Developments of microfluidic paper-based analytical devices (μPADs) for water analysis: A review

Highlights

• Microfluidic paper-based analytical devices (μPADs) for water analysis are reviewed.

• The detection methods used by these devices and their pros and cons are described.

• Precision and accuracy of µPAD-based water analysis is assessed critically.

• Ability of µPADs to meet legislative guidelines is discussed.

• Suitability of µPADs for in-field measurements is examined.

Abstract

Water pollution is a serious environmental problem affecting millions of people, and the demand for frequent water quality monitoring is increasing. The need for analytical platforms that combine high sensitivity, selectivity and accuracy with low cost, portability and user friendliness remains a challenge. Microfluidic paper-based analytical devices (μPADs) are recognised as a powerful analytical platform that can satisfy these requirements.

The aim of this review is to provide a detailed overview of the μPADs that have been developed for the determination of important water quality parameters, such as nutrients, metals and organic contaminants, in a range of waters. A description of the fabrication and detection methods selected for these applications is provided, and the performance of the μPADs with respect to their precision and accuracy is critically assessed. The potential of these devices for real-life applications is also critically examined, particularly if they can determine the concentrations specified in water quality guidelines or the maximum recommended concentrations for various waters, as well as if they are suitable for field applications.

Introduction

The fate of the aquatic environment has become a critical issue globally. Population growth, and urbanisation and industrialisation in most countries have had a deleterious effect on water quality. With more than 50% of the world's population living in cities, and almost 70% expected to be urban by 2050, the pressure on water resources is enormous [1]. While in developing countries, water scarcity is a serious threat to human health, in developed countries water is often taken for granted, and consumed and managed in a haphazard manner. Moreover, anthropogenic activities and poor management of agricultural and industrial wastes (e.g. illicit discharges, runoff) all contribute to the decline in water quality [2].

Hence, in parallel with socioeconomic and political measures, frequent water quality assessment and diligent water management are crucial in order to guarantee safe water supplies to humans, and to identify, control and prevent aquatic pollution. There is thus an urgent need for analytical platforms that can combine high-sensitivity, accuracy and rapid analysis with simplicity, portability and low cost, in order to make water analysis (e.g. of drinking water, freshwater, wastewater) accessible to water scientists and managers, as well as for interested citizen groups.

Microfluidic paper-based analytical devices (µPADs) are recognised as a potentially powerful analytical platform because they embody many of the features listed above. The use of paper as a substrate for analytical purposes has several advantages [3], namely it is readily available and inexpensive, can be easily patterned into discrete hydrophilic and hydrophobic zones using existing printing or cutting technologies, is able to wick fluids by capillary action without external power sources, is lightweight and easy to transport, and is disposable and biodegradable. The antecedents of the paper-based analytical devices date back to the mid-seventeenth century, when Boyle introduced litmus paper as an acid-base indicator [4]. However, since 2007 this methodology has grown exponentially [5] because of Whitesides and co-workers who described a simple method for patterning paper with hydrophobic barriers in order to create well-defined hydrophilic channels [6]. Even though most of the research on µPADs has been focused on point-of-care diagnostic devices [7], other applications have emerged [8], [9], namely in environmental analysis [10] as well as in food and water analysis [11]. A plethora of fabrication techniques and detection methods have been proposed, and are described in recent reviews [12], [13], [14].

While substantial advances have been made in μPAD development, the question remains whether they can replace traditional and well-established analytical methods commonly used for water analysis or are suitable for screening purposes only. Are they sufficiently sensitive and robust to be applied in the field? With these questions in mind, the present article aims to provide a critical review of μPADs that have been developed and applied to water analysis. Each μPAD will be highlighted in terms of its main features (e.g. fabrication and detections methods) and analytical figures of merit, along with an individual pictorial description. A critical assessment of their precision, accuracy and real-life applicability will also be the focus of this review.