Highly Sensitive and Selective Fluorescent Probe for Ascorbic Acid with a Broad Detection Range through Dual-Quenching and Bimodal Action of Nitronyl-Nitroxide

MOFs as versatile scaffolds to explore environmental contaminants based on their luminescence bustle

Metal-Organic Frameworks (MOFs) with luminescent properties hold significant promise for environmental remediation. This review critically examines recent research on these materials design, synthesis, and applications, mainly focusing on their role in combating environmental pollutants. Through a comprehensive analysis of metal ions, ligands, and framework compositions, the review discusses the importance of tailored design and synthesis approaches in achieving desired luminescent characteristics. Key findings highlight the effectiveness of luminous MOFs as fluorescent sensors for a wide range of contaminants, including heavy metals, reactive species, antibiotics, and explosives. Considering all this, the review discusses future research needs and opportunities in the field of luminous MOFs. It emphasizes the importance of developing multifunctional materials, refining design methodologies, exploring sensing mechanisms, and ensuring environmental compatibility, scalability, and affordability. By providing insights into the current state of research and outlining future directions, this review is a valuable resource for researchers seeking to address environmental challenges using MOF-based solutions.

Dual functional profluorescent nitroxides for the detection of reactive oxygen species and inhibition of collagen degradation during reassembly

High content of reactive oxygen species (ROS) in the human body leads to oxidative stress and serious health problems, such as cancer and cardiovascular or bone diseases. It is also one of the agents that cause collagen damage. Herein, detection of ROS, scavenging of formed carbon-centered radicals and inhibition of collagen fragmentation were performed in a single operation using newly synthesized profluorescent nitroxide PN1via a switch-on approach. Reassembly of acid soluble collagen (ASC) in the presence of hydroxyl and hydroperoxyl radicals, representatives of ROS, was monitored to study the efficiency of the PN1 probe. Self-assembly curves of collagen fibril solution were in accordance with differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) observations, and indicated that PN1 efficiently inhibited the collagen chain scission. In order to prevent the leakage of the probe in materials, a PN2 monomer was successfully incorporated with MMA to form a profluorescent copolymer probe. Furthermore, PN1 and PN2-MMA copolymer probes offered high sensitivity of detection of ROS in the presence of collagen fibrils with detection limits of 1.1 and 2.7 μM, respectively. The mechanism of ROS detection and inhibition of collagen degradation by profluorescent nitroxides was proposed.

An irreversible paper-based profluorescent nitroxide probe for the selective detection of ascorbic acid

Ascorbic acid (AA) or vitamin C plays multiple crucial roles, particularly as an antioxidant. This essentially biologically active molecule was selectively detected over other reductants by the synthesized profluorescent nitroxide probe ProN6via a switch-on method. After either a hydrogen atom or single electron transfer from AA to nitroxide, the resulting diamagnetic hydroxylamine was rapidly cyclized to form a fluorescent O-acylalkoxyamine. This cyclization prevented the reoxidation of the corresponding hydroxylamine to the nitroxide, leading to a high precision of detection. A kinetic fluorescence study indicated that ProN6 exhibited higher reactivity than ProN7. Density functional theory (DFT) calculations indicated that the Gibbs free energy of the AA-induced cascade reductive lactonization of ProN6 was lower than that of ProN5 and ProN7. The designed probe achieved the sensitive and specific detection of AA with detection limits of 77.9 nM and 195.9 μM in solution and on paper, respectively. The utilization of the probe as a paper-based fluorescent sensor demonstrated the good accuracy of the quantitative analysis of AA in commercial supplements.

2D Phthalocyanine-Assembled Porous Nanostructure-Based Electrochemical Platform for High-Efficiency Detection of Ascorbic Acid

In this work, a novel two-dimensional (2D) porous nanostructure is constructed upon air/water interfacial assembly of 12-crown-ether-4-incorporated double-decker phthalocyanine (Pc2). The combination of the good electroconductivity of phthalocyanine and the great surface area of the porous structure endows the assembled film with excellent chemical sensing property for ascorbic acid (AA). The low limit of detection can be 0.15 μM with a large linear concentration range and strong anti-interfering ability, which can be comparable to the best results of tetrapyrrole-based electrochemical sensors for AA. Furthermore, the obtained 2D porous assembled film sensor can be applied in real-time monitoring of AA in commercial drinks, indicating its application potential in accurate detection of AA in real samples.

