Current advances in lanthanide ion (Ln(3+))-based upconversion nanomaterials for drug delivery.

Lanthanide ion (Ln(3+))-based upconversion nano/micromaterials that emit higher-energy visible light when excited by low-energy NIR light have aroused considerable attention in the forefront of materials science and biomedical fields, which stems from their unique optical and chemical properties including minimum photodamage to living organisms, low autofluorescence, high signal-to-noise ratio and detection sensitivity, and high penetration depth in biological or environmental samples. Thus, Ln(3+)-based upconversion materials are rising new stars and are quickly emerging as potential candidates to revolutionize novel biomedical applications. In this review article, we mainly focus on the recent progress in various chemical syntheses of Ln(3+)-based upconversion nanomaterials, with special emphasis on their application in stimuli-response controlled drug release and subsequent therapy. Functional groups that are introduced into the stimuli-responsive system can respond to external triggers, such as pH, temperature, light, and even magnetic fields, which can regulate the movement of the pharmaceutical cargo and release the drug at a desired time and in a desired area. This is crucial to boost drug efficacy in cancer treatment while minimizing the side effects of cytotoxic drugs. Many multifunctional (magnetic/upconversion luminescence and porous) composite materials based on Ln(3+) have been designed for controlled drug delivery and multimodal bioimaging. Finally, the challenges and future opportunities for Ln(3+)-based upconversion materials are discussed.

Efficient gene delivery and multimodal imaging by lanthanide-based upconversion nanoparticles.

Nanoparticles have been explored as nonviral gene carriers for years because of the simplicity of surface modification and lack of immune response. Lanthanide-based upconversion nanoparticles (UCNPs) are becoming attractive candidates for biomedical applications in virtue of their unique optical properties and multimodality imaging ability. Here, we report a UCNPs-based structure with polyethylenimine coating for both efficient gene transfection and trimodality imaging. Cytotoxicity tests demonstrated that the nanoparticles exhibited significantly decreased cytotoxicity compared to polyethylenimine polymer. Further, in vitro studies revealed that the gene carriers are able to transfer the enhanced green fluorescence protein (EGFP) plasmid DNA into Hela cells in higher transfection efficiency than PEI. Gene silencing was also examined by delivering bcl-2 siRNA into Hela cells, resulting in significant downregulation of target bcl-2 mRNA. More importantly, we demonstrated the feasibility of upconversion gene carriers to serve as effective contrast agents for MRI/CT/UCL trimodality imaging both in vitro and in vivo. The facile fabrication process, great biocompatibility, enhanced gene transfection efficiency, and great bioimaging ability can make it promising for application in gene therapy.

Multifunctional upconversion mesoporous silica nanostructures for dual modal imaging and in vivo drug delivery.

Incorporating the agents for magnetic resonance imaging (MRI), optical imaging, and therapy in one nanostructured matrix to construct multifunctional nanomedical platform has attracted great attention for simultaneous diagnostic and therapeutic applications. In this work, a facile methodology is developed to construct a multifunctional anticancer drug nanocarrier by combining the special advantages of upconversion nanoparticles and mesoporous silica. ß-NaYF4 :Yb(3+) , Er(3+) @ß-NaGdF4 :Yb(3+) is chosen as it can provide the dual modality of upconversion luminescence and MRI. Then mesoporous silica is directly coated onto the upconversion nanoparticles to form discrete, monodisperse, highly uniform, and core-shell structured nanospheres (labeled as UCNPs@mSiO2 ), which are subsequently functionalized with hydrophilic polymer poly(ethylene glycol) (PEG) to improve the colloidal stability and biocompatibility. The obtained multifunctional nanocomposites can be used as an anticancer drug delivery carrier and applied for imaging. The anticancer drug doxorubicin (DOX) is absorbed into UCNPs@mSiO2 -PEG nanospheres and released in a pH-sensitive pattern. In vitro cell cytotoxicity tests on cancer cells verify that the DOX-loaded UCNPs@mSiO2 -PEG has comparable cytotoxicity with free DOX at the same concentration of DOX. In addition, the T1 -weighted MRI that measures in aqueous solutions reveals that the contrast brightening increases with the concentration of Gd(3+) component. Upconversion luminescence images of UCNPs@mSiO2 -PEG uptaken by cells show green emission under 980 nm infrared laser excitation. Finally, the nanocomposites show low systematic toxicity and high in vivo antitumor therapy efficacy. These findings highlight the fascinating features of upconversion-mesoporous nanocomposites as multimodality imaging contrast agents and nanocarrier for drug molecules.

