Simple, sensitive, colorimetric detection of pyrophosphate via the analyte-triggered decomposition of metal-organic frameworks regulating their adaptive multi-color Tyndall effect

This paper describes initially the application of the Tyndall effect (TE) of metal-organic framework (MOF) materials as a colorimetric signaling strategy for the sensitive detection of pyrophosphate ion (PPi). The used MOF NH2-MIL-101(Fe) was prepared with Fe3+ ions and fluorescent ligands of 2-amino terephthalic acid (NH2-BDC). The fluorescence of NH2-BDC in MOF is quenched due to the ligand-to-metal charge transfer effect, while the NH2-MIL-101(Fe) suspension shows a strong TE. In the presence of PPi analyte, the MOFs will undergo decomposition because of the competitive binding of Fe3+ by PPi over NH2-BDC, resulting in a significant decrease in the TE signal and fluorescence restoration from the released ligands. The results demonstrate that the new method only requires a laser pointer pen (for TE creation) and a smartphone (for portable quantitative readout) to detect PPi in a linear concentration range of 1.25-800 μM, with a detection limit of ~210 nM (3σ) which is ~38 times lower than that obtained from traditional fluorescence with a spectrophotometer (linear concentration range, 50-800 µM; detection limit, 8.15 µM). Moreover, the acceptable recovery of PPi in several real samples (i.e., pond water, black tea, and human serum and urine) ranges from 97.66 to 119.15%.

A reliable and selective ratiometric sensing probe for fluorometric determination of P2O74- based on AIE of GSH@CuNCs-assisted by Al-N@CQDs

In this study, a novel composite material was developed for the ratiometric detection of pyrophosphate anion (P2O74-). This composite consisted of Al and nitrogen co-doped carbon dots (Al-N@CQDs) and glutathione-capped copper nanoclusters (GSH@CuNCs). The Al-N@CQDs component, with its high reserved coordination capacity of Al3+, induced the non-luminescent behavior of GSH@CuNCs, resulting in an aggregation-induced emission (AIE) effect. The hybrid material (Al-N@CQDs/GSH@CuNCs) exhibited dual-emission signals at 620 nm and 450 nm after integrating the two independent materials utilizing the AIE effect and the fluorescence resonance energy transfer (FRET) approach. This approach represents the first utilization of this composite for ratiometric detection. Nevertheless, upon the addition of P2O74-, the AIE and FRET processes were hindered due to the higher coordination interaction of Al3+ towards P2O74- compared to the amino/carboxyl groups on Al-N@CQDs. This successful interference of the AIE and FRET processes allowed for the effective estimation of P2O74-. The response ratio (F450/F620) increased with increasing the concentration of P2O74- in the range of 0.035-160 µM, with an impressive detection limit of 0.012 µM. This innovative approach of utilizing hybrid CQDs/thiolate-capped nanoclusters as a ratiometric fluorescent sensor for analytical applications introduces new possibilities in the field. The as-fabricated system was successfully applied to detect P2O74- in different real samples such as water, serum, and urine samples with acceptable results.

Metal-free auxiliary pyrophosphate detection based on near-infrared carbon dots

The excessive use of pyrophosphate (PPi) anions as additives poses a serious threat to human health and the environment. Considering the current status of PPi probes, the development of metal-free auxiliary PPi probes has important applications. In this study, a novel near-infrared nitrogen and sulfur co-doped carbon dots (N,S-CDs) were prepared. The average particle size of N,S-CDs was 2.25 ± 0.32 nm with average height was 3.05 nm. The probe N,S-CDs showed a special response to PPi, and a good linear relationship was obtained with PPi concentrations ranging from 0 to 1 μM, with the limit of detection being 0.22 nM. Tap water and milk were used for practical inspection, and ideal experimental results were acquired. In addition, the probe N,S-CDs also showed good results in biological systems, such as cell and zebrafish experiments.

Metal-organic framework-based nanomaterials as opto-electrochemical sensors for the detection of antibiotics and hormones: A review

Increasing trace levels of antibiotics and hormones in the environment and food samples are concerning and pose a threat. Opto-electrochemical sensors have received attention due to their low cost, portability, sensitivity, analytical performance, and ease of deployment in the field as compared to conventional expensive technologies that are time-consuming and require experienced professionals. Metal-organic frameworks (MOFs) with variable porosity, active functional sites, and fluorescence capacity are attractive materials for developing opto-electrochemical sensors. Herein, the insights into the capabilities of electrochemical and luminescent MOF sensors for detection and monitoring of antibiotics and hormones from various samples are critically reviewed. The detailed sensing mechanisms and detection limits of MOF sensors are addressed. The challenges, recent advances, and future directions for the development of stable, high-performance MOFs as commercially viable next-generation opto-electrochemical sensor materials for the detection and monitoring of diverse analytes are discussed.

