BODIPY-conjugated thermoresponsive copolymer as a fluorescent thermometer based on polymer microviscosity

Thermal Probing Techniques for a Single Live Cell

Temperature is a significant factor in determining and characterizing cellular metabolism and other biochemical activities. In this study, we provide a brief overview of two important technologies used to monitor the local temperatures of individual living cells: fluorescence nano-thermometry and an array of micro-/nano-sized thin-film thermocouples. We explain some key technical issues that must be addressed and optimised for further practical applications, such as in cell biology, drug selection, and novel antitumor therapy. We also offer a method for combining them into a hybrid measuring system.

Development of a novel colorimetric thermometer based on poly(N-vinylcaprolactam) with push–π–pull tricyanofuran hydrazone anion dye

Thermochromic poly(N-vinylcaprolactam-co-tricyanofuran hydrazone) [poly(VC-co-TCFH)] gel labeled with a halochromic chromophore was developed using traditional free radical polymerization.

Complete microviscosity maps of living plant cells and tissues with a toolbox of targeting mechanoprobes

Mechanical patterns control a variety of biological processes in plants. The microviscosity of cellular structures effects the diffusion rate of molecules and organelles, thereby affecting processes such as metabolism and signaling. Spatial variations in local viscosity are also generated during fundamental events in the cell life cycle. While crucial to a complete understanding of plant mechanobiology, resolving subcellular microviscosity patterns in plants has remained an unsolved challenge. We present an imaging microviscosimetry toolbox of molecular rotors that yield complete microviscosity maps of cells and tissues, specifically targeting the cytosol, vacuole, plasma membrane, and wall of plant cells. These boron-dipyrromethene (BODIPY)-based molecular rotors are rigidochromic by means of coupling the rate of an intramolecular rotation, which depends on the mechanics of their direct surroundings, with their fluorescence lifetime. This enables the optical mapping of fluidity and porosity patterns in targeted cellular compartments. We show how apparent viscosity relates to cell function in the root, how the growth of cellular protrusions induces local tension, and how the cell wall is adapted to perform actuation surrounding leaf pores. These results pave the way to the noninvasive micromechanical mapping of complex tissues.

Temperature- and Structure-Dependent Optical Properties and Photophysics of BODIPY Dyes

We report on the temperature- and structural-dependent optical properties and photophysics of a set of boron dipyrromethene (BODIPY) dyes with different substitution patterns of their meso-aryl subunit. Single-crystal X-ray diffraction analysis of the compounds enabled a classification of the dyes into a sterically hindered and a unhindered group. The steric hindrance refers to a blocked rotational motion of the aryl subunit around the bond connecting this moiety to the meso-position of the BODIPY core. The energy barriers related to this rotation were simulated by DFT calculations. As follows from the relatively low rotational barrier calculated to about 17 kcal/mol, a free rotation is only possible for sterically unhindered compounds. Rotational barriers of more than 40 kcal/mol determined for the sterically hindered compounds suggest an effective freezing of the rotational motion in these molecules. With the aid of temperature-dependent spectroscopic measurements, we could show that the ability to rotate directly affects the optical properties of our set of BODIPY dyes. This accounts for the strong temperature dependence of the fluorescence of the sterically unhindered compounds which show a drastic decrease in fluorescence quantum yield and a significant shortening in fluorescence lifetime upon heating. The optical properties of the sterically hindered compounds, however, are barely affected by temperature. Our results suggest a nonradiative deactivation of the first excited singlet state of the sterically unhindered compounds caused by a conical intersection of the potential energy surfaces of the ground and first excited state which is accessible by rotation of the meso-subunit. This is in good agreement with previously reported deactivation mechanisms. In addition, our results suggest the presence of a second nonradiative depopulation pathway of the first excited singlet state which is particularly relevant for the sterically hindered compounds.

Boron-based stimuli responsive materials

Representative types of boron-based molecular systems that respond to external stimuli such as temperature, pressure, light, or chemicals (oxygen, acid, base etc.) are described in this review article. The boron molecules are classified according to their operating mechanisms, with emphasis on systems, which are based on switchable boron-donor bonds and switchable excited states.

