Knut Rurack

Flipping the light switch 'on'--the design of sensor molecules that show cation-induced fluorescence enhancement with heavy and transition metal ions.

Real-time and real-space analysis of heavy and transition metal ions employing fluorescent sensor molecules has received much attention over the past few years. Since many of these cations possess intrinsic properties that usually quench the fluorescence of organic dye molecules, a lot of research has lately been devoted to designing fluorescent probes that show complexation-induced fluorescence enhancement. Such an analytical reaction would be highly desirable in terms of increased sensitivity and selectivity. However, in this particular field of sensor research, the photophysical and photochemical mechanisms involved as well as the chemical constitutions of the sensor molecules employed are rather diverse and up to now, very few attempts have been made to establish some general concepts for rational probe design. By analyzing various systems published by other researchers as well as own work, this contribution aims at an elucidation of some of the underlying principles of heavy and transition metal ion-enhanced emission.

Molecular Switching in the Near Infrared (NIR) with a Functionalized Boron–Dipyrromethene Dye

The highly fluorescent, unsymmetrically substituted boron-dipyrromethene dye 1 shows emission features that are strongly dependent on the solvent polarity. Thus, 1 can be used for highly sensitive fluorometric probing of solvent polarity and acidity and furthermore can be switched chemically or electrochemically in the near infrared.

Fluorescence quantum yields of a series of red and near-infrared dyes emitting at 600-1000 nm.

The determination of the fluorescence quantum yields (QY, Φ(f)) of a series of fluorescent dyes that span the absorption/excitation and emission ranges of 520-900 and 600-1000 nm is reported. The dyes encompass commercially available rhodamine 101 (Rh-101, Φ(f) = 0.913), cresyl violet (0.578), oxazine 170 (0.579), oxazine 1 (0.141), cryptocyanine (0.012), HITCI (0.283), IR-125 (0.132), IR-140 (0.167), and four noncommercial cyanine dyes with specific spectroscopic features, all of them in dilute ethanol solution. The QYs have been measured relative to the National Institute of Standards and Technologys standard reference material (SRM) 936a (quinine sulfate, QS) on a traceably characterized fluorometer, employing a chain of transfer standard dyes that include coumarin 102 (Φ(f) = 0.764), coumarin 153 (0.544), and DCM (0.435) as links between QS and Rh-101. The QY of Rh-101 has also been verified in direct measurements against QS using two approaches that rely only on instrument correction. In addition, the effects of temperature and the presence of oxygen on the fluorescence quantum yield of Rh-101 have been assessed.

A highly efficient sensor molecule emitting in the near infrared (NIR): 3,5-distyryl-8-(p-dimethylaminophenyl)difluoroboradiaza-s-indacene

The spectroscopic properties and the photophysical behaviour of difluoroboradiaza-s-indacene 1, especially designed for the near infrared (NIR) spectral region and equipped with a p-dimethylaminophenyl group at the meso-position, were studied by steady-state and time-resolved optical spectroscopy. Solvent-dependent measurements revealed that for 1, excited state deactivation is governed by population of a non-emissive charge transfer excited state (1CT) as the solvent polarity increases, whereas reference compound 2 shows strong fluorescence from a locally excited state (1LE) in all the solvents employed. Accordingly, protonation of 1 completely suppresses the quenching excited state charge transfer process and leads to strong enhancement of fluorescence in the NIR, distinguishing 1 as a very sensitive fluorescent sensor molecule for pH or solvent acidity in this favourable wavelength region.

A Mesoporous 3D Hybrid Material with Dual Functionality for Hg2+ Detection and Adsorption

