Cost-Effective Plasmonic Platforms: Glass Capillaries Decorated with Ag@SiO₂ Nanoparticles on Inner Walls as SERS Substrates

A cost-effective method for the fabrication of a glass capillary based plasmonic platform for the selective detection and identification of analytes of importance in health, environment, and safety is demonstrated. This was achieved by coating Ag@SiO₂ nanoparticles (Ag ∼ 60 nm) having silica shell of varying thickness (∼2 and ∼25 nm) on the inside walls of glass capillaries, over 2 cm in length, with uniform coverage. It was found that the particle density on the surface plays a decisive role on the enhancement of Raman signals. Multiple hot spots, which are essentially junctions of amplified electric field, were generated when ∼30 Ag@SiO₂ particles/μm² were bound onto the walls of glass capillaries. The pores of the silica shell allow the localization of analyte molecules to the vicinity of hot spots resulting in signal enhancements of the order of 10¹⁰ (using pyrene as analyte; excitation wavelength, 632.8 nm). The applicability of Ag@SiO₂ coated capillaries for the detection of a wide range of molecules has been explored, by taking representative examples of polyaromatic hydrocarbons (pyrene), amino acids (tryptophan), proteins (bovine serum albumin), and explosives (trinitrotoluene). By increasing the thickness of the silica shell of Ag@SiO₂ nanoparticles, an effective filtration cum detection method has been developed for the selective identification of small molecules such as amino acids, without the interference of large proteins

Ag@SiO₂ Core–Shell Nanostructures: Distance-Dependent Plasmon Coupling and SERS Investigation

Enhancement of Raman signals of pyrene due to the enhanced electric fields on the surface of silver nanoparticles has been investigated by controlling the thickness of the silica shell. Dimeric nanostructures having well-defined gaps between two silver nanoparticles were prepared, and the gap size (<i>d</i>) was varied from 1.5 to 40 nm. The molecules trapped at the dimeric junctions showed higher Raman signal enhancements when the gap was less than 15 nm due to the presence of amplified electric field, in agreement with our theoretical studies. The experimental Raman enhancement factors at the hot spots follow a 1/<i>d</i>^<i>n</i> dependence, with <i>n</i> = 1.5, in agreement with the recent theoretical studies by Schatz and co-workers. Experimental results presented here on the distance dependence of surface enhanced Raman spectroscopy (SERS) enhancement at the hot spots can provide insight on the design of newer plasmonic nanostructures with optimal nanogaps

Simple and Cost-Effective Quantum Dot Chemodosimeter for Visual Detection of Biothiols in Human Blood Serum

An emission “turn-off” chemodosimeter for the naked-eye detection of biothiols using silica-overcoated cadmium selenide quantum dots is developed. Hole scavenging by the thiol group of cysteine, homocysteine, or glutathione on interaction with quantum dots resulted in an instant and permanent emission quenching under physiologically relevant conditions. Also, the emission suppression is so specific that thiols and substituted thiols (methionine and cystine) can easily be distinguished. A pilot experiment for the visual detection of serum thiols in human blood was also conducted. Densitometry analysis proved the potential of this system as a new methodology in clinical chemistry and research laboratories for routine blood and urine analyses using a simple procedure. This method enables one to visually distinguish biothiols and oxidized biothiols, whose ratio plays a crucial role in maintaining “redox thiol status” in the blood