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Papers by Nathan Greeneltch

The Analyst, 2016

Simplicity and low cost has positioned inkjet paper- and fabric-based 3D substrates as two of the... more Simplicity and low cost has positioned inkjet paper- and fabric-based 3D substrates as two of the most commonly used surface-enhanced Raman spectroscopy (SERS) platforms for the detection and the identification of chemical and biological analytes down to the nanogram and femtogram levels. The relationship between far-field and near-field properties of these 3D SERS platforms remains poorly understood and warrants more detailed characterization. Here, we investigate the extremely weak optical scattering observed from commercial and home-fabricated paper-, as well as fabric-based 3D SERS substrates. Using wavelength scanned surface-enhanced Raman excitation spectroscopy (WS-SERES) and finite-difference time-domain (FDTD) calculations we were able to determine their near-field SERS properties and correlate them with morphological and far-field properties. It was found that nanoparticle dimers, trimers, and higher order nanoparticle clusters primarily determine the near-field properties of these substrates. At the same time, the far-field response of 3D SERS substrates either originates primarily from the monomers or cannot be clearly defined. Using FDTD we demonstrate that LSPR bands of nanoparticle aggregates near perfectly overlap with the maxima of the near-field surface-enhanced Raman scattering responses of the 3D SERS substrates. This behaviour of far-field spectroscopic properties and near-field surface-enhanced Raman scattering has not been previously observed for 2D SERS substrates, known as nanorod arrays. The combination of these analytical approaches provides a full spectroscopic characterization of 3D SERS substrates, while FDTD simulation can be used to design new 3D SERS substrates with tailored spectral characteristics.

The Analyst, Jan 21, 2014

The potential for discriminating between analytes by their unique vibrational signature makes sur... more The potential for discriminating between analytes by their unique vibrational signature makes surface-enhanced Raman scattering (SERS) extremely interesting for chemical detection. However, for molecules that weakly adsorb to non-functionalized plasmonic materials, detection by SERS remains a key challenge. Here we present an approach to SERS-based detection where a polycrystalline metal-organic framework (MOF) film is used to recruit a range of structurally similar volatile organic compounds for detection by SERS. MOF films were grown on the surface of Ag "films-over-nanospheres" (FONs), which have previously been shown to enhance Raman signals of surface adsorbates by a factor of 10(7). Upon exposing the MOF-coated FON to benzene, toluene, nitrobenzene, or 2,6-di-tert-butylpyridine, the MOF film traps the vapors at the FON surface, allowing the unique Raman spectrum of each vapor to be recorded. By contrast, these analytes do not adsorb to a bare FON surface and thus can...

The Journal of Physical Chemistry B, 2003

This work presents the first systematic study of the surface-enhanced Raman-scattering (SERS) pro... more This work presents the first systematic study of the surface-enhanced Raman-scattering (SERS) properties of nanosphere lithography (NSL) derived Ag nanoparticles. Furthermore, it demonstrates the necessity of correlating nanoparticle structure and localized surface plasmon resonance (LSPR) spectroscopic data in order to effectively implement SERS on nanofabricated surfaces that have narrow (∼100 nm) LSPR line widths. Using nanoparticle substrates that are structurally well characterized by atomic force microscopy, the relationship between the LSPR extinction maximum (λ max ) and the SERS enhancement factor (EF) is explored in detail using the innovative approach of plasmon-sampled surface-enhanced Raman excitation spectroscopy (PS-SERES). PS-SERES studies were performed as a function of excitation wavelength, molecular adsorbate, vibrational band, and molecule-localized resonance or nonresonance excitation. In each case, high S/N ratio spectra are achieved for samples with an LSPR λ max within a ∼120-nm window that encompasses both the excitation wavelength and the scattered wavelength. These results unambiguously demonstrate a systematic approach to the optimization of SER spectra on nanoparticle substrates with large interparticle spacings and consequently, weak or no electromagnetic coupling. In fact, this work demonstrates the largest SERS, EF > 1 × 10 8 , and SERRS, EF > 7 × 10 9 , enhancement factors measured to date on nanostructured substrates. The observation that EF SERRS /EF SERS ∼ 40 and E RRS /EF Pre-RRS ∼ 40 in the Fe(bpy) 3 2+ system illustrates that this adsorbate, with its molecule-localized electronic transition, does not damp the nanoparticle-localized LSPR and that the SERS and RRS effects are strictly multiplicative. † Part of the special issue "Arnim Henglein Festschrift".

The Journal of Physical Chemistry A, 2014

The Analyst, 2014

Microfluidic sensing platforms facilitate parallel, low sample volume detection using various opt... more Microfluidic sensing platforms facilitate parallel, low sample volume detection using various optical signal transduction mechanisms. Herein, we introduce a simple mixing microfluidic device, enabling serial dilution of introduced analyte solution that terminates in five discrete sensing elements. We demonstrate the utility of this device with on-chip fluorescence and surface-enhanced Raman scattering (SERS) detection of analytes, and we demonstrate device use both when combined with a traditional inflexible SERS substrate and with SERS-active nanoparticles that are directly incorporated into microfluidic channels to create a flexible SERS platform. The results indicate, with varying sensitivities, that either flexible or inflexible devices can be easily used to create a calibration curve and perform a limit of detection study with a single experiment.

