Sensitive, Stable, and Recyclable ZnO/Ag Nanohybrid Substrates for Surface-Enhanced Raman Scattering Metrology

Surface-enhanced Raman scattering is a practical, noninvasive spectroscopic technique that measures chemical fingerprints for varieties of molecules in multiple applications. However, synthesizing appropriate substrates for practical, long-term applications of this method has always been a challenging task. In the present study, we show that ZnO/Ag nanohybrid substrates may act as highly stable, sensitive, and recyclable substrates for surface-enhanced Raman scattering, as illustrated by the detection of methylene blue, selected as a test dye molecule with self-cleaning functionalities. Specifically, we demonstrate the detection enhancement factor of 3.7 × 107 along with exceptional long-term stability explained in terms of the localized surface plasmon resonance from the Ag nanocrystals embedded into the chemically inert ZnO nanoparticles, constituting the nanohybrid. Significantly, these substrates can be efficiently cleaned and regenerated while maintaining their high performance upon recycling. As a result, using these substrates, up to 10–12 M detection sensitivity has been demonstrated, enabling the accuracy required in modern environmental monitoring, bioassays, and analytical chemistry. Thus, ZnO nanoparticles with embedded Ag nanocrystals constitute a novel class of advanced nanohybrid substrates for use in multiple applications of surface-enhanced Raman scattering metrology.


Figure S6(c)
shows the Raman spectra of the Ag:ZnO nanohybrid within the spectral range between 100 -800 cm -1 .Peaks around 97 and 433 cm -1 in both samples corresponds to E 2 (low) and E 2 (high) fundamental phonon mode of ZnO.These are the characteristic modes of ZnO on Ag doping they become broader 4 .In addition, peaks at 289 and 547 cm -1 are the salient modes referred as B 1 low and B 1 high respectively.Furthermore, it is believed that the O-C-O symmetric bends in the acetate groups utilised as reactants throughout the synthesis process are the source of the Raman mode detected at 731 cm −1 .Similar, results were reported in literature 4,5 .Table S2.Normal and SERS Raman shifts and their assignments for MB molecules 2,6,7 .

Enhancement Factor Calculation :
The SERS enhancement factor (EF) is given by: EF= N Normal I SERS /N SERS I Bulk where N Normal and N SERS are the number of molecules probed in the aqueous sample and on the SERS substrates, respectively.I SERS and I Bulk are the corresponding normal Raman and SERS intensities.
where πr 2 h is the optical excitation volume; ρ is the density of MB dye (1.23 g/cm 3 ) in the bulk; N A is the Avogadro number, and M is the molecular weight of MB molecule (319.85 g/mol).
The laser spot volume was determined by multiplying its (πr 2 , r=0.61×λ/N.A.) and depth of focus , where λ=514 nm is the excitation wavelength and N.A.=0.5 is the numerical

Figure S1
Figure S1XRD spectra of different concentrations of Ag:ZnO.

Figure S2
Figure S2 TEM images of Ag:ZnO.

Figure S3
Figure S3HRTEM images of Ag:ZnO.

Figure S4
Figure S4The Particle size distribution curve formed from HRTEM images of 10AZ, 15AZ, and 20AZ samples of ZnO:Ag nanohybrid.

Figure S5
Figure S5Shows the FESEM and EDX mapping images of 0AZ and 15AZ.

Figure
Figure S6 FTIR, TGA and Raman spectra of Ag:ZnO.

Figure S7
Figure S7 Normal Raman spectra of MB in powder form and MB in aqueous solution

Figure
Figure S1 (a) XRD spectra of ZnO:Ag nanohybrid showing a shift in (101) peak with Ag doping.

Figure
Figure S2(a-b) TEM images of 0AZ and 15AZ samples of ZnO:Ag nanohybrids.

Figure
Figure S3 (a-b) HRTEM images of 10AZ and 20AZ samples of ZnO:Ag nanohybrid.

Figure
Figure S4 (a-c) The Particle size distribution curve formed from HRTEM images of 10AZ, 15AZ, and 20AZ samples of ZnO:Ag nanohybrid.

Figure S7 :
Figure S7: Normal Raman spectra of MB in powder form and MB in aqueous solution.
(and C-N-C) ring stretching aperture8,9 .The number of molecules being probed (N Normal ) was calculated to be 0.028 x 10 13 .The value of N SERS was obtained by dividing the laser surface area (6.28 µm 2 ) by the effective cross-sectional area per molecule (6 × 10 −7 ).The calculated value of N SERS to obtain the number of molecules on Ag:ZnO substrate are 1.04 × 107 10, 11  .We used the strongest signature stretching modes for I SERS and I Bulk at 1622.50 cm -1 , which are 1672.32and 901.20 respectively.The calculated EF value of the SERS-active 15AZ is 3.3 ×10 6 .

Figure S8 .
Figure S8.Measurements showing the long term stability of ZnO:Ag nanohybrid after 1 year of storage at room temperature: (a) Raman (b) Optical absorption spectra (c) XPS and (d) SERS spectra.

Table S1 :
Average Ag NCs size estimated by using Mie theory calculations with increasing Ag concentration.

Table S2 .
Normal and SERS Raman shifts and their assignments for MB molecules.