Surface-Enhanced Raman
Scattering Detection of pH
with Silica-Encapsulated 4‑Mercaptobenzoic Acid-Functionalized
Silver Nanoparticles
- Publication date
- Publisher
Abstract
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@SiO<sub>2</sub>) 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