Dirhodium tetracarboxylate scaffolds as reversible fluorescence-based nitric oxide sensors

We report the synthesis and characterization of dirhodium tetracarboxylate complexes [Rh(2)(mu-O(2)CR)(4)(L)(2)], with R = Me and L = dansyl-imidazole (Ds-im) or dansyl-piperazine (Ds-pip). The fluorophores coordinate to the axial sites of the dirhodium core through the imidazole or piperazine N-atom and emit only weakly when excited at 365 or 345 nm for the Ds-im and Ds-pip complexes, respectively. These fluorophore-containing complexes were investigated for their ability to elicit a fluorescence response in the presence of NO. An immediate increase in fluorescence emission of greater than 15-fold occurs when NO is admitted to solutions containing [Rh(2)(mu-O(2)CMe)(4)] and Ds-pip or Ds-im. In both systems, the fluorescence response, which arises by NO-induced displacement of the axially coordinated fluorophore, is reversible with a sensitivity of approximately 4 microM. The related dinitrosyl complexes [Rh(2)(mu-O(2)CR)(4)(NO)(2)], where R = Me, Et, or n-Pr, were prepared, structurally characterized, and found to be air-stable, losing NO upon standing in solution. Sequestration of a methylene chloride solution of the Ds-pip complex from aqueous media by a NO-permeable membrane allows for fluorescence detection of NO for potential applications in biological fluids.     

Novel arene receptors as nitric oxide (NO) sensors

Nitric oxide sensors are based on the reversible, specific, and efficient binding to various types of arene receptors via intermolecular complexes, which are characterized by complete electron delocalization and strong interaction between the donor and acceptor moieties.     

Copper(ii) complexes as turn on fluorescent sensors for nitric oxide

Two copper(ii) complexes, 1 and 2, of two tridentate N-donor ligands, L(1) and L(2) [L(1) = dansyl derivative of bis-[3-(dimethylamino)-propyl]amine; L(2) = dansyl derivative of dipropylenetriamine] were synthesized and characterized. The quenched fluorescence intensity of complexes 1 and 2, in degassed methanol or aqueous (buffered at pH 7.2) solution, was found to reappear on exposure to nitric oxide. This is attributed to the reduction of paramagnetic Cu(ii) center by nitric oxide to diamagnetic Cu(i).     

Robotic sequential analysis of a library of metalloporphyrins as electrocatalysts for voltammetric nitric oxide sensors

A library of 83 metalloporphyrins with varying substitution pattern at the meso-position of the porphyrins and different central metal ions in the core region has been synthesized in small quantities using a parallel synthesis strategy. By means of a specially designed electrochemical robotic device integrating a 96-well microtitre plate and an easily movable assembly of working, counter and reference electrodes, the different porphyrins were automatically applied in sequence to an in-well electrochemical preparation and testing of NO sensors. Screening the entire compound collection suggested initial considerations concerning the influence of varied functionalities of the metalloporphyrins on their electrocatalytic properties for the oxidation of NO and helped to identify the quality of the investigated catalyst candidates. As compared to manually performed quality tests, the proposed strategy of automation has advantages in convenience, rapidity and especially reproducibility avoiding any inaccuracies introduced by manually performing all steps of the complex sensor formation and testing sequence.     

Transition metal-based chiroptical switches for nanoscale electronics and sensors

Efficient metal-based chiroptical switches have been designed that are capable of achieving multiple stable and reversible states. Studies in this field have yielded a variety of complex molecular devices whose conformations are controllable by many triggering mechanisms including pressure, solvent, counter ion, redox state, and photoinduction. Many of the systems are monitored with precision using circular dichroism spectroscopy. This review aims to provide a brief background of the development of these systems and a comprehensive overview of recently developed metal-based chiroptical switches. Potential applications in electronics and sensor technologies are discussed.     

Dendrimers as a scaffold for nitric oxide release

The preparation and characterization of nitric oxide (NO)-releasing dendrimer conjugates are reported. Generation 3 and 5 polypropylenimine dendrimers (DAB-Am-16 and DAB-Am-64) were modified at the exterior to impart different amine functionalities. The ability to store NO on a dendritic scaffold using N-diazeniumdiolate NO donors was examined via the reaction of primary amine, secondary amine, and amide functionalities with high pressures of NO (5 atm). The secondary amine dendrimer conjugates exhibited a high storage capacity for NO (up to 5.6 micromol NO/mg), greatly increasing the "payload" of released NO over existing macromolecular NO donors. The mechanism of diazeniumdiolate decomposition was proton initiated, generating NO spontaneously under physiological conditions (pH 7.4, 37 degrees C). The NO release durations (>16 h) observed for the secondary amine dendrimers were significantly longer compared to small molecule alkyl secondary amine diazeniumdiolates, thus illustrating a dendritic effect on NO release kinetics. The multivalent exterior of dendrimers allows for the future combination of NO donors and other functionalities on a single molecular scaffold, enabling diverse utility as NO storage/delivery systems.     

