Highly Sensitive Electrochemical Sensor for the Detection of Anions in

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Highly sensitive electrochemical sensor for the detection of anions in water based on redox active monolayer incorporating anion receptor Balwinder Kaur, cristiane Erdmann, Mathias Daniels, Wim Dehaen, Zbigniew Rafi#ski, Hanna Radecka, and Jerzy Radecki Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03001 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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Analytical Chemistry

Highly sensitive electrochemical sensor for the detection of anions in water based on redox active monolayer incorporating anion receptor Balwinder Kaura, Cristiane Andreia Erdmannb, Mathias Daniëlsc, Wim Dehaenc, Zbigniew Rafińskid, Hanna Radeckaa, and Jerzy Radeckia,* a

Institute of Animal Reproduction and Food Research, Polish Academy of Sciences, Tuwima 10,

10-747 Olsztyn, Poland b

Universidade Estadual de Ponta Grossa - UEPG, Setor de Ciências Exatas e da Terra,

Departamento de Química, Av. Carlos Cavalcanti, 4748, CEP 84030-900 - Ponta Grossa/ PR, Brasil c

University of Leuven, Chemistry Department, Celestijnenlaan 200F, B-3001 Leuven, Belgium

d

Nicolaus University in Toruń, Faculty of Chemistry, Gagarina 7, 87-100 Toruń, Poland

______________________________________________________________________________ *

Corresponding author: E-mail: [email protected];

Fax: +48-895240124; Tel: +48-895234612

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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 pM range). The results showed that both systems, incorporating Cu(II) as well as Co(II) redox centres, exhibited highest sensitivity towards 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 Clwith 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 1.0 pM level for both sensing systems. The electrodes based on Co(II) redox centres displayed better selectivity towards Cl- anion detection than those based on Cu(II) centres which can be attributed to the stronger electronic interaction between the receptor-target anion complex and the Co(II)/Co(III) redox centres in comparison to 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.

Keywords: Gold electrode, Redox active layer, Dipodal anion receptor, Electrochemical anion recognition, Water medium


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Introduction In 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. 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 P.D. Beer 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,

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The drawback of ion channel sensors 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 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 3 ACS Paragon Plus Environment


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the electrochemical molecular recognition based on bi-functional 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. Based on 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

•

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-2ylidene)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-2-yl)(2H-pyrrol-2-ylidene)methyl)benzoate (0.275 mmol) was stirred with 100 mg of 1,1'-(((2-aminoethyl)azanediyl)-bis(ethane-2,1diyl))bis(3-hexylurea) (0.25 mmol) in 2.5 mL of dichloromethane at room temperature overnight. The crude product was purified by column chromatography (SiO2, eluent dichloromethane: 7 N NH3 in methanol 95:5) (Figure S1 and S2). The details are provided in Supporting Information. Successive steps of the modification of gold electrode. The procedure used for the cleaning of gold electrodes is described in Supporting Information. After cleaning, the electrodes were rinsed thoroughly with water, methanol, and dichloromethane and dipped into a 180 µL 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 4 ACS Paragon Plus Environment

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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 thiol derivative of DPM are provided in Figure S3. Detailed experimental data concerning materials and chemicals, instrumentation, modification of electrode with DPM-Co(II)-DPM, electrochemical measurements, and preparation of artificial matrix mimicking physiological sample is described in Supporting Information. The presented error bars illustrates the average error from different freshly modified electrodes. They were prepared for each set of experiment.

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 intermolecular recognition process at the interface (water/solid). In the present study, efforts were made to develop a highly sensitive electrochemical device for the ultra-trace 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 analytical active layer. The first step represents the modification of the gold electrode surface 5 ACS Paragon Plus Environment


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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 centres 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 Supporting Information (Figure S5 and S6). The presence of small amount of the weak acid H3BO3 stabilizes the reduction of Cu(II) to 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 quasi reversible behaviour. The electrochemical parameters (peak positions and peak current values) obtained for this study are summarised 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 S4a,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 centres. 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 S4c,d and Table S1). Using OSWV technique, we can observe a small shift in the peak potential in case of Co(II) system also which was not clearly visible in the CV. After dipodal anion receptor immobilization, the potentials of the redox peak were shifted to higher values, ca. 41 mV 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 S4c,d and Table S1).


