Solid-state surface-enhanced Raman spectroscopic (SERS) substrates surmount the limitations of colloidal nanoparticles for many practical chemical and biomolecular sensors. The main bottom-up approaches adopted for SERS substrate fabrication include preparing plasmonic nanoparticles and their deposition to selected surfaces via chemical self-assembly, inkjet printing, spray coating, dip-coating, etc. In contrast, the top-down strategy is to create nanostructures on surfaces via different lithographic techniques (nanoimprint lithography, electron beam lithography), laser ablation, or chemical etching and then depositing a nanolayer of coinage metals. The ease and reproducibility of production, high enhancement, and uniformity of performance are the requirements for a reliable SERS substrate. This work describes an easy process for preparing SERS substrates from a less expensive and readily available material, brass. The treatment of brass with hydrochloric acid caused preferential etching of Zn from the surface, leaving a uniform Cu nanoporous substrate (Cu NPS ). The subsequent galvanic displacement of Cu NPS with AgNO 3 resulted in silver nanocrystal overgrowth, as evidenced by microscopic, spectroscopic, and elemental studies. The corresponding SERS studies with 4-mercaptophenylboronic acid (4-MPBA) as the probe molecule revealed ∼30 and ∼300 times improved performance compared to Cu NPS and the parent brass substrates, respectively. Further, we explored the possibility of sensing creatinine, the biomarker for kidney functioning. Creatinine followed a Freundlich type adsorption, and the SERS substrate developed herein exhibited a sensitivity down to 1.7 pg/mL.
Herein, we combine the ideas of concerted emission from fluorophore ensembles and its further amplification through FRET in an organic-inorganic hybrid approach. Spherical and highly fluorescent organic nanoparticles (FONPs, Φ =0.38), prepared by the self-assembly of oligo(phenylene ethynylene) (OPE) molecules, were selected as a potential donor material. This organic core was then decorated with a shell of fluorescent CdSe/ZnS core-shell quantum dots (QDs; ≅5.5 nm, Φ =0.27) with the aid of a bifunctional ligand, mercaptopropionic acid. Its high extinction coefficient (ϵ≈4.1×10 m cm ) and good spectral match with the emission of the FONPs (J(λ)≈4.08×10 m cm nm ) made them a better acceptor candidate to constitute an efficient FRET pair (Φ =0.8). As a result, the QD fluorescence intensity was enhanced by more than twofold. The fundamental calculations carried out indicated an improvement in all the FRET parameters as the number of QDs around the FONPs was increased. This, together with the localization of multiple QDs in a nanometric dimension (volume≈1.8×10 nm ), gave highly bright reddish luminescent hybrid particles as visualized under a fluorescence microscope.
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