Microelectrode arrays-based double electrode systems possess the property of diffusion layer overlapping between the two sets of electrodes. They have received increased attention in recent years in the field of electrochemistry and electroanalytical chemistry. This review introduces the fabrication, characterization, and applications of microelectrode arrays-based double electrode systems. The progress of photolithography enables and benefits microfabrication of the electrode systems with various geometries. As an essential step before electrochemical applications, various characterization methods are addressed to monitor the quality of the fabricated electrodes. Following that, applications as electrochemical sensors in generator-collector mode are discussed. Then, electrochemical sensors in bipolar mode, an interesting phenomenon existing in double electrode systems, are also introduced. Finally, applications of double electrode systems to the measurement of fundamentally electrochemical parameters are demonstrated.
Microelectrodes are important because they possess various advantages arising from small dimensions. Small charging current and reduced iR drop make them suitable for fast voltammetry (Wightman 1981, Wightman et al. 1988, Wipf and Wightman 1989), which is very important in measuring kinetic parameters and capturing reaction intermediates or species with short lifetimes (Basha and Rajendran 2006). And they are ideal candidates for monitoring neurotransmitter fluctuation in vivo (Cahill et al. 1996, Suzuki et al. 2007). Another attractive characteristic for microelectrodes is the steady-state electrochemical response at moderate scan rate because of radial diffusion (Wightman 1981). This point benefits analytical measurement greatly. But the current of single microelectrode is too low, which goes against highly sensitive detection, and it requires higher instrumental equipment to separate signal from noise.
It has been proven that as collection of hundreds or even thousands of microelectrodes, microelectrode arrays (MEAs) wired in parallel are much advantageous and convenient for many practical purposes (Chidsey et al. 1986). Compton and coworkers give detailed descriptions on the electrochemical behaviors of MEAs at different timescales versus electrode geometries (Davies and Compton 2005, Davies et al. 2005), as depicted in Figure 1: (i) When the scan rate is fast, i.e. the timescale is short, the diffusion layer of each microelectrode is independent and presents linear diffusion (Figure 1A), the overall current is the sum of currents from each unit in the array; (ii) when the timescale is moderate, radial diffusion dominates each microelectrode (Figure 1B), and thus, MEAs retain the steady-state response of microelectrodes with additive voltammetric signal, which improves detection sensitivity by orders of magnitude; (iii) the third stage refers to partial overlapping between diffusion layers of microelectrodes (Figure 1C), which is usually avoided; and (iv) When the scan rate is too slow, the diffusion layers of microelectrodes overlap heavily (Figure 1D), planar diffusion over the whole surface makes the voltammetric response of MEAs similar to the macroelectrode with the same electrode area, but with lower background charging current. Among the above cases, the second is the most favorable and has been intensively used in the electroanalytical field (Seddon et al. 1994, Feeney and Kounaves 2000, Lowinsohn et al. 2006, Ordeig et al. 2006a, Xu et al. 2008, Huang et al. 2010, Kokkinos et al. 2011, 2012, Moraes et al. 2012, Wang et al. 2013, 2014, Ben-Amor et al. 2014, Wu et al. 2014).
Coupling with double electrode systems enriches the configuration and function of MEAs further and makes them more powerful. Double electrode systems have been studied for a long time and have been systematically reviewed elsewhere (Barnes et al. 2012). The notable features for double electrode systems are the existence of two sets of electrodes and the potential could be controlled alternatively, They have been successfully used to control local microenvironment by producing or depleting some species based on the fact of diffusion layer overlapping (Wang et al. 2005a,b, ChenMing et al. 2007, Jia et al. 2007, Hasnat et al. 2014, Read et al. 2014). Generator-collector working mode is most classical and popular for double electrode systems. When one or two sets of electrodes in double electrode systems are MEAs, the systems gain intensive concern because they retain the advantages of MEAs, and they provide scope for operation versatility. Here, double electrode systems with MEAs refer to dual electrode systems and at least one of them is the form of MEAs, while the other can be the form of MEAs or just a macroelectrode.
Generally speaking, for double electrode systems with MEAs, it should be ensured that diffusion layers of two sets of electrodes overlap each other considering the electrode geometries and experimental parameters (Tomčík 2013). The overlapping of diffusion layers makes the substance interactive between the two electrodes. The thinner diffusion layer of double electrode systems with MEAs evidences that it is more suitable than the MEAs for achieving higher electrode density without losing the steady-state response of microelectrodes. In addition, MEAs-based double electrode systems have a more rapid steady-state response than MEAs do. Moreover, the current at collector electrode held at a constant potential is not influenced by charging current during potential change, and it allows faster sweep of cyclic voltammetry (CV) and will not be distorted by the charging current. There are different configurations of double electrode systems with MEAs reported in the literature.
