Paper-Based Enzymatic Electrochemical Sensors for Glucose Determination: History
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Contributor:
Olaya Amor-Gutiérrez
,
Estefanía Costa-Rama
,
M. Teresa Fernández-Abedul

The general objective of Analytical Chemistry is to obtain best-quality information in the shortest time to contribute to the resolution of real problems. In this regard, electrochemical biosensors are interesting alternatives to conventional methods thanks to their great characteristics, both those intrinsically analytical (precision, sensitivity, selectivity, etc.) and those more related to productivity (simplicity, low costs, and fast response, among others). The scientific community has made continuous progress in improving glucose biosensors, being this analyte the most important in the biosensor market, due to the large amount of people who suffer from diabetes mellitus. The sensitivity of the electrochemical techniques combined with the selectivity of the enzymatic methodologies have positioned electrochemical enzymatic sensors as the first option.

  • paper-based analytical devices
  • electrochemical biosensors
  • µPADs
  • enzymatic sensors
  • glucose
  • enzymes

1. Introduction

Glucose is one of the most important biological compounds in nature. It is a monosaccharide responsible for the generation of most of the energy required for growth and reproduction, due to its important role in the pathway of aerobic and anaerobic respiration in living organisms. In plants and bacteria, it is usually produced from water and carbon dioxide through photosynthesis, and it is condensed forming starch. Glucose can be stored as an energy reserve or used as a substrate for the synthesis of a great range of saccharides, such as sucrose or cellulose. In the food industry, it is used to enhance texture and flavor [1]. However, the role of glucose in society cannot be explained without mentioning the disease diabetes mellitus, a disorder derived from high glucose concentration in blood due to different reasons: abnormal insulin secretion, resistance to insulin, or both [2]. Due to these and its strong relationship with obesity problems, glucose monitoring is an important global issue in health control and food safety [3][4].
In fact, glucose was the target of the first enzymatic biosensor, developed by Clark and Lyons in 1962 [5]. After that, the first commercial blood glucose analyzer was released by Yellow Springs Instruments (YSI) in 1975 and the first glucose meter by Medisense years later, in 1987. Since then, the interest in developing glucose biosensors and their impact in the biosensor market have grown enormously (glucose sensors or glucose meters represent an 85% share of the biosensor market) [6] and continue to rise, being one of the point-of-care devices par excellence in the biosensor world [7]. This increasing interest in achieving even smaller and cheaper glucose sensors, keeping or even improving their characteristics, such as long-term stability, specificity and biocompatibility, among others, promotes the implementation of the latest sensing trends and technologies in their development (e.g., 3D printing or wearable approaches) [8][9][10].
The most popular glucose biosensors use enzymes as biorecognition elements, and electrochemical techniques for detection. Although, in most of the cases, more than one enzyme is needed, this strategy is very simple and highly specific and sensitive (because of the enzymatic amplification produced). Regarding transduction, amperometry is the electrochemical technique most widely used in glucose electrochemical biosensors and it is characterized by its small energy consumption, ease of use and simplicity (just applying a constant potential for a time is required). [11]. In terms of material, one of the strategies that has been used to develop simpler and cheaper glucose sensors is the use of paper, with a dramatic increase in the last ten years. This is due to its numerous advantages, apart from low cost: its presentation in light sheets, which makes easy its transportation and storage; its foldability that allows multilayer/origami designs; its porosity that enables bio/nanomodifications, as well as fluid transportation; and its biodegradability, which converts it into an environmentally friendly material.