Methyl viologen induced fluorescence quenching of CdTe quantum dots for highly sensitive and selective "off-on" sensing of ascorbic acid through redox reaction

A turn-on fluorescent sensor based on CdTe quantum dots (QDs) is designed for highly sensitive and selective ascorbic acid (AA) detection. CdTe shows a strong emission centered at 578 nm. When assembled with poly(sodium 4-styrenesulfonate) (PSS) and methyl viologen (Mv²⁺) through electrostatic interaction, the emission is found to be effectively quenched. In the presence of AA, Mv²⁺ is reduced to Mv⁺, making the fluorescence of CdTe QDs restored. Under the optimal conditions, the proposed AA sensing method shows a linear proportional response from 0.8 µM to 20 µM, with the detecting limit as low as 50 nM. The developed method was successfully applied in the analysis of AA in human serum samples and cell lysates with satisfactory results.

Organic radical materials in biomedical applications: State of the art and perspectives

Owing to their unique chemical reactivities and paramagnetism, organic radicals with unpaired electrons have found widespread exploration in physical, chemical, and biological fields. However, most radicals are too short-lived to be separated and only a few of them can maintain stable radical forms via stereochemical strategies. How to utilize these raw radicals for developing stable radical-containing materials have long been a research hotspot for many years. This perspective introduces fundamental characteristics of organic radical materials and highlights their applications in biomedical fields, particularly for bioimaging, biosensing, and photo-triggered therapies. Molecular design of these radical materials is considered with reference to their outstanding imaging and therapeutic performances. Various challenges currently limiting the wide applications of these organic radical materials and their future development are also discussed.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Inner filter effect between upconversion nanoparticles and Fe(ii)-1,10-phenanthroline complex for the detection of Sn(ii) and ascorbic acid (AA)

Dual-function and multi-function sensors can use the same material or detection system to achieve the purpose of detection of two or more substances. Due to their high sensitivity and specificity, dual-function and multi-function sensors have potential applications in many fields. In this article, we designed a dual-function sensor to detect Sn(ii) and ascorbic acid (AA) based on the inner filter effect (IFE) between NaYF₄:Yb,Er@NaYF₄@PAA (UCNPs@PAA) and Fe(ii)–1,10-phenanthroline complex. Fe(ii)–1,10-phenanthroline complex has strong absorption in most of the ultraviolet-visible light range (350 nm–600 nm), and this absorption band overlaps with the green emission peak of UCNPs@PAA at 540 nm; Fe(ii)–1,10-phenanthroline complex can significantly quench the green light emission of UCNPs@PAA. When Sn(ii) or AA is added to the UCNPs@PAA/Fe(iii)/1,10-phenanthroline, they can reduce Fe(iii) to Fe(ii). Fe(ii) can react with 1,10-phenanthroline to form an orange complex, thereby quenching the green light emission of UCNPs@PAA. And the quenching efficiency is related to the concentration of Sn(ii) and AA; there is a linear relationship between quenching efficiency and the concentration of Sn(ii) and AA, within a certain concentration range the detection limits of this dual-function sensor for Sn(ii) and AA are 1.08 μM and 0.97 μM, respectively. In addition, the dual-function sensor can also detect Sn(ii) and AA in tap and spring water.

Based on the inner filter effect (IFE), we use UCNPs to develop a dual-function sensors, which can realize sensitive and selective detection for the Sn(ii) and ascorbic acid (AA).

