Highly Sensitive Electrochemical Sensor for the Detection of Anions in

Article pubs.acs.org/ac

Highly Sensitive Electrochemical Sensor for the Detection of Anions in Water Based on a Redox-Active Monolayer Incorporating an Anion Receptor Balwinder Kaur,† Cristiane Andreia Erdmann,‡ Mathias Daniel̈ s,§ Wim Dehaen,§ Zbigniew Rafiński,∥ Hanna Radecka,† and Jerzy Radecki*,† †

Department of Biosensors, Institute of Animal Reproduction and Food Research of the Polish Academy of Sciences, Tuwima 10, 10-747 Olsztyn, Poland ‡ Departamento de Química, Setor de Ciências Exatas e da Terra, Universidade Estadual de Ponta Grossa, Avenida Carlos Cavalcanti, 4748, CEP 84030-900 Ponta Grossa, Paraná, Brazil § Chemistry Department, University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium ∥ Faculty of Chemistry, Nicolaus University in Toruń, Gagarina 7, 87-100 Toruń, Poland S Supporting Information *

ABSTRACT: In the present work, gold electrodes were modified using a redox-active layer based on dipyrromethene complexes with Cu(II) or Co(II) and a dipodal anion receptor functionalized with dipyrromethene. These modified gold electrodes were then applied for the electrochemical detection of anions (Cl−, SO42−, and Br−) in a highly diluted water solution (in the picomolar range). The results showed that both systems, incorporating Cu(II) as well as Co(II) redox centers, exhibited highest sensitivity toward Cl−. The selectivity sequence found for both systems was Cl− > SO42− > Br−. The high selectivity of Cl− anions can be attributed to the higher binding constant of Cl− with the anion receptor and the stronger electronic effect between the central metal and anion in the complex. The detection limit for the determination of Cl− was found at the 1.0 pM level for both sensing systems. The electrodes based on Co(II) redox centers displayed better selectivity toward Cl− anion detection than those based on Cu(II) centers which can be attributed to the stronger electronic interaction between the receptor−target anion complex and the Co(II)/Co(III) redox centers in comparison to the Cu(II)/Cu(I) system. Applicability of gold electrodes modified with DPM-Co(II)-DPM-AR for the electrochemical determination of Cl− anions was demonstrated using the artificial matrix mimicking human serum.

I

The study on the electrochemical properties of supramolecular systems is an area of intense current interest.5 A significant part of this research concerns redox-active molecular receptors capable of sensing charged or neutral substrates and reporting their presence by means of an electrochemical response. Such systems require that the guest binding site and the redox-active group can communicate to each other. Evidently, selective binding of a particular guest coupled with an electrochemical response is of paramount importance for future potential prototypes of new amperometric molecular sensing devices. A pioneering research work has been carried out by Beer et al. using the artificial anion receptors for the anion recognition.6−9 A number of ion channel sensors have been developed based on various artificial receptors for organic and inorganic ions.10,11 The drawback of ion channel sensors is

n the last decades, the intermolecular recognition of anions in water has been attracting the attention of numerous scientific groups involved in supramolecular chemistry.1,2 In biological systems, remarkable selectivity and sensitivity of molecular recognition processes are occurring. These processes are mostly taking place in water, and they remain an inexhaustible source of inspiration for researchers dealing with supramolecular and analytical chemistry.1 In general, the intermolecular recognition processes rely on the formation of hydrogen bonds, electrostatic interactions, van der Waals interactions, Lewis acid−base interactions, and π−π interactions.3 Water molecules can act both as hydrogen-bond donor and acceptor. Therefore, polar molecules are highly hydrated in solution. This feature provides a major challenge for the development of functional systems in aqueous media. Many neutral anion receptors are insoluble in water, which prevents binding studies in aqueous environment.4 One solution for this problem is the functionalization of the surface of solid electrodes with receptors and the observation of recognition processes at the solid/water interface. © 2017 American Chemical Society

Received: July 28, 2017 Accepted: November 7, 2017 Published: November 7, 2017 12756

DOI: 10.1021/acs.analchem.7b03001 Anal. Chem. 2017, 89, 12756−12763

Article

Analytical Chemistry Scheme 1. Schematic Representation of the Steps Involved in the Preparation of Electrochemical Sensora

a

M represents Cu(II) or Co(II).