Electrospun upconversion composite fibers as dual drugs delivery system with individual release properties.

Novel multifunctional poly(ε-caprolactone)-gelatin encapsulating upconversion core/shell silica nanoparticles (NPs) composite fibers as dual drugs delivery system (DDDS), with indomethacin (IMC) and doxorubicin (DOX) releasing in individual release properties, have been designed and fabricated via electrospinning process. Uniform and monodisperse upconversion (UC) luminescent NaYF4:Yb(3+), Er(3+) nanocrystals (UCNCs) were encapsulated with mesoporous silica shells, resulting in the formation of core/shell structured NaYF4:Yb(3+), Er(3+)@mSiO2 (UCNCs@mSiO2) NPs, which can be performed as DOX delivery carriers. These UCNCs@mSiO2 NPs loading DOX then were dispersed into the mixture of poly(ε-caprolactone) (PCL) and gelatin-based electrospinning solution containing IMC, followed by the preparation of dual drug-loaded composite fibers (DDDS) via electrospinning method. The drugs release profiles of the DDDS were measured, and the results indicated that the IMC and DOX released from the electrospun composite fibers showed distinct properties. The IMC in the composite fibers presented a fast release manner, while DOX showed a sustained release behavior. Moreover, the UC luminescent intensity ratios of (2)H(11/2)/(4)S(3/2)-(4)I(15/2) to (4)F(9/2)-(4)I(15/2) from Er(3+) vary with the amounts of DOX in the system, and thus drug release can be tracked and monitored by the luminescence resonance energy transfer (LRET) mechanism.

Poly(acrylic acid)-modified Fe3O4 microspheres for magnetic-targeted and pH-triggered anticancer drug delivery.

Monodisperse poly(acrylic acid)-modified Fe(3)O(4) (PAA@Fe(3)O(4)) hybrid microspheres with dual responses (magnetic field and pH) were successfully fabricated. The PAA polymer was encapsulated into the inner cavity of Fe(3)O(4) hollow spheres by a vacuum-casting route and photo-initiated polymerization. TEM images show that the samples consist of monodisperse porous spheres with a diameter around 200 nm. The Fe(3)O(4) spheres, after modification with the PAA polymer, still possess enough space to hold guest molecules. We selected doxorubicin (DOX) as a model drug to investigate the drug loading and release behavior of as-prepared composites. The release of DOX molecules was strongly dependent on the pH value due to the unique property of PAA. The HeLa cell-uptake process of DOX-loaded PAA@Fe(3)O(4) was observed by confocal laser scanning microscopy (CLSM). After being incubated with HeLa cells under magnet magnetically guided conditions, the cytotoxtic effects of DOX-loaded PAA@Fe(3)O(4) increased. These results indicate that pH-responsive magnetic PAA@Fe(3)O(4) spheres have the potential to be used as anticancer drug carriers.

Manganese oxide nanomaterials boost cancer immunotherapy.

Immunotherapy, a strategy that leverages the host immune function to fight against cancer, plays an increasingly important role in clinical tumor therapy. In spite of the great success achieved in not only clinical treatment but also basic research, cancer immunotherapy still faces many huge challenges. Manganese oxide nanomaterials (MONs), as ideal tumor microenvironment (TME)-responsive biomaterials, are able to dramatically elicit anti-tumor immune responses in multiple ways, indicating great prospects for immunotherapy. In this review, on the basis of different mechanisms to boost immunotherapy, major highlighted topics are presented, covering adjusting an immunosuppressive TME by generating O2 (like O2-sensitized photodynamic therapy (PDT), programmed cell death ligand-1 (PD-L1) expression downregulation, reprogramming tumor-associated macrophages (TAMs), and restraining tumor angiogenesis and lactic acid exhaustion), inducing immunogenic cell death (ICD), photothermal therapy (PTT) induction, activating the stimulator of interferon gene (STING) pathway and immunoadjuvants for nanovaccines. We hope that this review will provide holistic understanding about MONs and their application in cancer immunotherapy, and thus pave the way to the translation from bench to bedside in the future.