2. Paper as a Substrate for Biosensing

Paper is a widespread material, essential for communication and for documenting our past. Therefore, paper fabrication is one of the greatest landmarks in human history, whose origin is traced back to China in the 2nd century AD. In addition to this, paper has been employed as substrate for analytical tests for centuries [12]. The first paper-based chemical sensor was probably the litmus paper for pH measurement, assumed to be derived from the essays, dated to 1664, of Robert Boyle, which related the color change of flower and vegetable dyes with the acidity of solutions [13]. However, the pH test strips were not patented and commercialized until the 1920s [14]. Before this, in the 1860s, the first urine test was reported for monitoring glucose levels of diabetes patients [15]. After these early reports, since 1940s, paper has become increasingly used in Analytical Chemistry for chromatography applications [16][17]. In 1956, the first latex agglutination was developed by Plotz and Singer, which is the basis of the lateral flow immunoassay (LFA) for the diagnosis of rheumatoid arthritis [18]. From that date, the basic principles of lateral flow technology were refined and, in the latter years of the 1980s, companies such as Unilever and Carter Wallace filed several important patents [19][20]. Currently, the simplicity of LFAs has increased enormously the interest in their use, as demonstrated by the well-known pregnancy test and the now widely used COVID-19 diagnosis test. Despite being a material employed for analytical tests long ago, paper has been rediscovered in recent years as substrate material for developing analytical devices since, in 2007, Whitesides et al. introduced the concept of Microfluidic Paper-based Analytical Devices (µPADs) [21]. In that work, paper was patterned by photolithography, defining hydrophilic flow channels to perform multiplexed assays. Although it was not strictly a new idea, since, in 1937, Yagoda reported the first filter paper with a hydrophobic pattern, using paraffin to delimit the detection area [22], and, in 1949, Clegg and Müller also used paraffin to design a fluidic channel in filter paper with chromatographic purposes [23], it was Whitesides’ work that opened up a large new area in Analytical Chemistry. From that moment, the development of paper-based devices has experienced a huge growth [14][15][24][25][26]. The intrinsic properties of paper (e.g., biocompatibility, porous nature, flatness, and presence of capillary forces) make new advances possible, not only in clinical and diagnostic fields, but also in food and environmental applications. Apart from these features, the success of paper-based analytical devices is mainly associated with their low cost, simplicity of fabrication and operation, portability, and disposability. The great versatility, demonstrated in lateral and vertical flow devices, single or multi-layered devices, which can be combined with other flat substrates (e.g., tape), together with the possibilities of being cut, pressed, folded, or printed or serving as excellent storage for biocomponents, conductive elements or nanomaterials, makes this material extremely appropriate for the decentralization of analysis.

3. Enzymes

Enzymes are biological catalysts able to accelerate any chemical reaction, not being consumed in the process nor being part of the final products. Enzymes decrease the activation energy of reactions, showing a great specificity, in contrast to what happens with inorganic catalysts. Enzymes only act on one specific substance (or group of substances), also called the substrate. Their high specificity makes them capable of differentiating even between different stereoisomers of one compound; for instance, between D- or L-glucose [27].

3.1. Types

Their name is usually built by adding “-ase” to the name of the substrate whose reaction they catalyze. They can also be named by defining the type of reaction, being this principle the most logical one for the classification of enzymes. In order to unify all the types of enzymes, the International Union of Biochemistry and Molecular Biology published a general classification in which six main groups (European Comission, EC numbers) were distinguished [28]: oxidoreductases (EC1), which catalyze redox reactions, and include oxidases, dehydrogenases, peroxidases and oxygenases; transferases (EC2), which catalyze the transfer of functional groups (amino, carboxyl, methyl, etc); hydrolases (EC3), which catalyze the cleavage of bonds such as C–O, C–S, C–N or O–P; lyases (EC4), catalyzing the cleavage of bonds such as C–C, C–S or C–N, excluding peptide bonds; isomerases (EC5), that interconvert any type of isomer, both optical, geometric or positional; and ligases (EC6), which catalyze the binding of two different molecules, performing hydrolysis of a high-energy bond of nucleoside triphosphate, forming bonds such as C–C, C–S, C–O or C–N [29]. In 2018, a seventh group of enzymes was discovered, known as translocases, which catalyze transfers from trans- to cis-conformation, or vice versa [29]. This type has also been added to the general classification.
Other types of catalysts that, in recent years, have attracted great attention are pseudoenzymes, which are proteins with a sequence homologous to that of enzymes, but without enzymatic activity, due to mutations in catalytic amino acid residues. Although they do not show catalytic activity, they can be found in many metabolic and signaling reactions combined with other enzymes, by different mechanisms. Regulating catalytic yields of conventional enzymes or binding substrates to assure the reaction between them and the enzymes are some examples. The most well-known are pseudokinases, although other such as pseudophosphatases, pseudolyases or pseudoproteases can be found [30].
Natural enzymes are widely used in many fields, but, sometimes, they present low stability, poor biological variety, and high costs. Due to this, a great effort to find new artificial reagents which mimic the activity of enzymes has been made. An example is nanozymes, which are usually made of nanomaterials with catalytic properties, and combine the advantages of typical chemical catalysts and biocatalysts.

