Over twenty years have elapsed since the first report of single molecule surface enhanced Raman spectroscopy (SERS) yet quantitative sensing in the single molecule regime remains elusive. We have recently introduced a new self-assembly method for fabricating sensor surfaces capable of single-molecule SERS with uniform SERS enhancements of 109 over 100s of μm. Yet, the main challenge in quantitative single molecule SERS is the complex, system dependent, hotspot occupancy rate of analyte. In this work, we solve this problem using a new big data method for quantifying analyte concentration in the single molecule regime. Specifically, deep convolutional neural networks are trained with SERS spectra acquired across large sensor surfaces. The universal approximation property of neural networks is used to automatically fit the hotspot occupancy rate of any molecule. We demonstrate quantitative detection of rhodamine 800 as low as 1 femtomolar, where the single molecule regime begins at approximately 100 picomolar concentrations. This method is validated by comparison with traditional, non-quantitative methods of single molecule SERS detection. Further, we use SERS’s rich spectral information and label free detection to demonstrate the simultaneous quantification of multiple analyte molecules. Finally, this new quantification method is used to sense small molecules produced by bacterial biofilms. The proposed method is not system specific and is thus broadly applicable to any SERS sensor capable of large-area, uniform single molecule detection.
We demonstrate the advantage of using machine learning for surface enhanced Raman scattering (SERS) spectral analysis for quantitative detection of pyocyanin in Luria-Bertani media. Planar Au nanoparticle clusters were selfassembled on PS-b-PMMA diblock copolymer template using EDC crosslinking chemistry and electrohydrodynamic flow to fabricate SERS substrates. Resulting substrates produce uniform SERS response over large area with signal relative standard deviation of 10.8 % over 50 μm × 50 μm region. Taking advantage of the uniformity, 400 SERS spectra were collected at each pyocyanin concentration as training dataset. Tracking the intensity of pyocyanin 1350 cm-1 vibrational band shows linear regime beginning at 10 ppb. PLS analysis was also performed on the same training dataset. Without being explicitly “told” which spectrum to look for, PLS analysis recognizes the SERS spectrum of pyocyanin as its first loading vector even in the presence of other molecules in LB media. PLS regression enables quantitative detection at 1 ppb, 1 order of magnitude earlier than univariate regression. We hope this work will fuel a push toward wider adoption of more sophisticated machine learning algorithms for quantitative analysis of SERS spectra.
Colloidal self-assembly combined with templated surfaces holds the promise of fabricating large area
devices in a low cost facile manner. This directed assembly approach improves the complexity of assemblies that
can be achieved with self-assembly while maintaining advantages of molecular scale control. In this work,
electrokinetic driving forces, i.e., electrohydrodynamic flow, are paired with chemical crosslinking between
colloidal particles to form close-packed plasmonic metamolecules. This method addresses challenges of obtaining
uniformity in nanostructure geometry and nanometer scale gap spacings in structures. Electrohydrodynamic flows
yield robust driving forces between the template and nanoparticles as well as between nanoparticles on the surface
promoting the assembly of close-packed metamolecules. Here, electron beam lithography defined Au pillars are
used as seed structures that generate electrohydrodynamic flows. Chemical crosslinking between Au surfaces
enables molecular control over gap spacings between nanoparticles and Au pillars. An as-fabricated structure is
analyzed via full wave electromagnetic simulations and shown to produce large magnetic field enhancements on the
order of 3.5 at optical frequencies. This novel method for directed self-assembly demonstrates the synergy between
colloidal driving forces and chemical crosslinking for the fabrication of plasmonic metamolecules with unique
Plasmonic nano antennas like dimers, have been investigated for their capability to provide a strong near-field enhancement when illuminated by external light. Traditionally these nano antennas, isolated or arrayed, are placed on a substrate and used in spectroscopy techniques. Surfaces made of such plasmonic nano antennas have been very useful for applications like surface enhanced Raman scattering in which it provides various orders of magnitude of enhanced sensitivity. These instruments however are not economic and are often not mobile since surfaces require an external beam illumination and the Raman scattering is detected by a large aperture microscope. The goal of this paper is to combine nano antennas made of gold dimers with integrated waveguide to make a spectrometer which has low cost and volume in comparison with the structure mentioned above. A technique in which optical plasmonic nano antennas are located in proximity of silicon nitride waveguide is proposed that is useful both for illumination and detection channels. The waveguide evanescent field, which is decaying outside of the waveguide, excites the dimer and causes it to resonate which results in a very strong electric field enhancement of approximately 25 times in the antenna gap. Also the coupling effect of dimer resonance on waveguide modes is investigated. To show the efficiency of the proposed structure, full wave analysis has been done and its results are compared with the multilayer structure case. The simulation results demonstrate that this structure can be designed and fabricated for the purpose of spectroscopy application.
