Sers system employing nanoparticle cluster arrays with multiscale signal enhancement

Abstract

Defined nanoparticle cluster arrays (NCAs) with dimensions up to 25.4 μm square are fabricated on a 10 nm gold film using template guided self-assembly. Structural parameters are precisely controlled, allowing systematic variation of the number of nanoparticles in the clusters (n) and edge to edge separation (∧) between 1<n<20 and 50 nm≤∧≤1000 nm, respectively. Rayleigh scattering spectra and surface enhanced Raman scattering (SERS) signal intensities as functions of n and ∧ reveal direct near-field coupling between the particles within individual clusters, whose strength increases with cluster size (n) until it saturates at around n=4. Strong near-field interactions between clusters significantly affects the SERS signal enhancement for edge-to-edge separations ∧<200 nm. The NCAs support multiscale signal enhancement from simultaneous inter- and intra-cluster coupling and |E|-field enhancement. Applications include SERS-based spectral identification of bacteria.

What is claimed is: 
     
         1 . A surface-enhanced Raman spectroscopy (SERS) system, comprising:
 a SERS substrate having a nanoparticle cluster array on a surface of a planar substrate layer, the nanoparticle cluster array including an array of clusters of metal nanoparticles characterized by a cluster size n and a cluster separation Λ, n being a nominal number less than 10 of tightly-packed nanoparticles in each cluster determined by a nominal size of the nanoparticles and a deterministic binding site width D, and Λ being a deterministic distance less than 200 nm between adjacent clusters, n and Λ together defining a multiscale enhancement of a resonant Raman optical signal induced in the SERS substrate upon excitation with an incident optical signal of a corresponding wavelength;   an optical source configured and operative to generate the incident optical signal and direct it at the SERS substrate, causing the SERS substrate to emit a multi-scale enhanced resonant Raman optical signal; and   an optical detector configured and operative to receive the multi-scale enhanced resonant Raman optical signal and generate a corresponding detector output signal indicative of amplitude of the multi-scale enhanced resonant Raman optical signal.   
     
     
         2 . The SERS system of  claim 1 , wherein the multi-scale enhancement of the resonant Raman optical signal arises from plasmon coupling on two distinct length scales, a first length scale being an inter-particle length scale within the clusters and providing a first stage of enhancement, a second length scale being an inter-cluster length scale between the clusters and providing a further increase of enhanced field. 
     
     
         3 . The SERS system of  claim 1 , wherein the wavelength of the incident optical signal is matched with an absorption band of the SERS substrate to maximize enhancement of the resonant Raman optical signal. 
     
     
         4 . The SERS system of  claim 3 , wherein the wavelength of the incident optical signal is lower than a resonance wavelength of a reference array of gold nanoparticles of a predetermined size, and the metal nanoparticles have a size and material providing
 energetically lower particle resonances matched with the wavelength of the incident optical signal.   
     
     
         5 . The SERS system of  claim 1 , wherein the incident optical signal from the optical source is non-polarized, wide-spectrum light. 
     
     
         6 . The SERS system of  claim 1 , wherein the optical detector is an imaging detector responding to a two-dimensional pattern of the multi-scale enhanced resonant Raman optical signal from the SERS substrate. 
     
     
         7 . The SERS system of  claim 6 , wherein the imaging detector is an imaging spectrometer providing spectral analysis of the multi-scale enhanced resonant Raman optical signal from the SERS substrate. 
     
     
         8 . The SERS system of  claim 7 , wherein the imaging spectrometer performs background correction of the multi-scale enhanced resonant Raman optical signal by subtracting a scattering signal from an equal-size, non-patterned adjacent area of the SERS substrate. 
     
     
         9 . The SERS system of  claim 8 , wherein the imaging spectrometer performs additional correction by normalizing with a scattering spectrum of an ideal scatterer on top of the SERS substrate. 
     
     
         10 . The SERS system of  claim 1 , wherein the SERS substrate includes a noble metal film on which the nanoparticle clusters are disposed, and an optical response of the nanoparticles on the noble metal film is determined by local plasmons of the particles, interactions between the plasmons through space, and coupling to delocalized plasmon modes supported by the noble metal film. 
     
     
         11 . The SERS system of  claim 10 , wherein the noble metal film is periodically corrugated by the nanoparticle clusters to act as a grating coupler enabling photons incident on the SERS substrate to excite a propagating surface plasmon in the noble metal film, the surface plasmon being Bragg-scattered from the nanoparticle cluster arrays. 
     
     
         12 . The SERS system of  claim 10 , wherein the multi-scale enhanced resonant Raman optical signal has a spectrum including a first peak attributable to a nanoparticle cluster resonance coupled via propagating surface plasmons in the noble metal film, and a separate smaller peak in a shorter-wavelength band. 
     
     
         13 . The SERS system of  claim 12 , wherein a periodic two-dimensional structure of nanoparticle clusters on the noble metal film acts as a transmission grating for components of the incident optical signal, the transmission grating inducing diffraction of the incident optical signal according to a grating relationship among center-to-center separation of nanoparticle clusters, a detection angle, a diffraction order, and refractive index, such that all wavelengths fulfilling the grating relationship are included in a scattering spectrum. 
     
     
         14 . The SERS system of  claim 1 , wherein the nanoparticle clusters assume a rhombohedral geometry in which inter-particle distances are minimized, resulting in maximal plasmon coupling. 
     
     
         15 . The SERS system of  claim 1 , wherein the nanoparticle clusters are three-dimensional to provide interactions between nanoparticles along an out-of-place spatial axis, shifting plasmon resonance further into the red than for two-dimensional clusters. 
     
     
         16 . A method of performing surface-enhanced Raman spectroscopy on a sample, comprising:
 placing the sample on a surface of a SERS substrate to create a mounted sample, the SERS substrate having a nanoparticle cluster array on a surface of a planar substrate layer, the nanoparticle cluster array including an array of clusters of metal nanoparticles characterized by a cluster size n and a cluster separation Λ, n being a nominal number less than 10 of tightly-packed nanoparticles in each cluster determined by a nominal size of the nanoparticles and a deterministic binding site width D, and Λ being a deterministic distance less than 200 nm between adjacent clusters, n and Λ together defining a multiscale enhancement of a resonant Raman optical signal induced in the SERS substrate upon excitation with an incident optical signal of a corresponding wavelength;   locating the mounted sample at a location for imaging in a Raman microscope; and   operating the Raman microscope with the mounted sample at location for imaging to capture a Raman spectrograph of the mounted sample.