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@SiO 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@SiO particles/μm 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 10 (using pyrene as analyte; excitation wavelength, 632.8 nm). The applicability of Ag@SiO 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@SiO 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.

Surface-enhanced Raman scattering (SERS) is observed solitarily for analytes that are placed in the vicinity of plasmonic nanoparticles since the amplitude of the electric field on their surface decays with distance. Taking this idea forward, we have designed core−shell plasmonic systems for SERS sensing, consisting of silver nanoparticles coated with mesoporous silica (Ag@m-SiO 2 ) having an average pore size of 2.4 nm. Studies presented herein show that a Ag nanoparticle core of ∼55 nm and an m-SiO 2 shell of ∼40 nm represent a preferred combination for sieving and sensing, established by following the SERS of a standard marker, namely, rhodamine 6G. However, under identical conditions, Ag nanoparticles capped with microporous silica (Ag@SiO 2 ) inhibit the passage of analyte molecules into the plasmonic field. Yet another level of selectivity is provided by the negative surface charge of Ag@m-SiO 2 (ζ = −33 mV), eliminating negatively charged molecules from SERS sensing due to strong electrostatic repulsion. These aspects are confirmed using pyrene molecules, which are neutral, and pyrene derivatives carrying positive and negative charges. Thus, the SERS signal arises only from the neutral and positively charged molecules, which can penetrate into the pores, and not from the negatively charged analytes. The practical application of Ag@m-SiO 2 having a shell thickness of ∼40 nm for SERS sensing has been established using two commonly used organophosphorus pesticides (quinalphos and triazophos) directly from various vegetable matrices after the removal of plant pigments. The mesoporous silica shell of Ag@m-SiO 2 having a thickness of ∼40 nm sieves large molecules such as proteins and keeps them away from the electric field generated by the Ag nanoparticle, thus enabling the sensing of small molecules such as pesticides that penetrate into the shell.

Assembling plasmonic nanoparticles on metal films is an elegant method for the design of SERS platforms with dense hot spots and amplified electric fields. Interaction between the localized surface plasmons of metal nanoparticles within assemblies induces collective plasmonic modes which can further couple with the propagating surface plasmons prevailing on the metal films. Herein, we report on the electric field effects as well as Raman signal enhancements arising due to the sandwiching of Au and Ag core−shell nanoparticle assemblies on Au films with varying thicknesses of the underlying metal film. The sandwich plasmonic platforms are prepared by linking Ag@SiO 2 as well as Au@SiO 2 nanoparticles on Au films using (3-mercaptopropyl)trimethoxysilane (3-MPTS). The interaction between the SiO 2 shell on the Ag/Au nanoparticles and free silanol groups on 3-MPTS provides a monolayer of core−shell systems on the Au films, as corroborated by SEM images. Finite-difference time-domain simulations with heptamer models of Ag@SiO 2 and Au@SiO 2 particles on Au films confirm an enhancement in the electric field upon sandwiching the nanoparticle aggregates on the Au films. The Raman signal enhancement factors for the dye Rhodamine 6G are estimated, and the enhancement in the Raman signal intensities on Ag@SiO 2 over Au@SiO 2 assembled on a 20 nm Au film is attributed to the higher Q-factor of Ag. The largest measured Raman signal intensity on Ag@SiO 2 on a 60 nm thick Au film, ∼10 7 , is reasoned based on the variation of the electric field intensity of the Au film as a function of its thickness.

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