Removal of Surface Contamination and Self-Assembled Monolayers (SAMs) from Silver (Ag) Nanorod Substrates by Plasma Cleaning with Argon

Abstract

Surface contamination of surface-enhanced Raman (SERS)-active metallic substrates has been a limitation to the utility of SERS as an analytical technique, potentially affecting surface coverage, spectral reproducibility, and analytical limits of detection. We have developed a simple and versatile cleaning method for SERS-active Ag nanorod arrays that consists of a short (4 min) exposure of the substrate to an Ar(+) plasma in a low-pressure environment. The findings presented here demonstrate that this cleaning procedure essentially eliminates organic background contamination. This procedure works equally well for self-assembled monolayers of thiolates that strongly adsorb onto Au and Ag surfaces. For SERS-active surfaces composed of arrays of Ag nanorods prepared by oblique-angle vapor deposition, we investigated the (1) Raman band intensities, (2) nanorod morphology via scanning electron microscopy, and (3) surface hydrophobicity via static contact angle measurements, as a function of exposure time of the Ag nanorods to the Ar(+) plasma. Short (4 min) exposure to Ar(+) plasma eliminated background contamination but decreased the observed SERS intensity for re-adsorbed analytes by approximately a factor of 2 while leaving the nanorod morphology essentially unchanged. Prolonged exposure to Ar(+) plasma (>10 min) resulted in substantial morphological changes of the Ag nanorod lattice and led to a decrease in the observed SERS intensities by a factor of 10. The results presented here suggest that Ar(+) plasma cleaning is an efficient process for removing carbonaceous and organic contamination as well as thiolate monolayers from SERS-active Ag surfaces, as long as the plasma conditions and exposure times are carefully monitored.

Characterization of polycyclic aromatic hydrocarbons using Raman and surface‐enhanced Raman spectroscopy

“…[28,30] To remove the organic contaminants accumulated during fabrication and storage, before SERS measurements, the as-deposited AgNR substrates were cleaned for 2 min in a custom-built inductively coupled radio frequency plasma chamber, which was operated at 30 W under a constant flow of ultra-pure argon with a chamber pressure of~600 mTorr. [31] Bulk Raman spectra…”

Ag nanorod based surface‐enhanced Raman spectroscopy applied to bioanalytical sensing

“…A mild and efficient method has been reported for removing carbonaceous and organic contaminants from Ag nanorod arrays [52]. The results from this investigation demonstrated that under controlled plasma conditions and exposure times (<4 minutes), the Ar + plasma cleaning procedure essentially eliminated any detectable background organic and carbonaceous contamination from Ag nanorod substrates without substantially changing their morphology.…”

Ag Nanorod Arrays for SERS: Aspects of Spectral Reproducibility, Surface Contamination, and Spectral Sensitivity

“…Moreover, the surface contaminants (or impurities) manifest themselves, even in very small concentrations, by their anomalous SERS bands that overlap the SERS signal of a studied molecule. Such problems were previously reported in the case of both colloidal NPs [12,13] and solid substrates [9,14,15]. Thus, special care must be exercised when analyzing any spectral pattern in an effort to reliably distinguish the bands coming from the analyte from those of the contaminants or, better still, to develop a method for preventing the contamination.…”

“…Thus, special care must be exercised when analyzing any spectral pattern in an effort to reliably distinguish the bands coming from the analyte from those of the contaminants or, better still, to develop a method for preventing the contamination. There have been numerous ways to clean SERS-active surfaces from contaminants under ambient conditions, including electrochemical [9], plasma [14,15], or ozone cleaning [16], but none have been uniformly successful [11]. Unfortunately, a side result of some electrochemical desorption techniques can be the introduction of surface defects where the carbon contamination was present or strong oxidation due to the high chemical reactivity of Ag [16,17].…”