Simultaneous Recognition of Dopamine and Uric Acid in the Presence of Ascorbic Acid via an Intercalated MXene/PPy Nanocomposite

Two-dimensional (2D) MXenes have shown a great potential for chemical sensing due to their electric properties. In this work, a Ti₃C₂T_x/polypyrrole (MXene/PPy) nanocomposite has been synthesized and immobilized into a glassy carbon electrode to enable the simultaneous recognition of dopamine (DA) and uric acid (UA) under the interference of ascorbic acid (AA). The multilayer Ti₃C₂T_x MXene was prepared via the aqueous acid etching method and delaminated to a single layer nanosheet, benefiting the in-situ growth of PPy nanowires. The controllable preparation strategy and the compounding of PPy material remain great challenges for further practical application. A facile chemical oxidation method was proposed to regulate magnitude and density during the forming process of PPy nanowire, which promotes the conductivity and the electrochemical active site of this as-prepared nanomaterial. The MXene/PPy nanocomposite-modified electrode exhibited the selective determination of DA and UA in the presence of a high concentration of AA, as well as a wide linear range (DA: 12.5–125 μM, UA: 50–500 μM) and a low detection limit (DA: 0.37 μM, UA: 0.15 μM). More importantly, the simultaneous sensing for the co-existence of DA and UA was successfully achieved via the as-prepared sensor.

A dual luminescent sensor coordination polymer for simultaneous determination of ascorbic acid and tryptophan

Simultaneous high sensitivity detection of biomolecules is important for research in medicine, living cells and environmental samples. In this work, a water stable coordination polymer, [Cd₂(bptc)(4,4'-bpy)(H₂O)₃]ˑH₂O 1 (H₄bptc = 2,3,3',4'-biphenyl tetracarboxylic acid, 4,4'-bpy = 4,4'-bipyridine), was designed and successfully synthesized as a luminescent sensor for simultaneous recognition of Ascorbic Acid (AA) and L-Tryptophan (L-Trp) based on luminescent -OFF and -ON, respectively. Importantly, the proposed sensing system showed an excellent performance with high K_SV values of 4.85 × 10⁴ M^-1, 9.60 × 10⁷ M^-1 and low limit of detection (LOD) of 0.28 nM, 63 nM, respectively. In addition, the probable mechanisms are also discussed. The luminescent quenching behavior by AA can be mainly attributed to the static resonance energy transfer between complex 1 and the analytes. Whereas the enhancing effect of L-Trp comes from the intrinsic strong luminescence for L-Trp itself and photo-competitive mechanism between CP 1 sensor and L-Trp, supposedly. In addition, the repeatability of both systems were also investigated.

In situ self-assembled cationic lanthanide metal organic framework membrane sensor for effective MnO4- and ascorbic acid detection

Developing portable membrane sensors to accurately detect the biomolecule ascorbic acid (AA) is extremely important for food safety and human health. Herein, we successfully design and synthesize a novel cationic metal organic framework (Eu-pbmc, Hpbmc = 2-(pyridine-2-yl)-1H-benzimidazole-5-carboxylic acid) and assemble polyacrylonitrile/Eu-pbmc membrane (PEM) by an in-situ growth strategy. Benefiting from the appreciable loading of Eu-pbmc nanoparticles and high water permeation flux, PEM possesses effective detection for MnO₄^- with a limit of detection (LOD) of 17 nM. Utilizing the cationic porous framework, we load MnO₄^- into PEM and construct a "on-off-on" system for effective AA detection. The oxidative MnO₄^- can be reduced by AA and the resulting turn-on luminescence can reflect the concentration of AA. Compared with pure Eu-pbmc crystals, PEM exhibits improved AA detection performance with LOD of 48 nM and detection time of 1 min via a concise detection operation. The stable membrane sensor realizes an accurate detection in real biological samples, meeting the practical requirement. Moreover, an IMP logic gate is helpful to analyze MnO₄^- and AA in water. The proposed novel luminescence platform as well as reasonable "on-off-on" luminescence mode provide a promising method for AA detection.

A turn-on luminescent probe for Fe3+ and ascorbic acid with logic gate operation based on a zinc(ii)-based metal–organic framework

A zinc(ii)-based metal–organic framework exhibits fluorescence turn-on behaviour for Fe³⁺ and AA with high sensitivity and selectivity.

