Sensing the glass transition in thin and ultrathin polymer films via fluorescence probes and labels

Fluorescence was used to characterize the glass transition in thin and ultrathin supported polymer films with common chromophores. The temperature dependence of the fluorescence intensity exhibits a transition or break upon cooling from the rubbery state to the glassy state, and this is identified as the glass transition. A variety of chromophores are investigated including pyrene, anthracene, and phenanthrene either as dopants, covalently attached to the polymer as a label, or both. The particular choice of the chromophore as well as the nature of the attachment, in the case of labels, have significant impact on the success of this method. Problematic cases include those in which the excited‐state chromophore undergoes significant photochemistry in addition to fluorescence or those in which the particular attachment of the chromophore as a label may allow for conformational interactions that affect the fluorescence quantum yield in a nontrivial way. Polymers that have an intrinsic fluorescence unit, for example, polystyrene, may allow for the fluorescence sensing of the glass transition without added dopants or labels. Finally, it is demonstrated that this technique holds promise for the study of the glass transition in polymer blends and within specific locations in multilayer films. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2745–2758, 2002     

Electrochemical biosensors for monitoring the recognition of glycoprotein-lectin interactions

Despite the wide applicability and specificity of lectins to carbohydrate moieties, there are few lectin specific biosensors. This is attributed to the difficulty in defining the relevant experimental parameters to measure for sensing. We hereby describe the development of direct and indirect electrochemical sensors to determine the exact trace amounts of probarley lectin (ProBL) and its conversion product wheat germ agglutinin (WGA). In addition to WGA, the antigens (ProBL) employed in this study were over expressed in bacteria, isolated from protein bodies, and purified using immobilized N-acetylglusamine in order to obtain correctly folded active lectins. The amperometric immunosensor uses cell lines producing monoclonal antibody (mAB) to the pro-region of ProBL over expressed from Escherichia coli. The efficacy and sensing characteristics of the lectin were optimized using monoclonal antibody to WGA and the resulting sensor was found to detect only ProBL in the linear range 10⁻³-10² μg mL⁻¹ and a detection limit of 10⁻³ μg mL⁻¹.     

A concave fluorescent sensor for anions based on 6-methoxy-1-methylquinolinium

The ligand 2, in which three fluorogenic 6-methoxy-1-methylquinolinium fragments are appended to a mesityl platform, in MeCN forms 1:1 adducts with halides and other inorganic anions. (1)H NMR studies and molecular modelling indicate that 2 provides a cavity for anion inclusion and establishes electrostatic interactions with the guest. Anion inclusion induces quenching of the fluorogenic fragments with an efficiency decreasing along the series Br(-)>I(-)>NCS(-)>Cl(-)>NO(3) (-)>HSO(4) (-). The fluorimetric response of 2 to anions is orders of magnitude more sensitive than that of just 6-methoxy-1-methylquinolinium, ligand 1.     

Supramolecular assembly of 2,7-dimethyldiazapyrenium and cucurbit[8]uril: a new fluorescent host for detection of catechol and dopamine

The formation of a highly stable inclusion complex between 2,7-dimethyldiazapyrenium (Me(2)DAP(2+)) and the cucurbit[8]uril host (CB8) was demonstrated by X-ray crystallography; MALDI-TOF mass spectrometry; and (1)H NMR, electronic absorption, and emission spectroscopy. The equilibrium association constant was determined to be 8.9(+/-0.2)x10(5) L mol(-1) from UV-visible data and 8.4(+/-1.5) x 10(5) L mol(-1) from fluorescence data. The Me(2)DAP(2+).CB8 inclusion complex acted as a host to bind compounds containing aromatic pi-donor moieties (D), such as catechol and dopamine. This point was demonstrated by (1)H NMR spectroscopy, and electrochemical and emission measurements. Fluorescence detection of the Me(2)DAP(2+).D.CB8 ternary complexes was evident in aqueous solution and on the surface of silica particles, to which fluorescent diazapyrenium units had been covalently immobilized.     

