ABSTRACT
This chapter highlights some promising aspects of using plasmon-enhanced infrared (IR) adsorption spectroscopy for monitoring trace amounts of analytes in water by utilizing its strong enhancement of the electromagnetic field and its near-field nature of signal amplification. In contrast to plasmon-enhanced Raman scattering spectroscopy, plasmon-enhanced IR absorption spectroscopy uses broadband incident light, and signal-amplifying media are also required to exhibit broadband plasmon resonance in the entire mid-IR region. Here we demonstrate some examples of in situ high-sensitivity IR sensing of analytes in an aqueous environment using broadband plasmon enhancement in combination with attenuated total reflection (ATR) geometry. A monolayer of deoxyribonucleic
acid (DNA) aptamers adsorbed on a Au surface and trace amounts of pathogenic enzymes trapped by a DNA aptamer are sensitively detected in situ by using this method in aqueous solution. 3.1 IntroductionPlasmonic nanoantennas are a central research topic in modern nanophotonics as well as in biosensing and energy-harvesting technology [1-12]. As the simplest example of an optical antenna, using spherical metallic nanoparticles, a resonance frequency in the visible region is readily achievable [1, 5]. Its plasmon frequency, that is, Mie resonance frequency, can be tuned by changing the shape of the object and its size. For example, by changing the particle shape to an ellipsoid the plasmon resonance splits into two modes with higher and lower energies than that of the Mie plasmon of a sphere [5, 6]. As the aspect ratio of the ellipsoid increases, this splitting becomes larger and the lower-energy mode approaches near-infrared (IR) and mid-IR regions [5-11]. Also, when two nanoparticles are placed close together and they interact electromagnetically, plasmon hybridization takes place, and red-shifted as well as the blue-shifted modes appear. When the number of particles increases, this hybridization effect yields multiple split modes, and eventually the spectrum of the whole system becomes very broad, ranging from the visible region down to the mid-IR region. These split and broadband plasmonic modes constitute an ideal platform for sensing molecular vibrations by IR absorption spectroscopy, for which broadband blackbody radiation is normally used as the incident light source [13]. This is because the spectral ranges of 1) the incident light, 2) the vibrational frequencies of the molecules, and 3) the plasmon resonance frequencies of the island ensemble all lie in the same mid-IR spectral region. While most man-made plasmonic antennas fabricated lithographically exhibit only a narrowband resonance frequency and their resonance spectra are complicated [6-12], ensembles of polymorphic islands or porous metals mostly exhibit broadband resonance and their resonance spectra are smoother and flatter [14-23]. The remarkable broadband resonant characteristics of polymorphic islands make them highly advantageous for carrying out molecular spectroscopy since the plasmon resonance can hybridize with arbitrary vibrational modes of molecules appearing
in a wide frequency range from 100 cm-1 to 3600 cm-1. The smooth and flat plasmonic spectra are also advantageous for facilitating the identification of vibrational signals compared to lithographic antennas, which mostly exhibit sharper, narrow resonant features that require special care in identifying the molecular vibrational signal from the plasmonic signal [8-12]. In this chapter we give an example of the fabrication and application of a prototypical broadband plasmonic substrate that is composed of polymorphic Au islands separated by a high-density network of nanogaps [16-18]. In the first part, we describe the fabrication of the plasmonic nanostructure and then discuss its optical characteristics as well as its mechanism of signal amplification. In the second part, some examples of the high-sensitivity detection of analytes in water are introduced. The in situ adsorption measurements of deoxyribonucleic acid (DNA) and pathogenic enzymes in phosphate buffered saline (PBS) solution are described. Since plasmon-enhanced IR spectroscopy detects the analytes from the interaction between the molecular vibrations and the “near-field” electromagnetic radiation of the plasmon resonance, the huge background signal from bulk water can be minimized, and the signals from the analytes trapped inside the nanogaps between the islands are predominantly amplified. DNA oligonucleotides and pathogenic enzymes in aqueous solution are readily detected by using this method. 3.2 Fabrication of a Broadband Plasmonic
SubstrateFigure 3.1 shows the setup used to fabricate a broadband plasmonic substrate [15-17]. Polymorphic Au islands are grown on the bottom surface of a hemicylindrical Si crystal by electroless deposition. This Si crystal constitutes part of the optical setup for attenuated total reflection (ATR) measurement in which an IR beam is incident from the hemicylindrical surface and is totally reflected at the bottom surface [24-26]. The cell is made of Teflon, and a Si ATR crystal is mounted on top of the flow cell, which is sealed by a Kalrez O ring. An evanescent field is generated at the Si-crystal interface, and the IR beam interacts with the solution near the Si surface in a region with a thickness on the order of 1 μm. When metallic nanostructures
are placed at the Si-solution interface, the IR evanescent field is further concentrated at the surfaces (especially in the nanogaps) of the metal objects in the form of localized surface plasmons (Fig. 3.2).