Interdigitated array electrodes (IDAs) originate from dual-band or triple-band electrodes and are advantageous in the aspect of sensitivity (Aoki et al. 1990), they are usually fabricated by microfabrication technique. To date, IDAs have been intensively studied and applied in various fields. Planar IDAs have two sets of comb-like microband electrodes (Sanderson and Anderson 1985), the diffusion layer is open, and key parameters influencing IDA performance include gap width (Wg) and electrode width (We). These are determined by the resolution of the microfabrication technique, which is generally on the scale of μm. The Zaretsky convention defines the meander length, M=N·l, where N is the number of digit pairs and l is the digit length of the array. The serpentine length S=2M (Zaretsky et al. 1988). Vertical IDA is advantageous to planar IDA in smaller electrode gaps (Niwa et al. 1989, 1991) because gap width is determined by the thickness of the insulating material and could be below 1 μm. Equally important, vertical IDA is a semi-open structure, which is beneficial for diffusion layer interaction between two sets of electrodes. Moreover, the chip size of the vertical IDA could be reduced relative to planar structure (Niwa et al. 1989).
Another preferable structure refers to ring-recessed microdisk electrode arrays first proposed by Compton (Menshykau et al. 2009); the recessed depth is the thickness of the insulating layer, and the recessed microdisk electrode is completely surrounded by micro-ring electrode. In this case, less generated species diffuse away to the bulk solution due to the configuration. An alternative is plane-recessed microdisk electrode arrays, with the plane electrode occupying the whole plane except for the recessed section (Horiuchi et al. 1990, Menshykau et al. 2010), so the plane electrode is not the form of array, which simplifies the fabrication process and provides high collection efficiency. More importantly, the progress of the microfabrication technique allows the fabrication of nanoscale dual electrode array systems (Hayashi et al. 2008, Wolfrum et al. 2008, Goluch et al. 2009), and their applications bring higher sensitivity and selectivity. In the following, we will introduce various types of MEAs-based double electrode systems, which present the features of both the MEAs and dual electrode systems.
The approaches for fabricating MEAs are abundant and have been reported intensively (Chidsey et al. 1986, Fiaccabrino and Koudelka-Hep 1998, Feeney and Kounaves 2000, Ordeig et al. 2007, Huang et al. 2009). For single MEAs, some simple physical construction methods are feasible (Penner and Martin 1987, Odell and Bowyer 1990, Seddon et al. 1994), including wired method, sandwich method, formation of composition on bulk electrode, etc., but for MEAs-based double electrode systems, the lithographic-based technique is the only available method to define various electrode parameters and photolithography is the most widely used form. Photolithography refers mainly to the process of film deposition and patterning through various techniques on substrate (Fiaccabrino and Koudelka-Hep 1998). Generally, film deposition can be realized by thermal oxidation, sputtering, and spin-coating, whereas patterning processes are fulfilled by pattern transfer from photoresist to substrate and subsequent wet or dry etching.
The fabrication of non-planar structured electrode is more complicated than that of planar structured electrode systems (Huang et al. 2010). For non-planar double electrode systems with MEAs, the fabrication processes require alignment between two layers of electrodes. And it has higher requirement for insulating ability of insulating material between them.
The following is an overview for fabrication processes of non-planar double electrode systems with MEAs, as illustrated in Figure 2.
After a standard cleaning step, the silicon wafer is thermally oxidized to form silicon oxide dielectric film for preventing current leakage. Then, adhesive layer and electrode material are sputtered or evaporated onto the wafer (Figure 2A). Here, the adhesive layer promotes the adhesion force between the substrate and the metal electrode layer; it usually adopts Ti or Cr. The metallized wafer is subject to the spin-coating of positive or negative photoresist and patterned using the first mask (Figure 2B), and wet etching is followed to etch the exposed region by placing the wafer to etchant of electrode material and adhesive layer, respectively (Figure 2C). When each etching procedure is complete, it is necessary to wash the wafer by a lot of water. These steps define the region of the first layer electrode that would be the bottom electrode and the contact pad and are noted as etching process. An alternative way to produce the bottom electrode is to pattern the substrate first and then deposit the adhesive layer and electrode material, which could be called “lift-off” process. To some extent, the lift-off process has better resolution than the etching process does because the latter is restricted to possible chemical reaction nonuniformity over the whole wafer and effective judgment of end point.
