Metal Film over Nanosphere (MFON) Electrodes for Surface-Enhanced Raman Spectroscopy (SERS):  Improvements in Surface Nanostructure Stability and Suppression of Irreversible Loss

Abstract

The stability and reproducibility of most SERS-active electrode surfaces are far from ideal. We have focused on this problem by developing and characterizing a metal film over nanosphere (MFON) electrode which solves these shortcomings. Atomic force microscopy (AFM), cyclic voltammetry, and surface-enhanced Raman spectroscopy (SERS) of representative molecules were used to characterize and evaluate the electrochemical and SERS performance of MFON electrodes. Tremendous stability to extremely negative potential excursions is observed for MFON electrodes as compared to standard metal oxidation reduction cycle (MORC) roughened electrodes. Consequently, irreversible loss of SERS intensity at negative potentials is not observed on these MFON electrodes. We conclude that MFON electrodes present a significant advantage over MORC electrodes because SERS enhancement is not lost upon excursion to extremely negative potentials. This work demonstrates that the MFON substrate, while easily prepared and temporally stable, offers unprecedented stability and reproducibility for electrochemical SERS experiments. Furthermore, one can conclude that irreversible loss is not a distinguishing characteristic of electrochemical SERS and consequently cannot be used as evidence to support the chemical enhancement mechanism.

Electrochemical SERS at a structured gold surface

“…The closest example in the literature to the work reported here is that of Tessier et al [24] and Dick et al [7]. Tessier et al prepared inverse gold opals by the co-deposition of 25 nm gold particles and 630 nm latex microspheres to produce thick multilayer samples which they used as a SERS substrate.…”

“…Firstly, the surface enhancement is highly surface selective so the technique is sensitive to molecules adsorbed at, or very close to, the electrode surface and it thus discriminates against molecules in the bulk solution. Secondly, SERS gives information about the molecular structure of the adsorbate, its orientation at the surface [7,8] and the dependence of orientation and coverage on applied potential. Finally, the Raman cross section for water is low so that SERS can be easily used to study electrodes in aqueous solution.…”

“…Consequently a number of alternative SERS substrates have been proposed over the past 30 years. These include the use of grating structures [9], colloidal particle arrays [10], and geometrically structured metal surfaces [7,8] It is generally accepted that there are two contributions to the surface enhancement [11]; a charge transfer (CT) enhancement [12] arising from chemisorption of the adsorbate at the metal surface and an electromagnetic (EM) enhancement [13]. Of these two the EM contribution is usually the more significant and is not adsorbate specific.…”

O efeito SERS na análise de traços: o papel das superfícies nanoestruturadas

“…As ilhas de formato piramidal se formaram em uma geometria hexagonal e o espaçamento entre elas pôde ser controlado pelo diâmetro da esfera polimérica 72 . O mate- rial monodisperso obtido foi construído sobre vidro, silício ou o próprio eletrodo metálico liso 73 . Estes eletrodos metálicos ativados por litografia com esferas poliméricas mostraram-se muito mais estáveis em potenciais negativos extremos que os eletrodos ativados por ciclos de oxidação-redução.…”

Colloidal Metal Surfaces as Biosensors of Biological Samples

“…Although platinum has recently been applied as an alternative plasmonic material, its plasmon band is in the UV region [10]. These metals are most-often used in the form of colloids [11], surfaces obtained in the repetitive electrochemical oxidation-reduction cycles (ORC) [12], vapor-deposited metal island films [13], and lithographyproduced nanostructures [14].…”