The optical properties of metal nanoparticles have long been of interest in physical chemistry, starting with Faraday's investigations of colloidal gold in the middle 1800s. More recently, new lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes, and dielectric environments. In this feature article, we describe recent progress in the theory of nanoparticle optical properties, particularly methods for solving Maxwell's equations for light scattering from particles of arbitrary shape in a complex environment. Included is a description of the qualitative features of dipole and quadrupole plasmon resonances for spherical particles; a discussion of analytical and numerical methods for calculating extinction and scattering cross-sections, local fields, and other optical properties for nonspherical particles; and a survey of applications to problems of recent interest involving triangula...

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The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment

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Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

Recent developments have greatly improved the sensitivity of optical sensors based on metal nanoparticle arrays and single nanoparticles. We introduce the localized surface plasmon resonance (LSPR) sensor and describe how its exquisite sensitivity to size, shape and environment can be harnessed to detect molecular binding events and changes in molecular conformation. We then describe recent progress in three areas representing the most significant challenges: pushing sensitivity towards the single-molecule detection limit, combining LSPR with complementary molecular identification techniques such as surface-enhanced Raman spectroscopy, and practical development of sensors and instrumentation for routine use and high-throughput detection. This review highlights several exceptionally promising research directions and discusses how diverse applications of plasmonic nanoparticles can be integrated in the near future.

Biosensing with plasmonic nanosensors

Localized surface plasmon resonance (LSPR) spectroscopy of metallic nanoparticles is a powerful technique for chemical and biological sensing experiments. Moreover, the LSPR is responsible for the electromagnetic-field enhancement that leads to surface-enhanced Raman scattering (SERS) and other surface-enhanced spectroscopic processes. This review describes recent fundamental spectroscopic studies that reveal key relationships governing the LSPR spectral location and its sensitivity to the local environment, including nanoparticle shape and size. We also describe studies on the distance dependence of the enhanced electromagnetic field and the relationship between the plasmon resonance and the Raman excitation energy. Lastly, we introduce a new form of LSPR spectroscopy, involving the coupling between nanoparticle plasmon resonances and adsorbate molecular resonances. The results from these fundamental studies guide the design of new sensing experiments, illustrated through applications in which researchers use both LSPR wavelength-shift sensing and SERS to detect molecules of chemical and biological relevance.

https://www.photonics.ethz.ch/fileadmin/user_upload/Courses/NanoOptics/willets.pdf

Localized Surface Plasmon Resonance Spectroscopy and Sensing

Noble metal nanostructures attract much interest because of their unique properties, including large optical field enhancements resulting in the strong scattering and absorption of light. The enhancement in the optical and photothermal properties of noble metal nanoparticles arises from resonant oscillation of their free electrons in the presence of light, also known as localized surface plasmon resonance (LSPR). The plasmon resonance can either radiate light (Mie scattering), a process that finds great utility in optical and imaging fields, or be rapidly converted to heat (absorption); the latter mechanism of dissipation has opened up applications in several new areas. The ability to integrate metal nanoparticles into biological systems has had greatest impact in biology and biomedicine. In this Account, we discuss the plasmonic properties of gold and silver nanostructures and present examples of how they are being utilized for biodiagnostics, biophysical studies, and medical therapy. For instance, takin...

Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine

The wavelength corresponding to the extinction maximum, λmax, 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 SiOx. 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...

Nanosphere lithography: Tunable localized surface plasmon resonance spectra of silver nanoparticles

In this paper, we explore the optical properties of Ag nanoparticles chemically modified with alkanethiol self-assembled monolayers (SAMs) by measuring the localized surface plasmon resonance (LSPR) spectrum using UV−vis extinction spectroscopy. For all the experiments presented here, the Ag nanoparticles were fabricated using the technique of nanosphere lithography (NSL) and had in-plane widths of 100 nm and out-of-plane heights of 50 nm. We first demonstrate that unmodified nanoparticles are extremely susceptible to slight changes in 3-dimensional structure when exposed to various solvents. These structural effects can have dramatic effects on the extinction maximum, λmax, of the LSPR shifting it to the blue by over 100 nm. The significant discovery reported here is that λmax for NSL fabricated Ag nanoparticles is extremely sensitive to the SAM properties. We will demonstrate the following new features:  (1) λmax of the LSPR linearly shifts to the red 3 nm for every carbon atom in the alkane chain; (2) ...

Chain Length Dependence and Sensing Capabilities of the Localized Surface Plasmon Resonance of Silver Nanoparticles Chemically Modified with Alkanethiol Self-Assembled Monolayers

In this paper, we explore the optical contributions of the substrate to the localized surface plasmon resonance (LSPR) spectrum of surface confined Ag nanoparticles produced by nanosphere lithography (NSL). We present optical extinction spectra of Ag nanoparticles fabricated on the following substrates:  fused silica, borosilicate optical glass, mica, and SF-10a high refractive index specialty glass. For all the experiments discussed here, the Ag nanoparticles were approximately 100 nm in in-plane width and 25 nm in out-of-plane height. In a controlled N2 environment, the wavelength corresponding to the extinction maximum, λmax, shifts to the red with increasing refractive index of the substrate, nsubstrate. The sensitivity factor, Δλmax/Δnsubstrate, was measured to be 87 nm per refractive index unit (RIU). Experimental extinction spectra were modeled using the discrete dipole approximation (DDA). The DDA theory qualitatively predicts the experimentally observed trend that λmax is linearly dependent on ns...

Nanosphere Lithography: Effect of Substrate on the Localized Surface Plasmon Resonance Spectrum of Silver Nanoparticles

This paper develops expressions for the Raman power emitted by liquid core optical fiber (LCOF) sample cells in six simple excitation/collection geometries and the fraction of that power that can be utilized in a conventional Raman spectrometer From these expressions a "figure of merit" is developed that can be used to predict the relative intensity enhancement provided by LCOFs having different inside diameters and loss characteristics For the apparatus used here, we show theoretically and experimentally that the figure of merit takes the simple form (αd)-1 where α and d are, respectively, the loss coefficient and inside diameter of the LCOF Given the varying optical quality of Teflon®-AF LCOFs currently in use, the analysis presented here should be useful in optimizing the performance of LCOF accessories in Raman and related spectroscopic applications

Michelle Duval Malinsky

Papers

Nanosphere lithography: Tunable localized surface plasmon resonance spectra of silver nanoparticles

Chain Length Dependence and Sensing Capabilities of the Localized Surface Plasmon Resonance of Silver Nanoparticles Chemically Modified with Alkanethiol Self-Assembled Monolayers

Nanosphere Lithography: Effect of Substrate on the Localized Surface Plasmon Resonance Spectrum of Silver Nanoparticles

Intensity Considerations in Liquid Core Optical Fiber Raman Spectroscopy