Design and Characterization of Hybrid Morphology Nanoarrays as Plasmonic Raman Probes for Antimicrobial Detection

Abstract

Advances in nanofabrication have allowed the production of new and more reproducible substrates for the Raman detection of trace antimicrobials in water. The superior substrate uniformity combined with the ability to control surface morphology represents a significant step forward in the design of substrates with improved enhancement factors and trace-detection capabilities. The work presented herein successfully combines electron-beam lithography (EBL) and reactive ion-etching (RIE) protocols for the construction, testing, and validation of plasmonic hybrid morphology nanoarrays for the detection of arsenic antimicrobials in water. The fabricated substrates consist of 2500 μm(2) Ag-coated silicon dioxide (SiO2)/Si pillar nanoarrays of alternating hexagonal and elliptical features. Control of simple fabrication parameters such as inter-particle spacing (gap) and its orientation relative to the laser polarization vector (parallel or orthogonal) result in over a tenfold improvement in the apparent Raman response under optimized conditions. At a 633 nm excitation frequency, the best substrate performance was observed on parallel-oriented features with a 200 nm gap, with over one order of magnitude increase in the apparent surface-enhanced Raman scattering (SERS) signal relative to standard silver-polydimethylsiloxane (Ag-PDMS) nanocomposites. Monitoring of the characteristic As-C stretching band at 594 cm(-1) allowed the detection of arsenic antimicrobials in water well within the parts per million range. Calculated surface-enhancement factors (SEF) for this substrate, employing 532, 785, and 633 nm excitation wavelengths, was within five, six, and seven orders of magnitude, respectively. The effect of substrate morphology and nanofabrication process on the Raman enhancement factor is presented.

Performance Characteristics of Bio-Inspired Metal Nanostructures as Surface-Enhanced Raman Scattered (SERS) Substrates

“…The standard operating procedure for signal normalization and EF calculation, particularly the number of molecules present in the sample volume ( N vol ), is readily available in the literature. 60 Similarly, the numerical density of molecules present at the surface ( N surf ) was determined by calculating the total surface area, for instance an ellipsoid, for the elliptical patterns (acacia leaf, sunflower, acacia pinnate, orchid and Asian Lilium ). The top area was empirically calculated using the average length and width of a particle based on using the SEM images (Figure 2).…”

“…9,59 For instance, the first generation of arrays are elliptical prisms, 150 nm long to 50 nm wide, in a ratio in which the longest axis aligned with the polarization vector of laser source produces the most intense radiation scattering. 28,60 Likewise, larger particle sizes and densities have been explored; 60 and although proven to be SERS active, their enhancement is yet far from that suggested for denser arrays with potential inter-particle coupling. 28,61 Finally, ZEP-520A (Zeon Chemicals L.P.) was selected as the standard EBL photoresist since it was currently validated at the EBL facility to enable reproducible nanofabrication at the 50 nm threshold of the LSP responses.…”

Preparation of Plasmonic Au-TiO2 Thin Films on a Transparent Polymer Substrate

“…To be integrated into lab-on-a-chip [47] or other portable T-LSPR devices [21], including SERS chips [48,49], the LSPR thin films might be combined with microfluidic chips [50][51][52], significantly reducing the consumption of analytes, among other advantages [44,46,53]. The microfluidic LSPR chip might be composed of two main components, where the plasmonic thin film, e.g., deposited on glass, is chemically bonded to a microfluidic module, usually a polymer like poly(dimethylsiloxane) (PDMS), using plasma treatment to functionalize its surface [21,50,54].…”

Surface-Enhanced Raman Scattering (SERS) Studies of Disc-on-Pillar (DOP) Arrays: Contrasting Enhancement Factor with Analytical Performance

“…Significant strives have been made on substrate development since it has been observed that adjustments in simple geometrical parameters (i.e., interparticle space, gap and morphology) can tune plasmonic responses to specific excitation frequencies thus optimizing substrate performance and reproducibility. 6–13…”