Colorimetric metal ion sensors

Colorimetric sensing for translational applications: from colorants to mechanisms

Colorimetric sensing offers instant reporting via visible signals. Versus labor-intensive and instrument-dependent detection methods, colorimetric sensors present advantages including short acquisition time, high throughput screening, low cost, portability, and a user-friendly approach. These advantages have driven substantial growth in colorimetric sensors, particularly in point-of-care (POC) diagnostics. Rapid progress in nanotechnology, materials science, microfluidics technology, biomarker discovery, digital technology, and signal pattern analysis has led to a variety of colorimetric reagents and detection mechanisms, which are fundamental to advance colorimetric sensing applications. This review first summarizes the basic components (e.g., color reagents, recognition interactions, and sampling procedures) in the design of a colorimetric sensing system. It then presents the rationale design and typical examples of POC devices, e.g., lateral flow devices, microfluidic paper-based analytical devices, and wearable sensing devices. Two highlighted colorimetric formats are discussed: combinational and activatable systems based on the sensor-array and lock-and-key mechanisms, respectively. Case discussions in colorimetric assays are organized by the analyte identities. Finally, the review presents challenges and perspectives for the design and development of colorimetric detection schemes as well as applications. The goal of this review is to provide a foundational resource for developing colorimetric systems and underscoring the colorants and mechanisms that facilitate the continuing evolution of POC sensors.

The complexometric behavior of selected aroyl-S,N-ketene acetals shows that they are more than AIEgens

Using the established synthetic methods, aroyl-S,N-ketene acetals and subsequent bi- and multichromophores can be readily synthesized. Aside from pronounced AIE (aggregation induced emission) properties, these selected examples possess distinct complexometric behavior for various metals purely based on the underlying structural motifs. This affects the fluorescence properties of the materials which can be readily exploited for metal ion detection and for the formation of different metal-aroyl-S,N-ketene acetal complexes that were confirmed by Job plot analysis. In particular, gold(I), iron(III), and ruthenium (III) ions reveal complexation enhanced or quenched emission. For most dyes, weakly coodinating complexes were observed, only in case of a phenanthroline aroyl-S,N-ketene acetal multichromophore, measurements indicate the formation of a strongly coordinating complex. For this multichromophore, the complexation results in a loss of fluorescence intensity whereas for dimethylamino-aroyl-S,N-ketene acetals and bipyridine bichromophores, the observed quantum yield is nearly tripled upon complexation. Even if no stable complexes are formed, changes in absorption and emission properties allow for a simple ion detection.

A novel dual-function fluorescent probe for the detection of cysteine and its applications in vitro

A fluorescent probe of both colorimetric and ratiometric type for highly selective and sensitive detection of Cys (cysteine) is very important in biological analysis. In this work, a new colorimetric and ratiometric fluorescent probe ((E)-2-(2-(5-(4-(acryloyloxy)phenyl)furan-2-yl)vinyl)-3-methylbenzo[d]thiazol-3-ium iodide, LP-1) was designed and synthesized for the detection of Cys. The reaction mechanism of LP-1 toward Cys involves a conjugate addition reaction between Cys and the α,β-unsaturated carbonyl group, leading to the formation of an intermediate thioether, followed by intramolecular cyclization to produce the desired compounds LP-1-OH. At this point, the ICT process is activated, significantly increasing the fluorescence intensity of the molecules. Meanwhile, LP-1 is highly selective and sensitive to Cys identification under optimized experimental conditions. LP-1 shows a good linear relationship in the range of Cys concentration from 0.40 μM to 40 μM (R2 = 0.9942) and the limit of detection (LOD) of Cys is 0.19 μM. In addition, we have developed a simple, portable and low-cost smartphone-based high-sensitivity Cys detection method based on naked eye obvious color detection. LP-1 also has low cell toxicity and can be successfully used for biological imaging of Cys, suggesting that it is a promising biological application tool for Cys detection.

