Tunable, Ultrasensitive pH-Responsive Nanoparticles Targeting Specific Endocytic Organelles in Living Cells

The Dynamic Range of Acidity: Tracking Rules for the Unidirectional Penetration of Cellular Compartments

Labeled ammonium cations with pK_a∼7.4 accumulate in acidic organelles because they can be neutralized transiently to cross the membrane at cytosolic pH 7.2 but not at their internal pH<5.5. Retention in early endosomes with less acidic internal pH was achieved recently using weaker acids of up to pK_a 9.8. We report here that primary ammonium cations with higher pK_a 10.6, label early endosomes more efficiently. This maximized early endosome tracking coincides with increasing labeling of Golgi networks with similarly weak internal acidity. Guanidinium cations with pK_a 13.5 cannot cross the plasma membrane in monomeric form and label the plasma membrane with selectivity for vesicles embarking into endocytosis. Self‐assembled into micelles, guanidinium cations enter cells like arginine‐rich cell‐penetrating peptides and, driven by their membrane potential, penetrate mitochondria unidirectionally despite their high inner pH. The resulting tracking rules with an approximated dynamic range of pK_a change ∼3.5 are expected to be generally valid, thus enabling the design of chemistry tools for biology research in the broadest sense. From a practical point of view, most relevant are two complementary fluorescent flipper probes that can be used to image the mechanics at the very beginning of endocytosis.

A pH-Responsive Nanoparticle Library with Precise pH Tunability by Co-Polymerization with Non-Ionizable Monomers

Precise monitoring of the subtle pH fluctuation during biological events remains a big challenge. Previously, we reported an ultra-pH-sensitive (UPS) nanoprobe library with a sharp pH response using co-polymerization of two tertiary amine-containing monomers with distinct pK_a . Currently, we have generalized the UPS nanoparticle library with tunable pH transitions (pH_t ) by copolymerization of a tertiary amine-containing monomer with a series of non-ionizable monomers. The pH_t of nanoparticles is fine-tuned by the non-ionizable monomers with different hydrophobicity. Each non-ionizable monomer presents a constant contribution to pH tunability regardless of tertiary amine-containing monomers. Based on this strategy, we produced two libraries of nanoprobes with continuous pH_t covering the entire physiological pH range (5.0-7.4) for fluorescent imaging of endosome maturation and cancers. This generalized strategy provides a powerful toolkit for biological studies and cancer theranostics.

Harnessing Thorpe-Ingold Dialkylation to Access High-Hill-Percentage pH Probes

Sensitivity is an important parameter for a molecular probe. Hill-type pH probes exhibit improved detection sensitivity compared to the traditional pH probes following the Henderson-Hasselbalch equation. Exploiting positive cooperativity, we recently devised a novel molecular scaffold (PHX) to offer such an unconventional Hill-type pH titration profile. We previously confirmed that PHX is not a pure Hill-type probe yet. Only 64% of its absorbance/fluorescence turn-on is the result of a Hill-type pathway. The remaining 36% is from an undesired Henderson-Hasselbalch-type pathway (HH pathway). In this work, the Thorpe-Ingold dialkylation was harnessed to further suppress the percent contribution of the HH pathway down to 16%. We also propose that PHX is a viable molecular model for assessing the efficacy of the steric compressing effect induced by different Thorpe-Ingold dialkylations.

A Cationic Micelle as In Vivo Catalyst for Tumor-Localized Cleavage Chemistry

The emerging strategies of accelerating the cleavage reaction in tumors through locally enriching the reactants is promising. Yet, the applications are limited due to the lack of the tumor-selectivity for most of the reactants. Here we explored an alternative approach to leverage the rate constant by locally inducing an in vivo catalyst. We found that the desilylation-induced cleavage chemistry could be catalyzed in vivo by cationic micelles, and accelerated over 1400-fold under physiological condition. This micelle-catalyzed controlled release platform is demonstrated by the release of a 6-hydroxyl-quinoline-2-benzothiazole derivative (HQB) in two cancer cell lines and a NIR dye in mouse tumor xenografts. Through intravenous injection of a pH-sensitive polymer micelles, we successfully applied this strategy to a prodrug activation of hydroxyl camptothecin (OH-CPT) in tumors. Its "decaging" efficiency is 42-fold to that without cationic micelles-mediated catalysis. This micelle-catalyzed desilylation strategy unveils the potential that micelle may act beyond a carrier but a catalyst for local perturbing or activation.

