A dual-emission Förster resonance energy transfer nanoprobe for sensing/imaging pH changes in the biological environment

Nanobio Interface Between Proteins and 2D Nanomaterials

Two-dimensional (2D) nanomaterials have significantly contributed to recent advances in material sciences and nanotechnology, owing to their layered structure. Despite their potential as multifunctional theranostic agents, the biomedical translation of these materials is limited due to a lack of knowledge and control over their interaction with complex biological systems. In a biological microenvironment, the high surface energy of nanomaterials leads to diverse interactions with biological moieties such as proteins, which play a crucial role in unique physiological processes. These interactions can alter the size, surface charge, shape, and interfacial composition of the nanomaterial, ultimately affecting its biological activity and identity. This review critically discusses the possible interactions between proteins and 2D nanomaterials, along with a wide spectrum of analytical techniques that can be used to study and characterize such interplay. A better understanding of these interactions would help circumvent potential risks and provide guidance toward the safer design of 2D nanomaterials as a platform technology for various biomedical applications.

Remodeling tumor immunosuppressive microenvironment via a novel bioactive nanovaccines potentiates the efficacy of cancer immunotherapy

The clinical outcomes of cancer nanovaccine have been largely impeded owing to the low antigen-specific T cell response rate and acquired resistance caused by the immunosuppressive tumor microenvironment (TME). Here, we reported a tumor acidity-responsive nanovaccine to remodel the immunosuppressive TME and expand the recruitment of tumor infiltrating lymphocytes (TILs) using hybrid micelles (HM), which encapsulated colony stimulating factor 1 receptor (CSF1-R) inhibitor BLZ-945 and indoleamine 2,3-dioxygenase (IDO) inhibitor NLG-919 in its core and displayed a model antigen ovalbumin (OVA) on its surface (denoted as BN@HM-OVA). The bioactive nanovaccine is coated with a polyethylene glycol (PEG) shell for extending nanoparticle circulation. The shell can be shed in response to the weakly acidic tumor microenvironment. The decrease in size and the increase in positive charge may cause the deep tumor penetration of drugs. We demonstrated that the bioactive nanovaccine dramatically enhance antigen presentation by dendritic cells (DCs) and drugs transportation into M1-like tumor-associated macrophages (TAMs) and tumor cells via size reduction and increasing positive charge caused by the weakly acidic TME. Such bioactive nanovaccine could remodel the immunosuppressive TME into an effector T cells favorable environment, leading to tumor growth inhibition in prophylactic and therapeutic E.G7-OVA tumor models. Furthermore, combining the bioactive nanovaccine with simultaneous anti-PD-1 antibody treatment leads to a long-term tumor inhibition, based on the optimal timing and sequence of PD-1 blockade against T cell receptor. This research provides a new strategy for the development of efficient cancer immunotherapy.

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A bioactive nanovaccine (BN@HM-OVA) was adopted for synergistic immunotherapy of E.G7-OVA tumors.

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BN@HM-OVA exhibited superior ability to induce DCs maturation and robust antigen-specific T cell responses.

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BN@HM-OVA contributed to a homeostasis in the tumor microenvironment ideal for antitumor vaccination.

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The combination treatment of BN@HM-OVA and αPD-1 achieved maximum therapeutic benefits.

Genetically encoded RNA nanodevices for cellular imaging and regulation

Nucleic acid-based nanodevices have been widely used in the fields of biosensing and nanomedicine. Traditionally, the majority of these nanodevices were first constructed in vitro using synthetic DNA or RNA oligonucleotides and then delivered into cells. Nowadays, the emergence of genetically encoded RNA nanodevices has provided a promising alternative approach for intracellular analysis and regulation. These genetically encoded RNA-based nanodevices can be directly transcribed and continuously produced inside living cells. A variety of highly precise and programmable nanodevices have been constructed in this way during the last decade. In this review, we will summarize the recent advances in the design and function of these artificial genetically encoded RNA nanodevices. In particular, we will focus on their applications in regulating cellular gene expression, imaging, logic operation, structural biology, and optogenetics. We believe these versatile RNA-based nanodevices will be broadly used in the near future to probe and program cells and other biological systems.

