biosensors Review Enzyme-Based Electrochemical Biosensors for Microfluidic Platforms to Detect Pharmaceutical Residues in Wastewater Ana Lucia Campaña 1 , Sergio Leonardo Florez 1 , Mabel Juliana Noguera 1 , Olga P. Fuentes 1, Paola Ruiz Puentes 2, Juan C. Cruz 2 and Johann F. Osma 1,* 1 Department of Electrical and Electronics Engineering, Universidad de los Andes, Cra. 1E No. 19a-40, Bogotá, DC 111711, Colombia; [email protected] (A.L.C.); sl.fl[email protected] (S.L.F.); [email protected] (M.J.N.); [email protected] (O.P.F.) 2 Department of Biomedical Engineering, Universidad de los Andes, Cra. 1E No. 19a-40, Bogotá, DC 111711, Colombia; [email protected] (P.R.P.); [email protected] (J.C.C.) * Correspondence: [email protected]; Tel.: +57-1-339-4949 Received: 12 February 2019; Accepted: 8 March 2019; Published: 15 March 2019 Abstract: Emerging water pollutants such as pharmaceutical contaminants are suspected to induce adverse effects to human health. These molecules became worrisome due to their increasingly high concentrations in surface waters. Despite this alarming situation, available data about actual concentrations in the environment is rather scarce, as it is not commonly monitored or regulated. This is aggravated even further by the absence of portable and reliable methods for their determination in the field. A promising way to tackle these issues is the use of enzyme-based and miniaturized biosensors for their electrochemical detection. Here, we present an overview of the latest developments in amperometric microfluidic biosensors that include, modeling and multiphysics simulation, design, manufacture, testing, and operation methods. Different types of biosensors are described, highlighting those based on oxidases/peroxidases and the integration with microfluidic platforms. Finally, issues regarding the stability of the biosensors and the enzyme molecules are discussed, as well as the most relevant approaches to address these obstacles. Keywords: enzymes; biosensors; pharmaceutical residues; electrochemistry; microfluidics 1. Introduction Many places around the world currently face water pollution due to the increasing volumes of waste discharged to superficial waters. This is in turn, the result of intensified industrial, agricultural and household activities. Consequently, the world has witnessed a growing scarcity of potable water and a decrease of aquatic biodiversity. A number of treatment processes have been devised to tackle these issues, which are mainly based on a proper determination of physicochemical parameters, the presence of microorganisms, and the possible uses of the recovered waste [1–4]. A complete characterization of polluted water resources is still elusive mainly due to the absence of tools for in situ, rapid, and sensitive monitoring of field samples. Pharmaceutical derivatives are some of the most difficult contaminants to detect and remove from water. One of the main pharmaceutical products found in wastewater are antibiotics, which are generally originated from both human and animal therapeutic and nutraceutical preparations [5]. In recent decades, various methods have been reported for the detection of pharmaceutical waste in wastewater including optical methods, immunoassay tests, molecular spectroscopy-based techniques and devices that rely on optical fiber technology [6]. In general, these methods incorporate antibodies, antigens, and enzymes as active components for the detection of specific antibiotics. This Biosensors 2019, 9, 41; doi:10.3390/bios9010041 www.mdpi.com/journal/biosensors Biosensors 2019, 9, x FOR PEER REVIEW 2 of 21 Biosensors 2019, 9, 41 2 of 21 is mainly done by tracking changes in the intensity of the reflected light and/or the fluorescence of conjugated markers that become active upon triggering specific recognition mechanisms [6]. isElectrochemical mainly done bystrategies tracking for changes the detection in the intensity of pharmaceutical of the reflected residues light have and/or been the reported fluorescence to be ofsensitive, conjugated selective, markers robust, that efficient become and active rapid upon altern triggeringatives in wastewater. specific recognition In general, mechanisms these strategies [6]. Electrochemicalrely on the measurement strategies forof thechanges detection in ions of pharmaceuticalor electrons generated residues by have enzymes been reported or antibodies to be sensitive,immobilized selective, on electrodes robust, efficient [7–9]. Over and rapidthe past alternatives few years, in wastewater. there has been In general, increasing these interest strategies in relysimultaneously on the measurement detecting several of changes water in contaminants. ions or electrons This generatedhas been intended