PhosphaSense: A fully integrated, portable lab-on-a-disc device for phosphate determination in water

https://doi.org/10.1016/j.snb.2016.12.040Get rights and content

Highlights

  • Development of a chemical sensor for on-site phosphate determination in water.

  • System contains a centrifugal disc for automation of the colourimetric method.

  • A complementary system facilitates disc rotation and absorbance measurements.

  • A long optical path length facilitated optimisation for low level PO43− in water.

  • The system was applied to phosphate measurement in river water and WWTP samples.

Abstract

A portable, compact, centrifugal microfluidic system for the in situ quantitation of phosphate in water is reported. The device uses the ascorbic acid method, a colourimetric absorbance based assay, for phosphate determination. The integrated system consists of two components; the disposable centrifugal microfluidic disc and the complementary system. The microfluidic disc is designed to have similar dimensions to that of a compact disc, with a slightly thicker composition. Capillary active micro-channels are integrated internally, through which small and precise volumes of fluids can flow. Upon loading of the disc with a water sample and chemical reagents, the fluids can be moved through the disc using centrifugal force. This is created by rotation of the disc by the motor in the complementary system. The loaded fluids are then mixed due to rapid expansion and contraction as they are forced through the microfluidic channels and significantly larger reservoirs. Once mixing has occurred, this force will then drive the fluid into the optical detection zone. The low-cost optical detection system incorporated into the complementary system consists of an LED-photodiode transducing pair that measures the absorbance of light by the molybdenum blue complex formed at 880 nm. The total mass of 2 kg and dimensions of 20 cm × 18 cm × 14 cm make this system portable and convenient for analysis at the sampling site. The limit of detection (LOD) and limit of quantitation (LOQ) of this device were 5 and 14 μg L−1 PO4-P, respectively. The linear range of 14–800 μg L−1 and sensitivity of 0.003 AU L μg−1 make it suitable for analysing water bodies with low levels of phosphate.

Introduction

Phosphorus (P) is an essential nutrient for life. It is a growth limiting nutrient, which makes it an important parameter to monitor in water [1]. Elevated levels of growth-limiting nutrients lead to algal blooms [2]. These blooms can be a nuisance, however some algal species release toxins which are harmful to humans and animals. Aside from toxicity, decay of the large amounts of organic matter associated with algal blooms leads to hypoxic or anoxic waters, forming ‘dead zones’ where aquatic animals cannot survive [3]. These harmful algal blooms can have devastating effects on the local ecosystem, as well as on the fishing industry, water sports and leisure activities, and drinking water supplies.

Major sources of P entry into fresh water systems include fertiliser run-off from farmlands, and effluent from waste water treatment plants and industrial plants. It exists in many different chemical forms in water [4]. The simplest method for estimating bioavailable phosphorus in water is to analyse for soluble reactive phosphate (SRP). Orthophosphates are the most abundant forms of SRP at pH levels typically encountered in natural waters [5], [6].

Phosphate cannot be measured directly in water, introducing the need for reagent based detection. A number of different strategies have been adapted for phosphate measurements on-site, including colourimetry [7], [8], [9], [10], [11], [12], electrochemistry [13], [14], [15], [16], [17] and fluorescence emission spectroscopy [18], [19], [20].

Lab-on-a-disc (LOAD) devices are an ideal means for rapid on-site measurements as they allow for miniaturisation and automation of laboratory based analytical protocols, towards the development of inexpensive, portable and compact devices [21]. The use of microlitre volumes results in a reduction in reagent consumption, waste production and analysis times compared to standard laboratory protocols. This coupled with reduced cost, work flow and lowered sample contamination risk makes disposable microfluidic devices an attractive option for water analysis [22].

Low reagent volume requirements improve portability of the system, as larger volumes of liquid reagents are cumbersome to transport, particularly when the sampling sites are difficult to access. Centrifugal microfluidics offers the added advantages of simplicity and low cost. In place of multiple microfluidic pumps, which are often expensive and require considerable power input, a simple motor is used for disc rotation, to generate a centrifugal force which acts from the centre of the disc, radially outwards [23]. This centrifugal force propels fluid through the microfluidic channels.