3.2. Immobilization Procedures

A critical step for designing enzymatic electrochemical biosensors is the immobilization of the enzymes on the surface of the transducer because the chosen immobilization method depends on the required characteristics of the sensor, not only the analytical ones (e.g., sensitivity, precision, stability), but also others such as simplicity and the final cost of the device. Different immobilization methods used to develop enzymatic biosensors [31][32][33] are commented on in the following sections.

3.2.1. Adsorption

Adsorption is the simplest method of immobilization. It is generally carried out by dissolving the enzymes in a buffer solution and putting it in contact with the solid support for a period of time. The bonds formed in this immobilization approach are weak, such as Van der Waals’ forces, and electrostatic or hydrophobic interactions. The main drawback of this strategy is that any change in the conditions of the medium, such as pH, temperature, or ionic strength, can produce enzyme desorption, affecting the stability of the biosensor. Adsorption can be carried out in three different ways: (i) physical adsorption, when the enzyme solution is directly deposited on the surface of the electrode; (ii) electrostatic interactions, when the enzymes are immobilized onto a charged surface; and (iii) retention in a lipidic microenvironment, using Langmuir–Blodgett (LB) technology, with accurately controlled thickness and molecular organization, where enzymes can be easily adsorbed [34].

3.2.2. Entrapment

Immobilization of enzymes can be carried out inside a three-dimensional matrix, by an easy methodology in which enzymes, mediators and additives can be simultaneously deposited on the surface of the electrode. The activity of the enzymes is preserved since there is no modification of the biological elements. The entrapment can be carried out by different methods, but the most used is electrochemical polymerization. It consists of applying an appropriate current or potential on an electrode with a mixture of the enzyme and monomer, in a way that the enzyme is physically incorporated within the polymer network. The most widely used polymers are conducting polymers, such as polyaniline, polypyrrole or polythiophene. Enzymes can also be entrapped in polysaccharide-based gels, carbon paste or sol-gel matrixes, forming solid metal or semimetal oxides by using hydrolytically labile precursors, such as polysilicic acid, halides of aluminium or polysilicic acid.

3.2.3. Cross-Linking

Another useful approach for the immobilization of enzymes is cross-linking, using reagents such as glutaraldehyde or other bifunctional agents, such as glyoxal or hexamethylenediamine. It is a very simple method which enables a strong chemical binding. Other cross-linking agents, such as functionally inert proteins (e.g., bovine serum albumin) can also be used.

3.2.4. Embedding and Encapsulation

The encapsulation of enzymes in liposomes is another approach used in enzymatic sensors. This strategy allows the stabilization of the enzyme, maintaining its activity for longer times and, consequently, improving the performance and the stability of the sensor [35][36]. Enzymes can also be embedded in metal–organic frameworks (MOFs), which have recently gained a lot of popularity as porous hybrid materials, because of their great stability, low density, crystallographic structure and high pore volume. Apart from that, it has shown to have great potential as an enzyme support due to its high surface area and the strong interactions between the organic part and the enzymes, preventing them from leaking. The design of MOFs for immobilizing enzymes can follow two different strategies, depending on the need of having co-precipitation agents. On the one hand, the co-precipitation method requires stabilizers to ensure that the enzyme is in its active form during the preparation. In this case, the enzyme is encapsulated with a co-precipitating agent (such as polyvinyl pyrrolidone, PVP). On the other hand, biomimetic mineralization can be carried out in the absence of co-precipitating agents to form enzyme–MOF biocomposites [37].

3.2.5. Affinity Reactions

Oriented immobilization of enzymes can be carried out creating affinity bonds between activated supports and specific groups of the peptide sequence of the enzyme. A great amount of affinity approaches has been described to immobilize enzymes, being avidin–biotin, lectin–carbohydrates or metal cation–chelator interactions the most used. The enzyme can contain affinity tags naturally within its sequence, or it can be inserted prior using chemical or genetic engineering methods.

