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Electrochemical Analysis of Anionic Analytes in Weakly Supported Media

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Electrochemical Analysis of Anionic Analytes in Weakly Supported Media www.nature.com/scientificreports OPEN Electrochemical analysis of anionic analytes in weakly supported media using electron transfer promotion efect: a case study on nitrite Alireza Khoshroo1 & Ali Fattahi1,2* In this study, a simple technique was developed for the electrochemical detection of anionic analytes in weakly supported media. This was conducted by the use of electrochemical paper-based analytical devices (ePADs). A sensing platform was modifed with nereistoxin and used to determine nitrite as a case study. The electrochemical response was improved due to the accelerated electron transfer between the sensing platform and the nitrite through the electrostatic interaction of the amino group of nereistoxin and the nitrite. The electrocatalytic current of the nitrite in the presence of nereistoxin was enhanced in the weakly supported media. By using nereistoxin as a signal enhancer, 97% of the electrochemical signal was obtained at the low ionic strength of the electrolyte, while less than 35% of this signal was obtained in the absence of nereistoxin. The limit of detection was as low as 20 nM using an ePAD. Generally, the proposed ePAD serves as a promising, efcient and low-cost device for sensing applications in weakly supported media. Te development of analytical methods for the detection of various analytes is essential in environmental analysis and clinical research1–3. Any novel method for this purpose needs to be sensitive, accurate, easy to practice and inexpensive4–7. Among various analytical methods, the use of electrochemical paper-based analytical devices (ePAD) has attracted signifcant attention. Tis is due to the unique characteristics of those devices such as simple fabrication processes as well as economical and compact systems8–10. So far, various methods have been used to fabricate paper-based systems such as inkjet printing11, photolithography12, wax printing13, PDMS plotting14, plasma etching15 and laser treatment 16. Most of these methods require harmful chemical materials and compli- cated fabrication processes to create hydrophobic areas on the paper17,18. Hence, a simple and rapid prototyping technique is essential for the construction of ePAD, specifcally in practical applications. Besides, providing user-friendly and low-cost conductive inks is a crucial component in the process of constructing ePADs. To date, many materials, e.g., graphite19, graphene20, carbon nanotube (CNT)21 and silver nanoparticles22, have been widely used to prepare conductive inks. Among these materials, carbon-based electrodes prove to have low background currents and broad electrochemical windows. In addition to a proper method of fabricating ePADs, a new detection procedure should be introduced to enhance the sensitivity of ePAD or facilitate sample preparation. Despite the signifcance of developing new electrochemical sensors for various analysts, most papers in the feld have just reported the application of avail- able electrochemical sensors in weakly supported media23,24. Compton et al.23 showed that the kinetics of elec- trochemical reactions might be afected by the concentration of the supporting media. Te electric feld on the electrode surface decreased with the decrease of the supporting medium concentration, which led to a change in the peak current and the ΔEp25. In another study24, a laccase-based microscale sensor was designed to determine phenol in weakly supported media. Te advantage of this microsystem was the formation of convergent difusion on the surface of microelectrodes rather than the planar mass transfer on macroscale electrodes. 1Pharmaceutical Sciences Research Center, Health Institute, Kermanshah University of Medical Sciences, Bākhtarān, Iran. 2Medical Biology Research Center, Health Technology Institute, Kermanshah University of Medical Sciences, Bākhtarān, Iran. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:14511 | https://doi.org/10.1038/s41598-020-71365-4 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1. (A) ePAD with the fexible paper, SEM images of cross-section (B,C) and top view (D) of ePAD. Te present study aims at the development of an electrochemical paper-based analytical device (ePAD) for the detection of anionic analytes in weakly supported media. Although nitrite is widely used for various applications, it is considered toxic by the World Health Organization (WHO), and its maximum recommended concentra- tion in drinking water should be 13 mg L −126. Terefore, it is crucial to develop an analytical method for nitrite monitoring in real samples. To serve the purpose, this paper reports the application of a straightforward, low-cost, and highly sensitive ePAD for the reliable