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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 centres for counter ions 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 DPM-Co(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 centres on the surface of the electrode.28 The relationship between scan rate vs 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)-DPM-AR and DPM-Co(II)-DPM-AR assembled on the gold surface was found to be 5.4 x 10-11 mol/cm2 and 3.9 x 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 vs. the anodic or cathodic peak potential using Laviron’s equation.31,

32

The values of the electron transfer

coefficient (α) were found to be 0.47±0.02 and 0.58±0.01 for DPM-Cu(II)-DPM-AR and DPMCo(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 (ks) 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)-DPM-AR system is more reversible and the redox process is faster compared to DPM-Co(II)-DPM-AR.33



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Thermodynamic parameters concerning anion recognition by electrodes modified with DPM-M(II)-DPM-AR 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 minutes (Figure S8, details in Supporting Information). Figure 1 shows the CVs recorded for gold electrodes modified with DPM-Cu(II)-DPMAR and DPM-Co(II)-DPM-AR 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 mV, 40 mV 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 mV, 61 mV and 47 mV for Cl-, SO42and Br-, respectively (Figure 1 and Table S3). The reduction peak potentials were also moved into lower values, 54 mV, 11 mV and 9 mV for Cl-, SO42- and Br-, respectively, for the DPMCu(II)-DPM-AR layer, whereas for DPM-Co(II)-DPM-AR this shift was observed ca. 102 mV, 17 mV 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) centres. Cu(II) first goes by reduction to Cu(I), and then Cu(I) undergoes oxidation to Cu(II). However, in the case of Co(II) system, the centre 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 into stronger electrostatic interaction between cobalt redox centres and


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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 DPM-Co(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 centre is coupled with the complexation of the receptor with the target anion. The reaction coupling efficiency (RCE) of the system [DPM-Cu(II)-DPM-ARH] or [DPM-Co(II)-DPM-ARH] towards the target anions can be calculated using equation 134 and the results are summarised in Table S4 and Table S5. KII/KI = exp{-nF(Eobound-Eofree)/RT}

(1)

where KI and KII are the stability constants of the complex in different redox states, Eofree is the reduction potential in the absence of the anion, Eobound 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 reaction coupling efficiency (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 equation 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)-DPM-ARH] 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 [DPM-Co(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)-DPM-AR 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 Supporting Information (Figure 9 ACS Paragon Plus Environment


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S9 and Table S6). The results show that the stronger Cl- binding was obtained for DPM-Co(II)DPM-AR (1.1x1012 M-1) than for DPM-Cu(II)-DPM-AR layer (6.0x1011 M-1). SO42- and Brbinding were one or two magnitudes lower than Cl- (Table S6). The explanation for such unexpected high values of binding constants is fact that 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 interface was observed by Toyoki Kunitake35 and Yoshio Umezawa.10 The high stability of the interfacial complexes results from the low dielectric constant of 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 host molecule located at the gold electrode. Figure S10 and Figure S11 show the CVs at different scan rates (0.01 V/s to 1 V/s) at the electrodes modified with DPM-Cu(II)DPM-AR and DPM-Co(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 (α), 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 DPM-Co(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 (Table S2 and Table S5). These results show that the rate of 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)-DPMAR, M=Cu or Co. OSWV has been applied for exploring the interaction between dipodal anion receptor attached to gold electrode via 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. 10 ACS Paragon Plus Environment