Novel Biomedical Sensors for Flow Injection Potentiometric Determination of Creatinine in Human Serum

Coated‐wire (CW) and tubular (Tu) type membrane sensors for creatinine are developed. These consist of creatinine tungstophosphate(CTP), creatinine molybdophosphate (CMP) and creatinine picrolonate (CPC) ion‐pair complexes as electroactive materials dispersed in plasticized poly(vinyl chloride) matrix membranes. Electrochemical evaluation of these sensors under static (batch) mode of operation reveals near‐Nernstian response with slopes of 62.9, 58.1, and 55.2 mV decade^?1 over the concentration range 1×10^?2–5.0×10^?6, 1×10^?2–7.5×10^?5, and 1×10^?2?3.1×10^?5 mol L^?1. The lower detection limits are 0.39, 3.49, and 2.20 μg mL^?1 creatinine with CTP, CMP and CPC membrane based sensors plasticized with o‐NPOE, respectively. Tubular and coated wire CTP membrane sensors are incorporated in flow‐through cells and used as detectors for flow injection analysis (FIA) of creatinine. The intrinsic characteristics of the detectors under hydrodynamic mode of operation in a low dispersion manifold are determined and compared with data obtained under static mode of operation. With 10^?2 mol L^?1 phosphate buffer of pH 4.5 as a carrier solution, the tubular and coated wire CTP detectors exhibit rapid response of 58.9 and 50.7 mV decade^?1 over the concentration range 1×10^?2–1×10^?5 mol L^?1 and detection limits of 0.39 μg mL^?1 and 0.85 μg mL^?1, respectively. Validation of the assay methods with the proposed sensors by measuring the lower detection limit, range, accuracy, precision, repeatability and between‐day‐variability reveals good performance characteristics confirming applicability for continuous determination of creatinine. The sensors are used for determining creatinine in human blood serum at an input rate of 40 samples per hour. No interferences are caused by creatine, most common anions, cations and organic species normally present in biological fluids. The results favorably compare with data obtained using the standard spectrophotometric method.     

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.     

Nuclear magnetic resonance and optosensing properties of di-2-thienyl ketone p-nitrophenylhydrazone (DSKNPH) in non-aqueous media

The compound di-2-thienyl ketone p-nitrophenylhydrazone (DSKNPH) melting point 168-170 °C was isolated in good yield from the reaction between di-2-thienyl ketone (DSK) and p-nitrophenylhydrazine in refluxing ethanol containing a few drop of concentrated HCl. Nuclear magnetic resonance studies on DSKNPH in non-aqueous solvents revealed strong solvent and temperature dependence due to solvent-solute interactions. Optical measurements on DSKNPH in DMSO in the presence and absence of KPF₆ gave extinction coefficients of 83,300±2000 and 25,600±2000 M⁻¹ cm⁻¹ at 612 and 427 nm at 295 K. In CH₂Cl₂, extinction coefficient of 34,000±2000 M⁻¹ cm⁻¹ was calculated at 422 nm. When DMSO solutions of DSKNPH were allowed to interact with DMSO solutions of NaBH₄ the low energy electronic state becomes favorable and when DMSO solutions of DSPKNPH where allowed to interact with DMSO solutions of KPF₆ or NaBF₄, the high energy electronic state becomes favorable. The reversible BH₄⁻/BF₄⁻ interconversion points to physical interactions between these species and DSKNPH and hints to the possible use of DSKNPH as a spectrophotometric sensor for a variety of physical and chemical stimuli. Thermo-optical measurements on DSKNPH in DMSO confirmed the reversible interconversion between the high and low energy electronic states of DSKNPH and allowed for the calculations of the thermodynamic activation parameters of DSKNPH. Changes in enthalpy (ΔH^∅) of +57.67±4.20; 27.15±0.90 kJ mol⁻¹, entropy (ΔS^∅) of +160±12.88; 83±2.91 J mol⁻¹ and free energy (ΔG^∅) of −8.52±0.40; 2.66±0.25 kJ mol⁻¹ were calculated at 295 K in the absence and presence of NaBH₄, respectively. Manipulation of the equilibrium distribution of the high and low energy electronic states of DSKNPH allowed for the use of these systems (DSKNPH and surrounding solvent molecules) as molecular sensors for group I and II metal ions. Group I and II metal ions in concentrations as low as 1.00×10⁻⁵ M can be detected and determined using DSKNPH in DMSO.     

