. Copyright 2016, National Academy of Sciences

  下载: 全尺寸图片 幻灯片
  

  Figure 7  A) Schematic diagram of UV light induced NPs reduction. B) Images of photo (up) and fluorescence signal (below). Reproduced with permission [61]. Copyright 2012, American Chemical Society

  下载: 全尺寸图片 幻灯片
  

  Figure 8  A) Schematic illustration of NPs transformation at acidic pH. B) Timedependent ex vivo fluorescence images of tumor and organs (heart, liver, spleen. lung and kidney) 96h post injecting PBS and NPs. C) Average fluorescence signal of tumor and liver at different time point. D) TEM images of tumor tissue slices from MCF-7 xenografted mice after injection of NPs. Blue arrows indicated the formation of NFs. E) 3D reconstructed green channel of CLSM image of NFs in tumor tissue slice. Reproduced with permission [80]. Copyright 2017, Wiley Publishing Group

  下载: 全尺寸图片 幻灯片
  

  Figure 9  A) SEM images of surface of cell treated with 1-NPs, 2-NPs and PBS (control). B) Fluorescence images in vivo of mice injected by transformable 1-NPs, nontransformable2-NPs and PBS wereacquiredat different time point. Reproduced with permission [82]. Copyright 2017, American Chemical Society

  下载: 全尺寸图片 幻灯片
  

  Figure 10  A) Schematic illustration of NPs' fusion and degradation. B) TEM image of NPs incubated for different time at pH 5.0. Scale bar: 100nm (5min, 1h and 4h); 400nm (72h). C) TEM images of HeLa cells cultured by NPs for 1h and 4h. Red arrows referred to fused nanospheres; green arrows referred to dispersion of single nanosphere in the cytosol. Scale bar: 2 μm. Reproduced with permission [83]. Copyright 2015, Institute of Physics Publishing

  下载: 全尺寸图片 幻灯片
  

  Figure 11  A) NPs' motion pathway during the whole microenvironment. Cross-linking was induced by UV light. B) PA images of corresponding quantitative results of NPs at control group (up) and tumor site (down) in different time interval (0 h and 1 h) after injection. C) In vivo tumor volume changing by time after injecting NPs and with stimulation. Reproduced with permission [84]. Copyright 2016, Nature

  下载: 全尺寸图片 幻灯片
  

  Figure 12  A) TEM images of NPs (left) and larger aggregates (right) after treatment by enzyme. B) Size distribution of NPs from DLS before and after enzyme activation. C) Image of healthy myocardium, scale bar: 1 mm. D) Time-dependent fluorescence signals of tissue after injection enzyme responsive NPs (top) and non-responsive NPs (Bottom), scale bar: 50 μm. Reproduced with permission [89]. Copyright 2015, Wiley Publishing Group

  下载: 全尺寸图片 幻灯片
  

  Figure 13  Under the mediation of DNA, shape change of NPs was shown as schematic diagram and TEM images. A) Basic building block such as different scales of DNAfunctionalized gold NPs and linker DNA strands. Structure model of DNA-linked NPs and its shift through adding different DNA chains. B) Corresponding TEM images of morphology changes. Reproduced with permission [90]. Copyright 2015, Science

  下载: 全尺寸图片 幻灯片
•