pH-Responsive Rhodamine Nanotube Capable of Self-Reporting the Assembly State

Nanomaterials that respond to intracellular signals, such as pH, have the potential for many biomedical applications, such as drug delivery, because the assembly/disassembly process can be tailored to respond to a stimulus characteristic of a specific subcellular location. In this work, two rhodamine-peptides that form stable nanotubes at physiological pH but dissociate into highly fluorescent monomers within the acidified interior of endosomal/lysosomal cellular compartments have been developed. The rhodamine dipeptide conjugates, NH2-KK(RhB)-NH2 (RhB-KK) and NH2-EK(RhB)-NH2 (RhB-KE) with rhodamine B chromophores appended at the ε-amino position of a lysine residue, were shown to assemble into well-defined nanotubes at pH values above ∼4-5 and to dissociate into a fluorescent monomer state at lower pH values. The pH dependence of the assembly process was investigated using circular dichroism (CD) and fluorescence spectroscopy along with transmission electron microscopy (TEM), atomic force microscopy (AFM), and confocal imaging. Although the ring opening/closing transition of the rhodamine chromophore took place at pH 4.1 for both peptides, the onset of assembly began at pH 4.6 for RhB-KE and at a comparatively more basic pH (5.8) for RhB-KK. Accordingly, the rhodamine-peptides interconverted between three pH-dependent states: an open-ring, monomeric state (λmax 580 nm, λex 550 nm) at pH values at or below ∼4.6; a closed-ring, nanotube form that exhibits AIEE (λmax 460 nm, λex = 330 nm) at higher pH values; a closed-ring, nonemissive monomeric state that emerged below the critical micelle concentrations (CMC). The pH-responsive features of the peptides were evaluated by live-cell imaging in three cancer cell lines using confocal laser scanning microscopy (CLSM). Visualizing the cells after incubation with either RhB-KE or RhB-KK produced CLSM images with a punctate appearance in the Texas Red channel that colocalized with the lysosomes. These experiments indicate that the nanotubes were rapidly trafficked into the acidic lysosomal compartments within the cells, which induced dissociation into a monomeric, open state. Uptake inhibition studies suggested that cellular uptake was mediated by either caveolae- or clathrin-mediated endocytosis, depending on the cell line studied.

Advances on fluorescence chemosensors for selective detection of water

Water, although an important part of everyday life, is acts as one of the most significant contaminants in various applications such as biomedical monitoring, chemical production, petroleum-based fuel and food processing. In fact, the presence of water in other solvents is a huge concern. For the quantification of trace water content, different methods such as Karl-Fischer, electrochemical, nuclear magnetic resonance, chromatography, and thermogravimetric analysis have been used. Although every technique has its own benefit, each one suffers from several drawbacks that include high detection costs, lengthy procedures and specialized operations. Nowadays, the development of fluorescence-based chemical probes has become an exciting area of research for the quick and accurate estimation of water content in organic solvents. A variety of chemical processes such as hydrolysis reaction, metal ions promoted oxidation reaction, suppression of the -C═N isomerization, protonation and deprotonation reactions, and molecular aggregation have been well researched in the last few years for the fluorescent detection of trace water. These chemical processes eventually lead to different photophysical events such as aggregation-induced emission (AIE), aggregation-induced emission enhancement (AIEE), aggregation-caused quenching (ACQ), fluorescent resonance energy transfer (FRET), charge transfer, photo-induced electron transfer (PET), excited state intramolecular proton transfer (ESIPT) that are responsible for the detection. This review presents a summary of the fluorescence-based chemosensors reported in recent years. The design of water sensors, sensing mechanisms and their potential applications are reviewed and discussed.