dichloromethane at room temperature overnight. The crude product was purified by column chromatography (SiO2, eluent dichloromethane/7 N NH3 in methanol 95:5) (Figures S1 and S2). The details are provided in the Supporting Information. Successive Steps of the Modification of the Gold Electrode. The procedure used for the cleaning of gold electrodes is described in the Supporting Information. After cleaning, the electrodes were rinsed thoroughly with water, methanol, and dichloromethane and dipped into 180 μL of a dichloromethane modification solution based on a mixed solution of 0.01 mM thiol derivative of dipyrromethene (DPM) and 1 mM 4-mercaptobutanol (MBT) at room temperature for 3 h. Next, the electrodes were washed with dichloromethane and a mixture of dichloromethane and methanol (ratio 1:1 v/v). Then, the electrodes were dipped in 180 μL of 1 mM Cu(II) or Co(II) acetate solution in dichloromethane and methanol (ratio 1:1 v/v) at room temperature for 1 h. The electrodes were then washed with dichloromethane and methanol (ratio 1:1 v/v), methanol, and water−methanol mixture (ratio 4:1 v/v). Finally, a 10 μL droplet of 0.1 mM dipyrromethene-modified anion receptor in water−methanol mixture (ratio 4:1 v/v) was placed on the surface of each electrode. The electrodes were covered with tubes and stored for 6 h at room temperature. After 6 h, the electrodes were rinsed with water−methanol mixture (ratio 4:1 v/v), water, and 0.1 M NaNO3 + 0.01 M H3BO3 (pH 4), respectively, and stored overnight at room temperature. The electrode modified with copper or cobalt complex is designated as DPM-Cu(II) or DPM-Co(II), whereas the gold electrode modified with the Cu(II) or Co(II) DPM complex and dipyrromethene-modified anion receptor (DPM-AR) is designated as DPM-Cu(II)-DPM-AR or DPM-Co(II)-DPM-AR, respectively. The molecular structures of dipyrromethene (DPM), and the thiol derivative of DPM are provided in Figure S3. Detailed experimental data concerning materials and chemicals, instrumentation, modification of electrodes with DPM-Co(II)-DPM, electrochemical measurements, and preparation of artificial matrix mimicking physiological sample is

that the redox-active species has to be added to the sample solution. Numerous examples of redox-active sensors for anion recognition have been reported so far in the literature.12−17 Most of them have been referring to recognition processes in an organic phase. Only a few reports described the recognition of anions at the border of water and a solid phase.18−21 In the present paper, we introduce the development of an anion sensor based on a gold electrode functionalized with a new electromediating layer consisting of functionalized Cu(II) or Co(II) dipyrromethene complexes. A very interesting aspect of the above research area concerns the electrochemical molecular recognition based on bifunctional receptor molecules which contain not only binding sites but also redox-active centers, whose electron-transfer reaction is coupled to the complexation of the receptor. The prepared layers, simultaneously applied as a molecular connector and transducer in new amperometric tools, are destined for study of the electrochemical recognition processes at the border of two phases (aqueous/solid) and the determination of anions. The structure of the proposed sensing layers is depicted in Scheme S1. On the basis of the results obtained, we have discussed the following processes: intermolecular recognition (receptor− analyte), chemical and electrochemical communication of the receptor−analyte complex located at the interface with the redox center located at the surface of modified gold electrode, chemical and electrochemical communication of the redox center with the electrode surface, mechanism of the analytical signal generation, and comparison of the analytical parameters of the electrodes modified with the electrochemical layer based on copper or cobalt complexes as redox centers.

■

EXPERIMENTAL SECTION The dipyrromethene-modified dipodal anion receptor (Z)-4((1H-pyrrol-2-yl)(2H-pyrrol-2-ylidene)methyl)-N-(2-(bis(2(3-hexylureido)ethyl)amino)ethyl)benzamide was synthesized as 100 mg of (Z)-2,5-dioxopyrrolidin-1-yl4-((1H-pyrrol-2yl)(2H-pyrrol-2-ylidene)methyl)benzoate (0.275 mmol) was stirred with 100 mg of 1,1′-(((2-aminoethyl)azanediyl)-bis(ethane-2,1-diyl))bis(3-hexylurea) (0.25 mmol) in 2.5 mL of 12757


