pH-responsive Fluorescence Research Articles

Polysaccharide enabled biogenic fabrication of pHsensing fluorescent gold nanoclusters as a biocompatible tumor imaging probe.

A biocompatible natural polysaccharide (PSP001) isolated from the fruit rind of Punica granatum was conjugated with L-cysteine (Y) to be used as a skeleton for the fabrication of fluorescent gold nanoclusters (AuNCs) represented as PSP-Y-AuNCs. With an average size of ~ 6nm, PSP-Y-AuNCs demonstrated high quantum yield (31%), with a pH-sensitive fluorescence emission behavior. An emission maximum of 520nm was obtained at acidic pH, which was blue shifted with increasing pH. This feature providesthe possibilities for accurate ratiometric pH imaging. The PSP-Y-AuNCs not only demonstrated excellent biocompatibility with cancer cells and isolated peripheral lymphocytes and red blood cells but also demonstrated to be an active molecular imaging probe with appealing cellular uptake efficiency. The investigations with BALB/c mice further confirmed the non-toxic nature and in vivo imaging potential of the AuNCs. Estimation of the bio-distribution on solid tumorbearing syngeneic murine models revealed a tumor-targeted enhanced fluorescence emission pattern which isattributed to the pH responsive fluorescence behavior and the acidic microenvironment of the tumor. These findings were further confirmed with an impressive tumor accumulation pattern displayed in a xenograft of human cancerbearing nude mice. On account of their impressive biocompatibility and photophysical features, PSP-Y-AuNCs can exploited for the real-time fluorescence imaging of cancer tissues. Graphical abstract Fluorescent gold nanoclusters (PSP-Y-AuNCs) fabricated using a non-toxic natural polysaccharide (PSP001) demonstrated pHsensitive fluorescence emission pattern. The increased fluorescence readouts at acidic conditions and excellent biocompatibility made the PSP-Y-AuNCs an appealing candidate for in vivo tumor imaging applications.

Carbon dots with pH-responsive fluorescence: a review on synthesis and cell biological applications.

This review summarizes state of the art synthesis and applications of carbon dots (CDs) with pH-responsive fluorescence. Following an introduction, the first section covers methods for the preparation of pH-responsive CDs, with subsections on general methods for preparing CDs (by hydrothermal, solvothermal, electrochemical, microwave, laser ablation, pyrolysis or chemical oxidation polymerization methods), and on precursors for synthesis. This is followed by a section on the mechanisms of pH-responsivity (by creating new functional groups, change of energy levels, protonation and deprotonation, aggregation, or by introduction shells). Several Tables are presented that give an overview of the wealth of methods and materials. A final section covers applications of carbon dots (CDs) with pH-responsive fluorescence for sensing, drug delivery, and imaging. The conclusion summarizes the current status, addresses challenges, and gives an outlook on potential future trends. Graphical abstract The synthesis and biological applications of carbon dots(CDs) with pH-responsive fluorescence are summarized. Precursors and methods for preparation of pH-responsive CDs, mechanisms of pH-responsivity, and biological applications of CDs with pH-responsive fluorescence for sensing, drug delivery, and imaging are discussed.

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

Wound infection is a major challenge in the clinic that greatly hinders the wound healing process. It is highly important to develop smart wound dressings that can sense bacterial infection at early stages and provide on-demand treatment. In this work, a smart hydrogel-based wound dressing capable of monitoring bacterial infection via a pH-responsive fluorescence resonance energy transfer (FRET) transition of Cyanine3 (Cy3) and Cyanine5 (Cy5) in a bacterial environment and providing on-demand treatment of infection via near infrared (NIR) light-triggered antibiotic release was developed. The smart hydrogel was prepared by physical crosslinking of polyvinyl alcohol (PVA) and an ultraviolet (UV)-cleavable polyprodrug (GS-Linker-MPEG), in which Cy3 and Cy5-modified silica nanoparticles (SNP-Cy3/Cy5) were loaded and acted as a pH-responsive fluorescent probe to detect bacterial infection based on the FRET effect between Cy3 and Cy5. Also, up-conversion nanoparticles (UCNP) were loaded into the hydrogels to cleave the UV-responsive GS-Linker-MPEG and achieve NIR-responsive release of GS in the bacterial environment. The in vitro studies proved that the smart hydrogels present good water absorption ability and excellent mechanical properties as well as good biocompatibility, which are necessary for their application in wound dressings. Moreover, the hydrogels showed obvious FRET transitions in both acidic buffer and bacteria solution. Upon irradiating the hydrogels with NIR light, UCNP were able to convert NIR light to UV light to trigger the release of GS from the hydrogels for antibacterial treatment. This research is expected to provide a new strategy for self-reporting and effective treatment of wound infection.

