Fluorescence Color Change Research Articles

Fluorescent hydrogel actuators with simultaneous morphing- and color/brightness-changes enabled by light-activated 3D printing

Gradient cross-linking hydrogels capable of shape-morphing and fluorescence-color changes have practical significance in the fields of anti-counterfeiting, implantable sensors and smart wearable devices. However, it remains challenging to fabricate the gradient hydrogel actuators in a green, efficient and controllable manner, limited in the design and processing conditions. Herein, we merge the intriguing characteristics of light-activated 3D printing process with shape-morphing properties to fabricate a new fluorescent hydrogel actuator that can self-actuate in response to temperature, meanwhile, the synchronous color/brightness changes as a function of temperature can be well realized by those hydrogels doped with lanthanide ions. Remarkably, this kind of fluorescent hydrogel actuator can also perform as an information carrier to reveal the concealed information in two ways: i) specific UV light irradiation to read out the information, ii) self-morphing with the increase of temperature to display the information. Based on these excellent properties, the 3D-printed fluorescent hydrogel actuators with gradient cross-linking structures provide new insights in designing and fabricating novel soft robots. In addition, the light-activated 3D printing should guide the future intelligent manufacturing for fluorescent hydrogel actuators with the advantages of low cost, high efficiency, high accuracy, as well as the tailorable shapes and programmable actuations.

Fluorophore dyes based on 9-thienylanthracene: AIEE behaviors, performance in solid state, and bioimaging in living cells

Four new 9-thienylanthracene fluorescent dyes (TAs) bearing 1-methylindole/triphenylamine moieties were successfully synthesized. They all displayed typical aggregation-induced emission enhancement (AIEE) features with bright and varying aggregative-state emission involving multiple fluorescence colors containing cyan, green, and jacinth. Meanwhile, their solid state fluorescence could be effectively switched by mechanical grinding and dichloromethane fuming except for TPATA-Py12. Specifically, ITA, TPATA and ITA-Py12 showed reversible high contrast mechanofluorochromic (MFC) behaviors involving distinct fluorescent color changes from cyan/yellow to green/red. Notably, these TA dyes displayed excellent biocompatibility and have been successfully realized in living cell imaging. Particularly, ITA-Py12 and TPATA-Py12 can steadily immobilize in the mitochondria of HeLa cells by strong hydrophobic interaction between the C12-alkyl chain and lipid bilayer, resulting in high specificity mitochondrial staining. These good properties of them could provide a reference for the design and development of multi-functional AIEE dyes based on TA-skeleton.

A simple AIE-active triphenylamine derivative for supersensitive detection of water in organic solvents with noticeable fluorescence color change

In this work, an extremely simple triphenylamine-based luminogen 1-(4-(diphenylamino)phenyl)-2-phenylethanone (TPACO) with aggregation-induced emission (AIE) and twisted intramolecular charge transfer (TICT) characteristic was readily synthesized as a super-sensitive fluorescent water sensor. TPACO exhibits remarkable solvatochromic effect and ultrahigh sensitivity to trace water in organic solvents with good linear“intensity-to-content” relationships. The detection limit of TPACO for water in tetrahydrofuran and 1,4-dioxane can be as low as 76 ppm (0.0068 vol%) and 31 ppm (0.0032 vol%), respectively. Under UV irradiation, the probe can signify water content in organic solvents with distinguishable fluorescence color change observable by naked eye. In addition, the emission of TPACO can be quenched by alkali via keto-enol isomerization.

Fluorimetric quantification of picric acid in aqueous medium via smartphone and invisible ink applications using pyrene based sensor

• Simple imidazole-pyrene based sensor for quantification of picric acid in aqueous medium. • The fluorescent probe became non-fluorescent in presence of picric acid. • Addition of picric acid caused bare-eye color change from colorless to yellow. • Colorimetric detection of picric acid using smartphone ‘Color detector App’. • Detection of picric acid by the means of invisible ink applications. A simple imidazole-pyrene based sensor (1) has been developed by simple one step reaction and characterized by using common spectroscopic techniques. When various nitroaromatic compounds were incorporated to CH 3 CN:H 2 O (1:2, v/v) solution of 1 , the fluorescent probe turned non-fluorescent in presence of picric acid, upon irradiation at 310 nm. This was accompanied by negligible changes with other molecules along with fluorescent color change from bluish-green to colorless in case of picric acid. Also, the fluorimetric sensing of picric acid was done using smartphone and invisible ink applications. The sensor has been found efficient for the recognition of picric acid in aqueous medium.

