In the last 10 years, we have witnessed a rapid growth in the development of highly selective and sensitive optical biosensors for the medical diagnosis and monitoring of diseases, drug discovery, and the detection of biological agents. Among the many advantages of optical biosensors, sensitivity and simple detection systems allow them to be applied widely. Optical sensing techniques are based on various sensing transduction mechanisms, fluorescence, light absorption and scattering, Raman scattering, and surface plasmon resonance (SPR) [1–3]. Especially, sensing systems using localized SPR (LSPR) have received significant research attention in recent years as a result of their potential for use as highly sensitive, simple, and label-free bio/chemical binding detection devices [4–6].
In the emergence of plasmonics, there has been much research interest in various kinds of metal nanostructures, in which confined free electrons are forced to oscillate by an incident light; the resulting collective oscillation of electrons can exhibit strong local field enhancement at a particular frequency. In the LSPR, the incoming light is absorbed or scattered by the nanostructures, and concurrently, there is an electromagnetic field enhancement close to the nanostructures. It is well established that the peak extinction wavelength, λmax, of the LSPR spectrum is dependent upon the size, shape, spacing, and dielectric properties of materials and the local environment [7–9]. LSPR has been explored in a range of nanostructure shapes such as spheres, triangles, or cubes. Major efforts have gone into studying the sensitivity of such structures to changes in the local environments and refractive index. The potential for their use as ultrasensitive detectors comes from both their high sensitivity and the short range of the associated optical fields. Therefore, this property opens a route to the sensing of local biomolecular recognition events where adsorbate-induced changes in the local dielectric environment around the nanostructures are utilized.
There is a significant demand for the development of simple, robust, and accurate optical biosensors for deployment in a wide range of applications such as the analysis of molecular structures or the detection of disease agents. Considering the use of LSPR sensing systems in the medical front, it is not satisfied only by evaluating sensitivities to the changing of the bulk refractive index or surface environment. It is noted that the detection of chemical systems including those targeting and proving molecules have to be done by LSPR sensing for practical purposes. For simple research on the present LSPR biosensor study on immunoassay, we focused on bovine serum albumin (BSA) binding onto the surface of metal nanostructures.
Such bioapplications with good performances require an excitation within 800 to 1,100 nm (the so-called optical window) to provide a deeper tissue penetration of photons with reduced photodamage effects. Several authors have taken advantage of the high permeability of the human skin and tissue to near-infrared (NIR) radiation to develop diagnostic detection tec-hniques. The use of NIR light is a promising approach for biomedical detection based on LSPR. Thus, metal nanoparticles with various shapes have been proposed to respond to NIR light. In shell-type geometries such as nanoshells and nanorings , interactions among electrons bound to the inner and outer surfaces of the shell give rise to the so-called plasmon hybridization [11–13], resulting in a wide range of tenability and higher sensitivities for sensing. It is well known that NIR light provides LSPR in nanoshells as the simplest nanostructure. Since sensing systems using NIR light, however, are required to improve their detection sensitivity, it is necessary to arrange as many nanostructures as possible as sensing units on the substrate.
From the abovementioned background, we propose nanoshell arrays for LSPR sensing platforms combining the best features of NIR light response. In recent years, there exist a lot of reports on various metals generating LSPR, while few researchers describe a systematic comparison to optimize sensing performance by changing the materials. In this study, we use Au, Ag, and Cu, typical materials for the plasmonic research field, for metal nanoshell arrays and experimentally and quantitatively demonstrate a suitable metal for LSPR sensing.