Two pH-responsive fluorescence probes based on indole derivatives

Two new pH fluorescent probes 2,8-bis((E)-2-(1,1-dimethyl-1H-benzo[e]indol-2-yl)vinyl)-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (1) and 2,8-bis((E)-2-(3,3-dimethyl-3H-indol-2-yl)vinyl)-6H,12H-5,11-methanodibenzo[b,f] [1,5]diazocine (2) were designed and synthesized via ethylene bridging of the Tröger's Base (TB) framework and indole derivatives. As the protonation of nitrogen atoms of indole derivatives, both probes displayed the significant pH-dependent spectral properties. The probing behavior toward pH variations indicated that probes exhibited excellent pH dependent behavior in the pH ranges of 2.07–7.01 (1) and 1.96–7.01 (2), especially possessing good linear response in the pH ranges of 2.50–4.00. The emissions of 1 can be reversibly switched between brilliant yellow and dark states by protonation and deprotonation. Also, the yellow-green fluorescence emission of 2 could be quenched upon protonation. Such behaviors enable them to work as turn-off fluorescent pH sensors in the solution state. Furthermore, protonation and deprotonation on indole moiety nitrogen of the probe 1 and 2 provided a sensing mechanism for pH changes. In addition, both probes performed good photostability, high selectivity and excellent reversibility.

Anti-quenching NIR-II molecular fluorophores for in vivo high-contrast imaging and pH sensing

The contrast and sensitivity of in vivo fluorescence imaging has been revolutionized by molecular fluorophores operating in the second near-infrared window (NIR-II; 1000-1700 nm), but an ongoing challenge is the solvatochromism-caused quenching in aqueous solution for the long-wavelength absorbing fluorophores. Herein, we develop a series of anti-quenching pentamethine cyanine fluorophores that significantly overcome the severe solvatochromism, thus affording stable absorption/emission beyond 1000 nm with up to ~ 44-fold enhanced brightness and superior photostability in aqueous solution. These advantages allow for deep optical penetration (8 mm) as well as high-contrast and highly-stable lymphatic imaging superior to clinical-approved indocyanine green. Additionally, these fluorophores exhibit pH-responsive fluorescence, allowing for noninvasive ratiometric fluorescence imaging and quantification of gastric pH in vivo. The results demonstrate reliable accuracy in tissue as deep as 4 mm, comparable to standard pH electrode method. This work unlocks the potential of anti-quenching pentamethine cyanines for NIR-II biological applications.

The synthesis, self-assembly and pH-responsive fluorescence enhancement of an alternating amphiphilic copolymer with azobenzene pendants

In this work, a novel alternating amphiphilic copolymer (AAC) P(EG4-a-NAzoOMe) with azobenzene pendants can selfassemble into large compound micelles, and it features fantastic pH-enhanced aggregate induced emission property.

Site-Selective in Situ Growth-Induced Self-Assembly of Protein-Polymer Conjugates into pH-Responsive Micelles for Tumor Microenvironment Triggered Fluorescence Imaging.