Electrode materials for flexible supercapacitor with real-time visual monitoring of potential

Novel intelligent materials with multiple functions that can be used in a diverse range of practical material applications are still in urgent demand. Herein, we have developed novel multifunctional and intelligent materials with excellent electrofluorescence (EFC), electrochromic (EC) and electrochemical properties that are suitable for the preparation of flexible supercapacitors. The novel intelligent polymer P1 has a high quantum efficiency of 10.75% in the solid-state and the colouring and bleaching times of P1 are 1.80 and 3.93 s, respectively. The polymer film with three colour variations changed from yellow to green and dark blue in the range of 0–1.0 V applied potential. The area ratio capacitance of the electrode material is 21.59 mF cm−2 at a current density of 0.075 mA cm−2. A flexible supercapacitor with a full polymer containing EC and EFC groups has been constructed and exhibits obvious colouring, bleaching and fluorescence colour changes with potential. The supercapacitor also responds quickly to the applied potential. In addition, recent studies may offer a significant guideline for multifunctional molecular design and provide a vital step forward in extending the energy storage and display sensor.

A rhodol-derived probe for intracellular biothiols imaging and rapid labelling of sulfhydryl-containing proteins

Biological thiols are crucial in biological processes. Therefore, a probe capable of targeting and imaging biothiols in living cells, as well as rapidly labelling the thiols in proteins, is desirable and imperative. Herein, we designed and synthesized FRB-T containing α, β-unsaturated ketones to recognize thiols for detecting biothiols and selectively labelling the sulfhydryl-containing proteins. The probe initially exhibited weak fluorescence and then performed 1, 4-Michael addition reaction between its α, β-unsaturated ketone and thiols, along with the spiro ring-opening, leading to remarkable fluorescence enhancement and colour changes. The specificity of FRB-T towards thiols was high, and its detection limits were 70 nM for cysteine (Cys) and 1.51 μg/mL for ovalbumin (OVA). Furthermore, FRB-T has also been successfully applied to biothiols imaging in Hela cells, and rapid labelling of the sulfhydryl-containing proteins without washing step.

Polymer Mechanochromism from Force-Tuned Excited-State Intramolecular Proton Transfer.

Real-time monitoring of strain/stress in polymers is a big challenge to date. Herein, we for the first time report an ESIPT (excited-state intramolecular proton transfer)-based mechanochromic mechanophore (MM). The synthesis of target MM PhMz-4OH [(2-hydroxyphenyl)benzimidazole with four aliphatic hydroxyls] is quite facile. PhMz-4OH possesses characteristic dual emissions, and its ESIPT activity is greatly affected by steric hindrance. Then, PhMz-4OH was covalently linked into polyurethane chains (PhMz-4OH@PU). Upon stretching, the PhMz-4OH@PU films showed fluorescence color change and spectral variation with the increase in enol emission and blueshift of keto emission due to the force-induced torsion of the dihedral angle between the proton donor and the proton acceptor. The PhMz-4OH@PU films with high mechanophore concentrations (>0.36 mol %) might undergo a two-stage force-responsive process, including torsion of the dihedral angle via force-induced disaggregation and direct chain-transduced force-induced torsion of the dihedral angle. The intensity ratio of enol emission to keto emission (IE/IK) shows a quantitative correlation with elongation, and real-time strain sensing is achieved. PhMz-4OH is a successful type II MM (without covalent bond scission) and displays high sensitivity and excellent reversibility to stress. Two control structures PhMz-NH2 and PhMz-2OH were also embedded into PU but no spectral or color changes were detected, further confirming that mechanochromism of PhMz-4OH@PU films arises from the chain-transduced force. Density function theory (DFT) calculation was performed to study the force-tuned ESIPT process theoretically and rationalize the experimental results. This study might lay the foundation for real-time stress/strain sensing in practical applications.