Fluorescent Poly(glycerol-co-sebacate) Acrylate Nanoparticles for Stem Cell Labeling and Longitudinal Tracking

The stable presence of fluorophores within the biocompatible and biodegradable elastomer poly(glycerol-co-sebacate) acrylate (PGSA) is critical for monitoring the transplantation, performance, and degradation of the polymers in vivo. However, current methods such as physically entrapping the fluorophores in the polymer matrix or providing a fluorescent coating suffer from rapid leakage of fluorophores. Covalent conjugation of fluorophores with the polymers and the subsequent core-cross-linking are proposed here to address this challenge. Taking rhodamine as the model dye and PGSA nanoparticles (NPs) as the model platform, we successfully showed that the synthesized rhodamine-conjugated PGSA (PGSAR) NPs only released less than 30% rhodamine at day 28, whereas complete release of dye occurred for rhodamine-encapsulated PGSA (PGSA-p-R) NPs at day 7 and 57.49% rhodamine was released out for the un-cross-linked PGSAR NPs at day 28. More excitingly, PGSAR NPs showed a strong quantum yield enhancement (26.24-fold) of the fluorophores, which was due to the hydrophobic environment within PGSAR NPs and the restricted rotation of (6-diethylamino-3H-xanthen-3-ylidene) diethyl group in rhodamine after the conjugation and core-cross-linking. The stable presence of dye in the NPs and enhanced fluorescence allowed a longitudinal tracking of stem cells both in vitro and in vivo for at least 28 days.

The Mechanisms and Biomedical Applications of an NIR BODIPY-Based Switchable Fluorescent Probe

Highly environment-sensitive fluorophores have been desired for many biomedical applications. Because of the noninvasive operation, high sensitivity, and high specificity to the microenvironment change, they can be used as excellent probes for fluorescence sensing/imaging, cell tracking/imaging, molecular imaging for cancer, and so on (i.e., polarity, viscosity, temperature, or pH measurement). In this work, investigations of the switching mechanism of a recently reported near-infrared environment-sensitive fluorophore, ADP(CA)₂, were conducted. Besides, multiple potential biomedical applications of this switchable fluorescent probe have been demonstrated, including wash-free live-cell fluorescence imaging, in vivo tissue fluorescence imaging, temperature sensing, and ultrasound-switchable fluorescence (USF) imaging. The fluorescence of the ADP(CA)₂ is extremely sensitive to the microenvironment, especially polarity and viscosity. Our investigations showed that the fluorescence of ADP(CA)₂ can be switched on by low polarity, high viscosity, or the presence of protein and surfactants. In wash-free live-cell imaging, the fluorescence of ADP(CA)₂ inside cells was found much brighter than the dye-containing medium and was retained for at least two days. In all of the fluorescence imaging applications conducted in this study, high target-to-noise (>5-fold) was achieved. In addition, a high temperature sensitivity (73-fold per Celsius degree) of ADP(CA)₂-based temperature probes was found in temperature sensing.

Intracellular temperature measurements with fluorescent polymeric thermometers

Intracellular temperature can be measured using fluorescent polymeric thermometersviatheir temperature-dependent fluorescence signals.

Thermo-responsive fluorescence of AIE-active poly(N-isopropylacrylamides) labeled with highly twisted bis(N,N-dialkylamino)arenes

A thermo-responsive fluorescent polymer materials were synthesized fromN-isopropylacrylamides with AIE-active 9,10-bis(N,N-dialkylamino)arene monomers.

Influence of the Near Molecular Vicinity on the Temperature Regulated Fluorescence Response of Poly(N-vinylcaprolactam)

A series of new fluorescent dye bearing monomers, including glycomonomers, based on maleamide and maleic esteramide was synthesized. The dye monomers were incorporated by radical copolymerization into thermo-responsive poly(N‑vinyl-caprolactam) that displays a lower critical solution temperature (LCST) in aqueous solution. The effects of the local molecular environment on the polymers' luminescence, in particular on the fluorescence intensity and the extent of solvatochromism, were investigated below as well as above the phase transition. By attaching substituents of varying size and polarity in the close vicinity of the fluorophore, and by varying the spacer groups connecting the dyes to the polymer backbone, we explored the underlying structure⁻property relationships, in order to establish rules for successful sensor designs, e.g., for molecular thermometers. Most importantly, spacer groups of sufficient length separating the fluorophore from the polymer backbone proved to be crucial for obtaining pronounced temperature regulated fluorescence responses.

Thermo-responsive rheological behavior of borinic acid polymer in dilute solution

Borinic acid polymer is rheologically thermo-responsive in solution, which expands its application as a new type of “smart” polymer.

