Enhancing the optical path inside a capillary without sacrificing light intensity: application to a compact and highly sensitive photometer

In order to increase the optical path and related sensitivity of photometers, multiple axial-reflection of parallel light-beam inside a capillary cavity is one of the most effective ways. However, there is a non-optimum trade-off between optical path and light intensity, e.g., smaller aperture on cavity mirror can increase multiple axial-reflection times (i.e., longer optical path) due to the lower cavity-loss, but it would also reduce coupling efficiency, light intensity, and related signal-to-noise ratio. Herein, an optical beam shaper, which is composed of two optical lenses with an apertured mirror, was proposed to focus the light beam (i.e., increasing coupling efficiency) without deteriorating beam parallelism and related multiple axial-reflection. Thus, by combining the optical beam shaper with a capillary cavity, large optical path enhancement (10-fold of capillary length) and high coupling efficiency (>65%) can be realized simultaneously, where the coupling efficiency was improved 50-fold. An optical beam shaper photometer (with a 7 cm long capillary) was fabricated and applied to detect water in ethanol with a detection limit of 12.5 ppm, which is 800-fold and 32∼80 fold lower than that of the commercial spectrometer (1 cm cuvette) and previous reports, respectively.

A new fluorescent probe for sensing Al3+ ions in the solution phase and CH3COO- in the solid state with aggregation induced emission (AIE) activity

An AIE (aggregation induced emission) active probe DFP-AMQ was designed and synthesized as a hexa-coordinated N₂O donor chelator for the selective sensing of Al³⁺ colorimetrically as well as fluorimetrically with a 27-fold fluorescence enhancement for CH₃CN-H₂O (9 : 1, v/v, pH 7.2, HEPES buffer). The fluorescence enhancement occurred through the blocking of ESIPT, chelation enhanced fluorescence effect (CHEF) arose, and as a result fluorescence enhancement was observed through 1 : 1 complexation with Al³⁺ ions. Detailed spectroscopic studies including UV-Vis, FTIR, ¹H NMR, and HRMS studies were carried out to characterize the probable structure of DFP-AMQ including the complexation of DFP-AMQ with Al³⁺ ions. The spectrophotometric and spectrofluorimetric titrations revealed strong binding towards Al³⁺ and the K _d values were obtained from UV-Vis (3.26 × 10^-5 M^-1) and fluorescence titration (2.02 × 10^-5 M^-1). The limit of detection of Al³⁺ by DFP-AMQ was 1.11 μM. The quantum yields of DFP-AMQ and [DFP-AMQ-Al]⁺ were calculated to be 0.008 and 0.211, respectively. Dynamic light scattering (DLS) studies showed that the sizes of the particles increased with increasing water percentage due to aggregation. SEM (scanning electron microscopy) studies revealed interesting morphological changes in microstructures in which DFP-AMQ demonstrated a rod-like shape, which was converted to a spherical-like shape in the presence of Al³⁺ and when DFP-AMQ aggregated in H₂O it showed aggregated block-like shape. In the solid phase, DFP-AMQ with nitrate has no particular shape, but in the presence of acetate, it converts to stone-like shape. This probe (DFP-AMQ) could be employed for on-site Al³⁺ ion detection in the solid state.

A novel fluorescence probe for the detection of water content in organic solvents and the distinction between deuterated and nondeuterated reagents

A novel D-π-A type fluorescent probe (probe 1) was developed for water content detection in organic solvents. By analyzing the relationship between fluorescence and water content, the probe was successfully applied to determine trace water content in tetrahydrofuran, ethyl acetate, 2-butanone, acetone, dimethylformamide, and acetonitrile. High water content in THF and ethyl acetate was associated with a gradual colour change from yellowish green to earthy yellow. The red/green value had a linear relationship with the water content in THF and ethyl acetate. There was a linear relationship between the red/blue value and water content in 2-butanone and acetone. Furthermore, probe 1 could be used for human serum albumin detection. Unexpectedly, probe 1 had a different colour response in deuterated and nondeuterated solvents, and had different fluorescence intensity and fluorescence emission wavelength. Probe 1 is rare tool that can distinguish between deuterated and nondeuterated reagents.

