Characterization and detection of uranyl ion sorption on silver surfaces using surface enhanced Raman spectroscopy.

The study of the chemical behavior of uranyl species and its rapid detection is of primary environmental and nonproliferation concern. Herein, we report on a surface enhanced Raman spectroscopic study of uranyl ion (UO(2)(2+)) sorption onto the thermally vapor deposited silver particle surface. The ability of vibrational spectroscopy to characterize surface phenomenon and the remarkable sensitivity of surface enhanced Raman spectroscopy (SERS) have been introduced as an appropriate combination for the surface characterization and detection of UO(2)(2+) onto the silver surface. The appearance of symmetric stretching frequency of UO(2)(2+) around 700 cm(-1) and the disappearance of the 854 cm(-1) band are attributed to the development of a chemical bond between silver surface and uranyl species. The effects of temperature, solute-surface interaction time, and pH have been studied using silver modified polypropylene filter (PPF) substrates. Results show that under appropriate conditions, the concentration of uranyl ion as low as 20 ng/mL can be easily detected using the discussed SERS approach without any surface modification of silver nanoparticles. Moreover, an alternative SERS approach of uranyl detection is demonstrated using nanolithographically fabricated SERS substrates.

Silicon nanopillars for field-enhanced surface spectroscopy.

Silicon nanowire and nanopillar structures have drawn increased attention in recent years due in part to their unique optical properties. Herein, electron beam lithography combined with reactive-ion etching is used to reproducibly create individual silicon nanopillars of various sizes, shapes, and heights. Finite difference time domain analysis predicts local field intensity enhancements in the vicinity of appropriately sized and coaxially illuminated silicon nanopillars of approximately 2 orders of magnitude. While this level of enhancement is modest when compared to plasmonic systems, the unique advantage of the silicon nanopillar resonators is that they enhance optical fields in substantially larger volumes. By analyzing experimentally measured strength of the silicon Raman phonon line (500 cm(-1)), it was determined that nanopillars produced local field enhancements that are consistent with these predictions. Additionally, we demonstrate that a thin layer of Zn phthalocyanine on the nanopillar surface with a total amount of <30 attomoles produced prominent Raman spectra, yielding enhancement factors (EFs) better than 2 orders of magnitude. Finally, silicon nanopillars of cylindrical and elliptical shapes were labeled with different fluorophors and evaluated for their surface-enhanced fluorescence (SEF) capability. The EFs derived from analysis of the acquired fluorescence microscopy images indicate that silicon nanopillar structures can provide enhancements comparable or even stronger than those typically achieved using plasmonic SEF structures without the limitations of the metal-based substrates, such as fluorescence quenching and an insufficiently large probe volume. It is anticipated that dense arrays of silicon nanopillars will enable SEF assays with extremely high sensitivity, while a broader impact of the reported phenomena is anticipated in photovoltaics, subwavelength light focusing, and fundamental nanophotonics.

Efficient disc on pillar substrates for surface enhanced Raman spectroscopy.

In this work, geometrical optimizations of Ag disc on pillar (DOP) hybrid plasmonic nanostructures were conducted and allowed us to achieve reproducible average enhancement factors of 1 × 10(9) and greater.

Controllable nanofabrication of aggregate-like nanoparticle substrates and evaluation for surface-enhanced Raman spectroscopy.

The development of new and better substrates is a major focus of research aimed at improving the analytical capabilities of surface-enhanced Raman spectroscopy (SERS). Perhaps the most common type of SERS substrate, one consistently exhibiting large enhancements, is simple colloidal gold or silver nanoparticles in the 10-150 nm size range. The colloidal systems that are used most for ultrasensitive detection are generally aggregated clusters that possess "hot spot(s)" within some of the aggregates. A significant limitation of these synthetic substrates is that the "hot" aggregates are extremely difficult to create consistently or predict. Electron beam lithography (EBL) along with combinatorial spectral mapping can be used to overcome this limitation. Our previous work, and that of other researchers, invokes the special capabilities of EBL to design and fabricate periodic, highly ordered nanoparticle arrays for SERS. Building on this work, EBL, in conjunction with ancillary fabrication steps, can be used to create complex patterns that mimic random aggregates. These aggregates, unlike those created by colloidal deposition methods, can be uniquely reproduced within the resolution limits of EBL. In the work reported herein, we use a unique approach to create substrates containing a large number of randomly generated cells with different morphologies that are arrayed on silicon wafers. Instead of isolated metal nanoparticles, these structures resemble the aggregates of colloid. By spectral mapping, we investigate the SERS activity of the combinatorial arrays of cells using probe analytes. Two general categories of shapes are randomly designed in different sizes and densities into several hundred different 5 mum square cells. Following fabrication, it is shown that a SERS performance contrast of more than a factor of 44 is achieved among these cells and that the best performing cells can be cloned into uniformly high performing macropatterns of lithographically defined nanoaggregates (LDNAs). In this manner, extended LDNA surfaces with uniform 5 x 10(8) enhancement factors are created. Furthermore, the LDNAs can be further dissected and studied in an effort to increase the SERS enhancement per unit geometric substrate area.