A rapid response "Turn-On" fluorescent probe for nitroreductase detection and its application in hypoxic tumor cell imaging

A novel "Turn-On" fluorescent probe, quaternarized 4-pyridinyl-substituted BODIPY dye by incorporating a 5-nitrofuran moiety, was developed and applied for imaging the hypoxic status of tumor cells by the indirect detection of nitroreductase. The design was based on a nitroreductase-catalyzed reduction of the nitrofuran moiety in the presence of reduced nicotinamide adenine dinucleotide (NADH) as an electron donor and followed by the 1,6-rearrangement-elimination and the release of free 4-pyridinyl-substituted BODIPY dye . This probe displayed desired properties such as high specificity, "Turn-On" fluorescence response with suitable sensitivity, appreciable water solubility and rapid response time (within 5 min). Moreover, as a biocompatible molecule, the probe has been successfully applied for imaging the hypoxic status of tumor cells (e.g. A549 cells) and especially used for real-time determination of nitroreductase produced by Escherichia coli. Therefore, we hope to apply this novel method in the biomedical research fields for the imaging of disease-relevant hypoxia and detection of pathogenic microorganisms.

Multistimuli Sensitive Behavior of Novel Bodipy-Involved Pillar[5]arene-Based Fluorescent [2]Rotaxane and Its Supramolecular Gel

Fluorescent [2]rotaxane BC12P5 is successfully constructed with 1,4-diethoxypillar[5]arene as wheel over a long alkyl axle with Bodipy chromophore as one stopper for the first time. NMR spectra clearly reveal its molecular shuttle nature triggered by multiple external stimuli including solvent polarity and temperature. In particular, the fluorescence nature introduced into [2]rotaxane BC12P5 renders it a good sensor for the external stimuli. Nevertheless, the supramolecular gel successfully fabricated from this novel rotaxane system via self-assembly in dimethyl sulfoxide (DMSO) also shows reversible gel-sol phase transition upon multiple external stimuli such as heating/cooling, shaking/resting, or the addition of different anions. Interestingly, exposure of the supramolecular gel film to HCl or ammonia vapor induces the change in the film fluorescence intensity, endowing this system with a potential application in gas detecting.

BODIPY-BODIPY dyad: assessing the potential as a viscometer for molecular and ionic liquids

A symmetrical BODIPY-BODIPY dyad with a diyne linker was prepared in two steps; the lifetime decay of this rotor appeared to correlate with the viscosity of the media, thus making this dyad a suitable small molecule viscometer for molecular solvents. The potential of using the rotor to probe the viscosity of ionic liquids was also investigated.

Multi-responsive behavior of highly water-soluble poly(3-hexylthiophene)-block-poly(phenyl isocyanide) block copolymers

Highly water-soluble block copolymers containing conjugated poly(3-hexylthiophene) and hydrophilic poly(phenyl isocyanide) segments were found to be responsive to multiple stimuli, such as temperature, pH and solvents.

Synthesis of multifunctional poly(1-pyrenemethyl methacrylate)-b-poly(N-isopropylacrylamide)-b-poly(N-methylolacrylamide)s and their electrospun nanofibers for metal ion sensory applications

PPy-b-PNIPAAm-b-PNMA multifunctional triblock copolymers based electrospun nanofibers detected temperature variation or metal-ions with a high sensitivity.

Fluorescent nanothermometers based on mixed shell carbon nanodots

Nanothermometers composed of a carbon nanodot core and thermo-sensitive polymeric mixed shell are prepared. Solution temperature can be traced through monitoring the fluorescence intensity variation of carbon nanodot.

High resolution imaging beyond the acoustic diffraction limit in deep tissue via ultrasound-switchable NIR fluorescence

Fluorescence imaging in deep tissue with high spatial resolution is highly desirable because it can provide details about tissue's structural, functional, and molecular information. Unfortunately, current fluorescence imaging techniques are limited either in penetration depth (microscopy) or spatial resolution (diffuse light based imaging) as a result of strong light scattering in deep tissue. To overcome this limitation, we developed an ultrasound-switchable fluorescence (USF) imaging technique whereby ultrasound was used to switch on/off the emission of near infrared (NIR) fluorophores. We synthesized and characterized unique NIR USF contrast agents. The excellent switching properties of these agents, combined with the sensitive USF imaging system developed in this study, enabled us to image fluorescent targets in deep tissue with spatial resolution beyond the acoustic diffraction limit.

Activation of BODIPY fluorescence by the photoinduced dealkylation of a pyridinium quencher

The photoinduced cleavage of a 2-nitrobenzyl group from a pyridinium quencher covalently attached to the meso position of a BODIPY fluorophore activates the emission of the latter. This photochemical transformation prevents the transfer of one electron from the BODIPY platform to its heterocyclic appendage upon excitation and, as a result, permits the radiative deactivation of the excited fluorophore. This versatile mechanism for fluorescence switching can translate into the realization of an entire family of photoactivatable fluorophores based on the outstanding photophysical properties of BODIPY chromophores.

