2.1.1 pH-sensitive bonds or linkers

A number of pH-responsive nanomaterials have been developed in response to the acidic microenvironment based on their physicochemical properties. The potential mechanisms of these nanomaterials could be mainly classified as the cleavage by protonation of chemical groups and pH-sensitive bonds/linkers, including hydrazone bonds, imine bonds, ester bonds, amide bonds, metal ion coordination bonds, and noncovalent interactions (e.g., hydrogen bonds, π–π stacking, and electrostatic forces).^54-65 These pH-sensitive bonds are comparatively stable at physiological condition, but could break down under acid condition, further triggering the drug release from nanomaterials.

Hydrazone bonds could serve as an ideal linkage by formation of hydrazine and aldehyde/carbonyl groups. Compared with other pH-sensitive bonds, hydrazone bonds are more stable under physiological condition, but will rapidly hydrolyze in acid environment of endo/lysosomes.⁶⁶ Doxorubicin (DOX) is a broad-spectrum anticancer drug containing carbonyl groups and can be chemically conjugated with hydrazide groups on polymer or pectin through hydrazone bonds formation.^{67, 68} After self-assembling into micelles, approximately 65% of DOX was released from copolymer within 72 h at pH 5.4, whereas only 31% at neutral condition.²⁵ Lu et al.²⁶ purposefully developed a multicomponent self-assembly nanocomplex (Ang–PEG-g-PLL@CPT–RT@IR783; APCI) through electrostatic, π–π stacking and hydrophobic interactions, which were self-assembled toluenesulfonyl protected arginine-conjugated camptothecin (CPT–RT) with canine dyes (IR783). Due to the protonation of sulfonic groups of IR783 under acidic conditions, a large amount of CPT–RT could be released from APCI at pH 5.0 after 24 h compared with that in physiological environment and exhibited a pH-responsive drug release property.

Recently, metal ion coordination bonds have been investigated for the design of pH-responsive nanomaterials. Metal ions, such as manganese, iron, or calcium, can self-assemble with negative ligands to form coordination bonds, which could be attributed to the competitive binding between metal ions and protons.^69-71 Driven by metal ion coordination as well as noncovalent forces, a full-active pharmaceutical ingredient nanodrug (FAND) was designed. FAND kept excellent stability under physiological conditions, with only 12.8% of drugs was released over 12 h. But the cumulative release was dramatically enhanced to 75.5% at pH 5.0, owning to the protonation of carboxyl group and cleavage of metal ion coordination bonds.²⁷

2.1.2 pH-triggered charge conversion

As the cell membrane is negatively charged, nanomaterials with positive charge exhibit enhanced cellular uptake efficiency via electrostatic interactions compared with nanomaterials with negative charge. However, an excessively positive charge would increase cytotoxicity and decrease stability, as well as lower systemic circulation time.⁷² Therefore, it is valuable to design pH-responsive nanomaterials with variable surface charges for sensing pH variations between normal and diseased tissues.

Charge-conversion strategy has been constructed for drug delivery, inspired by the pH-triggered charge reversal from negative or neutral charge under physiological environment to positive charge at acidic condition.^{28, 73, 74} By incorporating acylsulfonamide-based pH-responsive zwitterionic ligands on the surface of nanomaterials, the formed pH-responsive nanomaterials can reverse charge in response to the decreased pH at diseased site, but the structure will not be altered.⁷⁴ Liu et al.²⁸ constructed a charge conversional biomimetic nanoplatform (Ang–RBCm–CA/siRNA) by leveraging Angiopep-2 peptide, red blood cells membrane, and citraconic anhydride grafted poly-L-lysine (PLL-CA) as charge conversional component for siRNA delivery. Owing to the charge conversion ability of CA, the membrane of biomimetic nanoplatform Ang–RBCm–CA/siRNA was gradually destroyed with the pH decreased and time prolonged, but the nonsensitive nanoplatform remained a spherical structure under both conditions (Figure 2A,B). Agarose gel electrophoresis assay proved that siRNA was released from Ang–RBCm–CA/siRNA at pH 5.0, further demonstrating the pH responsive and charge-conversional behavior of CA (Figure 2C).

2.1.3 pH-responsive carriers

Polymers containing H⁺ labile linkages will occur protonation under lower pH conditions, leading to a change of physical structures and subsequent release of loaded drugs. Polymers capable of reversible ionization with amino and carboxylic groups are commonly used as pH-responsive carriers for drug delivery, such as alginate, hyaluronic acid (HA), carboxymethylcellulose, and chitosan.⁷⁵ For example, chitosan, a pH-responsive polymer, is proved to have excellent biodegradable, biocompatible, and nontoxic properties.⁷⁶ Under physiological environment, chitosan is insoluble, whereas the amino groups within chitosan become protonated and soluble under acidic pH condition, further promoting drug release.^29-31

In addition to pathological microenvironment, physiological environment might be acid as well, such as the stomach. Oral administration is a commonly used medication strategy in clinical practice due to its low cost, simplicity, and convenience. For oral administration systems, they are supposed to protect drugs from harsh conditions of gastrointestinal tract, enhance absorption into circulation system, target specific sites, and achieve controlled release.⁷⁷ One design approach is to prepare pH-responsive swelling nanomaterials based on the carboxyl protonation. Surface-functionalization with acid-stable targeting ligands (e.g. polyanions, polycations and inorganic materials) can also improve the stability of nanomaterials.⁷⁸

Overall, through surface modification of nanomaterials, pH-sensitive bonds or linkers can be conjugated with polymers to endow nanomaterials with pH-responsive properties. Upon exposure to pathological environment, these nanomaterials will occur cleavage, protonation, or charge conversion in response to altered pH conditions. Besides, a variety of pH-responsive carriers and derivatives have been developed to facilitate insoluble-to-soluble transformation at acid conditions, leading to controlled release of loading drugs. However, pH-responsive nanomaterials have not been approved for clinical applications. Several challenges of these nanomaterials should be overcome, such as the pH heterogeneity in the tumor site, complex preparation technology, and systemic toxicity of nanomaterials.⁵⁵ Therefore, to facilitate further translational study on pH-responsive nanomaterials, the nontoxicity, simple preparation, and specificity of nanomaterials should be taken into consideration.