An AIE-Active probe for detection and bioimaging of pH values based on lactone hydrolysis reaction.

Cellular pH homeostasis is essential for many physiological and pathological processes. pH monitoring is helpful for the diagnosis, treatment and prevention of disorders and diseases. Herein, we developed a ratiometric fluorescent pH probe (TCC) based on a coumarin derivative containing a highly active lactone ring. TCC exhibited a typical AIE effect and emitted blue fluorescence under weak acidic condition. When under weak basic condition, the active lactone moiety underwent a hydrolysis reaction to afford a water-soluble product, which gave red-shifted emission. The emission color change from blue through cyan and then to yellow within pH 6.5-9.0 which is approximate to the biological pH range. And the fluorescence color change along with pH value is reversible. Furthermore, TCC was successfully utilized in the detection of the intracellular pH change of live HeLa cells, which indicated that TCC had practical potential in biomedical research.

Rigidity-Tuned Full-Color Emission: Uncommon Luminescence Change from Polymer Free-Volume Variations.

Probing the rigidity change of microenvironments via tracking embedded molecular fluorophore emissions represents a robust approach to monitor various polymer microstructural evolutions and biomolecular events with a high spatiotemporal resolution. However, reported fluorophores exclusively blueshift their emissions (termed as "rigidochromism") or merely alter intensities upon rigidification, suffering from inferior sensitivities, low-contrast outputs, and attenuated biocompatibilities. Here, phenanthridine-fused triazatruxene fluorophores (PTFs) with pronounced bathochromic emission (up to 135nm) toward rigidifying media at a low loading of 5ppm without sacrificing the quantum yields and lifetime are developed. PTFs effectively interact with polymeric matrixes through polar-π interactions and form charge-transfer complexes, resulting to a remarkable fluorescent color change from blue to red-orange over matrix rigidifying. Such a unique anti-rigidochromism enables a highly sensitive rigidity detection (i.e., a subtle polymer molecular-weight change (as low as 1000Da vs up to 10kDa for conventional probes) can result to obvious emission color changes). PTFs are able to noninvasively detect polymerization kinetics and in situ optically report polymer degradations. The broadly (nearly full-spectrum) tunable emission and the efficient coupling between anti-rigidochromism and polymer hierarchical structures/topologies render fluorescence with controlled wavelength and chirality, leading to an unprecedented free-volume-based data encryption and anti-counterfeiting technology with a superhigh security level.

Multiple fluorescence color transitions mediated by anion-π interactions and C-F covalent bond formation

The emission changes of fluorescent dyes under the influence of environmental changes or interaction with analytes are the basis for designing ratiometric fluorescent probes and logic gates. However, it is rare that only one external stimulus induces continuous fluorescent color changes in a fluorescent dye. In this paper, we report a cage-like molecule formed by two benzene rings and three imidazolium salts which produces continuous fluorescence wavelength changes when interacted with fluoride ions. Fluoride ions are first bound to the center of the cage under the action of anion-π interaction, and the (C−H)+···F– type ionic hydrogen bonds induce the blue-shift of fluorescence. The subsequent formation of C-F covalent bonds with fluoride ions makes the fluorophore wavelength continue to blue-shift, and finally obtains continuous multiple fluorescence changes caused by a single external stimulus. According to the fluorescence wavelength and intensity, six different fluorescence signal channels can be obtained, which can be encoded as six numbers from 0 to 5. We expect that this reaction process can find applications in quantitative anion recognition and molecular counters.

Synthetic Supramolecular Host for D-(-)-Ribose: Ratiometric Fluorescence Response via Multivalent Lectin-Carbohydrate Interactions.

Easily synthesizable, carbazole-based organic nanoaggregates have been designed for selective detection of D-(-)-ribose at physiological pH. The addition of D-ribose results in a ratiometric change in fluorescence color from green to cyan (LOD: ∼12 μM). The mechanistic studies indicate the presence of multipoint noncovalent interactions, such as hydrogen bonding and CH⋅⋅⋅π interactions between D-ribose and acyl hydrazone and terminal pyridyl units of the probe molecule. However, such multipoint interactions dissociate the preformed self-assembled nanoclusters and induce change in optical response. The probe molecule was further exploited in analyzing D-ribose content in biological fluids (diluted human urine and blood serum) and oral supplements. The small standard deviation values (2-3.8 %) with nearly quantitative recovery (93.5-105.5 %) indicate the high accuracy of the presented method. Further, low-cost portable device based coated paper strips were designed for 'on-location' rapid, detection of D-ribose even at remote locations.