Water bridges as the trigger in an amino functionalized Zn-MOF for highly selective and sensitive fluorescent sensing of water

Water is a fundamental element for life. The highly selective and sensitive sensing of water is always attractive for mankind in activities such as physiological processes study and extraterrestrial life exploration. Fluorescent MOFs with precise channels and functional groups might specifically recognize water molecules with hydrogen-bond interaction or coordination effects and work as water sensors. As a proof of concept, herein, an amino functionalized Zn-MOF (named as complex 1) with pores that just right for water molecules to form hydrogen bond bridges is revealed for highly selective and sensitive fluorescent sensing of water. The single-crystal X-ray diffraction analysis indicates that the 3D framework of complex 1 is functionalized with free amino groups in the channels. Hydrogen bonds formed in the channel along b-axis as water bridges to connect two adjacent NH2bdc ligands and result in the restriction of intramolecular motions (RIM) which could responsible for the selective turn-on fluorescence response to water. Complex 1 exhibits high sensitive to trace amount of water in organic solvents and could be used for water detection in a wide range water contents. Take advantages of complex 1, portable sensors (complex 1@PMMA) were prepared and used in the highly sensitive water sensing.

Nonpolar selective emission (NPSE) of carbonyl-bridged rhodols

Polarity-responsive luminophores (PRLs), whose emission properties change in response to the polarity of the surrounding environment, are used for the fluorescence sensing of intracellular environments and various chemical compounds. Herein, we propose a concept called nonpolar selective emission (NPSE) for the development of a new PRL family. Unlike the conventional emission of PRLs, the NPSE luminophore can switch to a completely non-emissive state upon a slight increase in solvent polarity. The NPSE concept offers a new means of distinguishing between nonpolar and low-polarity environments. Moreover, the NPSE property is little affected by the viscosity of the surrounding medium. We demonstrate that NPSE dyes can be used as emission sensors for molecular gases. Furthermore, we discovered the potential use of NPSE dyes as a time-dependent security ink triggered by the volatilization of polar molecules.

Determination of water content in dimethyl sulfoxide/N,N-dimethyl formamide and methanol content in ethanol by solvatochromism of azo dye, 2-(tert-butyl)-4-methoxy-6-(naphthalen-1-yldiazenyl) phenol

A novel 2-(tert-butyl)-4-methoxy-6-(naphthalen-1-yldiazenyl)phenol (NAP) was synthesized by coupling reaction of 2-tert-butyl-4-methoxyphenol with diazotized naphthylamine as diazo component. The azo dye was characterized by NMR, FT-IR and UV-vis spectroscopic techniques. The visible spectrum of NAP was recorded in different solvents and at different pHs. NAP exhibited a large wavelength shift with increasing solvent polarity, showing significant color change over a wide range in different solvents. The determination of water content in organic solvents miscible with water such as dimethyl sulfoxide (DMSO)/N,N-dimethyl formamide (DMF) and methanol content in ethanol, which is also a common mixture were investigated with NAP which is azo dye. The present reported solvatochromic compound for the determination of water content in DMSO/DMF and methanol content in ethanol showed a fairly wide linear range compared to some previously reported solvatochromic compounds in the literature. In addition, the solvatochromism of NAP allows the determination of methanol content in ethanol, which has caused many deaths, with a fast, cheap and easy method.

Fluorescence sensors for detection of water based on tetraphenylethene–anthracene possessing both solvatofluorochromic properties and aggregation-induced emission (AIE) characteristics

TPE-(An-CHO)4 has been developed as an SFC (solvatofluorochromism)/AIEE (aggregation-induced emission enhancement)-based fluorescence sensor for detection of water over a wide range from low to high water content regions in solvents.

An inquisitive fluorescence method for the real-time detection of trace moisture in polar aprotic solvents with the application of water rancidity in foodstuffs

A simple fluorometric approach to quantify atmospheric moisture incorporation in polar aprotic solvents with application for moisture sensitive oil-based foods is reported.