Improving the biocompatibility of in vivo sensors via nitric oxide release

The continuous, real-time monitoring of clinically important analytes (e.g., PO2, PCO2, pH, K+, Na+, glucose, and lactate) is of great importance to human health care. Despite considerable efforts spanning several decades, the use of in vivo sensors clinically remains limited due to inadequate biocompatibility. The discovery of nitric oxide (NO) as an effective inhibitor of platelet and bacterial adhesion has opened a new direction of research related to designing the next generation of in vivo sensors. In this Highlight article, recent progress in designing more biocompatible in vivo sensors is described, with a particular focus on preparing interfaces that resist biofouling via controlled NO release.     

Preparation and characterization of implantable sensors with nitric oxide release coatings

The widespread use of miniaturized chemical sensors to monitor clinically important analytes such as PO₂, PCO₂, pH, electrolytes, glucose and lactate in a continuous, real-time manner has been seriously hindered by the erratic analytical results often obtained when such devices are implanted in vivo. One major factor that has influenced the analytical performance of indwelling sensors is the biological response they elicit when in contact with blood or tissue (e.g. thrombus formation on the device surface, inflammatory response, encapsulation, etc.). Nitric oxide (NO) has been shown to be a potent inhibitor of platelet adhesion and activation as well as a promoter of wound healing in tissue. Herein, we review recent work aimed at the development of hydrophobic NO-releasing polymers that can be employed to coat catheter-type amperometric oxygen sensors without interfering with the analytical performance of these devices. Such modified sensors are shown to exhibit greatly enhanced hemocompatibility and improved analytical performance when implanted within porcine carotid and femoral arteries for up to 16 h. Further, results from preliminary studies also demonstrate that prototype fluorescent oxygen sensors, catheter-style potentiometric carbon dioxide sensors and subcutaneous needle-type enzyme-based amperometric glucose sensors can also be fabricated with new NO-release outer coatings without compromising the analytical response characteristics of these devices. The NO-release strategy may provide a solution to the lingering biocompatibility problems encountered when miniature chemical sensors are implanted in vivo.     

Comparative studies of ONNO-based ligands as ionophores for palladium ion-selective membrane sensors

Palladium sensors based on two neutral ionophores, N,N′-bis(acetylacetone) cyclohexanediamine (L₁) and N,N′-bis(o-hydroxyacetophenone)-1,2-cyclohexanediamine (L₂) for quantification of palladium ions are described. Effect of various plasticizers (o-NPOE, DBP, DEP, DOP, TBP, and CN) and anion excluder, sodium tetra phenyl borate (NaTPB) has been studied. The best performance is obtained with a membrane composition of PVC:o-NPOE:ionophore (L₁):NaTPB of 150:300:5:5 (%, w/w). The sensor exhibits significantly enhanced selectivity towards palladium ion over the concentration range 1.0 × 10⁻⁸ to 1.0 × 10⁻¹ M with a lower detection limit of 4.0 × 10⁻⁹ M and a Nernstian compliance (29.1 ± 0.3 mV decade⁻¹ of activity) within pH range 2.0-6.0 and fast response time of 10 s. Influence of the membrane composition and possible interfering ions has also been investigated on the response properties of the electrode. Fast and stable response, good reproducibility and long-term stability of the sensor are demonstrated. The sensor has been found to work satisfactorily in partially non-aqueous media up to 20% (v/v) content of methanol, ethanol and acetonitrile and could be used for a period of 4 months. Selectivity coefficients determined with fixed interference method (FIM) indicate high selectivity for palladium. The proposed electrode shows fairly good discrimination of palladium from other cations. The application of prepared sensor has been demonstrated in determination of palladium ions in spiked water sample.     