Dual-response ratiometric fluorescence based ligand-functionalized CsPbBr3 perovskite quantum dots for sensitive detection of trace water in edible oils

The sensitive and accurate detection of trace water in edible oils is of significance for the protection of their quality and safety. Herein, a kind of novel dual-emission CsPbBr3 perovskite quantum dots modified with a functional ligand dimethyl aminoterephthalate (CsPbBr3 @DMT-NH2 QDs) was synthesized at low temperature. The QDs display highly-sensitive fluorescence turn-on/off dual response and distinct fluorescence color change from green to blue in the presence of water. Thus, both the ratiometric fluorescence sensing and the visual ratiometric chromaticity detection method were established for water assay with the ultralow limit of detection of 0.006% (v/v) and 0.01% (v/v), respectively, and they can realize the accurate detection (recovery of 93.0%–108.0%, relative error ≤ 5.33%) of trace water in edible oils. Furthermore, it is found that the high polarity levels of protic solvents disintegrate the QDs, and the relatively-low polarity levels are conducive to the aggregation of the QDs, which ultimately lead to green fluorescence quenching. Meanwhile, the deprotonation and polarity of water promote the excited-state intramolecular proton transfer and intramolecular charge transfer of DMT-NH2, so enhancing its blue fluorescence. To our knowledge, this is the first report on dual-response ratiometric fluorescence perovskite QDs for ultrasensitive water detection in edible oils.

RhB-encapsulated metal-organic cage as a dual-emission fluorescence sensor for detection of malachite green and glycine

RhB@ZrT-1-OH composite was constructed by introduction of Rhodamine B (RhB) into the cages of zirconium-based metal–organic cage that had two fluorescence emission peaks at 466 and 612 nm upon excitation at 327 nm. The dual-emission fluorescence sensor exhibits ultra-high sensitive detection for malachite green (MG) and glycine (Gly) in phosphate buffer solution (pH = 6.86). RhB@ZrT-1-OH as a ratiometric fluorescence probe was applied to detect MG with a low LOD of 0.2879 μM and presented obvious fluorescence visual changes from orange to purple to blue under 254 nm UV–vis lamp. Moreover, RhB@ZrT-1-OH also can be utilized as a “turn-on” fluorescence sensor to recognize Gly with a low LOD of 0.3747 μM and exhibits fluorescence color changes from orange to pink to purple. Notably, the corresponding test papers for sensing MG and Gly were designed for recognize the concentration of MG and Gly. Furthermore, the dual-emission fluorescence sensor can be used to detect MG and Gly in fish and human serum with high sensitivity and reliable. The possible detecting mechanisms of RhB@ZrT-1-OH for sensing MG and Gly were detailedly explored.

Lab-on-a-Molecule Probe: Multitarget Detection of Five Aromatic Pesticides Using a Supramolecular Probe under Single Wavelength Excitation.

In order to prevent and control the effects of pesticide residues on human health and the ecological environment, the rapid, highly sensitive, and selective detection of multiple pesticide residues has become an urgent problem to be solved. Herein, a lab-on-a-molecule probe based on a host-guest complex (ThT@Q[8] probe) has been developed to simultaneously analyze multiple aromatic pesticides under single wavelength excitation, such as fuberidazole, thiabendazole, carbendazim, thidiazuron, and tricyclazole. The fluorescence titration spectra of the ThT@Q[8] probe with the five pesticides mentioned above showed that the fluorescence intensity exhibited a good linear correlation with the pesticide concentration and the limit of detection was as low as 10-7 M. Because the ThT@Q[8] probe exhibits diverse fluorescence color changes to the five pesticides studied under a 365 nm ultraviolet lamp, we fabricated a single probe used to detect multiple analytes in the RGB triple channel by extracting the RGB variations. Principal component analysis and linear discriminant analysis proved that the ThT@Q[8] probe can recognize and distinguish five pesticides and can be applied at different concentrations. In real samples, the ThT@Q[8] probe recognized and distinguished five pesticides in tap water and Huaxi River water. The 1H NMR spectra results proved that a charge-transfer complex of ThT and pesticides in the Q[8] cavity may be formed. Moreover, we selected a test strip as a carrier to detect pesticides. The results indicate it can be used to quickly and conveniently detect different pesticides due to the rapid color change. Besides, the ThT@Q[8] probe has good cell permeability and can be used to detect pesticide residues in living cells. This work has laid the foundation for the qualitative and quantitative multitarget detection of pesticide residues.

