An electronic device based on gold nanoparticles and tetraruthenated porphyrin as an electrochemical sensor for catechol

The aim of this study was to obtain an electrochemical device between the electrostatic interaction of the electropolymerized porphyrin {CoTPyP[RuCl3(dppb)]4}, where TPyP = 5,10,15, 20-tetrapyridilphorphyrin and dppb = 1,4-bis(diphenylphosphino)butane, and gold nanoparticles (AuNPsn−), to be used as a voltammetric sensor to determine catechol (CC). The modified electrode, labelled as [(CoTPRu4)n8+-BE]/AuNPsn− {where BE = bare electrode = glassy carbon electrode (GCE) or indium tin oxide (ITO)}, was made layer-by-layer. Initially, a cationic polymeric film was generated by electropolymerization of the {CoTPyP[RuCl3(dppb)]4} onto the surface of the bare electrode to produce an intermediary electrode [(CoTPRu4)n8+-BE]. Making the final electronic device also involves coating the electrode [(CoTPRu4)n8+-BE] using a colloidal suspension of AuNPsn− by electrostatic interaction between the species. Therefore, a bilayer labelled as [(CoTPRu4)n8+-BE]/AuNPsn− was produced and used as an electrochemical sensor for CC determination. The electrochemical behaviour of CC was investigated using cyclic voltammetry at [(CoTPRu4)n8+-GCE]/AuNPsn− electrode. Compared to the GCE, the [(CoTPRu4)n8+-GCE]/AuNPsn− showed higher electrocatalytic activity towards the oxidation of CC. Under the optimized conditions, the calibration curves for CC were 21–1357 µmol l−1 with a high sensitivity of 108 µA µmol l−1 cm−2. The detection limit was 1.4 µmol l−1.


Materials
CC was purchased from Vetec and all the chemicals used were of reagent grade or comparable purity. Methanol, dichloromethane and ether were distilled prior to use. All the aqueous solutions were prepared with ultra-pure water (ASTM type I, 18 MΩ cm of resistivity). Ruthenium(III) chloride hydrate, 1,4-bis(diphenylphosphino)butane (dppb), chloroauric acidH [AuCl 4 ], tetrabutylammonium hexafluorophosphate (TBAH) and cobalt (II) acetate salt were purchased from Aldrich and they were used as received. Acetate buffer solutions (HAc-NaAc, 0.1 mol l −1 ) with different pH values were prepared by mixing 0.1 mol l −1 of acetic acid (HAc) and 0.1 mol l −1 of sodium acetate (NaAc) and the pH was adjusted by NaOH or HCl. Purified argon atmosphere was used in all procedures described to remove the dissolved oxygen.

Apparatus
Voltammetric experiments were performed on a µAUTOLAB type III potentiostat/galvanostat with a conventional three-electrode system. A bare or modified GCE (geometric diameter = 0.2 cm) or ITO electrode was used as a working electrode. A KCl saturated Ag/AgCl (AgCl (sat) ) electrode was used as a reference electrode and a platinum wire as a counter electrode. All the experiments were carried out at room temperature. The cyclic voltammetry (CV) was performed from −0.4 to 1.0 V versus Ag/AgCl (s) with a scan rate of 100 mV s −1 , sample interval, 0.0001 V and quiet time of 2 s. Under the conditions used, E 0 for the one-electron oxidation of [Fe(η 5 -C 5 H 5 ) 2 ], added to the test solutions as an internal standard, is +0.43 V.
UV/Vis spectra were recorded on a Shimadzu spectrophotometer, model UV-1800, coupled with a thermoelectrically temperature controlled cell TCC-100 (at 25 ± 0.1°C), using a quartz cell (1 cm), between 200 and 800 nm. Elemental analyses were performed with a Thermo Scientific CHNS-O FLASH 2000 micro analyzer. AFM measurements were performed using a Nanoscope V Veeco/Bruker with a scan assist.

Preparation of gold nanoparticles
AuNPs n− with an average diameter of 5 nm were prepared by reduction of H[AuCl 4 ] in a biphasic mixture of water/toluene (1 : 1) with sodium citrate tribasic dihydrate and sodium tetrahydridoborate. Hexadecyltrimethylammonium bromide (HDTBr) was used as a transfer reagent phase by modifying a procedure described in the literature [48]. In brief, 1.0 ml of 1% H[AuCl 4 ] was added to 90 ml of H 2 O at room temperature (20-23°C). After 1 min of stirring, 2.0 ml of sodium citrate tribasic dihydrate (38.8 mmol l −1 ) was added. The solution was stirred for another 1 min, and then 1.0 ml of fresh 0.075% NaBH 4 was added to the solution of sodium citrate tribasic dihydrate (38.8 mmol l −1 ). After this, 20 ml of HDTBr in toluene solution (0.13 mol l −1 ) was mixed with aqueous AuNPs n− . The resulting solution was kept under stirring for 20 min to move the AuNPs n− swiftly from an aqueous phase to an organic phase. Afterwards, the colloidal solution was allowed to stand for phase separation, and then the mixture was separated using an analytical funnel and the organic phase was stored in a dark bottle at 4°C.