A Novel Rhodamine-Phenolphthalein Architecture for Selective Mercury Ion Detection in Aqueous Media

In this study, the primary objective is to synthesize a novel fluorescent Rh-PP-Rh compound and explore its extensive range of photochemical behaviors. Initially, the synthesis of the novel Rh-PP-Rh was carried out for this purpose. Subsequently, UV-Vis and fluorescence spectroscopy were employed to investigate the interactions between Rh-PP-Rh and a diverse array of ions in aqueous solvent systems. Through fluorescence and UV-Vis studies, it was observed that Rh-PP-Rh demonstrated turn-on sensor properties in the presence of Hg2+ ions. Furthermore, the limits of detection (LOD) and association constant (Ka) values for Rh-PP-Rh/Hg2+ were determined as 334 nM and 9.13×1011 M-2, respectively. Additionally, the reversible studies demonstrated a switchable on/off response upon alternate addition of HgCl2 and [Bu4N]F to Rh-PP-Rh. These findings suggest that the probe Rh-PP-Rh also possesses specific sensor properties for F- ions in the presence of mercury. In addition, the investigation encompassed an assessment of the visual analysis of the color alterations of Rh-PP-Rh both on filter paper and in an EtOH/H2O solution. The findings demonstrated that Rh-PP-Rh can be successfully utilized in solutions containing mercury, as it generates significant color transformations.

An Indole-based Chromofluorogenic Probe for Detection of Trivalent Al3+, Ga3+, In3+ and Fe3+ Ions

An easily synthesizable indole-derived chromofluorogenic probe InNS has been demonstrated for recognition of trivalent metal ions (i. e., Al3+, Ga3+, In3+ and Fe3+). Both UV-Vis and emission spectral studies have been employed to assess the cation sensing ability of InNS in semi-aqueous medium. This probe exhibited a chromogenic response for these metal ions, and the related change was accompanied with the appearance of a new absorption near 376 nm. An obvious color change from pale yellow to dark yellow could also be noticed upon addition of the aforementioned metal ions to the probe's solution. Distinctively from the UV-Vis analysis, the fluorescence behavior of InNS was completely different; it displayed a 'turn-on' fluorescence response for only Al3+ among all the studied cations. The detection limit and the association constant (Ka) for Al3+ were determined to be 12.5 nM and 6.85×106 M-1, respectively. A potential 1 : 1 binding mode of Al3+-InNS has been established based on Job's plot, 1H NMR and DFT analyses. The reversibility experiment was conducted using strongly chelating EDTA ion, and a corresponding logic gate has been devised. In terms of practical applications, the InNS has been utilized to detect Al3+ in human breast carcinoma (MCF-7) cell lines displaying promising 'turn-on' bioimaging experiments.

Pyrene Functionalized Luminescent Phenylalanine for Selective Detection of Copper (II) Ions in Aqueous Media

A novel pyrene-based fluorescent chemosensor 1 (pyren-1-ylmethyl)-L-phenylalanine was designed and synthesized by combining 1-pyrenecarboxyaldehyde and L-phenylalanine. 1 was characterized by several analytical methods and used as a fluorescent chemosensor for the selective and sensitive detection of Cu²⁺ ions through "turn-off" mechanism with a detection limit of 2 × 10^-8 M. 1 can also be used to detect Cu²⁺ ions in a natural water sample and exhibits gelation properties with high thermal stability.

Rapid colorimetric discrimination of cyanide ions - mechanistic insights and applications

In this work, we have employed an intramolecular charge transfer-based DMN colorimetric probe for the rapid naked-eye detection of cyanide ions in solution as well as real water samples. The intermolecular interaction between the DMN probe and cyanide ions in solution was investigated using a combination of spectroscopic and computational methods in this study. The DMN probe exhibited a selective colorimetric response for cyanide ions over the other anions exposed. The cyanide sensing mechanism of the probe has been investigated by ¹H NMR titration and density functional theory calculations. The results reveal that the colorimetric response of the DMN probe is due to the Michael adduct formation in the β-conjugated position of the dicyanovinyl group with cyanide, which blocks intramolecular charge transfer transition. Under optimized experimental conditions, the DMN probe showed a linear plot in the concentration range of 0.01-0.25 μM, with a detection limit of 23 nM. Further, a 3D printed portable accessory for the smartphone and an open-source android application is developed to suit the DMN probe for on-site work. In addition, we have developed the microfluidic paper-based analytical device that could selectively detect cyanide ions at very low concentration using a colorimetric DMN probe. In addition, the DMN probe was effectively used to determine the cyanide ion in a variety of water samples.

A simple approach based on transmetalation for the selective and sensitive colorimetric/fluorometric detection of copper(II) ions in drinking water

The search for feasible and efficient methods for sensing cations in the environment is a challenge of current scientific interest. Among colorimetric and fluorometric methods, those allowing a direct and...