Naked eye detection of moisture in organic solvents and development of alginate polymer beads and test cassettes as a portable kit

New dabsyl-thiophene based receptor DABT and its mercury complex DABT-Hg is reported as a colorimetric sensor for rapid and sensitive detection of trace amount of water in aprotic solvents. Based on intramolecular charge transfer in the excited state, the receptor dabsyl-thiophene (yellow color) binds with the mercury ions (magenta color) to stimulate a colorimetric response. The mercury complex is used as a moisture sensor in THF, acetone, and acetonitrile due to its instability in moisture containing organic solvents. The probe exhibits higher sensitivity towards water in THF (LOD = 0.0041% w/w), acetone (LOD = 0.0144% w/w) and acetonitrile (LOD = 0.1008% w/w). The dissociation of mercury from probe DABT-Hg in the presence of water is accountable for the colorimetric response as proven by the ¹H NMR and ESI-MS studies. DABT-Hg is the first mercury based complex for the detection of moisture in organic solvents. Test paper strip and PVA thin film doped with the probe were successfully used to detect moisture content in organic solvents. DABT-Hg incorporated alginate beads are prepared to determine the water content in triethylamine and ethylene glycol. Portable test cassettes are developed for the on-site detection of distilled and undistilled wet solvents in the chemical laboratory through naked-eye detection.

Development of optical sensor for water in acetonitrile based on propeller-structured BODIPY-type pyridine-boron trifluoride complex

A propeller-structured 3,5,8-trithienyl-BODIPY-type pyridine–boron trifluoride complex, ST-3-BF₃, which has three units of 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile at the 3-, 5-, and 8-positions on the BODIPY skeleton, was designed and developed as an intramolecular charge transfer (ICT)-type optical sensor for the detection of a trace amount of water in acetonitrile. The characterization of ST-3-BF₃ was successfully determined by FTIR, ¹H and ¹¹B NMR measurements, high-resolution mass spectrometry (HRMS) analysis, thermogravimetry-differential thermal analysis (TG-DTA), photoabsorption and fluorescence spectral measurements, and density functional theory (DFT) calculations. ST-3-BF₃ showed a broad photoabsorption band in the range of 600 to 800 nm, which is assigned to the S₀ → S₁ transition of the BODIPY skeleton with the expanded π-conjugated system over the 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile units at the 3-, 5-, and 8-positions onto the BODIPY core. In addition, a photoabsorption band was also observed in the range of 300 to 550 nm, which can be assigned to the ICT band between the 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile units at 3-, 5-, and 8-positions and the BODIPY core. ST-3-BF₃ exhibited a characteristic fluorescence band originating from the BODIPY skeleton at around 730 nm. It was found that by addition of a trace amount of water to the acetonitrile solution of ST-3-BF₃, the photoabsorption band at around 415 nm and the fluorescence band at around 730 nm increased linearly as a function of the water content below only 0.2 wt%, which could be ascribed to the change in the ICT characteristics due to the dissociation of ST-3-BF₃ into ST-3 by water molecules. Thus, this work demonstrated that the 3,5,8-trithienyl-BODIPY-type pyridine–boron trifluoride complex can act as a highly-sensitive optical sensor for the detection of a trace amount of water in acetonitrile.

Propeller-structured 3,5,8-trithienyl-BODIPY-type pyridine–boron trifluoride complex, ST-3-BF₃, has been developed as an intramolecular charge transfer (ICT)-type optical sensor for the detection of a trace amount of water in acetonitrile.

Visual monitoring of trace water in organic solvents based on ecofriendly b/r-CDs ratiometric fluorescence test paper

Developing a green, non-toxic and easy to synthesize of fluorescence probe for fast and visual detecting trace water in various organic solvents was an important task. Here, a novel dual-emission fluorescence probe (b/r-CDs) was designed based on the red CDs (r-CDs) and blue CDs (b-CDs) to detect the trace water and enhance the visualization for naked-eye observation in different organic solvents. Among, the red fluorescence carbon dots (CDs) was found to have the capability to monitor trace amounts of water, which synthesized with green tea by facile ultrasonic method. Further, Such a dual-emission probe could fast monitor trace water in various organic solvents with high stability and fast response. Importantly, a synergistic mechanism of the dynamic process (b-CDs) and static quenching (r-CDs) was proved for the study of water detection. Moreover, the test paper was made for detecting trace water in different organic solvents, achieving convenient and effective detection.