Next, a layer of insulating material is deposited by plasma-enhanced chemical vapor deposition (inorganic material, like SiOx, SixNy, or the combination of the two) or spin-coating (organic material, like polyimide and parylene) onto the wafer (Figure 2D); the thickness of the insulating layer determines the gap between two sets of electrodes. The second mask is subsequently used to pattern the top electrode (correspondingly, the insulating layer). For the top electrode, both the lift-off procedure (Figure 2E–F) and the etching procedure as described above could be used like the bottom electrode. The exposed insulating layer is dry etched using the patterned electrode material as mask (Figure 2G). In this case, dual structured MEAs are obtained. Figure 3A is a digital photo of the real device using the above fabrication processes; the electrode region consists of two layers of electrode insulated by polyimide insulator. The bottom electrode is the form of MEAs and the upper electrode is a macro-electrode occupying the whole plane except the recessed holes.
Recently, progress in the ability to control matter in nanoscale has promoted the fabrication of nanoelectrode arrays (Wolfrum et al. 2008, Goluch et al. 2009, Kang et al. 2012, Ma et al. 2013, Huske et al. 2014), which aids in the observation of new information and gives opportunity for higher performance and integration with nanofluidics or other miniaturized devices.
Characterization is a necessity before the use of MEAs-based double electrode systems. Various methods have been proposed to monitor the quality and status of the electrode.
Physical characterization is the first choice to check the size and morphology of a fabricated electrode system. Conventional tools include optical microscopy and scanning electron microscopy (Nagale and Fritsch 1998, Buss et al. 1999, Aguiar et al. 2007), and confocal microscopy can provide more information than normal optical microscopy. Figure 3B gives an optical view of the electrode region in Figure 3A, where recessed microelectrodes are arranged in a square configuration, and Figure 3C is a display of confocal image, where the three-dimensional profile presents straightforward information about recessed electrode systems and depth information could be further analyzed from the profile. Atomic force microscopy (AFM) aids in providing more detailed information about the surface condition of electrodes (Feeney et al. 1998, Nagale and Fritsch 1998), which is very important for surface-sensitive electrochemical reactions. Figure 4 displays the contour of plane-recessed microdisk electrode arrays (Figure 4A) and the morphology of upper Au (Figure 4B). From Figure 4B, uniform particle composition could be observed. Moreover, profilometer is much useful in probing recess depth related with recessed microelectrodes (Menshykau et al. 2009, Huang et al. 2010). Besides, electrochemical characterization is a necessary procedure to evaluate electrode validity because there is a possibility of failure during complicated fabrication processes and after usage. Electrochemical signal of probe molecules can provide plentiful information about the performance of double electrode systems. They will be given in the following section in terms of fabrication efficiency and electrochemical performance, respectively.
Due to imperfect fabrication processes, there exist a few electrochemically inactive electrodes, which mainly result from insulating layer residues or disconnected pad line. In order to estimate fabrication efficiency, a series of procedures have been proposed, where traditional electrochemical techniques form the basis, such as CV and electrochemical impedance spectroscopy. These give overall electrochemical responses of the whole array; therefore, detailed information about fabrication efficiency cannot be drawn from the data of electrochemical techniques alone. A solution is to combine traditional electrochemical techniques with optical microscopy, which gives the opportunity to count the number of inactive microelectrodes. Modifications of related materials onto microelectrode unit such as copper (Ordeig et al. 2006a,b), gold nanoparticles, or gold nanoparticles conjugated functional molecules (Orozco et al. 2007) are usually adopted to facilitate the observation, this kind of method is not suitable when the number of microelectrode is large, to several thousands. Another effective way is to combine electrochemical techniques with theoretical formula calculation or simulation process using the designed ideal electrode geometries; the difference between experimental and theoretical results reflects the fraction of electrochemically inactive electrodes (Ordeig et al. 2006b, Aguiar et al. 2008). This method is popular and has been widely applied to verify obtained results.
Meanwhile, as a tool possessing the ability of spatial resolution, scanning electrochemical microscopy (SECM) is effective in observing localized electrochemical signal and gives information about individual microelectrode (Köster et al. 2001, Zoski et al. 2004). It has been explored to check track connections (Zoski et al. 2004) and utilized to reveal failure mechanism (Köster et al. 2001).
