Application-Specific Adaptable Coatings for Sensors: Using a Single Polymer–Plasticizer Pair to Detect Aromatic Hydrocarbons, Mixtures, and Interferents in Water with Single Sensors and Arrays

Abstract

A relatively simple design procedure is presented for new, adaptable chemical sensor coatings made from a single polymer−plasticizer pair to detect single or a mixture of chemical compounds (e.g., BTEX, the small aromatic hydrocarbon family). Affinity between coating components and target analytes, expressed through Hansen solubility parameters and relative energy difference values, describes the sensitivity of the resultant coatings to each analyte. While analyte affinity is paramount for plasticizer selection, for the aqueous-phase sensing application described here, it must be traded off with the permanence in the host polymer, i.e., resistance to leaching into the ambient aqueous phase; deleterious effects including coating creep must also be minimized. By varying the polymer:plasticizer mixing ratio, the physical and chemical properties of the resultant coatings can be tuned across a range of sensing properties, in particular the differential response magnitude and rate, for multiple analytes. Together with the measurement of multiple sensor response parameters (relative sensitivity and response time constant) for each coating, this approach allows for identification and quantification of target analytes not previously separable using commercial off-the-shelf (COTS) polymer sensor coatings. Sensing results using a five-sensor array based on five different mixing ratios of a single plasticizer polymer pair (plasticizer: ditridecyl phthalate; polymer: polystyrene) demonstrate unique identification of mixtures of BTEX analytes, including differentiation of the chemical isomers ethylbenzene and total xylene (or "xylenes"), something not previously feasible for separation-free liquid-phase sensing with commercially available polymer coatings. Ultimately, the response of a single optimized sensor coating identified and quantified the components of various mixtures, including identification of likely interferents, using a customized estimation-theorybased multivariate signal-processing technique.

Identification and Quantitation of Aqueous Single- and Multianalyte Solutions of the Isomers Ethylbenzene, m-, p-, and o-Xylene Using a Single Specifically Tailored Sensor Coating and Estimation Theory-Based Signal Processing

“…A single sensor coating has been selected for this work from among the set of previously designed coatings. 7 Polystyrene (35,000 g/mol) was purchased from Sigma-Aldrich, and DTP (530.8 g/mol) was purchased from Scientific Polymer Products. Chloroform, used as a solvent to cast the coating layer, was purchased from Sigma-Aldrich (purity > 99.5%).…”

“…23 The cleaning procedures for sensor devices and glass slides (acting as sensor analogs for coating thickness characterization) have been described in detail. 7 Following proper cleaning, coatings were deposited onto the surface of each sensor device and analogue via spin coating at 5000 rpm, followed by baking for 1 h at 60 °C to evaporate the solvent and remove internal stress from the coatings. Thickness was characterized using an α-Step IQ surface profilometer and was measured to be h = 1.30 ± 0.03 μm using the conditions specified above.…”

“…24 The experimental setup, measurement procedures, and transient response analysis (both for single-and multianalyte solutions) are described in further detail elsewhere. 7,11 For the analysis, sensor responses to single-analyte and multianalyte samples were modeled as a single exponential curve and as a sum of exponential functions, respectively. 11…”

“…8−11 We showed, however, that using polymer-plasticizer blends as sensor coatings significantly increased the sensitivity to each individual BTEX analyte and also provided partial isomer selectivity, in particular, the capability to differentiate ethylbenzene from "total xylene" (the sum of its m, p, and o isomers). 7,12 This result piqued our interest in the development of a sufficiently sensitive and BTEX-optimized chemical sensor coating to utilize this relatively simple approach to identify and quantify all three chemical isomers of xylene.…”

“…5 We recently reported the design of a set of sensor coatings made from a single polymer-plasticizer pair with different mixing ratios as a means to tailor the sensitivity to specific aromatic hydrocarbons such as BTEX. 7 We have also reported on sensor coatings made from a variety of commercial, off-theshelf (COTS) polymers 8−11 and from other polymerplasticizer blends. 12 A significant challenge in these investigations has been the limitation of the primary mechanism of detection in reversible liquid-phase chemical sensing, namely partially selective absorption into the bulk of the chemically sensitive coating.…”