Matthew J. Banholzer

Rationally designed nanostructures for surface-enhanced Raman spectroscopy.

Research on surface-enhanced Raman spectroscopy (SERS) is an area of intense interest because the technique allows one to probe small collections of, and in certain cases, individual molecules using relatively straightforward spectroscopic techniques and nanostructured substrates. Researchers in this area have attempted to develop many new technological innovations including high sensitivity chemical and biological detection systems, labeling schemes for authentication and tracking purposes, and dual scanning-probe/spectroscopic techniques that simultaneously provide topographical and spectroscopic information about an underlying surface or nanostructure. However, progress has been hampered by the inability of researchers to fabricate substrates with the high sensitivity, tunability, robustness, and reproducibility necessary for truly practical and successful SERS-based systems. These limitations have been due in part to a relative lack of control over the nanoscale features of Raman substrates that are responsible for the enhancement. With the advent of nanotechnology, new approaches are being developed to overcome these issues and produce substrates with higher sensitivity, stability, and reproducibility. This tutorial review focuses on recent progress in the design and fabrication of substrates for surface-enhanced Raman spectroscopy, with an emphasis on the influence of nanotechnology.

The role radius of curvature plays in thiolated oligonucleotide loading on gold nanoparticles.

We show that by correlating the radius of curvature of spherical gold nanoparticles of varying sizes with their respective thiol-terminated oligonucleotide loading densities, a mathematical relationship can be derived for predicting the loading of oligonucleotides on anisotropic gold nanomaterials. This mathematical relationship was tested with gold nanorods (radius 17.5 nm, length 475 nm) where the measured number of oligonucleotides per rod (3330 +/- 110) was within experimental error of the predicted loading of 3244 oligonucleotides from the derivation. Additionally, we show that once gold nanoparticles reach a diameter of approximately 60 nm the local surface experienced by the oligonucleotide is highly similar to that of a planar surface.

On-wire lithography: synthesis, encoding and biological applications

The next step in the maturing field of nanotechnology is to develop ways to introduce unusual architectural changes to simple building blocks. For nanowires, on-wire lithography (OWL) has emerged as a powerful way of synthesizing a segmented structure and subsequently introducing architectural changes through post-chemical treatment. In the OWL protocol presented here, multisegmented nanowires are grown and a support layer is deposited on one side of each nanostructure. After selective chemical etching of sacrificial segments, structures with gaps as small as 2 nm and disks as thin as 20 nm can be created. These nanostructures are highly tailorable and can be used in electrical transport, Raman enhancement and energy conversion. Such nanostructures can be functionalized with many types of adsorbates, enabling the use of OWL-generated structures as bioactive probes for diagnostic assays and molecular transport junctions. The process takes 13–36 h depending on the type of adsorbate used to functionalize the nanostructures.

Correlating nanorod structure with experimentally measured and theoretically predicted surface plasmon resonance

The extinction spectra of Au nanorods electrochemically synthesized using anodic aluminum oxide templates are reported. Homogeneous suspensions of nanorods with average diameters of 35, 55, 80, and 100 nm and varying lengths were synthesized, and their resultant surface plasmon resonances were probed by experimental and theoretical methods. Experimental extinction spectra of the nanoparticles exhibit good overall agreement with those calculated using the discrete dipole approximation. We determine the dependence of the dipole plasmon wavelength on both rod length and diameter, and we then utilize these results to derive an equation for predicting longitudinal dipole resonance wavelength for nanorod dimensions beyond the quasistatic limit. On average, the equation allows one to predict plasmon resonance maxima within 25 nm of the experimentally measured values. An analysis of factors that are important in determining the plasmon width is also provided. For long rods, the width decreases with increasing length in spite of increased radiative damping due to increased frequency dispersion in the real part of the metal dielectric function.

Surprisingly Long-Range Surface-Enhanced Raman Scattering (SERS) on Au–Ni Multisegmented Nanowires†

Very long range surface-enhanced Raman scattering is observed from a nickel nanowire that is separated by 120 nm from a pair of gold nanodisks. The excitation of the surface-plasmon resonance (SPR) from the gold nanodisk pair generates an enhanced electromagnetic field near the nickel segment (SEM, left), leading to Raman intensity greater than the nickel alone (right).

Plasmonic Focusing in Rod−Sheath Heteronanostructures

This paper describes the fabrication of plasmonic focusing, free-standing rod-sheath hetero-nanostructures based on electrochemical templated synthesis and selective chemical etching. These hetero-nanostructures take advantage of plasmon interference together with field enhancements due to sharp junction structures to function as stand-alone SERS substrates containing Raman hot spots at the interface of the rod and sheath segments. This result is investigated with empirical and theoretical (discrete dipole approximation, DDA) methods, and we show how plasmon interference can be tuned by varying the sheath and rod lengths.

Designing nanostructures with optimized surface-enhanced Raman scattering behavior

Investigations in the burgeoning field of nanophotonics have demonstrated that certain nanostructures act in previously unanticipated ways. One such behavior is surface-enhanced Raman scattering (SERS), an area of substantial interest, as demonstrated by the hundreds of scientific papers published on the subject during the past 5 years. SERS was discovered over 3 decades ago.1–3 It transforms the basic Raman technique (a measure of the structure and properties of molecules based on their interaction with light that normally has very weak intensity relative to other spectroscopies) into one that has highly sensitive detection capabilities. Whereas Raman scattering is useful for analyzing the structure of small molecules and for addressing traditional analytical issues such as a material’s composition in bulk, SERS promises advances in ultrasensitive detection of a variety of molecule classes. The most important among these are biomolecules such as DNA, RNA, and proteins. Such detection capabilities (which can approach single-molecule limits) could lead to very sensitive and even mobile medical diagnostic assays. Yet the SERS phenomenon is still perplexing to scientists. The enhanced effect is most commonly attributed to electromagnetic or chemical mechanisms.4–6 The latter involves charge transfer excitation between analyte molecules (i.e., molecules of interest) and the metal particles that they coat in nanostructures such as nanowires. (The closer an analyte is to the SERSenhancing metal nanostructure, the stronger the SERS effect, hence the coating. Coating is used for detection beyond SERS as well.) The electromagnetic mechanism is dominated by plasmon excitation, leading to ‘hot spots’ of Raman signals around nanosized metal particles. In SERS, these signals are highly ampliFigure 1. The OWL process.7 HNO3: Nitric acid. KOH: Potassium hydroxide. SiO2: Silicon dioxide. PECVD: Plasma-enhanced chemical vapor deposition. SiH4: Silane. N2O: Nitrous oxide. HAu(CN)2: Aurocyanic acid. AgCN: Silver cyanide. Ni: Nickel.