detection of nitrite. Te ePAD is modifed with a nereistoxin and works through the electrostatic interaction between the amino group of nereistoxin and nitrite. Te catalytic signals are highly improved in the presence of nereistoxin, leading to an accelerated electron transfer. Te fabricated ePAD has proved to be applicable for the detection and analysis of nitrite in weakly supported media. Results and discussion Fabrication of the ePAD. An ePAD was fabricated by the sticking of a patterned self-adhesive glossy paper onto the surface of a commercial type of wax paper. Te diferent steps of the fabrication process are shown in Fig. S1 (Supporting Information). Te patterned self-adhesive glossy paper was used as a stencil to control the size and the shape of the electrode system. Furthermore, this layer could provide a hydrophobic area on the ePAD. Basically, the reproducibility of the surface area of an electrode is a critical parameter for the development of an electrochemical sensor. Te laser cutting technique provides a precise stencil from glossy adhesive paper. Te reproducibility of the surface area of electrodes also depends on laser cutter parameters, including power and speed. In this study, the efects of these parameters on the reproducibility of the patterns were evaluated by the ImageJ sofware. Te mean calculated for the surface area in ten patterns and the relative standard deviations (RSD) are listed in Tables S1 and S2. Te RSD values calculated for the optimum parameters (i.e. the power of 12.5% and the speed of 10 mm/s) were around 4%. Terefore, this method can be applied in the reproducible construction of ePADs. In the next step (Fig. S1e,f), the electrodes and the connections were fabricated by a manual screen-printing process. Te as-fabricated ePAD was mechanically stable and fexible (Fig. 1A). Te thickness of the layer of graphite was investigated using SEM (Fig. 1B–D). It could be controlled easily by adjusting the pattern thick- ness. Te SEM image showed the uniform printing of the graphite layer on the paper surface with a thickness of 40.5 ± 1.9 μm. Te reproducible paper-based electrochemical system could, thus, be fabricated using the proposed method. Te surface morphology of the working electrode before and afer the deposition of AuNPs was observed by SEM (Fig. S2). Te AuNPs were successfully distributed onto the graphite surface with a sphere-like structure. Figure S3a shows the X-ray difraction spectra of the Au/ePAD. Te difraction peaks located at 2θ = 43.2° cor- respond to (100) crystal planes of graphite, while the difraction peaks at around 2θ = 36.3°, 38.1°, 50.4°, and 74.1° can be assigned to (111), (200), (220) and (311) planes of the Au nanoparticles respectively 27,28. Tese results suggest the formation of Au nanoparticles on the graphite working electrode surface. Furthermore, as SCIENTIFIC REPORTS | (2020) 10:14511 | https://doi.org/10.1038/s41598-020-71365-4 2 Vol:.(1234567890) www.nature.com/scientificreports/ Figure 2. (A) CVs of ePAD in the absence (a) and presence (b) of 1.0 mM Fe(CN)3−/4− containing 0.1 M KCl at a scan rate of 100 mV s−1. (B) 1st cycle and the 20th cycle of ePAD in 1.0 mM Fe(CN)3−/4− containing 0.1 M KCl at a scan rate of 100 mV s−1. Figure 3. (A) CVs ePAD, Au/ePAD and NTX–Au/ePAD in 1.0 mM Fe(CN)3−/4− containing 0.1 M KCl at a scan rate of 100 mV s−1, and (B) EIS of ePAD, Au/ePAD and NTX–Au/ePAD in 1.0 mM Fe(CN)3−/4− containing 0.1 M KCl, (C) CVs of NTX–Au/ePAD in 1.0 mM Fe(CN)3−/4− containing 0.1 M KCl at diferent scan rates, from 10 to 140 mV s−1, Inset: Variation of Ip vs. logarithm of ν1/2. the energy-dispersive X-ray spectrum of the Au/ePAD suggests in Fig. S3b, the working electrode surface was composed of Au and carbon. Electrochemical investigation of the ePAD. Cyclic voltammetry was conducted to investigate the elec- trochemical performance of the ePAD in the presence of 1.0 mM Fe(CN)3−/4− containing 0.1 M KCl at a scan rate of 100 mV s−1. Te cyclic voltammogram in the absence of electroactive species (curve a, Fig. 2A) showed a low background current without an oxidation or reduction peak. Tis indicates that graphite ink components 4−/3− are not electroactive in the CVs potential range. However, in the presence of 1.0 mM [Fe(CN)6] , a pair of 4−/3− redox peaks appeared with the ΔEp of 176 mV (curve b in Fig. 2A). Tis suggests that [Fe(CN)6] has a quasi- reversible behavior. In the literature, ΔEp values higher than 150 mV have been reported for graphite-based 19,29,30 31 electrodes , which is attributed to the high electrical resistance of the binder . Furthermore, the ratios of I pa to Ipc for the 1st and the 20th cycles were 1.05 and 1.02, indicating the stability of ferricyanide ions produced at 4−/3− the surface of the electrode. It is noteworthy that no absorption or adsorption of [Fe(CN)6] ions occurred on the graphite working electrode of the ePAD.
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