Design strategies for lab-on-a-molecule probes and orthogonal sensing

Excitation wavelength-dependent lanthanide-disalicylaldehyde coordination hybrid capable of distinguishing D2O from H2O

The increasing demands in fields of anti-counterfeiting, fluorescence analysis, clinical therapy and LED illumination are urgently eager for more excellent optically switchable luminescent materials with the stable and multimodal fluorescence in single-component matrix. Herein, the lanthanide-disalicylaldehyde coordination hybrid H2Qj4/TbxEuy is proposed as an efficient luminescent matrix to connect terbium sensibilization with ESIPT (excited-state intramolecular proton transfer) effects, and three multi-emission hybrids are finally designed and synthesized by regulating Tb3+ and Eu3+ ratios. Surprisingly, the H2Qj4/Tb0.91Eu0.09 shows the excitation wavelength-dependent luminescence in solution which originates from two energy transfer ways of terbium sensibilization effect. It exhibits green and red lights under the 369 and 394 nm UV lamp, respectively. Three hybrids are further used as lab-on-a-molecule fluorescent probes to perform multianalyte detection for various solvents by selected fluorescent sensing channels. By means of PCA (principal component analysis) and HCA (hierarchical cluster analysis), all of them can successfully detect and discriminate17 common solvents, especially the H2O and D2O. Moreover, the H2Qj4/Tb0.91Eu0.09 also shows the wide linear responses of H2O content in D2O, discrimination of two-component solvent mixtures, hygroscopicity evaluation of D2O and information encryption which will advance the progress of multimodal luminescent materials and multianalyte chemosensors.

Multiple-Emitting Luminescent Metal-Organic Framework as an Array-on-a-MOF for Rapid Screening and Discrimination of Nitroaromatics

Fluorescence array technologies have attracted great interest in the sensing field because of their high sensitivity, low cost, and capability of multitarget detection. However, traditional array sensing relies on multiple independent sensors and thus often requires time-consuming and laborious measurement processes. Herein, we introduce a novel fluorescence array strategy of the array-on-a-metal-organic framework (MOF), which integrates multiple array elements into a single MOF matrix to achieve facile sensing and discrimination of multiple target analytes. As a proof-of-concept system, we constructed a luminescent MOF containing three different emitting channels, including a lanthanide ion (europium/Eu3+, red emission), a fluorescent dye (7-hydroxycoumarin-4-acetic acid/HCAA, blue emission), and the MOF itself (UiO-66-type MOF, blue-violet emission). Five structurally similar nitroaromatic compounds (NACs) were chosen as the targets. All three channels of the array-on-a-MOF displayed rapid and stable fluorescence quenching responses to NACs (response equilibrium achieved within 30 s). Different responses were generated for each channel against each NAC due to the various quenching mechanisms, including photoinduced electron transfer, energy competition, and the inner filter effect. Using linear discriminant analysis, the array-on-a-MOF successfully distinguished the five NACs and their mixtures at varying concentrations and demonstrated good sensitivity to quantify individual NACs (detect limit below the advisory concentration in drinking water). Moreover, the array also showed feasibility in the sensing and discrimination of multiple NACs in real water samples. The proposed "array-on-a-MOF" strategy simplifies multitarget discrimination procedures and holds great promise for various sensing applications.

Lab-on-a-molecule and multi-analyte sensing

The concept of a lab-on-a-molecule, which was proposed just short of two decades ago, has captured the imagination of scientists. From originally being proposed as an AND logic gate driven by three chemical inputs as a direct way of detecting congregations of chemical species, the definition of what constitutes a lab-on-a-molecule has broadened over the years. In this review, molecules that can detect multiple analytes by fluorescence, among other techniques, are reviewed and discussed, in the context of molecular logic and multi-analyte sensing. The review highlights challenges and suggestions for moving the frontiers of research in this field to the next dimension.

