Review Article
Unusual metals as electrode materials for electrochemical sensors

https://doi.org/10.1016/j.coelec.2019.05.005Get rights and content

Abstract

Although noble metals are still widely used in electroanalysis, a plethora of different nonconventional metals is now enriching the panorama of materials acting as the electrochemical transducer in sensing systems. In particular, Ti, Cu, Co, Fe, Mo, Ta, W, Rh, Bi, Sb, Te and Pb are discussed here in view of their peculiar physicochemical properties and of the interesting electrocatalytic activities ascribable to these elements and to the relevant metal oxide ultrathin films that spontaneously form at the electrode–solution interface. This behaviour, exploitable in electroanalysis for the detection of a number on analytes, is often accompanied by low price and high resistance to corrosion and to abrasion characterising these materials. These peculiarities encourage the possible use of the cited metals in a wide number of analytical frames, ranging from process control to bioimplantable sensing systems.

Introduction

The present article aims at discussing the electrochemical behaviour of nonconventional metals, addressing their applications in amperometric sensing for liquid samples. The term ‘amperometric’ refers to ‘a detection method in which the current is proportional to the concentration of the species generating the current’, in agreement with International Union of Pure and Applied Chemistry (IUPAC) [1]. Hence, this family of analytical methods includes sensors based on voltammetry and on amperometry at a constant potential value.

In particular, Ti, Cu, Co, Fe, Mo, Ta, W, Rh, Bi, Sb, Te and Pb are considered (Figure 1). Although the literature dealing with these metals is spread over the last few decades, seminal articles dated back to the 1980s, the number of publications has increased significantly in the last few years. The reinforced interest in these metals arises from their peculiar chemical nature, leading to quite a different electrochemical behaviour with respect to conventional electrode materials, such as Au or Pt, Pd, Ni and Hg. As an example, the ferricyanide/ferrocyanide redox couple does not show the well-known reversible voltammetric behaviour at Ti electrodes, although Ti electrodes show suitable conductivity. However, their performance as amperometric sensors rivals that of conventional materials, for example, in the case of the determination of strong oxidants. The peculiar electrochemical behaviour is often accompanied by their low price, especially when compared with electrode devices consisting of noble metals; this opens to the possibility to massively produce disposable systems for a number of sensing applications.

Many metals are acknowledged to possess peculiar mechanical properties that can be exploited in the fabrication of microelectrodes, as in the case of W, or in sensing systems suitable for analysis in abrasive media. Some of them, for example, Ti and Mo, are also chemically inert, which implies the occurrence of very limited corrosion even in harsh environments and favours biocompatibility. In several cases, the resistance to corrosion, as well as the characteristic electrochemical behaviour previously described, is ascribed to the spontaneous formation of an outermost layer of metal oxides, possessing a not-well-defined stoichiometry and containing metal ions at various oxidation states; the layer is a few nanometres thick, and it is well adherent to the underlying metal surface. In other cases, metal surfaces, for example, consisting of Fe, are more reactive with respect to water and oxygen, leading to the formation of thicker films based on different oxides. These films are reported to be catalytically active towards a number of species, for example, glucose and neurotransmitters, as better discussed in the following section.

The nature of these oxide layers is widely discussed in the literature (see, e.g., Saji and Lee [2] and Liu et al [3]). It mainly depends on the procedure followed for the preparation of the surface so that standardisation of these procedures is mandatory. Activation processes are often required, but the evolution of the chemical nature of the electrode surface during this process is still basically unknown. Actually, this issue is common even in the case of conventional metal electrodes such as Cu.

Characterisation of the resulting electrode surface is also quite challenging, especially when trying to bring to light the electrochemical behaviour of the oxide layer in the absence and in the presence of target analytes. The metal oxide surface is often self-healing: a passive oxide layer is immediately formed when the electrode surface is scratched, exposing the fresh metal. It is noteworthy to mention that the nature and thickness of the oxide layer does not prevent from effective charge conduction, although the quantitative estimation of the conductance is not well feasible.