Self-assembly of site-selective protein-polymer conjugates into stimuli-responsive micelles is interesting owing to their potential biomedical applications, ranging from molecular imaging to drug delivery, but remains a significant challenge. Herein we report a method of site-selective in situ growth-induced self-assembly (SIGS) to synthesize site-specific human serum albumin-poly(2-(diisopropylamino)ethyl methacrylate) (HSA-PDPA) conjugates that can in situ self-assemble into pH-responsive micelles with tunable morphologies. Indocyanine green (ICG) was selectively loaded into the core of sphere-like HSA-PDPA micelles to form pH-responsive fluorescence nanoprobes. The nanoprobes rapidly dissociated into protonated individual unimers at a transition pH of around 6.5, that is the extracellular pH of tumors, which resulted in a sharp fluorescence increase and markedly enhanced cellular uptake. In a tumor-bearing mouse model, they exhibited greatly enhanced tumor fluorescence imaging as compared to ICG alone and pH-nonresponsive nanoprobes. These findings suggest that pH-responsive and site-selective protein-polymer conjugate micelles synthesized by SIGS are promising as a new class of tumor microenvironment-responsive nanocarriers for enhanced tumor imaging and therapy.

Design, synthesis and evaluation of HepDirect fluorescence probes mediated by asialoglycoprotein receptor

A series of HepDirect pH responsive fluorescence probes of glycosyl-rhodamine are synthesized. The recognition trend of glucoside < lactoside < galactoside illuminates that the galactose is a targeting group toward HepG2 cells and its recognition is affected by the bulk of targeting group. The HepG2 cells binding trend of RDGal1< RDGal2< RDGal3 supports the cluster effect of galactose. RDGal3 with the optimum targeting potential is verified to selectively recognize HepG2 cells via a galactose dependent ASGPR binding mechanism. These nontoxic probes could be controllably activated by the acidic microenvironment of cancer cells, which is very significant to achieve diagnosis of hepatocellular carcinoma and provide a novel HepDirect carrier for precision treatment of hepatocarcinoma.

Multicolor fluorescent carbon nanodots@zeolitic imidazolate framework-8 nanoparticles for simultaneous pH-responsive drug delivery and fluorescence imaging

Multicolor fluorescent carbon nanodots@zeolitic imidazolate framework-8 nanoparticles for simultaneous pH-responsive drug delivery and fluorescence imaging

A zwitterionic pH responsive ESIPT-Based fluorescence “Turn-On” Al3+ ion sensing probe and its bioimaging applications

A zwitterionic compound (L) as a specific detector for aluminum ion in methanol/HEPES buffer (5mM, pH 7.2; 1:5v/v) by fluorescence “turn on” was synthesized by the condensation of propargylated 2,4-dihydroxybenzophenone with ethanolamine. The compound L was characterized by various spectral methods and also crystalographically. L was found to be in equilibrium with the deprotonated form with a pKa value of 8.63. During photophysical process, L exhibits weak fluorescence with low quantum yield (Φ=0.069) at 440nm due to the quick conversion of excited keto-form into the excited enol-form by transfer of a proton (ESIPT). The interaction of L with Al3+ inhibits the ESIPT process and the emission enhanced band (20 fold) appears at shorter wavelength (427nm) with increased quantum yield (Φ=0.217). The binding of L with Al3+ and stoichiometry (1:1) has been established by UV–vis, NMR, emission and ESI–MS spectral methods L.Al3+ is reversible with EDTA and can be reused. The effect of pH, time, temperature and solvent concentrations on fluorescence intensity were studied. Fascinatingly, the fluorescence microscopic studies established that L could be used as an efficient probe for detection of intracellular Al3+ ions in HeLa cells without any toxicity.

Quenching of pH-Responsive Luminescence of a Benzoindolizine Sensor by an Ultrafast Hydrogen Shift.

Fluorescent-sensor design requires consideration of how photochemical dynamics control properties of a sensing state. Transient absorption (TA) spectroscopy reveals an ultrafast net [1,3]-hydrogen shift following excitation of a protonated methoxy benzoindolizine (bzi) sensor in solution. These photochemical dynamics explain a quenched pH-responsive fluorescence shift and dramatically reduced fluorescence quantum yield relative to other (e. g. methyl) bzi compounds that do not tautomerize. Calculations predict the energetic and structural feasibility for rearrangement in protonated bzi compounds, such that interaction between the pi-network and strongly electron-donating methoxyl must lower the barrier for suprafacial H or H+ shift across an allylic moiety. As bzi compounds broadly exhibit pH-responsive emission shifts, chemical interactions that modulate this electronic interaction and suppress tautomerization could be used to facilitate binding- or surface-specific acid-responsive sensing.