Article

Analytical Chemistry

immobilization, the potentials of the redox peak were shifted to higher values, ca. 41 and 11 mV, for layers incorporating Cu(II) and Co(II), respectively. The shifts of the redox peak potential were assisted with a decrease of the current intensity (Figure S4, parts c and d, and Table S1). The decrease of the faradic current after bonding of the anion receptor to the DPM-Cu(II) or DPM-Co(II) complex is a consequence of changes in accessibility of the redox centers for counterions present in the supporting electrolyte solution which neutralize the charge appearing as a consequence of the redox reaction.24 Small additional peaks observed in the CV at 0.638 ± 0.01 V in case of the electrode modified with Cu(II) complex and at 0.659 ± 0.02 V for the electrode modified with Co(II) complex are due to catalytic water oxidation (Figure S4).25−27 These peaks are no longer visible in OSWV, and they did not interfere with the detection of anions and, hence, can be neglected. Further, to confirm that DPM-Cu(II)-DPM-AR and DPMCo(II)-DPM-AR complexes are located at the electrode surface, the effect of the scan rate on the peak currents was investigated (Figure S7). Figure S7 shows that both systems exhibited a linear relationship between the scan rate and the anodic or cathodic peak currents. This indicates that the redox process is not diffusion-controlled and confirmed the localization of redox centers on the surface of the electrode.28 The relationship between scan rate versus anodic and cathodic peak currents was used to calculate the density of redox-active layer (Γ). The density (Γ) of redox-active layers DPM-Cu(II)-DPMAR and DPM-Co(II)-DPM-AR assembled on the gold surface was found to be 5.4 × 10−11 and 3.9 × 10−11 mol/cm2, respectively. The density of Cu(II) was 1.4 times larger than density of Co(II) units. Nevertheless, these values are in a similar range as reported for the other redox-active probes located at the electrode surface.29,30 In addition, the data presented in Figure S7 also confirm that the anodic peak shifts positively, whereas the cathodic peak shifts negatively with an increase in the scan rate. Electrochemical parameters such as the electron-transfer coefficient (α) and the electrode reaction standard rate constant (ks) were calculated from the relationship between the natural logarithm of the scan rate versus the anodic or cathodic peak potential using Laviron’s equation.31,32 The values of the electron-transfer coefficient were found to be 0.47 ± 0.02 and 0.58 ± 0.01 for DPM-Cu(II)-DPM-AR and DPM-Co(II)-DPM-AR, respectively (Table S2). These values are close to the theoretical value 0.5 for a completely reversible redox process. The electrode reaction standard rate constants for DPM-Cu(II)-DPM-AR and DPM-Co(II)-DPM-AR were found to be 1.82 ± 0.03 and 0.64 ± 0.05 s−1, respectively (Table S2). These parameters show that DPM-Cu(II)-DPMAR system is more reversible and the redox process is faster compared to DPM-Co(II)-DPM-AR.33 Thermodynamic Parameters Concerning Anion Recognition by Electrodes Modified with DPM-M(II)-DPMAR Systems, M = Cu or Co. The main driving forces for the formation of the complex of the anion with the anion receptor are (a) electrostatic interaction between the protonated nitrogen atom of the anion receptor and the negatively charged anion, (b) hydrogen bonds between the amide −NH groups present in the anion receptor and the target anion, and (c) four hydrogen bonds between two urea groups of the anion receptor and target anion. The response time of the developed sensors was optimized as 10 min (Figure S8, details in the Supporting Information).

described in the Supporting Information. The presented error bars illustrate the average error from different freshly modified electrodes. They were prepared for each set of experiments.

■

RESULTS AND DISCUSSION Fabrication of Electrochemical Sensors for Anion Recognition. Most synthetic anion receptors are insoluble or weakly soluble in water.22 This prevents the application of these anion receptors as host molecules for recognition of anions in water. The functionalization of the electrode surface with proper anion host molecules allows the observation of the intermolecular recognition process at the interface (water/ solid). In the present study, efforts were made to develop a highly sensitive electrochemical device for the ultratrace detection of anions (Cl−, SO42−, and Br−) in a highly diluted aqueous solution. The present sensing system is based on a gold electrode modified with a redox-active layer consisting of a copper or cobalt dipyrromethene complex functionalized with a dipodal anion receptor. Scheme 1 is an illustration of the functionalization of the gold electrode surface with the analytical active layer. The first step represents the modification of the gold electrode surface with a mixture of 4-mercaptobutanol and the thiol derivative of dipyrromethene. These are attached to the surface of electrode by a direct covalent Au−S bond. In the second step, Cu(II) or Co(II) complexes with dipyrromethene were formed at the surface of the electrode. In the last step of modification, these complexes were treated with the dipyrromethene-functionalized dipodal anion receptor which acts as a host molecule for the recognition of guest anions. Characterization of the Redox-Active Layers. The presence of redox-active centers on the electrode surface was confirmed electrochemically using cyclic voltammetry (CV). Figure S4 shows the CV recorded for electrodes modified with DPM-Cu(II) and DPM-Co(II) complexes. The electrochemical measurements have been performed in 0.1 M NaNO3 and 0.01 M H3BO3 as supporting electrolyte (pH 4.0 adjusted using 0.1 M HNO3). These conditions have been selected experimentally, for which the details are provided in the Supporting Information (Figures S5 and S6). The presence of a small amount of the weak acid H3BO3 stabilizes the reduction of Cu(II) to the Cu(I) state, and Cu(II)/Cu(I) redox couple peaks are clearly visible in Figure S5.23 The electrode modified with both DPM-Cu(II) and DPM-Cu(II)-DPM-AR showed quasireversible behavior. The electrochemical parameters (peak positions and peak current values) obtained for this study are summarized in Table S1. After the immobilization of the dipodal anion receptor via a DPM unit, the intensity of the oxidation and reduction peaks current decreased for both systems studied (Figure S4, parts a and b, and Table S1). Potential shifts of the peak current after dipodal anion receptor attachment were observed only in the case of a system incorporating Cu(II) redox centers. The potentials were moved into higher values, 45 mV for oxidation, and 23 mV for reduction (Table S1). In order to eliminate the capacitive current, Osteryoung square wave voltammetry (OSWV) has been performed. Using this technique, for the gold electrode modified with DPMCu(II) and DPM-Co(II) complexes, well-defined redox peaks were obtained (Figure S4, parts c and d, and Table S1). Using the OSWV technique, we can observe a small shift in the peak potential in the case of the Co(II) system also which was not clearly visible in the CV. After dipodal anion receptor 12758