Dynamic Anticounterfeiting Through Novel Photochromic Spiropyran-Based Switch@Ln-MOF Composites.

Fluorescent materials presenting unique color changes in response to external stimuli have wide applications in information storage and anticounterfeiting. However, developing intelligent fluorescent materials with high security levels and dynamically displaying encrypted information is still a challenge. Herein, we report a new method for constructing excellent fluorescent materials by loading the photochromic molecule spiropyran into a lanthanide metal-organic framework. Controlling the isomerization of the spiropyran unit regulates the fluorescence resonance energy transfer (FRET) mechanism between the spiropyran acceptor and the lanthanide donor, leading to an exceptional reversible absorption/luminescence modulation ability. As the irradiation time is extended, the fluorescent color changes continuously from yellow-greenish to orange and then to red through the FRET process within 60 s. This composite system has great potential in anticounterfeiting because of the following advantages: (1) the materials have different fluorescence emissions and optical colors regulated by ultraviolet radiation, which is convenient for designing complex anticounterfeiting patterns; (2) the system can be repeatedly verified quickly and exhibit dynamic fluorescence color within 60 s, having great potential in advanced anticounterfeiting, where time is key in encryption/decryption. These unique advantages will greatly enhance the reliability of anticounterfeiting measures and increase the difficulty of anticounterfeiting.

Metal-Ligand Interactions and Oligo(p-phenylene vinylene) Derivatives Based Supramolecular Polymer Possessing Variable Fluorescence Colors.

Fluorescent supramolecular polymers combine the benefits of supramolecular polymers in terms of dynamic nature with the optoelectronic features of incorporated fluorophores. However, the majority of fluorescent supramolecular polymers can only exhibit a single fluorescent state, restricting their applications. Incorporating J-type dyes into supramolecular monomers is expected to impart supramolecular polymers with variable fluorescence colors, because the aggregation mode of J-type dyes is closely related to the formation of supramolecular polymers. Herein, the authors report a supramolecular polymer [M1·Zn(OTf)2 ]n , in which the monomer M1 contains a J-type dye, oligo(p-phenylene vinylene) derivative, and two terpyridine ends. The M1 + Zn(OTf)2 solutions exhibit fluorescence color changes varying from cyan to yellow-green in the monomer concentration ranging from 0.04 to 1.00mm. Moreover, based on the outputs from laser scanning confocal microscopy, the fluorescence color transition during the formation of supramolecular polymers is intuitively proven. Additionally, considering the close relationship between the supramolecular polymer structure and the fluorescence color, the fluorescence color can be regulated by introducing tetraethylammonium hydroxide that can bind with Zn2+ competitively to break up the structure of the supramolecular polymer.

Histamine-responsive dye-incorporated carbon dots for visual monitoring of food spoilage

With food safety problems abounding, it is essential to explore some efficient and effective methods for monitoring food quality. To this end, we have developed a histamine-responsive carbon dots (CDs)-based sensing system for visual monitoring of food spoilage. In this system, the aldehyde group on the surface of CDs serves as the recognition sites to combine with the amino group on histamine to form a Schiff base that effectively enhances the fluorescence. On the other hand, 1-aminoanthraquinone (ANQ) was introduced as a reference signal to endow the fluorescent probe with self-calibrating capacity. As the histamine concentration increases, linear ratiometric response was obtained accompanied with a distinguishable fluorescence color change from red to blue. Besides, test strips-based smartphone visual assays were successfully used to evaluate and monitor the histamine levels in fish samples. Our proposed sensing strategy has been demonstrated to be efficient and robust along with great potential for real-time visual monitoring of food spoilage.