A copper ion-mediated on-off-on gold nanocluster for pyrophosphate sensing and bioimaging in cells

Herein, gold nanoclusters (AuNCs@EW@Lzm, AuEL) with the bright red fluorescence at 650 nm were prepared by egg white and lysozyme as double protein ligands, which exhibited good stability and high biocompatibility. The probe displayed highly selective detected pyrophosphate (PPi) based on Cu²⁺-mediated AuEL fluorescence quenching. Specifically, the fluorescence of AuEL was quenched once the Cu²⁺/Fe³⁺/Hg²⁺ is added to chelate with amino acids on the AuEL surface, respectively. Interestingly, the fluorescence of quenched AuEL-Cu²⁺ was significantly recovered by PPi, but not the other two. This phenomenon was attributed to the stronger bond between PPi and Cu²⁺ than that of Cu²⁺ with AuEL nanoclusters. The results demonstrated a good linear relationship between PPi concentration and the relative fluorescence intensity of AuEL-Cu²⁺ in the range of 131.00-685.40 μM with a detection limit of 2.56 μM. In addition, the quench AuEL-Cu²⁺ system can also be recovered in acidic environments (pH ≤ 5). And the as-synthesized AuEL showed excellent cell imaging and target the nucleus. Thus the fabrication of AuEL offers a facile strategy for efficient PPi assay and offers the potential for drug/gene delivery to the nucleus.

Multi-stimuli-responsive molecular fluorescent probes for bioapplications

Stimuli-responsive fluorescent probes have been widely utilized in detecting the physiological and pathological states of living systems. Numerous stimuli-responsive fluorescent probes have been developed due to their advantages of good sensitivity, high resolution, and high contrast fluorescent signals. In this feature article, the progress of multi-stimuli-responsive probes, including organic molecules and metal complexes, for the detection of various biomarkers for bio-applications is summarized. The feature article focuses on the applications of organic-molecule- and metal-complex-based molecular probes in biological systems for detecting different biomarkers of cancer or other diseases. The current challenges and potential future directions of these probes for applications in biological systems are also discussed.

Substituents make a difference: 6,6″-modified terpyridine complexes with helix configuration and enhanced emission

A series of complexes L22-M (L2: 6,6″-bis(4-methoxyphenyl)-4'-phenyl-2,2':6',2″-terpyridine, M: Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) were synthesized by coordinating p-methoxyphenyl 6,6″-substituted terpyridine ligand with first-row transition metal ions and characterized by NMR, ESI-MS, and X-ray single crystal diffraction techniques. Single-crystal structures demonstrated that the steric hindrance of p-methoxyphenyl substituents endowed complexes L22-M with obvious longer coordination bond lengths and larger bond angles and dihedral angles compared with unmodified L12-M (L1: 4'-phenyl-2,2':6',2″-terpyridine). The chiral helix geometry was observed for L22-M, in which 2,2':6',2″-terpyridine moiety dramatically twisted to a spiral form in comparison to the nearly coplanar structure of the parent L12-M, resulting in plentiful intramolecular and intermolecular π-π interactions. Also, the appealing racemic (P and M) double helix packed structure for 6,6″-modified bisterpyridine complex L22-Cu was formed in the crystal. The consequent appealing charge transfer (CT) emission for L22-Zn in the solution and solid were investigated via UV-vis and fluorescence spectroscopy techniques and time-dependent density functional theory (TD-DFT) calculations. This work afforded a new method to achieve intriguing chiral geometry and CT optical properties via the subtle design and modification of terpyridine ligands.

Monitoring nucleic acid amplification process by UiO-66-NH2-based fluorescence sensor

Herein, we developed a nucleic acid amplification process monitoring scheme by use of UiO-66-NH2, in which pyrophosphate ion (PPi) released from the amplification can competitively coordinate with Zr to weaken...

Sensing and Imaging of PPi in vivo using Lanthanide-based Second Near-infrared Luminescent Probes

pathological changes. Here, we reported a luminescent nanoprobe for the detection and imaging of PPi in vivo based on a lanthanide nanoparticle with luminescence at the second near-infrared window modified...