Article


Figure 1. Representative cyclic voltammograms obtained for gold electrodes modified with (A) DPM-Cu(II)-DPM-AR and (B) DPM-Co(II)-DPMAR in the absence and presence of 9.0 pM of the following anions: Cl−, SO42−, and Br−. Scan rate: 0.1 V/s. Supporting electrolyte: 0.1 M NaNO3 and 0.01 M H3BO3, pH 4.0.

Figure 1 shows the CVs recorded for gold electrodes modified with DPM-Cu(II)-DPM-AR and DPM-Co(II)-DPMAR in the absence and presence of 9.0 pM of target anions (Cl−, SO42−, and Br−) in 0.1 M NaNO3 and 0.01 M H3BO3 solution (pH 4.0). The pKa value of the tertiary amines is in the range of 9−10. Therefore, under such conditions (at pH 4.0), the tertiary nitrogen atom in the anion receptor is protonated. The obtained results clearly show that addition of target anions to the electrochemical cell causes a shift in the corresponding redox peak potentials of Cu(II)/Cu(I) or Co(II)/Co(III) assisted with a decrease of the redox current intensity (Figure 1). These two parameters indicated the interaction between the proposed layers and the target anions. The oxidation potentials were shifted to the lower values, ca. 57, 40, and 17 mV for Cl−, SO42−, and Br−, respectively, for the DPM-Cu(II)-DPM-AR layer, whereas in case of DPM-Co(II)-DPM-AR this shift was observed ca. 154, 61, and 47 mV for Cl−, SO42−, and Br−, respectively (Figure 1 and Table S3). The reduction peak potentials were also moved into lower values, 54, 11, and 9 mV for Cl−, SO42−, and Br−, respectively, for the DPM-Cu(II)DPM-AR layer, whereas for DPM-Co(II)-DPM-AR this shift was observed ca. 102, 17, and 7 mV for Cl−, SO42−, and Br−, respectively (Figure 1 and Table S4). The maximum shift in the oxidation and reduction peak potentials was obtained for Cl−

anion for both the layers incorporating Cu(II) and Co(II) units. A higher shift in the peak potentials was observed for the layer incorporating Co(II) units when compared with the analogous layer incorporating Cu(II) centers. Cu(II) first goes by reduction to Cu(I), and then Cu(I) undergoes oxidation to Cu(II). However, in the case of the Co(II) system, the center undergoes first oxidation from Co(II) to Co(III), and then reduction from Co(III) to Co(II). Cobalt possesses higher oxidation state in comparison to copper resulting in stronger electrostatic interaction between cobalt redox centers and dipodal receptor−Cl− complex. This might be the reason why the DPM-Co(II)-DPM-AR layer interacts more strongly with the target Cl− compared to the DPM-Cu(II)-DPM-AR system. Schematic representation of the reactions occurring at the gold electrode modified with (1) DPM-Cu(II)-DPM-AR and (2) DPM-Co(II)-DPM-AR complex are shown in Scheme S2. The protonated forms of DPM-Cu(II)-DPM-AR and DPMCo(II)-DPM-AR are abbreviated as DPM-Cu(II)-DPM-ARH and DPM-Co(II)-DPM-ARH, respectively. The electron-transfer reaction of the redox-active center is coupled with the complexation of the receptor with the target anion. The reaction coupling efficiency (RCE) of the system [DPMCu(II)-DPM-ARH] or [DPM-Co(II)-DPM-ARH] toward the 12759


Article


Figure 2. Representative Osteryoung square wave voltammograms recorded at gold electrodes modified with DPM-Cu(II)-DPM-AR and DPMCo(II)-DPM-AR with varying concentrations of Cl− anion in 0.1 M NaNO3 and 0.01 M H3BO3 (pH 4.0), and the plots of difference in intensity of the redox currents in relation to the concentration of anions (Cl−, SO42−, and Br−) at the different electrodes modified with DPM-Cu(II)-DPM-AR and DPM-Co(II)-DPM-AR (n = 4). Concentration of anions: (i) 0, (ii) 1.0, (iii) 2.0, (iv) 3.0, (v) 6.0, (vi) 9, (vii) 12.0, (viii) 15.0, and (ix) 18 pM.

