**Abstract**

The presence of contaminants in water generates a great concern worldwide. As contaminants, we can refer different classes of chemicals, such as pharmaceuticals, personal care products, heavy metals, and also microorganisms, such as waterborne pathogens. Some of the chemical compounds have the potential to bioaccumulate in the aquatic biota. Hence, the development of simple and portable methods for the detection of contaminants in the aquatic environment can improve their monitoring and, consequently, the study of their environmental impact. In this context, the development of paper-based analytical tools and also of biosensor devices has been exploited for quantitative and semiquantitative analysis of several contaminants in different water matrices. The association of these two analytical strategies can provide the implementation of low-cost, portable, and easily handled methods for detecting chemical and biological contaminations in water. In this chapter, we provide a review of the developed paper-based analytical biosensors, highlighting the features of the paper-based (paper substrate and fabrication procedures) and biosensor devices (transducers and biorecognition elements). Moreover, the application of the referred paper-based biosensors for the detection of different water contaminants (pathogens, pharmaceuticals, and heavy metals) in environmental and wastewater samples is discussed.

**Keywords:** microfluidic, paper-based devices, water analysis, water contaminants, biosensing

### **1. Introduction**

The contamination of the different water compartments with several chemicals and by-products has become a major concern for human health and aquatic biota [1–3]. The environmental contaminants of major concern are pharmaceuticals, personal care products, pesticides and herbicides, heavy metals, and waterborne pathogens. Some of the chemical compounds are persistent to degradation and, therefore, can accumulate in aquatic organisms and sediments. Thus, the monitoring of contaminants in the aquatic environment is crucial to study their impact [4].

Currently, there is no regulation about the allowed levels of pharmaceutical compounds, including ethinylestradiol and antibiotics, in water. Concerning the maximum contaminant levels (MCL) in drinking water for the target metals presented in this work, their MCL are between 2 and 30 μg L<sup>−</sup><sup>1</sup> , according to the chemical species [5, 6]. Arsenic presents an MCL value of 10 μg L<sup>−</sup><sup>1</sup> [5, 6], while the MCL value

for mercury is 2 or 6 μg L<sup>−</sup><sup>1</sup> according to United States Environmental Protection Agency (EPA) and World Health Organization (WHO), respectively. Moreover, the MCL for lead is 15 [5] and 10 μg L<sup>−</sup><sup>1</sup> [6]. Uranium presents the higher MCL, which is 30 μg L<sup>−</sup><sup>1</sup> [5, 6]. Cadmium's maximum contaminant level corresponds to 3 and 5 μg L<sup>−</sup><sup>1</sup> according to WHO and EPA, respectively. With respect to the pathogens targeted in the paper-based biosensors, the EPA [7] recommends that *Escherichia coli* cannot exceed 126 CFU per 100 mL in fresh recreational water, while *Enterococcus* should present a maximum of 35 CFU per 100 mL in marine and freshwater.

In this context, paper-based biosensor devices combine the main features of paper substrates (cost-effectiveness, easy manipulation, and compatibility with proteins and biomolecules), with the high specificity and selectivity of the biorecognition systems of biosensors [1, 8, 9]. Furthermore, paper-based assays can be a solution in resource-limited contexts, as both sample and reagents can be introduced without any flow device, through imbibition and filtration via capillary action [10].

The first types of paper-based devices were related to semiquantitative analysis of glucose in urine and immunoassays on chromatographic paper test strips (or lateral flow) [11]. In the last decade, a new fabrication method based on wax patterning was introduced, allowing the design of well-defined channels on paper surface, which provided microfluidic features to the paper-based devices [12].

Reviews concerning the application of paper-based devices in different fields such as food, water analysis, environmental monitoring, and health diagnostics are available [8, 11, 13]. Furthermore, the application of biosensors has been extensively discussed regarding both their usefulness on assessing environmental and urban pollutants [1], and also their role as part of portable biochemical detection systems [14]. However, gathering information about the implementation of paperbased techniques coupled with biosensor devices to water analysis is still lacking. Hence, the aim of this work is to provide a description of the state of the art about the development and application of paper-based analytical biosensors to detect contaminants in water, focusing on work developed in the last 3 years.