Halochromic Polymer Nanosensors for Simple Visual Detection of Local pH in Coatings

Replacing metallic structures before critical damage is beneficial for safety and for saving energy and resources. One simple approach consists in visually monitoring the early stage of corrosion, and related change of pH, of coated metals. We prepare smart nanoparticle additives for coatings which act as a pH sensor. The nanoparticles are formed with a terpolymer containing two dyes as side chains, acting as donor and acceptor for a FRET process. Real time monitoring of the extent of localized corrosion on metallic structures is then carried out with a smartphone camera. Colored pH mapping can be then manually retrieved by an operator or automatically recorded by a surveillance camera.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Highly swelling pH-responsive microgels for dual mode near infra-red fluorescence reporting and imaging

Near infra-red (NIR) fluorescence is a desirable property for probe particles because such deeply penetrating light enables remote reporting of the local environment in complex surroundings and imaging. Here, two NIR non-radiative energy transfer (NRET) fluorophores (Cy5 and Cy5.5) are coupled to preformed pH-responsive poly(ethylacrylate-methacrylic acid-divinylbenzene) microgel particles (PEA-MAA-5/5.5 MGs) to obtain new NIR fluorescent probes that are cytocompatible and swell strongly. NIR ratiometric photoluminescence (PL) intensity analysis enables reporting of pH-triggered PEA-MAA-5/5.5 MG particle swelling ratios over a very wide range (from 1-90). The dispersions have greatly improved colloidal stability compared to a reference temperature-responsive NIR MG based on poly(N-isopropylacrylamide) (PNP-5/5.5). We also show that the wavelength of maximum PL intensity (λ _max) is a second PL parameter that enables remote reporting of swelling for both PEA-MAA-5/5.5 and PNP-5/5.5 MGs. After internalization the PEA-MAA-5/5.5 MGs are successfully imaged in stem cells using NIR light. They are also imaged after subcutaneous injection into model tissue using NIR light. The new NIR PEA-MAA-5/5.5 MGs have excellent potential for reporting their swelling states (and any changes) within physiological settings as well as very high ionic strength environments (e.g., waste water).

Spectroscopic Investigation of DCCH and FTSC as a potential pair for Förster Resonance Energy Transfer in different solvents

Two molecules, 7-(diethylamino)coumarin-3-carbohydrazide (DCCH) and fluorescein-5-thiosemicarbazide (FTSC) were investigated in different solvents, under varying pH conditions regarding their spectroscopic properties for the usage as a Förster Resonance Energy Transfer (FRET) pair to study the molecular interaction between cellulosic surfaces. All the relevant spectroscopic properties to determine the Förster distance were measured and the performance as a FRET system was checked. From the results, it is clear that the environmental conditions need to be accurately controlled as both, but especially the FTSC dyes are sensitive to changes. For high enough concentrations positive FRET systems were observed in DMF, DMSO, H2O, THF and alkaline DMF. However due to the low quantum yield of the unmodified DCCH throughout the investigated parameter range and the strong environmental dependency of FTSC, both dyes are not preferable for being used in a FRET system for studying interaction between cellulosic surfaces.

Smart wound dressing for infection monitoring and NIR-triggered antibacterial treatment

Hydrogel-based wound dressings can monitor infection via pH-responsive FRET changes and provide on-demand antibacterial treatment via NIR-triggered antibiotic release.

"Smart" Nanoprobes for Visualization of Tumor Microenvironments

Physiological parameters in tumor microenvironments, including hypoxia, low extracellular pH, enzymes, reducing conditions, and so on, are closely associated with the proliferation, angiogenesis, invasion, and metastasis of cancer, and impact the therapeutic administrations. Therefore, monitoring the tumor microenvironment is significant for diagnosing tumors, predicting the invasion potential, evaluating therapeutic efficacy, planning the treatment, and cancer prognostics. Noninvasive molecular imaging technologies combined with novel "smart" nanoparticle-based activatable probes provide a feasible approach to visualize tumor-associated microenvironment factors. This review summarizes recent achievements in the designs of "smart" molecular imaging nanoprobes responding to the tumor microenvironment-related features, and highlights the state of the art in tumor heterogeneity imaging.

Ratiometric optical nanoprobes enable accurate molecular detection and imaging

Exploring and understanding biological and pathological changes are of great significance for early diagnosis and therapy of diseases. Optical sensing and imaging approaches have experienced major progress in this field. Particularly, an emergence of various functional optical nanoprobes has provided enhanced sensitivity, specificity, targeting ability, as well as multiplexing and multimodal capabilities due to improvements in their intrinsic physicochemical and optical properties. However, one of the biggest challenges of conventional optical nanoprobes is their absolute intensity-dependent signal readout, which causes inaccurate sensing and imaging results due to the presence of various analyte-independent factors that can cause fluctuations in their absolute signal intensity. Ratiometric measurements provide built-in self-calibration for signal correction, enabling more sensitive and reliable detection. Optimizing nanoprobe designs with ratiometric strategies can surmount many of the limitations encountered by traditional optical nanoprobes. This review first elaborates upon existing optical nanoprobes that exploit ratiometric measurements for improved sensing and imaging, including fluorescence, surface enhanced Raman scattering (SERS), and photoacoustic nanoprobes. Next, a thorough discussion is provided on design strategies for these nanoprobes, and their potential biomedical applications for targeting specific biomolecule populations (e.g. cancer biomarkers and small molecules with physiological relevance), for imaging the tumor microenvironment (e.g. pH, reactive oxygen species, hypoxia, enzyme and metal ions), as well as for intraoperative image guidance of tumor-resection procedures.