Designed boronate ligands for glucose-selective holographic sensors

In this study, 2-acrylamidophenylboronate (2-APB) was synthesised and its ability to bind with glucose was investigated both in solution and when integrated into a holographic sensor. Multiple forms of 2-APB, resulting from the neighbouring effect of the amido group with the boronic acid through an intramolecular B--O-coordinated interaction, were shown to exist in solution by using multinuclear NMR spectrometry. It was found that 2-APB predominantly adopts a zwitterionic tetrahedral form at physiological pH values. The complex formation of 2-APB with glucose and lactate was investigated in DMSO; 2-APB favours binding with glucose rather than lactate and generates a five-membered-ring complex. Furthermore, a 2-APB-based holographic sensor displayed a significant response to glucose with little interference from lactate, and with no dependence on pH in the physiological pH range. These features suggest that the new ligand 2-APB is a potential candidate for the development of glucose-selective sensors.     

Metal oxide electrodes as sensors in complexometric titrations

EDTA titrations of lead and manganese, and of some other ions by using these as indicator ions, have been followed potentiometrically with PbO(2) and MnO(2) electrodes. Explanations are put forward for the anomalies observed in the titration curves. With the MnO(2) electrode, formation of an Mn(III)-EDTA complex is responsible for the diminished potential breaks. With the PbO(2) electrode the decrease in the potential break and the distortion of the curves in acid solution may be due to reaction between electrode coating and titrant. The MnO(2) electrode has been used for titrations of Ba(2+) Ca(2+), Cd(2+) and Cu(2+) with errors < 1%.     

Metal organic frameworks as nitric oxide catalysts

The use of metal organic frameworks (MOFs) for the catalytic production of nitric oxide (NO) is reported. In this account we demonstrate the use of Cu(3)(BTC)(2) as a catalyst for the generation of NO from the biologically occurring substrate, S-nitrosocysteine (CysNO). The MOF catalyst was evaluated as an NO generator by monitoring the evolution of NO in real time via chemiluminescence. The addition of 2, 10, and 15-fold excess CysNO to MOF-Cu(II) sites and cysteine (CysH) resulted in catalytic turnover of the active sites and nearly 100% theoretical yield of the NO product. Control experiments without the MOF present did not yield appreciable NO generation. In separate studies the MOF was found to be reusable over successive iterations of CysNO additions without loss of activity. Subsequently, the MOF catalyst was confirmed to remain structurally intact by pXRD and ATR-IR following reaction with CysNO and CysH.     

Nitromethane as a substitute for nitric oxide in chemical ionization

Nitromethane chemical ionization mass spectra of some alkenes, alkynes and alcohols show that the spectra are similar in many respects to those obtained under nitric oxide chemical ionization conditions.     

Fluorescent iminodiacetamide derivatives as potential ionophores for optical zinc ion-selective sensors.

Fluorescent sensor molecules were synthesized by conjugation of iminodiacetamide derivatives with fluorescent moieties of different structures and their UV-visible and fluorescent properties were characterized in acetonitrile solvent. The fluorescent measurements revealed that the N-(2-naphthyl) and N-phenyl derivatives exhibit a distinct zinc ion-selectivity over alkali and alkaline earth metal ions, while N-(anthrylmethyl) and N-(3-methoxyphenyl) derivatives do not possess any ion-selectivities. In contrast to the fluorescent measurements, all ligands show Zn(2+) selectivity over Ca(2+) and Mg(2+) ions in plasticized PVC membranes using potentiometric signal transduction. This observation found for N-(anthrylmethyl) and N-(3-methoxyphenyl) derivatives can be ascribed to the more hindered interaction between the signalling group of the ionophore and the central metal ion in PVC membranes than in acetonitrile solution upon complexation. From the fluorescent measurements it can also be concluded that the ligands with metal ions form complexes mainly with 2:1 stoichiometry (L(2)M). On complex formation a considerable decrease in the fluorescent intensity was observed for all ligands except the N-(anthrylmethyl) derivative, where a 25 - 30 fold fluorescence enhancement was found, which is explained by the photoinduced electron transfer (PET) mechanism. All ionophores exhibited serious hydrogen ion interference, therefore complexation-induced spectral changes were measured in aprotic acetonitrile solution.     

