Teaching old indicators new tricks

Turning fluorescent dyes into Cu(II) nanosensors

There is great interest in the self-organization of the proper subunits as a new strategy for the realization of fluorescent chemosensors. In this article, it is shown that commercially available fluorescent dyes, functionalized with triethoxysilane moieties, can be converted into fluorescent chemosensors by simple inclusion into silica nanostructures. Dye-doped silica nanoparticles and thin films detect Cu(II) ions in the micromolar range by the quenching of fluorescence emission. The different response toward Zn(II), Ni(II), and Co(II) metal ions was also investigated and is reported. The self-organization of the silica structures leads, at the same time, to the formation of metal ion binding sites as well as to the linking of a fluorescent reporter in their proximity. Structural features of the materials, particularly particle size and network porosity, strongly affect their ability to act as fluorescent sensors.

Revisiting Noncovalent SO2−Amine Chemistry: An Indicator−Displacement Assay for Colorimetric Detection of SO2

A supramolecular approach for potential detection of SO2 is presented, which is based on the "old" donor-acceptor chemistry between SO2 and amines and includes an indicator-displacement assay. When amines were added to Zn-tetraphenylporphyrin 1 in CHCl3, the solution changed from red to dark green. A bathochromic shift of Deltalambda approximately 10 nm was observed for the Soret band, indicating the formation of 1*amine complexes. After this, SO2 gas was introduced, and the original red color of the solution was restored. The Soret band returned to its position for free porphyrin 1. The 1*amine complexes dissociated, and new SO2*amine adducts formed. Porphyrin 1 thus served as an indirect colorimetric indicator for SO2. The system discriminates between SO2 and such typical exhaust gases as COX, NOX, and H2O. From the indicator-displacement assay, the Kassoc values between 1000 and 30 000 M-1 for SO2*amine complexes were determined, which are comparable to those obtained by direct titration experiments between SO2 and the amines. Spectroscopic features of SO2*amine complexes are also presented.

“Naked-Eye” Detection of Histidine by Regulation of CuII Coordination Modes

The association of various alpha-amino acids with four new, coordinatively unsaturated metal complexes ([Cu(5)]2+, [Cu(6)]2+, [Cu(7)]2+, and [Zn(8)]2+) was examined. The receptors [Cu(5)]2+ and [Cu(7)]2+ were found to discriminate histidine (His) from other zwitterionic alpha-amino acids by means of indicator-displacement assays (IDAs) using 5(6)-carboxyfluorescein as an indicator in buffered methanol/water (3:1) solvent. The colorimetric detection of His was achieved by using this IDA method, which appears to owe its selectivity to a unique process involving disruption of the host complex to form a 2:1 His/Cu(II) complex rather than simple indicator displacement. The occurrence of distinct intermolecular coordination processes in response to the introduction of a different amino acid is observed. X-ray crystal structures of the host metal complexes were obtained and exhibit the adoption of a variety of coordination geometries about the metal center.

A colorimetric titration method for quantification of millimolar glucose in a pH 7.4 aqueous phosphate buffer

3-Pyridinylboronic acid is identified as a key sensing element for reversible sugar complexation in an aqueous solution at the physiological pH. The utility of a sensing element has been demonstrated through a simple colorimetric titration of glucose using absorption spectroscopy in the visible region. The mechanism of the high diol/triol binding affinity of pyridine boronic acid in the neutral pH is discussed based on the 1H and 11B NMR spectroscopic studies.

Recognition of Anionic Carbohydrates by an Artificial Receptor in Water

A tris-cationic artificial receptor 1 efficiently binds anionic carbohydrates even in aqueous solution as shown by NMR and UV titration experiments. Complex formation involves both ion pair formation and H-bonds to the sugar, explaining the preference for saccharides compared to simple anions and the observed selectivities among different sugars. [structure: see text]

Efficient Complexation of N-Acetyl Amino Acid Carboxylates in Water by an Artificial Receptor: Unexpected Cooperativity in the Binding of Glutamate but Not Aspartate

A new tris-cation 1 binds N-acetyl amino acid carboxylates in water even at millimolar salt concentrations with Kass approximately 103 M-1 due to a clustering of electrostatic interactions. Binding is efficient enough to allow a naked-eyed detection using an indicator displacement assay. Furthermore, receptor 1 shows an unexpected 2:1 complex formation with strong positive cooperativity with glutamate but not aspartate.

Membrane Media Create Small Nanospaces for Molecular Computation

Molecular computation is fashionable because of the small device sizes that should be possible. For instance, information processing in nanospaces is widespread in biology. However, experimental cases with computing molecules occupy rather large volumes until now (ca. 1012 nm3). Now we demonstrate small-scale computation using a designed logic gate in spheres of 3 nm radius (volume = ca. 102 nm3) which are produced by detergent micelles. The logic gate accommodated in the small nanospace fluoresces only when both H+ and Na+ ions are available.

