A novel approach for the synthesis of colloidal silver nanoprisms (AgNPrs) with controllable localized surface plasmon resonance (LSPR) via a chemical shape transformation of silver nanospheres (AgNSs) is presented. The shape conversion is carried out by feeding hydrogen peroxide (H 2 O 2 ) solution into a starchstabilized AgNS colloid under ambient conditions. Oxidative dissolution and the mild reducing action of H 2 O 2 under alkaline conditions serve as the principal reactions for the shape transformation process. After addition of H 2 O 2 , the instantaneous shape transformation events can be visualized by the naked eye through the color change of the colloid. Initial concentration of AgNSs, molar ratio of H 2 O 2 : AgNSs, H 2 O 2 injection rate, and mixing efficiency are the key parameters for controlling the LSPR wavelengths of AgNPrs as the in-plane dipole plasmon resonance can be selectively tuned across visible and near infrared regions (i.e., 460-850 nm). The obtained AgNPrs exhibited mixed geometries e.g. hexagonal, truncated triangular, rounded-tip triangular prisms, and circular disks with average bisector lengths of 30 to 120 nm and the thickness of 10 to 20 nm. A colloid of highly concentrated AgNPrs having a final concentration up to 11 mM can be produced within 10 min.

A nanoscale pH-profile on a 4×4 µm 2 area of NH 2 -anchored glass slide in an aqueous solution is constructed using chemically modified tip-enhanced Raman scattering (TERS). Para-mercaptobenzoic acid (pMBA) and para-aminothiophenol (pATP) are bonded to the tip surface. A pH change can be detected from a peak at 1422 cm -1 due to the -COOstretching vibration from pMBA and that at 1442 cm -1 due to the N=N stretching vibration arising from the formation of 4,4′-dimercaptoazobenzene (DMAB) on the pATP-modified tip.The pMBA-and pATP-modified tip can be used to determine pH in the range of 7-9 and 1-2, respectively. The spatial resolution to differentiate pH of two areas can be considered as ~400 nm. The measured pH becomes the pH of the bulk solution when the tip is far by ~200 nm from the surface. This technique suggests a possibility for the pH sensing in wet biological samples.TERS tips could also be chemically modified with other molecules to determine other properties in a solution.

In this report, we propose a novel starch-mediated photochemical reduction method for synthesizing micrometer-sized gold nanoplates and the possibility of using them as a tip-enhanced Raman scattering (TERS) substrate. To reduce gold ions, a starch chain firstly forms a complex with AuCl(4)(-), and the gold ion is subsequently reduced by receiving an electron from a chloride ion and generating a chloride radical when the [AuCl(4)(-)-starch] complex is irradiated by sunlight. Due to the slow reaction rate and the capability of starch as a template, gold structure can thermodynamically grow along the (111) facet which is the lowest energy facet of the gold face-centered cubic (fcc) crystal. This method can provide various shapes of gold plates such as triangle, truncated triangle, hexagon, polygon, etc. The plate size can be controlled in the range from a few micrometers to more than one hundred micrometers by increasing the acidity of solution while the plate thickness is less than 100 nm. Potential application of the gold plates as TERS substrates is demonstrated by collecting Raman signals while approaching a silver-coated tungsten tip to the surface of the micrometer-sized gold nanoplate covered by crystal violet (CV) molecules. The results show that less than one hundred CV molecules can be detected in our study.

In this report, we propose a novel technique for the determination of the concentrations of iodide and thiocyanate by surface-enhanced Raman scattering (SERS) of starch-reduced gold nanoparticles. Starch-reduced gold nanoparticles show an intrinsic Raman peak at 2125 cm(-1) due to the -C≡C- stretching mode of a synthesized byproduct. Because of the high adsorptivity of iodide on a gold surface, the intensity of the SERS peak at 2125 cm(-1) decreases with an increase in the iodide concentration. Thiocyanate also strongly adsorbs on a gold surface, and a new peak appears at around 2100 cm(-1), attributed to the -C≡N stretching vibration in a SERS spectrum of starch-reduced gold nanoparticles. These two peaks were successfully used to determine the iodide and thiocyanate concentrations separately, even in their mixture system. The detection limit of this technique for iodide is 0.01 μM with a measurement range of 0.01-2.0 μM, while the detection limit of this technique for thiocyanate is 0.05 μM with a measurement range of 0.05-50 μM. This technique is highly selective for iodide and thiocyanate ions without interference from other coexisting anions such as other halides, carbonate, and sulfate.