Electrochemical Investigation of Anthraquinone-Based Chemodosimeter for Cu2+ Metal Ion
The electrochemical properties of an anthraquinone-containing Cu2+-promoted desulfurization chemodosimeter (AQCD) in the presence of various metal ions were investigated by voltammetry. AQCD showed voltammetric changes toward divalent metal ions (M2+), which are similar to those of anthraquinone (AQ) and 1-aminoanthraquinone (AQNH2) with the metal ions except Cu2+. This can be explained on the electrostatic interaction (AQCD-M2+) and chemodosimetric reaction (AQCD-Cu2+).
Chemodosimeters are used for sensing an analyte through a usually rapid response, highly selective, and irreversible chemical reaction between dosimeter molecule and the target material, leading to an observable signal [1–5]. Most have been utilized as optical sensors to monitor the target by UV/Vis and fluorescence spectroscopic measurements [1, 2]. These methods are usually sensitive, easy to perform, and inexpensive, but they have limitations in portability and mobility in terms of practical sensor devices.
Alternatively, analytical techniques based on electrochemical detection are suitable for the development of convenient, sensitive, selective, and low cost tools that could be exploited for rapid monitoring, ultimately applicable to hand-held or autonomous operation. However, only a few examples of electrochemical sensors using chemodosimeters have been reported [6–11]. These studies utilized ferrocene [6, 7], iridium(III) complex [8, 9], and pyridine-4-thione  as redox units. Previously, we reported an anthraquinone-containing chemodosimeter (AQCD) responsible for Cu2+ ion-induced desulfurization to exhibit highly selective UV and fluorescence changes (Scheme 1) .
The anthraquinone unit can serve not only as a chromofluorescent unit but also as a redox center. It has largely proved to be an effective, remarkable and promising redox-signaling unit for electrochemical applications due to its robust electrochemistry [13–15].
Here, we report now the redox properties as well as cation sensing behavior of AQCD in the absence and the presence of alkali, alkaline earth, Cu2+, and other metal ions by voltammetry. The present study aims to investigate the electrochemical influence of various metal ions on AQCD behavior in terms of chemodosimetric reaction confirmed previously by optical methods  and consequently to check the feasibility of anthraquinone-based chemodosimeter as an electrochemical sensor for metal ions. In the pursuit of this goal, electrochemical behavior of AQCD was also compared with the results of anthraquinone (AQ) and 1-aminoanthraquinone (AQNH2) in the various metal ions.
Electrochemical measurements were carried out with a Model 660D electrochemical workstation (CH Instruments, Austin, TX, USA). The three-electrode system consists of a glassy carbon working electrode, an Ag/Ag+ (in 0.1 M AgNO3) reference electrode, and a Pt wire counter electrode. The surface of the working electrode was polished with 0.03 and 0.05 μm alumina (CH Instruments, Austin, TX, USA) and rinsed with deionized water. Residual alumina particles were thoroughly removed by positioning the electrode in an ultrasonic bath for 10 min. Then, the electrode was dried and washed with pure acetonitrile before use. The supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile solution (CH3CN). The concentration of AQCD was 0.5 mM and stock solution of metal perchlorate salts with various concentrations was prepared using acetonitrile. Test solutions were prepared by placing 2 mL of the AQCD solution into an electrochemical cell and adding appropriate aliquot of each metal stock solution with a microsyringe. All experiments were carried out in a nitrogen atmosphere at room temperature. Synthesis and identification of AQCD were described in the previous paper . All reagents were purchased from Aldrich and used without further purification.