Pseudomonas aeruginosa (PA), a biofilm forming bacterium, commonly affects cystic fibrosis, burn victims, and immunocompromised patients. PA produces pyocyanin, an aromatic, redox active, secondary metabolite as part of its quorum sensing signaling system activated during biofilm formation. Surface enhanced Raman scattering (SERS) sensors composed of Au nanospheres chemically assembled into clusters on diblock copolymer templates were fabricated and the ability to detect pyocyanin to monitor biofilm formation was investigated. Electromagnetic full wave simulations of clusters observed in scanning electron microcopy images show that the localized surface plasmon resonance wavelength is 696 nm for a dimer with a gap spacing of 1 nm in an average dielectric environment of the polymer and analyte; the local electric field enhancement is on the order of 400 at resonance, relative to free space. SERS data acquired at 785 nm excitation from a monolayer of benzenethiol on fabricated samples was compared with Raman data of pure benzenethiol and enhancement factors as large as 8×109 were calculated that are consistent with simulated field enhancements. Using this system, the limit of detection of pyocyanin in pure gradients was determined to be 10 parts per billion. In SERS data of the supernatant from the time dependent growth of PA shaking cultures, pyocyanin vibrational modes were clearly observable during the logarithmic growth phase corresponding to activation of genes related to biofilm formation. These results pave the way for the use of SERS sensors for the early detection of biofilm formation, leading to reduced healthcare costs and better patient outcomes.
We demonstrate the fabrication of a highly nonlinear sub-micron silicon nitride trench waveguide coated with gold nanoparticles for plasmonic enhancement. The average enhancement effect is evaluated by measuring the spectral broadening effect caused by self-phase-modulation. The nonlinear refractive index n2 was measured to be 7.0917×10-19 m2/W for a waveguide whose Wopen is 5 μm. Several waveguides at different locations on one wafer were measured in order to take the randomness of the nanoparticle distribution into consideration. The largest enhancement is measured to be as high as 10 times. Fabrication of this waveguide started with a MEMS grade photomask. By using conventional optical lithography, the wide linewidth was transferred to a <100> wafer. Then the wafer was etched anisotropically by potassium hydroxide (KOH) to engrave trapezoidal trenches with an angle of 54.7º. Side wall roughness was mitigated by KOH etching and thermal oxidation that was used to generate a buffer layer for silicon nitride waveguide. The guiding material silicon nitride was then deposited by low pressure chemical vapor deposition. The waveguide was then patterned with a chemical template, with 20 nm gold particles being chemically attached to the functionalized poly(methyl methacrylate) domains. Since the particles attached only to the PMMA domains, they were confined to localized regions, therefore forcing the nanoparticles into clusters of various numbers and geometries. Experiments reveal that the waveguide has negligible nonlinear absorption loss, and its nonlinear refractive index can be greatly enhanced by gold nano clusters. The silicon nitride trench waveguide has large nonlinear refractive index, rendering itself promising for nonlinear applications.
Periodic arrays of sub-wavelength structures have garnered significant interest for surface enhanced Raman
spectroscopy (SERS) and metal enhanced fluorescence (MEF), and for anti-reflective coating properties. For SERS
and MEF, coupling metal nanoparticles with nanometer scale spacing can induce strong local electromagnetic field
enhancements at the plasmon resonance, significantly increasing the Raman signal or fluorescence of a molecule.
