Abstract
Surface enhanced Raman scattering (SERS), in which sample molecules are placed in the proximity of conducting nanostructures, subjects the molecules to intense electron oscillation (plasmon) field. The intense field, however, may cause heavy distortion and thermal damage to the molecule as well as non-separable and heavy convolution with the metal electronic structure. We utilized 3-nm red luminescent Si nanoparticles decorating the DNA molecules (drawn electrostatically) to enhance Raman scattering in solution at 532 nm. We demonstrated that the nanoparticles enhance the spectral resolution and intensity of vibrations of DNA by two orders of magnitude and reveal vibrations that are otherwise weak or forbidden. Theoretically, we conducted calculations of Mie scattering and three-dimensional finite-difference time-domain scattering and obtained the wavelength dependence of the near-field distribution from single or dimer Si particles. The simulations show moderate intensity enhancement (25–40-fold) and exciton resonances. Moreover, it shows that the near field is highly confined, extending only to 3–5 Å from the Si particle (atomic scale) compared to several nanometers for metal nanoparticles. The observed SERS-type characteristics are understood in terms of polarization-based light scattering, which is possible by the use of Si of highly reduced size for which the polarizability and exciton processes are strong. However, multilayers contribute to metal SERS, and monolayers/single molecules dominate the Si case. Weaker but highly confined, ultra-short range polarization-based scattering provides an alternative to plasmon and Mie scattering, while providing practical, straightforward interpretation of vibration printing of bio-medical species without compromising the molecular structure.
Highlights
Surface enhanced Raman scattering (SERS),[1,2] in which sample molecules are placed within atomic distances from sharp nanoscale metal structures, increases scattering of external light by several orders of magnitude
We demonstrated that the nanoparticles significantly increase the resolution and intensity of Raman scattering from calf thymus DNA by two orders of magnitude
We demonstrated increases in the resolution and intensity of Raman scattering from calf thymus DNA by two orders of magnitudes while eliminating Mie and plasmonic resonance contributions, which cause heat damage as well as distortion to the molecular sample
Summary
Surface enhanced Raman scattering (SERS),[1,2] in which sample molecules are placed within atomic distances from sharp nanoscale metal structures, increases scattering of external light (laser light, for example) by several orders of magnitude. We conduct experimental and theoretical studies to examine the feasibility of using intrinsic ultra-small sub 3-nm luminescent silicon nano-structures/nano-particles instead of metal particles as an enhancer in Raman scattering measurements under wet solution conditions. These nanoparticles are interesting because they are high K materials (high refractive index). The reduced conductivity and reduced size shift plasmon and Mie resonance scattering out of the visible range, alleviates strong couplings, distortions, and thermal damage, as well as non-separability and heavyconvolution of the electronic structures of the molecule and the silicon nanoparticle. The luminescent nano-silicon exhibits some aspects of surface enhanced Raman scattering (SERS)-type characteristics while providing practicality and straightforward interpretation of vibration fingerprinting of sensitive bio-medical and chemical species without compromising the luminescence
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