Prussian blue (PB; iron (III) hexacyanoferrate), is a mixed-valent polynuclear complex widely used in electrochromic display, biosensors, ion-selective electrodes, electrocatalysis and energy storage systems like batteries and fuel cells [1,2]. For the PB preparation, an electrochemical deposition using an equimolar concentration of Fe²⁺ and Fe(CN)₆^3- in 0.1 M KCl-electrolyte condition was reported. In another case, a template approach, wherein either Fe²⁺ (or ironic species) or Fe[(CN)₆]^3- immobilized on the electrode surface followed by interaction with counter ion were reported [3]. For instance, a deliberately added ironic species (>20 wt.%) into a multiwalled carbon nanotube (MWCNT) modified electrode has been utilized as a template for the PB functionalized MWCNT preparation by potential cycling of the precursor electrode with Fe[(CN)₆]^3- [3]. Meanwhile, an intrinsic iron species that originated from its catalytic source has been referred an influencing parameter in altering the electronic conductivity of the MWCNT (MWCNT-Fe*, Fe*-Instrinsic iron) [4]. Our group has utilized the pristine MWCNT as a template for in-situ preparation MWCNT@Fe*(bpy)₃²⁺ and as an electro-Fenton catalyst for electroanalytical applications [5]. However, the in-situ derivation of iron impurity (2-4 wt.%) into the Prussian blue complex has not been explored yet. The inaccessibility of the iron and its low concentrations are the likely reason for the limitation. This report demonstrates an in-situ electrochemical PB formation using MWCNT-Fe* modified glassy carbon electrode as a template and Fe(CN)₆^3- as a counter ionic species. This modified electrode (GCE/MWCNT-Fe@PB) showed a characteristic redox peak for the high-spin ironic state of PB at about 0.2 V vs Ag/AgCl in addition with a post-peak at 0.3 V vs Ag/AgCl [Figure 1A]. The nickel impurity in the pristine MWCNT is suspected to be involved in the complexation with Fe(CN)₆^3- ion along with the ironic species.

In-situ formation of PB on MWCNT-Fe* was probed using the scanning electrochemical microscope technique (SECM). In a typical experiment, MWCNT-Fe* modified GCE was taken as a base-electrode along with 5 mM Fe(CN)₆^3- in pH 2 KCl-HCl electrolyte and the potentials were set as 0.1 V and 0.5 V vs Ag/AgCl accordingly to reduce Fe^III(CN)₆^3- + e^-® Fe^II(CN)₆^4- and parallely reoxidize it as Fe^II(CN)₆^4- ® Fe^III(CN)₆^3- + e^- on the Pt-tip surface (Figure 1B). The Fe^II(CN)₆^4- ion formed as an intermediate species interacts chemically with Fe²⁺ ion that might be stripped out from the MWCNT-Fe* on the interface for the in-situ PB formation [Figure 1C]. An island-like morphology with average particle size, few micrometers of PB crystallites on the MWCNT-Fe* surface was noticed. Typical SECM surface imaging of a controlled carbon, carbon nanofiber (CNF), which doesn’t have any metal impurity, was displayed in Figure 1D. A feature-less surface morphology was noticed under the identical imaging condition. Physico-electrochemical characterization of GCE/MWCNT-PB modified electrode using Raman, FT-IR, UV visible spectroscopy and Field emission scanning electron microscopy (FESEM) supported the formation of a hybrid complex on MWCNT-Fe* electrode surface. It has been confirmed that intrinsic iron content was responsible for the PB-formation. As an independent electroanalytical study, the selective electrocatalytic reduction of H₂O₂ was carried out using a newly developed MWCNT-PB system [Figure 1B]. As an electroanalytical application, amperometric i-t sensing of H₂O₂ was carried out, which showed a calibration plot wth linearity upto ~5mM of H₂O₂ at an operating potential, 0.1 V vs Ag/AgCl in pH 2 solution. There is no interference by common biochemical such as ascorbic acid (AA), uric acid (UA), dopamine (DA), cysteine (CySH), nitrite, nitrate and glucose etc.

In conclusion, we have prepared a Prussian blue functionalized MWCNT modified glassy carbon electrode using an intrinsic iron-containing MWCNT as a template in an acidic solution. The in-situ formation of PB on the MWCNT-Fe* has been probed and imaged using the scanning electrochemical microscope technique. The selective electrocatalytic and electroanalytical applications of the GCE/MWCNT-Fe@PB towards H₂O₂ have been demonstrated. This study will provide unique info about the intrinsic iron in the MWCNT and its in-situ derivatization process.

Acknowledgement

The authors acknowledge the Department of Science and Technology – Science and Engineering Research Board and Technology Development Program, DST-SERB-EMR/2016/002818, IDP/MED/04/2017 and CRG/2021/001048 Schemes for the funding.

References

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