2.2.1 GSH-responsive nanomaterials

As a ubiquitous reductive agent, GSH is a tripeptide capable of antioxidant properties and other vital functions, including redox homeostasis, cell proliferation detoxification, and so on.⁸³ It has been reported that GSH level is higher in diseased tissues than that in healthy ones, especially elevated in tumors, which can achieve a fourfold higher concentration (2–10 mM) than in normal tissues.⁸⁴ Disulfide bonds (S─S), prone to be stable under physiological environment, but swiftly cleaved in the presence of GSH, are widely applied to prepare GSH-responsive nanomaterials.^85-89 The cleavage mechanism of GSH-responsive nanomaterials under high GSH concentration relies on the thiol-disulfide exchange reaction between disulfide bonds and free thiols of GSH.⁹⁰

Disulfide bonds can work as GSH-responsive ligands linking drug molecules to form nanoprodrugs, such as paclitaxel (PTX), CPT, and DOX.^{32, 91, 92} Jiang et al.³² proposed PTX-SS-C₁₈-conjugated self-assembled nanoparticles (PSNPs) for glioblastoma (GBM) treatment. PTX-SS-C₁₈ was synthesized by mixing dithiodiglycolic acid, anhydrous acetic anhydride, N,N-diisopropylethylamine, and PTX. ¹H NMR and MS spectra showed that octadecanol was conjugated with PTX via S-S. After incubation with 10 mM GSH, the spherical morphology and size distribution demonstrated the disassembly of the nanoparticles. Likewise, PTX could be released rapidly from nanoparticles in response to elevated GSH in the tumor site.

Several prodrugs were synthesized by introducing a disulfide bond between two different drug molecules to regulate the corelease of both drugs and realize an enhanced antitumor effect.^{33, 93} For example, the dimer of lonidamine and NLG919 was connected via a disulfide bond, which could be cleaved by GSH to realize drug release (Figure 2D,E). To enhance the transport of the dimer, Pluronic F127 was chosen to promote the formation of nanoprodrugs (LSN). When encountered with GSH, the disulfide bond in the dimer was broken, leading to the release of two drug molecules for their respective functions in cancer treatment (Figure 2F).³³

Additionally, the doping of disulfide bonds will confer nanomaterials with GSH-responsive biodegradability, and lower their potential toxicity.⁹⁴ The surface disulfide linker on mesoporous silica nanoparticles (MSN-S-S-NH₂) was introduced by mercaptopropyl-derivatized MSN reacting with S-(2-aminoethylthio)-2-thiopyridine hydrochloride. After that, RGD peptide was capped onto MSN using click chemistry for tumor-targeting and controlled drug release. Under reductive conditions, over 33 and 78% of DOX were released within 90 min at the GSH concentration of 2 and 10 mM, respectively, but less than 18% was released over 24 h in the absence of GSH.³⁴

2.2.2 ROS-responsive nanomaterials

Biologically, as secondary messengers in modulating cellular functions, ROS is a class of oxygen-containing molecules, including hydrogen peroxide (H₂O₂), hydroxyl radicals (∙OH), singlet oxygen (¹O₂), and superoxide (O₂⁻). Distinct from the physiological environment, the concentration of ROS is overproduced and up to around 50−100 μM at pathological conditions, such as in tumor and inflammation. The disparity in ROS concentration between healthy and diseased tissues has fueled the development of nanomaterials with ROS-responsive ability for targeting diseased regions and minimizing the toxicity to normal tissues. Hence, numerous ROS-responsive nanomaterials have been developed by using ROS-responsive linkages, such as thioethers or sulfides, thioketal (TK), phenylboronic ester group for targeted and controllable drug release.^{35, 95-98}

TK is a biodegradable and nontoxic thioether group, which can be incorporated into the nanostructure or serve as a ROS-reactive linker for ROS-responsive nanomaterials.^{7, 99-101} The stability of the TK group in physiological condition allows for its easy cleavage under oxidative environment. By taking the ROS-responsive property of TK, Wen et al.³⁵ constructed an angiopep-2 peptide modified and ROS-cleavable nanocarrier for siRNA delivery. The TK linker was prepared by mixing 3-mercaptopropionic acid with anhydrous acetone, following with stirring, crystallizing, and drying. The results showed that high concentration of H₂O₂ could significantly enhance the cumulative release of siRNA from the complexes and promote siRNA escaping from endosomes through the ROS-responsive drug release effect, confirming the excellent ROS-responsive effect of TK (Figure 2G–I).

However, due to the low intrinsic concentration of ROS, Chu et al.³⁶ developed an ROS-cleavable dual prodrug self-assembly nanoparticle via conjugating CPT with poly(ethylene glycol) methyl ether (MPEG) by TK linkage. Photosensitizer pyropheophorbide-a (PPa) was connected to MPEG via lipid linkage for ROS generation through photodynamic therapy (PDT). With the assistance of laser illumination, the released CPT from MPEG–(TK-CPT)–PPa exhibited enhanced performance, and about 40.4% CPT was released from the nanoparticles within 48 h. Besides, the addition of extrinsic ROS source through Fenton reaction via Fe²⁺ and H₂O₂ further enhanced the release of CPT. These results showed that ROS is an effective endogenous triggered factor for controlling drug release and can bridge the link between nanomaterials and drugs for disease treatment.

2.2.3 GSH/ROS dual responsive nanomaterials

Besides, GSH/ROS dual responsive nanocarriers such as manganese dioxide (MnO₂), PTX-TKN, and PMPC-P (Se-co-RB)-P can also serve as smart materials to control drug release and protect the drugs from premature leaking.^{102, 103} As a typical TME dual-responsive nanocarrier, MnO₂ has aroused widespread attention in cancer treatment.^{37, 38} Through biocompatible functionalization, the toxicity induced by manganese ion (Mn²⁺) could be reduced in MnO₂ nanomaterials.¹⁰⁴ Notably, MnO₂ can not only undergo a redox reaction in the presence of high GSH concentration of TME to yield Mn²⁺ and simultaneously consume GSH, but also convert H₂O₂ into oxygen (O₂) and ·OH through the Mn²⁺-mediated Fenton-like reaction.^{39, 40} Therefore, MnO₂-based nanomaterials with good biocompatibility and dual GSH/ROS-responsive ability make them to promisingly applicable in cancer treatment.^105-107

3.1.1 Drug delivery in CVDs

Drug delivery systems are considered as a technology using a number of functionalized nanomaterials to precisely target pathogenic sites.¹⁹⁴ Stimuli-responsive nanomaterials have gained great progress for designing controllable drug delivery systems, owing to their excellent spatiotemporal and controllable capabilities. Following administration, stimuli-responsive drug delivery systems will passively or actively target to diseased sites. Once triggered by endogenous or exogenous stimulus, the chemical or physical structural changes of these nanomaterials may induce on-demand drug release, thereby improving the efficiency of drug delivery.²³ Consequently, the utilization of stimuli-responsive nanomaterials in drug delivery of therapeutics is a promising approach for diseases treatment.