Cl− binding was obtained for DPM-Co(II)-DPM-AR (1.1 × 1012 M−1) than for the DPM-Cu(II)-DPM-AR layer (6.0 × 1011 M−1). Sulfate and bromide binding were 1 or 2 magnitudes lower than Cl− (Table S6). The explanation for such unexpected high values of binding constants is the fact that the process of anion recognition in our case takes place at the aqueous/modified electrode interface. Such phenomena of enhancement of binding constant between anions and receptor at an interface was observed by Ariga and Kunitake35 and Xiao et al.10 The high stability of the interfacial complexes results from the low dielectric constant of the monolayer phase. In addition, they concluded that the interface anion bindings did not require their full dehydration. This strongly facilitates their complexation by the receptors attached to the electrodes. CV was further used to study the kinetics of the electrode reaction for the guest anion complexation with the host molecule located at the gold electrode. Figures S10 and S11 show the CVs at different scan rates (0.01−1 V/s) at the electrodes modified with DPM-Cu(II)-DPM-AR and DPMCo(II)-DPM-AR in the presence of 9.0 pM of anions (Cl−, SO42−, or Br−). The electrochemical parameters of described layers such as the electron-transfer coefficient and the electrode reaction standard rate constant (ks) were also calculated from the effect of the scan rate on the redox peaks of DPM-Cu(II)DPM-AR and DPM-Co(II)-DPM-AR in the presence of target anions, and the results are summarized in Table S2. The α values increased after the addition of the target anions in the electrochemical cell for both DPM-Cu(II)-DPM-AR and DPMCo(II)-DPM-AR systems, indicating a decrease in the reversibility of the system after the interaction between the anions and redox layers.33 The values of ks for both anion sensing systems decreased in the presence of anions when compared with ks in the absence of anions, i.e., in the blank solution (Tables S2 and S5). These results show that the rate of

target anions can be calculated using eq 1,34 and the results are summarized in Tables S4 and S5. KII/KI = exp{−nF(E° bound − E°free )/RT }

(1)

where KI and KII are the stability constants of the complex in different redox states, E°free is the reduction potential in the absence of the anion, E°bound is the reduction potential in the presence of the anion, n is the number of electrons transferred during the process, F is Faraday’s constant, R is the gas constant, and T is the temperature. Equation 1 links the thermodynamically important stability constants KI and KII of a complex in different redox states with experimentally measurable redox potentials. Therefore, it provides an easy way to obtain the ratio KII/KI which is a theoretically useful parameter known as the RCE. Electron insertion (reduction) or withdrawal (oxidation) from the host molecule will change the stability constant of the complex formed resulting in a change in the ratio of KII/KI. This change in the stability constant will cause a change in the redox potential of the host according to eq 1. The RCE for [DPM-Co(II)-DPM-ARH] with Cl− was found to be highest. The RCE for [DPM-Co(II)-DPM-ARH] is almost 8.6 times higher than that for the [DPM-Cu(II)-DPMARH] in the presence of Cl− (Table S4). This indicates a stronger binding affinity in the Co(III) state than in the Cu(I) state. The RCE of [DPM-Cu(II)-DPM-ARH] or [DPMCo(II)-DPM-ARH] with Br− was found to be the smallest (Table S4). These results shows that Cl− binds and interacts strongly with DPM-Co(II)-DPM-AR or DPM-Cu(II)-DPMAR systems in comparison to Br− and SO42−. Similar results were obtained when 12 pM concentration of the anions was used (Table S5). The interface binding constants for the target anions with the anion receptor complex formation were also calculated, for which details are provided in the Supporting Information (Figure S9 and Table S6). The results show that the stronger 12760


