Plasmonic nano-protrusions: hierarchical nanostructures for single-molecule Raman spectroscopy

Abstract

Classical methods for enhancing the electromagnetic field from substrates for spectroscopic applications, such as surface-enhanced Raman spectroscopy (SERS), have involved the generation of hotspots through directed self-assembly of nanoparticles or by patterning nanoscale features using expensive nanolithography techniques. A novel large-area, cost-effective soft lithographic technique involving glancing angle deposition (GLAD) of silver on polymer gratings is reported here. This method produces hierarchical nanostructures with high enhancement factors capable of analyzing single-molecule SERS. The uniform ordered and patterned nanostructures provide extraordinary field enhancements that serve as excitatory hotspots and are herein interrogated by SERS. The high spatial homogeneity of the Raman signal and signal enhancement over a large area from a self-assembled monolayer (SAM) of 2-naphthalenethiol demonstrated the uniformity of the hotspots. The enhancement was shown to have a critical dependence on the underlying nanostructure via the surface energy landscape and GLAD angles for a fixed deposition thickness, as evidenced by atomic force microscopy and scanning electron microscopy surface analysis of the substrate. The nanostructured surface leads to an extremely concentrated electromagnetic field at sharp nanoscale peaks, here referred to as 'nano-protrusions', due to the coupling of surface plasmon resonance (SPR) with localized SPR. These nano-protrusions act as hotspots which provide Raman enhancement factors as high as 10 over a comparable SAM on silver. Comparison of our substrate with the commercial substrate Klarite™ shows higher signal enhancement and minimal signal variation with hotspot spatial distribution. By using the proper plasmon resonance angle corresponding to the laser source wavelength, further enhancement in signal intensity can be achieved. Single-molecule Raman spectra for rhodamine 6G are obtained from the best SERS substrate (a GLAD angle of 60°). The single-molecule spectrum is invariant over the substrate, due to the patterned ordered nanostructures (nano-protrusions).

Surface Plasmon Enhanced Fluorescence Temperature Mapping of Aluminum Nanoparticle Heated by Laser

“…Plasmonic grating substrates were fabricated using a microcontact lithography stamping process [48,50,[54][55][56][57][58]. A high-definition digital video disk (HDDVD) grating structure was replicated by curing 5:1 Sylgard ® 184 polydimethylsiloxane (Gelest) over a halved, cleaned HDDVD for 24 h at 50 • C and 55% relative humidity.…”

“…Both R6G excitation and emission spectra overlap strongly with the surface plasmon resonance coupling peaks of the grating at near-normal incidence, resulting in enhanced fluorescence intensity of the fluorophores and enhanced capture through directed emission toward the photodetector (i.e., camera) [52]. Excitation may be enhanced via plasmon resonance energy transfer (PRET), while emission is enhanced due to aforementioned SPCE [50,69,70]. The grating lines are apparent in the fluorescence micrograph as the plasmon-induced electric field varies between the peaks and valleys of the grating.…”

“…As an initial control experiment, the laser was irradiated on a sample area without Al NPs for 250 ms. No increase in intensity from the initial temperature was observed, indicating that the THV, which has a low optical absorption at 808 nm, and the grating substrate do not absorb the laser energy shown in Figure 4E,F. Silver is widely used in plasmonic applications due to its inherently low loss [50,77]. In contrary, aluminum, is a high loss material and not an attractive plasmonic material with interband transition located in the near-IR.…”

“…Unfortunately, longduration exposure to exciting radiation may lead to irreversible photobleaching of R6G, and the relatively low signal-to-noise ratio of R6G has been insufficient to measure fast-dynamic events and low-temperature differences, limiting use of rhodamines in nanothermometry. Recently, excitation of fluorophores involving surface plasmon resonance (SPR) coupling of the incident excitation energy at a metal-dielectric interface (e.g., Ag-Al 2 O 3 ) was shown to enhance the intensity of excited dyes by a factor of 100 compared to glass [47][48][49][50][51]. The free-space SPR effect provided by a dielectric-coated metal grating may also enhance the photothermal heating rate experienced by a nanoparticle near the grating surface [9,10].…”

Self-assembled peptide nanostructures for electrical, optical, and magnetic applications

“…[55][56][57] There have been recent reports of various plasmonic substrates that allow wavelength independent SERS. [58][59] The scattering enhancement primarily arises from the local electromagnetic field of surface plasmons, excited by an incident laser light.…”