On the basis of effective fabrication processes, it is an important issue to address the electrochemical performance of MEAs-based double electrode systems from the aspects of collection efficiency and redox cycling using probe molecules (usually ferrocene, aq-ferrocene, ferrocyanide, and Ru(NH3)6Cl3)). High values of collection efficiency and redox cycling are preferred (Sanderson and Anderson 1985, Bard et al. 1986, Niwa et al. 1990, Huske et al. 2014).
The two parameters could be obtained through experiment of generator-collector dual working mode. The detailed process is illustrated in Figure 5A and is described as follows: one segment of the electrodes is biased to oxidize or reduce original electroactive species (generator), while the other is controlled to convert generated molecules to original state, which is similar to the phenomenon in SECM feedback mode (Kwak and Bard 1989). Here, the assurance of the diffusion layer overlapping between the two sets of electrodes makes two important contributions (taking oxidant as initial species). First, species reduced at generator electrode diffuse to the collector electrode, and the collector electrode gives oxidative current response relative to diffusive reduced species; thus, collection efficiency could be deduced by comparing the steady-state current at the generator and collector electrode (Bard et al. 1986), as illustrated in Eq. (1).
A higher value of collection efficiency indicates better situation of diffusion layer overlapping. And a value of 1.0 means that all reduced species are collected at the adjacent collector electrode and cannot diffuse away. Second, oxidized species regenerated at the collector electrode could diffuse back to the generator, leading to steeper concentration gradient and producing amplified current compared with that working in single mode (when collector electrode is left floating). Amplification factor or redox number is used to give quantitative information about reversible or quasi-reversible redox species shuttling between two sets of electrodes. Figure 5B shows the CVs of plane-recessed MEAs in the single mode and generator-collector mode. It is easy to estimate the performance of double electrode systems through comparison of CVs in dual mode and in single mode. High collection efficiency is a guarantee for high degree of redox cycling. The common way for defining redox cycling amplification factor is illustrated by dividing generator current in the dual mode by that working in the single mode of the same device [Eq. (2)]. The formula is applicable to most systems but should be used with caution for confined systems, such as lab-on-chip device, because the current in single mode will decay with reaction time in that case, and the influence of electrochemical reaction on bulk concentration cannot be ignored. On other occasions, if the current responses at generator electrode in the single mode do not achieve steady-state, the comparison between two working modes could be ambiguous and problematic. Nevertheless, it is convenient to estimate device performance in most common cases.
Niwa et al. showed that the collection efficiency and redox cycling of redox species at IDA depend strongly on average diffusion length, expressed as We /4+Wg . With decreasing average diffusion length, the collection efficiency approaches unity and amplification factor reaches more than 40, while experiments using probe molecules with different diffusion coefficients (aq-ferrocene, ferrocene in organic solvent, ferrocyanide, and Ru(NH3)6Cl3) revealed that collection efficiency and redox cycling are almost irrelevant with diffusion coefficient (Niwa et al. 1990). The above conclusions demonstrate that electrode configuration plays a decisive role in the electrochemical performance of MEAs. For other double electrode systems with MEAs, similar effects of electrode diameter and gap between two electrodes on electrochemical behaviors have also been revealed. These findings inspire the exploit of abundant electrode systems, which are advocated by optimization of configuration design (Niwa et al. 1989, Dam et al. 2007) or adoption of novel or advanced fabrication technique (Goluch et al. 2009, Kang et al. 2012, Ma et al. 2013, Huske et al. 2014, Partel et al. 2014).
Double electrode systems with MEAs are very useful tools in electrochemical measurements. For electrochemical sensing, the ability to control potential independently enables highly sensitive and selective detection. Various working principles based on potential control have been proposed to realize selective and sensitive detection for different target analytes. Interestingly, for macro-micro double electrode systems, even when the macro electrode is left floating, i.e. it does not connect with potentiostat, MEAs can still present enhanced current response. In this case, the macro electrode functions as a bipolar electrode. The term bipolar electrode has been introduced in the literature (Chow et al. 2009, Loget and Kuhn 2011). Briefly, electrochemical oxidation and reduction occur simultaneously at different areas of the bipolar electrode. According to the mechanism, it can be categorized as electric-field-induced bipolar electrode or concentration-gradient-induced bipolar electrode. In this review, only concentration-gradient-induced bipolar electrodes are discussed. The bipolar behaviors facilitate electrochemical sensing and provide opportunities for combining redox cycling with other electrochemical techniques or spectroscopic methodology. Besides, the interaction between two sets of electrodes allows the investigation of some other processes, such as determination of diffusion coefficient of redox species and monitoring of electrogenerated intermediates with a short lifetime to reveal reaction mechanism. In this section, we will introduce the applications of MEA-based double electrode systems as electrochemical sensors in generator-collector mode and in bipolar mode and also their applications in fundamentally electrochemical research.