A dynamic Eu(III)-macrocycle served as the turn-on fluorescent probe for distinguishing H2O from D2O

H2O and D2O are an important pair of analogues, and their high-efficient detections are closely related to fields of chemical industry, food processing, semiconductor, environmental monitoring, etc. Because of their extremely similar physical and chemical properties, H2O and D2O can be mutually soluble in any ratios, and it is generally thought that the discrimination of H2O and D2O is an enormous challenge. Herein, upon the fact that vibrational frequency of O-H is greater than O-D, we design a dynamic Eu(III)-macrocycle Eu-2a with two emitters which exhibits the imine bond breakage of macrocycle emitter H2L2a in H2O or D2O, resulting in the turn-on fluorescence of Eu(III) emitter. For their differential fluorescence sensing signals of Eu-2a on three emission bands (433, 500 and 615 nm), the statistical analysis method is employed to produce fully separated fingerprints and thus high-throughput discrimination of 13 common solvents, especially the H2O and D2O. Fluorescent titration experiments by instrumental or smartphone-based analysis method also prove the successful determination of proportional H2O/D2O mixtures together with the good sensitivity and wide linear response range. Moreover, this H2O-triggered fluorescent complex Eu-2a used as the fluorescence ink also shows its potential in information encryption application. This article must be a valuable reference for the areas of lanthanide-based luminescent material, multianalyte detection and information encryption.

A novel intramolecular charge transfer-based near-infrared fluorescent probe with large Stokes shift for highly sensitive detection of cysteine in vivo

Cysteine (Cys) distribute widely in organisms as the crucial components of proteins, and play important roles in pathophysiological processes of human body. Low level of Cys might induce hepatic injury, edema and growth retardation, while superfluous level of Cys is found to be closely relevant to Alzheimer's and Parkinson's diseases. In this work, a novel near-infrared (NIR) fluorescent probe PFQ-C was developed for highly selective detection of Cys in living cells and mice by utilizing the cyclization removal reaction between acrylate group and Cys. The superior sensitivity (limit of detection, 0.036 μM), NIR emission (655 nm), large Stokes shift (135 nm) and low cytotoxicity of the probe highlight its broad potential for future clinical applications. The response mechanism of the probe towards Cys was clarified by spectroscopy, chromatography and theoretical calculation. In addition, results of fluorescence imaging of cells and mice revealed the good performance of the probe for monitoring the distributions and variations of Cys activity in vivo, which is very useful for the researches on diseases associated with Cys.

A susceptible coordination hybrid based terbium sensibilization coupled ESIPT effects for pattern discrimination of analogues

Multianalyte detection and analogue discrimination are extremely valuable frontier areas for their wide applications in environmental, medical, clinical and industrial analyses. Nowadays, researchers rack their brains on how to develop excellent multianalyte chemosensors that have presented huge challenges in designing high-efficient fluorescent sensing materials and constructing high-throughput detection methods. In this paper, we propose a novel strategy to utilize the dual-emission fluorescent detection platform as a lab-on-a-molecule, arising from the disalicylaldehyde-coordinated hybrid H₂Q_j3/Tb based terbium sensibilization coupled excited-state intramolecular proton transfer effects. Using the statistical analysis (PCA and HCA) for sensing signals of three fluorescence channels (431, 543 and 583 nm), we demonstrate this elaborate chemosensor with multianalyte detection of three species (solvents, anions and cations) and pattern discrimination of analogues. As a result, the H₂Q_j3/Tb shows great lab-on-a-molecule characters for each set of species, resulting in the easier identification of many critical analytes (e.g., H₂O, NO₂^- and Fe³⁺) and discrimination of analogues. In addition, it is also proven to be able to provide reliable content determination for an analyte, especially the NO₂^- (LOD = 0.37 μM), and discrimination for mixed analogues. A combination of easy-to-implement preparation procedure and data analysis technique makes this work promising for not only designing similar lanthanide-based materials but also realizing more high-efficient multianalyte sensing systems towards various potential applications.

Polymer-based lab-on-a-molecule for the selective and sensitive recognition of fluoride and cyanide

A polymer chemical sensor based on lab-on-a-molecule was synthesised by amine-aldehyde polymerisation for selective detection of fluorine ions (F^-) and cyanide (CN^-). The polymer chemical sensor shows significant absorption and fluorescence changes upon the addition of F^- and CN^-, which can be observed by the naked eye and optical reactions. The polymer has a higher fluorescence enhancement effect than its monomer and the distance of wavelength of each anion increased which could be applied to better distinguish the two anions.

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.