Fluorescent nitric oxide detection by copper complexes bearing anthracenyl and dansyl fluorophore ligands

Anthrancenyl and dansyl fluorophore ligands [AnCH2pipCS(2)K (1), Ds-Hen (2), Ds-HAMP (3), Ds-HAQ (4), and Ds-HAPP (5)] were prepared for coordination to Cu(II). Five Cu complexes, [Cu(AnCH2pipCS2)2] (6), [Cu(Ds-en)2] (7), [Cu(Ds-AMP)2] (8), [Cu(Ds-AQ)2] (9), and [Cu(Ds-APP)(OTf)] (10), were synthesized as candidates for detecting nitric oxide (NO) by fluorescence and characterized by X-ray crystallography. A decrease in fluorescence compared to that of the free ligands (1-5) was measured following the formation of the corresponding five Cu(II) complexes 6-10. Fluorescence was restored in the presence of NO in CH3OH/CH2Cl2 solutions of the compounds. Complexes 7, 8, and 10 exhibited a fluorescence response to NO in pH 7.0 or 9.0 buffered aqueous solutions. Spectroscopic studies revealed that NO-induced fluorescence enhancement in these Cu(II) complexes occurs by reduction to Cu(I). The present studies demonstrate that Cu(II) complexes are effective as fluorescent probes for detecting NO in both organic and aqueous environments.     

Pyrrole functionalised metalloporphyrins as electrocatalysts for the oxidation of nitric oxide

Pyrrole-functionalised tetracarboxyphenyl porphyrin and trimethoxyphenylcarboxy-phenyl porphyrin containing Ni, Mn and Pd as the central metal ion were used to modify Pt-disk microelectrodes (∅ 50 μm) (by repetitive cyclic voltammetry, dip-dry and pulse-amperometry methods) for the detection of nitric oxide (NO). Electrodes modified with Mn(II) trimethoxyphenylcarboxyphenyl porphyrin using the pulse amperomery approach, were found to be sensitive, stable and fast in response towards the oxidation of NO. Thus, they were used for the detection of NO release from a population of transformed human umbilical vein endothelial cells (T-HUVEC) into a droplet of electrolyte solution following stimulation with vascular endothelial growth factor (VEGF). The electrode surface was covered with an additional layer of Nafion® to prevent interference from anionic molecules such as nitrite.     

Bisthioxanthylidene biscrown ethers as potential stereodivergent chiral ligands

The concept of bisthioxanthylidene biscrown ethers as potential stereodivergent chiral ligands in asymmetric synthesis is introduced. Substituted bisthioxanthylidenes may be chiral and can exist as stable enantiomers due to their folded structure. As a result, both a right-handed helix (P) and left-handed helix (M) are present in this type of molecule. This offers the unique possibility to construct two crown ether moieties, attached to the same molecule, of which one exhibits (P)-helicity and the other (M)-helicity. When the crown ether moieties differ in size they can be complexed selectively with a base containing a cation of appropriate diameter. In this manner the (P)-helix and the (M)-helix can be activated selectively to serve as a chiral environment for base catalyzed asymmetric synthesis. Thus, we envisioned the new concept of a single chiral ligand to separately synthesize two enantiomers of a chiral product just by varying the added base. For this purpose, four new bisthioxanthylidene monocrown ethers and two new bisthioxanthylidene biscrown ethers were synthesized. Two biscrowns and two monocrowns were separated into their respective enantiomers (HPLC) and optical data (UV and CD) were collected to ensure stability of enantiomers at ambient temperatures. Ion complexation of one mono- and two biscrown ethers with potassium and sodium cations was investigated.     

Water soluble extended naphthalene diimides as pH fluorescent sensors and G-quadruplex ligands

Extended naphthalene diimides (NDIs) fused to 1,4-dihydropyrazine-2,3-dione, containing two solubilizing moieties, have been synthesized. Fluorescence spectra of the new NDIs were remarkably affected by pH, as the second deprotonation of the dihydropyrazinedione moiety (pK(a) 6.9) switched off the emission. Binding to a G-quadruplex folded oligonucleotide and stoichiometry were evaluated by FRET melting assay and CD analysis. G-quadruplex binding was strongly enhanced shifting from pH 7.4 to pH 6.0 as a consequence of the dihydropyrazinedione moiety protonation. Cytotoxicity studies using two human telomerase-positive cell lines (HT29 and A549) revealed that the best G-quadruplex ligand was very active against the colon cell line, with an EC(50) of 300 nM.     

Nitrate esters as nitric oxide donors: SS-nitrates

[structure: see text]. The important biological secondary messenger NO can be generated from exogenous nitrovasodilators and NO donors. Nitrate esters are nitrovasodilators and NO mimetics, believed to be biotransformed to NO in vivo. On the basis of a mechanistic hypothesis, nitrates have been synthesized that release NO at significant rates in neutral aqueous solution in the presence only of added thiol. The novel masked beta-mercaptonitrates reported (SS-nitrates), provide information on possible sulfhydryl-dependent biotransformation mechanisms for nitrates in clinical use.