Electrochemical sensors in generator-collector mode
MEAs-based double electrode systems demonstrate powerful merits in electrochemical analysis; they render high sensitivity and selectivity as sensors for biological, medical, and environmental issues. They are attractive when used as electrochemical detectors coupled in flow injection analysis and high-performance liquid chromatography (Aoki et al. 1990). The advantages come from the fact that the potentials of the two electrodes could be controlled independently relative to the same reference electrode using bipotentiostat instrument. With respect to different target systems, various principles have been proposed and utilized to realize selective and sensitive detection of target analytes. Redox cycling aids to signal amplification directly. Based on the process, sensitive determination of many systems are realized, such as Fe in spectral carbon (Bustin et al. 1995), iodide in mineral water (Tomčík and Bustin 2001), p-aminophenol (Kim et al. 2004), and paracetamol (Goluch et al. 2009). For many electrochemical sensors, selectivity is another important issue to be addressed, as well as sensitivity. An important issue is to distinguish the target analyte from the interfering species in terms of electrochemical response by seeking for the difference between them, such as chemical (ir)reversibility, reaction potential, etc. (Zhu et al. 2011, Hu and Fritsch 2015).
An example that can demonstrate the advantage of double electrode systems with MEAs is selective detection of chemically reversible or quasi-reversible electroactive species against electroactive interferents, which display chemically irreversible property. As described in the “Characterization” section, when the two sets of electrodes work in the mode of generator-collector for detection, more dedicated potential control is involved as a result of mixed electroactive compounds. For CV, the generator is cycled between two potentials to oxidize original species, including the target analyte and interfering species (here, reductive species is taken as initial state). The product of interfering species experiences a short lifetime and vanishes before reaching the collector electrode, while the oxidized target diffuses across the gap and reaches the collector electrode, the potential of which is held constant to induce reverse reaction and regenerated target reoxidized at generator electrode, thus establishing redox cycling for target analyte, and each molecule contributes more than one copy of electrons depending on electrode geometries.
The above strategy is intensively applied for selective detection of dopamine (DA) in the presence of ascorbic acid (AA). DA is an important neurotransmitter, and an abnormal concentration level indicates the possibility of diseases, while AA usually coexists in fluid and has a similar oxidation potential with DA, thus giving interfering additive signal to the detection of DA. Using the above-mentioned strategy, DA and AA oxidize at the generator; oxidized AA undergoes fast hydration process, rendering it inactive for further electrochemical reaction, whereas oxidized DA diffuses across the gap and is reduced to its original state at the collector electrode, and the regenerated DA diffuses back to generator electrode. Therefore, selectivity is achieved, and sensitivity is further enhanced due to redox cycling besides signal amplification from electrode array.
Different electrode geometries are attempted for detection. Using IDA with both planar and vertical geometries, 10 nmol/l DA could be selectively detected in the presence of 10-fold excess of AA (Niwa et al. 1991). The combination of IDA with Nafion/polyester selective film by the casting method achieves a higher selectivity of 100-fold excess of AA in the detection of DA (Niwa et al. 1994). Fabrication and utilization of nanoscale electrode systems are expected to present better electrochemical performance.
On the other hand, carbon-material-based electrodes are dominative in electrochemical analysis. However, limited by sputtering process, most of MEAs-based double electrode systems are made of Au or Pt film, which restricts the enrichment of electrode performance. An alternative choice is to obtain carbon source through pyrolysis of organic compounds; in this case, carbon-film-based double electrode systems could be successfully fabricated, which exhibit a wider potential window than that of Au and Pt, especially in the cathodic region. As already known, pretreatment of carbon-material-based electrodes is popular to increase sensitivity (Engstrom and Strasser 1984, Fagan et al. 1985, Qiao et al. 2008, Thiagarajan et al. 2009), yet it will yield larger background current. Nevertheless, the situation could be improved using double electrode systems because current response at the collector electrode is closely related to diffusive species generated at the generator electrode rather than from bulk solution. The improved electrochemical response at the generator electrode will give promotion to the collector electrode; thus, only by pretreatment of carbon generator electrode, current response at the collector increased without increasing background current, which is beneficial for sensing analyte. Adopting the above strategy, a detection limit of 10 nmol/l DA and a wide linear range from 10 nmol/l to 1 mmol/l were achieved (Niwa and Tabei 1994).