A novel ratiometric fluorescent probe for water content in ethanol and temperature sensing

In this paper, two tetrahydo[5]helicene-based dyes (THH1 and THH2) were studied on their twist intramolecular charge transfer. And the water-soluble blue carbon dots (N-CDs) were synthesized. Considering that their different optical propeties, the ratiometric fluorescence probes N-CDs/THH1 and N-CDs/THH2 were constructed by mixing N-CDs and dyes simply. It was found that N-CDs/THH1 and N-CDs/THH2 could quickly and sensitively detect water content in ethanol, with linear range of 0.500-25.0 vol% and 0.500-30.0 vol%, respectively. What's more, through the actual sample test, it showed that the detection had good accuracy and precision. At the same time, it was found that two ratiometric probes could also be applied to the thermometry with good reversibility based on optical temperature sensors.

Spectroscopic and Computational Studies on a Dansyl Based Luminescent Probe: Detection of Water Contaminant in Hygroscopic Deuterated Solvents

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A Dansyl functionalized fluorescent probe (DFFP) has been intended, synthesized, and well-characterized (NMR, IR, Mass, SEM, SCXRD), capable of sensing trace amounts of water contaminant in hygroscopic deuterated solvents by changing colour under UV irradiation. A distinct bathochromic shift in emission spectra of probe DFFP and the visual colour change (Green to Yellow) under UV lamp are the key evidence of the presence of water contaminant. To prove the potentiality of the probe while detecting the remnant water, we did some experimental studies along with exhaustive theoretical evaluation. DFT (Energy optimization and other calculations) helped in better understanding the sensing mechanism and the mode of interactions among probe-water-solvent. Total electron density mapped over Electrostatic Potential Surface and calculation of ESP charges helped in locating more electron-dense regions in the ground state. The involvement of TD-DFT studies helped in finding the possible electronic transitions and corresponding absorption bands. Moreover, the probe is capable of sensing ethanolic water vapour in the gaseous phase. Due to high fluorescence and being nontoxic to cells, probe DFFP could be used as a potential cell imaging dye. It has been employed in a human cancer cell line (A549), and fluorescent confocal microscopic images were obtained.

A chromo-fluorogenic probe for detecting water traces in aprotic solvents based on C2-symmetry dianthrimide-hydroxide complexes: experimental and theoretical studies

A C₂-symmetry dianthrimide based probe (D) and its hydroxide complex (D-OH) are reported as a chromo-fluorogenic sensor for rapid and sensitive detection of trace amounts of water in polar aprotic solvents. Based on intramolecular charge transfer in the excited state, the pink-coloured probe binds with the hydroxide ions to induce a colorimetric response of the resulting complex (D-OH), green in colour. The hydroxide based complex is used as a H₂O or moisture sensor, tested in DMF and DCM, due to its high instability in moisture-containing organic solvents and paper materials/fabrics. The probe exhibits higher sensitivity towards pure H₂O in DMSO with the LOD measured at 0.0067% v/v, perhaps even lower, in DMF (LOD = 0.100% v/v) and DCM (LOD = 0.013% v/v). The dissociation of OH^- from D in the presence of H₂O is responsible for the colorimetric and fluorometric responses. The litmus test paper strips prepared by adsorbing or coating them with the D-OH complex in DMSO could not be entirely achieved in an open system, due to the highly unstable state of the complex in the presence of water traces or the atmospheric moisture accumulated in the paper materials. The complex D-OH is also highly suspected to compete for adsorbed water in silica gel crystals in desiccators, due to its high affinity towards water molecules. The experimental studies were complemented by theoretical calculations using the Spartan'14 software package, and the computed data are in good agreement with the spectral data.