Thermometry at the nanoscale

Non-invasive precise thermometers working at the nanoscale with high spatial resolution, where the conventional methods are ineffective, have emerged over the last couple of years as a very active field of research. This has been strongly stimulated by the numerous challenging requests arising from nanotechnology and biomedicine. This critical review offers a general overview of recent examples of luminescent and non-luminescent thermometers working at nanometric scale. Luminescent thermometers encompass organic dyes, QDs and Ln(3+)ions as thermal probes, as well as more complex thermometric systems formed by polymer and organic-inorganic hybrid matrices encapsulating these emitting centres. Non-luminescent thermometers comprise of scanning thermal microscopy, nanolithography thermometry, carbon nanotube thermometry and biomaterials thermometry. Emphasis has been put on ratiometric examples reporting spatial resolution lower than 1 micron, as, for instance, intracellular thermometers based on organic dyes, thermoresponsive polymers, mesoporous silica NPs, QDs, and Ln(3+)-based up-converting NPs and β-diketonate complexes. Finally, we discuss the challenges and opportunities in the development for highly sensitive ratiometric thermometers operating at the physiological temperature range with submicron spatial resolution.

Synthesis, morphology, and sensory applications of multifunctional rod-coil-coil triblock copolymers and their electrospun nanofibers

We report the synthesis, morphology, and applications of conjugated rod-coil-coil triblock copolymers, polyfluorene-block-poly(N-isopropylacrylamide)-block-poly(N-methylolacrylamide) (PF-b-PNIPAAm-b-PNMA), prepared by atom transfer radical polymerization first and followed by click coupling reaction. The blocks of PF, PNIPAAm, and PNMA were designed for fluorescent probing, hydrophilic thermo-responsive and chemically cross-linking, respectively. In the following, the electrospun (ES) nanofibers of PF-b-PNIPAAm-b-PNMA were prepared in pure water using a single-capillary spinneret. The SAXS and TEM results suggested the lamellar structure of the PF-b-PNIPAAm-b-PNMA along the fiber axis. These obtained nanofibers showed outstanding wettability and dimension stability in the aqueous solution, and resulted in a reversible on/off transition on photoluminescence as the temperatures varied. Furthermore, the high surface/volume ratio of the ES nanofibers efficiently enhanced the temperature-sensitivity and responsive speed compared to those of the drop-cast film. The results indicated that the ES nanofibers of the conjugated rod-coil block copolymers would have potential applications for multifunctional sensory devices.

Luminescent Diazaborolyl-Functionalized Polystyrene

We present two different procedures for the synthesis of poly[4-(1',3'-diethyl-1',3',2'-benzodiazaborolyl)styrene] (3a) and poly[4-(1',3'-diphenyl-1',3',2'-benzodiazaborolyl)styrene] (3b). The new polymers were fully characterized by GPC, multinuclear NMR, and elemental analysis. The thermal properties and stability were studied by DSC and TGA, and the optical characteristics were examined by absorption and time-resolved fluorescence spectroscopy. Remarkably high quantum yields of up to 77% were measured. In comparison to molecular species we found significantly shorter lifetimes, likely as a result of incorporation of the chromophores into the polymer structure.

Polymeric assemblies and nanoparticles with stimuli-responsive fluorescence emission characteristics

Fluorescent polymeric assemblies and nanoparticles (NPs) of nanoscale dimensions have become a focus of intensive investigations during the past few decades due to combined advantages such as improved biocompatibility, water dispersibility, stimuli-responsiveness, facile integration into optical detection devices, and the ability of further functionalization. In addition, the chemical composition and morphology of polymeric assemblies and NPs can be modulated via synthetic approaches, leading to the precise spatial organization of multiple fluorophores. Thus, polymeric assemblies and NPs have been utilized to optimize the photoluminescent properties of covalently or physically attached fluorophores and facilely modulate the fluorescence resonance energy transfer (FRET) processes when the polymeric matrix is endowed with stimuli-responsiveness. These fascinating fluorescent polymeric assemblies and NPs offer unique and versatile platforms for the construction of novel detection, imaging, biolabeling, and optoelectronic systems. This feature article focuses on the recent developments of polymeric assemblies and NPs-based stimuli-tunable fluorescent systems and highlights their future practical applications with selected literature reports.