3. Results and Discussion
The redox chemistry of AQCD was investigated in CH3CN with TBAPF6 as a supporting electrolyte in a nitrogen atmosphere. Cyclic voltammogram (CV) of 0.5 mM AQCD is presented in Figure 1, and, for comparison, CVs of AQ and AQNH2 are also exhibited in Figure 1. The reason for the comparison between AQCD, AQ, and AQNH2 is to clarify the electrochemical results of AQCD by using AQ as a useful reference and to confirm the role of 1-amino substituent (1-NHR′) of AQCD in the absence and the presence of metal ions. AQCD shows reversible first and second waves, of which shape agrees with those of AQ and AQNH2. But only a slight negative shift is observed in the redox potentials of AQCD and AQNH2, compared with those of AQ. This is thought to be due to the existence of electron-donating amine groups (1-NHR′) in AQCD and AQNH2. The first and second waves are separated by about 525 mV for AQCD, 475 mV for AQNH2, and 489 mV for AQ. The cyclic voltammetric data for the two waves for all three anthraquinones in 0.1 M TBAPF6/CH3CN are summarized in Table 1. Two reduction peaks of AQCD, AQNH2, and AQ correspond to two single reductions to give the semiquinone and then dianion in agreement with well-known redox chemistry of quinone compounds . But for a shade difference, electrochemical behaviors of all three anthraquinones show typical redox chemistry of quinone compounds. This shows that neither amine group and electroinactive lariat influence the redox of anthraquinone. Therefore, there is no factor affecting the electrochemical reduction and oxidation of AQCD, AQNH2, and AQ except the characteristic interaction between the anthraquinones and metal ions.
Electrochemical properties of AQCD in the presence of alkali, alkaline earth, Cu2+, and other metal ions were also investigated in 0.1 M TBAPF6/CH3CN by voltammetry. As a chemodosimeter, AQCD showed high selectivity toward Cu2+ over other competitive cations in aspects of colorimetric and fluorometric chemodosimetry . Addition of Cu2+ ion to AQCD solution exhibited a rapid color change along with its blue-shifted absorption band. This is caused by the fact that a planar intramolecular charge-transfer (ICT) structure of AQCD becomes difficult to be adopted due to the cyclization assisted by Cu2+, resulting in reducing the ICT character in the anthraquinone moiety (Scheme 1). One can expect the same behavior of AQCD toward Cu2+ and other metal ions in the electrochemistry. Thus, we first investigated an electrochemical reaction between AQCD and Cu2+ ion by cyclic voltammetry. Figure 2 shows voltammetric variation of 0.5 mM AQCD upon gradual addition of Cu2+ (0.25, 0.5, and 0.75 mM). AQCD shows significant voltammetric changes and increase of the intensity of redox wave at a potential between −0.1 and −1.1 V as the concentration of Cu2+ increases. Redox waves, however, assigned to anthraquinone unit at a potential between −1.1 and −2.0 V exhibit no significant change even with the increase of Cu2+ concentration. This indicates that transformation of the aminoethylthiourea unit of AQCD into imidazoline (Scheme 1) causes no influence on redox chemistry of anthraquinone unit. The redox waves at a potential between −0.1 and −1.1 V are presumably due to the redox reaction of copper species (CuS).
To gain an insight into the voltammetric properties of AQCD with Cu2+, CV experiments have been also performed with AQ and AQNH2 in the presence of Cu2+ (Figure 3). The CV data of AQ and AQNH2 also show two distinctive patterns. Firstly, the characters of the redox curves of anthraquinone unit remain intact even when the concentration of Cu2+ ions increases. This confirms that Cu2+ leads to no significant effect on redox reaction of anthraquinone unit, indicating no structural change of anthraquinone moiety in AQCD, as described in Scheme 1. The second redox features at a potential between −0.5 and −1.0 V in AQ and AQNH2 are caused by copper reduction (Cu2+ → Cu0) and oxidation (Cu0 → Cu2+), which is confirmed by the CV of Cu2+ itself (Figure 3). But compared with those in AQ and AQNH2, the copper redox reaction in AQCD shows negative shift in reduction and positive shift in oxidation, meaning more difficult redox reactions, attributable to the production of CuS resulting from chemodosimetric reaction. This also confirms the mechanism described in Scheme 1.