Journal of Raman Spectroscopy, 2013

The Journal of Physical Chemistry Letters, 2013

ABSTRACT Normal and surface-enhanced Raman spectra for a set of substituted benzenethiols were me... more ABSTRACT Normal and surface-enhanced Raman spectra for a set of substituted benzenethiols were measured experimentally and calculated from static polarizability derivatives determined with time-dependent density functional theory (TDDFT). Both silver and gold cluster–thiolate complexes were studied to investigate how the chemical enhancement varies with substituent. The experimental relative peak intensities and positions are well-matched by their theoretical counterparts. The static chemical enhancement of the ring stretching modes near 1600 cm–1 is determined experimentally and computationally for each derivative, and it is found that the experimental enhancement varies by a factor of 10 as a result of chemical substitution, with stronger electron donating groups on the benzene unit leading to higher enhancements. The calculated trends with substitution match experiment well, suggesting that TDDFT is describing the chemical effect qualitatively, if not quantitatively, in the static (low-frequency) limit. A two-state model is developed, providing qualitative insight into the results in terms of the variation of ligand-to-metal charge-transfer excitation energy with substitution.

Electrochimica Acta, 2011

Nanocrystalline Pt/CeO2 composite electrodes were fabricated to study the electrochemical oxidati... more Nanocrystalline Pt/CeO2 composite electrodes were fabricated to study the electrochemical oxidation of methanol and ethanol. The performance of the electrodes was tested as the ceria solutions aged over time. It was observed that the performance oscillated with time, ...

Catalysis Letters, 2007

A nanocrystalline Pt/CeO 2 composite electrode is fabricated to study the electrochemical oxidati... more A nanocrystalline Pt/CeO 2 composite electrode is fabricated to study the electrochemical oxidation of methanol and ethanol. Three different methods have been developed for the preparation of the composite electrode and are compared in terms of their oxidation efficiencies. In general, all the electrode preparation methodologies show a great enhancement of the oxidation current for the alcohol. However, methods based on the electrodeposition of a ceria rich Pt matrix show the greatest effectiveness of alcohol oxidation. The enhancement of the anodic current is dependent on the particle loading and size distribution in the composite matrix. In general, ceria appears to be an effective alternative to ruthenium in direct alcohol fuel cells.

Applied Spectroscopy, 2013

The surface-enhanced coherent anti-Stokes Raman scattering (SECARS) from a self-assembled monolay... more The surface-enhanced coherent anti-Stokes Raman scattering (SECARS) from a self-assembled monolayer (SAM) of benzenethiol on a silver-coated surface-enhanced Raman scattering (SERS) substrate has been measured for the 1574 cm(-1) SERS mode. A value of 9.6 ± 1.7×10(-14) W was determined for the resonant component of the SECARS signal using 17.8 mW of 784.9 nm pump laser power and 7.1 mW of 895.5 nm Stokes laser power; the pump and Stokes lasers were polarized parallel to each other but perpendicular to the grooves of the diffraction grating in the spectrometer. The measured value of resonant component of the SECARS signal is in agreement with the calculated value of 9.3×10(-14) W using the measured value of 8.7 ± 0.5 cm(-1) for the SERS linewidth Γ (full width at half-maximum) and the value of 5.7 ± 1.4×10(-7) for the product of the Raman cross section σSERS and the surface concentration Ns of the benzenethiol SAM. The xxxx component of the resonant part of the third-order nonlinear optical susceptibility |3 χxxxx((3)R)| for the 1574 cm(-1) SERS mode has been determined to be 4.3 ± 1.1×10(-5) cm·g(-1)·s(2). The SERS enhancement factor for the 1574 cm(-1) mode was determined to be 3.6 ± 0.9×10(7) using the value of 1.8×10(15) molecules/cm(2) for Ns.

Applied Spectroscopy, 2012

Analytical Chemistry, 2013

We describe the fabrication of optimized plasmonic substrates in the form of immobilized nanorod ... more We describe the fabrication of optimized plasmonic substrates in the form of immobilized nanorod assemblies (INRA) for surface-enhanced Raman spectroscopy (SERS). Included are highresolution scanning electron micrograph (SEM) images of the surface structures, along with a mechanistic description of their growth. It is shown that, by varying the size of support microspheres, the surface plasmon resonance is tuned between 330 and 1840 nm. Notably, there are predicted optimal microsphere sizes for each of the commonly used SERS laser wavelengths of 532, 633, 785, and 1064 nm.

MRS Bulletin, 2013

ABSTRACT Surface-enhanced Raman spectroscopy (SERS) is highly dependent upon the substrate, where... more ABSTRACT Surface-enhanced Raman spectroscopy (SERS) is highly dependent upon the substrate, where excitation of the localized metal surface plasmon resonance enhances the vibrational scattering signal of proximate analyte molecules. This article reviews recent progress in the fabrication of SERS substrates and the requirements for characterization of plasmonic materials as SERS platforms. We discuss bottom-up fabrication of SERS substrates and illustrate the advantages of rational control of metallic nanoparticle synthesis and assembly for hot spot creation. We also detail top-down methods, including nanosphere lithography for the preparation of tunable, highly sensitive, and robust substrates, as well as the unique benefits of tip-enhanced Raman spectroscopy for simultaneous acquisition of molecular vibrational information and high spatial resolution imaging. Finally, we discuss future prospects and challenges in SERS, including the development of surface-enhanced femtosecond stimulated Raman spectroscopy, microfluidics with SERS, creating highly reproducible substrates, and the need for reliable characterization of substrates.