Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta ChromiSense: A colourimetric lab-on-a-disc sensor for chromium speciation in water G. Duffy a , I. Maguire a , Brendan Heery a , Pauline Gers a , J. Ducrée b , F. Regan a, ⁎ a Water Institute, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland b School of Physical Sciences, Fraunhofer Project Centre at Dublin City University (FPC@DCU), Glasnevin, Dublin 9, Ireland ARTICLE INFO Keywords: Centrifugal Chromium Optical Sensor Water Microfluidic ABSTRACT The development of a centrifugal device for quantitative analysis of both chromium (III) and (VI) species in water is reported. ChromiSense is a colourimetric sensor system that has been applied to the measurement of chromium in spiked river water samples. For analysis, the sample is loaded into a reservoir on the disposable microfluidic disc, along with reagents. A centrifugal force is created by spinning the disc to pump liquids through microchannels, causing them to mix and react to form a coloured product. The coloured product is then pre- sented to a low-cost optical detection system, where absorbance measurements can be recorded. The optical detection system consists of a light emitting diode (LED) and photodiode (PD) couple. Chromium (III) was measured using 2,6-pyridine dicarboxylic acid as a ligand, forming a complex that was measured at 535 nm and at 335 nm. While measuring at 535 nm allowed for the use of a low cost LED, the sensitivity was improved 2.5 times by measuring at 335 nm. However, 335 nm also yielded a diminished linear range with little improvement in limit of deteciton (LOD), and required a lengthier manufacturing process due to the need for a UV-transparent material. Chromium (VI) was detected using 1,5-diphenyl carbazide (DPC). This standard analysis method was simplified for automation on-disc, and optimised to achieve a low LOD. The LOD for trivalent and hexavalent chromium using this device were 21 mg L -1 and 4 μgL -1 , respectively. The linear range for quantitative ana- lysis was found to be 69–1000 mg L -1 for Cr(III) and 14–1000 μgL -1 for Cr (VI). While this range is high for Cr (III), incorporation of an off-disc pre-concentration method would make this technology suitable for environ- mental sample analysis. The device is simple to use, low in cost, and could provide rapid on-site measurements, with results comparable to those obtained using a benchtop spectrophotometer. 1. Introduction Chromium speciation refers to the quantitative analysis of chro- mium species. Although chromium exists in oxidation states from 0 to VI, it is primarily found in the (III) and (VI) states [1]. Chromium enters the environment through effluent discharged from industry (e.g. tex- tile/electroplating) and cooling towers. It can enter drinking water supplies through corrosion inhibitors used in pipes or through con- tamination leaching from sanitary landfill [2]. Changes in the oxidation state of an element can greatly change its bioavailability and toxicity [2]. Cr (III) is an essential micronutrient in the human diet, and is considered essential for maintenance of glucose, lipid and protein metabolism [3]. Cr (VI) is strongly oxidizing, ex- hibiting high toxicity, with carcinogenic and mutagenic properties [4]. According to the World Health Organisation (WHO), the recommended maximum allowable concentration for chromium (VI) is 0.05 mg L -1 in drinking water [5]. Traditional laboratory-based measurements of chromium in fresh- water are time and labour intensive, requiring expensive instrumenta- tion and trained personnel. Commonly used techniques include atomic absorbance or emission spectroscopy, inductively coupled plasma mass spectrometry, ion chromatography, spectrophotometry, potentiometry and capillary electrophoreses. These methods can incorporate sample pre-treatments such as solid or liquid phase extraction, cloud point extraction, adsorbents or ion exchange [2]. Handheld colourimeters for on-site measurements are a more con- venient option for frequent monitoring; however the limit of detection (LOD) of these devices is typically higher than laboratory-based methods. A low LOD is essential for a chromium sensor as the re- commended Cr (VI) limit is so low. A centrifugal ‘lab-on-a-disc’ (LoaD) approach is employed for the development of an optical sensor for chromium speciation in water. This approach was proven successful for a different analyte in a pre- vious publication [6]. By making changes to the sensor and disc design, http://dx.doi.org/10.1016/j.talanta.2017.09.066 Received 14 July 2017; Received in revised form 16 September 2017; Accepted 22 September 2017 ⁎ Corresponding author. E-mail address: Fiona.regan@dcu.ie (F. Regan). Talanta 178 (2018) 392–399 Available online 23 September 2017 0039-9140/ © 2017 Published by Elsevier B.V. MARK