The comprehension of the electrocatalytic behaviour of these metals and, as a consequence, their performance as sensors could exploit more of less conventional theoretical frameworks, for example, the incipient hydrous oxide/adatom mediator model developed by Burke [4]. Actually, these approaches, well established in the case of noble metals, are difficult to use in the case of the metal discussed in the present article because of the nature of the oxide layer. On the other hand, calculations based on density functional theory, widely used in the case of fuel cells, may be an invaluable tool, especially in the case of the multimetallic system 5, 6, 7. The main obstacles in the use of such approaches may be the complexity of the electrochemical reactions of the species of interest in the frame of sensing. As an example, the oxidation mechanism of glucose is still debated, even in the case of conventional Au surfaces. In addition, the high number of species present in real matrices constitutes a further obstacle in the developing of a theoretical framework. As a result, combinatorial approaches could be highly effective in the identification of the best procedure for the preparation of electrocatalysts based on the cited metals.

The reactivity towards electroactive species, that is, towards specific analytes, can be also widely tuned, thanks to the possibility to prepare a variety of multimetallic systems 8, 9 and to modify the electrode surface with coatings based on the already cited nonconventional metals. Most of the studies deal with the use of systems containing a large fraction of noble metals. They have been investigated in the frame of electrocatalysis applied to fuel cells, H2 production and CO2 reduction; a minor portion of these studies involves amperometric sensing. On the contrary, the use of systems free of expensive metals is still in its infancy.

In multimetallic systems, synergic effects between the various components of the electrode, for example, although more or less strong chemical interactions among them [10], were exploited to enhance electrocatalytic properties of the device to be used in sensing applications. The electrochemical performance of multimetallic systems may also depend on the crystalline phase and on the grain orientation [11]. Unfortunately, these aspects are poorly investigated.

Some multimetallic systems are deliberately prepared in the amorphous phase as it is the case of CuTi systems [12]. These systems are realised by following special procedures, consisting of melting and fast cooling processes. Their electrocatalytic activity is often superior with respect to conventional crystalline alloys, although the factors inducing this effect are not completely clear.

Porous electrodes, that is, devices exhibiting particularly a high surface area, were also studied for possible application in the sensor field. They were obtained by powder metallurgy or through dealloying procedures aimed at selectively removing one of the components of a multimetallic system. Actually, the removal of one of the components may not be quantitative so that peculiar multimetallic systems are formed. As an example, it was proved that dealloying of a Ti–Cu bimetallic electrode leads to formation of nanoporous Ti-based electrodes, exhibiting a surface that still retains Cu in the form of nanostructures at the electrode–solution interface [12]. As a result, the systems are effective in anodic oxidation, that is, detection, of ethylene glycol.

Additional exploitation of unconventional metals and the relevant alloys in amperometric sensing may be boosted by the commercial availability of low-cost nanostructures, for example, Ti nanoparticles. This possibility allows the researchers to avoid complex chemical synthesis.

Besides this overview of the general physicochemical characteristics of nonconventional metals in electroanalysis, in the following sections, we will discuss the advantages and critical aspects of the use of the most significant ones. It is meaningful to clarify that the present review does not deal with thick films of oxide-based materials, oxide microstructures and nanostructures and compounds such as sulphides, nitrides and carbides.

Section snippets

Titanium

Ti possesses exceptional mechanical properties and high resistance to corrosion when alloyed. Many properties of Ti, such as corrosion resistance, are due to the presence of a thin layer of TixOy, ca. 4-nm thick, which spontaneously forms on the surface. It consists of Ti(II), Ti(III) and Ti(IV) species, well adherent to the underlying Ti(0). The exact chemical nature of this layer has been widely discussed in the literature [3].

Ti is not extensively used as an electrode material [13] because

Copper

Due to the very high number of studies dealing with Cu based electrodes, this metal can not be considered “nonconventional”. However, the applications in the electroanalytical frame have been so strongly increased in the last few years to deserve a peculiar attention. In particular, literature reports from 2017 onwards focus their attention on the use of Cu oxide nanostructures, for example, Cu2O- and CuO-based materials (see, e.g., Figiela et al [21]). In addition, the use of pristine metal or

Cobalt and iron

The number of studies dealing with Co has significantly increased in the recent years: Co has been reported to be an effective electrocatalyst towards the oxidation and thus detection of different organic species, for example, glucose [24], paracetamol [25], neurotransmitters [26], hydrogen peroxide [27] and nitrite ions [28]; in addition, it was demonstrated that Co is an effective electrode material in the determination of As(III) [29]. Finally, different metals have been alloyed with Co

Molybdenum, tantalum and tungsten

Mo, Ta and W, as refractory metals, exhibit peculiar properties in terms of corrosion resistance (even when electrycally polarized), wear resistance and biocompatibility.