Near-infrared cell-permeable Hg2+-selective ratiometric fluorescent chemodosimeters and fast indicator paper for MeHg+ based on tricarbocyanines

Three tricarbocyanine dyes (IR‐897, IR‐877, and IR‐925) with different thiourea substituents that function as dosimeter units through specific Hg²⁺‐induced desulfurization have been demonstrated in a fast indicator paper for Hg²⁺ and MeHg⁺ ions. In comparison with available Hg²⁺‐selective chemodosimeters, IR‐897 and IR‐877 show several advantages, such as convenient synthesis, very long wavelengths falling in the near‐infrared (NIR) region (650–900 nm) with high molar extinction coefficients, a ratiometric response, and quite low disturbance with Ag⁺ and Cu²⁺ ions. They exhibit large redshifts, which result in a clear color change from deep blue to pea green that can be easily monitored by the naked eye for a convenient indicator paper. In emission spectra, they display a characteristic turn‐off mode at 780 nm and turn‐on mode at 830 nm with titration of Hg²⁺ ions. Remarkably, the signal/noise (S/N) ratio with other thiophilic metal ions (Ag⁺ and Cu²⁺) is greatly enhanced with ratiometric measurement of two channels: excitation spectra mode (I_{810 nm}/I_{670 nm}, monitored at 830 nm) and emission spectra mode (I_{830 nm}/I_{780 nm}, isosbestic absorption point at 730 nm as excitation). The distinct response is dependent upon the electron‐donating effect of the thiourea substituents; that is, the stronger the electron‐donating capability of the thiourea substituents, the faster the Hg²⁺‐promoted cyclization. Additionally, experiments with living SW1116 cells show that these three tricarbocyanine dyes with low toxicity can exhibit special characteristics that are favorable for visualizing intracellular Hg²⁺ and MeHg⁺ ions in biological systems, including excellent membrane permeability, minimal interfering absorption and fluorescence from biological samples, low scattering, and deep penetration into tissues.     

Thin films composed of multiwalled carbon nanotubes, gold nanoparticles and myoglobin for humidity detection at room temperature.

Optically transparent and electrically conductive nanocomposite thin films consisting of multiwalled carbon nanotubes (MWCNTs), gold nanoparticles (GNPs) and myoglobin molecules that glue GNPs and MWCNTs together are fabricated for the first time on glass substrates from aqueous solution. The nanocomposite thin film is capable of varying its resistance, impedance or optical transmittance at room temperature in response to changes in ambient humidity. The conductometric sensitivity to relative humidity (RH) of the nanocomposite thin film is compared with those of the pure and Mb-functionalized MWCNT layers. The pure MWCNT layer shows a small increase in its resistance with increasing RH due to the effect of p-type semiconducting nanotubes present in the film. In contrast, a four times higher sensitivity to RH is observed for both the nanocomposite and Mb-functionalized MWCNT thin films. The sensitivity enhancement is attributable to swelling of the thin films induced by water absorption in the presence of Mb molecules, which increases the inter-nanotube spacing and thereby causes a further increase of the film resistance. A humidity change as low as DeltaRH=0.3 % has been readily detected by conductometry using the nanocomposite thin film.     

Peptide Modified Electrodes as Electrochemical Metal Ion Sensors

Sensors for the detection of metal ions are of considerable importance for enabling the monitoring of environmental samples for metal ion contamination directly in the field. This review outlines the use of peptides and amino acids as the recognition element of electrochemical sensors for metal ion detection. Initially the complexation of metals by peptides is discussed followed by the immobilization of peptides on electrode surfaces. Subsequently, the application of peptide modified electrodes for detecting metals is reviewed and finally challenges and future prospects are outlined.