Boosting the antitumor efficacy over a nanoscale porphyrin-based covalent organic polymer via synergistic photodynamic and photothermal therapy.

Porphyrin-based covalent organic polymer nanoparticles were synthesized via a Schiff base reaction at room temperature. The resulting product showed a good photodynamic effect and a high photothermal conversion efficiency (34.88%). Both in vitro and in vivo experiments demonstrated the enhanced antitumor efficacy via synergistic photodynamic and photothermal therapy.

O2-Cu/ZIF-8@Ce6/ZIF-8@F127 Composite as a Tumor Microenvironment-Responsive Nanoplatform with Enhanced Photo-/Chemodynamic Antitumor Efficacy.

Hypoxia and overexpression of glutathione (GSH) are typical characteristics of the tumor microenvironment, which severely hinders cancer treatments. Here, we design a novel biodegradable therapeutic system, O2-Cu/ZIF-8@Ce6/ZIF-8@F127 (OCZCF), to simultaneously achieve GSH depletion and O2-enhanced combination therapy. Notably, the doped Cu2+ doubles the O2 storage capacity of the ZIF-8 matrix, which makes OCZCF an excellent pH-sensitive O2 reservoir for conquering tumor hypoxia, enhancing the photodynamic therapy (PDT) efficiency of chlorin e6 (Ce6) under 650 nm laser irradiation. Moreover, the released Cu2+ can act as a smart reactive oxygen species protector by consuming intracellular GSH. The byproduct Cu+ will undergo highly efficient Fenton-like reaction to achieve chemodynamic therapy (CDT) in the presence of abundant H2O2. The accompanying O2 will further alleviate hypoxia. The in vitro and in vivo experimental data indicate that OCZCF could cause remarkable tumor inhibition through enhanced synergetic PDT and CDT, which may open up a new path for cancer therapy.

MnFe2O4-decorated large-pore mesoporous silica-coated upconversion nanoparticles for near-infrared light-induced and O2 self-sufficient photodynamic therapy.

The limited light penetration depth and tumor hypoxia are two natural shortcomings of photodynamic therapy (PDT). Overcoming these two issues within a single system is still a great challenge. Herein, photosensitizer (PS)-loaded and PEG-modified MnFe2O4-decorated large-pore mesoporous silica-coated ß-NaYF4:20%Yb,2%Er@ß-NaYF4 upconversion nanoparticles (UCMnFe-PS-PEG) as excellent PDT agents are successfully prepared for NIR light-mediated and O2 self-sufficient PDT. The large mesoporous structure observably increases PS loading efficiency (11.33 wt%) and the green luminescence from upconversion nanoparticles activated by NIR is able to activate PSs to generate ROS effectively. In addition, sub-10 nm MnFe2O4 nanoparticles work as a Fenton catalyst to generate O2in situ. In vivo experiments further prove that UCMnFe-PS-PEG not only provides magnetic guidance to the tumor, but also overcomes tumor hypoxia and dramatically enhances PDT efficiency. Furthermore, in vivo MR and UCL imaging are performed for accurate cancer therapy. We believe that the successful construction of the multifunctional UCMnFe-PS-PEG provides more revelations for developing advanced nano-drug systems for cancer therapy.

The release behavior of doxorubicin hydrochloride from medicated fibers prepared by emulsion-electrospinning.

The release behavior of a water-soluble small molecule drug from the drug-loaded nanofibers prepared by emulsion-electrospinning was investigated. Doxorubicin hydrochloride (Dox), a water-soluble anticancer agent, was used as the model drug. The laser scanning confocal microscopic images indicated that the drug was well incorporated into amphiphilic poly(ethylene glycol)-poly(L-lactic acid) (PEG-PLA) diblock copolymer nanofibers, forming "core-sheath" structured drug-loaded nanofibers. The drug release behavior of this drug-loaded system showed a three-stage diffusion-controlled mechanism, in which the release rate of the first stage was slower than that of the second stage, but both obeyed Fick's second law. Based on these results, it is concluded that the Dox-loaded fibers prepared by emulsion-electrospinning represent a reservoir-type delivery system in which the Dox release rate decreases with the increasing Dox content in the fibers.

Self-Templated Stepwise Synthesis of Monodispersed Nanoscale Metalated Covalent Organic Polymers for In Vivo Bioimaging and Photothermal Therapy.