by enzymes to reduce or antibodies analysis immobilizedtimes and production on electrodes costs of [7 –biosensors.9]. Over the past few years, there has been increasing interest in simultaneouslyThis chapter detecting is dedicated several to waterdescribe contaminants. both electrochemical This has been biosensors intended ba tosed reduce on enzymes analysis such times as andlaccase production and oxidase/peroxidase, costs of biosensors. and amperometric microfluidic biosensors. Additionally, we provide a set Thisof recommendations chapter is dedicated for the to describedesign and both testing electrochemical of biosensor biosensors prototypes. based on enzymes such as laccase and oxidase/peroxidase, and amperometric microfluidic biosensors. Additionally, we provide a2. set Enzymatic of recommendations Biosensors Applied for the design to Microfluidic and testing Systems of biosensor prototypes. 2. EnzymaticSensing platforms Biosensors are Applied generally to Microfluidiccomposed of Systemsa recognition element and a transducing device as seen in Figure 1 [10–12]. In the particular case of biosensors, the recognition element is a biomolecule Sensing platforms are generally composed of a recognition element and a transducing device as with stimuli-responsive capabilities. The transducing device converts, therefore, a measurable seen in Figure1[ 10–12]. In the particular case of biosensors, the recognition element is a biomolecule biological response or physicochemical change into an electrical signal [11,12]. Among the most with stimuli-responsive capabilities. The transducing device converts, therefore, a measurable common recognition molecules are: enzymes, antibodies, phages, or single-stranded DNA [10,11,13]. biological response or physicochemical change into an electrical signal [11,12]. Among the most The recognition element in affinity biosensors binds permanently or semi-permanently with the common recognition molecules are: enzymes, antibodies, phages, or single-stranded DNA [10,11,13]. analyte of interest, while for catalytic biosensors, the element maintains a non-permanent interaction The recognition element in affinity biosensors binds permanently or semi-permanently with the analyte in the form of a chemical reaction or electron transfer [10]. of interest, while for catalytic biosensors, the element maintains a non-permanent interaction in the form of a chemical reaction or electron transfer [10]. Figure 1. Schematic of a typical biosensor system architecture. The target analyte is detected by theFigure bioreceptor 1. Schematic and of translated a typical into biosensor a signal system for analysis architec withture. theThe aid target of aanalyte transducer. is detected (A) Sample. by the (bioreceptorB) Biosensor and electrode: translated composed into a signal by the fo bioreceptor,r analysis thewith immobilization the aid of a transducer. surface and (A the) Sample. transducer (B) element.Biosensor (C electrode:) Physicochemical composed reaction. by the (Dbiorecepto) Data analysis.r, the immobilization surface and the transducer element. (C) Physicochemical reaction. (D) Data analysis. Enzyme-based sensors are highly specific catalytic biosensors where the recognition elements are extremelyEnzyme-based selective sensors enzyme are highly molecules specific immobilizedcatalytic biosensors on transducing where the recognition surfaces known elements as electrodesare extremely[10, 14selective,15]. Under enzyme the presencemolecules of immobilized a suitable substrate, on transducing the enzymes surfaces catalyze known electrochemical as electrodes reactions[10,14,15]. involving Under the electroactive presence of productsa suitable orsubstr measurableate, the enzymes electric changes catalyze on electrochemical the transducer reactions and the sampleinvolving [15 electroactive,16]. The concentration products or or measurable redox potential electric of an changes analyte on in athe sample transducer can be and determined the sample via measurements[15,16]. The concentration of potential, or charge, redox or potential current. of The an devices analyte to in conduct a sample these can measurements be determined apply via potentiometric,measurements of impedimetric potential, charge, and amperometric or current. The techniques, devices respectivelyto conduct [these14,16 ,measurements17]. apply potentiometric,The amperometric impedimetric methods and are amperometr the most suitableic techniques, to perform respectively the transduction [14,16,17]. of the enzymatic responseThe amperometric into a quantifiable methods signal are [ 14the]. most In this suitable case, theto perform number the of speciestransduction involved of the in enzymatic the redox processresponse is into related