Article


Scheme 2. Schematic Representation of the Processes Occurring at the Gold Electrode Modified with (A) DPM-Cu(II)-DPMAR and (B) DPM-Co(II)-DPM-AR Complexes

modified with DPM-Cu(II)-DPM-AR was used, and 1.1, 2.3, and 3.0 pM, respectively, when an electrode modified with DPM-Co(II)-DPM-AR was used. The selectivity sequence toward the target anion detection for both Cu(II) and Co(II) complex systems presented in this study was found as Cl− > SO42− > Br− (Figure 2). The high selectivity of Cl− anions can be attributed to the higher binding constant of Cl− with the anion receptor and the stronger electronic effect between the central metal and Cl− anion in the complex. Furthermore, the electrochemical device based on the DPM-Cu(II)-DPM-AR complex is more sensitive, whereas the system based on DPMCo(II)-DPM-AR complex is more selective toward Cl− anion detection among the two systems presented in this paper. The sensor based on DPM-Co(II)-DPM-AR is able to detect Cl− selectively in the presence of Br − and SO 4 2− . The reproducibility, stability of the developed sensors, and control experiments from other anions that do not bind to the anion receptor were also evaluated in the sensing studies, the details for which are provided in the Supporting Information. The developed sensor is highly reproducible and stable with relative standard deviation (RSD) less than 5% (Figure S13). To confirm that interaction between the dipodal anion receptor and the anion is responsible for the generation of the analytical signal, we tested the response of the electrode modified with a DPM-Co(II)-DPM layer, free of anion receptor, toward the chloride anion detection. The control experiment was performed using gold electrodes functionalized with DPM-Co(II)-DPM in the presence of 0.1 M NaNO3 and 0.01 M H3BO3 (pH 4.0) with different concentrations of Cl−. The results showed that, in the case of the electrode free of anion receptor, a negligible current decrease was observed in

electrochemical redox processes slows down for both sensing systems in the presence of the target anions. Electrochemical Recognition of Anions Using Electrodes Modified with DPM-M(II)-DPM-AR, M = Cu or Co. OSWV has been applied for exploring the interaction between the dipodal anion receptor attached to the gold electrode via the DPM-M(II) redox-active layer and target anions. The representative OSWV curves are displayed in Figure 2 and Figure S12. The addition of anions (Cl−, SO42−, or Br−) caused a decrease of the Cu(II)/Cu(I) or Co(II)/Co(III) redox current. The decrease in the current is proportional to the concentration of anions in the electrochemical cell. Linear ranges were obtained from 1.0 to 12.0 pM for all anions. The relatively narrow dynamic range of the proposed sensors is a consequence of the limited number of anion receptor molecules attached to the redox layer (Supporting Information). Second, the formation of anion receptor−target anion complex caused the changes of the sensing layer structure as well as its charge. This influences accessibility of counterions (supporting electrolyte) to the redox centers. The lack of an appropriate number of counteranions in the vicinity of redox centers leads to a slow down of the redox reaction, and the faradaic current decreases gradually with increase in the concentration of target anions in the electrochemical cell (Figure 2).24 Therefore, even not all anion receptors are occupied by target anions, the complexation of the next target anions cannot be observed because of very low faradaic current (Figure 2), which results in the narrow dynamic range of the developed sensor. The limits of detection for Cl−, SO42−, and Br− were found to be 1.0, 1.4, and 1.6 pM, respectively, when an electrode 12761


Article

Analytical Chemistry the presence of Cl− in comparison with a system incorporating the anion receptor DPM-Co(II)-DPM-AR (Figure S14). These control experiment results clearly demonstrate the importance of the presence of the anion receptor in the sensitive detection of anions in the aqueous medium. These control experiment results also proved that the anions are not adsorbed on the electrode surface and analytical signal change is a consequence of the complex formation between the anion receptor and the target anions. Determination of Cl− Anions in the Presence of an Artificial Matrix Mimicking Physiological Sample. To show the practical application of the developed sensor based on DPM-Co(II)-DPM-AR, experiments were performed to determine the concentration of Cl− anions in the artificial matrix mimicking human serum, and the results are presented in Figures S15 and S16 and Table S7. The OSWV and calibration plot recorded in the artificial matrix are similar as recorded with Cl− anion solution (Figure 2 and Figure S15). This shows that the presence of artificial matrix did not influence the detection of Cl− anions. The values of recovery were in the range from 98% to 102%, suggesting the accuracy of the DPM-Co(II)-DPM-AR modified gold electrode based sensor. These results confirm that the proposed sensor is reliable and sensitive enough for the determination of Cl− anions in biological samples. Furthermore, high dilution eliminates the influence of ion pairing and matrix effects in the biological samples. Mechanism of Anion Recognition by the Proposed Systems. On the basis of our results, we have proposed the mechanism of the analytical signal generation. The decrease of faradaic current and shift in the redox peak potential after complexation of anions with the anion receptor is the consequence of two phenomena: (i) changes in the accessibility of counterions (supporting electrolyte) to the redox center (Scheme 2) and electron-transfer efficiency to/from the underlying electrode,24 and (ii) the electrostatic interaction between the central metal and target anion in the complex. The flexibility of the spacer between the receptor and redox center allows bringing the anion−receptor complex enough close to the redox center to communicate with each other by means of electrostatic interaction through space (Scheme 2). This causes changes in the electron density of the redox center through space electrostatic interaction.34 Similar phenomenon is described by Beer et al. in the literature.3,34 In consequence of this, a shift in the peak potential position and a decrease of current intensity are observed. The difference in selectivity between the Cu(II) complex modified electrode and the analogous complex incorporating Co(II) centers is a consequence of possibilities of approach of receptor−anion complex toward the redox center. In case of the cobalt redox center, because of counterions (anions), the distance between the anion−receptor complex and redox center is longer than that in the case of the copper one in which the counterions are cations (Scheme 2). As a result of this, the Co(II) complex system is able to detect Cl− anions selectively in the presence of SO42− and Br− because Cl− anions have a stronger interaction with the redox centers present at the electrode surface compared to SO42− and Br−. Comparison of the Anion Sensing Systems Presented with Those Already Reported. In the literature, only a few reports concerning the detection of anions in the aqueous medium are available. The data collected in Table S8 clearly show that the presented systems are superior to others already