Size- and shape-controlled growth of nanoscale microporous organic polymers (MOPs) is a big challenge scientists are confronted with; meanwhile, rendering these materials for in vivo biomedical applications is still scarce. In this study, a monodispersed nanometalated covalent organic polymer (MCOP, M=Fe, Gd) with sizes around 120 nm was prepared by a self-templated two-step solution-phase synthesis method. The metal ions (Fe3+ , Gd3+ ) played important roles in generating a small particle size and in the functionalization of the products during the reaction with p-phenylenediamine (Pa). The resultant Fe-Pa complex was used as a template for the subsequent formation of MCOP following the Schiff base reaction with 1,3,5-triformylphloroglucinol (Tp). A high tumor suppression efficiency for this Pa-based COP is reported for the first time. This study demonstrates the potential use of MCOP as a photothermal agent for photothermal therapy (PTT) and also provides an alternative route to fabricate nano-sized MCOPs.

Rational design of a comprehensive cancer therapy platform using temperature-sensitive polymer grafted hollow gold nanospheres: simultaneous chemo/photothermal/photodynamic therapy triggered by a 650 nm laser with enhanced anti-tumor efficacy.

Combining multi-model treatments within one single system has attracted great interest for the purpose of synergistic therapy. In this paper, hollow gold nanospheres (HAuNs) coated with a temperature-sensitive polymer, poly(oligo(ethylene oxide) methacrylate-co-2-(2-methoxyethoxy)ethyl methacrylate) (p(OEGMA-co-MEMA)), co-loaded with DOX and a photosensitizer Chlorin e6 (Ce6) were successfully synthesized. As high as 58% DOX and 6% Ce6 by weight could be loaded onto the HAuNs-p(OEGMA-co-MEMA) nanocomposites. The grafting polymer brushes outside the HAuNs play the role of "gate molecules" for controlled drug release by 650 nm laser radiation owing to the temperature-sensitive property of the polymer and the photothermal effect of HAuNs. The HAuNs-p(OEGMA-co-MEMA)-Ce6-DOX nanocomposites with 650 nm laser radiation show effective inhibition of cancer cells in vitro and enhanced anti-tumor efficacy in vivo. In contrast, control groups without laser radiation show little cytotoxicity. The nanocomposite demonstrates a way of "killing three birds with one stone", that is, chemotherapy, photothermal and photodynamic therapy are triggered simultaneously by the 650 nm laser stimulation. Therefore, the nanocomposites show the great advantages of multi-modal synergistic effects for cancer therapy by a remote-controlled laser stimulus.

Multifunctional NaYF4:Yb, Er@mSiO2@Fe3O4-PEG nanoparticles for UCL/MR bioimaging and magnetically targeted drug delivery.

A low toxic multifunctional nanoplatform, integrating both mutimodal diagnosis methods and antitumor therapy, is highly desirable to assure its antitumor efficiency. In this work, we show a convenient and adjustable synthesis of multifunctional nanoparticles NaYF4:Yb, Er@mSiO2@Fe3O4-PEG (MFNPs) based on different sizes of up-conversion nanoparticles (UCNPs). With strong up-conversion fluorescence offered by UCNPs, superparamagnetism properties attributed to Fe3O4 nanoparticles and porous structure coming from the mesoporous SiO2 shell, the as-obtained MFNPs can be utilized not only as a contrast agent for dual modal up-conversion luminescence (UCL)/magnetic resonance (MR) bio-imaging, but can also achieve an effective magnetically targeted antitumor chemotherapy both in vitro and in vivo. Furthermore, the UCL intensity of UCNPs and the magnetic properties of Fe3O4 in the MFNPs were carefully balanced. Silica coating and further PEG modifying can improve the hydrophilicity and biocompatibility of the as-synthesized MFNPs, which was confirmed by the in vitro/in vivo biocompatibility and in vivo long-time bio-distributions tests. Those results revealed that the UCNPs based magnetically targeted drug carrier system we synthesized has great promise in the future for multimodal bio-imaging and targeted cancer therapy.

Multifunctional core-shell structured nanocarriers for synchronous tumor diagnosis and treatment in vivo.