Metal-Containing Trifurcate Receptor that Recognizes and Senses Citrate in Water

[reaction: see text] The binding tendencies toward carboxylates of a trifurcated receptor containing three copper(II)-cyclam subunits have been investigated in pure water, through the displacement of a fluorescent indicator. The receptor is tailor-made for the recognition of tricarboxylates, e.g., citrate, whose three negatively charged oxygen atoms interact with the three coordinatively unsaturated Cu(II) centers.

Colorimetric Enantiodiscrimination of α-Amino Acids in Protic Media

A new method is described for the determination of optical purity of alpha-amino acid samples in protic media. No derivatization of the analyte or multistep synthesis is required, and high accuracy is obtained from the colorimetric output. Chiral discrimination is achieved through the use of an optically pure trans-1,2-diaminocyclohexane-derived Cu(II)-containing host that differentiates amino acid enantiomers by a factor of about 2. Enantioselective signaling arises from the implementation of an indicator displacement assay based on competitive Cu(II) coordination involving the chiral Cu(II)-containing host, the amino acid guest, and a metal ion indicator. The molecular structure of the host/guest complex was determined by X-ray analysis and exhibits chelation of the Cu(II) center by the amino acid to provide substrate organization.

Selective Detection of Cysteine and Glutathione Using Gold Nanorods

A unique strategy for the selective detection of micromolar concentrations of cysteine/glutathione in the presence of various other alpha-amino acids through the plasmon coupling of Au nanorods is reported.

Anthryl-Substituted Heterocycles as Acid-Sensitive Fluorescence Probes

[reaction: see text] Four pH-sensitive fluorescence probes are presented which consist of an anthracene fluorophore and a pi-conjugated oxazoline, benzoxazole, or pyridine substituent. The protonation of the heterocycles increases their acceptor properties and results in significant red-shifts of the absorption and emission maxima of the anthracene chromophore. The comparison between 2-[2'-(6'-methoxyanthryl)]-4,4-dimethyl-2-oxazoline and 2-[2'-(anthryl)]-4,4-dimethyl-2-oxazoline reveals that the donor-acceptor substitution pattern of the fluorophore is not required to achieve a red shift upon protonation. The benzoxazole and pyridine substituents offer a particular advantage due to their persistence under acidic conditions. Thus, these compounds may be used as efficient pH-sensitive fluorescence switches. Nevertheless, the switching of benzoxazole 2c requires relatively strong acidic conditions. The anthrylpyridinium exhibits a red-shifted emission in chloroform; however, it is nonfluorescent in aqueous or alcoholic solution. Although the oxazoline is not persistent under permanent acidic conditions, this heterocycle may be useful as a substituent in fluorescence indicators since it may be used to detect acid concentrations of 10(-4)-10(-5) M, which are close to the biologically relevant range.

Design and Synthesis of a Neutral Fluorescent Macrocyclic Receptor for the Recognition of Urea in Chloroform

[structure: see text] An artificial macrocyclic fluorescent receptor 1 has been designed and synthesized for the recognition of urea. 1 shows significant fluorescence quenching on complexation with urea and thiourea in chloroform and thus may be used as a synthetic fluorescent molecular sensor for their determination in a nondegradative way.

Guidelines in Implementing Enantioselective Indicator-Displacement Assays for α-Hydroxycarboxylates and Diols

Enantioselective association events between boronic acid receptors and bifunctional substrates such as alpha-hydroxycarboxylates and vicinal diols were exploited to develop enantioselective indicator-displacement assays. It was found that the sensitivity of the assay could be tuned by using either colorimetric or fluorescent indicators. Mathematical principles were established for concurrent determination of concentration and enantiomeric excess (ee) of a chiral analyte, which has been a challenge to the high-throughput analysis of chiral samples. Furthermore, experimental protocols were optimized to maximize the chiral resolution of the assay and to improve the accuracy of the ee measurements.

Switching of Intramolecular Charge Transfer in Cruciforms: Metal Ion Sensing

Three different cruciforms containing the 1,4-bis-4'-pyridylethynyl-2,5-distyrylbenzene or the 1,4-bis-phenylethynyl-2,5-distyrylbenzene unit were synthesized and investigated with respect to their metal sensing properties. Upon addition of metal cations to these cruciforms, either a bathochromic or hypsochromic shift in emission and absorption is observed. The shift depends on whether the metal coordinates preferentially to the pyridine or to the dibutylaniline branches of the cruciforms. The three cruciforms were exposed to a selection of metal cations in dichloromethane, and their emission was examined. The cruciforms show differential sensing of metal cations, that is, different metals can be specifically discerned upon exposure to three cruciforms but not when exposed to only one. The system can discern calcium from magnesium cations and silver from mercury or lithium cations.

