We report a cost effective and facile way to synthesize flexible, uniform, and large area surface enhanced Raman scattering (SERS) substrates using an oblique angle deposition (OAD) technique. The flexible SERS substrates consist of 1 μm long, tilted silver nanocolumnar films deposited on flexible polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET) sheets using OAD. The SERS enhancement activity of these flexible substrates was determined using 10(-5) M trans-1,2-bis(4-pyridyl) ethylene (BPE) Raman probe molecules. The in situ SERS measurements on these flexible substrates under mechanical (tensile/bending) strain conditions were performed. Our results show that flexible SERS substrates can withstand a tensile strain (ε) value as high as 30% without losing SERS performance, whereas the similar bending strain decreases the SERS performance by about 13%. A cyclic tensile loading test on flexible PDMS SERS substrates at a pre-specified tensile strain (ε) value of 10% shows that the SERS intensity remains almost constant for more than 100 cycles. These disposable and flexible SERS substrates can be integrated with biological substances and offer a novel and practical method to facilitate biosensing applications.
Label-free surface-enhanced Raman spectroscopy (SERS) detection of nucleic acid hybridization is impeded by poor spectral reproducibility and the fact that the chemical signatures of hybridized and unhybridized sequences are highly similar. To overcome these issues, highly reproducible silver nanorod SERS substrates along with a straightforward least-squares (LS) technique have been employed for the quantitative determination of the relative ratios of the four nucleotide components A, C, G, and T/U before and after hybridization using a clinically relevant micro-RNA sequence.
Only a few remaining technical hurdles currently prevent the implementation of SERS as a mainstream detection technology. Although oblique-angle deposited silver nanorod arrays provide superior analytical figures of merit for SERS sensing, stability issues of silver surfaces can impede their use for real-world sensing applications within certain environments. To circumvent this issue, silver nanorod arrays are modified with a straight-forward, inexpensive Au-coating via a galvanic replacement reaction. The morphological, structural, compositional, and optical properties of the Au-modified Ag nanorod arrays are studied by multiple ex situ morphological characterization techniques and in situ optical absorbance spectroscopy. Depending on the reaction time, the Au coating experiences five different stages of the morphological and compositional changes. The porosity of the Au layer and the content of Ag decrease with reaction time. The optical measurements show that the representative localized plasmon resonance peak of the nanorod red-shifts as the reaction proceeds. The surface enhanced Raman scattering (SERS) intensity, tested using 4-mercaptophenol, decreases exponentially with reaction time, due to the compositional evolution of the nanostructure from pure Ag to a Au-Ag alloy with increasing Au content. We show that the Au-modified Ag nanorod is very stable in NaCl solution compared to the as-deposited Ag nanorod, and the 20 or 30 minute Au-modified Ag nanorod substrate shows an improved SERS sensitivity for air contamination detection. Such an improved SERS substrate can be used in more hostile environments where a pure Ag nanorod substrate cannot be used, and is good for practical sensing applications.
We demonstrate that silver nanorod (AgNR) array substrates can be used for on-chip separation and detection of chemical mixtures by combining ultra-thin layer chromatography (UTLC) and surface enhanced Raman spectroscopy (SERS). The UTLC-SERS plate consists of an AgNR array fabricated by oblique angle deposition. The capability of the AgNR substrates to separate the different compounds in a mixture was explored using a mixture of four dyes and a mixture of melamine and Rhodamine 6G at varied concentrations with different mobile phase solvents. After UTLC separation, spatially-resolved SERS spectra were collected along the mobile phase development direction and the intensities of specific SERS peaks from each component were used to generate chromatograms. The AgNR substrates demonstrate the potential for separating the test dyes with plate heights as low as 9.6 μm. The limits of detection are between 10(-5)-10(-6) M. Furthermore, we show that the coupling of UTLC with SERS improves the SERS detection specificity, as small amounts of target analytes can be separated from the interfering background components.
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