Fabrication of Co3O4 NPs-graphene oxide nanocomposites as an efficient catalyst towards oxygen reduction and its catalytic applications

Co₃O₄ nanoparticles-graphene oxide (Co₃O₄ NPs-GO) nanocomposites with good solubility were successfully synthesized and well characterized. As a nanocatalyst with oxidase-mimicking activity, the nanocomposites can catalyze the oxidation of 3',5,5'-tetramethylbenzidine (TMB) with high efficiency. The catalyzing reaction was rapid and no extra H₂O₂ was needed compared with other similar TMB oxidation reactions. The catalyzing reaction mechanism of the system was investigated in detail. And it was demonstrated that the oxygen involved in the reaction came from the oxygen absorbed on the nanocomposites which oxidized TMB. Based on this reaction, a colorimetric system for vitamin C detection in vegetable and fruits was established. Under the optimum conditions, the detection can be achieved within 10 min and a linear relationship in concentration range of 2.5 × 10^-6 to 1.8 × 10^-5 M and 3.4 × 10^-5 to 1.7 × 10^-4 M was obtained with a detection limit of 7.0 × 10^-7 M. The colorimetric system exhibited good selectivity, sensitivity and satisfactory recoveries ranged from 93.1% to 101.1%.

A MnO2 nanosheet-based ratiometric fluorescent nanosensor with single excitation for rapid and specific detection of ascorbic acid

Ascorbic acid (AA) detection in biological sample and food sample is critical for human health. Herein, a MnO₂ nanosheet (MnO₂-NS)-based ratiometric fluorescent nanosensor has been developed for high sensitive and specific detection of AA. The MnO₂-NS presents peroxidase-like activity and can oxidize non-fluorescent substrate of o-phenylenediamine (OPDA) into fluorescent substrate, presenting maximum fluorescence at 568 nm (F₅₆₈). If MnO₂-NS is premixed with AA, the MnO₂-NS is then decomposed as Mn²⁺ by AA, decreasing the fluorescent intensity of F₅₆₈. Meantime, AA is oxidized as dehydroascorbic acid (DHAA), which can react with OPDA to generate fluorescent substrate. A new fluorescence response is found at 425 nm (F₄₂₅). The dual fluorescent responses can be excited with a universal excitation wavelength, simplifying the detection procedure. With F₄₂₅/F₅₆₈ as readout, limit of detection for AA reaches as low as 10.0 nM. Satisfactory recoveries are found for AA detection in serum and diverse beverages. The ratiometric strategy significantly eliminates false-negative and false-positive results, providing a cost-effective, rapid, and reliable way for AA detection in real sample.

Dual-Emitting Eu(III)-Cu(II) Heterometallic-Organic Framework: Simultaneous, Selective, and Sensitive Detection of Hydrogen Sulfide and Ascorbic Acid in a Wide Range

As important biomolecules, the deficiency or maladjustment of hydrogen sulfide (H₂S) or ascorbic acid (AA) is associated with the symptoms of the same disease (e.g., cardiovascular disease or cancer). There is an urgent need to develop a fluorescent probe capable of distinguishing between H₂S and AA simultaneously. Here, we report the syntheses, structure, and property of the first dual-detection fluorescent probe which can differentiate H₂S or/and AA in aqueous media. Accordingly, a novel [EuCu(pydc)₂(ox)_0.5(H₂O)₃·1.5H₂O]_{2 n} (1, H₂pydc = 2,3-pyridinedicarboxylic acid and ox = oxalic acid) for selective and sensitive detection of H₂S and AA in a wide range has been constructed (H₂S: [130 nM, +∞); AA: [55 nM, +∞)), exhibiting excellent catalytic activity comparable to horseradish peroxidase. In addition, the highly efficient detection in human serum sample also proves the potential application in medical diagnosis. Meanwhile, a combinatorial logic gate (AND(INH)-OR) based on activated 1 has also been constructed. Furthermore, this approach for simultaneous H₂S and AA detection suggests that the current work will expand the potential application of metal-organic frameworks for dual or multiple detections in biomedical fields.