A Molecular Flytrap for the Selective Binding of Citrate and Other Tricarboxylates in Water

The synthesis and binding properties of a new tricationic guanidiniocarbonyl pyrrole receptor 7 are described. Receptor 7 binds citrate 9 and other tricarboxylates such as trimesic acid tricarboxylate 8 with unprecedented high association constants of K(assoc) > 10(5) M(-1) in water as determined by UV and fluorescence tritration studies. According to NOESY experiments and molecular modeling calculations, the tricarboxylates are bound within the inner cavity of receptor 7 by ion pairing between the carboxylate groups and the guanidiniocarbonyl pyrrole moieties, favored by the nonpolar microenvironment of the cavity. Hence, receptor 7 can be regarded as a molecular flytrap. In the case of the aromatic tricarboxylate 8, additional aromatic interactions further strengthen the complex. The complexes with the tricarboxylates are so strong that even the presence of a large excess of competing anions or buffer salts does not significantly affect the association constant. For example, the association constant for citrate changes only from K(assoc) = 1.6 x 10(5) M(-1) in pure water to K(assoc) = 8.6 x 10(4) M(-1) in the presence of a 170-fold excess of bis-tris buffer and a 1000-fold excess of chloride. This makes 7 one of the most efficient receptors for the binding of citrate in aqueous solvents reported thus far.

Selective Calcium Ion Sensing with a Bichromophoric Squaraine Foldamer

Several squaraine tethered bichromophoric podand systems 1a-d and a monochromophoric analogue 2 were prepared and characterized. Among these, the bichromophore, 1b, containing five oxygen atoms in the flexible podand moiety was found to specifically bind Ca(2+) in the presence of other metal ions such as K(+), Na(+), and Mg(2+). The selective binding of Ca(2+) is clear from the absorption and emission spectral changes as well as by the visual color change of 1b from light-blue to an intense purple-blue. Benesi-Hildebrand and Job plots confirmed a 1:1 binding between 1b and Ca(2+). Signaling of the binding event is achieved by the cation-induced folding of the bichromophore and the resultant exciton coupling between the squaraine chromophores. The monochromophoric squaraine dye 2 failed to give optical signals upon Ca(2+) binding, due to the absence of exciton interaction in the bound complex. Titration of the folded complex 9 with EDTA released the metal ion from the complex, thereby regaining the original absorption and emission properties of the bichromophore. The squaraine foldamer 1b reported here is the first example of a selective chromogenic Ca(2+) sensor, which works on the principle of exciton interaction in the folded Ca(2+) complex of a bichromophore, the optical properties of which are similar to those of the "H"-type aggregates of analogous squaraine dyes.

Piezoluminescence at the air-water interface through dynamic molecular recognition driven by lateral pressure application

The steroid cyclophanes with a cyclic core consisting of a 1,6,20,25-tetraaza[6.1.6.1]paracyclophane connected to four steroid moieties (cholic acid or cholanic acid) through a flexible l-lysine spacer were spread on water as Langmuir monolayers. The pi-A isotherm of the cholic-type steroid cyclophane includes a transition to the condensed phase with a limiting area of approximately 2 nm(2). This value is close to the cross-sectional area of the steroid cyclophane with a standing-up conformation of the cholic acid moieties, strongly suggesting that the cavity converts from a two-dimensional cavity to a three-dimensional cavity upon compressing the monolayer. Surface-reflective fluorescence spectroscopy of the monolayer using an aqueous fluorescent probe (6-(p-toluidino)naphthalene-2-sulfonate (TNS)) showed an abrupt increase in the TNS fluorescence intensity at a molecular area of 2 nm(2). Efficient binding of the guest probe would occur upon the completion of the three-dimensional cavity. Repeated compression and expansion induces periodic changes in the fluorescence intensity. This indicates a piezoluminescence effect through the catch and release of the TNS guest upon dynamic cavity formation. Analyses of the binding behavior of TNS to the steroid cyclophane resulted in binding constants in the range of approximately (5-9) x 10(4) M(-1) which are similar to that observed in lipid bilayer media (K = 5.1 x 10(4) M(-1)). The fluorescence intensity within the condensed phase was significantly increased with increasing pressure, suggesting that suppression of the molecular motion of the bound TNS may retard the nonemission process. Similar monolayer experiments were carried out with the monolayer of the cholanic-type steroid cyclophane that cannot form an open conformation on water. Both the phase transition in the pi-A isotherm and the change in the fluorescence intensity were negligible, confirming that the dynamic characteristic of the cavity is indispensable for the efficient pressure-induced binding of the guest and the consequent luminescence.