AS is the central inflammatory disease of CVDs, which caused by the accumulation of lipids on the wall of blood vessels, followed by thickening and clogging.^{195, 196} A series of stimuli-responsive nanomaterials have emerged as crucial tools for targeted drug delivery and controlled release in AS treatment.^{197-199, 137, 138, 200} He et al.¹³⁹ proposed ROS-responsive nanoassemblies comprising β-cyclodextrin (β-CD)-anchored discoidal recombinant high-density lipoprotein (NP³_ST) with HA–ferrocene (Fc) conjugation (HA–Fc/NP³_ST). Under the excessive ROS environment of AS, the hydrophobic Fc would be oxidized into hydrophilic ferrocenium ion, resulting in the disintegration of HA–Fc/NP³_ST and NP³_ST release (Figure 4A). In this study, we investigated the oxidation properties of HA–Fc/NP³_ST. Nonresponsive nanoassemblies based on HA–amantadine (Am)/NP³_ST were constructed as the unresponsive control. Upon labeling with fluorescence dyes (Rho123–HA–Fc/NP³_ST–RITC), the emission intensity of Rho123 was decreased, and that of RITC was increased, suggesting fluorescence resonance energy transfer in the presence of ROS (1 mM H₂O₂). However, for Rho123–HA–Am/NP³_ST–RITC, the emission intensity of either Rho123 or RITC showed no obvious changes after ROS intervention. Subsequently, the ROS-responsive drug release of HA–Fc/NP³_ST was investigated. About 55% of NP³_ST was released from HA–Fc/NP³_ST after 72 h incubation with ROS, whereas only 38.2% in the absence of ROS (Figure 4B). Conversely, the drug release profiles of HA–Am/NP³_ST exhibited no discernible differences with or without ROS intervention, further confirming that HA–Fc was an excellent ROS-responsive nanomaterial for targeted drug delivery in AS therapy.

Specifically, owing to the narrowing blood vascular in AS, the wall shear stress in the plaques (31.9–136.09 dyn cm⁻²) was higher than that in normal vessels (1–10 dyn cm⁻²).²⁰¹ Hence, this enhanced shear stress presents opportunities for designing shear-stress-responsive nanomaterials for AS treatment.^{202, 203} Shear-stress-sensitive lenticular vesicles of an artificial 1,2-diaminophospholipid were constructed by Holme et al.²⁰⁴ for targeted drug delivery to AS plaques. The designed Pad–PC–Pad vesicles were stable under normal stress but could break down and release their loading cargos when subjected to higher shear-stress levels. This shear-stress-responsive changes could possibly be attributed to the lenticular morphologies of Pad–PC–Pad, leading to their instability along equator. However, the hemodynamic variations are influenced by factors such as the blood circulatory system, blood vessel bifurcations and arterial openings, which enquired that the shear-stress force should be sensitive enough to fight against these interfering factors and precisely control the drug release.²⁰⁵ From this point of view, shear-stress stimulus could be combined with other stimulus such as overexpressed ROS, realizing dual-responsive drug release and improving the therapeutic efficiency against AS.²⁰⁶

3.1.2 Imaging in CVDs

Owing to the slow progression and mild symptoms of CVDs, it is difficult to diagnose the pathological changes in time.²⁰⁷ In recent years, computed tomography (CT), MRI, fluorescence imaging, ultrasonography (US), and so on have been utilized as a real-time, accurate, and continuous auxiliary for CVDs diagnosis.^{140, 208-210} Numerous stimuli-responsive approaches have been developed for imaging AS plaques on the basis of their pathology like lipid droplets (LDs) and unregulated ROS levels,^141-143 which are considered as two hallmarks of AS. LDs in foam cells within AS lesions are formed by the triggering of oxidized low-density lipoprotein.²¹¹ Labeling LDs with fluorescence probes can help us to detect the AS plaques.²¹² Ye et al.¹⁴⁴ proposed a three-in-one fluorescent probe (iSHERLOCK) capable with dual-target sequentially activated properties for precise detection of AS plaques. LDs and hypochlorous acid (HClO, excessively produced in inflammatory diseases) were used as respective indicators for plaques detection. The iSHERLOCK probe MTB-B-CF₃ was prepared by combining LDs-responsive tracker (BODIPY) and HClO-responsive ligand (MTB). In the presence of LDs, MTB-B-CF₃ showed about 160-fold fluorescence enhancement. In contrast, there was no obvious fluorescence change in biological conditions. Likewise, upon addition of HClO, the maximal emission band of MTB-B-CF₃-LD mimetic (MTB-B-CF₃-LDM) shifted from 615 to 600 nm with increasing concentration of HClO (Figure 4C). It is worth mentioning that these processes could be completed within a few minutes. In vitro cells studies demonstrated that MTB-B-CF₃ was colocated with the LDs and HClO distribution in foam cells (Figure 4D). In the ApoE^−/−, increased fluorescence signal was observed in the AS plaques, further suggesting the potential application of iSHERLOCK for precise detection of AS lesions (Figure 4E).

It has been reported that approximately 90% of strokes are ischemic and caused by AS, and continuous ischemia will result in serious injury when untreated timely.¹⁹⁹ IS accounts for 85% of all strokes, which is caused by the abruption of cerebral perfusion.²¹³ When ischemia occurs, the pH at cerebral site decreased to 5.9, whereas the pH of ischemic core and peri-ischemia penumbra ranges from 6.0 to 6.9.²¹⁴ Besides, ROS is another biomarker in IS processes with altered expression levels.¹⁴⁵ The variations in pH and ROS levels between pathological and normal sites give an opportunity for designing pH and ROS-responsive nanomaterials for IS treatment.^{145, 147} Based on these characterizations of the IS microenvironment, Yang and coauthors¹⁴⁶ proposed a pH-responsive micelle encapsulated with Fe₃O₄ nanoparticles for the detection of cerebral ischemic areas. Ischemic tissues are more acidic pH than physiological tissues owing to the accumulation of lactic acid, thereby offering potential for the development of pH-responsive nanomaterials for sensitive imaging. After intravenous injection of Fe₃O₄-loaded mPEG-P(DPA-DE)LG micelles into the mice with middle cerebral artery occlusion (MCAO) pretreatment, the T₂-weighted MRI of ischemic section was performed on the ischemic section. The results showed that an obviously decreased signal was caught with pH-responsive micelles administration, which caused by the released Fe₃O₄ following pH-sensitive demicellization. Conversely, no signal decreases were observed in the nonresponsive group, proving the potentiality of pH-active MR diagnostic imaging in ischemic diseases.

3.1.3 Theranostics in CVDs

As CVDs progress and deteriorate, it could cause serious damages to life quality of patients. Although clinical imaging approaches provide moderate information on the severity of vascular stenosis, identifying high-risk lesions associated with CVDs remains challenging.²¹⁵ Hence, accurate diagnosis and effective therapy are important for risk stratification and reducing mortality rates related to CVDs.¹⁴⁸ Stimuli-responsive nanomaterials have garnered considerable attention in the field of the theranostics for CVDs, especially in AS, due to their on-demand release, lesion-targeting specificity, and excellent bioavailability.^{205, 216} After combination with imaging toolbox, these nanomaterials could precisely detect the pathological condition, as well as monitoring the drug targeting efficiency and accumulation.