Preparation of the modified electrode
The modified electrode was obtained by modifying a procedure described in the literature [46]. Initially, the GCE surface was polished in alumina slurry (  The electropolymerized porphyrin produces a cationic species of mixed-valence {CoTPyP[Ru(dppb) 4 (Cl 3 ) 2 } 2n 8n+ , which adsorbs on the electrode surface ( figure 1). The mechanism of the electropolymerization, including all characterizations of the film, was previously published by our research group [52] and involves the reduction of 'RuCl 3 (dppb)' moiety (Ru III →Ru II ) at 0 V. The quasi-reversible redox process of Ru II Ru III →Ru III Ru III couple is observed at E 1/2 = 0.55 V, assigned to the formation of the films of the mixed-valence complex {CoTPyP[Ru(dppb) 4 (Cl 3 ) 2 } 2n 8n+ . The aggregation of AuNPs n− on the cationic polymeric film occurs by physical adsorption using electrostatic interactions [42,44]. The two basic steps described above were carried out threefold, which produced a total of three bilayers, that presented the optimal condition for CC sensing. In figure 1, it can be observed that the peak currents increase with the number of cyclic voltammograms, owing to the increase in the immobilized , which is a similar system without AuNPs n− , with a roughness value of 4.5 nm, even after five cyclic voltammograms [45]. This result shows that the immobilization of the AuNPs n− on the surface of the electrode causes an increase in the roughness of the film. Undoubtedly, the increasing electroactive area is directly associated with the presence of AuNPs n− .  Figure 4 shows the cyclic voltammograms of CC (0.30 mmol l −1 ), using the nude GCE as a work electrode, exhibiting the anodic peak potential (E pa ) at 516 mV and cathodic peak potential (E pc ) at −126 mV. The potential peak difference ( E p ) was 642 mV, which means that the oxidation of the CC is an irreversible process in the uncovered GCE. However, the work electrode was altered to the [(CoTPRu 4 ) n 8+ -GCE], the electrochemical behaviour of the CC was changed from irreversible to quasi-reversible, with the E pa at 251 mV and E pc at 126 mV, and a significant decrease in the E p = 125 mV. Finally, in the third circumstance, when the [(CoTPRu 4 ) n 8+ -GCE]/AuNPs n− was used as a sensor for CC detection, well-resolved redox processes were also observed, E pa = 334 mV; E pc = 209 mV; and E p = 125 mV, related to the exposed GCE. The main advantage of AuNPs n− on the electrode was a significant increase in the peak currents (I p ), which was approximately seven times higher than the I p obtained with the [(CoTPRu 4 ) n 4+ -GCE]. These results show that the AuNPs n− increases the sensitivity of the electrode.

Influence of pH and scan rate
The effect of the pH solution and scan rate on the electrochemical behaviour of CC with [(CoTPRu 4 ) n 8+ -GCE]/AuNPs n− as a work electrode was investigated by CV. Figure 5 shows conditions. Consequently, the acetate buffer solution was chosen as the supporting electrolyte in the protocol of the quantitative analyses. The influence of the pH solution on the peak potentials of CC was also studied. It was observed that the E pa of CC decreases with the increase of the pH. A linear relationship can be noted between the E pa and pH, described as follows (equation (3.1)): E pa (mV) = −52.5 pH + 0.64 (R 2 = 0.885). (3.1) A plot of E pa versus pH from equation (3.1) (inside figure 5) provides a slope = −52. 5 mV pH −1 , which is consistent with the theoretical value of −59 mV pH −1 [53], indicating that the redox processes of CC require the same number of protons and electrons. Figure 6 shows the result for CC detection, where the intensity of the redox peak currents increases linearly with the increase in the square root of the scan rate (ν 1/2 ). Therefore

Determination of catechol
The determination of CC was carried out by CV using the [(CoTPRu 4 ) n 8+ -GCE]/AuNPs n− . Figure 7 shows    of its initial response obtained in the first day.

Conclusion
This work reports on the construction of an electronic device, using a bilayer based on AuNPs n− and tetraruthenated porphyrin, with the aim of applying it as an electrochemical sensor. Having the electrostatic interaction between the cationic polymeric film of porphyrin and anionic surface of AuNPs n− , the modified electrode based on self-assembly bilayers could be built, labelled as environmental samples analysis. In addition, it exhibits excellent stability over a period of 15 days, with a response of about 97%, owing to its initial response on the first day. The results suggest that the aggregation of AuNPs n− for the polymeric film of porphyrin provides a significant increase in the electroactive area of the electrode.
Data accessibility. All primary data collected and analysed for this study have been deposited with Figshare and can be accessed at https://doi.org/10.6084/m9.figshare.5514994.