A nitroreductase and acidity detecting dual functional ratiometric fluorescent probe for selectively imaging tumor cells

A dual functional ratiometric fluorescent probe can obviously distinguish acidity, nitroreductase, and nitroreductase in an acidic environment. Confocal fluorescence imaging of A549 cells indicates the probe can detect acidity and expressed nitroreductase in living cells.

Ratiometric fluorescent pH nanoprobes based on in situ assembling of fluorescence resonance energy transfer between fluorescent proteins

pH-dependent protein adsorption on mesoporous silica nanoparticle (MSN) was examined as a unique means for pH monitoring. Assuming that the degree of protein adsorption determines the distance separating protein molecules, we examined the feasibility of nanoscale pH probes based on fluorescence resonance energy transfer (FRET) between two fluorescent proteins (mTurquoise2 and mNeonGreen, as donor and acceptor, respectively). Since protein adsorption on MSN is pH-sensitive, both fluorescent proteins were modified to make their isoelectric points (pIs) identical, thus achieving comparable adsorption between the proteins and enhancing FRET signals. The adsorption behaviors of such modified fluorescent proteins were examined along with ratiometric FRET signal generation. Results demonstrated that the pH probes could be manipulated to show feasible sensitivity and selectivity for pH changes in hosting solutions, with a good linearity observed in the pH range of 5.5-8.0. In a demonstration test, the pH probes were successfully applied to monitor progress of enzymatic reactions. Such an "in situ-assembling" pH sensor demonstrates a promising strategy in developing nanoscale fluorescent protein probes. Graphical abstract Working principle of the developed pH sensor TNS; and FRET Ratio (I₅₂₈/I₄₆₀) as a function of pH under different protein feed ratios (mNeonGreen to mTurquoise2).

Imaging Lysosomal pH Alteration in Stressed Cells with a Sensitive Ratiometric Fluorescence Sensor

The organelle-specific pH is crucial for cell homeostasis. Aberrant pH of lysosomes has been manifested in myriad diseases. To probe lysosome responses to cell stress, we herein report the detection of lysosomal pH changes with a dual colored probe (CM-ROX), featuring a coumarin domain with "always-on" blue fluorescence and a rhodamine-lactam domain activatable to lysosomal acidity to give red fluorescence. With sensitive ratiometric signals upon subtle pH changes, CM-ROX enables discernment of lysosomal pH changes in cells undergoing autophagy, cell death, and viral infection.

FRET-trackable biodegradable HPMA copolymer-epirubicin conjugates for ovarian carcinoma therapy

To develop a biodegradable polymeric drug delivery system for the treatment of ovarian cancer with the capacity for non-invasive fate monitoring, we designed and synthesized N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-epirubicin (EPI) conjugates. The polymer backbone was labeled with acceptor fluorophore Cy5, while donor fluorophores (Cy3 or EPI) were attached to HPMA copolymer side chains via an enzyme-cleavable GFLG linker. This design allows elucidating separately the fate of the drug and of the polymer backbone using fluorescence resonance energy transfer (FRET). The degradable diblock conjugate (2P-EPI) was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization using a bifunctional chain transfer agent (Peptide2CTA). The pharmacokinetics (PK) and therapeutic effect of 2P-EPI (Mw ~100 kDa) were determined in mice bearing human ovarian carcinoma A2780 xenografts. Compared to 1st generation conjugate (P-EPI, Mw <50 kDa), 2P-EPI demonstrated remarkably improved PK such as fourfold terminal half-life (33.22 ± 3.18 h for 2P-EPI vs. 7.55 ± 3.18 h for P-EPI), which is primarily attributed to the increased molecular weight of the polymer carrier. Notably, complete tumor remission and long-term inhibition of tumorigenesis (100 days) were achieved in mice (n=5) treated with 2P-EPI. Moreover, in vitro cell uptake and intracellular drug release were determined via FRET intensity changes. The results establish a solid foundation for future in vivo tracking of drug delivery and chain scission of polymeric conjugates by FRET imaging.