β-CD serves as a lipid solubilizer for lipid removal of AS.²¹⁷ Ma et al.¹⁴⁸ constructed an MMP-9/ROS dual-responsive therapeutic complex (PLCDP@PMH) based on β-CD for both imaging and treatment of AS (Figure 4F). Liver X receptor ligand T0901317 was bridged with β-CD, and glucocorticoid prednisolone (Pred) was loaded into β-CD via host–guest interaction. After that, this LCDP complex was encapsulated with a photoacoustic imaging (PA) probe (PMeDTDPP-EDOT), followed by coating of oxidized HA, ROS-responsive polymer PMEMA and MMP-9 sensitive peptide for targeting and on-demand release in AS lesions. In vitro studies demonstrated that over 92% of Pred was released after 48 h, suggesting the excellent MMP-9/ROS dual-responsive ability of PLCDP@PMH (Figure 4G). Upon administration, PLCDP@PMH actively accumulated in the AS lesions based on the interactions between HA and overexpressed CD44 on endothelium cells. The overexpressed ROS and MMP-9 within plaques would trigger the breakage, disintegration, and subsequent release of cargos from PLCDP@PMH. In ApoE^−/− model, PA signals of the plaques were stronger than normal tissues, and the plaques formation and progression were efficiently inhibited, realizing in vivo diagnosis, plaque-formation inhibition and lipid removal for AS theranostics (Figure 4H).

In another study, Cheng and associates¹⁴⁷ developed a pH-responsive theranostic nanoplatform for the delivery of rapamycin (RAPA) in IS treatment. Ce6-labeled pH-sensitive segments (PDPA) self-assembled with RAPA and Gd³⁺ to form RAPA/Gd³⁺@NPs for dual-modal imaging including MRI and NIR fluorescence imaging. These nanomaterials were stable and self-quenched at physiological condition. Under pathological condition at pH 6.0, over 80% of RAPA was released within 4 h, revealing the pH-responsive property of RAPA/Gd³⁺@NPs. On the contrary, only 20% of Gd³⁺ was released within 24 h, which is mainly due to the strong chelation between Gd³⁺ and Ce6 molecules. In vitro MRI and fluorescence imaging study showed that the longitudinal correlation coefficient and fluorescence intensity of RAPA/Gd³⁺@NPs at pH 6.0 were higher than that at pH 7.4, demonstrating the acid-activated MR and fluorescence behavior of RAPA/Gd³⁺@NPs. Moreover, in vivo MR and fluorescence imaging were performed on transient MCAO rat models. The signal and fluorescence intensity in the right hemisphere of cerebral of mice injected with RAPA/Gd³⁺@NPs increased, indicating the efficient accumulation of these nanomaterials in the ischemic region. Besides, the released RAPA exerted excellent neuroprotective effects for IS treatment. Therefore, the use of RAPA/Gd³⁺@NPs nanomaterials offer a promising approach to tackle IS by theranostics.

Collectively, stimuli-responsive nanomaterials have demonstrated distinct advantages in drug delivery, imaging, and theranostics for CVDs by responding to both endogenous and exogenous stimulus. Especially, abnormal shear stress in AS gives potential of constructing shear-stress-responsive nanomaterials. However, the therapeutic outcomes of these nanomaterials may be further hindered by their sensitivity toward the diseased microenvironment, and discrepancies between animal models used for testing and human.

3.2.1 Drug delivery in cancer

Owing to the complexity of TME, a series of stimuli-responsive nanomaterials have been developed for drug delivery. Size-changeable strategy is designed for delivering drugs to achieve effective tumor targeting, accumulation and retention at tumor sites. The changeable size of these nanomaterials with either size-shrinkable or size-increasing property is mainly relied on their structural properties to respond to endogenous and exogenous stimulus.²²⁵ Cheng et al.¹⁶⁰ reported a size-changeable nanotheranostic agent (AtkCPTNPs) using polyprodrug-modified iron oxide nanoparticles (IONPs), which was prepared by assembly of pH-responsive polymer, ROS-responsive prodrug-modified IONPs, and a photosensitizer (Figure 5A). After passively targeting to the tumor site, this nanomaterial was aggregated under acidic environment (size growth from 90 to 300 nm). Then, the aggregates were transformed into small-sized IONPs under the stimuli effects of produced ROS with white light irradiation (size reduction to 17 nm) (Figure 5B). The programmed size changes could effectively enhance tumor retention and drug release and decrease the toxicity of nanotheranostic agent via accelerated elimination.

Stimuli-responsive nanocarriers have been extensively employed for drug delivery for cancer treatment. Methotrexate (MTX) is limited in poor solubility and weak targeting ability in clinical cancer treatment. To address these challenges, Dong et al.¹⁰⁷ proposed a hollow MnO₂ nanoparticles loaded with chemotherapeutic drug MTX. Opca, an outer membrane invasion protein for brain targeting, was modified on the surface of MnO₂ nanoparticles to form a bionic nanotherapeutic system (MTX@MnO₂–Opca) (Figure 5C). In TME-mimicking environment (pH 6.5), about 50% of MTX was released within 12 h, which was higher than that at pH 7.4 of 15% (Figure 5D). Besides, the MTX@MnO₂–Opca was catalyzed by H₂O₂ to produce O₂, confirming the effective H₂O₂-responsive ability of MnO₂. Therefore, MnO₂ nanocarriers exhibited good drug encapsulation, degradability, and TME-responsive properties, which could be further used as a potential nanomaterial for advanced drug delivery applications.

3.2.2 Imaging in cancer

Medical imaging technology is an associated strategy in clinical diagnosis and treatment, enabling real-time monitoring of the biological process of cancer and drug efficiency.^{18, 226, 227} Currently, diverse diagnostic modalities including optical imaging, MRI, and PA, have been proved their effectiveness in cancer imaging.

Optical imaging is commonly used imaging technique based on fluorescent agents, owing to its simplicity, high sensitivity, and intuitive results. But the strong and nonspecific binding with plasma proteins, nonspecific targeting, and unsatisfactory photostability of fluorescent agents hinder the broader their applications.^{228, 229} To solve these challenges, stimuli-responsive nanomaterials can improve the in vivo tumor-specific targeting ability and pharmacodynamics of fluorescent agents for cancer imaging. A recent study reported the use of γ-glutamyl traspeptidase (GGT) as a biomarker to develop GGT-responsive NIR nanomaterials (NRh-G-NPs), by conjugating GGT-specific substrate γ-glutamic acid (γ-Glu) with cyanine fluorophore (NRh-NH₂), which enables switchable fluorescence imaging and glioma therapy.¹⁵⁶ Triggered by the overexpressed GGT in the TME, nonfluorescent NRh-G-NPs were cut off to form NRh-NH₂-NPs and showed a strong fluorescence along with photothermal ability, suitable for both cancer diagnosis and photothermal therapy (PTT). This strategy offers potential benefits such as reduced toxicity toward normal tissues during the blood circulation. It was anticipated to improve the therapeutic efficiency to tumor, additionally facilitating on-demand fluorescence imaging.

