Conferring selectivity to chemical sensors via polymer side-chain selection: thermodynamics of vapor sorption by a set of polysiloxanes on thickness-shear mode resonators

Entropy of mixing is shown to be the driving interaction for the endothermic physisorption process of organic vapor partitioning into seven systematically side-chain-modified (polar, acidic, basic, polarizable side groups and groups interacting via H-bridges) polysiloxanes on thickness-shear mode resonators. Each sensor was exposed to seven analytes, selected for their diversity of functional groups. This systematic investigation of sorption yields benchmarking data on physisorption selectivity: response data and modeling reveal a direct correlation of partition coefficients with interactions between specific polymer side chains and analyte functional groups. Partition coefficients were determined for every polymer/analyte pairing over the 273-343 K range at 10 K intervals; from partition coefficient temperature dependence, overall absorption enthalpies and entropies were calculated. By subtracting the enthalpy and entropy of condensation for a given pure analyte, its mixing entropy (primarily combinatorial) and mixing enthalpy (associated with intermolecular interactions) with each polymer matrix were determined. These two crucial thermodynamic parameters determine the chemical selectivity patterns of the polymers for the analytes. Simple molecular modeling based on the polymer contact surface share of the modified side group or the introduced functional group reveals a direct correlation between the partition coefficients and the side-group variation.

Effective use of molecular recognition in gas sensing: results from acoustic wave and in situ FT-IR measurements.

To probe directly the analyte/film interactions that characterize molecular recognition in gas sensors, we recorded changes to the in situ surface vibrational spectra of specifically functionalized surface acoustic wave (SAW) devices concurrently with analyte exposure and SAW measurement of the extent of sorption. Fourier transform infrared external-reflectance spectra (FT-IR-ERS) were collected from operating 97-MHz SAW delay lines during exposure to a range of analytes as they interacted with thin-film coatings previously shown to be selective: cyclodextrins for chiral recognition, nickel camphorates for Lewis bases such as pyridine or organophosphonates, and phthalocyanines for aromatic compounds. In most cases where specific chemical interactions--metal coordination, "cage" compound inclusion, or pi-stacking--were expected, analyte dosing caused distinctive changes in the IR spectra, together with anomalously large SAW sensor responses. In contrast, control experiments involving the physisorption of the same analytes by conventional organic polymers did not cause similar changes in the IR spectra, and the SAW responses were smaller. For a given conventional polymer, the partition coefficients (or SAW sensor signals) roughly followed the analyte fraction of saturation vapor pressure. These SAW/FT-IR results support earlier conclusions derived from thickness-shear mode resonator data.

Chiral discrimination of inhalation anesthetics and methyl propionates by thickness shear mode resonators: new insights into the mechanisms of enantioselectivity by cyclodextrins.

The discrimination of the enantiomers of methyl lactate, methyl 2-chloropropionate, and the inhalation anesthetics enflurane, isoflurane, and desflurane in the gas phase has been performed using thickness shear mode resonators. The selective coating was a modified perpentylated gamma-cyclodextrin derivative dissolved in a polysiloxane matrix. A new model for the sorption of the chiral compounds into the cyclodextrin cavities and into the polymer matrix was established for the purpose of characterizing the sensor responses. This characterization included the fitting of the sensor responses (preferential and nonpreferential sorption) according to the model and extracting the characteristic parameters. In particular we attempted to explain the observed variation of the chiral discrimination factor alpha with changing analyte or cyclodextrin concentrations and search for an invariable parameter, characteristic for a certain analyte-cyclodextrin combination. The process of chiral or "molecular" recognition was thoroughly investigated.

Chiral discrimination using piezoelectric and optical gas sensors.

Odour perception in humans can sometimes discriminate different enantiomers of a chiral compound, such as limonene. Chiral discrimination represents one of the greatest challenges in attempts to devise selective and sensitive gas sensors. The importance of such discrimination for pharmacology is dear, as the physiological effect of enantiomers of drugs and other biologically active molecules may differ significantly. Here we describe two different sensor systems that are capable of recognizing different enantiomers and of qualitatively monitoring the enantiomeric composition of amino-acid derivatives and lactates in the gas phase. One sensor detects changes in mass, owing to binding of the compound being analysed (the 'analyte'), by thickness shear-mode resonance; the other detects changes in the thickness of a surface layer by reflectometric interference spectroscopys. Both devices use the two enantiomers of a chiral polymeric receptor, and offer rapid on-line detection of chiral species with high selectivity.

Chiral discrimination in the gas phase using different transducers: thickness shear mode resonators and reflectometric interference spectroscopy.

The discrimination of optical isomers (enantiomers) in the gas phase has been performed using two different analytical tools: thickness shear mode resonators (TSMRs) and reflectometric interference spectroscopy (RIFS). The selective coatings included both enantiomers ((S)- and (R)-receptor) of a Chirasil-Val derivative (stationary phase material in GC) with octyl side chains. Successful discrimination of the enantiomers of different types of analytes (amino acids and lactates) was achieved. The results of both transduction methods were consistent and in good agreement with GC measurements. In addition, different mixtures of both enantiomers of the respective analyte were measured, and the enantiomeric composition could be quantitatively determined with excellent reliability. Since the sensors allow on-line monitoring (not possible with GC) of enantiomeric purity, an application in industrial synthesis (process control) of such compounds represents an interesting feature, especially with regard to the tested derivatives of lactic acid.

Performances of mass-sensitive devices for gas sensing: thickness shear mode and surface acoustic wave transducers.

In this work we investigated different thickness shear mode resonators (TSMRs) with fundamental frequencies of 10 and 30 MHz and surface acoustic wave devices with fundamental frequencies of 80 and 433 MHz. Four aspects were of primary interest in this comparison: noise levels and signal-to-noise ratios (S/N), influence of the polymer film thickness, influence of temperature on the transducer signal before and after coating, and minimum threshold values for monitoring different volatile organic compounds in the environment. We limited our investigations to a temperature range between 298 and 308 K, with 303 K the routine measuring temperature. Analyte concentrations (n-octane, tetrachloroethene) were chosen from the minimum detection limit up to 5000 µg/L. The temperature was found to strongly affect the performance of all the devices. The sorption of the analyte vapors into the polymeric films was demonstrated to be transducer-independent (identical partition coefficients for all the devices). The 30 MHz TSMRs showed very satisfying results in terms of S/N and limits of detection.