Redox reactivity at silver microparticle—glassy carbon contacts under a coating of polymer of intrinsic microporosity (PIM)

Silver microparticles (ca. 1 μm average size clustered into cage-like aggregates of 10–20 μm diameter) are shown to adhere to a glassy carbon electrode surface to give voltammetric current responses, which are considerably enhanced/stabilised when applying a coating with a molecularly rigid polymer of intrinsic microporosity (PIM-EA-TB). In preliminary voltammetric experiments characteristic Ag(0/I) surface oxidation and back-reduction processes are observed in aqueous phosphate buffer (associated with silver phosphate layer formation on the silver surface). In contrast to the oxidation, which is dominated by a nucleation process causing a sharp well-defined current signal, for the back-reduction stochastic current responses are observed possibly associated with density fluctuations in the surrounding liquid phase (“Brownian activation”) as an essential part of the mechanism of conversion of surface-oxidised silver back to silver metal. Graphical abstract Graphical abstract


Introduction
Voltammetric analysis of microparticles [1] has been developed as analytical tool in the study of redox active materials which are "mechanically attached" [2] or adhered [3] or simply deposited from solution onto suitable electrode surfaces. For silver-based materials, voltammetric microparticle analysis has been applied to silver tarnish products [4] and silver alloys [5]. In recent work on silver nanoparticle electrochemistry in solution the "impact" of these nanoparticles [6] and the resulting redox conversion [7] were employed to obtain nanoparticle size, shape [8], and reactivity [9] information. Due to the small size of these nanoparticles (and the appropriate choice of reaction conditions) complete/quantitative conversion of silver to soluble or insoluble species is usually possible [10]. In contrast, for macroscopic silver samples, for example a silver wire or silver-coated textiles, in contact to a glassy carbon electrode surface [11] more complex behaviour has been reported with incomplete conversion and formation of an electrically insulating layer between silver metal and electrode surface. The electrical contact of metallic silver to the underlying glassy carbon electrode has been shown to be broken during oxidation when an insulating film is produced. As a result only a small amount of the available silver was oxidised [11]. The back-reduction to metallic silver was suggested to be associated with nucleation of silver metal on glassy carbon and a "re-connection" from glassy carbon to the metallic silver-coated textile. Very similar processes are observed here for silver microparticles immobilised at a glassy carbon electrode surface and immersed in aqueous electrolyte media. or during potential cycling. This can be crucial for example for application in electrocatalysis where microparticles have to remain in contact to the electrode surface.
Microparticles have to withstand significant mechanical forces during wetting (during immersion) and during drying (for re-use). Figure 1 shows a schematic drawing of a silver microparticle immobilised at a glassy carbon electrode surface. When in the reduced form, the metallic silver makes electrical contact to the glassy carbon. However, when oxidised, the silver is coated with an insulator and electrically disconnected. The silver microparticles are studied here when mechanically "fixed" by application of an intrinsically microporous polymer (see PIM-EA-TB in Figure 1B). The application of a polymer of intrinsic microporosity (PIM) as new class of molecularly rigid and microporous ion-conducting materials in electrochemistry has been suggested recently [12]. It has been demonstrated that metal nano-particle catalysts can be PIMcoated and thereby protected against poisoning [13] and against detrimental loss and corrosion processes [14]. Here, we employ an intrinsically microporous polymer (PIM) material based on an ethano-anthracene (EA) building block that was synthesised employing a Tröger base (TB) method (PIM-EA-TB [15], see molecular structure in Figure   1B). This polymer material exhibits 70 kDa average molecular weight and N2-adsorption surface area of typically 1027 m 2 g -1 [15]. The inherent microporosity of PIM-EA-TB allows ion and small molecule transport without inhibiting the electrode reaction [16]. The rigid molecular structure ensures a stable attachment of the microparticles to the electrode surface as well as sufficient access of electrolyte through the microporous film.
In this study, silver microparticles are investigated at glassy carbon electrode surfaces and when immersed into aqueous phosphate buffer solution. During oxidation, silver microparticles are shown to only react only partially (similar to silver wire in contact to glassy carbon [11]) due to surface coating with an insulating silver phosphate film. The process is chemically reversible and after back-reduction the silver microparticles remain at the electrode surface (protected by a PIM-EA-TB film). Stochastic events during the reduction are suggested to be associated with activation by interfacial momentum transfer from Brownian motion in the liquid phase.