Design and fabrication of fluorescence resonance energy transfer-mediated fluorescent polymer nanoparticles for ratiometric sensing of lysosomal pH

The design of effective tools capable of sensing lysosome pH is highly desirable for better understanding its biological functions in cellular behaviors and various diseases. Herein, a lysosome-targetable ratiometric fluorescent polymer nanoparticle pH sensor (RFPNS) was synthesized via incorporation of miniemulsion polymerization and surface modification technique. In this system, the donor: 4-ethoxy-9-allyl-1,8-naphthalimide (EANI) and the acceptor: fluorescein isothiocyanate (FITC) were covalently linked to the polymer nanoparticle to construct pH-responsive fluorescence resonance energy transfer (FRET) system. The FITC moieties on the surface of RFPNS underwent structural and spectral transformation as the presence of pH changes, resulting in ratiometric fluorescent sensing of pH. The as-prepared RFPNS displayed favorable water dispersibility, good pH-induced spectral reversibility and so on. Following the living cell uptake, the as-prepared RFPNS with good cell-membrane permeability can mainly stain in the lysosomes; and it can facilitate visualization of the intracellular lysosomal pH changes. This nanosensor platform offers a novel method for future development of ratiometric fluorescent probes for targeting other analytes, like ions, metabolites,and other biomolecules in biosamples.

Surface patterned pH-sensitive fluorescence using β-cyclodextrin functionalized poly(ethylene glycol)

This paper reports the development of a pH-responsive molecular pattern that shows specific and selective affinity for particular host-guest interactions, and its use as a pH fluorescent sensor. The pH-responsive boronate ester is formed via interactions between the diol group of β-cyclodextrin (CD) and phenylboronic acid of poly(ethylene glycol), and is strategically designed to allow reversible formation of a molecular lining pattern. Printing on a versatile substrate provides a method to monitor the positioning of different molecules by using a pH-responsive boronate ester, allowing specific host-guest interactions on any surface. Confocal laser scanning microscopy, fluorescence spectroscopy, and 1H NMR results indicate that the assembled CD monolayer can be removed by washing with an acidic pH buffer, demonstrating the presence of a boronate ester connective bridge, which is acid labile. Therefore, visualization of the pH-responsive fluorescence sensor using a rhodamine-CD complex allows straightforward discrimination between different molecules on any substrate, thus facilitating application of this sensor in clinical diagnostics and environmental monitoring.

Aggregation-induced emission polymer nanoparticles with pH-responsive fluorescence

A convenient and efficient method was developed to prepare stable polymer nanoparticles with varied morphology and pH-responsive AIE properties, based on the amphiphilic block copolymer self-assembly, crosslinking, and post-functionalization techniques.

Latent pH-responsive ratiometric fluorescent cluster based on self-assembled photoactivated SNARF derivatives

We have developed a self-assembled fluorescent cluster comprising a seminaphthorhodafluor (SNARF) derivative protected by a photoremovable o-nitrobenzyl group. Prior to UV irradiation, a colorless and nonfluorescent cluster was spontaneously assembled in aqueous solution. After UV irradiation, the self-assembled cluster remained intact and showed a large enhancement in pH-responsive fluorescence. The unique pH responsive fluorescent cluster could be used as a dual-emissive ratiometric fluorescent pH probe not only in the test tube but also in HeLa cell cultures.

An aggregation-induced emission star polymer with pH and metal ion responsive fluorescence

A one-pot strategy from ring-opening metathesis polymerization was employed to prepare AIE-active star polymers, which were designed to have multi-responsive fluorescence to varied stimuli including pH, CO2, and metal ions.

