Nanosphere lithography (NSL) is an inexpensive, simple to implement, inherently parallel, high throughput, materials general nanofabrication technique capable of producing an unexpectedly large variety of nanoparticle structures and well-ordered 2D nanoparticle arrays. This article describes our recent efforts to broaden the scope of NSL to include strategies for the fabrication of several new nanoparticle structural motifs and their characterization by atomic force microscopy. NSL has also been demonstrated to be well-suited to the synthesis of size-tunable noble metal nanoparticles in the 20−1000 nm range. This characteristic of NSL has been especially valuable for investigating the fascinating richness of behavior manifested in size-dependent nanoparticle optics. The use of localized surface plasmon resonance (LSPR) spectroscopy to probe the size-tunable optical properties of Ag nanoparticles and their sensitivity to the local, external dielectric environment (viz., the nanoenvironment) is discussed in detail. More specifically, the effects of nanoparticle size, shape, interparticle spacing, nanoparticle-substrate interaction, solvent, dielectric overlayers, and molecular adsorbates on the LSPR spectrum of Ag nanoparticles are presented. This systematic study of the fundamentals of nanoparticle optics promises to find application in the field of chemical and biological nanosensors; herein, the initial data demonstrate that LSPR spectroscopy of Ag nanoparticles can be used to sense specifically bound analytes with zeptomole per nanoparticle detection limits and no detectable nonspecific binding.
The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.
Two-dimensional carbon-based nanomaterials, including graphene oxide and graphene, are potential candidates for biomedical applications such as sensors, cell labeling, bacterial inhibition, and drug delivery. Herein, we explore the biocompatibility of graphene-related materials with controlled physical and chemical properties. The size and extent of exfoliation of graphene oxide sheets was varied by sonication intensity and time. Graphene sheets were obtained from graphene oxide by a simple (hydrazine-free) hydrothermal route. The particle size, morphology, exfoliation extent, oxygen content, and surface charge of graphene oxide and graphene were characterized by wide-angle powder X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, dynamic light scattering, and zeta-potential. One method of toxicity assessment was based on measurement of the efflux of hemoglobin from suspended red blood cells. At the smallest size, graphene oxide showed the greatest hemolytic activity, whereas aggregated graphene sheets exhibited the lowest hemolytic activity. Coating graphene oxide with chitosan nearly eliminated hemolytic activity. Together, these results demonstrate that particle size, particulate state, and oxygen content/surface charge of graphene have a strong impact on biological/toxicological responses to red blood cells. In addition, the cytotoxicity of graphene oxide and graphene sheets was investigated by measuring mitochondrial activity in adherent human skin fibroblasts using two assays. The methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay, a typical nanotoxicity assay, fails to predict the toxicity of graphene oxide and graphene toxicity because of the spontaneous reduction of MTT by graphene and graphene oxide, resulting in a false positive signal. However, appropriate alternate assessments, using the water-soluble tetrazolium salt (WST-8), trypan blue exclusion, and reactive oxygen species assay reveal that the compacted graphene sheets are more damaging to mammalian fibroblasts than the less densely packed graphene oxide. Clearly, the toxicity of graphene and graphene oxide depends on the exposure environment (i.e., whether or not aggregation occurs) and mode of interaction with cells (i.e., suspension versus adherent cell types).
The wavelength corresponding to the extinction maximum, λ max , of the localized surface plasmon resonance (LSPR) of silver nanoparticle arrays fabricated by nanosphere lithography (NSL) can be systematically tuned from ∼400 nm to 6000 nm. Such spectral manipulation was achieved by using (1) precise lithographic control of nanoparticle size, height, and shape, and (2) dielectric encapsulation of the nanoparticles in SiO x . These results demonstrate an unprecedented level of wavelength agility in nanoparticle optical response throughout the visible, near-infrared, and mid-infrared regions of the electromagnetic spectrum. It will also be shown that this level of wavelength tunability is accompanied with the preservation of narrow LSPR bandwidths (fwhm), Γ. Additionally, two other surprising LSPR optical properties were discovered: (1) the extinction maximum shifts by 2-6 nm per 1 nm variation in nanoparticle width or height, and (2) the LSPR oscillator strength is equivalent to that of atomic silver in gas or liquid phases. Furthermore, it will be shown that encapsulation of the nanoparticles in thin films of SiO x causes the LSPR λ max to red shift by 4 nm per nm of SiO x film thickness. The size, shape, and dielectric-dependent nanoparticle optical properties reported here are likely to have significant impact in several applications including but not limited to the following: surfaceenhanced spectroscopy, single-molecule spectroscopy, near-field optical microscopy, nanoscopic object manipulation, chemical/biological sensing, information processing, data storage, and energy transport in integrated optical devices.
mekal theoretically predicted inelastic light scattering in 1923 (1). Raman and Krishnan first experimentally observed the phenomenon and reported in their 1928 Nature paper that the inelastic scattering effect was characterized by "its feebleness in comparison with the ordinary scattering" (2). This "feeble" phenomenon is now known as Raman scattering. The change in wavelength that is observed when a photon undergoes Raman scattering is attributed to the excitation (or relaxation) of vibrational modes of a molecule. Because different functional groups have different characteristic vibrational energies, every molecule has a unique Raman spectrum. In accordance with the Raman selection rule, the molecular polarizability changes as the molecular vibrations displace the constituent atoms from their equilibrium positions. The intensity of Raman scattering is proportional to the magnitude of the change in molecular polarizability. Thus, aromatic molecules exhibit more intense Raman scattering than aliphatic molecules.Even so, Raman scattering cross sections are typically 14 orders of magnitude smaller than those of fluorescence; therefore, the Raman signal is still several orders of magnitude weaker than the fluorescence emission in most cases. Because of the inherently small intensity of the Raman signal, the sensitivity limits of available detectors, and the intensity of the excitation sources, the applicability of Raman scattering was restricted for many years. However, its utility as an analytical technique improved with the advent of the laser and the evolution of photon detection technology.In 1977, Jeanmaire and Van Duyne demonstrated that the magnitude of the Raman scattering signal can be greatly enhanced when the scatterer is placed on or near a roughened noble-metal substrate (3). Strong electromagnetic fields are generated when the localized surface plasmon resonance (LSPR) of nanoscale roughness features on a silver, gold, or copper substrate is excited by visible light. When the Raman scatterer is subjected to these intensified electromagnetic fields, the magnitude of the induced dipole increases, and accordingly, the intensity of the inelastic scattering increases. This enhanced scattering process is known as surface-enhanced Raman (SER) scattering-a term that emphasizes the key role of the noblemetal substrate in this phenomenon.
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