An effective dual sensor for Cu2+ and Zn2+ with long-wavelength fluorescence in aqueous media based on biphenylacrylonitrile Schiff-base

Although some sensors for Cu²⁺ and Zn²⁺ had been reported, the sensor with long-wavelength emission in aqueous media for in-situ detecting Cu²⁺ and Zn²⁺ was always expected. Herein, a biphenylacrylonitrile Schiff-base (OPBS) with large aromatic conjugated system was designed and synthesized in yield of 82%. OPBS possessed excellent long-wavelength fluorescence at 550-750 nm in aqueous media, which selectively response to sense Cu²⁺ with quenched fluorescence and Zn²⁺ with chromotropic fluorescence from red to yellow. The detection of Cu²⁺ and Zn²⁺ were realized without mutual interference in their coexistence system by means of the assistance of ATP. The detection limits were 2.3 × 10^-7 M for Cu²⁺ and 1.8 × 10^-6 M for Zn²⁺, respectively. The sensing mechanism was elucidated by binding MS spectra, fluorescence Job's plot and ¹H NMR spectra. Moreover, OPBS exhibited good bioimaging performance and the in-situ sensing abilities for Cu²⁺ and Zn²⁺ in living cells, suggesting the application potential for detecting Cu²⁺ and Zn²⁺ in both vitro assay and vivo environment.

N-alkylation briefly constructs tunable multifunctional sensor materials: Multianalyte detection and reversible adsorption

A series of N-alkyl-substituted polybenzimidazoles (SPBIs), synthesized by simple condensation and N-alkylation, act as functional materials with tunable microstructures and sensing performance. For their controllable morphologies, the formation of nano-/microspheres is observed at the n(RBr)/n(PBI) feed ratio of 5:1. Products with different degrees of alkylation can recognize metal ions and nitroaromatic compounds (NACs). For example, SPBI-c, obtained at the feed ratio of 1:1, can selectively detect Cu2+, Fe3+, and NACs. By contrast, SPBI-a, obtained at the feed ratio of 0.1:1, can exclusively detect Cu2+ with high sensitivity. Their sensing mechanisms have been studied by FT-IR spectroscopy, SEM, XPS, and DFT calculations. Interestingly, the SPBIs can adsorb Cu2+ in solution and show good recyclability. These results demonstrate that polymeric materials with both sensing and adsorption applications can be realized by regulating the alkylation extent of the main chain, thus providing a new approach for the facile synthesis of multifunctional materials.

Aggregation-induced fluorescent response of urea-bearing polyphenyleneethynylenes toward anion sensing

A π-conjugated urea-bearing phenyleneethynylene polymer (Poly-2) was rationally designed by the Sonogashira coupling condensation reaction and had been demonstrated to have a unique fluorescent quenching effect for the optical detection of all determined anions, especially for CN^-. The fluorescent emission of Poly-2 was significantly quenched upon adding CN^-, together accompanied with a continuous red shift of the emission peak from 442 to 464 nm with the cyanide concentration increased from 0 to 1.0 mM. On the contrary, its precursor polymer, Poly-1, itself also displayed fluorescent responsibility with all selected anions but had no obvious selectivity and tendency. For instance, the addition of highly basic CN^-, N₃ ^-, AcO^-, or F^- to Poly-1 solution in DMF/H₂O (v/v = 1:1) led to the photoluminescence amplification, while the addition of weakly basic anions like Cl^-, I^-, and Br^- showed a fluorescence quenching effect. Both polymers were in a seriously self-aggregated state in solution no matter in the absence or presence of an anion. Interestingly, it was found that Poly-2 exhibited an aggregation-induced emission behavior, while Poly-1 had an aggregation-caused quenching effect, based on the relationship between photoluminescence and polymer aggregation state. The structural characterizations were carried out by NMR spectroscopy and size exclusion chromatography measurements; the photoluminescence properties of Poly-1 and Poly-2 together with anion sensing properties were followed by fluorescence spectroscopy, and the relationship between photoluminescence and aggregation behavior of both polymers in solution was investigated by dynamic light scattering measurements.