A gold nanocluster-based fluorescent probe for simultaneous pH and temperature sensing and its application to cellular imaging and logic gates.

Metal nanocluster-based nanomaterials for the simultaneous determination of temperature and pH variations in micro-environments are still a challenge. In this study, we develop a dual-emission fluorescent probe consisting of bovine serum albumin-stabilized gold nanoclusters (BSA-AuNCs) and fluorescein-5-isothiocyanate (FITC) as temperature- and pH-responsive fluorescence signals. Under single wavelength excitation the FITC/BSA-AuNCs exhibited well-separated dual emission bands at 525 and 670 nm. When FITC was used as a reference fluorophore, FITC/BSA-AuNCs showed a good linear response over the temperature range 1-71 °C and offered temperature-independent spectral shifts, temperature accuracy, activation energy, and reusability. The possible mechanism for high temperature-induced fluorescence quenching of FITC/BSA-AuNCs could be attributed to a weakening of the Au-S bond, thereby lowering the charge transfer from BSA to AuNCs. Additionally, the pH- and temperature-responsive properties of FITC/BSA-AuNCs allow simultaneous temperature sensing from 21 to 41 °C (at intervals of 5 °C) and pH from 6.0 to 8.0 (at intervals of 0.5 pH unit), facilitating the construction of two-input AND logic gates. Three-input AND logic gates were also designed using temperature, pH, and trypsin as inputs. The practicality of using FITC/BSA-AuNCs to determine the temperature and pH changes in HeLa cells is also validated.

Zwitterionic N2O2-Type Protonated Dipyrrin Bearing a Phosphate Anionic Moiety as a pH-Responsive Fluorescence Indicator.

Zwitterionic protonated-dipyrrin 1 bearing a phosphate unit was synthesized from the N2O2-type tetradentate dipyrrin ligand. Compound 1 is in equilibrium with the deprotonated form 1' with a pKa value of 5.8. Compound 1 exhibited a pH-responsive fluorescence under physiological conditions; the fluorescence intensity increased in aqueous media as the pH increased. In living cells, 1 also exhibited emission responsive to pH. Thus, 1 should be applicable as a pH probe for detecting tumor cells.

PH-responsive fluorescence chemical sensor constituted by conjugated polymers containing pyridine rings.

Poly(p-pyridinium phenylene ethynylene)s (PPyPE) functionalized with alternating donor-acceptor repeat units were synthesized by a Pd-catalyzed Sonogashira coupling reaction between diethynyl monomer and di-iodopyridine for use as a pH-responsive fluorescence chemical sensor. The synthesized PPyPE, containing pyridine units, was characterized by FT-IR, (1)H and (13)C NMR, UV-visible and fluorescence spectroscopies. We investigated the relationship between changes of optical properties and protonation/deprotonation of PPyPE containing pyridine units in solution. Addition of HCl decreased and red-shifted the fluorescence intensity of the conjugated polymers that contained pyridine rings; fluorescence intensity of the polymers increased upon addition of NaOH solution. The synthesized PPyPE was found to be an effective and reusable chemical sensor for pH sensing.

A sialic acid-targeted near-infrared theranostic for signal activation based intraoperative tumor ablation.

Agents enabling tumor staging are valuable for cancer surgery. Herein, a targetable sialic acid-armed near-infrared profluorophore (SA-pNIR) is reported for fluorescence guided tumor detection. SA-pNIR consists of a sialic acid entity effective for in vivo tumor targeting and a profluorophore which undergoes lysosomal acidity-triggered fluorogenic isomerization. SA-pNIR displays a number of advantageous biomedical properties in mice, e.g. high tumor-to-normal tissue signal contrast, long-term retention in tumors and low systemic toxicity. In addition, SA-pNIR effectively converts NIR light into cytotoxic heat in cells, suggesting tumor-activatable photothermal therapy. With high performance tumor illumination and lysosome-activatable photothermal properties, SA-pNIR is a promising agent for detection and photothermal ablation of surgically exposed tumors.