Investigation of the Use of Nanocrater-decorated Anodic Aluminum Oxide Membranes as Substrates for Reproducibly Enhanced SERS Signals

IntroductionSurface-enhanced Raman spectroscopy (SERS) is an optical phenomenon yielding enhanced Raman signals on nano-decorated conducting materials. It provides label-free analysis of molecules and has the potential to... [ view full abstract ]

Introduction

Surface-enhanced Raman spectroscopy (SERS) is an optical phenomenon yielding enhanced Raman signals on nano-decorated conducting materials. It provides label-free analysis of molecules and has the potential to detect down to single molecule. Despite the potential sensitivity and the wide range of applications for SERS, it can not be used as a routine diagnostic tool due mainly to the poor reproducibility of the SERS signals. To obtain reproducibly strong SERS signals, both lithographic and non-lithographic approaches are investigated to produce large-area nanopatterned SERS-active substrates displaying periodical arrays of nanostructures. These methods can provide controllable periodicity of plasmonic nanostructures as well as tune hot-spot density and geometry which are known to influence the electromagnetic enhancement, the major contributor to SERS signal intensities. Also the control over the structure and the periodicity results in minimum sample-to-sample variation ensuring reproducibility.

Methods

We studied a non-lithographic method for fabricating periodically decorated nanoparticle arrays by utilizing the barrier sides of anodic aluminum oxide(AAO) membranes. These membranes are a class of special biomaterials that are produced from high purity aluminum via two step anodization. The production of the substrates are easy and highly controllable and compared to lithography it is cost-effective. The obtained barrier side of AAO membranes which are periodically nanobump-decorated(NBDS) are further treated with wet etching to create periodic arrays of nanocraters(NCDS).(Fig.1) After gold-coating on the surfaces, the intensity of SERS signals for both fabricated surfaces were compared by using two different dyes, Methylene Blue and Congo Red.

Results
The optimized thickness of gold was found to be 20 nm for both surfaces.(Fig.2) NCDS displayed intensified SERS signals compared with the NBDS counterparts.(Fig.3) This result was also confirmed with computer simulation studies and it was related to the increased surface roughness for the NCDS substrates. The fabricated Au@NCDS nanoplatforms were stable for extended periods and allowed enhanced (2.3x10⁵ enhancement factor) and reproducible SERS signals(Fig.4) with RSD values 10% from independently prepared samples and LOD levels down to 10^-7 M for Methylene Blue.

Discussion

Our current studies are focused on the potential use of these SERS substrates for sensing biomarker molecules including myoglobin and troponin-T.