The microfluidic platform for this system was fabricated from poly(methyl methacrylate) (PMMA) and pressure sensitive adhesive (PSA). It is rotated at ∼8 Hz to create the centrifugal force for fluid manipulation. The on-board microfluidic architecture, including an air ventilation system for performance enhanced mixing of sample and reagents, also enables precise fluidic manipulation. Each disc has three separate analytical zones, allowing for three samples to be analysed before the disc can be disposed of. The ascorbic acid method for SRP determination was incorporated onto this device due to its high sensitivity compared to other colourimetric methods. A long optical path length of 75 mm was included in order to maximise absorbance signal, producing improved sensitivity and LOD.

There are currently few examples of LOAD devices for water quality analysis. Some parameters for water quality assessment that have been automated on-disc include pH, turbidity, nitrate, nitrite, ammonium, silicate and hexavalent chromium [21], [24], [25], [26], [27]. These devices are compared in Table 1.

Section snippets

Chemicals

All solutions were prepared using ultra-pure water (Elga Maxima®, 18.2 MΩ) and ACS grade reagents purchased from Sigma Aldrich, Arklow, Ireland. Working standards were prepared by dilution of a 50 μg PO4-P mL−1 stock solution, prepared from potassium dihydrogen phosphate monobasic. A 0.032 M solution of ammonium molybdate tetrahydrate, a 0.004 M solution of potassium antimonyl tartrate and a 0.1 M solution of L-ascorbic acid were prepared. A 5 M sulphuric acid solution was prepared by adding 7 mL

Calibration and evaluation of analytical performance

The calibration curve obtained using the fully optimised PhosphaSense system is shown in Fig. 4. This system displayed a linear response signal to phosphate concentration from 14 to 800 ug L−1 PO4-P. The LOD and LOQ values achieved were 5 and 14 μg L−1, respectively.

The sensor’s performance was compared to that of the same colourimetric method performed using a spectrophotometer. This comparison is shown in Fig. 5, with a more detailed comparison shown in Table 2.

From Fig. 5, the close agreement

Conclusions

A fully integrated, centrifugal, microfluidic optical sensor for phosphate determination in water has been developed. The use of a microfluidic disc was advantageous as it allowed the use of a long optical path length for improved sensitivity, while also enabling low reagent and sample volumes to be used. The detection limit achieved by the standard spectrophotometric ascorbic acid method is 10 μg L−1 PO4-P [5]. By adapting this method onto a microfluidic device, using a simple design to

Acknowledgements

The authors would like to thank the Naughton Graduate Fellowship Program 2013 in the University of Notre Dame, USA, and DCU Educational Trust and Faculty of Science & Health for funding this project.

Gillian Duffy B.Sc. is a final year PhD candidate at Dublin City University (DCU), under the supervision of Prof. Fiona Regan and Prof. Dermot Diamond. Her research is focused on the development of low cost, wet chemistry based optical sensors for water quality monitoring. Gillian was awarded a Naughton Fellowship from the University of Notre Dame, USA to complete this research. She received a B.Sc. (Hons) in Analytical Science in 2013 from DCU, and completed research on paper based

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    Gillian Duffy B.Sc. is a final year PhD candidate at Dublin City University (DCU), under the supervision of Prof. Fiona Regan and Prof. Dermot Diamond. Her research is focused on the development of low cost, wet chemistry based optical sensors for water quality monitoring. Gillian was awarded a Naughton Fellowship from the University of Notre Dame, USA to complete this research. She received a B.Sc. (Hons) in Analytical Science in 2013 from DCU, and completed research on paper based microfluidic systems with electrochemical detection for separation and quantitation of electroactive analytes. She also received a Hamilton Research Scholarship in 2011 on microfluidic technologies under the supervision of Prof. Jens Ducrée in the National Centre for Sensor Research (NCSR) at DCU. Her research interests include analytical chemistry, method development, optical sensor development and technology for environmental and medical applications, and microfluidic technologies.