Dual-function hybrid material U1 was designed for simultaneous chromofluorogenic detection and removal of Hg(2+) in an aqueous environment. Mesoporous material UVM-7 (MCM41 type) with homogeneously distributed pores of about 2-3 nm in size, a large specific surface area exceeding 1000 m(2) g(-1), and nanoscale particles was used as an inorganic support. The mesoporous solid is decorated with thiol groups that were treated with squaraine dye III to give a 2,4-bis(4-dialkylaminophenyl)-3-hydroxy-4-alkylsulfanylcyclobut-2-enone (APC) derivative that is covalently anchored to the inorganic silica matrix. The solid was characterised by various techniques including X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and nitrogen adsorption. This hybrid solid is the chemodosimeter for Hg(2+) detection. Hg(2+) reacts with the APC fragment in U1 with release of the squaraine dye into the solution, which turns deep blue and fluoresces strongly. Naked-eye Hg(2+) detection is thus accomplished in an easy-to-use procedure. In contrast, U1 remains silent in the presence of other thiophilic transition metal ions, alkali and alkaline earth metal ions, or anions ubiquitously present in water such as chloride, carbonate, sulfate, and phosphate. Material U1 acts not only as chemodosimeter that signals the presence of Hg(2+) down to parts-per-billion concentrations, but at the same time is also an excellent adsorbent for the removal of mercury cations from aqueous solutions. The amount of adsorbed mercury ranges from 0.7 to 1.7 mmol g(-1), depending on the degree of functionalisation. In addition, hybrid material U1 can be regenerated for both sensing and removal purposes. As far as we know, U1 is the first example of a promising new class of polyfunctional hybrid supports that can be used as both remediation and alarm systems by selective signalling and removal of target species of environmental importance. Model compounds based on silica gel (G1), fumed silica (F1), and micrometre-sized MCM-41 scaffolds (M1) were also prepared and studied for comparative purposes.

The determination of methylmercury in real samples using organically capped mesoporous inorganic materials capable of signal amplification.

Mercury exists in the environment in a variety of compounds, and the toxicity depends on the chemical species. Organomercury derivatives, especially methylmercury (CH3Hg ), are more toxic than inorganic or elemental mercury. Methylmercury is rarely emitted anthropogenically, but usually formed naturally through biomethylation of mercury, often of anthropogenic origin. Methylmercury subsequently bio-accumulates through the food chain, for example in the tissue of fish, in which methylmercury concentrations are frequently found that exceed the maximum levels recommended by the Environmental Protection Agency (EPA) and the World Health Organization (WHO) for human consumption (0.1 and 0.23 mg (kg body weight) 1 d ). Methylmercury exposure in adults has been linked to cardiovascular diseases, autoimmune effects, hearing impairment, blindness, and death. In a number of cases, mercury intoxication is related to the consumption of fish. Several analytical methods have been described for the determination of methylmercury in biological samples. For example, gas chromatography (GC) with electron capture detection (ECD) or inductively coupled plasma mass spectrometry (ICP-MS) and high performance liquid chromatography (HPLC) with elemental or ICP-MS detection have been extensively used. As an alternative to these technically sophisticated methods, which require a laboratory setting, the development of more simple procedures for in situ and rapid screening applications that are based on optical, electrochemical, or gravimetric procedures have recently received considerable attention; these methods involve in part biological species as active sensing elements. Regarding the development of chromoand fluorogenic indication systems for mercury derivatives, a large number of examples have been reported for the inorganic form (Hg), but few studies have targeted CH3Hg . Furthermore, most of these studies were unable to discriminate between Hg and CH3Hg + and did not involve the determination of the analyte(s) in relevant samples or matrices such as fish. Chemically, the great majority of the reported approaches rely on indicator molecules that either bind 17a] or react with Hg to yield the desired change in color or fluorescence. Only very recently, alternative procedures involving organic, inorganic, or hybrid materials have been proposed, which are promising in their performance. Our interest in the latter type of materials motivated us to explore bioinspired strategies toward new signaling models. For mercury indication, we combined our experience in Hg sensing and supramolecular hybrid materials design and developed an organically capped mesoporous inorganic material for selective CH3Hg + determination through signal amplification. Inspired by gated ion channels and pumps, the proposed sensing mechanism relies on the opening of a pore that is controlled by the interaction of a certain molecular stimulus (the target species, CH3Hg ) at the receptors that close the gate. Although this reaction itself can already induce an optical response, a second process is implemented in the system that leads to strong signal amplification: the pores of the hybrid are loaded with a large amount of dye molecules, which are only liberated upon analyte-induced opening of the pores. To date, apart from a few examples of analyte-induced pore blockage, pore-opening methods for sensing applications have not been reported. The sensing procedure is shown in Scheme 1. The inorganic support is a calcined MCM-41 mesoporous solid that features homogeneous porosity, facile surface functionalization, inertness, and a high loading capacity. The solid is first loaded with a dye (safranine O) and is then capped with 2,4-bis(4-dialkylaminophenyl)-3-hydroxy-4-alkylsulfanylcyclobut-2-enone (APC) groups. The APC moieties are [*] E. Climent, Dr. M. D. Marcos, Prof. R. Mart nez-M ez, Dr. F. Sancen n, Dr. J. Soto Instituto de Reconocimiento Molecular y Desarrollo Tecnol gico Centro Mixto Universidad Polit cnica de Valencia—Universidad de Valencia, Departamento de Qu mica Universidad Polit cnica de Valencia Camino de Vera s/n, 46022 Valencia (Spain) Fax: (+ 34)96-387-9349 E-mail: rmaez@qim.upv.es and CIBER de Bioingenier a, Biomateriales y Nanomedicina (CIBER-BBN)