Employing heterogeneous electrode material is also an effective way to increase sensitivity and selectivity. Hayashi’s group proposed a scheme where the two sets of electrodes were composed of different electrode materials (Hayashi et al. 2005). Using metal oxide (ITO) as subtrate, they fabricated ITO-Au interdigitated MEAs, and ITO in PBS can suppress the signal of anionic electroactive species such as uric acid (UA) and AA. Meanwhile, Au is a good candidate to reduce oxidized DA and thus resulting in redox cycling. Therefore, the combination of ITO and Au provides both high selectivity and sensitivity.
Different from the above-mentioned working principle, we proposed a selective detection strategy based on interferent depleting and redox cycling using plane-recessed MEAs (Zhu et al. 2011, Oleinick et al. 2013). For DA detection in the presence of AA, the CV at a slow sweep rate of 10 mV/s aids in choosing the potential region where AA consumes while DA does not react. By holding the potential of the plane electrode in the above potential region, an interferent-depleted microenvironment for microelectrodes recessed to the plane electrode could be created, then CV was applied to MEAs for sensing DA. The potential applied to the plane electrode also corresponds to reductive potential of oxidized DA, leading to redox cycling of DA between the plane and the recessed microdisk electrodes, as illustrated in Figure 6. Here, in one sense, the plane electrode functions as a “diffusional Faraday cage” to block access of interferent species AA to the cavities. In other sense, the recessed MEAs functions as a generator electrode and the plane electrode as a collector electrode; redox cycling between the two sets of electrodes enhances sensitivity further. The detection strategy breaks the limitation of the aforementioned reversible-irreversible systems; it could also be used to detect reversible-reversible redox systems, provided that there exists reacting potential difference between the target analyte and the interferent species. Selective detection of pyrocatechol in the presence of hydroquinone accounts for the point. Simulation results elucidate the origin of successful performance and enable other geometries and operating modes to optimize the performance of the device (Oleinick et al. 2013). In our subsequent work, a strategy based on generator-collector mode concerning plane-band-recessed microdisk array electrodes was proposed. As illustrated in Figure 7, the plane and band electrode are used to oxidize both DA and AA, while the recessed microelectrode is swept negatively to detect oxidized DA. In this special electrode system, the addition of band electrode increases the implementation of redox cycling between DA and oxidized DA (Pang et al. 2014).
Controlling appropriate potential for selective detection can be also applied to IDA. Dam showed that IDA could be used to electrochemically distinguish the signal of weak reductant (DA) from that of a stronger one (K4[Fe(CN)6]). In the experiment, the generator oxidizes both species at high oxidative potential; for the collector electrode, potential could be selected to only reduce oxidized DA. In this case, regenerated DA shuttles between the two electrodes and thus enables selective detection (Dam et al. 2007, Odijk et al. 2008).
Mikkelsen’s group reported cyclic biamperometry (Rahimi and Mikkelsen 2010, 2011), which can be also achieved by employing IDA (Rahimi and Mikkelsen 2011). Ferri/ferrocyanide redox couple was used as a model analyte to demonstrate efficiency. The close proximity of the two working electrodes and their configuration result in signal amplification. Amplification factor of almost 20 was achieved due to redox cycling. The analytical method has feature of not changing the net contents of the sample, which is very important for the application of IDA to the electrochemical cell with a very small volume.
It is worth mentioning that the concept of titration was also applied in double electrode systems, i.e. the so-called diffusion layer titration (Tomčík et al. 1998, 2001, Paixao et al. 2003). Based on diffusion layer titration, double electrode systems could be used to detect electroinactive species indirectly. The method involves initiating an electrochemical reaction at the generator electrode, which usually adopts oxidation of Br- or I-, the product (titrant, usually halogens) of which proceeds a chemical reaction with the target analyte in the vicinity of the electrode quantitatively, while the collector electrode collects the unreacted generated flux and gives a corresponding current. The kind of chemical reactions is various; it could be inorganic redox (Bustin et al. 1996, Tomčík et al. 1997, 1998), organic redox (Tomčík et al. 2001), and organic addition reaction (Bustin et al. 1996, Paixao et al. 2003), It is proposed that the chemical reaction is quantitative and kinetic rapid. By analyzing the current difference between the generator and the collector, quantitative analysis of electroinactive species could be achieved. Up to now, the method is successfully used in the determination of thiosulfate, allyl alcohol, As(III), ammonium, iodide, tetraethylthiuram disulfide (a popular drug used for treatment of alcoholism), AA, and dyprone. The advantage of diffusion layer titration is that one type of titrant could be used to titrate many electroinactive species, yet the method refers to many steps and is much complicated and requires design of specific chemical reaction.