Carbon dots as naked eye sensors

Optical sensors are always fascinating for chemists due to their selectivity, sensitivity, robustness and cost-effective nature.

New chalcone-substituted metallophthalocyanines: Synthesis, characterization, and investigation of their properties

The synthesis of novel metallophthalocyanines (M = Zn, Mg, and Co) derived from ( E)-3-(3-bromophenyl)-1-(3-hydroxyphenyl)prop-2-en-1-one and ( E)-3-(3-fluorophenyl)-1-(3-hydroxyphenyl)prop-2-en-1-one is achieved. These complexes and the synthesized novel phthalonitrile derivatives are characterized by FTIR, 1H NMR, 13C NMR, UV-Vis, mass spectrometry, and elemental analysis. The aggregation properties of the metallophthalocyanines ZnPc, MgPc, and CoPc are investigated in different solvents and at different concentrations in dimethyl formamide. The electrochemical behavior is also investigated. The cyclic voltammograms give one oxidation reaction for all the bromo-derived metallophthalocyanines and one reduction reaction for all the fluoro-derived metallophthalocyanines. Fe3+, Cd2+, Hg2+, Cu2+, Ni2+, and Co2+ ions are titrated fluorometrically with the phthalocyanines. The bromo- and fluoro-substituted phthalocyanine compounds show different effects on the metal ion titrations.

Fluorescent, colourimetric, and ratiometric probes based on diverse fluorophore motifs for mercuric(II) ion (Hg2+) sensing: highlights from 2011 to 2019

Though it has not been shown to deliver any biological importance, mercuric(II) ion (Hg²⁺) is a deleterious cation which poses grievous effects to the human body and/or the ecosystem, hence, the need for its sensitive and selective monitoring in both environmental and biological systems. Over the years, there has been a great deal of work in the use of fluorescent, colourimetric, and/or ratiometric probes for Hg²⁺ recognition. Essentially, the purpose of this review article is to give an overview of the advances made in the constructions of such probes based on the works reported in the period from 2011 to 2019. Discussion in this review work has been tailored to the kinds of fluorophore scaffolds used for the constructions of the probes reported. Selected examples of probes under each fluorophore subcategory were discussed with mentions of the typically determined parameters in an analytical sensing operation, including modulation in fluorescence intensity, optimal pH, detection limit, and association constant. The environmental and biological application ends of the probes were also touched where necessary. Important generalisations and conclusions were given at the end of the review. This review article highlights 196 references.

Design and optimization of a single-use optical sensor based on a polymer inclusion membrane for zinc determination in drinks, food supplement and foot health care products

A single-use optical sensor was designed for Zn(II) determination based on the immobilisation of the colorimetric reagent 2-acetylpyridine benzoylhydrazone (2-APBH) in a polymer inclusion membrane (PIM) adhered on the surface of an inert rectangular strip of polyester (Mylar). Different components for the membrane preparation were tested and those resulting in membrane with good appearance, proper physical and optical properties and ease of preparation were selected. Factorial design 23 with three replicates of the central point was applied for the optimisation of the membrane composition. The optimal composition consisted of 2.5 g of poly(vinyl chloride) (PVC), 4 mL of tributyl phosphate (TBP) and 0.04 g of 2-APBH. The optode showed a linear dynamic range from 0.03 (detection limit) to 1 mg L-1 of Zn(II) ions with a response time of 30 min in aqueous solution at pH 6 and a relative standard deviation of 3.90% for 0.4 mg L-1 of Zn(II). The sensor exhibited good selectivity to Zn(II) over other commonly ions. It was successfully applied to the determination of Zn(II) in a water certified reference material, spiked tap water, vitamin-mineral drink, food supplement and foot health care products, as contribution to the concern about this heavy metal due to its significant role in many biological and physiological processes although toxicant at high doses.