Multifunctional, mesoporous, silica-coated upconversion luminescent/magnetic NaGdF4:Yb/Er@NaGdF4:Yb@mSiO2-PEG (referred to as UCNPS; PEG=polyethylene glycol) nanocomposites were fabricated through a phase-transfer-assisted surfactant-templating coating process, followed by hydrophilic polymer (PEG) functionalization to improve the stability and biocompatibility. The UCNP core imparts the nanomaterials with luminescence and magnetic properties for simultaneous upconversion optical and magnetic resonance (MR) imaging, whereas the mesoporous shell affords the nanomaterials the ability to load the anticancer drug doxorubicin. Proof-of-principle in vitro and in vivo experiments are presented to demonstrate that the resultant composite nanomaterials can serve as nanotheranostics for synchronous upconversion luminescence/MR dual modal imaging and anticancer drug delivery; this finally realizes the integration of diagnostics and the treatment of cancers.

Poly(acrylic acid) modified lanthanide-doped GdVO4 hollow spheres for up-conversion cell imaging, MRI and pH-dependent drug release.

In this study, multifunctional poly(acrylic acid) modified lanthanide-doped GdVO(4) nanocomposites [PAA@GdVO(4): Ln(3+) (Ln = Yb/Er, Yb/Ho, Yb/Tm)] were constructed by filling PAA hydrogel into GdVO(4) hollow spheres via photoinduced polymerization. The up-conversion (UC) emission colors (green, red and blue) can be tuned by changing the codopant compositions in the matrices. The composites have potential applications as bio-probes for cell imaging. Meanwhile, the hybrid spheres can act as T(1) contrast agents for magnetic resonance imaging (MRI) owing to the existence of Gd(3+) ions on the surface of composites. Due to the nature of PAA, DOX-loaded PAA@GdVO(4):Yb(3+)/Er(3+) system exhibits pH-dependent drug releasing kinetics. A lower pH offers a faster drug release rate. Such character makes the loaded DOX easily released at cancer cells. The cell uptake process of drug-loaded composites was observed by using confocal laser scanning microscopy (CLSM). The results indicate the potential application of the multifunctional composites as theragnostics (effective bimodal imaging probes and pH-responsive drug carriers).

Core-shell structured luminescent and mesoporous ß-NaYF4:Ce3+/Tb3+@mSiO2-PEG nanospheres for anti-cancer drug delivery.

Multifunctional nanocomposites integrating mesoporous and luminescence properties into a single entity are synthesized via a facile and effective approach. Oleic acid capped ß-NaYF4:Ce(3+)/Tb(3+) nanoparticles (NPs) are transferred into aqueous solution by cetyltrimethylammonium bromide (CTAB) surfactants, and further encapsulated with uniform mesoporous silica shell followed by the surface modification with poly(ethylene glycol) (PEG), leading to the formation of water-dispersible and biocompatible core-shell structured ß-NaYF4:Ce(3+)/Tb(3+)@mSiO2-PEG (denoted as NPs@mSiO2-PEG) nanospheres. The as-synthesized nanospheres show a typical mesoporous structure and a green emission under UV irradiation. Doxorubicin hydrochloride (DOX), a widely used anti-cancer drug, is used as a model drug to evaluate the loading and controlled release behaviors of the NPs@mSiO2-PEG in phosphoric acidic buffer solutions (PBS) at different pH values (pH = 7.4 and 5.0). The composite carriers provide a pH-sensitive drug release pattern and the drug releases faster under lower pH value. The endocytosis process of fluorescein isothiocyanate (FITC)-labelled nanospheres is characterized using flow cytometry and confocal laser scanning microscopy (CLSM) against A549 cells. The in vitro cytotoxic effect against A549 cells of the DOX-loaded carriers is investigated in detail. In addition, the extent of drug release can be monitored by the variation of photoluminescence (PL) intensity of ß-NaYF4:Ce(3+)/Tb(3+). Considering the good biocompatibility and pH-dependent drug release pattern, such core-shell structured luminescent NPs@mSiO2-PEG nanospheres have potential applications in controlled drug delivery.

Rational design of multifunctional upconversion nanocrystals/polymer nanocomposites for cisplatin (IV) delivery and biomedical imaging.

By combining upconversion nanoparticles with the cisplatin (IV) prodrug we have demonstrated that a stable and multifunctional drug delivery system can be designed that will both reduce the drawbacks of cisplatin and give insight in to its in vitro/in vivo imaging. The up/down-conversion fluorescence are detectable and show obvious co-localization, demonstrating that the nanoparticles are rather stable inside cells and retain the UCNPs and block copolymer.

