A voltammetry biosensor based on self-assembled layers of a heteroleptic tris(phthalocyaninato) europium triple-decker complex and tyrosinase for catechol detection

Highlights

• An enzymatic electrochemical sensor based on Tyrosinase and a triple-decker complex was developed.

• Remarkable specificity, sensitivity and stability for catechol was demonstrated.

• Mechanisms are proposed for ectrocatalyzed catechol oxidation.

Abstract

A highly efficient enzyme-based sensor was made from a heteroleptic tris(phthalocyaninato) europium triple-decker complex Eu₂(Pc)[Pc(OPh)₈]₂, for the first time. The ITO working electrode was modified by a mixture of hybrid multi-layers consisting of Eu₂(Pc)[Pc(OPh)₈]₂, stearic acid (SA) and tyrosinase (Tyr) (Eu₂(Pc)[Pc(OPh)₈]₂/SA/Tyr-ITO) using the Langmuir-Blodgett (LB) technique. The microstructure and morphology of the resulting LB films can be characterized by their π–A isotherms, UV–vis absorption spectra, X-ray diffraction and atomic force microscopy (AFM) analysis. The experimental results revealed that the triple-decker molecules of Eu₂(Pc)[Pc(OPh)₈]₂ take a H-type molecular stacking mode in both pure and mixed LB film, and the microstructures of films were effectively improved by mixing SA within the triple-decker Eu₂(Pc)[Pc(OPh)₈]₂ molecules. The excellent electrocatalytic effect of the Eu₂(Pc)[Pc(OPh)₈]₂/SA/Tyr LB films, leads to a good linear increase from 5.26 × 10⁻⁷ to 2.1 × 10⁻⁴ M for catechol. It also leads to an excellent sensitivity of 2.19 μA/μM, and a detection limits of 6.29 × 10⁻⁸ M (S/N = 3) of catehol at the oxidation peak, achieving best catechol sensing performance among the phthalocyanine-based biosensing mediators. The reduction peak also showed a good linear increase from 5.26 × 10⁻⁷ to 1.60 × 10⁻⁴ M for catechol with a sensitivity of 0.615 μA/μM, and a detection limit of 1.69 × 10⁻⁷ M (S/N = 3). Moreover, the Eu₂(Pc)[Pc(OPh)₈]₂/SA/Tyr-ITO electrode are easy to reproduce, stable and resistant to interference when it comes to detection for catehol, and this indicates great potential of industrial application of tris(phthalocyaninato) rare earth complexes in ultrasensitive and specific biosensors.

Introduction

In general, unique electronic characteristics, high chemical stability and great electroactivity of phthalocyanines are well known and documented [1], and as a result, phthalocyanine-based devices have been applied in the electrochemical sensors [2], OFETs [3] and gas sensors [4,5]. Depending on their electroactivity, some electrochemical sensors based on phthalocyanines [6] and biphthalocyanines [7] for the detection of phenolic compounds have been researched widely. Very recently, solution-processed quasi-Langmuir–Shäfer (QLS) films of the functionalized sandwich mixed (phthalocyaninato) (porphyrinato)europium derivatives have been revealed to display specific electrochemical recognition for H₂O₂ [8], dopamine, uric acid, tyrosine, tryptophan and acetaminophen [9,10], etc. depending mainly on intermolecular interaction between triple-deckers and the analytes. However, to the best of our knowledge an electrochemical sensor based on tris(phthalocyaninato) rare earth triple-deckers towards catechol has not yet been reported.

Catechol is a phenolic compound which is widely used in many chemical industries for the production of pesticides, plastics, herbicides, pharmaceuticals, cosmetics, antiseptics and synthetic products. Nevertheless, catechol is also a toxic environmental pollutant due to its poor biodegradability in waste water [11]. Up to now,many kinds of methods have been used for the determination of catechol, such as flow injection analysis [12], electrochemical methods [13], high performance liquid chromatography (HPLC) [14], electro-chemiluminescence [15], and gas chromatography/mass spectrometry [16]. However, the practical applications of these methods still have some limitations such as complicated operating conditions, time-consuming processes, expensive equipment, etc. Therefore, much efforts have been devoted to develope the simple, sensitive, and cost-effective electrochemical method for detection of catechol. It has been reported that catechol can be detected by using the DMPA (phospholipid dimyristoyl phosphatidic acid) mixed either with FePc (iron phthalocyanine) or with LuPc₂ (lutetium bisphthalocyanine) as voltammetry sensors, achieving the limit of detection (LOD) of 4.30 × 10⁻⁷ and 3.34 × 10⁻⁷ M for FePc/DMPA and LuPc₂/DMPA, respectively [17]. The biomimetic sensors containing LuPc₂ and tyrosinase nano-aggregates was developed for the voltammetry detection of catechol with a LOD as low as 3.75 × 10⁻⁷ M [18]. By using CoPc as an electron mediator and tyrosinase as a biocatalyst, the-CoPc/tyrosinase-based sensor exhibited a highly sensitive response to catechol with a low LOD of 1.66 × 10⁻⁶ M [19]. Compared to metal phthalocyanines (MPcs) and bis(phthalocyaninato) rare earth double-decker complexes, tris(phthalocyaninato) rare earth triple-decker complexes possess more extended systems in the normal direction of the macrocycle plane. Therefore, it would be high promising for these triple-deckers as electroactive candidates in electrochemical sensing application [[8], [9], [10]]. With this background in mind, herein, the self-assemblied hybrid films of a tris(phthalocyaninato)europium(III) complex, Eu₂Pc[Pc(OPh)₈]₂ [Pc = unsubstituted phthalocyaninate; Pc(OPh)₈ = 2,3,9,10,16,17,23,24-octaphenoxyphthalocyaninate] (Scheme 1), mixed with stearic acid (SA) (denoted as Eu₂Pc[Pc(OPh)₈]₂)/SA are prepared by using the Langmuir-Blodgett (LB) technique. The sensitive and selective electrochemical responses of the Eu₂Pc[Pc(OPh)₈]₂)/SA LB films modified by tyrosinase (Tyr) toward catechol have been revealed.