Straightforward nano patterning on optical fiber for sensors development.

A simple method to prepare a nano pattern along the surface of an optical fiber is applied in this Letter to develop a pH sensor. The template is made of a block copolymer that defines specific locations where gold nano particles are adsorbed on forming clusters. The average diameter of the resulting agglomerates is 121 nm, and the mean distance between the centers is 182 nm. The morphology of the gold cluster array produces localized surface plasmon resonance. The absorbance spectrum is affected by pH variations, and the ratio between the absorption at two different wavelengths is used to characterize the response, which is repetitive and reversible. This Letter highlights the potentiality of this type of chemical nano patterning for the development of optical fiber sensors.

Changes in fluorescent emission due to non-covalent interactions as a general detection procedure for thin-layer chromatography.

Changes in fluorescence emission due to non-covalent analyte-fluorophore interactions in silica gel plates are studied and used as a general detection procedure for thin-layer chromatography (TLC). The presence of the analyte modifies the microenvironment of the fluorophore and thus changes the balance between radiative (k(r)) and non-radiative (k(nr)) emission constants. A model is proposed for analyte-fluorophore induced electrostatic interactions, which depend on analyte polarizability and are responsible for fluorescence enhancements. As consequence of these induced interactions, the analyte creates an apolar environment that prevents non-fluorescent decay mechanisms, decreasing k(nr). On the other hand, the effect of an increase in refractive index on k(r) is investigated, as it contributes to some extent to fluorescence enhancements in silica gel medium. Changes in fluorescence emission should be regarded as a general property of fluorophores in the presence of analytes, and criteria that fluorophores should meet to be used as sensitive TLC probes are discussed here.

Fluorescence detection by intensity change based sensors: a theoretical model.

According to Fluorescence Detection by Intensity Changes (FDIC) the fluorescence intensity of many fluorophores depends on the non-covalent (specific and/or non-specific) interactions these fluorophores would be able to establish with the solvent and, more interestingly, with other surrounding molecules. This latter effect is the basis of FDIC for analytical purposes. In this paper, a preliminary study of FDIC applications using a fluorophore supported in a solid medium (sensor film) is presented. First, a mathematical model relating the analyte concentration with the immobilized fluorophore fluorescence is deduced. The model includes all the different mechanisms explaining this relationship: index of refraction or dielectric constant modification, scattering coefficient alteration and sensor film volume increase. Then, the very first experimental results are presented, using different fluorophores and solid supports. The best results were obtained using polyacrylamide (PAA) polymers and coralyne as the fluorophore. This sensor film is applied for albumin and polyethylenglycol determination and the results are compared with those obtained using coralyne in solution. Albumin quenches the coralyne fluorescence in both cases (solution and film), while PEG quenches coralyne fluorescence in films but increases it in solution. These results suggest that the outstanding fluorescence change mechanism is sensor films is the film volume increases, which is different than those observed in solution.

Tandem [8 + 2] cycloaddition--[2 + 6 + 2] dehydrogenation reactions involving imidazo[1,2-a]pyridines and imidazo[1,2-a]pyrimidines.

The reaction between benzynes and imidazo[1,2-a]pyridines (pyrimidines) to form benzo[a]imidazo[5,1,2-cd]indolizines and 2,3,9c-triazocyclopenta[j,k]fluorenes has been studied computationally and experimentally. It is found that these reactions take place via tandem [(pi)8(s) + (pi)2(s)] and [(sigma)2(s) + (pi)6(s) + (sigma)2(s)] processes. The [8 + 2] cycloaddition steps are essentially barrierless, and the aromatization steps occur via highly synchronous aromatic transition structures. From an experimental standpoint, the reaction is feasible under microwave irradiation and using 2-(trimethylsilyl)phenyl triflates as benzyne precursors. Depending on the substitution pattern in the starting triflate a complete regiocontrol of the reaction can be achieved. The tetracyclic compounds thus prepared emitted blue light when excited at 365 nm and exhibited interesting photophysical properties.

A theoretical approach for designing fluorescent reagentless biosensors: The optical model.

In this paper a mathematical model describing the analytical signal obtained in fluorescence sensors is presented and compared with other commonly used models. The model starts from the Kubelka-Munk theory for solid surfaces but incorporates new theoretical improvements, being principally: (a) the increase in the effective optical pathlength due to the Scattering Induced Path Variation (SIPV), the incorporation of this parameter allows us to deduce that the fluorescence intensity from solid surfaces does not linearly change with the fluorophore concentrations; (b) the influence of the inner filter effect and how the error can be rectified and (c) the calculation of the scattering coefficients in sensor films for this kind of sensor. From this model it is possible to predict the effect of the fluorophore concentration, the sensor film scattering coefficient and the sample inner filter effect on the fluorescence signal. The conclusions obtained can be extended to other types of fluorescence measurements from solid surfaces.