A highly sensitive micrometer-sized optical fiber affinity biosensor is reported based on the localized surface plasmon resonance in gold nanoparticles adsorbed at an endface of an optical fiber. This sensor probes the affinity between biological molecules in real time without any labeling of the analyte. The highest resolution of 10−5 in refractive index units is demonstrated with a red-light-emitting diode used as a light source. This value corresponds to an adsorption amount of 10−12g∕mm2 in the case of the affinity measurement. This sensitivity is similar to that of the conventional surface plasmon resonance biosensors that use attenuated total reflection optics.

Protein microarray technology has gone through numerous innovative developments in recent decades. In this review, we focus on the development of protein detection methods embedded in the technology. Early microarrays utilized useful chromophores and versatile biochemical techniques dominated by high-throughput illumination. Recently, the realization of label-free techniques has been greatly advanced by the combination of knowledge in material sciences, computational design and nanofabrication. These rapidly advancing techniques aim to provide data without the intervention of label molecules. Here, we present a brief overview of this remarkable innovation from the perspectives of label and label-free techniques in transducing nano-biological events.

Vacuum-deposited films of a long-chain n-alkane, n-C44H90, tetratetracontane (TTC), on a Au(111) surface were investigated by infrared reflection absorption spectroscopy. We found four different structures for TTC on the Au surface. One is a “flat-on” structure formed by molecules in an all-trans conformation with the molecular plane composed of carbon atoms parallel to the surface. This structure was found in the first monolayer physisorbed on the surface. The second structure includes the gauche conformation, which shows a change in the orientation of the methylene groups with an increase in the thickness of TTC. The third structure is the crystalline state formed by all-trans molecules, which was found in the third and thicker layers with the molecular long axis parallel to the surface. These three structures were found for films deposited and measured at room temperature. With an increase in temperature after deposition at room temperature, the first and second structures were easily converted to another structure. This fourth structure consists of TTC molecules in the vicinity of the surface with their molecular axes parallel to the surface and their molecular planes tilted from the surface. The molecules in this structure interact with the Au(111) surface through some of their methylene groups, while the molecules in the flat-on structure contact the surface through all of their methylene groups. The appearance of these two structures is controlled by the balance of two kinds of interaction:  methylene group−Au surface interaction and the interaction among TTC molecules. In the second layer, the formation of TTC molecules including a gauche conformation is ascribed to the geometrical incompatibility between flat-on TTC and crystalline TTC.

Nanoisland films prepared by annealing thin gold films at high temperatures were imaged using scanning electron microscopy (SEM) and atomic force microscopy, and optically characterized through absorption spectroscopy. Thin gold films of effective thicknesses 2, 5 and 7 nm annealed at 500, 700 and 900 degrees C were fabricated and studied experimentally. The measured absorption characteristics in support of theoretical calculations showed that the shapes of gold islands were partial spheres. The position of the peak absorption wavelength measured with s-polarized light or at normal incidence confirmed that the island shape grew from a near-hemisphere towards a sphere with increasing annealing temperature. The SEM images confirmed that the size of islands increased from 15 nm in diameter to 40 nm in diameter as film thickness increased from 2 to 5 nm. The affect of the index of the substrate material on absorption characteristics were also studied by comparing the absorption spectra of gold island films on quartz and LaSF15 glass substrates. The use of gold nanoisland films for preparing localized surface plasmon resonance substrates was suggested as they held advantages over the gold colloid films.