Two 8-hydroxyquinoline-based fluorescent chemosensors for ultra-fast and sensitive detection of water content in strong polar organic solvents with large Stokes shifts

It is of great significance to detect the moisture in organic solvents before used in water-sensitive reactions. Herein, two Schiff base quinoline derivatives, 8-hydroxyquinoline-2-carboxaldehyde thiosemicarbazone (HQCT) and 8-hydroxyquinoline-2-carboxaldehyde (pyridine-2-carbonyl)-hydrazine (HQPH), were designed and synthesized by a simple one-step reaction, and used as fluorescent chemosensors for ultra-fast and sensitive detection of water content in strong polar organic solvents. Based on excited-state intramolecular proton transfer (ESIPT) process, HQCT and HQPH exhibited strong fluorescence emissions with large Stokes shifts in dimethyl sulfoxide (DMSO) and N, N-dimethylformamide (DMF) solvents compared to other various organic solvents, and their fluorescence quenching and fluorescent color changes were obviously observed with increasing water content. The experimental results revealed that the hydroxyl groups substituted at the 8-position of HQCT and HQPH played a key role in the fluorescence emission processes. Dynamic light scattering (DLS) and ¹H NMR titration indicated that the sensing mechanism for the detection of water was based on inhibition of the ESIPT by H₂O via forming hydrogen bonds. In the range of 0.0-1.8 wt%, the fluorescence intensity of chemosensors changed as a linear function of water content. The detection limits of water in DMSO by HQCT and HQPH were as low as 0.0220 wt% and 0.0274 wt%, respectively. Moreover, HQCT and HQPH are successfully applied for the detection of moisture content in real commercial organic solvents.

[Development of External Stimuli-responsive Organic π Molecules and the Creation of Novel Photo-functions]

We have found that the spiro form of aminobenzopyranoxanthene (ABPX) exhibits dual solvatochromic and nanoaggregate fluorescence in organic solvents by spectrophotometric and theoretical analyses. The dual fluorescence properties of ABPX were adjustable in response to water content, and served as a new detection principal for naked-eye visualization (above 0.5 wt%) and quantification (0.010-0.125 wt%) of water in tetrahydrofuran. In investigating the optical properties of a dicationic form of ABPX, ABPX containing linear n-alkyl chains at amino groups were then synthesized. These ABPX exhibited fluorescence emission in the far-red and NIR wavelength regions, and we noted an increased fluorescence quantum efficiency with increasing n-alkyl chain length. To further improve the fluorescence quantum yields, we have designed and synthesized ABPX with different nitrogen-containing fused rings. It was kinetically demonstrated that the structurally rigid conjugation of the xanthene moiety is an effective molecular design for the drastic enhancement of fluorescence emission efficiency.

Colorimetric and ratiometric fluorescence sensing of water based on 9-methyl pyrido[3,4-b]indole-boron trifluoride complex

In this work, 9-methyl pyrido[3,4-b]indole-boron trifluoride complex, 9-MP-BF3, was designed and developed as a colorimetric and ratiometric fluorescent sensor for the detection of water in the low- and high-water-content regions in solvents. In the low-water-content region, a new photoabsorption band at around 360 nm and a fluorescence band at around 370 nm gradually appeared due to the dissociation of 9-MP-BF3 into 9-methyl pyrido[3,4-b]indole (9-MP) by water molecules with a simultaneous decrease in the photoabsorption band at around 390 nm and the fluorescence band at around 460 nm originating from 9-MP-BF3. In the moderate-water-content region, the photoabsorption band at around 360 nm and the fluorescence band at around 370 nm gradually shifted to a longer wavelength region with an increase in the fluorescence intensity, which could be ascribed to the formation of a hydrogen-bonded complex (9-MP-H2O) with water molecules. Furthermore, in the high-water-content region, two photoabsorption bands at around 305 nm and 390 nm and one fluorescence band at around 460 nm gradually reappeared with simultaneous decrease in the photoabsorption band at around 290 nm and the fluorescence band at around 370 nm, which was attributed to the formation of a hydrogen-bonded proton transfer complex (9-MP-H+) with water molecules. Thus, this work revealed the mechanism of a colorimetric and ratiometric fluorescent sensor based on pyrido[3,4-b]indole-boron trifluoride complex for the detection of water over a wide range from low water content to high water content in solvents.