Multianalyte azo dye as an on-site assay kit for colorimetric detection of Hg2+ions and electrochemical sensing of Zn2+ ions

A new tailor-made colorimetric chemosensor, (E)-1-(benzo[d]thiazol-2-yl)-3-(pyridin-3-yldiazenyl)naphthalen-2-ol (1), containing benzothiazole and pyridine moieties connected through an azo (-N=N-) linkage has been designed and synthesized. The synthesized chemosensor displayed an eye-catching color change upon binding to acetate [AcO^-;colorless to russet] and mercury (II) [Hg²⁺;colorless to greenish blue] ions in 9:1 (v/v) aqueous CH₃CN (pH 7.0 HEPES buffer).The mechanism of interaction between the chemosensor and the Hg²⁺/AcO^- ions has been confirmed by ¹H NMR titration experiments. Moreover, the colorimetric chemosensor 1 displayed potential in-field applications as on-site assay kit and detection of Hg²⁺ ions in real water samples. Importantly chemosensor 1 gave selective electrochemical response towards Zn²⁺ ions, enabling simple azo-dye 1 as multichannel chemosensor for colorimetric detection of Hg²⁺ ions and electrochemical detection of Zn²⁺ ions.

A multi-responsive crown ether-based colorimetric/fluorescent chemosensor for highly selective detection of Al3+, Cu2+ and Mg2

A novel multi-response chemosensor L based on coumarin-chalcone-crown ether was designed and synthesized, which exhibited a high selectivity for the colorimetric detecting Al³⁺ and Cu²⁺ and fluorescent recognizing Al³⁺ and Mg²⁺ in ethanol. L can monitor Al³⁺ and Cu²⁺ via distinct color changes from a slight yellow to pink and to orange, respectively. The sensor L can also monitor Al³⁺ and Mg²⁺ by fluorescence emission responses at 592 nm and 547 nm with low detection limits of 0.31 μM and 0.23 μM, respectively. The selectivity of L toward Al³⁺, Cu²⁺ and Mg²⁺ was not interfered by a large number of coexisting ions and was found to be reversible. By means of spectrometric titration, Job's plot, mass spectrometry, ¹H NMR titration and IR spectroscopy analysis, it was unanimously confirmed that the sensor L had a stoichiometric ratio of 1:1 with Cu²⁺ and Mg²⁺, and 1:2 with Al³⁺. The order of the stability of the complexes formed by L and Al³⁺, Cu²⁺, Mg²⁺ was as follows: L-Al³⁺ > L-Cu²⁺ > L-Mg²⁺. At the same time, some possible bonding modes and sensing mechanisms were further proposed, and the optimized structure of the sensor L and its sensing mechanism for Al³⁺, Cu²⁺ and Mg²⁺ were confirmed by the calculations of DFT/B3LYP and TD-DFT methods in a suite of Gaussian 09 programs.

A new hydrazone-based colorimetric chemosensor for naked-eye detection of copper ion in aqueous medium

A new hydrazone-based colorimetric Cu2+ chemosensor is synthesized. Its structure was confirmed by single-crystal X-ray diffraction. Its binding properties towards various metal ions are examined through absorption spectroscopy. In aqueous THF solution, the chemosensor exhibits selective recognition towards Cu2+ over other metal ions with a colour change from colourless to pink. The complex formed between the chemosensor and Cu2+ ions forms a 2:1 stoichiometry with an association constant of 2.46 × 108M−2. The analytical detection limit for Cu2+ by the naked eye is as low as 10.0 μM.

A chitosan-based fluorescent hydrogel for selective detection of Fe2+ ions in gel-to-sol mode and turn-off fluorescence mode

A novel chitosan-based fluorescent hydrogel has been synthesized by chemically bonding terpyridine-bearing aldehydes onto chitosan via an acid condensation reaction.

Smart, chiral, and nonconjugated cyclohexane-based bis-salicylaldehyde hydrazides: multi-stimuli-responsive, turn-on, ratiometric, and thermochromic fluorescence, single-crystal structures via DFT calculations

Multidentate and environmentally sensitive dyes show turn-on, ratiometric, and thermochromic fluorescence.

Recent developments in nanotechnology transforming the agricultural sector: a transition replete with opportunities

The applications and benefits of nanotechnology in the agricultural sector have attracted considerable attention, particularly in the invention of unique nanopesticides and nanofertilisers. The contemporary developments in nanotechnology are acknowledged and the most significant opportunities awaiting the agriculture sector from the recent scientific and technical literature are addressed. This review discusses the significance of recent trends in nanomaterial-based sensors available for the sustainable management of agricultural soil, as well as the role of nanotechnology in detection and protection against plant pathogens, and for food quality and safety. Novel nanosensors have been reported for primary applications in improving crop practices, food quality, and packaging methods, thus will change the agricultural sector for potentially better and healthier food products. Nanotechnology is well-known to play a significant role in the effective management of phytopathogens, nutrient utilisation, controlled release of pesticides, and fertilisers. Research and scientific gaps to be overcome and fundamental questions have been addressed to fuel active development and application of nanotechnology. Together, nanoscience, nanoengineering, and nanotechnology offer a plethora of opportunities, proving a viable alternative in the agriculture and food processing sector, by providing a novel and advanced solutions. © 2017 Society of Chemical Industry.