reported when taking into account the sensitivity (picomolar range) and water medium.

■

CONCLUSIONS In this manuscript, we have described a novel electrochemical anion recognition system based on gold electrodes functionalized with dipodal anion receptor attached to dipyrromethene Cu(II) or dipyrromethene Co(II) redox-active layers. This system was suitable for sensitive and selective anion detection in highly diluted (picomolar range) aqueous medium. In these systems, the redox centers responsible for the analytical signal generation are located relatively close to the electrode surface and their positions remained unchanged after anion recognition. It may be assumed that the dipodal receptor attached to the redox centers is flexible. Therefore, after anion encapsulation the complex could change its position closer to the redox centers. This creates the possibility for the “space” electrostatic interactions between Cu(II) or Co(II) redox centers and dipodal anion receptor−anion complexes. The consequence of this phenomenon is that the redox potential position shift and redox current decrease, which is the base of the generation of the analytical signal. Both systems DPMCu(II)-DPM-AR and DPM-Co(II)-DPM-AR showed better selectivity toward Cl− anions than Br− and SO42−. The selectivity sequence correlates well with the binding affinity. The electrodes modified with DPM-Cu(II)-DPM-AR exhibited better anion sensitivity, whereas the electrode modified with DPM-Co(II)-DPM-AR exhibited better selectivity. This can be attributed to the stronger interaction between the dipodal anion receptor−target anion complexes and the Co(II)/ Co(III) redox centers in comparison to Cu(II)/Cu(I) systems and the different approach of anion receptor−anion complexes toward the redox center.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b03001. Detailed experimental data concerning materials and chemicals, instrumentation, synthesis of the anion receptor, cleaning of gold electrodes, electrode modification with DPM-Co(II)-DPM, electrochemical measurements, preparation of artificial physiological matrix, optimization of measuring conditions, binding constants and number of receptor molecules at the electrode surface, CV and OSWV related to the developed sensor, steps of sensor preparation, processes occurring at the electrode surface, and comparison of electrochemical parameters and electrochemical sensors reported in literature (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +48-895240124. Phone: +48-895234612. ORCID

Wim Dehaen: 0000-0002-9597-0629 Jerzy Radecki: 0000-0002-7975-2250 Notes

The authors declare no competing financial interest. 12762


Article


■

(32) Laviron, E. J. Electroanal. Chem. Interfacial Electrochem. 1979, 100, 263−270. (33) Bard, A. J.; Faulkner, L. R. Electrochemical Methods and Applications, 2nd ed.; John Wiley & Sons Inc.: New York, 2001. (34) Beer, P. D.; Gale, P. A.; Chen, G. Z. Coord. Chem. Rev. 1999, 185-186, 3−36. (35) Ariga, K.; Kunitake, T. Acc. Chem. Res. 1998, 31, 371−378.

ACKNOWLEDGMENTS This work was supported by the National Science Centre, Poland, no. 2016/21/B/ST4/03834. C.A.E. was supported by the National Council of Scientific and Technological Development-Brazil (CNPq), with a fellowship within the program Science without Borders.