Probing Dynamic Features of Phagosome Maturation in Macrophage using Au@MnOx @SiO2 Nanoparticles as pH-Sensitive Plasmonic Nanoprobes

Phagosome maturation in macrophage is essential to the clearance of pathogenic materials in host defence but the dynamic features remain difficult to be measured in real time. Herein, we reported the multilayered Au@MnO_x @SiO₂ nanoparticle as a robust pH-sensitive plasmonic nanosensor for monitoring the dynamic acidification features over the phagosome maturation process in macrophage under darkfield microscopy. For this multilayered nanosensor, the gold nanoparticle core plays a role of signal reporter, the MnO_x shell and the outmost SiO₂ act as the sensing layer and the protecting layer, respectively. After subject to the acidic buffer solution, the MnO_x layer in the multilayered nanoprobe could be decomposed rapidly, resulting in a remarkable spectral shift and color change under darkfield microscopy. We demonstrated this nanosensor for the investigation of single phagosome acidification dynamics by monitoring the color changes of nanoprobes after phagocytosis over time. The nanoprobes after phagocytosized in macrophage displayed a slight color change within the first hour and then cost several minutes to change from red to green in the next stage, indicating the phagosome undergoes a slow first and then fast acidification feature as well as a slow-to-fast acidification translation over the phagosome maturation process. Moreover, we validated that the slow-to-fast acidification translation was dependent on the activation of V-ATPase from the ATP depletion assay. We believed that this nanosensor is promising for studying the dynamic acidification features as well as disorders in phagosome maturation in phagocytic cells, which might provide valuable information for understanding the disease pathogenesis related to phagosome dysfunctions.

Constructing Adaptive Photosensitizers via Supramolecular Modification Based on Pillararene Host-Guest Interactions

In order to promote the development of photodynamic therapy (PDT), undesired side effects like low tumor specificity and the "always-on" phenomenon should be avoided. An effective solution is to construct an adaptive photosensitizer that can be activated to generate reactive oxygen species (ROS) in the tumor microenvironment. Herein, we design and synthesize a supramolecular switch based on a host-guest complex containing a water-soluble pillar[5]arene (WP5) and an AIEgen photosensitizer (G). The formation of the host-guest complex WP5⊃G quenches the fluorescence and inhibits ROS generation of G. Benefitting from the pH-responsiveness of WP5, the binding site between G and WP5 changes in an acidic environment through a shuttle movement. Consequently, fluorescence and ROS generation of the host-guest complex can be switched on at pH 5.0. This work offers a new paradigm for the construction of adaptive photosensitizers by using a supramolecular method.

Renal Clearable Luminescent Gold Nanoparticles: From the Bench to the Clinic

With more and more engineered nanoparticles (NPs) being translated to the clinic, the United States Food and Drug Administration (FDA) has recently issued the latest draft guidance on nanomaterial-containing drug products with an emphasis on understanding their in vivo transport and nano-bio interactions. Following these guidelines, NPs can be designed to target and treat diseases more efficiently than small molecules, have minimum accumulation in normal tissues, and induce minimum toxicity. In this Minireview, we integrate this guidance with our ten-year studies on developing renal clearable luminescent gold NPs. These gold NPs resist serum protein adsorption, escape liver uptake, target cancerous tissues, and report kidney dysfunction at early stages. At the same time, off-target gold NPs can be eliminated by the kidneys with minimum accumulation in the body. Additionally, we identify challenges to the translation of renal clearable gold NPs from the bench to the clinic.