The electrochemical behaviour of Mo and of the relevant surface oxides was recently reviewed [2]. Bulk Mo electrodes were used for the determination of phosphate ions, which are of key importance in monitoring and for investigation of rivers and lakes [33]. In particular, Mo electrodes were anodically oxidised to produce

Nonconventional platinum group metals

In the frame of amperometric sensors, the metal belonging to the platinum group mostly used so far has been Pt, followed by Pd. However, some examples of application of pristine Ru, Ir, Os and Rh are also reported, although dated between the 1990s and the 2010. An example is the use of a composite based on Rh nanoparticles and Si nanowires: the performance in the determination of H2O2 appeared superior with respect to the pristine Rh nanoparticles deposited on glassy carbon [39]. It is worth

Other elements: Bi, Sb, Te and Pb

Bi and Sb are still widely studied as electrode materials for the determination of heavy metals by stripping analytical procedures, aiming at substituting the toxic Hg-based electrochemical devices. The electrode devices that have been developed so far consist of bulk materials, as well as of thin films and nanoparticles, deposited on most conventional electrode surfaces. Owing to the great interest in environmental controls of heavy metals affecting human health, the relevant literature was

Conclusions

In the present review, different analytical applications of unconventional metals as electrode materials for sensor applications have been reported. Despite the potentialities of these electrode materials, they are largely unexploited. One of the main obstacles in this direction is the lack of systematic studies on these materials, allowing a proper definition of the property (i.e. electrochemical behaviour)–structure (i.e. composition, surface structure and so on) relationship. For this

Conflict of interest statement

Nothing declared.

References (46)

  • J. Liu et al.

    Sensitivity enhancement of electrochemical biosensor via cobalt nanoflowers on graphene and protein conformational intermediate

    J Electroanal Chem

    (2017)
  • S. Premlatha et al.

    Fabrication of Co-Ni alloy nanostructures on copper foam for highly sensitive amperometric sensing of acetaminophen

    J Electroanal Chem

    (2018)
  • B. Amanulla et al.

    A non-enzymatic amperometric hydrogen peroxide sensor based on iron, nanoparticles decorated reduced graphene oxide nanocomposite

    J Colloid Interface Sci

    (2017)
  • C. Forano et al.

    Recent trends in electrochemical detection of phosphate in actual waters

    Curr Opin Electrochem

    (2018)
  • M. Zhao et al.

    Synthesis of Ta/Ni microcavity array film for highly sensitive uric acid detection

    J Electroanal Chem

    (2019)
  • A. Jo et al.

    Electrodeposition of tantalum on carbon black in non-aqueous solution and its electrocatalytic properties

    Anal Chim Acta

    (2016)
  • M. Roham et al.

    Wireless amperometric neurochemical monitoring using an integrated telemetry circuit

    IEEE Trans Biomed Eng

    (2008)
  • Z. Song et al.

    Rhodium nanoparticle-mesoporous silicon nanowire nanohybrids for hydrogen peroxide detection with high selectivity

    Sci Rep

    (2015)
  • K. Dhara et al.

    Electrochemical nonenzymatic sensing of glucose using advanced nanomaterials

    Microchim Acta

    (2018)
  • K. Tyszczuk-Rotko et al.

    A lead film electrode for adsorptive stripping voltammetric analysis of ultratrace tungsten(VI) in acidic medium

    Electroanalysis

    (2012)
  • Compendium of chemical terminology

  • V.S. Saji et al.

    Molybdenum, molybdenum oxides, and their electrochemistry

    ChemSusChem

    (2012)
  • L.D. Burke

    Scope for new applications for gold arising from the electrocatalytic behaviour of its metastable surface states

    Gold Bull

    (2004)
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