1 Bimetallic 3D Nanostar Dimers in Ring Cavities: Recyclable and Robust Surface-Enhanced Raman Scattering Substrates for Signal Detection from Few Molecules Anisha Gopalakrishnan § , Manohar Chirumamilla § , Francesco De Angelis, Andrea Toma, Remo Proietti Zaccaria, and Roman Krahne Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italia § These authors contributed equally to the work Corresponding author email: [email protected]SUPPORTING INFORMATION Figure SI1: a) Normal incidence SEM image of AgAu nanoring cavities. b) Magnified image of a ring cavity with 52° tilted view.
Transcript
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Bimetallic 3D Nanostar Dimers in Ring Cavities:
Recyclable and Robust Surface-Enhanced Raman
Scattering Substrates for Signal Detection from Few
Molecules
Anisha Gopalakrishnan§, Manohar Chirumamilla§, Francesco De Angelis, Andrea Toma, Remo
Proietti Zaccaria, and Roman Krahne
Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italia
Figure SI1: a) Normal incidence SEM image of AgAu nanoring cavities. b) Magnified image of a ring cavity with 52° tilted view.
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Figure SI2: Contour profiles in x-z- plane of the electromagnetic near field distribution for a Au Nanostar dimer (a), a bimetallic AgAu nanostar dimer (b), a bimetallic AgAu nanoring cavity (c), and a bimetallic AgAu nanostar dimer in a ring cavity (3D-NSDiR).
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Table SI 1. Assignment of peak energies to molecular vibration modes of pATP.1
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Figure SI3: (a) SERS intensity of the peak at 1077 cm-1 of pATP (10 µM) molecules with respect to pillar height recorded from S8 3D-NSDiR structures coated with a Au layer. The excitation source, power and accumulation time were 830 nm, 1.4 mW and 30 sec, respectively. (b) SEM images of S8 Au 3D-NSDiR structures with different pillar height (30, 60, 80 and 150 nm depicted in A-D, respectively). Images were recorded with 52° tilted view, and the corresponding dimers at higher magnification are shown in the right panel.
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Figure SI4: (a, c) SEM images of a bimetallic nanoring and 3D-NSDiR structure with S5
geometry. The scale bar corresponds to 100 nm. (b) SERS spectra of pATP (10 µM) molecule
recorded from empty ring cavities (green trace) and 3D-NSDiR structures (red trace). Experimental
parameters were excitation wavelength at 830 nm with 1.4 mW power, and 30 sec acquisition time.
Figure SI5: (a, c) Contour plots of the calculated electric field distribution in S5 AgAu 3D-NSDiR structures for light polarization parallel and perpendicular to the IPS axis. (b) SERS spectra of pATP (10 µM) molecule recorded from 3D-NSDiR structures with incident laser polarization parallel (red trace) and perpendicular (blue trace) to the IPS axis. Experimental parameters were excitation wavelength at 830 nm with 1.4 mW power, and 30 sec acquisition time.
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Figure SI6. Typical SERS spectra of pATP molecule (10 µM) chemisorbed on S5 3D-NSDiR structures coated with Au, AgAu and Ag metal layers. The laser wavelength was 830 nm at 1.4 mW power, and accumulation time was 30s.
Figure SI7. Calculated extinction spectrum of 3D-NSDiR structures with S5 geometry. The maximum around 820 nm is in good agreement with the experimental SERS spectra reported in Figure 4 of the main text. The inset shows a contour plot of the calculated electric field distribution under excitation at 820 nm.
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SERS enhancement factor calculation The SERS enhancement, G, is calculated using the following expression,
⎟⎟⎠
⎞⎜⎜⎝
⎛=
SERS
Raman
SERS
Raman
SERS
Raman
Raman
SERS
tt
PP
AA
IIG *** (eq. S1)
Here I, A, P and t are the peak height, the area of the structure, the laser power and the accumulation time, where the subscripts Raman and SERS stand for measurements performed on planar metal films and 3D-NSDiR, respectively. In both the cases, time and power are kept constant for calculating the enhancement factor. In order to evaluate ASERS we have considered the local surface area[5] of the hot spot resulting from the simulation in Figure 2(d), which is 23.2 nm2 for each tip. We consider a single nanostar dimer in the laser illumination spot. Therefore we obtain 46.4 nm2 as the active SERS area. ASERS = 23.2 nm2 × 2 = 46.4 nm2 = 46.4 × 10-18 m2, For the reference spectra we take the illumination spot size: ARaman = 3.14 × (0.5 µm)2 = 0.785 µm2 = 0.785 × 10-12 m2
The experimental values of I, P and t for SERS and Raman are taken from Figure 3 (a), and result in: ISERS = 2.58232 × 105 (counts) and IRaman = 110 (counts); With these values we obtain:
𝐺 = 2.58232×10! ×0.785 × 10!!"
110 ×46.4 ×10!!"