Low-level detection of water in polar aprotic solvents using an unusually fluorescent spirocyclic rhodamine

Detection of trace amount of water in polar aprotic solvents (acetonitrile) by novel fluorescent spirocyclic rhodamine (sDRh).

Fluorescent sensor for water based on photo-induced electron transfer and Förster resonance energy transfer: anthracene-(aminomethyl)phenylboronic acid ester-BODIPY structure

An anthracene-(aminomethyl)phenylboronic acid ester-BODIPY (DJ-1) was designed and developed as a fluorescent sensor based on photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET) for the detection of a trace amount of water in solvents, where the anthracene skeleton and BODIPY skeleton are the donor fluorophore and the acceptor fluorophore in the FRET process, respectively. It was found that the addition of water to organic solvents containing DJ-1 causes both the suppression of PET in the anthracene-(aminomethyl)phenylboronic acid ester as the PET-type fluorescent sensor skeleton and the energy transfer from the anthracene skeleton to the BODIPY skeleton through a FRET process, thus resulting in the enhancement of the fluorescence band originating from the BODIPY skeleton. This work demonstrates that the PET/FRET-based fluorescent dye composed of the donor fluorophore possessing PET characteristics and the acceptor fluorophore in the FRET process can act as a fluorescent sensor with a large SS for the detection of a trace amount of water in solvents.

An anthracene-(aminomethyl)phenylboronic acid ester-BODIPY (DJ-1) structure was developed as a fluorescent sensor based on photo-induced electron transfer (PET) and Förster resonance energy transfer (FRET) for detection of water in solvents.

Light-induced morphological transition between unconjugated bilirubin photoisomers

Morphology switching by an external stimulus creates the possibility to detect and control the activity and functionality of bio-molecules. Unconjugated bilirubin (UCB), a waste product resulting from heme catabolism, is highly sensitive towards blue light induced configurational conversion from (4Z,15Z) to (4Z,15E)-bilirubin. UCB has a distinct elongated nanostructure which is readily photoswitchable to spherical by external blue light (470 nm) irradiation. Herein, the morphology alteration by blue light was nicely correlated with the photoisomerisation of UCB, using different microscopic and spectroscopic techniques. To restrict the other photo-incidents during phototreatment on UCB, a suitable time frame was calibrated by monitoring the absorption, HPLC, lifetime distribution and ¹H NMR studies. Furthermore, by the help of this morphological transition as a marker, UCB early stage photoisomerisation has also been triggered by two-photon irradiation (940 nm).

A sensitive fluorescent sensor for the detection of trace water in organic solvents based on carbon quantum dots with yellow fluorescence

The quantitative analysis of trace water in organic solvents has always been a research hotspot, and it is still in the development stage and needs to be continuously developed. In this study, a facile and rapid approach was developed for the preparation of carbon quantum dots (CQDs) with yellow fluorescence emission and ultrahigh absolute fluorescence quantum yields (92.6%). Compared to traditional organic fluorescent molecules, the preparation of CQDs is simpler, faster and more environmentally friendly. It is found that the fluorescent properties of CQDs are excellent in organic solvents and could be quenched by trace water, which makes them a promising material used without any modification for the detection of water in organic solvents. As a result, the as-prepared CQDs were adopted as fluorescent probes for the detection of water in organic solvents (ethanol, tetrahydrofuran, and 1,4-dioxane). The limit of detection was as low as 0.01%. To the best of our knowledge, this is the first time that CQDs have been used as water sensing fluorescent probes in organic solvents. The possible mechanism for trace water detection of the as-prepared CQDs in organic solvents is attributed to the specific water-fluorophore interaction and partially to the increase in polarity of the solvent caused by an increase in water concentration.

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.