A smartphone integrated ratiometric fluorescent sensor for point-of-care testing of fluoride ions.

In view of the high toxicity and widespread availability, it is of great importance to develop accurate, sensitive, and convenient assays for fluoride ion (F-) detection. Herein, a ratiometric fluorescent system is established for point-of-care testing (POCT) of F- with a smartphone as analyzer. The sensing system of calcein-QDs-Eu3+ contains two fluorescent probes of calcein (green emission) and ZnCdSe/ZnS QDs (red emission). The sensing system only presents red emission in that the calcein emission is quenched due to the combination between calcein and Eu3+. When F- is introduced, the fluorescence of calcein is recovered due to the stronger interaction between F- and Eu3+, which changes the emission from red to green. The ratiometric strategy offers an obvious fluorescence color change of the system, which eliminates interference and improves the detection accuracy. Specifically, the sensing system has excellent selectivity in that Eu3+ is more inclined to bind with F- rather than other anions. The developed assay was further used to prepare a test paper and hydrogel for POCT. To further improve the detection sensitivity and realize quantitative analysis, a smartphone installed with a color scan app is integrated as signal reader and analyzer, which is used for POCT of F- in real samples, showing great application potential in environmental protection and food safety evaluation.

Lanthanide coordination polymer nanoparticles as a ratiometric fluorescence sensor for real-time and visual detection of tetracycline by a smartphone and test paper based on the analyte-triggered antenna effect and inner filter effect

Lanthanide coordination polymer nanoparticles (LML-Eu3+-GMP CPNPs) were fabricated as a ratiometric fluorescence sensor for real-time and visual assays of tetracycline (TC). The LML-Eu3+-GMP CPNPs were prepared by spontaneous self-assembly of Eu3+, luminol (LML) and guanosine 5′-monophosphate (GMP) at room temperature, which only emitted blue fluorescence dominated by LML, and no red characteristic fluorescence of Eu3+ was observed. In the presence of TC, TC can trigger the antenna effect (AE) and inner filter effect (IFE), the blue fluorescence of LML-Eu3+-GMP CPNPs is quenched by TC via IFE, and the red characteristic fluorescence of Eu3+ can be turned on through AE. When sodium citrate (Cit) is added to the sensing system, it can further enhance the red fluorescence of Eu3+. The dual-ligand CPNP sensor has a rapid fluorescence response, noticeable fluorescence color change, good selectivity, and high sensitivity with a detection limit of 3.4 nM. It was successfully applied to detect TC in honey, milk, lake water and tap water. Meanwhile, the portable smartphone and test paper were applied to develop POCT devices for real-time and visual detection of TC, which could offer a promising application for real-time and semiquantitative assays of TC.

Fluorescence-color-changing biomimetic tough films with simultaneous shape-deformation triggered by pH

Stimuli-responsive materials with complex practical functions that could imitate certain behaviors of living organisms have wide potential applications in frontier fields. However, the fabrication of stimuli-responsive films with simultaneous change in fluorescence color and shape in response to a single stimulus, especially by a simple fabrication strategy, remains a challenge. Herein, a simple hydrogel-film-post crosslinking strategy is proposed to prepare films that exhibit fluorescence change accompanying with shape deformation under pH stimulus. The synchronous changes are realized by the combination of dynamic boron chemistry and pH-sensitive lanthanide complexes. Under different pH conditions, the protonation/deprotonation of the ligands coordinated to Eu3+ ions could generate switchable on/off fluorescence behavior, while the association/dissociation of borate ester bonds between 3-aminobenzeneboronic acid (SA-PBA) and polyvinyl alcohol (PVA) could lead to macroscopic deformation. The existence of borate ester bonds is beneficial to the mechanical properties of composite films. Under pH stimulus, the film-based artificial flower could simulate the flowering of natural flowers accompanying with color changes and the film-based gripper could grasp object with a visual signal, which demonstrates that the SA-PBA/PVA/EuL films have great potential for wide application.