Self-Assembled α-Cyanostilbenes for Advanced Functional Materials

In the specific context of condensed media, the significant and increasing recent interest in the α-cyanostilbene (CS) motif [ArCHC(CN)Ar] is relevant. These compounds have shown remarkable optical features in addition to interesting electrical properties, and hence they are recognized as very suitable and versatile options for the development of functional materials. This progress report is focused on current and future use of CS structures and molecular assemblies with the aim of exploring and developing for the next generations of functional materials. A critical selection of illustrative materials that contain the CS motif, including relevant subfamilies such as the dicyanodistyrylbenzene and 2,3,3-triphenylacrylonitrile shows how, driven by the self-assembly of CS blocks, a variety of properties, effects, and possibilities for practical applications can be offered to the scientific community, through different rational routes for the elaboration of advanced materials. A survey is provided on the research efforts directed toward promoting the self-assembly of the solid state (polycrystalline solids, thin films, and single crystals), liquid crystals, nanostructures, and gels with multistimuli responsiveness, and applications for sensors, organic light-emitting diodes, organic field effect transistors, organic lasers, solar cells, or bioimaging purposes.

Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides

The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through p-ter-phenyl (PPP), ter-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of 3, 5, 6, PPP-NNSi, PTP-NNSi, and final biradicals of TTT-NN, TPT-NN, and PPP-NN, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in TTT-NN and TPT-NN than for NN and phenyl units (∼29°) in PPP-NN were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals TTT-NN, TPT-NN, PTP-NN, and PPP-NN were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for TTT-NN and TPT-NN. The intramolecular exchange interactions for TPT-NN and TTT-NN were found in good agreement with the ones calculated by broken symmetry DFT calculations.

Biological Relevance of Free Radicals and Nitroxides

Nitroxides are stable, kinetically-persistent free radicals which have been successfully used in the study and intervention of oxidative stress, a critical issue pertaining to cellular health which results from an imbalance in the levels of damaging free radicals and redox-active species in the cellular environment. This review gives an overview of some of the biological processes that produce radicals and other reactive oxygen species with relevance to oxidative stress, and then discusses interactions of nitroxides with these species in terms of the use of nitroxides as redox-sensitive probes and redox-active therapeutic agents.

A luminescent cerium metal–organic framework for the turn-on sensing of ascorbic acid

Based on a simple redox reaction, the cerium metal–organic framework (ZJU-136-Ce) exhibits highly sensitive and selective turn-on sensing of AA.

Synthesis and Straightforward Quantification Methods of Imino Nitroxide-Based Hexaradical Architecture on a Cyclotriphosphazene Scaffold

The synthesis of a homogeneous neutral hexaradical architecture consisting of six imino nitroxide radical moieties covalently bonded on a cyclotriphosphazene scaffold was reported. The synthesis of hexaradical imino nitroxide compounds follows the Ullman procedure involving the condensation of 2,3-bis(hydroxylamino)-2,3-dimethylbutane with hexa-(4-formylphenoxy)cyclotriphosphazene (3) followed by oxidation of the condensation product hexa-[4-(1-hydroxy-4,4,5,5-tetramethyl-2-imidazoline-2-yl)phenoxy]cyclotriphosphazene (2) by NaIO₄. Characterization of hexaradical was performed by X-ray and SQUID in solid state and by EPR, absorption spectroscopy, and electrochemistry in solution. CV of 1 shows an oxidation peak at 1.184 V (vs SCE) and a reduction peak at -0.883 V, both characteristics of the presence of phenyl imino nitroxide (7) moieties, suggesting that the contribution of the cyclotriphosphazene core is negligible. Attention was particularly focused on developing methods, UV-vis spectroscopy and square-wave voltammetry, to quantify the number of radicals in a way to confirm easily and rapidly the polyradicals' structure.

Application of graphene quantum dots decorated with TEMPO-derivatized zinc phthalocyanine as novel nanoprobes: probing the sensitive detection of ascorbic acid

A nanocomposite of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl substituted zinc phthalocyanine and graphene quantum dots is a selective fluorescence sensor for ascorbic acid.