3.2.6. Covalent Immobilization

The covalent coupling between enzymes and polymeric substrates is very popular in the development of enzymatic biosensors. It consists of the binding of the enzymes to the surface by means of functional groups which are not involved in the catalytic activity. In order to perform a covalent immobilization, a previous activation of the surface is usually carried out using multifunctional reagents, such as glutaraldehyde or carbodiimide. The enzyme reacts with the activated support, binding covalently, either to the transducer surface or onto an activated membrane made of synthetic polymers, such as Nylon®, or natural matrixes, such as cellulose.

3.3. Enzymatic Sensors

Among the different possibilities within enzymatic assays (for saccharide determination), different “generations” of enzymatic sensors have been described, depending on how enzymes are used [38].
  • First generation: their principle is based on the use of oxygen as a co-substrate, producing H2O2 as the enzymatic reaction product, which is measured. However, the electrochemical monitoring of H2O2 requires high operating potentials, which can affect the selectivity, due to the high probability of interferences, i.e., electroactive species that can be reduced at those high potentials.
  • Second generation: they overcome the limitation coming from the high potential needed for the determination of the H2O2 by using redox mediators as co-substrates, which ensure the efficient electron transfer at lower potentials and are regenerated on the surface of the electrode. Mediators are artificial electron transfer agents which can participate in the reaction with the enzyme and increase the electron transfer rate. An ideal mediator should: (i) be able to react quickly with the enzyme; (ii) have reversible kinetics; (iii) be pH independent; (iv) have stable reduced and oxidized forms and (v) do not react with oxygen in any of its forms [39]. Different electron mediators have been used, being ferrocene, ferrocyanide, cobalt phtalocyanine and methylene blue among the most common.
  • Third generation: they do not need mediators since they are based on the direct transfer between the enzyme and the surface of the electrodes at very low operating potentials, achieving high selectivity.
  • Fourth generation: they are enzyme-free sensors in which the glucose is directly oxidized on the electrode surface [40]. Although numerous enzyme-free sensors for glucose have been reported, mainly using electrocatalytic nanostructures based on transition elements, the performance of enzymatic glucose sensors is still better in terms of sensitivity and biocompatibility.
Among all the enzymes that can be used for glucose determination, the most common is glucose oxidase (GOx), followed by glucose dehydrogenase (GDH) [41][42], both belonging to the EC1 (oxidoreductases) group. GOx is a dimeric enzyme with flavin adenine dinucleotide (FAD) as its redox prosthetic group. GOX oxidizes glucose to gluconic acid using oxygen as the electron acceptor and producing H2O2 [43]. This reaction involves the reduction of the flavin group to give the reduced form of the enzyme (FADH2). GDH is an oxidoreductase enzyme which also oxidizes glucose into gluconic acid, but it is unable to use oxygen as an electron acceptor; therefore, it requires the cofactors (e.g., nicotine adenine dinucleotide (NAD), nicotine adenine dinucleotide phosphate (NADP) or pyrroloquinoline quinone (PQQ)) [41][42]. Since the potential needed to detect the products of these enzymatic reactions is high (e.g., +0.6 V for H2O2), it is common to use auxiliary reactions forming a cascade of enzymatic reactions that enhance the sensitivity of the sensor. The most commonly used enzymes for this purpose are peroxidases which catalyze the decomposition of H2O2. Moreover, the use of redox mediators, such as ferrocyanide [44], Prussian Blue or cobalt phthalocyanine [45], is also common to decrease even more the overpotential required, with the aim of minimizing the oxidation of endogenous species [42].