Wang et al.¹⁵⁷ developed an H₂O₂-responsive nanoplatform derived from IONPs (IONPs–ICG–HA), which was decorated with ICG and HA via electrostatic interactions for multimodal imaging-guided cancer therapy (Figure 5E). In this nanoplatform, the combination of ICG and IONPs could not only improve the photostability of ICG, but also enhance the antitumor effect on H₂O₂-responsive triggering Fenton-reaction. Under upregulated H₂O₂ condition, IONPs exhibited improved catalytic activity to produce ∙OH. After administration for 4 h, both PA and fluorescence signals enhanced over time (Figure 5F). Besides, IONPs–ICG–HA owned superior light-to-heat conversion ability under laser irradiation, enabling in-depth imaging. The results showed that the temperature of tumor sites could reach a temperature of 50.9°C within 5 min under irradiation and exhibited excellent photothermal efficacy in vitro and in vivo experiments. This study presents a novel strategy for designing stimuli-responsive nanomaterials functionalized with PA/infrared thermal/fluorescence imaging.

3.2.3 Theranostics in cancer

As a novel technology in clinical cancer therapy, theranostics integrates diagnosis and therapeutic abilities into one system, generating real-time signals to promote the therapeutic processes of cancer treatment.²³⁰ Early diagnosis, timely therapy, and real-time monitoring are crucial in the fight against cancer. It has been reported that TME-responsive nanomaterials in theranostics could specifically amplify imaging signals, enhance drug accumulation at tumor sites, and promote deep penetration.^{231, 232}

CDs are enzymatically degraded products possessing highly branched hydroxyl moieties and hydrophobic cavities. As a building block, CDs can be used in the preparation of stimuli-responsive CD-based nanomaterials through incorporating ideal responsive functionalities. Stimuli-responsive CD-based nanoplatforms for cancer theranostics, including pH-responsive, redox-responsive, enzyme-responsive, and so on, have been summarized by Yao and associates.²³³ Through stimuli-responsive design, CD-based nanoplatforms can provide sensitive and real-time imaging of the biodistribution of drugs and realize imaging-guided cancer treatment.

In another study, a pH/GSH/glucose-responsive nanozyme was designed by preparing a bismuth-manganese core–shell nanoflower loading with glucose oxide (BDS–GO_x@MnO_x) (Figure 5G).¹⁵⁹ The MnO_x shell could serve as a “smart switch” with endogenous microenvironment-responsive properties, including glucose, H₂O₂, and GSH-responsiveness. Under weak acidic environment at pH 6.5, about 80% of GO_x was released within 48 h, but only 21% of GO_x at pH 7.4. The similar acidic-dependent release performance was observed in the Mn releasing profiles. As an effective catalytic enzyme, GO_x could exhaust glucose in the TME to produce H₂O₂ via starvation therapy. Under glucose condition, the consumption of glucose gradually enhanced under pH-activated effects at pH 6.5 compared with pH 7.4, confirming its efficacy. Besides, the H₂O₂ and GSH dual-responsive properties of MnO_x further enhanced the generation of ∙OH and exhaustion of GSH. Both in vitro and in vivo results suggested that BDS–GO_x@MnO_x nanozymes could enhance the antitumor effects (Figure 5H). Furthermore, owing to the high X-ray attenuation coefficient of bismuth and longitudinal relaxivity of Mn, BDS–GO_x@MnO_x nanozymes could serve as a TME-responsive agent for both CT and MRI imaging (Figure 5I). In conclusion, this study provided a potential strategy that is multiresponsive for cancer diagnosis and treatment.

Owing to the complex characteristics exhibited by TME, such as changes in pH levels, oxidative stress, and protein expression patterns, stimulus-responsive nanomaterials have been widely applied in cancer therapy. However, there are a few challenges need to be addressed: (1) due to the tumor heterogeneity and mutative TME, used tumor models are drastically differ from patients with cancer; (2) the potential toxicity and long-term biosafety of these nanomaterials are mainly relied on their targeting capabilities.

3.3.1 Drug delivery in neurological disorders

Epilepsy is a common neurological disorders characterized by recurrent seizures, affecting approximately 10 million people in China.²⁴⁴ The microenvironment of epilepsy can be described as excitotoxicity of glutamate and oxidative environment, as well as abnormal electrical activity of neurons. Based on the specific property of abnormal electrical activity, Ying et al.¹⁶¹ and Wang et al.²⁴⁵ developed electro-responsive hydrogen nanoparticles (ANG–PHT–ERHNPs) for targeted delivery and on-demand drug release of phenytoin sodium (PHT), a conventional antiepileptic drug, into the brain for epilepsy treatment (Figure 6A). The electro-responsive nanoparticles (ERHNPs) were synthesized through a soap-free emulsion copolymerization strategy (Figure 6B). Under an electric field, ERHNPs could swell owing to the ionization of sulfonate groups in their structure, resulting in increased particle size from 102.3 ± 16.8 to 388.0 ± 20.4 nm under the current of 500 μA within 1 min. In recent decades, PHT has been replaced as first-line medication for epilepsy treatment due to its potential side effects at high dosage. Hence, specifical targeting of PHT to seizure focus is essential for improving antiepilepsy efficiency. After being loaded into ERHNPs, the release rate of PHT increased from 34.6 to 87.3% triggered by a current of only 200 μA (Figure 6C). These results suggested that these electro-responsive ANG–ERHNPs have the capability for on-demand drug release, while avoiding premature leakage.

Moreover, the biosafety of nanomaterials is of importance for drug delivery. Inspired by the electrical stimuli of epilepsy, Wu et al.¹⁶² reported a nanoengineered on-demand drug delivery nanosystem capable of excellent electric-responsiveness and biocompatibility for epilepsy therapy. This nanosystem was fabricated through one-pot copolymerization of pyrrole and dopamine to form conductive polymer (PPY)–functionalized PDA nanomaterials (PPY–PDA). PHT and ANG were encapsulated and modified onto the PPY–PDA hybrid materials as antiepileptic drug and brain-targeting peptide, respectively. The electrical conductivity of PPY was significantly enhanced with a dopamine mass ratio of 5%. In vitro drug release study showed the electric-stimuli drug release behavior in response to the intermittent discharges. Interestingly, after three “on-off” cycle of electric stimulus, PPY–PDA–PHT exhibited obvious drug release during the “on” state, while showing negligible drug leaking at “off” state, further implying their advantages of electro-responsiveness in epilepsy treatment.

