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Nanoimprint lithography of Al nanovoids for deep-UV SERS.

Ding T, Sigle DO, Herrmann LO, Wolverson D, Baumberg JJ - ACS Appl Mater Interfaces (2014)

Bottom Line: Deep-ultraviolet surface-enhanced Raman scattering (UV-SERS) is a promising technique for bioimaging and detection because many biological molecules possess UV absorption lines leading to strongly resonant Raman scattering.Here, Al nanovoid substrates are developed by combining nanoimprint lithography of etched polymer/silica opal films with electron beam evaporation, to give a high-performance sensing platform for UV-SERS.Enhancement by more than 3 orders of magnitude in the UV-SERS performance was obtained from the DNA base adenine, matching well the UV plasmonic optical signatures and simulations, demonstrating its suitability for biodetection.

View Article: PubMed Central - PubMed

Affiliation: Nanophotonics Centre, Cavendish Laboratory, University of Cambridge , Cambridge CB3 0HE, United Kingdom.

ABSTRACT
Deep-ultraviolet surface-enhanced Raman scattering (UV-SERS) is a promising technique for bioimaging and detection because many biological molecules possess UV absorption lines leading to strongly resonant Raman scattering. Here, Al nanovoid substrates are developed by combining nanoimprint lithography of etched polymer/silica opal films with electron beam evaporation, to give a high-performance sensing platform for UV-SERS. Enhancement by more than 3 orders of magnitude in the UV-SERS performance was obtained from the DNA base adenine, matching well the UV plasmonic optical signatures and simulations, demonstrating its suitability for biodetection.

No MeSH data available.


Related in: MedlinePlus

UV Raman spectraof adenine: (a) 1 mM of adenine aqueous solution on Al nanovoids (redline) and flat Al films (black line), (b) bulk adenine powders.
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fig3: UV Raman spectraof adenine: (a) 1 mM of adenine aqueous solution on Al nanovoids (redline) and flat Al films (black line), (b) bulk adenine powders.

Mentions: To investigate the potential forUV-SERS, we used adenine as a test molecule. An aqueous solution (1mM) of adenine was drop-cast onto the nanovoid substrate which wassubsequently covered by a quartz coverslip for UV-SERS measurements,forming a thin film of analyte solution on the substrate. The measurementswere performed in the solution phase to reduce any molecular degradation,and simulate biochemical sensor conditions. The Raman signal was collectedby a Raman system with an excitation laser at 244 nm with total integrationtime of 30 s (Renishaw). Since higher laser powers induce some photodegradationof the substrate, laser powers below 0.3 mW were used. The UV laserspot diameter was 5 μm. Given the lattice parameter of our array(∼200 nm), we estimate that a typical spectrum contains ∼500nanovoids. At the pump wavelength (244 nm), both SERS and conventionalRaman measurements are in the resonant Raman condition. The SERS enhancementfactors quoted below are therefore stated on top of the electronicresonant enhancement. The adenine SERS and Raman spectra (Figure 3a) clearly show the four Raman peaks of adenine(1248, 1332, 1497, and 1620 cm–1) with intensityaround 5 counts/mW/s for the 1332 cm–1 peak, whilefor the flat Al film these peaks are barely seen. Moreover, the UV-SERSsignal is highly reproducible across the entire Al nanovoid substrate(see the Supporting Information, FigureS5), which reflects the uniformity of Al nanovoid arrays across thesubstrate. The relatively larger noise for UV SERS compared with ourprevious report16 is mainly due to theshorter accumulation time needed to avoid photodegradation of thismolecular sample. To further confirm the key contribution from UVresonant plasmons, we checked with excitation wavelengths of 633 nm,and no SERS peaks were observed at all due to excitation far off resonance.The UV-SERS signal of 1 mM adenine obtained from these Al nanovoidsis almost of the same intensity as the signals obtained from bulkadenine powder (Figure 3b). The powder formallows the most accurate estimation of the enhancement factors whichare rarely stated in the literature in the deep-UV regime. Previousreports for Pd, Rh, and Ru provide only 2 orders of magnitude enhancementfactors.10,11 Sigle et al.16 give enhancements of up to 6 orders of magnitude, but this is onlygiven for the hot-spot region rather than averaged over the entiresample. A similar value inside a bow-tie antenna is given by Li etal.17 Here, we state a global enhancementvalue over the entire sample which we believe is more relevant fora sensing platform. Taking into account the adenine extinction coefficientand the corresponding optical penetration depth in the adenine crystalsallows an estimate of the number of molecules in the probe volume.From an adenine extinction coefficient of 1.5 × 104 M–1 cm–1 and molecule diameterof ∼2 nm, we estimate that the number of molecules in the powderform interacting with the laser is ∼4 × 1012. This can be compared to the number of molecules contributing tothe SERS signal (given by the molecular concentration of 1 mM, a voidvolume of 2 × 106 nm3 and a total numberof ∼500 probed voids, which gives 5.9 × 108 molecules contributing to the SERS) and allows determination ofthe SERS enhancement factor (on top of resonant electronic enhancementsalways obtained at this wavelength). We estimate this enhancementto be ∼5 × 103 averaged over the plasmonicsurface, which is much higher than previously reported values on suchlarge areas16,18 and is attributed to the near-defect-freefabrication of the surface. These results clearly demonstrate thecapability of using aluminum for plasmonic UV-sensing platforms ona large scale.