Electrochemical sensors in bipolar mode
Niwa’s group pioneered the work of bipolar behavior in the field of MEAs (Horiuchi et al. 1991, 1992, Tabei et al. 1992), which is similar to the phenomenon observed in positive feedback mode at unbiased conducting substrate in the SECM configuration (Bard et al. 1989, Kwak and Bard 1989). In SECM, the conducting substrate should be much larger than the size of the tip to completely behave as a bipolar electrode, which was elucidated clearly by Amatore et al. (Oleinick et al. 2011). Similar to the above requirement, the area of macroelectrode should be much larger than that of microelectrodes; the existence of bipolar behavior allows redox cycling between the two electrodes by just potentiostating one electrode. The schematic process is illustrated in Figure 8, which adopts the configuration of plane-recessed MEAs; microelectrodes at the bottom are set to the reductive potential of oxidative species (taking oxidant as initial species), and the large plane conducting film is left floating. When oxidative species is reduced at recessed microdisks of the MEAs, the rim of floating upper Au film at the edge of the cavities senses the change in concentration. The concentration difference induces a reverse reaction, regenerating original redox species; thus, the rim of the upper metal film functions as an anode, and a part of the upper conducting film away from the rim functions as a cathode to neutralize the floating conducting film. Therefore, the concentration gradient of oxidative species at the bottom microelectrodes increases, leading to current enhancement at microdisk electrodes, which is similar to the electrochemical process when the device is operated in generator-collector mode and redox cycling occurs between the floating metal film and microdisk electrodes. This kind of redox cycling does not require bipotentiostat, which simplifies experimental equipment greatly.
The concept is smartly utilized to highly sensitive detection using substitutional stripping voltammetry (SSV) supplementary to conventional stripping voltammetry (Horiuchi et al. 1992). Conventional stripping voltammetry provides very high sensitivity in electroanalysis, but it is restricted to an inherent range of species, such as metal ions, halides, and adsorptive compounds. Applying bipolar electrode systems, SSV extends the scope of stripping voltammetry to the detection of other redox species. The experiment was arranged in two compartment cells connected through a salt bridge; redox species was placed in the microelectrode side, which was in the form of IDA; and one set of IDA was connected to a macroelectrode placed in a second metal ion contained cell. By controlling the potential of only one set of IDA, the preelectrolysis at the macroelectrode was induced to happen due to bipolar behavior between the connected IDA and macroelectrode, and at the stage of stripping, potential was applied to strip deposited material. The stripping peak correlated with the concentration of target redox species, and a detection limit of 100 pmol/l was successfully achieved (Morita and Iwasaki 1995). The method renders much higher detection signal than that obtained from CV under redox cycling at IDA.
From another point of view, our group proposed the strategy for increasing electrode density by utilizing the bipolar behavior of a metallic film (Zhu et al. 2014). For commonly used microdisk electrode arrays, the radius of the microelectrode (r) and the distance (d) between the adjacent microelectrodes are key parameters affecting electrochemical responses because of different diffusion modes. And it has been a focus interest from both experimental (Sandison et al. 2002, Rahman and Guiseppi-Elie 2009) and theoretical or simulation (Saito 1968, Amatore et al. 1983, Bartlett and Taylor 1998, Lee et al. 2001, Davies et al. 2005, Guo and Lindner 2009) aspects. When the value of d/r is sufficiently large, the MEAs present steady-state property of microelectrode. The steady-state response is expected, with total signal being the sum of signals from individual microelectrode, which is the result of independent diffusion layer. In contrast, when the value of d/r is small, the current response of MEAs loses steady-state characteristic because of the overlapping of diffusion layers. A recent work reveals that the relationship of d>23r at moderate scan rate would satisfy the requirement of independent diffusion layer without overlapping (Guo and Lindner 2009), and for recessed MEAs, because of confined effect in the cavity, the smallest d/r is a little smaller than that of inlaid MEAs and a modified diffusion zone diagram and a formula for predicting necessary value are proposed (Guo and Lindner 2009). In our work, fixing the diameter of the microelectrode, a series of recessed MEAs and plane-recessed MEAs with different d values were fabricated and used to evaluate the influence of bipolar behaviors of the floating metal film on diffusion layer development and electrochemical responses. The results reveal that the diffusion profiles of microdisk electrodes are remarkably altered. Under investigated electrode configuration, a d/r value of around 20 for recessed MEAs should be satisfied to avoid the overlapping of diffusion layers, while for plane-recessed MEAs (plane metal film is left floating), the smallest d/r for observing steady-state is around 8. The representative effect of floating metallic film on diffusion mode is shown in Figure 9. For densely packed MEAs, at a moderate scan rate, the heavy overlap between diffusion layer leads to planar diffusion over the entire array (Figure 9A). With the addition of plane metal film, radial diffusion dominates at each microelectrode due to the effect of bipolar behavior. Therefore, by just adding a thin layer of metallic film on the plane and leaving it floating, both the current response of the individual microelectrode and electrode density could be increased, and the detection limit increases by more than one order of magnitude. The theory about the generality of bipolar behavior and its limit has been proposed recently (Oleinick et al. 2015). Redox couples with relatively fast electron transfer rate constants aid in the presentation of bipolar behavior at densely packed electrode arrays. Conversely, when the electron rate constant is small, decreased electrode density for efficient bipolar behavior is required.