Multi-mode fluorescence sensing detection based on one core-shell structure quantum dots via different types of mechanisms

Multi-target sensing completed with single chemosensor is cost-effective and pretty attractive for the analysis of multiple analytes in single sample. CdTe/CdS QDs show high fluorescence quantum yield as a probe for the sensing purpose, but only single target is often detected. Here we proposed that three kinds of different response mechanisms were realized using CdTe/CdS QDs with the simultaneous sensing of Fe³⁺, Cr₂O₇^2-, and IO₃^- as model. The specific binding affinity was used to detect Fe³⁺ with the photo-induced electron transfer (PET) mechanism. CdTe/CdS QDs showed intensive fluorescent response to Cr₂O₇^2- owing to the inner filter effect (IFE). IO₃^- is an oxidant and reacts to surface functional groups of CdTe/CdS QDs with oxidation quenching (OQ) mechanism for the selective detection of IO₃^-. The three targets recorded with the three mechanisms were obviously clustered even they were tested at different concentrations. Thus, the targets could be differentiated with each other through their different responses of PET, IFE, and OQ. The linear ranges for the selective detection of Fe³⁺, Cr₂O₇^2-, and IO₃^- were 5.0-100.0, 20.0-140.0 and 1.0-80.0 μM, with the limits of detection as low as 4.1, 9.7, and 0.9 μM, respectively. The proposed methods were satisfactorily applied to accurately detect Fe³⁺, Cr₂O₇^2-, and IO₃^- in real samples and it was further detected iodate and the total amount of iodine in table salt samples. The proposed multi-target detection strategy with the single fluorescence single from single QDs will stimulate the application of the other matrix with abundant response with enhance sensing efficiency.

A novel near-infrared turn-on fluorescent probe for the detection of Fe3+ and Al3+ and its applications in living cells imaging

In this study, a new hemicyanidine-based colorimetric-fluorescent probe L has been synthesized and characterized by X-ray single crystal diffraction, NMR, HRMS and other technologies. The probe L serves as a "turn-on" probe for the detection of Fe³⁺ and Al³⁺ ions in DMF-HEPES system with a high sensitivity and an excellent selectivity. The probe L manifesting the color of the solution containing L turns red on the addition of Fe³⁺, and turns pink on the addition of Al³⁺. The fluorescence turn-on detection of Fe³⁺ and Al³⁺ ions is attributed to the photo-induced electron transfer (PET) process and the exertion of the chelation-enhanced fluorescence effect (CHEF) mechanism. The results of thin layer silica gel plate coloration experiments also present the same characteristics. Additionally, we further demonstrate that the probe L exhibit good cell permeability and could be employed to monitor Fe³⁺ and Al³⁺ ions in the living cells.

A novel dual functional pyrene-based turn-on fluorescent probe for hypochlorite and copper (II) ion detection and bioimaging applications

A novel dual-function fluorescent probe PPH was conveniently synthesized by linking pyrazinehydrazide unit and pyrenyl fluorophore. The present probe PPH could be used for simultaneous detection of ClO^- and Cu²⁺ through a fluorescence turn-on response. The probe PPH displayed excellent selectivity and sensitivity towards ClO^- and Cu²⁺ over other anions and metal ions. The detection limits of PPH for ClO^- and Cu²⁺ were calculated to be as low as 4.02 nM and 157 nM, respectively. The probe PPH had good pH stability and was highly responsive to ClO^- and Cu²⁺ in a wide pH range. Furthermore, this probe could be readily applied to visualize ClO^- and Cu²⁺ on the test paper. Additionally, this probe was successfully utilized to the fluorescence imaging of ClO^- and Cu²⁺ in HeLa cells.

A novel "off-on" rhodamine-based colorimetric and fluorescent chemosensor based on hydrolysis driven by aqueous medium for the detection of Fe3