A New Lysosome-Targetable Turn-On Fluorogenic Probe for Carbon Monoxide Imaging in Living Cells

A lysosome-targetable fluorogenic probe, LysoFP-NO₂, was designed and synthesized based on a naphthalimide fluorophore that can detect selectively carbon monoxide (CO) in HEPES buffer (pH 7.4, 37 °C) through the transformation of the nitro group into an amino-functionalized system in the presence of CO. LysoFP-NO₂ triggered a "turn-on" fluorescence response to CO with a simultaneous increase of fluorescence intensity by more than 75 times. The response is selective over a variety of relevant reactive nitrogen, oxygen, and sulfur species. Also, the probe is an efficient candidate for monitoring changes in intracellular CO in living cells (MCF7), and the fluorescence signals specifically localize in the lysosome compartment.

A novel fluorescent sensor for water in organic solvents based on dynamic quenching of carbon quantum dots

A novel fluorescent sensor for water in organic solvents was proposed based on dynamic quenching of carbon quantum dots.

A naphthalimide-based fluorescence ‘‘turn-on’’ chemosensor for highly selective detection of carbon monoxide: imaging applications in living cells

A naphthalimide-based fluorescence chemosensor, COFP, was designed and synthesized for the detection of carbon monoxide (CO) in HEPES buffer (pH 7.4, 37 °C).

A turn-on green channel Zn2+ sensor and the resulting zinc(ii) complex as a red channel HPO42− ion sensor: a new approach

A turn-on green channel Zn²⁺ sensor and the resulting zinc(ii) complex as a red channel HPO₄²⁻ ion sensor: a new approach.

Fluorescent and colorimetric sensors for the detection of humidity or water content

In this tutorial review, we describe the current state of the art in water sensors and provide an overview of the major advances made in this field post 2000. The field is currently still in its early development stages and subject to continuous improvements, and the current work provides a structured approach describing different sensing mechanisms and potential future applications associated with each of these. With these developments and their potential implications for the diverse scientific fields requiring tight control over the water content, we strongly believe the discipline is potentially at the threshold of translation into more widespread application and we hope the current review might allow for an expedited process thereof.

Development of a cell permeable red-shifted CHEF-based chemosensor for Al(3+) ion by controlling PET

A structurally modified quinazoline derivative (L) acts as highly selective chemosensor for Al(3+) ions in DMSO-H2O (1:9, v/v) over the other competitive metal ions. L shows a red shifted fluorescence after the addition of Al(3+) ions and later the further fluorescence enhancement is due to chelation enhanced fluorescence (CHEF) through inhibition of photoinduced electron transfer (PET). This probe (L) detects Al(3+) ions as low as 9nM in DMSO-H2O (1:9, v/v) at biological pH. The non-cytotoxic probe (L) can efficiently detect the intercellular distribution of Al(3+) ions in living cells under a fluorescence microscope to exhibit its sensible applications in the biological systems.

Effect of metal oxidation state on FRET: a Cu(i) silent but selectively Cu(ii) responsive fluorescent reporter and its bioimaging applications

A new structurally characterized cell permeable rhodamine-cinnamaldehyde hybrid (HL) behaves as a Cu(ii) ions selective chemosensor through FRET process which depends on +2 state of copper ion exclusively.

Selective and sensitive turn-on chemosensor for Al(iii) ions applicable in living organisms: nanomolar detection in aqueous medium

A new crystallographically characterized non-cytotoxic CHEF based highly sensitive Al(iii) ion selective chemosensor (L) is useful to detect the Al(iii) ions' distribution in A549 cell lines and also able to sense Al(iii) ions in tea extract.

A water soluble copper(ii) complex as a HSO4− ion selective turn-on fluorescent sensor applicable in living cell imaging

A water soluble non-fluorescent biofriendly cell permeable structurally characterised copper(ii) complex (1) selectively senses HSO₄⁻ ions as low as 3.18 × 10⁻⁷ M in water : DMSO (9 : 1, v/v) HEPES buffer at biological pH.

A new fluorogenic probe for the selective detection of carbon monoxide in aqueous medium based on Pd(0) mediated reaction

A coumarin-based fluorogenic probe, PCO-1, detects carbon monoxide selectively in buffer at pH 8.0 through the intramolecular cyclization–elimination pathway based on Pd(0) mediated reaction.