An improved approach to poly(ester-carbonate) conjugates.

A novel poly(ester-carbonate) (P(LA-co-NPC)) with activated pendant carboxyl groups was synthesized by ring-opening polymerization of L-lactide (LA) and 2-methyl-2-(4-nitrophenoxycarbonyl)-propylene carbonate (NPC) with diethyl zinc (ZnEt(2)) as catalyst. GPC and NMR studies confirmed the co-polymer structure. The pendant 4-nitrophenyl carboxylate groups can react with amino-containing molecules, such as 3,6,9-trioxa-1,11-undecanediamine, doxorubicin and chitosan under mild conditions. Therefore, any amino-containing molecules of biomedical interests can be conjugated to P(LA-co-NPC) efficiently and easily. The biocompatibility of the co-polymer was tested using L929 cell line, indicating that P(LA-co-NPC) is a promising biomedical material.

Electrospinning preparation and drug delivery properties of Eu3+/Tb3+ doped mesoporous bioactive glass nanofibers.

Luminescent Eu(3+)/Tb(3+) doped mesoporous bioactive glass nanofibers (MBGNFs) with average diameter of 100-120 nm were fabricated by electrospinning method. Pluronic P123 and N-cetyltrimethylammonium bromide (CTAB) were used as co-surfactants to generate porous structure of the nanofibers. N(2) adsorption-desorption measurement reveals that the MBGNF:Eu(3+) have a surface area of 188 m(2) g(-1), a pore volume of 0.246 cm(3) g(-1) and average pore size of 4.17 nm, and the MBGNF:Tb(3+) have a surface area of 171 m(2) g(-1), a pore volume of 0.186 cm(3) g(-1) and average pore size of 3.65 nm. Photoluminescence measurements reveal that the MBGNF:Eu(3+) show strong red emission dominated by the (5)D(0)→(7)F(2) transition of Eu(3+) at 614 nm with a lifetime of 1.356 ms, and MBGNF:Tb(3+) show strong green emission dominated by the (5)D(4)→(7)F(5) transition of Tb(3+) at 544 nm with a lifetime of 1.982 ms. The biocompatibility tests on L929 fibroblast cells using MTT assay reveal low cytotoxicity of MBGNF. These luminescent nanofibers show sustained release properties for ibuprofen (IBU) in vitro. The emission intensities of Eu(3+) in the drug delivery system vary with the released amount of IBU, thus making the drug release be easily tracked and monitored by the change of the luminescence intensity.

Up-conversion cell imaging and pH-induced thermally controlled drug release from NaYF4/Yb3+/Er3+@hydrogel core-shell hybrid microspheres.

In this study, we report a new controlled release system based on up-conversion luminescent microspheres of NaYF(4):Yb(3+)/Er(3+) coated with the smart hydrogel poly[(N-isopropylacrylamide)-co-(methacrylic acid)] (P(NIPAM-co-MAA)) (prepared using 5 mol % of MAA) shell. The hybrid microspheres show bright up-conversion fluorescence under 980 nm laser excitation, and turbidity measurements show that the low critical solution temperature of the polymer shell is thermo- and pH-dependent. We have exploited the hybrid microspheres as carriers for Doxorubicin hydrochloride (DOX) due to its stimuli-responsive property as well as good biocompatibility via MTT assay. It is found that the drug release behavior is pH-triggered thermally sensitive. Changing the pH to mildly acidic condition at physiological temperature deforms the structure of the shell, causing the release of a large number of DOX from the microspheres. The drug-loaded microspheres exhibit an obvious cytotoxic effect on SKOV3 ovarian cancer cells. The endocytosis process of drug-loaded microspheres is observed using confocal laser scanning microscopy and up-conversion luminescence microscopy. Meanwhile, the as-prepared NaYF(4):Yb(3+)/Er(3+)@SiO(2)@P(NIPAM-co-MAA) microspheres can be used as a luminescent probe for cell imaging. In addition, the extent of drug release can be monitored by the change of up-conversion emission intensity. These pH-induced thermally controlled drug release systems have potential to be used for in vivo bioimaging and cancer therapy by the pH of the microenvironment changing from 7.4 (normal physiological environment) to acidic microenvironments (such as endosome and lysosome compartments) owing to endocytosis.