A novel "off-on" colorimetric and fluorescent chemosensor N²,N⁶-bis(2-(rhodamine B amido)ethyl)pyridine-2,6-dicarboxamide (RhBEP) has been designed and synthesized, which can selectively detect the presence of Fe³⁺ with about 75-fold enhancement in fluorescence emission intensity at 585 nm over various environmentally relevant metal cations such as Hg²⁺, Cu²⁺, Cr³⁺, Li⁺, Na⁺, K⁺, Ag⁺, Zn²⁺, Mg²⁺, Ca²⁺, Ba²⁺, Fe²⁺, Cd²⁺, Ni²⁺, Mn²⁺, Al³⁺, Bi³⁺ and Au³⁺ in PBS reaction media. The remarkable color change from UV-Vis titration experiments indicates that RhBEP can be used as a colorimetric chemosensor for Fe³⁺. The ultrasensitive detection limit for Fe³⁺ in the fluorescence measurement is down to 2.0 × 10^-8 mol·L^-1. The recognition mechanism of RhBEP to Fe³⁺ was analyzed by Job's plot and mass spectrometry analysis. In addition, the data from fluorescent cell imaging experiments confirmed that the chemosensor RhBEP has a promising application for the detection of Fe³⁺ in biological systems.

1 H NMR Chemosensing of Potassium Ions Enabled by Guest-Induced Selectivity Switch of a Gold Nanoparticle/Crown Ether Nanoreceptor

A sensing protocol to detect potassium ions in water by ¹ H NMR spectroscopy is described. The method exploits the K⁺ -modulated affinity of 18-crown-6 functionalized gold nanoparticles towards organic ions, combined with NOE magnetization transfer. Binding of K⁺ to the crown ether moieties switches the nanoreceptor preference (and its ability to transfer magnetization) from organic cations (tyramine) to organic anions (phloretate). In this way, a ratiometric NMR signal is produced with a detection limit of 0.6 mM. Detection can be performed in 20 min with standard instruments and with little interference from other alkali and alkaline earth metal ions present in the sample.

Single-Molecule Topochemical Analyses for Large-Scale Multiplexing Tasks

Multitasking is the pivotal feature in next-generation chemo- or bioanalyses. However, simultaneous analyses rarely exceed over three different tasks, which is ascribed to the limited space to accommodate analyzing units and the compromised signal-to-noise (S/N) level as the number of tasks increases. Here, by leveraging superior S/N of single-molecule techniques, we analyzed five microRNA biomarkers by spatially encoding miRNA recognition units with nanometers resolution in a DNA template, while decoding the analyte binding temporally in seconds. The hairpin stem is interspersed by internal loops to encode recognition units for miRNA. By mechanical unfolding of the hairpin, individual internal loops are sequentially interrogated for the binding of each miRNA. Using this so-called topochemical spatiotemporal analysis, we were able to achieve subpicomolar detection limits of miRNAs. We anticipate that this new single-molecule topochemical analysis can massively analyze single-molecule targets.

Rapid detection of TNP based on a commercial fluorescent probe

A simple, rapid and low cost sensing method for the fluorescent detection of TNP in 100% aqueous media was successfully developed based on a commercial probe. Under the non-covalent interactions between TNP with probe, a ratiometric output signal was achieved, which can be applied in real samples test.

Molecular engineering of logic gate types by module rearrangement in 'Pourbaix Sensors': the effect of excited-state electric fields

Two types of fluorescent logic gates are accessed from two different arrangements of the same modular components, one as an AND logic gate (1) and the other as a PASS 0 logic gate (2). The logic gates were designed with an 'electron-donor-spacer1-fluorophore-spacer2-receptor' format and demonstrated in 1 : 1 (v/v) methanol/water. The molecules consist of ferrocene as the electron donor, 4-aminonaphthalimide as the fluorophore and a tertiary alkylamine as the receptor. In the presence of high H+ and Fe3+ levels, regioisomers 1a and 1b switch 'on' as AND logic gates with fluorescence enhancement ratios of 16-fold and 10-fold, respectively, while regioisomers 2a and 2b are functionally dormant, exhibiting no fluorescence switching. The PASS 0 logic of 2a and 2b results from the transfer of an electron from the excited state fluorophore to the ferrocenium unit under oxidising conditions as predicted by DFT calculations. Time-resolved fluorescence spectroscopy provided lifetimes of 8.3 ns and 8.1 ns for 1a and 1b, respectively. The transient signal recovery rate of 1b is ∼10 ps while that of 2b is considerably longer on the nanosecond timescale. The divergent logic attributes of 1 and 2 highlight the importance of field effects and opens up a new approach for regulating logic-based molecules.