Micelles, consisting of a hydrophobic core and hydrophilic shell, have been widely utilized as nanocarriers for drug delivery, which can greatly improve the aqueous solubility of hydrophobic and insoluble drugs.²⁴⁶ In recent years, stimuli-responsive micelles have gained considerable attention in the field of neurological disorders. For instance, Lu et al.¹⁶³ constructed a ROS-responsive polymeric micelle system (Ab–PEG–LysB/CUR) for AD treatment, by using an amphiphilic polymer (poly(ethylene glycol)(PEG)–LysB) as the ROS-responsive section. After incubation with H₂O₂, these micelles were disassociated with irregular size distribution, which might be attributed to their conversion from amphiphilic to hydrophilic state in response to the oxidative environment associated with AD.²⁴⁷

3.3.2 Imaging in neurological disorders

With the development of imaging technology, the clinical imaging of neurological disorders is largely hampered by the protection of BBB and the depth and accuracy of imaging.²⁴⁸ Hence, it is imperative to design nano-imaging agents with high resolution and contrast, responsiveness to pathological environments, as well as BBB-transporting ability for precise imaging of neurological disorders.

AD is one of the most occurring neurodegenerative disease characterized by progressive decline and cognitive capacity.^{249, 250} It has been reported that the concentration of metal ions (such as Cu²⁺, Fe³⁺, Al³⁺, etc.) in the AD environment is higher than that in normal tissue. Hence, detection of excessive metal ions holds potential for diagnosing and understanding the pathological conditions of AD.²⁵¹ For instance, Wang et al.¹⁶⁴ designed an activable PA probes (RPS1) for visualization of Cu²⁺ in AD brains (Figure 6D). As a small molecule, RPS1 with electron-donating groups showed the longest absorption wavelength at 713 nm after specifically chelation with Cu²⁺ (Figure 6E). In vitro competition experiment showed that RPS1 displayed superior selectivity toward Cu²⁺ over other metal ions such as Fe²⁺, Fe³⁺, and Ni²⁺ and exhibited the strongest PA signal (Figure 6F). Inspired by these favorable results obtained from RPS1, in vivo PA imaging was demonstrated on AD mice (Figure 6G). Weak PA signals were observed in both normal mice brains following RPS1 injection, and AD mice treated with PBS. But a strong PA signal was captured specifically in the cortex region of AD mice after RPS1 treatment, proving that RPS1 could selectively detect Cu²⁺ concentration in the brain of AD mice through PA imaging.

Amyloid-β (Aβ) plaques, assembled by Aβ monomers, are another pathological hallmark of AD, which will deposit to induce neurological disorders.²⁵² Hence, the detection of Aβ plaques is another approach for predicting AD progression. Mao et al.¹⁶⁵ reported an activatable NIR-II fluorescent probes (DMP) for specifically binding to Aβ plaques. The fluorescence intensity of DMP was activated after incubation with Aβ fibrils, whereas no obvious change in fluorescence signal was observed with Aβ monomers or oligomers. Furthermore, both in vitro and in vivo NIR-II fluorescence imaging showed the excellent Aβ-activatable property of DMP. Notably, the fluorescence signal of DMP could be retained in the brain on AD mice reached up to 60 min, enabling real-time longitudinal imaging in vivo.

3.3.3 Theranostics in neurological disorders

MnO₂ nanomaterials are one kind of the star biomaterials in the field of theranostics for neurological disorders, particularly in GBM treatment, due to their remarkable pH, ROS, and GSH-responsive properties. The released Mn²⁺ can serve as MRI contrast agents for monitoring the drug delivery process, as well as enhancing therapeutic efficacy. Tan et al.¹⁶⁶ designed a theranostic nanomaterial iRPPA@TMZ/MnO by loading drugs into MnO nanocarriers, and encapsulated them within polymeric micelles for glioma treatment. After systematic administration, iRPPA@TMZ/MnO can cross the BBB to reach the glioma site with the guidance from iRGD peptide targeting ligands. Upon excessive H₂O₂ levels at TME, iRPPA@TMZ/MnO will discompose to release TMZ, Mn²⁺, and O₂, thereby improving therapeutic effects against glioma. Additionally, iRPPA@TMZ/MnO exhibited TME-activatable T₁-weighted signal intensity under the trigger of acid pH and H₂O₂. After administration, the MRI signal in glioma site was stronger than normal site, indicating efficient tumor-targeting and TME-responsive properties of iRPPA@TMZ/MnO.

In our previous work, Zhang et al.¹⁶⁷ fabricated an electro-responsive nanomaterial by assembly Fc-conjugated D-a-tocopherol polyethylene glycol succinate and amphiphilic Poloxamer 407, and encapsulating fluorescent dye Cy5.5 or antiepileptic drugs (TFP@cargo) for epileptic foci imaging and antiepileptic treatment (Figure 6H). The ex vivo fluorescence imaging showed that higher fluorescence intensity was observed on the epileptic foci compared with that on contralateral side in epilepsy mice (Figure 6I). Moreover, no obvious difference was obtained between both sides of the normal mice group. Besides, three antiepileptic drugs were utilized as substitutes for Cy5.5 in epilepsy treatment. The results showed that TFP micelles could improve the antiepileptic efficacy of drugs in two epilepsy models (Figure 6J,K). Combined with the results of electro-responsive drug release, TFP@cargo could realize precise imaging of pathological sites, electro-responsive drug release and enhanced antiepileptic effects.

The BBB, which serves as a protective shield against toxins and pathogen, poses a significant impediment to the delivery of nanomaterials for neurological disorders therapy. Therefore, improved BBB transport efficiency is imperative for enhancing the therapeutics of neurological disorders.

3.4.1 Drug delivery in inflammation

IBD is a chronic and nonspecific inflammation that influence the mucosa and submucosa of the large intestine. Aminosalicylates, steroids, immunosuppressants and biological agents are commonly used therapeutics in IBD therapy.¹⁶⁸ As a synthetic steroid, budesonide (BDS) is applied in a number of inflammatory diseases. In order to solve the problem of poor solubility and quick elimination of BDS, Sun et al.¹⁶⁸ proposed a pH/redox dual-responsive nanomaterial (ATP–CMI) for efficient BDS delivery. This nanomaterial was obtained by self-assembly of carboxymethyl inulin (CMI) modified with 4-aminothiophenol (4-ATP) derivatives via aromatic moieties. In vitro drug release results showed that more than 80 wt% BDS was released in the presence of GSH at pH 6.0, which was higher than in the GSH-free physiological conditions. The mechanism of pH/ROS-responsive manner might be due to the cleavage of disulfide bonds within the nanomaterial structure. The treatment efficiency against colitis was investigated using a dextran sulfate sodium (DSS)-induced colitis model. Following BDS-NPs treatment, the mice displayed reduced colon weight-to-length ratio, less weight loss, and decreased histological scoring compared with PBS-treated mice and BDS-sus-treated mice. These improvements might be attributed to targeted accumulation of BDS-NPs at inflammatory regions, followed by the pH/ROS-responsive drug release for enhanced drug delivery.

