Electrocatalysis with a Dirhodium-Substituted Polyoxometalate Anchored in a Xerogel-Based Composite: Application to the Oxidation of Methionine and Cystine at Physiological pH

Abstract

Replacing a tungsten oxide group of phosphotungstic acid with a Rh(II) acetate dimer yielded a species, Rh 2 POM, that catalyzed the electrochemical oxidation of L-methionine (L-Met), L-cystine, and As(III). The electrocatalytic activity of Rh 2 POM was greater than that of the simple Rh(II) acetate dimer in homogeneous solution. A conducting carbon-composite electrode (CCE) comprising Rh 2 POM and carbon powder in a silica matrix prepared by sol-gel chemistry was catalytically active over the pH range 2-10. The CCEs produced voltammetric responses toward L-Met and L-cystine that were pH independent over the above range and were stable for at least 4 months under dry storage. If passivated, the catalytic activity was reproducibly generated by polishing. For example, a relative standard deviation of 2.9 % (5 points) was obtained when the surface was polished between cyclic voltammetric experiments on 5.0 mM L-Met at pH 7.4. A least squares fit of flow injection amperometric data obtained over the range 5-156 mM L-Met (7 points) yielded the following: slope, 0.85 mA mM ¹1 ; intercept, 0.05 mA; and r = 0.995. The detection limit was 2.3 mM.

Electrocatalytic oxidation of small organic molecules in acid medium: Enhancement of activity of noble metal nanoparticles and their alloys by supporting or modifying them with metal oxides

“…In addition, by changing their chemical composition the acid-base and redox behavior can be modified and adjusted to a desired level. This fact makes HPAs an attractive component of redox catalysts in electrochemical processes [13,61-69]. The role of the acid character of HPAs and their salts in their catalytic efficacy has been the topic of extensive investigations [13,61-63,70-72].…”

Preparation and electrocatalytic application of composites containing gold nanoparticles protected with rhodium-substituted polyoxometalates

“…Chemical modification of polyoxometalates to include the dirhodium moiety extends their use to include catalysis of electrochemical oxidations [19-21]. Here, these studies were further expanded to the synthesis and application to electrocatalysis of gold nanoparticles protected by Rh 2 PMo 11 .…”

“…The bis(aceto)dirhodium -11-tungstophosphate (Rh 2 PW 11 ) and its molybdenum analogue, Rh 2 PMo 11 were synthesized in accord with prior reports [18,19]. In summary, Li 2 CO 3 was added to 10 mL of 1.13 × 10 -3 mol dm -3 PW 12 (or PMo 12 ) in a quantity to give a pH of 4.8.…”

“…The synthesis of Rh II -substituted polyoxometalates by first forming the lacunary structure by removing a metal and its terminal oxygen from phosphotungstic acid and then substituting dirhodium acetate to yield [(PO 4 )W 11 O 35 {Rh 2 (OAc) 2 }] 5- , Rh 2 PW 11 , has been described [18]. The Rh 2 PW 11 formed from this Keggin anion or the analogous molybdenum compound, Rh 2 PMo 11 , was demonstrated to catalyze the electrochemical oxidation of methionine and cystine [19], phosphatidylcholine [20], and various peptides [21]. …”

“…The CP served as a trapping agent that allowed preconcentration of the phosphatidylcholine at the electrode surface. Because of the weaker electrostatic interaction of Rh 2 PW 11 than of Rh 2 PMo 11 with cationic counter- layers, the former species was immobilized on electrodes as a component of a composite with a silica sol-gel [19,21]. …”