Combining 3-D plasmonic gold nanorod arrays with colloidal nanoparticles as a versatile concept for reliable, sensitive, and selective molecular detection by SERS

Abstract

The detection of molecules at an ultralow level by Surface-Enhanced Raman Spectroscopy (SERS) has recently attracted enormous interest for various applications especially in biological, medical, and environmental fields. Despite the significant progress, SERS systems are still facing challenges for practical applications related to their sensitivity, reliability, and selectivity. To overcome these limitations, in this study, we have proposed a simple yet facile concept by combining 3-D anisotropic gold nanorod arrays with colloidal gold nanoparticles having different shapes for highly reliable, selective, and sensitive detection of some hazardous chemical and biological warfare agents in trace amounts through SERS. The gold nanorod arrays were created on the BK7 glass slides or silicon wafer surfaces via the oblique angle deposition (OAD) technique without using any template material or lithography technique and their surface densities were adjusted by manipulating the deposition angle (α). It is found that gold nanorod arrays fabricated at α = 10° exhibited the highest SERS enhancement in the absence of colloidal gold nanoparticles. Synergetic enhancement was obviously observed in SERS signals when combining gold nanorod arrays with colloidal gold nanoparticles having different shapes (i.e., spherical, rod, and cage). Due to their ability to produce localized surface plasmons (LSPs) in transverse and longitudinal directions, utilization of colloidal gold nanorods as a synergetic agent led to an increase in the enhancement factor by about tenfold compared to plain gold nanorod arrays. Moreover, we have tested our approach to detect some chemical and biological toxins namely dipicolinic acid (DIP), methyl parathion (MP), and diethyl phosphoramidate (DP). For all toxins, Raman spectra with high signal-to-noise ratios and reproducibility were successfully obtained over a broad concentration range (5 ppm-10 ppb). Our results suggest that the slightly tangled and closely-packed anisotropic gold nanorod arrays reinforced by the gold nanoparticles may serve as an ideal SERS substrate to detect any analyte in trace amounts.

Micro‐/Nanostructured Highly Crystalline Organic Semiconductor Films for Surface‐Enhanced Raman Spectroscopy Applications

“…Note, the enhancement factors achieved in the current organic semiconductorgold bilayer substrates are signifi cantly higher (>1000×) than those of pristine rough gold fi lms and 3D gold nanorod arrays previously reported through oblique-angle deposition methods (≈10 4 -10 5 ). [ 42,51 ] These data demonstrate the key role of the underlying organic semiconductor in the observed high SERS activity of the fi nal substrates. Therefore, in addition to electromagnetic enhancements, micro-/nanoscale charge-transfer processes in the C8-BTBT/gold/MB interfaces may be another important contributor to SERS signal enhancement possibly by magnifying the molecular polarizability.…”

“…In the case of C8-BTBT fi lm fabricated at α = 90°, a 3D morphology results in the generation of hot spots with an extremely high electric fi eld enhancement as a result of the tip-focusing, cavity resonances and antenna effects. [ 41,42 ] This factor is the dominant contributor to the observed high SERS signal enhancement for this particular fi lm. On the other hand, for the C8-BTBT fi lms fabricated at α = 10°, island like gold formations, which are very effective to create hot spots as a result of stronger enhancement of the local electromagnetic fi eld by closely placed gold nanostructures, should be the main contributor to the observed electromagnetic enhancement.…”

“…A closer look at these fi lms via crosssectional SEM shows that platelet microstructures with thicknesses of 70-500 nm are stacked on top of each other to adopt a face-on orientation relative the substrate surface. In sharp contrast, when the deposition angle was changed from 10° to 90°, high density arrays of vertically aligned ribbon-like micro-/ nanostructures, which are highly favorable for SERS applications, [ 41,42 ] were observed (Figure 1 a,b). The heights of these edge-on features are 0.8-3.0 µm.…”

“…All collected spectra are well defi ned and have acceptable signal-tonoise ratios, consistent with our earlier reports for MB. [ 41,42 ] For all cases, the most prominent peaks in the spectra of MB can be summarized as follows: ring stretch (ν(C C)) at ≈1620 cm −1 , symmetric and asymmetric C N stretches (ν sym (C N) and ν asym (C N)) at ≈1395 and ≈1433 cm −1 , respectively, and the C N C skeletal deformation mode (δ(C N C)) at ≈449 cm −1 . Interestingly, gold-coated C8-BTBT fi lms fabricated at α = 90° exhibit a remarkable increment of the SERS signal intensities relative to that of the smooth gold fi lm.…”

Light‐Driven Unidirectional Liquid Motion on Anisotropic Gold Nanorod Arrays

“…Furthermore, the structural properties including length, density, and tilt angle of the fabricated AuNRs could be easily manipulated by controlling the deposition parameters in order to tune their plasmonic behaviors. [ 24,25 ] Using a spectroscopic ellipsometry, we evaluated the plasmonic properties of fabricated AuNR arrays. Figure 2 shows the variations of ellipsometric variables (Ψ and Δ) and pseudo-dielectric constants (real (〈 ε real 〉) and imaginary (〈 ε Im 〉) parts) for AuNR arrays before PNIPAAm coating with respect to the direction of incoming light at 65° of incident angle.…”