Inspired by moth eyes, metal nanoparticle arrays can reduce the reflection of incident light, shown useful for
improving the efficiency of solar cells. Here, we present fabrication of robust, tunable, inexpensive and quickly
reproducible gold coated, nanopillar arrays for applications in enhancing Raman/fluorescence signals or antireflective
surfaces for efficient solar cells. To create homogenous metallic nanostructures with controllable sizes and
interparticle spacings, we have integrated conventional nanosphere lithography techniques with thermally
responsive polyolefin (PO) films. Spin coating 500 nm PS beads onto PO substrates, followed by oxygen plasma
etching, is used to vary the size and periodicity of the resulting PS nanopillar bead array. A 50 nm thick gold film
can then be added using chemical vapor deposition (CVD). Nanostructures were characterized with scanning
electron microscopy and atomic force microscopy. When heated from room temperature up to 115oC, structures on
PO films undergo a reduction in feature size and interparticle spacing by up to 35 % in length and 50% in surface
Selective patterning of chemical functional groups on polymer surfaces is utilized for controlled placement of monodisperse noble metal nanoparticles. Self-assembled diblock copolymer films deposited on hydrophobic silicon substrates are used as a template for metal nanoparticle organization. By varying the processing conditions of polymer templates, micelle and cylindrical polystyrene-b-poly(methyl methacrylate) diblock copolymer templates were fabricated. Functional groups on the surface of poly(methyl methacrylate) domains in the diblock copolymer films were chemically modified from an ester group to a carboxylate using a base catalyzed hydrolysis step. Gold and silver nanoparticles were fabricated in solution in order to achieve size and shape control. After gold nanoparticle synthesis, a ligand exchange reaction was performed to produce nanoparticles with amine functional groups for chemical attachment on chemically modified poly(methyl methacrylate) surfaces. Atomic force microscopy and scanning electron microscopy images demonstrate that this fabrication route results in preferential attachment of metal nanoparticles on poly(methyl methacrylate) thin films and on poly(methyl methacrylate) domains in polystyrene-b-poly(methyl methacrylate) diblock copolymer thin films.
Parallel arrays of self-assembled rare earth disilicides (erbium and dysprosium) nanowires were grown on Si(001) substrates with nanowire width between 3-10 nm and used as a template for fabricating noble metal (platinum and gold) nanostructure arrays. Submonolayer coverage of platinum and gold were deposited on the nanowire/Si(001) surface post rare earth disilicide growth. Scanning tunneling microscopy and reactive ion etching showed that platinum and gold preferentially deposited on the nanowire surface versus the Si surface. Reactive ion etching of erbium disilicide nanowires with and without platinum on the surface demonstrated that platinum acted as a more resistant etch mask than erbium disilicide. By varying the platinum coverage on the surface we demonstrate the ability to select arrays of nanowire or nanocrystal arrays as a function of platinum coverage.
Transmission electron microscopy studies in both the scanning and parallel illumination mode on samples of two generic types of self-assembled semiconductor quantum dots are reported. III-V and II-VI quantum dots as grown in the Stranski-Krastanow mode are typically alloyed and compressively strained to a few %, possess a more or less random distribution of the cations and/or anions over their respective sublattices, and have a spatially non-uniform chemical composition distribution. Sn quantum dots in Si as grown by temperature and growth rate modulated molecular beam epitaxy by means of two mechanisms possess the diamond structure and are compressively strained to the order of magnitude 10 %. These lattice mismatch strains are believed to trigger atomic rearrangements inside quantum dots of both generic types when they are stored at room temperature over time periods of a few years. The atomic rearrangements seem to result in long-range atomic order, phase separation, or phase transformations. While the results suggest that some semiconductor quantum dots may be structurally unstable and that devices based on them may fail over time, triggering and controlling structural transformations in self-assembled semiconductor quantum dots may also offer an opportunity of creating atomic arrangements that nature does not otherwise provide.