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Linear ranges were obtained from 1.0 pM to 12.0 pM for all anions. The relatively narrow dynamic range of proposed sensors is consequence of limited number of anion receptor molecules attached to the redox layer (Supporting Information). Secondly, the formation of anion receptor-target anion complex caused the changes of sensing layer structure as well as its charge. This influence on accessibility of counter ions (supporting electrolyte) to the redox centres. The lack of appropriate number of counter anions in the vicinity of redox centres lead to 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 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 limit of detections for Cl-, SO42-, and Br- were found to be 1.0 pM, 1.4 pM, 1.6 pM, respectively, while an electrode modified with DPM-Cu(II)-DPM-AR was used, and 1.1 pM, 2.3 pM, 3.0 pM, respectively, when an electrode modified with DPM-Co(II)-DPM-AR was used. The selectivity sequence towards 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 DPM-Co(II)-DPM-AR complex is more selective towards 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 presence of Br- and SO42-. 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 Supporting Information. The developed sensor is highly reproducible and stable with 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, towards 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 11 ACS Paragon Plus Environment


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concentrations of Cl-. The results showed that in case of the electrode free of anion receptor, a negligible current decrease was observed in 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)-DPMAR, 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-S16 and Table S7. The OSWV and calibration plot recorded in the artificial matrix is similar as recorded with Clanion solution (Figure 2 and Figure S15). This shows that 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. Based on 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 consequence of two phenomenon: (i) Changes in the accessibility of counter ions (supporting electrolyte) to the redox centre (Scheme 2) and electron transfer efficiency to/from the underlying electrode.24 (ii) The electrostatic interaction between the central metal and target anion in the complex. The flexibility of spacer between receptor and redox centre allows bringing the anionreceptor complex enough close to the redox centre to communicate with each other by means of electrostatic interaction through space (Scheme 2). This causes changes in the electron density of the redox centre through space electrostatic interaction.34 Similar phenomenon is described by 12 ACS Paragon Plus Environment

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P.D. Beer and Z. Chen in the literature.3, 34 In consequences 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) centres is a consequence of possibilities of approach of receptor-anion complex towards the redox centre. In case of cobalt redox centre because of counter ions (anions), the distance between the anionreceptor complex and redox centre is longer than that in case of copper one in which the counter ions are cations (Scheme 2). As a result of this 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 centres 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 shows that the presented systems are superior to others already reported when taking into account the sensitivity (pM range) and water medium.

Conclusions In this manuscript, we have described a novel electrochemical anion recognition systems 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 (pM range) aqueous medium. In these systems, the redox centres 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 centres is flexible. Therefore, after anion encapsulation the complex could change its position closer to the redox centres. This creates the possibility for the “space” electrostatic interactions between Cu(II) or Co(II) redox centres and dipodal anion receptor-anion complexes. The consequence of this phenomenon is the redox potential position shift and redox current decrease, which is the base of the generation of the analytical signal. Both systems DPM-Cu(II)-DPM-AR and DPM-Co(II)-DPM-AR showed better selectivity towards 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



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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 receptortarget anion complexes and the Co(II)/Co(III) redox centres in comparison to Cu(II)/Cu(I) systems and different approach of anion receptor-anion complexes towards the redox centre.

Acknowledgements This work was supported by National Science Centre, Poland, No 2016/21/B/ST4/03834. Cristiane Andreia Erdmann was supported by National Council of Scientific and Technological Development-Brazil (CNPq), with a fellowship within the program Science without Borders.

Supporting Information The supporting information contains detailed experimental data concerning materials and chemical, instrumentation, synthesis of 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 no. of receptor molecules at the electrode surface, figures showing CV and OSWV related to developed sensor, two schemes showing steps of sensor preparation and processes occurring at the electrode surface, and eight tables listing comparison of electrochemical parameters and electrochemical sensors reported in literature.