Periodontitis is a chronic inflammatory disorder, affecting approximately 90% of population worldwide. Hydrogels are polymeric networks of hydrophilic polymer chains with high swelling and distending properties. Yu et al.¹⁶⁹ designed a pH-responsive chitosan-based hydrogel loaded with N-phenacylthiazolium bromide (PTB). In vitro drug release results showed that more PTB could be released at pH 5.5–6.5 compared with pH 7.4, demonstrating the excellent pH-responsive ability of this hydrogel formulation. Besides, inflammatory cell infiltration and the loss of collagen matrix were distinctively reduced both in the induction and recovery phases. Moreover, this chitosan-based hydrogel, with simple synthesis and good stimuli-responsive property, is a promising nanomaterial for targeted drug delivery and on-demand drug release in the treatment of inflammatory disorders.

3.4.2 Imaging in inflammation

As an endogenous gas molecule, hydrogen sulfide (H₂S) is intricately associated with a variety of diseases, such as inflammation, cancer, gastrointestinal, and so on.^254-256 H₂S is a double-edged sword with protection or damage function against pathological processes. Monitoring H₂S levels in the microenvironment is essential for comprehending pathological conditions. Liu et al.¹⁷⁰ reported an innovative NIR-II luminescent strategy by introducing absorption competition-induced effect for real-time imaging of hepatic inflammation. The NIR-II luminescence probe (1-PEI-DCNPs) was synthesized by conjugation of H₂S-responsive chromophore with NIR-II luminescent lanthanide nanoparticles using polyethylenimine (PEI). In the presence of H₂S, the absorption peak at 820 nm of chromophore decreased, whereas the NIR-II luminescence of nanoparticles at 1060 nm gradually recovered within 10 min, suggesting the good H₂S-responsive property of 1-PEI-DCNPs probe. Compared with NIR-I, NIR-II irradiation possesses longer wavelength, and therefore exhibit higher tissue penetration.²⁵⁷ Lipopolysaccharide-induced liver inflammation was utilized as an ideal model to investigate the effectiveness for in vivo H₂S detection. The results demonstrated that 1-PEI-DCNPs could sensitively reflect the damage and recovery of liver by H₂S imaging, providing a distinct approach for detecting liver injury.

In another study, a chitosan-based pH-responsive nanomaterial was prepared by Jing et al. for acute pancreatitis (AP) imaging.¹⁷¹ The bimetallic ions of Ce and Ga were doped into carbon dots through the hydrothermal method. After that, chitosan was applied as a responsive nanocarrier for loading Ce/Gd–carbon dots and resveratrol (RES) loading. Encouraged by the pH-responsive properties of chitosan, the carbon dots/RES@CS NPs (CRCS) could achieve responsive cargo release at inflammatory sites. The released carbon dots and Ce/Gd were employed for fluorescence and MR imaging, respectively, demonstrating the potential of CRCS in dual-modal inflammation imaging.

3.4.3 Theranostics in inflammation

Osteoarthritis (OA) is considered as the most common chronic joint disease characterized by acid environment and overexpressed MMP-13.^258-261 Based on the unique microenvironment of OA, Lan et al.¹⁷² developed a pH/MMP-13 dual-responsive nanoplatform for OA theranostics. This nanoplatform was constructed by conjugating a pH-responsive polymer (PPL) with an MMP-13-responsive peptide (H₂N–GPLGVRGC–SH). To achieve fluorescence imaging, Cy5.5 dye was labeled onto the nanoplatform, and coupled with black hole quencher-3 to effectively quench its fluorescence signal. The obtained pH/MMP-13 dual-responsive nanoplatform (MRC–PPL@PSO) exhibited excellent stability under physiological environment. Upon exposure to the acidic condition of OA, the nanoplatform was disassembled to release encapsulated drugs for anti-inflammatory therapy. Besides, the overexpressed MMP-13 in OA microenvironment cleaved the MMP-13-responsive peptide on the nanoplatform, triggering Cy5.5 release and subsequent fluorescence activation. Collectively, MRC–PPL@PSO has excellent dual-responsiveness and potent antiarthritic effects.

Compared with traditional single-photon fluorescence imaging, two-photon excited fluorescence technology demonstrates superior dimensional resolution and enhanced tissue penetration capabilities.^{262, 263} Another theranostic compound has been constructed by bridging a two-photon AIE fluorophore and anti-inflammatory Pred via a ROS-responsive linker.¹⁷³ After packaged into polymeric micelles, the obtained TPP@PMM was capable with two-photon bioimaging and ROS-responsive release properties. After administration, a more obvious lesion of arthritis could be obtained using two-photon confocal images than that of single-photon imaging, proving that TPP@PMM is adaptable for two-photon bioimaging and antiarthritis treatment, as well as theranostics for AS.

The acid pH and elevated ROS levels within the inflammatory microenvironment are commonly utilized triggers for stimuli-responsive nanomaterials construction. However, it is crucial to address bacterial infection that often accompany inflammation.¹⁹

3.5.1 Drug delivery in bacterial infection

Stimuli-responsive nanomaterials have been widely utilized in drug delivery in bacterial infection owing to their ability to achieve on-demand drug release and targeted properties at the infection regions.²⁷¹ Drugs can be encapsulated in nanocarriers to improve their local concentrations and minimize systematic side effects. Bacteria infected microenvironment is considered to be acidic with the pH values ranging from 5.0 to 5.5 caused by the anaerobic glycolysis during metabolism.²⁷² Hence, the slight acidic conditions associated with bacterial infections can serve as a switch for drug delivery. The design of pH-responsive nanomaterials can also be realized through pH-sensitive chemical bonds or linkers, pH-triggered charge conversion, and pH-responsive carriers.²⁷³ Sun et al.¹⁷⁴ constructed a pH-responsive antibacterial surface by coating polycationic polymer brushes and forming pH-sensitive enamine bonds via yne-amine click reaction. Once bacteria colonize on the surface of this nanomaterial, the pH-sensitive bond would be broken down in response to local acid conditions leading to the exposure of polycations that exert antibacterial effects. In contrast, bacteria thrived wherever on the model strain of Staphylococcus aureu (S. aureus) or Escherichia coli (E. coli) in pH 7.4 conditions.

