Absorption of a chemical analyte into a polymer coating results in an expansion governed by the concentration and type of analyte that has diffused into the bulk of the coating. When the coating is attached to a microcantilever, this expansion results in bending of the device. Assuming that absorption ͑i.e., diffusion across the surface barrier into the bulk of the coating͒ is Fickian, with a rate of absorption that is proportional to the difference between the absorbed concentration and the equilibrium concentration, and the coating is elastic, the bending response of the coated device should exhibit a first-order behavior. However, for polymer coatings, complex behaviors exhibiting an overshoot that slowly decays to the steady-state value have been observed. A theoretical model of absorption-induced static bending of a microcantilever coated with a viscoelastic material is presented, starting from the general stress/strain relationship for a viscoelastic material. The model accounts for viscoelastic stress relaxation and possible coating plasticization. Calculated responses show that the model is capable of reproducing the same transient behavior exhibited in the experimental data. The theory presented can also be used for extracting viscoelastic properties of the coating from the measured bending data.

A liquid chromatography-atmospheric pressure chemical ionization mass spectrometry [LC-(APCI)MS] method was developed to identify and quantify the carotenoids present in fresh, pasteurized, and freeze- and spray-dried egg yolk in two independent batches. The egg yolk powders in each batch were stored in the dark for 6 months at -18 or 20 degrees C. Carotenoids were isolated by solvent extraction without saponification and analyzed by HPLC using a C(30) column coupled to a photodiode array and mass detector. The most abundant carotenoids were all-E-canthaxanthin, all-E-lutein, all-E-zeaxanthin, 9-Z-canthaxanthin, and beta-apo-8'-carotenoic acid ethyl ester. Pasteurization of the egg yolk caused no critical changes in the carotenoid content. On the contrary, drying to a dry matter of 98-99% led to higher carotenoid contents, induced by a denaturation of binding proteins, and a destabilization of the cell matrix. After the 6 months of storage, the contents of all main carotenoids in the egg yolk powder were significantly lower. The synthetic carotenoids canthaxanthin and beta-apo-8'-carotenoic acid ethyl ester showed a higher retention rate, and the greatest losses occurred within the first 8 weeks. Statistical tests (ANOVA, P < 0.05) also proved that after 26 weeks, the egg yolk powders stored at -18 degrees C showed only a slightly higher retention of carotenoids when compared to the powders stored at 20 degrees C.

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Abstract-Sensor drift from slowly changing environmental conditions and other instabilities can greatly degrade a chemical sensor's performance, resulting in poor identification and analyte quantification. In the present work, estimation theory (i.e., various forms of the Kalman filter) is used for online compensation of baseline drift in the response of chemical sensors. Two different cases, which depend on the knowledge of the characteristics of the sensor system, are studied. First, an unknown input is considered, which represents the practical case of analyte detection and quantification. Then, the more general case, in which the sensor parameters and the input are both unknown, is studied. The techniques are applied to simulated sensor data, for which the true baseline and response are known, and to actual liquid-phase SH-SAW sensor data measured during the detection of organophosphates. It is shown that the technique is capable of estimating the baseline signal and recovering the true sensor signal due only to the presence of the analyte. This is true even when the baseline drift changes rate or direction during the detection process or when the analyte is not completely flushed from the system.

Expressions describing the resonant frequency and quality factor of a dynamically driven, polymer-coated microcantilever in a viscous liquid medium have been obtained. These generalized formulas are used to describe the effects the operational medium and the viscoelastic coating have on the device sensitivity when used in liquid-phase chemical sensing applications. Shifts in the resonant frequency are normally assumed proportional to the mass of sorbed analyte in the sensing layer. However, the expression for the frequency shift derived in this work indicates that the frequency shift is also dependent on changes in the sensing layer's loss and storage moduli, changes in the moment of inertia, and changes in the medium of operation's viscosity and density. Not accounting for these factors will lead to incorrect analyte concentration predictions. The derived expressions are shown to reduce to well-known formulas found in the literature for the case of an uncoated cantilever in a viscous liquid medium and the case of a coated cantilever in air or in a vacuum. The theoretical results presented are then compared to available chemical sensor data in aqueous and viscous solutions.Polymer-coated microcantilevers have been extensively investigated for use as chemical sensor platforms. [1][2][3][4] Microcantilevers have shown high sensitivities in chemical vapor detection. In particular, polymer-coated cantilevers are often utilized in the dynamic (resonant) mode for detection in gas.2,5-9 Several investigators have analyzed such devices by considering only the mass loading effect of the chemical analyte without consideration of the coating viscoelastic effects.3,4,10 Others have explicitly included the effects of the coating properties, 2 while assuming operation in a vacuum. In this work, the effects of both the medium and the coating will be taken into account.Application of microcantilevers to liquid-phase detection has mostly focused on static-mode detection because dynamically driven microcantilevers suffer from low frequency stability in viscous liquid media.11 The characteristics of uncoated dynamically driven microcantilevers have previously been investigated in viscous liquid media accounting for the properties of the liquid media, including density and viscosity.12 Recently, work has been done on characterizing the behavior of polymer-coated microcantilevers in a vacuum, which indicated significant chemically induced coating plasticization effects.1 However, the effects of the polymer coating in a viscous liquid environment have not been studied, which includes plasticization and the properties of the viscous liquid. The present derivation extends previous work on uncoated and coated microcantilevers to obtain generalized formulas for the characteristics NOT THE PUBLISHED VERSION; this is the author's final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.Analytical Chemistry, Vol 80, No. 15 (August 1, 2008): pg. 5760-5767...