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Figures and Scheme captions Figures Figure 1

Representative cyclic voltammograms obtained for gold electrodes modified with (A) DPM-Cu(II)-DPM-AR and (B) DPM-Co(II)-DPM-AR in the absence and presence of 9.0 pM of 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 2

Representative Osteryoung square-wave voltammograms recorded at gold electrode modified with DPM-Cu(II)-DPM-AR and DPM-Co(II)-DPM-AR with varying concentrations of anion Cl- 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 pM, (ii) 1.0 pM, (iii) 2.0 pM, (iv) 3.0 pM, (v) 6.0 pM, (vi) 9 pM, (vii) 12.0 pM, (viii) 15.0 pM, (ix) 18 pM.

Scheme Scheme 1

Schematic representation of the steps involved in the preparation of electrochemical sensor. M represents Cu(II) or Co(II).

Scheme 2

Schematic representation of the processes occurring at the gold electrode modified with (A) DPM-Cu(II)-DPM-AR, and (B) DPM-Co(II)-DPM-AR complex.



(B)

(A) 0.3

DPM-Cu(II)-DPM-AR 0.2 0 pM Cl 9 pM Cl 0.1

Current (µA)

Current (µA)

0.3

0.0 -0.1 -0.2

DPM-Co(II)-DPM-AR 0 pM Cl 0.2 9 pM Cl 0.1 0.0 -0.1 -0.2

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-0.2

0.0

0.4

0.6

0.8

0.3

0.3

DPM-Cu(II)-DPM-AR 20 pM SO4 0.2 29 pM SO4 0.1

Current (µA)

Current (µA)

0.2

Potential (V)

Potential (V)

0.0

DPM-Co(II)-DPM-AR 20.2 0 pM SO4 2-

9 pM SO4

0.1 0.0

-0.1

-0.1

-0.2

-0.2 -0.2

0.0

0.2

0.4

0.6

0.8

-0.2

1.0

0.0

0.2

0.4

0.6

0.8

Potential (V)

Potential (V) 0.3

0.3

Current (µA)

DPM-Cu(II)-DPM-AR

Current (µA)

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Page 18 of 22

-

0.2

0 pM Br 9 pM Br

0.1 0.0

-0.1 -0.2

DPM-Co(II)-DPM-AR 0 pM Br 0.2 9 pM Br 0.1 0.0 -0.1 -0.2

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-0.2

Potential (V)

0.0

0.2

0.4

0.6

0.8

Potential (V) Figure 1


Page 19 of 22

Cl

Cl

(i)

Current (µA)

(i)

Current (µA)

-

0.8 DPM-Co(II)-DPM-AR

-

1.6 DPM-Cu(II)-DPM-AR 1.2 (ix) 0.8 0.4

0.6 (ix) 0.4 0.2 0.0

0.0 0.0

0.2

0.4

0.6

0.8

0.0

1.0

0.2

Potential (V)

0.4

0.6

0.8

Potential (V)

0.0

0.0

DPM-Cu(II)-DPM-AR -0.2

In-Io (µA)

-0.2

In-Io (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.4

-0.4 -0.6

-0.6 -

-

Cl 2SO4

-0.8

Cl 2SO4

-0.8

-

-

Br

-1.0 0

3

-1.0

6

9

12

15

18

21

Concentration (pM)

DPM-Co(II)-DPM-AR

Br 0

3

6

9

12

15

18

21

Concentration (pM) Figure 2



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MBT: 4-mercaptobutanol, DPM: Dipyrromethene, M(OOCH3 )2 : Cu(OOCH3 )2 or Co(OOCH3 )2

Page 20 of 22

HN

HN

NH

O

NH O

N

Gold electrode

NH O

MBT/thiol derivative of DPM (100:1)

OH2

H2O

N

N

HN

HO

N

N

N

HO

HO

HO

DPM modified anion receptor

M(OOCH3 )2

S

S

S

S

S

N M

M

HO

HO

S

S

N

S

S

Scheme 1


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Scheme 2



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Table of Contents

For TOC only


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Highly Sensitive Electrochemical Sensor for the Detection of Anions in

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