Triphenylamine based reactive coloro/fluorimetric chemosensors: Structural isomerism and solvent dependent sensitivity and selectivity

Triphenyl amine based chemosensors, (2-(((2-(9H-carbazol-9-yl)phenyl)imino)methyl)-5-(diphenylamino)phenol (ortho-CPDP) and 2-(((4-(9H-carbazol-9-yl)phenyl)imino)methyl)-5-(diphenylamino)phenol (para-CPDP), showed solvent and isomerism dependent selective coloro/fluorometric sensing of multiple metal ions (Fe³⁺, Al³⁺ and Zn²⁺) with distinguishable responses. In CH₃CN, ortho and para-CPDP selectively produced yellow color upon addition of Al³⁺ and Fe³⁺ that was slowly disappeared. The yellow color of ortho and para-CPDP in DMF was decolourised selectively by adding Al³⁺ and Fe³⁺. Both ortho and para-CPDP in CH₃CN showed nearly similar rate of decolourization for Fe³⁺ and Al³⁺. However, the rate of decolourization of ortho and para-CPDP in DMF was different for Fe³⁺ (10μM, 8min) and Al³⁺ (5×10^-4M, 40min) ions. The limit of detection of para-CPDP for Fe3+ is 10μM and Al³⁺ 500μM. The mechanistic studies revealed the imine hydrolysis of ortho and para-CPDP in presence of Lewis acidic Fe³⁺ and Al³⁺. The reactivity based sensing lead to high selectivity for Al³⁺ and Fe³⁺ ions. Further, para-CPDP exhibited selective fluorescence turn-on for Zn²⁺ in DMF (λ_max=513nm) and detection limit of 6.0μM. Thus, reactive chemosensors, ortho and para-CPDP, exhibited selective and distinguishable colorimetric sensing of Fe³⁺ and Al³⁺ ions and isomerism and solvent dependent fluorescence sensing of Zn²⁺.

A dual-responsive colorimetric and fluorescent chemosensor based on diketopyrrolopyrrole derivative for naked-eye detection of Fe3+ and its practical application

A novel dual-responsive colorimetric and fluorescent chemosensor L based on diketopyrrolopyrrole derivative for Fe³⁺ detection was designed and synthesized. In presence of Fe³⁺, sensor L displayed strong colorimetric response as amaranth to rose pink and significant fluorescence enhancement and chromogenic change, which served as a naked-eye indicator by an obvious color change from purple to red. The binding constant for L-Fe³⁺ complex was found as 2.4×10⁴ with the lower detection limit of 14.3nM. The sensing mechanism was investigated in detail by fluorescence measurements, IR and ¹H NMR spectra. Sensor L for Fe³⁺ detection also exhibited high anti-interference performance, good reversibility, wide pH response range and instantaneous response time. Furthermore, the sensor L has been used to quantify Fe³⁺ ions in practical water samples with good recovery.

Water-Soluble Colorimetric Amino[bis(ethanesulfonate)] Azobenzene pH Indicators: A UV-Vis Absorption, DFT, and 1H-15N NMR Spectroscopy Study

Water-soluble azobenzene derivatives containing amino[bis(ethanesulfonate)] groups are demonstrated as colorful pH indicators in water and on filter paper. Vibrant color changes were observed from yellow/orange to pink between pH 1 and 4, which are attributed to an intramolecular charge-transfer mechanism. The pK _as of the indicators range from 2.1 to 2.6. ¹H/¹H-¹⁵N NMR studies in deuterium oxide reveal that the protonation of the azobenzene pH indicators occurs predominantly at the β-azo nitrogen atom, in agreement with the density functional theory calculations. Excellent selectivity for protons was confirmed in water over common biologically relevant metal ions. Studies in methanol, however, indicate that the pH indicator with a methoxy group ortho to the amino[bis(ethanesulfonate)] group facilitates the selective coordination of Cu²⁺ with a binding constant pβ_Cu2+ of 4.6 ± 0.1. The indicators complement the existing library of azobenzene indicator dyes and may be useful for measuring the environmental pH at higher proton concentrations.