Highly Sensitive Hill-Type Small-Molecule pH Probe That Recognizes the Reversed pH Gradient of Cancer Cells

A hallmark of cancer cells is a reversed transmembrane pH gradient, which could be exploited for robust and convenient intraoperative histopathological analysis. However, pathologically relevant pH changes are not significant enough for sensitive detection by conventional Henderson-Hasselbalch-type pH probes, exhibiting an acid-base transition width of 2 pH units. This challenge could potentially be addressed by a pH probe with a reduced acid-base transition width (i.e., Hill-type probe), appropriate p K_a, and membrane permeability. Yet, a guideline to allow rational design of such small-molecule Hill-type pH probes is still lacking. We have devised a novel molecular mechanism, enabled sequential protonation with high positive homotropic cooperativity, and synthesized small-molecule pH probes (PHX1-3) with acid-base transition ranges of ca. 1 pH unit. Notably, PHX2 has a p K_a of 6.9, matching the extracellular pH of cancer cells. Also, PHX2 is readily permeable to cell membrane and allowed direct mapping of both intra- and extracellular pH, hence the transmembrane pH gradient. PHX2 was successfully used for rapid and high-contrast distinction of fresh unprocessed biopsies of cancer cells from normal cells and therefore has broad potentials for intraoperative analysis of cancer surgery.

Smart Self-Assembled Nanosystem Based on Water-Soluble Pillararene and Rare-Earth-Doped Upconversion Nanoparticles for pH-Responsive Drug Delivery

Exploring novel drug delivery systems with good stability and new structure to integrate pillararene and upconversion nanoparticles (UCNPs) into one system continues to be an important challenge. Herein, we report a novel preparation of a supramolecular upconversion nanosystem via the host-guest complexation based on carboxylate-based pillar[5]arene (WP5) and 15-carboxy-N,N,N-trialkylpentadecan-1-ammonium bromide (1)-functionalized UCNPs to produce WP5⊃1-UCNPs that can be loaded with the chemotherapeutic drug doxorubicin (DOX). Importantly, the WP5 on the surface of the drug-loaded nanosystem can be efficiently protonated under acidic conditions, resulting in the collapse of the nanosystem and drug release. Moreover, cellular uptake confirms that the nanosystem can enter human cervical cancer (HeLa) cells, resulting in drug accumulation in the cells. More importantly, cytotoxicity experiments demonstrated the excellent biocompatibility of WP5⊃1-UCNPs without loading DOX and that the nanosystem DOX-WP5⊃1-UCNPs exhibited an ability of killing HeLa cells effectively. We also investigated magnetic resonance imaging and upconversion luminescence imaging, which may be employed as visual imaging agents in cancer diagnosis and treatment. Thus, in the present work, we show a simple yet powerful strategy to combine UCNPs and pillar[5]arene to produce a unified nanosystem for dual-mode bioimaging-guided therapeutic applications.

pH-Responsive Natural Polymeric Gene Delivery Shielding System Based on Dynamic Covalent Chemistry

A novel pH-responsive system based on aldehyde-bearing dextran derivatives (ODEX or FDEX) was designed to use as gene carrier shielding. Through pH sensitive Schiff base bonds between amino groups of PEI (in PEI/DNA polyplex) and aldehyde groups of dextran derivatives, PEI/DNA polyplex could be shielded and further condensed to obtain an effectively decreasing ζ-potential with smaller size. Schiff base bonds were pH-responsive, which were relatively stable in neutral environment but were deformed in slightly acidic and acidic environments. Through use of this characteristic, the PEI/DNA polyplex was effectively shielded during circulation in the body, and upon arrival at the tumor, the slightly acid pH triggered the breaking of Schiff base bonds to expose the positive PEI/DNA polyplex, which further interacted with tumor cell membranes, achieving efficient gene expression. Use of such characteristics could effectively address the high transfection efficiency versus stability dilemma of gene carriers. FDEX/PEI/DNA nanoparticles not only mediate higher cellular uptake and transfection efficiency in vitro but also effectively accumulate in tumors with gene expression in vivo higher than that of the ODEX analogues. As a result, this pH-responsive system is a promising strategy for cancer therapy.

