Miniaturized Separation Technology Enabling Selective Chemical Sensors

Abstract

Introduction Chemical recognition with sensors offers tremendous potential in emerging applications including the next generation of non-invasive medical diagnostics [1], smart indoor air control [2], exhaust gas analysis and food quality assessment. Nowadays, chemical gas sensors offer low price, compact size, easy applicability and they typically can detect target analytes at low part-per-billion (ppb) concentration. A major obstacle, however, in the realization of such sensors is their lack of selectivity [3]. They often fail in the application due to the complexity of the gas matrix (ambient air or breath) containing hundreds of interferants. Strategies to overcome this limitation include sensor material design and combining different selective sensors to arrays [4]. Often, however, the achieved selectivity is insufficient for the stringent requirements in the applications. Here, we present the use of compact separation columns upstream of sensors to achieve unprecedented selectivity to target analytes. Method The separation column consists of a packed bed of Tenax TA particles (60–80 mesh, ~35 m2 g-1) inside a Teflon tube (4 mm inner diameter) at loading of 150 mg, secured on both ends with silanized glass wool and tension springs. Columns were flushed over night with 100 mL/min synthetic air at 50% relative humidity (RH) before use to desorb impurities that might be present on Tenax. Sensors were prepared by depositing flame-made Pd-doped SnO2 (1 mol% Pd) nanoparticles directly onto micromachined sensor substrates. These microsensors were bonded onto leadless chip carriers and placed inside a Teflon sensor chamber. The separation column was secured upstream of the sensor and connected to it with Teflon connections. The performance of the column–sensor system was characterized with a dynamic gas mixing setup, where it was continuously flushed with synthetic air at varying relative humidity (RH). Analyte gases, supplied from calibrated gas standards, were admixed to the synthetic gas air stream to generate well-defined analyte exposures. Analyte retention times (tR ) were defined as the time elapsed between the analyte exposure and sensor’s maximum response, analogous to gas chromatography. Results and Conclusions When exposing the non-specific sensor without separation column to analytes typically contained in ambient air [5] and breath [6] such as ethanol, methanol or acetone, it quickly responds to them (response time <5 s) with high responses at low ppb concentrations [7]. However, it cannot accurately measure single compounds in gas mixtures, especially when interferants are present at elevated concentrations. This can be solved by a simple and compact separation column. In Figure 1a, the sensor resistance with separation column to 10 s pulses of methanol, ethanol and acetone is shown. The analytes are retained depending on their specific interaction with the Tenax surface. As Tenax is non-polar and adsorbs molecules primarily due to nonspecific van-der-Waals forces, the analyte retention times typically increase with higher molecular weight. As a result, methanol is detected first (tR = 1.7 min), followed by ethanol (8.7 min) and acetone (33 min). Analytes that are retained longer are thereby released over a longer time period, leading to lower maximum concentration and lower sensor response. Similar responses are thus obtained for all compounds although concentrations vary greatly (1, 5, and 20 ppm). When exposing the column–sensor system now to the gas mixture (Figure 1b), all analytes are detected individually at their corresponding retention time, i.e., with very high selectivity. This concept can be utilized for selective detection of individual target analytes, as recently demonstrated for selective detection of toxic methanol over ethanol in breath and liquor [7], or to detect multiple analytes with a single sensor. By flushing with air, the separation column regenerates, which can be easily sped up by slight heating of the column and increasing the flow rate (e.g., acetone from ~60 min to 20 s when increasing column temperate and flow rate briefly to 80 °C and 100 mL min-1).As a result, separation columns demonstrate how to possibly address chemical sensors’ long-standing challenge of selectivity. They give comparable performance to classical gas chromatographic columns, however, are much simpler in design (packed particle bed), inexpensive (<10 $) and compact (4 cm length). A broad variety of different commercial sorbent materials is readily available to design columns for specific applications and combine them freely with different sensor technologies typically suffering from low selectivity (e.g., metal-oxide, optical or electrochemical sensors). Based on their low price and compact size, they could enable the next generation of highly selective portable breath analyzers and environmental monitors.