Fabrication of a 2D metal-organic framework (MOF) nanosheet colloidal system and investigation of its fluorescence response to pesticide molecules

Pesticides, as a type of toxic chemicals widely used for a long time, not only pollute the environment but also affect people's health and cause serious harm to the human body, soil and environment. Therefore, it is very necessary to exploit a portable and environmentally friendly method to detect pesticides with high sensitivity. Herein, a new luminescent metal-organic framework ([Zn(TPYBDC)·H₂O]_n, TPYBDC^2- = 4'-(pyridin-4-yl)-[2,2':6',2''-terpyridine]-4,4''-dicarboxylate) with 2D coordination layers has been designed and assembled using 4'-(pyridin-4-yl)-[2,2':6',2''-terpyridine]-4,4''-dicarboxylic acid as the ligand. The as-synthesized Zn-LMOF was exfoliated to ultrathin 2D nanosheets (4-5 nm) to form a luminescence colloidal sensor by destroying the weak interaction between the coordination layers such as H-bonding between the matrix H₂O and the coordination carboxyl oxygen, and the π-π interactions among the interlayer conjugated aromatic rings. Investigation of its recognition and detection ability towards chemical pesticides shows that it can sensitively detect pesticides such as imidacloprid, nitenpyram and dinotefuran via fluorescence quenching effect with very low detection limit (LOD). Using imidacloprid as a typical case, a LOD value of 0.562 μM and recoveries for the simulated agricultural environmental samples in the range of 94-115% suggests that the as-fabricated 2D Zn-MOF nanosheet colloidal sensor (Zn-LMOF probe) is a most promising candidate for sensing chemical pesticides.

Metal-Organic Frameworks for Liquid Phase Applications

In the last two decades, metal-organic frameworks (MOFs) have attracted overwhelming attention. With readily tunable structures and functionalities, MOFs offer an unprecedentedly vast degree of design flexibility from enormous number of inorganic and organic building blocks or via postsynthetic modification to produce functional nanoporous materials. A large extent of experimental and computational studies of MOFs have been focused on gas phase applications, particularly the storage of low-carbon footprint energy carriers and the separation of CO2-containing gas mixtures. With progressive success in the synthesis of water- and solvent-resistant MOFs over the past several years, the increasingly active exploration of MOFs has been witnessed for widespread liquid phase applications such as liquid fuel purification, aromatics separation, water treatment, solvent recovery, chemical sensing, chiral separation, drug delivery, biomolecule encapsulation and separation. At this juncture, the recent experimental and computational studies are summarized herein for these multifaceted liquid phase applications to demonstrate the rapid advance in this burgeoning field. The challenges and opportunities moving from laboratory scale towards practical applications are discussed.

Two Cd(ii)-based metal–organic frameworks for the highly effective detection of Fe3+ ions and levofloxacin in aqueous media

Two new Cd(ii)-MOFs are hydrothermally synthesized and can be applied as dual-response sensors to identify Fe3+ and levofloxacin (LVX) with high selectivity, sensitivity and excellent reusability.

A new 7-diethylamino-4-hydroxycoumarin based reversible colorimetric/fluorometric probe for sequential detection of Al3+/PPi and its potential use in biodetection and bioimaging applications

A new 7-diethylamino-4-hydroxycoumarin appended acylhydrazone probe was prepared and utilized for the sequential detection of Al³⁺/PPi in a reversible off–on–off emissive manner. The various practical applications of the probe were established.

C3-symmetric triaminoguanidine based colorimetric and fluorometric chemosensor: Sequential detection of Zn2+/PPi, its RGB performance for detection of Zn2+ ion and construction of IMPLICATION logic gate

In this work, new ethyl(E)-2-cyano-3-(1H-pyrrol-2-yl)acrylate appended C₃-symmetric star-shape triaminoguanidine based Schiff base (LH₃) was designed and synthesized from simple synthons. New probe, LH₃ was completely analyzed by ¹H NMR, ¹³C NMR and mass spectrum. In the present probe LH₃, effective π-conjugated ethyl(E)-2-cyano-acrylate unit was introduced on the periphery of the pyrrole-triaminoquanidine conjugates by using carefully chosen building units. The probe LH₃ shows high selectivity and sensitivity towards Zn²⁺ ion via colorimetric and fluorometric changes. The yellowish orange color of LH₃ solution turned to wine red color upon addition of Zn²⁺ solution, along with red shifted absorption maxima from 450 nm to 550 nm, this indicates the formation of LH₃-Zn²⁺ species. Job's plot and mass spectrum analysis confirms the formation of 1:3 stoichiometric complex between the LH₃ and Zn²⁺ ions. Further this ensemble shows selective detection towards PPi anion over the other anions based on displacement metal ion approach. Hence, reversible colorimetric/emission response of LH₃ towards Zn²⁺ and PPi ions via "on-off-on" manner could allow the construction of IMPLICATION logic gate functions. The practical efficacy of the probe LH₃ was established by utilization of the probe for the detection of Zn²⁺ ions in real water sample analysis. Further, the significant noticeable colorimetric changes of the probe LH₃ upon addition of Zn²⁺ ion have been successfully integrated with a smartphone app RGB color value to construct a real-time analysis of Zn²⁺ ions.