Experimental
Chemical Reagents. Chloroform, isopropanol, sodium hydroxide, phosphoric acid (85 %) were purchased from Aldrich and used without further purification. PIM-EA-TB was prepared following a literature recipe [17]. Solutions were prepared with filtered and deionized water of resistivity 18.2 MΩ cm from a Thermo Scientific water purification system (ELGA).
Instrumentation. A μAutolab III system (Ecochemie, NL) was employed for electrochemical measurements in a conventional three-electrode cell with a platinum wire counter electrode and a KCl-saturated calomel (SCE) reference (Radiometer, Kopenhagen).
All experiments were performed with a 3 mm diameter glassy carbon electrode (Bioanalytical Systems, IN, USA). Morphology of the silver sample was analyzed with a JEOL FESEM6301F scanning electron microscope (SEM).
Silver Microparticles Synthesis. The synthesis of Ag microparticles is based on the nonhydrolytic sol-gel method [18,19] that has been recently applied for the synthesis of Ag antimicrobial coatings [20,21]. Briefly, 642 mg of silver acetate (99%, Aldrich) along with 20 cm 3 of benzylamine (99%, Aldrich) were used for the synthesis of the particles. The

Results and Discussion
Initial experiments were performed with 2 g silver microparticles (previously synthesised for application as anti-microbial agent [20,21], see Figure 2) drop-cast deposited onto a glassy carbon electrode surface and immersed in aqueous 0.1 M phosphate buffer pH 12. Figure 3A shows voltammograms with an oxidation response at 0.26 V vs. SCE. Silver has been reported to undergo oxidation (anodic dissolution) in neutral aqueous phosphate buffer media to switch from Ag(0) to Ag(I) accompanied by formation of poorly soluble phosphate films [11,22,23] (see equation 1). This equation is here tentatively assigned to the process observed in Figure 3, although the true chemical nature of "Ag3PO4(s)" under these conditions may be more complicated.
3 Ag(m) + HPO4 2-(aq) Ag3PO4(s) + H + (aq) + 3 e - The area under the oxidation peak suggests approximately 5 C charge has passed, which corresponds to only 0.2 % of the silver present. When applying a film of 4 g PIM-EA-TB from a chloroform solution over the silver microparticles on the glassy carbon surface, the oxidation signal in the voltammogram can be improved by one order of magnitude in terms of charge (see Figure 3A). The PIM-EA-TB polymer is proposed to act here as a porous matrix that allows oxidation and reduction to occur whilst stopping losses due to dislodged silver microparticles. As a result voltammetric responses for the silver maicroparticles are more stable and repeatable. Within the potential range studied here PIM-EA-TB has no direct electrochemical activity [13,14] and it can be considered both cation and anion conducting [15]. Generally, with the PIM-EA-TB coating applied features such as the position of oxidation and back-reduction voltammetric responses are maintained and also the complex peak shape observed during the reduction is retained. Figure 3B    independent of the scan rate, which is believed to be associated with a nucleation of silver phosphate (equation 1) on the silver surface. Upon scanning the potential more negative the reduction peak appears complex and random without well-defined peak potential and/or onset potential. With increased scan rate peaks move in average to more negative potentials.
When adding nitrate into the solution ( Figure 4C) the well-known silver-catalysed twoelectron reduction of nitrate to nitrite (equation 2 [24,25]) is observed.  recently for other types of silver | glassy carbon contacts [19]. The underlying reasons are likely to be associated with a trade-off between electrolyte conductivity and solubility and will need further study.
For all concentrations of phosphate buffer significant randomness in the reduction signal is observed. This is most clearly seen for experiments in 0.01 M NaOH. Figure 5B shows a multi-cycle voltammetric experiment performed in pH 12 NaOH solution showing the stochastic nature of the reduction current peaks, which occur around 0.0 V vs. SCE and with an average charge complementary to that observed for the oxidation peak, however, in short "bursts" which may be linked to individual particles or regions on the electrode surface "re-connecting" to the silver microparticles. The trigger responsible for these processes could be associated at least in part with a density fluctuation in the liquid phase adjacent to the electrode surface or "Brownian activation".
This type of phenomenon is closely linked to Brownian motion and the associated diffusional transport in the liquid phase. Brownian motion [26] as proposed by Einstein and by von Smoluchowski [27] is induced by natural fluctuations in the density of the liquid, which are comparable to the more collective fluctuations caused by lattice phonons in solid materials. Artificial Brownian motors [28] are of interest in nanoscale systems that exploit these density fluctuations. Brownian motion of objects at liquid | solid interfaces can affect the rate of physical processes such as electron transfer, but these effects require micron-sized particles to become apparent. The idea of "Brownian activation" has been postulated previously for example for the inter-nanoparticle electron transfer within hybrid films of redox active TiO2 nanoparticles [29]. Related effects of Brownian motion at liquid | solid interfaces may also play a role in the voltammetry of "impact" processes [30,31] where stochastic effects are assigned to bulk transport (motion) rather than to surface processes (activation). For cyclic voltammograms shown in Figure 5A nucleation processes are believed to be responsible for the sharp oxidation peak observed during the oxidation of silver microparticles in the presence of phosphate buffer. In contrast, the reduction is governed by a stochastic process with multiple events that occur of a period of time. When scanning the potential of the electrode more quickly these current responses distribute to more negative potentials indicative either (i) of irreversible electron transfer (microscopic) and/or (ii) irreversible activation of microparticles (macroscopic). The stochastic nature of "Brownian activation" is suggested to be associated with these processes but further investigation of the mechanism will be necessary.

Summary and Conclusion
Silver