Lanthanide-Based T2ex and CEST Complexes Provide Insights into the Design of pH Sensitive MRI Agents

The CEST and T1 /T2 relaxation properties of a series of Eu3+ and Dy3+ DOTA-tetraamide complexes with four appended primary amine groups are measured as a function of pH. The CEST signals in the Eu3+ complexes show a strong CEST signal after the pH was reduced from 8 to 5. The opposite trend was observed for the Dy3+ complexes where the r2ex of bulk water protons increased dramatically from ca. 1.5 mm-1  s-1 to 13 mm-1  s-1 between pH 5 and 9 while r1 remained unchanged. A fit of the CEST data (Eu3+ complexes) to Bloch theory and the T2ex data (Dy3+ complexes) to Swift-Connick theory provided the proton-exchange rates as a function of pH. These data showed that the four amine groups contribute significantly to proton-catalyzed exchange of the Ln3+ -bound water protons even though their pKa 's are much higher than the observed CEST or T2ex effects. This demonstrated the utility of using appended acidic/basic groups to catalyze prototropic exchange for imaging tissue pH by MRI.

Optical molecular imaging for tumor detection and image-guided surgery

We have witnessed rapid development of fluorescence molecular imaging of solid tumors for cancer diagnosis and image-guided surgery in the past decade. Many biomarkers unique to cancer cells or tumor microenvironment, such as cell surface receptors, hypoxia, secreted proteases and extracellular acidosis have been characterized, and can be used to distinguish cancer from normal tissue. A variety of optical imaging probes have been developed to target these biomarkers to improve tumor contrast over the background tissue. Unlike conventional anatomical and molecular imaging technologies, fluorescent imaging method benefits from its safety, high-spatial resolution and real-time capability, and therefore, has become a highly adoptable imaging method for tumor detection and image-guided surgery in clinics. In this review, we summarize recent progress in 'always-ON' and stimuli-activatable fluorescent imaging probes, and discuss their potentials in tumor detection and image-guided surgery.

Interactions of Renal-Clearable Gold Nanoparticles with Tumor Microenvironments: Vasculature and Acidity Effects

The success of nanomedicines in the clinic depends on our comprehensive understanding of nano-bio interactions in tumor microenvironments, which are characterized by dense leaky microvasculature and acidic extracellular pH (pHe ) values. Herein, we investigated the accumulation of ultrasmall renal-clearable gold NPs (AuNPs) with and without acidity targeting in xenograft mouse models of two prostate cancer types, PC-3 and LNCaP, with distinct microenvironments. Our results show that both sets of AuNPs could easily penetrate into the tumors but their uptake and retention were mainly dictated by the tumor microvasculature and the enhanced permeability and retention effect over the entire targeting process. On the other hand, increased tumor acidity indeed enhanced the uptake of AuNPs with acidity targeting, but only for a limited period of time. By making use of simple surface chemistry, these two effects can be synchronized in time for high tumor targeting, opening new possibilities to further improve the targeting efficiencies of nanomedicines.

Dimerization of Organic Dyes on Luminescent Gold Nanoparticles for Ratiometric pH Sensing

Synergistic effects arising from the conjugation of organic dyes onto non-luminescent metal nanoparticles (NPs) have greatly broadened their applications in both imaging and sensing. Herein, we report that conjugation of a well-known pH-insensitive dye, tetramethyl-rhodamine (TAMRA), to pH-insensitive luminescent gold nanoparticles (AuNPs) can lead to an ultrasmall nanoindicator that can fluorescently report local pH in a ratiometric way. Such synergy originated from the dimerization of TAMRA on AuNPs, of which geometry was very sensitive to surface charges of the AuNPs and can be reversely modulated through protonation of surrounding glutathione ligands. Not limited to pH-insensitive dyes, this pH-dependent dimerization can also enhance the pH sensitivity of fluorescein, a well-known pH-sensitive dye, within a larger pH range, opening up a new pathway to design ultrasmall fluorescent ratiometric nanoindicators with tunable wavelengths and pH response ranges.