4. Transduction Step

Colorimetric detection is very appropriate for white paper-based analytical devices. However, this type of detection is not usually sensitive enough, and it depends on many factors, such as, for example, the environmental light. By contrast, electrochemical detection is an alternative that provides higher sensitivity and does not depend on illumination or sample color. In addition to this, electrochemical techniques show several advantages for µPADs: (i) electrodes can be easily miniaturized and manufactured, (ii) the instrumentation needed is not very complex nor expensive, (iii) portable potentiostats are nowadays commercially available, and (iv) electrochemical detection can be applied in many different fields, such as food, medical or environmental [46]. The excellent integration between paper as substrate and electrochemistry for detection has stimulated the development of electrochemical paper-based analytical devices (ePADs), with a huge growth in the last 10 years.
In this context, the first ePAD, reported by Henry’s group, used photolithography to design hydrophobic channels and screen printing to fabricate the electrodes [47]. In the development of ePADs, apart from the design and the patterning, electrode fabrication is a critical step, since the electron transfer is an interfacial phenomenon, and the electrode surface determines the analytical characteristics of the device. An electrochemical transducer usually consists of a cell with two electrodes (reference and indicator, mainly used for potentiometric measurements) or three electrodes (reference (RE), counter (CE) and working (WE) electrodes, mainly for voltammetric techniques). The use of paper, with extremely interesting properties, allows many design possibilities.
The electrochemical techniques that are used for detection in electrochemical biosensors include a wide range of measurement possibilities [48]. Thus, electrodic techniques, those based on the measurement of electrical properties at the electrolyte–electrode interface, allow either static (current-zero measurements as in potentiometry) or dynamic (with current flowing through the system as in amperometry) approaches. The most common are amperometric techniques, where the current is measured at a given potential (named amperometry or chronoamperometry, depending on the mass transport regime) or during a potential scan (in this case, they are known as voltametric techniques). In the last case, a linear scan can be made (cyclic voltammetry or linear sweep voltammetry) or pulses can be superimposed, as in the two most common modes: differential pulse voltammetry (DPV) and square wave voltammetry (SWV). The current is sampled before and after the pulse in DPV or before the end of forward/backward pulses in SWV, which decreases the capacitive current, and, in turn, increases the sensitivity (proportional to the if/ic ratio). In those cases, the measurement is made under a diffusional mass transport regime, but convection is possible (hydrodynamic voltammetry). On the other hand, amperometry can be performed under diffusional or convective regimes. In the first case, chronoamperograms (i-t curves) are recorded and the current is usually measured at the stationary and more precise part of the curve (e.g., 20 s), where capacitive current is almost negligible. Injection analysis either under flow (FIA) or static systems (BIA, batch injection analysis) produces transitory signals, i.e., fiagrams or biagrams. The measurement of the current requires the use of a potentiostat to apply the excitation signal to the working electrode (usually included in a potentiostatic system of three electrodes, together with RE and CE) and to record the response signal (i-E or i-t curve). Regarding potentiometric techniques, as mentioned before, they require a two-electrode system to measure the difference of potential with a potentiometer.
The transducer and its design are key components of an electrochemical biosensor. In this section, different designs of transducers are briefly summarized, distinguishing between 2D and 3D paper-based devices, and describing different approaches to include or integrate electrodes on paper. Most of these approaches take advantage of the porous nature of paper, modifying it with a conductive material, usually in the form of ink (made of a carbon or metallic material), through different techniques such as screen/stencil printing, inkjet printing, drawing, or sputtering. Other strategies combining metallic (micro)wires and paper have also been reported [44][49][50][51]. The incorporation of nanomaterials for improving the performance of ePADs is a very common strategy since these materials can enlarge the electroactive surface and enhance the electron transfer and the immobilization of biomolecules. The modification of paper-based carbon electrodes with nanomaterials such as nanoparticles, including metallic (e.g., gold, silver, platinum) and metallic oxide (e.g., zinc or nickel oxide), to improve their electrochemical characteristics has been widely reported. Carbon nanomaterials are usually used as transduction materials on ePADs because of their excellent conductivity, but also due to their high surface area, which boosts the anchoring of biomolecules. The high porosity and tunable structures of MOFs have made them attractive for developing ePADs. An example is the origami ePAD based on a cobalt metal–organic framework of modified carbon cloth/paper developed by Wei et al., for non-enzymatic glucose determination, in which the Co–MOFs act as an oxidase-mimicking nanozyme [52]. Several interesting articles have reviewed the different reported strategies to fabricate electrodes on paper [53][54][55] and the main usages of nanomaterials on ePADs [56][57][58].

This entry is adapted from the peer-reviewed paper 10.3390/s22166232

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