    Ivan Maguire, currently a research PhD student since 2014, is a member of the Marine and Environmental Sensing Technology Hub (MESTECH), Dublin City University (DCU), supervised by Prof. Fiona Regan. He studied Physics with Biomedical Sciences at the DCU and graduated top of his class in 2014. During his degree, he underwent a final year project which was aimed at the sorting and detection of circulating tumour cells (CTCs) by using the in-house developed, strategic microfluidic obstacle architecture designs, and was supervised by Dr. Charles Nwankire and Prof. Jens Ducrée. He continued his microfluidic research in the form of a PhD program sponsored by an FP7-funded called ‘MARIABOX’, SmartBay Ireland and MESTECH, DCU. His primary technical challenge is the development of a centrifugal microfluidic platform capable of detecting targeted marine pollutants, with other interests in the integration and automation of both marine and non-marine based monitoring systems.

    Brendan Heery, (B.Eng, PgDip) is a Mechatronic Engineer, completing his PhD on Marine Sensing with Mestech, DCU. Brendan specialises in sensor instrumentation for chemical analysis, including optical and electrochemical systems. He is currently employed as an R&D Engineer by PalmSens BV in the Netherlands.

    Dr. Charles Ezenwa Nwankire is an Irish Research Council Fellow at University College Dublin, Ireland. He obtained his BSc and MSc degrees from St. Petersburg State Technical University, Russia; and PhD degree from University College Dublin, Ireland with international scholarships. He is internationally recognised for his work in biomedical engineering, material science and design technology. He is a co-inventor of 2 patents, published 1 scientific book, 3 book chapters and over 25 peer-reviewed research and review articles. For over 10 years, he has developed cutting edge technologies with leading institutions and multinational corporations including EMD Millipore USA, Medtronic Inc. USA, Alfa Laval Sweden, Suomen Karbonatti, Finland, etc. Dr. Nwankire has won awards in Science communication and Commercialisation in Life Sciences and has made poster and invited oral presentations at international scientific conferences. He has research interests in point-of-use devices and materials for biomedical and environmental monitoring.

    Jens Ducrée Dr. Jens Ducrée’s main scientific research interests are in the fields of micro- and nanofluidic lab-on-a-chip technologies, underlying micro- and nanofabrication schemes, handling and processing of complex (bio-)fluids including blood and cell suspensions, detection technologies, instrumentation and system integration. The fields of application he has been involved in are cell research, systems biology, immunoassays, molecular diagnostics, integrated sample preparation, bioprocess engineering, water analysis, energy harvesting, microprocess engineering and polymer microfabrication. Dr. Ducrée has been very active in technology transfer, e.g. by engineering research tools for the life sciences, systems biology and biomedical, point-of-care diagnostic devices compliant with clinical environments, doctor’s offices and resource-poor settings in home care and global health.

    Fiona Regan, Professor in Chemistry since 2015, established the Marine and Environmental Sensing Technology Hub (MESTECH), DCU in 2010. She studied Environmental Science and Technology at the Institute of Technology in Sligo and graduated in 1991. After completing her PhD in analytical chemistry in 1994, and postdoctoral research in optical sensing in 1996 at DCU, she took up a position at Limerick Institute of Technology as lecturer in Environmental and Analytical Science. In 2002 Fiona returned to the School of Chemical Sciences, DCU, as a lecturer in analytical chemistry, in 2008 she became senior lecturer and in 2009 became the Beaufort PI in Marine and Environmental Sensing. Her research is in the area of separations and sensors, materials for sensing and anti-biofouling applications on aquatic deployed systems, including novel sensors and sensor networks and decision support systems. Fiona is Director of MESTECH and coordinates the Marine ICT SmartBay research under PRTLI V and the International SmartOcean Graduate Enterprise Initiative (ISGEI).

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