Placebo white tablet cores (lactose anhydrous [47.6%], corn starch [23.8%], microcrystalline cellulose [19.1%], polyvinylpyrrolidone [7.9%], magnesium stearate [0.8%], and talcum powder [0.8%]) were coated with a colorant (hydroxypropyl methylcellulose [8% w/v], titanium dioxide [0.2% w/v], FD&C yellow No. 6 with aluminum lacquer [0.8% w/v], polyethylene glycol 4000 [0.4% w/v], and purified water [q.s.p. 100 mL]) using a random spraying method during 130 minutes. During the coating process, batches of 21 samples were extracted every 10 minutes and measured with a DigiEye imaging system. The initial cores showed very similar and uniform colors (Mean Color Difference from the Mean [MCDM] of 0.8 CIELAB units), but partially coated tablets showed lower uniformity (MCDM below 2.0 CIELAB units). There was a high color variability (MCDM about 4.0 CIELAB units) among tablets of the same batch in the period between 10 and 30 minutes, which decreased as the coating process progressed, until achieving a final acceptable value (MCDM below 2.0 CIELAB units). During the coating process, L* decreased, C*ab strongly increased, and h ab remained nearly constant (disregarding results at 0 and 10 minutes). CIELAB and CIEDE2000 color differences (mainly chroma differences) with respect to the initial color of the tablets were modeled as a function of time by exponential functions with three coefficients. The color change in the interval from 90 to 130 minutes (4.3 CIELAB units, or 2.6 CIEDE2000 units), may be considered negligible bearing in mind the color variability in the batches of 21 samples and typical values of visual color thresholds.
Se investiga la variación con el pH de las propiedades ópticas (absorción y luminiscencia) de moléculas de fluoresceína incorporadas a recubrimientos porosos sol-gel en un intervalo de concentraciones comprendido entre 0,5 y 3, % en peso. Los espectros de absorción de los recubrimientos presentan bandas principales en 435 nm y 47 nm para pH ácido y básico, respectivamente. Al comparar con los espectros de las disoluciones, la posición de ambos máximos se desplaza hacia el rojo y aparecen en 43 nm y 492 nm. La dependencia de la absorción de la fluoresceína en recubrimientos y disoluciones se ajusta a la ley de Lambert-Beer. Al excitar los recubrimientos y las disoluciones con fotones de la región en que la absorción es importante, los espectros de fotoluminiscencia muestran una banda ancha principal en 520 nm, tanto para pH ácido como para básico. En este caso y contrariamente a lo observado en los espectros en disolución, la intensidad a 520 nm decrece con el pH al aumentar éste desde 2 a 12. Palabras clave: fluoresceína, pH, recubrimientos sol-gel, absorción, fotoluminiscencia Study of optical properties of fluorescein-doped sol-gel coatings as a function of concentration and pHOptical properties (absorption and luminescence) of fluorescein molecules encapsulated in porous sol-gel coatings for concentrations ranging from 0.5 to 3. wt % are investigated as a function of pH. Absorption spectra of the coatings show main bands at 435 nm and 47 nm for acid and basic pH, respectively. Both peaks appear in the solutions of fluorescein redshifted at 43 nm and 492 nm. However, the dependence in intensity of both peaks as a function of pH is found essentially similar between coatings and solutions. In both cases, the absorption dependence on concentration can be fitted to the Lambert-Beer law. On the other way, photoluminescent spectra of coatings and solutions upon excitation in the peak absorption region, show a dominant broad band peaked at 520 nm for both acid and basic pH. The intensity of the 520 nm-coating band displays a decreasing behaviour with increasing pH from 2 to 12, in contrast with those found for the same peak when the corresponding solutions were analysed.
Thin coatings optically sensitive against pH are obtained by entrapping erythrosin B molecules in a sol-gel silica matrix. Comparisons between optical spectra of erythrosin B in solutions and those once entrapped in silica coatings are performed for pH ranging from 0.5 to 12.0. Significant differences between both kinds of spectra (solutions and coatings) of erythrosin B are observed. Main changes are displayed in the absorption spectra for neutral to basic pH. Furthermore, the erythrosin B doped coatings show memory effect or hysteresis in their spectral response against pH. This means that absorption spectra depend on the direction of the pH change. Likewise, the photoluminescent spectra of erythrosin B in buffered solutions exhibit pH dependence and a maximized response in the basic pH range. On contrast, no significant photoluminescence from the erythrosin B entrapped in the silica coatings is observed. Results are mainly discussed on the basis of different optical response of polyprotic forms of erythrosin B and possible interactions between the molecule and oxygen as well as the porous walls of the sol-gel silica matrix. In addition, the presence of diffusional barriers, which hamper the flowing of protons and other associated ions, is taken into account. Good agreement with previous observations about low absorption of polyprotic cationic and neutral forms of erythrosin B is found. Keywords: A. Optical materials, B. Glassy coatings, C. Sol-gel chemistry, D. Optical properties, D. LuminescenceDependencia espectral de la eritrosina B con el pH en soluciones tampón y en recubrimientos sol-gel de silice.Se han obtenido recubrimientos delgados ópticamente sensibles al pH mediante atrapamiento de moléculas de eritrosina B en matrices de sílice obtenidas por sol-gel. Se compara la respuesta espectral de la eritrosina B en solución y atrapada en los recubrimientos de sílice para un intervalo de pH comprendido entre 0,5 y 12. Se observan apreciables diferencias entre ambos tipos de espectros (en solución y en recubrimiento) de la eritrosina B. Los principales cambios aparecen en los espectros de absorción para el intervalo neutro y básico de pH. Además, los recubrimientos dopados con eritrosina B muestran respuestas espectrales frente al pH con efecto de memoria o histéresis. Esto significa, que el espectro de absorción depende del sentido del cambio del pH. De forma análoga se ha visto que los espectros de fotoluminiscencia para la eritrosina B en soluciones tampón dependen del pH, mostrando su máxima respuesta en el intervalo básico de pH. Por el contrario, para la eritrosina B atrapada en los recubrimientos de sílice no se observa señal de fotoluminiscencia. Se discuten los resultados teniendo en cuenta principalmente, la diferente respuesta óptica de las formas polipróticas de la eritrosina B y sus posibles interacciones con el oxígeno así como con las paredes de los poros de la matriz sol-gel. Además, se considera la presencia de barreras difusionales hacia la molécula que impiden el flujo de protones y otros...
Porphyrin-doped silica coatings were prepared by sol-gel. Films optical properties were studied and compared to the behavior of the molecule in solution. A quasi-linear behavior throughout the pH range studied was found. Luminescent emission peak and molar absorptivity were maximum at slightly lower wavelength than molecule in solution.
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