Printed Disposable Colorimetric Array for Metal Ion Discrimination M. Ariza-Avidad, † A. Salinas-Castillo, † M. P. Cue ́ llar, ‡ M. Agudo-Acemel, † M. C. Pegalajar, ‡ and L. F. Capita ́ n-Vallvey* ,† † ECsens. Department of Analytical Chemistry, Faculty of Sciences, University of Granada, Campus Fuentenueva, Granada 18071 Granada Spain ‡ Department of Computer Science and Artificial Intelligence, E.T.S. Ingenierías Informa ́ tica y de Telecomunicació n, University of Granada, C/Periodista Daniel Saucedo Aranda s/n, Granada 18071, Spain * S Supporting Information ABSTRACT: One of the main limiting factors in optical sensing arrays is the reproducibility in the preparation, typically by spin coating and drop casting techniques, which produce membranes that are not fully homogeneous. In this paper, we increase the discriminatory power of colorimetric arrays by increasing the reproducibility in the preparation by inkjet printing and measuring the color from the image of the array acquired by a digital camera, using the H coordinate of the HSV color space as the analytical parameter, which produces robust and precise measurements. A disposable 31 mm × 19 mm nylon membrane with 35 sensing areas with 7 commercial chromogenic reagents makes it possible to identify 13 metal ions and to determine mixtures with up to 5 ions using a two-stage neural network approach with higher accuracy than with previous approaches. T he identification and determination of metallic ions, mainly multivalent transition metals with potential toxicity or ecotoxicity, 1 in the lab is a well-established subject that uses an ample panoply of analytical techniques, including inductively coupled plasma/atomic emission spectrometry (ICP-AES), inductively coupled plasma/mass spectrometry (ICP-MS), atomic absorption spectroscopy (AAS), and atomic fluores- cence spectroscopy (AFS). However, the persistence of these metals in both natural and contaminated environments and their involvement in the development of human cancer and neurodegenerative diseases, encourages the development of fast, cheap procedures that can be performed in situ by untrained personnel, able to manage a large number of samples that provide qualitative and quantitative information on metal ions, particularly in drinking and natural waters and wastewaters. 2 One of the most interesting approaches for rapid multi- analyte analysis is a sensor arrays designnot those based on specific receptors because of selectivity problemsbut rather those based on nonspecific sensors producing a set of high dimensional analytical signals. The processing of signals through advanced mathematical procedures of pattern recog- nition and/or multivariate analysis 3,4 makes it possible to obtain qualitative and quantitative information. From the existing sensing array schemes, optical arrays (also known as optical electronic tongues) offer a reasonable combination of discriminatory power, sensitivity and simplicity to convey the requirements of these analytical devices. These are mainly combined with imaging devices for simultaneous signal acquisition, such as scanners and digital cameras and more recently, with digital cameras integrated into portable devices such as tablets and smartphones, because the built-in microprocessors can be programmed to carry out the image processing and mathematical data treatment. Different approaches have been described for optical electronic tongues, 5-7 but with regard to metal ion discrimination alone, three different formats have been devised: membranes, 8,9 microtiter plates, 8,10-15 and paper-based sensing devices. 16-18 The microtiter plate format arranges the reagents in the wells either immobilized as a membrane at the bottom 10,11,15 or as a solution placed in vials that are arranged in an array before acquiring the analytical signal. 12 That format needs only a small volume of sample, even at submicroliter level, 14 although in the usual application with water analysis it is not the main problem. Membrane and paper-based devices offer the additional advantage of preconcentration, lowering the detection limit. For instance, the arrangement of metal ion reagents in ormosils coated on a cellulose acetate/nitrate membrane has been proposed for the discrimination of eight metal ions with detection limits of 50 nM. 9 Different types of paper-based devices have been described: Hossain and Brennan 16 used a flower design with wax-printed hydrophobic channels and inkjet-printed detection areas in which the stem is the sampling area. Feng et al. 17 described an eight detection area wheel design paper device with a sampling area in the middle that improves the preconcentration, including a water absorbent cell on each detection area and a water absorbent resin placed at the end of each hydrophilic channel. In this way it is possible to Received: April 21, 2014 Accepted: August 4, 2014 Published: August 4, 2014 Article pubs.acs.org/ac © 2014 American Chemical Society 8634 dx.doi.org/10.1021/ac501670f | Anal. Chem. 2014, 86, 8634-8641