Nanoparticle-based luminescent probes for intracellular sensing and imaging of pH

Fluorescence imaging microscopy is an essential tool in biomedical research. Meanwhile, various fluorescent probes are available for the staining of cells, cell membranes, and organelles. Though, to monitor intracellular processes and dysfunctions, probes that respond to ubiquitous chemical parameters determining the cellular function such as pH, pO2 , and Ca(2+) are required. This review is focused on the progress in the design, fabrication, and application of photoluminescent nanoprobes for sensing and imaging of pH in living cells. The advantages of using nanoprobes carrying fluorescent pH indicators compared to single molecule probes are discussed as well as their limitations due to the mostly lysosomal uptake by cells. Particular attention is paid to ratiometric dual wavelength nanosensors that enable intrinsic referenced measurements. Referencing and proper calibration procedures are basic prerequisites to carry out reliable quantitative pH determinations in complex samples such as living cells. A variety of examples will be presented that highlight the diverseness of nanocarrier materials (polymers, micelles, silica, quantum dots, carbon dots, gold, photon upconversion nanocrystals, or bacteriophages), fluorescent pH indicators for the weak acidic range, and referenced sensing mechanisms, that have been applied intracellularly up to now. WIREs Nanomed Nanobiotechnol 2016, 8:378-413. doi: 10.1002/wnan.1366 For further resources related to this article, please visit the WIREs website.

Ratiometric detection of pH fluctuation in mitochondria with a new fluorescein/cyanine hybrid sensor

The homeostasis of mitochondrial pH (pH_m) is crucial in cell physiology. Developing small-molecular fluorescent sensors for the ratiometric detection of pH_m fluctuation is highly demanded yet challenging. A ratiometric pH sensor, Mito-pH, was constructed by integrating a pH-sensitive FITC fluorophore with a pH-insensitive hemicyanine group. The hemicyanine group also acts as the mitochondria targeting group due to its lipophilic cationic nature. Besides its ability to target mitochondria, this sensor provides two ratiometric pH sensing modes, the dual excitation/dual emission mode (D_ex/D_em) and dual excitation (D_ex) mode, and its linear and reversible ratiometric response range from pH 6.15 to 8.38 makes this sensor suitable for the practical tracking of pH_m fluctuation in live cells. With this sensor, stimulated pH_m fluctuation has been successfully tracked in a ratiometric manner via both fluorescence imaging and flow cytometry.

Injectable microbeads with a thermo-responsive shell and a pH-responsive core as a dual-switch-controlled release system

Treating inflammation with a dual-switch-controlled release system: The release of a drug from the developed microbead system occurs only in response to both an increase in local temperature and an acidic environmental pH. This dual-switch-controlled release system has the advantages of distinguishing between inflamed and healthy tissues to improve treatment efficacy.

FRET-based dual-emission and pH-responsive nanocarriers for enhanced delivery of protein across intestinal epithelial cell barrier

The oral route is a convenient and commonly employed way for drug delivery. However, therapeutic proteins have poor bioavailability upon oral administration due to the impermeable barrier from intestinal epithelial tight junction (TJ). Moreover, the pH of the small intestine varies among different regions of the intestinal tract where digestion and absorption occur at different levels. In this study, a tunable dual-emitting and pH-responsive nanocarrier that can alter the fluorescent color and emission intensity in response to pH changes and can trigger the opening of intestinal epithelial TJ at different levels were developed from chitosan-N-arginine and poly(γ-glutamic acid)-taurine conjugates. As pH increased from 6.0 to 8.0, the binding affinity of the oppositely charged polyions decreased, whereas the ratio of the intensity of the donor-to-acceptor emission intensity (ID/IA) increased by 27-fold. The fluorescent and pH-responsive nanocarrier was able to monitor the pH change of intestinal environment and to control the release of an anti-angiogenic protein in response to the pH gradient. The nanocarrier triggered the opening of intestinal epithelial TJ and consequently enhanced the permeation of the released protein through the intestinal epithelial barrier model (Caco-2 cell monolayer) to inhibit tube formation of human umbilical vein endothelial cells.