Chemosensing properties and logic gate behaviors of graphene quantum dot-appended terpyridine

Graphene quantum dot-covalently appended terpyridine, GQDs-tpy, was synthesized and characterized by X-ray photoelectron spectroscopy, FTIR spectroscopy and transmission electron microscopy. GQDs-tpy was found to act as multifunctional chemosensors: a highly selective colorimetric chemosensor for Fe²⁺ as evidenced by an obvious color change from colorless to pink, and a typical fluorescence enhanced probe for Zn²⁺ over 13 metal cations even in practical water samples. Moreover, two-input XOR, INHIBIT and IMPICATION logic gates as well as four-input OR and NOR logic gates were constructed according to the characteristic responses of GQDs-tpy to a sequence of cations.

Sequential displacement strategy for selective and highly sensitive detection of Zn2+, Hg2+ and S2- ions: An approach toward a molecular keypad lock

A thiocarbonohydrazone locked salicylidene based macrocycle ligand L has been synthesized and its ion sensing properties were examined by UV-visible and fluorescence spectroscopy. The macrocycle serves as a highly selective colorimetric sensor for Hg²⁺ ions while it acts as an excellent fluorescent sensor for Zn²⁺ ions by exhibiting a green fluorescence at 498 nm even in the presence of interfering ions. A detailed analysis of binding characteristics such as complex stoichiometry, association constant and detection limits of L toward Hg²⁺ and Zn²⁺ ions were evaluated by UV-visible and fluorescence experiments which revealed a stronger binding affinity and higher detection limit of L toward the mercury ions. Further, the sequential displacement strategy for the chromofluorogenic detection of Zn²⁺, Hg²⁺ and S^2- ions by ligand L, has been studied comprehensively. Finally, the ion-responsive fluorescence output signal of L were employed to design a molecular keypad lock which could be accessible by two users having two different set of chemical passwords (inputs) through distinguishable optical trajectories.

Novel dual-functional fluorescent sensors based on bis(5,6-dimethylbenzimidazole) derivatives for distinguishing of Ag+ and Fe3+ in semi-aqueous medium

Three novel bisbenzimidazole derivatives have been synthesized and developed as dual-functional fluorescent sensors for the rapid and highly selective detection of Ag⁺ and Fe³⁺ ions in semi-aqueous medium with distinct spectral response for the first time. The absorption intensity is drastically decreased after the addition of Ag⁺. Contrarily, it is markedly increased upon the addition of Fe³⁺. And there is a good linear relation at low concentration of both Ag⁺ and Fe³⁺, which provides a quantitative method for their detection. Similarly, the sensors show a distinct fluorescence response towards Ag⁺ and Fe³⁺ with a different fluorescence color change under UV light. In addition, no significant changes and interference can be observed with other metal ions. The sensing mechanism studies confirm that the N atom in CN of benzimidazole ring of sensor 4a may bind with Ag⁺ or Fe³⁺ ion to form metal complex. And there is only a static quenching process for the 4-Ag⁺ complex system, but both dynamic and static quenching processes occur in the 4-Fe³⁺ complex system. Moreover, sensors 4 can steadily work in solution with a wide range of pH 4-13 and rapidly respond to Ag⁺ and Fe³⁺ with a response time of 10 s. Finally, the sensors have been successfully applied to the visual detection of Ag⁺ and Fe³⁺ not only in solution, but also in test paper.

A lab-on-a-molecule with an enhanced fluorescent readout on detection of three chemical species

The first naphthalimide-based three-input AND logic gate detects a congregation of three cations in aqueous methanol with a 25-fold enhanced fluorescence.

Near-infrared turn-on fluorescent probe for discriminative detection of Cys and application in in vivo imaging

Near-infrared (NIR) fluorescent probes are widely employed in biological detection because of their lower damage to biological samples, low background interference, and high signal-to-noise ratio. Herein, a highly water-soluble NIR probe (NIRHA) based on a hemicyanine skeleton and bearing an acrylate moiety was synthesized. The probe showed high selectivity toward cysteine (Cys) over homocysteine (Hcy) and glutathione (GSH). The probe also had low cytotoxicity and was successfully applied in HeLa cells and mouse experiments. Results of bioimaging experiments indicated that the probe was effective for visualizing endogenous Cys in vitro and in vivo.

