Plasmonic Silver Nanobelts via Citrate Reduction in the Presence of HCl and their Orientation-Dependent Scattering Properties

Silver nanobelts were synthesized without any external seeds or templates using a facile hydrothermal route via citrate reduction in the presence of hydrochloric acid. Both H⁺ and Cl^– ions in HCl are important for the generation of nanobelts. The morphology can be controlled by varying the concentration of HCl and citrate. The obtained nanobelts have 3D orientation-dependent optical properties. By tracking the transportation and rotation of the nanobelts in cells or other biosystems, useful information related to the physical or chemical surroundings may be obtained

Surface-Enhanced Raman Scattering Detection of pH with Silica-Encapsulated 4‑Mercaptobenzoic Acid-Functionalized Silver Nanoparticles

Sensors based upon surface-enhanced Raman spectroscopy (SERS) are attractive because they have narrow, vibrationally specific spectral peaks that can be excited using red and near-infrared light which avoids photobleaching, penetrates tissue, and reduces autofluorescence. Several groups have fabricated pH nanosensors by functionalizing silver or gold nanoparticle surfaces with an acidic molecule and measuring the ratio of protonated to deprotonated Raman bands. However, a limitation of these sensors is that macromolecules in biological systems can adsorb onto the nanoparticle surface and interfere with measurements. To overcome this interference, we encapsulated pH SERS sensors in a 30 nm thick silica layer with small pores which prevented bovine serum albumin (BSA) molecules from interacting with the pH-indicating 4-mercaptobenzoic acid (4-MBA) on the silver surfaces but preserved the pH-sensitivity. Encapsulation also improved colloidal stability and sensor reliability. The noise level corresponded to less than 0.1 pH units from pH 3 to 6. The silica-encapsulated functionalized silver nanoparticles (Ag-MBA@SiO₂) were taken up by J774A.1 macrophage cells and measured a decrease in local pH during endocytosis. This strategy could be extended for detecting other small molecules in situ