Methicillin-resistant Staphylococcus aureus (MRSA) biofilms-associated infection poses a significant threat to human health. As a defensive barrier, MRSA biofilms may undermine the penetration of antibiotics and cause local inflammation. To address this problem, Zhang et al.¹⁷⁵ designed a pH-responsive nanomaterial loaded with ICG as the photosensitizer (ICG–ZnS NPs). Upon acidic environment, ZnS NPs would decompose and release Zn²⁺ and H₂S for cell metabolism prevention and gas therapy, respectively. Combined with ICG-mediated hyperthermia therapy, this nanomaterial exhibit enhanced antibacterial efficacy both in vitro and in vivo, suggesting that ICG–ZnS NPs have great potentials as pH-responsive nanomaterials for MRSA biofilms and wound infection eradiation.

3.5.2 Imaging in bacterial infection

To effectively combat bacterial infection, it is of importance to timely diagnose and monitor infectious process. Nanomaterials integrating rapid and noninvasive diagnosis strategies have been developed for antibacterial therapy.²⁷⁴ Compared with “always-on” nanomaterials, “turn-on” nanomaterials triggered by bacterial microenvironment or exogenous stimulus can activate their own signal output for precise diagnosis.²⁷⁵ A variety of imaging strategies, such as MRI, CT, and US, have been developed to accurately distinguish bacterial infections in clinic.^{276, 277}

As a noninvasive imaging approach, MRI has been exploring for the diagnosis of bacterial infection, especially utilizing distance-dependent magnetic resonance tuning (MRET)-based MRI, which can be activated by pathological changes. Inspired by the activation properties of MRET, Li et al.¹⁸⁰ constructed a pH-responsive MRET MRI probe (MDVG-1) through the self-assembly of paramagnetic enhancer (Gd–DNA₃–Gd) and superparamagnetic quencher (MDV). The T₁-weighted signal was quenched in physiological environment. Under the acidic condition infected by bacteria, MDMG-1 would disassemble into monomers, resulting in an increase in T₁-weighted signal (r₁) of Gd–DNA₃–Gd from 5.13 at pH 7.4 to 35.2 mM⁻¹ s⁻¹ at pH 5.5. In the meantime, the T₂-weighted signal (r₂) of MDMG-1 decreased from 187.1 to 86.2 mM⁻¹ s⁻¹. In vivo MRI images showed that MDVG-1 had good accuracy for S. aureus-infected site imaging. These results revealed that MDMG-1 owned switchable abilities from T₂ to T₁-weighted imaging under the bacterial infection, providing a potential approach for accurately diagnosing bacterial infection.

In another study, Li et al. proposed a peptide-modified MRET probe (MPD-1) via assembly of MNPs with Gd³⁺ modified MMP-2 responsive peptide and bacteria-targeting peptide.¹⁸¹ After exposure to MMP-2 at the bacterial infection site, MPD-1 disassembled into MNPs and Gd³⁺, and the r₂/r₁ ratio was decreased from 46.14 to 2.33, suggesting the switchable properties of MPD-1 from T₂ to T₁-weighted imaging owing to its response toward MMP-2. Furthermore, after administration of MPD-1 into S. aureus-infected mice, the T₁-weighted signal in infection sites was significantly increased, whereas no obvious change in the sterile inflammation, demonstrating the excellent MMP-2 responsive properties of MPD-1.

3.5.3 Theranostics in bacterial infection

Theranostics integrates real-time diagnosis and in situ therapy into a single system for rapid, effective, and noninvasive detection and treatment.²⁷⁸ Bacterial biofilms, which are more resistant to antibiotics, can form during the development of bacterial infection.^279-281 Hence, detection and eradication of bacteria in time could avoid the formation of biofilms, reduce the drug-resistant of bacteria, and enhance antibacterial efficiency.^{230, 282} Liu et al.¹⁸² constructed a pH-responsive theranostic nanomaterial through self-assembly of poly(poly(ethylene glycol) methyl ether methacrylate)-b-poly(2-(diisopropylamino) ethyl methacrylate-co-2-hydroxyethyl methacrylate)–chlorin e6 (PPEGMA-b-P(DPA-co-HEMA)–Ce6). Under pH 6.0 condition, PPEGMA-b-P(DPA-co-HEMA)–Ce6 NPs underwent a charge conversion from negative of −1.45 mV at pH 7.4 to highly positive of +11.6 mV, which could be ascribed to the protonation of PDPA ligand. The nanoparticle size also increased from 78 to 90 nm with the decrease of pH value. In addition, the pH-dependent interaction of nanoparticles with E. coli bacteria has been investigated via flow cytometry. The results showed that higher fluorescence signals could be observed after NPs treatment under pH 6.2, whereas no obvious fluorescence change was obtained after free Ce6 incubation at pH 6.0–7.4. These results demonstrated that PPEGMA-b-P(DPA-co-HEMA)–Ce6 NPs exhibited excellent pH-responsive properties for on-demand binding with E. coli and fluorescence imaging. Besides, owing to the synergetic effects of the cationic property of P(DPA-co-HEMA)–Ce6 and Ce6-mediated PDT, PPEGMA-b-P(DPA-co-HEMA)–Ce6 NPs demonstrated effective antibacterial activity in an acidic environment.

Aggregation-induced emission (AIE)-based nanomaterials are capable of overcoming aggregation-caused quenching effect of traditional fluorescence probes, thereby enabling ROS generation for bacterial imaging and ablation.²⁸³ In order to specifically track the in vivo process of bacteria and precisely kill bacteria, H₂O₂-responsive MIL-100 (Fe) NPs were utilized for delivering 3-azido-_D-alanine (_D-AzAla).¹⁸³ In the presence of H₂O₂ (50 μM) simulated inflammatory environment of bacteria, about 60% of _D-AzAla was released within 10 h, which was higher than that incubated with low H₂O₂ levels of 0.5 μM in physiological condition. The TEM images and increased size of NPs also illustrated the H₂O₂-responsive properties of NPs. After that, AIE photosensitizer PS NPs were injected to react with bacteria through in vivo click chemistry. The released _D-AzAla selectively integrated into the bacteria and exhibited fluorescence upon reaction with DBCO-Cy5 for fluorescence imaging. With the assistance of PDT, the number of bacteria was sharply reduced. This study provided a bacteria metabolic labeling strategy for timely diagnosis and precise antibacterial therapy.

The applications of stimuli-responsive nanomaterials displayed great promise in antibacterial therapy, including ameliorating the resistance and sides effects of antibiotics. However, these nanomaterials are still in their early stages, necessitating the resolution of several challenges including technology limitations, appropriate antibiotic selection, and nanomaterial toxicity.²⁶⁹