Nanoimprint lithography of Al nanovoids for deep-UV SERS.

Ding T, Sigle DO, Herrmann LO, Wolverson D, Baumberg JJ - ACS Appl Mater Interfaces (2014)

UV Raman spectraof adenine: (a) 1 mM of adenine aqueous solution on Al nanovoids (redline) and flat Al films (black line), (b) bulk adenine powders.
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Related In: Results  -  Collection

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fig3: UV Raman spectraof adenine: (a) 1 mM of adenine aqueous solution on Al nanovoids (redline) and flat Al films (black line), (b) bulk adenine powders.
Mentions: To investigate the potential forUV-SERS, we used adenine as a test molecule. An aqueous solution (1mM) of adenine was drop-cast onto the nanovoid substrate which wassubsequently covered by a quartz coverslip for UV-SERS measurements,forming a thin film of analyte solution on the substrate. The measurementswere performed in the solution phase to reduce any molecular degradation,and simulate biochemical sensor conditions. The Raman signal was collectedby a Raman system with an excitation laser at 244 nm with total integrationtime of 30 s (Renishaw). Since higher laser powers induce some photodegradationof the substrate, laser powers below 0.3 mW were used. The UV laserspot diameter was 5 μm. Given the lattice parameter of our array(∼200 nm), we estimate that a typical spectrum contains ∼500nanovoids. At the pump wavelength (244 nm), both SERS and conventionalRaman measurements are in the resonant Raman condition. The SERS enhancementfactors quoted below are therefore stated on top of the electronicresonant enhancement. The adenine SERS and Raman spectra (Figure 3a) clearly show the four Raman peaks of adenine(1248, 1332, 1497, and 1620 cm–1) with intensityaround 5 counts/mW/s for the 1332 cm–1 peak, whilefor the flat Al film these peaks are barely seen. Moreover, the UV-SERSsignal is highly reproducible across the entire Al nanovoid substrate(see the Supporting Information, FigureS5), which reflects the uniformity of Al nanovoid arrays across thesubstrate. The relatively larger noise for UV SERS compared with ourprevious report16 is mainly due to theshorter accumulation time needed to avoid photodegradation of thismolecular sample. To further confirm the key contribution from UVresonant plasmons, we checked with excitation wavelengths of 633 nm,and no SERS peaks were observed at all due to excitation far off resonance.The UV-SERS signal of 1 mM adenine obtained from these Al nanovoidsis almost of the same intensity as the signals obtained from bulkadenine powder (Figure 3b). The powder formallows the most accurate estimation of the enhancement factors whichare rarely stated in the literature in the deep-UV regime. Previousreports for Pd, Rh, and Ru provide only 2 orders of magnitude enhancementfactors.10,11 Sigle et al.16 give enhancements of up to 6 orders of magnitude, but this is onlygiven for the hot-spot region rather than averaged over the entiresample. A similar value inside a bow-tie antenna is given by Li etal.17 Here, we state a global enhancementvalue over the entire sample which we believe is more relevant fora sensing platform. Taking into account the adenine extinction coefficientand the corresponding optical penetration depth in the adenine crystalsallows an estimate of the number of molecules in the probe volume.From an adenine extinction coefficient of 1.5 × 104 M–1 cm–1 and molecule diameterof ∼2 nm, we estimate that the number of molecules in the powderform interacting with the laser is ∼4 × 1012. This can be compared to the number of molecules contributing tothe SERS signal (given by the molecular concentration of 1 mM, a voidvolume of 2 × 106 nm3 and a total numberof ∼500 probed voids, which gives 5.9 × 108 molecules contributing to the SERS) and allows determination ofthe SERS enhancement factor (on top of resonant electronic enhancementsalways obtained at this wavelength). We estimate this enhancementto be ∼5 × 103 averaged over the plasmonicsurface, which is much higher than previously reported values on suchlarge areas16,18 and is attributed to the near-defect-freefabrication of the surface. These results clearly demonstrate thecapability of using aluminum for plasmonic UV-sensing platforms ona large scale.

Bottom Line: Deep-ultraviolet surface-enhanced Raman scattering (UV-SERS) is a promising technique for bioimaging and detection because many biological molecules possess UV absorption lines leading to strongly resonant Raman scattering.Here, Al nanovoid substrates are developed by combining nanoimprint lithography of etched polymer/silica opal films with electron beam evaporation, to give a high-performance sensing platform for UV-SERS.Enhancement by more than 3 orders of magnitude in the UV-SERS performance was obtained from the DNA base adenine, matching well the UV plasmonic optical signatures and simulations, demonstrating its suitability for biodetection.

View Article: PubMed Central - PubMed

Affiliation: Nanophotonics Centre, Cavendish Laboratory, University of Cambridge , Cambridge CB3 0HE, United Kingdom.

ABSTRACT
Deep-ultraviolet surface-enhanced Raman scattering (UV-SERS) is a promising technique for bioimaging and detection because many biological molecules possess UV absorption lines leading to strongly resonant Raman scattering. Here, Al nanovoid substrates are developed by combining nanoimprint lithography of etched polymer/silica opal films with electron beam evaporation, to give a high-performance sensing platform for UV-SERS. Enhancement by more than 3 orders of magnitude in the UV-SERS performance was obtained from the DNA base adenine, matching well the UV plasmonic optical signatures and simulations, demonstrating its suitability for biodetection.

No MeSH data available.


Related in: MedlinePlus