**1. Introduction**

The impurity dopants in zinc oxide (ZnO) produce interesting optical, electrical and magnetic functionalities. For example, photoluminescence behaviors are observed by doping with rare-earth and transition-metal atoms [1–4]. Ferromagnetic properties are found by the incorporation of transition-metal atoms into the host [5–7]. Above all, the donor dopants, such as Ga and Al ions, lead to the metallic conductivity of ZnO which has been widely used as transparent electrodes [8, 9]. The metallic properties of ZnO are also prospective candidates for alternatively plasmonic materials in the infrared (IR) range [10–13]. Unlike classical plasmonic materials such as Ag and Au metals, plasmonic resonances can be controlled by changing electron density in ZnO. Additionally, the electronic structure of ZnO is composed of 4 *s* and 2*p* orbitals, providing no inter-band transition such as those shown by Ag and Au metals [14]. The inter-band transition of ZnO only shows in the ultra-violet range. This band system produces a low optical loss in the IR range. Thus far, different geometries such as dots, wires, and films have been chosen to study surface plasmon resonances (SPRs) [15–17]. These SPR-related studies have shown the unprecedented capabilities of ZnO for use as alternatives to metals in IR applications.

**Figure 1.**

*Schematic figures of SPR sensing platforms of (a) single, (b) IMI and (c) hybrid samples.*

It is known that ZnO is one of the interesting materials in biological sensing.

Optical and electrical techniques detect biomolecular interactions on ZnO film surfaces. The piezoelectric responses of ZnO have been applied to biosensors based on surface acoustic waves [18]. Electrochemical impedance is needed to use transparent electrodes based on ZnO [19]. These interesting properties of ZnO as biocompatible materials have generated much attention as solid oxide substrates for highly sensitive biosensing platforms. Our group has investigated layer samples of ZnO to improve the optical technology associated with SPRs. In particular, layer samples could readily be employed in biological sensing based on propagation-type SPRs. The benefits of layer samples with large surface areas render film platforms more attractive for industrial applications. Recently, SP waves, which are excited by traverse magnetic polarization, have been produced in ZnO-based optical fibers as biochemical sensing probes [20, 21]. Surface-enhanced infrared absorptions have also been confirmed on ZnO film surfaces using a prism-based SPR method [22, 23]. These previous studies indicate the potential of ZnO-based SPR (ZnO-SPR) biosensors and have motivated the study of biological sensing.

In this chapter, we discuss the structural and optical properties of ZnO-SPR from experimental and theoretical approaches. We first outline single layers' SPR properties based on ZnO: Ga, which is referred to as a single sample. The single sample is excited by Kretschmann-type SPRs using attenuated total reflection (ATR) optics. Second, the utilization of an insulator–metal–insulator (IMI) structure to a ZnO-SPR (IMI sample) is introduced in relation to the long-range SP mode. Finally, we fabricate hybrid layer structures with the capping of thin dielectric layers to ZnO-SPR (dielectric-assisted ZnO-SPR; hybrid sample) to enhance SPR properties in the IR range. Schematic pictures of single, IMI and hybrid samples are shown in **Figure 1**. Each sample's detection sensitivity is discussed from the viewpoint of electric-field distribution (*E*-field), propagation and penetration depths of SP waves. Finally, biological sensing on ZnO-SPR is experimentally demonstrated by measuring biological interactions between biotin and streptavidin. This study introduces SPR devices based on ZnO for biological sensing.