A Novel Colorimetric and Fluorescent Chemosensor for Anions Involving PET and ICT Pathways

A novel colorimetric and fluorescent chemosensor ADDTU-1 bearing dual receptor sites, which shows specific optical signaling for AcO-, H2PO4-, and F- over other anions and dual response toward AcO- and F- via PET and ICT mechanisms, is described. [structure: see text]

‘Twisted’ isophthalamide analogues

Steric interactions in 1,3-dicarboxamidoanthraquinones have been employed to 'twist' isophthalamide-like anion binding sites; the crystal structure of the fluoride complex of a bis-3,5-dichlorophenylamide derivative shows the receptor acting as a 'hydrogen-bonding corner' in a '2 + 2' fluoride containing molecular box.

Detection of anions using a fluorescent alizarin–phenylboronic acid ensemble

Selective anion-induced organization of phenylboronic acids and alizarin results in a new TURN-ON fluorescent sensor for anions in MeOH.

Binding of inorganic cations by p-sulfonatocalix[4]arene monitored through competitive fluorophore displacement in aqueous solution

A new working principle for detecting inorganic cation binding by water-soluble calix[4]arenes involves the displacement of a fluorescent azoalkane as guest. Fluorescence regeneration is observed for various metal ions, and binding of monovalent cations (alkali and ammonium) to p-sulfonatocalix[4]arene is detected and quantified for the first time.

Subphthalocyanines as fluoro-chromogenic probes for anions and their application to the highly selective and sensitive cyanide detection

The use of a subphthalocyanine derivative as a selective chromo-fluorogenic reporter for the anion cyanide in mixed aqueous solutions is reported.

Fluorescence probes for thiol-containing amino acids and peptides in aqueous solution

A new fluorescence probe is described using the "chemosensing ensemble" method. The probe shows high selectivity and sensitivity for thiol-containing amino acids and peptides, and can detect cysteine and homocysteine from healthy to abnormal levels under physiologically-relevant conditions.

Development ofN-Benzamidothioureas as a New Generation of Thiourea-Based Receptors for Anion Recognition and Sensing

A series of neutral N-(substituted-benzamido)-N'-phenylthioureas (substituent = p-OC(2)H(5), p-CH(3), m-CH(3), H, p-Cl, p-Br, m-Cl, and p-NO(2)) were designed as anion receptors, in which the thiourea binding site was attached to the benzamido moiety via an N-N bond. The absorption spectra of these N-benzamidothioureas in acetonitrile peaked at ca. 270 nm were found to show unprecedented red shifts by 7 373 to 14 325 cm(-1) in the presence of anions such as AcO(-), F(-), and H(2)PO(4)(-). Under the same conditions, the classic neutral thiourea receptors, N-(substituted-phenyl)-N'-phenylthioureas, showed absorption spectral shifts in most cases of less than 800 cm(-1) with one exception of 6501 cm(-1). Control experiments, effects of protic solvent, and (1)H NMR titration confirmed the formation of hydrogen-bonding complexes between the new N-benzamidothiourea receptors and anions. The binding constants with AcO(-), for example, are at 10(5)-10(7) mol(-1) L order of magnitude, which are 13 to 590 times those of the corresponding classic N-phenylthioureas in the same solvent. It was found that, whereas the absorption of the N-benzamidothiourea receptors showed essentially no dependence on the substituent, the substantially red-shifted new absorption band of the N-benzamidothiourea-anion binding complex was sensitively subject to the substituent. A linear relationship was found between the absorption energies of the N-benzamidothiourea-acetate binding complexes and the Hammett constants of the substituents with a negative slope of -0.34 eV. This led to the assignment that the substantially red-shifted absorption band was the ground-state intramolecular charge-transfer absorption with the substituent locating in the electron acceptor moiety. It was concluded that anion binding to the thiourea moiety of the N-benzamidothiourea receptors switched on their ground-state charge transfer. An anion-binding induced structural change was suggested to occur around the N-N bond in N-benzamidothioureas, which resulted in a substantially increased electron donating ability of the electron donor in the receptor molecules. As a consequence, the ground-state charge transfer takes place in the N-benzamidothiourea-anion binding complexes, leading to unprecedented red shifts in the absorption spectra and substantially enhanced anion binding affinities than those of the corresponding N-phenylthiourea receptors. N-Benzamido-N'-phenylthioureas represent a new generation of neutral thiourea-based anion receptors that show substantially improved anion binding performance important for anion sensing and recognition.

Using Indicator-Displacement Assays in Test Strips and To Follow Reaction Kinetics

[structure: see text] An indicator-displacement assay was used to study the kinetics of reactions that produce tartrate. The same assay was also found to be useful in developing tartrate test strips, by coating filter paper with the same receptor and indicator.