■

REFERENCES

(1) Langton, M. J.; Serpell, C. J.; Beer, P. D. Angew. Chem., Int. Ed. 2016, 55, 1974−1987. (2) Kubik, S. Chem. Soc. Rev. 2010, 39, 3648−3663. (3) Beer, P. D.; Gale, P. A.; Chen, Z. Adv. Phys. Org. Chem. 1999, 31, 1−90. (4) Gale, P. A.; Busschaert, N.; Haynes, C. J.; Karagiannidis, L. E.; Kirby, I. L. Chem. Soc. Rev. 2014, 43, 205−241. (5) Busschaert, N.; Caltagirone, C.; Van Rossom, W.; Gale, P. A. Chem. Rev. 2015, 115, 8038−8155. (6) Beer, P. D.; Szemes, F.; Balzani, V.; Salà, C. M.; Drew, M. G.; Dent, S. W.; Maestri, M. J. Am. Chem. Soc. 1997, 119, 11864−11875. (7) Beer, P. D.; Drew, M. G.; Hesek, D.; Nam, K. C. Chem. Commun. 1997, 107−108. (8) Beer, P. D. Chem. Commun. 1996, 689−696. (9) Beer, P. D.; Hesek, D.; Kingston, J. E.; Smith, D. K.; Stokes, S. E.; Drew, M. G. Organometallics 1995, 14, 3288−3295. (10) Xiao, K. P.; Bühlmann, P.; Umezawa, Y. Anal. Chem. 1999, 71, 1183−1187. (11) Sugawara, M.; Kojima, K.; Sazawa, H.; Umezawa, Y. Anal. Chem. 1987, 59, 2842−2846. (12) Gale, P. A. Acc. Chem. Res. 2011, 44, 216−226. (13) Busschaert, N.; Gale, P. A.; Haynes, C. J.; Light, M. E.; Moore, S. J.; Tong, C. C.; Davis, J. T.; Harrell, W. A., Jr. Chem. Commun. 2010, 46, 6252−6254. (14) Willener, Y.; Joly, K. M.; Moody, C. J.; Tucker, J. H. J. Org. Chem. 2008, 73, 1225−1233. (15) Clare, J. P.; Statnikov, A.; Lynch, V.; Sargent, A. L.; Sibert, J. W. J. Org. Chem. 2009, 74, 6637−6646. (16) Cao, Q. Y.; Pradhan, T.; Kim, S.; Kim, J. S. Org. Lett. 2011, 13, 4386−4389. (17) Zhao, L.; Davis, J. J.; Mullen, K. M.; Chmielewski, M. J.; Jacobs, R. M.; Brown, A.; Beer, P. D. Langmuir 2009, 25, 2935−2940. (18) Bayly, S. R.; Gray, T. M.; Chmielewski, M. J.; Davis, J. J.; Beer, P. D. Chem. Commun. 2007, 2234−2236. (19) Beer, P. D.; Cormode, D. P.; Davis, J. J. Chem. Commun. 2004, 414−415. (20) Ngo, T. H.; Nastasi, F.; Puntoriero, F.; Campagna, S.; Dehaen, W.; Maes, W. J. Org. Chem. 2010, 75, 2127−2130. (21) Langton, M. J.; Beer, P. D. Acc. Chem. Res. 2014, 47, 1935− 1949. (22) Caltagirone, C.; Gale, P. A. Chem. Soc. Rev. 2009, 38, 520−563. (23) Shaikh, A. A.; Badrunnessa, M.; Firdaws, J.; Rahman, M. S.; Pasha, N. A.; Bakshi, P. K. J. Bangladesh Chem. Soc. 2011, 24, 158−164. (24) Kurzątkowska, K.; Sirko, A.; Zagórski-Ostoja, W.; Dehaen, W.; Radecka, H.; Radecki, J. Anal. Chem. 2015, 87, 9702−9709. (25) Lu, X.; Zhang, L.; Li, M.; Wang, X.; Zhang, Y.; Liu, X.; Zuo, G. ChemPhysChem 2006, 7, 854−862. (26) Wang, Q.; Zhi, F.; Wang, W.; Xia, X.; Liu, X.; Meng, F.; Song, Y.; Yang, C.; Lu, X. J. Phys. Chem. C 2009, 113, 9359−9367. (27) Wang, D.; Groves, J. T. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 15579−15584. (28) Finklea, H. O.; Hanshew, D. D. J. Electroanal. Chem. 1993, 347, 327−340. (29) Zhuang, J.; Fu, L.; Tang, D.; Xu, M.; Chen, G.; Yang, H. Biosens. Bioelectron. 2013, 39, 315−319. (30) Grabowska, I.; Singleton, D. G.; Stachyra, A.; Góra-Sochacka, A.; Sirko, A.; Zagórski-Ostoja, W.; Radecka, H.; Stulz, E.; Radecki, J. Chem. Commun. 2014, 50, 4196−4199. (31) Laviron, E. J. Electroanal. Chem. Interfacial Electrochem. 1979, 101, 19−28. 12763


Highly Sensitive Electrochemical Sensor for the Detection of Anions in

Recommend Documents