In Situ Surface-Enhanced Raman Scattering Studies of the Nitrosyl Adduct of Hemin Adsorbed on Roughened Silver Surfaces in Aqueous Electrolytes

Abstract

Modifications in the electrochemical and spectral properties of hemin (Hm) adsorbed on roughened Ag electrodes in an aqueous electrolyte (phosphate buffer, pH 3) induced by brief exposure to mildly acidic solutions containing NO have been examined by in situ surface-enhanced Raman scattering (SERS) using Q band excitation (λexc = 532 nm). Two lines of evidence support the formation of the adsorbed nitrosyl adduct of Hm (NO−Hm) under the conditions employed for these experiments: the complete disappearance of the characteristic voltammetric redox peaks of Hm centered at ca. −0.25 V versus SCE, and a significant drop in the intensity, I, of the ring-based ν30 mode at 1165 cm-1 in the in situ SERS spectrum compared to that of adsorbed Hm under the same conditions. Such a decrease in the intensity of the I(ν30) mode was also found in the solution phase Raman spectrum of Hm in THF and DMF upon NO exposure. Notably absent from the SERS spectrum were features attributable to vibrational modes due to NO and Fe−NO, which are clearly discernible for NO iron macrocyclic adducts in solution phase using Soret band excitation (λexc = 413 and 406 nm). Scanning to sufficiently negative potentials led to the reemergence of the Hm redox peaks and restored the I(ν30) mode in the SERS spectrum recorded at potentials that were positive enough for the reduced form of Hm to undergo full oxidation. These observations clearly indicate that, to the level of sensitivity of these measurements, the formation and subsequent reduction of the adduct can be effected without compromising the integrity of Hm. Careful inspection of the data showed that the SERS spectrum of the NO−Hm adduct resembles that of adsorbed (axially uncoordinated) Hm, rather than its reduced counterpart, measured under otherwise identical conditions, suggesting that the electronic environment within the ring in the NO−hemin adduct and the (NO-free) oxidized Hm are similar. This behavior is in agreement with information derived from Mossbauer effect spectra for NO−Fe macrocyclic adducts.

Food additives characterization by infrared, Raman, and surface‐enhanced Raman spectroscopies

“…18 Thus, the use of an extremely sensitive technique, namely, surface-enhanced Raman spectroscopy (SERS), to study the interaction of a molecule with the metal surface on the nanoscale, permits one to solve the structure of the adsorbed species and thus, in some cases, allows understanding the mechanism of binding of different substrates to their receptors. 19,20 SERS spectroscopy is a very sensitive technique that employs rough substrates at the nanometer scale to enhance the Raman signal produced by the adsorbed and immobilized species. 21 This signal is too weak to be detected for solute concentrations below 10 1 M with conventional RS.…”

Optical Determination of Differential Coverage−Potential Relations of Redox-Active Species Immobilized on Electrode Surfaces

“…As evidenced by the electrochemical and optical results, the redox activity associated with CoTSPc does not seem to be affected by the presence of hydrazine in the electrolyte. It may be speculated that formation of a stable intermediate adduct, of the type found for the NO|Hemin system on silver, for example, would lead to changes both in the electrochemistry and in the optics . It must be stressed, however, that the fact that no such changes can be observed does not rule out that possibility, as the actual concentration of such intermedi-ate species may be below the detection limit at the wavelength selected for this study.…”

A Study of NO Adducts of Iron Protoporphyrin IX Adlayer on Au Electrode with in Situ ATR-FTIR Spectroscopy

“…In terms of model heme−NO complexes, both Fe III −NO ({FeNO} 6 ) and Fe II −NO ({FeNO} 7 ) complexes have been characterized spectroscopically and structurally. Historically, studies on NO binding with iron porphyrins were mainly limited to bulk-phase reactions, − ,− with a variety of techniques including electron paramagnetic resonance (EPR), UV−vis absorption, , X-ray absorption fine structure (XAFS), nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), −, and resonance Raman spectroscopes. ,, …”

“…As for electrochemical systems, cyclic voltammetry in conjunction with optical methods has been applied to monitor the formation and conversion of dissolved iron nitrosyl adducts in both aqueous and nonaqueous media. ,− Recently, efforts have been extended to study the coordination of NO to an iron porphyrin and its subsequent conversion with an expectation to create or modify versatile functional solid−liquid interfaces. ,− Specifically, iron(III) protoporhyrin IX (FePP) (or hemin, a prothetic group of many hemoproteins; for its basic structure, please refer to Chart 1), irreversibly adsorbed on graphite and metal surfaces, exhibits excellent electrocatalytic effects on the reduction of dioxygen, hydrogen peroxide, nitrite, ,, and carbon dioxide as well as potential sensing for trace amounts of CO and NO , because of its extraordinary binding ability.

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