We investigated the influence of the mixed n-alkanethiolate self-assembled monolayer (SAM) formed on gold nanoparticles (AuNPs: 50.0 ± 3.2 nm in diameter) on their assembly into colloidal films. Dodecanethiol and octadecanethiol were selected as the short- and long-chain alkanethiols, respectively. The mixed SAMs were formed by immersing AuNPs in a mixed alkanethiol solution at different molar ratios. Au colloidal films were fabricated on indium tin oxide substrates by our previously reported hybrid method. The composition of the two alkanethiolates in the SAM was deduced from the intensity ratio of two Raman bands at 1080 and 1105 cm–1. The surface coverage of the colloidal films increased by forming equimolar or dodecanethiolate-dominant mixed SAMs on AuNPs instead of a pure dodecanethiolate or octadecanethiolate SAM. The highest coverage exceeded 80%. This improvement is attributed to the high dispersion stability of AuNPs covered with equimolar or dodecanethiolate-dominant mixed SAMs.

We report a sandwich-type SERS-based immunoassay using a two-dimensional (2D) array of gold core@silver shell (Au@Ag) nanoparticles (NPs) as the SERS substrate and antibody-conjugated gold NPs labeled with 4-mercaptobenzoic acid (MBA) as the SERS probes. To achieve highly sensitive detection, the size of the SERS probes was first optimized for the immunoassay of Human-IgG (H-IgG), where the Au core size of the SERS probes was varied from 26 to 110 nm in diameter. The maximum SERS intensity was observed at an Au core size of 53 nm. Then, the influence of the size of the sandwich immunocomplexes on the sensitivity was examined by performing sandwich SERS immunoassays for H-IgG and prostate-specific antigen (PSA) using SERS probes with 53 nm Au core size. The sensitivity improvement by using the SERS substrate (2D-array of Au@Ag NPs) instead of an Au evaporated film, which was used as a reference substrate, was evaluated for each immunoassay. The sensitivity improvement for H-IgG and PSA detection was 2.3-fold and 6.4-fold, respectively. The larger sensitivity improvement for the PSA system can be attributed to the smaller immunocomplex of PSA; the shorter separation distance between the SERS probes and the SERS substrate induces stronger plasmon coupling. This result indicates that the sensitivity of the sandwich-type immunoassay performed on the SERS substrate increases with decreasing size of sandwich immunocomplex, suggesting that the sensitivity can be improved by adopting an antibody-fragment with the same affinity for the target antigen as that of the antibody.

SrTiO3(100) – (√5 × √5) – R26.6 surfaces were studied by means of high-resolution scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. By varying the bias voltage in the occupied state, it was possible to observe the arrangement of titanium and oxygen atoms in the unit cells of a (√5 × √5) surface superstructure, which revealed that the TiO2 layer is the terminating plane of the (√5 × √5) surface. In the STM images, peculiar protrusions were seen at the oxygen fourfold hollow site responsible for √5 × √5 periodicity. The protrusions are asymmetrical in contrast, which would be an important consideration in proposing accurate structural models for (√5 × √5) surface superstructures.Research Highlights► We reported the surface structure of SrTiO3(100)-√5×√5-R26.6. ► High resolution scanning tunneling microscope observations were conducted at room temperatures in ultrahigh vacuum. ► Atomic orbitals of Ti and O atoms were observed and it is confirmed that √5×√5 surface is terminated by TiO2 plane. ► Asymmetrical structures appear at the O fourfold hollow site with √5×√5 periodicity. ► The asymmetrical structures are not simply consistent with the Sr-adatom model.

Bi nanolines form at around 570 °C during the anneal of a Bi-rich surface, which is composed of a mixture of Bi dimers, Si dimers, and missing dimer defects. High-temperature STM observations find that as well as the stable nanolines previously reported, a second Bi-related linear feature forms in the background, which has not previously been identified. Using quench experiments, and by comparison with tightbinding simulations of candidate surface structures, the structure of this linear feature has been identified. It is concluded that the linear defective feature comprises a single missing dimer defect (1DV) decorated with Bi, and forms as a result of cooperative strain relaxation between the Bi dimers and the 1DV.