Microfluidic electronic tongue

Abstract

Fast, simple inspection of liquids such as coffee, wine and body fluids is highly desirable for food, beverage and clinical analysis. Electronic tongues are sensors capable of performing quantitative and qualitative measurements in liquid substances using multivariate analysis tools. Earlier attempts to fulfil this task using only a few drops (microliters) of sample did not yield rational results with non-electrolytes e.g. sucrose (sweetness). We report here the fabrication and testing of a microfluidic e-tongue able to distinguish electrolytes from non-electrolytes, covering also the basic tastes relevant to human gustative perception. The sensitivity of our device is mainly attributed to the ultrathin nature of an array formed by non-selective sensing units. The electronic tongue is composed of an array of sensing units designed with a microchannel stamped in a poly(dimethylsiloxane) (PDMS) matrix and sealed onto gold interdigitated electrodes (IDEs). The IDEs are then coated in situ with a 5-bilayer film deposited by the layer-by-layer (LbL) technique. The cationic layer is derived from polyallylamine chloride (PAH). The anionic layer is either poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) PEDOT:PSS, polypyrrole or nickel tetrasulfonated phthalocyanine. When compared to a conventional electronic tongue our system is three times faster and requires only microliters of sample. Applying Principal Component Analysis to the data yields a high correlation for all substances tested. This microfluidic e-tongue has the potential for producing low-cost, easily integrated, multi-functional sensor for food, beverages, in addition to clinical and environmental applications.

Introduction

Microfluidics is at the interface of Physics, Medical Sciences, Engineering and Chemistry, bringing the benefits of reduced size and, hence small test volumes, low waste and fast response as well as the potential for small, portable and integrated devices [1], [2], [3]. Within this context, it is highly desirable to develop platforms incorporating micro-analytical tools for food, beverage, pharmaceutical, clinical and environmental investigations in liquid samples. An electronic tongue (e-tongue) is a multisensory system using multivariate analysis for the quantitative and qualitative inspection of a solution [4]. Microfluidic systems incorporating the e-tongue concept reported to date have not been used with non-electrolyte substances [5], [6]. We present here a method that benefits from the manipulation of nanostructured layers and samples inside a microchannel to create a microfluidic electronic tongue able to distinguish all basic tastes (sour, salt, sweet, bitter and umami) at sensitivities below the human threshold.

Several types of e-tongue systems have been reported in the literature [7]. The device reported here is based on measuring the impedance of a sensor array composed of ultrathin films deposited onto interdigitated electrodes [8], [9]. Briefly, the distinct electrical characteristic of the nanostructured materials used to form individual sensing units creates a fingerprint of the solution analyzed, with the ultrathin nature of the films yielding a highly sensitive sensor. Since we do not perform potential-dependent measurements, there is no need to use the three-electrode cell normally employed in electrochemistry.

Various methods can be used to assemble nanostructures. The layer-by-layer technique (LbL) is a simple, versatile, bottom-up procedure for multilayer formation on free surfaces [10] and in confined geometries e.g. inside microchannels [11], [12], [13]. There are several reports on nanostructured thin films inside microchannels using dynamic layer-by-layer (LbL) deposition, but only one is related to electronic tongues or taste sensors. Jacesko et al. [5] reported a surface acoustic wave microfluidic sensor having a 10 mL chamber; however, our system requires only 200 μL of sample and uses impedance spectroscopy as the detection method, thus obviating the need for a piezoelectric substrate. Zou et al. [14] used a system analogous to ours to check protein binding at an electrode surface and observed a significant reduction in time analysis and reagent volumes. Kim et al. [15] reported a bioelectronic super-taster, incorporating the human bitter-taste receptor protein immobilized on a single-walled carbon-nanotube field-effect-transistor (SWCNT-FET) with a lipid membrane which exhibited a surprising human tongue-like sensitivity. Nonetheless, fabrication of the field-effect device requires many assembly steps including the immobilization of a composite lipid membrane between source and drain followed by the integration into a microfluidic system. Hossein-Babaei and Nemati [6] reported the concept of a microfluidic electronic tongue, which was used only for strong electrolytes (sourness and saltiness). Measurements for bitterness and sweetness, such as sucrose (non-electrolyte) and caffeine, were not performed.

In this study, we describe the fabrication and testing of a microfluidic taste sensor, in which sensing is undertaken by measuring the impedance between co-planar, interdigitated gold electrodes coated with nanostructured films. The device is formed by integrating individual sensing units comprising 5-bilayer films deposited in situ within each microchannel using the layer-by-layer approach. Every bilayer is composed of polyallylamine chloride (PAH) as the cationic layer and either poly(3,4-ethyenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), polypyrrole (PPy) or nickel phthalocyanine (NiTsPc) as the anionic layer. Each sensing unit requires only 200 μL of samples and presents a much faster analysis than a conventional setup (needs ∼ 50 mL for sampling), providing also high correlation between data acquired in triplicate. Indeed, different devices integrated into microfluidic systems enable a variety o distinct designs in an e-tongue system for producing low-cost, easily integrated, multi-functional sensors that can be directed for a specific application for food, beverages, clinical and environmental analysis.