Synthesis and fluorescence properties of N-substituted 1-cyanobenz[f]isoindole chitosan polymers and nanoparticles for live cell imaging

Highly fluorescent N-substituted 1-cyanobenz[f]isoindole chitosans (CBI-CSs) with various degrees of N-substitution (DS) were synthesized by reacting chitosan (CS) with naphthalene-2,3-dicarboxaldehyde (NDA) in the presence of cyanide under mild acidic conditions. Introduction of 1-cyanobenz[f]isoindole moieties into the CS backbone resulted in lowering of polymer thermal stability and crystallinity. The fluorescence quantum yield (Φf) of CBI-CS was found to be DS- and molecular-weight-dependent, with Φf decreasing as DS and molecular weight were increased. At similar DS values, CBI-CS exhibited 26 times higher Φf in comparison with fluorescein isothiocyanate-substituted chitosan (FITC-CS). CBI-CS/TPP nanoparticles were fabricated using an ionotropic gelation method in which pentasodium triphosphate (TPP) acted as a cross-linking agent. CS and CBI-CS exhibited low cytotoxicity to normal skin fibroblast cells over a concentration range of 0.1-1000 μg/mL, while an increased cytotoxicity level was evident in CBI-CS/TPP nanoparticles at concentrations greater than 100 μg/mL. In contrast with CBI-CS polymers, the CBI-CS/TPP nanoparticles exhibited lower fluorescence; however, confocal microscopy results showed that living normal skin fibroblast cells became fluorescent on nanoparticle uptake. These results suggest that CBI-CS and fabricated nanoparticles thereof may be promising fluorescence probes for live cell imaging.

Nanoparticles with dual responses to oxidative stress and reduced ph for drug release and anti-inflammatory applications

Oxidative stress and reduced pH are involved in many inflammatory diseases. This study describes a nanoparticle-based system that is responsive to both oxidative stress and reduced pH in an inflammatory environment to effectively release its encapsulated curcumin, an immune-modulatory agent with potent anti-inflammatory and antioxidant capabilities. Because of the presence of Förster resonance energy transfer between curcumin and the carrier, this system also allowed us to monitor the intracellular release behavior. The curcumin released upon triggering could efficiently reduce the excess oxidants produced by the lipopolysaccharide (LPS)-stimulated macrophages. The feasibility of using the curcumin-loaded nanoparticles for anti-inflammatory applications was further validated in a mouse model with ankle inflammation induced by LPS. The results of these studies demonstrate that the proposed nanoparticle system is promising for treating oxidative stress-related diseases.

Macro-/micro-environment-sensitive chemosensing and biological imaging

We have summarized the research progress on fluorescent sensors responsive to environmental factors, including local viscosity, polarity, temperature, hypoxia and pH.

Fabrication and characterization of electrospun semiconductor nanoparticle-polyelectrolyte ultra-fine fiber composites for sensing applications

Fluorescent composite fibrous assembles of nanoparticle-polyelectrolyte fibers are useful multifunctional materials, utilized in filtration, sensing and tissue engineering applications, with the added benefits of improved mechanical, electrical or structural characteristics over the individual components. Composite fibrous mats were prepared by electrospinning aqueous solutions of 6 wt% poly(acrylic acid) (PAA) loaded with 0.15 and 0.20% v/v, carboxyl functionalized CdSe/ZnS nanoparticles (SNPs). The resulting fluorescent composite fibrous mats exhibits recoverable quenching when exposed to high humidity. The sensor response is sensitive to water concentration and is attributed to the change in the local charges around the SNPs due to deprotonation of the carboxylic acids on the SNPs and the surrounding polymer matrix.

Tunable dual-emitting shell-crosslinked nano-objects as single-component ratiometric pH-sensing materials

Dual-emitting photonic nano-objects that can sense changes in the environmental pH are designed based on shell-crosslinked micelles assembled from amphiphilic block copolymers and crosslinked with pH-insensitive chromophores. The chromophoric crosslinkers are tetra-functionalized pyrazine molecules that bear a set of terminal aliphatic amine groups and a set of anilino amine groups, which demonstrate morphology-dependent reactivities towards the poly(acrylic acid) shell domain of the nano-objects. The extent to which the anilino amine groups react with the nano-object shell is shown to affect the hypsochromic shift (blue-shift). The ratio of fluorescence intensity at 496 nm over that of 560 nm is dependent upon the solution pH. We report, herein, observations on the pH-sensitive dual-emission photophysical properties of rod-shaped or spherical nano-objects, whose shell domains offer two distinct platforms for amidation reactions to occur-through formation of activated esters upon addition of carbodiimide or pre-installation of activated ester groups. We demonstrate that physical manipulations (changes in morphology or particle dimensions) or chemical manipulations of the crosslinking reaction (the order of installation of activated esters) lead to fine tuning of dual-emission over ca. 60 nm in a physiologically relevant pH range. Rod-shaped shell-crosslinked nanostructures with poly(p-hydroxystyrene) core show blue-shift as a function of increasing pH while spherical shell-crosslinked nanostructures with polystyrene core and poly(ethylene oxide) corona exhibit blue-shift as a function of decreasing pH.