Acylsulfonamide-Functionalized Zwitterionic Gold Nanoparticles for Enhanced Cellular Uptake at Tumor pH

A nanoparticle design featuring pH-responsive alkoxyphenyl acylsulfonamide ligands is reported herein. As a result of ligand structure, this nanoparticle is neutral at pH 7.4, becoming positively charged at tumor pH (<6.5). The particle uptake and cytotoxicity increase over this pH range. This pH-controlled uptake and toxicity makes this particle a promising tool for tumor selective therapy.

Delivering colloidal nanoparticles to mammalian cells: a nano-bio interface perspective

Understanding the behavior of multifunctional colloidal nanoparticles capable of biomolecular targeting remains a fascinating challenge in materials science with dramatic implications in view of a possible clinical translation. In several circumstances, assumptions on structure-activity relationships have failed in determining the expected responses of these complex systems in a biological environment. The present Review depicts the most recent advances about colloidal nanoparticles designed for use as tools for cellular nanobiotechnology, in particular, for the preferential transport through different target compartments, including cell membrane, cytoplasm, mitochondria, and nucleus. Besides the conventional entry mechanisms based on crossing the cellular membrane, an insight into modern physical approaches to quantitatively deliver nanomaterials inside cells, such as microinjection and electro-poration, is provided. Recent hypotheses on how the nanoparticle structure and functionalization may affect the interactions at the nano-bio interface, which in turn mediate the nanoparticle internalization routes, are highlighted. In addition, some hurdles when this small interface faces the physiological environment and how this phenomenon can turn into different unexpected responses, are discussed. Finally, possible future developments oriented to synergistically tailor biological and chemical properties of nanoconjugates to improve the control over nanoparticle transport, which could open new scenarios in the field of nanomedicine, are addressed.

Chaotropic-anion-induced supramolecular self-assembly of ionic polymeric micelles

Traditional micelle self-assembly is driven by the association of hydrophobic segments of amphiphilic molecules forming distinctive core-shell nanostructures in water. Here we report a surprising chaotropic-anion-induced micellization of cationic ammonium-containing block copolymers. The resulting micelle nanoparticle consists of a large number of ion pairs (≈60,000) in each hydrophobic core. Unlike chaotropic anions (e.g. ClO4(-)), kosmotropic anions (e.g. SO4(2-)) were not able to induce micelle formation. A positive cooperativity was observed during micellization, for which only a three-fold increase in ClO4(-) concentration was necessary for micelle formation, similar to our previously reported ultra-pH-responsive behavior. This unique ion-pair-containing micelle provides a useful model system to study the complex interplay of noncovalent interactions (e.g. electrostatic, van der Waals, and hydrophobic forces) during micelle self-assembly.

Boronic Acid functionalized core-shell polymer nanoparticles prepared by distillation precipitation polymerization for glycopeptide enrichment

The boronic acid-functionalized core-shell polymer nanoparticles, poly(N,N-methylenebisacrylamide-co-methacrylic acid)@4-vinylphenylboronic acid (poly(MBA-co-MAA)@VPBA), were successfully synthesized for enriching glycosylated peptides. Such nanoparticles were composed of a hydrophilic polymer core prepared by distillation precipitation polymerization (DPP) and a boronic acid-functionalized shell designed for capturing glycopeptides. Owing to the relatively large amount of residual vinyl groups introduced by DPP on the core surface, the VPBA monomer was coated with high efficiency, working as the shell. Moreover, the overall polymerization route, especially the use of DPP, made the synthesis of nanoparticles facile and time-saving. With the poly(MBA-co-MAA)@VPBA nanoparticles, 18 glycopeptides from horseradish peroxidase (HRP) digest were captured and identified by MALDI-TOF mass spectrometric analysis, relative to eight glycopeptides enriched by using commercially available meta-aminophenylboronic acid agarose under the same conditions. When the concentration of the HRP digest was decreased to as low as 5 nmol, glycopeptides could still be selectively isolated by the prepared nanoparticles. Our results demonstrated that the synthetic poly(MBA-co-MAA)@VPBA nanoparticles might be a promising selective enrichment material for glycoproteome analysis.