A simple colorimetric sensor for the detection of moisture in organic solvents and building materials: applications in rewritable paper and fingerprint imaging

A simple off-the-shelf dye molecule, 1,4-dihydroxy-9,10-anthraquinone or quinizarin (1), has been investigated for the effective detection of moisture in organic solvents and building materials. Anion-induced deprotonation of 1 to 1.F followed by re-protonation with water is the working principle of the sensor system. Changes in colour, UV-Vis spectra and emission intensity indicate the moisture detection of 1.F in various organic solvents. The probe 1.F is more effective at the detection of water in acetonitrile and THF with a LOD of 0.0011 and 0.0026 wt%. Probe 1.F is reversible, reusable, highly selective, and sensitive and has a fast response time both in solution phase and in test papers. Probe 1.F is also applied for the detection of unknown moisture content in raw building materials such as cement, fly ash, foundry sand, and limestone. 1.F incorporated cellulose-based papers are applicable for inkless writing and stamping in the read-erase manner. Furthermore, these papers are also suitable for fingerprint imaging and sweat pore mapping by the simple colour change method.

Development of an intramolecular charge transfer-type colorimetric and fluorescence sensor for water by fusion with a juloidine structure and complexation with boron trifluoride

An intramolecular charge transfer-type optical sensor fused with a juloidine structure and complexed with boron trifluoride can detect and determine water over a wide concentration range.

Colorimetric and fluorometric dual sensing of trace water in methanol based on a Schiff Base-Al3+ ensemble probe

A new julolidine based Schiff base receptor (L) was synthesized and characterized. L forms a 1:1 complex with Al³⁺ in methanol, resulting in an immediate color change from chartreuse to orange and a remarkable enhancement in its emission intensity along with a bathochromic shift from 540 nm to 570 nm. Addition of trace amounts of water significantly quenches the fluorescence emission, where a decomplexation of Al³⁺ from the L-Al³⁺ complex takes place. The significant quenching effect indicated that the L-Al³⁺ ensemble system can be used to detect trace water in commercial methanol. From the fluorescence titration, the detection limit for sensing water in methanol was estimated to be 0.0047%. We have also made an easy-to-prepare test strip of L-Al³⁺ to detect water in methanol through naked-eye observation, which is possible to realize in situ monitoring.

Tetraphenylethene– and diphenyldibenzofulvene–anthracene-based fluorescence sensors possessing photo-induced electron transfer and aggregation-induced emission enhancement characteristics for detection of water

RS-1 and RS-2 have been developed as PET/AIEE hybrid fluorescence sensors for detection of water in the low and high water content regions in solvents.

Gaining insight into the photophysical properties of a coumarin STP ester with potential for bioconjugation

The photophysical properties of a coumarin 392 4-sulfotetrafluorophenyl ester, C392STP (sodium (E/Z)-4-(4-(2-(6,7-dimethoxycoumarin-3-yl)vinyl)-benzoyl)-2,3,5,6-tetrafluoro-benzenesulfonate), an amine reactive coumarine with potential for bioconjungation, have been studied in different solvents.

Ratiometric fluorescence detection of trace water in organic solvents based on aggregation-induced emission enhanced Cu nanoclusters

A novel dual-emission fluorescent nanocomposite material, CDs/Cu NCs, was fabricated for detecting trace water in organic solvents (DMSO, DMF, THF, and ACN).

Synthesis of Aminobenzopyranoxanthenes with Nitrogen-Containing Fused Rings

An efficient and practical method for the synthesis of a variety of aminobenzopyranoxanthenes (ABPXs) with different nitrogen-containing fused rings was developed. On the basis of the mechanistic studies of the formation of the xanthene framework, the presented methodology was developed to facilitate access to previously inaccessible asymmetric ABPXs.