A dual emission metal-organic framework for rapid ratiometric fluorescence detection of CO3 2- in seawater

A dual emission metal-organic framework (IRMOF-10-Eu) was prepared and used as a ratiometric fluorescent sensor for CO₃ ^2- detection. IRMOF-10-Eu had good stability and excellent luminescence in aqueous solution. IRMOF-10-Eu showed dual fluorescence emission from the ligand and Eu³⁺ with single excitation. Upon treatment with CO₃ ^2-, the fluorescence ratio (I ₆₂₄/I ₃₅₈) of the probe displayed significant change. The relative fluorescence intensity ratio (I ₆₂₄/I ₃₅₈) and CO₃ ^2- concentration had a linear relationship in 50-300 μM range with a low detection limit of 9.58 μM. And the luminescence probe of CO₃ ^2- showed a fast detection time. The possible mechanism was investigated. CO₃ ^2- changed the structure of IRMOF-10-Eu and interrupted the energy transfer process. Thus, the fluorescence emission intensity of the ligand was increased and Eu³⁺ was decreased with the addition of CO₃ ^2-. IRMOF-10-Eu was used to detect CO₃ ^2- in seawater, which showed good prospect in practical application. Subsequently, a highly selective and sensitive probe, IRMOF-10-Eu, may pave an efficient way for CO₃ ^2- detection in seawater.

Ultrasonic assisted functionalization of MWCNT and synergistic electrocatalytic effect of nano-hydroxyapatite incorporated MWCNT-chitosan scaffolds for sensing of nitrofurantoin

In the present work, we report the fabrication of stable composite of chitosan hydrogels (CHI) on multiwalled carbon nanotubes (MWCNT) using a simple ultrasonic-assisted method. Also, rod-like hydroxyapatite nanoparticles (HA NPs) were synthesised using a hydrothermal route and were incorporated into the highly conductive MWCNT-CHI scaffolds using an ultrasonication method. The functionalization of MWCNT and preparation of HA NPs on MWCNT-CHI nanocomposite were done using the sonication over the frequency of 37 kHz with the ultrasonic power capable of 150 W (Elmasonic Easy 60H bath sonicator). The resulting hybrid HA NPs/MWCNT-CHI nanocomposites have an excellent surface area and high surface to volume ratio, which leads to the sensitive detection of nitrofurantoin than pristine MWCNT and HA NPs. The complete elemental and morphological analyses of the HA NPs/MWCNT-CHI nanocomposites were characterised by XRD, FTIR, RAMAN, FESEM, TEM, EDX, and elemental mapping techniques. Electrochemical analysis of the HA NPs/MWCNT-CHI nanocomposites was carried out by cyclic voltammetry, electrochemical impedance spectroscopy and amperometry methods. The modified glassy carbon electrode (GCE) of HA NPs/MWCNT-CHI nanocomposites exhibit the nitrofurantoin detection activity at the linear range of 0.005-982.1 µM with the detection limit of 1.3 nM. The synergistic electrocatalytic activity of HA NPs/MWCNT-CHI nanocomposites modified GCE is correlated to the sensitivity of 0.16 µAµM^-1 cm^-2 with excellent precision and accuracy towards the sensing of nitrofurantoin.

Design and Construction of a Metal-Organic Framework as an Efficient Luminescent Sensor for Detecting Antibiotics

We first design and synthesize a dendritic aromatic 6-carboxyl linker (H₆TDCPB), which is successfully assembled with Cd(II) ion to construct a porous metal-organic framework with a raw Cd₆ cluster, {[Cd₃(TDCPB)·2DMAc]·DMAc·4H₂O}_n (namely, complex 1). More interestingly, six adjacent linkers are packed together by π-π-stacking interactions to form an amazing six-molecule accumulation in the crystal structure. By virtue of high stability and luminescent properties, the as-synthesized sample not merely owns an excellent detectable ability but also possesses an outstanding selectivity for nitrofurans with remarkable recursitivity.

One-pot synthesis of luminol–gallium nanoassemblies and their peroxidase-mimetic activity for colorimetric detection of pyrophosphate

Luminol–Ga nanoassemblies exhibit peroxidase-mimetic activity. Colorimetric detection of PPi is developed owing to the formation of a complex between PPi and Ga³⁺.

A white-light-emitting lanthanide metal–organic framework for luminescence turn-off sensing of MnO4− and turn-on sensing of folic acid and construction of a “turn-on plus” system

A white-light-emitting lanthanide MOF shows recyclable and dual-responsive sensing for MnO₄⁻ and folic acid in an aqueous system with high selectivity and sensitivity.

Amino-functionalized copper-based metal–organic frameworks for highly selective and sensitive detection of hypochlorite

In this work, we have developed amino-functionalized copper-based metal–organic frameworks (NH₂-Cu-MOFs) for the detection of hypochlorite (ClO⁻).