Very recently, Bohn’s group reported that ultrasensitive electrochemical measurements can be realized by coupling fluorescence microscopy and voltammetry on the basis of bipolar behavior (Ma et al. 2015). Reduction of 1 nm Ru(NH3)63+ was detected by monitoring the fluorescence intensity. The work demonstrates that there is great potential for the development of techniques by combining bipolar behavior with spectroscopic methodology.
Applications in fundamentally electrochemical research
MEAs-based double electrode systems have also been utilized to measure diffusion coefficient (Feldman et al. 1987, Paeschke et al. 1995a,b, Menshykau et al. 2009, Liu et al. 2014). Three methods are primarily developed to derive the basic parameter. The first involves calculation from formula concerning steady-state limiting current using readily known solution concentration and electrode geometries (Aoki et al. 1988). Up to now, for MEAs-based dual electrode systems, only configuration of IDA has acknowledged expressions, so applications of the first method are limited. And deviation between experimental results and theoretical estimation may introduce errors. The second method refers to estimation of diffusion coefficient from the Einstein equation [Eq. (3)].
In this equation, tmax is the time to reach steady-state response at the collector electrode and θ is the numerical factor. Transient time-of-flight experiment allows measurement of the diffusion time between the generation of the species and the collection at the neighboring electrode; the value is highly dependent on the diffusion coefficient of oxidized and reduced species and the distance between the generator and the collector electrode (Feldman et al. 1987, Menshykau et al. 2009). θ could be derived from simulation or determined from control experiment using a system with readily known diffusion coefficient (Paeschke et al. 1995a,b). The third one was proposed recently; it allows the diffusion coefficient to be calculated by detecting the time delay between the generation reaction and the onset of reverse reaction at the collector without knowing concentration, electrode area, and other geometric values except the gap between the generator and the collector electrode (Liu et al. 2014).
In addition, MEAs-based double electrode systems could also be used to investigate reaction mechanism by detecting intermediates or electrogenerated species with much shorter lifetime (Bard et al. 1986, Wollenberger et al. 1994, Postlethwaite et al. 1996, Menshykau et al. 2009, 2010), and heterogeneous electron transfer rate constants could also be determined (Yang and Zhang 2007, del Campo et al. 2014, Hu and Fritsch 2015).
Conclusions and outlook
Significant progress on MEAs-based double electrode systems has been achieved in the field of electrochemical analysis. This review has summarized the versatile functions of double electrode systems based on a part of published work in the literature, highlighting fabrication processes, characterization in view of collection efficiency and redox cycling amplification and a variety of applications. The ability to control potential independently for the two sets of electrodes makes them suitable candidate as electrochemical sensors with high sensitivity and selectivity.
The existence of bipolar behavior in macro-micro double electrode configuration benefits the integration of MEAs and improvement of analyte detection. Some information concerning electrochemical reaction, such as diffusion coefficient and reaction intermediates, could also be drawn through generator-collector experiment.
Further decreasing the size of double electrode systems with MEAs will undoubtedly promote their performance and will benefit the integration of micro- or nano-devices with fluidics, which can remarkably widen the applications of MEAs. Meanwhile, coupling of MEA-based double electrode systems with other techniques such as spectroscopy will open the field for further studies and applications in analytical science, imaging field, and physical electrochemistry.
This work was supported by Funds from JingGangShan University (no. JZB1325), the Natural Science Foundation of Fujian Province of China (no. 2012J01054), and the National Natural Science Foundation of China (nos. 21373174, 21321062).
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