G = 4 × 107 We note that the planar AgAu films are already functioning as SERS substrates with an enhancement factor G' (evaluated with respect to a non-enhancing Raman substrate) which can be calculated based on the following expression:
⎟⎟⎠
⎞⎜⎜⎝
⎛=
SERS
Bulk
SERS
Bulk
SERS
Bulk
Bulk
SERS
tt
PP
NN
IIG ***' (eq. S2)
where I, N, P and t represent the peak height, the number of molecules, the laser power and the accumulation time. The subscripts “Bulk” and “Raman” indicate measurements performed a on non-enhancing Raman substrate and a planar metal film, respectively. The estimation of the number of molecules NBulk = 3.3× 1010 excited within the laser spot was based on the focusing volume and on a pATP density of 1.06 g/cm3. 2,3On the other hand, NRaman was calculated assuming the same active area employed in eq. S1 (ARaman) and a packing density of 0.2 nm2/molecule for the p-ATP.2,3 Accordingly, the estimated enhancement factor is G' = 3.2 × 104. Based on this result we can conclude that the absolute enhancement of the AgAu 3D-NSDiR structure is around 1 × 1012.
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Figure SI8. SERS intensities of pATP (10 µM) molecule as a function of 3D-NSDiR structure size for the bands at 1077, 1140, 1390, 1438, and 1590 cm-1 recorded with laser wavelengths at 532, 633 and 830 nm.
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Table SI2. Assignment of peak energies to molecular vibration modes of adenine. 4-7
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Figure SI9. (a) SERS spectra of R6G molecules chemisorbed on S5 3D-NSDiR structures from 1 µM, 1 nM and 1 pM concentrations (shown as black, red and green traces, respectively). (b) SERS spectra of R6G (1 pM) at 10 different spatial positions of the same substrate. Data was recorded with excitation at 830 nm (14 mW) and 3 s acquisition time.
The black trace in Figure SI9a, recorded from molecules chemisorbed from 1 µM
concentration manifests the clearly distinguishable characteristic peaks of R6G at 1360, 1510 and
1610 cm-1 that can be assigned to C–H bending, a combination of C–N stretching, C–H and N–H
bending and combination of ring stretching of the C–C vibration and C–Hx bending of the
xanthenes ring, respectively), which are in good agreement with reports in Refs.4, 8 Table SI3 lists
the assignments of the various peaks for R6G. Prominent peaks associated to R6G vibration modes
are clearly visible in spectra recorded from lower molecule densities (red and green traces) that
reach the limit of single/few molecules per hot spot. Figure SI9b shows several spectra obtained
from R6G molecule chemisorbed from1 pM concentration recorded from 10 different 3D-NSDiR
on the same substrate. The characteristic vibrational band at 1610 cm-1 (yellow bar) is clearly
evident in all spectra, showing some variation in peak intensity and position which is characteristic
for single molecule detection.4, 9 The intensity and position of the peaks originating from the
molecular vibrational modes are dominated by the orientation of the analyte molecules adsorbed on
the metal surface. From large molecule ensembles an average over all possible conformations is
observed in the SERS spectra, while in single molecule experiments, the differences in
conformation (orientation of molecule with respect to the substrate and polarization of the laser
beam) become evident. Therefore the spectra presented in Fig. SI9b are an indirect proof of single
molecule detection capability of the 3D-NSDiR.
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Table SI3. Assignment of peak energies to molecular vibration modes of R6G.10, 11
Comparison with oxygen plasma and piranha etch cleaning
Figure SI 10: (a) SERS spectra recorded from pATP molecules that were chemisorbed at 10 µM concentration on 3D-NSDiR structures recorded after oxygen plasma treatment for different times. Clearly, also after 20 min of plasma treatment the molecules could not be removed completely. Plasma Parameters, 100 W Power, 15 SCCM O2 flux. The laser wavelength was 830 nm at 1.4 mW power, and accumulation time was 30s. (b) SERS spectra recorded from a sample treated with 10 µM pATP molecules that was then cleaned by etching in cold piranha solution (30% hydrogen
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peroxide and concentrated (95%) sulfuric acid in the ratio 1:3) for 5s and 30 s. Here the dominant peak is reduced to 2% in intensity due to the piranha cleaning, but still clearly detectable and therefore showing incomplete removal of the molecules. (c) SERS spectra recorded from a sample treated with 10 µM pATP solution, after cleaning and metal re-deposition as described in our recycling process, and after new molecule chemisorption.
SERS experiments on recycled substrates that were functionalized with adenine and R6G molecules at 1 pM concentration
Figure SI 11. SERS spectra recorded from adenine (a) and R6G (b) molecules chemisorbed on 3D-
NSDiR structures at concentration of 1 pM. The green spectra were obtained after one recycling
process. The laser wavelength was 830 nm at 14 mW, and accumulation time was 3s.
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