Near-infrared (NIR) fluorescent probes are widely employed in biological detection because of their lower damage to biological samples, low background interference, and high signal-to-noise ratio.

Color-tunable lanthanide metal-organic framework gels

MOF gels with intrinsic emission color are prepared with 5-boronoisophthalic acid and Eu³⁺, Tb³⁺, and/or Dy³⁺. Single-metal gels exhibit trichromatic fluorescence, so full color emissions are readily obtained by tuning the type and/or ratio of Ln³⁺ ions to prepare mixed-metal gels. Nano-ribbons form from the precursors and then entangle together to generate the gels.

Fluorescence significantly improves the performance of gels. Various strategies, such as embedment and crosslinking, have been used to integrate extrinsic luminophores into gel systems, but the procedures are usually complex. Herein, for the first time, we report gels with intrinsic and tunable emission color prepared with 5-boronoisophthalic acid (5-bop) and Eu³⁺, Tb³⁺, and/or Dy³⁺ similar to the procedure for the preparation of metal–organic frameworks (MOFs). The single-metal gels exhibit intrinsic trichromatic fluorescence, due to which full-color emissions are readily obtained by tuning the type and/or ratio of Ln³⁺ ions to prepare mixed-metal gels. The emission is governed by an antenna effect and is thus excited with single-wavelength at 275 nm. The nucleation-growth mechanism reveals that the Ln³⁺ ions and 5-bop produce separated layers, which then grow anisotropically to form nanoribbons by high coordinated valence of Ln³⁺ ions and biased carboxyl distribution as well as steric hindrance and hydrogen bonds of the boric acid group in 5-bop. The nanoribbons entangle together to generate chemical-physical hybrid gels. To the best of our knowledge, this is the first example of gels with inherent and tunable emission color. Due to their optical and viscoelastic properties, the gels have numerous potential applications such as tunable emission and multi-target detection.

Triple-Indicator-Based Multidimensional Colorimetric Sensing Platform for Heavy Metal Ion Detections

Heavy metals are highly toxic at trace levels and their pollution has shown great threat to the environment and public health worldwide where current detection methods require expensive instrumentation and laborious operation, which can only be accomplished in centralized laboratories. Herein, we report a low-cost, paper-based microfluidic analytical device (μPAD) for facile, portable, and disposable monitoring of mercury, lead, chromium, nickel, copper, and iron ions. Triple indicators or ligands that contain ions or molecules are preloaded on the μPADs and upon addition of a metal ion, the colorimetric indicators will elicit color changes observed by the naked eyes. The color features were quantitatively analyzed in a three-dimensional space of red, green, and blue or the RGB-space using digital imaging and color calibration techniques. The sensing platform offers higher accuracy for cross references, and is capable of simultaneous detection and discrimination of different metal ions in even real water samples. It demonstrates great potential for semiquantitative and even qualitative analysis with a sensitivity below the safe limit concentrations, and a controlled error range.

Detection and identification of designer drugs by nanoparticle-based NMR chemosensing

Properly designed monolayer-protected nanoparticles (2 nm core diameter) can be used as nanoreceptors for selective detection and identification of phenethylamine derivatives (designer drugs) in water. The molecular recognition mechanism is driven by the combination of electrostatic and hydrophobic interactions within the coating monolayer. Each nanoparticle can bind up to 30-40 analyte molecules. The affinity constants range from 105 to 106 M-1 and are modulated by the hydrophobicity of the aromatic moiety in the substrate. Detection of drug candidates (such as amphetamines and methamphetamines) is performed by using magnetization (NOE) or saturation (STD) transfer NMR experiments. In this way, the NMR spectrum of the drug is isolated from that of the mixture, allowing broad-class multianalyte detection and even identification of unknowns. The introduction of a dimethylsilane moiety in the coating monolayer allows performing STD experiments in complex mixtures. In this way, a detection limit of 30 μM is reached with standard instruments.

Optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals

Optically-active nanocrystals (such as quantum dots and plasmonic noble metal nanoparticles) have received great attention due to their size-tunable optical properties. The indicator displacement assay (IDA) with optically-active nanocrystals has become a common practice for optical sensor development, since no sophisticated surface functionalization of nanoparticles is required. Among the IDA-based optical sensors, the use of metal ions as receptors seems to be attractive. Therefore, in this review, the research progress of optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals has been summarized. In particular, metal ion-mediated selective recognition has been summarized here based on the classical Hard-Soft-Acid-Base (HSAB) principle, which has been seldom mentioned before. Most of the references were therefore categorized according to their located place based on the HSAB theory. Besides, several metal ion modulation strategies that were not related to the HSAB theory (e.g., redox modulation) were also included. Finally, due to the cross-talk of metal ions in selective recognition, we have also summarized sensor array development based on multiple metal ion receptors in IDA sensing with optically-active nanocrystals. Several interesting applications of the IDA sensing with metal ions as receptors and optically-active nanocrystals as indicators are presented, with specific emphasis on the design principles and photophysical mechanisms of these probes.

β-Carboline–imidazopyridine hybrids: selective and sensitive optical sensors for copper and fluoride ions

Two rationally designed β-carboline–imidazopyridine hybrid chromofluorescent sensors S1 and S2 have been successfully synthesized and evaluated for the selective sensing of metal ions and anions.

Recent progress and developments of iridium-based compounds as probes for environmental analytes

Metal complexes based on iridium metal centers have attracted attention as probes due to their tunable biological and chemical characteristics.

AIE-doped poly(ionic liquid) photonic spheres: a single sphere-based customizable sensing platform for the discrimination of multi-analytes

By simultaneously exploiting the unique properties of ionic liquids and aggregation-induced emission (AIE) luminogens, as well as photonic structures, a novel customizable sensing system for multi-analytes was developed based on a single AIE-doped poly(ionic liquid) photonic sphere. It was found that due to the extraordinary multiple intermolecular interactions involved in the ionic liquid units, one single sphere could differentially interact with broader classes of analytes, thus generating response patterns with remarkable diversity. Moreover, the optical properties of both the AIE luminogen and photonic structure integrated in the poly(ionic liquid) sphere provide multidimensional signal channels for transducing the involved recognition process in a complementary manner and the acquisition of abundant and sufficient sensing information could be easily achieved on only one sphere sensor element. More importantly, the sensing performance of our poly(ionic liquid) photonic sphere is designable and customizable through a simple ion-exchange reaction and target-oriented multi-analyte sensing can be conveniently realized using a selective receptor species, such as counterions, showing great flexibility and extendibility. The power of our single sphere-based customizable sensing system was exemplified by the successful on-demand detection and discrimination of four multi-analyte challenge systems: all 20 natural amino acids, nine important phosphate derivatives, ten metal ions and three pairs of enantiomers. To further demonstrate the potential of our spheres for real-life application, 20 amino acids in human urine and their 26 unprecedented complex mixtures were also discriminated between by the single sphere-based array.

Light-induced self-assembly of bi-color CdTe quantum dots allows the discrimination of multiple proteins

We have found that the addition of proteins can greatly influence the light-induced self-assembly (LISA) behavior of bi-color thioglycolic acid (TGA)-capped CdTe Quantum Dots (QDs) and thus cause significant changes of their fluorescence (FL) signals (color and intensity), according to which a dual-channel FL sensor can be established for simultaneous discrimination of multiple proteins. The sensor is successfully used for the identification of ten native proteins and ten thermally denatured proteins and eight native proteins artificially added in human urine, respectively, during which process principal component analysis (PCA) is utilized to differentiate the targets based on their corresponding FL change patterns. This assay has provided a visual and simple method for the discrimination of various analytes, which may have great potential in the study of conformational changes of biomacromolecules and the analysis of real biological fluids.

A highly selective and sensitive rhodamine-derived fluorescent probe for detection of Cu2

A novel water-soluble and reversible fluorescent probe was designed and synthesized based on a rhodamine B derivative. It was used for detection of Cu²⁺ in drinking water and in living cells with high sensitivity and excellent selectivity. The tested concentration range and the limit of detection (LOD) of the probe were 0-15.00μmolL^-1 and 0.085μmolL^-1, respectively. In addition, the mode of binding and mechanism of interaction between the probe and Cu²⁺ were analyzed by density functional theory (DFT) calculations.