Traceability in Fluorometry: Part II. Spectral Fluorescence Standards

The need for the traceable characterization of fluorescence instruments is emphasized from a chemist’s point of view, focusing on spectral fluorescence standards for the determination of the wavelength- and polarization-dependent relative spectral responsivity and relative spectral irradiance of fluorescence measuring systems, respectively. In a first step, major sources of error of fluorescence measurements and instrument calibration are revealed to underline the importance of this issue and to illustrate advantages and disadvantages of physical and chemical transfer standards for generation of spectral correction curves. Secondly, examples for sets of traceable chemical emission and excitation standards are shown that cover a broad spectral region and simple procedures for the determination of corrected emission spectra with acceptable uncertainties are presented. With proper consideration of the respective measurement principle and geometry, these dye-based characterization procedures can be not only applied to spectrofluorometers but also to other types of fluorescence measuring systems and even to Raman spectrometers.

Test-Strip-Based Fluorometric Detection of Fluoride in Aqueous Media with a BODIPY-Linked Hydrogen-Bonding Receptor†

The measurement of biologically relevant anions, such as fluoride, is an important task in analytical chemistry, in particular, for dental health and osteoporosis. Although a large number of fluoride probes are known, the applicability under relevant conditions is limited to a few examples. To improve this situation, BODIPY-amidothiourea dyes with varying hydrogen-bond donating strengths were developed, the most H-acidic of which (1 c) could detect F(-) from an inorganic source (NaF) in 50 % aqueous solution (DMSO/water 1:1, v/v) with 0.01 ppm sensitivity through selective fluorescence quenching by a photoinduced electron-transfer (PET) process. Use of the probe and a reference dye with a test-strip assay and a portable and rapidly recording lateral-flow fluorescence reader made determination of F(-) in neat aqueous solutions, such as spiked water samples and toothpaste extracts, possible in a self-referenced manner, achieving a detection limit of 0.2 ppm.

Sialic Acid-Imprinted Fluorescent Core–Shell Particles for Selective Labeling of Cell Surface Glycans

The expression of cell surface glycans terminating with sialic acid (SA) residues has been found to correlate with various disease states there among cancer. We here report a novel strategy for specific fluorescence labeling of such motifs. This is based on sialic acid-imprinted core-shell nanoparticles equipped with nitrobenzoxadiazole (NBD) fluorescent reporter groups allowing environmentally sensitive fluorescence detection at convenient excitation and emission wavelengths. Imprinting was achieved exploiting a hybrid approach combining reversible boronate ester formation between p-vinylphenylboronic acid and SA, the introduction of cationic amine functionalities, and the use of an NBD-appended urea-monomer as a binary hydrogen-bond donor targeting the SA carboxylic acid and OH functionalities. The monomers were grafted from 200 nm RAFT-modified silica core particles using ethylene glycol dimethacrylate (EGDMA) as cross-linker resulting in a shell thickness of ca. 10 nm. The particles displayed strong affinity for SA in methanol/water mixtures (K = 6.6 × 10(5) M(-1) in 2% water, 5.9 × 10(3) M(-1) in 98% water, B(max) ≈ 10 μmol g(-1)), whereas binding of the competitor glucuronic acid (GA) and other monosaccharides was considerably weaker (K (GA) = 1.8 × 10(3) M(-1) in 98% water). In cell imaging experiments, the particles selectively stained different cell lines in correlation with the SA expression level. This was further verified by enzymatic cleavage of SA and by staining using a FITC labeled SA selective lectin.

Red/Near‐infrared Boron–Dipyrromethene Dyes as Strongly Emitting Fluorophores

We present an overview of the state of the art in long‐wavelength boron–dipyrromethene (BODIPY) fluorophores, focusing on strategies to shift the absorption and emission bands into the red/near‐infrared (NIR) range of the spectrum. This report also discusses chemical modifications of the chromophoric core to obtain analyte‐responsive fluorophores, including examples of pH and metal ion indicators. Finally, we present a new series of phenanthrene‐fused BODIPY dyes, emitting with high efficiency in the red/NIR region of the spectrum, as well as discussing potential applications thereof as probes.