The addition of alkali metal ions to free AQCD solution leads to little change in both CVs and square wave voltammograms (data not shown). But the presence of alkaline earth and other metal ions (Cd2+, Fe2+, Pb2+, and Zn2+) alters both the reduction/oxidation peak potentials and currents of AQCD, which is different behavior in the presence of Cu2+ in terms of redox chemistry of anthraquinone unit. Typical example of CVs of AQCD with divalent metal ions (alkaline earth and other metal ions except Cu2+) is presented in Figure 4(a), and for comparison, CVs of AQ and AQNH2 with the same metal ion are also illustrated in Figures 4(b) and 4(c). Significant changes in potentials and currents of AQCD were observed in the presence of Mg2+ ion with change in concentration. As the concentration of Mg2+ increases, a new reduction wave appears at a potential less negative than that of the original waves (Figure 4(a)). At high concentration of Mg2+, the new wave dominates and the peak at the more negative potential disappears. A similar behavior also occurs in the redox reaction of AQ and AQNH2 with Mg2+. This indicates that the electrochemical reaction of AQCD and divalent metal ions except Cu2+ is associated only with the redox chemistry of anthraquinone unit and not with electroinactive lariat in AQCD, which confirms the chemodosimetric mechanism for selective detection of Cu2+ in Scheme 1.
Typically, by reduction of quinone in aprotic media, addition of increasing concentrations of a strong proton donor such as a strong acid causes new prior wave and leads to positive shift of wave with irreversibility, until merging with the new wave and disappearing of wave . This is generally due to fast protonation of semiquinone (Q−•)/dianion (Q2−) and reduction of the more easily reduced QH•/QH2 at the same potential. As shown in Figure 4, the electrochemical characteristics of anthraquinones in the presence of divalent Mg2+ ion are very similar to those of quinone with proton donor, which is so called proton-coupled reaction. That is, the electrochemistry of anthraquinone in the presence of divalent metal ion shows divalent metal ion-coupled reaction.
It is noted that with divalent Cu2+ AQ does not show the coupled reaction because Cu2+ ions are already reduced to Cu0 at a more positive potential before interacting with AQ, as shown in Figure 3. In case of other divalent metal ions (alkaline earth, Cd2+, Fe2+, Pb2+, and Zn2+ metal ions) the reduction occurs at a more negative potential than that of the reduction of AQ, thus leading to divalent metal ion-coupled redox reaction of AQ. As shown in Figure 5, Ca2+ in 0.1 M TBAPF6/CH3CN does not exhibit reduction at a potential between 0 and −2.0 V, thus resulting in the coupled reaction of anthraquinones. Except for subtle differences, other divalent metal ions also show similar voltammetric behaviors of divalent metal ion-coupled reaction with AQ, AQNH2, and AQCD (data not shown). This means, in essence, that AQCD interacts with Cu2+ ion via chemodosimetric reaction and with other divalent ions via metal ion-coupled reaction. Even with the difference of electrochemical mechanisms between AQCD with Cu2+ and AQCD with other divalent metal ions, it seems inappropriate to apply AQCD to electrochemical sensor for Cu2+ ion due to signal changing in the presence of other divalent metal ions, as well as Cu2+.
We have investigated the electrochemical behaviors of an anthraquinone bearing chemodosimeter for Cu2+ ion-induced desulfurization with metal ions. Differently from the results by optical measurements showing only colorimetric and fluorometric changes toward Cu2+, the electrochemical measurements of AQCD gave two different types of voltammetric changes toward divalent metal ions and Cu2+. This is attributed to the chemodosimetric reaction for Cu2+ and the divalent metal ion-coupled electrochemical reaction for other metal ions in the redox chemistry of AQCD. Therefore, to develop the electrochemical chemodosimeter for metal ions using anthraquinone derivatives, one should consider the metal ion-coupled electrochemical reaction as the interference for selective detection.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the Soonchunhyang University Research Fund. This work (Grant no. C0119167) was also supported by Business for Cooperative R&D between Industry, Academy, and Research Institute funded Korea Small and Medium Business Administration in 2013.
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