Utilizing 3D SERS Active Volumes in Aligned Carbon Nanotube Scaffold Substrates

Abstract

Gold and silver nanoparticles and nanostructures exhibit plasmon resonances that result in strong scattering and absorption of light, as well as enhanced optical fi elds near the metal surface. The resonant fi eld enhancement dramatically enhances the weak Raman scattering signals from molecules near the metal surface. [ 1 ] This effect, called surface enhanced Raman scattering (SERS), has been widely pursued as a molecular sensing technology over the past decade. [ 2 , 3 ] SERS is non-destructive, suffi ciently sensitive for single molecule detection, and provides inherent molecular specifi city since it yields molecular vibrational spectra. While fi eld enhancement occurs over an entire nanostructure surface, SERS signals are strongest from small gaps between nanoparticles referred to as "hot spots" where the fi eld enhancement is maximal. [ 4 , 5 ] Most SERS work to date has focused on detection with substrates that are designed to maximize the density, sensitivity, and reproducibility of hot spots in order to give the strongest possible SERS signal. Many substrates have been developed which refl ect the variety of nanofabrication, synthesis, and assembly strategies that have emerged over the past decade. These include semiconducting nanowires, [ 6 ] aggregated colloids, [ 7 , 8 ] colloidal lithography, [ 9 , 10 ] soft lithography, [ 11 , 12 ] e-beam lithography, [ 13 ] and colloidal assembly. [ 14 , 15 ] Most of the substrates consist of nanostructured gold or silver on a fl at substrate. Some reports describe substrates with increased surface roughness to increase the number of hot spots, including aligned carbon nanotube substrates that support silver nanoparticles. [ 16 ] These substrates were found to provide highly sensitive detection. However, optical scattering and light collection occur in a three dimensional focal volume. Therefore, to maximize the quantity of scattered light generated and detected, SERS substrates should contain hot spots in a large three dimensional volume that is matched to the optics of the SERS instrumentation. Three dimensional substrates have been fabricated and tested based on porous silicon, microfabricated silicon, and porous gold. [17][18][19][20] These have indeed improved the detection limit for small molecules like trinitrotoluene (TNT). Therefore, it is desirable to create densely packed metal nanostructures with nanogaps to form plentiful hot spots for better SERS performance. [21][22][23] In the present study, we describe the fabrication of a structurally tunable 3D SERS substrate based on vertically aligned CNTs. Vertically aligned CNTs provide a new paradigm to realize three dimensional SERS substrates with high nanoparticle density. The vertically aligned CNTs were synthesized on a SiO 2 substrate by water-assisted chemical vapor deposition (CVD). The resulting vertically aligned CNTs were several millimeters long and were composed of a mixture of double-and triple-walled nanotubes. [ 24 , 25 ] A gold fi lm (50 nm thickness) was deposited on top of the CN...

Cicada wing decorated by silver nanoparticles as low-cost and active/sensitive substrates for surface-enhanced Raman scattering

“…40 Besides, the number of molecules within a detection area affected the performance of a SERS substrate. 10 Ag NPs on the surface of the cicada wing can offer high density 'hot spots' to increase the number of probe molecules within a detection area, compared to conventional 2D nanostructures. To further verify this point, the 3D finite-difference time-domain (3D-FDTD) simulation was introduced, which as a well-known theoretical simulation method could be displayed more clearly to show the electromagnetic source of Raman signal amplification at noble metal NPs surfaces.…”

“…Especially, Ag nanostructures with strong and tunable LSPRs from visible to near-infrared spectral regions have spurred efforts to fabricate more effective SERS active substrate. [8][9][10][11] Nowadays, because three-dimensional (3D) nanostructured arrays with nanogap-rich Ag nanoparticles (NPs) could generate high density SERS "hot spots" within a detecting area and further improve the SERS effect, the fabricating processes of this structure have been pursued. 10,11 And wide variety of 3D nanostructured arrays, such as silicon pillar/wire array, ZnO nanowire array, carbon nanotube array and glass nanopillar array, etc., have been widely used to fabricate the large-scale 3D active/sensitive SERS substrates.…”

“…[8][9][10][11] Nowadays, because three-dimensional (3D) nanostructured arrays with nanogap-rich Ag nanoparticles (NPs) could generate high density SERS "hot spots" within a detecting area and further improve the SERS effect, the fabricating processes of this structure have been pursued. 10,11 And wide variety of 3D nanostructured arrays, such as silicon pillar/wire array, ZnO nanowire array, carbon nanotube array and glass nanopillar array, etc., have been widely used to fabricate the large-scale 3D active/sensitive SERS substrates. 9,[11][12][13] Usually, the 3D structures after Ag NPs growth will provide more "hot spots."…”

Surface-enhanced Raman scattering of silver thin films on as-roughened substrate by reactive ion etching

“…Surface-enhanced Raman scattering (SERS) has been extensively studied for the last decades and considered as an important ultrasensitive analysis method in nanoscience, biological, medicine, chemistry, and environment fields [1][2][3][4][5]. In SERS, the Raman signal of the probe molecule is amplified through excitation of localized surface plasmon resonances (LSPRs) of the substrate, usually in the form of metal particles or a roughened metal film.…”

Review—Surface-Enhanced Raman Scattering Sensors for Food Safety and Environmental Monitoring

“…Periodic nanostructure arrays on solid substrates have been developed using self-assembly, [63][64][65][66][67][68][69] electrodeposition, [70][71][72][73] template-assisted fabrication, [74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89] lithography, 70,[90][91][92][93][94][95] and skeletonassisted methods. [96][97][98][99][100][101][102][103][104][105] Resonance with excitation light.-When LSPR band is overlapped the wavelength of the excitation light, that is, the plasmon is in resonance with the excitation light, it can enhance SERS activity. Fortunately plasmonic band can be tuned by tailoring the material component, size, shape of metal nanoparticles and the medium around the metal.…”