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

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(2)) 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.

Nanoscale pH Profile at a Solution/Solid Interface by Chemically Modified Tip-Enhanced Raman Scattering

“…[40][41][42][43][44][45] For pMBA, a -COOH group transforms to a -COO -moiety when an acidic solution becomes alkaline. For pATP, in an alkaline solution, two pATP molecules react each other to form an azo (N=N) dimer, i.e., 4,4′-dimercaptoazobenzene (DMAB) while a pATP molecule is stable in acidic environments.…”

Coordination chemistry of 3- and 4-mercaptobenzoate ligands: Versatile hydrogen-bonding isomers of the thiosalicylate (2-mercaptobenzoate) ligand

“…Ph3SnOC(O)C6H4SH-4; analysis of this complex by 13 C and 119 Sn NMR spectroscopies was used to provide evidence that the complex exists in a monomeric form in solution. [25] The bis(triphenyltin) derivative of 3MBA, [Ph3SnOC(O)C6H4SSnPh3-3], has also been prepared by reaction of H23MBA with Ph3SnCl and NaOEt in benzene.…”

“…These functional silver nanoparticles were successfully taken up by living J774A.1 macrophage cells, permitting determination of the pH of the local environment during endocytosis. [119] As previously described for metal chalcogenide materials, MBA-functionalised metal nanoparticles can also be coupled through the carboxylic acid moiety onto other materials. For example, raspberry-like gold microspheres have been derivatised with 4MBA, and the consequent particles then immobilised on an indium tin oxide (ITO) surface that had been functionalised with amine groups by derivatisation with H2N(CH2)3Si(OMe)3, with the interaction of the carboxylic acid and amine groups providing the immobilisation pathway.…”

Endoscopic sensing of alveolar pH

“…1). The 1.2 mm wide optrode operates using 150 nm gold nanoshells deposited on the distal-end of the optrode, functionalized with para-mercaptobenzoic acid (p-MBA), a molecule that has been well characterized as being sensitive to pH changes in the physiological range [11][12][13]. The pH response of the p-MBA molecule can be interrogated by Fig.…”